1 Introduction
Chapter 1 Introduction Topic
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Cellular concept ................................................................................................. 7 Analogue and digital signals ............................................................................ 16 PCM/E1 Link ................................................................................................... 21 Wireline transmission ...................................................................................... 22 Microwave radio relay ..................................................................................... 24 Switching ......................................................................................................... 26
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1 Introduction
Cellular concept In 1940s the first fully commercial mobile telephony systems DPLM Domestic Public Land Mobile were launched in United States. However, these systems were not cellular networks. There was just one Base Station (BS) that was using the whole available bandwidth.
f1,f2,f3,f4
Figure 1-1 Non-cellular mobile system The only way to increase the capacity (the number of simultaneous call connections) was to add extra radio channels. This solution is very inconvenient, as the licenses for radio frequencies are very expensive and also the bandwidth for any application is limited as radio resources are also used for other services than a mobile telephony.
f1,f2,f3,f4+f5,f6
Figure 1-2 Non-cellular mobile system (capacity increase) The second disadvantage of single cell systems is a limited coverage. The only way of increasing the coverage is to increase the output power, both of 7
GSM/UMTS/LTE Basics
Base Station (BS) and Mobile Station (MS). However it is only possible to increase the output power of the MS to a certain level defined by the national regulations, as around very powerful transmitters there should be the closed zone, to protect people against radiation.
P P
P
P f1,f2,f3,f4+f5,f6
P
Figure 1-3 Non-cellular mobile system (coverage increase) Due to limited capacity and coverage, the single cell systems were very expensive and they have never been popular (less than 1 million of users around the world) The cellular concept was envisioned at Bell Laboratories in 1947. However due to its complexity, the first such systems were putted into operation in the late 70s.
Figure 1-4 Cellular mobile system 8
1 Introduction
Cellular network comprises of a number of BSs. The radio coverage areas given by the neighbouring BSs are partially overlapping each other, allowing for the handover of the call connection from one cell to another, when the subscriber is roaming through the network. The available radio channels are organized in sets. The same set of frequencies can be allocated to a number of cells if only the distance between them is far enough to prevent interferences. This way, it is possible to cover an unlimited area by using a limited number of radio channels. The maximum number of simultaneous calls in the cell still is equal to the number of channels allocated to that cell.
Figure 1-5 Cellular mobile system (frequency re-use) However by decreasing the cell size, it is possible to place at the same area more cells. Assuming that the number of frequencies per cell is constant, the number of simultaneous calls increases. The unlimited coverage and a very high capacity are the great advantages of the cellular networks.
Figure 1-6 Cellular mobile system (capacity increase)
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GSM/UMTS/LTE Basics
At the same time the cellular networks have same disadvantages; these are: difficult radio network planning, necessity of automatic tracing of the subscriber movement in the geographical cell structure, necessity of handovers and the complex technology.
Cells A cell is defined as the area of radio coverage given by one BS antenna system. Typically, cells are represented graphically by hexagons. However these are the type of antenna and the environment parameters, which determines the actual BS radiation pattern.
equal signal strength from both BS
Figure 1-7 Cell – hexagon shape
Figure 1-8 Cell – hexagon and real radiation pattern example 10
1 Introduction
There are two main types of cells: •
Omini directional cell – served by antenna, which transmits equally in all directions,
•
Sector cell – served by antenna, which transmits in a given direction only (e.g. 120º or 180º).
omni directional
directional
Figure 1-9 Antenna types The group of BSs that are serving sector cells can be installed at one site, leading to terms such as two-sectored site and more commonly used threesector site. In such case usually the functionality of all BSs at the site resides in a single hardware network node.
Omni directional cell
3 sector cells
Figure 1-10 Types of cells
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GSM/UMTS/LTE Basics
Figure 1-11 3-sectors site
Frequency reuse and clusters Groups of frequencies can be placed together into pattern of cells called a cluster. A cluster is a group of cells in which all available frequencies have been used once and only once. Clusters have such shapes that they fit one to another so it is possible to cover the whole area of the network without any gaps. The frequency re-use distance between cells that are using the same set of frequencies in two neighbouring clusters are kept constant, therefore the interference conditions in all cells within the network are the same.
f1
f1
Figure 1-13 4/12 Frequency re-use pattern
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1 Introduction
In Fig. 1-13 the 4/12 frequencies re-use pattern is shown. 4/12 means that there are four three-sector sites supporting twelve cells using twelve frequency groups. The 4/12 cell pattern is commonly used by GSM operators. GSM also can work with other patterns providing higher capacity. The example of such pattern is 3/9 where the entire set of available frequencies is divided only between nine cells in the cluster in comparison to 12 in 4/12 pattern. Unfortunately at the same time the signal quality in the network using 3/9 pattern is lower due to decreased frequency re-use distance.
