Mạng và viễn thông P15

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Mobile Telephone Networks Being away from a telephone, a telex or a facsimile machine has become unacceptable for many individuals, not only because they cannot be contacted, but because they may be deprived also of the opportunity to refer to others for advice or information. For these individuals, the advent of mobile communications promises a era, one in which there will never be excuse for being new an ‘out-of-touch’.

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Networks and Telecommunications: Design and Operation, Second Edition. Martin P. Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic) 15 Mobile Telephone Networks Being away from a telephone, a telex or a facsimile machine has become unacceptable for many individuals, not only because they cannot be contacted, but because they may be deprived also of the opportunity to refer to others for advice or information. For these individuals, the advent of mobile communications promises a era, one in which there will never be excuse for being new an ‘out-of-touch’. This chapter discusses modern mobile radio communication technologies, covering the principles of ‘radio telephone service’, ‘trunk mobile radio (TMR)’ cordless telephones, the ‘global system for mobilecommunication (GSM)’, as well as describing telephone communication withships,aircraft, and trainsandtheemergingsatellitemobilenetworks(e.g.Iridiumand Globalstar). Mobile datacommunication networks are covered in Chapter 24. 15.1 RADIO TELEPHONE SERVICE The first radio telephone services were manually operated in the high frequency radio band. These supported international telephone services, as well as communication with ships and aircraft. Immediately after World War 2, a new breed of VHF (very high frequency) radio transmitters and receivers (transceivers) were developed. First used for applications such as the police, the fire service and in taxis, later development lead to their use as full radio telephones, connected to the public switched telephone network (PSTN) for the receipt and generation of ordinary telephone calls. The technology used a radio mast located on a hill and equipped with a powerful multi-channel radio transceiver. The mobile stations were weighty but were nonetheless popular for commercial ‘car telephone’ service, which grew rapidly in popularity in the mid-1970s. For calls made to or from the radiotelephone user, the public telephone network is connected via mobile switching centres (MSCs)to a number of transmitter base stations which emit and receive radio signals from the mobile telephones. Two radio channels (using different frequencies)are needed to connect each telephone during conversation, one radio channel for each direction of conversation. Figure 15.1 illustrates a typical automatic radio telephone system of the late 1970s. 297
298 NETWORKS TELEPHONE MOBILE Mobile swltchlng centre (MSC) Public switched telephone network LOO circutts to PSTN Figure 15.1 A simple radio telephonesystem Figure 15.1 illustrates a single mobile switching centre and a single transmitter base station, serving 3600 mobile customers within an area of 1200 km2, within a radius of about 20 km from the base station. Calls are made orreceived by the mobile telephone station by monitoring a particular radio channel,called the signalling or call up channel. When making a call, the mobile station signals a message to the base station in much the same way as an ordinarytelephone sends an off-hook signal and a dialled digit train (Chapter 7). The base station next allocates a free pair of radio channels to be used between the base station and themobile for the subsequent period of conversation, and the two switch over to the allocated channels simultaneously. Attheendofthe call theradiochannelsare released for use onanother call. Incoming calls to the radio telephone connect in the same way. An ordinary telephone number is dialled by an ordinary customer connected to the public switched telephone network (PSTN). A particular area code within the normaltelephone numbering plan is usually allocated to identify the mobile network, and all calls with this dialled area code arerouted the to mobileswitchingcentre and via the appropriate base station transmitter to reach the desired mobile receiver. A hurdle to be overcome in the design of radio telephone systems is the need for incoming calls to be routed only to the nearest base station. Failure to so causes the do failure of incomingcalls, because the mobile cannot guarantee tobe within the coverage area of a particular transmitting base station. Early radio telephone systems overcame the problem by requiring the caller to know the whereabouts of the mobile that he wished to call. The same customer number would be used on all incoming calls, but a different area code would have to be used, depending on which base station the caller wished the call to be routed to (i.e. according to which zone the caller thought the mobile was in). Figure 15.2 illustrates this principle. We see here that if the mobile customer’s number is ‘12345’, then when a caller believes that the mobile in zone 1, the number033 1 12345 should be dialled. Similarly for zones is 2 and 3, the numbers0332 12345 and 0333 12345 are appropriate. Some more advanced
