Ch 01

WIRELESS LOCAL LOOPS AND A COMPARATIVE STUDY BETWEEN ITS’ CAPACITY AND CELLULAR SYSTEMS’ CAPACITY USING “DIFFERENT MULTIPLE ACCESS METHODS”

Md. Nazmul Hassan Md. Arifuzzaman Chapter 1 Introduction 1. Overview Wireless Technology is now recognized as an important option for delivering mobile, fixed and broadband services. Wireless Local Loop (WLL) is an emerging technology that allows rapid connection to the wired network for remote locations. WLL systems deploy access technologies based either on existing standards(such as IS-95, CDMA,GSM,TDMA etc) or proprietary radio link technologies to provide reliable and cost effective local telephone service for replacement or displacement of traditional copper wire line. Major differences between the WLL and the mobile cellular environment include a usually strong line-of-sight (LOS) component and a stationary subscriber unit with a directional antenna. WLL can shorten the time to deploy a communication infrastructure, it can reduce the cost of the communication

system and can adapt to the changes of needs and environment A WLL is sometimes called a “fixed cellular system”. WLL are low capital cost, fast network deployment and lower maintenance costs clearly attractive considerations. WLL networks have been proven to have the capability to function as core communications systems in times of disaster; for disaster recovery, service providers have the option of paridly deploying a WLL system during an emergency. This way, communications downtime caused by natural disasters such as flood, earthquakes, and hurricanes and so on can be kept to a minimum. The term WLL has been in use for sometime and is well-ingrained in the Literature. WLL is often called the radio local loop (RLL) or the “fixed wireless access” (FWA).Using advanced digital radio technologies, WLL can provide a variety of data services and multimedia services as well as voice.

1.1 Historical Background of WLL The user of radio telephony to provide basic communication service to distant, isolated communities in such countries as Canada and Australia has been attempted since 1950’s using single channel very high frequency (VHF) radios.

The user capability and large scale commercial deployment of wireless access in the 1950 and 1960s was hindered by such factors as: •

Lack of suitable technologies for sharing frequency spectrum

•

High-cost and low-performance of the radio equipment

•

Lack of user-friendly operational procedures

•

Lack of reliability

•

Poor quality of service encountered by the users

The technological advances in wireless access system to provide mobile services during 1970s and 1980s leading to the analog cellular technology and subsequently (in1990s) to digital cellular technologies provided the

underpinnings for a viable business case for WLL systems. Since the early 1990s there has been a rapid proliferation of WLL systems based on cellular and cordless radio technologies as well as those based on proprietary radio technologies. These systems are being deployed worldwide.

1.1 Definition Wireless local loop (WLL) is the use of radio to provide a telephone connection to the home. WLL is proposed for a range of environments; and the range of possible telecommunications delivery is widening.

Figure 1.1 is a simplistic diagram of the role of WLL in the world. In a simple world, a house is connected to a switch first via a local loop, then through a distribution node onto a trunked cable going back to the switch. Historically, the local loop was copper cable buried in the ground or carried on overhead pylons, and the trunked cable was composed of multiple copper pairs. WLL replaces the local loop section with a radio path rather than a copper cable. It is concerned only with the connection from the distribution point to the house; a l l other parts of the network are left unaffected. In a WLL system, the distribution point is connected to a base station; a transmit/receive radio device is mounted on the side of the house, in much the same manner as a satellite receiver dish; and a cable is run down the side of the house to a socket inside the house. The socket is identical to the one into which users currently plug their home telephones. Hence, apart from a small transceiver on the side of their house, the home subscriber does not notice any difference [1].

Figure 1.1 The role of WLL 1.2 WLL Service Requirements The services offered depend strongly on the customer segment. The ADSL, cable network upgrades for data services and developments in 3rd-Generation mobile all impact the WLL service. They drive the minimum data rate needed for a fixed wireless solution to remain competitive in the residential segment.

With the introduction of broadband wireless technologies, data rates of more than l0Mbit/s are now possible, accommodating bandwidth intensive applications such as video-on-demand or LAN interconnect. [2] A summary of service needs for different customer types is shown in Table 1.1.

• Communications Quality: Since a WLL system serves as an access line for fixed telephone sets, it must provide the same level of quality as conventional telephone systems with respect to such aspects as speech quality, grade of service (GOS), connection delay and speech delay.

Table 1.1 Service need per customer type

Basic

Internet

BRA

Customer Telephony Data/fax ISD \Service Very

n×64/56 n×E1/T1 LAN

MPEG2 IN

Kbps

PRA

ATM

functions

■

ISDN ■

■

■

■

■

N ■

■

■

■

■

■

■

■

Medium

■

■

■

■

▼

▼

■

business Small

■

■

■

■

■

SOHO

■

■

■

■

▼

High

■

■

■

▼

■

■

■

■

▼

large business Large business

business

■

spending resident Med spending resident Low spending resident ■ means ▼means partial use full use

▼

• Secure Transmission: WLL must be secure to give the customer confidence that conversation remains confidential. The system should also include authentication to prevent fraudulent use.

• ISDN Support: The system should support integrated services digital network (ISDN) when appropriate to provide voice and data service.

• Easy Environment Adaptation: The system should be capable of smallcell or large-cell operation to serve dense urban or rural areas respectively.

• Absence of Interference with Other Wireless Systems: A WLL system must not cause any interference with the operation of existing systems, such as microwave communications and broadcasting systems.

• High Traffic Volume: One characteristic of a WLL system is that it must support a larger traffic volume per subscriber than mobile or even wire line communications systems.

• High Capacity and Large Coverage The maximum system range and base station capacity should be large to make the 'cost per subscriber' as low as possible and minimize the entry cost for an operator.

• Developing Regions In many developing regions, the infrastructures for basic telephone services are still insufficient. For these areas, the requirements of WLL services can be summarized in the following: •

In terms of service coverage, a wide area should be covered within relatively short period.

•

Especially, for the regions with dense population, a high-capacity

system is indispensable. •

The service fee per subscriber must be low.

•

The system should be implemented rapidly so that the services might be launched quickly.

• Developed Regions In the developed regions, the service requirements contain not only POTS but also other advanced services. The WLL channel of the second provider should be superior to or, at least, comparable with the first operators' one in quality and data rate. Therefore, WLL should provide toll quality voice and at least medium-rate data corresponding to the integrated services digital network (ISDN). WLL can be a useful alternative for their network expansion. Most countries impose the universal service obligation (USO) upon the first operators. In this case, WLL can be considered as a supplementary means to wire line networks, for covering areas with sparse population, e.g. islands. The first service requirement for this application of WLL is the compatibility with and the transparency to the existing wire line network. On the other hand, the cellular mobile service providers can offer easily WLL services by using their existing infrastructure for mobile services. In this case, fixed WLL service may have competitiveness by combining with the mobile services 1.3 Advantages of WLL Using radio rather than copper cable has a number of advantages. It is less expensive to install a radio than to dig up the road, it takes less time, and radio units are installed only when the subscribers want the service, unlike copper, which is installed when the houses are built. That begs the question as to why copper was ever used. As will be seen, it is only in the last six years that advances in radio technology have brought the cost of radio equipment below

the cost of copper cabling. Hence, since around 1994, WLL has steadily become the most appropriate way to connect subscribers. Some other advantages are also mentioned [3].

• Lower network costs o Cost of wireless is dropping o Cost of digging trenches for fiber & cable is not necessary • Lower operating costs o Cost of maintaining a wireless network is less than a fiber, cable or copper line network • Enhanced Performance o Excellent voice quality and security o Rapid access to Internet o Low output power o Improved system reliability o Less sensitive to environmental disasters o Optimum spectral efficiency • Enhanced Performance o Excellent voice quality and security o Rapid access to Internet o Low output power o Improved system reliability o Less sensitive to environmental disasters o Optimum spectral efficiency • Rapid user hookup

o Connecting a wireless user is fast & simple o Self installation is possible o No trunk roll is necessary • Economies of scale o Open Standard - not proprietary o A standardized air interface allows cost reductions due to “off-theshelf” component availability and mass produced silicon circuits • Economies of scope o Mobile/fixed services convergence: easy expansion of WLL network to enable limited and full mobile services. o WLL network equipment and sites are compatible with mobile network equipment. o Easy transition to mobility.

• Flexible network planning o Increased coverage and capacity o Flexible (modular) approach to expanding capacity o N=1 spectrum reuse allows for unexpected growth in for unexpected growth in demand o Indifferent to topography and distance • Faster deployment and quicker ROI o Dramatically reduced right reduced right-of of- way approvals o Shorter per loop installation time • Toll quality voice o Enhanced Voice Rate Vocoder EVRC offers highest quality voice at a low 8 kbps rate

o Selectable Mode Vocoder SMV will enhance MOS • Subscriber

Equipment

Lower

capital

expense

and

financing

requirements o Wireless networks reach break-even point sooner than wire line networks o Wire line networks will have excess capacity o Price of subscriber equipment is driven by mobile device market • High Speed Data o 3G CDMA enables data rates up to 2.4 Mbps o Capable of providing wireless ISP services • Backward compatible evolution to 3G services o Allows multiple 1.25 MHz channel migration to CDMA2000 1x/1xEV o Migration to IMT-2000 on a common ANSI- 41, IP or GSM/MAP platform.

1. 4 The Potential Limitations The potential limitations of wireless access and WLL systems are as follows: • At least four to six hours availability of AC power is prime requirement for satisfactory working of FWT at customer premises which is causing slow deployment in rural areas.

• As power is key requirement, proper working of WLL depends on availability of AC power in rural areas.

• Portability of WLL handsets / FWT’s is difficult between different service providers.

• Restrictions on Spectrum Availability are a major problem while installing a WLL system which limits many things.

• The availability of bandwidth must be balanced between the customers and the service provider. • The lack of agreed standards means that the manufacturers are offering WLL service but the frequency spectrum may already be in use and will lock all the dealings. • Radio Link Planning must be made as many WLL systems uses line-ofsight (LOS) transmission and the customer installs line-of-sight (LOS) radio link to provided acceptable service quality [4].

The limitation on voice quality service must be maintained so that it does not get distorted due to multipath fading, co-channel interference and signal loss due to rain 1.5 Objectives The primary objective of this paper is to provide a system level view of the technical and deployment aspect of WLL systems, as an alternative technology to copper-based local loop plant for the provision of basic telecommunication services such as voice and voice-band data to end user. However this paper also includes fixed wireless access (FWA) that are emerging as an important alternative to fiber, cable modem, and copper (digital subscriber loop or DSL) to meet the increasing demand for such services as video on demand, internet access ,and high-speed data services.

A major part of this paper is devoted to the background for and discussion of WLL systems and rest of the part is some measurement and simulation of WLL and cellular systems capacity.

1.6 Motivation This thesis proposes wireless local loop as a superior alternative to traditional wireline access to telephone services. Showing a tremendous need for additional telephone service around the world in a brief local loop market overview, the paper presents the original WLL terminal technology combined

with the network management system. International Telecommunications Union (ITU) predicted that by the end of the year 2006, there will be a demand for almost 250 millions new telephone lines to be installed around the world, especially in remote and isolated areas.

1.7 Organization of the report Several technical parts have been studied related to these topics and later simulation done based on this theories. This thesis paper consists of eleven chapters.

In this chapter covers the definition of WLL, some historical background of WLL, also describe about the services required for WLL system. There are some advantages and disadvantages are also described in this chapter. Overall it is the introductory chapter.

Chapter two consider the different type of access technology.There different types of standards, methode of there work.mobile radio access technology.

In chapter three, satellite based system, different types of cellular system, like analog digital system, IS-95, PCS, PACS system and there services, also describe about FWA technology.

WLL reference model ,interface which are needed for WLL ,system architecture ,different types of functional blocks all are describe in chapter four.

Chapter five describe about the allocated frequency for WLL system.

Chapters six introduce different types of interface like network interfaces, radio interfaces. Protocols for radio interface are also describe in this chapter Chapter seven provide protocol operation and V5 interfaces.

In chapter eight, described about multiple accesses method like FDMA, TDMA, CDMA, some advantage of CDMA technology.

Planning for Line-of-sight LOS path is described in chapter nine.

Chapter ten is the technical part. In this part spectrum efficiency measured, also the capacity of WLL and cellular system is measured .Then the capacity is compared between these systems.

Chapter eleven is the conclusion chapter. This is also the concluding section.

Chapter 3 WLL System Technologies 3. Overview Early WLL systems used standard cellular and cordless technologies to gain access to spectrum. These are at low frequencies, which have become congested and expensive, as mobile operators are able to pay premium rates.

In our days, however, WLL deployments also utilize other proprietary systems, narrowband or broadband in frequency bands that have been provided by ITU on a worldwide basis. In general, the frequency bands, which have been used or standardized for WLL service, are described in Table 3.1 [2].

Table

3.1

Frequencies

used

or

standardized

for

WLL

Frequency Use 400-500 MHz Rural applications with mostly analogue cellular systems 800-1000 MHz Digital cellular radio in most countries 1.5 GHz Typically for satellites and fixed links 1.7-2 GHz Cordless and cellular bands in most countries 2.5 GHz Typically for Industrial, Scientific and Medical (ISM) equipment 3.4-3.6GHz Standardized for WLL around the world 10 GHz Newly standardized for WLL in some countries 28 GHz and 40 For microwave distribution systems around the GHz world The WLL revolution is underway WLL suppliers and operators are flocking to emerging markets, using whatever available wireless and line interface technologies are at hand to achieve fast time to market.

