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

Time synchronization in communication networks provides a common time frame among all nodes, thussupporting various network functions such as message transmission, channel scheduling and resourcesharing in real-time and in correct order. In vehicular ad-hoc networks (VANETs) for connected andautomated vehicles, network nodes must be synchronized to exchange time-critical vehicle locationsand warning messages for various road safety applications. However, this is challenging as the datacommunication systems need to achieve low latency and high reliability under the conditions ofhigh user dynamics and density. While many synchronization techniques have been developed forgeneral communications networks, it is necessary to understand the requirements for VANET timesynchronization 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 addressessome key factors in VANET time synchronization such as requirements analysis, precision, accuracy,availability, scalability and compatibility, and highlights the advantages of Global Navigation SatelliteSystem (GNSS) in VANET time synchronization. Through this survey, some insights are developed intoexisting and emerging protocols for time synchronization in VANETs.