RIS-Assisted Secure Communication In Non-Orthogonal Multiple Access Systems

Reconfigurable intelligent surface(RIS)is a new communication-assisting technology that has emerged in recent years and has received significant attention due to its ability to control electromagnetic waves.RIS can reconfigure incident signals effectively to enhance signal power for legitimate users;it can also reconfigure incident signals destructively to avoid information leakage to eavesdroppers,and it can create additional reflective links to aid communication.Non-orthogonal multiple access(NOMA)techniques can serve multiple users in the same resource block by actively introducing interference information,which is a good way to improve spectrum efficiency,but increases the complexity of the transmission scheme as it requires continuous interference cancellation at the receiver.In RIS-assisted nonorthogonal multiple access networks,the RIS needs to achieve a trade-off between ensuring successful successive interference cancellation decoding and channel enhancement or suppression for the user,which may result in the legitimate communication channel being weaker than the eavesdropping channel,leaving legitimate information vulnerable to eavesdropping.As the research on RIS-assisted secure communication for uplink nonorthogonal multiple access is not sufficient at home and abroad,this thesis conducts an indepth study of it,which mainly includes the following aspects:(1)Considering the information transmission security of a single-RIS-assisted multi-user communication system,we investigate maximizing the total secrecy rate of the system by optimizing the reflected power of the users and the phase shift of the reflected elements of the RIS,while satisfying the users' transmit power constraints,the users' quality-of-service constraints,and the rank-one constraint of each reflected element.Although the optimization problem involved is nonconvex and the optimized variables are highly coupled with each other and difficult to solve,this thesis proposes an alternating optimization and semidefinite relaxation-based algorithm to find the suboptimal solution to it.Finally,it is verified by computer simulation results that the proposed algorithm significantly improves the total secrecy rate of the system,as compared to several benchmark solutions.(2)We consider a multi-user secure communication system with the assistance of multiple RISs.The problem of maximizing the total secrecy rate of the system is investigated to ensure the quality of communication services for multiple users.This problem can be solved by decoupling the optimized variables into three subproblems using an alternating optimization method.Furthermore,each subproblem is further transformed into a convex optimization problem by applying the semidefinite relaxation and successive convex approximation methods.Finally,computer simulation results verify that the proposed algorithm can improve the total secrecy rate of the system better than other benchmark schemes.And it is demonstrated that multiple RISs cooperatively assisting a secure communication system can guarantee the system's secure communication better than a singleRIS-assisted secure communication system.