The challenge for WLL vendors is to identify the optimal wireless protocol for their unique application needs, then reduce cost per subscriber and deliver integrated solutions to the marketplace.

WLL will be implemented across five categories of wireless technology. They are Digital Cellular, Analogue Cellular, Personal Communication System (PCS) Digital

European/Enhanced

Cordless

Telecommunication

(DECT)

and

Proprietary Implementations. Each of these technologies has a mix of strengths and weakness for WLL applications.

3.1

Satellite-based

Systems

These systems provide telephony services for rural communities and isolated areas such as islands. Satellite systems are designed for a Gaussian or Rician channel with K factor greater than 7dB. These systems can be either of technology designed specifically for WLL applications or of technology piggybacked onto mobile satellite

systems as an

adjunct service.

Of these, the former offers quality and grade of service comparable to wireline access, but it may be expensive. The latter promises to be less costly but, due to bandwidth restrictions, may not offer the quality and grade of service comparable to plain old telephone service (POTS). An example of a satellitebased technology specifically designed for WLL is the Hughes Network Systems (HNS) telephony earth station (TES) technology. This technology can make use of virtually any geostationary earth orbit (GEO) C-band or Ku-band satellite.

Satellite technology has been used to provide telephony to remote areas of the world for many years. Such systems provide an alternative to terrestrial telephony systems where landlines are not cost-effective or where an

emergency backup is required. There are several proposed systems for mobile satellite service, including the Inmarsat International Circular Orbit (ICO) system, Globalstar, and American Mobile Satellite Corporation (AMSC) system.

These systems are specialized to support low-cost mobile terminals primarily for low bit rate voice and data applications. Fixed applications are a possible secondary use to mobile applications. There is a great deal of difference between these systems, especially when considering the orbit and the resultant propagation delay. The number of satellites and the propagation delay pose very different constraints on system design, so that there is no true representative system. For example, GEO satellite systems are not required to support handover even for most mobile applications. Mid-earth orbit (MEO) and low earth orbit (LEO) satellite systems require handover capability for all fixed and mobile applications because the satellites are in motion relative to the earth's surface even when the terrestrial terminal is fixed. This can be problematic if the handover is supported in the switch because mobile switching centers (MSCs) support sophisticated mobility functions such as link handover, but do not typically support ordinary switching functions such as hunt groups, for example, which are highly desirable in a WLL system. [10] 3.2 Cellular Based System Cellular phones have been immensely successful, first in the mid-late 1980s with analog systems and then in the early to mid-1990s with secondgeneration digital systems. Typically, they operate in the mobile frequency bands at 800-900 MHz, 1.8-1.9 MHz and sometimes at 450 MHz or 1.5 GHz. The key strengths of cellular include good coverage, excellent immunity to channel errors, excellent economies of scale as a result of high sales, and advanced and fully featured phones. Given the use of cellular in some of the

first WLL applications, it is only natural that cellular should find itself a key WLL technology.

Cellular technologies also might seem the natural choice where cellular operators have been given WLL licenses (although the provision of dual licenses has not been a widespread occurrence). The use of cellular technology for WLL would bring benefits in terms of economies in operations and maintenance. However, mobile operators typically will not want to use their mobile spectrum for fixed applications and for that reason may select a different technology in a different band.

When applied to WLL, cellular brings the advantage of immediate availability, inexpensive subscriber equipment, and proven systems. It has good range and can offer mobility if required. However, it also brings a host of problems, including relatively poor speech quality (although this is improving); expensive network infrastructure configured to handle mobility, handover, and so on, which is no longer required; and a need to use spectrum dedicated to cellular services, which tends to be expensive.

Telecommunications Industry Association (TIA) group TR.45 is considering IS136 (US Digital TDMA),IS-95 (US Digital CDMA) and PCS-1900 (GSM) based system for WLL. These systems are all optimized for cellular telephony, that is, for a Rayleigh fading channel with millisecond fade durations and with 5-10 ms of delay speed. The Cellular System consists of Analog and Digital Cellular system.

3.2.1 Analog Cellular

Analog cellular has been deployed widely throughout most of the first world countries since the mid-1980s. Its key attraction for WLL lies in the fact that it is readily available and relatively inexpensive. Its key problems are that it lacks facilities and has poor voice quality. The wide deployment of analog cellular is expected to decrease in the future as digital and proprietary equipment becomes more readily available. As far as WLL operations are concerned, all the analog cellular systems are more or less identical. There are currently three main analogue cellular system types operating in the world: AMPS (Advanced Mobile Phone Systems), NMT (Nordic Mobile Telephone) and TACS (Total Access communications Systems). AMPS dominate the analogue cellular market with 69% of subscribers, TACS has 23% and NMT has only 8 % of the global subscribers. The total access communications system (TACS) is described here as an example.

A TACS network consists of a number of base stations connected directly to a switch. Switches are interconnected and are connected into the PSTN. The TACS system allows for registration, tracking, and handoff.

TACS operates in the 900-MHz band using 25-kHz channel spacing and a 45MHz duplex spacing. Each TACS network also requires at least 21 channels to be reserved for control purposes. The control channels provide paging information, allow random access, and provide the mobiles with information on the system status. Radio channel using analog FM with a peak deviation of 9.5 kHz. One major problem of TACS is that when a mobile is in a voice call, signaling also must be sent over the same channel. When a signaling message is being sent, the radio momentarily mutes the audio path, which results in occasional

loss of a syllable of voice. The effect is a slight, but noticeable, degradation in the speech quality.

Some of the key parameters of TACS are provided in Table 3.2 In calculating the capacity, a cluster size of 12 was assumed, which, if anything, results in an optimistic assessment [1].

Table

3.2Key

Parameters

of

TACS

Services Telephony ISDN Fax Data Videophone Supplementary services Multiple lines Performance

Relatively poor quality NO NO Limited, perhaps 1.2 Kbps NO Limited range NO

Range (radius)

Up to 35 km where topography allow 2 Cells per 100 km 0.026 Capacity per cell, 2x1 3.3 voice calls MHz 3.2.2 Digital Cellular These systems have seen rapid growth and are expected to outpace analogue cellular over the next few years. Major worldwide digital cellular standards include GSM (Global System for Mobile Communications), Hybrid solution of TDMA and FDMA and CDMA. GSM dominates the digital cellular market with 71% of subscribers.

Digital Cellular is expected to play an important role in providing WLL. Like analogue cellular, digital cellular has the benefit of wide availability. Digital

cellular can support higher capacity subscriber than analogue cellular, and it offers functionality, that is better suited to emulate capabilities of advanced wireline networks.

There are two key digital cellular systems, those based on TDM A technology and those based on CDMA technology. Examples of both are provided in this section [3].

3.2.2.1GSM/DCS 1800 GSM is a TDMA digital cellular system deployed in more than 120 countries around the world. It is the nearest piece of equipment yet to become a global mobile standard.

A GSM network consists of base stations connected to base station controllers (BSCs), which are connected in turn to switches. The switches in the network are interconnected, and there is a single O&M and billing platform located in the network.

GSM divides the spectrum into 200-kHz TDMA channels. Each channel is modulated with the use of Gaussian minimum shift keying (GMSK), resulting in significant adjacent channel interference.

Each 200-kHz channel is divided into 8 full-rate voice channels or 16 half-rate voice channels. GSM employs 13-Kbps speech coding using a complex RPE-LTP coder. That provides speech quality that is better than analog cellular but typically inferior to ADPCM coders. Each voice channel alternatively can be used as a data channel. GSM provides a range of data rates depending on the error rate that can be tolerated. The services available are described in Table 3.3 GSM also can transport group 3 fax information transparently over the air interface. GSM currently is being enhanced to provide packet data and higher

speed data. That would allow data rates of up to 64 Kbps at a 0.3% error rate and more efficient use of the air interface resources. Table 3.2 lists the key parameters of GSM. 3.2.2.2 IS-95 The IS-95 standard for cellular communications is based heavily on the CDMA technology designed by the American company Qualcomm. The standard has gained some success in the United States and Asia-Pacific regions and is now entering commercial deployment. Its proponents have claimed that its major advantage is a significant capacity increase over other cellular systems. That increase has proved impossible to calculate or simulate.

Table 3.3 GSM Available Services Data Rate 9.6 Kbps 4.8 Kbps 2.4 Kbps

Channel Type Full Full Half Full Half

Error Rate 0.3% 0.01% 0.3% 0.01% 0.001%

Table 3.4 Key Parameters of GSM

Service Telephony ISDN Fax

Yes, with digital voice quality slightly inferior to wireline No, but 64-Kbps support may be available in 1999 Yes

Data Videophone Supplementary service Multiple lines Performance

Yes, up to 9.6 Kbps No Wide range

Range (radius) Cells per 100 km2 Capacity per cell, 2xl MHz

Up to 30 km where topography allows 0.035 Full rate: 10 voice channels; half rate: 20 voice channels

No

The system uses single cell clusters, in which each base station transmits its CDMA signal on the same carrier. The carrier bandwidth is 1.228 MHz, and there is a duplex spacing of 45 MHz. The downlink transmission consists of a permanent pilot tone and a number of radio channels. The pilot tone is used by the mobile to estimate the path loss, so as to set power control initially, and to acquire synchronization to the codeword generator. Other channels are set aside for paging and other downlink information. Speech is encoded using a variable-rate codec, where the bit rate depends on the talker activity. That reduces interference to other users of the CDMA channel. Channel coding then is applied to generate a transmitted bit rate of 19.2 Kbps, which then is spread by a 64-chip Walsh code sequence to generate the 1.228-Mbps transmitted waveform.

The uplink transmission is slightly different. Speech is generated in the same manner, but higher rate coding is used to give a bit stream of 28.8 Kbps. The interleaved bits then are grouped into 6-bit symbols, and each symbol addresses a lookup table containing a particular Walsh code. That spreads the signal to 307.2 kchips/s. Because other mobiles could select the same Walsh

code, further spreading is required. Each mobile generates a unique PN sequence at 1.228 Mbps, which is used to spread the data to the full transmitted bandwidth.

Table

3.5

Key

Parameters

of

IS-95

Service Telephony ISDN Fax Data Videophone Supplementary service Multiple lines Performance Range (radius) Cells per 100 km2 Capacity per cell, 2xl MHz

Yes, with voice quality similar to or slightly better than GSM No Yes, using a modem Up to 9.6 Kbps No Limited range No 30 km under good topographic conditions 0.035 12 per sector

Calculation of the capacity of a CDMA system is extremely difficult. CDMA has a key advantage over TDM A in that when a new sector is added, the capacity is increased by an extra 15 channel.

The IS-95 standard is relatively poor at supporting fax and data. Both can be carried with modem support up to around 9.6 Kbps, but the range of services provided by GSM is not available. It may be that those services will be introduced in the future. Table 3.5 lists the key parameters of IS-95

3.3

Future

cellular

systems

Substantial work is being carried out on the next generation of mobile radio systems, which are expected to appear between 2002 and 2005. Although no information is available on whether those systems will be designed such that they are applicable to WLL, it seems likely that the technology eventually will be suggested for WLL. In Europe the next generation has been named the Universal Mobile Telecommunications System (UMTS); at a worldwide level, it has been termed the future public land mobile telecommunications system (FPLMTS).

The vision of UMTS, which is shared to a large extent by those outside Europe, has been articulated in many slightly differing ways but can be summarized as "communication to everyone everywhere. To achieve that goal, the system essentially must provide the following features-

• An extensive feature set to appeal to all the current disparate users of different types of radio system such as: Cellular users, who require high voice quality and good coverage; Users who currently deploy their own private systems (often known as PMR users), who require group and broadcast calls, rapid channel access, and low cost; Paging users, who require a small terminal and good coverage; Cordless users, who require excellent communications with high data rates when in the office or home; Satellite users, who require truly worldwide coverage;

Users in airplanes, who currently have limited system availability; Data users, whose requirements range from telemetry to remote computer network access.

• Ubiquitous coverage with a wide range of cells such as: Satellite cells covering whole countries; Macrocells covering a radius of up to 30 km; Minicells covering up to around 3 km; Microcelis covering a few streets; Picocells covering an office, a train, an airplane, and so on.

Major players in this area postulate that that will be achieved through i system with the following characteristics: •

An adaptive air interface such that access methods such as CDMA, TDMA and FDMA can be selected as appropriate with bandwidths dependent on the service required and a data rate of up to 2 Mbps available in some locations;

•

Mobiles that can have their "operating system" downloaded to allow for network evolution.

•

An architecture based on intelligent network principles.

The system also must integrate seamlessly with the fixed network such that users receive nearly identical services whether they arc using fixed or mobile phones. Current predictions as to the time scales of third-generation systems vary slightly, but typically the following assumptions are made: •

Standardization completed, 1999;

•

First product available, 2002;

•

Product

widely

available,

2005.

One of the prerequisites of the third-generation system was that it was agreed worldwide as a single global system, achieving the design aim of international roaming. There are three main key players in third-generation work: Europe, the United States, and Japan. Europe would like to see GSM evolve to become the third-generation system. The Japanese have a different agenda: they have failed to make an impression on the world scene with their first- and secondgeneration systems and want to make sure they do not fail with the thirdgeneration system. They plan to do that by launching a new system as early as possible, based on something quite different from GSM. Finally, the United States has the view that as little as possible should be standardized, because standards prevent innovation and consumer choice and because standards making often is not performed by the most appropriate bodies.

