Combined MMSE equalization and multiuser detection for high-speed CDMA forward link with sparse multipath channels

This paper investigates the performance of an adaptive, low-rank, chip-level structured, MMSE equalizer in a 3G wideband CDMA forward link in the presence of frequency-selective multipaths, in single and multiuser detection schemes. The equalizer exploits the underlying channel structure, specifically the fact that the channel impulse response is sparse, to project the full-rank data vectors onto a much lower dimensional subspace spanned by a few columns of the pulse shaping filter convolution matrix, as determined by the multipath delays. This low-rank MMSE equalizer exhibits significantly lower error rates than both standard MMSE equalizers and RAKE receivers. We propose to utilize the equalizer coefficients to reconstruct the synchronous chip-level signal of the intracell users, and implement subtractive interference cancellation to further improve the performance. The performance of the combined MMSE equalizer and multiuser detection scheme is presented using one stage of parallel interference cancellation.