Daryl Reynolds, H. Vincent Poor, and Xiaodong Wang
The three primary characteristics that distinguish wireless communications from wireline communications are dynamism, fading, and interference. In this chapter, we address all three of these characteristics by considering adaptive methods for multiuser detection in fading channels.
Fading refers to variations in the gain of a communications channel. The primary source of fading is multipath, which leads to constructive and destructive self-interference of signals at a communications receiver. Depending on various properties of the communication link (bandwidth, mobility, etc.) fading can vary with time (i.e., time-selective fading) and frequency (frequency-selective fading). Of course, given the fact that the multipath profile depends on geometry, fading is also dependent on position and angle of arrival.
A key issue in the mitigation of fading is that the fading is caused by the channel, which is not known a priori to the receiver. There are two approaches to dealing with this issue. One is to treat the channel gains as random quantities with known distributions (e.g., Rayleigh, Rician, etc.) and to devise optimal receiver algorithms based on this statistical model. Examples of this approach can be found in [27,42,59]. An alternative approach, which is considered here, is for the receiver to use the outputs of the channel to adapt to the fadings, i.e., adaptive systems ...