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One of the most promising techniques for mobile communication systems is DS-CDMA. However, the DS-CDMA system is prone to multiple access interference (MAI) due to the nonorthogonality of the users' signatures. This problem can be mitigated by the use of multiuser detection techniques. Several multiuser receivers have been proposed that require knowledge only of the parameters of a specific desired user. In CDMA systems, users spread their signals by modulating the information bitstreams with codes called signature sequences. Since user delays are random, maintaining the orthogonality of users' signatures requires codes that are pairwise orthogonal for any arbitrary delay. This is a difficult task, and even with the best design, the codes will have nonzero cross-correlations for an arbitrary delay. The nonorthogonality of users' signatures will cause a problem known as the near-far effect, in which nearby users who have higher powers degrade the performance of faraway users who have lower powers. Two techniques are proposed to defeat the near-far problem: multiuser detection and power control. Loosely speaking, multiuser detection involves designing receiver filters to suppress the interference of other users relative to the user of interest. Power control is concerned with balancing the users' powers for a given receiver filter. This chapter addresses the design of different schemes for multiuser detection with fixed received powers.
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