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OPTO ELECTRONI CS 500
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IMTS (ISO 9001-2008 Internationally Certified) OPTO ELCTRONICS
OPTO ELCTRONICS
CONTENT
OPTO ELECTRONICS UNIT – I
01-16
GLASS FIBER FABRICATION
UNIT 2
17-26
OPTICAL SOURCE AND TRANSMITTER CIRCUITS
UNIT 3
27-35
THEORY OF SOLID STATE PHOTODIODE STATISTICAL VIEW POINT OF OPTICAL DETECTION. UNIT – IV
36-51
PHOTONIC SWITCHING
UNIT-V
52-108
INTRODUCTION TO SATELLITE COMMUNICATION
OPTO ELECTRONICS
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UNIT – I GLASS FIBER FABRICATION
Glass is made by fusing mixtures of metal oxides, sulfides. Pr semodes. The resulting material is a randomly connected molecular network rather than a well defined ordered structure as found in crystalline materials. A consequence of this random order is that glasses do not have well-defined melting points. When glass is heated up from room temperature, it remains a hard solid up to several hundred degrees centigrade. As the temperature increases further, the glass gradually begins to soften until at very high temperatures it becomes a vicious liquid.
The expression “melting temperature” is
commonly used in glass manufacture. This term refers only to an extended temperature range in which the glass becomes fluid enough to free itself fairly quickly of gas bubbles.
The largest category
of optically transparent glasses from which optical fibers
are made consists of the oxide glasses. Of these, the most common is silica (SiO2), which has a refractive index of 1.458 at 850nm. To produce two similar materials that have slightly different indices of refraction for the core and cladding, either fluorine or various oxides (referred to as dopants), such as B2O3, GeO2, or P2O5, are added to the silica. As shown in Fig. 2-26 the addition of GeO2 or P2O5 increases the refractive index, whereas
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doping the silica with fluorine or B2O3 decreases it. Since the cladding must have a lower index than the core, examples of fiber compositions are 1. GeO2 – SiO2 core; SiO2 cladding 2. P2O5 – SiO2 core; SiO2 cladding 3. SiO2 core; B2 O3 –SiO2 cladding 4. GeO2 – B2O3 –SiO2 core; B2O3 –SiO2 cladding Here, the notation GeO2 –SiO2 – SiO2, for example denotes a GeO2 – doped silica glass. The principal raw material for silica is sand. Glass composed or pure silica is referred to as either silica glass, fused silica, or vitreous silica. Some of its desirable properties are a resistance to deformation at temperatures as high as 1000C, a high resistance to breakage from thermal shock because of its low thermal expansion, good chemical durability, and high transparency in both the visible and infrared regions of interest to fiber optic communication systems.
Its high melting temperature is a
disadvantage if the glass is prepared from a molten state. However. This problem is partially avoided when using vapor deposition techniques.
Halide Glass Fibers
In 1975 researchers at the Universite de Rennes discovered fluoride glasses that have extremely low transmission losses at mid-infrared wavelengths (0.2-8µm, with the lowest loss being around 2.55µm). Fluoride glasses belong to a general family of halide glasses in which the anions are from elements in group VII of the periodic table, namely fluorine, chlorine, bromine, and iodine.
The material that researchers have concentrated on is a heavy metal fluoride glass, which uses ZrF4 as the major component and glass network former. Several other constituents need to be added to make a glass that has moderate resistance to crystallization lists the constituents and their molecular percentages of a particular fluoride glass referred to as ZBLAN (after its elements ZrF4 BaF2, LaF3, AlF3 and NaF).
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OPTO ELECTRONICS This material forms the core of a glass fiber. To make a lower – refractive –index glass, one partially replaces ZrF4 by HaF4 to get a ZHBLAN cladding. Although these glasses potentially offer intrinsic minimum losses of 0.01 -0.001 dB/km, fabricating long lengths of these fibers is difficult. First, ultra pure materials must be used to reach this low loss level. Second, fluoride glass is prone to diversification. Fiber-making techniques have to take this into account to avoid the formation of microcrystallines, which have a drastic effect on scattering losses.
Active Glass Fibers
Incorporating rare-earth elements (atomic numbers 57-71) into a normally passive glass gives the resulting material new optical and magnetic properties. These new properties allow the material to perform amplification, attenuation, and phase retardation on the light passing through it58-60. Doping can be carried out for both silica and halide glasses.
Two commonly used materials for fiber lasers are erbium and neodymium. The ionic concentrations of the rare-earth elements are low (on the order of 0.005 -0.05 mole percent) to avoid clustering effects. By examining the absorption and fluorescence spectra of these materials, one can use an optical source which emits at tan absorption wavelength to excite electrons to higher energy levels in the rare-earth dopants. When these excited electrons drop to lower energy levels, they emit light in a narrow optical spectrum at the fluorescence wavelength. Chapter 11 discusses the applications of erbium – doped fibers to optical amplifiers.
Chalgenide Glass fibers
In addition to allowing the creation of optical amplifers, the nonlinear properties of glass fibers can be exploited for other applications, such as all-optical switches and fiber lasers. Chalgenide glass is one candidate for these uses because of its high optical
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OPTO ELECTRONICS nonlinearity and its long interaction length.61,
4 62
These glasses contain at least one
chalogen element (S, Se, or Te) and typically one other element such as P, I, Cl, Br, Cd, Ba, Si, or Tl for tailoring the thermal, mechanical, and optical properties of the glass. Among the various chalgenide glasses, As2S3 is one of the most well-known materials. Single –mode fibers have been made using As40S58Se2 and As2S3 for the core and cladding materials, respectively. Losses in these glasses typically range around 1 dB/m.
DIRECT MELTING
Silica, chalgenide, and halide glass fibers can all be made using a direct-melt double-crucible technique8.57. In this method, glass rods for the core and cladding materials are first made separately by melting, mixtures of purified powders. To make the appropriate glass composition. These rods are then used as feedstock for each of two concentric crucibles, the inner crucible contains molten core glass and the outer one contains the cladding glass. The fibers are drawn from the molten state through orifices in the bottom of the two concentric crucibles in a continuous production process.
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Although this method has the advantage of being a continuous process, careful attention must be paid to avoid contaminants during the melting. The main sources of contamination arise from the furnace environment and from the crucible. Silica crucibles are normally used in preparing the glass feed rods, whereas the double concentric crucibles used in the drawing furnace are made from platinum. A detailed description of the crucible design and an analysis of the fiber-drawing process is given by Midwinter.
High purity silica fibers by vapor deposition
The OVPO process described in is a lateral deposition method. Another OVPOtype process is the vapor-phase axial deposition (V AD) method,76,77 illustrated in. In this method, the Si02 particles are formed in the same way as described in the OVPO process. As these particles emerge from the torches, they are deposited onto the end surface of a silica glass rod which acts as a seed. A porous preform is grown in the axial. Direction by moving the rod upward. The rod is also continuously rotated to maintain cylindrical symmetry of the particle deposition. As the porous preform moves upward, it is transformed into a solid, transparent rod preform by zone melting (heating in a narrow localized zone) with the carbon ring heater the resultant preform can then be drawn into a fiber by heating it in another furnace. Both step- and graded-index fibers in either multimode or single-mode varieties can be made by the VAD method. The advantages of the VAD method are (I) the preform has no central hole as occurs with the OVPO process, (2) the preform can be fabricated in continuous lengths which can affect process costs and product yields; and (3) the fact that the deposition chamber and the zone-melting ring heater are tightly connected to each other in the same enclosure allows the achievement of a clean environment.
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CABLE DESIGN In any practical application of optical waveguide technology, the fibers need to incorporated in some type of cable structure.
95-98
The cable structure will vary greatly,
depending on whether the cable is to be pulled into underground intra building ducts, buried directly in the ground, installed on outdoor poles or submerged under water. Different cable designs are required for each type of application, but certain fundamental cable design principles will apply in every case. The objectives of cable manufacturers have been that the optical fiber cables should be installable with the same equipment, installation techniques, and precautions as those used for conventional wire cables. This requires special cable designs because of the mechanical properties of glass fibers.
One important mechanical property is the maximum allowable axial load or; the cable, since this factor determines the length of cable that can be reliable installed. In copper cables the wires themselves are generally the principal load bearing members of
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OPTO ELECTRONICS the cable, and elongations of more than 20 percent a possible without fracture. On the other hand, extremely strong optical fibers tend to break at 4-percent elongation, whereas typical good-quality fibers exhibit long-length breaking elongations of about 0.5-1.0 percent. Since static fatigue occurs very quickly at stress levels above 40 percent of the permissible elongation and very slowly below 20 percent, fiber elongations during cable manufacture and installation should be limited to 0.1-0.2 percent. Steel wire which has a Young's modulus of 2 x 104 MPa has been extensively used for reinforcing conventional electric cables and can also be employed :or optical fiber cables. For some applications it is desirable to use nonmetallic constructions, either to avoid the effects of electromagnetic induction or to reduce cable weight. In this case, plastic strength members and high-tensile-strength organic yarns such as Kevlar (a product of the DuPont Chemical Corporation) are used. With good fabrication practices, the optical fibers are isolated from other cable components, they are kept close to the neutral axis of the cable, and room is provided for the fibers to move when the cable is flexed or stretched.
Another factor to consider is fiber brittleness. Since glass fibers do not deform plastically, they have a low tolerance for absorbing energy from impact loads. Hence, the outer sheath of an optical cable must be designed to protect the glass fibers inside from impact forces. In addition, the outer sheath should not crush when subjected to side forces, and it should provide protection from corrosive environmental elements. In underground installations, a heavy-gauge-metal outer sleeve may also be required to protect against potential damage from burrowing rodents, such as gophers.
In designing optical fiber cables, several types of fiber arrangements are possible and a large variety of components could be included in the construction. The simplest designs are one- or two-fiber cables intended for indoor use. In the, hypothetical twofiber design, a fiber is first coated with a, buffer material and placed loosely in a tough, oriented polymer tube, such as polyethylene. For strength purposes this tube is surrounded by strands of aramid yarn am and, in turn, is encapsulated in a polyurethane
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OPTO ELECTRONICS jacket. A final outer jacket of polyurethane, polyethylene, or nylon binds the two encapsulated fiber units together.
Larger cables can be created by stranding several basic fiber building blocks around a central strength member. This is illustrated in Fig 2-36 for a six-fiber cable. The fiber units are bound onto the strength member, with paper or plastic binding tape, and then surrounded by an outer jacket. If repeaters are required along the route where the cable is to be installed, it may be advantageous to include wires within the cable structure for powering these repeaters. The wires can also be used for fault isolation or as an engineering order wire for voice communications during cable installations.
SPLICING METHODS
A fiber splice is a permanent or semipermanent joint between two fibers. These typically used to create long optical links or in situations where frequent connection and disconnection are not needed. In making and evaluating such splices, must take into
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OPTO ELECTRONICS account the geometrical differences in the two fibers, fiber alignments at the joint, and the mechanical strength of the splice. This section fir addresses general splicing methods and then examines the factors contributing loss when splicing single-mode fibers.
Splicing Techniques Fiber splicing techniques include the fusion splice, the V-groove mechanical splice, and the elastic-tube splice.58,59 The first technique yields a permanent joint, whereas the other two types of splices can be disassembled if necessary.
Fusion splices are made by thermally bonding together prepared fiber ends pictured in. In this method, the fiber ends are first prealigned and butted together. This is done either in a grooved fiber holder of under a microscope with micromanipulators. The butt joint is then heated with an electric arc or a laser pulse so that the fiber ends are momentarily melted and hence bonded together. This technique can produce very low splice losses (typically averaging less than 0.06 dB). However, care must be exercised in this technique, since surface damage due to handling, surface defect growth created during heating, and residual stresses induced near the joint as a result of changes in chemical composition arising from the material melting can produce a weak splice60, 61
In the V -groove splice technique, the prepared fiber ends are first butted together in a V-shaped groove,. They are then bonded together with an adhesive or are held in place by means of a cover plate. The V-shaped channel can be either a grooved silicon, plastic, ceramic, or metal substrate. The splice loss in this method depends strongly on the fiber size (outside dimensions and core-diameter variations) and eccentricity (the position of the core relative to the center of the fiber).
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The elastic-tube splice shown cross-sectionally is a unique device that automatically performs lateral, longitudinal, and angular alignment. It splices multimode fibers to give losses in the same range as commercial fusion splices, but much less equipment and skill are needed. The splice mechanism is basically a tube made of an elastic material. The central hole diameter is slightly smaller than that of the fiber to be spliced and is tapered on each end for easy fiber insertion. When a fiber is inserted, it expands the hole diameter so that the elastic material exerts a symmetrical force on the fiber. This symmetry feature allows an accurate and automatic alignment of the axes of the two fibers to be joined. A wide range of fiber diameters can be inserted into the elastic tube. Thus, the fibers to be spliced do not have to be equal in diameter, since each fiber moves into position independently relative to the tube axis.
Splicing Single –Mode Fibers
As is the case in multimode fibers, in single-mode fibers the lateral (axial) offset loss presents the most serious misalignment. This loss depends on the shape of the propagating mode. For gaussian –shaped beams the loss between identical fibers is62.
LSM;lat = -10log exp [- ( ) ]d W
2
Where the spot size W is the mode-field radius defined in Eq. (2-74), and d is the lateral displacement. Since the spot size is only a few micrometers in single-mode fibers, lowloss coupling requires a very high degree of mechanical precision in the axial dimension.
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OPTICAL FIBER CONNECTORS
A wide variety of optical fiber connectors have evolved for numerous different applications. Their uses range from simple single-channel fiber-to-fiber connectors in a being location to multi channel connectors used in harsh military field environments. Some of the principal requirements of a good connector design are as follows:
1. Low coupling losses. The connector assembly must maintain stringent alignment tolerances to assure low mating losses.
These low losses must not change
significantly during operation or after numerous connects and disconnects. 2. Interchangeability. Connectors of the same type must be compatible from one manufacturer to another. 3. Ease of assembly. A service technician should readily be able to install the connector in a field environment; that it, in a location other than the connector factory. The connector loss should also be fairly insensitive to the assembly skill of the technician. 4. Low environmental sensitivity. Conditions such as temperature, dust, and moisture should have a small effect on connector – loss variations. 5. Low-cost and reliable construction. The connector must have a precision suitable to the application, but its cost must not be a major factor in the fiber system. 6. Ease of connection. Generally, one should be able to mate and demate the connector, simply, by hand.
Connector Types
Connectors are available in screw-on, bayonet-mount, and push-pull configurations. These include both single-channel and multichannel assemblies for cable-tocable and for cable-to-circuit card connections. The basic coupling mechanisms used in these connectors belong to either the butt-joint or the expanded-beam classes.
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Butt-joint connectors employ a metal, ceramic, or molded-plastic ferrule for each fiber and a precision sleeve into which the ferrule fit. The fiber is epoxied into a precision hole which has been drilled into the ferrule. The mechanical challenges of ferrule connectors include maintaining both the dimensions of the hole diameter and its position relative to the ferrule outer surface.
Two popular butt-joint alignment designs used in both multimode and singlemode fiber systems. These are the straight-sleeve and the tapered-sleeve (or biconical) mechanisms. In' the straight-sleeve connector, the length of the sleeve and a guide ring on the ferrules determine the end separation of the fibers. The biconical connector uses a tapered sleeve to accept and guide tapered ferrules. Again, the sleeve length and the guide rings maintain a given fiber-end separation.
An expanded-beam connector, employs lenses on the ends of the fibers. These lenses either collimate the light emerging from the transmitting fiber, or focus the expanded beam onto the core of the receiving fiber. The fiber-to-lens distance is equal to the focal length of the lens. The advantage of this scheme is that, since the beam is collimated, separation of the fiber ends may take place within the connector. Thus, the connector is less dependent on lateral alignments. In addition, optical processing elements, such as beam splitters and switches, can easily be inserted into the expanded beam between the fiber ends.
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OPTO ELECTRONICS FIBER MEASUREMENT ATTENUATION
Attenuation of a light signal as it propagates along a fiber is an important consideration in the design of an optical communication system, since it plays a major role in determining the maximum transmission distance between a transmitter and a receiver or an in-line amplifier. The basic attenuation mechanisms in a fiber are absorption, scattering, and radiative losses of the optical energy.1-5 Absorption is related to the fiber material, whereas scattering is associated both with the fiber material and with structural imperfections in the optical waveguide. Attenuation owing to radiative effects originates from perturbations (both microscopic and macroscopic) of the fiber geometry.
In this section we shall first discuss the units in which fiber losses are measured and then present the physical phenomena giving rise to attenuation
DISPERSION
In the design of single-mode fibers, dispersion behavior is a major distinguishing feature, since this is what limits long-distance and very high-speed transmission. Comparing we see that whereas the dispersion of a single mode silica fiber is lowest at 1300 nm, its attenuation is a minimum at 1550 nm, where the dispersion is higher. Ideally, for achieving a maximum transmission distance of a high-capacity link, the dispersion null should be at the wavelength of minimum attenuation. To achieve this, one can adjust the basic fiber parameters to shift the zero-dispersion minimum to longer wavelengths
The basic material dispersion is hard to alter significantly, but it is possible to modify the waveguide dispersion by changing from a simple step-index core profile design to more complicated index profiles. Researchers have thus examined a variety of core and cladding refractive-index configurations for altering the behavior of single-mode fibers. representative refractive-index profiles of die four main categories: l300-nm-
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OPTO ELECTRONICS optimized fibers, dispersion-shifted fibers, dispersion-flattened fibers, and largeeffective-core-area fibers. To get a better feeling of their geometry, the three dimensional index profiles for several different types of single-mode fibers.
The most popular single-mode fibers used in telecommunication networks are near-step-index fibers, which are dispersion-optimized for operation at 1300 nm. These 1300-nm-optimized single-mode fibers are of either the matched cladding or the depressed-cladding design. Matched-cladding fibers have a uniform refractive index throughout the cladding. Typical mode-field diameters are 9.5 j1-m and core-to cladding index differences are around 0.37 percent. In depressed-cladding fibers the cladding portion next to the core has a lower index than the outer cladding region. Mode-field diameters are around 9Âľm, and typical positive and negative index differences are 0.25 and 0.12 percent, respectively.