Whatever the route, by the year 2005 it is expected that there will be a new generation cellular systems. The systems will have a much wider range of capabilities, support higher data rates, and be able to communicate with satellites when out of the range of cellular systems. It is likely that they also will be used for some WLL deployments, perhaps by the year 2005.

3.3 Low –Tier PCS or Microcellular –Based Systems.

PCS (Personal Communication System) incorporates elements of digital cellular and cordless standards as well as newly developed radio-frequency (RF) protocols. Its purpose is to offer low-mobility wireless service using low-power antennas and lightweight, inexpensive handsets. PCS is primarily seen as a city communication system with far less range than cellular. They are typically

operated at 800 MHz, 1.8 MHz and 1.9 GHz frequency bands. Compared with the cellular based WLL, more base stations are needed to cover the same service area. Operators may consider low-tier WLL technologies when an existing infrastructure is in place to support backhaul from many base stations to the switch or when wireline –like service and quality are essential. Microwave backhaul should be seriously considered to reduce the transmission cost.

Low- tier systems such as PACS and PHS are designed to operate in a Rayleigh channel and can tolerate intermediate delay spreads up to 500 ns. The basic user channel is typically 32 kb/s. Low-tier PCS and high-tier cellular air interfaces intended for WLL can be connected to conventional switches and do not require an MSC. The advantage of the high-tire radio system is the large coverage area of the base stations and the user velocities at which access can be supported the trade-off, however, is low –quality voice and limited data service capabilities with high delays. The low-tire systems are disadvantaged as far as coverage area size and user speeds. The advantages include high-quality, low –delay voice and data capabilities [11].

For the WLL, we have assumed that the low-tier radio technologies operating in the licensed PCS spectrum are more suitable to provide a quality service (QoS)

that

is

expected

traditionally

from

local

exchange

(LECs).Therefore, we have focuses on PWT-E and PACS. Table 3.6 A comparison of high-tier and low-tier radio technologies.

Base Stations

High Tier

Low Tier

Large expensive

and Small inexpensive

and

carriers

Coverage/Base station Capacity

Up to several < 1/3 mile radius miles Low to medium Very high

Talk time for Short (1hr) portables Vechicular service >70 mph

Long(>4 hours)

Quality

<Wireline

Wireline

User acceptance

Low

High

Expected usage

Low

High

35 mph

Principal “ designed Outdoor vehicular Indoor/outdoor for” application pedestrian Representative DCS 1900,IS- PWT,PACS,PHS systems 136,IS-95 3.3.1 PERSONAL WIRELESS TELECOMMUNICATIONS – ENHANCED Version of DECT is PWT for unlicensed use and PWT-E for licensed use. For the United States, the protocol has been modified to support both the licensed and unlicensed PCS bands. DECT and PWT were originally designed as a radio system interface between a fixed part (FP) and a portable part (PP) (Fig. 3.1).

Figure

3.1

PWTE

reference

The FP is generically understood to have three major functions:

architecture

•

Radio fixed part (RFP) — terminates the common interface (CI) air interface protocol.

•

Central system — provides a cluster controller functionality managing a number of RFPs.

•

Interworking unit (IWU) — provides all the necessary function for the PWT/DECT radio system to interwork with the attached wireline network;

For the WLL application (Fig. 3.2), the portable party (PP) in the PWT reference model is a component attached to the building and typically called the cordless terminal adapter (CTA). The CTA contains all the functionality of a PP and an additional interface to the inside wiring.

Figure 3.2 PWT-E reference model for WLL applications As shown in Fig.3.3 the PWT reference architecture for WLL may contain a controller/concentrator and an IWU. A network management system (NMS) may also be part of the architecture.

Figure-3.3 More formal architecture for PWT-E WLL

reference

While other PCS technologies separate the band into a handset transmit band and a base station transmit band, PWT uses time-division duplex (TDD) with both the handset and base station transmitting on the same frequency (at different times).

PWT has 24 time slots in 10 ms. Twelve slots are defined for base-to-handset transmission, and 12 are defined for handset-to-base transmission. The overall data rate for voice for handset/base is 32 kb/s using adaptive differential pulse code modulation (ADPCM), which provides toll-quality voice. The transmission path between handset and base station uses a pair of time slots on the single RF channel.

3.3.1.1 SUPPLEMENTARY SERVICES PWT standards recommend the use of supplementary services defined for ISDN. The following supplementary services are explicitly defined in various standards: •

Malicious call identification (MCID)

•

Call forwarding busy (CFB)

•

Call forwarding unconditional (CFU)

•

User-to-user signaling (UUS)

•

Calling line Identification presentation (CLIP)

•

Calling line identification restriction (CLIR)

•

Connected line identification presentation (COLP)

•

Connected line identification restriction (COLR)

•

Completion of calls to busy subscribers (CCBS)

•

Freephone (FPH)

•

Advice of charge (AOC)

•

Subadressing (SUB)

•

Terminal portability (TP)

•

Call waiting (CW)

•

Direct dialing in (DDI)

•

Multiple subscriber number (MSN)

•

Closed user group (CUG)

•

Explicit call transfer (ECT)

•

Single-step call transfer (SCT)

•

Call forwarding no reply (CFNR)

•

Call deflection (CD)

•

Conference call add-on (CONF)

•

Call hold (CH)

•

Three-party (3PTY)

Supplementary services in DECT/PWT are treated In two separate categories.

3.3.1.2 DATA CAPABILITIES PWT provides data capabilities in a number of different areas to facilitate various data applications. The connection-oriented message service (COMS) includes point-to-point connection-oriented packet service, which supports only packetmode calls.

The connectionless message service (CLMS) Includes fixed- and variable-length message services. The fixed-length message service is used to support for group paging and broadcast messages from FP to PP only. The variable-length message service operates in both directions.

The following are the optional user-plane data services defined for PWTE/DECT: • Transparent unprotected service (TRUP): to transmit unprotected transparent data

• Frame relay service (FREL): reliable transmission of SDUs, performs, checksum, segmentation and reassembly • Frame switching service (FSVVI): for further study • Forward error correction service (FEC): for further study • Basic rate adaptation service (BRAT): provides for the transparent transport of synchronous continuous data rate at the following rates: 64, 32. 16, and 8 kb/s, allowing "transparent" interworking with ISDN B and D channels • Secondary rate adaptation service (SRAT): operates only in conjunction with BRAT service; rate-adapts data terminal equipment with V-series interfaces (async or sync from 50 b/s to 56 kb/s) to be interfaced to one of the input rates provided by the BRAT; uses the procedures definedinV.110 • Escape service (ESC): allows an implementation-specific nonstandard service 3.3.2PACS PACS is a low-power radio system for both PCS applications and for fixed wireless loop applications. Bellcore started research on PACS technology in 1983 and started detailed formulation of system architecture and requirements for PACS in 1989.

PACS, a North American standard (ANSI J-STD-014), is optimized to provide basic capabilities to support WLL and additional capabilities to support mobility. It is integrated into the public telephone network to take advantage of the public network's rich ISDN/advanced intelligent network (AIN) feature set, including support for mobility. The service capabilities of PACS include voice, fax, voiceband data, and wireless digital data.

PACS design has been optimized from the beginning to support WLL with additional capabilities to provide mobility, such as roaming and handoff. PACS relies on the public network switches' ISDN and AIN capabilities.

Table 3.7 PACS system Parameters Frequency range

1850-1910 MHz uplink 1930-1990 MHz downlink RF channel spacing 300 kHz Channel bit rate 384 kb/s Access method TDM/TDMA Voice encoding 32 kb/s ADPCM Frame duration 2.S ms (8 time slots) Channel coding Error detection (15-bit CRC) Handoff/automatic link Portable/subscriber unit transfer controlled Radio port TX power 800 mW peak Subscirber TX power 200 mW peak Receiver sensitivity -101 dBm Frequency planning Quasi static automatic frequency assignment(QSAFA) 3.3.2.1 Supplementary Services The PACS network delivers the same voice quality that subscribers receive from wiring service providers. Also, it offers most of the custom calling and convenience features, such as: â&#x20AC;˘

Call waiting, to alert the customer to an incoming call while a call is in progress.

o Call forwarding, so that calls can be automatically forwarded from a business office, for example, to the PACS handset

o Three-way calling, allowing PACS to subscribe to add a third party to an ongoing call directly from their handsets o Caller ID. which reveals the name or number of the calling party before the call is answered o Call screening, so that incoming calls can be blocked or rerouted depending on the calling number or a private access number o Three-way calling o Call waiting o Call forwarding (busy, no answer, variable) o Single number o Calling number blocking o Caller ID presentation o Voice messaging o Distinctive ringing o Time-of-day routing

3.3.2.2 DATA CAPABILITIES PACS supports circuit mode and packet mode data services. In addition, individual message service (IMS) and interleaved speech/data services are also supported.

PACS supports full p.-law PCM encoding through aggregating two 32 kb/s time slots to form a 64 kb/s channel. A 64 kb/s channel can support 28.8 kb/s voiceband data using existing customer premise equipment (modems).

Circuit mode data capabilities are supported via a reliable Link Access Procedure-D (LAPD)-like protocol called Link Access Procedures for Radio

(LAPR). This is basically V.42 optimized for the PACS radio environment (e.g., snorter frame length). PACS provides "real-time" data transport service using LAPR, with full rate or subrate channels at up to 29 kb/s (106 error rate in full rate (32 kb/s) channel). PACS supports higher-layer circuit or packet protocols (circuit, e.g., data and fax modem; packet, e.g., X.25, Transmission Control Protocol/Internet Protocol, TCP/IP; asynchronous transfer mode, ATM).

The IMS is a two-way point-to-point messaging service supporting large file transfers. The messages can contain text, image, audio, and video files It supports variable-length messages up to 16 Mbytes.

PACS' interleaved speech and data service enables a call to have two modes: speech and data. This service defines signaling to switch modes.

PACS supports a packet mode data service. The packet mode uses different layer 2 and layer 3 protocols to allow optimization of radio resources for this type of data transport. The packet mode data service supports asynchronous shared access, which provides reliable variable asymmetrical bandwidth. It supports subscriber units using single slot (32 kb/s) or multiple slots (256 kb/s max).

Layer 2 uses the unique PACS Packet Channel (PPC) Data Link and Security protocol. The packet mode also supports higher-layer circuit or packet protocols (e.g., data and fax modem, X.25. TCP/IP, ATM).

3.4 Fixed Wireless Access System 3.4.1 What is FWA?

FWA (Fixed Wireless Access) is the use of radio spectrum to provide an alternative in the so-called 'last mile' connectivity between the subscriber and the fixed telecommunications network. Wireless access systems provide an opportunity to increase competition in the telecommunication market by giving more choice and innovation to consumers. Fixed Wireless Access removes the need to drape wires across the country or dig up roads to provide fixed telecommunication links, as is the case for fixed telephony and cable networks. As a result, it can easily also provide an effective platform from which to expand existing infrastructure, or serve to provide infrastructure in hitherto under-served areas [12].

There are mainly two ways in which FWA can be implemented: In point-to-multipoint systems, a single base station will provide telecommunications services within a specifically defined geographical service area. The service area will be centered at the geographic location of the proposed base station.

On the other hand, “Mesh” networks do not utilize a central base station but smaller nodes, thus the service area is centered at an operator defined geographic location rather than the location of a specific base station.

3.4.2 FWA Spectrum Overview Fixed wireless Access can typically be implemented in the following spectrum bands: 3.4GHz – 3.6GHz, 10.5GHz, 26GHz, 28GHz and 42GHz. Broadband wireless technology at any of these frequencies has the capacity to offer exactly the same set of services, however, the capabilities of 3.5GHz networks differ from those of the other frequencies because of the larger

geographical reach of tried and tested equipment for use at 3.5GHz. These favorable properties made this frequency band the preferred choice for vendors.

Thus significant technological advances have been made in this area, which enabled the production of mass-market radio products at this frequency. This in turn resulted in cheaper products being made available for prospective operators and customers alike.

There are mainly two ways in which the download and upload bands can be implemented: â&#x20AC;˘ Time-division duplex (TDD): refers to duplex communications links where the uplink is separated from the downlink by the allocation of different time slots in the same frequency band. â&#x20AC;˘ Frequency division duplex (FDD): is a technique in which one frequency band is used to transmit and another used to receive.

In the case of Frequency Division Duplex (FDD) systems will need to be licensed for a frequency channel consisting of an upper and lower frequency (e.g. 2 x 28MHz in the 3.5GHz band). Time Division Duplex systems (TDD) will need to be licensed for one half of the FDD channel as outlined above (e.g. 1 x 28MHz in the 3.5GHz band).

3.4.3The Drive behind the Technology In the world of telecommunications, two of the fastest growing areas are fixed and mobile IP services. The main reasons for this are deregulation, increased competition, market demand for broadband services, and the introduction of new technology.

New fixed wireless broadband technologies give operators an opportunity to address the ever-increasing demand for bandwidth caused by the surge in Internet and intranet traffic, LAN-to-LAN interconnect and the mounting interest in voice over IP (VoIP). Thus the market is demanding an access network solution that can carry different kinds of broadband multimedia services.

