Space-time equalization using conjugate-gradient algorithm in WCDMA downlink

In the downlink of the wideband code division multiple-access (WCDMA) system users are separated by means of orthogonal variable spreading factor codes. The orthogonality of the users is lost in multipath channels causing multiple access interference which limits the system capacity and the availability of high data rate services. To suppress the interference in the downlink, it has been proposed to utilize linear channel equalization to partially restore the orthogonality of the users´ spreading codes. The well-known direct linear minimum mean-square error (LMMSE) solution for the equalizer suffers from high computational complexity mainly due to the required matrix inversion. The problem is especially severe in case of a high channel delay spread and multi antenna receivers. In this paper, we apply the conjugate-gradient algorithm to iteratively solve the linear space-time equalizer. The performance of the proposed LMMSE space time equalizer is evaluated in the UTRA FDD downlink.