A Multipath Detection Scheme for CDMA Systems With Space–Time Spreading

Space-time spreading (STS) is an appealing open-loop transmit diversity scheme, which has recently been included into the cdma2000 standard. It has been shown that the performance of the STS scheme is highly sensitive to fading coefficient estimation errors, particularly when the channel is highly time dispersive. In practical systems, channel estimation is normally performed after the multipath components are resolved, which suggests that improving multipath detection reduces such estimation errors. Motivated by this, we address, in this paper, the problem of multipath detection in STS-based code division multiple access (CDMA) systems. We first extend the conventional energy-based multipath detection scheme (EMDS) to cope with the spatial channel structure. We derive approximate expressions for the probability of detection and probability of false alarm. It is shown that the errors produced by the conventional scheme in detecting the potential multipath components severely impact the performance of the receiver. To improve upon the EMDS, we introduce and analyze an improved multipath detection scheme (IMDS) based on the estimation of the interference power in the individual resolved multipath components. The efficacy of the proposed scheme stems from the fact that the interference in each potential path is estimated and subtracted before that path is detected. We also present a simple and realizable version of the proposed IMDS detection scheme. Our results show that the proposed scheme not only improves the bit-error-rate performance significantly but also utilizes the pilot power much more efficiently.