Timing-free code-aided blind adaptive joint MAI and ISI suppression in dispersive CDMA channels

The problem of joint multiaccess interference (MAI) and intersymbol interference (ISI) suppression in DS/CDMA systems operating over fading dispersive channels is considered. A timing-free, blind and code-aided detection algorithm is proposed, i.e. the procedure requires knowledge of neither the interfering users´ parameters (spreading codes, timing offsets and propagation channels), nor the timing and channel impulse response of the user of interest, but only of its spreading code. The proposed structure is a two-stage one. The first stage is aimed at suppressing the multiuser interference, while the second-stage performs channel estimation and data detection. The proposed system is shown to incur a very limited loss with respect to the non blind MMSE detector; moreover, it outperforms other previously known blind systems, and is near-far resistant. A major advantage of the new structure is that it admits an adaptive implementation with quadratic (in the processing gain) computational complexity. This adaptive algorithm, which couples a recursive-least-squares estimation of the blocking matrix and subspace tracking techniques, achieves effective steady-state performance.