CELLULAR RADIO 299 Dial area code Mobile as below plus zone 2 0332 zone 3 0 333 Figure 15.2 Area code routing for incoming radio telephone calls radio telephone systems were developed with the ability to ‘remember’ the last location from which acall was made and route the callthere.Others used ‘trial-and-error’ methods, butthese systems were all superseded by the advent of cellular radio, which has eliminated the market for old-style radio telephones used by roaming users. Another drawback of the early radio telephone systems was their low user capacity, and their ‘unfriendly’ usage characteristics. Not only did the automatic systems rely on the caller to know where themobile was, but somedemandedthe‘press-to-speak’ action of the mobile user. The systems allowed the mobile user to continue to move about during the call, but if he or she moved out of the coverage area of the base station, the call would be terminated. There were no facilities for transferring to other base stations during the course of a call. Together, these drawbacks lead to the demise of radiotelephonyinitsoriginal guise andtoits replacement with cellularradio, discussed next. The technologyhas, however, survived for localized radionetwork coverage, such as requiredby taxi companies or regional haulage companies. The terms private mobile radio ( P M R ) or trunk mobile radio ( T M R ) (in Germany Bundelfunk) are more commonly used nowadays. Some interest has been shown to extend the usage and life of the technology, as demonstrated in the recent development by ETSI of new standards for T E T R A (trans-European trunk radio), which aims to provide relatively low speed integrated voice and data networking capability to and from mobile stations located within relatively large geographical radio coverage areas. T E T R A is discussed more fully in Chapter 24. 15.2 CELLULAR RADIO A difficulty with early radio telephone systems was the small capacity in number of users. Thisarose because theearly systems were designed to have very wide area coverage, buthad a limited number of radiochannelsavailable within eachzone. Furthermore, the re-use of radio channels in other zones was precluded by the risk of
300 NETWORKS MOBILE TELEPHONE interference except where base stations were separated by large distances. Because the 1970s saw a boom in radio telephone demand, and because radio channel availability was (and still is) alimitedresource,therewaspressure for development of revised methods, more efficiently using the radio bandwidth, therebygreatlyincreasingthe available user capacity. The system that evolved has become known as cellular radio. The basic components of cellular radio networks are mobile switching centres (MSCs), base stations, and mobile units, as Figure 15.3 shows. Cellular radio networks make efficient use of the radio spectrum, re-using the same radio channel frequencies in a large number of base stations or cells. Oriented in a 'honeycomb'fashion,each cell is keptsmall, so thattheradiotransmitting power required at the transmitting base station can be kept low. This limits the area over whichthe radio signal is effective, and so reduces the area overwhichradiosignal interference can occur. Outside the interfering zone of the transmitter, i.e. in a non- adjacent cell at a sufficient distanceaway fromthe first,thesame radio channel frequencies may be re-used. Figure 15.5 illustrates the interfering zone of a given cell base station, and shows another cell using the same radio channel frequencies. A whole honeycomb of cells is established,re-using radiochannelsbetweenthe various cells accordingto a pre-determined plans. A seven cell re-usepattern is shown in Figure 15.6. Seven different radio channel frequency schemes are repeated over each cluster of seven hexagonal cells, each cell using a different set of frequencies. By such planning the same radio frequency can be used for different conversations two or three cells away. Figure 15.6 shows a 7-cell re-use pattern. Other patterns, some involving as few as three cells, and some more than thirty cells, can also be used. Large repeat patterns are necessary to cater for heavy traffic demand in built-up areas where small non-adjacent cells may still interfere with one another. Each cell is served initially by a single base station at its centre and is complemented as trafficgrowswithdirectional antennasandmoreradiochannels. Usingdirec- tional antennas helps to overcome radio wave shadows. For example, to locate three Cell coverage area / 'Cells' ? / MSCs or centres; /\" I / / " Mobile handset Other base statlons Figure 1 . The basic components of a cellular radio network 53
CELLULAR RADIO 301 Figure 15.4 Cellular radio carphone: mounted and in use in a car Cell re-using channels 1-400 v Figure 15.5 Cellular radio channel interference and re-use. BS = Base Station
