Research On Key Techniques Of Vehicular Ad Hoc Network QoS

In vehicular ad-hoc network, the characteristic of the vehicle moving at high speed not only makes the network topology complicated, which causes great difficulties in designing MAC layer protocols and routing protocol, but also has influence on the wireless signal. When the vehicle drives in a tunnel, or densely built-up and complex multi-obstacle environment, the wireless signals reflects off the surrounding obstacles, which produces inter symbol interference because of multipath transmission. And severe multipath fading exists in the channel between the sender and the receiver, resulting in the fact that the unstable wireless link becomes more unreliable, which affects the quality of communication in the vehicular ad-hoc network. Considering all of these factors, it is necessary to ensure the vehicular ad-hoc network QoS of high real-time and reliability of the security communication services. This paper focuses on researching the key techniques to meet the above two requirements, including the architecture of the vehicular ad-hoc network, the access control mechanism of the MAC layer channel, and the routing protocol.Firstly, this paper analyses the design idea of the hierarchical and integrated network architecture, compares the advantages and disadvantages with each other. In a conclusion, it is more suitable for the vehicular ad- hoc network not to select single method in this kind of mobile ad-hoc networks but to take the hierarchical functional decomposition into account, which realize the flexible layer-by-layer or cross-layer transmission of the dataflow.Secondly, the channel access control mechanism EDC A adopted by the MAC layer is analyzed, which is more effective in comparison with DCF, however, has problems in the channel competition and ensuring the real time of the network. An improvement strategy is proposed in this paper. The approaches are taken that the AC priority is further elaborated, the AC priority factor is set, the contention window is dynamically regulated based on collision rate and priority factor, and the inner and external competition mechanism is improved. With a series of measures above, the A-EDCA channel competition mechanism is obtained, which meets the requirement of high real time in the application of VANET. The Simulation results show that the A-EDCA mechanism is more effective in average delay, average delay jitter, and throughput.Finally, this paper elaborates the advantages and disadvantages of these three typical routing protocol: TBR, PBR and MBR. By analyzing deeply, the GPSR routing protocol is considered to be more suitable for the vehicular ad-hoc network. But without ignorance, the defect in greedy forwarding of GPSR routing protocol might cause the packet loss. In this paper, the GPSR routing protocol is improved based the position and time: 1) the neighbors are predicted by the position, speed and direction angle of the vehicle node; 2) An appropriate next hope is screened; 3) the routing maintenance time is considered while packets forwarding. These three strategies are combined, the PMPT-GPSR routing protocol is realized. The Simulation results show that PMPT-GPSR has more advantages over GPSR in reducing the time delay and packet loss rate.