WASY: A Distributed Time Slot Synchronization Mechanism for Direct-to-Satellite IoT Systems

The Direct-to-Satellite Internet of Things (DtS-IoT) enables direct data transmission between remote ground terminals and low Earth orbit (LEO) satellites using low-power and long-range communications. However, the performance of communication protocols that use time division multiple access decreases in LEO satellite environments. This decline is caused by the high-speed movement of the satellites and the large and constantly changing number of ground terminals within their coverage area. Thus, time synchronization mechanisms that consider these aspects are necessary in DtS-IoT to improve the channel access reliability and scalability of the system. This paper proposes WAit-and-SYnchronize (WASY), a novel time slot synchronization mechanism for LEO satellite networks that considers the maximum orbital altitude. In addition, it operates without external infrastructure, allowing distributed synchronization at ground terminals without relying on global navigation satellite systems or Internet connectivity. The results show that the performance of our synchronization mechanism is sensitive to orbital altitude. In addition, our mechanism offers two operational alternatives. The first approach reached a synchronization accuracy of less than 1 ns; in contrast, the second approach introduced a computational cost that is up to 6 times lower than that generated by the first approach, but with an accuracy in the order of tens of microseconds.