In this respect, FWA is a good alternative to DSL and cable modem technologies and offers several benefits: • Time to market: By deploying an efficient radio-access system, operators can reduce the time to market for new services, thereby enabling them to remain successful in this business.

• Ownership of infrastructure: This implies independence from other operators as well as from the unbundling process of the local loop.

• Low cost: Fiber is expensive to deploy and copper is expensive to maintain.

The main cost of radio access is the equipment, which is falling in pace with the development of new technologies. Furthermore, maintenance costs are lower for radio access since they don’t involve the additional cost of digging up and there is no associated erosion and decay with the transmission medium.  Flexibility: The infrastructure can be expanded as the operator’s business grows Chapter 4 WLL System Components 4. Overview As opposed to the fixed network, the subscriber equipment in a WLL system generally consists of complex electronic components to support the radiorelated functions and ensure adequate service quality. The radio interfaces

available for WLL applications include those based on existing and emerging cellular and cordless telecommunication system radio standards, as well as those based on proprietary technologies. There are also a number of options for network interfaces for interconnecting the WLL radio access network to the PSTN switch. This chapter describes typical WLL system components and available options for radio and network interfaces.

4.1 Approaches for a Wireless Local Loop System In general there are two approaches for a WLL system: A direct connection to the wireline network provided adequate capacity exists on the central office (CO) switch. With this type of application, base stations are connected to the CO and the CO switch continues to provide billing and database functions as well as the numbering plan and progress tones (Fig.4.1)

co Base station Base station

Figure 4.1.Cell sites connected to central office (CO) Base Stations are connected to a private branch exchange (PBX) which connects to the CO (Fig 4.2)

co P B X

Base station Base station

Figure 4.2.Cell sites connected to a PBX In a WLL system no handoffs occur because it is a fixed-to-fixed link. Also, the air line from each building to the cell site can be customarily installed to reduce the interface. Since the links remain unchanged after installation, the design of the WLL system is much simpler than that of a mobile system.

WLL systems do not need to offer mobile services basically, even if some systems provide limited mobile services. Thus, for example, there is no home and visitor location register (HLR/VLR) in a WLL system and its overall architecture may be simpler than that of the mobile systems [13].

4.2 WLL Reference Model The WLL reference model, which is independent of the technology applied, was defined by ETSI as Figure 4.3 shows ETSI ETR 139. In general terms, ETSI proposed that a WLL system might consist of the following elements and interfaces: [2] • Local Exchange (LE): In this model ‘local exchange’ is intended to represent a number of different elements of the PSTN network, according to operator requirements. These include the telephone network, leased line network and data network.

• Base Station: One or more base stations may be connected to the controller. Each of them receives and transmits information and signaling from/to a customer terminal; they must also monitor the radio path.

• Radio Termination: The radio termination has the ability to access the airinterfaces. It should be possible to support standard ISDN, or leased line terminals via the radio termination.

• Customer Terminal A standard: ISDN or PSTN terminal.

• Network Management Agent (NMA): This element handles configuration data, customer system and radio parameters.

Figure 4.3 General reference model for a WLL system.

Controller: The functions of this entity are to control the base stations, interface to the NMA element, and connect the WLL into LE/ PSTN. • LE to Controller Interface, IF1: This interface connects the WLL access network to the public fixed network. This information carried by the IF1 interface is related to the services accessed by the WLL users.

• NMA Interface, IF 2: Interfaces the NMA and the controller.

• Controller to BS Interface, IF3: Connects one or more BSs to the controller; information related to the call handling , radio resource management , O&M messages, and mobility management specific for WLL. • Radio Interface, IF4: This interface carries the same information as the IF3 interface. In addition, it may be used to carry supervisory messages to the radio termination.

• Radio Termination to Customer Terminal Interface, IF5: Information related to the services accessed by a user or an application is carried in the interface.

â&#x20AC;˘ O&M Interfaces, IF6: Information related to the configuration, performance and fault management of the WLL system is carried in the interface.

4.3 WLL Interfaces 4.3.1 Radio Interfaces Based on Current Cellular/Cordless Standards The radio channel that provides a communication path between the end-user terminals and the base station is governed by the radio interface deployed for the WLL system. The radio interface specifications not only define the frequency band of operation and the duplexing (FDD/TDD) and multiple access (FDMA/TDMA/ CDMA) methods, but also specify the detailed channel rasters, framing structures, timing requirements, source and channel coding schemes, and modulation methods.

The radio interfaces used in WLL systems fall in two basic categories: those based on current and emerging cellular and cordless telecommunication system standards or those which are proprietary and are developed by individual WLL equipment vendors to meet specific applications and markets.

It is possible to utilize radio interfaces based on first-generation analog cellular systems operating in the 450-MHz band (e.g., NMT-450) for WLL systems.

Since these analog cellular systems are now almost all replaced by secondgeneration digital cellular systems like GSM, the frequency band may be available for use in WLL applications. Because of the lower frequency of operation. WLL systems based on NMT-450 also have the advantage of larger cell size and wider coverage due to lower path losses. However, deployment of these systems poses the potential risk of the WLL operator being locked into an obsolete technology for which equipment for replacement or expansion purposes may not be easily available. [4]

Table 4.1 Summary of Digital Cellular and Cordless Standards Used in WLL Systems Radio Standard GSM (900) GSM (1800) GSM (1900) DAMPS (800) DAMPS (1900)

Frequency Band (MHz) 890-915 935-960 1710-1785 1805-1880 1850-1910 1930-1990 824-849 869-894 1850-1910 1930-1990

Access Method TDMA/F DD TDMA/F DD TDMA/F DD TDMA/F DD TDMA/F DD

CDMA (800) CDMA (1900)

824-849 869-894 1850-1910 1930-1990

CDMA/F DD CDMA/F DD

DECT

1880-1900 TDMA/T DD 1895-1918 TDMA/T DD

PHS

Referenc e Standard ETS300,500 ETS300,500 700 ANSI007A IS-136 ANSI009, 010,011 IS-95 ANSI008, 018,019 ETS300 175 RCR-28

Relevant SDO* ETSI (Europe) ETSI (Europe) Joint Tl/TIA (U.S.A.) TIA (U.S.A.) Joint Tl/TIA (U.S.A.) TIA (U.S.A.) Joint Tl/TIA (U.S.A.) ETSI (Europe) RCR (Japan)

4.3.2 Radio Interfaces Based on Proprietary Radio Technology There are a significant number of proprietary WLL systems which utilize radio interfaces that do not conform to any existing radio standards and are designed to operate in frequency bands that are not currently assigned to cellular or cordless applications. As mentioned earlier, the advantages of such proprietary systems is that they are designed specifically for WLL applications (rather than being a byproduct of existing cellular/cordless systems), so that they can provide better efficiency and coverage and can accommodate frequency bands that are different from those already in use for existing cellular or cordless systems.

Examples of these proprietary interfaces and their radio characteristics are summarized in Table 4.2. A W-CDMA WLL radio interface is likely to become

the

South

Korean

national

(Telecommunications

standard

Technology

under

the

Association),

auspices

the

South

of

TTA

Korean

telecommunications standards organization.

Table 4.2 Radio Interface Characteristics for some Proprietary WLL Systems Parameter Air Loop System (Lucent) Access DS-CDMA method Duplex FDD method Frequency 3.4-3.45 GHz bands 3.5-3.55 GHz. or options 3.45-3.5GHz 3.55-3.6 Duplex 100 MHz separation CH 5 MHz/FA Total RF 115 (max) channels Modulatio QPSK Speech LD-CELPoptions ADPCM-32 kb/s PCM-

Internet FWA (Nortel) TDMA

SWING (Lucent)

W-CDMA WLL (LGE)

TDMA

DS-CDMA

FDD

TDD

FDD

3.4-3.6 GHz

2.3-2.33 GHz 2.37-2.4 GHz

100 MHz

1.88-1.9 GHz 1.9-1.92 GHz 1.911.93GHz N.A.

2 MHz 264

2 MHz 120

10 MHz/FA 500 (max)

pi/4ADPCM

GFSK ADPCM PCM

QPSK ADPCM

70 MHz

4.4 System Architecture The WLL system described in this article is shown in Fig. 3. Briefly, the WLL system includes: a WLL gateway switching system that connects the radio system to the PSTN; the radio port controller (RPC), which provides concentration and control functions to a number of base stations called the radio port (RP); and the radio interface unit (RIU), which are the fixed units attached to the residential or commercial buildings. Also included are the radio port operation and maintenance (RPOM) unit, which is responsible for maintaining and managing the radio network elements, and the inter-working

function (IWF) unit, which is used as a gateway to data services such as the Internet and the public switched packet data network (PSPDN).

The WLL system's various components can be configured according to market needs. The RIU can connect up to 32 lines. The RP has a separate outdoor-type that can facilitate network expansion. The RPC can accommodate up to 20,000 subscribers. As the WLL gateway, a commercially available central office switch is used to accommodate up to 250,000 subscribers [14].

Figure 4.4 Overall WLL system architecture.

As shown in Fig.4.4 subscribers can gain access to the PSTN, ISDN, Internet or other data networks via a regular phone, a facsimile terminal, a modem, an ISDN phone, or a PC, which are connected to the RIU. The RIU can be configured to support 1, 2, 4, or 32 subscriber lines and provides various services requested by the subscribers. The RIU is equipped with LEDs allowing the subscribers to check operation status. The RIU uses normal AC voltage (110 ~ 220V AC) as power that can be supplied from a battery during power failure.

The RIU can be placed on a desktop or be wall-mounted. The RP with an omnitype antenna can be deployed indoors or outdoors, depending on the needs.

Alternatively, the RP can be equipped with a sector-type antenna, which is more suitable for urban areas. The RP can support up to 80 channels simultaneously. The RPC can be expanded to support up to 20,000 subscribers and controls up to 32 RPs. The RPC interacts with the IWF and the switch. The interface between the switch and the RPC can be configured as E1/V5.2 interfaces, two-wire analog subscriber lines, or multiple ISDN (2B+D) lines. The interface between the switch and the public network can be configured as PSTN or ISDN trunks. .

Figure 4.5 Typical architecture of WLL Table 4.3 Characteristics of WLL system.

RPC

RP RIU

RPOM IWF Switching system

Subscriber line capacity: Max. 20,000 subscribers (0.1 Erlang) Number of RPs supported: Max. 32 RPs Number of Els supported:: Max. 64 E1s Channel capacity: 80 channels RF transmit power: Max. 20W 1 line: POTS or Data Service 2 lines: 2 POTS or 1 POTS+1 Data 4 lines RIU: 4 POTS or 3 POTS+1 Data or 3 32 lines RIU: 32 POTS or 24 POTS+4 Data+4 Subscriber line capacity: Max. 500,000 Number of RPCs supported: Max: 16 RPCs Data service line capacity: Max. 8,100 ports Subscriber line capacity: Max. 250,000 Trunk capacity: Max. 60,000 trunks

4.5 WLL Functional Blocks

4.5.1 Fixed Subscriber Unit The fixed subscriber unit (FSU) is an interface between subscriber's wired devices and WLL network. The wired devices can be computers or facsimiles as well

as

telephones.

FSU

performs

channel

coding/decoding,

modulation/demodulation, and transmission/ reception of signal via radio, according to the air-interface specification. If necessary, FSU also performs the source coding/decoding.

FSU also supports the computerized devices to be connected to the network by using voice-band modems or dedicated data channels.

Figure: 4.6 FSUs serving multiple subscribers There are a variety of FSU implementations. In some types of commercial products, an FSU is integrated with handset. The basic functions of this integrated FSU are very similar to those of handset for mobile communications, except that it does not have a rich set of functions for

mobility management. Another example of FSU implementation is a highcapacity, centralized FSU serving more than one subscriber.

FSU is connected with the base station via radio of which band is several hundreds of MHz or around 2 GHz. Since WLL is a fixed service, high-gain directional antennas can be used between FSU and the base station, being arranged by line-of-sight (at least, nearly) [2].

4.5.2 The Base Station System The other major component in a WLL system is the base station system (BSS) whose chief function is to provide radio coverage for end users within a single or multiple cells. Radio traffic passes from the end-user terminal to the BSS in the up-link direction and from the BSS to the terminal in the down-link direction.

The BSS manages all aspects of the RF links. It allocates the radio channels (frequency and time slot in TDMA systems and frequency and spreading codes in CDMA systems) for each terminal, and it controls the power level the terminal should use. During the call, it monitors the radio channel and releases it when the call is completed. Other ancillary functions of the BSS relate to efficient functioning of the cells. These functions include the definition of radio channels into traffic and signaling channels and collection and analysis of signal quality measurements. Thus the key functions performed by the BSS include Radio channel management Configuration of radio channels Channel selection, allocation, release Channel blocking indication

Monitoring of idle channels Automatic transmit power control (if implemented) Digital signal processing for Transcoding and rate adaption unit (TRAU) Channel coding and decoding Termination of control channels

Figure 4.7 Typical base station system (BSS) architecture In typical WLL implementations, the base station system is divided into two functional blocks: base station controllers (BSCs) and base transceiver station (BTSs) or base station (BS). In a hierarchical architecture shown in Figure 4.7, the BSC generally serves a number of BTSs which are located at the cell sites where each BTS provides radio coverage for a number of end-user terminals within the cell. The BSS functions in the architecture shown in Figure 4.6 are therefore partitioned into the BSC and the BTS. 4.5.3 End-user equipment Figure 4.8 illustrates the basic components of the equipment at one end of a radio link. The three basic components (indoor unit, outdoor unit, and the

antenna) are present in all types of radio terminals, independent of whether it is in the local loop (point-to-multipoint) part or in the backhaul radio (point-topoint) part, or whether the terminal is a radio base station or is a radio remote terminal.

