s enhanced with the desired signal seven times. This is the output of the modulo-2 adder, which has an output voltage seven times greater than the input voltage of one chip. CELLULAR AND PCS RADIO SYSTEMS Cross- Cross- Cross- correlate correlate correlate with (s)t with (s)t with (s)t Diversity combiner Figure 18.13 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. FREQUENCY REUSE Because of the limited bandwidth allocate