A directional data dissemination protocol for vehicular environments

This paper presents a simple and robust dissemination protocol that efficiently deals with data dissemination in both dense and sparse vehicular networks. Our goal is to address highway scenarios where vehicles equipped with sensors detect an event, e.g., a hazard and broadcast an event message to a specific direction of interest. In order to deal with broadcast communication under diverse network densities, we design a dissemination protocol in such a way that: (i) it prevents the so-called broadcast storm problem in dense networks by employing an optimized broadcast suppression technique; and (ii) it efficiently deals with disconnected networks by relying on the store-carry-forward communication model. The novelty of the protocol lies in its simplicity and robustness. Simplicity is achieved by only considering two states (i.e., cluster tail and non-tail) for vehicles. Furthermore, vehicles in both directions help disseminating messages in a seamlessly manner, without resorting to different operation modes for each direction. Robustness is achieved by assigning message delivery responsibility to multiple vehicles in sparse networks. Our simulation results show that our protocol achieves higher delivery ratio and higher robustness when compared with DV-CAST under diverse road scenarios.     

Dissemination of safety messages in IEEE 802.11p/WAVE vehicular network: Analytical study and protocol enhancements

Multi-channel IEEE WAVE 1609.4 protocol has been proposed to guarantee the co-existence of safety and non-safety applications over the same Vehicular Ad hoc NETwork (VANET) scenario. While the usage of multi-channel avoids the risk of collisions between applications allocated on different frequencies, its implementation on a single-radio transceiver poses some major concerns about the effective utilization of the channel resources. In this paper, we study the performance of safety applications over multi-channel single-radio VANETs, and we present three novel contributions in this regard. First, we propose an analytical analysis and a simulation study of IEEE 1609.4. We show the harmful impact of synchronous channel switching on the message delay and delivery ratio. Second, we investigate the problem of dissemination of safety broadcast messages over multi-channel VANETs, where the network is intermittently disconnected, due to the alternation of control and service intervals. Finally, we propose a WAVE-enhanced Safety message Delivery (WSD) scheme to enable fast dissemination of safety messages over multi-channel VANETs, while guaranteeing compatibility with the existing WAVE stack. To this aim, we formulate the dissemination problem as a multi-channel scheduling problem. We further introduce cooperation among vehicles to reduce the dissemination latency. Simulation study shows the ability of the WSD scheme to enhance the performance of IEEE 1609.4 in terms of message delay and delivery ratio under different topologies and various applications.     

一种用于高速公路上防车辆连环碰撞的V2V广播协议

目前,V2V网络主要通过周期性广播紧急预警消息(emergency warning message,EWM)来解决高速公路上经常发生的连环碰撞事件,但是周期性广播EWM容易产生广播风暴,造成大量消息的传输失败和传输延时,从而影响了预警网络的可靠性和效率.通过研究V2V网络中的各种无线广播协议,提出了一种用于高速公路上防止车辆连环碰撞的广播协议.协议在方向性广播的基础上,通过发送ACK帧选择广播车辆并由广播车辆负责广播EWM来解决广播协议中的EWM冗余问题.仿真实验表明:协议能有效地控制EWM的冗余问题,提高EWM传输的可靠性并降低传输延时.     

微博客蕴含交通信息的提取

微博客消息中可能蕴含大量描述城市道路的交通信息,如交通状况、交通事件、交通管制等,提取这些交通信息能够为传统的固定式传感器和浮动车采集交通信息手段提供有效补充.然而,微博客消息描述的模糊性、差异性及非结构化特征,使得从海量微博客消息中快速准确地提取和甄别交通信息成为难题.提出一种从微博客消息中快速提取和融合交通信息的技术方法,首先对采集到的微博客消息进行分词解析和路网匹配,然后采用基于神经网络的模糊C聚类方法对描述路段交通状态的微博客消息定量化结果进行分析,获取各路段置信度最高的交通状态描述,最后得到各路段的交通畅通度水平.基于新浪微博客和北京路网的实验过程验证了本文技术方法的有效性.     

Exploiting skewed access and energy-efficient algorithm to improve the performance of wireless data broadcasting

As wireless networks become an integral component of the current communication infrastructure, energy efficiency is a crucial design consideration because of the limited battery life of mobile terminals. Data broadcast is an effective data dissemination method in mobile environments. The current air indexing schemes for data broadcast focused on energy efficiency (reducing tuning time) only, and current broadcast scheduling schemes reduce access latency through nonflat data broadcast to improve only responsiveness. Few studies have addressed energy efficiency and responsiveness issues concurrently. This study proposes a fast data access scheme that concurrently supports the energy saving protocol, which constructs broadcast channels according to the access frequency of each type of message to improve energy efficiency in mobile devices. The windmill scheduling algorithm that is presented in this paper was used to organize all types of messages in the broadcast channel in the most symmetrical distribution, to reduce tuning and access time. The performance of the proposed mechanism was analyzed, and the efficiency improvement over existing methods was demonstrated numerically. Results indicate that the proposed mechanism is capable of improving both tuning and access time because of the presence of skewness in the access distribution among disseminated messages.     

