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Advances in Communications-Based Train Control Systems by F. Richard Yu

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81
Chapter 5
Modeling of theWireless
Channels with Leaky
Waveguide for
Communications-Based
Train Control Systems
Hongwei Wang, F.Richard Yu, Li Zhu, and Tao Tang
Contents
5.1 Introduction ...............................................................................................82
5.2 Leaky Waveguide in CBTC Systems ..........................................................82
5.2.1 Overview of CBTC Radio Channel with Leaky Waveguide ...........82
5.3 Measurement Campaign ............................................................................83
5.4 Modeling of the CBTC Radio Channel with Leaky Waveguide ............... 84
5.4.1 Measurement Results ..................................................................... 84
5.4.2 Determination of the Path Loss Exponent ......................................85
5.4.3 Determination of the Small-Scale Fading .......................................88
5.5 Conclusion .................................................................................................91
References ...........................................................................................................91
82 Advances in Communications-Based Train Control Systems
5.1 Introduction
As urban rail transit systems are deployed in a variety of environments (subway tunnels,
viaducts, etc.), there are dierent wireless network congurations and propagation
media. For the viaduct scenarios, leaky rectangular waveguide is a popular approach,
as it can provide better performance and stronger anti-interference ability than the free
space [1]. For example, leaky waveguide has been applied in Beijing Subway Yizhuang
Line. In addition, due to the available commercial o-the-shelf equipment, wireless
local area networks (WLANs) are often adopted as the main method of train–ground
communications for communications-based train control (CBTC) systems [2].
For general applications, leaky waveguide is taken as a leaky wave antenna with
the length of several operation wavelengths. However, in CBTC systems, the length
of leaky waveguide is several hundred meters and the distance between the leaky
waveguide and the receiving antenna is short (about 30cm). Due to the specicity
of the CBTC application, there are only a few research works about leaky wave-
guide in CBTC systems. e author of [1] gives a description of leaky waveguide
used in CBTC systems, and the advantages of leaky waveguide are demonstrated
by comparisons with natural propagation. e characteristics of leaky waveguide
are shown in [3] through laboratory measurements.
In this chapter, based on the measurement results in Beijing Subway Yizhuang Line,
we use the polynomial tting and equivalent magnetic dipole method to build the path
loss model. In addition, the Akaike information criterion with a correction (AICc) is
applied to determine the distribution model of the small-scale fading. e proposed
path loss model of the channel with leaky waveguide in CBTC systems is linear, and the
path loss exponent can be approximated by the transmission loss of leaky waveguide.
We show that the small-scale fading follows log-normal distribution, which is often
referred to as the distribution model of the small-scale fading in body area communi-
cation propagation channels [4,5]. In addition, the corresponding parameters of log-
normal distribution μ
dB
and
σ
dB
are also determined from the measurement results.
e rest of this chapter is organized as follows: Section 5.2 describes an over-
view of CBTC systems and the application scenario of leaky waveguide in CBTC
systems. Section 5.3 discusses the real eld measurement conguration and sce-
nario. en, Section 5.4 presents the path loss model and the small-scale fading
model. Finally, Section 5.5 concludes the chapter.
5.2 Leaky Waveguide in CBTC Systems
5.2.1 Overview of CBTC Radio Channel
with Leaky Waveguide
Generally speaking, the radio waves of CBTC systems are often transmitted in
the free space, especially in tunnels. However, in the viaduct scenarios, the per-
formance of wireless communication could be aected by the interference from

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