302 NETWORKS TELEPHONE MOBILE 'Cluster'of Adjacent seven cells 'cluster' W Figure 15.6 Cellular radio in re-use pattern obstacle Radio 'shadow' Transmitter Mobilestation Alternative radio path \ Figure 15.7 Location of multiple base stations directional antennas, one at each alternate corner of the cell, helps to overcome the shadow effects that might otherwise occur near tall buildings by giving an alternative transmission path, as Figure 15.7 shows. A feature of cellular radio networks their ability to cope withan increasing level of is demand first by using more radio channels and more antennas in the cell, and then by reducing the size of cells, splitting the old cells to form a multiplicityof new ones. Only a limited number of radio channels can be made available in acell at the same time, and thislimitsthenumber of simultaneoustelephoneconversations. By increasingthe number of cells, the overall call capacity can be increased. The number of channels needed in a given cell is determined by the normal Erlung formula (Chapter 30). The total call demand during the busy hour of the day depends on the number of callers within the cell at the time. Reducing the size of the cell has the effect of reducing the number of mobile stations that is likely to be in it at any time, and so relieves
RADIO MAKING CELLULAR CALLS 303 R--- / 1 / Metropolitan Country zone zone l \ \ \ ‘--A Figure 15.8 Cell splitting to increase cell capacity congestion. Figure 15.8 shows a simple splitting of cells and a gradual reduction in cell size in the transitional region between a low traffic (country) area and the high traffic region surrounding a major metropolitan zone. When splitting cells in this manner, due care needs to be taken when allocating radio frequencies to the new cells, and a new frequency re-use plan may be necessary to prevent inter-cell interference. 15.3 MAKINGCELLULAR RADIO CALLS One or more control orpuging radio channels used to make or are receive calls between the mobile station. If a mobile user wants to makea call, the mobile handset scans the pre- determined channelsto determine the strongest control channel, monitors to receive and it network status andavailability information. When the telephone number the destina- of tion has been dialled by the customer and the send key has been pressed, the mobile handset finds a free control channel and broadcasts a request for a user (i.e. radio telephone) channel. All the base stations use the same control channels, and monitor them for call requests. On the receipt of such a request by any base station, amessage is sent to the nearest mobile switching centre ( M S C ) , indicating both the desire of the mobile station to place a call and the strength of the radio signal received from the mobile. The MSC determines which basestationhas received the greatest signal strength, and, based on this, decides which cell the mobile is in. It then requests the mobile handset to identifyitself with an authorization number that canbe used for call charging. The authorization procedure eliminates any scope for fraud. Following authorization of an outgoing call, a free radio channel is allocated in the appropriate cell forthecarriage of the call itself, andthe call is extended to its destination on the public switched telephone network. Incidentally, the appropriate cell need not necessarily imply the nearest base station; more sophisticated cellular net- worksmight also choose to use adjacent base stations if this will help to alleviate
304 NETWORKS TELEPHONE MOBILE Newstronger signalradiopath 7 p a t hw Ne / I switching l centre) Old ell c P a t h re1 e a s e d on h a n d off- Figure 15.9 ‘Hand-off’ during a call channel congestion. At the end of the call, the mobile station generates an on-hook or end o call signal which causes release of the radio channel, and f reverts the handset back to monitoring the control and paging channel. Each mobile switching centre controls a numberof radio base stations. If, during the course of a call, the mobile station moves from one cell to another (as is highly likely, because the cells are small), then the MSC is able to transfer the call to route via a different base station, appropriate to the position. Theprocess of changeover to the new new cell occurs without disturbance to the call, and is known as hand-oflor handover. This is one of the most importantcapabilities of a mobile telephone network. Hand-off is initiated either by the active base station, or by the mobile station, depending upon the network and system type. The relative strengths of signals received at all the nearest- base-stationsarecompared withone anothercontinuously,and whenthecurrent station signal strength falls below a pre-determined threshold, or is surpassed by the signal strength available via an adjacent base station, then hand-ofSis initiated. The mobile switching centre establishes a duplicate radio and telephone channel in the new cell, and once established, the call is transferred to the new radio path by a control message to the mobile handset. When confirmed on the new channel and base station, the original connection is cleared, as Figure 15.9 illustrates. 15.4 TRACING CELLULAR RADIOHANDSETS Whenever the mobile handset is switched on, and at regular intervals thereafter, it uses the control channel to register its presence to the nearest mobile switching centre. This
