【作者】 刘源；

【导师】 邓晓华； 王玉皞；

【作者基本信息】 南昌大学 ， 信号与信息处理， 2012， 硕士

【摘要】 快速移动性与负载多样性的约束条件对智能交通系统中车辆通信网络的信道接入机制提出了新的挑战。传统接入协议的性能表现难以满足新时期智能车辆通信网络对信道接入机制提出的需求,因此必须在多种传统信道分配方式的基础上进行混合设计,使之能够动态、自适应的针对多载荷与车辆移动性进行信道的优化分配,实现在低负载条件下保持对信道的高效利用,高负载条件下保持网络性能的稳定与可靠,对节点的高移动性具有较强的鲁棒性。针对以上问题,本文提出CSMA与TDMA相融合的混合接入协议方案,将网络的接入协议设计要求与上层的多载荷应用综合考虑,通过合理的时隙规划与信道接入规则设计,实现以网络负载为控制因子的自适应调节机制。本文主要贡献与创新点如下：(1)深入调研了多载荷条件下VANET网络发展背景,分析了多载荷与与快速移动性对无线信道接入协议的性能要求,详细评估了CSMA与TDMA接入技术在相关交通通信网络中的应用现状,论证了两者的性能特点与互补性,明确了接入协议设计所面临的问题与挑战。(2)通过分布式时隙分配算法为节点进行初始化时隙分配并建立区域范围内帧结构,设置计时器功能模块,将CSMA竞争机制融入时隙中,采用退避机制区分节点优先级,利用广播竞争通告消息作为冲突解决方案。通过一系列功能模块的设计与信道接入规则的制定,形成在高负载时倾向以时隙分配信道为主,低负载条件下以竞争接入信道为主的自适应接入机制。(3)根据VANET网络应用的调研结果建立了高速公路环境下仿真场景,在NS2网络仿真软件中对相关设计方案进行了仿真实现。以仿真数据为依据,从信道利用率、丢包率和网络开销三个方面对混合型接入协议进行性能评估。结果表明,混合型接入协议在由低到高的动态负载条件下均具有较高的信道利用率,虽然由于竞争机制的固有缺陷出现了少量的丢包,但是与传统TDMA相比网络开销所占比例大大降低,提高了可用带宽。与此同时,混合协议对节点速度的变化表现出了很高的稳定性,表现更加均衡。最后通过辅助技术的仿真评估,对协议的进一步研究提供了理论基础。更多还原

【Abstract】 Multiple load data traffic and high speed mobility bring a new challenge to the medium access control protocol of VANET in intelligent transportation system. It is hard for the traditional MAC protocols to meet the need of the new network. Thus it is very important to improve the performance of the traditional MAC protocol. A new MAC protocol which can keep efficient and reliable in both low and high load networks is needed, which should also keep robust in the network with high mobility nodes.A hybrid MAC mechanism is proposed in this paper, which takes the advantages of both TDMA and CSMA mechanism. This hybrid mechanism is based on TDMA, while CSMA mechanism is added in time slots to improve the slot utilization. The main contribution and innovation of this paper are as follows:(1) A conclusion about the development of VANET and analysis on the demand of the multiple-load VANET are made in this paper. The performances of CSMA and TDMA in the related scene are evaluated. The challenge and problems which brought by the new constraints of multiple load are especially concerned in this paper.(2) The key functional modules in the new mechanism are as follows:Slots are assigned to the nodes in the network with distributed slot assignment algorithm. Clock module is set for every node, and a hybrid protocol with backoff mechanism is carried out. Contention Notify Message is set to be the collision solution. Through these series of designs and access control rules, the new MAC mechanism becomes to be flexible and stable, which tend to be a TDMA mechanism when the load is high on the network, a CSMA mechanism in the low contention network.(3) Based on the data of simulation, the performance of the hybrid MAC protocol is evaluated in channel utilization, packet loss ratio and the overhead. It is can be concluded that the new protocol have a good performance in channel utilization whenever the load is high or low, though there are some packet lost because of the problem of contention-based mechanism, the overhead of the hybrid protocol has been reduced remarkably. The hybrid protocol also keeps a good robustness when the node speed changed. Through the research on the assistance mechanism, an idea for the future research is provided.更多还原