【作者】 王文杰；

【导师】 罗涛；

【作者基本信息】 北京邮电大学 ， 通信与信息系统， 2017， 博士

【摘要】 车载自组织网络VANET (Vehicular Ad-hoc Network)利用无线通信网络技术构建高效智能交通系统以提高道路交通安全和效率,具有重要的研究价值和社会意义,近年来受到全球范围内学术界和工业界的广泛关注。在车联网的诸多应用中,安全应用对数据传输的服务质量要求越来越高,亟需高效的广播协议来满足不断增长的需求。而车辆高速移动、网络拓扑高度动态变化以及无线信道的衰落和干扰,使得车联网中数据传输面临着严峻挑战。本文分别从物理链路、网络拓扑和应用业务三个层面研究了车联网中安全消息的广播优化算法。针对城市和高速路场景的不同特点,分别设计了基于发送方和基于接收方中继决策的广播路由,本文的主要研究内容和贡献如下:首先,针对多跳广播最基本的中继转发节点选择问题,结合发送方主动指定和接收方分布式协作的优势,提出一种动态预估最小期望时延的可靠广播方案。该方案在发送方选择中继节点时,利用城市道路全局的距离和方向等静态因素来指导中继选择的范围,利用局部的链路质量、车辆移动、信道竞争和干扰等动态因素来建模分析节点的一跳转移概率和转移时延,以预估选择某个邻居为中继时的最小期望时延度量节点的转发质量来优选中继。针对广播可靠性不稳定的问题,提出了非指定中继分布式协作转发的策略,确保广播的连续性。仿真结果表明所提算法兼顾了广播的时效性和可靠性,同时具有较低冗余。其次,针对基于发送方的中继策略在拓扑快速变化场景下可靠性急剧下降的问题,提出一种基于接收方分布式竞争转发的快速广播方案。针对现有的分布式转发方案在稀疏网络中存在“慢反应”、在密集网络中存在“局部广播风暴”的问题,提出一种低负载、自适应的感知策略。该策略利用网络中已有的BSM消息收集发送节点的局部拓扑信息,嵌入到广播报文中协助接收节点认知先验拓扑结构来统一调整转发等待时延;还利用车流密度与行车速度的线性关系跟踪网络状态的变化,自适应地调整转发概率。针对密集网络中广播发送节点的拓扑信息导致网络负载剧增的问题,感知策略提炼了拓扑的特征信息用于接收节点转发优先级调度。仿真结果表明所提算法具有较低的时延和冗余以及较高的广播效率。自适应的局部拓扑感知技术完全是自组织方式,具有良好的拓展性。最后,针对现有的广播协议大多数只考虑消息类型来区分信道接入优先级、而忽略了消息紧急程度随时间和空间动态变化的问题,提出了一种新颖的基于安全消息消息量化的高效广播方案。分析了网络中消息的安全信息量随时间、空间的变化规律,通过紧急程度来形象地度量消息所含有的安全信息量,并推导了消息紧急程度量化表达式。针对车流密度和节点分布动态变化特性,研究了消息的一跳有效覆盖增益,结合安全信息量化值,推导了消息的一跳广播收益,并基于此收益值设计了消息的优先级调度机制。所提广播方案旨在实现安全信息量全网的最大化覆盖,满足更多的节点接收更紧急安全消息的期望。仿真结果表明所提算法具有较高的车辆覆盖率和广播效率以及更低的投递时延。更多还原

【Abstract】 VANET, Vehicular Ad-hoc Network, can be used to build efficient Intelli-gent Traffic System based on the developed wireless and network technology,for the purpose of improving the safety and efficiency of road. VANET is one of the most potential subject that is valuable to science and society, attracting the increasing attention from the academia and industry all over the world. Among the numerous applications in VANET, safety services are the most important,which is the original intention that VANET is proposed by researchers. For the life related purpose, safety applications demand highly QoS of data transmis-sion, which needs efficient broadcast protocol satisfying the increasing require-ment. However, speedy movement of vehicles, the highly dynamic topology,and the fading of wireless channel and interference, the transmission of safety data in VANET are facing serious challenges.Based on the existed one-hop and multi-hop broadcast technology, the pa-per studies the broadcast protocols for disseminating safety messages in VANET on three perspectives, that are on the level of physical link, based on the cog-nition of local topology and according to the application services. Based on the analysis of characters of VANET scenarios, broadcast protocols that are sender-oriented and receiver-oriented are proposed, separately. The main re-search contents and contribution of this paper are following:Firstly, a reliable broadcast protocol based on prediction of the minimum expected delay (DPCast) in urban VANET is studied on the level of physicallink. According to the characters of varying traffic density with time and zone and the uneven distribution, a sender-based forwarding scheme for fast dissem-inating messages is proposed in Chapter Ⅱ. The global static route attributes and the local dynamic route attributes of city roads are analyzed. The global static route attributes are used to guide the range of forwarder selection, while the local dynamic route attributes are used to model the quality of forwarding of nodes based on the link quality, vehicle movement, channel contention and data interference, which is acting as the metric of selecting the optimal forwarders.In order to alleviate the problem of unstable link, a scheme that receivers are cooperating forwarding packets on the failure of rebroadcasting for specified nodes is proposed, insuring the continual dissemination of safety data. The fi-nal simulation results show that DPCast achieves faster broadcasting with little redundancy as compared to the other represented protocols.Secondly, a fast broadcast protocol based on the sensing of local topology information for normal scenarios (LISCast) is studied based on the cognition of dynamic topology. In order to mitigate the problem of reliability decline in the highly dynamic network for sender-based forwarding scheme, a receiver-based forwarding scheme is proposed in Chapter III. Since waiting-based broadcast protocol may face the problem of forwarding slowly in the sparse network and local broadcast storm in the dense network, a low load scheme is studied to adaptively adjust the parameters for candidates scheduling forwarding priority.Periodical beacons are used to collect local topology information of sender,which will be embedded in the head of safety packet to inform the receivers.On checking up the uniform characteristic information of sender, receivers con-figure the waiting time and probability for forwarding the packet in a distributed way, for the purpose of reduce channel contention and collision. The final simu-lation results show that LISCast improves the broadcast performance in latency and broadcast efficiency as compared to other represented protocols. Besides,LISCast is working in an ad hoc way, which can be expanded to other scenarios.Thirdly, an efficient broadcast protocol based on the safety information quantity of messages in the normal scenario is studied orienting on the appli-cation services. Since most of existing safety protocols schedule the priority of packets for channel access only based on the type of message, ignoring the dy-namic changing emergency of message with time and space, a novel broadcast protocol based on the dynamic emergency degree of messages (EDCast) is pro-posed in Chapter Ⅳ. The dynamic information quantity of message, which is measured by the metric of emergency degree, is analyzed in the term of tempo-ral and spatial value. According the distribution and density of vehicles, the one hop coverage gain of message are deduced. Combining the emergency value and one hop coverage, the one hop broadcast utility of message is shown, based on which the priority of channel access of message is designed, for the purpose of maximize the coverage of safety information quantity. In this way, the ex-pectation for vehicles to collect more emergent messages can be satisfied. The final simulation results show that EDCast provides a significant improvement on per-vehicle coverage and delay with little redundancy.更多还原