Research On MIMO Techniques For Multi-Beam Satellite Communications

As the multimedia service develops and the transmission rate in terrestrial communication systems increases rapidly,satellite comuunication which is an important complement to terrestrial communication networks and an essential part of the integrated air-space-ground network needs to improve the transmission rate and throughput.Multiple input multiple output(MIMO)techniques can increase the system capacity siginificantly by introducing the space dimension,as the system bandwith keeps the same.Therefore,utilization of MIMO techniques for satellite communications to increase the throughput is an important research topic.Considering that the multibeam satellite system is an important scheme in the future high-throughput satellite communications due to the efficient frequency reuse,MIMO techniques for multibeam satellite communications are researched in the dissertation.The multibeam satellite system and MIMO system models are presented at first,based on which the multibeam satellite MIMO system model is established and the characteristic of the multibeam satellite MIMO system is analyzed.Then,MIMO techniques for the forward link and reverse link of multibeam satellite systems and multibeam dual-polarized satellite systems are studied.The main contents are summarized as follows.Firstly,the precoding techniques for the forward link of multibeam satellite systems are studied.The forward link model of the multibeam satellite MIMO system is established and the performance of three conventional precoding algorithms is evaluated.Considering the high complexity of the lattice reduction aided Tomlinson-Harishima precoding(LR-THP)algorithm,a signal-to-leakage-plus-noise-ratio(SLNR)based low-complexity group precoding algorithm is proposed based on a high-performance group precoding.The proposed algorithm can take advantage of the multibeam satellite MIMO channel characteristics to reduce the dimension of precoding matrices and decrease the computational complexity.However,the generalized eigendecomposition is necessary in the proposed algorithm which results in high complexity when the number of groups is large.In order to reduce the computational complexity further,a generalized minimum mean squared error channel inversion(GMI)based group precoding algorithm is proposed.The GMI and QR decomposition are used to calculate the precoding matrix and the generalizedeigendecomposition is avoided which results in lower complexity.Simulation results show that both of the proposed algorithms have lower computational complexity than the LR-THP and original group precoding algorithms.Besides,the GMI based group precoding algorithm has the lowest complexity and its performance gets close to that of the original group precoding algorithm.Secondly,the multiuser detection techniques for the reverse link of multibeam satellite systems are researched.The reverse link model is established and the performance of conventional multiuser detection algorithms is analysed.The successive interference cancellation multiuser detection algorithm has high complexity and long processing latency when applied to the multibeam satellite MIMO system.In order to deal with this problem a low-complexity group-wise interference cancellation(GIC)algorithm is proposed for the multibeam satellite MIMO system.The users are grouped according to the distance between spot beams in the proposed algorithm.Some groups are detected in parallel after a crucial group-wise interference cancellation.Simulation experiments indicate that the proposed algorithm can reduce the computational complexity and processing latency effectively.The maximum likelihood(ML)detection algorithm has the optimal performance,but its computational complexity is high.To reduce the computational complexity of the ML algorithm,a hybrid ML/GIC detection algorithm is proposed.Users are also grouped based on the distance of different beams.Then,the ML detection is applied in each group and the iner-group interference is mitigated by the hybrid successive/parallel interference cancellation structure.Simulation results prove that the proposed algorithm can achieve a near-optimal performance as the computational complexity is reduced significantly.Thirdly,the MIMO techniques for multibeam dual-polarized satellite systems are researched.The full frequency reuse and dual-polarization per beam are combined in the multibeam satellite system and a larger-size multibeam dual-polarized satellite MIMO system is established.The capacity of the forward and reverse link in multibeam dual-polarized satellite MIMO systems is studied and the performance of block diagonalization(BD)type precoding algorithms in multibeam dual-polarized satellite MIMO systems is evaluated.Then,a transmission scheme based on the BCH-LDPC concatenated code for the forward link is designed and iterative detection and decoding algorithms are applied to achieve a good performance.However,the conventional Turbo iterative detection and decoding algorithm hasa high complexity.To decrease the computational complexity,an iterative detection and decoding algorithm with outer code decision feedback is proposed for the BCH-LDPC coded MIMO transmission scheme.A feedback structure is added after the outer decoder in the proposed algorithm.The feedback information is exploited to reduce the computational complexity of the MIMO detector at each iteration and improve the reliability of the extrinsic information,which results in a lower complexity and better performance.The advantage of the proposed algorithm in complexity and bit error rate(BER)performance is verified by simulations.