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

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 waterfall behavior and is very close to the corresponding sum capacity when the system load is just below the transition point of the 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. Index TermsMultiple-input multiple-output (MIMO) systems, direct-sequence code-division multiple access (DS-CDMA), asymptotic analysis, multiuser detection, replica method.K. Takeuchi is with the

Large-System Analysis of Joint Channel and Data Estimation for MIMO DS-CDMA Systems

“…Our result is an extension of the decoupling results for randomly-spread MIMO DS-CDMA channels with perfect CSI at the receiver [11], [20] to the no-CSI case at the receiver. The difference from the previous decoupling results appears in the receiver structure for the SIMO channel (27), which depends on the receiver structure of the original MIMO DS-CDMA systems.…”

“…The equivalent channel between the users in K and their decoders looks like a bundle of the single-user SIMO channels (27) with the original channel estimator (11).…”

“…The receiver consists of the original LMMSE channel estimator providing the marginal posterior pdfs (11), an optimal detector, and the decoder of user k (See Fig. 4).…”

“…In symbol period t(> τ ), the optimal detector uses the information about the received vectors Y t,k = {y t,k,m : for all m} of the single-user SIMO channel (27) in symbol period t and about the posterior pdf (11) provided by the original channel estimator to construct the posterior pdf…”

“…Our large-system analysis is based on the replica method [37]- [39], which is a powerful method for analyzing randomly-spread DS-CDMA systems [11], [18]- [20], [40] and MIMO systems [41]- [44]. The replica method is based on several non-rigorous procedures at present time.…”