f1
f1
Figure 1-14 3/9 Frequency re-use pattern
pattern
4/12
3/9
# frequencies # frequencies per cell
1/3
1/1
36/3=12
36/1=36
36 36/12=3
36/9=4
tighter frequency re-use → higher capacity tighter frequency re-use → shorter re-use distances → higher interferences Figure 1-15 Frequency re-use pattern and capacity
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GSM/UMTS/LTE Basics
Other frequency re-use patterns, such as 7/21, with long frequency re-use distance, are recommended for analogue networks, which are more sensitive to interference. In contrast, the newest cellular systems, like UMTS, LTE and the future 4G system (currently known under name advanced-LTE) are using commonly 1/1 frequency re-use pattern. In those systems the interferences caused by lack of frequency re-use distances are minimised by other methods than frequency planning (more in the following chapters).
4/12
3/9
1/3
1/1
GSM UMTS LTE Figure 1-16 Frequency re-use patterns for cellular systems In the real network the allocation of channels to cells and the cell size will not be as uniform as in the presented examples. Cells where locally there is a higher traffic will require more frequencies than the cells with smaller traffic than average level in the network. In the big cities where there is a huge population density the cell size will be reduced to increase the capacity, whereas in the rural areas the cells can be quite large.
Increase of capacity in cellular system If the number of subscribers in a system continues to increase, at some point it becomes necessary to increase the capacity of the system. There are several ways to increase the capacity of the system: •
increase the frequency band (for example, a GSM 900 operator might buy GSM 1800 licenses),
•
make frequency reuse tighter (for example, going from a 4/12 reuse pattern to a 3/9 reuse pattern),
•
make the cells smaller and smaller (cell split).
The examples of a cell split are shown in Fig. 1-17, 1-18 and 1-19. Initially, the omni directional cells, giving the maximum range are used (Fig. 1-17). The next step is to introduce three cells per site (Fig. 1-18), using the original sites and feeding the cells from the corners. Now, the number of sites is still the same, but there are three times as many cells as before. In the following step it is possible to split existing cells again, but it is necessary to add new sites (Fig. 1-19).
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1 Introduction
Figure1-17 Before cell split
Figure 1-18 Cell split phase 1
Figure 1-19 Cell split phase 2
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GSM/UMTS/LTE Basics
Cellular network The cellular network is not only comprising from BS and cells but also it includes fixed elements and interfaces to which the BS are connected to. The role of fixed elements, including, exchanges, switches, routers, databases, servers and interfaces is to trace the terminal location and establish connection between terminals in case one is calling to another.
VLR BSC
MSC HSS
BSC
SGSN
VLR RNC
GGSN
GMSC
MSC
Figure 1-20 Cellular network
Analogue and digital signals A signal is an impulse or a fluctuating electric quantity, such as voltage, current, or electric field strength, whose variations represent coded information. Signal to be transmitted through any transmission medium can be analogue or digital. Analogue signal is continuous and can take any value over time. An example of analogue signal is human voice. Digital signal is not continuous, as it changes its value only at the predefined time instances. The values of digital signal are taken from the finite set of values. An example of digital signal is the light in case when the lamp is used for transmission of Morse Code symbols. There are only two possible values (shine or darkness) and the change of value can be done only at certain time instances as the duration of dot, dash and space between these two symbols is specified.
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1 Introduction
analogue
digital
Figure 1-21 Analogue and digital signal Both analogue and digital signals are distorted during the transmission process. In case of the analogue signal it is not possible to regenerate the original signal at the receiver side, as it can take any value over the time, which the receiver system cannot predict. In case of the digital binary signal, the receiver can make a decision to which of two possible values the received signal level is closer. If only the distortions are not very severe, by doing this the receiver can regenerate the original signal. transmitted
received
regenerated
Figure 1-22 Regeneration of digital signal The possibility of regeneration is a great advantage of using digital transmission. In digital system it is possible to implement additional coding in order to protect the information against transmission errors and ciphering to protect information against overhearing by third parties. Very often the analogue information such as human speech or music is transmitted or stored in the digital form (digital telephony ISDN/GSM, CD), because digital signals provide better quality for transmission of analogue information than analogue signals.
Analogue to digital conversion One of the primary functions of an mobile terminal is to convert the analogue speech information into digital form for transmission using a digital signal. The Analogue to Digital (A/D) conversion process outputs a collection of bits: binary ones and zeros which represent the speech input.