AR EARLY 305 enables the local mobile switching centre at least to have some idea of the location of the mobile user. If outside the geographical area covered by the base stations controlled by its home MSC, the local MSC (i.e. the nearest to the mobile user) undertakes a registrationprocedure, in which itinterrogatesthe home MSC (orthe intelligent network database associated with it) for details of the mobile, including the authoriza- tion number and other information. The information is held by the home MSC in a database called the home location register, or HLR. It contains the mapping informa- tion necessary for completing calls to the mobile user from the PSTN (its network identity, authorization and billing information). The local MSC duplicates some of this information in a temporary visitor location register or VLR, until the caller leaves the MSC area. Once the visiting location register has been established in the local MSC, outgoing calls may be made by the mobile user. The registration procedure is a crucial part of the mechanism used for tracing the whereabouts of mobile users, so that incoming calls can be delivered. Incoming calls are first routed to the nearest mobile switching centre (MSC) to the point of origin (i.e. the caller). This MSC interrogates the home location register for the last known location of the mobile user (this is known as a result of the most recent mobile registration). The call can then be forwarded to the mobile switching centre where the mobile was ‘last heard’, whereupon a paging mechanism, using the base station control channels, can locate the exact cell in which themobile is currently located. A suitablefreeradio channel may then be selected for completion of the call. Both the handsets and the network infrastructure needed to supportcellular radio are complex and expensive, although the increase in user demand is reducing the cost. The mobile handsets come in a number different forms, from the traditional car-mounted of telephone, to the pocket versions. The latter areexpensive not only because of the feats ofelectronicsminiaturizationthathas been necessary butalso because thetrends towards pocket telephones hasnecessitated advanced battery technology and theuse of sophisticated battery conservation methods (which, in effect, turn much of the unit off forasmuch of the time as possible, to save power). The mobileswitchingcentres (MSCs) rely on advanced computers capable of storing and updating large volumes of customer information, and alsoof rapidly interrogating other MSCs for the location of out-of-area, or roaming mobiles. The interrogation relies on the use of the mobile applicationpart ( M A P ) andthe transactioncapability ( T C A P ) user parts o f SS7 signalling (which we discussed in Chapter 12). 15.5 EARLY CELLULAR RADIO NETWORKS A number of different cellular radio standards have evolved, with the result that hand portables purchased for use on one system are unsuitable for use on another. The most common o f the systems currently in use worldwide are as follows. A M P S (Advanced Mobile Telephone System) This was first introduced in Chicago in 1977 by Illinois Bell, at that time one o f the operatingcompanies of AT&T in theUnitedStates.Developed by AT&T’s Bell
306 MOBILE TELEPHONE NETWORKS Laboratories, this became the most commonly used system in North America up to the early 1990s, when digital systems began to take over. It operates in the 800 MHz and 900 MHz radio bands, at a channel spacing of 30 kHz. N M T (Nordic Mobile Telephone Service) This commenced service in the Nordic countries in 1981 and is used in other countries in Europe (Austria, Belgium, Czechoslovakia, France, Hungary, Netherlands, Spain, Switzerland). The original system was 450 MHz based but has been gradually extended and to some extent replaced after 1986 with the later NMT-900 version. C (Network C ) Network C was a developmentof Network B, a digitally controlled mobile radio system introduced in the Federal Republic Germany in 1971. Network Bwas 450 MHz based. of Its replacement, Network C can be either 450 MHz or900 MHz based (hence C-450 and C-900); it is still in use in Germany and Portugal, butbeing replaced by GSM systems. TACS (Total Access Communication System) This is a derivative of the American AMPS standard, modified to operate in the 900 MHz band, with a more efficient 25 kHz radio channel spacing. TACs was the system introduced into the UK and Ireland during 1985. E T A C S or extended T A C S is a com- patible derivative of TACs which gives greater channel availability, particularlyin very congested areas like the metropolitan London area. The system is also used in Austria, Italy and Spain. Other mobile telephone standards include N A M T S (Nippon Automatic Mobile Telephone System used in Japan), Radiocom 2000 (used in France), R T M S (second generation Mobile Telephone used in Italy), UNZTAX (used in China and Hong Kong) and Comvik (used in Sweden). Table 15.1 Comparison of analogue cellular radio network types AMPS C 450 NMT 450 NMT 900 TACS (USA) (Germany) (Scandinavia) (Scandinavia) (UK) Uplinkband 824-849 MHz 450-455 MHz 453-458 890-915 890-915 MHz MHz MHz Downlink 869-894 461-466 463-468 935-960 935-960 MHz MHz MHz MHz MHz band Channel 30 kHz kHz 20 25 kHz 25 kHz 25 kHz spacing Multiplexing FDMA FDMA FDMA FDMA Modulation FSK PSK PSK FSK FSK Number of 833 222 180 1000 1000 channels
