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Multi-hop broadcast is an important means to disseminate safety information like time-sensitive emergency messages (EMs) in Vehicular Ad hoc Networks (VANETs). Providing low-latency, high-coverage and scalable multi-hop EM broadcast is a hard task in VANET with unreliable links. The major challenge comes from that fact that the link-layer broadcast uses unreliable transmissions, i.e., no positive feedback to acknowledge the reception of the message. Many existing works have used redundant relay nodes to enhance the reliability of broadcast packet reception. However they often involve more relays than it is necessary, which increases the network load and undermines the scalability of the protocol. Moreover, large latency is often incurred due to coarse protocol design. In this paper, we propose a new EM broadcast scheme that uses a small number of relays to achieve fast multi-hop EM propagation, at the same time to maintain a high level of transmission reliability, i.e., a minimum packet reception probability (PRP). Two types of relays are introduced to provide fast EM propagation and to enhance PRP simultaneously, so that low-latency, the desired reliability level and small overhead can be achieved at the same time. The opportunistic broadcast protocol (OBP) is based on opportunistic broadcast (OB), a MAC-layer mechanism to select single relay distributively, which features an effective redundant relay suppressing mechanism and very small rebroadcast delay for high priority nodes. Simulation study shows that OBP achieves close to 100% PRP, while using a small number of relays with very low broadcast latency under a wide range of road traffic conditions.

Opportunistic Broadcast of Emergency Messages in Vehicular Ad Hoc Networks with Unreliable Links