In wireless communications, fading is the deviation of the signal attenuation affecting a certain propagation media. Discoloration may vary with time, the geographical position or frequency of the radio, which is often modeled as a random process. A fading channel is a communication channel experiencing fading.
In wireless systems, fading can be either due to multipath, called as multipath fading or due to shadowing from obstacles affecting the wave propagation, known as shadow fading. Here in this chapter, we will discuss how multipath fading affects the reception of signals in CDMA.
CDMA systems use a signal fast chip rate for spreading the spectrum. It has a high time resolution, due to which it receives a different signal from each path separately. The RAKE receiver prevents signal degradation by summing all the signals.
Because CDMA has high time-resolution, different paths delay the CDMA signals, which can be discriminated. Therefore, energy from all paths can be summed by adjusting their phases and path delays. This is a principle of RAKE receiver. By using a RAKE receiver, it is possible to improve the loss of the received signal due to fading. It can ensure a stable communication environment.
In CDMA systems, multi-path propagation improves the signal quality by using RAKE receiver.