Cellurar Radio Systems

47 Cellular radio systems

PART 1 (47.1 – 47.3.5) 47.1 Introduction Cellular radio systems are by far the most common of all public mobile telephone networks, the earlier (pre-cellular) networks now all being in decline. The basic principles of cellular systems were established by Bell Laboratories in 1949, but it was not until the early 1980s that technology allowed real commercial networks to be built and service offered to the public. Systems were developed at different times in different countries and subject to a variety of different constraints such as frequency band, channel spacing etc. As a result, a number of different and incompatible cellular standards are in use throughout the world, and the more important standards are summarised later in this chapter. Work is already well in hand to specify and develop second generation cellular systems, for which the opportunity is being taken to develop common standards and systems across several countries. One such notable system, GSM, has been developed in Europe, and is described in some detail later in this chapter. 47.2 Principles of operation 47.2.1 Network configuration In a cellular radio system, the area to be covered is divided up into a number of small areas called cells, with one radio base station (BS) positioned to give radio coverage of each cell. Each base station is connected by a fixed link to a mobile services switching centre (MSC), which is generally a digital telephone exchange with special software to handle the mobility aspects of its users. Most cellular networks consist of a number of MSCs each with their own BSs, and interconnected by means of fixed links. The MSCs interconnect to the public switched telephone network (PSTN) for both outgoing calls to, and incoming calls from fixed telephones. Figure 47.1 shows a typical network arrangement. [pic] Figure 47.1 Cellular network configuration A cellular network will be allocated a number of radio frequencies, or channels, for use across its coverage area, this number being dependent upon the amount of spectrum made available by the licensing authority and the channel spacing of the technical standard used by the network. The radio channels are grouped together into a number of channel sets, and these sets are then allocated to the cells, one set per cell, on a regular basis across the whole coverage area. Each channel will therefore be re-used many times by the network. The method of radio planning and allocation of channels to the cells is described later in this chapter. 47.2.2 Signalling Generally, one radio channel is set aside in each cell to carry signalling information between the network and mobile stations. In the land to mobile (L-M) direction, overhead information about the operating parameters of the network, including an area identifier code, is broadcast to all mobiles located in the cell's coverage area. In addition specific commands are transmitted to individual mobiles in order to control call setup and mobiles' location updating. In the mobile to land (M-L) direction, the signalling channel is used by the mobiles to carry location updating information, mobile originated call setup requests, and responses to land originated call setup requests. 47.2.3 Location registration When a mobile is not engaged in a call, it tunes to the signalling channel of the cell in which it is located and monitors the L-M signalling information. As the mobile moves around the network, from time to time it will need to retune to the signalling channel of another cell when the signal from the current cell falls below an acceptable threshold. When the mobile retunes in this way, it reads the overhead information broadcast by the new cell and updates the operating parameters as necessary. It also checks the location information being broadcast by the new cell and, if this differs from the previous cell, the mobile automatically informs the network of its new location by means of an interchange on the signalling channel (Figure 47.2). By means of this location registration procedure, the network is able to keep updated a database of the location area of all mobiles. This information is used in the call setup procedure for land to mobile calls.

[pic]
Figure 47.2 Mobile location registration 47.2.4 Call set up The signalling procedures for mobile to land (M-L) and land to mobile (L-M) call set up depend upon the technical standard of the particular network. However the general procedure described below holds true for many networks. When the user wishes to make a call, the telephone number to be called is entered followed by a 'call initiation' key (eg pressing the SEND button). The mobile will transmit an access request to the network on the M-L signalling channel; this may be preceded by the mobile rescanning to ensure it is operating on the signalling channel of the nearest base station. If the network can process the call the base station will send a voice channel allocation message which commands the mobile to switch to a designated voice channel, namely one of the channels allocated to that cell. The mobile retunes to the channel indicated and the network proceeds to set up the call to the desired number. As part of the call set up procedure, the network will validate the mobile requesting the call to ensure that it is a legitimate customer. Many networks incorporate specific security features to carry out this validation. When the network receives a call for a mobile (eg from the PSTN) it will first check the location database to determine in which location area the mobile last registered. Paging calls to the mobile are transmitted on the L-M signalling channels of all the base stations in the identified location area and a response from the mobile awaited. If the mobile is turned on and receives the paging call it will acknowledge to its nearest base station on the M-L signalling channel. The base station receiving the acknowledgement sends a voice channel allocation message to the mobile and informs the network so that the two halves of the call can be connected.

[pic]

Figure 47.3 In-call handover
47.2.5 In-call handover At all times during a call (whether L-M or M-L) the base station currently serving the mobile monitors the signal (strength and/or quality) from the mobile. If the signal falls below a predesignated threshold, the network will command neighbouring base stations to measure the signal from the mobile (Figure 47.3(a)). If another base station is receiving the mobile with a stronger signal than the current base station, a signalling message is sent to the mobile on the voice channel from the current base station commanding the mobile to a new voice channel, namely a free voice channel from those allocated to the neighbouring cell. The mobile changes frequency (and thereby the serving base station) and simultaneously the network connects the call to the new base station (Figure 47.3(b)). The measuring process and new cell selection may take several seconds, but the user will only be aware of a brief break in transmission as the mobile tunes to the new voice channel. 47.2.6 Power control Since the size of a cell may be anything from one kilometre to tens of kilometres across, it is not necessary for a mobile to transmit on full power at all times in order to maintain a satisfactory signal level at the base station's receiver. Most cellular standards therefore incorporate mobile power control, the base station commanding the mobile to transmit at one of a number of power levels. As the mobile moves closer to or further from the base station, further commands are issued to keep the received signal level to prescribed limits. By reducing the average mobile power level, co-channel interference is reduced, improving overall system quality. 47.3 Radio planning As previously described, cellular radio re-uses the same radio channels in different cells and because of this re-use, two mobiles using the same channel in different cells may interfere with each other, a phenomenon known as co-channel interference. The key objective of planning a cellular radio system is to design the cell repeat pattern and frequency allocation in order to maximise the capacity of the system whilst controlling co-channel interference to within acceptable limits.

[pic]

Figure 47.4 Cell repeat patterns: (a) four cell repeat; (b) seven cell repeat; (c) twelve cell repeat

47.3.1 Cell repeat patterns The cell plan has to be chosen such that the number of channel sets (N) fit together in a regular fashion without gaps or overlaps. Only certain values of N achieve this, and typical arrangements of interest to cellular radio are N = 4, 7 and 12 as shown in Figure 47.4. The value of N has a major effect on the capacity of the cellular system. As the number of channels sets is decreased, the number of channels per cell increases, hence the system capacity increases. For example, if there are a total of 140 channels available, a 4 cell repeat pattern would provide 35 channels per cell, whilst a 7 cell would provide 20 channels per cell. On this basis, the smallest possible value of N seems desirable. However, as N decreases, so the distance between cells using the same channels reduces, which in turn increases the level of co-channel interference. The repeat distance D and the cell radius R are both related by the geometry of the cell pattern. These are shown in Figure 47.5 and Equation 47.1.
[pic]
In practice, in a real network, it is not possible to achieve a regular cell pattern. This is because radio propagation at the frequencies used by cellular radio systems is affected by the terrain and by buildings, trees and other features of the landscape.
[pic]
Figure 47.5 Frequency re-use - D/R ratio

