Integrative Modelling And Analysis Of Radio Frequency Link For Wireless Communication In Railway Environments

The wireless communication system of GSM-R at 900 MHz band is used in China Railway as the main way of information transmission in train dispatching and controlling systems. The wireless RF link of this communication system includes three parts:base-station antenna, radio propagation in space and antenna upon the train. The characteristics of the link have a significant influence on the reliability of the whole communication system and even the safety of train operation. Available works are mainly focused on the second part of the radio frequency link—radio wave propagation in space. Hence the results based on these would have great difference from the actual characteristics of the whole link, especially in the high-speed railway environment. The research of this dissertation originates from the National Natural Science Foundation of China under Grant No.60674010 and 60825101, and the National High-Tech. Research & Development Program (863 Program) of China under Grant 2008AA01Z224. Mainly directing towards the trend of railway development, the work is to do the research on the integrative modelling methods and characteristic analyses of the wireless RF link under high-speed railway environments, including the transceiver antennas and their environments together. It is of great significance for the performance estimation of railway wireless communication system, the optimization design of train-to-wayside wireless communication system and so on. The main innovative achievements are listed in the following aspects:1. Aiming at relatively complicated environment in high-speed railway wireless communication, the research idea of integrative modelling and analysis is proposed to consider transceiver antennas and their surrounding environments, and radio propagation in space together. For the open areas based on base-station antennas, the full-wave method, ray-tracing method and random statistical methods are combined to complete the integrative modelling of the RF link within long distance. For the confined areas based on leaky waveguide, only the full-wave method is used to accomplish the integrative modelling of the link within short distance.2. In the procedure of integrative modelling, firstly, the influence of train length on receiving characteristic of the antenna mounted upon locomotive is analyzed. Thus the minimum train length used in simulation is determined so as to satisfy the calculation accuracy requirement while the calculation amount is reduced. Then the train antenna and its surrounding railway environments including the locomotive body are simulated as a whole. The receiving-transmitting characteristics of the vehicular antenna with influence of its environment are studied. The corresponding patterns are given.3. In the simulation process, for introducing signal source, a square coaxial line feeding structure is put forward to feed the vehicular monopole antenna. In the situation of rectangular meshing in FDTD, the using of this model could be easy and effective to feed the antenna or obtain the output voltage and current signals of the antenna. According to the characteristic of the research problems in this dissertation, the method of introducing plane wave source into FDTD computation domain is improved. The validity of the improved method is verified. An approximation method of using magnetic dipole array for simulating radiating leaky coaxial cable in FDTD computation domain is also proposed.4. By involving the one-path model and two-path model with the FDTD method, the large-scale fading characteristics of the wireless link within long distance under the railway environment in flat and open areas are integratively modeled and analyzed. Within very long distance along the railway line under the coverage of base-station signal, the variations of output signals of train antenna with train location are studied.5. By involving the multi-path statistical distribution models for predicting scattered fields, two-path deterministic propagation model together with the FDTD method, the small-scale fading characteristics of the wireless link are integratively modeled and analyzed. Here the small-scale fading is caused by the multi-path effect under railway environments. Within very short distance along the railway line under the influence of multi-path time-variant scattered waves, the variations of output signals of train antenna with train location are studied.6. The research idea of introducing modulation-demodulation module into the integrative model of the whole wireless RF link is put forward to study the influence of the railway environments on error rate of data transmission. The situation of the influence of environmental factor on the transmission process of BPSK modulated signal is preliminarily investigated. Based on this, the interference of electromagnetic pulse noise generated by the sparkover between pantograph and conducting wire for power supply on the BPSK modulated signals received by the train antenna is studied.7. The characteristics of wireless RF link composed of the leaky coaxial cable and vehicular antenna in mountain tunnel are studied. The field strength coverages in tunnel with and without train passing through are both analyzed. The corresponding field distributions in tunnel are given. The influence of installation locations of leaky coaxial cable on the field distributions in tunnel is also discussed.