Research On Reliable Broadcast Schemes For VANET

With the development of wireless communication technology, Vehicular Ad Hoc Network (VANET) communication has become a hot spot of research in the world. As a kind of distributed wireless communication network based between vehicles, VANET is a extensional network of the point to point wireless communications in vehicles area. As the basic scheme of message transmission in VANET, wireless broadcast scheme can spread emergency warning messages and entertainment messages to all neighboring nodes timely and effectively. However, since neighboring nodes cannot return an ACK frame when they received one broadcast message, the lost MAC frame can’t be perceived and retransmitted when a collision occurs, thus the reliability of broadcast scheme is difficultly guaranteed.For the problem of unreliability in traditional wireless broadcast scheme, a virtual destination based reliable one-hop broadcast scheme (VDB-ROBS) is proposed in this dissertation. Though the statistics and analysis of all the received frames from neighboring nodes, each broadcasting source node selects a suitable neighboring node as its virtual destination node (VD). Then the broadcast source node will send frames to VD based on unicast transmission scheme, while the other neighboring nodes still receive this MAC frame as a broadcast frame. Therefore this scheme can enable multiple adaptation mechanism effectively, such as RTS/CTS, ACK, retransmission and contention window adjustment, which improves the reliability performance of one-hop broadcast. Simulation results show that the proposed VDB-ROBS outperforms IEEE 802.11 broadcast scheme and probabilistic retransmission based reliable broadcast scheme in reliable performance.Besides, on account of the irrationality and unfairness of traditional binary exponential back-off process in VDB-ROBS scheme, the paper proposes a VANET density index based contention window adaptive scheme named DIB-CWAS. In this scheme, the broadcast node can firstly calculate the current density of the local network situation, then obtain the density index by a lookup table, finally adjust the contention window dynamically according to the corresponding density index in the stage of the channel competition. This scheme makes the adaptive process of the contention window accord with the real-time network conditions. Simulation results show that the proposed DIB-CWAS can enhance the communication performance further compared with the VDB-ROBS in wireless broadcast service.