As we saw from Eqs. (3-20) and (3-26), whereas material dispersion depends only on the composition of the material, waveguide dispersion is a function of the core radius, the refractive-index difference, and the shape of the refractive-index profile. Thus, the waveguide dispersion can vary dramatically with the fiber design parameters. By creating a fiber with a larger negative waveguide dispersion and assuming the same values for material dispersion as in a standard single-mode fiber, the addition of waveguide and material dispersion can then shift the zero dispersion point to longer. wavelengths. The resulting optical fibers are known as dispersion-shifted fibers. Examples of refractiveindex profiles for dispersion-shifted fibers. A typical waveguide dispersion curve for this type of fiber is depicted. The resultant total dispersion curve is for fibers with a zerodispersion wavelength at 1550 nm.
An alternative is to reduce fiber dispersion by spreading the dispersion minimum out over a wider range. This approach is known as dispersion jlattening. Dispersionflattened fibers are more complex to design than dis- persion-shifted fibers, because dispersion must be considered over a much broader range of wavelengths. However, they offer desirable characteristics over a wide span of wavelengths. Typical cross-sectional
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OPTO ELECTRONICS and three-dimensional refractive-index profiles, respectively. A typical waveguide dispersion curve for this type of fiber is depicted gives the resultant· total dispersionflattened characteristic.
The advent of optical fiber amplifiers for operation in the 1550-nm region the accompanying demand for long-distance high-capacity links led to the development of a single-mode optical fiber with a larger effective core area. The impetus for larger core areas is the need to reduce the effects of fiber nonlinearities, which limit system capacities, gives two examples of the index profile for these large-effective area (LEA) fibers. Whereas standard single-mode fibers have effective core areas of about 55 µm2, these profiles yield values greater than 100 µm2
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UNIT 2
OPTICAL SOURCE AND TRANSMITTER CIRCUITS INTRODUCTION The principal light sources used for fiber optic communications applications are heterojunction – structured semiconductor laser diodes (also referred to as injection laser diodes or ILDs) and light – emitting diodes (LEDs). A heterojunciton consists of two adjoining semiconductor materials with different band-gap energies. These devices are suitable for fiber transmission systems because they have a high efficiency, and their dimensional characteristics are compatible with those of the optical fiber. Comprehensive treatments of the major aspects of LEDs and laser diodes are presented in various books.1-6 Review articles and book chapters covering the operating principles of these devices are also available, 7-12 and the reader is referred to these for details.
The intent of this chapter is to give an overview of the pertinent characteristic of fiber-compatible luminescent sources. The first section discusses semiconductor material fundamentals that are relevant to light source operation. The next two sections present the output and operating characteristics of LEDs and laser diodes, respectively. These are followed by sections discussing the temperature responses of optical sources, their linearity characteristics, and their reliability under various operating conditions.
We shall see in this chapter that the light-emitting region of both LEDs and laser diodes consists of a pn junction constructed of direct-band-gap III-V semi conductor materials. When this junction is forward biased, electrons and holes are injected into the p and n regions, respectively. These injected minority carriers can recombine either radiatively, in which case a photon of energy hυ is emitted, or nonradiatively, whereupon the recombination energy is dissipated in the form of heat. This pn junction is thus known as the active or recombination region.
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OPTO ELECTRONICS A major difference between LEDs and laser diodes is that the optical output from an LED is incoherent, whereas that from a laser diode is choherent. In a optical energy released from this cavity has spatial and temporal coherence, which means it is highly monochromatic and the output beam is very directional. In an incoherent LED source, no optical cavity exists for wavelength selectivity. The output radiation has a broad spectral width, since the emitted photon energies range over the energy distribution of the recombining electrons and holes, which usually lie between 1 and 2kBT (kB is Boltzmann’s constant and T is the absolute temperature at the pn junction). In addition, the incoherent optical energy is emitted into a hemisphere according to a cosine power distribution and thus has a large beam divergence.
In choosing an optical source compatible with the optical waveguide, various characteristics of the fiber, such as its geometry, its attenuation as a function of wavelength, its group delay distortion (bandwidth), and its modal characteristics, must be taken into account. The interplay of these factors with the optical source power, spectral width, radiation pattern, and modulation capability needs to be considered. The spatially directed coherent optical output from a laser diode can be coupled into either single-mode or multimode fibers. In general, LEDs are used with multimode fibers, since normally it is only into a multimode fiber that the incoherent optical power from an LED can be coupled in sufficient quantities to be useful. However, LEDs have been employed in high-speed local-area applications in which one wants to transmit several wavelengths on the same fiber. Here, a technique called spectral slicing is used.13-15 This entails using a passive device such as a waveguide grating array (see Chap. 10). To split the broad spectral emission of the LED into narrow spectral slices. Since these slices are each centered at a different wavelength, they can be individually modulated externally with independent data streams and simultaneously sent on the same fiber.
LIGHT-EMITTING DIODES (LEDs)
For optical communication systems requiring bit rates less than approximately 100-200Mb/s together with multimode fiber-coupled optical power in the tens of
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microwatts, semiconductor light-emitting diodes (LEDs) are usually the best light source choice. These LEDs require less complex drive circuitry than laser diodes since no thermal or optical stabilization circuits are needed (see Sec. 4.3.6), and they can be fabricated less expensively with higher yields.
The edge emitter depicted consists of an active junction region, which is the source of the incoherent light, and two guiding layers. The guiding layers both have a refractive index which is lower than that of the active region but higher than the intex of the surrounding material. This structure forms a waveguide channel that directs the optical radiation toward the fiber core. To match the typical fiber-core diameters. ( 5010/am), the contact stripes for the edge emitter are 50-70/am wide, Lengths of the active regions usually range from 100 to 150/am. The emission pattern of the edge emitter is more directional than that of the surface emitter, In the plane parallel to the junction, where there is no waveguide effect, the emitted beam is Lambert Ian (varying as cos )with a half-power width of
=120 .in the plane perpendicular to the junction, the half –
power been made as small as 25-35 by a proper choice of the waveguide thickness.
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OPTO ELECTRONICS LASER DIODES
Laser come in many forms with dimensions ranging from the size of a grain of salt to one that will occupy an entire room, The lasing medium can be a gas, a liquid, an insulating crystal (solid state), or a semiconductor. Laser diodes. They are similar to other lasers, such as the conventional solid-state and gas lasers, in that the emitted radiation has spatial. And temporal coherence: that is the output radiation is highly monochromatic and the light beam is very directional
Despite their differences, the basic principle of operation is the same for each type of laser. Laser action is the result of three key processes: photon absorption, spontaneous emission, and stimulated emission, These three processes are represented by the simple two-energy –level diagrams in, where Ei is the ground –state energy and E2 Is the excited-state energy. According to planck” s law , a transition between these two states involves the absorption or emission of a photon of energy hv12 =E2-E1. Normally, the system is in the group state. When a photon of energy and be excited to state e2,as shown in since this is an unstable state, the electron will shortly return to the ground state, thereby emitting a photon of en energy hv12.This occurs without any external stimulation and is called spontaneous emission. These emissions are isotropic and of random phase, and thus appear as a narrowband Gaussian output.
The electron can also be induced to make a downward transition from the excited level to the ground-state level by an external stimulation if a photon of energy hv12 impinges on the system while the electron is still in its excited state, the electron is immediately stimulated to drop to the ground state and give off a photon of energy hv12. This emitted photon is in phase with the incident photon, and the resultant emission is known as stimulated emission.
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OPTO ELECTRONICS In thermal equilibrium the density of excited electrons is very small. Most photons incident on the system will therefore be absorbed, so that stimulated emission is essentially negligible. Stimulated emission will exceed absorption only if the population of the excited states is greater than that of the ground state. This condition is known as population inversion. Since this is not an equilibrium condition, population inversion is achieved by various “pumping” techniques. In a semiconductor laser, population inversion is accomplished by injection electrons into the device contacts to fill the lower energy states of the conduction band.
These can conveniently be separated into two independent sets of transeverse electric (TE) and transverse magnetic (TM) modes. Each set of modes can be described in terms of the longitudinal, lateral, and transverse half-sinusoidal variations of the electromagnetic fields along the major axes of the cavity. The longitudinal modes are related to the length L of the cavity and determine the principal structure of the frequency spectrum of the emitted optical radiation, Since L is much larger than the lasign wavelength of approximately 1 µm, many longitudinal modes can exist. Lateral modes lie in the plane of the pn junction. These modes depend on the side well preparation and the width of the cavity, and determine the shape of the lateral profile of the laser beam. Transverse modes are associated with the electromagnetic field and beam profile in the direction perpendicular to the plane of the pn junction. These modes are of great importance, since they largely determine such laser characteristics are the radiation pattern (the angular distribution of the optical output power) and the threshold current density.
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Developments in Laser Diode Structures For Photonic Systems Apart from the availability of low loss fibers, low threshold and long life laser diodes were very much responsible for the onset of optical communication systems. Laser diode structures which were developed in the initial stages largely resemble traditional semiconductor bulk devices. These devices produced an optical output whose properties differed from the spectral properties of analogous electrical sources with respect to line width and coherence. So much so, well known techniques in communication like heterodyne detection, FDM and coherent systems were not possible with optical systems. It was also a fact that optical communication system technology in the early days had not matured enough for these techniques. However, with the demand and necessity for high bit rate and long haul systems there has been a need to improvise LD structures which have narrow line width and high coherence and accordingly accept extremely large modulation bandwidths. Achieving narrow linewidth and tenability at such high carrier frequencies (1014 Hz) involves extremely sophisticated techniques and very high dimensional tolerances in device fabrication process. These devices take care of both material parameters and structural parameters to overcome several factors which contribute to the degradation of optical output in conventional devices. Distributed Feed Back(DFB) lasers, Distributed Bragg Reflection(DBR)lasers, Multiple Quantum Well(MQW) lasers, External
Cavity(EC) lasers are prominent among many such
developments. Accordingly, we emphasise on the principle behind these devices which are an integral part of the high speed, high performance lightwave systems, it must be noted that these devices are alse being increasingly used in photonic switching, optical computing and other high speed optical storage applications.
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Limitations of Conventional Structures Semiconductor
LDs employ a double-hetero(DH) structure and a Fabry-Perot
resonant cavity for optical oscillation and carrier confinement. Active layer thickness is of the order of a few hundred nanometers. Fabrication technology used is Liquid phase Epitaxy(LPE )and Molecular Beam Epitaxy (MBE). The optical output of the devices will have more than one longitudinal mode spectrum. When high frequency modulation is applied onto these devise, they give rise to frequency chirps and mode partition noise. This leads to a broadening of the spectrum. Mode partition quite pronounced in direct intensity modulation. These effects, couples with fiber dispersion, limit the transmission bit rate capacity. Improvements in device structures are needed even when the devices are used in relatively short haul or low bit rate applications under situations where i.
Dense WDM/OFDM is required
ii.
Heterodyne detection is employed
iii.
Wavelength tunability is required
Hence conventional devise are not suitable for high performance communication systems.
DFB Lasers The gain difference in an LD is the difference between cavity gain and the cavity loss. The value of this among different longitudinal modes does not differ significantly in Fabry-Perot LDs. Hence, many longitudinal modes are supported which degrades the spectral purity of the optical output.
In a modified structure called a distributed
feedback(FD) LD, this gain difference is increased for a required mode with respect to the other modes, by introducing a frequency dependent loss. This loss is uniform for all modes in a conventional device and is mainly due to the end face mirrors. A corrugated structure called DF is incorporated into the multi-layer structure and lasing action is obtained from the Bragg reflectors (gratings) unlike the used of cleaved mirrors in FabryPerot diodes. Such corrugations introduce a periodic variation of refractive index. The corrugated layer acts as a distributed reflector.
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If the refractive index of the medium is n and Bragg wavelengths is b the period of corrugation will be = (b/2n)m where m corresponds to the order of diffraction. =b r r depends on n and groove depth of grating.
The diffraction grating is made of a material with higher bandgap than that of the active layer for low absorption. The corrugated gratings can be placed above or below the active layer. Only the mode nearest to /2n is reflected while other modes are suppressed. The end facets are anti-reflection (AR) coated. Although two modes located symmetrically on either side of Bragg wavelength are to be supported, the lack of exact similarly between the two end face cleavings supports one of the modes. Besides, this asymmetry is deliberately introduced in the AR coatings.
A DFB structure.
In a quarter wave shifted DFB LD, a phase shift of /2
corresponding to a path difference of /4 is introduced at the centre of the laser cavity. For this phase shift, the main mode corresponding to the Bragg wavelength will have maximum gain difference. The structures exhibit lower mode partition noise, higher mode stability, larger modulation speeds and narrower linewidths. Linewidths which could e as much as 100 MHz in conventional devices can be brought down to a few MHz and below.
Longer cavity lengths or the use of extended cavities help reduce the
linewidth. Longer the cavity, smaller the linewidth. By introducing frequency tunable loss elements like diffraction gratings and
electro optic/ acousto optic filters, it is
possible to make these devise tunable over a range of wave lengths.
To improve the tenability characteristics, multiple electrode DFB lasers are used. A larger current is applied to one electrode while a smaller current is applied to the other electrode. The region near the output port is pumped at current densities near or just
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above threshole density. This region serves as a Bragg reflector. Larger change in refractive index is possible at a lower current, which aids in wavelength tuning. The other region which is pumped at higher current, provides the gain. Tuning ranges of the order of 2-3 nm are reported with a line width of 15 MHz.
Quantum Well Structures The active region in DH structure LDs has a thickness of about 150nm.
This
thickness is adequate for carrier confinement. Nevertheless, the properties are same as in bulk material. When active layer thickness is reduced by using sophisticated fabrication technologies tosuch values which compare with the deBroglie wavelength of thermalised electrons, the electronic and optical properties undergo a drastic change. The energymomentum or the e-k diagram would be different and the density of states will be quantized. Active layer thickness could be reduced below 50 nm right upto 10 nm. Such structures are called quantum well structures [4.8].
Depending on the number of
dimensions in which thckness reduction is achieved, we have three structures-quantum wells, quantum wires and quantum dots(fig.4.8). Laser action is derived form stimulation electron hole recombination between discrete (QW) states.
The advantages of quantum structures compared to bulk active layer devices are
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Narrower linewidth, higher device efficency
Smaller threshold current
Higher differential quantum efficiency
Improved noise/dynamic performance
Tenability
Lower chirp, higher differential gain
The schematic of a quantum well along with its energy level diagram for holes and electrons. A thin layer, (<50 nm) of GaAs is surrounded by AIGaAs. The bandgap of GaAs is smaller than that of surrounding layers. Rectangular potential wells are formed in conduction and valence bands in which electrons and holes are conofined. Along the x direction, carriers are confined within a distance dx which is much smaller than dy and dz in y and z directions, respectively. Hence, along y and z directions the behavious is similar to that in bulk devices.
When multiple layered structures of different semiconductor materials are fabricated which are a grown alternatively, we have a multiple quantum well structure. Multiple Quantum Well (MQW) structures find application in both laser sources and detectors. The number of different layers can as much as 100. MQW structures can be grown by alternating layers of ASGaAs and GaAs. The gain of the MQW laser in which there are N layer is N times the gain each well could provide.
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UNIT – III
THEORY OF SOLID STATE PHOTODIODE STATISTICAL VIEW POINT OF OPTICAL DETECTION. The most common semiconductor photodetector is the photodiode, shown schematically in the device structure consists of p and n regions separated by a very lightly n-doped intrinsic (i) region. In normal operation a sufficiently large reverse – bias voltage is applied across the device so that the intrinsic region is fully depleted of carriers. That is, the intrinsic n and p carrier concentrations are negligibly small in comparison with the impurity concentration in this region.
When an incident photon has an energy greater than equal to the band-gap energy of the semiconductor material, the photon can give up its energy and excite an electron from the valance band to the conduction band. This process generates free electron-hole pairs, Which are know as photocarriers since they are photon-generated charge carriers. The photodetector is normally designed so that these carriers are generated mainly in the depletion region (the depleted intrinsic region where most of the incident light is a absorbed. The high electric and be collected across the reverse-biased junction. This gives rise to a current flow in an external circuit, with one electron flowing for every carrier pair generated. This current flow is known as the photocurrent.
As the charge carriers flow through the material, some electron-hole pairs will recombine and hence disappear. On the average, the charge carriers move a distance Ln or Lp for electrons and holes, respectively. This distance is known as the diffusion length. The time it takes for an electron or hole to recombine is known as the carrier lifetime and is represented by and p, respectively. The lifetimes and the diffusion lengths are related by the expressions Ln =(Dn n )1/2
and
LP = (DPp )1/2
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Where Dn and DP are the electron and hole diffusion coefficients (or constants), respectively, which are expressed in units of centimeters squared per second.
Optical radiation is a absorbed in the semiconductor material according to the exponential law P(x) = P0 (I – e-αs(λ)x) Here, as (λ) is the absorption coefficient at a wavelength λ, P0 is the incident optical power level, and P(x) is the optical power absorbed in a distance x.
The dependence of the optical absorption coefficient on wavelength is shown in Fig. 6-3 for several photodiode materials.
13
As the curves clearly show, αs depends
strongly on the wavelength. Thus, a particular semiconductor material can be used only over a limited wavelength range. The upper wavelength cutoff λc is determined by the band-gap energy Eg of the material. If Eg is expressed in units of electron (Ev), then λc is given in units of micrometers (µm) by λ c(µm) =
hc = Eg
1.24 Eg (eV)
The cutoff wavelength is about 1.06 am for Si and 1.6 am for Ge. For longer wavelength, the photon energy is not sufficient to excite an electron from the valance to the conduction band.
Example 6-1. A photodiode is constructed of GaAs, which has a band-gap energy of 1.43 Ev at 300 K. From Eq. (6.2), the long-wavelength cutoff is
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hc λc =
29
= Eg
= 869 nm (1.43eV) (1.6 x 10-19 J/ev)
This GaAs photodiode will not operate for photons of wavelength greater than 869 nm.
At the lower-wavelength end, the photoresponse cuts off as a result of the very values of αs at the shorter wavelengths. In this case, the photons are absorbed very close to the photodetector surface, where the recombination time of the generated electron – hole pairs is very short. The generated carriers thus recombine before they can be collected by the photodetector circuitry.
If the depletion region has a width w, then, from Eq. (6-1), the total power absorbed in the distance w is P (w) = P0 (1 – e-αs w) If we take into account a reflectivity Rf at the entrance face of the photodiode, then the primary photocurrent Ip resulting from the power absorption of Eq. (6-3), is given by q P0 (1-e- αs w) (1-Rf)
Ip = h
Where P0 is the optical power incident on the photodetector, q is the electron charges, and h is the photon energy.
