Codebook Design And Multi-User Detection Algorithm In SCMA System

At present,5th generation mobile communication technology(referred to as 5G)has officially entered the commercial stage in China.Compared with the fourth-generation mobile communication technology(referred to as 4G),5G requires more user access and higher spectrum resource utilization efficiency.Sparse Code Multiple Access(SCMA)as a nonorthogonal multiple access(Non-orthogonal Multiple Access,NOMA)technology,can better meet the demand.SCMA technology is an extension of Low-Density Signature(LDS)technology.SCMA technology integrates Quadrature Amplitude Modulation(QAM)and LDS coding into one step,each user has their own specific codebook,and the codebook is sparse.At the sending end,users directly map the bit data stream into multi-dimensional codewords,and different codewords of multiple users are superimposed on the same resource.At the receiving end,multi-user detection is performed using Message Passing Algorithm(MPA).In this thesis,the codebook design and multi-user detection algorithm of sparse code multiple access system are investigated concretely.The main research contents and achievements are as follows:(1)According to the hexagonal lattice formed by Eisenstein integer in the complex plane as the optimal lattice,a new SCMA codebook design method is proposed.First,the total constellation is constructed with the Eisenstein integer points on the complex plane,and secondly,the subset constellation is directly selected on the total constellation to generate the user codebook.The codebook designed by this method has better shaping gain.Simulation results show that the codebook proposed in this paper has better BER performance under high signal-to-noise ratio.(2)In order to solve the problem of low BER performance when the threshold is low in the threshold MPA algorithm,an improved threshold MPA algorithm is proposed.After each message update iteration,the credibility of the function node corresponding to the user variable node is judged.When the position of the credibility of all function nodes of a user is the same,and the credibility of all function nodes meets the preset threshold,the user is decoded in advance.The users who have decoded will not continue to update in subsequent iterations,thereby reducing the complexity of the algorithm.Simulation results show that the proposed algorithm is better than the threshold MPA algorithm in terms of convergence speed and BER performance.Compared with the original MPA algorithm,the algorithm greatly reduces the computational complexity while ensuring BER performance.(3)In order to solve the problem of low BER performance in the user-stabilized MPA algorithm,an improved user-stabilized MPA algorithm is proposed.After each iteration of the message update iteration,the location of the most credible function node corresponding to the undecoded user variable node is found.If all the function nodes of a user have the same position with the highest credibility,and the positions of the most credibility of the function nodes in the current iteration and the previous iteration are also the same,the user is decoded in advance.Simulation results show that the algorithm's BER performance and convergence speed are better than the user-stabilized MPA algorithm.Compared with the original MPA algorithm,the algorithm greatly reduces the computational complexity while ensuring BER performance.