Multiple-Input Multiple-Output (MIMO) systems that realize high-speed data transmission with multiple antennas at both transmitter and receiver are drawing much attention. In line-of sight (LOS) environments, the performance of MIMO systems depends largely on the difference of the phase difference of direct paths from transmit antennas to each receive antenna. When the phase difference of direct paths are close to each other, the spatial division multiplexing (SDM) channels are not orthogonal to each other so signal detection becomes difficult. In this paper, we propose a MIMO system with relative phase difference time-shift modulation (RPDTM) in Rician fading environments. The proposed scheme transmits independent signals from each antenna at each time slot where the relative phase difference between signal constellations used by transmit antennas varies in a pre-determined pattern. This transmission virtually changes the phase difference of direct paths from transmit antennas to each receive antenna without lowering data rate and without knowledge of the channels. In addition, forward error correction coding (ECC) is applied to exploit the time slots where the receiver can detect the signals easily to improve the detection performance. If there are time slots where the receiver can separate the received signal, the receiver can decode the data by using the time slots and the correlation between data. From the results of computer simulation, we show that MIMO system with RPDTM can achieve the better bit error rate (BER) than the conventional MIMO system. We also show that the MIMO system with RPDTM is effective by about Rician factor K = 10 dB.

Multiple-Input Multiple-Output (MIMO) systems can achieve high data-rate and high capacity transmission. In MIMO systems, eigen-beam space division multiplexing (E-SDM) that achieves much higher capacity by weighting at the transmitter based on feedback channel state information (CSI) has been studied. Early studies for E-SDM have assumed perfect CSI at the transmitter. However, in practice, the CSI fed back to the transmitter from the receiver becomes outdated due to the time-varying nature of the channels and feedback delay. Therefore, an outdated E-SDM cannot achieve the full performance possible. In this paper, we evaluate the performance of E-SDM with methods for reducing performance degradation due to feedback delay. We use three methods: 1) method that predicts CSI at future times when it will be used and feeds the predicted CSI back to the transmitter (denoted hereafter as channel prediction); 2), 3) method that uses the receive weight based on zero-forcing (ZF) or minimum mean square error (MMSE) criterion instead of those based on singular value decomposition (SVD) criterion (denoted hereafter as ZF or MMSE-based receive weight). We also propose methods that combine channel prediction with ZF or MMSE-based receive weight. Simulation results show that bit error rate (BER) degradation of E-SDM in the presence of feedback delay is reduced by using methods for reducing performance degradation due to feedback delay. We also show that methods that combine channel prediction with ZF or MMSE-based receive weight can achieve good BER even when the large feedback delay exists.