Combined Space-Time Coding and Multiuser Detection for DS-CDMA Systems

Digital Communications In a downlink DS-CDMA system with orthogonal signature sequences the performance is limited by multipath propagation, which introduces both multi-access interference (MAI) and intersymbol interference (ISI). Multi-user detection (MUD) techniques, which to a large extent remedy these effects, are well known. They combine interference rejection and equalization by designing appropriate receivers only. We consider joint optimisation of both signature sequences and receivers filters in order to improve the overall performance. The joint optimisation is performed to minimize the mean-squared-error (MSE) from the input to the output while keeping the total transmitting power fixed. In the formulation of the optimisation problem the transmitter (the base-station) requires knowledge of all the downlink propagation channels, which has clear practical limitations. However, for a time-division-duplex (TDD) system the channel impulse responses can be estimated from uplink transmission using the channel reciprocity. Alternatively, the estimated channel impulse responses can be fed back from the receivers to the transmitter, which requires that the channels are sufficiently slowly time varying so that the amount of feedback information is low compared to the data-rate. In our formulation of the problem we use a filtering approach to accommodate for the ISI and to avoid processing complete data bursts together. We minimize the sum of the MSEs at the output of the receiver filter while keeping the transmitting power fixed. By means of simulations we demonstrate a large performance gain over a traditional CDMA system where only the receivers are optimised, see CPK Annual Report 2001 - Figure 4.4.1.2 (Signal-to-noise and interference ratio (SNIR) versus the signal-to-noise ratio achieved by the scheme performing "joint optimisation", the conventional matched filter (MF) receiver, and the minimum mean-square error (MMSE) receiver). (Joachim Dahl, Bernard H. Fleury, Søren H. Jensen)