摘 要

随着我国经济水平的不断提高，交通行业迅速发展，但随之而来的就是道路交通安全问题日趋严峻，智能交通系统的提高在一定程度上改善了交通问题，提高了交通效率和减少交通事故的发生。并且无线通信技术与交通业的发展日益密切，车载自组织网络（Vehicle Ad-hoc Network，VANET）的作用越来越显著。

但由于车辆的快速移动使得网络拓扑结构频繁发生变化，在车载自组织网络中易引起隐藏终端问题，造成数据包的丢失。数据包中的协同感知消息（Cooperative Awareness Message，CAM）包含着车辆的行驶状态信息——行驶速度，即时的地理位置，安全警报等消息，车辆通过自组织网络进行广播信息的传输。可以有利于驾驶员更加清晰的了解周围交通环境状况，并对可能出现的交通事故作出相应的调整，在一定程度上降低了交通事故发生的风险，提高交通运行效率。所以这些信息对于交通安全行驶有着至关重要的意义，但由于隐藏终端问题的产生，造成数据的延迟发送甚至丢包使得车辆无法及时接受消息，最终影响人生财产安全和道路交通安全。

针对其问题，首先需要考虑如何检测出隐藏终端问题，并在检测出来的基础上制定有效的解决方案及验证其可行性。本文在分析位图的结构和功能上，探讨了如何检测隐藏终端问题，并根据不同的避碰案列，提出了一种基于分散位图的避碰方案，利用车辆的本地网络信息处理隐藏数据冲突。主要取得以下几个方面结果：

首先，位图最初用于指示车辆从其单跳邻居接收数据包的最新状态，可以进一步利用这些信息检测CAM广播中隐藏的数据冲突。具体而言，每个车辆v_i通过检查这些邻居如何在其位图中标记车辆v_i的位来确定邻居是否已接收到其数据包，然后挖掘隐藏的数据。通过推断是出是自身还是其他车辆的加入导致这些邻居无法接收到这个数据包而产生的碰撞。

其次，当使用位图检测隐藏的数据冲突时，每个车辆v_i只需考虑其在接收位图中的传输时隙的位状态，将这个位的状态表明发送车辆是否已接收车辆v_i的数据包。根据其他车辆v_j为已知邻居还是新邻居，并且车辆v_i通过最新接收的CAM中的地理位置和速度检查车辆v_j的地理位置，并且比较车辆v_i传输范围的半径，决定是丢弃数据包还是调到另一个时隙进行信息传输。

最后，如两个车辆在同一个时隙中进行广播，如果它们在彼此的传输范围内，则会造成隐藏的数据冲突。由于无法接收到数据包，两台车辆都无法实现彼此之间的存在，从而导致CAM广播中连续隐藏的数据冲突。为了解决这个问题，当没有位图可以被接收时，使用随机时隙跳变来检测潜在的隐藏数据冲突。具体地，每个车辆v_i保持具有随机值的计时器，并且在计时器期满时随机跳跃到空闲时隙。其目的是窃听是否有任何其他车辆在同一插槽中传输。如果没有检测到同步传输，v_i跳回原来的时隙，为下一次窃听重新设置一个大值的定时器；否则，它停留在新的时隙中，并用一个小值重置计时器，以便下一次窃听。

本文利用MATLAB软件，模拟了一个十字路口，模拟车辆在真实道路上行驶时，使用位图前后冲突率的大小，在仿真测试中表明，该方案降低了隐藏的数据冲突，提高了网络吞吐率。

关键词：研究背景及意义；隐藏终端；位图；冲突避免

Research on Hidden Terminal in Vehicle Ad hoc Network

ABSTRACT

With the continuous improvement of the economic level of our country, the transportation industry is developing rapidly, but the road traffic safety problem is becoming more and more serious, and the improvement of intelligent transportation system has improved the traffic problem to a certain extent. It improves traffic efficiency and reduces the occurrence of traffic accidents. With the development of wireless communication technology and transportation industry, the role of vehicle self-organizing network (Vehicle Ad-hoc Network, VANET) is becoming more and more significant.

However, due to the rapid movement of vehicles, the network topology changes frequently, so it is easy to cause hidden terminal problems in the vehicle ad hoc network, resulting in the loss of data packets. The cooperative awareness message (Cooperative Awareness Message, CAM )in the packet contains the driving state information of the vehicle-driving speed, real-time geographical location, security alarm and so on. The vehicle transmits the broadcast information through the ad hoc network. It can help drivers to understand the surrounding traffic environment more clearly, and make corresponding adjustments to the possible traffic accidents, reduce the risk of traffic accidents to a certain extent, and improve the efficiency of traffic operation. Therefore, these information is of great significance for traffic safety, but due to the generation of hidden terminal problem, the delay of data transmission and even packet loss make the vehicle unable to accept the message in time, and finally affect the safety of life property and road traffic safety.

In order to solve the problem, it is necessary to consider how to detect the hidden terminal problem, and to formulate an effective solution and verify its feasibility on the basis of the detection. In this paper, the structure and function of bitmap are analyzed, and how to detect hidden terminal is discussed. According to different collision avoidance cases, a collision avoidance scheme based on decentralized bitmap is proposed, which uses the local network information of vehicles to deal with hidden data conflicts. The main results are as follows:

First of all, bitmaps are originally used to indicate the latest status of vehicles receiving packets from their single-hop neighbors, and this information can be further used to detect hidden data conflicts in CAM broadcasting. Specifically, each vehicle v_i determines if the neighbor has received its data packet by checking the bits of the vehicle v_i in its bitmap, and then mining the hidden data. It is concluded that the addition of itself or other vehicles results in collisions that these neighbors are unable to receive for this data packet.

Secondly, when using bitmap to detect hidden data conflicts, each vehicle v_i only needs to consider the bit state of its transmission time slot in the received bitmap, and the status of this bit indicates whether the sending vehicle has received the packet of vehicle v_i. according to whether the other vehicle v_j is a known neighbor or a new neighbor, and the vehicle v_i checks the geographic position of the vehicle v_j by the geographic position and speed in the newly received cam and compares the radius of the transmission range of the vehicle v_i, It is determined whether to discard the data packet or to transfer it to another time slot for information transmission.

Finally, some special cases, such as two vehicles broadcasting in the same time slot, if they are within each other's transmission range, will cause hidden data conflicts. Because the data packets can not be received, neither of the two vehicles can realize the existence of each other, resulting in continuous hidden data conflicts in CAM broadcasting. In order to solve this problem, when no bitmap can be received, random slot jump is used to detect potential hidden data conflicts. Specifically, each vehicle v_i maintains a timer with random values and randomly jumps to the idle slot when the timer expires. The aim is to eavesdrop on whether any other vehicles are transmitted in the same slot. If synchronous transmission is not detected, v_i jumps back to the original slot and resets a large timer for the next wiretapping; otherwise, it stays in the new slot and resets the timer with a small value for the next wiretapping.