Linear-Aided Decision-Feedback Equalization for the CDMA Downlink

In the CDMA downlink chip-level equalization at the receiver can restore the orthogonality lost in the time-dispersive channel. Direct application of decision-feedback equalization is ineffective, as chip decisions are degraded by extremely low SNR and the high-order composite constellation. We propose a two-stage technique that consists of: linear equalization followed by despreading, non-linear symbol amplitude estimation, and re-creation of the approximate transmitted chip sequence; then, a second DFE stage using the reconstructed chip sequence as the input to the feedback filter. We obtain the optimum filter coefficients for a finite-length two-sided (non-causal) DFE in closed-form. The proposed technique is especially beneficial when detecting higher-order modulations.