Research On Channel Reconstruction Of High-Speed Visible Light Communication Based On RIS

To meet the high demand of data traffic for the future 6G communications,Visible Light Communication(VLC)has been widely concerned by scholars for its characteristics of environmental protection,no electromagnetic interference,good confidentiality,rich spectrum resources and so on.However,the unique mode of Intensity-Modulation and Direct-Detection(IM/DD)adopted by VLC makes it highly dependent on line-of-sight transmission,which limits the communication performance of VLC.Therefore,in order to break through the bottleneck of VLC technology,Reconfigurable Intelligent Surface(RIS)emerging as a new technology candidate is expected to solve the problem of insufficient performance improvement for VLC caused by changing the coding mode in traditional transceivers,and therefore,break through the bottleneck of VLC technology development.This thesis does some research on RIS-assisted VLC technology.The main research contents and innovations are as follows:1.We propose a design scheme of RIS position in SISO VLC system to maximize the achievable data rate.Based on the optical RIS composed of a mirror array,we construct the indoor RIS-aided SISO VLC system with a fixed receiver and a mobine receiver,respectively.Aiming at the two system models,the achievable data rate maximizations in these two different scenarios are taken as the utility functions to optimize the position of RIS according to the corresponding positions of transmitter and receiver.By analyzing the problem,we can obtain that this kind of optimization problem is NP-hard and non-convex,which is complex and difficult to solve.To solve these two non-convex optimization problems,a meta-heuristic intelligent algorithm,particle swarm optimization(PSO)algorithm,is adopted to determine the optimal location of RIS array.Different from the traditional particle swarm optimization algorithm,this thesis adds an adaptive mechanism in the process of solving optimization problems to ensure the randomness in the optimization process for each particle.The complexity analysis indicates that the proposed algorithm decreases the complexity of solving such problems to some extent compared with the exhaustive search algorithm,and the simulation results confirm that suitable location of RIS can improve the communication performance of VLC system to a greater degree.2.We proposed an optimal designment scheme for the parameters of RIS for asymptotic capacity maximization in an indoor MISO VLC system.We regard the high-SNR asymptotic capacity with the inputs subject to peak-intensity constraints as a goal and formulate an asymptotic capacity maximization problem to find the optimal orientations of mirrors.As for the non-convex optimization problem,we convert it into a quadratic programming(QP)problem with hemispherical constraints and prove that it can be solved by computing the maximum eigenvalue of an equivalent matrix.Simulation results indicate that the asymptotic capacity is able to be improved significantly by adopting RIS in MISO VLC systems.Meanwhile,we observe that the proper deployment scheme of RIS is able to enhance the degree of improvement through several simulations.3.We propose the optimal allocation matrix for RIS in parallel MIMO VLC to maximize the asymptotic capacity.A RIS-aided parallel MIMO VLC system model is constructed under the point source hypothesis.Different from the scheme of optimizing parameters of RIS,an allocation matrix is designed to determine the service relationship between RIS units and transceivers.Aiming at maximizing the asymptotic capacity of parallel MIMO VLC system with high signal-to-noise ratio(SNR),the RIS allocation matrix was considered as the decision variable and the optimization problem was therefore constructed.It can be observed that this problem is a non-convex problem of integer programming.In order to solve this problem,convex relaxation is carried out first.The original optimization problem can be approximately transformed into a convex problem,and then,the convex optimization toolbox in MATLAB is used to solve the convex problem.A relaxed particle swarm search algorithm(RPSO)is proposed to solve the loss problem caused by relaxation.After comparing with several benchmark allocation matrix schemes,the simulation results confirm the effectiveness of the proposed RPSO algorithm in solving such optimization problems.Meanwhile,it can be found that the addition of RIS can greatly improve the communication performance for parallel MIMO VLC system.4.An optimal refractive index designment scheme is proposed to maximize the asymptotic capacity of liquid crystal RIS-aided MISO VLC systems.With the continuous development of metamaterials,the study of optical RIS is no longer limited to the reflection of a mirror array.The function of dynamically changing refractive index provided by liquid crystal(LC)RIS has become a new research field.LC RIS is usually deployed in front of the receiver of the traditional VLC system,and the field of view in the receiver can be effectively improved by dynamically changing the refractive index.Meanwhile,the transmitted signal can be amplified to achieve the improvement of communication performance.Based on the liquid-crystal RIS-assisted receiver,an LC RIS-aided MISO VLC system was proposed and the optimization problem was constructed with the asymptotic capacity maximization as the optimization objective and the tunable refractive index of liquid-crystal RIS as the optimization variable.It can be analyzed that this problem is a low dimensional non-convex optimization problem,which can be solved by using the particle swarm optimization algorithm(PSO).Simulation results confirm the effectiveness of PSO algorithm in solving such optimization problems,and meanwhile,it can be found that the application of liquid crystal RIS can greatly improve the communication performance of MISO VLC system.At the same time,it can be observed that the addition of liquid crystal RIS improves the performance gain of the receiver in the corner of MISO VLC system more significantly,which can effectively solve the problem that the dead zone cannot be covered in traditional VLC.