| In 2020,5G communication technology was commercially available,in which the non-orthogonal multiple access(NOMA)system plays an important role.NOMA systems can allocate different power based on the users’ channel quality,so as to serve more users at the same time and frequency.However,due to the broadcast characteristics of wireless communications,hidden eavesdroppers will affect the physical layer security performance of the system.In addition,the traditional radio frequency(RF)links’ resources are limited,so it is difficult to realize the transmission of the massive data in the future.Therefore,researchers focus on the free space optical(FSO)communication technology with the advantages of high bandwidth and low cost.However,the performance of FSO communication links will be deteriorated by fading caused by the atmospheric turbulence and extreme weather.Therefore,scholars have proposed the FSO/RF hybrid system.When FSO links are interrupted,RF links are used as auxiliary links to complete signal transmissions,but only RF links can not meet the needs of the future communication.In recent years,the communication academic community has started a wave of research on the reconfigurable intelligent surface(RIS).The RIS is widely used in the field of wireless communication due to its advantages of low loss,easy deployment,and no complex coding and decoding.In particular,the application of RISs to RF links can further improve the received signal-to-noise ratio,resulting in the improvement in performance.Therefore,aiming at the physical layer security problem of NOMA systems and the poor performance of RF links in FSO/RF hybrid systems,this thesis proposes two RIS-assisted communication system models.The main research contents are as follows:(1)The physical layer security performance of an RIS-assisted multi-user NOMA system is simulated,modeled and theoretically analyzed.Assuming the worst case,an eavesdropper can also take advantage of the RIS and receive the signals reflected from the RIS.The secrecy outage probability and the SOP asymptotic expression are derived,and the accuracy of the analysis is verified by MATLAB simulations.The results show that the use of RISs can improve the security performance of the physical layer compared with the traditional NOMA systems.However,due to the existence of an eavesdropper,increasing the number of RIS reflecting elements will deteriorate the security performance,while increasing the number of users can improve the security performance through the group selection.(2)The system performance of an RIS-assisted FSO/RF hybrid transmission system is simulated,modeled and theoretically analyzed.The FSO link is regarded as the main link.When the main link is interrupted,the RIS-assisted RF link is used as the backup link for transmission.The accurate expressions of the outage probability,the average bit error rate and the channel capacity are derived,and the accuracy of the analysis is verified by MATLAB simulation.The results show that the performance of the RIS-assisted FSO/RF hybrid system is significantly improved compared with the traditional hybrid FSO/RF system. |