Two important characteristics of a photodetector are its quantum efficiency and its response speed, These parameters depend on the material band gap, the operating wavelength, and the doping and thickness of the p, i , and n regions of the device. The
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quantum efficiency n is the number of the electron – hole carrier pairs generated per incident photon of energy h and is given by number of electron –hole pairs generated =
Ip / q =
Number of incident photons
P0 / h
Here, Ip is the average photocurrent generated by a steady – state average optical power P0 incident on the photodetector.
Avalanche Photodiode
Avalanche photodiodes (APDs) internally multiply the primary signal photocurrent before it enters the input circuitry of the following amplifier. This increases receiver sensitivity, since the photocurrent is multiplied before encountering the thermal noise associated with the receiver circuit. In order for carrier multiplication to take place, the photogene rated carriers must traverse a region where a very high electric field is present. In this high-field region, a photogenerated electron or hole can gain enough energy so that it ionizes bound electrons in the valence band upon colliding with them. This carrier multiplication mechanism is known as impact ionization. The newly created carriers are also accelerated by the high electric field, thus gaining enough energy to cause further impact ionization. This phenomenon is the avalanche effect. Below the diode breakdown voltage a finite total number of carriers are created, whereas above breakdown the number can be infinite.
A commonly used structure for achieving carrier multiplication with very little excess noise is the reach-through construction
7,13-16
shown in Fig. 6-5. The reach –
through avalanche photodiode (RAPD) is composed of a high- resistivity
p – type
material deposited as an epitaxial layer on a p+ (heavily doped p-type ) substrate. A ptype diffusion or ion implant is then made in the high-resistivity material, followed by the construction of an n+ (heavily doped n-type ) layer. For silicon, the dopants used to from
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these layers are normally boron and phosphorus , respectively, This configuration is referred to as p+πpn+reach-through structure. The π layer is basically an intrinsic material that inadvertently has some p doping because of imperfect purification . Describes more complex structures used for In GaAs APDs. The term “reach – through” arises from the photodiode operation. When a low reverse –bias voltage is applied, most of the potential drop is across the pn+ junction. The depletion layer widens with increasing bias until a certain voltage is reached at which the peak electric field at the pn+ junction is about 5-10 percent below that needed to cause avalanche breakdown. At this point, the depletion layer just “reaches through” to the nearly intrinsic π region.
In normal usage, RAPD is operated in the fully depleted mode. Light enters the device through the p+ region and is absorbed in the material, which acts as the collection region for the photogenerated carriers. Upon being absorbed, the photon gives up its energy, thereby creating electron – hole pairs, which are the separated by the electric field in the region. The photogenerated electrons drift through the region in the pn+ junction, where a high electric field exists. It is in this high-field region that carrier multiplication takes place. The average number of electron – hole pairs created by a carrier per unit distance traveled is called the ionization rate. Most materials exhibit different electron ionization rates α and hole ionization rates . Experimentally obtained values of α and for five different semiconductor materials The ratio k = / α of the two ionization rates is a measure of the photodetector performance. As we shall see in Sec.6.4, avalanche photodiodes constructed of materials in which one type of carrier largely dominates impact ionization exhibit low noise and large gain-bandwidth products. Of all the materials shown in Fig. 6-6, only silicon has a significant difference between electron and hole ionization rates. The multiplication M for all carriers generated in the photodiode is defined by IM M =
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Where IM is the average value of the total multiplied output current and Ip is the primary unmultiplied photocurrent defined.In practice, the avalanche mechanism is a statistical process, since not every carrier pair generated in the diode experiences the same multiplication. Thus, the measured value of M is expressed as an average quantity.
Receiver sensitivity and bit error rate
The main function of an optical receiver is to extract the optical signal from various noise disturbances and reconstruct the information correctly. An optical receiver has to satisfy many different requirements depending upon the particular application. These requirements are often conflicting and much attention needs to be paid toe the receiver design to evaluate many trade – offs that are involved. Some of the important requirements of an optical receiver are
High sensitivity.
Wide dynamic. Range.
Bit rate transparency.
Fast acquisition time.
Bit pattern independence.
All these requirements are not expected to be satisfied by a single receiver. Depending upon the application, some of the requirements are less critical than others, The transmitted power from the transmit end undergoes attenuation along the fiber cable path. In addition the signal also suffers other impairments such as dispersion, corruption by noise, crosstalk, etc. In digital communication, the received signal is sampled at the rate of dock frequency and in each pulse recovered a decision is made as to whether a bit “1” or “0” has been received according to whether the sampled voltage exceeds a preset threshold or not. The threshold setting is done according to a certain optimum procedure based on noise
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statistics. However because of the statistical nature of detection process, an error can occur. Let us define P(0/1) as the probability that an actual bit “1” is misdirected as “0” and similarly P(1/0) be the error due to transmitted “0” is detected as “1”. If p(1) and p(0) are the probabilities of occurrence of “1” and “0” in a data stream, the bit error rate is given by
BER= p(1) p (0/1) + p(0) p(1/0) If we assume that “1” and “0” occur with equal probability, (each equal to 1/2) the above equation reduces to 1 BER =
[P(0/1) + P (1/0)] 2
We have seen earlier that shot noise and thermal noise are two important noise sources which affect the receiver performance.
An incident optical power of p gives rise to a photocurrent I = RP. The shot noise associated with this be 2 = 2qIB and the thermal noise is 4kTB R If P1 and I1 represent the power and the total current associated with bit “1”, and P0 and I0 represent the power and total current associated with bit “0”, then the variance in the detected signal currents will be 1 and 0 and is given as
4kTB 1 = 2q11B + R
4kTB 0 = 2q10B +
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For an ideal on –off keying (OOK), no power is present during a “0” bit. In this case P0 = I0 = 0. However in practical cases there will be some power during the “0” bit period. The threshold current for decision – making assuming Gaussian noise statistics is set as 0 I1 + 1 I1 Ith = 0 + 1
The probabilities of occurrence of errors P(0/1) and P(1/0) are obtained from the following equations.
1
(I-I1)2
I 0 exp -
P (0/1) = 1 2
1 dI =
221
0 exp -
P (0/1) = 1 2
1 BER =
I1 – I0 erfc
2
12
Ith – I0
(I-Ith) erfc -
12
1 dI =
220
I
erfc 2
(I-I0)2
1
I1 - Ith
+ erfc
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4
02
Let a parameter Q be defined as (I1 – Ith) / 1 = (Ith – I0) / 0 = Q
1
exp (-Q2 / 2)
Q
BER =
erfc 2
= 2
Q2
For a given BER, Q and hence required optical power level can be obtained from the above equations. The value of Q is about 6 for a BER of 10-9.
Let us now illustrate how the performance of a practical receiver is limited by the noise source. In any time interval T, the number of electrons emitted by the photon counter, in responses to a known incident light level follows a passion.
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UNIT – IV
PHOTONIC SWITCHING In modern communication systems it is expected that together with narrow band information, high speed data and video will also be integrated in a common network. Optical transmission and switching systems are expected to be widely used in such a network. bit-rate independent future communication systems.
Any communication system is incomplete if one does not have proper switching devices to either route the information carrying signals or to control the communication system itself. Optical switching technology is highly promising and can be used for solving many of the problems encountered in electronic switching. It may be said that switching systems using photonic technologies will most likely provide the future high speed, broadband services, all the way to the customer’s premises.
In fiber optic communication, switching can be implemented in the electronic domain by converting the optical signal to electrical signal, switching electronically and converting back to the optical signal to electrical signal, switching electronically and converting back to the optical form for transmission purposes. However a more elegant way would be to use a photonic switch which handles the signal in the optical domain. It is hence seen that once photonic switches are easily made available, the need for a lot of signal processing which goes with electronic switching gets eliminated. With n increasingly large section of signals – either voice, video or date – being carried by optical fibers, there arises a necessity- both economic and otherwise- to achieve an all-optical path from the source node to the destination node without having intermediate optical-to- electrical (O/E) and electrical- to –optical (E/O) conversions.
The need to develop and establish the technology for photonic switching is driven by the demands on system bandwidth: the high bit rates needed place a severe demand on FOR MORE DETAILS VISIT US ON WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554621
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The switching of signals in the optical domain is achieved by photonic switching devices, typically two-input/two-output (2x2) devices, connected in a network so as to functionally emulate an N x N network. Sometimes, these networks consist of stages of switches and are referred to as multistage interconnection networks (MINs), a term commonly used in electronic switching. However, as we will soon see, at times it is not possible to classify photonic networks built of photonic switching elements as photonic switching architectures (PSAs).
Several devices have been proposed in recent years with the capacity to meet these goals. One proposed classification of these devices divides them into two classesthe relational devices and the logic devices. Relational devices establish a relation or mapping between the inputs and outputs of the device under external control. In the case of logical devices, the data controls the state of the device in such a way that a Boolean function is performed on the inputs.
Further, the control signal in a switch may either be electrical (the electro-optic effect) or optical (the opto-optic effect). Severe bottlenecks may occur if electronics and associated electronic processing would always limit the speed of the switching device. Optical self-routing. Where the control information for setting the state of the photonic switch is derived from the data being routed (i.e, the data pulls itself through to the correct output), removes these bottlenecks.
Photonic switching has the following limitations. Firstly, since the power required to achieve switching is the ratio of the switching energy to the switching time, it follows that a shorter switching time will require more power. Secondly, spontaneous switching may often occur due to the background thermal energy present in a device. To prevent
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OPTO ELECTRONICS such random transitions, therefore, the switching energy must be appreciably larger than the maximum random background energy. From an analysis by smith {10.1} to differentiate statistically between two states, each bit should be composed of at least 1000 photons, which constitutes an approximate lower bound of the switching energy to prevent spontaneous switching. Devices operate in this region in the pulsed mode rather than the continuous mode (since thermal energy cannot be removed faster than μw/μm2.).
Due to the development of integrated optics (IO), a host of IO-based photonic switching elements (IOBPSES) are available today, A review of some of these devices can be found in {10.2}. Novel switches (both passive and active), new concepts and new architectures continue to be reported, which combine a high switching efficiency with a low substrate area utilization. The basic features of photonic switching are given below.
Basic Switch Elements 1. Control mechanisms- electrically controlled, e.g. directional couplers based on the electro –optic effect: optically controlled, e.g. optical bistable devices. 2. Guided wave or free space types. 3. Bulk optic form or integrated optic form. 4. Input-output configuration- may be 1 x 1, 1 x 1, 2 x 2. Switch Arrays 1. Logic interconnection of basic switch elements leads to switches of higher order (larger number of inputs and outputs) 2. Architectures –include cross-point, tree, planar, benes, clos, etc. 3. Bit switching or packet switching 4. Space division, time division , wavelength division or frequency division switching 5. Centralised or distributed routing mechanisms
Desirable Properties 1. Low insertion loss 2. Low crosstalk/ high isolation
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3. Low switching voltage or low optical energy per change 4. Low switching speed/wide bandwidth 5. Good reliability and fault tolerance 6. Capability for highly integrated implementation
PHOTONIC SWITCHING ARCHITECTURE
An N X M PSA has N inputs and M outputs. Any interconnection request from the N inputs to the M outputs is called a permutation request – since it usually achieves the mapping between the N inputs and a permutation of the M outputs. Further, if this permutation request is an one-one and onto mapping from the set of inputs to the set of outputs, it is termed an access request since it define a legitimate access connection.
The setting up of the network to achieve a particular access request is called a Pass. An access request may be successfully completed in one or more passes. For instance, if the calls in progress have been set up in such a manner that some of the connections in the access request cannot be met, those connections are completed after the calls in progress are over, i.e, in the next pass. If any input can request for and establish a connection to any output in a network in one or more passes, the criterion of dynamic full access (DFA) is said to have been met. If there are Z switches in the network, and each switch is capable of assuming states, there can be, at the most, (i
=
Z-1
states, It is possible that some of the Fi connections
0 ….Z-1 ) may be identical since they imply that a particular access request is
possible with a different set of switches or with the same set of switches set to different states either of these is useful in the case of occurrence of faults. Assuming that the N inputs are denoted by a N-tuple,{i0 ,........i N-1}, the state of the network may be uniquely described by a (N + Z) tuple {P(O0…..Oz), So----SN-1},where P{00-----0Z)denotes a permutation of the outputs oi…..Oz and Si denotes the state of the ith switch.
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Characteristics of Photonic Switching Architectures SXR This is the ratio (in dB) of the signal power to the noise power due to crosstalk. Photonic switching devices usually have a finite extinction ratio. The extinction ratio is the ratio of the power measured as present at one output of the switching device to the power present at the same output port when the input power is switched.
Worst Case SXR
Let Lb be the insertion loss for a given device in the bar state, Le be the insertion loss for a given device in the cross state and Xx be the crosstalk (expressed in dB and assumed equal for the cross and bar states). If the signal path passes through Ne switches in the cross state and through Nb switches in the bar state, the total attenuation (in dB) would be given by L1 = Nb Lb + Nc Lc Thus, the total power that arrives in an output channel j is given by Pout = Pout – (NbLb + Nc Lc), where Pout is the power entering the input x (in dB).
The noise that enters a signal path is the total of the cross talk power at every switch in the optical signal path. The noise that enters the output j from any output k is given by Pnk = Nb, k Xx – Nc,k Lc where Nc.k is the number of switches in the cross state and Nb,k is the number of switches in the bar state through which the signal path passes. Thus the total noise in the output channel is given by
Pin
x
10-LeNe, k 10
Pjn = Xx
k=1
Nc, k
Converting back to describes, we obtain, x
10-LeNe, k 10
Pjn (dB) = Pn (dB) – 10log
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Nc, k
Therefore the worst case SXR is given by x
10-LeNe, k 10
k=1
Nc, k
SXR word case = Xx – NbLb – NcLc – 10 log
Insertion Loss
This loss, ILij, is the difference in the input power at the input I and the power output at output j. Usually this figure of merit does not convey much information since the insertion loss, in general, varies for different values of
I and J. Another
figure of merit, the worst case insertion loss, given by max
(ILij)
undesirable since it implies that signals of widely varying intensity and SXR are present at the output and that signal regeneration may be necessary for some of the signals.
Switching Time
Each switch has a transition time (Tt) during which it changes states, and a holding time (Ht) during which it allows data to pass through the switching fabric. This offers a means to classify switches based on the relative durations of their T r and Hr- the range includes the high Tr /low Hr opto-mechanical
switches(as are used in submarine
repeaters and maybe switched once in several years if a fault occurs), medium T1/medium Hr electro-optic directional couplers(10-6 s) to low tr/low Hr opto-switches(10-12 s).
Fault Tolerance
The fault tolerance of an architecture is defined as the minimum number of switches that must be faulty for dynamic full access connectivity criteriono to be destroyed. Since this figure may differ for different architectures, we use the normalized
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fault tolerenace, defined as the ratio of the minimum number or switches that must be faulty (to destroy DFA) to the total number of switches.
Blocking Characteristics Every PSA is either blocking (it does not allow and input-output connection to be establi8shed in any sequence-it blocks those calls)or non-blocking (it allows every inputoutput combination too be established in one or more passes). Blocking architectures may be non-blocking in the wide sense. Wide-sense(or serially) non blocking networks may be rearrangeably non-blocking, i.e. the new connection may be established when the established connections are selected according to some specific switching or packing rule or if he existing connections are rearranged, to allow the new call to be established along with those already in progress.
Number of Switch Element The number of switches required to implement an architecture is an important figure of marit. Apart from the higher cost associated with having a larger number of switches, the reliability of the PSA decreases as the number of switches increases. Often switches are added to a PSA which is already non-blocking to increase the fault tolerance by increasing the redundancy. An associated issue is the number of electronic drives in electro-optic switches. This becomes significant, if the implementation cost/complexity, power requirement and substrate real estate associated with each drives is not regligible. Control/set-up Algorithm
A general heuristic is that the strictly non-blocking PSAs
require simpler control algorithm than the rearrangeably non-blocking architectures. Consider for instance, the simplicity of the crossbar where, to connect the i th input to the jth output , only one jth switch in the ith row nees to be put to the bar state. In the case of other architecures the set-up algorithm could be more complex. Physical Layout and Fabrication Considerations
Since most PSAs are monolithically integrated (i.e. all the switches in the architecture are physically fabricated on the same substrate) and the interconnections between them are optical waveguides, a leakage of
power between waveguides is
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inevitable when two waveguides intersect cross-over. It is important, therefore, to minimize the number of waveguide cross-overs.
Planar PSAs, therefore, are more
desirable. However, though the crosstalk between them and the attenuation in each waveguide due to power leakage increases. Planar PSAs, while not strictly required, are preferred. One way of reducing the number of waveguide cross-overs is to partition the architecture into sub-architectures which are planar, fabricate these sub-architectures on separate substrates and interconnect multiple substrates using optical fibers. As the number of substrates increase, the cost increases and complications in the alignment of the interconnecting fibers arise.
While monolithic integration is preferable, in electro-optic directional coupler (EODC) based architectures, as the number of EODCs increase, it becomes difficult to design and fabricate a PSA with a satisfactory device yield. Due to the large radiation losses associated with passive optical interconnection having a mall radius of curvature, interconnection between switches are either straight or have gradual curvature (usually circular). This implies that switches must be placed end-to-end in a PSA unlike in electronic integration, where the devices may be arranged more efficiently due to the electrical connections betweens them. If the EODCs are placed end-to-end, then having a large number of EODCs implies that each EODC is shorter, requiring a higher voltage to achieve switching-this reduces the switching speed since faster slew rates are more difficult to achieve with higher electrode voltages.
Other Issues
Several other issues also drive the choice of a PSA such as- total
cost, fault-tolerance (the occurrence of a fault does not lead to catastrophic degradation in the PSA connectivity and DFA is still maintained with increased number of passes), fault isolation (it must be easy to isolate faults in switches which may arise either during fabrication or in operation so that other sections of PSA are not affected), partitionability (the PSA may be logically partitioned to function as a number of smaller PSAs) and modularity (so that it can be easily service and upgraded).
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TYPES OF PHOTONIC SWITCHES
An optical switch or cross point is a device which allows an incoming optical signal to be routed to one(or more) of several outgoing paths.
There are several
approaches which have been demonstrated to accomplish this function.
Photonic
switches can be broadly classified as-mechanically operated, magneto-optic, acoustooptic and electro-optic switches.
