Modeling And Analysis Of Leaky Waveguide In Communication-based Train Control Systems

ABSTRACT:CBTC adopts Wireless Local Area Network based on IEEE802.11protocols, which can supply the bidirectional train-ground communication. Train-ground communication of CBTC systems can be easily affected by the signal interference when the free space is used as the propagation medium. Leaky waveguide, with the advantages of low transmission loss, high anti-interference ability, as the wireless transmission medium, greatly improves the reliability and security of the system and has been gradually applied in CBTC systems.Transmission principles of the rectangular waveguide and the slot theory are studied in the thesis, and a leaky waveguide model is developed through the equivalence magnetic dipole theory. The characteristics of the signal strength distributions above the leaky waveguide through the HFSS software are obtained. Combined with the field test and the simulation results, the rationality of the leaky waveguide model is verified.The research includes:(1) The study of the transmission principles of rectangular waveguide. The thesis introduces the basic rectangular waveguide transmission theory, and transmission modes of the waves in the rectangular waveguide and the electromagnetic field distributions are studied.(2) The analysis of the slot theory of leaky waveguide. Based on the transmission principles of rectangular waveguide and the electric current distribution on a post in the waveguide, the influence of the location of slots on the radiation characteristics is analyzed, which provides the theoretical basis for the design of leaky waveguide slots.(3) The radiation characteristics of the leaky waveguide through the establishment of the model based on the magnetic dipole equivalence theory. A model of leaky waveguide of CBTC systems is proposed and analyzed based on the magnetic dipole equivalence principle and the antenna array theory. The theoretical method to calculate the signal strength above the leaky waveguide and the analysis of the radiation characteristics are present.(4) The verification of the leaky waveguide model through the simulation software and the field test. In order to verify the leaky waveguide model, radiation characteristics of the leaky waveguide are analyzed through the different parameters including the antenna height, the slot size and the forms of the slot through the HFSS software and the field test.The results show that the trend of the signal strength distributions of the leaky waveguide using the equivalent method of magnetic dipole is in the consistent with simulation results and experimental results, which verifies the rationality of the model. The height of the receiver, the size of the slot and the forms of the slot influence radiation characteristics of leaky waveguide. In the case of the same area, it is obvious that the radiation strength of the long slot form perpendicular to center axis is much better than that of the circular form. The research about the leaky waveguide will serve as reference for CBTC systems.