Carphone 1981

System 4 Automatic Radiophone - British Telecom Engineering Vol 6 April 1987 

Jeremy Newton

British Telecom (BT) opened its System 4 automatic mobile radio service, formerly marketed under the name of Radiophone, in 1981 About 10,000 customers benefit from a very high frequency (VHF) network which extends further into rural areas than either of the competing cellular systems is able to do economically. The service has a total of 110 duplex channels at 12·5 kHz channel spacing and uses the system known as the mobile automatic telephone system (MATS-B)Customers are able to dial national and international calls, and to receive calls from any UK telephone, but at present they cannot receive international calls. Investment in the system continues: a new regional structure, with new exchanges was introduced in 1986, and further possible developments are being studied. Although it was originally a London-only network, System 4 is now planned and marketed as a low-density nationwide system with good coverage of rural areas, as opposed to cellular systems which are better situated to high-density urban operation.


MATS-B was developed by TeKaDe (now part of Philips Kommunikations Industrie AG) in West Germany. The BT system is one of about six MATS-B systems in use in the world, but is the only one to use 12·5 kHz channel spacing: other administrations, notably in the Netherlands and West Ger­many itself, use the more conventional 25 kHz channels. Unlike the West German system, which Is based on individual cities, System 4 is operated in seven independent regions or zones, with exchanges in each region connected to a large number of base stations to give extensive coverage.System 4 is operated by BT under two licences held by British Telecommunications plc. BT is allowed, under a licence issued by the Home Office under the Wireless Tele­graph Act, 1949, to make radio transmissions from, and to, base stations and mobiles. The Home Office charges £5000 a year for each of the 110 channels which are in the 160 MHz band. The second licence, under the Telecommunications Act, 1984, is issued by the Department of Trade and Industry and can be revoked if four years' notice in writing is given. Unlike the cellular systems, BT is both the network operator and system provider for System 4.

System 4  compared with German B- Netz as described at

Channel Spacing 12.5 Khz 20 Khz

Duplex distance 4.5 Mhz 4.6 Mhz

Base Station ERP 25w 10w-30w

uk  "160Mhz band"

Down link freq xxxx 153.01 - 153.73 MHz

Up link freq xxxxx 148.41 - 149.13

network opened 1981 in London 1972 in Germany

network closed xxxxxx 1994

Max voice channels 110 38 then 75

Base stations 60 158

subs peaked in london in May 1985 27,000 subs in 1986


The first UK Radiophone service opened in South Lancashire in 1959, with a London service following in 1965. This was a manual system, later designated System 1, and catered for only 320 customers in London[I].A more advanced manual service, System 3,opened in 1971. System 3 used an initial 37 half-duplex channels with 25 kHz channel spacing. In 1973, a further 18 speech channels were introduced. One of the 37 channels was used for control signalling, with the other 36 available for speech. Capacity was increased by replacing the earlier mobiles, which were restricted to one signalling and nine speech channels, with more advanced equipment able to access all 55 channels[2]. But even so, the system quickly reached full capacity, particularly in London where 3264 mobiles were accommodated in December 1978-over half the nation wide total of 6052. The next biggest area was the Midlands with 819. The Grampian area, running up the east coast of Scotland from Dundee to Inverness, had 53.The need for an automatic system was seen with the aim of increasing efficiency and reducing operating costs. In addition, more than 55 channels were required, but the Home Office would not allocate more of the radio spectrum for an automatic system, so it was necessary to adopt 12·5 kHz channels, which would ultimately allow 110 channels to be used. The decision was taken to run System 3 and System 4 in parallel until System 3 could be closed. 


Several of System 3's 25 kHz channels were taken out of service in preparation for the introduction of automatic working. This process had to be carried out carefully, as many of the old nine-channel mobiles were still in use and it had to be arranged that each base station continued to radiate channels that were available to all users. The released channels were used to inaug­urate the System 4 service, initially in the London area only, with the new spacing of 12· 5 kHz between channels and 4· 5 MHz separation between the two transmission paths. In January 1983, dealers were given three years' notice that System 3 would close existing customers were told a year later. The system closed in England and Wales on 31 January 1986, but was maintained in Scotland for almost a year longer to enable System 4's coverage to be extended beyond that of System 3, which then closed in Scotland on 30 December 1986.


