| With the iterative development of wireless communication technologies,especially the commercialization of 5G networks,wireless communication is facing more complex application scenarios.For further research on 6G networks,researchers are increasingly focusing on reducing energy consumption,reducing system complexity,and reducing path loss during over-the-horizon transmission.In recent years,researchers have proposed a new technology,Reconfigurable Intelligent Surface(RIS),which in some ways compensates for some of the shortcomings of traditional relaying techniques such as amplification and forwarding,decoding and forwarding.When relaying is required to enhance the transmission of signals,the low cost and low power consumption of RIS allows it to be deployed on a large scale,reducing hardware costs compared to traditional relaying methods.RIS is composed of an artificial surface made of electromagnetic(EM)materials.Compared with traditional relay methods,RIS is characterized by its low cost and low power consumption,making it suitable for large-scale deployment and application,thereby reducing hardware costs.At the same time,each reflection unit of RIS is phase-adjustable,which can be used to enhance or cancel directional signals,giving RIS a significant advantage in physical layer security(PLS).Therefore,in-depth research on RIS-related technologies has important significance for solving existing problems and future wireless communication development.This article focuses on the relay selection problem of RIS as a relay node to assist wireless communication.Firstly,the practical significance and research background of RIS are introduced,and the theoretical knowledge that needs to be paid attention to when RIS is used as a relay node is briefly introduced.These include wireless channel characteristics,common fading channels,physical layer security issues,RIS-related knowledge,duplexing mode,and reliability indicators of cooperative communication systems.Secondly,this article proposes the relay selection problem of RIS in full-duplex bidirectional systems.Three mathematical analysis methods are used to analyze the interruption probability of the system when RIS is used as a relay,including gamma distribution approximation,non-central chi-square distribution(NCCS)precise solution,and precise solution of distribution.The advantages and disadvantages of the three methods are compared and analyzed.In addition,RIS optimization for relay selection problems with multiple RIS links and multiple AFR links is studied.Finally,the accuracy of the proposed expressions is proven through Monte Carlo simulation.Finally,this article proposes the optimization of RIS for physical layer security in full-duplex communication systems.When using security performance as the evaluation metric,the relay selection problem for multiple RIS links is studied.For eavesdropping models,analysis is conducted on both the ideal phase error-free and phase error models.At the same time,the relay selection problem of RIS is studied with secrecy capacity as the selection criterion.The simulation results confirm that RIS has a significant optimization effect on PLS,and the correctness of the maximum secrecy capacity selection strategy is verified through Monte Carlo simulation. |
