Low-complexity turbo equalization and multiuser decoding for TD-CDMA

We propose a low complexity multiuser joint Parallel Interference Cancellation (PIC) decoder and Turbo Decision Feedback Equalizer for CDMA. In our scheme, an estimate of the interference signal (both MAI and ISI) is formed by weighting the hard decisions produced by conventional (i.e., hard-output) Viterbi decoders. The estimated interference is subtracted from the received signal in order to improve decoding in the next iteration. By using asymptotic performance analysis of random-spreading CDMA, we optimize the feedback weights at each iteration. Then, we consider two (mutually related) performance limitation factors: the bias of residual interference and the ping-pong effect. We show that the performance of the proposed algorithm can be improved by compensating for the bias in the weight calculation, and we propose a modification of the basic PIC algorithm, which prevents the ping-pong effect and allows higher channel load and/or faster convergence to the single-user performance. The proposed algorithm is validated through computer simulation in an environment fully compliant with the specifications of the time-division duplex mode of 3rd generation systems, contemplating a combination of TDMA and CDMA and including frequency-selective fading channels, user asynchronism, and power control. The main conclusion of this work is that, for such application, Soft-Input Soft-Output (SISO) decoders (e.g., implemented by the forward-backward BCJR algorithm) are not needed to attain very high spectral efficiency, and simple conventional Viterbi decoding suffices for most practical settings.