摘要
车载网络(Vehicular Ad Hoc Networks, VANETs)是由配备了无线通信设备的车辆节点或路边单元(Roadside Unit, RSU)构成的特殊移动自组织网络,是智能交通系统的重要组成部分,具有车辆节点高速移动、网络拓扑频繁变化、通信链接不稳定、消息类型多样等特点。网络中的节点具有一定的计算能力、存储能力和短距离通信能力等。另外,车辆只能沿着道路方向移动,因此节点的运行轨迹具有一定的规律性和可预测性。车载网络中的消息传递是通过车辆节点间相遇时的信息交换和资源同享来实现,这些信息可以包括节点位置、运动方向、节点密度以及其他车辆感知信息等。
在众多车载网络的学术研究中,路由算法及相关研究占据了相当大的比例,是车载网络的重点研究内容。其研究目标是提高车载网络的数据传输成功率,降低传输时延和网络负载等。
由于车载网络中消息传递类型多样,消息对传输的要求也复杂多样,因此研究为不同的消息类型提供对应质量的数据传输服务意义重大；车载网络中的节点具有高速移动的特点,这会大大缩短节点间的可通信时间,研究网络中车辆节点的移动规律,适当通过路边设施的辅助来完成数据传输是车载网络的一个重要研究方向；车载网络中的拓扑结构频繁变化,交通流量密度随着地理位置和时间的不同有较大差异,交通流量密度对车载网络中的数据传输有着巨大的影响,因此研究交通流量对数据传输的影响可以有效提高路由算法的效率；分析网络中的节点对网络性能的影响是进一步优化各类算法的必要条件,对网络节点的分析包括如何统计节点整体/个体的性能参数、以及这些参数对网络传输的影响等,这部分的研究目前仍需深入进行。基于上述车载网络路由研究中依然存在的问题,本文对基于机会通信的车载网络路由技术进行了深入的研究,得出了如下的研究成果：
1.针对车载网络中目标为移动的车辆节点时数据传输成功率低下的问题,提出了一种基于移动车辆轨迹计算的机会路由算法(Moving target ORiented opportuNistic routing algorithm, MORN)。 MORN算法中,由当前携带数据的节点来机会选择数据转发的车辆节点,其选择依据为目标车辆的运行轨迹与备选中继节点轨迹之间的相似度；MORN算法为数据传递提供最短时间和最高传输成功率两种性能选择参数。通过仿真实验和实际的交通数据实验证明,与其他算法相比,MORN算法可以提供较高的数据传输成功率,较低的传输时延和网络负载。
2.为了在目标车辆轨迹未知的情况下向移动目标车辆传输数据,提出了一种基于区域覆盖的机会转发算法(Coverage Area based oPportunistic message forwarding algorithm, CAP)。 CAP的主要思想是使数据尽可能的覆盖目标车辆的可能途径区域,从而使得数据最大可能的传输给目标车辆。为了达到这一目的,CAP会预测目标车辆的可能途径区域,并计算候选中继节点传输数据的可能覆盖范围。在此基础上,选择候选中继节点中其数据覆盖范围与目标车辆可能途径区域重合度最高的候选节点作为下一跳的中继节点。与其他现有的研究相比,CAP的特点在于对目标车辆的可能途径区域进行预测,充分利用目标车辆运动模型、出发位置和目标位置以及道路分布等可以获取的信息。仿真测试的结果表明,CAP取得了较好的实验效果(传输成功率和传输时延),对车载网络中多跳传输有明显的改进。
3.提出了一种基于地理和交通信息的机会路由算法(An opportunistic routing based on geography and traffic for VANET, ORRIS),不仅仅考虑地理位置、车辆运动矢量特征对消息传输的影响,同时也考虑了道路上车辆流量对消息传输的影响等。在算法中,我们利用反方向车辆节点的相遇车辆历史数据来预估不同路段的交通流量密度,并在此基础上进行实时的中继节点机会选择。仿真实验的结果表明,本算法性能要优于其他算法,尤其是在交通流量较高或是道路流量密度差异性较大时,ORRIS算法的优势将更为明显。
4.车载网络中消息的产生常常是突发性的,而消息数量的激增对算法数据传输成功率有很大的消极影响。针对这种情况,我们提出了一种ORRIS算法的改进算法(ORRIS+),对ORRIS算法中车辆节点的消息转发策略进行了改进,增加消息传输备选道路的数量,分散突发消息的传输途径。另外,ORRIS+算法还引入了有效的消息队列管理机制,使之适用于网络中消息突发的情况。仿真实验的结果表明,在消息突发的情况下ORRIS+算法可以有效缓解突发消息传输成功率的下降。
The Vehicular Ad Hoc Networks (VANETs) is an important component of intelligent traffic system, which is composed of vehicular nodes or Roadside Units (RSUs) equipped with short distance communication devices. The highly mobility of nodes, frequently changing topology, intermittent communication among the vehicles and complicated applications'requirements are some of big challenges in VANETs. Usually, VANETs nodes also have processing and storage capabilities, which have enabled vehicular network the ability to form a new intelligent network. The vehicles'trajectories are predictable for that their movements are limited by traffic pattern and road layout. The message transfer is happened only when the nodes meet.