System 4 started in London in July 1981.For almost two years, it was considered as essentially a London-based system with limited access outside the capital. Roaming facilities were introduced to allow London customers to make calls from other areas; base stations were installed in the Solent and Bir­mingham areas in early-1982, but local mar­keting was kept to a minimum. Demand for service was so high that the saturation point was reached in late-1982, and a waiting list for service began to grow. By 1984, there was a black market in cars equipped with System 4 Radiophone equipment. The pressure on London eventually eased in the spring of 1985, a few months after the launch of the two cellular radio systems. The waiting list was cleared in March 1985, though the number of customers in London continued to grow until it peaked in May 1985.In July 1983, the system was extended to include such cities as Aberdeen, Edinburgh, Glasgow, Manchester, Leeds and Cardiff. However, in this second phase of development, there were still wide gaps in coverage along the routes between cities and in other rural areas. The concept of System 4 as the first nationwide Radiophone service developed and went into operation from 1983 onwards. The number of base stations was increased from 30 to 60 to provide the extension of service to the major urban areas, and work began to provide coverage along the major motorways. This enabled customers based in the main conurbations to use their radio telephones more extensively across the country, which provided a further stimulation for sales. The process of installing new base stations has continued in order to bring mobile telephony to areas which had previously no access to such a service and to improve the service in some fringe areas. Figure 1 shows one of System 4's more remote base stations installed in December 1986 to cover the A9 A86 and A889 roads. In late-1986 and early 1987, coverage was extended to wide areas of Cornwall, North and South Wales Lin­colnshire, and Northumberland. Further decisions have to be taken about extending coverage to the Lincolnshire and north Nor­folk coastlines and to include the east-west transport links in northern England. System 4 base stations, with an effective radiated power (ERP) of 25 W, can providec overage over an area up to 55 km from the base, depending on the nature of the surrounding environment. However, traffic needs and topography generally combine to restrict the radial range of a typical base station to about 20 km.Unlike cellular radio systems, System 4 currently has no hand-off facility to switch calls to the next base station when a mobile starts to move out of range. However, a trial hand-over facility is currently being tested between Reigate and Brighton. If successful, this facility will be extended to some other areas, but as System 4 base stations, in both rural and urban areas, generally cover a far greater area than their cellular counter parts, a mobile is much less likely to move out of range during an average call. On longer calls, it is up to the callers to decide the point at which the signal strength, and therefore the audio quality, have deteriorated beyond an acceptable level. At that point, a new call must be set up: the system will find the best possible channel available. System 4 is rugged enough to hold a link once set up until well after conversation becomes inaudible. There is no risk, as there is with cellular systems, that the call will suddenly cut off without warning as the mobile moves away from a base station. There are ways in which a mobile customer, when beginning a call, can ensure that transmission is maintained for as long as possible. This is achieved by over-riding the normal automatic selection procedure. The customer can select instead a base station towards which the mobile is moving. Each base station transmits continuously an idle marking signal(I MS) code on all vacant channels used insetting up calls. There are seven different IMS codes, which allow base stations to be identified within a particular restricted area. The customer enters the appropriate IMS code to instruct the radiophone to select a channel only from the chosen base station. This procedure is more complicated than making a normal radiophone call and requires additional knowledge about the operation of System 4. Few customers understand the method of operation of System 4 sufficiently well for this facility to be used more than rarely. In addition, in areas of overlapping coverage, it limits the number of channels available to the radiophone. In practice, the coverage map is probably drawn conservatively and operation is possible beyond the advertised boundaries. By late-1986, about 93% of the population of England, Scotland and Wales was within range of a System 4 base station, and BT believes that it might be possible to extend this coverage profitably to 96-97%. System 4, like System 3 before it, has never served Northern Ireland, though there were once plans to build two base stations in the Belfast area.


A zonal structure was introduced from 1983 onwards as System 4 was developed into a national network. In many ways, this resembles the regional structure of the earlier System 3, which was managed as a series of discrete areas, but it has the added benefit of allowing customers who opt for the facility to roam; that is, to make and receive calls away from their primary zone. At first, the zone structure was fairly simple outside London. One zone covered South-East England, another the Midlands with parts of South Wales and South-West England, and there were two linked zones covering Northern England and Scotland. Customers in Scotland also had full access to the network in the north of England and vice versa. Separate TeKaDe exchanges, known as Radiophone control exchanges (RCEs), were located in each zone to provide service: at Esher, Surrey, for South-East England (a shared site with the London System 4 exchange); in Birmingham for the Midlands; and in Bradford, with a line concentrator in Dundee, for the north of England and Scotland. All base stations in a particular zone are linked by leased line to the zonal exchange. Each RCE can serve no more than 42 base stations and 96 radio channels. (See Figure 2).

