Connecting Battery-free IoT Tags Using LED Bulbs

Connecing Baery-free IoT Tags Using LED Bulbs Domenico Giusiniano IMDEA Neworks Insiue Madrid, Spain domenico.giusiniano@imdea.org Ambuj Varshney Uppsala Universiy Uppsala, Sweden ambuj.varshney@i.uu.se Thiemo Voig Uppsala Universiy and RISE SICS Uppsala, Sweden hiemo@sics.se ABSTRACT We inroduce BackVLC, a sysem o connec baery-free IoT ags using LED bulbs. We make use of bulbs beyond illuminaion. We send daa o he ags wih visible ligh communicaion (VLC), and rerofi he bulbs wih simple circuiry o enable he uplink channel curren VLC sysems lack, using Radio Frequency (RF) backscaer communicaion from he ags. Tags process and send daa, harvesing energy from ligh and radio. We presen our sysem design and implemenaion, evaluae i in preliminary simulaion sudies and experimens, and discuss he research challenges o develop a complee nework archiecure. BackVLC is he firs work ha combines VLC wih RF backscaer. 1 INTRODUCTION Ligh Emiing Diodes (LEDs) are aking over he illuminaion marke due o heir energy efficiency and lifeime wih respec o radiional lighing sources. Indusry effor has shown ha LED bulbs could embed IoT sensors wih he main objecive of reducing he carbon fooprin of lighing sysems and providing he mos comforable ligh condiions, being managed, configured and conrolled remoely [3, 21]. Recen works have challenged he assumpion ha bulbs are solely an endpoin for IoT conneciviy, and may raher communicae wirelessly o baery-free IoT devices hrough LED bulbs. However, hey eiher include an RFID reader in he LED bulb for uplink Radio Frequency (RF) backscaer, which increases he cos and allows only for a shor communicaion range [5], or use uplink Visible Ligh Communicaion (VLC) [8, 29], wih ligh sources embedded in IoT ags ha disurb he uniform illuminaion in he environmen. In order o esablish baery-free IoT conneciviy hrough bulbs, we presen BackVLC, a sysem where we rerofi LED infrasrucure for RF backscaer communicaion. The goal of BackVLC is o ake he bes of VLC and RF-based echnologies for communicaing o baery-free IoT devices. Firs, Permission o make digial or hard copies of all or par of his work for personal or classroom use is graned wihou fee provided ha copies are no made or disribued for profi or commercial advanage and ha copies bear his noice and he full ciaion on he firs page. Copyrighs for componens of his work owned by ohers han ACM mus be honored. Absracing wih credi is permied. To copy oherwise, or republish, o pos on servers or o redisribue o liss, requires prior specific permission and/or a fee. Reques permissions from permissions@acm.org. HoNes-XVII, November 15 16, 218, Redmond, WA, USA 218 Associaion for Compuing Machinery. ACM ISBN 978-1-453-612-/18/11... $15. hps://doi.org/1.1145/328662.328677 LED Bulb Wireless/Wired connecion VLC 1 RF carrier 1 1 Baery-free IoT ag RF backscaer Backscaer Reader Figure 1: In BackVLC, downlink communicaion from he LED bulb o he IoT ag occurs wih Visible Ligh Communicaion (VLC). The bulb provides energy o he baery-free IoT ag and conrols he uplink RF backscaer communicaion wih an embedded radio carrier generaor. The IoT ag uses a solar cell boh for harvesing energy and receiving VLC daa. The backscaer signal is sen o he reader, ha connecs bulbs and IoT ags, and serves as edge conroller of BackVLC. LEDs are pervasively deployed and modulaing hem allows o send VLC downlink daa. Ye, as menioned above, VLC is undesired for uplink communicaion. This is a broad problem ha affecs VLC in general. This is no he case of RF backscaer, used in our design. Second, IoT ags suffer from he low sensiiviy of passive RF envelope deecors. In conras, receiving daa from a modulaed ligh source allows o replace he envelope deecor wih a simple and low-cos VLC receiver based on he direc deecion (DD) mehod. Third, using VLC downlink allows o use he solar cell no only for harvesing energy, as in ypical IoT ags employed in RF backscaer [14], bu also o receive VLC daa. BackVLC is he firs work ha combines VLC wih RF backscaer. We presen a high-level view of our sysem in Figure 1. Downlink communicaion from he LED bulb in he ceiling o he IoT ag occurs