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Safe, Efficient, and Fair---A Top-Down Approach to Inter-Vehicle Communication

Author(s): Daniel Baselt.
Title: Safe, Efficient, and Fair---A Top-Down Approach to Inter-Vehicle Communication
Published: PhD thesis, Heinrich Heine University, Düsseldorf, Germany, Juli 2013
Keyword(s): Car-to-car communication, Inter-vehicle communication, Top-down, Protocol design
Abstract: Since the development of the first automobiles, individual traffic changed in many respects while the main objectives of transportation---safety and efficiency---have stayed the same. Perhaps the most obvious change is that traffic got denser. To support drivers coping with more and more cars that occupy the roads, car manufacturers developed aids of various kinds that assist driving according to those objectives.The assistants deployed today rely on information made available by a vehicle's on-board sensors and the decisions based on that data are made isolated from other cars. Cooperative decisions of vehicles are enabled by the active exchange of information using inter-vehicle communication. For example, drivers can coordinate with each other about route selections or can warn about hazards like the rear end of a traffic jam in time.The availability of low-cost wireless technology in recent years again inspired the network research community to develop novel protocols and algorithms for inter-vehicle communication. These were then used to build applications supporting the transportation objectives. The developed protocols are based on available or slightly modified standards; their suitability to the applications is limited by the technologies' constraints. Currently, car-to-car networks are designed as general-purpose networks, although the objectives for employing communication for exchanging car information are clear and very specific: optimize traffic safety and efficiency. In addition, the understanding of the application domain was not in focus in the depth that it deserves. With the knowledge available at the time work on this thesis was commenced, it was not possible to measure how well a protocol performs in view of the above objectives.There was in fact no information at all on how to relate available information in vehicles, how such information is exchanged, and the application objectives; the potential that inter-vehicle communication bears was unknown. But without a solid understanding of how these aspects interrelate, a protocol developer cannot be sure that what is pursued is the best---or even just a reasonably good---way of supporting road traffic by means of communication.This thesis proposes to complement the existing research on inter-vehicle communication with a top-down approach. Instead of starting out with a given technology and then developing protocols bottom-up, this approach begins with a formal discussion of what does not change: the objectives of traffic. For these, an optimal behavior of cars and, with this, information demands are deduced that good protocols have to satisfy. Thereafter, according protocols are developed, followed by a suitable communication technology. Because this is an enormous task, a roadmap is presented that describes how to properly divide it. In this thesis, the approach is applied to two basic road topologies up to the point a protocol is obtained.Followed by a comprehensive discussion of related work in the different areas of research that affect the top-down approach, the first steps of the road map are applied to a simplistic scenario. The influence of available information on the vehicles' behavior is considered. Different schemes for information exchange are analyzed including beaconing, a periodic information exchange. It turns out that a car's optimal behavior with beaconing is to drive with alternating periods of acceleration and deceleration which converge to a steady state.The scenario is extended to multiple cars and analyzed for the optimal sending times of information for a cruise control application. This provides us with detailed knowledge about how beaconing should work in a reliable communication environment. The discussion continues with considering the effects of packet losses and it is found that not only the direct follower of a sending car leaves its steady state, but a chain reaction affects numerous upstream cars. To withstand this effect, and in particular to cope with consecutive losses, the steady state distances are enlarged by multiples with beaconing. The then proposed algorithm Carrot is able to detect losses implicitly without a need for acknowledgments and to react via a fast repetition of the missing beacon. Analytical and simulative evaluations show that Carrot is able to repeat beacons in fractions of a beaconing interval, so that the steady state distances can be chosen very close to the minimum.Communication enables cars to cooperate and thereby allows for a third objective besides safety and efficiency that is not yet in focus of research for inter-vehicle communication: fairness. Towards this, this thesis contributes the definition of a fairness criterion for cooperation. This criterion is applied to the ordering of cars at a merging of two lanes. An analysis of the zipper merge, the only relevant merging scheme for today's cars if no lane is given the right of way, clearly shows an inherent unfairness. A coordination scheme is proposed that creates an optimal fair merge order. The scheme is adapted for distributed decisions with local knowledge and communication using a beacon-based approach. Evaluations which also accounts for unreliable wireless communication describe the influence of the ratio of participants on merging order fairness. The results show that the algorithm yields very good fairness even if only a small percentage of about 1% of the cars follows the algorithm's guidance.
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