Comparative performance evaluation of three MMSE equalizers for the CDMA forward link in frequency selective multipath

We derive a subspace constrained "symbol-level" equalizer for the CDMA downlink over a frequency selective multipath channel and compare its performance with the chip-level equalizer, the unconstrained symbol-level equalizer, and the RAKE receiver. The constrained equalizer must lie in a subspace determined by shifted versions of the user\´s Walsh-Hadamard code multiplied by the long code. Due to the presence of the long code, the constrained equalizer varies from symbol to symbol, as the symbol-level equalizer does. Since the symbol is being estimated directly, the constrained equalizer performs better than the chip-level equalizer but at the expense of greatly increased computational complexity. The unconstrained equalizer performs better than the subspace constrained equalizer and is computationally less demanding. We summarize the advantages and disadvantages of each equalizer.