as in radiional VLC. In his way, he environmen is illuminaed and he bulb can communicae downlink daa o he IoT ags. Communicaion in he uplink occurs wih RF backscaer ransmissions. The bulb conrols he uplink communicaion, wih an embedded radio carrier generaor ha is used by any IoT ag o enable low-power backscaer ransmissions. The bulb provides energy o he baery-free IoT ag hrough illuminaion and hrough he embedded carrier generaor. As energy efficiency and cos are essenial no only in he ags, bu also in he LED bulbs, we decouple he reader from he radio energy source, similarly o he concep of bi-saic configuraion presened in [27]. The main propery of bi-saic configuraions is ha he highes

C2R C2R = Illuminaion + VLC = RF carrier = RF Sensing = Conrol uni = Connecion o he reader Figure 2: High-level represenaion of he LED bulb in BackVLC. The bulb is used for illuminaion, downlink VLC, conrolling uplink backscaer communicaion and providing energy o he baery-free IoT ag. I is also conneced o he reader. signal-o-noise raio (SNR) a he receiver is achieved when eiher he ag is close o he source or close o he reader. As we use VLC for downlink, he LED bulb and he ag are confined in he same area (e.g., a room). Therefore, here is a high likelihood ha he ag is close o he LED bulb, and, subsequenly, of a high SNR a he backscaer reader. The reader is placed in he local or wide area nework, depending on he frequency used. I receives he radio backscaer signals from he IoT ag and serves as edge conroller of BackVLC. BackVLC can impac several IoT applicaions. The main requiremens are fixed illuminaion infrasrucure, generaing (almos) coninuous arificial ligh, and IoT ags ha can deec his ligh. Poenial scenarios are smar offices, smar ciies, and greenhouses. The realizaion of a nework archiecure ha efficienly inegraes ligh and RF for IoT requires he invesigaion of mehods a he inersecion of communicaion, energy and muli-echnology inegraion in sofware and hardware. The nex secion inroduces our bulb. We hen presen our baeryfree IoT ag design and our backscaer reader. Afer inroducing he principles of he communicaion proocol, we show preliminary resuls, including experimens wih our cusomized prooype hardware, and we finally describe he research challenges o realize a full nework archiecure. 2 LED BULB FOR IOT CONNECTIVITY We envision o rerofi LED bulbs o connec baery-free IoT ags o he cloud. We use downlink VLC o ransmi daa o he IoT ags and backscaer communicaion o suppor uplink ransmissions from he backscaer ag. As shown in Figure 2, he bulb includes five main unis we discuss below. The hardware componens of he bulb can be affordable for ypical form facors. Compared o a design ha rerofis bulbs only for VLC [18], ours enails only wo addiional componens ha are dedicaed o conrol uplink RF backscaer. Illuminaion and Downlink VLC. The LED in he bulb is used for illuminaion and downlink VLC. A consan bias curren flows hrough he LED o achieve he desired illuminaion level. A variable curren I around he bias is adoped o send daa. Communicaion causes an addiional power consumpion a he bulb. Large I increases he received signal srengh a he IoT ag, bu also consumes more power. The ligh inensiy emied from he LED is modulaed o ransmi daa using On-Off-Keying (OOK) modulaion. We use OOK raher han more complex modulaions (like OFDM), as i is he simples form o send VLC daa, wih he lowes consumpion a he IoT ag for recepion. RF Carrier Generaor. We build on LoRea [24] o suppor ulra-low power backscaer ransmissions. BackVLC explois he following properies of LoRea: i) spaial separaion beween RF carrier generaion and he recepion, ii) separaion in frequency beween carrier and backscaered signals, and iii) narrowband ransmissions for sufficien communicaion range. The RF carrier generaor emis he energy required by he IoT ag o ransmi RF backscaer signals. Carrier generaors are no expensive, wih ypical prices of a few USD [24]. In his paper, we use commodiy wireless chipses o generae carrier signals. In BackVLC, downlink VLC and RF carrier generaion do no occur concurrenly. This allows o use a simpler power supply, wih lower volage and curren requiremens. RF