Mechanically Activated Optical switching Cross point
Here mechanical forces move an incoming fiber to one of many positions, causing it to couple to one of the corresponding outgoing fibers(Fig. 10.1). The mechanical force can be generated by an electrically activated solenoid. Any number of possible ways of realizing these switches can be thought of; the rotating mirror technique, the moving prism methods, etc. These switches have a good cross talk figure of merit but the major limitation is their switching speed, which is of the order of milliseconds.
Magneto-optic Switch
These switches utilize the magneto-optic effect in an Yttrium Iron Garnet (YIG) single crystal and other materials. The switching principle is as follows. The linearly polarized light is first rotated through a 45ď&#x201A;° Faraday rotator of the YIG single crystal. The direction of rotation is controlled by inverting the direction in which the magnetic field is applied to the rotator. Then the optical path is determined by an poloaisation separator, which functions as an optical switch. The Faraday rotator now used, is in the form of a thin plate, making it possible to operate it with a low strength magnetic field. To prevent internal cross talk the surfaces of the thin crystal plate are coated with optical interference films. A semi-hard magnetic bistable switch is one that requires no power except during switching thus resulting in high reliability and feasibility. The polarization state of light is randomized when it is transmitted through a fiber. To make a switch
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whose characteristics are independent of the polarization state of the incident light, special ploarising prisms made from single crystal TiO2 used as polarizing separators.
Acousto-optic Switching
In this type of switching the incoming optical beam is allowed to interact with an acoustic wave.
In planar form, the acoustic wave are generated using interdigital
electrodes. The angle of diffraction is dependent on the grating spacing, which in turn depends on the acoustic wave frequency. Hence switching can be done by controlling the acoustic frequency. One of the most important applications of acousto-optic interactions is in the deflection of optical beams. This can be achieved by changing the sound frequency while operating near the Bragg-diffraction condition.
Electro-optic Switch
Here optical waveguides and the electro-optic effect are combined too achieve switching. Guided optical waves propagate in a thin propagate in a thin film with a thickness of the order of optical wavelengths. The index of refraction f the film is required to be higher than that of the substrate material so that light can be trapped in the film by total internal reflection(TIR).
Efficient electro-optic switches and modulators utilise single mode strip optical waveguides. Typically three refractive indices define the waveguide: substrate index (ns), the
waveguide strip index (ng) and
the cladding or cover layer index (nc).
The
dimensions of the guide and the value of the indices, determine the number of guided modes and their propagation constant (ď ˘). The propagation constant can be written as
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OPTO ELECTRONICS 2n where is the free space optical wavelength, no is the optical effective index of the guided mode. The value of no lies between ns and ng.
Optical Directional Coupler
An integrated optic directional coupler is formed by fabricating two parallel waveguides in proximity (for example, two identical parallel single-mode optical waveguides fabricated by Ti-indiffusion in LiNbO3 that are placed at a distance g apart) so that the light in one waveguide can couple to the other via the evanescent fields. An input and output section wherein the waveguides separate and go further apart are added to the central coupling region. These sections are necessary not only to avoid coupling beyond central region but also to enable connection of the IO waveguides to input/output fibers. Typically the length of the coupling region is about 2-5 mm with a gap of about 2-5 µm. The separation between the waveguides at the input / output ends is about the diameter of a single mode fiber, i.e. 125 µm. In fig. 10.2, a waveguide directional coupler circuit is illustrated.
If the two waveguides are separated by a distance g over a length l, then the waveguides are said to be coupled so that optical energy can transfer between the two guides, when g is very small. If the waveguides have the same propagation constant and energy is incident in only one guide, it will transfer completely to the other guide over a distance le = / 2K, called the transfer length or coupling length, where K is the coupling coefficient, which describes the strength of the inter-guide coupling. Typically, the coupling coefficient is related to the separation between the guides g by an exponential relation of the form K=K0 exp (-g/) where K0 and are coefficients that depend upon the various waveguide parameters. Thus as the waveguides move away, they are decoupled within a short distance.
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Directional Coupler Switch
The directional coupler switch consists of an optical directional coupler with electrodes placed over the waveguides in the coupler region (Fig.10.3) in this case the applied electric field is normal to the crystal surface (and hence the waveguide length equal to the coupling or transfer length. The straight – through state is obtained by applying an appropriate voltage to the electrodes to satisfy the phase mismatch condition. For one transfer length coupler, the required phase mismatch is 1 = 3.
It is difficult to fabricate such devices with good cross – over power transfer because of the necessity of maintaining close fabrication tolerance in order that l = lc. As a result, cross state cross – talk is limited by fabrication tolerance. A direction – al coupler switch that allows electrical adjustment to achieve both switch states with low cross –talk is shown in Fig. 10.3 (b). This technique requires two set of electrodes over the coupler. The electrodes divide the coupler into two equal length sections in which electrical adjustment of both switching states are obtained. Such a configuration is called a reversal switch. To place the switch in the cross state, two equal but opposite voltages are applied to the electrode whereas, to place the switch in the bar state, one of the voltages is reversed to a required value.
Ti In diffused LiNbO3 Optical Switch
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OPTO ELECTRONICS Titanium diffused lithium niobate waveguide has held considerable promise for the development of active integrated optical devices, for the following reasons. 1. Diffusion temperatures are below the curie temperature (1125C for Ti). 2. Diffused waveguide is electro-optically active. 3. Tight optical confinement can be obtained with low transmission loss. 4. Channel waveguides can be fabricated by lithography methods.
LiNbO3 is a ferro-electric crystal with excellent electro-optic properties. A strip waveguide is easily fabricated in LiNbO3 by first photo-lithographically delineating the waveguide circuit in Ti metal. The metal is then diffused into the LiNbO3 substrate in an oxygen atmosphere at about 1000C for several hours. For = 0.6328 µm, a 4 µm Ti strip 300 A thick, diffused for seven hours creates a single mode waveguide with a depth of 2 µm. the refractive index in the volume where the metal is diffused is increased proportionally to the Ti concentration. The loss in Ti diffused waveguides is typically, less than 1 dB/cm at 0.6328 µm. The cover layer could be a layer of SiO2 about 200nm in thickness. The switching speed of the directional coupler is limited by the capacitance of the electrodes. For the electrodes geometry shown in fig.10.3, the capacitance could be approximated to be C= , L where , is the dielectric permittivity of the crystal and L is the electrode length. The capacitance of these electrodes is of the order of 1 pF. A higher order optical switch like a 4 x 4 switch matrix is readily formed using a combination of directional couplers and interconnecting waveguides as shown in fig.10.4. To establish the following connectivity, no voltage is applied on any of the switches, i.e. all switches are in the cross – states.
14; 22; 33; 41. In this case inputs 1 and 4 are exchanged while 2 and 3 are not. By placing voltages on some or all of the switches similar connection paths can be worked out.
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Opto –optic Switches In the earlier sections, the switching of light is achieved by a control mechanism that is non-optical, namely, electro-optic or acousto-optic. It is desirable that optical radiation be used to switch or reconfigure signal paths, for example, by non-linear optical effects. Self – electro-optic effect device (SEED) is a one such opto-optic device. However opto-optic switching and control is at present very inefficient and calls for high intense optical beams for control. This function may became practical only when more efficient highly non-linear materials such as polymers are developed.
Photonic Switching Architectures Photonic switching architectures may be classified as follows. 1. Space switching architectures 2. Time division switching – includes time slot interchange and time division multiplexed bus switching. 3. Code division switched architectures. The Crossbar The simplest example of a single stage space switching architecture is the crossbar as shown in Fig.10.5 with N2 switches arranged in N rows of N switches each. Each of these switches (denoted as Sij for the ith switch in the jth row where i,j, <N) may be either open or closed (achieving eth connection of the ith input to the jth output). Also, only one switch in a particular row may be in the through state at a time in any valid access request. The crosser suffers from several shortcomings – the failure of any switch SH forever disconnects the input k from the output l and thus the crossbar is not fault – tolerant. Moreover, having N switches is expensive particularly when N is large. An 8 x 8 strictly non-blocking PSA using stepped reversal type directional couplers has also been reported [10.4]. Implemented by Titanium indiffusion on z-cut ypropagating LiNbO3 with an operating wavelength of 1300 nm, the 60 mm long PSA is a crossbar structure. Interconnections between switches are by circular ares of radius 30
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OPTO ELECTRONICS mm. The drive voltages are 18.4 V (± 2.4V) for the bar state and 26.4V (± 1.6V) for the cross state. Extinction ratios range from 18.6 dB to 37.2 dB.
Three Stage Space Switching Architectures A three state space switching Architecture, also referred to as a Clos network, may be constructed as shown in Fig.10.6. The architecture is not planar and is usually constructed on multiple substrates. The first state has r substrates of order n x m each, the second stage has m substrates of dimension r x r and the third stage has r substrates of dimension m x n. This PSA, is non-blocking in the wide sense and gives a reduction in the number of cross- points for switches of order greater than 23 x 23. it is strictly nonblocking if m ≤ 2n-1 and rearrangeably non –blocking if m ≤ n.
Multistage Interconnection Networks A planar N x N PSA, known as the Taylor network (shown in Fig.10,7), is a rearrangeably non-blocking architecture with N stages with a total number of [N(N-1)]/2 switches. An advantage of this architecture is that the worst – case optical signal path is shorter than in the crossbar PSA.
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51
A non-planar rearrangeably non-blocking architecture, called the Benes network, is shown in Fig. 10.8 with 2 log2 (N-1)
stages. The Benes network requires the
fewest number of switching elements N/2[2 log2 (N-1) to implement an N x N PSA. On the same substrate. The worst rase (longest) signal path passes through four switches in this PSA. Assuming that each switch has an SXR of x Db, the SXR at the end of the signal path passing through n switches in given by S
-1
1
n = 10 log (-1)i+1 10
10 log N
10 i=1 With a power budget of 40 dB, assuming a LD pin FET combination at the transmitting and receiving ends, and assuming x = 30 dB, and using Eqn. 10.2 the worst – case crosstalk with the signal path passing through 4 switches is 0.0004 dB – which is negligible. Assuming that each switch introduces an insertion loss of 1 dB per switch, the worst – case insertion loss in 4 dB. Assuming a worst – case connection loss of 2 dB per connector, we have 4 dB of power lost due to the connectorizaiton of the fiber at either end of the PSA. Thus the total power lost in the worst – case signal path is 8 dB, which leaves us with a comfortable power margin of 36 dB.
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UNIT-V
INTRODUCTION TO SATELLITE COMMUNICATION Satellite communication networks are now an indispensable part of most major telecommunication systems. Satellites have a unique capability for providing coverage over large geographical areas. The resulting interconnectivity between communication sources provides major advantages in applications such as interconnecting large traffic nodes (e.g. telephone exchanges), provision of
end-to-end connections directly to
users, mobile communications, television and sound broadcasts directly to the public. The advantages offered have enabled this technology to mature within just three decades. To date, most benefits within the telecommunications area have been achieved for pointto-point communication within the international and domestic systems, direct television broadcasts and mobile communications. In recent years, satellite communication systems have begun to face competition from optical fibre systems for point-to-point communication between large concentrated traffic sources. To retain a competitive edge, it has been necessary to develop various new techniques. Thus the phenomenal growth of satellite technology continues, the major growth now being in those areas where satellites can provide unique advantages. Such applications include service provision directly to customers using small, low-cost earth stations; mobile communication to ships, aircrafts, land vehicles and individuals; and direct-to-public television/sound broadcasts and data distribution/gathering from widely distributed terminals. In many applications, such as video
distribution,
service-providers
are
combining
the
benefits
of
satellite
communications with optical fibre systems to produce the best solution to users' needs. Similarly, systems have emerged which combine the advantages of satellite mobile communication systems with those of terrestrial mobile systems to provide a seamless coverage - terrestrial systems providing service in populated areas and satellite systems in those areas unserved by the terrestrial system. Recent advances in system architecture have permitted the introduction of satellite personal communication services.
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The purpose of the book is to give the reader a sound understanding of the various issues involved in designing satellite communication systems, be it for a new system design, a trade-off study between various transmission media, an understanding of an existing satellite system or academic pursuit. This chapter begins with a brief history of satellite communications, and an overview of the main components of a satellite communication system follows. Major design considerations are then discussed and the present status and future trends of the technology are briefly reviewed.
Background
The first known use of a device resembling a rocket is said to have been in China in the year 1232. A number of instances of the use of such devices were subsequently recorded over the next few centuries. However, significant progress in the field was not made until the Russian school teacher, Konstantin
E. Tsiolkovsky
(1857-1935), propounded the basis of liquid propelled rockets. He went further and put forward ideas for multi-stage launchers, manned space vehicles, space walks by astronauts and large platforms that could be assembled in space complete with their own biological life support systems. His theoretical work on liquid propelled rocket engines was verified when in 1926 Robert H. Goddard launched the first liquid propelled rocket in the United States. Other rocket pioneers also corroborated and extended Tsiolkovsky's ideas. However, the work of a small German amateur group provided the breakthrough which laid the foundations of the present rocket technology. Their work was later supported by the German military, leading to the successful launch of V-2 rockets in 1942 (Gatland, 1975). Recognizing the potential of V-2 rockets, in 1945 Arthur C. Clark suggested the use of geostationary satellites for world-wide voice broadcasts (Clark, 1945). The work on V-2 rockets was extended in the United States and the former Soviet Union after the second world war. This led to the development of the first satellite launchers. The satellite era began in October 1957 with the launch of Sputnik-I, a former Soviet Union satellite. This was soon followed by the launch of a US satellite, Explorer-I, in 1958.
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Communication by moon reflections was demonstrated and used in the United States in the 1940s and 1950s. The early years of satellite communications exploited similar techniques. Satellites were used as passive reflectors of radio waves for establishing communication. An immediate problem with the use of satellites as passive reflectors was the extremely low level of signal strength, resulting in a need for very sensitive and hence costly receivers. The main reason favouring the use of passive satellites at the time was the lack of space-qualified electronics. It was recognized that the use of satellites capable of amplifying the received signal on-board before retransmission could greatly enhance the capability of satellites for communications. Therefore considerable research and development effort was spent during the next few years in the development of space-qualified electronics, eventually leading to the introduction of active repeaters. In 1963 the first geostationary satellite, Syncom III, was successfully launched. The first commercial satellite for international communication, Early Bird, was launched in 1965.
The breakthrough provided by satellites in telecommunications resulted in a major research and development effort in all the related technologies. Most of the early work concentrated on international point-to-point telecommunications applications. Later, the application of satellite communication extended to direct satellite broadcasts (1970s), mobile communications (1980s) and personal communications (1990s). There was a phenomenal increase in international telecommunications traffic in the period 1965-90, and growth is predicted to continue (Bennet and Braverman, 1984). The increase in the number of global circuits in the INTELSAT network was about a thousand-fold in the same period. The revenue generated has funded large projects for research and development into both space and ground segment hardware. As a result, there has been a steady reduction in cost per circuit. Similarly, in the area of satellite mobile communications, the total space segment capacity of the International Mobile Satellite Organization (Inmarsat) has increased several-fold during the period from 1981 to 2000. INTELSAT and Inmarsat are international organizations which operate global fixed and mobile communications networks respectively. Most countries of the world are
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OPTO ELECTRONICS members of these organizations.
In addition to providing international telecommunications, satellite technology is used in regional and domestic telecommunication networks. Regional networks refer to those networks formed to serve a group of countries in a region (such as Europe), and domestic networks refer to those designed for intra-country use.
At present, international point-to-point communication is mainly provided by INTELSAT which was founded in 1964 to develop the commercial uses of satellite communications. Examples of other international operators are INTER-SPUTNIK which provides communication to ex-Soviet block countries, and the International Mobile Satellite Organization (Inmarsat) which provides worldwide mobile communications. Examples of regional system operators are the European Telecommunication Satellite Organisation (EUTELSAT) and the Arabian Satellite Communication Organisation (ARABSAT) which provide services in the European and the Middle East regions, respectively. Several countries now use domestic satellite systems - for example, North America, Canada, India and Indonesia. A summary of the major milestones in the development of satellite communications is given at the end of the chapter.
The fact that satellites have a wide view of the Earth makes them useful for a variety of applications besides telecommunication. These applications are in the fields of meteorology, navigation, astronomy, management of earth resources such as forestry and agriculture, military reconnaissance, amateur radio and others. To coordinate the working of various radio systems, an international body, the International Telecommunications Union (ITU), has categorized these services and set guidelines for the design and operation of each satellite service. Telecommunication is provided by the Fixed Satellite Service (FSS) used for communication between fixed points on Earth; Mobile Satellite Service (MSS) used for communication with moving terminals; and the Broadcast
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OPTO ELECTRONICS Satellite Service (BSS) which deals with television and sound broadcasts directly to customers. In this book we are concerned with the design principles of telecommunications services. However, the system design concepts developed are generally also applicable to other services.
Basic Satellite System A basic satellite system consists of a space segment serving a specific ground segment. The characteristics of each segment depend on whether the system is for fixed, mobile or direct broadcast applications. The main features of these services and the main system related issues are briefly addressed in this section.
The main elements of a satellite communication system Ground stations (or earth stations) in a network transmit radio frequency (RF) signals to the operational satellite. The received signals are processed, translated into another radio frequency and, after further amplification, retransmitted towards the desired regions of the Earth. Communication can be established between all the earth stations located within the coverage region.
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The main elements of a satellite communications network. Ground segment of a fixed satellite service is shown.
Space Segment The space segment consists of one or more satellite(s) and suitable orbit(s). Satellite and orbital characteristics depend on the application needs. The satellite is controlled and its performance monitored by the Telemetry Tracking and Command (TT &C) stations. In an operational system the satellite in use is usually backed up by one or more in-orbit spare satellites. Many of the present communication satellites are in the geostationary orbit. A circle at an altitude of approximately 35786 km above the equator
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OPTO ELECTRONICS is known as the geostationary arc. Satellites orbiting the Earth in such an orbit rotate in unison with the Earth, appearing almost stationary to an observer on the ground. A satellite which appears stationary minimizes the operational requirements of earth stations. The design of ground terminals is simplified because simple tracking systems can be used and RF signals do not suffer significant Doppler frequency shift (chapter 2). Further, a single geostationary satellite can provide communication to large areas (over about one-third of the Earth), permitting easy interconnection between distant ground terminals. Thus three geostationary satellites placed 120° apart can provide coverage to almost all the populated areas of the world, as shown in figure 1.2. It is therefore not surprising that many communication satellite systems use the geostationary orbit.
