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.