SYSTEM GLOBAL COMMUNICATIONS FOR MOBILE 307 The trend of the above systems to migrate to the 900 MHz follows the allocation of frequencies in the band by the World Administrative Radio Council ( W A R C ) in 1979. Subsequently there has been pressure to digitalize the radio speech channels, as well as the control channel, to improve radio spectrum usage. Digital channels allow closer channel spacing, dueto reduced interferencebetween channels. Theadoption of digital technology enabled, greater first, efficiency in the use of available radio bandwidthandtherefore higher traffic volumes;second, dramatic pricereductions resulting from miniaturization and large scale production of components. Further, the introduction of competing cellular radio telephone network operators as the first stage of deregulation and competition for the traditional monopoly telephone companies simultaneouslyresulted in muchreducedpricesformobiletelephone services. The result has been aworldwide boom in mobile telephony. The predominant technical standardsarethose of GSM (globalsystemfor mobilecommunication) and PCN (personal communications network, also known as DCS-1800: digital cellular system/ 1800 MHz). 15.6 GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS (GSM) In 1982, CEPT (the European Conference of Posts and Telecommunications) decided to start work onspecifying new technical standards for the support of a pan-European mobile telephone service, based entirely on digital radio transmission. The task group set up to performthespecificationwork was theCEPTiGSMgroup, GSM being initially an acronym of Groupe Speciale-Mobiles. It was an ambitious programme which targetted a network spanning the whole of Europe by 1991, which would allow mobile handset users to roam anywhere on the European land mass and still be able to make and receive calls. The GSM programme was adopted by the European Commission of the European Union, who recognized its potential for spurring standardization and deregulation of the telecommunications marketin Europe. In particular, GSM became the potential for standardization of common networks across Europe, thus opening a common market for end user telephone equipment. In parallel, it presented an opportunity for govern- ments of the European Union member countries to commence deregulation of their monopoly telephone markets without undue threat the vast established terrestrial (or to f i x e d ) telephone networks. Finally, the programme was further reinforced European by Commission pressure on member countries to participate in a memorandum o under- f standing ( M O U ) initially signed in mid-1988, committingthenetworkoperators in European countries to ensure operation of the networks by 1991, allowing in addition, full roaming of mobile users between the networks and countries. Although the 1991 target was not met by all countries, it led to a flurry of second operator licensing across Europe, and to the rapid deployment of GSM networks. In most European countries, at least two GSM networks have been established, one owned by the monopoly operator, one operated by a new competing carrier. Most of the networks have been fully operational since 1993, with extensive coverage equalling or bettering the previous analogue mobile networks. With call prices under pressure from competition, the GSM mobile telephone boom took hold in late 1993.
308 NETWORKS TELEPHONE MOBILE A major attraction of the GSM system has been the ability of its users to make and receive telephone calls anywhere in the GSM world (anywhere in Europe, Australia, New Zealand, South Africa, Hong Kong, etc.) without having to advise callers of any change in telephone number (this is called international roaming). 15.7 GSM TECHNOLOGY Figure 15.10 presents the structure and main elements of a GSM network. In general, the system works exactly as described earlier in the chapter. The mobile switching centres (MSCs) are the main controllingelements of the networks (a national network typically comprises 10-25 MSCs). Each controls a given geographic area over which a number of base transmitter stations ( B T S ) are spread (typically 5-8 km radius cells). The BSC (base station switching centre) is the control element for the base trans- mitter stations, but need not be collocated with the BTS. Thus in a dense metropolitan area, several antenna sites may be used, but they require only one small BSC switching radio part mobile B! I m- I- interface U, A 0 AuC authorization centre HLR Home locationregister BSCbasestationswitchingcentre MSCmobileswitchingcentre BSS station base sub-system NSS networkandswitchingsub-system BTS transmitter base station OMCoperationsandmaintenancecentre EIR equipment identity register OSS operationsandsupportsub-system Figure 15.10 Main components of a GSM network
GSM TECHNOLOGY 309 site. Home location registers ( H L R ) and visitor location registers ( V L R ) have the func- tions described earlier in this chapter. Typically one HLR and one VLR is associated with each MSC. As explained in Chapter 12, the mobile application part ( M A P ) is used in communications between MSC, HLR and VLR. The MSC provides for call control and switching, and for gateway functions to other mobile or fixed networks.TheGSM system hasanumber of additional security features compared with its predecessors. These aim to reduce the problem of stolen handsets.A SZM card, containinga small user identificationchip and information concerning the users configuration, must be inserted into the user’s handset before it will operate. This chip enablesan authorization procedure to be carried out at each call set up between mobile station, MSC and the equipment identfication register ( E I R ) . Should a handset or a card lost or stolen, then the EIR may be updated to record this be information.TheEIRcontains a blacklist of barredequipmentand