TABLE OF CONTENTS The conventional single-user direct-sequence receiver of Figure 2.14 is optimal against multiple-access interference only if the spreading sequences of all the interfering signals and the desired signal are orthogonal. Orthogonality is possible in a synchronous communication network, but in an asynchronous network, it is not possible to find sequences that remain orthogonal for all relative delays. Thus, the conventional single-user receiver, which only requires knowledge of the spreading sequence of the desired signal, is suboptimal against asynchronous multiple-access interference. The price of the suboptimality might be minor if the spreading sequences are carefully chosen and the noise is relatively high, especially if an error-control code and a sector antenna or adaptive array are used. If a potential near-far problem exists, power control may be used to limit its impact. However, power control is imperfect, entails a substantial overhead cost, and is not feasible for peer-to ?peer communication networks. Even if the power control is perfect, the remaining interference causes a nonzero error floor, which is a minimum bit error probability that exists when the thermal noise is zero. Thus, an alternative to the conventional receiver is desirable.
A multiuser detector is a receiver that exploits the deterministic structure of multiple-access interference or uses joint processing of a set of multiple-access signals. An optimum multiuser detector almost completely eliminates the multiple-access interference and, hence, the near-far problem, thereby rendering power control unnecessary, but such a detector is prohibitively complex to implement, especially when long spreading...
