Performance Study On Physical-layer Security For Df Based Dual-hop Mixed Rf-fso Systems

Free space optical(FSO)communication,which shows several superior characteristics such as high transmission rate,large system capacity,free and unlicensed spectrum,and immunity to electromagnetic interference,has become the research focus quickly.However,the quality of the FSO communication can be severely decreased by weather environment,and the transmission length is also limited.Furthermore,with the rapid increase of people's demand for communication,the problem of spectrum resources scarcity in traditional radio frequency(RF)communication is increasingly prominent.In order to enjoy the joint advantages of FSO and RF techniques,and to extend the transmission distance,mixed RF-FSO communication systems have been proposed in academic circles.On the other hand,due to the inherent security problem existing in wireless communication,the theory of physical-layer security(PLS)has been proposed under new situations and circumstances,and its main idea is to utilize the randomness and time-varying of wireless channel to achieve secure transmission.The issue of PLS has been also introduced into mixed RF-FSO systems.To that end,this thesis studies the PLS performance for a dual-hop hybrid RF-FSO system under different scenarios,which in turn can provide an insight for RF-FSO systems from both analytical and numerical point of views in the context of PLS.The main contributions are as follows:1.Analyze the PLS performance for a dual-hop mixed RF-FSO system under the scenario that the FSO link is wiretapped.In addition,the relay node is assumed with a decode-and-forward(DF)protocol,and the RF and FSO channels are subject to Rayleigh fading and Gamma-Gamma turbulence,respectively.Then,analytical expressions for secrecy performance in terms of secrecy outage probability(SOP)and the probability of strictly positive secrecy capacity(SPSC)are derived.Furthermore,the accuracy of the derived expressions is validated by Monte Carlo simulation,and the effects of pointing errors,turbulence,and detection techniques on SOP and the probability of SPSC have been discussed.The results show that a little increasing on turbulence strength would result in a big attenuation to secrecy performance,and the secrecy performance with Heterodyne Detection technique is better than that with Intensity Modulation/Direct Detection technique.2.Furthermore,this thesis also investigates the PLS performance for a dual-hop hybrid RF-FSO system in the presence of multi-relaying and multi-aperture.Compared with the first research content,the RF channel is subject to Nakagami-m fading model in the second research content.In addition,a multi-relaying scheme and a multi-aperture FSO receiver are employed.Hence,the system model in the second research content is more practical and challenging.In particular,the closed-form expressions for the lower bound of SOP and the probability of SPSC are derived with both a partial relay selection protocol and an equal gain combining scheme.Moreover,Monte Carlo simulation results are presented to validate the accuracy of the derived expressions,and the effects of the number of both relay and aperture on the performance of SOP and the probability of SPSC have been discussed.The results show that the multi-relaying scheme can greatly improve the ability of combating fading channel in the RF link,and the multi-aperture technique can remarkably reduce the influence of atmospheric turbulence on the FSO communication,and the system secrecy performance with multi-relaying and multi-aperture schemes outperforms that with single-relaying and single-aperture.