2.1 Introduction

Most CDMA modulators bear a structure similar to that depicted in Figure 1.3 and differ only in the manner in which they choose the spreading codes and timing. On the other hand, CDMA demodulators can be quite different depending on the multipath channel conditions and performance requirements. When the CDMA system is wideband, (1.8) indicates that the multipath induced interchip interference (ICI) can alter the spreading waveforms of transmitted signals. Without a proper receiver, the ICI may have a strong negative impact on the system capacity.

For CDMA with short codes, multiuser detection can be employed to enhance the reception performance by demodulating multiple signals at the same time. However, almost all multiuser detectors require explicit knowledge of users' spreading, timing, and, in the case of wideband CDMA, the frequency-selective channel coefficients. Because the system is time varying, this necessitates the use of training sequence or some computationally intensive channel parameter estimation algorithms. To save bandwidth and cost, there is an evident need for lower complexity receivers that can perform detection without explicit knowledge of the channel parameters.

An extreme choice would be the RAKE receiver, applicable to CDMA with any spreading codes, which coherently combines the signal-of-interest (SOI) scattered in the time domain. Despite its low complexity, we already point out in Chapter 1 that the conventional RAKE receiver relies exclusively on the statistics of the desired user and thus has fundamental difficulties in handling the MAI. For high-performance communication, it is necessary to find a better...