Research On Key Technologies Of Signal Optimization In Distributed Transmit Antenna Systems

MIMO with distributed transmit antennas is the prominent feature of the fourth generation mobile communication systems. In the distributed transmit antenna systems (DAS), the transmit antennas are deployed in different locations. This may cause that signals transmitted from different antennas asynchronously arrive at the receive antennas, and that the channels between different transmit antennas and the same receive antenna have dissimilar channel parameters. The characteristics of the DAS determine that the design and detection of MIMO signals in DAS are different from that in the centralized antenna systems (CAS).For this reason, this thesis investigates the optimization of transmit signals in DAS. On the one hand, we design the optimized transmit signals, which can utilize the characteristics of channels in DAS to enhance performance. On the other hand, we improve the traditional MIMO techniques so that the influence of separately-deployed transmit antennas is reduced.Firstly, the space-frequency power spreading in VBLAST-OFDM with distributed transmit antennas is proposed. By spreading the power of transmit symbol on different antennas and OFDM sub-carriers, this scheme utilizes the relative delays between different transmit antennas to improve transmit diversity. According to the analysis of pairwise error probability using maximum likelihood detection, the improvement of transmit diversity gain is proved. Simulation results of 2×2 VBLAST-OFDM using BPSK modulation in Rayleigh fading channels with two equal-gain taps each show that the proposed algorithm has 2dB signal-to-noise gain at BER of 10-3 compared to conventional method.Secondly, the sub-carrier grouping of STBC-OFDM with distributed transmit antennas is proposed in the scenario where the channels between different transmit antennas and the same receive antenna have unequal channel taps. According to minimizing the upper bound of pairwise error probability, the length of each sub-carrier group and the grouping interval are set. Simulation results of 2×1 STBC-OFDM using BPSK modulation (the channels are ITU-R M.1225 pedestrian test channel A and B, respectively) show that the proposed algorithm has 2dB signal-to-noise gain at BER of 10-3 compared to conventional method of CAS. Finally, a power assignment method for distributed transmit antennas using zero-forcing receiver in Rayleigh fading channel is proposed. By minimizing the average bit error rate of each symbol block at the constraint of the fixed total power, the optimal power allocation values are set. Simulation results of 4×4 VBLAST using BPSK modulation show that the proposed algorithm has more than 1dB signal-to-noise gain at BER of 10-3 compared to equal-power assignment method.This thesis studies the optimization of transmit signals in MIMO system with distributed transmit antennas, which are specified as follows: the improvement of transmit diversity of VBLAST-OFDM, the optimality of sub-carrier grouping of STBC-OFDM and the power assignment of VBLAST. The results can be applied to 4G using distributed transmit antennas.