Global Positioning System Reference
In-Depth Information
Figure 6.10
Simple phased array to determine angle of arrival of radio signal.
6.2.8
Trunked Radio and Cellular Radio
When cellular radio evolved from fixed telephony, the trunking (which was
performed in a hierarchy of exchanges or switches) had to be modified to deal
with the problems of radio. Radio frequency bands (which are inherently scarce
resources that are highly regulated, allocated, and more recently auctioned
commercially) must be tightly controlled and managed with maximal spectral
efficiency. In other words, operators and their systems seek to maximize the
number of uses per square kilometer for any given block of spectrum (without
disrupting by interference any other users).
The way this is achieved is to allocate radio channels dynamically under
computer control to those who need them on demand from a common pool. The
channels can be reused but only at distances where the energy from transmissions
has dropped to a level where it is negligible (or “in the noise”). This type of
interference is known as cochannel interference and can be engineered by
carefully sighting base stations, antennae, and controlling power levels. The term
“cellular” became common as the (theoretical) pattern of channel reuse can
resemble the cells of a honeycomb.