基于移动ad-hoc网络的车辆定位

当车辆不能通过GPS接收机获得自身位置信息时,就难以获得有用的交通提示风险服务.提出了一种新的基于车载移动ad-hoc网络车辆定位方法,该方法能够获取作为网络节点车辆的大致位置,并结合一种改进的警报信息优化传播算法,向即将处于危险或拥堵区域的不能通过GPS接收机定位的车辆发送警报信息.仿真实验表明只要车辆自组织网络中有40%的车辆可以获得GPS的定位信息,就可以将警报信息准确完整地送达处于网络中的所有车辆.当遇到浓雾天气、交通事故或者是其他拥塞时,该方法会防止车辆进一步拥堵并能提醒驾驶员防范危险.     

车联网中多跳广播转发节点选择研究

在车载自组织网络中(VANET),大部分应用采用多跳广播来交换道路交通信息,其中最典型的为泛洪广播,但是随着车辆和节点的增多泛洪广播会引起广播风暴,导致信道阻塞。结合模糊逻辑,提出一种基于接收节点的多跳广播方案,依靠接收节点的覆盖范围、信道忙闲比和移动因素,由模糊逻辑系统决定当前节点是否具备转发资格。通过于其它方案的仿真对比验证,所提出的方案在投递率与时延方面具有更好的性能,可有效提高信道利用率,降低广播风暴风险。     

Velocity calculation by automatic camera calibration based on homogenous fog weather condition

A novel algorithm for vehicle average velocity detection through automatic and dynamic camera calibration based on dark channel in homogenous fog weather condition is presented in this paper. Camera fixed in the middle of the road should be calibrated in homogenous fog weather condition, and can be used in any weather condition. Unlike other researches in velocity calculation area, our traffic model only includes road plane and vehicles in motion. Painted lines in scene image are neglected because sometimes there are no traffic lanes, especially in un-structured traffic scene. Once calibrated, scene distance will be got and can be used to calculate vehicles average velocity. Three major steps are included in our algorithm. Firstly, current video frame is recognized to discriminate current weather condition based on area search method (ASM). If it is homogenous fog, average pixel value from top to bottom in the selected area will change in the form of edge spread function (ESF). Secondly, traffic road surface plane will be found by generating activity map created by calculating the expected value of the absolute intensity difference between two adjacent frames. Finally, scene transmission image is got by dark channel prior theory, camera’s intrinsic and extrinsic parameters are calculated based on the parameter calibration formula deduced from monocular model and scene transmission image. In this step, several key points with particular transmission value for generating necessary calculation equations on road surface are selected to calibrate the camera. Vehicles’ pixel coordinates are transformed to camera coordinates. Distance between vehicles and the camera will be calculated, and then average velocity for each vehicle is got. At the end of this paper, calibration results and vehicles velocity data for nine vehicles in different weather conditions are given. Comparison with other algorithms verifies the effectiveness of our algorithm.     

An adaptive and opportunistic broadcast protocol for vehicular ad hoc networks

A primary goal of broadcasting in vehicular ad hoc network (VANET) is to improve the road safety by transmitting alert messages to all surrounding vehicles as soon as possible. In this paper, we adopt the concept of opportunistic routing and propose a multiple candidate relays opportunistic broadcast (MCROB) protocol for VANET. The MCROB protocol is a sender-driven broadcast scheme independent of node density. The packet delivery ratio (PDR) is derived and an expected transmission speed (ETS) for the MCROB is proposed. A priority rule for selecting a proper candidate relay and an adaptive algorithm for forwarding timers of candidate relays are also presented in this paper. Simulations show that MCROB is adaptive to the rapid changing of network conditions. It keeps a low communication overhead introduced by the broadcast and increases the average transmission speed by around 40%.     

车载自组织网络中基于停车骨干网络的数据传输

车载自组织网络(vehicular ad hoc networks,简称VANETs)具有网络间歇连通、节点高速移动及动态的网络拓扑结构等特性,如何有效地实现车辆间的数据传输,成为VANETs的重大挑战.现有研究工作基于历史交通流量或历史延迟预测路段当前交通状况的方法并不可靠.此外,要实现高效的数据路由传输,配置大量路边基础设施节点(deploying roadside unit,简称RSU)是一种可行方案,但通常需要额外开销.基于城市区域长时间拥有大量地上停放车辆这一事实,提出了基于停车骨干网络的数据传输策略PBBD(parking backbone based data delivery),不需要配置任何地面基础设施,而是把地面的停放车辆组成一个虚拟的停车覆盖网络,通过该停车覆盖网实现数据的传输.为此,首先,对于每一条道路,把路边和非路边停放车辆组成一个尽可能长的停车簇,并基于这些停车簇组织城市停车骨干网络.其次,设计基于停车覆盖网络的全新数据传输算法来实现车辆间的有效数据传输.基于真实城市地图和交通数据的模拟实验结果表明,与现有的几种数据传输算法相比,PBBD能够以较低的网络传输开销和较小的传输延迟获得较高的数据传输成功率.     