In the case of the remote terminal unit, the figure applies to remote units that are not mobile and are installed at the customer premises. If the WLL system supports

Figure 4.8 Components of a WLL subscriber radio terminal mobility, then the remote unit has to be portable, containing all the necessary electronics and power source (battery), and cannot use a directional antenna.

4.5.4 Indoor Unit (IDU) The basic function of the IDU is to convert the digital signal from the end-users equipment (voice, data, video terminal) into a suitable form for transmission [by the outdoor unit (ODU) and the antenna] over the radio channel and to process the received signals (from the ODU) for presentation to the end-user device.

The IDU functions comprise the radio transmission and reception functions for the outgoing and incoming user information and control signaling, telemetry, and network management functions for monitoring and configuration of the

end-user equipment including the IDU, the ODU, the directional antenna, the power supply for the IDU and ODU, and functions for multiplexing various incoming and outgoing signals (including power supply) over the IDU-ODU feeder cable.

4.5.4.1 Transmitter / Receiver functions These

include

functions

like

source

coding,

channel

coding,

interleaving/deinterleaving, and modulation/ demodulation For systems that deploy frequency division multiple access for defining radio channels, generally the modulator in the IDU converts the received signal either to the so-called base-band (BB) frequency or to an intermediate frequency (IF). In the case of base-band modulation, the modulating signal corresponds to a frequency that represents the channel bandwidth available on the radio path. This resulting signal, modulated to the available channel bandwidth, is called base-band signal.

The ODU will then up-shift the base-band signal to the appropriate frequency of the (up-link) channel allocated (by the base station) to the end-user terminal. For example, the 200-kHz up-link channel allocated to the terminal in the GSM example may be in the frequency range 890-890.2 MHz.

Radio Rx/Tx

NM Functions

IDU/ODU Power

Function s MUX

Figure 4.9 Functions associated with the indoor unit (IDU) Since FDMA channel structure defines a constant channel bandwidth (e.g., 200 kHz in GSM), the same base-band modulating frequency has to be used for both the up-link (IDU -> ODU) and the down-link (ODU -> IDU) signals. This means that the two signals cannot be multiplexed on a single IDU -> ODU cable, and two separate cables may be required.

The intermediate frequency is chosen somewhere in between the base-band and the RF center frequency. The IF modulated signal from the IDU is then upshifted by the ODU to an appropriate RF carrier for transmission. The choice between use of IF versus BB signal between the IDU and the ODU is a design parameter, and the factors that may dictate the choice include Better suitability of the IDU modulation method Potential for lower loss and interference on the IDU-ODU cable potential for multiplexing all signals between IDU and ODU on a single cable (however, this requires that two different IFs be used for up-link and down-link directions) 4.5.4.2 Network Management Functions The physical equipment to support network management functions (e.g., terminal control unit and telemetry modem) contains the electronics to monitor the radio terminal as a whole, and it responds to s/w commands from the NM port. It is generally designed to be software-based and to conform with standard s/w and network management interfaces (including SNMP and CMIP) and is capable of being monitored and configured from a remote location. It also provides a communication channel for network management, monitoring, and configuration purposes, which allows the ODU to be controlled from the control terminal in the IDU. At the base station, capability

can be provided to diagnose and reconfigure antenna equipment without having to climb the mast.

4.5.4.3 Power Supply Unlike telephones in a wireline system where a terminal is powered from the local exchange, the electronics involved in a WLL terminal needs to be powered from a power unit at the customer premises. Reliability of the inhouse power supply is therefore critical for continuous service. Typically, 48volt or 24-volt DC power is supplied from the mains supply with a standby battery. Solar power options are also available from some WLL equipment suppliers.

4.5.4.4 Cable Multiplexer Serves only to share the use of IDU-ODU connection cable between the modulated user data signal, the network management information, and control signals and for provision of power to the ODU. 4.5.5 Outdoor Unit (ODU) The ODU in a WLL radio terminal basically comprises of the RF transmitter and receiver (transceiver). Its main function is to modulate and amplify the lowerfrequency signal received from the IDU into a RF signal at the appropriate frequency for transmission over the radio channel, and to effectively alleviate the effects of signal degradation, and demodulate the received RF signal to a suitable frequency for further processing by the IDU. The ODU functionality does not alter the basic characteristics (like bit rate and synchronization) of the signals received or passed on to the IDU. Figure 4.5 illustrates the functional diagram of a typical ODU of a WLL radio.

4.5.5.1 Receiver Front End

The main function of the splitter is to separate the incoming (receive) and outgoing (transmit) signals at the antenna. Before the demodulation stage in the receiver, devices like a low-noise amplifier (LNA) and/or an automatic gain control (AGC) is used to strengthen and stabilize the received signal. The former (LNA) is intended to amplify weak signals while preserving the clarify and quality of the received signal. The latter (AGC) is a variable strength amplifier that is designed to present a constant-strength signal to the receiver.

It compensates for unpredictable radio signal losses over the radio channel caused by radio path attenuation that may be caused by adverse weather conditions like rain and snow.

For proper reception of high-bit-rate data or video signals that require highfidelity reception, an equalizer stage may be added before demodulation. The equalizer is intended to compensate for uneven losses or attenuation whereby the

Figure 4.10 Functions associated with the outdoor unit (ODU)

Frequencies at the lower end may be affected more (or less) than those at the higher Ld These unbalances are more pronounced when the incoming signal is spread over a wide frequency band and is modulated using a higher-order modulation Method like 8-QAM or higher.

4.5.5.2 Demodulators The first stage of demodulation in the receiver is used to shift the received signal to the down-link carrier frequency (frequency channel) assigned to the end-user terminal (by the base station). This shifting is achieved by using a frequency synthesizer that allows the receiver to be tuned to any of the downlink frequency channels defined for the operating frequency band allocated to the system (e.g., down-link channel 1 in GSM with the center frequency = 935.1 MHz). The second demodulation stage then down-shifts the signal to the appropriate intermediate frequency (IF-down) which is then passed on to the IDU for further processing. Thus the frequency synthesizer is a highly reliable tunable oscillator that is software-controlled, and the IF generator is a very stable and accurate fixed frequency oscillator.

4.5.5.3 Modulator The transmit part of the ODU works like a receiver in receiver. The up-link signal from the IDU is received over the IDU-ODU cable connection. It is the user signal modulated by the up-link intermediate frequency (IF-up) which is then up-shifted to the appropriate up-link RF channel using the frequency synthesizer. The frequency synthesizer ensures that the outgoing RF signal is modulated by the up-link frequency channel assigned by the base station. The resultant signal is then amplified at the high-power amplifier (HPA) and then conveyed by means of the splitter and waveguide to the antenna for transmission.

Chapter 5 Frequencies for WLL 5. Overview As has been mentioned, some propagation phenomena depend on the frequency of transmission. If this were a book on mobile radio, the variation with transmission frequency would be of little relevance, because across the world the same frequency bands are used for mobile radio. Unfortunately, that is not the case for WLL. To understand why, it is necessary to know a little more about the background to radio spectrum allocation. Radio spectrum is a scarce resource; as one American analyst noted, “Spectrum is like real estate—they just don’t make it any more.” Numerous applications require radio spectrum, including cellular, microwave links, satellites, radars, meteorology systems, government radio systems, defense equipment, garage door openers, and microwave ovens. The list is almost endless. As a result, new systems need to find parts of the spectrum that are little used and fit in with existing users.

Those little used parts tend to be at higher frequencies, where the propagation achieved is poor compared to lower frequencies.

The need for low frequencies (i.e., below 1 GHz) with their resultant long propagation ranges also varies across different WLL deployments. Where the subscribers are distributed with a low density across a large area, long range is important. Where users are clustered in a city, a long range can actually be problematic, resulting in interference to neighboring cells. Because of those differences, different WLL operators have looked to different frequencies to best meet their needs.

Attempts are ongoing to standardize some frequencies across the world for WLL operation. Standardization would enable manufacturers to achieve economies of scale and operatorstominimize interference with neighboring countries. Because around the world, the higher frequencies are lightly used, WLL frequency standardization must, by necessity, focus on frequencies above3GHz. The result is that operators requiring greater range than can be achieved at 3 GHz will use nonstandardized frequency bands. There is little problem with that, as long as an assignment can be gained in the country of operation and equipment obtained that operates at the required frequency.

It is not appropriate here to discuss the process of standardizing frequencies; readers who are interested should refer to. Suffice it to say that at the time of this writing it seemed likely that before the end of 1998 3.4 to 3.6 GHz and 10.15 to 10.65 GHz would emerge as standard bands. As will be seen in Part IV, only a fraction of the currently available WLL systems are designed to work at those frequencies.

Table 5.1 lists the frequencies currently used or proposed for WLL. From the discussion in this chapter, readers will have deduced that the lower the frequency the greater the range and the better the diffraction around obstructions. Broadly speaking, then, operators tend to try to obtain spectrum at as low a frequency as possible. A number of factors tend to put pressure on operators to agree to using spectrum at higher frequencies, including the following: â&#x20AC;˘

The higher cost of lower frequency spectrum where it is auctioned;

â&#x20AC;˘

The fact that more spectrum is available at higher frequency, providing a greater system capacity;

â&#x20AC;˘

The fact that the size of the cell may be limited not by the propagation distance but by the number of subscribers that can be supported in a cell; if

there are more subscribers than the maximum in a cell, the size of the cell must be reduced.

Table 5.1 Frequencies Used or Proposed for WLL Frequency Use 400–500 MHz 800–1,000 MHz 1.5 GHz

For rural application

For cellular radio in most countries Typically for satellites and fixed links 1.7–2 GHz For cordless and cellular bands in most countries 2.5 GHz Typically for industrial equipment 3.4–3.6 Being standardized for WLL GHz around the world 10 GHz Newly opened for WLL in some countries 28 GHz and For microwave distribution 40 GHz systems around the world Chapter 6 Interfaces for WLL 6. Overview WLL network interface specifies the transport and protocols associated with the base station to PSTN local switch connection, which is generally referred as the backhaul component. The various transport and protocol options available for the design of this link for WLL systems are shown in Figure 6.1.

6.1 Network interfaces for WLL systems 6.1.1

Coax

Cable

Coax cable: This may be a viable and perhaps economic option (as leaded lines) in metropolitan and urban applications. However, unless good-quality coax cable connectivity is available, it may be difficult to meet service quality targets.

6.1.2 Fiber Links Fiber links: Again this may be viable only in metropolitan and urban environments (where fiber connectivity is available). The transmission quality and reliability of this transport medium w i l l be excellent.

6.1. 3 Microwave Links Microwave Links: These are frequently deployed in a point-to-point mode at a range of frequencies. The coverage range of microwave systems reduces as the frequency of operation becomes higher; implying that more microwave links may be required to cover a given distance. However, higher-frequency microwave systems have some cost advantages in terms of smaller dishes and smaller antenna structures. Since these are radio links, their availability and reliability are critical design factors.

Figure 6.1 Interconnection of WLL to PSTN: transport and protocol options

6.2 Radio interfaces for WLL systems 6.2.1 Radio interfaces based on current cellular/cordless standards The radio channel that provides a communication path between the end-user terminals and the base station is governed by the radio interface deployed for the WLL system. The radio interface specifications not only define the frequency band of operation and the duplexing (FDD/TDD) and multiple access (FDMA/TDMA/ CDMA) methods, but also specify the detailed channel rasters, framing structures, timing requirements, source and channel coding schemes, and modulation methods.

The radio interfaces used in WLL systems fall in two basic categories: those based on current and emerging cellular and cordless telecommunication system standards or those which are proprietary and are developed by individual WLL equipment vendors to meet specific applications and markets.

The advantage of WLL systems based on cellular or cordless radio standards is that these systems are manufactured by cellular or cordless system vendors as a byproduct of their main business, with resulting potential for lower cost and easy availability of WLL equipment. However, these systems need to operate in the frequency bands standardized for the cellular or cordless applications, and the frequency band may already be in use in a geographic area where the WLL system is planned to be deployed. Alternatively, if the frequency band is available, the licensing fees for the band may be too high for the WLL business case to be viable [4].

It is possible to utilize radio interfaces based on first-generation analog cellular systems operating in the 450-MHz band (e.g., NMT-450|-for WLL systems.

Since these analog cellular systems are now almost all replaced by second-

generation digital cellular systems like GSM, the frequency band may be available for use in WLL applications. Because of the lower frequency of operation. WLL systems based on NMT-450 also have the advantage of larger cell size and wider coverage due to lower path losses. However, deployment of these systems poses the potential risk of the WLL operator being locked into an obsolete technology for which equipment for replacement or expansion purposes may not be easily available.

