Up-link SCMA Signal Detection In MMTC

In massive Machine Type Communications(mMTC),the existing orthogonal multiple access(OMA)technology has been unable to meet the needs of massive terminal access.Therefore,some non-orthogonal multiple access(NOMA)technologies emerge as the times require.Sparse code multiple access(SCMA)is a non-orthogonal multiple access technology in the 5th generation mobile communication system(5G).SCMA system can provide more services on the same time-frequency resources,which is in line with the characteristics of massive machine communication.However,the complexity of detection algorithm is high in SCMA system,so this thesis studies low complexity detection algorithm in SCMA system.The main research contents are as follows:1.In view of the high detection complexity of current algorithms in uplink synchronous SCMA system,based on the analysis of the existing detection algorithms,the optimal probability message passing algorithm(OP-MPA)is proposed.In this algorithm,the initial conditional probability value is reasonably selected,and a reasonable proportion of conditional probability value is selected for subsequent iteration.At the same time,a weight factor is set to allocate a larger initial probability to the superimposed codeword combination close to the received signal,so as to compensate performance loss caused by the previously discarded part conditional probability value which does not participate in the iteration.Finally,the proposed algorithm is verified by simulation,and the simulation results show that the computational complexity is greatly reduced while ensuring the detection performance.2.In the Grant-free(GF)mode,not only the interactive signaling is reduced,but also the terminal power consumption is reduced,which can better meet the needs of mMTC applications.However,due to the lack of time synchronization signaling,the problem of asynchronous reception in SCMA system cannot be ignored.Existing belief propagation message passing algorithms(BP-MPA)can be used for up-link asynchronous SCMA systems,but the complexity is high.Based on the BP-MPA algorithm,according to the judgment of each resource on the probability of users sending codewords,and selects some highly reliable user nodes not to participate in the iteration of the message passing algorithm,we propose an improved belief propagation message passing algorithm(IBP-MPA).Simulation results show that under the same conditions,IBP-MPA can greatly reduce the complexity while ensuring the detection performance.