Carphone 1985 - TACS

Editorial

Despite continued advances in technology, telecommunication services available to the mobile user in the UK have, in the past, been restricted to about 10,000 users by the small number of radiophone channels available. However, in January of this year, the cellular radio service became operational with a target of providing service covering 95% of the population by the end of the decade and the prospect of over half a million users. The use of the cell concept with low-power transmitters and sophisticated monitoring and hand-over arrangements permits the users to travel between cells without loss of service and enables the available channels to be reused in a different cluster of cells. Although cellular radio is not a new concept, it was first proposed some 40 years ago, technological developments and, in the UK in particular, changes in the regulatory environment have only recently made its introduction possible. An article on p. 62 of this issue of the Journal is the first in a series of three on the Cellnet cellular radio network. This article gives a general overview of the system; the subsequent articles will describe the operational features of the system and finally the signal transmission and subscriber equipment. Time-division multiple-access (TDMA) techniques are shortly to be introduced into some of the satellite communication links operated by the INTELSAT and EUTELSAT organisations. A general outline of TDMA systems, together with details of synchronisation and operating protocols necessary to operate such global systems, is given on p. 85 of this Journal.

The Cellnet Cellular Radio Network

Part 1-General System Overview

M. S. APPLEBY, M.A.t, and J. GARRETT, M.A., c.ENG., M.I.E.R.E.*

UDC 621.396.93 : 621.395.4

This article, to be published in three parts, describes the cellular radio network run by Telecom Securicor Cellular Radio Ltd. (TSCR). The TSCR service, the result of a joint venture between British Telecom and Securicor, is one of two cellular radio services that have recently begun operation in the UK. Part 1 begins by discussing the background behind the setting up of cellular radio services in the UK and goes on to describe the basic principles of cellular radio, and the features and operation of the standard that has been adopted for both services—the Total Access Communications System. Parts 2 and 3 will go on to describe the operational features of TSCR’s system and the equipment used.

INTRODUCTION

Previous Public Mobile Radiophone Services

The South Lancashire Radiophone Service opened in 1959 and operated from two base stations in the very-high-frequency (VHF) band. This was a pilot system, which used frequency modulation (FM) on 50 kHz spaced channels, and its success gave rise to the opening, in 1965, of the London Radiophone Service. This was also a manually controlled system, which spread to other parts of the country during the 1970s.

The first automatic service, known as Radiophone System 4, started in London in 1981 and now covers all the areas previously served by the manual service, plus many more. Well over 10,000 customers are using the service, which now operates in the VHF band on channels spaced at 12.5 kHz.

The ultimate capacity of these systems is limited by the small number of channels available and the relatively slow and limited signalling repertoire. In London, where the demand is highest, the system is unable to support all those who wish to use it and, since 1978, efforts have been made to obtain further frequency allocations to allow the existing service to be expanded or a new one to be opened. No further channels are available in the VHF band, but the 1979 World Administrative Radio Conference allocated a band near 900 MHz that is wide enough to accommodate 1000 channels with 25 kHz spacing. Part of this has been made immediately available in the UK for cellular radio. A further allocation will be made when other users have vacated the band; in addition, there will be a reserved segment for future systems.

Foreign Systems

In the USA, Japan and the Nordic countries, the capacity of their existing VHF and ultra-high-frequency (UHF) services was exhausted and development was proceeding on higher-capacity systems at UHF by adopting cellular techniques. In 1980, trial systems were operating in the USA, Japan and Sweden; these were studied for possible application by British Telecom (BT).

Competition in the Provision of Cellular Radio Services

Whilst BT studied the techniques that could be applied, and awaited the necessary licence for the cellular service, the UK Government moved towards a greater liberalisation in telecommunications. It was decided to license two competing cellular radio operating companies and allocate half of the available channels to each. One operator would be the BT and Securicor joint venture, now called Telecom Securicor Cellular Radio Ltd. (TSCR) and trading as Cellnet; the other was chosen from a number of applicants and is now known as Racal Vodafone. Each operator was required to commence operation by the end of March 1985 and to provide service covering 90% of the population (approximately 60% of the land area) by the end of the decade.

Whatever the merits of competition in speeding up the provision of service and, possibly, in the long run, making for cheaper mobile units, the technical implications of the decision to have two competing services are severe. The division of the small number of available channels into two groups, one for each operator, is not an efficient way of using scarce bandwidth. Not only are double the number of channels allocated as dedicated control channels, but the smaller groups of channels available at each base station cannot be used as efficiently as groups more than twice their size. In the USA, the Federal Communications Committee administers the allocation of radio channels and acts as a central co-ordination point to minimise inter-user interference. There is no such co-ordination for the UK channels and it is possible that neighbouring but competing base stations could cause interference (for example, intermodulation) to each other. To fully avoid such interference, artificial constraints on the allocation of channels would be needed which would have a knock-on effect to further reduce system capacity.

