Device-to-Device (D2D) communication is introduced for cellular networks allowing User Equipments (UEs) to communicate directly in close physical proximity. In traditional cellular networks, time synchronization is performed by the network infrastructure equipment to address the challenges of coordinating independent clocks and tasks of UEs to communicate with each other. These challenges are more critical in D2D as the network has no central point to coordinate UEs. The aim of this paper is twofold: (i) reviewing synchronization standards and techniques for D2D-enabled cellular networks and (ii) discussing different scenarios and contemporary cellular network generations. We review the main signals, channels, and entities used by D2D-enabled cellular networks for synchronization procedures in different scenarios. Moreover, we focus on the synchronization in Vehicle-to-Vehicle (V2V) and Vehicle-to-Everything (V2X). Finally, we conduct a literature study on effective proposed synchronization techniques in D2D and vehicular communications to highlight recent advances and provide future directions.

为蜂窝网络引入了设备到设备（D2D）通信，允许用户设备（UE）在紧密的物理距离内直接通信。在传统的蜂窝网络中，时间同步是由网络基础设施设备执行的，以解决协调UE的独立时钟和任务相互通信的挑战。这些挑战在D2D中更为关键，因为网络没有协调UE的中心点。本文的目的是双重的：（i）审查支持D2D的蜂窝网络的同步标准和技术，以及（ii）讨论不同的场景和当代蜂窝网络时代。我们回顾了启用D2D的蜂窝网络在不同情况下用于同步过程的主要信号，信道和实体。此外，我们专注于车对车（V2V）和车对一切（V2X）的同步。最后，我们对D2D和车辆通信中有效提出的同步技术进行了文献研究，以突出近期的发展并提供未来的方向。