47.3.2 Co-channel interference Generally, a mobile will receive a wanted carrier signal (C) from the base station serving the cell in which it is located, and in addition, interfering signals (1) from other cells. The carrier to interference ratio C/I is related to the re-use ratio D/R. Cellular radio systems are designed to tolerate a certain amount of interference, but beyond this, speech quality will be severely degraded. The TACS cellular system, for example, will work with a C/I down to around 17dB. This lower limit on C/I effectively sets the minimum D/R ratio that can be used. The two key factors in ensuring that good quality transmission can occur between a mobile and base station are that the wanted signal strength is sufficiently large, that is, above the receiver threshold sensitivity, and that the interference level is low enough to give an adequate C/I ratio. Both of these factors depend on the radio propagation between the mobile and base stations. 47.3.3 Radio propagation There are a number of elements which contribute to the received signal strength at a mobile. Firstly, for a line of sight path, there is a free space path loss which is related to the radial distance between base station and mobile. In addition to this loss, where there is no direct line of sight path, there will be a diffraction loss resulting from obstructions in the path. In general there will also be an effect due to multiple signals arriving at the mobile due to reflections from buildings and other terrain features. This multi-path effect will result in signals either adding constructively or destructively. As a mobile moves around within a cell it will experience varying signal, as shown in Figure 47.6, due to these factors. Fast fading is caused by the multipath effect, and occurs with only a small movement of the mobile. This is also known as Rayleigh fading. Slow fading is mainly caused by terrain features and occurs over large distances of hundreds of metres. In addition, the path loss is also dependent upon the type of terrain, for example, urban with dense buildings, or rural with trees, or even over water. The height of the mobile and base station above ground level also affects the propagation, although the mobile height is not usually a variable. Predicting path loss is an essential part of radio planning, and because of the large number of contributing factors, empirically based formulae are used. The most widely used formula is the Hata model (Hata, 1980) which is based on the propagation measurement results of Okumura et al. (1968). Hata's basic formula for the total path loss, Lp, is given by Equation 47.2 where f is the carrier frequency in MHz, k is the base station antenna height, h is the mobile antenna height, R is the radial distance in kilometres, and a(h ) is the mobile antenna height correction factor.
Lp (dB) = 69.55 + 26.16log(fc) – 13.82log(hb) – a(hm) + (44.9 – 6.55log(hb))logR (47.2)
Correction factors can be used to take into account the type of terrain [pic]

Figure 47.6 Fading effects

47.3.4 Practical radio planning Armed with a propagation model it is possible to calculate both the wanted signal strength and the interference level for all locations in a cell. Generally this is done using a computer based tool which can draw upon a database of cell site information and terrain data. Some advanced tools can also take account of diffraction losses. For practical purposes a planner will aim to achieve the required signal strength and C/I ratio over 90% of the cell coverage area, by varying antenna heights, transmitter powers, frequency allocations and other factors as appropriate. To simplify calculations, an allowance for Rayleigh fading and shadow fading is usually made within the system power budget. A typical power budget is shown in Table 47.1. 47.3.5 Adding capacity Once a cellular network has been planned to provide overall coverage, there are a' number of ways of adding additional capacity. A simple and cost effective option is to allocate further radio channels to existing cells. However, this can only be done by an extension band, for example the ETACS allocation in the UK. Other alternatives involve rearranging the cellular plan, either by cell splitting or by sectorisation. Cell splitting is achieved by dividing an existing cell up into a number of smaller cells, by adding additional base stations as shown in Figure 47.7; it is then necessary to reallocate the radio channels. By repeatedly splitting cells; the cell size, and hence the system capacity, can be tailored to meet the traffic capacity requirements demanded by customer behaviour in all areas.
Table 47.1 Typical power budget (TACS) (1 = Key planning parameters)
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Figure 47.7 Cell splitting

In rural areas, cells may be 20km to 30km in radius. In practice, as cell sizes decrease, propagation effects, particularly in city areas, cause an increase in co-channel interference, even if the repeat pattern is maintained. Also, as cell sizes decrease, it becomes increasingly difficult to find suitable base station sites, which need to be accurately positioned in order to keep to a regular pattern. The cost of providing and maintaining a large number of individual base stations is also a factor, such that in addition to cell splitting, sectorisation of cells is commonly used in urban areas. In a regular cellular layout, co-channel interference will be received from six surrounding cells which all use the same channel set. One way of cutting significantly the level of interference is to use several directional antennas at the base stations, with each antenna illuminating a sector of the cell, and with a separate channel set allocated to each sector. There are two commonly used methods of sectorisation, using three 120 degree sectors or six 60 degree sectors as shown in Figure 47.8, both of which reduce the number of prime interference sources to one. This is because, of the six surrounding co-channel cells, only one will be directed at the wanted cell. A disadvantage of sectorisation is that the channel sets are divided between the sectors such that there are fewer channels per sector, and thus a reduction in trunking efficiency. This means that the total traffic which can be carried for a given level of blocking is reduced. However, this effect is offset by the ability to use smaller cells, such that the end result is a significant increase in total capacity. [pic]

Figure 47.8 Sectorisation

Exercise 1 Learn the words and word combinations
|a cellular network |сеть радиосвязи с сотовой структурой |
|paging call |поисковой вызов, передача сигналов поискового вызова |
|predesignated threshold |предварительный обозначенный порог |
|frequency allocation |распределение частот (между службами) |
|ETACS (Extended Total Access Communication System) |расширенная система связи с полным доступом |
|overhead information |служебная информация |
|Cellular radio systems |сотовые системы связи |
|to be in decline |быть в состоянии упадка, идти на убыль |
|frequency band |диапазон частот; полоса частот |
|assigned (-frequency) band |полоса частот, выделенная для радиостанций |
|attenuation band |полоса ослабления, полоса затухания |
|broad band |широкий диапазон частот |
|broadcast band |радиовещательный диапазон частот (535 Гц – 160 кГц) |
|citizen band |диапазон частот, выделенный для частной и служебной связи (26, 965-27, 405МГц; |
| |460-4709-МГц) |
|communication band |диапазон (полоса) частот радиосвязи |
|exclusive band |диапазон частот, запрещенный для использования |
|channel spacing |разнесение каналов; канальный интервал |
|GSM (Global System for Mobile Communications) |глобальные системы мобильной связи |
|overlapping channel spacing |расстановка каналов с перекрытием по спектру |
|incompatible |несовместимый, невзаимозаменяемый |
|cell |сота, элемент, ячейка |
|radio transparent coverage |радиопрозрачное покрытие |
|frequency coverage |перекрываемый диапазон частот, перекрытие по частоте |
|a mobile services switching centre (MSC) |мобильный центр коммутации |
|a digital telephone exchange |цифровая телефонная станция |
|software |программное обеспечение |
|link |связь, соединение; линия связи |
|backbone link |магистральная линия связи; магистральный канал связи |
|bandwidth-limited link |линия связи с ограниченной полосой пропускания |
|bidirectional link |двусторонняя линия связи |
|communication(s) link |линия связи |
|data link |линия (передачи) данных; канал (передачи) данных |
|dedicated link |закрепленный (выделенный) канал связи |
|the public switched telephone network (PSTN) |телефонная сеть общего пользования |
|outgoing calls |исходящие звонки |
|incoming calls |входящие звонки |
|fixed telephones |стационарные телефоны |
|to be dependent upon |зависеть от |
|channel sets |группа (набор) каналов |
|signalling channel |канал сигнализации (тональной), |
| |канал передачи служебных сигналов |
|signalling |передача сигналов; телеграфирование, вызов (в телефонии) |
|an area identifier |определитель; устройство распознавания; устройство опознавания |
|an area code |код зоны |
|coverage area |зона обслуживания |
|self-checking code |код с самопроверкой |
|single error-correcting code |код исправления одиночных ошибок |
|standard code |правила эксплуатации |
|specific code |абсолютный код |
|termination code |код завершения |
|in the land to mobile direction |в направлении от станции к телефону |
|in the mobile to land direction |в направлении от телефона к станции |
|location updating |обновление (изменение), определение место нахождения |
|to re-use |использовать многократно |
|terrain diffraction |дифракция на рельефе местности |

