A Fast Sphere Decoding Algorithm for Space-Frequency Block Codes

The recently proposed space-frequency-coded MIMO-OFDM systems have promised considerable performance improvement over single-antenna systems. However, in order to make multiantenna OFDM systems an attractive choice for practical applications, implementation issues such as decoding complexity must be addressed successfully. In this paper, we propose a computationally efficient decoding algorithm for space-frequency block codes. The central part of the algorithm is a modulation-independent sphere decoding framework formulated in the complex domain. We develop three decoding approaches: a modulation-independent approach applicable to any memoryless modulation method, a QAM-specific and a PSK-specific fast decoding algorithm performing nearest-neighbor signal point search. The computational complexity of the algorithms is investigated via both analysis and simulation. The simulation results demonstrate that the proposed algorithm can significantly reduce the decoding complexity. We observe up to 75% reduction in the required FLOP count per code block compared to previously existing methods without noticeable performance degradation.