V2V Comm Protocol

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A Vehicle-to-Vehicle Communication Protocol
for Cooperative Collision Warning
Xue Yang
University of Illinois at Urbana-Champaign
Jie Liu
Microsoft Research
Feng Zhao
Microsoft Research
Nitin H. Vaidya
University of Illinois at Urbana-Champaign
nhv@uiuc.edu Abstract
This paper proposes a vehicle-to-vehicle communication
protocol for cooperative collision warning. Emerging wireless technologies for vehicle-to-vehicle (V2V) and vehicleto-
roadside (V2R) communications such as DSRC [1] are
promising to dramatically reduce the number of fatal roadway accidents by providing early warnings. One major
technical challenge addressed in this paper is to achieve
low-latency in delivering emergency warnings in various
road situations. Based on a careful analysis of application
requirements, we design an effective protocol, comprising
congestion control policies, service differentiation mechanisms and methods for emergency warning dissemination.
Simulation results demonstrate that the proposed protocol
achieves low latency in delivering emergency warnings and
efficient bandwidth usage in stressful road scenarios.
1. Introduction
Traffic accidents have been taking thousands of lives
each year, outnumbering any deadly diseases or natural disasters. Studies [18] show that about 60% roadway collisions
could be avoided if the operator of the vehicle was
provided warning at least one-half second prior to a collision. Human drivers suffer from perception limitations on
roadway emergency events, resulting in large delay in propagating emergencywarnings, as the following simplified example
illustrates. In Figure 1, three vehicles, namely , ,
This work was funded in part by Palo Alto Research Center while the first author worked there as a summer intern. The first author is also supported in part by Vodafone-U.S. Foundation Graduate Fellowship. and , travel in the same lane. When suddenly brakes

abruptly, both vehicles and are endangered, and being
further away from does not make vehicle any safer
than B due to the following two reasons:
Line-of-sight limitation of brake light: Typically, a
driver can only see the brake light from the vehicle directly in front1. Thus, very likely vehicle will not
know the emergency at until brakes. Large processing/forwarding delay for emergency events: Driver reaction time, i.e., from seeing the brake
light of A to stepping on the brake for the driver of vehicle B, typically ranges from 0.7 seconds to 1.5
seconds [6], which results in large delay in propagating
the emergency warning.
Figure 1. V2V helps to improve road safety
Emerging wireless communication technologies are
promising to significantly reduce the delay in propagating
emergency warnings. The Dedicated Short Range Communications (DSRC) consortium2 is defining short to
medium range communication services that support public
safety in vehicle-to-vehicle (V2V) communication
1 In favorable conditions, a driver may see brake lights further ahead. But we consider typical or worst-case scenarios.
2 IEEE P1609 Working Group is proposing DSRC as IEEE 802.11p standard.
Using V2V communication, in our previous example,
vehicle can send warning messages once an emergency
event happens. If vehicles and can receive these messages
with little delay, the drivers can be alerted immediately.
In such cases, has a good chance of avoiding the accident
via prompt reactions, and benefits from such warnings
when visibility is poor or when the driver is not paying
enough attention to the surroundings. Thus, the vehicleto-
vehicle communication enables the cooperative collision
warning among vehicles , and .
Even though V2V communication may be beneficial,
wireless communication is typically unreliable. Many factors, for example, channel fading, packet collisions, and
communication obstacles, can prevent messages...
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