Space-time iterative MMSE multiuser detection in frequency selective coded CDMA wireless mobile channels

Iterative multiuser detection in coded CDMA uplink channels is based on the iterative exchange of soft information between a (multiuser) demodulator and a bank of single-user soft decoders to mutually benefit them. From a practical perspective, using the MMSE multiuser interference suppressor for demodulation yields good performance over a wide range of received signal powers. However, the progressively increasing correlation between estimates (with an increasing number of iterations) limits the improvement that can be obtained via further iterations, implying that the suboptimality of the MMSE demodulator plays a role in the performance levels that can be achieved. An immediate enhancement of the demodulator´s performance is obtained by introducing multiple sensors at the receiver; the resulting space-time demodulator now operates on received signal mixtures with components whose spatial separation is accounted for, by virtue of their different directions of arrival. Since this separation is limited in practice, by restrictions on sensor count, array geometry, and the receiver´s inability to control arrival directions, a multisensor block modulation scheme is proposed. This scheme transmits cyclically block-shifted versions of bit sequences from additional sensors using different random spreading sequences, and exploits the variation in the channel response between various transmission instances of a given bit. An iterative multiuser channel state estimator is also developed. Performance studies indicate significant reductions in the decoded bit error rate-up to that of a single user at moderately low received signal powers-and hence vastly increased spectral efficiencies