MMSE equalization for forward link in 3G CDMA: symbol-level versus chip-level

We investigate a "symbol-level" MMSE equalizer for the CDMA downlink over a frequency-selective multipath channel meant to improve on the recently proposed "chip-level" downlink equalizers. Indeed the symbol-level equalizer performs better than the chip-level, but is computationally more demanding. The symbol-level equalizer is optimal for "saturated cells" where all Walsh-Hadamard channel codes are in use and have equal power. It performs very close to optimal even for relatively lightly loaded cells. We derive a bound on the off-diagonals of the covariance matrix of the transmitted data that helps explain why the equalizer works when there are fewer active channel codes than the spreading factor. Performance is evaluated through simulations to obtain the average bit error rate (BER) over a class of channels for two cases: no out-of-cell interference, and one equal power base-station. The symbol- and chip-level equalizers are compared to the conventional RAKE receiver.