Major digital cellular and cordless systems like GSM. IS-136 (DAMPS), IS-95 (CDMA), DECT, and PHS, which are commonly used as the basis for WLL systems. Table 6.1 provides a summary of these standards. These cellular and cordless radio standards represent the so-called second-generation (2G) mobile and personal communication systems developed and deployed since the beginning of the 1990s. In the meantime, work has been underway in the regional and international standards organizations to specify new and evolved radio interface standards to meet the needs of the new or third-generation (3G) mobile communications.

Third-generation mobile communication systems are already being planned around the world, and some initial systems based on W-CDMA and CDMA 2000 are now commercially available". Commercial WLL systems based on these new or enhanced CDMA technologies are being developed by many vendors and are expected to be available for deployment in the very near future. In fact, some vendors are already offering WLL systems based on these technologies.

Table 6.1 Summary of Digital Cellular and Cordless Standards Used in WLL Systems

Radio Standard GSM (900) GSM (1800) GSM

Frequency Band (MHz) 890915 17101785 1850-

Access Method TDMA/FD D TDMA/FD D TDMA/FD

Refere nce Stand ETS-300, 500, 700 ETS-300, 500, 700 ANSI-

Relevant SDO* ETSI (Europe) ETSI (Europe) Joint

(1900) DAMPS (800) DAMPS (1900)

1910 824849 1850-1910 1930-1990

D TDMA/FD D TDMA/FD D

007A IS-136

Tl/TIA TIA (U.S.A.)

ANSI009, 010,011

Joint Tl/TIA (U.S.A.)

CDMA (800) CDMA

824CDMA/FD IS-95 849 D 1850-1910 CDMA/FD ANSI008, 1930-1990 D 018,019

(1900)

TIA (U.S.A.) Joint Tl/TIA

(U.S.A.) DECT 1880-1900 TDMA/TD ETS300 ETSI D 175 (Europe) PHS 1895-1918 TDMA/TD RCR-28 RCR D (Japan) 6.2.2 Radio interfaces based on proprietary radio technologies There are a significant number of proprietary WLL systems which utilize radio interfaces that do not conform to any existing radio standards and are designed to operate in frequency bands that are not currently assigned to cellular or cordless applications. As mentioned earlier, the advantages of such proprietary systems is that they are designed specifically for WLL applications (rather than being a byproduct of existing cellular/cordless systems), so that they can provide better efficiency and coverage and can accommodate frequency bands that are different from those already in use for existing cellular or cordless systems. Examples of these proprietary interfaces and their radio characteristics are summarized in Table 6.2. A W-CDMA WLL radio interface is likely to become the South Korean national standard under the auspices of TTA (Telecommunications

Technology

Association),

telecommunications standards organization.

the

South

Korean

Table 6.2 Radio Interface Characteristics for Some Proprietary WLL Systems Parameter Access method Duplex Frequency bands options Duplex separation CH Total RF channels Modulation Speech options

Air Loop System DS-CDMA FDD 3.4-3.45 GHz 3.5-3.55 GHz. 3.45-3.5 GHz 3.55100 MHz

Internet (Nortel) TDMA FDD 3.4-3.6

WWLL DSCDMA FDD 2.3-2.33 2.37-2.4

100

SWING (Lucent) TDMA TDD 1.88-1.9 1.9-1.92 1.91-1.93 GHz N.A.

5 MHz/FA 115 (max)

2 MHz 264

2 MHz 120

10 500

QPSK LD-CELPADPCM-32 kb/s PCM-

pi/4ADPCM

GFSK ADPCM PCM

QPSK ADPCM

70 MHz

6.2.3 Protocols at the Radio Interfaces An interface represents the point of contact between two adjacent entitiesâ&#x20AC;&#x201D; for example, the end-user terminal and the BTS in the case of a radio interface.

However, the radio interface can carry information not only between the radio terminal and the BTS, but also between the radio terminal and the BSC and the MSC (or local switch) to support the required functions. These messages from the radio terminal (to BSC or MSC/LE) are carried transparently through the BTS. In cellular mobile systems, there may also be message flows between the radio terminal and the HLR for support of supplementary services. The flow of information across the radio interface in a cellular mobile system or a WLL system supporting mobility is shown in Figure 6.2.

The layered architecture for the radio interface and the functions supported at the different layers are illustrated in Figure 4.8. Within the layered protocol architecture, the radio channel with its key characteristics described in the

preceding few sections represents layer 1 or the physical layer. Layer 2 is required to support the acquisition and control of the radio link and is generally partitioned into media access control (MAC) and link layer control (LLC) functions. The LLC is generally based on a modified form of LAPD (link access protocol for D channel in ISDN) to support radio interface specific features. As illustrated in Figure 6.3, Layer 3 is divided into three sublayers that deal with radio resource (RR) management, mobility management is concerned with managing logical channelsâ&#x20AC;&#x201D;their assignment and performance measurements.

Mobility management functions like terminal registration, terminal location updating, and authentication are required where terminal mobility is to be supported. Call management sublayer functions are concerned with call and connection

control,

establishing

and

clearing

call/connections,

management of supplementary services.

Figure 6.2 Functions supported by the radio interface protocols.

and

Figure 6.3 Layered architecture for the radio interface.

Chapter 7 WLL Protocols 7. WLL Protocols 7.1 Protocol Options Independent of the transport medium used to connect the base stations to the PSTN switch, a common protocol needs to be supported at both ends of the link. If the base station and the switch are provided by the same vendor as a package, the vendor may implement a proprietary interconnection protocol.

Such proprietary protocols can be more efficient and can save bandwidth over the link. However, the down side is that the operator is locked into the products provided by the vendor and will have little flexibility in using other vendors' products (even if they are more economical). [4] In WLL systems where the base stations are to be connected to existing PSTN switches, there are a number of standard protocols that can be deployed. The choice will generally depend on the additional cost for upgrading the PSTN

switch (if necessary) and the-range of services that the WLL system needs to support.

• CAS (channel-associated signaling) can only carry simple voice traffic and thus limits the range of services available to the WLL subscribers. It further requires one dedicated circuits to be assigned to each subscriber and is therefore very inefficient I in terms of resource utilization.

• Q.931 or DSS1 is the ISDN user-network interface (UNI) protocol, and some WLL systems based on cordless radio (e.g., PHS) utilize Q.931 -based interface to the PSTN. It can only provide services supported by the basic rate ISDN (2B + D) link.

• SS7 (signaling system 7) is the network-to-network interface (NNI) protocol deployed in most modern digital networks. Many cellular systems utilize SS7-based: interfaces between the BSC and MSC (generally known as the A interface). Many WLL systems based on cellular standards (e.g., GSM) will support SS7 for backhaul connection to the PSTN. 7.2 V5 (V5.1 and V5.2) interface V5 (V5.1 and V5.2) interface is an open interface specially developed and standardized (by ETSI) between the local exchange (LE) and an access network (AN). The access network may be a remote switching unit (RSU), a private branch exchange (PBX), or a radio access network (RAN). The BSC in a WLL application represents a radio access network, and a V5 interface can be used to advantage in WLL system. The V5.1 interface requires one time slot per subscriber and offers no concentration or trunking of time slots. The V5.2

interface, on the other hand, allows time slots to be trunked or shared among subscribers on a demand basis which can lead to significant cost savings. The ITU version of the V5.2 interface is described in ITU-T Recommendation G.965; and the North American version, known as the TR303.V5.2 interface.

Chapter 8 Multiple Accesses 8. Overview Multiple accesses, however, is of greater importance because the available WLL technologies differ in the multiple-access method they use. Decisions about which technology to adopt will be influenced significantly by the access medium required.

Each operator has a given amount of radio spectrum to divide among its users.

There are broadly three ways to do that: Frequency division multiple access (FDMA), in which the frequency is divided into a number of slots and each user accesses a particular slot for the length of the call.

Time division multiple access (TDMA), in which each user accesses all the frequency but for only a short period of time.

Code division multiple access (CDMA), in which each user accesses all the frequency for all the time but distinguishes the transmission through the use of a particular code.

Figure 8.1 shows the different multiple-access methods in a diagrammatic form.

WLL technologies are available that make use of each of the different access methods. Each method and its advantages and disadvantages are described next in more detail [1].

8.1 Frequency Division Multiple Access (FDMA) In a typical FDMA system, the available bandwidth is divided into slots about 25 kHz wide as shown diagrammatically in Figure 8.1 Each slot

Figure 8.1 Diagrams of FDMA, TDMA, and CDMA access methods.

Figure 8.2 An ideal FDMA spectrum. contains a dedicated transmission. The problem lies in the fact that the transmitted power when plotted against bandwidth is not an idealized rectangle (i.e., equal power is not transmitted across the entire available bandwidth). To generate such a transmission would require that the input signal be filtered over an infinitely long time period, clearly not a practical option. With the use of appropriate pulse shaping, the spectrum transmitted

can approach the ideal requirement. Figure 8.3 shows the spectrum transmitted by the GSM mobile radio system, which uses one of the most complex filtering arrangements of any technology currently available. Two adjacent channels are shown, and it is clear that there is significant interference between the two channels. Because of that, it is not possible to use adjacent channels in the same cell. Had the channels been spaced farther apart so that adjacent channels could be used in the same cell, there would be substantially fewer carriers per megahertz and the spectrum efficiency of the overall system would be much lower.

It is clear that substantial energy extends outside the channel in which a single user is transmitting. The result is that adjacent channels cannot be used in the same cell because the interference between them would be too great. It is also clear that, compared to an ideal rectangular spectrum emission; the GSM system does not transmit as much power

Figure 8.3 GSM spectrums. within the band; the power levels fall away as the band edges are reached. That represents a lost opportunity, reducing from the ideal case the power available to

the

mobile.

A more pragmatic problem with FDMA is that at the base station each individual channel requires a separate power amplifier before passing through an expensive high-power combiner and then being transmitted from the antenna. It is possible to combine the signals before the amplifier if the amplifier is highly linear, but in practice such amplifiers are extremely expensive and inefficient in their use of power.

In summary, the advantage of FDMA is that it is the simplest access method to implement. The disadvantages are the following: â&#x20AC;˘

The loss of efficiency caused by imperfect filtering;

â&#x20AC;˘

The expensive radio frequency (RF) elements required at the base station.

Now that technology has advanced to where other access methods can be implemented at relatively low cost, the single advantage of FDMA is broadly negated. For that reason, virtually no digital WLL systems use FDMA, and it is in use only in the older analog WLL technologies.

8.2 Time Division Multiple Access (TDMA) In TDMA, a user has access to a wide bandwidth but only for a short period of time. Using the example of GSM, which is a TDMA system, a user has access to 200 kHz of bandwidth for one-eighth of the time. To be more precise, the user has access to the channel for 577 ms every 4.6 ms. During that period, the transmitter sends a burst of data that was previously buffered in the transmitter.

Like FDMA, TDMA has its inefficiencies. Those inefficiencies are caused by the need to allow the mobile time to increase its power from zero and to reduce it back to zero again. If time is not allowed for that transition, the nearinstantaneous change in power is in effect, close to transmitting a square wave,

resulting in a momentary use of an extremely high bandwidth, with resulting interference to a wide range of users. Guard bands are provided to allow for the powering up and down. The structure of a burst in TDMA is shown in Figure 8.4, where it can be

Figure 8.4 GSM ramping up for a burst. seen that around 30 ms is required to ramp up for a burst of 540 ms. (The time taken to ramp down also is used by the next mobile to ramp up, so it is not required in addition to the ramp-up time.) Hence, the inefficiency is around 30/540 = 5.5%.

TDMA systems require additional overhead because they have to send timing information so the subscriber units know exactly when to transmit. There is an additional problem. By transmitting over a wider bandwidth, the problem of ISI is exacerbated. In some systems, such as GSM, that has resulted in the need for a component called an equalizer, which removes the ISI in the receiver. For an equalizer to work, the channel impulse response needs to be measured periodically. The timing is performed by placement of a training sequence in the middle of each burst. By correlating the received training sequence with a local

copy of the training sequence, the channel impulse response can be measured.

However, the sending of the training sequence represents a substantial inefficiency in the use of radio spectrum.

In summary, TDMA has the following advantages over FDMA: •

It is more efficient.

•

It is less expensive to implement.

The disadvantages of TDMA are the following: •

More complex subscriber units are required.

•

ISI may become problematic.

TDMA is a widely used multiple-access method for many mobile and WLL systems. Strictly speaking, most TDMA systems actually are TDMA/FDMA. For example, GSM places eight users on a 200-kHz channel using TDMA but then divides the assignment into 200-kHz slots using FDMA. 8.3 Code Division Multiple Access (CDMA) This section goes into somewhat more detail, because CDMA is more complex and less intuitive than the other access technologies. There is significant debate as to whether CDMA or TDMA is more appropriate, and an understanding of that debate requires a good understanding of CDMA itself.

CDMA is the process of data transmission using a code. In that process, each user is allocated a particular codeword. The user first generates data, which could be, for example, the output of a speech coder. The data is generated at a rate known as the bit rate, or Rb. Each bit is multiplied by the code to achieve the final output stream. The output stream is at a data rate equal to Rb multiplied by the length of the codeword, G; that rate is the chip rate, or Rc.

The process of multiplication by a codeword is known as spreading and is shown for an example datastream and codeword in Figure 7.11.

