Pulsar: A Wireless Propagation-Aware Clock Synchronization Platform

In this paper, we introduce Pulsar, a wireless time transfer platform that can achieve clock synchronization to better than five nanosecond between indoor or GPS-denied devices. Nanosecond-level clock synchronization is a missing capability for many real-time applications like next-generation wireless systems that leverage spatial multiplexing to improve channel capacity and provide services like time-of-flight localization. With finegrained synchronization, both clock stability and propagation delays introduce significant sources of error. Pulsar leverages a stable clock source derived from a Chip-Scale Atomic Clock (CSAC) along with an Ultra-WideBand (UWB) radio able to perform sub-nanosecond packet timestamping to estimate and correct for clock offsets. We design and evaluate a proof-ofconcept network-wide synchronization protocol for Pulsar that selects low-jitter links to both estimate the location of nodes and reduce cumulative synchronization error across multiple hops. The Pulsar platform and protocol together provide a phase synchronized one pulse per second (1PPS) signal and 10 MHz reference clock that can be easily integrated with typical enduser applications like software-defined radios and communication systems. We experimentally evaluate the Pulsar platform in terms of clock synchronization accuracy, Allan deviation between pairwise clocks and ranging accuracy to show a clock synchronization of better than five nanoseconds per hop with an average of 2.12 ns and a standard deviation of 0.84 ns. The platform is able to identify and avoid clock error in cases where there is heavy multi-path or non-Line-of-Sight signals.