Channel Modeling Of Visible Light Communication In Tunnel Scenarios

In recent years,the amount of tunnel laying in China has increased rapidly.The communication system of tunnel scene should not only realize the operation and dispatch of the train,but also provide the function of passenger communication.The traditional RF communication technology can use less and less spectrum resources,and the cost of high-speed communication is higher and higher.License-free visible spectrum communication is a new research direction.The most important thing is to design a new tunnel visible light communication system to study and analyze the channel characteristics.Based on the principle of visible light communication and the actual characteristics of tunnel environment,this paper selects two appropriate channel modeling methods,simulates the channel model and analyzes the channel characteristics.The first method is a channel modeling method based on regular geometry,which uses a cylinder structure to simulate the tunnel environment.The cross section of the tunnel is round,and the scatterers in the tunnel are randomly dispersed in the cylinder in the form of clusters.The emitting end LED and receiving end PD are considered as MIMO systems.In the transmission process of visible light signal,there may be multiple transmission paths,and more than once through the scatterers,so both the direct path and the non-direct path are considered.In the simulation experiment,the direction of the train in the tunnel is far away from the transmitting end,the transmission delay becomes larger and larger,and the received power becomes smaller and smaller.In the received power,the direct path component accounts for the main part of the total received power,and the non-direct path takes into account the single cluster reflection path and the double cluster reflection path component,which has a small contribution to the received power.The RMS time delay spread and K factor of the direct path are both minimum,so the direct path part is mainly considered in the promotion of channel transmission.The second method is to use ZEMAX software channel modeling,also choose the cylinder as a tunnel structure,based on the principle of ray tracing,the ray tracing function using the sequence record visible light signal transmission in the tunnel path information,including the number of reflection,reflection Angle,the length of the path of each segment,arrival Angle and reach power,etc.According to the analysis of the simulation data,in the ideal tunnel visible light communication system,the direct beam component accounts for almost all of the total received power.However,it is difficult to achieve the ideal system in the short term because the LED needs to be continuously laid on the inner wall of the tunnel.Therefore,the channel model is improved on the basis of the piecewise LED laying method,and a section of the transmission environment of the tunnel is selected for channel modeling.In the simulation results,the direct path is the main contribution,the size of the tunnel,the size of the receiving detector,the Angle of the transmitting end,the bending of the tunnel and other parameters will have a certain influence on the signal transmission.The system capacity increases with the increase of signal-to-noise ratio,the spatial correlation decreases with the increase of the distance between transmitter and receiver,and PPM can reduce the bit error rate.These conclusions provide a certain theoretical basis for the application of visible light communication in tunnel scenarios.