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Abstract
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Due to the growing number of vehicles over the last five decades, we have now a saturation of the transportation infrastructure. Therefore, traffic congestion, accidents and transportation delays become unavoidable. For example, in European Union alone, there are more than 3.5 million injured people and more than 160 billion EURO in material loses reported each year. Several measures are implemented in order to solve these problems, such as implementation of safety systems (e.g., safety belts and airbags) and construction of more and better roads and highways. However, it is clear that building more roads is a solution limited by the large monetary investment required and also the lack of available space, which is particularly pronounced in urban areas.
Because of these concerns, Intelligent Transportation Systems research is chartered to alleviate highway safety problems, that is, develop safety applications that lead to a significant reduction of critical traffic events. Examples of safety applications are accident notification, traffic congestion warning and traffic congestion avoidance. These applications utilize a variety of data obtained from the host vehicle, the road and surrounding vehicles. Such data includes braking data from the cars ahead, road conditions sensed by surrounding cars, and data from other sensors in a vehicle.
We propose to use IEEE 802.11e in conjunction with retransmission mechanisms for priority-based Intervehicle Communication (IVC) for highway safety messaging in both vehicular infrastructure and ad hoc networks. Each IVC message, which is assigned a priority based on the safety event urgency, requires different quality of service in terms of communication reliability and average delay. To increase the communication reliability in a broadcast-based IVC, we apply retransmission mechanisms that can provide proportional reliability differentiation for each prioritized message. We evaluate the performance of our proposed protocol using OPNET Modeler, in terms of percentage of successful transmissions and average delay. The protocol performance is evaluated for (1) different system parameters such as the packet size, number of vehicles, and percentage of priority 1 vehicles, and (2) priority-based protocol parameters, such as contention window, interframe spacing and retransmission parameters. The results show that proper protocol parameter settings per priority message result in an efficient solution for priority based broadcast IVC. Moreover, the proposed protocol is fully compatible with both IEEE 802.11 and IEEE 802.11e standards. |