Opportunistic Broadcast For Safety Applications In VANETs

With the rapid development of modernization and the sharp increase of vehicles, traffic safety issue has caused great concerns among all countries around the world. Frequent traffic accidents have already posed a threat to people’s lives and property and brought about quite a dramatic economic loss, all of which pose a great challenge for the vehicular networks to tackle.The safety applications’hard real-time and high reliable requirements as well as the redundant broadcast problem and disconnected network problem caused by dynamic density of vehicles nodes make vehicular networking protocol design a really challenging job. At present, store-carry-forward (SCF) based opportunistic broadcast is one of the most promising techniques for safety messages multi-hop dissemination in vehicular environments. However, due to the implicit acknowledgement mechanism, most of present algorithms are still faced with large amount of redundant broadcasts. In addition, during sparse traffic situation pure SCF mechanism relies heavily on the meeting time of two vehicles. Both of these two problems heavily restrict the real-time performance and reliability of broadcast.To this end, we proposed a Roadside Units aided opportunistic broadcast scheme in this paper. In the network layer, a double-phase broadcast strategy is proposed to achieve fast message propagation, of which the first phase is to disseminate the message to roadside units in the affected area quickly with minimum routing algorithm and then broadcast the message periodically by the roadside units. The second phase is to ensure the reception reliability by retransmission. In the link layer, we propose an opportunistic contention scheme with broadcast request to avoid broadcast storm and hidden terminal problem. Simulation shows that our protocol can achieve higher dissemination rate and close to99%reception radio with significantly smaller number of transmissions compared with traditional protocols.