Time synchronization in vehicular ad-hoc networks: A survey on theory and practice

Time synchronization in communication networks provides a common time frame among all nodes, thus supporting various network functions such as message transmission, channel scheduling and resource sharing in real-time and in correct order. In vehicular ad-hoc networks (VANETs) for connected and automated vehicles, network nodes must be synchronized to exchange time-critical vehicle locations and warning messages for various road safety applications. However, this is challenging as the data communication systems need to achieve low latency and high reliability under the conditions of high user dynamics and density. While many synchronization techniques have been developed for general communications networks, it is necessary to understand the requirements for VANET time synchronization and the applicability of existing time synchronization techniques in VANET applications. This paper provides a survey on theory and practice of time synchronization in VANETs. It addresses some key factors in VANET time synchronization such as requirements analysis, precision, accuracy, availability, scalability and compatibility, and highlights the advantages of Global Navigation Satellite System (GNSS) in VANET time synchronization. Through this survey, some insights are developed into existing and emerging protocols for time synchronization in VANETs.