214 ◾ Advances in Communications-Based Train Control Systems
Urban rail transit systems have developed rapidly around the world in the recent
past. Due to the huge urban trac pressure, improving the eciency of urban rail
transit systems is in high demand. As a key subsystem of urban rail transit systems,
communications-based train control (CBTC) is an automated train control system
using train–ground communications to ensure the safe and ecient operation of
rail vehicles . CBTC can improve the utilization of railway network infrastruc-
ture and enhance the level of service oered to customers .
As urban rail transit systems are built in a variety of environments (e.g., under-
ground tunnels, viaducts, etc.), there are dierent wireless network congurations
and propagation schemes. For tunnels, the free space is generally adopted as the
propagation medium. However, the leaky coaxial cable is also an option, such as
Tianjin Subway lines 1 and 2 built by Bombardier. For the viaduct scenarios, leaky
rectangular waveguide is a popular approach, as it can provide higher performance
and stronger anti-interference ability than the free space . In addition, due to the
available commercial o-the-shelf (COTS) equipment, wireless local area networks
(WLANs) are often adopted as the main method of train–ground communications
for CBTC systems .
Building a train control system over wireless networks is a challenging task. Due
to unreliable wireless communications and train mobility, the train control perfor-
mance can be signicantly aected by wireless networks . Because CBTC systems
are safety critical, trains usually run according to the front train’s state, including
velocity and position. When a wireless network brings large communication latency
caused by unreliable wireless communications or handos, the current train may
not be able to obtain the accurate state information of the front train, which would
severely aect train operation eciency, or even cause train emergency brake.
e performance issues in railway environments have attracted a lot of interest
recently. A fast hando algorithm suitable for passenger lines is proposed in  by
setting a neighboring cell list to facilitate hando operations. In , a novel hand-
o scheme based on on-vehicle antennas is introduced. e authors of  propose
a cross-layer hando design in for multiple-input and multiple-output (MIMO)-
enabled WLANs in CBTC systems. In , energy-ecient train control schemes
are studied in CBTC systems.
Although these above excellent works have been done to study CBTC systems
from both train–ground communication and train control perspectives, these two
important areas have traditionally been addressed separately in the CBTC literature.
However, as shown in the following, it is necessary to jointly consider these two
advanced technologies together to enhance the level of safety and services in CBTC
systems. e motivation behind our work is based on the following observations.
◾ Most existing CBTC systems are based on the COTS equipment, in which
traditional design criteria (e.g., network capacity) are used in the design and