One disadvantage of geostationary satellites is that they appear almost at the horizon at latitudes above approximately 76°. Above about 84° latitude the satellites are no longer visible. They become unusable (or unreliable) below about 5° elevation because the received signal quality is inadequate for good quality communication. This is caused by a combination of increased degradation in the troposphere and ground reflections which cause rapid signal fluctuations.
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Three geostationary satellites can provide coverage within about ď&#x201A;ą 75ď&#x201A;° latitude. Locations at higher latitude may be served by Molniya orbit. Thus, for providing service to high-latitude locations other types of orbits are necessary. An elliptical orbit inclined at 63.4ď&#x201A;° to the equatorial plane has been in use for this purpose. Satellites in this type of orbit appear almost stationary to higher-latitude earth stations for periods of 8-12 hours. Therefore several satellites are necessary to provide a 24-hour service. The use of this type of orbit has also been proposed for providing land mobile communication in high latitude areas. Figure 1.2 shows the coverage areas provided by a geostationary satellite with a 63.40 inclined elliptical orbit. Chapter 2 covers the topic of orbit in detail.
Renewed interest in the use of low earth orbit for mobile communications has been generated recently (e.g. Richharia et al., 1989; Maral et al., 1991). The use of low and medium earth orbits was considered during the early stages of development of satellite communications, but abandoned because of the resulting complexity in the earth station and network architecture. The main problems are the requirement for complex earth stations; the need for a relatively large constellation of satellites to cover the Earth; complex handover procedures as satellites continuously appear and set at a particular location on the ground; and complex techniques to interconnect terminals which cannot view the same satellite simultaneously. However as technology has matured, solutions to many of these problems have become feasible. Some of the main advantages of low and medium earth orbit include: (a) the possibility ()f using hand-held receiver terminals because satellites are closer to the Earth and can therefore provide stronger signals at the receiver, and ground stations need to transmit at lower power; (b) the possibility of reusing the frequencies more often than is possible with geostationary orbit because the geographical area covered by low earth orbit satellites is much smaller; (c) the possibility
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OPTO ELECTRONICS of reduction in transmission delay. Chapters 2 and 11 discuss the topic in detail.
Various types of communication satellites are in use, from small satellites for domestic communications to large and complex satellites, serving international traffic. Chapter 9 discusses satellite technology in detail. In general, satellites serving the mobile and broadcast sectors need to transmit at higher power than do satellites serving the fixed network.
Ground segment The ground segment of each service has distinct characteristics. The ground segment of a fixed satellite service (FSS) consists of several types of fixed earth stations. The size and characteristics of earth stations depend on the application. For example, earth stations for handling international traffic use large antennas (11-30m) together with complex RF and baseband sub-systems, whereas terminals for providing communication directly to customers' premises employ non-tracking antenna of 1-3 m diameter with simple RF and baseband hardware. Each earth station is interfaced to the user either directly or through a regional or national public switched network. The interface between an earth station and the user is an important consideration in the design of a FSS network. The main elements of a mobile satellite service (MSS). The ground segment consists of several types of mobile terminals connected to the fixed telecommunication networks via the satellite. Mobile satellite communication is categorized into three classes according to the environment served - maritime, aeronautical and land based. Ground terminals serving each category of mobile differ to varying extents. Differences occur because: the physical space available for mounting the antenna and receiver depends on the type of mobile (e.g. a ship has more available space than does a truck); the communication needs and affordable terminal cost differ (e.g. a personal communicator must be low cost with a moderate communication capability relative to, say, an aeronautical terminal); the behaviour of the transmit/receive signals is significantly affected by the environment in the proximity of the mobile. However, this
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61
does not imply that a terminal designed for one environment cannot operate in the other. If the application so demands, a terminal designed for use in one environment can indeed be used in the other. For example, ship terminals can be readily altered to operate as a land terminal, or land terminals can be used in aircraft. In general, ship terminals use the largest antenna and land mobile terminals the smallest. Consequently, maritime terminals can provide the largest communication capability, followed by aeronautical terminals. At present, land mobile terminals provide a rather limited capability because of severe limitations on the antenna size, and the harsh propagation environment coupled with the technological limitations of the space segment. A radical change in space segment architecture
has
made
possible
voice
communication
directly
to
hand-held
communicators (resembling a cellular telephone) and portable multi-media terminals.
The main elements of a mobile satellite service. Satellite communications provide a unique advantage for mobile applications because of the wide geographical coverage made possible by satellites.
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The main elements of a direct broadcast system. Programmes transmitted by an earth station are received directly by the audience.
A rescue coordination centre can also be incorporated in. the MSS network, to respond to distress calls from mobiles in the network.
The main elements of a direct broadcast system (DBS). Programmes (originating in a studio or ‘live’) are transmitted through a large ‘gateway’ earth station and a highpower satellite to small terminals dispersed throughout the service area. Terminals typically use 50-100cm antennas with a facility for interfacing to television sets used to receive local terrestrial transmissions. In recent years the cost of direct broadcast receivers has become comparable to other home entertainment appliances. Cable television systems have also exploited the benefits of direct broadcasts. Cable television
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OPTO ELECTRONICS companies can afford to use larger and more sensitive receivers, and therefore provide a better signal quality and often a larger choice of programme to their subscribers. A recent addition has been sound broadcasts directly to personal sets.
Satellite Orbits
Although an infinite number of orbits are possible, only a very limited number of these are of use for satellite communications. Some of the terms used in describing an orbit are
Apogee. The point farthest from the earth. Perigee. The point of closest approach to the earth. Ascending node. The point where the orbit crosses the equatorial plane going from south to north. Inclination. The angle from the earth’s equatorial plane to the orbital plane measured counterclockwise at the ascending node.
The polar orbiting satellite follows an orbit that is close to the earth and passes over, or very close to, the poles; that is, the inclinations is close to 90. The average height of these orbits is typically 800 to 1000 km above the earth, and they are used mainly for earth observation and surveillance (weather, pollution monitoring, and the like), and for search and rescue work. More recently, trials have been conducted using small satellites for data communications and position determination (ORBICOMM System), which may provide low-cost services in these areas.
The inclined highly elliptical orbit is used where communications is desired to regions of high latitude. Kepler’s second law shows that the orbital velocity is least at the apogee, and hence by placing the apogee above the high latitude regions the satellite remains visible for a longer period from these regions. The Russian Molniya series of satellites use highly inclined orbits. One effect of the earth’s equatorial bulge is to cause the orbit to rotate, such that apogee and perigee move around the earth, this being
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referred to as rotation of the line of apsides. However, at one particular value of inclination, i=63.4, the rotation of the line of apsides is zero, and satellites that are required to have the apogee remain fixed over a particular region are launched into orbits with this value of inclination. These orbits are referred to as being in the 63 slot.
A recently introduced service that uses near-circular, non-geostationary orbits is the Global Positioning satellite (or GPS) services, which is essentially a navigation and position determination service. The GPS system utilizes 6 orbits with 4 satellites in each. The ascending modes of the orbits are separated by 60and the inclination of each orbit is 55.
Geostationary Orbit A geostationary satellite is one that appears to be stationary relative to the each. There is only one geostationary orbit, but this is occupied by a large number of satellites. It is the most widely used orbit by far, for the very practical reason that earth station antennas do not need to track geostationary satellites (except for certain very high gain earth station antennas that require a limited range of tracking, as will be described later).
The first and obvious requirement for a geostationary satellite is that it must have zero inclination. Any other inclination would carry the satellite over some range of latitudes and hence would not be geostationary. Thus the geostationary orbit must lie in the earth’s equatorial plane.
The second obvious requirement is the geostationary
satellites should travel eastward at the same rotational velocity as the earth. Since this velocity is constant, then from Kepler’s second law it can be deduced that the orbit must be circular, since as previously shown the velocity in an elliptical orbit varies from a maximum at perigee to a minimum at apogee and hence is not constant.
The earth maker one complete rotation, relative o thee fixed stars, in approximately 23h 56m. Notice that this is slightly less than the time required for one complete rotation about its own axis, which is 24h. Substituting P = 23h 56m in Eq. (19.4.1) for Kepler’s third law, along with the value for A given in Eq. (19.4.2), results in
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OPTO ELECTRONICS
65 agso = 42,164 km
The subscript gso is included to remind us that is the value for the geostationary orbit. It will be recalled that because the orbit is circular this is also the radius of the orbit measured from the center of the earth. The earth’s equatorial radius is approximately 6378 km, and hence the height of the geostationary orbit above the earth is h = 42,164 – 6378 = 35,786 km
This value is often rounded up to 36,000 km for use in calculations. It will be seen that there is only the one value of a that satisfies Kepler’s third law for the periodic time of 23h 56m, and hence there can only be one geostationary orbit.
Earth Stations Earth stations are a vital element in any satellite communication network. The function of an earth station is to receive information from, or transmit information to, the satellite network in the most cost-effective and reliable manner while retaining the desired signal quality. Depending on the application, an earth station may have both transmit and receive capabilities or may only be capable of-either transmission or reception. Further categorization can be based on the type of service. Usually, the design criteria are different for the Fixed Satellite Service (FSS), the Broadcast Satellite Service (BSS) and the Mobile Satellite Service (MSS). A fundamental parameter in describing an earth station is the G/T (antenna gain to system noise temperature ratio - see section 4.5). This figure of merit represents the sensitivity of an earth station. A higher value implies a more sensitive station. Thus, depending on the value of G/T and service provided, various categories of earth stations are possible. Table 10.1 shows a possible categorization of earth stations based on these criteria.
In the following sections we shall discuss basic design considerations of earth stations, followed by descriptions of the main sub-systems of a typical earth station and
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OPTO ELECTRONICS important features of various types of earth station. Design considerations The design of an earth station depends on a number of factors. Some of these are: Type of service: Fixed Satellite Service, Mobile Satellite Service or Broadcast Satellite Service Type of communication requirements: telephony, data, television, etc. Required baseband signal quality at the destination Traffic requirements: number of channels, type of traffic - continuous or bursty Cost, reliability. Two broad stages may be identified in the design process. The first stage is based on the overall system requirements from which the required earth station parameters such as G/T, transmit power, access scheme, etc. emerge. An earth station designer then engineers the most cost-effective configuration to achieve these specifications.
It is necessary to minimize the overall system costs which include costs (development and recurring costs) of space and ground segments. Several trade-offs are applied in the design optimization. During the earlier days of development, the available effective isotropic radiated· power (EIRP) from satellites was low and hence earth stations tended to be complex and expensive. Earth stations used large antennas (-30m) and were very expensive ($5-10 million). The current trend is to minimize earth station complexity at the expense of a complex space segment, especially in applications which are directed towards a large user population (e.g. direct broadcasts, mobile and business use). The availability of low-cost earth stations is vital to the economic viability of the overall system in such applications.
Some of the trade-offs applied can be understood by examining the equation developed in an earlier chapter. The link equation, in a rearranged form, can be written as
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OPTO ELECTRONICS G/T where C/Nď&#x201A;°
= C/Nď&#x201A;° -
67 EIRP + (Lp + Lm) + k
= carrier to total noise power spectral density
EIRP = satellite effective isotropic radiated power Lp
= path loss
Lm
= link margin
K
= Boltzmann constant (in dB).
A useful optimization criterion is to minimize the earth station sensitivity (quantified as G/T), as this results in a minimal-cost earth station. It is therefore necessary to minimize the parameters on the right-hand side for a specified baseband signal quality.
In general, this is achieved by using high satellite EIRP and modulation schemes which are robust to noise (in other words, the desired baseband quality at the receiver can be achieved with lower carrier-to-noise ratio). When digital baseband is used, further reduction in earth station G/T is possible by using coding. In fact, many small earth stations are only economically viable through the use of coding. Other factors which have an impact on earth station cost are earth station EIRP, satellite tracking requirements, traffic handling capacity, interface to user or terrestrial network and network architecture. For a given application, the optimization is constrained by international regulations (e.g. antenna patterns have to meet regulatory requirements) and the state of the technology. ]
International Regulations Most of the fixed satellite service frequency bands are shared with terrestrial systems. For systems to coexist the International Telecommunication Union (ITU) has specified certain constraints in the transmitted EIRP of satellites.
Such constraints have an impact on the design of earth stations. For example, consider the use of earth stations in applications where a small size of terminal is
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OPTO ELECTRONICS essential (e.g. an earth station located in a user's premises). To be able to provide good signal quality with a limited available flux density on the ground, the G/T cannot be reduced below a certain value. Moreover even if G/T were reduced by the use of a smaller antenna, a reduction in the antenna size also increases the antenna side lobe levels, resulting in more interference to and from adjacent satellite systems.
It should be noted that limiting the satellite EIRP for applications such as direct broadcast and mobile communication would preclude the use of small antenna diameters, essential for such applications. This fact is recognized by the ITU and hence the frequency band allocations for these services tend to be exclusive (i.e. not shared with other radio services) and tend to have a higher allocation status permitting much higher satellite EIRP (see section 3.2). The limitations (if any) in these applications are mainly due to technological constraints of the space and ground segments.
Technical constraints The transmitter power of a satellite is limited by the maximum DC power which a satellite can generate (at present -10 kW) and the upper power limit of reliable power amplifiers. The maximal spacecraft antenna gain is limited by the practical constraint imposed on the satellite antenna diameter. Note that for a given antenna size the gain falls with a decrease in frequency and therefore the EIRP limitation is more acute at lower frequencies (e.g. for the L band, commonly used for mobile satellite communication
Technical constraints apply to earth station hardware and software. Generally, the cost and size of earth stations have been steadily falling. The size reduction has been dramatic in certain applications. For example, in mobile applications portable telephones are commonplace, and pocket-sized pagers and pocket-sized telephones are now available. Similarly, direct-to-home receivers typically use a 30-50cm dish and are widely affordable.
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OPTO ELECTRONICS From the above discussion it is evident that several trade-offs are necessary in the optimization process. The use of computer-aided techniques simplifies the design process considerably (e.g. Richharia, 1985). It should be noted that design optimization criteria for services vary.
To ensure compatibility and maintain acceptable signal quality, service providers standardize earth station size for use in their network.
With the basic specifications laid out, an earth station designer's next task is to develop an optimum configuration. The specified G/T and EIRP may be achieved by a number of combinations of antenna, low-noise amplifier (LNA) and high-power amplifier (HPA). It is possible to choose a combination of either a small-diameter (low cost) antenna and a relatively low-noise (expensive) LNA, or a large antenna and a LNA with a higher noise figure. It should however be noted that the diameter of the antenna also has an impact on the EIRP of the station. A small value of gain may give rise to an unduly large HPA requirement. Therefore this variable must also be taken into account in the optimization.
Additional factors which must be considered include the cost of other equipment, the floor area, environmental factors (e.g. temperatures to which equipment may be subjected, wind load on antenna, etc.), interference considerations (sites must be well away from potential interfering sources, such as microwave radio relays) and recurring costs. For mobile and personal earth stations antenna and receiver space imposes a severe constraint. When considering direct broadcast receivers, very low cost is essential for commercial viability. Aesthetic antenna design is becoming increasingly important. Application Survellane Based on the unique advantages offered by satellite communication systems and taking into account the influencing factors summarized above, some of the possible future applications and emerging growth areas are identified as follows: ď&#x201A;ˇ Direct communication to customers' premises bypassing the public switched network FOR MORE DETAILS VISIT US ON WWW.IMTSINSTITUTE.COM OR CALL ON +91-9999554621
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OPTO ELECTRONICS providing broadband services such as multi-media. Interconnecting large traffic sources such as city telephone exchanges in regions with little or insufficient infrastructure . Restoration of outages in optical fibre systems. Providing ISDN facility to remote areas. Teleconferencing. Video distribution between fixed points. Communications and distress alert facility for ships, aircraft, land vehicles and individuals. Personal communications such as newspaper delivery, portable radios receiving highquality signals perhaps leading to ‘wrist-watch’ size radios early in the next century, newspaper and mail delivery to remote areas, extension of medical consultancy to remote/inaccessible areas, connection to Internet and others (see, for example, Ida et at., 1985). Direct-to-home, business and community broadcasts. Television and sound broadcasts to mobiles. High-definition television system.
With this background, we shall now examine each service individually.
(i) Fixed satellite service Impact of optical fibre links Let us first consider the impact of fibre optic links on satellite communication systems, as this topic has been a subject of considerable interest in recent years. Fibre optic systems are expected to influence mainly the growth of fixed the satellite service. It is now recognized that a portion of satellite communications’ traffic will migrate to the optical fibre medium where optical fibre systems are installed. To remain competitive, satellite systems must lower costs and improve quality. Quality improvement refers to improvement in factors such as the effect of echo and delay, and network reliability. The traffic environments most likely to be affected are international and inter-city routes of large developed countries such as the USA.
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It is interesting to note that during the past 20 years satellites and optical fibres have both grown by about 20 times. Let us compare the main feature: each.
At present, the main advantage of optical fibre vis-a-vis satellite communications are: less delay and echo; lower risk (e.g. no launch or satellite failure); less prone to noise; longer life (>25 years).
However, several recently proposed systems using LEO/MEO constellations are designed to provide fibre optic-like services.
The main advantages of satellite communication in this respect are: capability of point to multipoint broadcast; cost-effectiveness on thin routes; suitability for broadcast and mobile communications; minimal extra transportation cost associated with the 'last mile' (see the next paragraph); short restoration time (typically <1 hour) in case of a satellite failure; danger of physical damage to satellites non-existent (unlike optical fibres); cost and difficulty associated with cable laying non-existent. It is worth noting that when comparing optical fibre systems with satellite systems on international routes, the 'last mile' advantage of satellite communications is often forgotten. On such routes optical fibre terminations are often far away from the traffic points. As an example, consider the trans-oceanic optical fibre systems connecting the USA and Europe. At the European end, cable links can terminate only in countries adjacent to the Atlantic Ocean. Thus a country located inland has to incur extra transportation cost by the use of a terrestrial 'tail'. Further, not all the nations may.
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OPTO ELECTRONICS be willing to use terrestrial links through other countries. Satellite communication reduces this so-called 'last mile' problem by delivering the traffic much nearer to the destination.
Another interesting issue is related to the equitable development of regions (Casas and Fromm, 1988). Optical fibre systems tend to concentrate around the developed areas giving further impetus for growth to an already developed region, at the expense of less developed regions. However, satellites because of their inherent broadcasting capability give an equal weightage to all regions. Thus, for example, the advantages of ISDN could reach the remotest regions instantly using a satellite system whereas it is likely to be a long time before optical fibre links to the region can be economically justified.
