A new low-complexity Maximum Likelihood (ML) based detection scheme is presented in this paper for full-rate Quasi Orthogonal Space Time Block Code (QOSTBC) in Multi-Input Multi-Output (MIMO) systems with four-transmit antenna. The proposed detection scheme is developed by using both lower and upper triangular versions of QR decomposition of channel matrix, without need of utilizing interference cancellation (IC) method. Simulation results show that the symbol error rate performance of the new proposed detection scheme is the same as that of the QRD-IC-based detection method but with less complexity.

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