Exercise 2 Read the text

Exercise 3 Find the Russian equivalents for the following English words and word combinations

|an acceptable threshold |дифракционное затухание (ослабление) |
|to validate |внутриканальная помеха; помеха совмещенного канала |
|co-channel interference |допустимый порог |
|within acceptable limits |медленное затухание |
|cell splitting |быстрое затухание |
|reallocation |вызов, запрос; разговор (по радио, телефону) |
|diffraction loss |проверять правильность |
|call |разделение ячейки |
|long-term fading |в пределах допустимых норм |
|short-term fading |повторное распределение, повторная установка в заданную позицию |
|disadvantage |недостаток |

Exercise 4 Answer the following questions:
|1 |When and where were the basic principles of cellular systems established? |
|2 |Why are different and incompatible cellular standards in use throughout the world? |
|3 |What are the main principles of network configuration, signalling and location registration? |
|4 |Is it difficult for you to describe in English the signalling procedures for mobile to land and land to mobile call set up? |
|5 |Why do most cellular standards incorporate mobile power control? |
|6 |What is the key objective of planning a cellular radio system? |
|7 |Is it possible to achieve a regular cell pattern in practice (in a real network)? |
|8 |What are the two key factors in ensuring that good quality transmission can occur between a mobile and base station? |
|9 |What affects the propagation? |
|10 |What are the main ways of adding additional capacity to a cellular network? |
|11 |Do cell splitting and sectorisation have any advantages and disadvantages? What are they? |

Part II (47.4 – 47.4.5)
47.4 Overview of systems Cellular networks have been developed and deployed at various times and places in many countries across the world. In several cases, the local telecommunications authority or company was central to the specification and development of the standard with which the network complied. Since frequency allocations and other basic parameters (such as channel spacing) have often been set at national level and not coordinated between countries, these local factors resulted in different standards being adopted by different countries. In addition, in cases where the development of a standard has started later, the opportunity has been taken to introduce new features made possible by advances in technology, further increasing the diversity between standards. From this wide range of differing system standards, four have become largely dominant and have been adopted in many countries, albeit still with some variations. These four are AMPS, TACS, NMT (both NMT 450 and NMT 900) and C450, and their basic system parameters are shown in Table 47.2. Each of these is described in outline below, but in addition special mention must be made of the European designed GSM system which is set to become the dominant standard for Europe in the mid to late 1990's.

