Physical Layer Security For Hybrid RF/FSO Wireless Communication Systems

Free space optics(FSO)system is considered as an alternative technology for radio frequency(RF)systems because of its wide bandwidth,support for more users and cost-effectiveness.However,the performance of the system will be greatly reduced due to the influence of the fading and path loss caused by atmospheric turbulence.In order to solve these shortcomings,a relay-assisted FSO system is proposed,and its performance has been extensively studied in a large number of literatures.However,these results are applicable to symmetrical channel conditions where each hop of links undergoes similar channel fading.In fact,each hop may suffer from different or asymmetric communication environments,which occurs when signals on each hop are transmitted through different communication systems or physically different paths.Therefore,a hybrid RF/FSO system based on the characteristics of RF and FSO has been proposed Since the RF communication system can achieve perfect secure communication by using randomness and time-varying of wireless channel,the security performance of different RF system structures on various channel models has been extensively studied.However,there are very few jobs related to the physical layer security of FSO systems.Therefore,this thesis mainly studies the physical layer security of hybrid RF/FSO communication systems.More specifically,we assume that eavesdropping only occurs on RF link because optical links have high security.We assume that all RF channels suffer from ?-?fading,while the FSO link suffers from M-distribution fading.Then,a series of analysis results of average secrecy rate(ASR)and secrecy outage probability(SOP)are obtained According to the research content,the main work of this thesis is introduced as followsIn this thesis,a hybrid RF/FSO communication system is adopted,and it is assumed that there exists an eavesdropper who can eavesdrop on the information on the RF link Because the beam has high directivity,the FSO link is very secure,so it does not consider the existence of eavesdropping on the FSO link.In order to have a general assumption,all RF links and FSO link suffer from ?-? fading and M fading,respectively The ?-? distribution is able to accurately simulate the small-scale variation of the signals related to nonline-of-sight scenario and includes Hoyt and Nakagami-m fading as special cases.On the other hand,M-distribution is appropriate to the entire range of turbulent conditions and is capable to be easily transformed to the GG and K distributions under certain conditions.Since the variable gain relay needs to consider the instantaneous channel state information of the first link,the fixed gain is not needed,and the fixed gain relay is simple and easy to deploy,but the variable gain performance is better in theory,so the security performance of the two relay systems is considered in this thesis.The closed expression of the cumulative distribution function(CDF)of the system under fixed gain and variable gain conditions is derived.Then the physical layer security performance of the system is further analyzed,and the lower limit asymptotic expression of the system SOP and the closed expression of the ASC under the condition of fixed gain are obtained,as well as the lower limit asymptotic expression of the system SOP and the closed expression of the ASC under the condition of variable gain.A series of simulation analysis is carried out for these expressions under different parameters.