a grey list of equipment not functioningcorrectly or for which no services are registered. A white list contains all registered users and relevant subscription services. The radio part of the GSM system uses a 25MHz radio band. The uplink channels (mobile to base station) occupy the band 890-9 15 MHz. The downlink (base station to mobile) channels are between 935-960 MHz, whereby uplink and downlink channels of a particular channel pair are separated by 45 MHz. The radio bandis subdivided into 124 carrier frequency pairs, each of 200 kHz uplink and 200 kHz downlink bandwidth. Each carrier is coded digitally using TDMA (time division multiple access as explained in Chapter 8). The normal TDMA frame used in GSM is illustrated in Figure 15.1 1. The usable individual circuit bitrate is around 24 kbit/s (1 14 bits per circuit, every 4.61 5 ms). Frequency hopping (Chapter 8) provides for protection against radio fading caused by multipath effects, and also gives some protection against criminal overhearing of the radio signal. A further GSM measure against overhearing is a key-coded data encryption, whereby the key is held on the SIM card in the mobile station and the authorization centre ( A u C ) provides the encryption algorithm. - F 4.615 ms 4 - I O I 1 1 2_ -- 1 3 1- - _ 1 5 1 6 1 7 1 4 __--- __--- ---. _ _ - - - --.__ - - m _ _ - -_ _ - - __-- - -- - - - _ _ _ 1 - _ --- 0.577 ms --- 4 156.25 bits I I guard guard l bit I I bit guard training guard TB encrypted data encrypted data TB bits sequence bits 8.25 3 57 1 26 1 57 3 8.25 Figure 15.11 TDMA frame used in GSM
310 c+ NETWORKS TELEPHONE voice or data ISDN S-interface .-1q+ i MOBILE ISDN R-interface (V- or X-series terminals) Figure 15.12 Mobile station termination types for GSM The radio modulation is GMSK (Gaussian minimum shft keying), a form of phase shift keying(PSK, binaryphase modulation). The P C M (pulse codemodulation) coding of the speech signal uses an A D P C M (adaptive dyerentialP C M ) algorithm (see Chapter 38) with a bitrate around10 kbit/s. The remaining bitrate needed for signalling, error is correction, encryption and other protocol functions (e.g. hand-of, etc.). Figure 15.13 Modern GSM handheld telephone (handy). (Courtesy of Siemens A G )
PERSONAL COMMUNICATIONS NETWORK (PCN) AND DCS- 1800 31 1 The U,-interface is the mobile telephone network equivalent the ISDNU-interface of (Chapter 10). It is the standard radiointerface and protocol thatallows mobile handsets from manydifferent manufacturers access to the mobile network. Figure 15.12 illustrates the classification of the various ISDN-like end user termination tapes. The boom in number of GSM subscribers and traffic volumes has prompted fastfurther development of the G S M system. The initial offering of carphones. requiring 20 Watt battery power have been rapidly supplanted by 8 Watt and 5 Watt portable equipments, and finally by 2 Watt handies(handheldterminals). These are the palm-sized mobile telephones now available in most electrical stores. The lower output power of the mobilestationsequatesto lower range and thereforesmaller effective cell size. However, as the boom in traffic has demanded smaller cells any- way (thenumber of calls per cell being limited), this is no longerapractical or economic problem. New developments of the GSM system will increase the intelligence and functionality of the system. Already, sophisticated mailbox services, short message services (displayed on the display of the mobile station as a short text), fax and data services are becoming common network offerings. 15.8 PERSONALCOMMUNICATIONS NETWORK (PCN) ANDDCS-1800 The PCN system, nowalsoknown as DCS-I800 (digitalcommunications system at 1800 M H z ) , was originally intended to herald a new era of ‘personal communications’. In the late 1980s a mass-market was foreseen for handheld mobiletelephones (like GSM-handies), but the 9 0 0 M H z G S M standards were not thought at the time to be capable of satisfying the demand, because it was not thought that the mobile sets could be created small enough and with sufficient power and battery conservation to work effectively. An initiative for PCN commenced, and a new radio band at 1800 MHz was allocated. In the event, the PCN initiative was largely overtaken by GSM, and the DCS-1800 system is little mure than a modified version of G S M for the 1800 MHz radio frequency band. The network licensing programme of European governments then sealed PCN technology as a direct competitor to standard G S M by issuing further mobile network licences to new network operators. The interest of these network operators has largely been to ‘cash-in’ on the mobile telephone boom of the G S M operators, so that DCS- 1800 network operators such as E-plus in Germany and One-to-one and Orange in UK cannot bedistinguished by mostcustomersfromthe classical cellular and G S M operators D1 and D2 in Germany and Cellnet and Vodujone in UK. A further reduc- tion in market prices has resulted. Table 15.2 comparesthe DCS-1800 system with thatof G S M and the USand Japanese digital cellular systems. One of the major lessons learnedfrom both DCS-1800 and GSM has been how quickly viable alternativenetworkstothetraditional,terrestrial-basedpublic telephone can be established. GSM operators have become very adept at finding and buying or leasing small footprint sites where they may erect radio towers which serve
