Research On Sparse Code Multiple Access

The 5th generation of the mobile communication system(5G)should reach the interconnection of all things,requiring the air interface technology to be flexible and resilient.However,current multiple access technologies are restricted by the orthogonal resources and do not meet the massive connectivity needs in a 5G system.Compared with the orthogonal multiple access technology,the non-orthogonal multiple access technology could serve more users with the same time-frequency resources,and therefore it is more applicable to the massive network connection.Sparse code multiple access(SCMA)is a non-orthogonal multiple access technology based on codebook,and can provide performance similar to the optimization of spectrum efficiency.Nevertheless,due to the high complexity of the current SCMA system,reducing the computational complexity and improving system performance has become an urgent issue.This thesis proposes several SCMA optimal schemes aiming to reduce the signal detection complexity while maintaining the performance.The specific research topics are presented as follows:1.A new codebook design scheme based upon a subset segmentation is proposed to reduce the high complexity of codebook design in the SCMA system.This scheme improves the performance through increasing the Euclid distance of constellation points by combining subset segmentation with low density spending matrix.2.A novel message passing algorithm(NMPA)based upon weighted message delivery is proposed to improve the poor performance of a traditional message passing algorithm(MPA).This algorithm realizes a better detection accuracy by introducing weighting factors and optimizing the initial probability of different combinations of codeword.3.A serial MPA detection scheme is proposed to expedite the rate of iterative convergence of traditional MPA.This scheme significantly improves the rate of convergence by applying the cross-correlation between nodes.The grouping scheme is then proposed to cause some operating latency due to the serial iteration.In each group,the messages are updated in parallel,while the processing of groups still remains in serial processing.As a result,this reduces the latency and suits the practical engineering applications well.4.An ultimate implementation scheme of SCMA multiple access link is proposed.This thesis utilizes the characteristics of MIMO channel as well as SCMA system,and analyzes the optimal points from the transmitting terminal codebook design to receive end signal detection.The simulation results prove that the proposed optimized scheme has greatly improved both convergence rate and detection performance.