Sensing. The sensiiviy of ambien raffic RF sensing in he IoT ag is low, as i ypically uses an envelope deecor [16]. In addiion, i would increase he energy consumed by he ag. For hese reasons, we perform RF sensing in he bulb. This concep is similar o he one in [7], ha delegaes he power-consuming ask of sensing o he power plug device. The module for RF sensing senses boh normal WiFi ransmissions and oher RF carrier generaors in he frequency band of operaion (e.g. from oher bulbs). Conrol Uni and Connecion o he Reader. The conrol uni is regulaed by a microconroller. I receives inpus from he reader, and, based on he proocol logic (c.f. Secion 5), i decides a run ime which operaion o perform among illuminaion, VLC, RF carrier generaion and RF sensing. An Eherne or powerline inerface connecs he bulb o he reader. The connecion o he reader could also be wih wireless conneciviy. 3 IOT TAG BackVLC advocaes a hybrid RF and VLC design for baeryfree IoT ags, as hese mediums are complemenary. A key challenge in supporing he hybrid design is o suppor recepions and ransmissions wihin he consrains of he harvesed energy. The convenional radio ransceivers such as hose employed o suppor WiFi communicaion are energy expensive [7], which makes i challenging o operae hem on harvesed energy. Similarly, sae-of-he-ar VLC receivers ofen consume significanly higher power han wha is available from ambien energy harvesing sources and hey arge daa raes higher han needed by IoT ags (an efficien VLC receiver has recenly been proposed wih a power consumpion of 2.2 mw and a daa rae of 24 Mb/s [9]).

We presen a high level overview of our ag in Figure 3. The ag operaes by harvesing energy from eiher inciden carrier signal (RF) or hrough visible ligh using a solar cell. I sores harvesed energy in a small capacior. The harvesed energy supplies he processing uni, onboard sensors, and suppors communicaion. To suppor communicaion relying only on harvesed energy, our ag leverages an ulra-low power VLC receiver for downlink communicaion as well as RF backscaer for uplink ransmission. Our IoT ag suppors boh uplink and downlink ransmissions using only he energy harvesed from he credi card-sized solar cell. Receiver. We implemen he receiver on he backscaer ag based on he design of ulra-low power receiver by Varshney e al. [25, 27]. The receiver uses a solar cell o eliminae energy expensive phoodiodes and ransimpedance amplifier. Furher, o reduce he power consumpion for digiisaion, he receiver uses an energy-efficien hresholding circui, also widely used in envelope deecors [1, 16]. The receiver allows us o demodulae OOK signals a a peak power consumpion of.5 µws. The receiver uses a direc deecion (DD) mehod, which only deecs he inensiy of he opical wave and can be implemened wih a few elecronic componens. This replaces sae-of-he-ar RF based envelope deecors ha are limied by low sensiiviy (-68 dbm) [16]. Processing Uni. The processing uni of our IoT ag is responsible for ineracion wih sensors, performing local processing, conrolling he swichover beween differen energy harvesing sources, and he necessary logic required o suppor uplink and downlink communicaion. We implemen he processing uni using a low-power microconroller - Texas Insrumens MSP43 ha feaures low power consumpion, and has persisen sorage FRAM which enables reenion of daa even in he case of power failures, a commonly occurring phenomenon on baery-free sensors [11]. Due o is aracive feaures, he paricular microconroller is also used in oher recen sysems [1, 24]. Energy Harvesing. Visible ligh emied by he bulbs is an obvious source of energy o suppor he baery-free operaion of our ags. In addiion, embedding he carrier generaor in he bulb opens he possibiliy of harvesing energy from he RF carrier signal as well. This makes i possible for our ag o funcion even when he LEDs are urned off. Our ag can harves energy from visible ligh using a credi card-sized solar panel. To harves from he RF carrier signal, we employ a uned anenna wih a maching nework and recifier similar o he design presened by Talla e al. [2]. The oupu of he solar cell or RF harveser is fed o an efficien energy harveser Texas insrumens BQ2557 ha sores he volage in a small 1 mf capacior. 