Adaptive chip-rate equalization of downlink multirate wideband CDMA

We consider a downlink DS-CDMA system in which multirate user signals are transmitted via synchronous orthogonal short codes overlaid with a common scrambling sequence. The transmitted signal is subjected to significant timeand frequency-selective multipath fading. In response to this scenario, a novel two-step receiver is proposed that combines chip-rate adaptive equalization with error filtering. In the first step, a code-multiplexed pilot is used to adapt the equalizer. The use of error filtering implies a third-order LMS algorithm which has significant advantages over standard LMS in tracking the time-varying channel. In the second step, decision-direction is used to improve the error signal used in adaptation, resulting in improved tracking performance. The performance of the adaptive receiver is studied through analysis and simulation.