Carregamento e Wake-Up de Dispositivos IoT com Coleta de Energia RF e Consumo Quase Zero

IEEE Internet of Things Magazine March 2023 162 25763180232500 2023 IEEE ACCEPTED FROM OPEN CALL Ahmed Abed Benbuk Nour Kouzayha Joseph Costantine and Zaher Dawy Charging and WakeUp of ioT deviCes Using harvesTed rf energy WiTh nearZero poWer ConsUmpTion IntroductIon The sixth generation 6G of wireless systems is expected to enable sustainable societies with smart infrastructure through InternetofThings IoT deployments with nearzero power con sumption 1 This vision is constrained by the limited battery lifespan of wireless IoT devices and the difficulty to replace the batteries of the devices in hardtoreach areas Radio frequency RF energy harvesting and wakeup radio offer effective solu tions to wirelessly power IoT devices and reduce their overall energy consumption 2 3 The IoT devices can then recharge their energy storage unit using a passive harvesting system such as an antenna with a well matched rectifier With RF wakeup radio the device can also enter an ultralow power sleep mode and only wake up when it receives a special RF wakeup signal from an external transmitter 3 This allows the device to shut down its main components for energy consumption reduction except for the auxiliary wakeup circuit that monitors the recep tion of the wakeup signal Unlike RF energy harvesting circuits the wakeup receiver must contain a digital address decoding unit Towards this end effective ultralow power noiseimmune address detectors need to be designed to improve the energy efficiency of wakeup receivers 4 While RF energy harvesting and wakeup radio have been mostly considered as two standalone solutions to improve the performance of IoT networks 2 3 combining these two tech niques together brings great benefits in further decreasing the energy consumption and enabling fully passive IoT devices that can operate nearly selfsustainably The main advantage of com bining RF energy harvesting and wakeup stems from the fact that the device does not require a separate battery to be func tional Instead it can harvest energy from ambient RF signals while remaining in sleep mode then use the harvested energy when it switches to active mode Numerous challenges persist when combining RF energy harvesting and wakeup together for IoT devices including increased noise at the device front end reduced harvester efficiency due to varying input power and load in addition to the address detectors sensitivity and energy consumption requirements In this article we present a general framework for design ing and implementing combined RF energy harvesting and wakeup circuits with different address detector design options Unlike previous work concerned with circuit design and fabri cation under specific constraints 5 we describe the general architecture and the main components needed in combined schemes in addition to the technical challenges that should be addressed We further present the design of a dualpower mode pulse width detector PWD circuit and a multipower mode pulse width modulation PWM detector and compare their performance in terms of current consumption In addition we demonstrate the efficiency and practicality of the described framework with sample performance analysis to extract prac tical insights and we compare it with standalone RF energy harvesting and wakeup solutions We conclude the article with proposed future research directions ArchItecture of combIned rf energy hArvestIng And WAkeup systems A fundamental component in RF energy harvesting and wake up receivers is the rectifying circuit that converts the received RF energy into DC voltage 6 Relying on a shared rectifier for the combined system is crucial in order to reduce the complex ity of the RF frontend Systems that combine RF energy har vesting and wakeup can employ one rectifier for both tasks as shown in the general framework presented in Fig 1 This archi tecture has the advantage of eliminating the need for active components such as mixers or oscillators for down conversion AbstrAct Sixth generation 6G wireless systems are envisioned to support ubiquitous connection of a massive number of batterypowered Internetof Things IoT devices Radio frequency RF energy harvesting and wakeup radio have been extensively considered as standalone technolo gies to extend the battery lifetime of IoT devices In this article we present a general framework for designing efficient combined RF energy harvesting and wakeup circuits to further reduce the energy consumption of IoT devices and allow them to operate nearly selfsustainably Specifically we propose two address detector designs a dualpowermode pulse width detector PWD and a multipowermode pulse width modulation PWM detector that can be integrated in combined architectures with a single shared antenna and rectifier Furthermore we validate the practicality of the presented framework by testing sample designs under real operational conditions Finally we conclude the article by introducing future research directions for realizing large IoT networks with combined RF energy harvesting and wakeup radio Digital Object Identifier 101109IOTM0012200202 Ahmed Abed Benbuk Joseph Costantine and Zaher Dawy are with American University of Beirut Lebanon Nour Kouzayha is with King Abdullah University of Science