Studies On The Key Techniques Of Vehicular Ad-hoc Networks For Vehicle Safety Applications

Vehicular Ad hoc Networks (VANETs) are new types of Mobile Ad hoc Networks. They provide fundamental supports to active safety applications, which will significantly reduce occurrence of traffic accidents. The Wireless Local Area Network (WLAN) based Dedicated Short Range Communication (DSRC) technology will make information available beyond the drivers'horizon of awareness. The information helps them to avoid potential hazards. In this dissertation, we focus on fundamental communication issues of VANETs safety applications. We investigate DSRC's performance in realistic scenarios, analyze the challenges this type of communication is faced with, and develop the safety application's solution space. Based on the above theoretical preparation, we design a suite of protocols to improve the communication's performance and to extend the application's solution space.First, we build an analytical model to investigate the performance of DSRC's one-hop broadcasting, which is the dominant communication pattern for safety applications. The model describes the reception rate and delay of safety messages, which are the most important performance criteria for VANETs safety applications. The model considers multiple parameters, including traffic density, safety message's length and broadcasting frequency, communication's transmitting power and data rate, as well as the distance between the sender and the receiver. Through simulation studies, we found that the model is reasonably accurate. The average difference between theoretic results and simulation results is about 10%.Second, based on the above analytical model, we introduce a theoretical framework to study the VANETs safety application's solution space. First, we analyze the challenges this type of communication is faced with, including node mobility, transmitting collision and channel vulnerability. Then, we study the safety application's requirement towards wireless communication from the aspects of time, space and information. Finally, we describe the solution space of safety application and set our goal of solution space extension in this dissertation. That is, to achieve at least 80% reception rate within the distance of 300m from the sender of safety messages.Third, according to the above theoretical preparation, we design a channel adaptive one hop broadcasting protocol for VANETs safety applications. The protocol is transparent to the upper application layer and introduces little overhead. It leverages the reactive channel condition monitoring strategy to avoid extra bandwidth overheads and adjusts the transmitting parameters based on local observations. The adaptive protocol fully utilizes the scarce wireless bandwidth and assures its fare sharing between different nodes. Through simulation studies, we found that the proposed protocol is able to extend the 80% plus coverage to about 150m from the sender of safety messages. Compared with the original IEEE 802.11p standards, whose coverage is lower than 75m, the improvement is over 100%.Fourth, to address the problem that DSRC broadcasting only provides best effort delivery, we propose a piggybacked cooperative repetition approach for reliably broadcasting safety messages in VANETs. Repetitions by neighbors that receive the original transmission can effectively cover the areas that are missed in the original transmissions. The repetition process is controlled, scheduled and secured. It extends and improves safety messages'coverage and reception rate. Through simulation studies, we found that the proposed protocol combined with the adaptive one hop broadcasting protocol is able to extend the 80% plus coverage to about 300m from the sender of safety messages. Compared with the original IEEE 802.11p standards, whose coverage is lower than 75m, the improvement is over 300%.Fifth, to address the problem that the above cooperative repetition method introduces high bandwidth consumption, we adapt the idea of network coding into the protocol design. The key idea is to code the repetitions together to share the channel resource and achieve throughput gain. A poll-based scheme is developed to assist the coding process and reduce system's overheads. Through simulation studies, we found that the proposed protocol is able to contain more information into 25% shorter packets, achieving maximum 20% reception rate improvement while preserving wireless bandwidth. The formed suite of protocols comprehensively improves active safety communications in vehicular environments and achieves the dissertation's goal of solution space extension.