Structured interference cancellation at a WCDMA terminal

In third generation CDMA-based cellular mobile systems, loading fraction is going to be less than one due to interference limitations. In other words, to achieve an acceptable BER, the number of served users and hence used codes will be less than the spreading factor. Accordingly, the existence of unused codes implies the existence of a noise subspace, which can be exploited to partially cancel the intracell and intercell interference. When aperiodic scrambling is used, as in the FDD mode of UMTS, the noise subspace varies with symbol period. In this case, multi-user interference cancelling receivers should contain time varying parts. We discuss two-stage structured receivers for mobile terminals that contain (de)scrambling, (de)spreading, projection/hardlimiting/ tangent-hyperbolic decision operations and linear filters the time-varying parts of which are limited to descrambling and scrambling. The first stages estimate and delete the intracell and/or intercell interference whereas the second stages operate on the resultant improved SINR signal to detect the user symbols. Performances of various different schemes are compared via simulations.