MMSE equalization under conditions of soft hand-off

We examine the performance of two equalizers for CDMA downlink over a frequency-selective multipath channel: the traditional RAKE receiver, and a chip-level MMSE (minimum mean square error) equalizer. The mobile-station is assumed to be approximately midway between two base-stations. We examine system performance for normal operation versus soft hand-off mode with two receive antennas. MMSE is confirmed to significantly outperform RAKE. Depending on the target uncoded bit error rate (BER), soft hand-off operation allows at least 3.5 dB less SNR for the MMSE chip-equalizers. The RAKE receiver also may operate at much lower SNR in soft hand-off mode, but a much smaller range of target BERs is available due to the multi-user access interference (MAI) induced BER floor of the RAKE receivers at moderate to high SNR. A closed form expression for the SINR is developed, which when used with a Gaussian approximation for the interference. Very well approximates the average BER