The length of the codeword, and hence the chip rate, is a fundamental design parameter of a CDMA system. Spreading with a large codeword results in a large transmitted bandwidth, which, if there are only a small number of users, will prove inefficient. Spreading with a smaller codeword yields a smaller transmitted bandwidth, which may not be able to accommodate sufficient users. The spreading factor also is influenced by the size of the frequency assignment available to the operator.

The choice of the code is critically important. The code should have good autocorrelation properties such that when correlated with offset versions of itself it exhibits a large impulse like peak at zero offset and a small residual signal at other offsets. Such a property maximizes the probability of reception.

Many families of such sequences are known.

At the receiver a process of despreading is required to recover the data. The process involves the multiplication of the received signal with the codeword.

Such multiplication results in the original binary information being decoded but with an enhancement of the signal level by a factor of G. The enhancement allows interference to be tolerated on the link.

In CDMA WLL systems, the ability to tolerate interference is used to allow other users to send their transmissions on the same channel. Each of the other users also has a spreading code. It is important that each user have a different code and that the codes are orthogonal, or nearly so, with each other. If two

users had the same code, the receiver would not be able to differentiate between them and the interference would be severe. If two users employed codes that were different but not orthogonal, a certain component of the second userâ&#x20AC;&#x2122;s signal would be decoded by the first user, negating some of the advantages of the decoding process The shortage of codewords can be a problem in some systems, particularly cellular, where interference from adjacent cells can be expected. It is overcome by using nearly orthogonal code families. In such families, there is no shortage of codes, but the interference from other users becomes slightly more problematic.

Transmitter

Receiver

Figure 8.5 Example of the generation of a CDMA signal.

Based on the discussion so far, it is apparent that the capacity of a CDMA system is limited by the amount of interference generated by other users employing the same frequency. As such, it often is stated that “CDMA is interference limited.” While in a WLL system, all access schemes ultimately are interference limited, the link between CDMA capacity and interference is more direct and apparent than in other access techniques.

One of the advantages of CDMA is that by reducing the spreading factor G, a user can increase the transmitted data rate without changing the bandwidth of the signal being transmitted. That allows a form of dynamic bandwidth-ondemand allocation. However, for the user who reduces the spreading factor, the interference that can be tolerated reduces, and the overall system capacity in terms of number of users falls. Nevertheless, such flexibility is extremely useful in the WLL environment. One of the main concerns with CDMA is power control. The accuracy required for power control is high. In cellular CDMA systems, such accuracy is difficult to achieve as mobiles pass through a fastfading environment.

8.4 Advantage of CDMA As discussed in the previous section, there are multiple factors that need to be considered for selecting a suitable WLL technology. However, CDMA technology provides the following advantages when used in WLL systems:[4]

•

It has higher capacity (in terms of subscribers/knr/MHz).

•

It exhibits significant capacity gain through sectorization because the same frequency can be used in each sector leading to a high capacity/cost ratio for sectorization.

•

No frequency planning is needed because all cells/sectors use same frequency allocation.

•

Space diversity available in CDMA systems provides immunity from multipath fading and capability for soft handoff (where mobility is supported).

•

CDMA systems are capacity-limited, which leads to graceful overload behavior.

These factors are further explained below: • Higher Capacity: In a CDMA system, on the other hand, the entire available frequency spectrum can be used in each cell with increased efficiency. EVRC (enhanced variable rate coding) codec, because of voice activity detection, utilizes smaller bandwidth over the air interface and leads to increased capacity. Furthermore, because the WLL terminals are typically fixed, very accurate power control is feasible, resulting in significant capacity gain.

• Greater Range: Range is related to the path loss and the minimum signal level that the receiver can decode reliably. Since in the CDMA network the receiver has the capability to apply a gain factor to the received signal, it can decode weaker signals more successfully, thereby increasing the range.

• Sectorization: Because the sectors at a cell site in a CDMA system can use the same frequency, the process of sectorization can increase the system capacity by almost a factor of three (in a three sector antenna), with a modest outlay for additional antenna and RF equipment.

• Frequency Planning: As opposed to TDMA and FDMA systems, where extensive frequency planning needs to be deployed for assigning frequencies to various cells, the single-frequency reuse in CDMA systems

avoids this step. However, CDMA systems do require allocation (and planning) of long PN codes to individual cells.

• Space Diversity: Use of rake receivers in a CDMA system permits the subscriber terminal to simultaneously receive signals from two or more base stations and thus utilize the advantages of space diversity for mitigating effects of multipath fading as well as providing the capability for soft handoffs.

• Soft Capacity Limit: The capacity of a cell or a sector in a non-CDMA system is hard-limited in the sense that if all the available traffic channels in the cell/ sector are busy, the next attempt is rejected. In a CDMA system, the operator has the flexibility to admit additional users during peak periods by providing a somewhat degraded service (increased bit error rate). This capability is especially important when calls might be dropped during handoff because of a lack of free channels in WLL systems that support mobility.

Chapter 9 Planning of Line-of –sight (LOS) Path in WLL System 9. Overview As discussed in preceding sections, the signal loss on the radio link is caused by intangible causes (atmospheric, climatic, and geographic) as well as tangible causes (obstacles, interference from neighboring radio and electrical systems).

By proper planning of the radio access network, it may be possible to minimize the potential signal loss over the radio links—especially those caused by multipath fading effects. Most WLL systems require LOS radio links to ensure adequate radio range and will deploy one or more base stations (cell sites) to provide adequate radio coverage for their subscribers.

In order to maximize the probability that LOS paths to the base station will be available from all of the subscriber terminal locations to be served by the base station, radio planning needs to be carried out on a cell-by-cell basis. This requires close attention in the selection of base station site and antenna height as well as the antenna height at subscriber terminal locations. For WLL systems there is relatively much greater flexibility in the choice of base station locations compared to the location of radio subscriber outdoor units and antennas.

In principle, it is necessary to choose a base station site that provides good visibility to the desired coverage area of the base station. At the time of base station site selection the exact locations of the remote subscribers is generally unknown. The planner only has information on the broad geographic service area where the subscribers are located and radio coverage is required. It is therefore not possible to carry out a definitive LOS check between the base station and subscriber unitsâ&#x20AC;&#x201D;only a general LOS coverage check is possible.

For most WLL systems, such location planning can be carried out by checking for the following: There are no obvious obstacles between the base station and the intended geographic service area that may contribute to multipath fading.

There are no other base stations in the neighborhood that may lead to mutual interference.

The height of the base station antenna extends over surrounding buildings and terrain variations to avoid shadowing effects.

General terrain surrounding the base station site is sloping down to ensure maximum visibility for the base station. Besides the fact that the base station

needs to be located where it can efficiently and economically provide coverage for the potential subscribers, the exact location of the base station may also be constrained by such factors as site availability, cost of leasing the site, and local regulations.

Similarly, some basic LOS confirmation procedures can be employed during the installation of subscriber terminal equipment. These are especially required for W'LL applications in rural environments to maximize system range and coverage. In the case of short- to medium-length radio links that are less than 10 km. availability of LOS between the remote subscriber location and the serving base station can be confirmed visually (using binoculars where necessary).

In case the visual LOS check indicates the presence of an obstacle, it may be necessary to determine the height of the antenna mast in order to overcome the blockage caused by an obstacle. This may be achieved by sending up a helium balloon at the remote site and observing it visually from the base station antenna location.

Additionally, tests may be performed to check that the Fresnel zone between the remote subscriber location and the base station is clear of obstacles and that the area near the intended location of the antenna is clear of any reflecting surfaces.

If an LOS path is not possible between the remote station antenna and the intended serving base station, it may be necessary to seek an LOS path to an alternative base stationâ&#x20AC;&#x201D;if such an option is available.

Whereas good planning of base station locations and suitable antenna heights and placing will go a long way toward ensuring LOS paths and avoiding multipath fading effects, additional counter measures like adaptive equalization and/or space diversity can also be employed. An adaptive equalizer corrects any amplitude, frequency, or phase distortion introduced on the radio path and attempts to restore the signal to its original balance. The equalization function needs to be implemented at the front end of the receiver before the demodulation stage.

In the case of space diversity, two separate receiving antennas are deployed with a suitable mixing function in the receiver to instantaneously provide the strongest signal. CDMA radio technologies are inherently well-suited to provide this type of space diversity, and this feature of CDMA-based WLL systems is a distinct advantage over systems based on alternate radio technologies. In fact, existence of multipath fading may be an advantage in CDMA systems in that CDMA systems are able to select the best incoming signal. [4] 9.1 The responsible factors for path loss: WLL system systems typically need to be designed such that a LOS is more frequently achieved. This is because the frequency used for WLL do not diffract well around obstacles. Hence any obstacle tends to more or less block the signal. On the other hand some reflections also take place during the propagation of the signal. Figures 9.1 and 9.2 are examples of diffracted and reflected paths. [1]

9.1.1 Diffraction: Diffraction is a phenomenon caused by the fact that each point of waveform generates a new wave. As the wave grazes the top of a building, wavelets are emitted in all directions, including away from LOS path.

The further away from the LOS path the weaker is the signal. The extent of diffraction depends on two parameters: the diffraction angle and the frequency of the carrier wave.

The greater the angle the lower the received signal, and the higher the frequency the lower the received signal.

The variation of the strength of a received signal depends on the parameter V. The following figure shows the variation of the loss of a received signal with the parameter V and V is given by: v=h

Where,

d1

2( d1 + d 2 ) λ.d1 .d 2

………….. (1) [1]

= Distance between the transmitter and the obstacle

d 2 = Distance between the obstacle and the receiver.

h = Height of the obstacle.

Figure 9.1 Example of reflected and diffracted path.

Figure 9.2 Variation of diffraction loss with parameter V 9.2 Relation between the height of the obstacle and the path loss due to diffraction: We assume that: d1

= Distance between the transmitter and the obstacle=500meter

d 2 = Distance between the obstacle and the receiver=500meter

h = Height of the obstacle (varies between 0 to 20 meter) f= frequency=3 GHz From the above figure we see that the parameter v increases linearly with the increment of the height of the obstacle. Again from the graph 9.2 we see that the loss is increasing in an approximately exponential pattern with respect to V. So, relating graph 9.3 with 9.2 we can conclude that the path loss of signal

will increase in an approximately exponential pattern with the increment of the height of the obstacle.

9.3 Relation between the frequency and the path loss due to diffraction: The following graph shows a relation between frequency o the signal and path loss which is approximately liner. Relating this graph with the graph with graph 9.2 we can say that path loss increases approximately in an exponential form with

the

increment

of

requency.

Variation of Parameter V with the height of the obstacle 60

50

V

40

30

20

10

0

0

2

4

6 8 10 12 14 Height of the obstacle "h" in meter

16

18

20

Figure 9.3 Variation of parameter V with change of height of the obstacle

Variation of V with respect to Frequency 700 650 600 550

V

500 450 400 350 300 250 200

0

1000

2000

3000 4000 Frequency in Mhz

5000

6000

Figure 9.4 Variation of parameter V with change of frequency.

9.1.2 Reflection: Reflection is caused when a wave strikes an object and is reflected back from it The key difficulty for WLL with its directional antennas at its subscribers end , is finding the reflections. It is possible to draw an ellipsoids from the transmitter to the receiver .Each of these ellipsoids is known as Fresnel zone. If there is any obstacle in the first half of the first Fresnel Zone, there is a significant reduction in the signal strength.

The radius of the first first Fresnel Zone is given by: R=

λ.d 1 .d 2 d1 + d 2

………….. (2)

Chapter 10 Technical Approach: Capacity measure and comparison

10. Overview In this paper capacity of cellular and WLL are compared by using multipleaxing technique TDMA and CDMA. The formula have derived below and written in MATLAB to get the graphs which has been shown later this section.

By observing the system capacity of WLL, considering a path loss of 20 dB. Similarly, cellular system capacity has been observed considering a path loss of 40 dB/dec. The following observations are for measuring capacity of two multiplexing techniques, namely Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA).

In the first case, the number of users per cell per bandwidth varies with the increase in bandwidth in a TDMA system, which has shown in figure 10.9 later this section. Equation (21) was written in MATLAB to get this observation. The change of the number of users per cell per bandwidth with the change in SIR was observed in the second case for a WLL CDMA system. The CDMA capacity equation (24) was written in MATLAB to get this observation.

By comparing the capacity of WLL system with that of cellular system athematically in equation (10.a) and (10.b) where it is apparent that the capacity of a WLL system is twice the capacity of a cellular system The general key drivers of deploying a WLL system are capacity, coverage, and quality. However, the three key drivers are related among themselves. With an allocated spectral band as a given, then: capacity â&#x2C6;&#x17E; (quality)-1 coverage â&#x2C6;&#x17E;(quality)-1 and they are all function of C/I.(Carrier to interference ratio) 10.1 Measuring spectrum efficiency

To evaluate the spectrum efficiency in frequency reuse systems such as cellular, personal communication services (PCS), and WLL, we may use the radio capacity formula to express radio capacity m as follows: m=

M K

here

(1) M = total number of channels in 1 MHz K = frequency reuse factor.

K can be expressed as follows: K =

1D   3 R 

here

2

(2)

D = distance between two co-channel cells, R= radius of the cell.

In frequency-division multiple access (FDMA) or time division multiple access (TDMA), the value of M is given, but the value of K is a function of C/I. In code division multiple access systems (CDMA), D always equals 2R; thus, K is a constant, K = 1.33 from Eq (2), but M is a function of C/I. Both K in FDMA (or TDMA) and M in CDMA expressed as a function of C/I [15].

