Research For SCMA Multiuser Detection Algorithm

As a broad prospect for 5G wireless air interface technology,Sparse Code Multiple Access(SCMA)is a non-orthogonal multiple access technology of better spectrum efficiency.The existing SCMA multi-user detection algorithm mainly focuses on the Message Passing Algorithm(MPA),which is efficiently approximate to the optimum Maximum A Posterior(MAP).The Parallel MPA(PMPA)is the basic one,where in each iteration all Function Nodes(FNs),and subsequently all the Variable Nodes(VNs),pass new messages to their neighbors.The improvement of MPA algorithm comes from two perspectives,i.e,improving the Bit Error Rate(BER)performance and reducing the computational complexity.In fact,they consider the fact that the existing SCMA multi-user detection algorithms are based on PMPA,where the convergence rate is not ideal and the computational complexity is high.As a result,two types of improved algorithms are proposed,i.e.Serial Strategy based MPA algorithm(SMPA)and Dynamic message scheduling based MPA with Residual aided(R-DMPA).SMPA algorithm are further divided into two types,known as Function Node Serial MPA(FN-SMPA)and Variable Node Serial MPA(VN-SMPA),based on whether the function node or variable node order is followed in the detecting process.Since the updated messages can join the message propagation immediately in current iteration,the convergence rate of the SMPA is accelerated.Furthermore,the R-DMPA algorithm exploits the dynamic selection of the message which has the largest residual,whenever messages are updated sequentially.Theoretical and simulation results show that the R-DMPA algorithm has the fastest convergence rate.The convergence of the MPA-based algorithms is analyzed theoretically by using the Extrinsic Information Transfer(EXIT)mathematical tool.First,the SCMA receiver detector is divided into a FN decoder and a VN decoder,and then the EXIT theoretical analysis is presented.Finally,the number of iterations needed to converge is estimated by comparing the EXIT curve of the PMPA,VN-SMPA,and R-DMPA algorithms.The simulation results confirm the accuracy of the EXIT graph analysis.