Research On The Physical Layer Security Of NOMA System

Non-orthogonal multiple access(NOMA)system with the characteristics of large capacity,high frequency spectrum efficiency has become one of the research hotspots in the fifth generation mobile communication system(5G).Different from the traditional multiple access system,NOMA system adopts power multiplexing technology at the transmitter side,where multiple users' signals are superimposed in the same time-domain,frequency-domain and code-domain and at the receiver side multi-user signal separation is conducted based on successive interference cancellation(SIC)receiver.Currently,the researches on NOMA system mainly focus on the power allocation strategy,the user matching strategy,the SIC receiver and so on.However,there is little interest in physical layer security of NOMA system.Considering physical layer security is the future development direction of wireless communication security,this paper aims to promote the combination of physical layer security research and 5G non-orthogonal system,and actively explores the higher security transmission mechanism in NOMA system.This paper starts with the security analysis of NOMA system,and proposes some physical layer security schemes for different NOMA communication scenarios.The main work and innovation are listed as follows,1.The optimal power allocation is derived under three scenarios: no eavesdropper,the eavesdropper inside group and the eavesdropper outside group,and the system security is further analyzed.The self-interference property of NOMA system has some security,but its security is not clear so far.On the one hand,there are two kinds of eavesdropping scenarios including eavesdropper inside group and eavesdropper outside group,so it is necessary to analyze the security of the NOMA system under different eavesdropping scenarios.On the other hand,considering the intensity of self-interference is closely related to the power allocation,it is also crucial to analyze the optimal power allocation scheme under different scenarios.Simulation results demonstrate that the correctness of the theoretical analysis.2.We study the secure transmission problem of single-input multiple-output NOMA(SISO-NOMA)in large-scale networks,and propose a physical layer security scheme based on cooperative jamming.Firstly,we use the Poisson point process to model the location of the cooperative nodes and the eavesdroppers.Then,derive the closed-form solution of the system outage probability under stochastic cooperative jamming scheme.Stochastic cooperative jamming scheme show poor performance when cooperative nodes' density is too high,so this paper further proposed a cooperative jamming scheme based on limited channel gain,this scheme chooses the cooperative nodes which the Rayleigh coefficient is less than a certain threshold,so it will cause more serious impact on the eavesdroppers.Simulation results demonstrate that the cooperative jamming scheme can significantly improve the security performance of the SISO-NOMA system.3.We study security transmission problem in multiple-input multiple-output NOMA(MIMO-NOMA)system,and propose a secure beam-forming scheme aided by artificial noise.The main task is to design the legal users' beam-forming vectors and artificial noise vector with the goal of maximizing secrecy sum rate(SSRM)under the condition of limited power of base station.Considering the SSRM is a non-convex optimization problem,this paper utilizes semi-definite relaxation(SDR)technique and sequential parametric convex approximation(SPCA)algorithm to transform this problem to a convex optimization problem,which can gain the local optimal solutions by interior point method.When the number of legal users is less than antennas number of base station,an alternative scheme with low complexity is proposed.This scheme transmits noise vector in null-space of legal users' channel,therefore it can ensure that the legitimate users' signals will not affected by artificial noise.Simulation results demonstrate that the proposed two schemes can significantly improve the security performance of the MIMO-NOMA system.