Research On Physical Layer Security And Delay Guarantees In Visible Light Communications Networks

In next-generation wireless networks,providing secure transmission and delay quality of service(Qo S)guarantees are two critical goals.Owing to the unlicensed broadband visible light spectrum(400 to 800 THz),the achievable rate of visible light communications(VLC)can easily reach the Gbps level.Therefore,VLC is expected to provide strict delay Qo S guarantees.Besides,due to the fact that light can be blocked by walls or by opaque objects,VLC has the natural advantage of security compared to traditional radio frequency(RF)communications.Hence,VLC has remarkable market potential in next-generation indoor wireless networks.However,since the wireless optical channel has a broadcasting nature,user’s confidential information may still be intercepted by other users when VLC networks are deployed in public areas.Therefore,the wireless transmission security of VLC networks remains to be investigated.Besides,different from RF networks,the service process of the VLC network is easy to be affected by the random blockage,field-of-view of the receiver,users’ random mobility and so on.The statistical delay Qo S guarantee method for the VLC network is worth to be explored.Further,for the VLC network with both secure and delay guarantees requirements,either of them requires a concession on the transmission rate.Hence,it is interesting to study that how to support secure and delay guarantee by allocating the VLC network resource,and how to depict the trade-off between secure guarantee,delay guarantee and transmission rate.Based on the network information theory and the effective capacity theory,this dissertation investigates the impact of secure guarantee,delay guarantee and secure-anddelay guarantee on the transmission rate of VLC systems.Then,based on the convex theory and the stochastic optimization theory,we study the physical layer security(PLS)guarantee and statistical delay Qo S guarantee methods for VLC networks with several factors taken into consideration,such as user mobility,random blockage of VLC channel,users scheduling and so on.In addition,we propose several resource allocation algorithms for VLC networks with secure and delay guarantees.The main contributions and innovative points of this dissertation are as follows.(1)In the aspect of PLS guarantee of VLC networks,we pay attention to intelligent reflecting surfaces(IRS),which can provide a controllable wireless environment and improve the security for RF wireless transmission by intelligently optimizing the amplitude and phase of IRS.For the first time,we apply IRS to VLC secure system and propose the IRS aided VLC PLS technique in this dissertation.First,for the VLC IRS system implemented by an intelligent mirror array,we derive the IRS channel gain according to geometrical optics and then derive a lower bound of the achievable secrecy rate based on network information theory.Further,to enhance the secrecy performance of the VLC system,we formulate an achievable secrecy rate maximization problem to find the optimal orientations of each mirror.Our objective is to propose an IRS-aided PLS technique for the VLC system by enlarging the difference between the channel gain of the legitimate user and the eavesdropper.Finally,since the sensitivity of mirrors’ orientations on the IRS channel gain makes the optimization problem hard to solve,we transform the original problem into a reflected spot position optimization problem,which greatly reduces the complexity of the problem.Our simulation results show that the achievable secrecy rate can be significantly improved by adding an IRS in a VLC system.(2)In the aspect of delay guarantee of VLC networks,this dissertation investigates the statistical delay analysis method and the delay-guaranteed resource allocation algorithm.(1)In the indoor downlink VLC system,considering the impact of user’s mobility on delay guarantee,we propose a user mobility model for indoor VLC systems based on the discretetime Markov chain theory.With the help of the mobility model,we map the impact of mobility on the distance and angle of the VLC line-of-sight link to the service process of the VLC system.Then,we build the Markov service model for the mobile user in the VLC system.Further,based on effective capacity theory,we derive the effective capacity for indoor downlink VLC system with the Markov service process taken into consideration,which characterizing the statistical delay-constrained transmission rate of the mobile user.Finally,we analyze the effects of various factors on the effective capacity,including different delay demands,field-of-view of the receiver,the parameters of mobility model,etc.(2)In the uplink VLC system,since the effectiveness of carrier sensing is restricted seriously by the directionality of light in VLC,we adopt the multi-packet reception(MPR)technique to alleviate collisions for the uplink VLC system.Since the implementation of MPR in random access system is challenging,we design a two-period frame structure including the access request period and the data transmission period.Jointly considering the random blockage of the VLC channel and the random-access behavior of users,we derive the successfully requesting probability of access request period and the transmission rate of the data transmission period.Further,based on the effective capacity theory,we propose the aggregate effective capacity,which is defined as a system metric to characterize multi-user’s heterogeneous delay-constrained transmission rate.Additionally,we formulate the aggregate effective capacity maximization problem and obtain the resource allocation strategy for the uplink VLC system.(3)In the aspect of PLS-and-delay guarantee of VLC networks,for a secure VLC network consisting of multiple access points and multiple users,we propose a dynamic usercentric secure cell formation algorithm with both wireless security and statistical delay guarantee.First,considering the user scheduling,we propose a cross-layer VLC secure cell formation design.In each VLC secure cell,the customized hybrid PLS technique is adopted to enhance the security of optical wireless transmission Then,based on the information network theory,we derive the achievable secrecy rate for each user with the existence of multiple potential eavesdroppers.Further,combined with the effective capacity theory,we propose the effective secrecy rate to characterize the delay-and-secure-constrained achievable VLC transmission rate for each user.Effective secrecy rate can also show the trade-off between secure guarantee,delay guarantee and transmission rate.Then,we formulate the VLC network resource allocation problem with multiple long-term statistical delay guarantee constraints as an effective secrecy rate maximization stochastic optimization problem.Further,based on Lyapunov optimization theory,according to the history observable transmission results,we decompose the long-term stochastic optimization problem into short-term deterministic optimization problems,and then decouple the deterministic optimization problem for each slot into two sub-problems: the user-centric cell formation sub-problem and the intra-cell hybrid PLS parameters optimization sub-problem.Finally,we leverage the modified particle swarm optimization algorithm and the interference graph-based greedy scheduling algorithm to tackle the subproblems,respectively.The simulation results show that the proposed secure cell formation algorithm can achieve the higher transmission rate with both security and delay guaranteed.