Overhead-free congestion control and data dissemination for 802.11p VANETs

Direct radio-based vehicle-to-vehicle communication can be used to prevent accidents by letting vehicles exchange information about their state, intentions, and the road conditions. Although it is now a standard, the IEEE 802.11p protocol for vehicular ad hoc networks (VANETs) has known shortcomings. Dissemination is not reliable over distances higher than 300 m and congestion in the communication channel can lead to very low rates of safety messages delivery. Multi-hop routing and access channel techniques are well known approaches that were separately investigated to improve network effectiveness. However, achieving low end-to-end latency with multi-hop techniques is usually at the cost of lower data delivery rates, which in turn causes problems for the effectiveness of safety services embedded in vehicles. In this paper, we introduce a new dissemination and congestion avoidance scheme for safety messages over IEEE 802.11p VANETs. In order to ensure good delivery rates beyond 300 m, the approach propagates information over two hops while avoiding the resulting congestion by using a fully distributed asymmetrical transmit power adjustment technique. The scheme uses two time-dependent optimization-under-constraint processes to elect the best vehicle to act as a relay for data forwarding. The scheme can estimate the probability of reception rate (PRR), and adjust the forwarding distance to meet the minimum requirements of PRR and delivery distance to fit specific safety application requirements. The proposed solution, unlike previous dissemination techniques, works simultaneously on reducing congestion due to multi-hop relaying and on ensuring low end-to-end delay. Simulation results confirm the effectiveness of the proposed adaptation and relaying scheme and its advantageous network performance compared to others, under various traffic constraints.