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A hybrid tree search algorithm is described for maximum-likelihood symbol detection in spatial multiplexing (SM) systems_ Essentially, the search tree is iteratively expanded in the breadth-first (BF) manner until the probability that the current most likely path is correct exceeds a specified threshold, at which point the depth-first (DF) stage is initiated to traverse the rest of the tree. In contrast to the sphere decoding (SD) algorithm, the proposed algorithm uses the BF stage to enhance the accuracy of the initial DF search direction, by exploiting the diversity inherent in the SM scheme. Simulation results demonstrate that the proposed algorithm achieves a significantly lower complexity than the SD algorithm in many scenarios of practical interest.

Combating the intersymbol interference is an important issue in single-carrier block transmission systems. To exploit the advantages of frequency-domain (FD) processing, we propose in this paper a hybrid-domain sequence detector that combines FD prefiltering with time-domain tree search detection. The proposed algorithm achieves a detection performance that is very close to that of the conventional tree search equalizers (which employ the QRD-M or sphere decoding algorithm), but with a much lower complexity. In addition, the proposed algorithm performs significantly better than the decision-feedback equalizer, with only a modest increase in complexity. Index Terms-Single-carrier block transmission, equalization, prefilter, frequency-domain processing, tree search.1536-1276 (c)