17-11-2012, 05:12 PM
Design and Analysis of a Robust Broadcast Scheme for VANET Safety-Related Services
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Abstract
IEEE- and ASTM-adopted dedicated short-range
communications (DSRC) standards are key enabling technologies
for the next generation of vehicular safety communications. Vehicle-
safety-related communication services, which require reliable
and fast message delivery, usually demand broadcast communications
in vehicular ad hoc networks (VANETs). In this paper, we
propose and justify a distributive cross-layer scheme for the design
of the control channel in DSRC with three levels of broadcast
services that are critical to most potential vehicle-safety-related
applications. The new scheme for enhancing broadcast reliability
includes preemptive priority in safety services, dynamic receiver-
oriented packet repetitions for one-hop emergency warning
message dissemination, a multifrequency busy tone and minislot
within the distributed interframe space (DIFS) in IEEE 802.11,
and robust distance-based relay selection for multihop broadcast
of emergency notification messages. Compared with a current
draft of IEEE 802.11p and other schemes for DSRC safety-related
services, the scheme proposed in this paper is more robust and
scalable and easy to implement. Additionally, we investigate the
reliability and performance of the proposed broadcast scheme for
DSRC VANET safety-related services on the highway analytically
and by simulations. The analytic model accounts for the impact
of the hidden terminal problem, the fading channel conditions,
varied message arrival intervals, and the backoff counter process
on reliability and performance.
INTRODUCTION
TRANSPORTATION systems are designed to transport
people from one place to the next as safely and efficiently
as possible. Unfortunately, many accidents and fatalities do
occur on the road every day. The recent development of the intelligent
transportation system (ITS) [1] is an effort toward safe
and smooth driving without delay. Wireless communications
systems are expected to play a pivotal role in ITS safety-related
applications. Dedicated short-range communication (DSRC)
radio technology with a 75-MHz bandwidth at the 5.9-GHz
band [3], [6] is projected to support low-latency wireless data
communications among vehicles and from vehicles to roadside
units in North America. DSRC-based communication devices
are expected to be installed in future vehicles and to work
with sensors in the vehicles to enhance road safety. The draft
of the upcoming 802.11p standard, defining specifications of
the physical layer and the medium access control (MAC)
layer of the vehicular wireless communication networks based
on DSRC, has been created and distributed for discussions.
RELATED WORK
There have been active researches on the design and analysis
of real-time broadcast services in VANETs or MANETs.
Two broadcast mechanisms for real-time services are investigated
in this research: one-hop direct messaging and
multihop messaging for long-range coverage. Some direct (or
one-hop) broadcast solutions to safety-related message delivery
have been suggested and investigated. Xu et al. [5] proposed
several single-hop broadcast protocols to improve reception
reliability and channel throughput. Torrent-Moreno et al. [4],
[7] proposed a priority access scheme for IEEE-802.11-based
VANETs and showed that the broadcast reception probability
can become very low under saturation conditions. Jiang [26]
raised channel congestion control issues for vehicular safety
communications and introduced feedback information to enhance
system performance and reliability. Elbatt et al. [27]
discussed the suitability of DSRC periodic broadcast messages
for cooperative collision-warning applications. Sibecas et al.
[8] analyzed and improved the performance and the reliability
of IEEE 802.11a broadcast for safety-related vehicle-tovehicle
applications. Xing et al. [9] dealt with the problem
of reliable and fast broadcast of mission-critical data with
rich content over ad hoc networks.
MODELING AND ANALYSIS
To verify the effectiveness of the proposed schemes, we
study and compare the performance of the selected VANET
broadcast schemes for the safety-related services in a highway
environment. We analytically examine how a new packet from
a node is transmitted and propagated in the network under the
proposed broadcast schemes.
Assumptions for IEEE 802.11 1-D Broadcast VANET
Real-world radio networks are influenced by many factors.
In our study, we make some assumptions to give a tractable
yet reasonable model to characterize the performance of the
proposed schemes. In our model, we assume that IEEE-802.11-
based broadcast VANETs built along a highway are simplified
as 1-D MANETs, as shown in Fig. 4, which consist of a collection
of statistically identical mobile stations randomly located
on a line. The 1-D network model is a good approximation of
ad hoc networks in highway. In addition, the analysis of the
1-D network model provides insight into more complex 2-D
networks.
Performance of Multihop Class-Two Emergency
Message Broadcast
In the 1-D highway environment, the proposed robust relay
selection method for class-two emergency message based on
directional distance is also reduced to the selection of the
farthest node based on straight distance. Hence, if the farthest
node m fails to receive the broadcast message due to channel
transmission errors, node m − 1 will take charge of rebroadcasting
the message under the condition that it successfully
receives the message. The selection process will continue until a
node is selected to rebroadcast the message. Thus, the analysis
of the rebroadcast process is similar to that of the class-two
message repetitions, except that the number of repetitions is two
for this service.
CONCLUSION
In this paper, a novel protocol for reliable and fast delivery
of safety-related messages using the control channel for DSRC
VANET has been introduced and investigated. Several core
strategies are combined as an integral scheme to enhance the
performance and the reliability of the safety message broadcast,
which include cross-layer message priority setting, dynamic
receiver-oriented packet repetitions, farthest relay with
distance-based AD timer for multihop broadcast, etc. The
shaped protocol allows the control channel to deliver three
classes of safety related messages with the required QoS and
is easy to implement. In addition, analytic models, as well as
simulation models, are constructed to analyze the performance
and reliability of the proposed protocol.