We shall briefly illustrate the advantage of satellite systems over optical fibre systems on thin traffic routes. For point-to-point communication applications, a comparison between optical fibre and satellite systems can be made by estimating the cost versus transmission distance for each medium. The break-even point can be defined as the distance beyond which satellites begin to offer cost advantage. Satellite system costs are insensitive to distance but depend on the amount of traffic, whereas optical fibre system costs increase with distance but are relatively insensitive to traffic. A comparison of point-to-point links shows that satellites become increasingly cost-effective as traffic between the points reduces. Thus it can be concluded that satellites will be used increasingly on thin routes, but on heavy routes a portion of satellite traffic may migrate to optical fibre systems where these systems coexist.
We have noted above that in many applications service-providers could combine the advantages of each system to offer the most cost-effective solution. Some of the possible satellite-optical fibre synergistic applications are: i.
A satellite system is used for concentrating data from various low-density traffic sources. The concentrated traffic is then transferred to a central location via an optical fibre link.
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OPTO ELECTRONICS ii.
Satellites provide back-up to optical fibres in case of optical fibre outage (e.g. on a trans-Atlantic route).
iii.
Optical fibre links transport video programmes from the studio to an earth station for satellite broadcasts
Another area likely to influence satellite communications is the introduction of the ISDN. It is expected that signal quality and time delay requirements can be met by single hop satellite circuits. Thus satellite and optical fibre systems will be competing on an equal basis for the ISDN market on some routes. However, satellites have some notable advantages (Casas and Fromm, 1988), summarized below: i.
Possibility of rapid introduction of ISDN over large geographical areas.
ii.
Capability of broadband ISDN from the outset - whereas broadband capability using optical fibres can only be introduced gradually, giving satellite systems a clear lead.
iii.
Capability to shift traffic according to demand - on a long-term basis as terrestrial links are gradually introduced, or on a short-term basis as traffic demand changes seasonally, over a day or in response to an extraordinary event. This rapid switchover of capacity can be achieved in a number of ways. For example, in a TDMA system, time slots can be redistributed among earth stations according to demand.
New technology and applications The proliferation of digital traffic implies that in the FSS, TDMA technology is likely to grow at both extremes - high-bit rate (e.g. 220Mbps) and very-low-bit rate (e.g. 1-3 Mbps). The use of a low bit rate is consistent with the projected interest in the use of VSA Ts. In spite of a rapid growth in data traffic during the 1990s, voice traffic is expected to remain dominant throughout this decade. Therefore considerable research
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OPTO ELECTRONICS and development effort is being devoted to improving voice coding techniques. It may be recalled that reduction in coding bit rate permits a more efficient utilization of bandwidth and satellite power. For example, a reduction in voice coding bit rate/channel from 64kbps to 8kbps can theoretically increase the capacity of a transponder by approximately a factor of 8. Voice coding rates approaching 2.4kbps are widely available in the 1990s.
To improve the utilization of satellite transponders, higher-order modulation schemes are being considered. For example, the use of 8-phase modulation could permit transmission rates of over 220 Mbps in a transponder bandwidth of 72 MHz, making the capacity of one such transponder equivalent to an optical fibre link, with all the added advantages of satellites.
A number of new applications of FSS will appear in the next decade. For example, satellites could be utilized to communicate directly between small terminals using LEO, MEa or GEO constellations. A competition between portable VSA T systems and mobile systems operations at bit rates between 64kbps and l.4Mbps is likely to emerge, as a result. Several systems for personal multi-media type fixed satellite services have been proposed.
Extensive work has been carried out by INTELSAT (Pelton and Wu, 1987) to forecast traffic growth trends in the FSS. Trade-off studies modelling the total network (i.e. ground and space segments) show that small FSS earth stations provide lower cost/circuit. Thus it is expected that generally large FSS stations will be phased out gradually and replaced by medium-to-small earth stations. To be able to serve smaller earth stations and at the same time make efficient use of RF spectrum and geostationary orbital arc, satellites will have to use improved techniques. One possible approach is to use on-board processing satellites. On-board processing refers to a number of functions such as demodulation/modulation, switching, etc. performed either at RF or baseband. The advantages include interference resistance and the capability to optimize uplinks and downlinks separately permitting smaller earth stations. When onboard switching is
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OPTO ELECTRONICS employed at the baseband, the satellite resembles a telephone switching centre with all the traffic points visible sjmultaneously. Hence traffic routing is simplified considerably. For example, in a domestic system the traffic destined for each telephone switching centre can be combined on-board and transmitted on individual carriers. The advantages are simple earth station design together with simple and low-cost terrestrial routing. Remote areas without any terrestrial links can be readily accommodated in the network. A satellite with such an on-board processing facility is often called a 'switchboard in the skyâ&#x20AC;&#x2122;.
Other techniques with a potential of providing more efficient use of space segment are discussed later.
Communication At present in populated areas such as cities, terrestrial cellular mobile systems are far more cost-effective than satellite mobile communication systems. However, satellite systems offer the only viable solution for providing mobile communications to remote areas. Satellite systems are expected to continue to play a complementary role in mobile communications - that of providing service to remote areas such as oceans, air corridors, unpopulated or scantily populated land, and to those areas of the world unserved by terrestrial mobile systems.
The mobile satellite service is the last entrant to the public communication domain. Its introduction had to be deferred until the 1980s because of inadequate technical development, because of lack of commercial and political support earlier. Awareness of the capabilities of MSS is now growing rapidly, accompanied by a steady reduction in terminal and call charges. The MSS is therefore expected to be one of the largest growth areas in the next decade. Recent introductions of satellite hand-held telephones herald another milestone in the short history of satellite communications bringing satellite communications into the area of personal communications.
Several regional mobile satellite systems (e.g. in the USA and Canada) have
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OPTO ELECTRONICS emerged giving a wider choice to the users in the region. Several companies have introduced a variety of mobile communication systems offering services varying in range from low-bit rate messaging service to medium-data rate, voice and broadband service. Several proposed systems plan the use of low earth orbit or medium earth orbit satellite constellations in the space segment. Network concepts vary in complexity. At one end are architectures deploying low earth orbit satellite constellations, using on-board processing satellites interconnected in space via inter-satellite links (lSL) and providing voice capability (e.g. Richharia et al., 1989). At the other extreme are LEO networks using simple satellites to provide store-and-forward low-bit rate data communications without the need for using ISLs (Maral et al., 1991). The main problems with LEO/ MEa constellations appear to be the extreme complexity of such networks, especially if realtime worldwide networking via inter-satellite links is required, and the financial risk involved in fostering unproven technology too quickly. With the race for personal satellite telephones, new techniques, revolutionizing satellite communications, are emerging. Simple LEO networks to provide store and-forward communications are feasible with mature technologies, and have been the first systems to be deployed.
Several regional mobile satellite systems (e.g. in the USA and Canada) have emerged giving a wider choice to the users in the region. Several companies have introduced a variety of mobile communication systems offering services varying in range from low-bit rate messaging service to medium-data rate, voice and broadband service. Several proposed systems plan the use of low earth orbit or medium earth orbit satellite constellations in the space segment. Network concepts vary in complexity. At one end are architectures deploying low earth orbit satellite constellations, using on-board processing satellites interconnected in space via inter-satellite links (lSL) and providing voice capability (e.g. Richharia et al., 1989). At the other extreme are LEO networks using simple satellites to provide store-and-forward low-bit rate data communications without the need for using ISLs (Maral et al., 1991). The main problems with LEO/ MEa constellations appear to be the extreme complexity of such networks, especially if realtime worldwide networking via inter-satellite links is required, and the financial risk involved in fostering unproven technology too quickly. With the race for personal
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OPTO ELECTRONICS satellite telephones, new techniques, revolutionizing satellite communications, are emerging. Simple LEO networks to provide store and-forward communications are feasible with mature technologies, and have been the first systems to be deployed.
A recent interesting development has been the combination of navigation and communications capabilities into a single package. Hybrid terminals combining these two functions are already commercially available. These terminals utilize a navigation satellite system such as the global positioning system (GPS) of the USA or a local terrestrial system to obtain their location, and a communication system to transmit this information to a central location where these data are processed and used as necessary. An example of an emerging application in this area is fleet management by large trucking companies. Here a central location maintains and manages the movement of each vehicle in the fleet via a communication-navigation system, saving cost and time.
The aeronautical community has favoured satellites for transferring location and other safety-related data. A significant growth is also expected in aeronautical voice and data traffic.
One of the most difficult issues in mobile satellite service is that of spectrum sharing. The frequency range between ~ 800 MHz and ~ 2.5 GHz is best suited for mobile satellite communications. There are a number of other services, including terrestrial land mobile systems, radio determination services and radio astronomy, which also prefer the use of this band. Thus the main problems as far as the MSS serviceproviders are concerned are to obtain adequate spectrum for the MSS system from the international frequency management body and then to agree a mutually agreeable sharing arrangement of the allocated spectrum between themselves. International meetings, known as the World Radio Conference (WRC), convened in 1992 and 1995, and have allocated additional spectrum to the GEO and non-GEO MSS to meet rising demands. Despite these allocations, spectrum sharing between service providers is becoming
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OPTO ELECTRONICS progressively difficult. To maximize spectrum utilization the use of spot beam satellites is essential, even though generating spot beam at the L band is more difficult than at higher frequencies such as 11 GHz. Advanced techniques such as on-board processing are being employed by new systems. Spread spectrum modulation techniques are being introduced for operation in an interference-limited and multipath environment.
T.V. Broadcasting
In most developed countries, broadcast satellite systems are beginning to penetrate a market traditionally dominated by terrestrial systems. Broadcast satellite services have been introduced in several countries such as the USA, Japan, the UK and other European countries. BSS are also well suited to serve the developing world and communities isolated in remote areas of some developed countries. Satellite broadcast is sometimes the most cost-effective and quickest solution for such areas.
In Europe the customer base of broadcast channels transmitted by ASTRA satellites continues to grow steadily despite severe terrestrial competition. It is interesting to note that medium-power ASTRA satellites operating in the fixed satellite service have edged out a more powerful satellite designed specifically for direct-to-home broadcast. This happened as a consequence of a dramatic reduction in the cost of receivers brought about by the rapid development in terminal technology, together with the commercial advantage gained by the ASTRA system, through an early market entry. This is a good example of the increasing influence of commercial pressures on satellite communication technology. Another aspect of interest is the blurring of the fixed and broadcast satellite bands.
Among developing countries, India's satellite broadcast system is notable. The Indian domestic broadcast satellite system (INSAT) is used to distribute television programmes of national interest to widely separated terrestrial television and radio transmitters (Rao et al., 1987). Education and social awareness programmes are beamed
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OPTO ELECTRONICS daily to low-cost community receivers in remote towns and villages. The programmes are either received directly for community viewing or, where possible, received and retransmitted through terrestrial transmitters. The recent introduction of commercial channels by another operator is proving to be very popular. The main reason for this success is the wide choice of channels (not available until now) together with the affordable costs at which cable television companies are providing the service. This example indicates that the potential growth areas of BSS are those regions of the world where there is generally a lack of good viewing material - provided, of course, that the viewing cost is made affordable to the local community. Other regions which could benefit from this technology are African countries.
Summarizing, it can be stated that in the next decade the BSS can be expected to grow in various forms and continue to increase its customer base throughout the world. Digital television will gradually replace analog, and high- definition television is likely to be introduced. It is worth noting here that the worldwide W ARC plan for direct broadcast formulated in 1977 and 1981 has so far not been implemented to the extent anticipated.
WARC 92 has allocated spectrum for direct broadcast of sound to portable receivers. Direct-to-home sound broadcasts are already in use in some countries in Europe. Transmissions are made on television channels as sub-carriers and received by fixed receivers intended for receiving television broadcasts. However, W ARC has allocated spectrum specifically for broadcasts direct to portable radio receivers. Several organizations have shown interest in developing this technology further, and some companies in the USA have already put forward commercial proposals. Severe opposition and competition from existing terrestrial broadcast system operators, especially in the developed world, are possible. However, they are vast areas of the developing world which could benefit from this technology.
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OPTO ELECTRONICS NAVIGATION Global Navigation Satellite System (GNSS) is the standard generic term for satellite navigation systems that provide autonomous geo-spatial positioning with global coverage. GNSS allows small electronic receivers to determine their location (longitude, latitude, and altitude) to within a few meters using time signals transmitted along a lineof-sight by radio from satellites. Receivers on the ground with a fixed position can also be used to calculate the precise time as a reference for scientific experiments. As of 2009, the United States NAVSTAR Global Positioning System (GPS) is the only fully operational GNSS. The Russian GLONASS is a GNSS in the process of being restored to full operation. China has indicated it will expand its regional Beidou navigation system into the global COMPASS navigation system by 2015. The European Union's Galileo positioning system is a GNSS in initial deployment phase, scheduled to be operational in 2013. GNSS classification GNSS that provide enhanced accuracy and integrity monitoring usable for civil navigation are classified as follows: ď&#x201A;ˇ
GNSS-1 is the first generation system and is the combination of existing satellite navigation systems (GPS and GLONASS), with Satellite Based Augmentation Systems (SBAS) or Ground Based Augmentation Systems (GBAS). In the United States, the satellite based component is the Wide Area Augmentation System (WAAS), in Europe it is the European Geostationary Navigation Overlay Service (EGNOS), and in Japan it is the Multi-Functional Satellite Augmentation System (MSAS). Ground based augmentation is provided by systems like the Local Area Augmentation System (LAAS).
ď&#x201A;ˇ
GNSS-2 is the second generation of systems that independently provides a full civilian satellite navigation system, exemplified by the European Galileo positioning system. These systems will provide the accuracy and integrity
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OPTO ELECTRONICS monitoring necessary for civil navigation. This system consists of L1 and L2 frequencies for civil use and L5 for system integrity. Development is also in progress to provide GPS with civil use L2 and L5 frequencies, making it a GNSS2 system.
Core Satellite navigation systems, currently GPS, Galileo and GLONASS.
Global Satellite Based Augmentation Systems (SBAS) such as Omnistar and StarFire.
Regional SBAS including WAAS(US), EGNOS (EU), MSAS (Japan) and GAGAN (India).
Regional Satellite Navigation Systems such a QZSS (Japan), IRNSS (India) and Beidou (China).
Continental scale Ground Based Augmentation Systems (GBAS) for example the Australian GRAS and the US Department of Transportation National Differential GPS (DGPS) service.
Regional scale GBAS such as CORS networks.
Local GBAS typified by a single GPS reference station operating Real Time Kinematic (RTK) corrections.
History and theory Early predecessors were the ground based DECCA, LORAN and Omega systems, which used terrestrial long wave radio transmitters instead of satellites. These positioning systems broadcast a radio pulse from a known "master" location, followed by repeated pulses from a number of "slave" stations. The delay between the reception and sending of the signal at the slaves was carefully controlled, allowing the receivers to compare the delay between reception and the delay between sending. From this the distance to each of the slaves could be determined. The first satellite navigation system was Transit, a system deployed by the US military in the 1960s. Transit's operation was based on the Doppler Effect, the satellites traveled on well-known paths and broadcast their signals on a well known frequency. The
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OPTO ELECTRONICS received frequency will differ slightly from the broadcast frequency because of the movement of the satellite with respect to the receiver. By monitoring this frequency shift over a short time interval, the receiver can determine its location to one side or the other of the satellite, and several such measurements combined with a precise knowledge of the satellite's orbit can fix a particular position. Part of an orbiting satellite's broadcast included its precise orbital data. In order to ensure accuracy, the US Naval Observatory (USNO) continuously observed the precise orbits of these satellites. As a satellite's orbit deviated, the USNO would send the updated information to the satellite. Subsequent broadcasts from an updated satellite would contain the most recent accurate information about its orbit. Modern systems are more direct. The satellite broadcasts a signal that contains the position of the satellite and the precise time the signal was transmitted. The position of the satellite is transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses an atomic clock to maintain synchronization of all the satellites in the constellation. The receiver compares the time of broadcast encoded in the transmission with the time of reception measured by an internal clock, thereby measuring the time-of-flight to the satellite. Several such measurements can be made at the same time to different satellites, allowing a continual fix to be generated in real time. Each distance measurement, regardless of the system being used, places the receiver on a spherical shell at the measured distance from the broadcaster. By taking several such measurements and then looking for a point where they meet, a fix is generated. However, in the case of fast-moving receivers, the position of the signal moves as signals are received from several satellites. In addition, the radio signals slow slightly as they pass through the ionosphere, and this slowing varies with the receiver's angle to the satellite, because that changes the distance through the ionosphere. The basic computation thus attempts to find the shortest directed line tangent to four oblate spherical shells centered on four satellites. Satellite navigation receivers reduce errors by using combinations of signals from multiple satellites and multiple correlates, and then
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OPTO ELECTRONICS using techniques such as Kalman filtering to combine the noisy, partial, and constantly changing data into a single estimate for position, time, and velocity. Civil and military uses The original motivation for satellite navigation was for military applications. Satellite navigation allows for hitherto impossible precision in the delivery of weapons to targets, greatly increasing their lethality whilst reducing inadvertent casualties from misdirected weapons. (See smart bomb). Satellite navigation also allows forces to be directed and to locate themselves more easily, reducing the fog of war. Satellite navigation using a laptop and a GPS receiver In these ways, satellite navigation can be regarded as a force multiplier. In particular, the ability to reduce unintended casualties has particular advantages for wars where public relations are an important aspect of warfare. For these reasons, a satellite navigation system is an essential asset for any aspiring military power. GNSS systems have a wide variety of uses:
Navigation, ranging from personal hand-held devices for hiking, to devices fitted to cars, trucks, ships and aircraft
Time transfer and synchronization
Location-based services such as enhanced 911
Surveying
Entering data into a geographic information system
Search and rescue
Geophysical Sciences
Tracking devices used in wildlife management
Asset Tracking, as in trucking fleet management
Road Pricing
Location-based media
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OPTO ELECTRONICS Note that the ability to supply satellite navigation signals is also the ability to deny their availability. The operator of a satellite navigation system potentially has the ability to degrade or eliminate satellite navigation services over any territory it desires. Current global navigation systems The United States' Global Positioning System (GPS), which as of 2007 is the only fully functional, fully available global navigation satellite system. It consists of up to 32 medium Earth orbit satellites in six different orbital planes, with the exact number of satellites varying as older satellites are retired and replaced. Operational since 1978 and globally available since 1994, GPS is currently the world's most utilized satellite navigation system. Glonass The formerly Soviet, and now Russian, GLObal'naya NAvigatsionnaya Sputnikovaya Sistema, or GLONASS, was a fully functional navigation constellation but since the collapse of the Soviet Union has fallen into disrepair, leading to gaps in coverage and only partial availability. The Russian Federation has pledged to restore it to full global availability by 2010 with the help of India, who is participating in the restoration project. Global navigation systems China has indicated they intend to expand their regional navigation system, called Beidou or Big Dipper, into a global navigation system; a program that has been called Compass in China's official news agency Xinhua. The Compass system is proposed to utilize 30 medium Earth orbit satellites and five geostationary satellites. Having announced they are willing to cooperate with other countries in Compass's creation, it is unclear how this proposed program impacts China's commitment to the international Galileo position system. Galileo
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The European Union and European Space Agency agreed on March 2002 to introduce their own alternative to GPS, called the Galileo positioning system. At a cost of about GBP £2.4 billion, the system is scheduled to be working from 2012. The first experimental satellite was launched on 28 December 2005. Galileo is expected to be compatible with the modernized GPS system. The receivers will be able to combine the signals from both Galileo and GPS satellites to greatly increase the accuracy. Mobile Communication
The ideal solution for mobile users is the ability to use the same mobile phone throughout the world or at least over large parts of the world. The CCIR/CCITT are studying such a concept which was formerly called future public land mobile telecommunication systems (FPLMTS) and is now called International mobile Telecommunication 2000 (IMT-2000).
