3.8 Multipath Revisited and the Rake Receiver

As discussed in Chapter 1, RF signals traveling from the transmitter to the receiver rarely travel only a single path directly to the receiving antenna, but instead tend to travel multiple paths, bouncing off numerous surfaces scattered between the two locations. In Chapter 1 we pointed out that previous-generation (analog) receivers lacked any means of distinguishing one signal arriving via one path from any other. As a result, all these multipathed signals were simply summed together at the receiver, which often resulted in a weaker or a degraded signal due to incoherent phases between all these various "multipaths."

One technique we use in CDMA to help mitigate some of the effects of multipath mentioned in Chapter 1, is to isolate each separate signal as it arrives at the receiver by exploiting the fact that our bit stream has been effectively "tagged" by, or interwoven into, the spreading code. This allows us to isolate each signal arriving at the receiver from all the others, including the multipathed signals. If we could construct a receiver with several front ends, and dedicate each front end to tracking each multipath reflection, we could then tie their outputs together. Using this approach, we are not only able to reverse the degradation due to multipath, but also use it to recover a great deal more of the original signal's energy that previously was either lost or destructively interfered with our arriving signal. Granted, this approach requires a great deal...