Table 47.2 Comparison of system parameters
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47.4.1 AMPS AMPS stands for Advanced Mobile Phone System and was developed in the USA primarily by Bell Laboratories as a successor for the heavily congested IMTS (Improved Mobile Telephone System). Being designed for the north American market, AMPS uses the 800MHz band allocated to mobile services in ITU Region 2 (the Americas), with 30kHz channel spacing in common with established PMR practice. AMPS uses analogue FM for speech transmission, but with a wider frequency deviation (12kHz) than is the norm for a 30kHz channelling system. By adopting the wide deviation, the dynamic range of the speech channel is extended and protection against co-channel interference is increased. This, together with the use of speech compression/expansion (companders) yields a high quality voice circuit with the capability to maintain performance in a high capacity (poor interference ratio) configuration. Signalling between mobile and base station is at l0kbit/s, with Manchester encoding applied taking the bit rate to 20kbit/s. The data is modulated onto the radio carrier by direct frequency shift keying (FSK). Error control is achieved by multiple repetition (5 or 11 times) of each signalling word, with majority voting applied at the receiver to correct errors. A BCH block code is also applied to detect any uncorrected errors. Whilst a call is in progress, the base station transmits a low level supervisory audio tone (SAT) in the region of 6kHz. Three different SAT frequencies are used by the network, and are allocated to the base stations so that the nearest co-channel base stations (i.e. those most likely to cause interference) have a different SAT from the wanted base station. The mobile continuously monitors the received SAT and also transponds the signal back to the base station. If the mobile (or the base station) detects a difference between the received SAT and that expected, the audio path is muted to prevent the interfering signal from being overhead. If the condition persists, the call is aborted. AMPS underwent a long development period, and an extended trial (technical and commercial) which not only fixed the system parameters but also contributed to the basic planning rules which hold true for all cellular systems. The system design was comprehensively described in 1979 (Bell, 1979), but it was not until 1983 that operating licences were issued and true commercial exploitation of the system commenced. AMPS is in operation extensively across the north American continent (USA and Canada). Due to the regulatory conditions in force in the USA, deployment has been in the form of a patchwork of largely independent standalone systems, with two competing systems operating in each licence area. Although commercial roaming agreements exist to allow customers of one operating company to obtain service from another when they are in a different part of the country, a seamless nationwide service is, as yet, not available to the customer. AMPS is now also used in a number of a number of central and south American countries, in Australia and some far east countries. World-wide it is the dominant standard in terms of installed customer base. AMPS is being further developed to incorporate digital speech encoding, with TDMA techniques to give three digital voice channels per one radio channel. Digital AMPS (DAMPS) has the same basic architecture and signalling protocol as AMPS and is therefore more evolutionary than revolutionary (as is GSM in Europe). 47.4.2 TACS TACS stands for Total Access Communications System, and was adapted from the AMPS standard by the UK when cellular radio was licensed for operation from 1985. The adaptation was necessary to suit European frequency allocations which were at 900MHz, with 25kHz channel spacing. This meant a reduction in frequency deviation and signalling speed was necessary (BS, 1990). The signalling scheme of AMPS was retained largely unchanged, but some enhancements were introduced, particularly in the procedures for location registration, to make the standard more suitable for deployment in systems offering contiguous nationwide coverage. The opportunity was also taken to introduce extra features, such as signalling of charge rate information (e.g. for payphones). TACS was originally specified to use the full 1(X)O channels (2 x 25MHz) allocated to mobile services in Europe. However in the UK, only 600 channels (2 x 15MHz) were released by the licensing authority, the remainder being reserved for GSM. Subsequently an additional allocation of channels below the existing TACS channels was made, namely the Extended TACS (ETACS) channels, and the standard was modified accordingly. TACS equipment availability and cost have both benefitted from the standard's similarity to AMPS, and TACS systems have been adopted by several European countries (UK, Eire, Spain, Italy, Austria and Malta), in the middle east (Kuwait, UAE and Bahrain) and the far east (Hong Kong, Singapore, Malaysia and China). In Europe, TACS is on an equal footing with NMT in terms of installed customer base. A variant of TACS (called J-TACS) has also been adopted in Japan. 47.4.3 NMT NMT stands for Nordic Mobile Telephone (system), and was developed jointly by the PTTs of Sweden, Norway, Denmark and Finland during the late 1970's/early 1980's. The system was designed to operate in the 450MHz band, and was later adapted to also use the 900MHz band. Although NMT was developed after AMPS, it saw commercial service before it, opening in late 1981. NMT450 uses a channel spacing of 25kHz, speech modulation being analogue FM with a peak frequency deviation of 5kHz, the same as standard PMR practice. NMT900 also uses a frequency deviation of 5kHz, but with a 12.5kHz channel spacing to double the number of available channels, albeit with a degraded adjacent channel rejection performance which must be taken into account during frequency planning. Signalling is at 12(K) bit/s using audio fast frequency shift keying (FFSK). Error protection of the signalling information is by means of a Hagelbarger convolutional forward error correcting code. NMT was designed from the outset to support international roaming and was first implemented with full four nation roaming in the four participating countries (Norway, Sweden, Finland and Denmark). Since then NMT450 has been deployed in many other European countries (Austria, Spain, Netherlands, Belgium, Luxembourg, France, Iceland, Faroe Is., Turkey and Hungary) but due to differences in the frequency allocations in the 450MHz band between countries, not all networks are fully compatible to allow roaming. NMT900 was developed as a necessity as capacity became exhausted on the NMT450 networks, and has been deployed since 1987 as an overlay network in several countries, and in Switzerland as their main network. 47.4.4 C450 C450 (also known as Netz-C) was developed by Siemens during the early 1980's under the direction of the (West) German PTT, Deutsche Bundespost. Commercial service opened in 1985 following a trial period. C450 has a channel spacing of 20kHz, in common with other mobile services in Germany at 450MHz and speech modulation is analogue FM with a frequency deviation of 4.0kHz. Signalling for call control is transmitted at 5.28kbit/s by direct FSK. Error protection of the signalling is by bit interleaving with a BCH block code backed by an acknowledgement protocol. In addition, C450 uses continuous signalling between base station and mobile during a call, achieved by time compressing the speech in bursts of 12.5ms, each burst being compressed into 11.4ms. This process opens up slots of 1.1ms duration every 12.5ms and the signalling data is inserted into these slots and extracted by the receiver which also time expands the speech back to its original form. This continuous signalling serves several purposes:
It allows the base station to send power control and handover messages to the mobile without disturbing the voice channel.
The data is checked for jitter, and thereby the quality of the channel can be determined in order to indicate the need for a handover.
The time delay between a base station transmitting a data burst and receiving the response from the mobile is measured at the base station and used to calculate the distance between them.
This distance is also taken into account in handover determination.
The data is used as a timing reference by the mobile to lock its internal clocks. C450 contains a number of advanced features made possible by the application of current developments in technology. Although speech transmission is analogue, it can be regarded as a hybrid technology system, and several of its characteristics such as time slotted signalling channels and continuous signalling during call have been carried through into the GSM system design. Coming later to the European scene, C450 has chiefly only served the German market, although systems are also operating in Portugal and South Africa.
47.4.5 GSM The GSM standard was developed as a joint initiative by the members of the Conference of European Posts and Telecommunications administrations (CEPT) with the eventual aim of building a unified pan-European network, giving the user a near uniform service throughout all European countries. An added bonus of a common standard should be lower terminal equipment prices through economies of scale. Work on the standard started in 1982, and by 1987 all the basic architectural features were decided. The full Phase 1 specification was completed in 1990, but work continues on further phases incorporating new features and services. In 1987, the majority of operators participating in GSM signed a Memorandum of Understanding (MoU) committing them to make GSM a reality by installing networks and opening commercial service by 1991. Since that time further operators have signed the MoU, bringing the total to date to 25. The GSM technical standard makes full use of currently available levels of technology, incorporating features such as low bit rate speech, convolutional channel coding with bit interleaving and frequency hopping. The standard is intended to endure for many years to come.
Exercise 1 Learn the words and word combinations
|frequency allocation |распределение частоты (между службами) |
|to comply with |подчинятся правилам |
|albeit |хотя |
|frequency deviation |девиация частоты |
|a compander |компандер |
|yield |производить, вырабатывать |
|shift keying |манипуляция |
|frequency (-shift) keying |частотная манипуляция, манипуляция сдвигом частоты |
|a BCH block code |блочный (-блоковый) код БХЧ (код Боуза-Чоудхури-Хок-венгема) |
|(Bose-Chaudhuri-Hocquengem) | |
|to abort |прерывать, прекращать |
|to mute |подавлять |
|path |маршрут (в сети передачи данных) |
|adjacent channel rejection |подавление помех от соседнего канала |
|burst |пакетный сигнал; вспышка, всплеск, выброс) |
|burst of signal |выброс сигнала |
|channel spacing |разнос каналов |
|handover |переключение, переход |
|power control |регулирование мощности |
|bit interleaving |чередование битов (разных сообщений при уплотнении каналов) |
|frequency interleaving |чередование частоты |
|code interleaving |кодовое перемежение |
|a hybrid system |дифсистема |
|terminal equipment |терминальное оборудование; оконечное оборудование |
|frequency hopping |скачкообразная перестройка частоты; перескок частоты |
|to endure |выдерживать испытание временем |
|convolution coding |сверточное кодирование |
|AMPS (advanced mobile phone service) |перспективная служба (радио) телефонной связи с подвижными объектами |
|IMTS (improved mobile telephone system) |усовершенствованная система подвижной телефонной связи |
|ITU (International Telecommunication Union |Международный союз электросвязи |
|error correcting code |код с исправлением ошибок |
|convolution code |сверточный код |
|timing reference |эталон времени |
|PRM (premium (tariff) – additional service when the call is partly | |
|paid by calling party, e.g. 900- services in the USA | |
|voice circuit |тональная цепь, телефонная цепь |
|SAT (supervisory audio tone) |диспетчерский тональный сигнал |
|standard system |автономная система, функционально законченная система |
|TDMA (time- division multiple access |многостанционный доступ с временным разделением каналов |
|contiguous coverage |соприкасающиеся зоны обслуживания |

Exercise 2 Read the text

Exercise 3 Find the Russian equivalents for the following English words and word combinations
|interleaving |выброс сигнала |
|to be fully compatible with |голос; речевой сигнал |
|a trial period |сигнализация по общему каналу |
|a patchwork |развертывание, ввод в действие |
|burst of signal |коммутация с помощью штепсельного соединителя |
|deployment |уплотнение импульсных сигналов; чересстрочная развертка |
|frequency coverage |выдерживать испытание временем |
|customer |абонент |
|voice |передача сигналов с модуляцией на несущей |
|to endure |охват по частотам; частотный диапазон |
|carrier signaling |испытательный срок |
|common channel signalling |быть полностью совместимым с … |
|compelled signalling |принудительная передача сигналов |

Exercise 4 Answer the following questions:
|1 |What are the four dominant cellular standards adopted in many countries? |
|2 |Where was AMPS developed and where is it in operation now? |
|3 |What are the key features of this standard |
|4 |What can you tell about TACS? |
|5 |Was NMT designed to support international roaming? |
|6 |Does NMT operate only in the 450 MHz band? |
|7 |Why was NMT 900 developed? |
|8 |What do you know about speech modulation, channel spacing and a frequency deviation of C 450? |
|9 |What purposes does the continuous signalling of C 450 serve? |
|10 |Is C 450 widely used nowadays? |
|11 |Whom was the GSM standard developed by? |
|12 |When was a Memorandum of Understanding signed? |
|13 |What is the GSM standard intended to endure for many years? |

