Reverse Flooding: exploiting radio interference for efficient propagation delay compensation in WSN clock synchronization

TL;DR

This paper introduces a novel method for estimating and compensating propagation delays in WSN clock synchronization, leveraging constructive interference and without requiring a spanning tree, achieving sub-microsecond accuracy.

Contribution

It presents a new delay estimation technique that exploits constructive interference, enabling precise synchronization without network topology constraints.

Findings

Feasibility of measuring propagation delays with off-the-shelf hardware

Achieved sub-microsecond synchronization accuracy

Effective delay compensation in networks with significant propagation delays

Abstract

Clock synchronization is a necessary component in modern distributed systems, especially Wirless Sensor Networks (WSNs). Despite the great effort and the numerous improvements, the existing synchronization schemes do not yet address the cancellation of propagation delays. Up to a few years ago, this was not perceived as a problem, because the time-stamping precision was a more limiting factor for the accuracy achievable with a synchronization scheme. However, the recent introduction of efficient flooding schemes based on constructive interference has greatly improved the achievable accuracy, to the point where propagation delays can effectively become the main source of error. In this paper, we propose a method to estimate and compensate for the network propagation delays. Our proposal does not require to maintain a spanning tree of the network, and exploits constructive interference even to transmit packets whose content are slightly different. To show the validity of the approach, we implemented the propagation delay estimator on top of the FLOPSYNC-2 synchronization scheme. Experimental results prove the feasibility of measuring propagation delays using off-the-shelf microcontrollers and radio transceivers, and show how the proposed solution allows to achieve sub-microsecond clock synchronization even for networks where propagation delays are significant.