Iterative MMSE Multiuser Detection for Coded CDMA Channels with Multisensor Transmitter and Receiver Arrays
Iterative processing enormously improves the performance of multiuser demodulation in coded CDMA uplink channels. It is based on the idea that the iterative exchange of soft information between a multiuser demodulator and a bank of single-user soft decoders mutually benefits them. From a practical perspective, such schemes are particularly interesting when used with suboptimal demodulators such as the MMSE multiuser interference suppressor, which displays good performance at both low and high SNRs. However, after a certain number of iterations the estimates from successive iterations become increasingly correlated, so that performance gets limited by the inherent suboptimality of the demodulator itself. 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 new block modulation scheme employing multisensor transmitters is proposed. This technique, called space-time spreading, seeks to exploit the variation in the arrival angles of mobile users’ signals with time, by retransmitting block-shifted versions of symbol sequences from additional sensors using different random spreading sequences. Employing space-time processing at both the transmitter and the receiver yields significant performance gains in the decoded bit error rate or alternatively implies increased overload capability.
KeywordsMinimum Mean Square Error Code Division Multiple Access Soft Information Receiver Array Multiuser Interference
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