Part III (47.5.1 – 47.5.7)
47.5 Detailed description of GSM
47.5.1 GSM architecture The basic architecture of GSM is not dissimilar to other cellular radio systems and comprises base transceiver stations (BTS), Base Station Controllers (BSC), Mobile Switching Centres (MSC), a variety of registers and a network management system, as shown in Figure 47.9. The mobile station comprises a mobile equipment and a subscriber identity module (SIM). In addition to these functional entities, GSM also defines several interfaces, the Radio Interface (Um), the interface between the MSC and BSC (A interface) and the signalling interface which allows roaming between networks. This is based on the CCITT No.7 signalling standard and is defined as a Mobile Application Part (MAP). The BTS and BSC together form the Base Station Subsystem (BSS) and carry out all the functions related to the radio channel management. This includes the management of the radio channel configurations, allocating radio channels for speech, data and signalling purposes, and controlling frequency hopping and power control. The BSS also includes, as does the MS, the speech encoding and decoding, and channel coding and decoding. The MSC, VLR and HLR are concerned with mobility management functions. These include authentication and registration of the mobile customer, location updating, and call set up and release. The HLR is the master subscriber database and carries information about individual subscribers numbers, subscription levels, call restriction-s, supplementary services and the current location (or most recent location) of subscribers. The VLR acts as a temporary subscriber database for all subscribers within its coverage area, and contains similar information to that in the HLR. The provision of a VLR means that the MSC does not need to access the HLR for every transaction. The authentication centre (AUC) works closely with the HLR and provides information to authenticate all calls in order to guard against fraud. The equipment identity register (EIR) is used for equipment security and validation of different types of mobile equipment. This information can be used to screen mobile types from accessing the system, for example if a mobile equipment is stolen, not type approved, or has a fault which could disturb the network. Network management is used to monitor and control the major elements of the GSM network. In particular, it monitors and reports faults and performance data. It can also be used to re-configure the network.
[pic]
MS Mobile Station MSC Mobile Switching Centre
BTS Base Transceiver Station
BSC Base Station Controller
VLR Visited Location Register
HLR Home Location Register
EIR Equipment Identity Regsiter
AUC Authentication centre
Figure 47.9 GSM architecture

47.5.2 Air interface The GSM Air Interface (Urn) provides the physical link between the mobile and the network. Some of the key characteristics of the air interface are given in Table 47.3. As already described, GSM is a digital system employing time division multiple access (TDMA) techniques and operating in the 900MHz band. The CEPT have made available two frequency bands to be used throughout Europe by the GSM system, namely;
890MHz to 915MHz for the mobile to base station (uplink)direction.
935MHz to 960MHz for the base station to mobile (downlink)direction. These 25MHz bands are divided into 124 pairs of carriers spaced by 200kHz. In addition, consideration is now being given to specifying additional carriers in a pair of extension bands 872MHz to 888MHz and 917MHz to 933MHz. Each of the carriers is divided up into eight TDMA timeslots of length 0.577ms such that the frame length is 4.615ms. The recurrence of each timeslot makes up one physical channel, such that each carrier can support eight physical channels, in both the uplink and downlink directions. The timeslot allocation in either direction is staggered so that the mobile station does not need to transmit and receive at the same time. Data is transmitted in bursts within the timeslots and a number of different types of burst can be carried as shown in Figure 47.10. The normal burst has a data structure as shown. It consists of 148 bits of which 114 are available for data transmission, 26 are used for a training sequence which allows the receiver to estimate the radio propagation characteristics and to set up a dispersion equaliser, 6 bits as tail bits, and two stealing flags. These physical channels therefore provide a data throughput of 114 bits every 4.615ms or 24.7kbit/s. The bursts modulate one of the RF carriers using Gaussian Minimum Shift Keying (GMSK) modulation with a BT index of 0.3. The allocation of the carrier can be such that frequency hopping is achieved, i.e consecutive bursts of a physical channel will be carried by differing RF carriers. This "hopping" is performed every TDMA frame, or every 4.615ms and provides extra protection against channel fading and co-channel interference. A number of logical channels can be carried by the physical channels described above. These are summarised in Table 47.4. There are two categories of traffic channels; speech, whether full rate using 22.8kbit/s or half rate using 11.4kbit/s, and data, providing a variety of data rates. There are four basic categories of control channels, known as the broadcast control channel (BCCH), the common control channel (CCCH), the standalone dedicated control channel (SDCCH) and the associated control channel (ACCH). These are further divided into channels with specific purposes and for a detailed description of these channels the reader is referred to the GSM Recommendations published by ETS1. Each of these logical channels is mapped onto the physical channels, using the appropriate burst type as shown in Figure 47.10. TDMA frames are built up into 26 or 51 frame multiframes, such that individual timeslots can use either of the multiframe types, and then into superframes and hyperframes as shown in Figure 47.10. The TCH and the associated ACCH uses the 26 frame structure, whilst the BCCH and CCCH use the 51 frame structure. The SDCCH may occupy one physical channel, providing 8 SDCCH, or may share a physical channel with the BCCH/CCCH. Typical arrangements for allocating the 8 physical channels could be:
7 channels TCH and SACCH + 1 channelBCCH/CCCH/SDCCH
6 channels TCH and SACCH + 1 channel BCCH/CCCH + 1channel SDCCH. Each cell must have at least one physical channel assigned to the BCCH/CCCH, where there are 2 or more carriers per cell, the non-BCCH carriers may have all 8 channels allocated to TCH.

Table 47-3 GSM air interface parameters
[pic]

[pic]

Figure 47.10 GSM timeframes, timeslots and bursts (Extract from GSM Recommendation 05.01)

47.5.3 Speech coding and channel coding The speech coder is a regular pulse excited linear predictive coder (RPE-LPC) with long term prediction. This provides a net bit rate of 13kbit/s. It is a block based coder where the input samples are analysed in blocks with a 20ms duration. Work is also being carried out to specify a half rate speech coder which will effectively double the system capacity of GSM. Before being assembled into the timeslots and frames, the digital speech and signalling data is encoded and interleaved. The speech coder output is divided up into three classes of bits and the most sensitive bits are encoded by adding parity check bits followed by a convolutional coder. Signalling data is encoded using a FIRE code. A process of interleaving is then used to spread the data blocks over a number of bursts. For speech, an interleaving degree of 8 is used, i.e the speech block is spread over 8 bursts, whilst an interleaving degree of 4 is used for signalling. This overall process is shown in Figure 47.11, and the combined use of coding and interleaving provides good protection of channel data from the fading, dispersion and interference effects on the radio path. With the addition of frequency hopping and diversity techniques, the GSM air interface is particularly robust. One of the penalties to be paid for this is the overall transmission delay. The speech coder contributes about 25ms and the channel coding and interleaving a further 37ms. The rest of the transmission delay budget allows for analogue to digital conversions, 16kbit/s transmission and switching in various parts of the network. The overall one way transmission delay thus amounts to around 90ms. Such a delay means that echo control is necessary even on short national calls.