312 NETWORKS MOBILE TELEPHONE Table 15.2 Comparison of DCS-1800 (PCN) with GSM, USDC and PDC ~ ~ -~ ~ Parameter GSM DCS- 1800 USDC (ADC) PDC (JDC) (US or American (Japanese digital cellular personal digital system) cellular system) Uplink 890-915 band MHz 1710-1785MHz 824-849 MHz 940-960 MHz Downlink 935-960 MHz 1805-1 855 MHz 869-894 MHz 8 10-830 MHz band Channel kHz200 200 kHz 30 kHz 25 kHz spacing Duplex spacing 45 MHz 95 MHz 45 MHz 130 MHz Multiplexing TDMAiFDD TDMA/FDD TDMAiFDD TDMAiFDD Modulation GMSK GMSK DQPSK DQPSK Speech data 13 kbit/s 13 kbitis < 13 kbitis < 1 1 kbitis rate (6.5 kbit/s) Frequencies 124 374 832 800 Time slots per 8 (16) 8 3 3 radio channel Data service 9.6 kbit/s 9.6 kbit/s 4.8 kbit/s 4.8 kbit/s Maximum km/h 250 250 km/h 100 km/h 100km/h speed of mobile station output of 2, 5 , 8 or 0.25 or 1 Watt handheld unit 20 Watt simultaneously as base stations for the radio links to mobile stations and masts for the installation of point-to-point microwave radio links as backhaul links to mobile switching centres. The boom in demand for mobile telephones has caused operational problems for the cellular radio network operators, who in some cases have not been able to expand their networks in step with the growing demand. The result has congestion. At first, the been relief of congestion could be brought about by reducing cell sizes and so introducing more cells. However, as time has progressed, the radio bandwidth available has come to be aproblem.Initially, DCS was seen as asolution.Nowadays, DECT (digital European cordless telephony described in Chapter 16) technology is increasingly being seen as a complement to GSM. The idea is that a dual-mode telephone handset could log into whichever network was available in a particular area. The claimed advantage of DECT is that much higher subscriber density can be supported, so overcoming the hotspot problem currently existing in some pure GSM networks.
AERONAUTICAL AND MARITIME MOBILE COMMUNICATIONS SERVICES 313 15.9 AERONAUTICAL AND MARITIME MOBILE COMMUNICATIONS SERVICES High frequency radio communications have long been used in aeronautical and mari- time applications for navigational purposes and distress Furthermore, manypublic calls. telephone network operators have for many years offered ordinary telephone customers the opportunity to place or receive H F radio telephone calls to and fromships in coastal waters. Unfortunately high frequency radio systems, when used for communication over long distances at sea, suffer from poor signal propagation, atmospheric disturbance, interference, and signal fading. As a result, the ships’ telephone radio service is of poor quality, and unreliable. As a means of getting over this problem, a number of organizations during the late 1960s and early 1970s were studying the use of satellites for aeronautical and maritime communications. These studiesled in 1976 to the launchof M A R I S A T , the world’s first commercialmaritime satellite system. It was conceived by aconsortiumofUnited States companies, and it comprised three M A R I S A T satellites in geostationary orbit, providing communications services for military and civilian use. At the outsetheavy use of the system was made by the US Navy. Further worldwideinterest, followed up an initiative of theInter-Governmental Maritime Consultative Organisation (IMCO) in 1973, led to the signing of an agree- ment by 26 countries in 1976, and the establishment in 1979 of the International Mari- time Satellite Organization ( I N M A R S A T ) . I N M A R S A T ’ s satellites have revolutionized the world of communications with ships. Geostationary satellites now placed over the Indian, Atlantic and Pacific Oceans make possible communication to any suitably equipped ship, no matter where it is located around the globe. Automatic telephone calls, dialled directly by customers of some countries’public switched networks can be made to people aboard ships; theonly requirement is that the caller knows which ocean the ship is in, so that the appropriate ocean country code number (part the international telephone number) can dialled, of be directing the call via the appropriate ocean’s satellite. Similarly, persons aboard ships may make direct-dialled calls to other customers connected to the international public telephone network. Figure 15.14 illustratesthemaincomponents of themaritime satellite network, comprising earth station equipments both on shore and aboard ship, together with a sophisticated Access Control and Signalling Equipment. The A C S E performs similar functions to those of cellular radio network base stations and mobile switching centres (MSCs). It allocates radio channels for individual calls and carries out billing, account- ing and other administrative functions. A cheap form of satellite communication, allowing ‘telex-like’ communication to and from terminals on boardsmaller craft, vehicles and lorries is the INMARSAT Standard C service. This allows two-way communication using an omnidirectional antenna about the size of a saucepan. By the end of 1987 the number of INMARSAT signatory countries hadincreased to 50. INMARSAT’s originalaim was to providemaritimecommunications,thereby improving the safety and management of ships. In 1985 the charter was expanded to include aeronautical satellite communications.