The goal of this network is too provide
telecommunication service to mobile terminals through an integrated network.
The
network architecture consists of terrestrial and satellite system elements in which a variety of mobile terminals are connected too the network through standardized interfaces, as shown in figure 12.1. The ultimate goal is too have this all-pervading network operational throughout the globe.
It has been mentioned that the fundamental limitations of a geostationary satellite communication system are large transmission delay and path loss. Improvements to the existing echo control techniques and delay compensation protocols are underway for minimizing the problem of transmission delay. On a futuristic not, some radical solutions have been proposed (Pelton and Wu, 1987). The use of such concepts could virtually eliminate the inherent disadvantage of the geostationary orbit. The use of tethered satellites is one such concept. Here a satellite is made to appear stationary with respect to the Earth at a lower altitude by suspending it from a geostationary satellite with the help of a tether. To compensate the torque on the geostationary ‘anchor’ satellite, another tether is suspended upwards as a counter-weight. Some trials to tether satellites have been conducted by NANS with limited success. Delay is minimized because of the much
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shorter range of the satellite. Another concept is the use of very lightweight satellites maintained in a â&#x20AC;&#x2DC;geosynchronousâ&#x20AC;&#x2122; orbit at say 800 km, powered by energy beamed from the ground. The use of the LEO satellite constellation concept as proposed for mobile satellite service is another possible solution. Such zero delay satellites could provide nearly all the capabilities of optical fibre coupled with the added advantages of satellite systems.
We have noted earlier that satellite circuit costs are reduced by extending the operational lifetime of the satellite.
Several ideas have been proposed for satellite
lifetime extension. These include: 1.
The
use
of
space
platforms
with
the
capability
of
being
refurbished/retrofitted/repaired by robots and service vehicles. The lifetime is estimated to extend up to o40 years by such a system. 2. The use of satellite clusters with the capability of being activated and updated over time according to need.
Following the recent spate of proposals for low earth orbit systems, it is logical to contemplate launching a system consisting of a constellation of air platforms, as such a system would essentially be terrestrial and hence the platforms would be physically accessible, giving obvious advantages. These types of system could also be potentially less expensive. Not surprisingly, there is a growing interest in deploying such systems, as they could potentially compete with satellite services. As a consequence, the most recent World Radio Conference has allocated spectrum around 40 GHz for such stratospheric systems. The idea of using air platforms to replace/argument base sites in terrestrial cellular systems has been considered in the past; but is not yet accepted owing to the problems of maintaining the airborne platform, difficult security, possible hazards to aircraft, possible infringement of local air traffic regulations, and reduced spectrum efficiency compared with ground cells. Such problems are multiplied by several orders of magnitude when considering a global system. In some regions of the world it may not even be possible to launch such platforms owing to political factors and lack of infrastructure. Nevertheless, with due effort, most technical problems can be solved,
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OPTO ELECTRONICS though the financial viability of stratospheric systems is another matter. Table 11.1 includes the number of 40km altitude airborne platforms necessary to cover the earth using a dynamic polar constellation as well as stationary platforms. The altitude was chosen on the basis of a development being sponsored by NASA to launch a balloon at such an altitude in the year 2000 with the intention to support scientific studies from stratospheric balloons. If proven successful, balloon systems may be offered at a fraction of the cost of LEO Scientific satellites. For a 5 elevation the stationary platform system requires 1638 satellites, and the polar constellation consists of 2888 satellites for global coverage. The launch cost alone, at US$1m per launch, would be in the region of $1.6 to $2.8 billion, rising to ~$4.6 to $7.7 billion if the minimum elevation angle is increased to 10. The cost becomes astronomical if payload, maintenance and logistical costs are included and certainly more than the most expensive satellite system proposed recently. However, one could envisage localized communication systems which could be deployed within in practical constraints mentioned above. A similar approach for providing cellular – like coverage to ship using floating base stations on oceans has been proposed. Such a system could potentially compete directly with mobile satellite systems which provide services over oceans. TV BROADCASTING Satellite television is television delivered by the means of communications satellite and received by a satellite dish and set-top box. In many areas of the world it provides a wide range of channels and services, often to areas that are not serviced by terrestrial or cable providers. History The first satellite television signal was relayed from Europe to the Telstar satellite over North America in 1962. The first geosynchronous communication satellite, Syncom 2 was launched in 1963. The world's first commercial communication satellite, called Intelsat I (nicknamed Early Bird), was launched into synchronous orbit on April 6, 1965.
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OPTO ELECTRONICS The first national network of satellite television, called Orbita, was created in Soviet Union in 1967, and was based on the principle of using the highly elliptical Molniya satellite for re-broadcasting and delivering of TV signal to ground downlink stations. The first domestic North American satellite to carry television was Canada’s geostationary Anik 1, which was launched in 1972. ATS-6, the world's first experimental educational and Direct Broadcast Satellite, was launched in 1974. The first Soviet geostationary satellite to carry Direct-To-Home television, called Ekran, was launched in 1976.
Technology Satellites used for television signals are generally in either naturally highly elliptical (with inclination of +/-63.4 degrees and orbital period of about 12 hours, also known as Molniya orbit) or geostationary orbit 37,000 km (22,300 miles) above the earth’s equator. Satellite television, like other communications relayed by satellite, starts with a transmitting antenna located at an uplink facility. Uplink satellite dishes are very large, as much as 9 to 12 meters (30 to 40 feet) in diameter. The increased diameter results in more accurate aiming and increased signal strength at the satellite. The uplink dish is pointed toward a specific satellite and the up linked signals are transmitted within a specific frequency range, so as to be received by one of the transponders tuned to that frequency range aboard that satellite. The transponder 'retransmits' the signals back to Earth but at a different frequency band (a process known as translation, used to avoid interference with the uplink signal), typically in the C-band (4–8 GHz) or Ku-band (12–18 GHz) or both. The leg of the signal path from the satellite to the receiving Earth station is called the downlink. A typical satellite has up to 32 transponders for Ku-band and up to 24 for a Cband only satellite, or more for hybrid satellites. Typical transponders each have a bandwidth between 27 MHz and 50 MHz. Each geo-stationary C-band satellite needs to be spaced 2 degrees from the next satellite (to avoid interference). For Ku the spacing can
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OPTO ELECTRONICS be 1 degree. This means that there is an upper limit of 360/2 = 180 geostationary C-band satellites and 360/1 = 360 geostationary Ku-band satellites. C-band transmission is susceptible to terrestrial interference while Ku-band transmission is affected by rain (as water is an excellent absorber of microwaves at this particular frequency).
The down linked satellite signal, quite weak after traveling the great distance (see inverse-square law), is collected by a parabolic receiving dish, which reflects the weak signal to the dishâ&#x20AC;&#x2122;s focal point. Mounted on brackets at the dish's focal point is a device called a feed horn. This feed horn is essentially the flared front-end of a section of waveguide that gathers the signals at or near the focal point and 'conducts' them to a probe or pickup connected to a low-noise block down converter or LNB. The LNB amplifies the relatively weak signals, filters the block of frequencies in which the satellite TV signals are transmitted, and converts the block of frequencies to a lower frequency range in the L-band range. The evolution of LNBs was one of necessity and invention. The original C-Band satellite TV systems used a Low Noise Amplifier connected to the feed horn at the focal point of the dish. The amplified signal was then fed via very expensive 50 Ohm impedance coaxial cable to an indoor receiver or in other designs fed to a down converter (a mixer and a voltage tuned oscillator with some filter circuitry) for down conversion to an intermediate frequency. The channel selection was controlled, typically by a voltage tuned oscillator with the tuning voltage being fed via a separate cable to the head end. But this simple design evolved. Designs for micro strip based converters for Amateur Radio frequencies were adapted for the 4 GHz C-Band. Central to these designs was concept of block down conversion of a range of frequencies to a lower, and technologically more easily handled block of frequencies (intermediate frequency). The advantages of using an LNB are that cheaper cable could be used to connect the indoor receiver with the satellite TV dish and LNB, and that the technology for handling the signal at L-Band and UHF was far cheaper than that for handling the signal
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OPTO ELECTRONICS at C-Band frequencies. The shift to cheaper technology from the 50 Ohm impedance cable and N-Connectors of the early C-Band systems to the cheaper 75 Ohm technology and F-Connectors allowed the early satellite TV receivers to use, what were in reality, modified UHF TV tuners which selected the satellite television channel for down conversion to another lower intermediate frequency centered on 70 MHz where it was demodulated. This shift allowed the satellite television DTH industry to change from being a largely hobbyist one where receivers were built in low numbers and complete systems were expensive (costing thousands of Dollars) to a far more commercial one of mass production. Direct broadcast satellite dishes are fitted with an LNBF, which integrates the feed horn with the LNB. The satellite receiver demodulates and converts the signals to the desired form (outputs for television, audio, data, etc.). Sometimes, the receiver includes the capability to unscramble or decrypt; the receiver is then called an integrated receiver/decoder or IRD. The cable connecting the receiver to the LNBF or LNB must be of the low loss type RG-6, quad shield RG-6 or RG-11, etc. It cannot be standard RG-59. (A new form of omni directional satellite antenna, which does not use a directed parabolic dish and can be used on a mobile platform such as a vehicle, was recently announced by the University of Waterloo.) Standards Analog television distributed via satellite is usually sent scrambled or unscrambled in NTSC, PAL, or SECAM television broadcast standards. The analog signal is frequency modulated and is converted from an FM signal to what is referred to as baseband. This baseband comprises the video signal and the audio sub carrier(s). The audio sub carrier is further demodulated to provide a raw audio signal. If the signal is a digitized television signal or multiplex of signals, it is typically QPSK.
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OPTO ELECTRONICS In general, digital television, including that transmitted via satellites, is generally based on open standards such as MPEG and DVB-S. The conditional access encryption/scrambling methods include BISS, Conax, Dig cipher, Irdeto, Nagravision, PowerVu, Viaccess, Videocipher, and VideoGuard. Many conditional access systems have been compromised. Categories of usage There are three primary types of satellite television usage: reception direct by the viewer, reception by local television affiliates, or reception by head ends for distribution across terrestrial cable systems. Direct to the viewer reception includes direct broadcast satellite or DBS and television receive-only or TVRO, both used for homes and businesses including hotels, etc. Direct broadcast via satellite Direct broadcast satellite, (DBS) also known as "Direct-To-Home" is a relatively recent development in the world of television distribution. â&#x20AC;&#x153;Direct broadcast satelliteâ&#x20AC;? can either refer to the communications satellites themselves that deliver DBS service or the actual television service. DBS systems are commonly referred to as "mini-dish" systems. DBS uses the upper portion of the Ku band, as well as portions of the Ka band. Modified DBS systems can also run on C-band satellites and have been used by some networks in the past to get around legislation by some countries against reception of Ku-band transmissions. Most of the DBS systems use the DVB-S standard for transmission. With Pay-TV services, the DataStream is encrypted and requires proprietary reception equipment. While the underlying reception technology is similar, the Pay-TV technology is proprietary, often consisting of a Conditional Access Module and smart card.
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OPTO ELECTRONICS This measure assures satellite television providers that only authorized-, paying subscribers have access to Pay TV content but at the same time can allow free-to-air (FTA) channels to be viewed even by the people with standard equipment (DBS receivers without the Conditional Access Modules) available in the market. Television receive-only The term Television receive-only, or TVRO, arose during the early days of satellite television reception to differentiate it from commercial satellite television uplink and downlink operations (transmit and receive). This was before there was a DTH satellite television broadcast industry. Satellite television channels at that time were intended to be used by cable television networks rather than received by home viewers. Satellite TV receiver systems were largely constructed by hobbyists and engineers. These TVRO system operated mainly on the C band frequencies and the dishes required were large; typically over 3 meters (10 ft) in diameter. Consequently TVRO is often referred to as "big dish" or "Big Ugly Dish" (BUD) satellite television. TVRO systems are designed to receive analog and digital satellite feeds of both television and audio from both C-band and Ku-band transponders on FSS-type satellites. The higher frequency Ku-band systems tend to be Direct To Home systems and can use a smaller dish antenna because of the higher power transmissions and greater antenna gain. TVRO systems tend to use larger rather than smaller satellite dish antennas, since it is more likely that the owner of a TVRO system would have a C-band-only setup rather than a Ku band-only setup. Additional receiver boxes allow for different types of digital satellite signal reception, such as DVB/MPEG-2 and 4DTV. The narrow beam width of a normal parabolic satellite antenna means it can only receive signals from a single satellite at a time. Simulate or the Vertex-RSI TORUS, is a quasi-parabolic satellite earth station antenna that is capable of receiving satellite transmissions from 35 or more C- and Ku-band satellites simultaneously.
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OPTO ELECTRONICS Direct to home television Today, most satellite TV customers in developed television markets get their programming through a direct broadcast satellite (DBS) provider, such as DISH TV or DTH platform. The provider selects programs and broadcasts them to subscribers as a set package. Basically, the provider’s goal is to bring dozens or even hundreds of channels to the customer’s television in a form that approximates the competition from Cable TV. Unlike earlier programming, the provider’s broadcast is completely digital, which means it has high picture and stereo sound quality. Early satellite television was broadcast in Cband - radio in the 3.4-gigahertz (GHz) to 7 GHz frequency range. Digital broadcast satellite transmits programming in the Ku frequency range (10 GHz to 14 GHz ). There are five major components involved in a direct to home (DTH) satellite system: the programming source, the broadcast center, the satellite, the satellite dish and the receiver. Programming sources are simply the channels that provide programming for broadcast. The provider (the DTH platform) doesn’t create original programming itself; it pays other companies (HBO, for example, or ESPN or STAR TV or Sahara etc.) for the right to broadcast their content via satellite. In this way, the provider is kind of like a broker between the viewer and the actual programming sources. (Cable television networks also work on the same principle.) The broadcast center is the central hub of the system. At the broadcast center or the Playout & Uplink location, the television provider receives signals from various programming sources, compresses these signals using digital compression (scrambling if necessary), and beams a broadcast signal to the proper satellite. The satellite receives the signal from the broadcast station and rebroadcast them to the ground. The viewer’s dish picks up the signal from the satellite (or multiple satellites in the same part of the sky) and passes it on to the receiver in the viewer’s house. The receiver processes the signal and passes it on to a standard television. Programming Satellite TV providers get programming from two major sources: International turnaround channels (such as HBO, ESPN and CNN, STAR TV, SET, B4U etc) and
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OPTO ELECTRONICS various local channels (SaBe TV, Sahara TV, Doordarshan, etc). Most of the turnaround channels also provide programming for cable television, so sometimes some of the DTH platforms will add in some special channels exclusive to itself to attract more subscriptions. Turnaround channels usually have a distribution center that beams their programming to a geostationary satellite. The broadcast center uses large satellite dishes to pick up these analog and digital signals from several sources. Broadcasting centers The broadcast center converts all of this programming into a high-quality, uncompressed digital stream. At this point, the stream contains a vast quantity of data â&#x20AC;&#x201D; about 270 megabits per second (Mbit/s) for each channel. In order to transmit the signal from there, the broadcast center has to compress it. Otherwise, it would be too big for the satellite to handle. The providers use the MPEG-2 compressed video format â&#x20AC;&#x201D; the same format used to store movies on DVDs. With MPEG-2 compression, the provider can reduce the 270-Mbit/s stream to about 3 or 10 Mbit/s (depending on the type of programming). This is the crucial step that has made DTH service a success. With digital compression, a typical satellite can transmit about 200 channels. Without digital compression, it can transmit about 30 channels. At the broadcast center, the high-quality digital stream of video goes through an MPEG-2 encoder, which converts the programming to MPEG-2 video of the correct size and format for the satellite receiver in your house. Encryption and transmission After the video is compressed, the provider needs to encrypt it in order to keep people from accessing it for free. Encryption scrambles the digital data in such a way that it can only be decrypted (converted back into usable data) if the receiver has the correct decoding satellite receiver with decryption algorithm and security keys. Once the signal is compressed and encrypted, the broadcast center beams it directly to one of its satellites. The satellite picks up the signal, amplifies it and beams it back to Earth, where viewers can pick it up.
