Fast Maximum-Likelihood Decoding of 4X4 Full-Diversity Quasi-Orthogonal STBCs with QAM Signals

In this paper, we present the lowest-computational-complexity maximum-likelihood (ML) decoder known to-date for 4×4 full-diversity Quasi-Orthogonal Space-Time Block Codes (QO-STBC) with symbols from square or rectangular quadrature amplitude modulation (QAM) constellations. The complexity savings come from a simplified quadratic ML decoding statistic that is being presented and the utilization of the signal points of the QAM constellation. Comparative computational complexity analysis is carried out and a simulation study demonstrates the theoretical equivalence of the proposed and original ML implementation.