Table 47.4 GSM logical channels
[pic]

[pic]

Figure 47.11 GSM channel coding and interleaving

47.5.4 GSM signalling Figure 47.12 shows the overall signalling model. The Air Interface uses LAPDm Layer 2 signalling protocol and this is also used for the A-bis, BTS to BSC interface. The layer 3 protocol consists of three sublayers, dealing with radio resource management (RR), mobility management (MM), and connection management (CM). Radio resource management is concerned with managing the logical channels, including paging, channel assignments, handover, measurement reporting, and other functions. The mobility management layer contains functions necessary to support the mobility of the user which include authentication, location updating, attach and detach of IMSI (International Mobile Subscriber Identity), and registration. The connection management layer is concerned with call control, establishing and clearing circuits, management of supplementary services and the short message service. The BSC to MSC A-interface, and the various MSC to Register interfaces employ CCITT No.7 signalling using the Message Transfer Part (MTP), Signalling Connection Control Part (SCCP), Transaction Capabilities Part (TCAP) and Mobile Application Part (MAP). An example of the signalling messaging for establishing a mobile originated call is shown in Figure 47.13. The key events are:
Request and assignment of a channel, between MS and BSS.
A service request procedure which accesses the VLR.
An authentication and ciphering exchange which validates the mobile user and sets the encryption cipher.
Call set up which includes sending of dialled digits and establishing the connection. Location updating is shown in Figure 47.14 An update request is indicated by the mobile and passed to the VLR in the new location area. The new VLR requests the IMSI from the old VLR and then signals the new location to the HLR. The HLR provides the subscriber data to the new VLR and cancels the subscriber entry in the old VLR. Finally a confirmation message is set back to the mobile. There are, of course, many other signalling exchanges, dealing with mobile terminating calls, supplementary services, and short message service. There is not space in this chapter to deal with the detailed signalling for these cases; the examples above describe the general principle and illustrate the roles of the MS, BSS, MSC, VLR and HLR.
[pic]

Figure 47.12 GSM signalling model

47.5.5 Security features The information on the air interface needs to be protected, to provide user data (including speech) confidentiality and to prevent fraudulent use of subscriber and mobile identities. The basic mechanisms employed are user authentication and user data encryption. Each mobile user is provided with a Subscriber Identity Module (SIM) which contains the IMSI, the individual subscriber authentication key (Ki) and the authentication algorithm (A3). After the mobile user has made an access and service request, the network checks the identity of the user by sending a random number (RAND) to the mobile. The mobile uses the RAND, Ki and A3 algorithm to produce a signed response SRES. This response is compared with a similar response calculated by the network, and access only continues if the two responses match. The SIM also contains a cipher key generating algorithm (A8). The MS uses the RAND and A8 to calculate a ciphering key (Kc) which is used to encrypt and decrypt signalling and user data information. The authentication centre (AUC) is responsible for all security aspects and its function is closely linked with the HLR. The AUC generates the Ki's and associates them with IMSIs, and provides the HLR with sets of RAND, SRES and Kc for each IMSI. The HLR then provides the appropriate VLR with these sets and it is the VLR which carries out the authentication check. Authentication of mobile users can be carried out on call set up, both mobile originated and mobile terminated, on location updating, and on activation of supplementary services. As the authentication sets are used up in the VLR, further sets are requested from the HLR. An additional security feature of GSM is the equipment identity register (E1R). This enables monitoring of the mobile equipment 1MEI (International Mobile Equipment Identity) which is used to validate mobile equipments thus preventing non-approved, faulty or stolen equipment from using the system. This range of security features provide a high degree of protection to the user and the network operator.
[pic]
Figure 47.13 Mobile originating call

47.5.6 GSM services and features In addition to speech, GSM offers a wide range of data bearer services up to 9.6kbit/s suitable for connection to circuit switched or packet switched data networks. GSM also supports Group 111 facsimile as a data service by use of an appropriate converter. A comprehensive range of supplementary services are offered by GSM, including call forwarding, call barring, multi-party service, advice of charge and others. A full description is provided in the GSM Recommendations, and further detail of cellular services is provided later in this chapter. An important feature of GSM is the short message service (SMS). This allows transmission of alphanumeric messages of up to 160 characters to or from a mobile via a service centre. If the message cannot be delivered due to mobile being switched off, or outside of the coverage area, the message is stored at a service centre and re-transmitted when the mobile registers again. Received messages can be displayed on the mobile and stored in the SIM for future reference. A related service is cell broadcast which allows messages of up to 93 characters to be sent to all mobiles within a specific geographical area, for example to deliver traffic or weather reports. 47.5.7 Roaming Naturally, with a pan-European system, roaming of subscribers between networks is specified by GSM. When a mobile first switches on in a foreign PLMN (Public Land Mobile Network), the local MSC/VLR will determine the identity of the home PLMN from the mobile network code which is part of the IMSI. The home HLR will be interrogated to establish whether roaming is permitted and for authentication. The home HLR then passes the subscriber data to the local (foreign) VLR and registers the foreign location of the mobile. Calls to and from the roamed mobile can then take place.

Exercise 1 Learn the words and word combinations
|BTS (base transceiver station) |базовая приемопередающая станция |
|BSC (base station controller) |контроллер базовых станций |
|MSC (mobile switching center) |мобильный центр коммутации |
|SIM (subscriber identity module) |модуль идентификации пользователя |
|MAP (mobile application part) |часть мобильного применения |
|BSS (base station subsystem) |подсистема базовых станций |
|MS (mobile station) |мобильная станция |
|AUC (authentication center) |центр аутентификации, установление подлинности |
|EIR (equipment identity register) |регистр идентификации оборудования |
|VLR (visitor location register) |регистр перемещения |
|HLR (home location register) |регистр состояния |
|CCITT |МККТТ (международный консуль-тативный комитет по телеграфии и телефонии) |
|TCAP (transaction capabilities part) |часть передающих способностей |
|layer 2 signalling protocol |сигнальный протокол 2-ого уровня |
|sublayer |подуровень |
|logical channel |логически канал |
|channel assignments |назначение каналов |
|handover |передача, переключение |
|measurement reporting |сообщение измерения |
|location updating |обновление месторасположения |
|attach and detach |подключение (присоединение) и отключение |
|to establish (to clear) a circuit |смыкать и размыкать цепь |
|establishing and clearing circuits |установление и освобождение цепи |
|supplementary services |дополнительные услуги |
|short message service |служба коротких сообщений |
|request and assignment of a channel |запрос и назначение канала |
|service request procedure |процедура запроса услуги |
|to validate the mobile user |подтверждать мобильного пользователя |
|ciphering exchange |шифрующая станция |
|encrypting cipher |шифрующий символ (цифра) |
|an update request |обновленный запрос |
|a confirmation message |сообщение окончательного подтверждения |
|mobile terminating calls |мобильные завершающие звонки |
|general principle |основной принцип |
|frequency control |регулировка частоты; подстройка частоты |
|to report faults |сообщать об ошибках |
|to guard against fraud |обеспечивать защиту от обмана |
|to re-configure the network |перегруппировать, перераспределить сеть |
|to have a fault |иметь дефект |
|coverage area |охват; зона покрытия, зона обслуживания |
|transaction |входное сообщение; транзакция |
|validation |проверка правильности; подтверждение |
|on call set up |во время установления звонка |
|authenticate |установить подлинность, опознать |
|performance data |эксплуатационные данные, технические данные, характеристика работы |
|recurrence |1. повторение |
| |2. рекуррентное соотношение |
|data throughput |пропускная способность (канала передачи данных) |
|GMSK (Gaussian minimum-shift keying) |Гауссова манипуляция с минимальным сдвигом |
|channel fading |замирание канала |
|multiframe |сверхцикл |
|parity check bit |разряд контроля четности |
|robust |устойчивый, выносливый |
|fraudulent |обманный, мошеннический |
|IMSI (International Mobile Subscriber Identity |международная идентификация абонента мобильной связи |
|a cipher key |ключ к шифру |
|packet-switched network |сеть пакетной коммутации |
|circuit switching network |сеть коммутации каналов, сеть прямых соединений |
|call forwarding |передача вызова |
|call barring |преграда вызова |
|cell broadcast |(передача) вещание в системе сотовой ячейки |
|authentication |позывные, система паролей (при установлении связи) |
|to release |разъединение |

