MUI-suppression with lattice structures 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 the classical system with Rake-receivers. In our approach, we attempt to restore orthogonality again by using a Lattice equaliser. This type of equaliser is composed of two parts. The prewhitening part and the adaptive equaliser part. The lattice structure itself has the advantage that its backward prediction errors are orthogonal to each other. Therefore the lattice structure itself is working as a prewhitening filter. The second part is the adaptive algorithm for the equaliser coefficients. We take the stochastic gradient algorithm for this task and use the pilot channel in UMTS as a reference to calculate the equaliser coefficients. In order to show the feasibility of this approach, some simulation results for channels with low Doppler frequencies are presented and compared with a system using the classical Rake-receiver and a system with another prefilter approach, the LMS-like Griffith equaliser.