Choice of Standard

None of the existing standards was found to be suitable for direct application in the UK. A detailed technical evaluation of the various systems in service or under development was carried out by BT. Factors considered included speech quality, signalling reliability and system capacity. The outcome of the technical evaluation was included with other considerations, particularly the time required for system development and a recommendation was made to the Department of Industry (now Department of Trade and Industry (DTI)) by TSCR and Racal that a modified version of the USA standard be adopted for the UK, to be known as the Total Access Communications System (TACS). This recommendation was endorsed by the DTI in January 1983. Immediately, a joint committee of BT, Racal and TSCR was set up to define the interfaces necessary to ensure compatibility between the two systems and thus allow a mobile telephone to work on either, subject to suitable commercial arrangements. The committee expanded to include members from the DTI, the Electrical Engineering Association (representing manufacturers of mobile-telephone equipment), the British Approvals Board for Telecommunications (BABT) (the approval agents for mobile telephones), and the BABT-appointed test laboratory, ERA Technology Ltd.

The main areas covered by the joint committee were:

(a) the ‘air interface’, covering the radio specifications, signalling protocols and formats, and audio specifications;

(b) the options for the interface to the public switched telephone network (PSTN); and

PRINCIPLES OF CELLULAR RADIO

Cellular radio is not a new concept. The fundamental ideas were originally proposed by the Bell Telephone Laboratories soon after the Second World War, although, at the time, the technology available did not allow a system to be implemented. Only recently have working cellular radio systems been introduced into public service. Among the countries currently operating networks are the USA, Japan, the four Nordic countries and, since the start of this year, the UK.

Cell Planning

The main attractions of cellular radio are its ability to cater for a wide range of traffic loading and its ability, ultimately, to handle far more customers than non-cellular systems. The basic principle of cellular radio is to split the required coverage area into a number of smaller areas, or cells, each having its own radio base station. The cells are grouped together into clusters and the radio channels available are allocated to each cluster according to a regular pattern which repeats over the whole coverage area. In this way, each channel is used several times throughout the coverage area in a regular fashion.

The number of cells in a cluster has to be chosen so that the clusters fit together into contiguous areas. Only certain cluster configurations do this, and typical arrangements of interest to cellular radio are groups of 4, 7, 12, and 21 cells (Fig. 1).

The number of cells in each cluster has a significant effect on the capacity of the overall system. The smaller the number of cells per cluster, the larger is the number of channels per cell, and thus the traffic carried per cell is higher. However, there is a penalty to pay for using small clusters. For the same cell size, as the cluster size reduces, the distance between cells using the same channels reduces, and so the interference from adjacent clusters increases. This type of interference, called co-channel interference, is one of the most significant factors that has to be taken into account when a cellular network is planned. Appendix 1 shows how co-channel interference is related to cluster size, and how the cluster size is chosen. In practice, the most common size is the 7-cell cluster, which represents a useful compromise between capacity and interference levels.

In the TACS system, signalling between mobiles and base stations for the purposes of setting up calls is carried on different channels from those which carry speech. Since co- channel interference would cause more problems on signalling channels than on speech channels, 21 channels are reserved for signalling, allowing a different cell cluster size to be used. With a cluster size of up to 21, co-channel interference can be kept to low levels.

As the maximum number of channels in a cell is fixed by the total number of channels divided by the cell repeat pattern, the maximum number of simultaneous calls is correspondingly limited. However, if the size of each cell is reduced, there are more cells in a given area, and so the total number of available channels in the area is increased, and the traffic that can be handled is correspondingly increased. Thus, it is common in urban centres where the demand for service is high to have very small cells. Conversely, in rural areas the traffic demand is comparatively low, and so to provide service economically the cell size is increased, and the number of expensive base stations is kept to a minimum. The size of cells can be controlled by the choice of location for the base station, the height and type of aerial, the power transmitted, and the signal threshold levels for transferring calls dynamically from one cell to another.

System Growth

Initially, the cellular radio networks will be configured with relatively large cells, even in urban areas. However, as traffic demands increase towards the limits that such a configuration can handle, ‘cell splitting’ will become necessary, particularly in urban areas.

Cell splitting consists of adding extra base stations and reconfiguring the channel allocations to give a larger number of smaller cells (Fig. 2). As the cell size is reduced in this way, two problems arise. Firstly, as the total number of base stations becomes large, it becomes increasingly difficult to find suitable sites to house them, and increasingly expensive to equip them. Secondly, propagation considerations mean that the amount of co-channel interference increases, even though the same repeat pattern is used.

