Non-orthogonal STBC for four transmit antennas with high coding gain and low decoding complexity

Multiple antenna based technology enhances performance of communication system by increasing capacity and reliable detection of the transmitted data. Space-time block codes (STBCs) for multiple-input multiple-output (MIMO) systems are designed to achieve full diversity when transmitter does not have channel state information. Orthogonal space-time block codes (OSTBCs) provide symbol-wise decoding, while quasi orthogonal STBCs are used to achieve high data rates with pair-wise decoding complexity. However, most of the existing OSTBCs and QOSTBCs utilize repeated transmission of symbols which reduces capacity of the wireless communication system. In this paper, we propose a non-orthogonal STBC for four transmit antennas that avoids repeated transmission of symbols. The proposed code transmits optimally mapped symbols in place of repeated transmissions of a symbol. The proposed STBC provides improved capacity and significant coding gain as compared to the existing STBCs for four transmit antennas. We also derive a low-complexity decoder of the proposed STBC which performs close to the maximum likelihood decoder.