Challenge: Ultra-Low-Power Energy-Harvesting ActiveNetworked Tags (EnHANTs)Maria Gorlatova†, Peter Kinget†, Ioannis Kymissis†,Dan Rubenstein∗, Xiaodong Wang†, Gil Zussman††Electrical Engineering,∗Computer ScienceColumbia University, New York, NY, [email protected], [kinget, johnkym]@[email protected], [wangx, gil]@ee.columbia.eduABSTRACTThis paper presents the design challenges posed by a new classof ultra-low-power devices referred to as Energy-Harvesting Ac-tive Networked Tags (EnHANTs). EnHANTs are small, flexible,and self-reliant (in terms of energy) devices that can be attached toobjects that are traditionally not networked (e.g., books, clothing,and produce), thereby providing the infrastructure for various noveltracking applications. Examples of these applications include lo-cating misplaced items, continuous monitoring of objects (items ina store, boxes in transit), and determining locations of disaster sur-vivors. Recent advances in ultra-low-power wireless communica-tions, ultra-wideband (UWB) circuit design, and organic electronicharvesting techniques will enable the realization of EnHANTs inthe near future. In order for EnHANTs to rely on harvested energy,they have to spend significantly less energy than Bluetooth, Zig-bee, and IEEE 802.15.4a devices. Moreover, the harvesting com-ponents and the ultra-low-power physical layer have special char-acteristics whose implications on the higher layers have yet to bestudied (e.g., when using ultra-low-power circuits, the energy re-quired to receive a bit is an order of magnitude higher than the en-ergy required to transmit a bit). These special characteristics poseseveral new cross-layer research problems. In this paper, we de-scribe the design challenges at the layers above the physical layer,point out relevant research directions, and outline possible startingpoints for solutions.Categories and Subject DescriptorsC.2.1 [Computer Communication Networks]: Network Archi-tecture and Design - Wireless communicationGeneral TermsAlgorithms, Design, PerformanceKeywordsUltra-low power communications, energy efficient networking, en-ergy harvesting, energy scavenging, ultra-wideband (UWB)Permission to make digital or hard copies of all or part of this work forpersonal or classroom use is granted without fee provided that copies arenot made or distributed for profit or commercial advantage and that copiesbear this notice and the full citation on the first page. To copy otherwise, torepublish, to post on servers or to redistribute to lists, requires prior specificpermission and/or a fee.MobiCom’09, September 20–25, 2009, Beijing, China.Copyright 2009 ACM 978-1-60558-702-8/09/09 ...$10.00. Sensor NetworksRFIDsEnHANTsSizeComplexity ThroughputEnergy consumptionFigure 1: EnHANTs in comparison to Sensor Networks andRFIDs.1. INTRODUCTIONThis paper focuses on the networking challenges posed by anew class of ultra-low-power devices that we refer to as Energy-Harvesting Active Networked Tags (EnHANTs). EnHANTs aresmall, flexible, and self-reliant (in terms of energy) devices that canbe attached to arbitrary objects that are traditionally not networked:books, clothing, produce, etc. EnHANTs will enable novel objecttracking applications such as recovery of lost items and continu-ous monitoring of objects’ proximity to each other. The realiza-tion of EnHANTs is based on recent advances in the areas of solarand piezoelectric energy harvesting [24] as well as ultra-low-powerwireless communications [26, 42]. In particular, recent novel cir-cuit designs that employ ultra-wideband (UWB) communicationsprovide new levels of ultra-low-power operation (at the orders ofnJ/bit) at short ranges. Moreover, solar energy harvesting based onorganic semiconductors allows having flexible solar panels [19,25],thereby allowing a pervasive use of tags.The wireless industry is already taking the first steps towards thedesign of energy harvesting ultra-low-power tags [2,3]. Hence, fol-lowing the transition from barcodes to RFIDs, we envision a futuretransition from RFIDs to EnHANTs that:• Network – Actively communicate with one another and withEnHANT-friendly devices in order to forward informationover a multihop network.• Operate at ultra-low-power – Spend a few nano-Joules orless on every transmitted bit.• Harvest energy – Collect and store energy from sourcessuch as light, motion, and temperature gradients.• Are energy adaptive – Alter communications and network-ing to satisfy energy and harvesting constraints.• Exchange small messages – Exchange limited information(basically IDs) using low data rates, possibly in several trans-mission bursts.• Transmit to short ranges – Communicate only when in closeproximity (1 to 10 meters) to one another.• Are thin, flexible, and small (a few square cm at most).As shown in Figure 1, in terms of complexity, throughput, size,and energy requirements, EnHANTs fit between RFIDs and sen-sor networks. Similarly to RFIDs, the tags can be affixed to com-monplace objects. However, EnHANTs will have a power source,will be able to communicate in distributed multihop fashion, andwill not have to rely on high-power readers. Compared to sen-sor nodes, EnHANTs will operate at significantly lower data rates,and will consume less energy. Moreover, unlike sensor nodes, En-HANTs will transmit mostly ID information (either in order to an-nounce themselves or to query for specific EnHANTs). Despitethese differences, some of the results obtained for sensor networks(see [14,16]) should apply to EnHANTs.EnHANTs will be enablers for the Internet of Things and as suchwill support a variety of tracking and monitoring applications be-yond what RFID permits. While RFIDs make it possible to identifyan object, EnHANTs will make it possible to search for an object,and to continuously track objects’ whereabouts and their proxim-ity to each other. RFIDs are typically activated only when placednear a reader, and only report on themselves. EnHANTs, on theother hand, can operate continuously, achieve pervasive coveragedue to their networking capabilities, and can report on themselvesand other EnHANTs around them. These EnHANTs capabilitiesenable many exciting applications, such as, for example, continu-ous peer monitoring of merchandize in transit, where EnHANTswould be able to identify if a particular box has been taken out atany point during the journey.One application that we plan to