Fuzzy Inference Based Adaptive MMSE-MRC Multiuser Detection of DS-CDMA Signals

Abstract

Direct-sequence code-division multiple access (DS-CDMA), a specific form of spreadspectrum
transmission, has been adopted as the multiaccess technology for nonorthogonal
transmission in the third-generation (3G) mobile cellular systems. DS-CDMA techniques
does not requires time or frequency coordination among the mobile stations, rather a unique
spreading code is assigned to each user to identify the user to the system, which is used to
modulate and demodulate the user’s data.. However the performance of DS-CDMA system is
limited by multiple-access interference (MAI) and multipath channel distortion. The
conventional matched filter detector fails to combat these problems and optimal receivers are
far too complex for practical implementations and hence several suboptimal receivers [3l-33]
for multiuser communications have been proposed. One of the approaches to reduce the MAI
is adaptive multiuser detection [39] which employs adaptive technique called least mean
square (LMS) and recursive least square (RLS) algorithms [41]. The typical operation of
those adaptive multiuser detectors requires each transmitter to send a training sequence at
startup which the receiver uses for initial adaptation. After the training phase ends, adaptation
during actual data transmission occurs in decision-directed mode. However, any drastic
change in the interference environment, a fresh training sequence is required. Therefore the
diversity scheme called maximum ratio combining (MRC) [15] as preprocessor to the
adaptive MMSE detector of DS-CDMA system in multipath channel is proposed which
eliminates the use of training sequences.