Research On Influence Of The Train On Radio Channel Characteristics In Underground Rail Transit Tunnel

In recent years,underground rail transit has gradually become the optimum solution to alleviate urban traffic congestion problems by virtue of its easy operability and safety characteristics.As the key to smart transportation,the communication system is an important link to ensure the normal operation and scheduling of rail transit,status monitoring and high-quality experience of passenger information service.Such diverse business demands always challenge the design of wireless communication systems.Since wireless channel modeling is the basis of wireless communication system design,thus precise understanding of propagation characteristics of radio waves in a channel is crucial for the design of higher performance communication systems.In this context,the thesis is focused on the properties of wireless channel in the underground rail transit tunnel with the existence of train vehicle.The ray tracing simulation method verified by the actual measurement is adopted here to investigate the propagation of radio waves in the vehicle-ground and in-vehicle communication channels,when the communication method using relay coverage is selected.(1)In the vehicle-ground communication channel,by changing the conditions of the antenna position and carrier frequency,we get the comparative results:in the underground rail transit tunnel,when the carrier frequency is selected at 40 GHz instead of 1.4 GHz,the received signal has to undergo more attenuation.The path loss fluctuates more drastically with the increase of the interval between the transmitting and receiving antennas.Further,the attenuation of the high-order reflection path is faster and the distinctions of the propagation path loss experienced by different multipath signals become more evident.(2)In the vehicle-ground communication channel,from the comparison of large-scale fading and small-scale fading results of the radio waves in the cases of train incorporated scenario and no train scenario,it is observed that the train itself does not induce the path loss significantly,instead,it has a profound impact on the root mean square(RMS)delay spread.Especially when the receiving antenna is placed at the top of the train,the existence of the train leads to an approximate 5-fold and 10-fold increase in the maximum RMS delay spread of two different carrier frequencies(40 GHz and 1.4 GHz).However,when the distance between the transmitting and receiving antennas exceeds a certain distance,the influence of the train on the RMS delay spread reduces rapidly.(3)In the in-vehicle communication channel,when the radio wave is selected in the millimeter wave band,the delay spread is large and the multipath effect become obvious.What's more,the polarization mode of the antennas has a serious influence on the signal propagation result.When the transmitting and receiving antennas are cross-polarized,a breakpoint occurs at a certain distance from the transmitting antenna.The receiving power,path loss,and RMS delay spread results at the breakpoint position show a turning trend.Additionally,compared to the co-polarization of the transmitting and receiving antennas,the radio waves experience larger attenuation,with a deviation of about 10 dB,the RMS delay spreads more violently,and the range of variation is larger,but the average value is still maintained at about 28 ns.The reflection from the sides of the train compartment contributes more to the received power than the bottom and top of the train.It causes that the difference in angular spreads of the azimuth angle of departure(ASD)is the largest of the four angular spreads,when the antennas are in a combination of different polarization modes.