Chip-sparsification and symbol-equalization for WCDMA downlink

We consider SINR maximizing receivers based on the concept of chip-level filtering and symbol level equalization for WCDMA downlink. In this contribution we propose a new class of receivers based on channel sparsifying linear pre-processing at chip-rate followed by time-varying symbol level equalizers. Due to a sparse structure imposed on the channel (i.e. sparsification) by a chip level pre-equalizer filter which we call channel sparsifier, the effective channel after despreading presents itself as a symbol-level ISI channel. Time-varying equalization at symbol level is necessitated by the presence of aperiodic scrambler which is treated as deterministic. The optimal channel sparsifier maximizes SINR at the output of symbol level equalizer. We focus here on downlink channels that have significant dispersion in the temporal domain. Expressions for post processing SINRs (after channel sparsification, despreading and subsequent symbol level equalization) are derived for all receivers and used for performance evaluation. We show that improved receivers for WCDMA downlink can be designed benefiting from a combination of generalized channel sparsification, deterministic treatment of scrambler and non-linear equalization.