Research On RIS Aided Physical Layer Security Technology For Diverse Scenarios In 6G

The open nature of wireless channels constantly exposes communication data to the risk of eavesdropping.In 6G(6th Generation Mobile Communication System)networks,the number of connected devices is surging,the network architecture is becoming more distributed,and the security boundaries between network domains are increasingly blurred,leading to a significant increase in potential eavesdroppers in multi-user communication scenarios.Additionally,satellite communication,as a crucial component of the 6G space-air-ground integrated concept,will play an important role in future networks,and its security is also of paramount importance.Building on existing encryption systems,physical layer security technology leverages the inherent security properties of wireless channels to provide lightweight security assurances based on information theory.On the other hand,as a potential key technology for 6G,reconfigurable intelligent surfaces(RIS)can manipulate the propagation of wireless electromagnetic waves by adjusting their electromagnetic response characteristics,effectively enhancing physical layer security performance.Currently,RIS-assisted physical layer security technology is receiving considerable attention.However,several issues remain unresolved in multi-user scenarios,satellite communication scenarios,and non-ideal scenarios:(1)Existing RIS-assisted multi-user secure transmission schemes have low security rates and limited application scenarios;(2)Existing RIS-assisted multiuser secure transmission schemes lead to a sharp increase in energy consumption in the 6G era;(3)Residual hardware impairments in transceivers negatively impact key generation rates,and current research lacks countermeasures;(4)Existing RIS-assisted secure transmission schemes for satellite communications have low energy efficiency and limited application scenarios.In addressing these issues,this dissertation focuses on the technical pathways of ”leveraging intelligent reflecting surfaces(IRS)to manipulate wireless channel environments and enhance the upper limit of physical layer security performance” and ”utilizing intelligent transmissive surface(ITS)antennas to control information transmission and improve physical layer security energy efficiency,” aiming to explore the design and performance optimization of RIS-assisted physical layer security schemes for 6G multi-user scenarios,satellite communication scenarios,and nonideal scenarios.The specific research work and contributions of this dissertation are outlined as follows:1.Addressing the issues of low security rates and restricted usage scenarios in existing RISassisted multi-user secure transmission schemes,this dissertation proposes a cooperative double IRS-assisted multi-user secure transmission scheme.This scheme deploys two cooperative IRSs on the channel side to manipulate the reflected signal beams,while the base station injecting artificial noise into the transmitted information to disrupt eavesdroppers.Under constraints of maximum base station transmission power and double IRS unit modulus,a joint design of base station beamforming vector,artificial noise precoding vector,and double IRS phase shift vector is proposed to enhance the weighted sum secrecy rate(WSSR)of the system.Specifically,a cooperative double IRS-assisted multi-user secure transmission model is first established,followed by the formulation of a WSSR optimization problem,and the design of a low-complexity joint beamforming algorithm for solution.Initially,the successive convex approximation(SCA)technique is utilized to derive an approximate concave lower bound of the objective function.Then,the Lagrangian dual method and the alternating direction method of multipliers(ADMM)are employed for the alternating design of the base station beamforming vector,artificial noise precoding vector,and IRS phase shift vector,achieving an approximate optimal solution to the proposed optimization problem.Theoretical proof demonstrates that the proposed algorithm can converge to a locally optimal solution.Simulation results indicate that the performance of the proposed algorithm approaches the performance upper bound of the scheme,validating the advantages of the proposed cooperative double IRS scheme over single IRS and non-cooperative double RIS schemes.2.In response to the challenge of sharply increased energy consumption faced by existing RIS-assisted multi-user security schemes in the era of 6G,this dissertation proposes a joint IRS and ITS-assisted multi-user secure transmission scheme.In this scheme,the base station adopts ITS antennas to replace traditional antenna arrays,reducing power consumption and hardware costs.IRS is deployed on the channel side to control the wireless propagation environment,in coordination with base station secure communication.Under constraints of maximum base station transmission power,ITS beamforming,and IRS unit modulus,an optimization problem is formulated with multi-user power allocation factors,ITS beamforming vectors,and IRS phase shift vectors as optimization variables,with system WSSR as the optimization objective.Beamforming algorithms are designed to enhance system performance for both ideal channel state information(CSI)and non-ideal eavesdropping CSI scenarios.For the ideal CSI scenario,the SCA method is used to transform the original objective function into an easily handled approximate form.Subsequently,an alternating optimization process decouples the transformed problem into three sub-problems for alternate solution.For the multi-user power allocation sub-problem,a low-complexity algorithm based on the Lagrangian dual method is proposed to obtain a semi-closed-form solution.Two low-complexity beamforming algorithms are designed using the ADMM method and the majorization-minimization(MM)method to obtain local optimal solutions for the ITS beamforming sub-problem and IRS phase offset sub-problem,respectively.Simulation results validate that the proposed scheme achieves approximate optimal performance.For the non-ideal eavesdropping CSI scenario,considering line-of-sight obstruction scenario,a norm-bounded error model is used to model the non-ideal CSI.For WSSR optimization,a robust algorithm for maximizing WSSR is designed using SCA technique,Cauchy-Schwarz inequality,Taylor expansion formula,and penalty function method,then combined with Dinkelbach method to design a robust algorithm for maximizing secure energy efficiency(SEE)to optimize the system SEE.The computational complexity of the proposed robust algorithm is analyzed,and simulation results verify the security and energy efficiency of the proposed robust algorithm.3.Addressing the issue of reduced key generation rate due to residual hardware impairments,this dissertation proposes an IRS-assisted key generation scheme under hardware impairment conditions.This scheme utilizes IRS on the channel side to manipulate reflected signals to assist legitimate communication parties in completing key generation,and designs a robust beamforming algorithm to improve the key generation rate.Firstly,the key generation process assisted by IRS under hardware impairment conditions is analyzed,and a closed-form expression for the key generation rate is derived.Then,under constraints of maximum base station transmission power and IRS unit modulus,an optimization problem is formulated with base station beamforming vector and IRS phase shift vector as optimization variables to maximize the key generation rate.Semidefinite relaxation(SDR)and Gaussian randomization methods are employed to obtain the optimal beamforming vector,and penalty function method is used to obtain the optimal IRS phase shift vector.The computational complexity of the proposed algorithm is analyzed,and simulation results validate the robust security performance of the proposed algorithm.4.Addressing the issue of low energy efficiency and limited usage scenarios in existing RISassisted satellite communication security transmission schemes,this dissertation proposes a highenergy efficiency satellite security transmission scheme assisted by joint IRS and ITS.In this scheme,the satellite utilizes ITS antennas to replace traditional antenna arrays,reducing hardware costs and power consumption,while deploying IRS and cooperative interference nodes on the ground to respectively assist satellite secure communication and interfere with eavesdroppers.Under constraints of maximum satellite and interference machine transmission power,ITS beamforming,and IRS unit modulus,a SEE maximization problem is formulated.For this non-convex problem,the Dinkelbach method is first utilized to handle its fractional objective function,followed by using the SCA technique to transform the objective function.Subsequently,the Riemannian manifold optimization(RMO)method and element-wised block coordinate descent(EBCD)method are employed to design ITS beamforming vectors and IRS phase shift vectors.The Lagrangian dual method is used to design multi-user power allocation factors and interference machine precoding vectors,and a low-complexity beamforming algorithm based on alternating optimization is proposed.Simulation results demonstrate that the proposed algorithm achieves performance close to the performance upper bound of the scheme.