Keep the Beat: On-The-Fly Clock Offset Compensation for Synchronous Transmissions in Low-Power Networks

Emerging protocols for low-power wireless networks increasingly exploit constructive interference and the capture effect. The basic idea is that the synchronous transmission of identical packets by neighboring nodes leads to constructive interference - or at least do not cause destructive interference. This requires that the temporal displacement of packets at receiving nodes is lower than 0.5 μs when employing IEEE 802.15.4 radios. However, commonly used sensor nodes are equipped with cheap and imprecise clocks that show high frequency deviations across nodes, making constructive interference difficult to achieve. Such deviations further increase when individual nodes are exposed to different temperatures. In this paper we introduce Flock, a novel approach to compensate for differences in clock frequency across synchronously transmitting nodes. We implemented Flock in Contiki on the example of Glossy, a flooding protocol based on synchronous transmissions. Our results confirm that Flock can achieve constructive interference on real sensor nodes in over 98% of the cases. Overall, Flock makes protocols that exploit synchronous transmissions more robust to operate even in challenging environments.