Wireless Transmission Technology And Security Based On Non-orthogonal Multiple Access

With the rapid development of the fifth-generation mobile communication system,the traditional or-thogonal multiple access technology is difficult to meet the needs of large-scale user equipment connection and ultra-low latency.The uplink grant-free non-orthogonal multiple access(NOMA)system is a kind of code domain NOMA(CD-NOMA),which can effectively solve the problems of signaling overhead and excessive transmission delay caused by the surge in the number of users,making large-scale connections possible.On the other hand,physical layer security(PLS)issues have increasingly become the focus of attention.The specific research work of this article is as follows:This paper first studies the multi-user detection problem of the uplink grant-free NOMA system.Com-bining the existing frame by frame joint sparse model and burst sparse model,that is,considering the situation that some users keep their active state unchanged in the whole frame duration,and also considering the situa-tion that some users only keep active in several adjacent time slots,a more realistic model is proposed.Then a block subspace pursuit aided orthogonal matching pursuit(BSPA-OMP)is proposed for this model.First use the block subspace algorithm to find the users who are active in the entire frame duration as a priori support set for the single-slot solution,and then use the orthogonal matching pursuit algorithm to solve the users who are only active in adjacent time slots on a slot-by-slot basis.The simulation results show that under different signal-to-noise ratios,the bit error rate of BSPA-OMP is always lower than the existing prior information-assisted adaptive subspace pursuit algorithm and prior information-assisted block orthogonal matching pursuit algorithm;Under different sparsity,the BER of bspa-omp is still lower than the two existing algorithms.This paper also studies the PLS problem under the CD-NOMA system,and gives the secrecy outage probability of legitimate users in the CD-NOMA system.For a special kind of CD-NOMA,sparse code mul-tiple access(SCMA),this paper studies its uplink and downlink secure transmission mechanism respectively.In the SCMA system downlink security transmission,the encrypted constellation diagram is obtained by ran-domly rotating the original constellation diagram,thereby realizing the information asymmetry between the legitimate user and the eavesdropper.During the uplink security transmission of the SCMA system,by using the channel phase information between the base station and the legal user,the tent mapping is used to gener-ate a random sequence,and the original codewords are rearranged in the order of the random sequence.The eavesdropper cannot know the phase information between the user and the base station.Therefore,the user data cannot be decoded correctly,and this method realizes safe transmission.