| WEB PROFORUM TUTORIALS PRINCIPAL
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Cellular
Communications |
2. Mobile Telephone
System Using the Cellular Concept
Interference problems caused by mobile units using the same
channel in adjacent areas proved that all channels could not be
reused in every cell. Areas had to be skipped before the same
channel could be reused. Even though this affected the efficiency of
the original concept, frequency reuse was still a viable solution to
the problems of mobile telephony systems.
Engineers discovered that the interference effects were not due
to the distance between areas, but to the ratio of the distance
between areas to the transmitter power (radius) of the areas. By
reducing the radius of an area by 50 percent, service providers
could increase the number of potential customers in an area
fourfold. Systems based on areas with a one-kilometer radius would
have one hundred times more channels than systems with areas 10
kilometers in radius. Speculation led to the conclusion that by
reducing the radius of areas to a few hundred meters, millions of
calls could be served.
The cellular concept employs variable low-power levels, which
allow cells to be sized according to the subscriber density and
demand of a given area. As the population grows, cells can be added
to accommodate that growth. Frequencies used in one cell cluster can
be reused in other cells. Conversations can be handed off from cell
to cell to maintain constant phone service as the user moves between
cells (see Figure 3).
The cellular radio equipment (base station) can communicate with
mobiles as long as they are within range. Radio energy dissipates
over distance, so the mobiles must be within the operating range of
the base station. Like the early mobile radio system, the base
station communicates with mobiles via a channel. The channel is made
of two frequencies, one for transmitting to the base station and one
to receive information from the base station.
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