Research For Multiuser Detection Algorithms Based On SCMA Systems

Sparse Code Multiple Access(SCMA)is one of the new multiple access technologies proposed to face the fifth generation(5G)of mobile communications,which has great potential for promoting the development of future wireless communications.SCMA uses sparse spread spectrum,combined with pre-assigned codebooks,to achieve an overloaded system with more users than resources that greatly improves spectrum utilization.Due to the sparseness of the spreading code,the receiver can adopt an iterative loop based message passing algorithm,resulting in an SCMA system that is not complicated to implement and has superior performance.This paper first describes the basic implementation principles of SCMA system,and introduces the principle of SCMA's uplink and downlink transmitters and receivers,compared with the current 4G OFDM system.The key technologies of physical layer in SCMA system,such as the multidimensional codebook design techniques,were analyzed.We also introduced the basic theory of codebook design,such as constellation rotation.For the receiver of the SCMA system,the existing Maximum Likelihood(ML)and Message Passing Algorithm(MPA)detection techniques increase search spaces and the number of iterations with the number of access users increased,that will cause a sharp increase in complexity.The key to multi-user detection in the SCMA system lies in how to quickly and efficiently obtain the transmitted bits of each user from the received superimposed signals.After researching the mainstream MPA multiuser detection algorithm,it is found that combining the channel estimation technique,probability density function,Euclidean distance can reduce the detection complexity simply and effectively.The simulation results show that these detection algorithms can achieve a good balance between complexity and detection performance.Inspired by parallel MAX-Log MPA and serial MPA,Inspired by parallel MAX-Log MPA and serial MPA,this paper presents a serial MAX-Log MPA detection algorithm,and comprehensively analyzes the MPA performance under different strategies.Based on this,this paper also proposes an improved MAX-Log MPA,which can improve its detection performance while maintaining its low complexity.In addition,inspired by the partial external information transmission,in this paper,combined the non-orthogonality characteristics of SCMA,a low complexity and fast convergence SCMA multi-user detection algorithm with serial MPA based on partial codeword partial codewords transmission was also proposed.The simulation results show that the algorithm not only accelerates its convergence,but also reduces its complexity,especially at high signal to noise ratio(SNR).