radiophone dial 140
zonal structure uk radio phone

Each exchange has a separate area code, which means that callers need to know where a System 4 mobile is so that they can dial the right area code. The six-digit telephone number of the mobile, however, remains unchanged nationally. Customers could pay for service (covering both incoming and outgoing calls) in one zone, or any combination of zones. This allowed quarterly rentals to be introduced which were substantially lower than those that would apply to a nationwide service allowing universal access. South-east zone customers were allowed to make calls in London, but not to receive calls-only customers who had paid for nationwide service, which was equivalent to the original London-only service, could receive calls in London. In subsequent years, the zonal structure has been refined. The south and east of England are now separate zones. On 1 April 1986, the Scottish zone was split from the North of England. Since then, the consequent reduction in charges and the increase in locally available channels have helped the number of customers in the North to grow by 25%. A zone covering Wales and the West of England has been split from the Midlands (see Figure 3). Because of these changes, new area codes have been introduced, see Table 1.Access in London has also been modified. Customers who pay for service in the south zone can make but not receive calls in London south of the River Thames; those who pay for service in the east zone are similarly served north of the Thames. To receive calls in London, customers must opt for full nation­wide service, which covers all zones. As the typical ratio between outgoing and incoming calls is three to one, many London customers are happy simply subscribing to the south and east zones, at a cost of less than half the quarterly charge for nationwide service, see Table 2.

Because the range of the System 4 base stations depends on the local topography, the boundaries between zones are not rigidly defined in practice. In many cases, calls can still be made and received even when a user is some distance inside a different zone, as long as there is an adequate signal from abase station in a zone for which service has been paid.


Each base station transmits a continuous signal on all traffic channels that are currently free and available for use. When a mobile customer wishes to place a call, the mobile equipment automatically scans the 110 channel band to seek a free channel. If more than one is available (possibly from more than one base station), the mobile selects the strongest signal. This process typically takes about two seconds. The mobile transmits the number dialed plus its own number on the channel. The base station transmits all this information back on the other half of the duplex link, and the mobile carries out a comparison. If the received numbers do not match the transmitted numbers, the mobile abandons the link and searches for another channel. The process is repeated until a satisfactory link is acquired. Some mobiles are capable of scanning the full band twice in order to achieve a connection. The base station checks via its zonal RCE that the mobile is authorised for service in that zone. Once a satisfactory link is achieved, the number dialled is passed through the RCE into BT's trunk network. All System 4 exchanges are sited at group switching centres. Callers have to dial the full national number of the customer they require, even when making a local call. Reliability of the base stations is high, with service availability measured at about 99·7%.Only one exchange has so far suffered a major breakdown-a brief failure in 1982. The proportion of calls blocked during the busiest hour is targeted at not more than 20%. Traffic is monitored for each base station on tape, and where necessary the channel capacity ofa station can be increased. For greater growth in demand, new base stations can be installed to relieve congestion on neighbouring stations ,as long as there is remaining capacity in the TeKaDe exchange. The exchanges themselves are not capable of expansion beyond 96 channels.


All calls are charged at a uniform rate, although there is a reduction for off-peak calls. International calls are charged at the standard BT rates plus a percentage premium. Information about calls-time, duration, called number-is recorded on tape at the System 4 exchanges and then processed at a central billing centre. Customers get fully itemised quarterly bills, which show national and international called numbers, and identify destinations in the case of national calls.


As already stated, the zonal structure of System 4 means that, in order to make a call to a mobile, it is necessary to know the zone in which the mobile is located so that the appropriate area code can be dialled. The RCEs are not linked, and if a mobile cannot be contacted within the zone covered by the exchange dialled, there are no facilities for forwarding the call to other zones. However, in practice, this has not proved to be a problem, as most System 4 customers opt for only one or two zones: hence there is rarely much scope for doubt about the number to dial. The RCE then selects the base station within its zone through which to route the call. A process akin to radiopaging is used, on one dedicated channel out of the 110 avail­able. This channel is used on a time-division basis by all base stations and is kept free of normal telephony traffic. Each base station is allocated to one of seven groups, in a distri­bution designed to ensure that the paging signals of no two base stations of the same group can interfere. All mobiles, when switched on but not set­ting up or carrying a call, automatically tune to the paging channel, and listen for their own number. The paging sequence takes about two seconds for all seven groups. Each base station in a group transmits the number of the mobiles ought plus the designation of a channe1.whichis available for conversation and which it is monitoring. A paged mobile, on receiving its number, responds by switching to that channel and acknowledging the call. Occasionally, the traffic levels are high enough for the paging system to build up a brief queue. The caller then hears a recorded announcement which states that the call will be put through as quickly as possible. Once the link is established, the mobile rings. If the mobile customer does not answer within one minute, the call is abandoned without charge to the fixed caller. Calls which are connected are charged at a uniform rate independent of the location of the fixed cus­tomer or the mobile-the same rate as is charged to cellular customers. No charge is payable by the System 4 customer for incoming calls.


