MUI-suppression with reduced T-equalizer for UTRA FDD downlink

Wideband CDMA systems with orthogonal spreading codes suffer severely due to the loss of orthogonality by multipath propagation. This yields multiple user interference (MUI), which gravely reduces the performance of classical systems with RAKE-receivers. We attempt to restore orthogonality by using a modified T-equalizer. Classical T-equalizers are composed as FIR filters with equidistant delays and appropriate coefficients. Their main disadvantage is the high computational effort due to the large number of coefficients. The question arises whether we need to calculate all equally spaced coefficients, or if some of the coefficients can be neglected. Basically, we use only a subset of coefficients and set all others to zero. The main advantages are the reduced computational effort in calculating the filter and the decreased computational costs in using it. We show the feasibility with some simulation results for block fading channels. In practice, a T-equalizer is not very often used for time variant channels, due to its high computational costs. Instead, an adaptive algorithm is taken and modified accordingly. We take a least mean squares (LMS) algorithm for MUI-suppression in a time variant environment. In order to show the feasibility of this approach, some simulation results for channels with low Doppler frequency are presented and compared with the classical RAKE-receiver and a full version of the LMS/Griffith by M.J. Heikkila (see Proc. IEEE Int. Symp. on Personal, Indoor and Mobile Radio Commun., PIMRC, 2001).