Low-power clock synchronization using electromagnetic energy radiating from AC power lines

Abstract

Clock synchronization is highly desirable in many sensor networking applications. It enables event ordering, coordinated actuation, energy-efficient communication and duty cycling. This paper presents a novel low-power hardware module for achieving global clock synchronization by tuning to the magnetic field radiating from existing AC power lines. This signal can be used as a global clock source for batteryoperated sensor nodes to eliminate drift between nodes over time even when they are not passing messages. With this scheme, each receiver is frequency-locked with each other, but there is typically a phase-offset between them. Since these phase offsets tend to be constant, a higher-level compensation protocol can be used to globally synchronize a sensor network. We present the design of an LC tank receiver circuit tuned to the AC 60Hz signal which we call a Syntonistor. The Syntonistor incorporates a low-power microcontroller that filters the signal induced from AC power lines generating a pulse-per-second output for easy interfacing with sensor nodes. The hardware consumes less than 58µW which is 2-3 times lower than the idle state of most sensor networking MAC protocols. Next, we evaluate a software clock-recovery technique running on the local microcontroller that minimizes timing jitter and provides robustness to noise. Finally, we provide a protocol that sets a global notion of time by accounting for phase-offsets. We evaluate the synchronization accuracy and energy performance as compared to in-band message passing schemes. The use of out-of-band signals for clock synchronization has the useful property of decoupling the synchronization scheme from any particular MAC protocol. Our experiments show that over a 11 day period, eight nodes distributed across the floor of the CIC building on Carnegie Mellon's campus remained synchronized on an average to less than 1ms without exchanging any radio messages beyond the initialization phase.

Hardware Assisted Clock Synchronization for Real-Time Sensor Networks

“…In [4], the authors propose a scheme called Glossy, which utilizes constructive interference at the symbol level to boost the probability of successful time synchronization messages. All of these approaches could be used as part of the initial synchronization mechanism required by the Syntonistor [1].…”

“…In order for a system to remain synchronized, a global clock signal must be present from which each device estimates its internal drift. The work in this paper builds upon the device described in [1] (called a Syntonistor) that recovers a common global clock from the electromagnetic field generated by power lines. Since each node in the system can lock onto the same global clock source, the nodes can remain synchronized for long periods of time without exchanging messages.…”

“…Protocols like [2], [3], [4] exchange packets in a manner that allows them to reduce timing uncertainties. In [1] we presented a hardware device called a Syntonistor that is able to achieve a similar goal by tracking the 60 Hz magnetic field (common across a wide area) being emitted from power lines within a building.…”

“…With this scheme, each receiver is frequency-locked with each other, but there is typically a phase-offset between them. Since these phase offsets tend to be constant, a higher-level compensation protocol can be used to globally synchronize a sensor network (described in more detail in [1]). The first contribution of this paper is an improved Syntonistor sensing front-end that is based on electric field sensing, as opposed to magnetic resonance.…”

MAC2: A Multi-Hop Adaptive MAC Protocol with Packet Concatenation for Wireless Sensor Networks

“…[17], can help the sensor nodes synchronize accurately with low power consumption, so the synchronous protocols can relax the synchronous overhead. There are several approaches to reducing delivery latency, such as S-MAC with adaptive listening [18] and T-MAC [19].…”