• The Three Key Drivers Related to C/I The three key drivers (capacity, coverage, and quality) can be expressed as a function of C/I. C ↑=quality is improving I C ↓= capacity or coverage is increasing I

Quality, Q, is proportional to C/I

Q∝

C I

Radio capacity, m, is inversed proportional to C/I −1

m=

M C  ∝  K I 

channels/cell

(3)

Coverage, R, is inversed proportional to C/I −1

C  R ∝ D.  I 

(4)

• Impact of the Required C/I The required C/'l in each system is determined from the accepted voice quality or corresponds to the specific frame error rate. The required C/I of a WLL system under a nonfading fixed-to-fixed condition is always less than the required C/I of a cellular system under a mobile radio multipath fading condition. C  C    <  I  w  I c

(5)

It shows that the WLL system can tolerate more interference than the interference of the cellular system. Therefore, the frequencies reuse distance for WLL is supposedly shorter than for cellular if the propagation path losses for both systems are the same.

10.2 Impact of Propagation Path Loss

• Coverage Increases in WLL — The coverage of a WLL is based on a fixed-tofixed propagation. The path loss of the fixed-to-fixed propagation in a WLL is based on 20 dB/decade. However, the path loss of mobile radio propagation (fixed-to-mobile) is based on 40 dB/decade, which shows high excessive loss. Therefore, the same wireless communication system can cover more area for WLL services than for mobile radio services.

• Capacity Decreases in WLL — (if FDMA or TDMA is used). Based on the path loss of 20 dB/decade for WLL and 40 dB/decade for cellular, the formula of C/I and K of both systems can be obtained as follows:

Figure 10.1 Six interferers (co-channel cells) model for frequency reuse (wireless systems).

• WLL Systems Under a condition of six interferers 2

Rw−12 C  =   −2  I  1 6.Dw1

D    R  w1 = 6

(6)

Substituting Eq.( 6) into Eq. (2) yields

2

K w1

1D C  =   = 2  3  R w1  I w1

(7)

Under a condition of one interferer (figure above) Rw−22  D  2 C   = −2 =    I  w2 Dw2  R  w2

and

(8)

2

1D 1 C  K w2 =   =   3  R w2 3  I w2

(9)

• Cellular Systems Always under a condition of six interferers (Fig above), 4

R −4 C    = c −4  I c 6.Dc

D    R c = 6

(10)

Substituting Eq. 9 into Eq. 2 yields 2

1D Kc =   = 3  R c

2 C    3  I c

(11)

Figure 10.2 Single interference from co-channel sites in WLL using directional antennas at both ends.

• Capacity Comparison The ratio of Kw/Kc can be used to compare the capacity of two systems; if Kw <1 Kc

(12)

The capacity of WLL is greater than that of cellular. The ratio of K w/Kc under a condition of six interferers is

K w1 Kc

=

2

C  2   I  w1

C     I w = 2.45 2C  C      3  I c  I c

(13)

The ratio of Kw2/Kc under two different conditions, one interferer for WLL and six interferers for cellular, is

1  Kw =  1 Kc 6  Rc

K w1

  

(14)

Equations(13) and (14) can be expressed with the aid of a variable a

C  C    = a   I c  I w

(15)

where a is always greater than that shown in Eq. 5. Assume that the required (C/I)w of WLL is 6 dB or more; then several observations can be stated as follows: •

The region in which WLL capacity is greater than cellular capacity is below the line of K w1 Kc

•

1

A WLL system under the condition of six interferers cannot a have a capacity greater than that of cellular.

•

The WLL system under the condition of one interferer can most likely have a capacity greater than that of cellular.

•

When the value becomes greater, the ratio of Kw/Kc increases.

10.3 Advantage of implementation Due to the nature of the wireless communication medium and of WLL systems, there are advantages in implementing a WLL system: •

The coverage of WLL is larger due to its low propagation path loss (i.e.20 dB/dec).

•

The capacity of WLL can be larger than the capacity of cellular if the number of interferers in WLL can be reduced by multibeam directional antennas.

•

Interference decrease: In a WLL, the frequency reuse distance can be further reduced because the WLL fixed-to-fixed link uses directional antennas on both ends so that the interference area becomes "small.

Reducing the frequency reuse distance more means that the capacity is further increased.

•

In a WLL, no handoffs occur because it is a fixed-to-fixed link. Furthermore, the air link from each building to the cell site can customarily be installed to reduce the interference. This link remains unchanged after installation, and the design of a WLL system is much simpler.

We may compare the capacity of cellular system (TDMA or FDMA) with that of WLL system (CDMA) based on propagation path loss Eb /Io 10.4 Capacity is independent if CDMA is used

• WLL system For 20 dB/decade less, the total number of traffic channel is

Mw

   1 1 =  +1 3  C    I     w  

(16)

• Cellular system For 40 dB/decade less, the total number of traffic channel is

   1 1 Mc =  +1 3  C       I c 

(17)

• Capacity Comparison Comparing equation. (16) with (17), the two formulas, one based on a path loss of 20 dB/decade, one on a path loss of 40dB/dec, are identical. It shows that in CDMA systems different path losses do not affect the radio capacity formula. Since (C/I)c>(C/I)w is always true in Eq. (5), then

Mw>Mc

(18)

Therefore, equation (17) is always true in CDMA systems.

• WLL System Let us make some assumptions In WLL Channel bandwidth,

Bc = 1.25 MHz

R = 9.6 kbps K = Bc/R = 130 Based on the path loss of 20 dB/decade of WLL, we have the equation for capacity of WLL system is  k  k  M w = 1.875 = 1 = 1.875 + 1 2   Eb / I o 

(19)

We can achieve a graph by putting the above equation in MATLAB which is like following-

Figure 10.3 Coding for WLL capacity with 20 dB/dec path loss The above coding gave the following graph3

Mw Capacity for WLL system with allowable path loss

10

2

10

1

10

0

10

0

2

4

6 8 10 12 14 ZdB path loss for WLL system

16

18

20

Figure 10.4 WLL capacity decrease with a path loss of 20 dB/dec Comment: It is seen from the graph that the capacity decreases sharply with the path loss and it also been observe that beyond 20 dB path loss, there is no more capacity decrease in WLL , which indicates that the overall system

capacity of a WLL is more than the cellular system. The graph below shows the Mw Capacity for WLL system with allowable path loss

capacity of a WLL system with a path loss beyond 20 dB/ decade.

3

10

2

10

1

10

0

10

0

5

10

15 20 25 30 35 ZdB path loss for WLL system

40

45

50

Figure 10.5 WLL capacity decrease beyond 20 dB/dec Based on the path loss of 40 dB/decade for cellular, we have the equation for capacity of cellular system is  k  d  M c = 1.875 = 1 = 1.875 + 1 z   Eb / I o 

(20)

We can achieve a graph by putting the above equation in MATLAB which is like following-

Figure 10.6 Coding for cellular capacity based on 40 dB/dec path loss

The above coding gave the following graph -

Mc

capacity for celluler system with allowable path loss

3

10

2

10

1

10

0

10

0

5

10 XdB

15 20 25 30 Path loss in celluler system

35

40

Figure 10.7 Cellular capacity decrease with a path loss of 40 dB/dec Comment: It is seen from the graph that the capacity decrease exponentially with the path loss, the system capacity decrease till the path loss raises to 40 dB/decade.

Comparing the graph we can conclude that the system capacity of WLL is more than that of cellular system.

We further make a reasonable assumption that Eb/Io= 5 dB for WLL and Eb/Io= 8dB for Cellular Then  k  M w =1.875  E / I +1   b o 

 130  =1.875 +1  3.16 

=79.01 and  k  M c =1.875  E / I +1   b o 

130  =1.875 +1  6.3 

=40.56 Comparing Mw and Mc, the capacity of WLL is double the capacity of cellular.

• Capacity comparison of different multiple access techniques TDMA capacity The capacity of TDMA system is relatively simple to calculate. The number of channels per megahertz is given by N=

1/ B K

(21)

here B = bandwidth per channel in megahertz and K = cluster size.

For the purpose of an example, assume that the bandwidth is 8 Kbps and a cluster size of 3 is achieved through the use of directional antennas. The number of user, who can be accommodated, then, is around 40 user/cell/MHz. TDMA (cellular) or FDMA system is bandwidth limited (B). So if we vary the bandwidth per cluster par channel the performance can be calculated.

The above equation can be placed in MATLAB which is like the following-

Figure 10.8 Coding for measuring performance of cellular (TDMA) system The output of the coding of fig 10.8 given the graph bellow6

N number of users per cell per bandwidth

10

5

10

4

10

3

10

0

0.5

1 1.5 2 2.5 B increasing bandwidth for channels in MHz

3

3.5 -4

x 10

Figure 10.9 Performance of a TDMA Cellular system Comment: From the graph it is understood that the number of user decreases gradually with the increase in bandwidth.

WLL Capacity Consider N mobiles communication with a base station. The signal from a particular mobile arrives with a power S. The other mobiles have perfect

power control, so their total signal strength is (N-1) S. The SIR experienced by the first mobile is SIR=

1 N −1

(22)

The SNR is given by the processing gain, G, multiplied by the SIR. Substituting for the SNR and rearranging to determine the number of users, it can be seen that N=

G +1 SNR

(23)

Which is the basic WLL capacity equation. It is enhance as follows, approximating by removing the factor of 1: N=

G 1 f .h. p SNR a

(24)

Where a is the voice activity factor, f is the intercell interference, h is the handover loss, and p is a factor relating to power-control inefficiency. Cellular WLL has a capacity of approximately 30% more than WLL. In such a system, G is 125 and SIR around 5. Thus equation (23) predicts 25 users/cell/WLL before the other factors, a, f ,h and p are taken into account. In practice, the number of subscribers is around 30% more than WLL. After the factors a,f,h and p are taken into account, a capacity of 52 would be achieved; hence, the remaining factors total around 2. Mostly, that gain is achieved through the fact that when a user is speaking there is activity for only 40% of the time.

As far as WLL is concerned, however, the issue is much simpler. The intercell interference is minimal, because directional antennas are use pointing back at the base. There is no handover loss because there is no handover, and the power control is nearly precise. In a cellular system f=0.66, h=0.85, and p=0.5; hence f×h×p= 0.28. In a WLL system, f might rise to, say, 0.8, h=1, and p might rise to 0.7; thus f×h×p=0.56, approximately doubling the capacity of the

cellular system. Overall, it might be expected that be expected that a WLL system would have around twice or even higher the capacity of an equivalent WLL system when deployed in a WLL configuration.

Equation (23) is written in MATLAB which is shown below â&#x20AC;&#x201C;

Figure 10.10 Coding for measuring performance of WLL (CDMA) system The following graph is a output of the coding of figure 10.8 â&#x20AC;&#x201C; 3

N number of users per cell per bandwidth

10

2

10

1

10

0

2

4

6

8 SIR IN dB

10

12

Figure 10.11: Performance of WLL (CDMA) system

14

16

Comment: From the graph it is understood that the number of user decreases sharply with the SIR value up to 6 dB, the number of users decreases gradually with in SIR. Chapter 11 Conclusion 11.1 Conclusions Wireless Technology is now recognized as an important option for delivering mobile, fixed and broadband services. Wireless Local Loop (WLL) is an emerging technology that allows rapid connection to the wired network for remote locations. Major differences between the WLL and the mobile cellular environment include a usually strong line-of-sight (LOS) component and a stationary subscriber unit with a directional antenna. WLL can shorten the time to deploy a communication infrastructure, it can reduce the cost of the communication system and can adapt to the changes of needs and environment In a WLL system no handoffs occur because it is a fixed-to-fixed link. WLL systems do not need to offer mobile services basically, even if some systems provide limited mobile services. Thus, for example, there is no home and visitor location register (HLR/VLR) in a WLL system and its overall architecture may be simpler than that of the mobile systems.

Using radio rather than copper cable has a number of advantages. It is less expensive to install a radio than to dig up the road, it takes less time, and radio units are installed only when the subscribers want the service, unlike copper, which is installed when the houses are built.

At least four to six hours availability of AC power is prime requirement for satisfactory working of FWT at customer premises which is causing slow deployment in rural areas.

The coverage of a WLL is based on a fixed-to-fixed propagation. The path loss of the fixed-to-fixed propagation in a WLL is based on 20 dB/decade. However, the path loss of mobile radio propagation (fixed-to-mobile) is based on 40 dB/decade, which shows high excessive loss. Therefore, the same wireless communication system can cover more area for WLL services than for mobile radio services.

It has been observed from the Thesis work that during the transmitting and receiving of signals we face losses due to diffraction. The diffraction mainly depends on the height of the obstcle, distance between the receiver and transmitter.

11.2 Future Works WLL is now widely recognized as an economically viable technology for provision of telecommunication services to subscribers in sparsely populated as well as highly congested areas because of its many advantages over its wired counterpart. However, the preparation of the business case, choice of a suitable technology, deployment planning, and radio and network system design for a WLL system depend on a range of technical and strategic planning variables. In order to successfully manage a WLL system from initial concept to final implementation and operation, the engineers and technical manages need to have and appreciation of the range of technical and planning issues associated with WLL systems. The engineers are also trying to reduce the diffraction so that to improve the standard.