Exercise 2 Read the text

Exercise 3 Find the Russian equivalents for the following English words and word combinations
|co-channel interference |канальное затухание |
|broadcast control channel |перекрытие по частоте; перекрываемый диапазон частот |
|frequency coverage |канал контроля радиопередачи |
|the two responses match |выполнять аутентификационную проверку |
|random number |межканальная помеха |
|channel fading |предотвратить несанкционированное использование |
|on location updating |во время обновления местоположения |
|to carry out authentication check |приемопередатчик |
|to prevent fraudulent use |случайный номер |
|transceiver |два ответа совпадает |
|to map |размещать |

Exercise 4 Speak on the problems:

|1 |GSM architecture |
|2 |Air interface |
|3 |Speech coding and channel coding |
|4 |GSM signalling |

Part IV (47.6 – 47.8) 47.6 Services The primary purpose of all cellular radio networks is to offer speech telephony service to its customers. In addition most networks offer a range of supplementary and value added services to enhance the basic product. In analogue systems, basic telephony is provided directly by the audio path between mobile and network. Other than some linear speech processing to increase the channel's signal to noise performance, the audio path is transparent across the speech band, allowing other sounds (tones, non-voice signals etc) to pass through undistorted. By contrast GSM (and other digital systems) use a speech coder tailored to voice characteristics. They therefore provide a fully acceptable telephony service, but non-voice signals can suffer distortion across the non-audio transparent path.
[pic]
Figure 47.14 Location updating

47.6.1 Supplementary services Supplementary services are provided by means of enhancements to the basic call processing software in the MSCs. Many of these services have specific relevance to the cellular radio user, and in the main they parallel services which are becoming increasingly available on the fixed telephone networks (such as BT's Star Services in the UK). Typical services are as follows:
Call divert, where all calls are diverted to the specified number, which may be another mobile or a termination on another network. This is of use if the user wishes to make calls but not receive them.
Divert on no answer, where calls are diverted to the specified number when the user does not answer within (for instance) 20 seconds. This is of use if a mobile is left switched on in an unattended vehicle.
Divert on mobile unavailable, where calls are diverted to the specified number if the network cannot contact the mobile owing to its being turned off or out of range. This is of particular use in a cellular system where, in general, users are not avail able at all times, and where coverage is not universal. This service is often combined with the "divert on no answer" service.
Divert on busy, where calls are diverted to the specified number when the mobile is already engaged on a call. As an alternative, networks also provide call waiting.
Call waiting, where if a call is received when the mobile is already engaged on a call, the user is informed that a second call is waiting, and can choose to place the first call on hold whilst dealing with the second caller.
Three party calling, where the mobile user may set up calls to two other parties and connect them in a three way conference. This service can also be used to make enquiry calls whilst holding the original call.

