Asymptotic Analysis of General Multiuser Detectors in MIMO DS-CDMA Channels

TL;DR

This paper uses the replica method to analyze the asymptotic behavior of multiuser detectors in MIMO DS-CDMA channels, revealing decoupling into single-user channels and performance close to sum capacity.

Contribution

It provides a novel asymptotic analysis of general multiuser detectors in MIMO DS-CDMA channels, including nonlinear detectors, using the replica method.

Findings

Decoupling into single-user Gaussian channels with or without spatial spreading.

Spectral efficiency with spatial spreading comparable to optimal space-time coding.

MMSE detector exhibits waterfall behavior close to sum capacity for QPSK modulation.

Abstract

We analyze a MIMO DS-CDMA channel with a general multiuser detector including a nonlinear multiuser detector, using the replica method. In the many-user, limit the MIMO DS-CDMA channel with the multiuser detector is decoupled into a bank of single-user SIMO Gaussian channels if a spatial spreading scheme is employed. On the other hand, it is decoupled into a bank of single-user MIMO Gaussian channels if a spatial spreading scheme is not employed. The spectral efficiency of the MIMO DS-CDMA channel with the spatial spreading scheme is comparable with that of the MIMO DS-CDMA channel using an optimal space-time block code without the spatial spreading scheme. In the case of the QPSK data modulation scheme the spectral efficiency of the MIMO DS-CDMA channel with the MMSE detector shows {\it waterfall} behavior and is very close to the corresponding sum capacity when the system load is just below the transition point of the {\it waterfall} behavior. Our result implies that the performance of a multiuser detector taking the data modulation scheme into consideration can be far superior to that of linear multiuser detectors.