Physical Layer Security Of Intelligent Reflecting Surface Assisted NOMA Networks

With the development of science and technology,wireless terminal equipment is used more frequently,which leads to the growth rate of data traffic going beyond the capacity of existing wireless communication technology,which undoubtedly brings great challenges to the development of wireless communication field.By introducing non-orthogonal multiple access(NOMA)technology and using power domain multiplexing technology to provide services for multiple users with the same time spectrum resource block,spectrum resources can be effectively saved and information transmission efficiency can be improved.At the same time,due to the broadcast nature of wireless communication,the sensitivity of information also increases,which makes the physical layer security problem under NOMA technology more prominent.This thesis will take this as the background to study the physical layer security under NOMA technology.The research on physical layer security is mainly achieved by preventing eavesdroppers from stealing legitimate information.First,aiming at the problems existing in the traditional physical layer security research,this thesis proposes to use intelligent reflecting surface(IRS)to replace the traditional repeater to better achieve physical layer security.IRS can intelligently change the wireless transmission environment,so it can adjust its reflection coefficient intelligently to improve the signal enhancement of the receiver and reduce the signal attenuation of the eavesdropper,so as to realize the physical layer security.In the traditional research,active relay is usually used as the auxiliary relay,which will cause the loss of useful signals,and also has some negative problems,such as high energy consumption,not conducive to deployment and so on.IRS can not only enhance useful signals and weaken eavesdropping signals,but also is easy to deploy with low energy consumption.Therefore,the combination of IRS and NOMA can further improve the network security performance and reduce the system energy consumption.Secondly,this thesis studies the secure transmission in IRS assisted NOMA networks.Considering an actual eavesdropping scenario,a robust beamforming scheme based on artificial noise(AN)is proposed to ensure the secure communication of the NOMA network assisted by IRS.At the same time,an optimization problem of joint transmit beamforming and IRS phase shift is proposed to minimize the base station transmit power.Compared with the traditional research,this combination method can not only improve the physical layer security,but also effectively reduce the system energy consumption.The research value of physical layer security is considerable.However,because the proposed problem is a nonconvex quadratic programming problem,it is relatively difficult to solve.This thesis proposes an effective solution,alternating optimization(AO)algorithm to simplify the nonconvex problem,and use arithmetic geometric average algorithm based on continuous convex approximation and first-order Taylor approximation algorithm to solve the nonconvex problem.At the same time,for the rank one constraint problem encountered in the process of solving,this thesis proposes using the difference of convex(DC)programming algorithm is proposed to solve the rank one constraint problem,and the optimal solution of the problem is obtained.The proposed algorithm can simplify the complex constraints,solve the optimization problem,and get better results,which verifies the rationality and accuracy of the proposed scheme.Finally,through the analysis of simulation results,the proposed AO algorithm can reduce the transmission power of the base station better than the random phase(RP)algorithm,which proves the advantages of the proposed AO algorithm.Adding artificial noise to the transmitter can also reduce the eavesdropping ability of the eavesdropper.Moreover,with the increase of the number of base station antennas and the number of IRS reflection units,the transmission power of the base station can be reduced and the system performance can be improved.The combination of IRS and NOMA can improve the security efficiency of the system.According to the structural advantages of IRS,the original intention of lower energy consumption,lower cost and better performance of the system is realized.In short,the problems proposed in this thesis have achieved the desired results.The combination of IRS and wireless communication can better improve the system performance,which has farreaching research significance.