The total co-channel interference received by any cell is the result of the signals received from each of the six clusters surrounding the cell (Fig. 3). One way of reducing the interference is to use directional aerials so that interference is received from only one cell. This is called sectorisation as the directional aerials split each cell into a number of sectors.

Two methods of sectorisation are commonly used, 3-sector and 6-sector (Fig. 4). With a 3-sector arrangement, the base stations in a 7-cell repeat pattern are fitted with directional aerials to split each cell into 3 areas. The channels of the original cell are allocated accordingly to the new areas. The effect is to produce a larger number of corner-excited cells using the same number of cell sites. With a 6-sector arrangement, the effect is again to increase the number of cells whilst keeping the same number of sites. However, the 6-sector arrangement also allows the main cell repeat pattern to be reduced to 4, thus increasing the number of channels per cell beyond that of the 7-cell arrangement. Both methods of sectorisation allow smaller cells to be realised by reducing the interference levels.

Network Aspects

A typical cellular network configuration is shown in Fig. 5. Base stations are connected by permanent links to mobile switching centres (MSC), which are computer-controlled telephone exchanges specially designed or adapted for cellular radio service. MSCs are connected to BT's PSTN to give access to and from land customers. MSCs within the cellular radio network are also connected together so that incoming calls to a mobile can be completed wherever it is.

located. In this way, a complete cellular network can be built up to give continuous radio coverage over a wide geographical area. However, two important features are necessary for the cellular network to function effectively, and these arise because mobiles move from cell to cell as they move through the coverage area.

The first of these features is mobile tracking, or location. When a call is received for a mobile from the PSTN, the cellular network needs to find which base station the mobile is nearest to so that the call can be connected successfully. In a network consisting of several hundred base stations and hundreds of thousands of mobiles, it is not feasible to transmit the call to the mobile on every base station. If this were the case, the capacity of the signalling channels would quickly become exhausted, and the overall capacity of the network would be limited. Instead, the cellular network is organised into a number of traffic areas, each consisting of a group of cells. The MSCs keep a record of the current location of all mobiles, which is updated by a process known as registration. Whenever a mobile is not making a call, it constantly listens to one of the common signalling channels. As part of the information transmitted on the signalling channels, the base stations generate a code identifying the traffic area. If the mobile starts to receive errors in the data stream, indicating that the signal has dropped below a usable level, it rescans the signalling channels to search for a higher signal level. If a change in traffic area code is detected, indicating a new traffic area, the mobile automatically registers its new location by calling the new base station and identifying itself. The network then ensures that the location information is updated.

The second feature is in-call hand-off. When a mobile is engaged on a call, it often moves from the coverage area of one base station into another. So that the conversation is not interrupted, the call must be handed-off automatically to the next base station. The base station constantly checks the signal level received from all mobiles in communication and, when the level drops below a threshold, it informs the MSC that a hand-off may be required. The MSC commands all the surrounding base stations to measure the signal strength of the mobile, and then chooses the best cell to transfer the call to. Once the new base station has been informed of the hand-off request, and the radio channel allocation has been made, the original base station is commanded to send a control message to the mobile to move to the new channel. This all happens automatically within a few seconds, and the user of the mobile is aware of only a very brief break in transmission (about 400 ms) when the hand-off proper takes place.

TACS RADIO FEATURES AND PARAMETERS

The cellular system chosen for adoption in the UK, TACS, is an adaptation and enhancement of the Advanced Mobile Phone System (AMPS) adopted in North America. Adaptation of AMPS was necessary as the regulatory position in the UK was different from that in the USA, particularly with regard to radio frequency bands and channel spacing (Table 1).

Radio Channels

The TACS system operates in the CEPT† 900 MHz band and uses up to 1000 radio channels with a 25 kHz spacing between channels. Each TACS channel comprises a pair of frequencies spaced 45 MHz apart; the higher frequency is used for transmission from the base station to the mobile (forward direction), and the lower from the mobile to the base station (reverse direction). The TACS system allows for two entirely independent networks to share the band, each being allocated its own exclusive channels. In the UK, the two competing networks will, by 1989, be allocated a total of 600 channels (300 to each operator), with the remaining 400 being held in reserve for a future pan-European system. However, the allocation of channels has been complicated by the need to allow time for services currently using the band to move frequency. Fig. 6 shows the current allocation to the two networks.

† CEPT—European Conference of Posts and Telecommunications

The TACS system has two types of radio channel: control and speech. For each of the two networks, 21 channels are reserved for control and cannot be used as speech channels. Of the remaining channels, some may be used for control where traffic demands extra control channels; otherwise, they can all be used as speech channels (Fig. 7).

