In this paper, we deal with the design of high-rate, full-diversity, lowmaximum likelihood (ML) decoding complexity space-time block codes (STBCs) withcode rates of 2 and 1.5 complex symbols per channel use for multiple-inputmultiple output (MIMO) systems employing three and four transmit antennas. Wefill the empty slots of the existing STBCs from CIODs in their transmissionmatrices by additional symbols and use the conditional ML decoding techniquewhich significantly reduces the ML decoding complexity of non-orthogonal STBCswhile ensuring full-diversity and high coding gain. First, two new schemes withcode rates of 2 and 1.5 are proposed for MIMO systems with four transmitantennas. We show that our low-complexity rate-2 STBC outperforms thecorresponding best STBC recently proposed by Biglieri et al. for QPSK, due toits superior coding gain while our rate-1.5 STBC outperforms the full-diversityquasi-orthogonal STBC (QOSTBC). Then, two STBCs with code rates of 2 and 1.5are proposed for three transmit antennas which are shown to outperform thecorresponding full-diversity QOSTBC for three transmit antennas. We prove by aninformation-theoretic analysis that the capacities of new rate-2 STBCs forthree and four transmit antennas are much closer to the actual MIMO channelcapacity than the capacities of classical OSTBCs and CIODs.
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