| With the application of the new generation network technology in the field of intel-ligent transportation, the research on the key technology of internet-of-vehicles (IoV) has been widely concerned. IoV has important research significance and application value in alleviating the traffic congestion in cities, improving the efficiency of road use and promoting the informationization and intelligentization of traffic. In view of the fact that abnormal events on roads usually have a bad impact on urban traffic conditions, design-ing reliable emergency message transmission mechanism and transmitting the informa-tion about abnormal events to the target group timely and effectively will help to enhance the self-adjustment and emergency response capacities of the road traffic. The topology of vehicular networks will change dynamically with the high mobility of vehicles and is limited by road network topology. Moreover,the particularity of urban road environment will bring new challenges to the design of emergency message transmission mechanisms.This thesis mainly focuses on how to transmit emergency messages reliably and efficient-ly in the urban road environment. The objective is to improve the reliability of message transmission and ensure the transmission efficiency. In the vehicular networks, the hidden terminal problem and frequent breakage of links are the main reasons leading to packet loss, damage to the reliability of emergency message transmission. Based on these two as-pects, this thesis will first propose reliable emergency message transmission mechanisms and forwarding strategies in IoV for urban road. Then, in the case of roadside unit (R-SU) support, this thesis considers how to effectively use the RSU to deliver the messages,and proposes a hybrid emergency message multi-hop broadcast mechanism to solve the impact of RSU sparse deployment on the performance of the message transmission. The main contributions and innovations of this thesis are as follows:(1) A reliable emergency message transmission mechanism based on road layout is proposed for solving packet loss caused by the hidden terminal problem. Firstly, the rea-sons for the collision of emergency messages are studied, and a Markov chain model is built under unsaturated conditions to analyze the collision probability of the emergency message. Secondly, considering the characteristics of the urban road environment, the multi-directional transmission and the obstacle occlusion, a layout-aware collision avoid-ance mechanism is proposed. This scheme uses different transmission strategies for dif-ferent road layouts and solves the transition problem of the message transmission when the scenarios change. The RBEM/CBEM handshake mechanism is designed to solve hidden terminal problems, and can reduce the impact of the control messages on the efficiency of emergency message transmission by simplifying the handshake processes. On this basis,an adaptive redundant relay node adaptation mechanism is designed by using the feed-back information provided by the handshake mechanism. This method can dynamically adjust the broadcast redundancy according to the link quality, and further improve the re-liability of message transmission. The simulation results show that the proposed reliable transmission mechanism can provide high message coverage ratio while maintaining low transmission delay.(2) A message forwarding mechanism based on link lifetime prediction is proposed for solving packet loss caused by the instability of communication link in IoV. Firstly,the influence of urban road topology and vehicle movement on the link duration is ana-lyzed. Considering the position, velocity, direction, transmission range of vehicles and the traffic lights, a mobile scenario-aware remaining link lifetime prediction method is proposed. Secondly, in order to balance the reliability and efficiency of message transmis-sion, two kinds of link utility value calculation methods are designed to reflect the value of communication link between vehicles. By combining the link lifetime prediction and the utility value calculation, a new forwarding node selection method is proposed, which aims to select stable communication links and establish reliable transmission paths, and finally achieve reliable message transmission. Simulation results show that the proposed scheme can effectively mitigate the impact of link breakage on the message transmission and obtain high message coverage ratio and transmission efficiency.(3) An SDN-enabled hybrid emergency message multi-hop broadcast is proposed,which aims to improve the reliability of emergency message transmission by using RSU communication effectively. Firstly, the software defined networking (SDN) technolo-gy is introduced into IoV, and a novel software defined vehicular network architecture based on buses is established by deploying SDN-enabled controller, SDN-enabled hy-brid RSU and SDN-enabled bus. Secondly, with the logically centralized control and global knowledge of SDN controller, an integer programming model is constructed, and the destination RSU, VBT-RSU and destination bus selection algorithms are designed.The calculation methods of communication coverage ratio, vehicle migration ratio and mobility entropy are proposed, and the use of RSU is rationally planned to reduce the broadcast redundancy. Then, in order to solve the problem of sparse RSU deployment,a multi-hop broadcast trigger mechanism is designed. In the area where RSUs cannot be completely cover, inter-vehicle communication is used to disseminate emergency mes-sages to improve the message coverage ratio. Finally, a decentralized forwarding node selection method is proposed for the inter-vehicle communication process. Simulation results show that, the new hybrid multi-hop broadcast mechanism under the new archi-tecture has higher message coverage ratio and lower controller overhead compared with the traditional ITS architecture. |
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