Research On Key Techniques Of SCMA Systems For LEO Satellite Communications

Nowadays,the research on the sixth generation mobile network(6G)is in the initial stage.As an important part of 6G,the satellite Internet of Things cooperates with the ground Internet of Things to achieve full coverage and the real “Internet of Everything”.Benefiting from the low manufacturing costs and short transmission distance of low earth orbit(LEO)satellites,the deployments of LEO satellites are in progress at home and abroad.However,the available frequency and orbit resources are limited.How to support massive connections with scarce resources has become a research hotspot of LEO satellite communications.Sparse code multiple access(SCMA)technique can support multiple users at one resource to improve system capacity and spectral efficiency,which alleviates the shortage of spectrum resources in LEO satellite communications.Therefore,the key techniques of SCMA for LEO satellite communications are studied in this thesis.To begin with,a PAPR-reduction SCMA codebook design is proposed to meet the challenge of the limited transmission power in the downlink LEO satellite communication system.In this scheme,the SCMA symbols corresponding to the constellation points with large power are reassigned to the constellation points with small power in the mother constellation.Then,the constellation rotations of users are determined to maximize the phase difference among the superimposed users' constellation points with maximum power.The simulation results show that the SCMA codebooks designed by the proposed scheme can effectively reduce the PAPR of the superimposed constellation compared with the existing codebook.Then,the detection algorithm for the SCMA system is studied and a low-complexity multiuser detection algorithm is proposed.The calculations at nodes are simplified through the first-order approximation of the message passing algorithm,and a novel framework of SCMA mapper is designed.Thus,a soft-out approximate message passing algorithm is obtained.In addition,in order to offset the performance loss caused by the approximation,the damping factor is introduced to the proposed algorithm.Simulation results show that the proposed algorithms with the channel codes can achieve linear complexity at the expense of less performance,which reduce the processing delay of the receiver effectively.Referring to single-carrier and orthogonal multi-carrier systems,the carrier synchronization module and the timing synchronization module are utilized for the SCMA system to combat the large Doppler shift and transmission delay in LEO satellite communication systems.For the carrier synchronization module,data-aided algorithms are used to estimate frequency offset and phase offset,respectively.And the frame structure is optimized according to the characteristics of the algorithm,which can achieve the performance close to the ideal condition with less pilot overhead.For the timing synchronization module,the synchronization algorithm with the help of decoders' feedback is used,which can get accurate estimation of transmission delay with less external iterations.Finally,the hybrid automatic repeat-request(HARQ)scheme for SCMA system is studied to meet the demand of high reliable transmission.An optimized HARQ scheme is proposed based on the existing incremental redundant HARQ scheme.Compared with the original scheme,the optimized scheme can effectively improve the system throughput with almost the same performance.