MAC and application level performance evaluation of beacon message dissemination in DSRC safety communication

In this paper, an analytic model is proposed for both MAC and application-level performance evaluation of periodic beacon message dissemination in the dedicated short range communications (DSRC) system on highways. Each vehicle can periodically generate beacon messages, which contain the information including position, velocity, direction, etc.. Out-dated message is replaced by the newly generated message. In order to develop a tractable analytic approximation, we first develop a semi-Markov process (SMP) model for the tagged vehicle to capture the periodic message generation, out-dated message replacement, channel contention and backoff behavior in IEEE 802.11 broadcast ad hoc networks. Further, an SMP model with an absorbing state is constructed to derive the message service time distribution. The fixed-point iteration method is utilized to obtain the converged solutions to resolve interactions between the SMPs of different vehicles. Both MAC-level and application-level performance metrics are derived. MAC-level analytic–numeric results are verified through extensive simulations. Application-level analytic–numerical results are evaluated subsequently to provide insights on network parameter settings. To ensure the quality of service (QoS) for a VANET application, application requirements in terms of application-level metrics are specified. Three typical safety-applications are evaluated in order to assess whether their application requirements can be satisfied or not.     

Traffic congestion detection in large-scale scenarios using vehicle-to-vehicle communications

Cooperative vehicular systems are currently being investigated to design innovative ITS (Intelligent Transportation Systems) solutions for road traffic management and safety. Through the wireless exchange of information between vehicles, and between vehicles and infrastructure nodes, cooperative systems can support novel decentralized strategies for ubiquitous and more cost-attractive traffic monitoring. In this context, this paper presents and evaluates CoTEC (COperative Traffic congestion detECtion), a novel cooperative technique based on Vehicle-to-Vehicle (V2V) communications designed to detect road traffic congestion. CoTEC is evaluated under large-scale highway scenarios using iTETRIS, a unique open source simulation platform created to investigate the impact of cooperative vehicular systems. The obtained results demonstrate CoTEC's capability to accurately detect and characterize road traffic congestion conditions under different traffic scenarios and V2V penetration rates. In particular, CoTEC results in congestion detection probabilities higher than 90%. These results are obtained without overloading the cooperative communications channel. In fact, CoTEC reduces the communications overhead needed to detect road traffic congestions compared to related techniques by 88%.     

交通微观仿真中车辆走行死锁问题研究

模拟交通冲突是道路交通微观仿真中的实现难点。该文描述了由于机动车辆对固定冲突点竞争而引起的“死锁”现象,分析了这种现象的产生机理。在评述现有解决方法的基础上,结合交通冲突的方式和固定冲突点的分布情况,给出了死锁模型和典型情况下预防死锁的算法,同时设定时间参数和车流量参数以便进行路口通行能力评价。最后针对有交通信号控制的十字路口进行仿真编程实现,仿真结果表明本算法对于机动车辆在固定冲突点的死锁预防是可行的,仿真效果与实际的车辆走行情况相吻合。     

VANET中基于捎带确认机制的自适应数据传播策略

车载自组网（VANET）通过自组织分散环境传播交通数据,交通数据的特点使得VANET数据传播方法更适合依赖广播。然而,当在高密度场景下广播数据时,可扩展问题和数据传播冗余问题变得尤为突出,可能导致广播风暴的发生。针对数据传播冗余和广播开销等问题,以最远转发策略和捎带确认机制为基础,提出一种捎带确认的自适应最远转发策略（AckAMFR）,根据存储转发的概念进行信息的传输和存储,降低了数据冗余,具有较强的可靠性和较小的广播开销。仿真结果表明,该转发策略具有低广播开销和高数据传输率的特性。     

A data distribution scheme for VANET based on fountain code

Data dissemination is one of the applications used to provide infotainment to the end-users in vehicular ad hoc networks (VANET). During this process, the vehicles receive the data broadcast by the RoadSide Unit (RSU). However, it is difficult for vehicles to collect the complete content within the communication range of one RSU when the vehicle moves at a high speed and the amount of broadcast data is large. To solve this problem, a multi-RSU cooperative data distribution scheme based on fountain code (MRFC) is proposed in this paper. The source data are encoded by fountain code and poured into the VANET by multiple cooperative RSUs, then the vehicles in the area share data packets through the V2V resource compensation method, so that all vehicles can obtain enough encoded packets to reconstruct the source data. To improve channel resource utilization and reduce delivery delays, the RSUs use fuzzy logic to determine the number of fountain code packets according to their locations, the speed and density of surrounding vehicles. The experimental results show that on the premise of ensuring the delivery rate, the proposed scheme can reduce the delivery delay by 30–50%, and achieve a significant improvement in performance.

