Research On Messages Transmission Of Safety-Related Applications In VANET

Recent advances in new wireless communication technologies and the emergency of computationally rich vehicles are making vehicular ad-hoc network (VANET) research receive much attention from academic communities and major car manufactures around the world, which is considered as a potential technology for intelligent transportation system and improving the road traffic safety. The key for various safety-related applications is the real-time and reliable delivery of safety-related messages among vehicles, which include periodical beacons and risk-triggered warning messages. However, the variety of network topology and wireless channel caused by high mobility of vehicles bring more challenges to the delivery of safety-related messages in VANET.Based on the current research of messages delivery in VANET, we design and optimize novel protocols for the transmission of safety-related messages in the MAC and routing view two levels, where the cross-layer design is considered. Moreover, in order to verify the performance of transmission protocols effectively, we also deeply study the simulators used in VANET and develop an integrated simulation platform named DVR_Sim. The main contributions of this research are as follows.1. The performance of MAC protocols is affected seriously by the high mobility and unbalance of vehicles on roads. The performance of random and contention-based methods deteriorates significantly with increased traffic load, because of a corresponding increase in message collisions. However, the contention-free MAC methods require more coordination to perform allocation, especially when the network configuration changes rapidly. Thus, we study the adaptive collision-free MAC protocol based on dynamic TDMA mechanism for inter-vehicular communication in the second chapter. The RSU is exploited to maintain a dynamic slots assignment cycle for moving vehicles under its coverage. Then we redefine the frame form and slots assignment algorithm. Besides, the redundant slots reclamation and seamless handover between adjacent RSUs are also optimized carefully. Finally, the simulation results verify that our proposed scheme outperforms the IEEE802.11p MAC protocol in term of packet loss and transmission delay. What’s more, the goodput of beacon messages is improved by40%compared with IEEE802.11p.2. Most of the existing research always assumes that the risk-triggered messages have a higher priority to be transmitted than that of beacons. However, both of them are very important for road traffic safety in emergency situations. Thus, we study the efficiency and fairness of delivering beacons and warning messages on the medium access, and then design the risk-aware dynamic MAC protocol for vehicular cooperative collision avoidance system in the third chapter. We adopt the combination mechanism of TDMA and CSMA/CA and each frame is divided into two parts: TDMA segment for transferring beacons, and CSMA segment for delivering warning messages. Then, we propose a stochastic model to predict the average total number of potential collisions in a platoon of vehicles, which determines the size of CSMA segment in the R-MAC protocol meticulously. After that, extensive simulations under different traffic scenes show that our protocol improves the packets delivery ratio by42%and the Jain’s fairness index of the channel access by32%compared with IEEE802.11p, respectively.3. Although many geocast routing schemes are employed to widely propagate the warning messages among vehicles, it’s still difficult to meet the requirement of real-time and reliability for risk-triggered messages delivery. Therefore, we study the delay-aware reliable broadcast scheme based on transmit power control technology for VANET in the fourth chapter. The wireless channel fading, transmission delay and retransmissions characters occurring in the PHY/MAC layer are all considered when we design the cross-layer optimal relay selection algorithm. Finally, simulation results show that our protocol increases38%in packets delivery ratio and reduce53%in average transmission delay compared with traditional SIP broadcast protocol.4. Since the simulations of traffic and network protocol are conducted separately and the driver’s factor is not considered in most VANET simulations, we deeply study the simulators and develop an integrated simulation platform named DVR_Sim in the fifth chapter. The DVR_Sim can simulate the traffic, network protocol and driver’s factor simultaneously, which makes the experiment configuration more close to the real traffic scene.