It was recognised however that there were differences between the UK and US radio environments and operational requirements, so it was necessary for some adaptation before the system could be used in the UK. The modified system is now known as total access communications system (TACS). To ensure full compatibility between the two competing networks, a joint radiophone technical interfaces group was set up under the auspices of the Department of Trade and Industry to define the key interfaces for the T ACS system. The group has included representatives from the Telecommunications Division and the Radio Regulatory Division of the DTI, British Telecom, Racal and the British Telecom/ Securicor joint venture company. \'V or king to a very tight timescale the group has produced the required specifications so that work may proceed towards the development of the cellular systems and of the mobile equipment. By next year it is expected that there will be several manufacturers producing mobile equipment, giving customers a wide choice. Cellular radio will, without doubt, result in large growth in mobile telephony and should satisfy the demand for mobile services for some years to come. The handportable radiophone may well revolutionise attitudes towards the mobile service and in future years it may become common to see people using handportables in the street. It is also likely that the mobile service will become a carrier for a host of new services as customers start to demand more from the telephone network than simple telephony. Mr M. S. Appleby Is an executive engineer In Development and Procurement's Systems Evolution and Standards Department working on radiophone standards.  

British Telecom's first automatic radiotelephone service was opened

during the summer of 1981 to supplement the already popular

manual service which had been operating for some years. Initially

service was offered only to London customers with some access in the

provincial areas already covered by the manual service, but since then

the system has expanded and now covers many of the high population

centres of the country. Between them, the manual and automatic

systems offer service to thousands of customers.

The present British Telecom systems

have an excellent performance record,

reflecting the continuous programme of

refinement which has been followed over

the years. But while that process will be

continued, overall capacity in the largest

markets - London - will be ultimately

limited by the number of radio channels

available in the heavily used 160 MHz

band. The spectrum available to the

British Telecom Radiophone service is

used economically, with channel

spacings of 12.5 kHz and there is some

scope for expansion. But there has to be

an unwelcome compromise between

geographic coverage and the allocation·

of channels to London and its environs

leading to shortfalls which will persist

for the foreseeable future.

In 1979 the World Administrative

Radio Conference (WARC)- held every

25 years to allocate radio frequency spec·

trum on a global basis- made available a

new frequency band for mobile radio

around 900 MHz. The size of the new

band- 1,000 channels in the European

allocation - made its exploitation very

attractive. By comparison, the present

allocation for radiophone in the 160

MHz band is a little over 100 channels.

British Telecom decided therefore to

begin some preliminary work so that a

proposal for a new cellular radiophone

service could be submitted to the Depart·

ment oflndustry.

Cellular radio is not a new concept. The

fundamental ideas were originally pro·

posed by Bell Telephone Laboratories

soon after the second world war although

at the time the technology available did

not allow implementation. Indeed it is

only recently that working cellular

systems have been developed for public

use. Among the countries currently

operating systems arc Japan, the four

Nordic countries, and North America,

where the go·ahead has recently been

given for a large expansion in cellular

radio.

The attraction of cellular radio IS Its

ability to cater for a wide range of traffic

loading and ultimately, to handle far

more customers than present noncellular

systems. The cellular concept is

to divide the required coverage area for

the system into smaller areas, or cells,

each with its own radio base station.

These cells ar􀂋 then formed into groups,

or clusters, wnh typically seven or nine

cells to a cluster.

The available radio channels are divided

equally in a fixed pattern between the

cells in a cluster, and the pattern repeated

to fill the whole coverage area. In this way

each radio channel may be used several

times throughout the system, but

because the distance between base

stations using the same channel is large

compared with the size of the cells . '

mterfcrence can be kept to an acceptably

low level.

Within each cell the number of

simultaneous calls which can be handled

is limited by the number of radio channels allocated to the radio base station. If

the size of each cell is reduced, there will

be more cells in a given area, so the total

number of available channels within the

area will be increased with a corresponding

increase in the maximum number of

simultaneous cells that can be carried.

The smaller the cells, the higher will be

the overall capacity of the system. In

practice, however, variations in radio

propagation will tend to limit the

minimum achievable cell size to a radius

of about two kilometres.

The size of a cell is controlled by careful

planning of the base station location, the

height and type of aerial, and the powertransmitted. It is also necessary to control

the power transmitted by the mobiles so

that they do not cause unacceptable levels

of interference to nearby cells using the

same channels. This is normally done by

remote control from the base stations.

