Physical Layer Security For Dual-Hop VLC/RF Communication Systems

The wireless optical communication technology is mainly to solve the problem of spectrum crowding in the existing radio frequency band,and has the advantages of high communication rate,wide frequency bandwidth,good security performance and no government control.As the research of optical communication becomes more popular and in-depth,visible light is used as an indoor reachable diode light source.On the one hand,it can provide lighting functions in the environment;On the other hand,it can directly send messages to users by optical channels.At this stage,the research on visible light is gradually increasing,not only considering its application in indoors,but also expanding to outdoor,even exploring the underwater visible light.This article mainly applies visible light to communicate in an indoor environment.As long as the optical signal is prevented from reaching the window,the security of communication can be ensured to a large extent.However,considering that visible light does not guarantee 100% indoor coverage,resulting in the problem that some users cannot receive messages,this paper uses a relay collaboration method to build a dual-hop VLC / RF communication system model to expand the coverage of the communication system.The relay in this model is also equipped with an energy harvesting device,which is specially used to collect the DC bias separated from the visible light signal,and use this part of energy to provide power to the signal retransmission.Because the second link of the model belongs to the RF channel,there may be potential security hazards anywhere within its coverage.Based on the above analysis,this paper focuses on the physical layer security of the dual-hop VLC / RF communication system.The work is mainly as follows:Firstly,a system model is constructed in which neither the user nor the eavesdropper is within the coverage of visible light,and can only obtain messages through the RF channel.Under this model,the closed-form expressions of the secure outage probability and average secrecy capacity of the system are calculated,providing their exact analysis and asymptotic analysis,and the effects of different system parameters on the system performance are observed through MATLAB simulation.It can be known from the results that the more energy collected at the relay,the better the overall performance of the system.And when the user's average signal-to-noise ratio tends to positive infinity,the system's secure outage probability and average secrecy capacity are approximately a constant.Secondly,other three models are constructed in this paper,namely,the user is in the visible light coverage but the eavesdropper is outside the visible light coverage,the eavesdropper is in the visible light coverage but the user is outside the visible light coverage,and both the user and the eavesdropper are in visible light coverage.Of course,the prerequisites for these three models are that users and eavesdroppers can receive relayed RF signals.Then we calculate the secure outage probability under these three models separately,and compare them with each other.The difference of system performance and the influence of parameters on system performance under different models were observed by simulation.According to the results,when the user can directly receive the signal of visible light relative to the eavesdropper,the security performance of the system is significantly improved;When the eavesdropper can directly receive the signal of visible light relative to the user,the probability of system interruption becomes greater;When both the user and the eavesdropper can directly receive the signal of visible light,the performance of the system will depend on the communication quality of the user and the eavesdropper to a certain extend.