Theoretical Research On The Physical Layer Security Of Mixed RF-FSO Systems With Imperfect CSI

Free Space Optical(FSO)uses the laser as a carrier to propagate signals in the atmosphere with the high safety factor,which has the advantages of concentrated direction,non-interference,and no frequency permission.FSO communication is highly susceptible to atmospheric turbulence during the propagation process,resulting in reduced communication quality and limited propagation distance.On the other hand,as people's requirements for wireless communication are increasing,it is easy to cause interference and security problems when adding radio frequency(RF)links.The mixed radio frequency(RF)/FSO system can fully complement the advantages of the RF link and the FSO link to save resources,increase transmission distance,and improve system performance.During the process of signal transmission,it is impossible for the receiver to obtain Channel State Information(CSI)without estimation error.In addition,the channel changes fast causing the CSI accepted by the receiver to be outdated compared to the CSI at the time of feedback.Moreover,due to the excellent openness of wireless communication systems,the analysis on physical layer security performance of mixed RF/FSO system has received long-term attention.Based on the physical layer security theory,this thesis conducts mathematical modeling and theoretical analysis of the RFFSO system with imperfect CSI,which will pave the way for subsequent research.The main research content is summarized as follows:1.Under the Amplify-and-Forward(AF)relay scheme and the Decode-and-Forward(DF)relay scheme,the physical layer security performance of the mixed RF(SIMO)/FSO system with imperfect CSI is analyzed.The Cumulative Distribution Function(CDF)of mixed signal-to-noise ratio(SNR)is derived,and the closed-form expressions of the Secrecy Outage Probability(SOP)is derived.In addition,when the signal-to-noise ratio tends to infinity,the closed-form expressions and asymptotic expressions of SOP are derived by using the reasonable mathematical approximation and the results of the theoretical analysis are verified by Monte Carlo simulation.The results show that the larger the system pointing error,the smaller the signal-to-noise ratio achieved by different detection methods,the worse the weather factor and the higher the imperfection degree can affect the safety performance of the system.2.Under the Decode-and-Forward(DF)relay scheme,the physical layer security performance of the mixed RF-FSO system with imperfect CSI is analyzed.The thesis analyzes the distribution law of universal channel Málaga and derives the probability density function(PDF)and CDF of the signal-to-noise ratio of the link.In order to improve system performance,three different antenna selection strategies are proposed and the closed-form expressions and asymptotic expressions of SOP are derived.The Monte Carlo simulation results are used to compare different antenna selection strategies.The results show that the OTAS has the best security performance in the proposed antenna selection strategy.The TASR and TASE can be selected through the link channel state information.The proposed three TAS can improve the system security performance..