System Optimization and Protocol Design for Vehicular Networks

Vehicular network, aiming at cooperative road safety and transportation comfort, is a fundamental building block of the intelligent transportation systems (ITS). While much standardization progress have been made to enable vehicular networking, many open topics remain unresolved in both safety applications and journey related data services. In current wireless access in vehicular environments (WAVE) and 802.11p based vehicular network system, not all system parameters are optimized for operation in a vehicular environment and some networking functionalities (i.e., multi-hop delivery) are not fully supported. This thesis addresses three major open challenges in current WAVE/802.11p system, and tackles them through mathematical modeling and high fidelity system simulation.;We first optimize one-hop broadcasting which is the default message exchange pattern in WAVE system. We model the broadcast packet reception in a 1-D vehicular network, and for the first time, explicitly derive its broadcast efficiency and broadcast reliability. The tradeoff between efficiency and reliability of p-persistent carrier sensed multiple access (CSMA) strategy in 1-D vehicular network is of fundamental importance for system optimization. We derive the optimal transmission power and transmission probability that maximize one-hop broadcast efficiency when all other parameters are known. For highly dynamic scenarios in which vehicle density knowledge is very coarse, we also have a max-min solution which provides worst case performance guarantee. A new layer---congestion control sub-layer---in a WAVE communication protocol stack is proposed to implement both optimal and max-min solutions.;Safety applications---i.e., emergency warning---sometimes need multi-hop coverage with minimal latency. Since WAVE messages are not based on IP, routing capability as well as multihop end-to-end delivery do not apply. Our second work is the design of a layer-2 low latency warning message forwarding algorithm and its implementation in the current WAVE architecture. We design a MAC layer prioritized rebroadcast contention control (PBCC) algorithm to minimize forwarding delay and reduce packet collision probability during rebroadcast. A new module is added to existing WAVE protocol stack with minimum system modification. We use network simulation to study the multi-hop propagation delay of warning messages in different scenarios, especially in the coexistence of high priority heart beat beacon messages.;Beyond safety applications, data services for in-vehicle consumption (such as 'commerce- and entertainment-on-the-wheel') are expected to become a primary driver in the development of future vehicular networks. Peer-to-peer data sharing using vehicle-to-vehicle ad hoc communication greatly supplements direct cellular and hot-spot data downloading. We model the data dissemination problem---content is available at a subset of vehicles that is desired by (many) others in the network. We characterize the gain of applying linear network coding to data dissemination in a 1-D multi-hop vehicular network via analysis and simulation.;Within the content of current WAVE/802.11p vehicular networking standards, this thesis optimizes one-hop broadcast message exchange through broadcast congestion control, designs a layer-2 forwarding algorithm and a new module in WAVE architecture for low latency warning message forwarding, and provides a way to apply linear network coding to vehicle-to-vehicle ad hoc data dissemination. These are important steps towards the success of vehicular networks.