An Extremely Accurate Time Synchronization Mechanism in Fog-Based Vehicular Ad-Hoc Network

Fog-based Vehicular ad-hoc networks (VANETs) are new kinds of network that combine fog computing and mobile ad-hoc networks appeared in recent years, which can be used in monitoring traffic, smart driving, safe driving, and so on. Fog-based VANET is composed of a large number of dynamic nodes and static nodes, which is a heterogeneous network. Due to the heterogeneity of fog-based VANET, there is a large difference in clock rate between different nodes. It makes clock of nodes soon become inconsistent. The inaccurate clocks would lead to problems such as imprecise duty-cycle scheduling and wrong events sequence, etc. Synchronizing the clocks needs multiple message exchanges among the nodes. However, due to the mobility of the fog-based VANET, the network topology changes dynamically, which may cause synchronization packets to fail to be transmitted to a node that needs to be synchronized. In this paper, a precise time synchronization mechanism for fog-based VANET is proposed. The mechanism uses two-way message exchange method to estimate clock offset through analyzing the mobility of dynamic nodes. Clock skew is corrected using a carrier frequency offset (CFO) based hardware clock frequency estimation and considering the Doppler effect. Theoretical analyses and simulation results show that our time synchronization mechanism achieves highly accurate time synchronization.