Demo: Enabling Asynchronous Awakenings in Wireless Sensor Networks Towards Removing Duty-Cycle Barriers Giannis Kazdaridis * , Panagiotis Skrimponis * , Ioannis Zographopoulos * , Polychronis Symeonidis * , Thanasis Korakis * and Leandros Tassiulas † * Department of Electrical and Computer Engineering, University of Thessaly, Greece † Department of Electrical Engineering, Yale University, New Haven, USA {iokazdarid,skrimpon,zografop,posymeon,korakis}@uth.gr,leandros.tassiulas@yale.edu ABSTRACT Typical wireless sensor network applications follow duty-cycle mechanisms, yielding important energy savings by reducing the power consumption of idle listening. However, this approach still dictates predefned cycles of active operation, which in some appli- cation scenarios is meaningless. Extended lifetime can be achieved by asynchronously awakening sensor network’s nodes only when truly required. In this work we present NITOS wake-up receiver that can be employed by typical sensor nodes to provide asynchro- nous wake-ups and substantially reduce their energy expenditure. Our wake-up circuit operates in the 868 MHz band and is activated by LoRa frames using OOK modulation. The developed system supports selective awakenings with the aid of a low-power micro- controller dedicated to sample the acquired signal and identify the wake-up address. KEYWORDS Wake-Up Radios, Wireless Sensor Networks, Low-Power Consump- tion 1 INTRODUCTION The unprecedented growth of Wireless Sensor Networks (WSNs) has revolutionized the way we interact with the physical context, improving our everyday life in several aspects. Several WSN appli- cations require increased life-duration, while battery replacement is an impractical or sometimes an infeasible task. A typical principle that deals with the excessive energy consumption is duty-cycling. Duty-cycling [4], [3] suggests that sensor nodes enter into a low- power mode (sleep state), where they turn of all the electronics except of a clock circuit dedicated to providing a wake-up interrupt, in order to save as much energy as possible during their inactive periods. Although substantially reducing the energy expenditure, this technique implies several limitations and drawbacks. For ex- ample, idle-listening [10] is reduced considerably, but still remains present, since duty-cycle defnes fxed intervals of active operation. The research leading to these results has received funding from the European Union’s H2020 Programme under grant agreement no. 687983 (MAZI Project). Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for proft or commercial advantage and that copies bear this notice and the full citation on the frst page. Copyrights for third-party components of this work must be honored. For all other uses, contact the owner/author(s). WiNTECH’17, October 20, 2017, Snowbird, UT, USA © 2017 Copyright held by the owner/author(s). ACM ISBN 978-1-4503-5147-8/17/10. https://doi.org/10.1145/3131473.3133330 Figure 1: NITOS Wake-Up Radio Testing Prototype However, a wide range of sensing application scenarios are event- based and do not require fxed wake-ups that only account for energy wastage. Moreover, this approach fails to serve time-critical applications that require immediate response towards sensing an abrupt event. All the above, motivate the development of wake-up radios to allow asynchronous awakenings, towards eliminating duty-cycle constrains. Several works have proposed novel schemes exploit- ing wake-up radios [8], [7] that use external nodes to propagate wake-up frames to targeted nodes. Inspired by [9], we adopted a similar scheme and developed an ancillary radio module dedi- cated to providing an interrupt signal to the host sensor node, in order to activate it only when required. Our prototype uses OOK modulation and operates in the band of 868 MHz, engaging LoRa transceivers to achieve long-range distances. In the next section we present the development of the wake-up radio module along with the implementation details. 2 SYSTEM IMPLEMENTATION The developed prototype receiver consists of low-cost electronics and a low-power micro-controller. The schematic diagram of the receiver is illustrated in Fig. 2(a), while the actual board in Fig. 1. For the wake-up receiver a matching network, a passive rectifer and a comparator to grenade interrupts were used. Moreover, a low-power micro-controller is responsible for processing the received signal and identifying the acquired address to verify whether it should wake the host node. To awake the network’s nodes we utilize LoRa radio transceivers, by modulating the propagated information using On-Of Keying (OOK) modulation. Briefy, OOK scheme defnes that the presence of a carrier for a specifc duration represents a binary one, while its absence for the same duration represents a binary zero. On the receiver side, the matching network consists of a capacitor and an inductor element that together form an LC flter used to reject undesired transmissions. In essence, the flter is designed to tolerate signals in the band of 868 MHz. Then, a passive rectifer in the topology of an envelope detector is formed with the aid of two Schottky diodes, used to discard the high frequency signals