Space-Time Chip Equalization for WCDMA Forward Link with Code-Multiplexed Pilot and Soft Handover

In the forward link of WCDMA systems, multipath propagation destroys the orthogonality of the user signals and causes multi-user interference (MUI). Channel equalization can restore the orthogonality and suppress the MUI. However, adaptive implementations of the chip equalizer receiver that can track fast fading multipath channels are hard to realize in practice. On one hand, sending a training chip sequence at regular time instants in correspondence with the coherence time of the channel would reduce the system’s spectral efficiency significantly. On the other hand, using blind techniques that do not require any training overhead would reduce the system’s performance. In this paper, we propose new trainingbased and semi-blind space-time chip equalizer receivers for WCDMA systems employing long spreading codes and a continous code-multiplexed pilot. The proposed receivers exploit the presence of a continuous code-multiplexed pilot signal in forthcoming 3G systems and offer both a good performance and a high spectral efficiency. Moreover, they can track multiple base-station signals, whenever the mobile user terminal enters soft handover mode. Whereas the trainingbased receiver only assumes knowledge of the pilot symbols and the pilot code for each base-station, the semi-blind receiver additionally assumes knowledge of all user codes for each base-station. For both receivers, we derive a Least Squares algorithm for block processing and a QRD-based Recursive Least Squares algorithm for adaptive processing. Both receivers can track fast fading multipath channels and outperform the conventional RAKE receiver with perfect channel knowledge. For low to medium system load, the semi-blind approach outperforms the training-based approach.