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GSM/UMTS/LTE Basics
analogue
digital A/D
Digital System
Figure 1-23 A/D conversion The A/D conversion is performed by using a process called Pulse Code Modulation (PCM). PCM involves three main steps: •
sampling,
•
quantisation,
•
coding.
Sampling Sampling Sampling is measurement of the signal level at the specific time intervals. The signal is described by the set of samples that can still take any value, however the signal is discrete in time domain. Sampling introduces a loss of information. However, the more often the samples are taken, the closer the resulting digital values will be to a true representation of the analogue information. The sampling theory states that: “To reproduce an analogue signal without distortion, the signal must be sampled with at least twice the frequency of the highest frequency component in the analogue signal spectrum”. Speech contains frequencies from the range 300Hz - 3400Hz. That means that to truly represent speech in the digital system the sampling frequency must be at least equal to 2 x 3400kHz = 6800Hz or 6.8 kHz. Telecommunication systems usually use the sampling frequency of 8 kHz to assure good speech quality.
Figure 1-24 Sampling
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1 Introduction
Quantisation The next step in A/D conversion is quantisation. The amplitude of the signal at the time of sampling is measured and approximated to one of the finite set of values. Quantisation introduces errors, as the approximated value is hardly ever equal to the value of the sample. The degree of accuracy depends on the number of quantisation levels used. In common telephony, 256 levels are used, while in GSM/UMTS/LTE 8192 levels are used.
Quantisation
Figure 1-25 Quantisation
Coding Coding involves converting the quantised values into binary values. In case of fixed ISDN when 256 quantisation levels are used, 8 bits represent each value. In GSM due to the fact that 8192 quantisation levels are used, 13 bits are needed to represent each sample. Coding 00→0000 01→0001 02→0010 03→0011 04→0100 05→0101 06→0110 07→0111 08→1000 09→1001 10→1010 11→1011 12→1100 13→1101 14→1110 15→1111
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
1000 1100 1110 1111 1111 1111 1111 1110 1101 1011
0110 0011 0010 0001 0001 0001 0001 0010 0100 1000
Figure 1-26 Coding After coding, in the traditional telephony, human voice is represented by a bit stream of 64 kbits/s (8 bits * 8 kHz = 64 kbits/s) and in GSM/UMTS/LTE 104 kbits/s (13 bits * 8 kHz = 104 kbits/s)1. 1
for AMR-WB 224 kbps (14 bis * 16 kHz)
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GSM/UMTS/LTE Basics
Segmentation and speech coding The key to reducing the bit rate is to send information about the speech instead of the speech itself. In GSM, the speech coding process analyses speech samples and outputs parameters of what the speech consists of the tone, length of tone, pitch, etc. This is then transmitted through the network to another MS, which generates the speech based on these parameters. The human speech process starts in the vocal chords or speech organs, where a tone is generated. The mouth, tongue, teeth, etc. act as a filter, changing the nature of this tone. The aim of speech coding in GSM is to send only information about the original tone itself and about the filter.
FR speech Segmentation Given that the speech organs are relatively slow in adapting to changes, the filter parameters representing the speech organs are approximately constant during 20 ms. For this reason, when coding speech in GSM, a block of 20 ms is coded into one set of bits. In effect, it is similar to sampling speech at a rate of 50 times per second instead of the 8,000 used by A/D conversion.
Speech coding Instead of using 13 bits per sample as in A/D conversion, GSM FR speech coding uses 260 bits. This calculates as 50 x 260 = 13 kbits/s. This provides a speech quality which is acceptable for mobile telephony and comparable with wire line PSTN phones.
20 ms 010001010‌11101101 260 bits
Figure 1-27 Segmentation and speech coding
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1 Introduction
Other types of speech speech coders The GSM FR is not the only speech codec to be used in the mobile network environment. Other types of standard speech codes are presented in Fig. 1-28. codec
bitrate before
bitrate after
G.711
64 kbps
64 kbps
HR
104 kbps
5,6 kbps
FR
104 kbps
13 kbps
EFR
104 kbps
12,2 kbps
AMR
104 kbps
4,75 – 12,2 kbps
AMR-WB
224 kbps
6,6 – 23,85 kbps
PSTN /ISDN
GSM
UMTS
LTE
Figure 1-28 Speech codecs (used across air interface)
PCM/E1 Link In telephony network a number of users (subscribers) can share the same physical connection (e.g. coaxial cable), thanks to the time division multiplexing, see Fig. 1-29.
