Adaptive space-time chip-level equalization for WCDMA downlink with code-multiplexed pilot and soft handover

In the downlink of WCDMA systems, multi-path propagation destroys the orthogonality of the user signals and causes multi-user interference (MUI). Chip-level equalization can restore the orthogonality and suppress the MUI. However, adaptive implementations of the chip-level equalizer receiver that can track time-varying multi-path channels are hard to realize in practice for two reasons: loss of spectral efficiency when using a training sequence and loss of performance when using pure blind techniques. We propose new training-based and semi-blind space-time chip-level equalizer receivers for the downlink of WCDMA systems employing long spreading codes and a continuous code-multiplexed pilot. The proposed receivers exploit the presence of common pilot symbols in the so-called Common PIlot CHannel (CPICH) of the Universal Terrestrial Radio Access (UTRA) for 3G systems. Moreover, they can simultaneously track multiple base-station signals, whenever the mobile station enters soft handover mode. For both receivers, we derive a recursive least squares (RLS) algorithm for adaptive processing. The proposed receivers are compared with the conventional space-time RAKE receiver and the ideal space-time chip-level equalizer receiver both in terms of performance and complexity.