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ADJACENT AND CO-CHANNEL INTERFERENCE
Voice communication is carried on the speech channels. In addition, at certain times,
some signaling is also carried on the speech channels.
Speech is carried on the speech channel as an analog frequency-modulated (FM)
signal with a frequency deviation of 9.5 kHz. (More traditional radio telephone systems
using the same 25 kHz channel spacing generally have a maximum frequency deviation of
5 kHz. This approach was taken to minimize interference problems in adjacent channels.)
The use of higher deviation in cellular radio greatly improves the rejection of
unwanted signals on the same frequency (co-channel interference). Co-channel
interference is the most significant limiting factor determinir^ the cell-repeat pattern
used. However, there has to be a compromise. Increasing the deviation increases the
interference to adjacent channels, and if this effect becomes too large it will negate the
effect of using higher deviations. This can be controlled by careful channel allocation, for
example ensuring that adjacent channels are never allocated in the same cell.
CONCLUSIONS
This newsletter has given a brief overview of the general principles of cellular radio.
While cellular radio is not a recent idea,, the practicalities of planning and operating such
high-capacity systems have been rapidly learnt. Further advances in antenna design, for
example, have enabled capacity to be increased by further dividing urban cells
(sectorization).
The rapid move towards an all-digital technology will again increase the utilization
of the available frequency spectrum. Further techniques still to be honed into practical
solutions lie in the areas of modulation techniques and reduced channel spacing.
The number of cellular subscribers has grown considerably over the past seven years,
and is still growing—proof that there is a considerable futvire market to exploit this
technology further.
(This document is reprinted with the permission of Dataquest's European
Telecommunications Industry Service.)
ESAM Newsletter © 1989 Dataquest Incorporated July
Jim Eastlake
Ted Richardson