     

Contention-based forwarding with multi-hop connectivity awareness in vehicular ad-hoc networks

Recent vehicular routing proposals use real-time road traffic density estimates to dynamically select forwarding paths. Estimating the traffic density in vehicular ad hoc networks requires the transmission of additional dedicated messages increasing the communications load. These proposals are generally based on unicast sender-based forwarding schemes. The greedy nature of sender-based forwarding can result in the selection of forwarders with weak radio links that might compromise the end-to-end performance. To overcome these limitations, this paper presents TOPOCBF, a novel contention-based broadcast forwarding protocol that dynamically selects forwarding paths based on their capability to route packets between anchor points. Such capability is estimated by means of a multi-hop connectivity metric. The obtained results demonstrate that TOPOCBF can provide good packet delivery ratios while reducing the communications load compared to unicast sender-based forwarding schemes using road traffic density estimates.     

基于机器学习反馈的车辆自动路况识别

研究车辆路况自动识别的问题,提高识别的准确率和鲁棒性。针对车辆的路况自动识别系统极易受外界环境的影响,传统的基于PCA的路况识别方法在提取路况信息时无法避免恶劣天气等环境的影响,造成最终的识别不准确和鲁棒性不高的问题。为了克服这一难题,提出了基于机器学习的车辆路况自动识别系统。首先采用Haar小波特征提取方法,将受环境影响的路况图像中的有效特征准确提取并降维,然后利用支持向量机选择合适的特征参数,将特征参数输入到AdaBoost分类器中进行分类识别后就完成了最终的车辆路况自动识别,避免了传统方法自动识别受恶劣环境影响的问题。实验证明,这种方法能够有效克服外界环境的影响,准确完成车辆路况的自动识别,并且识别结果具有较好的鲁棒性和满意的效果。     

Concurrent Broadcast for Information Dissemination

Concurrent broadcast involves the dissemination of a database, consisting of messages initially distributed among the nodes of a network, so that a copy of the entire database eventually resides at each node. One application is the dissemination of network status information for adaptive routing in a communications network. This paper examines the time complexity and communication complexity of several distributed procedures for concurrent broadcast. The procedures do not use information depending on the network topology. The worst-case time complexity of a flooding procedure for concurrent broadcast is shown to be linear in the number of nodes plus the number of messages, and no other procedure for concurrent broadcast has a better worst-case time complexity. A variant of flooding is proposed to eliminate redundant message receipts from the flooding process by real-time signaling between neighbors concerning messages residing at each. This variant can reduce communication complexity, while having a worst-case time complexity similar in form to that of the flooding procedure. Special properties of concurrent broadcast in a tree are also given. The present time complexity results can be used to bound the time during which inconsistent databases may reside at different nodes, to evaluate and compare procedures for (or including) concurrent broadcast, and to schedule a sequence of instances of concurrent broadcast so that the instances do not overlap and there is no need for sequence numbers.     

Real-time vehicle tracking mechanism with license plate recognition from road images

Abstract Information about vehicles on the road is very important for the maintenance of traffic control in current complex traffic condition. Images of vehicles are captured by vehicle-directed cameras. This paper proposes a new vehicle tracking mechanism using license plate recognition technology, which is essential to having information about vehicles on the roads. The proposed method is a real-time processing system using multistep image processing, as well as recognition and tracking processes from 2D and 3D images. The experimental results of real environmental images in recognition and tracking using the proposed method are shown.     

A fault-tolerant protocol for atomic broadcast

A general protocol for atomic broadcast in networks is presented. The protocol tolerates loss, duplication, reordering, delay of messages, and network partitioning in an arbitrary network of fail-stop sites (i.e. no Byzantine site behavior is tolerated). The protocol is based on majority-concensus decisions to commit on unique ordering of received broadcast messages. Under normal operating conditions, the protocol requires three phases to complete and approximately 4N/V messages where N is the number of sites. This overhead is distributed among the messages of which the delivery decision is made and the heavier the broadcast message traffic, the lower the overhead per broadcast message becomes. Under abnormal operating conditions, a decentralized termination protocol (also presented) is invoked. A performance analysis of this protocol is presented, showing that this protocol commits with high probability under realistic operating conditions without invoking termination protocol if N is sufficiently large. The protocol retains its efficiency in wide-area networks where broadcast communication media are unavailable