On Adaptive MMSE Receiver Strategies for TD-CDMA

In this thesis a modified implementation of the adaptive minimum mean squared error (MMSE) receiver for a time division code division multiple access (TD-CDMA) system for third generation mobile communications is presented. This implementation can operate with spreading sequences which span over a few symbols and in environments where more than one spreading code is allocated to a single user. Two structures which combine the presented MMSE structure and the Rake receiver are also presented in an attempt to combine the advantages of both structures. After analysing the effect on a direct sequence spread spectrum system of multiple access interference and multipath fading induced inter-chip interference, the existing techniques for multiple access interference suppression capabilities are reviewed. Special attention is paid to the adaptive MMSE receiver, which takes into account the effect of multipath fading without requiring any additional channel estimation strategy. It is also suitable to be implemented in the mobile-station since it does not require any knowledge of other users’ parameters. The modified adaptive MMSE and MMSE-Rake structures operating in different modes are tested in a downlink multiuser environment under three different multipath Rayleigh propagation conditions for different values of the spreading factors. Its complexity and performance in terms of bit error rate are compared to those of other well known receiver structures, namely, the traditional DS-CDMA matched filter receiver (different phase estimation techniques are considered the linear equaliser and the Rake receiver. The thesis shows that considering a DSP implementation the adaptive MMSE receiver presents the best performance complexity ratio of the considered structures except in the case where multipath components lie outside the filter span. The adaptive MMSERake receivers present a high computational complexity but do not improve the performance significantly due to the interference caused by multipath components outside the filter span in the different fingers or to their slow convergence speed.