and Technology KAUST Saudi Arabia Authorized licensed use limited to Instituto Nacional De Telecomunicações INATEL Downloaded on March 152023 at 134046 UTC from IEEE Xplore Restrictions apply FIGURE 1 A framework architecture that combines RF energy harvesting and wakeup by sharing a single antenna and rectifier for both tasks Signal 1 shows a variable received signal power at the receive antenna Signal 2 shows the rectifiers DC output voltage which follows the peak of the signal at the input Signal 3 shows the regulated voltage at the output of the PMU Signal 4 shows the digitized wakeup signal containing the IoT devices address IEEE Internet of Things Magazine March 2023 164 nity to avoid jamming and false wakeup of the IoT device On the other hand the address detector contains active compo nents to perform analogtodigital conversion demodulation and correlation of the wakeup signal Therefore the current consumption of the address detector must be reduced during charging tasks or when listening to the wakeup signal Care fully devising the addressing scheme can augment the address detectors noise immunity and reduce its average current con sumption Furthermore innovative techniques are required to diff erentiate between charging and wakeup operations which allows for complete shutdown of the energy consuming com ponents of the address detector during the charging phase Address detector desIgns for combIned rf energy hArvestIng And WAkeup By considering the challenges introduced earlier we present a general framework for designing combined RF energy har vesting and wakeup circuits We focus mainly on the design of powerful address detectors that can process wakeup signals with minimal energy consumption and are immune to the noise caused by the charging phase The developed methodology builds on our extensive experience in the design implementa tion and prototyping of energy harvesting and wakeup circuits A detailed description of the proposed architectures at the cir cuit level can be found in previous work 5 8 desIgn of pulse WIdth detector pWd cIrcuIt Before combining RF wakeup and charging it is useful to discuss the attainable modulation schemes to devise a wake up signal in the combined system The rectifi er cannot detect phase and frequencymodulated wakeup signals 3 Never theless it can operate as an envelope detector to demodulate amplitude shift keying ASKmodulated wakeup signals A sample design can be achieved by considering a wakeup signal that comprises a single RF pulse with a distinct width where the width of this pulse determines the address of the IoT device The data slicer converts the rectifiers output into a digital pulse as shown in Fig 2 The data slicer is confi gured with a threshold that determines the sensitivity of the system and it represents a high impedance load for the rectifi er with a very small input current The Dlatch networks store the voltage levels as 0 or 1 bits and the XoR gate compares the two bits and generates a trigger only if the received pulse width is within a specifi c range The rise and fall time of the rectifi ers output voltage causes an error in detecting the true width of the RF pulse This detector design can be hardwareprogrammed to detect diff erent pulse widths by adjusting the time constant of the threshold detectors An autopower off timer detects the rising edge that initiates the wakeup signal to generate a control signal and deliver common collector voltage VCC to the PWD The main drawback of this scheme when compared to OOK or PWMmodulated bits is the limited scalability when adding more IoT devices in the network This requires extra addresses with larger pulse widths As such increasing the addresses pool requires further hardware changes in the PWD circuit which is not practically feasible Furthermore the wakeup delay in the circuit is equal to the pulse width because the entire pulse must be received by the PWD before performing the XoR operation The PWD can therefore be considered as a baseline approach for designing low energy address detectors for combined RF energy harvesting and wakeup and must be further optimized to realize simultaneous charging and wakeup especially in time and energy constrained massive IoT deployments combInIng rf WAkeup And chArgIng usIng pWd Combining charging with RF wakeup can be achieved by lever aging the idle periods of the PWD that separate consecutive wakeup signals To do so a PMU must be integrated at the rectifiers output in parallel with the PWD The PMUs input resistance is independent of the RF frequency 9 however it is a function of several factors such as the input and output volt age the switching frequency and the duty cycle of the buck boost converter We can defi ne the sensitivity of the charging component as the PMUs minimum threshold to charge the battery of the IoT device to the steady state Although the suggested architecture can realize simultane ous charging and wakeup the main disadvantage is that the Dlatch network and XoR gate of the PWD remain powered ON during charging Moreover there is a disparity between the sensitivity of the wakeup and charging components which encourages further work on using adaptive transmit power when performing charging and wakeup Another interest ing research direction is to use mobile transmitters such as unmanned aerial