Frequency Reuse

The concept and implementation of frequency reuse was an essential element
in the quest for cellular systems to have a higher capacity per geographic
area than an MTS or IMTS system. Frequency reuse is the core
concept defining a cellular system and involves reusing the same frequency in a system many times over. The capability to reuse the same radio frequency
many times in a system is the result of managing the C/I signal levels
for an analog system. Typically, the minimum C/I level designed for in a
cellular analog system is 17 dB C/I.

In order to improve the C/I ratio, the reusing channel should be as far
away from the serving site as possible so as to reduce the interferer component
of C/I. The distance between reusing base stations is defined by the
D/R ratio, which is a parameter used to define the reuse factor for a wireless
system. The D/R ratio, shown in Figure 2-9, is the relationship between
the reusing cell site and the radius of the serving cell sites. Table 2-2 illustrates
standard D/R ratios for different frequency reuse patterns, N.

As the D/R table implies, several frequency reuse patterns are currently
in use throughout the cellular industry. Each of the different frequency
reuse patterns has its advantages and disadvantages. The most common
frequency reuse pattern employed in cellular is the N
7 pattern, which is
shown in Figure 2-10.

The frequency repeat pattern ultimately defines the maximum amount
of radios that can be assigned to an individual cell site. The N
7 pattern
can assign a maximum of 56 channels that are deployed using a threesector
design.