Multipass channel estimation and joint multiuser detection and equalization for MIMO long-code DS/CDMA systems

Abstract

The problem of joint channel estimation, equalization, andmultiuser detection for a multiantenna DS/CDMA system operatingover a frequency-selective fading channel and adopting longaperiodic spreading codes is considered in this paper. First ofall, we present several channel estimation and multiuser datadetection schemes suited for multiantenna long-code DS/CDMAsystems. Then, a multipass strategy, wherein the data detectionand the channel estimation procedures exchange information in arecursive fashion, is introduced and analyzed for the proposedscenario. Remarkably, this strategy provides, at the price ofsome attendant computational complexity increase, excellentperformance even when very short training sequences aretransmitted, and thus couples together the conflicting advantagesof both trained and blind systems, that is, good performance andno wasted bandwidth, respectively. Space-time coded systems arealso considered, and it is shown that the multipass strategyprovides excellent results for such systems also. Likewise, it isalso shown that excellent performance is achieved also when eachuser adopts the same spreading code for all of its transmitantennas. The validity of the proposed procedure is corroboratedby both simulation results and analytical findings. In particular,it is shown that adopting the multipass strategy results in aremarkable reduction of the channel estimation mean-square errorand of the optimal length of the training sequence.