Adaptive MMSE equalization for wideband CDMA forward link with time-varying frequency selective channels

In recent years, receivers based on channel equalization on the chip-level prior to despreading have been proposed for the 3G wideband CDMA forward link. Of these the chip-level MMSE equalizer has attracted much interest. In this paper, we investigate the performance of training-based MMSE equalizers in moderate to rapid time-varying Rayleigh faded channels. We show that the standard MMSE equalizers fail to adequately track the time-variations as large Doppler shifts. We also demonstrate that chip-level MMSE equalizers that exploit the underlying channel structure, specifically the fact that the channel impulse response is sparse, are much more robust against time-selective fading. This equalizer is obtained by projecting 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. Simulations results show that this low-rank MMSE equalizer performs significantly better than both standard MMSE equalizers and the RAKE receiver in time-varying situations.