Abstract: IEEE 802.11p protocol is the MAC layer and physical layer standard designed for wireless access in vehicular environment. The backoff algorithm it utilized is Binary Exponential Backoff（BEB）. BEB doesn’t adjust along with the change of the vehicle speed and the network status, which will lead to serious unfairness and collision in vehicle-to-
infrastructure communications. So it can’t be well applied to the vehicular network. In order to solve these problems, a novel algorithm is proposed, which uses a constant optimal contention window in backoff process. The contention window can also be adjusted with the vehicle speed as well as the network status got from roadside units. The mathematical analysis and simulation experimental results show that the proposed algorithm in this paper can obviously improve the performance of channel access such as throughput, fairness, reliability and so on.

Key words: Vehicular Ad Hoc Networks（VANET）, Vehicle to Infrastructure（V2I） communication, IEEE 802.11p, fairness, Media Access Control（MAC） protocol

摘要： IEEE 802.11p协议是针对车载无线接入技术制定的MAC层和物理层标准，仍采用二进制指数退避算法，该算法不能根据车辆速度和密度作出调节，在V2I通信中，会造成严重的信道访问不公平和碰撞等现象，无法很好地适用于车载网络。针对上述问题，提出的算法在退避重传阶段，采用固定的最优竞争窗口，并能够根据车速和从路边单元获取的网络信息实时调整竞争窗口大小。模型分析和仿真实验表明，该算法可明显改善信道访问的吞吐量、公平性与可靠性。

关键词: 车载自组织网络, 车路通信, IEEE 802.11p, 公平性, 媒体访问控制协议