4.1 Introduction

For CDMA with long spreading codes where joint detection is problematic, the primary way to counter fading is to combine multipath signals from the desired user using a RAKE receiver. Since introduced in [1], RAKE receivers that constructively exploit the multipath signals have been demonstrated to be very effective in alleviating multipath fading. When multiple antennas are available at the basestation, 2D RAKE receivers (i.e., space-time RAKE receivers) can be employed to profit from both the spatial and the temporal diversities [2].

Previous work on RAKE reception often assumes timing information of all multipath reflections [2]. The underlying idea is to enhance the signal strength by coherently combining all multipath reflections that stemmed from the desired user. A RAKE receiver thus has the same front structure as the optimum receiver depicted in Figure 1.10, except in RAKE signal detection is performed without accounting for outputs from other matched filters. A practical difficulty of this scheme is that in the presence of a large number of active users, multipath delay estimation is nontrivial [3]. Secondly, even if all multipath delays, {? _l} are available at the basestation, signal processing on a particular multipath signal is prohibitive due to the resolution constraintof discrete samples. In addition to these limitations, the matched filter-based receiver may not be the most efficient way to use RAKE structure in the first place. Many other ways of combining (e.g., the frequency-domain approach by Zoltowski et al. [4]) are more effective in simultaneously incorporating the...