Research Of Radio Channel Character In Tunnels Based On The Ray-tracing Method

The urban underground railway transit has become more important component of urban transport as the accelerated pace of urbanization. The demand for efficient and reliable communication system is also gradually increasing. To research the propagation characteristics in the radio channel of underground tunnels is the foundation of designing wireless communication system. The paper combined measurement and simulation based on ray tracing method. Studied on large-scale fading and multipath channel characteristics of tunnel environments. Based on the current demand for higher capacity communication systems and data transfer speed, the paper studied the feasibility and impact factors of MIMO systems in tunnels. Using ray tracing method to simulate the channel.Firstly, the paper take an overview of the radio wave propagation mode and the main parameters of radio channel. The differences between underground tunnels and other common scenes are analyzed. Large-scale propagation of the tunnel environment showed a waveguide properties. The complex reflection in tunnel cause significant multipath effects. The keyhole effect and MIMO correlation in tunnel is different from other common scenes.Then the paper introduces the basic concept and application modes of ray tracing method. Ray tracing method is based on the principle of geometrical optics. By analyzing the propagation path between transmitter and receiver and calculating the propagation attenuation of each ray, the total signal energy at the receiver can be calculated. Due to the characteristic of tunnel environment, it is suitable to use mirroring method in ray tracing simulation. The calculation of electric field intensity is also introduced in this paper.Two measurements were taken to study the characteristics of the channel tunnel environment. The paper introduces this two measurements in detail and analyzes the results. The measurement scenarios are simulated by ray tracing method. As we can see from the analysis of large-scale fading, the path lass is significantly lower than free-space scene. The trends of the simulation results also confirm this. By compare the CIR of measurement and simulation results, it can be seen that both the power and the delay obtained by simulation can match the measurement results. So the accurate CIR information can be obtained by ray tracing method. The CIR data can be used-to calculate the parameters of MIMO channel such as angle extension and correlation parameters and channel capacity.The characteristics of MIMO channel in tunnels are studied by ray tracing method. Firstly the paper studied the change of channel correlation in different frequency. The simulation results show that the channel correlation coefficient changes periodically. The period of change decrease as the frequency of signal increase. The paper also studied the effects of different cross-sectional dimensions for MIMO channel characteristics. The results show that when cross-sectional dimensions increase, the angle spreading of received signal decrease, it cause the reduced of fluctuation period. At the same time, the spatial correlation coefficient reduced, The MIMO channel capacity increase.