Multiuser Quasi-Orthogonal Space-time Coded MIMO System With Transmit Beamforming

Multi-antenna technology is one of the key technologies in the next generation mobile communication systems, which can greatly improve the frequency spectrum utilization, capacity and data rate without increasing the system bandwidth in communications. multi-antenna technology includes spatial diversity technology, space multiplexing technology and smart antenna technology. Transmit diversity technology based on space-time block coded and beamforming technology are considered to be two effective means of improving system performance in MIMO system. Space-time block code technology can obtain significant diversity gain and increase capacity of system by using the independence of multiple antennas. And beamforming technology can restrain inter-symbol interference and multi-user interference through the high correlation of multiple array signals. It will be of great potential in enhancing system performance and increasing system capacity in wireless communication system that combing the two technologies by appropriate strategies.This thesis deals with the joint use of beamforming technology with quasi-orthogonal space-time coding (QOSTBC) in MIMO systems. The core details of the research are as follows.(1) Rotated quasi-orthogonal space-time coding is explored based on traditional quasi-orthogonal space-time codes, which will add a rotated angle on traditional QOSTBC to form a new constellation and then choose the best angle of rotation to achieve full diversity.(2) A beamforming scheme combined with QOSTBC is proposed for point to point single-user MIMO systems. Instead of using four independent transmit antennas in the traditional QOSTBC transmitter, four independent antenna arrays are employed. The scheme can offer full diversity and be superior to the classic transmit diversity scheme. The feasibility and effectiveness of the proposed strategy for OFDM systems is demonstrated by using MATLAB-based simulations. (3) A new scheme that combing beamforming technology with QOSTBC in multi-user MIMO systems is presented. In order to highlight that the beamforming technology can effectively restrain MAI, a composite algorithm based on convolution is proposed. The algorithm divided the weighted vector into two sub-weighted vector, which can be determined by using the ZF approach and the eigenvalue decomposition algorithm. By doing so, MAI can be efficiently suppressed and QOSTBC diversity gain can be achieved. Furthermore, we also combined OFDM to the scheme in broadband communication systems. Simulation results are also presented.