4 BSCATTER READER The hird componen of he sysem is he reader. Is role is more han he one of radiional backscaer readers. In BackVLC, i serves as edge conroller [17], connecing bulbs and IoT devices locally, and managing he daa collecion and RF Anenna Solar Cell and VLC Receiver RF/Ligh Harvesing and Power Managemen Backscaer Transmier Processing Uni Sensor Figure 3: Block diagram represenaion of he IoT Tag in BackVLC. The ag operaes wihou baeries on energy harvesed hrough inciden RF carrier signals, or hrough visible ligh. The ag receives daa using VLC, and ransmis using backscaer communicaion. connecion o he cloud. For backscaer signals reaching he local area nework, we expec he reader o be inegraed wih he WiFi rouer, o ease he communicaion of sensed daa o he cloud. The reader is conneced o he bulbs wih powerline, Eherne or wireless communicaion. I is responsible of a few operaions, deailed below. Recepion of Backscaer Signal. For receiving backscaered daa from he IoT ag, we build on LoRea [24]. We use a commodiy CC25 radio ransceiver ha is low cos and has a high receiver sensiiviy, which allows us o suppor a sufficien communicaion range so ha a single reader device can cover a ypical home or several rooms. Daa Processing and Transmission. The reader performs some daa processing. I receives acknowledgemens (s) from he IoT ag. Upon failure of recepion, i sends a reques o he bulb o re-ransmi he VLC daa. Finally, i forwards requess received from he IoT ag o he bulb. For insance, he IoT ag may reques addiional energy o ransmi is daa, and he bulb may subsequenly emi he RF carrier. Furhermore, i forwards received IoT daa o he cloud. 5 COMMUNICATION PROTOCOL Based on he sysem componens inroduced in he previous secions, we briefly presen he communicaion proocol in BackVLC. We refer o Figure 4. We disinguish four possible main evens for he communicaion: Case (a). The LED in he bulb is modulaed in order o send daa o he IoT ag hrough VLC. The ag receives he informaion. Subsequenly, he bulb sars o emi an RF carrier. The ag acknowledges he recepion of VLC daa wih an message ha backscaers he received RF signal. No shown in he figure, a N (Negaive ) is sen o he reader if he ag does no receive VLC daa correcly. Case (b). The LED is modulaed in order o send daa o he IoT ag hrough VLC. The ag receives he informaion. The RF sensing module in he bulb measures a busy channel and he RF carrier generaor is deferred. Once he channel is clear, he ag can acknowledge he recepion of VLC daa.

LED Bulb IoT Tag Illuminaion + VLC RF Carrier RF Backscaer DL Daa DL Daa Busy DL Daa UL Daa (a) (b) (c) (d) Figure 4: Communicaion proocol and main evens. The wireless communicaion is conrolled by he bulb. (No ligh) UL Daa Case (c). The bulb ransmis daa o he ag as before, and he ag includes a flag in he (or in he N) o inform he nework ha i has daa sored in memory o send, and o reques he carrier for addiional ime o ransmi his daa o he reader. Once he bulb sars emiing he RF carrier, he ag sends is sensed daa o he reader. Case (d). I is possible ha no bulb illuminaes he environmen. No daa is sen o he IoT ag under his condiion. During ligh-off period, he bulb can sill uses is RF carrier o reques daa from he ag. If he IoT ag has daa available, i sends an backscaer signal wih a flag o reques he carrier source for addiional ime. 6 FEASIBILITY ANALYSIS In his secion, we presen preliminary resuls ha demonsrae he feasibiliy of our proposed sysem. 6.1 Communicaion Range and Transmi Power Earlier backscaer sysems have used carrier generaor s wih high oupu power and neverheless achieved shor communicaion ranges in he order of meers [7]. In his secion, we firs invesigae he required ransmission power o cover rooms of differen sizes. We base our analysis on Friis pah loss model. Kellogg e al. have used i o compue he signal srengh P r a he reader [7]: P r = P G 4πd 2 1 }{{} Bulb o IoT ag λ2 Gpassive 2 Γ 2 1 λ 2 G r α 4π 4 4πd } {{ } 2 2 4π } {{ } Backscaer IoT ag o Reader The firs par of he equaion models he signal propagaion from he RF generaor in he bulb o he IoT ag. P denoes he oupu power of he ransmier, G he anenna gain