
Cellular Radio 
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Cellular radio is a radiotelephone communications system that began to develop
rapidly in the early 1980s. Basically it is a small-scale, "cellular" version of
the communication linkage provided by large radio-broadcasting systems. A
metropolitan area is divided up into smaller units, called cells, that have a
radius of about 13 to 19 km (8 to 12 mi); each cell has its own small radio
transmitter. If necessary, a cell can be further divided into smaller cells that
have a radius of about 1.5 to 2.4 km (1 to 1.5 mi). In this way, the honeycomb
pattern of cells can repeatedly use the same range of radio frequencies without
interfering with one another, so long as neighboring cells do not use precisely
the same radio channels. Persons equipped with small mobile phones can make
calls by means of this system in the same way that telephone calls are made
using standard carriers. 
Computerized switching is essential to the operation of cellular radio, so that
when mobile-unit calls are switched from one cell to the next, the transfer in
channels can take place without interruption or at most a brief delay. Two
separate radio channels are used for each pair of transmitters and receivers,
and three-way calls are sometimes also possible. Calls are initiated as are
standard telephone calls, using seven digits (or ten, if another area code is involved). 
The concept of mobile telephone units dates back to the earliest days of radio,
but for several decades such phones were restricted by the limited availability
of frequencies from large-scale carriers. Systems such as ham radio and
citizens'-band radio were also restricted to one-way communication. With the
growth of electonic switching systems and the development of MICROPROCESSORS,
however, the cellular approach to mobile telephone service began to be explored
in the 1970s. In the United States, the Federal Communications Commission (FCC)
granted a license to AT&T to test such a system in Chicago in 1978, and others
have been placed in operation since then. The FCC licenses two competitors (one
a telephone company and the other a radio communications company) to serve a
given area. It has set aside a radio spectrum of 40 megahertz (MHz) to divide
between them, each band of 20 MHz being able to carry 333 channels. 
Other telephone services in standard use today, such as message-waiting,
call-forwarding, and data transmission, are being introduced into cellular
radio, and mobile phones are finding increasing business and commercial
applications. During the 1980s a typical phone bill would amount to about $150
per month, although the minimum could be about half that cost. Cellular radio
has been in operation in major cities of several other countries for a number of
years.
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