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Inter-Vehicle Communications : Achieving Safety in a Distributed Wireless Environment : Challenges, Systems and Protocols
Autor: M. Torrent Moreno Links:
Quelle: Dissertation, ISBN: 978-3-86644-175-0, Universitätsverlag Karlsruhe, 2007
Vehicular 'active safety' can be enhanced by the wireless exchange of information among the vehicles driving along a road. In particular, inter-vehicle communications (IVC) can support safety systems designed to avoid road accidents by two means. First, periodic transmissions from all vehicles to inform their neighbors about their current status enables accident prevention by being capable of identifying dangerous road situations. Second, the fast dissemination of emergency information whenever a hazard has been detected can help drivers avoid the danger. The goal of this thesis is to design required communication protocols and systems in order to provide the means for a robust and effective transmission of safety-related information. We first analyze the performance of the underlying wireless technology via simulation in order to identify the most relevant challenges for IVC. The simulation tool consists of a significantly extended network simulator (ns-2.28) with more accurate and up-to-date models, including probabilistic radio wave propagation, wireless interface adjusted according to the IEEE 802.11p draft (the envisioned technology) and realistic vehicular movement corresponding to fast-moving German highway scenarios. The obtained results show that in IEEE 802.11 DCF-based vehicular networks, one has to make sure that interference and packet collisions do not lead to a failure in the reception of safety-critical information. This effort represents a challenging task particularly when all safety-related messages share one wireless channel and the resulting load of periodic messages leads to a saturated network, as could easily happen in many critical vehicular traffic conditions. Therefore, we propose two methods based on power control and contention mechanisms to shape data traffic such that messages are received with high probability where they are relevant. First, we propose a method based on a strict fairness criterion, D-FPAV, to control the load of periodic messages on the channel while ensuring a high probability of message reception within the safety distance of the sending vehicle. Second, we propose a method, EMDV, for fast and effective dissemination of emergency messages within a geographical area. Using the extended ns-2.28 simulator we show the merits of both approaches as well as of their synergies. Simulation results show that: i) D-FPAV is capable of improving the reception rates of periodic messages at close distances from the sender as well as increasing the probability of reception of emergency messages over a wide range of distances between sender and receivers, ii) EMDV can deliver the emergency information to all nodes located in a geographical area with short delay and iii) when EMDV is used in combination with D-FPAV, the dissemination efficiency and delay are considerably improved. Furthermore, we make use of the insight gained along the realization of this thesis to develop a set of design guidelines for an IVC protocol architecture. As a result, we obtain a system design tailored to the characteristics of safety-related IVC to be used as a basis for implementation. The communication protocols and the system design proposed in this thesis have been adopted by the project Network on Wheels for the development of an inter-vehicle communications platform in an European prototype.