Opportunistic broadcast of emergency messages in vehicular ad hoc networks with unreliable links
Ming Li, Wenjing Lou · 2008
Multi-hop broadcast is an important means to disseminate safety information like time-sensitive emergency messages (EMs) in Ve-hicular 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 mes-sage. Many existing works have used redundant relay nodes to en-hance 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. More-over, 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 propaga-tion, at the same time to maintain a high level of transmission re-liability, 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 op-portunistic broadcast (OB), a MAC-layer mechanism to select sin-gle 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.