A large number of researches have been devoted to the routing algorithm and related works in VANETs. The target of these researches are concentrated on improving the message delivery ratio, puling down the delivery delay and reducing the overhead of the whole VANETs.
The requirements for message delivery are complicated since differen-t kinds of message exist in VANETs. The research of providing different kind of message delivery strategy is significant. The nodes in VANETs are mov-ing with high speed, which will shorten the communication time when nodes meet. It is important to improve the message delivery assisted by RSUs, which is based on studying the vehicles'movement law. The topology in VANETs is frequently changing; and the traffic flow varies noticeably between different ar-eas or times. The traffic flow can affect the message delivery, thus considering the traffic flow can improve the efficiency of the routing algorithms. It is neces-sary to analyse the influence on the delivery efficiency from the nodes'setting. However, the relationship between nodes'setting and the network performance still need in-depth studies.Base on the above problems, this thesis studied the opportunistic routing in VANETs, and the research result are shown as follow.
1. A Moving target ORiented opportuNistic routing algorithm in vehicular networks (MORN) is proposed for message delivery from information source to a moving target vehicle. In order to adapt the constantly chang-ing topology of networks, the forwarding decisions are made locally by each intermediate vehicle based on the trajectory information of the tar-get vehicle. The simulation and real trace experiment show that MORN provides an efficient message delivery with a higher success ratio, short-er delivery delay, and lower transmission overhead compared with other reference approaches.
2. A novel multi-hop message delivery method which is called Coverage Area based opportunistic message forwarding algorithm (CAP) is pro-posed. The main idea of CAP is to cover the potential area of moving target vehicle as much as possible with the reachable area of required delivery messages, so that the message can be delivered successfully. To achieve this, CAP calculates and compares the candidature carriers' reachable area, and finds the best candidate with a maximum coincident area between the candidates'reachable area and the target vehicle's po-tential area. Being different from existing approaches, it makes use of target vehicle's potential area which is predicted according to the target vehicle's movement model, the source and destination location, and the road layout. The simulation results show that CAP provides an efficient message delivery with a higher success ratio and a shorter message delay.
3. The message delivery ratio and delay is affected deeply by traffic flow. An opportunistic routing based on geography and traffic for VANET (ORRIS) is proposed, which leverages the knowledge of the geography position, motion vector and the traffic flow. In order to estimate the traffic flow density, the history of encounter number for the vehicles in the opposite direction is considered in ORRIS. The forwarding are made locally based on the geography topology and the traffic flow.The simulation result show that, ORRIS has a better performance, especially when the traffic is busy or the flow rates have a great differences between roads.
4. The messages in VANETs would burst in many cases, which would have a greater negative impact on the message delivery ratio of routing algo-rithms. In order to mitigate the degraded performance, an advanced OR-RIS algorithm (ORRIS+) is proposed, which improves the forwarding s-trategy of VANETs nodes. The ORRIS+makes the message a diversion to other paths by increasing the path number for message delivery; and adapts queue management scheme for each node. The simulation result-s have validated the effectiveness of ORRIS+in mitigating the degraded delivery performance when messages burst.
引文
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