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- (Page 13) Cellular Telephone Basics
continued : Bell System Overview
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From: A History of Engineering and Science in the Bell
System: Communications Sciences (1925 -- 1980)
Advanced Mobile Phone Service (continued)
Cellular Concept. Although the MJ and MK automatic systems offer
some major improvements in call handling, the basic problems,
few channels and the inefficient use of available channels still
limit the traffic capacity of these conventionally designed systems.
Advanced Mobile Phone Service overcomes these problems be using
a novel cellular approach. It operates on frequencies in the
825- to 845 MHz and 870-to 890-MHz bands recently made available
by the FCC. The large number of channels available in the new
bands has made the cellular approach practical.
A cellular plan differs from a conventional one in that the
planned reuse of channels makes interference, in addition to
signal coverage, a primary concern of the designer. Quality calculations
must take the statistical properties of interference into account,
and the control plan must be robust enough to perform reliably
in the face of interference. By placing base stations in a more
or less regular grid (spacing them uniformly), the area to be
served is partitioned into many roughly hexagonal cells, which
are packed together to cover the region completely. Cell size
is based on the traffic density expected in the area and can
range from 1 to 10 miles in radius.
Up to fifty channels are assigned to each cell to achieve their regular reuse and to control interference between adjacent cells. This is illustrated in Figure 11-35, where cell A' can use the same channels as cell A. Because of the inverse power law of propagation, the spatial separation between cells A and A' can be made large enough to ensure statistically that a signal-to-interference ratio greater than or equal to 17 dB is maintained over 90 percent of the area. Maintenance of this ratio ensures that a majority of users will rate the service quality good or better.
Cellular systems also differ from conventional systems in
two significant ways:
High transmitted power and very tall antennas are not required.
Wide FM deviation is permissible without causing significant
levels of interference from adjacent channels.
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