Natural gradient based blind multi user detection in QPSK DS-CDMA system

Blind Multi User Detection (BMUD) is the process of simultaneously estimating multiple symbol sequences associated with all users in the downlink of a Code Division Multiple Access (CDMA) communication system using only the received data. We propose to apply Natural Gradient based online Blind Source Recovery (NGBSR) techniques using either the feedforward or the feedback symbol recovery structures to achieve the global task of BMUD for QPSK DS-CDMA systems. The quasi-orthogonality of the spreading codes and the inherent independence among the various transmitted user symbol sequences form the basis of the proposed BMUD methods. The application of these algorithms is justified since a slowly fading multipath CDMA environment is conveniently represented as a linear combination of convolved independent symbol sequences. The proposed structures and algorithms demonstrate promising results as compared to the conventional detection techniques comprised of matched filters (MF), RAKE, subspace LMMSE and LMMSE receivers. Illustrative simulation results compare the BER performance of the two proposed online BMUD structures to the conventional detectors.