The 4-wire System 4 radio system is connected to the 2-wire telephone network via a special interface. Electronic control is applied to ensure the speech level from both callers is constant and thus achieve maximum audi­bility by obtaining as good a signal-to-noise ratio on the radio circuit as possible. However, this does have disadvantages. The mobile cal­ler's voice is picked up from the interface and fed back to the mobile via the local control circuitry. Thus, when the fixed caller is not speaking, all noise on the mobile-to-base link is amplified and is also heard on the mobile. System 4 engineers are taking steps to overcome these problems. Customer awareness of these problems has arisen only since the introduction of cellular radio systems, allowing a comparison to be made. But the development of cellular tele­phones has potential spin-off, notably in the area of miniature companders. The extension of BT's digital trunk network-System 4 currently interfaces with Strowger exchanges should also bring improvements. The significance of these problems should not be overestimated because the single biggest cause of interference with both System 4 and cellular mobile telephony is in poor installation of customers' equipment. Particular care has to be taken with System 4 installation as vehicle panels and chassis parts can produce interference with duplex VHF transmission and reception.


When System 4 was launched, BT ran the network, but customers obtained their mobile equipment from independent suppliers, including Philips, Marconi and Storno. From 1983, however, when it was decided to make System 4 a nationwide service, BT started to market its own range of mobile telephones. The first, Emerald 1, manufactured by Mobira but sold by BT, was introduced in April 1983. A joint development with Philips led to the introduction of the Sapphire in 1984-85, and Quartz (see Figure 4) from Dancall was included in the range from mid-1985.Particular features of the Quartz include:

on or off-hook dialling, audio monitor, last-number recall,99 number store, visible and audible signal to indicate incoming call, service light that shows when mobile is in Radiophone coverage area, handset volume control, three-level electronic security lock to guard against unauthorised use, and dual-tone multi-frequency dialling to allow keypad to be used as access to facilities such as Voicebank®.

Because of the restrictions of the 4-wire/2-wire interface between the RCE and the GSC, hands-free telephones are not currently possible, as these would suffer from audio feed­back. The power requirements- 3·5 A at 12 V-also make portable telephones impractical. Some System 4 mobiles have been fitted into briefcases, but these have to be powered from a vehicle lighter socket. BT markets through its own Districts and independent dealers. Over 100 dealers registered users onto the system in 1986. About 40 dealers do significant levels of business, but it is hoped that the number can be increased to 70--80.After the relaunch of System 4 in April 1986, with a reorganised regional structure,it is hoped to expand the number of users in the short term to 12 000. In the longer term,a target of 15 000 is believed reasonable, and up to 18 000 could be accommodated without degrading the level of service.


With its extensive coverage of rural areas, considerable potential is seen for the use of System 4 in public transport. British Rail has experimented with System 4 coin-operated payphones on several Inter-City routes, but has recently decided to standardise on card­ operated cellular telephones. However, there is interest in providing System 4 radiophones on major provincial routes, where cellular networks have poorer coverage. System 4 payphones are already in use on Sealink Isle of Wight ferries. The telephone was developed by System 4 technical staff from the standard Payphone 200 table top unit. Its operation, apart from the coin-col­lecting mechanism, differs from normal mobiles in that it continually hunts for a free channel and provides a visual indication when service is available. If another mobile starts to use the selected channel, the service indicator goes out until another channel is found. Further applications are sought on long­ distance coaches, particularly those not using the motorways which are well covered by the cellular systems, as well as on other ferry routes and on commercial fishing and other coastal vessels. The range of System 4 base stations over sea appears quite extensive-upto 140 km has been reported by one dealer. Several installations have been made on ves­sels and on gas production rigs which are located close to the coast. There is also believed to be some scope for data transmission via System 4. The technical feasibility has been established: indeed, System 4 scores heavily over cellular radio in this respect. Hand-off between base stations on the cellular radio system, and other control signals, cause an interruption in the conversation, or data stream, for 300 ms. This puts particularly strenuous demands on the modem design.


The System 4 direct-dial radiophone service is a fast and efficient means of communication for business people on the move. The service is available nationwide through a series of seven zones, and utilisation can be tailored to suit individual needs. Calls can be received automatically from any telephone in the UK. The full range of outgoing calls, including international direct dialling facilities, are available. The service is complementary to the cellular radio systems, particularly in the less populated rural areas where it is able to offer excellent coverage.


Jeremy Newton joined BT in 1973 and was appointed Head of System 4 Radiophone in BT's Mobile Com­munications Division in June 1986. Previously he had been with the Consumer Product Division concerned with sales and marketing support for the distribution network. He is a member of the Institute of Mar­keting.