Research On Space-Time Block Coding For Multiple Input Multiple Output Wireless Communication Systems

Since the mid-1990's, the transmit diversity and spatial multiplexing have been a hot reach field of wireless communication, which can be achieved from space-time coding. Multiple-input multiple-output (MIMO) technique can exploit space resource adequately, and improve spectrum efficiency and link reliability. Space time coding is one of the effective ways to achieve these benefits of MIMO system. The problems channel encoding, modulation, transmit diversity and receiving diversity are jointly considered and combined by space time coding to improve the transmission performance of MIMO system.This dissertation focuses on the application of space-time coding in MIMO systems. The main contributions included in this dissertation can be summarized as follows:Firstly, a complex orthogonal design which provides full transmission rate for a space-time block code is not possible for more than two transmission antennas. The structure and character of Quasi-orthogonal Space-Time Block Coding (QOSTBC) are investigated, and a QOSTBC scheme for 4 transmit antennas is proposed. In quasi-static fading channels, the STBC designed by Alamouti is the unique coding scheme designed with complex orthogonal matrix. Due to the orthogonality between the columns of STBC transmit matrix, the full diversity gain and full transmission rate can be achieved. However, partial orthogonality of the transmit matrix is sacrificed to achieve full transmission rate, thus the QOSTBC scheme is applied. On the basis of STBC codes, several quasi-orthogonal STBC structures with full diversity gain and partial diversity gain are proposed, and the recursive method of the transmit matrix of QOSTBC is summarized. The decoding performance of the QR decomposition and zero-forcing decoding algorithm is also analyzed.Secondly, coherent demodulation of space-time block codes depends on accurate channel state information, thus estimation of MIMO channel characters is very important to signal detection and channel decoder. On the basis of the construction of the multiple input multiple output time-frequency double selective fading channel model, the estimation method based on superimposed training sequence was analyzed, and an estimation method based on data-dependent superimposed training and least square was proposed. The periodic exponential sequence with the same magnitude was used as the training sequence, and another periodic sequence based on the information sequence was added to eliminate the effect of the unknown information sequence on the training sequence. The channel matrix can be estimated at the receiver by the first order statics of the receiving signal. Compared with traditional superimposed training method, the proposed estimating method obviously reduces the mean square error and the bit error rate than the traditional superimposed training method, and the spectrum efficiency is maintained, and none extra bandwidth is required.Thirdly, differential coding for unknown frequency-selective channels is investigated in this dissertation. Differential coding is attractive alternative channel coding method, when the channel estimation is difficult. The conventional decoding scheme of the differential space time codes based on maximum-likelihood (ML) is analyzed, and a fast decoding method under high signal to noise ratio is proposed to avoid the problem of exponentially increasing complexity of conventional decoding scheme. The maximum spatial diversity multipath gain can be achieved without the channel estimation at the receiver. The decoding complexity is reduced, and the performance is similar to ML method. The calculation of the decoding complexity and simulation performance verified the effectiveness of proposed algorithm.Finally, cooperative diversity is investigated under virtual MIMO technique. The system capacity is effectively improved in MIMO, but the configuration of multi antennas in the uplink is impractical with the limitation of size and power constraints of mobile terminals. Cooperative diversity enables single-antenna mobile terminals in a multi-user environment to share their antennas, and a virtual MIMO system is formed to relay the transmission information, thus the spatial diversity is achieved at the mobile terminal with single antenna. On the basis of existing collaborative methods, a collaborative diversity scheme combined with the differential coding and space time coding is proposed. The differential demodulation is realized at the mobile terminal, and the orthogonal space time codes are applied in the uplink. The relationship of the transmitter and receiver of MIMO system with cooperative diversity is deduced, and the BER performance and system capacity of the proposed scheme are verified by the simulation.