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OPTO ELECTRONICS Dish A satellite dish is just a special kind of antenna designed to focus on a specific broadcast source. The standard dish consists of a parabolic (bowl-shaped) surface and a central feed horn. To transmit a signal, a controller sends it through the horn, and the dish focuses the signal into a relatively narrow beam. The dish on the receiving end can’t transmit information; it can only receive it. The receiving dish works in the exact opposite way of the transmitter. When a beam hits the curved dish, the parabola shape reflects the radio signal inward onto a particular point, just like a concave mirror focuses light onto a particular point. The curved dish focuses incoming radio waves onto the feed horn. In this case, the point is the dish’s feed horn, which passes the signal onto the receiving equipment. In an ideal setup, there aren’t any major obstacles between the satellite and the dish, so the dish receives a clear signal. In some systems, the dish needs to pick up signals from two or more satellites at the same time. The satellites may be close enough together that a regular dish with a single horn can pick up signals from both. This compromises quality somewhat, because the dish isn’t aimed directly at one or more of the satellites. A new dish design uses two or more horns to pick up different satellite signals. As the beams from different satellites hit the curved dish, they reflect at different angles so that one beam hits one of the horns and another beam hits a different horn. The central element in the feed horn is the low noise block down converter, or LNB. The LNB amplifies the signal bouncing off the dish and filters out the noise (signals not carrying programming). The LNB passes the amplified, filtered signal to the satellite receiver inside the viewer’s house. The receiver The end component in the entire satellite TV system is the receiver. The receiver has four essential jobs: It de-scrambles the encrypted signal. In order to unlock the signal, the receiver needs the proper decoder chip for that programming package. The provider can communicate with the chip, via the satellite signal, to make necessary adjustments to its decoding programs. The provider may occasionally send signals that disrupt illegal descramblers, as an electronic counter measure (ECM) against illegal users. It takes the
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OPTO ELECTRONICS digital MPEG-2 signal and converts it into an analog format that a standard television can recognize. Since the receiver spits out only one channel at a time, you canâ&#x20AC;&#x2122;t tape one program and watch another. You also canâ&#x20AC;&#x2122;t watch two different programs on two TVs hooked up to the same receiver. In order to do these things, which are standard on conventional cable, you need to buy an additional receiver. Some receivers have a number of other features as well. They pick up a programming schedule signal from the provider and present this information in an onscreen programming guide. Many receivers have parental lock-out options, and some have built-in Digital Video Recorders (DVRs), which let you pause live television or record it on a hard drive. While digital broadcast satellite service is still lacking some of the basic features of conventional cable (the ability to easily split signals between different TVs and VCRs, for example), its highquality picture, varied programming selection and extended service areas are features now seen as an alternative. With the rise of digital cable, which also has improved picture quality and extended channel selection, the TV war is really heating up. Just about anything could happen in the next 10 years as all of these television providers battle it out. Direct broadcast satellite
Direct broadcast satellite (DBS) is a term used to refer to satellite television broadcasts intended for home reception, also referred to more broadly as direct-to-home signals. The expression direct-to-home or DTH was, initially, meant to distinguish the transmissions directly intended for home viewers from cable television distribution services that sometimes carried on the same satellite. The term predates DBS satellites and is often used in reference to services carried by lower power satellites which required larger dishes (1.7m diameter or greater) for reception. SATELLITE RADIO A satellite radio (SR) is a digital radio signal that is broadcast by a communications satellite, which covers a much wider geographical range than terrestrial radio signals.
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OPTO ELECTRONICS For now, satellite radio offers a meaningful alternative to ground-based radio services in some countries, notably the United States. Mobile services, such as Sirius, XM, and World space, allow listeners to roam across an entire continent, listening to the same audio programming anywhere they go. Other services, such as Music Choice or Muzak's satellite-delivered content, require a fixed-location receiver and a dish antenna. In all cases, the antenna must have a clear view to the satellites. In areas where tall buildings, bridges, or even parking garages obscure the signal, repeaters can be placed to make the signal available to listeners. Radio services are usually provided by commercial ventures and are subscriptionbased. The various services are proprietary signals, requiring specialized hardware for decoding and playback. Providers usually carry a variety of news, weather, sports, and music channels, with the music channels generally being commercial-free. In areas with a relatively high population density, it is easier and less expensive to reach the bulk of the population with terrestrial broadcasts. Thus in the UK and some other countries, the contemporary evolution of radio services is focused on Digital Audio Broadcasting (DAB) services or HD Radio, rather than satellite radio. Applications Satellite radio, particularly in the United States, has become a major provider of background music to businesses such as hotels, retail chains, and restaurants. Compared to old-line competitors such as Muzak, satellite radio's significantly lower price, commercial-free channel variety, and more reliable technology make it a very attractive option.Both North American satellite radio providers offer business subscriptions, though given the merger of XM Satellite Radio with Sirius, the future of XM for Business is uncertain. Sirius's commercial services are provided nationally by third-party partner Applied Media Technologies Corporation. System design
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OPTO ELECTRONICS Satellite radio uses the 2.3 GHz S band in North America and generally shares the 1.4 GHz L band with local Digital Audio Broadcasting (DAB) stations elsewhere. It is a type of direct broadcast satellite and is strong enough that it requires no satellite dish to receive. Curvature of the earth limits the reach of the signal, but due to the high orbit of the satellites, two or three are usually sufficient to provide coverage for an entire continent. Local repeaters similar to broadcast translator boosters enable signals to be available even if the view of the satellite is blocked, for example, by skyscrapers in a large town. Major tunnels can also have repeaters. This method also allows local programming to be transmitted such as traffic and weather in most major metropolitan areas, as of March 2004. Each receiver has an Electronic Serial Number (ESN) Radio ID to identify it. When a unit is activated with a subscription, an authorization code is sent in the digital stream telling the receiver to allow access to the blocked channels. Most services have at least one "free to air" or "in the clear" (ITC) channel as a test. For example, Sirius uses channel 184, Sirius Weather & Emergency. Most (if not all) of the systems in use now are proprietary, using different codecâ&#x20AC;&#x2122;s for audio data compression, different modulation techniques, and/or different methods for encryption and conditional access. Like other radio services, satellite radio also transmits program-associated data (PAD or metadata), with the artist and title of each song or program and possibly the name of the channel. The sound quality with both satellite radio providers and DTR providers varies with each channel. Some channels have near CD-quality audio, and others use lowbandwidth audio suitable only for speech. Since only a certain amount of bandwidth is available within the licenses available, adding more channels means that the quality on some channels must be reduced. Both the frequency response and the dynamic range of satellite channels can be superior to most, but not all AM or FM radio stations, as most
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OPTO ELECTRONICS AM and FM stations clip the audio peaks to sound louder; even the worst channels are still superior to most AM radios, but a very few AM tuners are equal to or better than the best FM or satellite broadcasts when tuned to a local station, even if not capable of stereo. AM does not suffer from multipath distortion or flutter in a moving vehicle like FM, nor does it become silent as you go behind a big hill like satellite radio. Some satellite radio services and DTR services act as in situ repeaters for local AM/FM stations and thus feature a high frequency of interruption. Nonprofit stations and public radio networks such as CBC/Radio-Canada, NPR, and PRI-affiliated stations and the BBC are commercial-free. In the US, all stations are required to have periodic station identifications and public service announcements.
In the United States, the FCC regulates technical broadcast spectrum only. Program content is unregulated. However, the FCC has tried in the past to expand its reach to regulate content to satellite radio and cable television, and its options are still open to attempt such in the future. The FCC does issue licenses to both satellite radio providers (XM and Sirius) and controls that hold these licenses to broadcast. Degree of content regulation varies by country; however, the majority of industrialized nations have regulations regarding obscene and/or objectionable content. SATELLITE TELEPHONE Satellite phone, or sat phone is a type of mobile phone that connects to orbiting satellites instead of terrestrial cell sites. Depending on the architecture of a particular system, coverage may include the entire Earth, or only specific regions. Satellite phone
The mobile equipment, also known as a terminal, varies widely. Early satellite phone handsets had a size and weight comparable to that of a late 1980s or early 1990s
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OPTO ELECTRONICS mobile phone, but usually with a large retractable antenna. More recent satellite phones are similar in size to a regular mobile phone while some prototype satellite phones have no distinguishable difference from an ordinary smart phone. Sat phones are popular on expeditions into remote areas where terrestrial cellular service is unavailable. A fixed installation, such as used shipboard, may include large, rugged, rackmounted electronics, and a steer able microwave antenna on the mast that automatically tracks the overhead satellites. Satellite phones have notoriously poor reception indoors, though it may be possible to get a consistent signal near a window or in the top floor of a building if the roof is sufficiently thin. The phones have connectors for external antennas that are often installed in vehicles and buildings. Some systems also allow for the use of repeaters, much like terrestrial mobile phone systems. In some countries ruled by oppressive regimes, such as Burma, possession of a satellite phone is illegal as their signals will usually bypass local telecoms systems, hindering censorship and wiretapping attempts. In Australia, residents of remote areas y apply for a government subsidy for a satellite phone. Satellite phone networks Geosynchronous services Some satellite phones use satellites in geosynchronous orbit. These systems can maintain near-continuous global coverage with only three or four satellites, reducing the launch costs. However the satellites used for these systems are very heavy (approx. 5000kg) and therefore very expensive to build and launch. The satellites sit at an altitude of about 22,000 miles (35,000 km); a noticeable delay is present while making a phone call or using data services. The amount of bandwidth available on these systems is substantially higher than that of the Low Earth Orbit (LEO) systems; all three active systems provide portable satellite internet using laptop-sized terminals with speeds ranging from 60 kbits to 512 kbits.
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OPTO ELECTRONICS Another disadvantage of geostationary satellite systems is that in many areas—even where a large amount of open sky is present—the line-of-sight between the phone and the satellite is broken by obstacles such as steep hills and forest. The user will need to find an area with line-of-sight before being able to use the phone. This is not the case with LEO services: even if the signal is blocked by an obstacle, one can wait a few minutes until another satellite passes overhead.
ACeS: This small regional operator provides voice and data services in East Asia using a single satellite.
Inmarsat: The oldest satellite phone operator, founded in 1979. It originally provided large fixed installations intended for use on ships, but has only recently started to enter the market of hand-held phones in a joint venture with ACeS. The company operates eleven satellites with another planned for launch in 2008. Coverage is available on most of the earth's surface, notably excepting Polar Regions.
Thuraya: A system based in the UAE. Three satellites are currently in active service that provides coverage to the most of Eurasia, Africa and Australia. There is some degree of coverage overlap between adjacent satellites within the network.
MSAT / Mobile Satellite Ventures: An American satellite phone company which uses equipment similar to INMARSAT, but plans to launch a service using handheld devices in the Americas similar to Thuraya's.
Low Earth orbit LEO telephones utilizes (low Earth orbit) satellite technology. The advantages include providing worldwide wireless coverage with no gaps. LEO satellites orbit the earth in high speed, low altitude orbits with an orbital time of 70–100 minutes, an altitude of 640 to 1120 kilometers (400 to 700 miles), and provide coverage cells of about (at a 100-minute orbital period) 2800km in radius (about 1740mi). Since the satellites are not
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OPTO ELECTRONICS geosynchronous, they must fly complete orbits. At least one satellite must have line-ofsight to every coverage area at all times to guarantee complete coverage. Depending on the positions of both the satellite and terrestrial user, a usable pass of an individual LEO satellite will typically last 4–15 minutes on average; thus, a constellation of satellites is required to maintain coverage (as is done with Iridium, Global Star, GPS, and others). Two such systems both based in the United States started up in the late 1990s but soon went into bankruptcy after they failed to gain the number of subscribers required to fund the large satellite launch costs. They are now operated by new owners who bought the assets for a fraction of their original cost and are now both planning to launch replacement constellations supporting higher bandwidth. Data speeds for current networks are between 2200 bit/s and 9600 bit/s using a satellite handset.
Globalstar: A network covering most of the world’s landmass using 44 active satellites; however many areas is left without coverage since a satellite must be in range of an earth station. Satellites fly in an inclined orbit of 52 degrees; as such, Polar Regions cannot be covered. The network went into limited commercial service at the end of 1999.
Iridium: A network operating 66 satellites in a polar orbit that claims coverage everywhere on the earth's surface. Commercial service started in November 1998 and fell into bankruptcy soon after. Notably radio cross-links are used between satellites in order to relay data to the nearest satellite with a connection to an earth station.
Tracking LEO systems have the ability to track a mobile unit's location using Doppler shift calculations from the satellite. However, this method can be inaccurate by tens of kilometers. On some Iridium hardware the coordinates can be extracted using AT commands, while recent Global star handsets will display them on the screen.
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OPTO ELECTRONICS Proposed systems
ICO Satellite Management: A satellite phone company which has launched a single geosynchronous satellite which is not yet in active service.
Teledesic: An ill-fated company backed by Microsoft which planned to provide broadband internet using a network of 840 LEO satellites, it ended up launching only one test satellite.
Terrestar: Proposed satellite phone system for North America
Ellipso: Start up that entered a partnership with ICO
One-way services Some satellite phone networks provide a one-way paging channel to alert users in poor coverage areas of an incoming call. When the alert is received on the satellite phone it must be taken to an area with better coverage before the call can be accepted. Global star provides a one-way data uplink service, typically used for asset tracking. Iridium operates a one-way pager service as well as the call alert feature. INTERNET SERVICE An Internet service provider (ISP, also called Internet access provider, or IAP) is a company that offers its customer’s access to the Internet. The ISP connects to its customers using a data transmission technology appropriate for delivering Internet Protocol datagram’s, such as dial-up, DSL, cable modem, wireless or dedicated highspeed interconnects. ISPs may provide Internet e-mail accounts to users which allow them to communicate with one another by sending and receiving electronic messages through their ISPs' servers. (As part of their e-mail service, ISPs usually offer the user an e-mail
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OPTO ELECTRONICS client software package, developed either internally or through an outside contract arrangement.) ISPs may provide other services such as remotely storing data files on behalf of their customers, as well as other services unique to each particular ISP. End-User-to-ISP Connection ISPs employ a range of technologies to enable consumers to connect to their network. For users and small businesses, the most popular options include dial-up, DSL (typically Asymmetric Digital Subscriber Line, ADSL), broadband wireless, cable modem, fiber to the premises (FTTH), and Integrated Services Digital Network (ISDN) (typically basic rate interface). For customers with more demanding requirements, such as medium-to-large businesses, or other ISPs, DSL (often SHDSL or ADSL), Ethernet, Metro Ethernet, Gigabit Ethernet, Frame Relay, ISDN (BRI or PRI), ATM, satellite Internet access and synchronous optical networking (SONET) are more likely to be used. Typical home user connection
Dial-up
DSL
Broadband wireless access
Cable Internet
FTTH
ISDN
Wi-Fi
Typical business type connection
DSL
SHDSL
Ethernet technologies
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OPTO ELECTRONICS Locality When using a dial-up or ISDN connection method, the ISP cannot determine the caller's physical location to more detail than using the number transmitted using an appropriate form of Caller ID; it is entirely possible to e.g. connect to an ISP located in Mexico from the USA. Other means of connection such as cable or DSL require a fixed registered connection node, usually associated at the ISP with a physical address. ISP Interconnection Just as their customers pay them for Internet access, ISPs themselves pay upstream ISPs for Internet access. An upstream ISP usually has a larger network than the contracting ISP and/or is able to provide the contracting ISP with access to parts of the Internet the contracting ISP by itself has no access to. In the simplest case, a single connection is established to an upstream ISP and is used to transmit data to or from areas of the Internet beyond the home network; this mode of interconnection is often cascaded multiple times until reaching a Tier 1 carrier. In reality, the situation is often more complex. ISPs with more than one point of presence (PoP) may have separate connections to an upstream ISP at multiple PoPs, or they may be customers of multiple upstream ISPs and may have connections to each one of them at one or more point of presence. Peering ISPs may engage in peering, where multiple ISPs interconnect at peering points or Internet exchange points (IXs), allowing routing of data between each network, without charging one another for the data transmittedâ&#x20AC;&#x201D;data that would otherwise have passed through a third upstream ISP, incurring charges from the upstream ISP. Physical interconnections for peering Scheme of interconnection and peering of autonomous systems
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OPTO ELECTRONICS The physical interconnections used for peering are categorized into two types: ď&#x201A;ˇ
Public peering - Interconnection utilizing a multi-party shared switch fabric such as an Ethernet switch.
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Private peering - Interconnection utilizing a point-to-point link between two parties.
Public peering Public peering is accomplished across a Layer 2 access technology, generally called a shared fabric. At these locations, multiple carriers interconnect with one or more other carriers across a single physical port. Historically, public peering locations were known as network access points (NAPs); today they are most often called exchange points or Internet exchanges ("IXP" or "IX"). Many of the largest exchange points in the world can have hundreds of participants, and some span multiple buildings and collocation facilities across a city. Since public peering allows networks interested in peering to interconnect with many other networks through a single port, it is often considered to offer "less capacity" than private peering, but to a larger number of networks. Many smaller networks, or networks who are just beginning to peer, find that public peering exchange points provide an excellent way to meet and interconnect with other networks that may be open to peering with them. Some larger networks utilize public peering as a way to aggregate a large number of "smaller peers", or as a location for conducting low-cost "trial peering" without the expense of provisioning private peering on a temporary basis, while other larger networks are not willing to participate at public exchanges at all. A few exchange points, particularly in the United States, are operated by commercial carrier-neutral third parties. These operators typically go to great lengths to promote communication and encourage new peering, and will often arrange social events for these purposes.
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OPTO ELECTRONICS Private peering Private peering is the direct interconnection between only two networks, across a Layer 1 or 2 medium that offers dedicated capacity that is not shared by any other parties. Early in the history of the Internet, many private peers occurred across 'telco' provisioned SONET circuits between individual carrier-owned facilities. Today, most private interconnections occur at carrier hotels or carrier neutral collocation facilities, where a direct cross connect can be provisioned between participants within the same building, usually for a much lower cost than Telco circuits. Most of the traffic on the Internet, especially traffic between the largest networks, occurs via private peering. However, because of the resources required to provision each private peer, many networks are unwilling to provide private peering to "small" networks, or to "new" networks who have not yet proven that they will provide a mutual benefit. ISPs requiring no upstream and having only customers (end customers and/or peer ISPs) are called Tier 1 ISPs. Network hardware, software and specifications, as well as the expertise of network management personnel are important in ensuring that data follows the most efficient route, and upstream connections work reliably. A tradeoff between cost and efficiency is possible. Virtual ISP A Virtual ISP (VISP) is an operation which purchases services from another ISP (sometimes called a "wholesale ISP" in this context). which allow the VISPs customers to access the Internet using services and infrastructure owned and operated by the wholesale ISP. Free ISP Free ISPs are Internet Service Providers (ISPs) which provide service free of charge. Many free ISPs display advertisements while the user is connected; like
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OPTO ELECTRONICS commercial television, in a sense they are selling the users' attention to the advertiser. Other free ISPs, often called freenets, are run on a nonprofit basis, usually with volunteer staff. There are also free shell providers and free web hosts.
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OPTOELECTRONI CS
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