Time diversity is the process of sending the same signal or components of a signal through a communication channel where the same signal is received at different times. The reception of two or more of the same signal with time diversity may be used to compare, recover, or add to the overall quality of the received signal.

The cause for time diversity may be naturally created or it may be self-induced. Delayed signals may be created by the reflection of the same signals by objects (such as buildings or mountains). When multiple signals are received that take different paths (direct and/or reflected), it is called multipath. In some cases, it is desirable to self-create multiple delayed signals in the transmitter. The creation of multiple delayed signals can be used to overcome the effects of signal fading. In either case, the CDMA uses a receiver that is capable of decoding two or more signals that are delayed in time. This receiver is called a rake receiver.

Figure 1.8 shows how the IS-95 CDMA system can use a rake receiver to combine multiple time delayed (multipath) signals to help produce a higher quality received signal. In this example, the same signal is received at the mobile device because one of the signals has been reflected off a building during transmission. This reflected signal has to travel a longer distance so it is delayed slightly when the mobile device receives it. Because each signal is identified by a unique code, the...