LongShoT: Long-Range Synchronization of Time

Low-Power Wide Area Networks, such as LoRaWAN, are rapidly gaining popularity in the field of wireless sensing and actuation. While LoRaWan is heavily studied in applications and performance, the concept of time has rarely been characterized in such networks. Many applications will require synchronized local clocks with varying levels of precision in order to maintain consistency and coordination in the network. Traditional time synchronization protocols however do not fit LoRaWAN's delay-inherent, low duty cycle, network model and wide-area deployment topology. Meanwhile, relying on GPS for time is not an option for low-power applications. In this paper, we present LongShoT, a time synchronization scheme built on LoRaWan capable of synchronizing device clocks to within 10μs of a reference clock with a single network request. This is achieved by utilizing the deterministic properties of Lo-Ra Wan networks along with hardware- and MAC-level timestamping of packets. LongShoT was implemented on consumer off-the-shelf hardware and evaluated over physically distributed devices using GPS 1PPS as a reference. Our results show that LongShoT achieves an average synchronization error of less than 2μs and compensates oscillator drift to less than 0.1ppm with devices distributed within 4km of a gateway.