LMMSE-based iterative and turbo equalization methods for CDMA downlink channels

An approach to turbo and iterative channel equalization is proposed for downlink code division multiple access (CDMA) receivers. At each iteration, the method employs a linear minimum mean squared error (LMMSE) equalizer that incorporates feedback information obtained in the previous iteration. The LMMSE filter is updated iteratively to provide incremental improvements on the estimate of transmitted chip sequence. The update can utilize information fed back from the turbo decoder output, resulting in a turbo equalizer, or from a soft decision operation without involving the decoder, yielding an iterative equalizer. The technique achieves a significant performance gain compared to non-iterative LMMSE equalization. In some cases, the performance after 3-4 iterations is within 1 dB of the matched filter bound.