Control Channels

The TACS control channels are used for the co-ordination of mobiles and for all call set-up procedures. Functionally, there are three types of control channel: dedicated control channels (DCCs), paging channels and access channels. In practice, all three functions may be combined and carried on the same channels. As the network grows and the traffic load increases, some functions may be allocated separate blocks of channels to avoid overload.

The DCCs are the most fundamental of the three types of control channel. All mobiles are permanently programmed with the channel numbers of the DCCs for both TACS networks and scan these channels at switch-on. The DCCs carry basic information about the network and inform the mobiles about the channel numbers of the paging channels.

The paging channels are used both to transmit messages to specific mobiles (for instance, to alert them to incoming calls), and general network information such as traffic area identity, channel numbers of the access channels, access methods to be used by mobiles etc.

Finally, access channels enable mobile units to initiate calls, register locations, and respond to paging requests.

Speech Channels

All voice communication is carried on the speech channels; in addition, some signalling (for hand-off, power control, cleardown etc.) is carried.

The speech signal is carried on the speech channels by means of analogue FM with a peak frequency deviation of 9.5 kHz. By comparison, previous radiophone systems with the same channel spacing of 25 kHz have used a frequency deviation of only 5 kHz, in order to minimise interference problems in the adjacent channels. The use of this wider deviation in the TACS greatly improves the rejection of unwanted signals on the same frequency (co-channel interference). Co-channel interference is the most significant limiting factor that determines the cell repeat pattern used,nd thus the ultimate capacity of the system. Reducing the effects of co-channel interference by increasing the deviation thus results in an overall increase in the maximum number of customers.

Increasing the deviation to 9.5 kHz increases the interference to the adjacent channels and, if this effect is too great, the benefit from the use of a wider deviation is negated. However, the adjacent-channel interference can be reduced by careful channel allocation; that is, by ensuring that adjacent channels are never allocated in the same cell, and only to a limited extent in the adjoining cells. It can be shown that, by following these rules and by restricting the deviation to 9.5 kHz, interference can be kept to acceptable levels.

The signalling format used from the base to the mobile on the control channels is of particular interest since it contains three data streams multiplexed together. The complete data stream consists of blocks of 463 bits of information. Bit synchronisation enables mobiles to lock onto the bit stream, and word synchronisation indicates the start of the information frame. The remainder of the block consists of two 40 bit messages (word A and word B) repeated five times each, multiplexed with a BUSY/IDLE bit stream. Each mobile responds only to either the word A or word B stream, depending on whether the identity number of the mobile is odd or even. The BUSY/IDLE stream is used by the base station to indicate whether the reverse signalling channel is free or not, and is checked by mobiles before the system is accessed.

During some phases of call set-up, the mobile needs to inform the base station of the ON/OFF-HOOK status (for example, whilst a mobile is being rung, or during cleardown). Instead of sending a complete signalling frame to indicate simply this status, the mobile transmits an 8 kHz signalling tone when the mobile is required to indicate that it is ON-HOOK.

Supervision

Whilst a call is in progress, a supervisory audio tone (SAT) is transmitted by the base station and looped back by the mobile. Both base station and mobile require the presence of the SAT on the received signal to enable the audio path. Three different frequencies are used for the SAT, all around 6 kHz. During call set-up, the base station informs the mobile which SAT to expect on the speech channel. If the SAT is incorrect, the mobile does not enable the audio path, but starts a timer which, on expiry, returns the mobile to stand-by. Similarly, the base station expects to see its transmitted SAT returned and takes this as confirmation that the mobile is operating on the correct channel.

The three SAT frequencies are allocated to the cells so that the clusters form a three-cluster repeat pattern. Thus a cell in the adjoining cluster using the same radio channel uses a different SAT. The effect of this is to reduce the possibility of a co-channel interfering signal, which, for some reason, is stronger than the wanted signal, being overheard by a mobile.

TACS METHOD OF OPERATION

Mobile Scanning

Mobiles are programmed with the channel numbers of the DCCs of both the primary and secondary networks. When the mobile is switched on, it scans the 21 DCCs of the primary network and measures the signal strength of each. It then attempts to receive overhead messages from the channel with the strongest signal. If this is unsuccessful, it tries the channel with the second strongest signal. If this also fails, the mobile scans the DCCs of the secondary network.

Once the mobile has received the channel numbers of the paging channels, it scans these channels for the strongest signal and attempts to receive overhead messages. If successful, the mobile obtains traffic and network configuration information. If unsuccessful, it tries the second strongest paging channel. If this also fails, the mobile may restart the scanning process.

The process of locking onto a paging channel normally takes between 5 and 10 seconds, but may take up to about 17 seconds if the mobile has to revert to the second strongest DCC or paging channel.

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