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

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149
Chapter 8
Novel Handoff Scheme
with Multiple-Input and
Multiple-Output for
Communications-Based
Train Control Systems
Hailin Jiang, Victor C. M. Leung,
Chunhai Gao, and Tao Tang
Contents
8.1 Introduction .............................................................................................150
8.2 Overview of the CBTC Communication Hando Procedure ..................152
8.2.1 Hando Latency of 802.11 and the Communication Latency
in CBTC Systems .........................................................................152
8.2.2 Features of Hando in CBTC Communication System ...............154
8.3 Proposed MAHO Scheme ........................................................................154
8.3.1 MIMO Transmission in the HandoProcedure
intheMAHOScheme .................................................................155
8.3.1.1 Physical Layer Processing ................................................155
8.3.1.2 Synchronization in the Downlink...................................159
8.3.2 Communication Latency in WLANs ...........................................161
8.4 Analysis of Hando Performance ............................................................. 162
8.4.1 Wireless Channel Model ...............................................................162
150 Advances in Communications-Based Train Control Systems
8.1 Introduction
e IEEE 802.11 standard for wireless local area networks (WLANs) has been
developed to provide wireless access in the oce and campus environments.
However, the procedures for hando between access points (APs) are not well sup-
ported. When a train moves along the rail, its mobile station (MS) would need to
switch from one AP to the next frequently to guarantee continuous data transmis-
sions between the train and the wayside devices, because the coverage of each AP is
quite limited. In general, during the hando procedure data packets will be lost if
no AP forwarding schemes are implemented. is will have serious impacts on the
safety and eciency of train control.
A lot of research has been done on the WLAN hando algorithms. e hando
procedure is divided into three stages [1]: the probe stage, the searching stage, and
the executing stage. In [1], some 802.11b parameters are further adjusted to reduce
the hando interruption time. In the current version of the IEEE 802.11 stan-
dards[2], the formats of probe request and probe response frames are dened and
the fast basic service set transition schemes including authentication and reassocia-
tion schemes are given. In [3], a channel scanning scheme in WLAN hando is
proposed, where the neighbor cells were cached in the buer to reduce the hando
latency in the probe stage. It is proposed in [4] that each MS continuously tracks
nearby APs by synchronizing short listening periods with periodic transmissions
from each base station. In this way, the station can pre-associate with new APs to
reduce the hando latency. A location-based hando scheme is proposed in [5],
and some congurations of the parameters in 802.11 networks are discussed to
reduce the hando latency in the scanning stage. An integrated design approach
is proposed to jointly optimize hando decisions and physical layer parameters to
improve the train control performance in CBTC WLAN systems in [68].
ese schemes can reduce the hando latency eciently, but all of them are
so-called break-before-make schemes, where the station needs to dissociate with
the old AP rst, and then nd a new AP and associate with it. In this procedure,
the data transmissions will be interrupted and the transmitted data will be lost.
In a CBTC system employing WLAN technology, the data transmitted include
8.4.2 Optimal Hando Location ........................................................... 163
8.4.3 Error-Free Period ..........................................................................164
8.4.4 FER of the Hando Signaling ......................................................166
8.4.5 Impacts on Ongoing Data Sessions ...............................................167
8.5 Simulation Results and Discussions .........................................................167
8.5.1 Analysis of the Hando Latency ................................................... 167
8.5.2 Error-Free Periods of Traditional Hando Schemes ......................169
8.5.3 FER of Hando Signaling with Dierent Data Rates ................... 170
8.6 Conclusion ...............................................................................................173
References .........................................................................................................174

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