Intercell interference cancellation at a WCDMA mobile terminal 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. 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. In the case of aperiodic codes (such as in the FDD mode of UMTS), the noise subspace is time-varying due to the scrambling. This motivated us to 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 is discussed. Performance improvements are illustrated via simulations.