Cellular systems have two key features

which set them apart from non-cellular

systems and both are required because

mobiles move from one cell to another as

they move through the coverage area.

The first feature is mobile tracking, or

location. When a call for a mobile is

received from the telephone network, the

radiophone system must be able to find

which base station the mobile is nearestto so that the call can be connected successfully.

In a system which might consist

of several hundred cells it would not

be feasible to send our a call to the mobile

in every cell, as the high resultant loading

on the calling channels would limit the

capacity of the system. Instead the system

keeps a record of the current location of

each mobile so that calls can be sent

directly to the correct base station.

When the mobile is not making a call it

constantly listens to one of a number of

special channels designated for control

purposes, checking from rime to time

that it is listening to the best possible

channel. Part of the information transmitted by the base stations on these

control channels is a number signifying

the area in which the mobile is located.

When the mobile detects a change in this

number, indicating that it has moved into

a new area, it automatically tells the

radiophone system which will then up·

date the information held about the

mobile's location.

The second feature is its 'in-call handoff.

When a mobile is engaged on a call it

may move from one cell to another. So

that communication is not interrupted

the mobile must be 'handed-off from the

cell it is leaving to a new radio channel in

the cell it is approaching. The system

constantly checks the signal level received

from mobiles in conversation and

detects when the mobile is leaving a cell.

It then commands all surrounding cells

to measure the signal strength of the

mobile and chooses the best cell to which

to 'hand-off the mobile. A short control

signal to the mobile completes the

process, which happens entirely without

the user noticing.

Although the cellular system and the

interactions required with the mobiles

are complex, the use of sophisticated

microprocessor control within the

mobiles means that the customer is pres·

ented with easy-to-use equipment. The

processes of mobile tracking, hand-off

and power control are automatic and occur

without the customer's knowledge or

intervention. The only controls he needs

to use are the keypad for putting in

telephone numbers, and a 'send' key to

initiate the call.

Cellular radio mobiles will not be

restricted to use with cars. The cellular

system will also allow the use of genuine

handportable radiotelephones. The main

difficulties with hand portable design are

the conflicting requirements of small size

and low weight but with a useful battery

life. A small-cell cellular system where

mobiles are only required to use low

power levels is an ideal environment for

the portable telephone. Thus a device

little larger or heavier than a cordless

telephone handset can be used in a

cellular system as a carry-anywhere

telephone.

In 1982, the Secretary of State for

Industry (now Trade and Industry)

announced that Britain was to enter the

cellular radio field and further that the

UK would have two competing services,

one to be run by a joint venture company

formed by British Telecom and

Securicor, and the other by a consortium

called Racal-Millicom. It was also

specified that the two competing services

be compatible so that customers would

be free to choose between them. The

target date for introducing the systemswas set for the beginning of next year.

Following detailed technical discussions

on the type of cellular system best

suited for adoption in the UK, it was announced

last February that the British

system would be based on the North

American system AMPS (advanced

mobile phone system). It was recognised

however that there were diffefences

between the UK and US radio environments

and operational requirements, so

it was necessary for some adaptation

before the system could be used in the

UK. The modified system is now known

as tctal access commun ications system

(TACS).

To ensure full compatibility between

the two competing networks, a joint

radiophone technical interfaces group

was set up under the auspices of the

Department of Trade and Industry to

define the key interfaces for the T ACS

system. The group has included

representatives from the Telecommunications

Division and the Radio

Regulatory Division of the DTI, British

Telecom, Racal and the British Telecom/

Securicor joint venture company.

\'V or king to a very tight timescale the

group has produced the requiredspecifications so that work may proceed

towards the development of the cellular

systems and of the mobile equipment. By

next year it is expected that there will be

several manufacturers producing mobile

equipment, giving customers a wide

choice.

Cellular radio will, without doubt,

result in large growth in mobile

telephony and should satisfy the demand

for mobile services for some years to

come. The handportable radiophone

may well revolutionise attitudes towards

the mobile service and in future years it

may become common to see people using

handportables in the street. It is also likely

that the mobile service will become a

carrier for a host of new services as

customers start to demand more from the

telephone network than simple

telephony.

Mr M. S. Appleby Is an executive engineer In Development and Procurement's Systems Evolution and Standards Department working on radiophone standards.