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.

In recent years, Pre-Rake combining technique has become a hot topic of research as it decreases the complexity of the portable mobile unit, while achieving the same multipath diversity effect of the Rake receiver. The technique is based on precoding of the transmitted signal relying on knowledge of the channel estimation response before transmission. This a priori channel state information is available in Time Division Duplexing (TDD) systems, since the same channel is used both in uplink and downlink. In practice, the error in channel estimation in Pre-Rake system occurs due to time variability in mobile radio channel. Most previous works on Pre-Rake in TDD CDMA have not taken into consideration the effect of imperfect channel estimation. In this paper, we present Pre-Rake performance under imperfect channel estimation due to time variability in TDD system, depending on Doppler Frequency and compare it with the ideal Pre-Rake system. Numerical analysis and computer simulations were carried out to obtain the system error probability.