Downlink intercell interference cancellation in WCDMA by exploiting excess codes

The deployment of third generation CDMA-based wireless systems foresees a loading fraction that is smaller than one, i.e. the number of users per cell is scheduled to be significantly less than the spreading factor to attain an acceptable performance. This means that a base station can set apart a subset of the codes, the excess codes, that it will not use. In the case of periodic codes (such as in the TDD mode of UMTS) the existence of excess codes implies the existence of a noise subspace, which can be used to cancel the interference coming from a neighboring base station. We introduce a SINR maximising linear receiver to perform this interference reduction. In the case of aperiodic codes (such as in the FDD mode of UMTS) the noise subspace is time-varying. In this case, we introduce structured receivers that combine scrambling and descrambling operations with projections on code subspaces and linear time-invariant filters for equalization, interference cancellation and multipath combining. So the time-varying part of these receivers is limited to (de-)scrambling operations. The design of the various filter parts gets discussed. Performance improvements are illustrated via simulations