314 MOBILE TELEPHONE NETWORKS - Radlo transparent cover l radome 1 \ Satellite CF antenna Ship’s antenna (detail ) d Antenna Ship ACSE Access Controland Signalling Equipment ‘Earth station’ Figure 15.14 Maritime satellite communications A number of initiatives have provided aeronautical satellite services. These bring the public telephone network to passengers and crew aboard commercial airliners. 15.10 IRIDIUM, GLOBALSTAR ANDTHE EVOLUTION TOWARDS THE UNIVERSAL MOBILE TELEPHONE SERVICE (UMTS) With the mobile telephone boom of theearly 1990s in full swing inthedeveloped countries, attention swung towards extending the possibilities of mobile roaming to worldwide coverage and to the increased penetration of mobile telephone services in less developed parts of theworld. The result has been anumber of new satellite technology initiatives. In these, the base transmitter station ( B T S ) and base switching centre ( B S C ) functions of GSM are combined into one unit which is now carried by a low orbiting satellite. The low orbit is necessary to reduce the transmit power needed in the mobile stations and also to reduce the undesirable long propagation time of signals transmitted to and from geostationary orbits at 37 000 km altitude (Chapter S). The low orbit creates a significant technological challenge, because each individual satellite now moves at considerable velocity relative to the earth’s surface, being at best, ‘visible’ from a particular point on the ground for a maximum of 15 minutes per orbit. Like GSM, each of the proposed satellite systems, divides the coverage area (the of earth’s surface) into a number cells, within each of which a number of radio channels permit individual users to make telephone calls. The difference is that the cells are now typically the size of a European country, so that the capacity in terms of traffic density is much less than for a terrestrial-based GSM system. The advantage, of course, of the satellite systems, is the global coverage and roaming capability. Figure 15.16 illustrates the concept in simple terms and Table 15.3 compares four of the systems proposed to go into service during the 1998-2000 timeframe.
IRIDIUM GLOBALSTAR AND THE EVOLUTION TOWARDS UMTS - 315 Figure 15.15 Inmarsatship’santenna.Theradomecoverprotectingaship’sINMARSAT telecommunications satellite antenna. (Courtesy of British Telecom)
316 TELEPHONE MOBILE NETWORKS K-band 10.9-36 GHz L-band 1.6-2.1 GHz I \ / \ I I / \ . I I earth station mPaw car or house office mobile Figme 15.16 The Universal Mobile Telephone Service (UMTS) Table 15.3 Comparison of satellite mobile telephone systems Globalstar Iridium Teledesic Odyssey Consortium Leader Motorola Loran, Qualcom TRW-Matra Bill Gates Number of 66 48 10 840 satellites Orbit type and LEO (low earth, LEO (low earth, ME0 (medium (low LEO altitude circular orbit), circular orbit), earth, circular earth orbit) 780 km, 1414km, orbit) 10355 km, 840 km 6 orbit paths 14 orbit paths 3 orbit paths Satellite inclination 86.4" 47/65" 55" Minimum elevation 8" 40" 10" 18" 2.6 ms 4.7 ms 34.5 ms No of 48 beams spot 6 163 (cells/satellite) Multiplexing FDMAiTDMA FDMAjCDMA FDMAiTDMA TDMA/SDMA QPSK QPSK Modulation kbit/s 2.4/4.8 Bitrate 4.8/9.6 kbit/s 4.8 kbit/s switching transparent on-board Satellite transparent weight 262 Satellite kg 689 1130kg kg 200 1999 1998 Date of service 1998 1