Carphone 1983

BT Journal Winter 1983/84 - Towards Cellular Radio by Malcolm Appleby

See pdf in files section Files

British Telecom is playing a major role in developing cellular radio- a concept which will extend the availability of mobile telephony throughout the country.

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.
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