a he ransmier and d 1 he disance beween he ransmier and he IoT ag. The second par models he fracion of he signal ha is backscaered a he IoT ag. In his par of he equaion, λ is he wavelengh of he RF signal. For simpliciy, we do no model he fac ha we shif he wavelengh of he incoming signal when backscaering. G passive models he anenna gain of he IoT ag. Γ 2 is he backscaer coefficien and α models he loss he signal experiences due o synhesis of he backscaer signal. Finally, in he las erm, G r denoes Required Transm. Power (dbm) 1 5-5 grid of bulbs one bulb 5m x 5m 1m x 5m 1m x 1m Room size Figure 5: Required ransmission power of he RF carrier generaor inside he bulb. I is low enough so ha sandard WiFi modules are sufficien even for larger rooms. he reader s anenna gain and d 2 he disance beween he IoT ag and he reader. We are ineresed in he oupu power P required by he RF carrier generaor so ha he received power a he reader P r is above is sensiiviy hreshold. The laer is -11 dbm for sae-of-he-ar implemenaions a a frequency of 2.4 GHz (so λ 12.5 cm) [24]. As boh he ransmier and IoT ag are consrained in size, we assume lower anenna gains of dbi and 2 dbi, respecively, whereas for he reader s anenna we assume a gain of 6 dbi. As Kellogg e al., we se Γ 2 o 1.1 db and α o -4.4 db. We believe ha our sysem is more efficien due o he simpler modulaion scheme. Hence, using heir values is a lower bound for our sysem. We assume rooms wih a ypical heigh of 3 m and look ino hree differen room sizes: i) 5mx5m; ii) 1mx5m; and iii) 1mx1m. We assume one reader ha is placed on he ceiling in he corner of he room. For our analysis, we invesigae wo differen scenarios for he placemen of he LED bulbs: Firs, we assume one LED bulb placed in he cener of he room. Since here is only one LED his leads o longer RF communicaion links. Second, we assume LED bulbs ha are placed in a uniform square array of NxN LED bulbs wih a disance of one meer beween he bulbs. This seup mimics one wih uniform illuminaion [12]. Our resuls in Figure 5 show a required ransmi power of below dbm wih he square array of LEDs, and around 13 dbm when here is only one bulb, even for he larges room size we analyze. Given ha modern WiFi chipses such as he CC32 have an oupu power of 18 dbm and 82.15.4 lowpower radios such as he CC242 have a maximum oupu

VLC Transmier Carrier Generaor Figure 6: Prooype ransmier wih VLC and RF carrier generaor. power of dbm, we conclude ha off-he-shelf radios can be used as BackVLC carrier generaors in larger rooms or even small aparmens. This is imporan o keep he cos of he sysem low. In addiion, he low ransmi power required helps he coexisence wih oher wireless raffic. 6.2 Message Acknowledgemen messages are a vial par of mos communicaion proocols. However, i is challenging o suppor s on baeryfree sensors ha employ VLC for downlink communicaion. This is because commonly used mehods for uplink ransmission in VLC sysems such as infrared or convenional radio ransmissions are oo energy expensive o operae wihin he budge of harvesed energy. In his experimen, we demonsrae ha BackVLC suppors his essenial funcionaliy. Seup. We experimen in our lab in ligh condiions represenaive for acual deploymens: a nigh wih indoor lighs, and during he day wih naural ligh. We program a lowcos radio CC2538 ransceiver wih a CC2592 [6] as a carrier generaor. The ransceiver can generae an RF carrier a a maximum srengh of 22 dbm. We co-locae he RF carrier generaor wih a 32 lm LED ha operaes as VLC ransmier, as shown in Figure 6. We experimen wih he IoT ag locaed firs on he desk, and hen on he floor. The disance from he carrier generaor and he ligh source o he desk is 1.5 m, and 2.2 m o he floor. In he experimen, he ag harvess energy only from he ambien ligh. We keep rack of ligh levels in he experimen using a ligh meer. The ligh levels (including VLC ransmier) on he desk and he floor during dayime are approx. 