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A maximum likelihood sequence estimation (MLSE) and a maximum a posteriori (MAP) detector for DS-CDMA are proposed. The proposed receivers are attractive when the desired signal is in multi-code reception scenarios and when propagation channels are dispersive. Multipath results in loss of orthogonality among own-cell signals in the downlink and among same-user signals in the uplink. In these cases, receiver performance is limited by self-interference from the desired signal itself and interference from all other signals. The generalized RAKE (C-RAKE) receiver can be used to suppress both forms of interference, treating them as colored noise. To improve receiver performance further, we use an MLSE or a MAP detector to alleviate self-interference. The MAP detector can be used with the outer code decoder to form a turbo equalizer to further improve the receiver performance. Our numerical results show that gains larger than 2 dB over G-RAKE can be achieved by the proposed receivers for a WCDMA radio-access bearer of 4.6 Mbps using a 64-state sequence detector.

We present a computationally efficient soft-output equalizer for communication over dispersive channels. The proposed equalizer has a forward and backward recursion structure similar to the classic BCJR MAP algorithm, but its complexity is reduced by constructing a reduced-state trellis, as in the decision feedback sequence estimator (DFSE). We are able to achieve consistent state definitions by running the backward pass on the same trellis as the forward pass. The performance of this reduced-complexity equalizer is evaluated within an iterative receiver in which the intersymbol interference (ISI) channel and channel code are treated as serially concatenated codes. The EDGE air interface is used as an illustrative example, for which we are able to reduce the number of states from 4096 to 8.