Reduced Complexity Tree-Search Detector for Hybrid Space-Time Codes

Space-time block codes are a well-known technique to exploit the capacity of multiple-input, multiple-output wireless channels. A space-time code specifies how the transmitted symbols are spread over both space and time. The resulting spatial- and time-diversity are exploited by the receiver to overcome deep fades and to increase the data rate. The search for receiver algorithms with high performance, low complexity, and applicability to a variety of codes is an active research area. In this paper, we present a receiver algorithm for hybrid space-time codes, which combine layers from purely spatial codes with layers from orthogonal codes, and obtains both diversity and multiplexing gains. The proposed decoder works in systems with three or four transmit antennas, and at least two receive antennas. The decoder performs a low-complexity tree search and achieves a bit error rate within a fraction of a decibel of the optimum. The low complexity results from adopting an improved search stop criteria, used in recent sphere decoders. The reduction is especially significant in correlated channels.