Toward a Shared Sense of Time for a Network of Batteryless, Intermittently-powered Nodes

Wireless sensor nodes powered solely by energy-harvesting show promise in enabling truly pervasive, long-duration sensing by avoiding the fragility, cost, and maintenance limitations of batteries. Unfortunately, since the amount of energy harvested is often significantly less than the active consumption of the device, these devices operate intermittently with limited control of when they are on and how long they are off. Such uncontrollable intermittency first poses a challenge for traditional time synchronization error metrics, since nodes cannot reliably communicate time values at known intervals, resulting in the illusion that nodes are out-of-sync. Second, long duration off-times can exceed the inherent timing limit of persistent clocks that intermittent nodes rely on to measure off-times. Bursty groups of long off-times can cause traditional time synchronization mechanisms to re-converge slowly, incurring significant periods of high error in which nodes are effectively out-of-sync. In this paper, we define the meaning of a shared sense of time for intermittently-powered nodes, and propose two intermittency-aware synchronization error metrics. We then propose an intermittency-resilient time synchronization mechanism, called Levee, that exhibits more rapid re-convergence after losing time and a 2.12× reduction in maximum time synchronization error for a 48-hour period.