Codebook Design And Serial Multiple User Detection In Sparse Code Multiple Access System

Sparse Code Multiple Access(SCMA)is a new non-orthogonal multiple access technology aimed to 5G,which can access to more users in limited resources so that the utilization of resources can be raised.On the basis of Low-Density-Signature Multiple Access(LDS-MA),SCMA combines modulation module and sparse spread spectrum module into SCMA encoder,and directly maps user bit information to a multidimensional complex codeword.By using the sparse codebook,the SCMA system can obtain better coding and shaping gain than the LDS system,thus the better bit error rate performance of the system can be achieved.One of the key problems in the study of the SCMA system is to design a codebook with good performance at the transmitter of the SCMA system.Trellis Coded Modulation(TCM)codebook is a better design scheme in the existing SCMA codebook design scheme.This paper proposes an improved TCM codebook called Latin-TCM codebook to solve the problem that the mapping rules between the sub constellations and resource block of the TCM codebook are random and the system performance is unstable.The improved Latin-TCM codebook scheme first uses a "TCM" subset to get the sub constellation of each user on the subcarrier.Then each user codebook is designed based on the constellation matrix,which is got by mapping each user to each subcarrier by the characteristics of the "Latin" rectangular matrix.The constructed Latin-TCM codebook determines the mapping rules between the sub constellation and resource block,and overcomes the defects of the system performance instability.The Latin-TCM codebook combined with the optimized multi-user detection algorithm can reduce the decoding complexity of the receiver under the premise of guaranteeing the robustness of the SCMA system,so that the SCMA system can have better comprehensive performance.The simulation results show that the Latin-TCM codebook is better than the TCM codebook to improve the bit error rate performance of the SCMA system.In the SCMA system,the Message Passing Algorithm(MPA)with low complexity is used for multi-user detection,but as the number of users increases rapidly,the complexity of MPA algorithm will also increase.Aiming at the problem that the number of iterations of the serial MPA multi-user detection algorithm for SCMA system could not be adjusted dynamically,a serial MPA multi-user detection algorithm with low complexity,called R-THMPA(Residual Threshold-aided MPA)algorithm,which could adjust the number of iterations dynamically is proposed.The proposed R-THMPA algorithm defines the difference values of marginal function between the user nodes and the resource nodes before and after iterations as the residual value.Before the new iterative decoding process starts,all the residual value between the user nodes and the resource nodes are compared with the threshold values respectively.If the threshold condition is not satisfied,the new iteration is only iterated to the user node with maximum serial number whose residual value is greater than the threshold value.Repeat the iterative process until the threshold condition is met or the maximum iterations are achieved.Simulation results show that the bit error rate performance and detection complexity of the SCMA scheme are balanced better with the proposed R-THMPA algorithm.Combined with the Latin-TCM codebook,the system can achieve a better bit error rate performance with fewer iterations.