WATCH: A Distributed Clock Time Offset Estimation Tool for Software-Defined Radio Platforms

The synchronization of clocks used at different transceivers across space is of critical importance in next-generation wireless networks. Experimental testbeds must provide synchronization services so that users can test new efficient communication and accurate localization technologies; yet reconfigurability can sometimes introduce unintended asynchronicities. We introduce WATCH, a tool to monitor the time synchronization of nodes in a geographically distributed software-defined radio (SDR) testbed. WATCH estimates clock differences using cross-correlation between transmitted and received packets and considers both propagation delay and noise in the channel. We model pairwise measurements as a linear function of the difference between the nodes' local clocks from the network's global clock, as well as each node's additional delay for packet transmission. This model allows WATCH to simultaneously estimate all of the clock offsets and transmit delays. We compare using transmissions of modulated data packets and pseudo-noise (PN) code signals to experimentally test and verify the performance of WATCH on the Platform for Open Wireless Data-driven Experimental Research (POWDER), where it detected a firmware problem that led to a time synchronization error between SDR nodes on the platform.