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It has been shown that a quasi-orthogonal space-time block code (QOSTBC) provides full transmission rate and half of the maximum possible diversity for four transmit antennas. We can see that QOSTBC has high complexity at the decoder since pairs of transmitted symbols are decoded with two parallel maximum likelihood (ML) detectors. In this paper, we propose new decoding schemes for reducing complexity at the decoder. By using Quasi-ZF based on the traditional Zero Forcing technique and Quasi-MMSE based on the traditional minimum mean squared error (MMSE) technique, we can have channel interference parameter be zero. As a result, only diagonal components remain. Eventually, we can detect the transmitted symbols easily with single ML detector. The proposed scheme can be used in case that all channel interference parameters are pure imaginary values in QOSTBC.