47.6.2 Value added services Value added services are normally provided by means of peripheral units attached to the cellular network, or to the fixed network with which to cellular network interconnects. In some countries, the prevailing regulatory regime will influence what services may be offered and in what manner, however, the following are typical.
47.6.2.1 Messaging services Voice messaging is commonly available in association with cellular networks. Used in conjunction with the call divert supplementary services, the messaging service can pick up calls when the user cannot, and the caller can leave a message for later retrieval by the user. Some services allow the user to be alerted to the receipt of messages by means of a radiopaging service, or in some cases by a ringback on the cellular network itself. In addition to voice messaging, GSM networks will incorporate the 'Short Message Service' which effectively turns a GSM mobile into a two way alphanumeric pager with forced message delivery and message delivery confirmation.
47.6.2.2 1 nformation services Voice information services are commonly available on fixed networks, normally carrying some premium call charge. Some services (such as travel and weather information) are of particular value to a mobile user and some networks make these more readily accessible, for instance by using the mobile's current location to select the appropriate information for that area.
47.6.2.3 Private interconnect A large user of cellular can often gain economies by leasing a direct connection between the cellular network and their company's private network, thus bypassing the PSTN. Call charges for such direct connections are tariffed by the cellular operator at a level substantially less than that for PSTN calls. An extra benefit of private interconnect are that calls can be delivered direct to extensions on a company's network without having to be handled by the switchboard operator, saving time and labour. 47.6.3 Data services In analogue cellular systems, the transparent audio path between the network and mobile can be used not only for voice communication, but also for non-voice communication such as data using in-band modems, and facsimile. In order to be used in conjunction with a mobile, data modems and fax machines which are designed for PSTN use have to be adapted for connection to the mobile by means of a special interface. Such interfaces are available for a range of mobiles, and often permit automatic call establishment and clear down under the control of the modem or fax machine. The data rate achievable over a cellular radio channel will often be less than that over a direct PSTN path, mainly due to the more limited frequency response of the channel, and the delay spread characteristic which is affected by the audio processing in both mobile and base station. However data transmission at 120()bit/s (using CCITT V.22) and 48(K)bit/s (using V32) can be achieved quite commonly on cellular networks, as well as fax up to 7200 or 9600bit/s. The radio link between a cellular network's base stations and a mobile station is a notoriously hostile environment for data transmission. Disturbance and interruptions come from a variety of sources, such as variability of the radio signal strength, noise and interference, and 'intentional' breaks due to signalling interchanges between base station and mobile for handover and power control. In order to transmit data reliably over such a path, error control of some form is essential. The simplest form of error control is a layer 2 protocol, and the emergence of the CCITT V.42 standard has led to error correcting modems becoming readily available. Although V.42 (which contains two protocols, the 'open' LAP-M and 'proprietary' MNP4) was designed for fixed PSTN use, it has proved to perform sufficiently well over cellular paths, particularly to static mobiles, for the user to receive good service. Many proprietary protocols have been specifically developed to cope with the errors experienced over cellular radio channels. One such protocol is called Cellular Data Link Control (CDLC), and was developed in the UK by Racal Vodata. CDLC uses two levels of error correction with dynamic switching, and techniques such as forward error correction, bit interleaving and BCH block coding with a basic HDLC protocol to give a highly robust data transmission path, even over poor quality channels. Facsimile transmission over cellular has benefitted by the increasingly widespread adoption of Group 3 error correcting (ECM) fax machines and the availability of portable machines suitable for vehicle use. The GSM system does not provide a transparent audio path due to the voice coding techniques used, so data transmission in GSM is dealt with differently. When the data mode is selected, the speech coder is replaced by a rate adaptor and channel coder which apply forward error correction to the data bits, and the resulting bit stream is then transmitted across the radio path in the same burst structure as for voice transmission. At the receive end the bit stream is extracted and errors are corrected up to the limit of the forward error correction scheme. If there are any errors remaining, a higher layer protocol is needed to detect and correct them. GSM has defined two families of data services, termed transparent and non-transparent. The transparent service applies only forward error correction as described above, and the user application must be able to cope with the residual error rate. The characteristics of the transparent service are constant delay and throughput but variable error rate. The transparent service is of particular use in synchronous applications (eg X.25, IBM SDLC) where the higher layer protocol inherent in the application will correct the errors. Asynchronous applications may also use the transparent service, particularly at low bit rates where the forward error correction applied by GSM is stronger. The non-transparent service applies a GSM specific layer 2 protocol between the mobile and the network in order to correct all residual errors, resulting in a near zero error rate. The penalty, however, is variable throughput and delay, dependent upon the prevailing radio conditions. The non-transparent service is of particular application to simple asynchronous terminals, although provision in the standards is also made for protocol conversion to allow X.25 packets to be carried. Facsimile transmission over GSM is complicated by the use in the Group 3 standard of a number of data transmission rates and modem types (V.21, V.29, V.27). In order to carry the fax signals, GSM mobiles need a special adaptor to convert the multiple standards into a synchronous bit stream for transmission between mobile and network. A similar converter in the network then converts the signal back into the Group 3 protocol to interwork with fax machines in the fixed network.
47.7 Future developments The technology of cellular radio systems continues to develop very rapidly. The early 1980s saw the introduction of the first commercial analogue systems and by the end of the decade trials of second generation digital systems were already under way. Systems such as GSM are now entering into service and work is already starting on the specification of a third generation world wide standard system. These developments are not introducing technology for its own sake, but are aimed at improving the quality, capacity, and availability, and reducing the cost of mobile communications. In addition to these step changes in 'generations' of system there are technical advances which are applicable to current systems. These include techniques such as microcellular and intelligent networks. 47.7.1 Microcells As the capacity of cellular systems has increased, cell sizes have decreased, in some networks to as small as 0.5km radius, such that controlling co-channel interface becomes a major problem. The use of microcells, that is, very small cells, is a way of increasing capacity still further. In a microcellular layout, base station antennas are placed below the building height in urban areas, and low power is used such that the propagation characteristics between base station and mobile are dominated by the street layout. Interference from adjacent cells is blocked by buildings. Microcellular techniques allow significantly higher traffic densities to be achieved, and also enable smaller, lower power mobiles to be used. The use of microcells requires improved handover techniques, which allow for fast and reliable handoff, for example when turning a street corner. One way of easing handover problems is to employ an 'umbrella cell' arrangement using conventional cells overlaying the microcells such that handover can be made into the umbrella cell where no suitable adjacent microcell can be identified. This also avoids the need to plan a contiguous coverage of micro-cells in an urban area. New technology is now enabling the use of more compact and cheaper base stations. Conventional base sites have generally required a purpose built building, or rented space within an existing building for installation of base station racks of equipment. Now, base stations can be housed in small roadside or roof top mounted cabinets, and further reductions in size can be expected. Small base station equipment, and antennas, are essential to enable microcells to be built cost effectively. 47.7.2 Intelligent networks Intelligent Network techniques (IN), are not, of course unique to cellular systems and have already become well established in fixed networks for the provision of 'free fone' or 'toll-free' type services, for example. However, the ability of an IN architecture to provide customised services is particularly valuable to a mobile user, who can have improved control over the handling of incoming calls. IN techniques also provide the ability to create a wide variety of advanced services. Second generation cellular systems such as GSM are already designed around an architecture which can support IN type applications. In particular, the HLR function is closely related to the IN service control point. We can expect further developments in the near future which will bring a range of IN features to both the mobile user and the service provider. 47.7.3 Personal communications The term PCN, Personal Communications Network, is used widely in the UK, whilst PCS, Personal Communications Services is used in the USA. Both aim at the same objective of serving the mass consumer market with mobile communications. The key challenge is to provide a very high capacity network to support a large number of users at low cost. Microcellular techniques will certainly be needed, and in order to keep costs down, the concept of regional service, and local access to the PSTN is being considered. IN techniques may offer personal numbering across a variety of networks. PCN is dealt with in detail in Chapter 48. The standard in Europe, known as DCS1800 is based on the GSM standard but operating at 1800MHz. There is therefore unlikely to be a significant technical difference between Cellular GSM and PCN, with microcellular techniques being equally applicable to either system. In the USA, the use of CDMA, code division multiple access, is being trialled for PCS. CDMA works on the principle of transmitting unique (orthogonal) codes to identify different users. Detection of signals is achieved by using correlating receivers such that other users appear as pseudonoise. CDMA thus allows a large number of users to share the same (wideband) radio channel. There is considerable debate about the advantages and disadvantages of CDMA, in particular how to control near/far user interference; the extent to which this can be achieved is crucial to the ultimate capacity of CDMA. One of the key benefits of CDMA is the potential to share spectrum with other users, for example fixed links, and for this reason it is particularly attractive where additional spectrum for mobile systems cannot be made available. 47.8 Conclusion Cellular radio is a comparatively young technology. Networks employing analogue systems have developed rapidly and now provide high quality service and excellent coverage in many of the developed countries. Technology developments are now increasing the potential network capacity, reducing the size of mobiles, and bringing advanced features and services to the mobile user. The decade ahead with the opportunity to introduce new digital systems and create a world-wide land mobile standard looks particularly exciting.

Exercise 1 Write out of the text (47.6 – 47.8) all terms referring to cellular radio systems. Give their Russian equivalents

Exercise 2 Words to remember:
|HDLC protocol- (High Level-Data-Link Control) |высокоуровневое управление каналом (передачей) данных |
|forward error correction |прямое исправление ошибок |
|contiguous coverage |соприкасающиеся зоны обслуживания |
|CDMQ –(code division multiple access) |коллективный (многостанционный) доступ с кодовым разделением каналов |
|ultimate capacity |придельная способность |
|noise performance |шумовая характеристика |
|relevance |соответствие, отношение |
|to divert |отклонять, отводить, изменять маршрут |
|infomation service = enquery office |телефонная справочная служба |

Exercise 3 Read the text and translate it without a dictionary

Exercise 4 Speak on the problems (work in pairs):
|1 |supplementary services (the most typical ones) |
|2 |value added services |
|3 |data services |

Exercise 5 Answer the following questions:
|1 |What is the primary purpose of all cellular radio networks? |
|2 |Do they offer a range of supplementary and value added services? |
|3 |What are the typical supplementary cervices? |
|4 |Which of value added services are considered to be the most important? |
|5 |Can you ask your groupmates 4-6 questions about data services? (47.6.3 Data Services)? |
|6 |How can you prove that the technology of cellular radio systems continues to develop very rapidly? |
|7 |What do microcellular techniques allow to achieve? |
|8 |What are these developments aimed at? |
|9 |One way of easing handover problems is to employ an “umbrella cell”, isn’t it? |
|10 |What advanced services do intelligent networks provide? |
|11 |Is there any difference between the term PCN and the term PCS? |
|12 |What are the main advantages of CDMA? |