Direct radio-based vehicle-to-vehicle communication can help to prevent accidents by providing accurate and up-to-date local status and hazard information to the driver. In this paper, we assume that two types of messages are used for traffic safety-related communication: periodic messages (`beacons') sent by all vehicles to inform their neighbors about their current status (i.e., position), and event-driven messages sent whenever a hazard has been detected. In IEEE 802.11 DCF-based vehicular networks, interferences and packet collisions can lead to failure of reception of safety-critical information, particularly when the beaconing load leads to an almost saturated channel as it could easily happen in many critical vehicular traffic conditions.
In this paper we demonstrate the importance of transmit power control for avoiding saturated channel conditions and ensuring best use of the channel for safety-related purposes. We propose a distributed transmit power control method based on a strict fairness criterion, D-FPAV, to control the load of periodic messages on the channel. The benefits are twofold: i) bandwidth is made availabl ... mehre for higher priority data like dissemination of warnings; ii) beacons from different vehicles are treated with `equal rights' and best possible reception under the available bandwidth constraints is ensured. We formally prove the fairness of the proposed approach. Then we make use of the ns-2 simulator significantly enhanced by realistic highway mobility patterns, improved radio propagation and receiver models, and the IEEE 802.11p specifications, to show the beneficial impact of D-FPAV for safety-related communications. We finally put forward a method, EMDV, for fast and effective multi-hop information dissemination of event-driven messages and show that EMDV benefits of the beaconing load control provided by D-FPAV with respect to both probability of reception and latency.