CELLULAR AND PCS RADIO SYSTEMS
A typical RAKE receiver used with direct sequence spread spectrum reception.
In Figure 18.12c we show the correlation process collapsing the spread signal spectrum
to that of the original bit spectrum when the receiver reference signal, based on the
same key as the transmitter, is synchronized with the arriving signal at the receiver. Of
overriding importance is that only the desired signal passes through the matched filter
delay line (adder). Other users on the same frequency have a different key and do not
correlate. These "other" signals are rejected. Likewise, interference from other sources is
spread; there is no correlation and those signals are also rejected.
Direct sequence spread spectrum offers two other major advantages for the system
designer. It is more forgiving in a multipath environment than conventional narrowband
systems, and no intersymbol interference (ISI) will be generated if the coherent bandwidth
is greater than the information symbol bandwidth.
If we use a RAKE receiver, which optimally combines the multipath components as
part of the decision process, we do not lose the dispersed multipath energy. Rather, the
RAKE receiver turns it into useful energy to help in the decision process in conjunction
with an appropriate combiner. Some texts call this implicit diversity or time diversity.
When sufficient spread bandwidth is provided (i.e., where the spread bandwidth is
greater or much greater than the correlation bandwidth), we can get two or more indepen-
dent frequency diversity paths by using a RAKE receiver with an appropriate combiner
such as a maximal ratio combiner. Figure 18.13 is a block diagram of a RAKE receiver.
Because of the limited bandwidth allocated in the 800-MHz band for cellular radio com-
munications, frequency reuse is crucial for its successful operation. A certain level of
interference has to be tolerated. The major source of interference is cochannel interfer-
ence from a "nearby" cell using the same frequency group as the cell of interest. For the
30-kHz bandwidth AMPS system, Ref. 5 suggests that C/I be at least 18 dB. The primary
isolation derives from the distance between the two cells with the same frequency group.
In Figure 18.2 there is only one cell diameter for interference protection.
Refer to Figure 18.14 for the definition of
R and D. D is the distance between cell
centers of repeating frequency groups and
R is the "radius" of a cell. We let
a = D/R.
The D/R ratio is a basic frequency reuse planning parameter. If we keep the D/R ratio
large enough, cochannel interference can be kept to an acceptable level. Lee (Ref. 7) calls
a the cochannel reduction factor and relates path loss from the interference source to R