22 lx. In he nigh wih office lighs, he ligh levels on he desk and he floor are 84 lx and 44 lx, respecively. We program he VLC ransmier o beacon every: (.1,.2,.5, 1, 2, 1) seconds. The message wakes up he IoT ag ha responds wih an (3 byes) ha is received by he reader kep in anoher room abou 1 meer away from he IoT ag. The scenario is similar o wha we can expec in a ypical deploymen a home wih a WiFi rouer locaed several meers away from he IoT ag. This scenario corresponds o he Case (a) of he proocol described in Secion 5. We Ack. Recepion Rae [%] Dayligh desk Dayligh floor Officeligh desk Offliceligh floor 1.2 1..8.6.4.2. 1 2 1.5.2.1 VLC Transmier Beacon Rae Figure 7: Supporing acknowledgemen messages. Brigh ligh condiions provide good energy harvesing condiions ha allow high packe recepion rae of he s. perform hree ieraions of he experimen. As a meric for evaluaion, we keep record of s received a he reader. Resul. Figure 7 demonsraes he resul of he experimen. A low beacon raes, we are able o receive mos of he messages irrespecive of he ambien ligh condiions. However, as we increase he beacon rae, he Recepion Rae decreases rapidly. When he ambien ligh levels are low, he reader receives very few messages. The reason for his behaviour is ha he IoT ag mus ransmi acknowledgemen messages more frequenly when he VLC beacon rae is high, which increases he overall energy consumpion. Furher, his also gives he IoT ag less ime o harves and replenish he energy sored in he capacior. Possible soluions are o improve he rae a which energy is harvesed eiher by employing a larger solar cell, or by harvesing energy from he inciden RF carrier signal under low ligh condiions. 6.3 RF Carrier Generaor and WiFi Traffic The communicaion range of he IoT ag improves wih he srengh of he carrier signal [24]. Hence, he RF generaors in our bulbs need o generae a reasonably srong carrier signal o achieve sufficien range. In fac, sae-of-he-ar sysems such as PassiveWiFi [7] ofen generae he carrier signal a highes legally allowed srengh for evaluaion. However, his occurs in he unlicensed band shared wih wireless echnologies such as WiFi. The carrier signal could inerfere negaively wih heir communicaion. In his experimen, we invesigae he impac of he carrier signal on he WiFi raffic. Seup. We se up a WiFi rouer o generae WiFi raffic in our lab. We use a TP-Link TL-WR143ND rouer [4] wih DD- WRT firmware [1] and operae i a he maximum ransmi power of 2 dbm. We se up he rouer as an iperf server and a MacBook lapop as an iperf clien. The rouer and he clien are a a disance of 6 m. We generae a carrier signal of srengh 16 dbm using a USRP B2 sofware defined radio. In he experimen, we observe he impac of carrier generaion on he WiFi raffic beween he rouer and he clien. We se up he carrier generaor a a differen locaion away from he seup, and also program i o generae he signal a he cenre frequency wihin and ouside he WiFi band. Resul. We observe ha when he RF carrier is ouside he band of he WiFi raffic, i has lile impac on he ongoing

raffic. This is due o he significan adjacen channel rejecion of WiFi ransceivers. Insead, he presence of he RF carrier wihin he WiFi band negaively impacs he ongoing WiFi raffic: The presence of he RF carrier defers he ongoing WiFi raffic when he carrier generaor is nearby (4 m). As we move he carrier generaor a a disance of around 2 m, he WiFi raffic resumes as he RF carrier is oo weak o inerfere wih he WiFi ransmissions. We conclude ha BackVLC should sense he unlicensed WiFi band and dynamically selec a frequency ouside he band of mos of WiFi raffic. 7 RELATED WORK In order o provide conneciviy o baery-free IoT devices, backscaer communicaion explois radio frequency (RF) signals in he environmen o harves energy for compuaion and passive ransmissions [1, 28]. Recen works have also considered applying backscaer o LED bulbs ransmiing wih visible ligh communicaion (VLC), where he receiver replies using an LCD shuer o change he refleced signal in ime [8, 29]. However, hese works are negaively affeced by he visual channel from he IoT ag (undesired for he human eye), and he low daa rae of communicaion. Gummeson e al. have proposed o add dedicaed modules in he bulbs for RF backscaer [5]. Differenly from Back- VLC, hey use he LEDs only for illuminaion, and require an RFID reader in each bulb. Their design is cos prohibiive (abou $3 [5]), and resuls in a communicaion range of less han five meers