A/D
0101 1101
A/D
0111 0001
MUX A/D A/D
0101 0111 0101 1111 1101 0001 0100 0101
0101 0100
TS 1
TS 2
TS 4
TS 3
1111 0101
Figure 1-29 Time division multiplexing
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GSM/UMTS/LTE Basics
The physical resources of the transmission medium are divided in time domain into several time slots. A time slot is a time duration during each a certain user can send the data. In case of PCM/E1 link there are 32 timeslots, however the TS=0 is reserved for synchronisation to ensure that the transmitter and receiver work with the same frequency. The remaining time slots can be used for other purposes, for example for speech transmission. During one time slot the user can send 8 bits (one octet) of data. Each TS is transmitted 8000 times per second. This means that the bit rate for each PCM TS (PCM channel) is equal to 64 kbits/s. And the bit rate for the whole PCM/E1 link is 2048 kbits/s (32 * 64 kbits/s = 2048 kbits/s).
Time Slot 31
01 2
TDMA frame synchronisation
data
Figure 1-30 PCM/E1 link 2048 kbits/s
Wireline transmission There are two ways of transmitting the information over a wire. The signal can be either electrical or optical. In the first case the transmission medium has to consist of at least two metallic conductors to create closed loop for the current. In the second case, the transmission medium is optical fibre that makes propagation of the light possible.
Copper wires Copper wires are mainly used in the analogue telephony systems, although digital transmission with high bit rates for short distances is also possible. There are two main types of copper cables:
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•
Twisted pair,
•
Coaxial cable.
1 Introduction
A twisted pair consists of two insulated copper wires, typically about 0.4 mm to 1 mm thick. The wires are twisted together in the helical form. Twisting of the wires is done to reduce electrical interferences and losses of signal. Twisted pairs may be used individually, like in home telephone line connections, or may be grouped in bigger cables. Such cables may consist from 2 up to even 500 twisted pairs. Twisted pairs may run for several kilometres without amplification, but for longer distances repeaters are needed.
plastic covering outer conductor insulating material metal core
Figure 1-31 Twisted pair (left) and coaxial cable (right). If better performance of the cable is needed, a coaxial cable is usually used. It consists of copper wire as a core surrounded by an insulating material. The insulator is encased by a cylindrical conductor covered by a protective plastic sheath. Coaxial cable has better shielding than twisted pair, which results in lower attenuation and wider bandwidth. It allows the transmission for longer distances and with higher bit rates. For 1 km long cable a bit rate of 1 to 2 Gbps is feasible.
Optical fibre An optical transmission system has three components: the light source, the transmission medium and the detector. The transmission medium which is optical fibre (optical cable) has great transmission capacity. The optical cable consists of several thin glass fibres, as shown in the Fig. 1-32.
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GSM/UMTS/LTE Basics
jacket cladding (glass) core (glass)
Figure 1-32 Side view of the optical fibre In the centre of the cable, there is a glass core. Depending on the type of fibre, it has a diameter of 8, 10 or 50 Âľm. The core is surrounded by the glass cladding, of which diameter is usually 125 Âľm. The cladding with the core, are protected against chemicals, moisture etc. by the primary coating. This structure is surrounded by a secondary coating ensuring final mechanical protection. It may be fixed to the primary cladding or loose. A number of such single optical fibres is usually grouped and create optical cable. Such cables are additionally protected against excessive stress, bending, moisture etc. Metal, Kevlar or plastic solutions are used depending on the cable requirements.
Microwave radio relay Microwave radio relay is a technology for transmitting telecommunication signals between two locations on a line of sight radio path. In microwave radio relay, radio waves are transmitted between the two locations with directional antennas, forming a fixed radio connection between the two points. Because of the high frequencies used, an optical line of sight between the stations is generally required. Because a line of sight radio link is made, the radio frequencies used occupy only a narrow path between stations. Antennas used must have a high directive effect; these antennas are installed in elevated locations such as large radio towers in order to be able to transmit across long distances. Typical types of antenna used in radio relay link installations are parabolic reflectors, shell antennas and horn radiators, which have a diameter of up to 4 meters. Highly directive antennas permit an economical use of the available frequency spectrum, despite long transmission distances. 24
1 Introduction
Figure 1-33 Microwave radio relay
Figure 1-34 Microwave antenna at GSM site’s tower Obstacles, the curvature of the Earth, the geography of the area and reception issues arising from the use of nearby land (such as in manufacturing and forestry) are important issues to consider when planning radio links. High intensity rain and snow must also be considered as an impairment factor, especially at frequencies above 10 GHz.
