Codebook Design And Parallel Multiple User Detection For Sparse Code Multiple Access System

Sparse Code Multiple Access(SCMA)technology is one of the candidates for multiple access techniques in the fifth generation mobile communication system(5G).It is evolved from Low Density Signature(LDS)technology.Compared with the LDS technology,SCMA technology combines the quadrature amplitude modulation(QAM)modulator and the sparse spreader in the LDS technology to jointly optimize and map the users' data bits directly into the multi-dimensional codewords in SCMA codebook,which makes SCMA system obtain the shaping gain that does not exist in LDS.In addition,the sparsity of the SCMA codewords enables the SCMA system to perform a low complexity multiple user detection(MUD)based on message passing algorithm(MPA)to approximate the bit error rate(BER)performance of the maximum a posterior(MAP)detection.Considering SCMA technology as the research object,this thesis focuses on the SCMA low complexity MUD algorithms and the SCMA codebook design.The main contributions are as follows:For the problem that threshold-based MPA has a high BER when the threshold is low,an improved SCMA multiple user detection scheme is proposed in this thesis.Based on the threshold MPA scheme,the proposed scheme adds a judgment on the necessary conditions for the stability of user nodes.The users who not only satisfy the threshold criterion but also pass the judgment on the necessary conditions for the stability of user nodes can be decoded in advance.This improves the reliability of codewords which are judged in advance and reduces the loss of posterior soft information caused by the detection mechanism similar to the hard decision.Compared with the threshold-based MPA scheme,the proposed scheme allows the messages to be iterated more fully at low thresholds,which makes SCMA users achieve a better BER performance at low thresholds.The simulation results show that a better BER performance is achieved with the proposed scheme than that with the threshold-based MPA scheme for SCMA users.For the problem that all the constellation points of the total constellation in the SCMA codebook design based on subset segmentation method of Trellis Coded Modulation(TCM)are completely distributed on the ring and cannot make full use of the entire constellation space,an improved SCMA codebook design is proposed in this thesis.Based on the Lattice constellation,this improved scheme combines constellation rotation and constellation scaling with TCM subset segmentation,which not only makes full use of the constellation space,but also introduces the correlation among non-zero elements of the codewords and the energy difference among different codewords.Compared with TCM codebook design scheme,the improved codebook design proposed in this thesis not only can make more use of the constellation space,but also can make the MPA multiple user detector at the receiver of SCMA system perform more effectively,so that a better BER performance is achieved.The simulation results show that a better BER performance of SCMA users is achieved with the improved codebook design scheme proposed in this thesis than that with the TCM codebook design.The above work is of great value to the further development of SCMA technology.