even wih a high ransmission power of 3 dbm. The RFID reader is he mos expensive componen of backscaer. In fac, i inerferes wih is own generaed carrier, which hinders he recepion of he very weak signals from he IoT ags. Compared o a design ha embeds boh he RF carrier and an RFID reader in he bulb [5], we achieve a much longer range a much lower cos [27]. An alernaive from relaed work is o sense coarse ligh variaions o modulae he RF backscaer signal [27], bu he proposed soluion does no allow o send daa o he IoT ag. Alernaives for uplink in VLC sysems based on infrared ransmissions are fragile, because of ransmission power regulaions for eye safey and narrow heir field-of-view [13]. RF uplink can provide wider coverage han infrared link, and proocols wih acive uplink RF and aggregaion have been proposed o alleviae hroughpu degradaion problems for coexisence wih WiFi [13]. Ye, acive RF for uplink impacs he energy budge of IoT devices and is cosly. 8 DISCUSSION Harvesing RF and ligh. The amoun of harvesed energy in each IoT ag from boh RF and ligh can change over ime. The IoT ag should opimize is local processing in order o communicae sensed daa wih he available budge. An efficien hardware design is also needed o reuse elecronic componens in he RF and ligh harvesing funcionaliies. Muliple LEDs and muliple IoT ags. There is he need of a medium access conrol o deal wih muliple LEDs in he same area for uniform illuminaion [15, 23, 3] as well as muliple IoT ags. A medium access proocol may be used so ha bulbs do no ransmi immediaely afer a clear RF channel. As RF coverage is likely larger han VLC coverage, only one RF carrier may be generaed o cover a given area. The proocol depends also on he ype of backscaer signal. For insance, if each IoT ag performs a frequency shif in a dedicaed frequency [19, 26], daa may be requesed simulaneously o more IoT ags wihou generaing RF collisions. Communicaing o IoT ags in absence of ligh. If he ligh is off, he bulb may sill send some daa a very low-dimmed ligh o he ag. Similar mehods such as [22] may be invesigaed. New designs are needed o solve he low daa rae (of abou 1.6 kb/s) of previous approaches. WiFi-friendly archiecure. If he reader has a wired connecion o he LED bulbs, i may be convenien o reuse he WiFi rouer [2] as reader and perform some edge compuing. LED bulbs a home could be also conrolled by he WiFi rouer. I requires o modify our design, as he IoT ag should perform some processing o generae WiFi complian codewords [7]. The drawbacks are he addiional compuaional resources in he ag, and he shorer communicaion range (because of higher loss α in he synhesis of he backscaer signal and lower sensiiviy of he reader). Oudoor deploymens. Our archiecure can also be used in smar ciies as we can embed carrier generaors in he sree lighs and semaphores. These applicaions require he ags o communicae over longer disances when compared o indoor scenarios. We can suppor hem by operaing our archiecure in he sub-ghz band, where recen sysems have demonsraed he abiliy o ransmi over large disances wih a carrier source in proximiy [19, 24]. 9 CONCLUSION In his work, we have inroduced BackVLC, a new sysem ha addresses a few fundamenal problems of curren emerging pervasive wireless neworks: i) VLC has a long-sanding problem wih he uplink communicaion channel and ii) RF backscaer has a long-sanding problem wih he sensiiviy of he RF envelope deecor. We have presened he foundaion of our sysem, is componens, as well as preliminary resuls and have discussed key research direcions in differen layers of he nework sack. VLC has no aken off ye. BackVLC can provide he use case i needs, offering beer coverage han pure RF soluions for baery-free Inerne of Things. As he massive deploymens of LEDs and baery-free IoT ags are on heir way, hey could embrace a join journey in he nex generaion of neworks. NOWLEDGMENT Our sincere hanks o he reviewers and our shepherd Fadel Abib for all of heir grea commens and consrucive feedback. We also hank Deepak Ganesan for his valuable feedback. This work has been parly funded by he Swedish Research Council (Gran 217-45989).

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