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      车载自组织网络是由配备了无线通信设备的车辆节点和路边单元组成的移动自组织网络，是智能交通系统的重要组成部分。车辆间的实时通信不但可以帮助维护道路交通安全以及缓解交通拥堵等问题，还可以提供娱乐服务。由于具有广泛的应用前景，车载自组织网络的发展受到世界各国工业界和学术界的广泛关注。MAC层信道接入技术和网络层路由技术是车载自组织网络的关键技术，对网络的整体性能起决定性作用。但是，在传统的分层协议中，这两层之间信息的共享被严格得限制，阻碍了网络性能的提升。因此，本文致力于车载自组织网络的MAC层协议以及Network-MAC跨层设计的研究，具体研究内容如下：

      首先，针对现有的车载自组织网络协议进行分析，包括WAVE协议栈中的MAC层协议、现有的Network-MAC跨层设计方案。通过分析指出当前研究所面临的重要问题及可能的解决方案。

      其次，针对车载自组织网络MAC层协议，本文提出基于无干扰图的MAC层调度方案（IG-TDMA）。该方案采用基于TDMA的中心式调度方法，由路边单元（RSU） 统一管理无线信道资源。在方案设计时，为充分利用空间资源，本文提出基于顶点权重的链路选择算法。该算法可以在有效的干扰控制下，实现多通信链路的同时传输。IG-TDMA方案可以减少数据包碰撞和拥塞，并且避免由竞争型MAC协议引起的暴露终端和隐藏终端问题，因而可以明显提高网络的吞吐量，改善网络性能。
      最后，本文进一步研究Network-MAC跨层设计方案，提出基于TDMA的中心式Network-MAC跨层设计方案。因为在传统的分层协议中，网络层只能通过特定的接口与MAC层进行静态的交互，在进行路由选择时不能充分利用MAC层信道接入特性，所以网络数据传输性能受到限制。因此，本文将网络层路由选择与MAC层调度方法进行结合，提出了路由发现与链路调度联合优化算法（JoRS）。本文设计仿真实验，对该方案性能进行了评估。实验结果表明，这种中心式的跨层设计方案不但优于中心式的分层设计方案，也优于分布式的跨层设计方案。

 

    The vehicular ad hoc network (VANET) is a type of mobile ad hoc networks consisting of vehicles and roadside units with onboard wireless communication equipment. VANET is an important part of intelligent transportation systems. Besides the benefits in terms of traffic safety and traffic congestion alleviation etc., real-time inter-vehicle communications can also provide entertainment services. Because of the promising future of the applications of VANETs, it has been attracting significant interest from both the industry and academia all around the world. Channel access on the media access control (MAC) layer and routing on network layer are key technologies in VANETs, which play decisive roles in improving the overall performance of the network. However, in the traditional layered protocols, information sharing between layers is strictly confined, hindering the improvement of network performance. Therefore, this paper is devoted to the studies on MAC layer protocols and network-MAC cross-layer design in VANETs. The more detailed research activities include:

    Firstly, analyses on existing VANETs protocols are conducted, including the MAC-layer protocols in the WAVE protocol stack, the existing network-MAC cross-layer design in the literature. Important problems and possible solutions of the current research are pointed out subsequently.

    Secondly, an interference-free graph based TDMA scheduling protocol (IG-TDMA) is proposed for VANETs. In this protocol, a TDMA-based centralized scheduling scheme is adopted, in which the wireless channel resources are allocated by the RSU. In this protocol, to better exploit the spatial resources, we propose a link section algorithm based on vertex weights in the interference-free graph. In this algorithm interference between these links is limited and simultaneous transmission of multiple communication links is implemented. IG-TDMA can mitigate packet collision and packet congestion. Moreover, it can avoid the problems of exposed terminals and hidden terminals. Therefore, this scheduling protocol can significantly improve the network throughput.

     Thirdly, the design of network-MAC cross-layer protocols is studied and aTDMA-based centralized network-MAC cross-layer design is proposed. Because in traditional layered protocols, the network layer can only interact with the MAC layer through specific interfaces, which impairs the traditional network layer routing schemes' capability to exploit MAC layer channel access characteristics, data transmission performance is restricted. Therefore, a joint routing and scheduling (JoRS) algorithm is proposed in this paper, in which routing on the network layer and scheduling on the MAC layer are jointly optimized. The performance of the proposed centralized cross-layer design is evaluated by simulations. Results demonstrate that it outperforms representative schemes in both categories of the centralized layered schemes and the distributed cross-layer schemes.