A fast sphere decoding algorithm for space-frequency block codes

The recently proposed space-frequency-coded MIMO-OFDM systems havepromised considerable performance improvement over single-antennasystems. However, in order to make multiantenna OFDM systems anattractive choice for practical applications, implementationissues such as decoding complexity must be addressed successfully.In this paper, we propose a computationally efficient decodingalgorithm for space-frequency block codes. The central part of thealgorithm is a modulation-independent sphere decoding frameworkformulated in the complex domain. We develop three decodingapproaches: a modulation- independent approach applicable to anymemoryless modulation method, a QAM-specific and a PSK-specificfast decoding algorithm performing nearest-neighbor signal pointsearch. The computational complexity of the algorithms isinvestigated via both analysis and simulation. The simulationresults demonstrate that the proposed algorithm can significantlyreduce the decoding complexity. We observe up to 75% reductionin the required FLOP count per code block compared to previouslyexisting methods without noticeable performance degradation.