Simulation And Analysis Of Radio Channel Characteristic In Tunnel

With the expansion of the city and the increase of the population,the subway has become the main transport system.While the popularity of portable communication equipment makes people increasingly demand when traveling.Tunnel is the most common scenario in subway line,but the channel characteristic in tunnel is different from in open space.Electromagnetic waves in tunnel will experience reflection,transmission,refraction and key-hole effect,which lead to deterioration of channel.In order to establish wireless communication system with high speed and reliable quality,it is necessary to investigate the characteristic of radio channel in tunnel.We measured radio channel for two times in Beijing Jiaotong University’s tunnel center and ZTT’s tunnel laboratory in Nantong,then simulated the scenarios that we measured using ray tracing method.As we can see from the analysis of large-scale fading,the simulation results and measurement results are quite similar.The root mean square error is about 4dB between simulation results and measurement results.So it is thought that ray tracing method can predict the large scale fading in tunnel.By comparing the channel impulse response(CIR)of measurement with simulation results,it can be seen that both delay and power obtained by simulation results can match measurement results.We can get accurate CIR through ray tracing method in tunnel.The path loss exponent of 2.4GHz and 5.9GHz are 1.418 and 1.438 in tunnel which is built in this paper.The shadow fading follow normal distribution.By comparing path loss in straight tunnel with curved tunnel,the results show that the path loss in curved tunnel is larger than that in straight tunnel.The path loss is larger when the transmit antenna is on the inside than on the outside of the tunnel,the difference is proportional to the distance between transmitter and receiver.Multiple-Input Multiple-Output is a kind of important methods to increase channel capacity.By comparing with single antenna and 2×2 antennas,the results show that the Multiple-Input Multiple-Output technology can improve capacity obviously.In this paper,we build train model to research the impact of subway for MIMO system.Compared with the condition without train,the receiver’s angel spread is larger and the correlation between antennas is smaller in the condition with train in tunnel.And also the capacity of channel with train is larger than that without.The increasing of antennas’number contributes to the channel capacity,the gap between channel capacity with train and Rayleigh channel increasing quickly.It is also shown that the increasing of antenna’s spacing in a certain range also makes capacity larger in tunnel without train but makes no difference in tunnel with train.