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GSM/UMTS/LTE Basics
Switching Circuit switching Circuit switching is historically the oldest type of switching, so it may serve as an explanation of the term switching itself. The simplest definition of switching in telephony is: to set up a connection between two subscribers to enable them to talk to each other. We have to switch the communication path in the exchanges on the way in order to complete this operation. In the old days of telephony operators in the exchanges were setting up such connections by manually switching circuits (cables). Even with the introduction of automatic switching the concept of a circuit remained unchanged. However, the meaning of the word changed very much with the successive introduction of digital transmission in early 1970s. Nowadays, circuit switching is regarded as switching of timeslots; representing 64 kbits/s transmission channels in which user information is delivered to digital exchanges, see Fig. 1-35.
0
31
3
group switch 3
0
27
31
27
Figure 1-35 Telephone connections in a PSTN network The process of switching is performed in the switching part of the exchange, whose main functional part is the group switch, controlled by a digital processor. With the growth of complexity of switching systems this processor evolved very quickly into a set of processors, cooperating in performing various more or less complicated tasks.
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1 Introduction
The circuit-switched mode of communication is connection-oriented. This means that the communication phase is preceded by a set-up phase, when signalling is performed. The process of connection set-up between two subscribers is done according to information provided by the calling subscriber (A-subscriber) which includes the called subscriber (B-subscriber) telephone number and the routing tables defined in the exchanges involved in the connection.
B-no
84620 58543 74683
B-no
B-no 12488 32443 74783
76868 64388 68684
Figure 1-36 Connection set-up In a Circuit-Switched (CS) network the information is transferred with the constant bit rate 64 kbits/s. The communication channel is reserved for the entire time of the connection and it cannot be used for any other purpose even if the subscribers (or one of them) are not talking at the moment. This means that the bandwidth is used in a very inflexible way; however, thanks to this the transmission delay is kept constant. The CS connection is suitable for voice and video transmission (real-time applications). It is not very suitable for data applications (e.g. e-mail) because usually data traffic is transmitted in bursts (not continuous) and CS networks do not support any mechanisms for error detection and correction.
Packet switching The Packet Switching (PS) is a technology widely used in the data networks. The information to be transferred is divided into packets of variable length. Each packet includes the recipient address. When the data packet arrives at a network node, it is stored in a buffer. The address is read and the packet is forwarded to the recipient or to the next node according to the routing table defined in each network node.
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GSM/UMTS/LTE Basics
There is no dedicated channel for each user. The available transmission capacity may be shared by several connections without any of them having reserved capacity. If no bandwidth is available at the moment, the packets remain in the buffer until there is sufficient bandwidth. This results in delays in the transfer process, however the utilization of resources is very high.
2 1
1 2 2
1
1 2
1 2
1 2
2
Figure 1-37 Packet switching The PS is most suitable for the applications that do not require a constant data flow (e.g. e-mail, file transfer), due to variable bit rate and variable delay in PS networks. PS is not suitable for real time applications like voice and video transmission. In the packet-switched network error correction is possible. When the packet arrives corrupted, the receiving node can send the negative acknowledgement and ask for retransmission. GPRS – wireless packets
The data bearer services that are available in the traditional GSM network are based on the CS technology. However majority of subscribers used them for dial-in access to packet data networks such as Internet and other IP networks. In case of typical applications (e-mail, web browsing) the data transfer is not continuous. For example, subscriber that is browsing the web, downloads the content of the page, then reads it and downloads the next one. The data transfer in this case is suspended several times during the connection. Very often even during the downloading process there could be some longer gaps in transmission if the external network cannot deliver them in continuous way. It is also important to note that usually the amount of data send on downlink is larger then on uplink. In the described example the traffic channel is reserved for the whole duration of the session and cannot be used by another terminal even if it is not utilized at the moment. That situation is
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1 Introduction
disadvantageous for both network operator and subscriber, because the radio recourses are used in a very un-efficient way and the subscriber has to pay for the whole duration of session, rather then for the amount of transmitted data. The General Packet Radio Service (GPRS) is a data service that overcomes the disadvantages of old CS data services. In GPRS physical channel is assigned only for the duration of packet transfer, and immediately after it is finished, the same channel can be assigned for another terminal. This solution allows a number of terminals to share one physical channel (statistical multiplexing). The network also can allocate radio resources for a certain user on more than one channel, which allows higher transmission bit rates.
GSM
UMTS
LTE
circuit switching (CSD)
packet switching (GPRS)
telephony
from R96
telephony
telephony
Figure 1-38 CS and PS services in mobile networks
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