Research On Low Complexity Detection Algorithm Based On SCAM System

Sparse Code Multiple Access(SCMA)technology is a new technology of multi-dimensional codebook and non orthogonal multiplexing.Therefore,it has the advantages of large capacity,low delay,high transmission rate and many transmission channels.In SCMA,although the Message Passing Algorithm(MPA)has good system bit error ratio(BER)and algorithm complexity.However,due to the exponential algorithm used in this algorithm,the computation of multi-user detection algorithm in SCMA system is very large.The Message Passing Algorithm based on Maximum logarithm(Max-log-MPA)adopts approximate and maximum logarithm operations,and it will result in the loss of data transmitted by some users.It causes the problem of poor accuracy of user information transmission in SCMA system.The Threshold based Message Passing Algorithm(T-MPA)adopts hard decision mechanism,and it can reduce the computation of multi-user detection algorithm.However,there is a problem of poor bit error ratio(BER)when the threshold is low.The Serial based Message Passing Algorithm(S-MPA)combines the message update of variable node and function node in the process of user information transmission.It can effectively reduce the complexity of the detection algorithm.But it will affect the transmission accuracy of variable node information in SCMA system.In view of the problems of poor accuracy and high complexity of user information transmission in SCMA system,this paper presents a Threshold-based Max-log-MPA Message Passing Algorithm(T-Max-log-MPA).In this algorithm,the maximum logarithm operation is used to reduce the power,then the threshold value is set to judge whether the user information meets the transmission conditions,and finally the variable node stability is judged.Before updating the user message,judge whether the variable node codeword reaches the stability of the variable node,and then judge the threshold.When the variable node information satisfies the threshold condition and the decision of the variable node stability,it can be released from the iterative update process.Therefore,it can effectively improve the accuracy of user information transmission and the complexity of system detection algorithm.Although T-Max-log-MPA algorithm improves the transmission accuracy of SCMA system and reduces the complexity of algorithm to a great extent,the bit error ratio(BER)is still poor at low threshold,and the algorithm complexity effect is not the best.On this basis,this paper presents a Max-log-MPA Message Passing Algorithm based on Serial and Threshold(S-T-Max-log-MPA).The algorithm also uses approximation and maximum logarithm operation,and uses threshold condition judgment and variable node stability judgment.Then,the variable node message update is combined with the function node information.So the system can keep good bit error ratio,especially when the threshold is low.