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

Wireless communication has evolved from a single voice communication to the current data,images,video,and other communication methods,leading to an increasing demand for bandwidth.The scarcity of Radio Frequency(RF)spectrum resources and the broadcast characteristics of the transmission channel make it easy to generate interference and security problems when adding an RF link,which may degrade the communication quality of the system seriously.Free Space Optical(FSO)uses the laser as the transmission medium.It has high safety factor and directionality,energy concentration and is free from interference caused by frequency and adjacent frequency bands.But it is affected by environmental factors and not suitable for long-distance communication.To complement the two communications,a hybrid RF-FSO system was proposed.It can not only save resources and avoid unnecessary equipment movement but also can utilize the superior performance of FSO communication to improve the utilization of broadband.Based on the physical layer security theory,this paper conducts mathematical modeling and theoretical analysis of the uplink hybrid RF-FSO system and the energy harvesting downlink hybrid FSO-RF system,which will pave the way for subsequent research.The main research content is summarized as follows:1.Mathematical modeling and theoretical analysis of the physical layer security of the uplink hybrid RF-FSO system.The accurate and asymptotic closed-form expressions of the secrecy outage probability and the average capacity of the system when the relay node R is working under AF and DF regimes are deduced,respectively.The results show that the system performance under the AF strategy is obviously better than the DF strategy;by changing the system parameters,the system performance under weak turbulence is better than that under strong turbulence in a certain regime of modulation and demodulation;At the same time,compared with Intensity Modulation/Direct Detection(IM/DD)technology,Heterodyne Detection(HD)technology is better.2.Mathematical modeling and theoretical analysis of the physical layer security performance of the downlink hybrid FSO-RF system based on energy harvesting.Among them,both the destination node and the eavesdropping node are equipped with multiple antennas and can all perform energy harvesting.Compared with the first research content of the hybrid RF-FSO system,the second research content models the main channel and eavesdropping channel as dependent identical distributed channels,and the system model is more practical.The accurate and asymptotic closed-form analytical expressions of the security outage probability of the DF relay node R are deduced respectively based on the probability theory.The correctness of the theoretical derivation is verified through Monte Carlo simulation.The results show that the system performance under weak turbulence is better than that under strong turbulence when a specific parameter keeps unchanged while changing the other parameters.At the same time,HD technology is better than IM/DD technology.