Research On Iterative Detection In MIMO System

There is an urgent need for the service of high data rate transmission in the future mobile communication systems. For solving relative problem,Multiple Input Multiple Output (MIMO),Orthogonal Frequency Division Multiplexing(OFDM), channel coding and iterative detection technologies are taken as the key technologies in B3G and 4G. Wireless communication systems with MIMO have the potential of vastly improving spectral efficiency without expanding the bandwidth. The detection and decoding are considered in iterative detection technology. By iteration, the performance of system is improved with low complexity. In this thesis, the iterative detection in MIMO is studied. The main research results of this dissertation are as follows:1. A novel low-complexity iterative receiver based on QR decomposition in Multiple Input Multiple Output (MIMO) system is presented. In the receiver, the channel matrix is decomposed and the received signal is processed. According to the upper triangular matrix after QR decomposition, the signal is detected layer by layer. Partial parallel interference cancellation based on QR decomposition is used, in which different layers are grouped and jointly detected by MAP. The interference between different antennas is cancelled effectively through serially concatenating a soft input soft output (SISO) detector and Turbo decoder by iterative process. Three dimension extrinsic information transfer characteristics (3D EXIT charts) are used to analysis the processes of iterative. Compared with traditional QR based serial interference cancellation iterative receiver, the performance of the proposed receiver is improved greatly. And it is not sensitivity to the mismatched SNR.2. To find the optimal order of QR-decomposition, the columns of channel matrix should be permutated and it can only be obtained by search method. An improved QR based detection algorithm is proposed for use in the Bell Labs Layered Space-Time (BLAST) systems. It doesn't require pseudo inverse of matrix, then has less computational load in contrast to the standard detection algorithm with inverse of matrix. It has less computational complexity than ordered QR-based detection .The probability of error propagation is reduced through serial interference canceling; The signal of minimum error probability is removed layer by layer. Simulation shows that it outperforms the classic V-BLAST and ordered, suboptimal ordered QR based detection algorithms. Combined with the iterative decoding, the extrinsic information delivered by detector will be improved further.3. A novel BLAST transceiver named Turbo-Like BLAST (TLBLAST) for MIMO communications is proposed, which combines the characteristics of HBLAST and VBLAST with the structure of Turbo encoder. The high data rate transmission can be implemented and in each transmitted antenna, different encode schemes can be used to supply different protection levels. The system performance is improved effectively through serially concatenating a soft input soft output (SISO) detector and decoder by iterative process with comparable complexity of VBLAST. Simulation results show that the performance of TLBLAST is better than HBLAST and VBLAST in Rayleigh flat fading channels. Aimed at the iterative receiver based on serial cancellation, an irregular modulation scheme in MIMO is proposed. In each antenna, different modulation is assigned with the same transmission rate. Simulation shows that the bit error rate is improved greatly with the irregular modulation scheme.4 The performance of MIMO system under imperfect channel information is analyzed and the improved scheme is proposed. A robust zero-forcing receiver in Multiple Input Multiple Output (MIMO) channel with channel estimation error is presented. By considering the statistic of channel estimation error, the coefficient of zero-forcing filter is modified with the estimated channel information. The presented receiver is more robust than traditional ZF receiver in the presence of inaccurate channel information in the receiver.5. The performance of iterative receivers based on linear filter and interference cancellation in the present of imperfect channel information are analyzed. To achieve the best performance by iteration, the special mapping is used, such as anti-gray map-ping. It can only take effect when the tunnel of iteration is open, and it will be useless easily when the channel state information is mismatched. By analyzing, iterative sys-tem with gray mapping is more robust under imperfect channel information, and the better performance than anti-gray mapping will be achieved when the channel esti-mation error is large. An adaptive transmission system is presented, whose mapping scheme is switched based on channel estimation quality. It is shown that the output extrinsic information is more reliable by switching the mapping scheme or adopting mixed mapping scheme and the performance of system is improved. It is more robust with the imperfect channel information. Additionally, the improved sic-mmse iterative algorithm is proposed, the more reliable extrinsic information will be output with the modified variance of estimated signal. It is shown that the modified algorithm is better than traditional algorithm.