Coherent and differentially coherent multiuser detectors for asynchronous CDMA frequency-selective channels

The authors consider an asynchronous CDMA (code-division multiple-access) frequency-selective Rayleigh fading channel in which a central receiver has knowledge of the signature waveforms of all users. Low-complexity coherent and differentially coherent multiuser detectors are analyzed for this scenario. The receivers consist of a bank of matched filters followed by a multipath decorrelating detector, appropriate multipath diversity combining, and demodulation. The performance of the derived detectors is expressed by the single-user error rate. The conventional single-user RAKE receiver suffers from near-far effects in the multiuser scenario, due to the fading nature of the channel. This results in an error probability floor for high signal-to-noise ratios and limits the multiple-access capability of the system. In contrast, both multiuser detectors alleviate the near-far problem, providing significant performance improvement in the CDMA frequency-selective fading channel.