13-02-2013, 04:35 PM
Cross-Layer Design for Video Transmissions in Metro Passenger Information Systems
Cross-Layer Design for Video.pdf (Size: 1.02 MB / Downloads: 26)
Abstract—
In metro passenger information systems (PISs), frequent
train handoffs can cause severe video distortion. In this
paper, we take an integrated design approach to jointly optimize
application-layer parameters and handoff decisions to improve
video transmission quality over PISs. We present a train-ground
video communication network based on fountain codes and IEEE
802.11p for metro PISs. The metro PIS channel is modeled as a
finite-sate Markov chain, and the channel-state transition probabilitymatrix
is calculated fromthe data that weremeasured in real
field tests. The handoff decision and application-layer parameters
adaptation problem is formulated as a stochastic semi-Markov
decision process (SMDP). Minimizing the end-to-end total video
distortion is the objective in our model. Simulation results show
that the proposed SMDP-based optimization algorithm can significantly
improve the end-to-end video transmission quality inmetro
PISs.
INTRODUCTION
RECENT explosions in world population have caused severe
traffic jam problems in big cities. Many cities have
been devoted to building comfortable, convenient, and efficient
metro systems to relieve the pressure on already-busy roads.
Based on computer system and multimedia network technologies,
a passenger information system (PIS) provides passengers
with real-time dynamic multimedia information, e.g., train
schedules, live news, live matches, finance and economics,
and weather forecasts, in stations or on trains [1]. When a
fire, an earthquake, or a terrorist attack occurs, metro PISs
can broadcast various kinds of dynamic emergency evacuation
instructions.
IEEE 802.11p
IEEE 802.11p, which is also known as Wireless Access in
Vehicular Environment (WAVE), is a draft amendment to the
IEEE 802.11 standards that adds applications to fast-changing
vehicular networks [11]. It essentially deals with the data
link and physical layers of the Open Systems Interconnection
(OSI) model. The Media Access Control (MAC) protocol in
IEEE 802.11p uses the Enhanced Distributed Channel Access
(EDCA) mechanism, which was originally provided by IEEE
802.11e [15], whereas the physical layer is a variation of
the orthogonal frequency-division multiplexing (OFDM)-based
IEEE 802.11a standard, with a 10-MHz-wide channel instead
of the 20-MHz channel that was usually used by IEEE 802.11a
devices.
Wireless Channel Model in Metro PISs
In metro PIS networks, the train moves along the railway.
Fig. 3 gives the received signal-to-noise ratio (SNR) from two
successive APs that were measured in the field. As shown in the
figure, the received SNR rapidly changes when the train moves
from one AP to another, which is mostly due to the changing
path loss, Doppler spreads, shadowing, and antenna radiation
pattern.