 Section 2.3.3 showed research results on optimum receivers that simultaneously demodulate all multiple-access signals. Here, the receiver treats multiple-access signals as one larger composite signal and attempts to demodulate all the signals with one optimal processor. Theoretical Shannon limits show that spread-spectrum radios in a multiple-access system permit capacity beyond what is possible with completely orthogonal signals. That is, it is possible to exceed the capacity limit set by the number of orthogo- nal coordinates such as frequency slots (frequency dimensions) in FDMA and time slots (time dimensions) in TDMA. Most of these earlier studies were primarily theoretical, although some more practical suboptimal receiver structures were considered. With the recent renewed interest in spread-spectrum radios and their potential capac- ity advantage, there has been a dramatic increase in the number of papers examining optimal and suboptimal CDMA receiver structures with higher overall capacity [19, 27-40]. Interference is now viewed differently than in the past. Most early stud- ies assume a receiver design based on the classical theory of noise interfer- Summary ence. In most high-density radio networks, capacity is limited not by noise but by mutual interference. With spread-spectrum multiple-access signals, the so-called interference is not noise, but is other similar signals. Unlike clas- sic receivers designed to reduce noise, more complex optimal receivers take advantage of the specific nature of multiple-access signals to achieve much higher capacity. For example, even in its simplest form, one can take advantage of the fact that the interference signal, unlike noise, possesses some non-random com- ponents that remain constant for each bit time interval. Suggested approaches have included adaptive noise cancellation and adaptive filter- ing techniques such as those used in telephone channel equalization [39, 40]. More advanced but complex techniques can improve performance to increase overall capacity