vehicles UAVs to minimize the mismatch between the charging and wakeup ranges Our preliminary work validated that UAVs can wirelessly charge a capacitor with a value in the order of a few hundred mF 10 however it remains a challenge to charge larger batteries desIgn of multIpoWermode pWm detector cIrcuIt To scale up the addressing scheme beyond detecting a pulse width a multipowermode pulse width modulation PWM address detector is designed to perform sequential activation while processing the wakeup signal A general architecture of the multipowermode PWM detector is shown in Fig 3 and it is based on the PWD discussed previously The PWD acts as a preamble detector that receives an RF pulse and informs the PWM detector that a PWMmodulated bit pattern will fol low Therefore the bit correlator of the PWM detector is only activated for a short period to process the bit pattern and the current consumption reaches the maximum value The PWM detector features a scalable addressing scheme because the wakeup signal is composed of the combination Figure 2 Architecture of dualpowermode PWD The top timing diagram highlights the discrepancy between the width of the received RF pulse and the digital pulse generated by the data slicer Dlatch Dlatch Dlatch Dlatch XoR Autopoweroff IoT Board INT SER Sensor Antenna Battery Pulse Width Detector Threshold Detectors Data Slicer Rectifier C1 C2 V1 V2 RF Pulse VCC Dlatch Threshold V1 V2 Trigger Antenna Rectifers Output 26 V Data Slicers Threshold Autopower Off Threshold Detector Power States Listening Processing Pulse Width Data Slicers Output Figure 3 Multipowermode PWM detectors hardware showing the components needed to process the wakeup signal Shift Register Data Slicer Rectifier Antenna Charging Wakeup PWD Auto Power off Bit Correlator PWM Bit Sequence 1 1 1 0 Shift Register Threshold PWM to AM Demodulation Detector Power States Listening Processing Pulse Width Processing Bit Sequence SER CLK Q1n Demodulator Authorized licensed use limited to Instituto Nacional De Telecomunicações INATEL Downloaded on March 152023 at 134046 UTC from IEEE Xplore Restrictions apply FIGURE 4 a experimental setup of the PWD to trigger an agricultural sensor and measure soil moisture 11 b the capacitors charging time in a combined RF wakeup and charging system as a function of the received power FIGURE 5 Current consumption of the PWD and PWM detectors in a test scenario that includes an unmodulated charging signal for a limited period and several modulated wakeup signals FIGURE 6 A comparison of the total current consumption between a harvesting only system wakeup only system and a system implementing combined RF energy harvesting and wakeup IEEE Internet of Things Magazine March 2023 167 consumption and latency are reduced Furthermore frequent battery chargingreplacement is no longer required As a result the IoT device can reach nearzero power consumption with high responsiveness and selfsustainable operation conclusIon And future dIrectIons In this article we presented the challenges of implementing combined RF energy harvesting and wakeup where a single rectifier is used for RFDC power conversion and demodu lation We introduced a general design framework with two design architectures namely the PWD and the PWM detec tors to enable the combined architecture and overcome the encountered challenges The PWM detector includes the PWD in its hardware and it extends its capability by adding a bit correlator and a wakeupcharging mode switch We further validated the practicality of the proposed methodol ogy by testing prototype designs under practical operational conditions and comparing them with traditional RF wakeup only and energy harvesting only schemes We outline below additional future research directions AdvAnced hArdWAre desIgns To further reduce the energy consumption of IoT devices and enhance the efficiency of energy transfer in combined systems optimized signal structures must be designed to maximize the effi ciency of the implemented hardware For instance adaptive sig nal generation can be applied at the transmitters side to change the number size and interframe spacing of transmitted packets according to network conditions This action aims to optimize the power delivery efficiency and minimize wakeup delay On the other side a power and loadagnostic rectifier design is required at the receivers side to overcome fluctuations in input power and variations in load impedance Employing the rectifier for wakeup also necessitates a fast rectifier response to support the demod ulation of wakeup signals at a high bit rate Decoupling the address detector during charging tasks is essential to reduce its current consumption and false wakeup probability On addition jamming avoidance at the address detector is an essential factor to handle in highly dense networks where the device shuts down wakeup signal processing during charging tasks uAvenAbled combIned rf energy hArvestIng And WAkeup In addition to the hardwarerelated enhancements UAVs can be effectively used to enable combined RF energy harvesting and wakeup of IoT devices in remote and hardtoreach areas 10 The use of UAVs can reduce the gap between the charging range and wakeup range since a UAV can easily relocate to get closer to the IoT device for charging tasks and remain distant during wakeup tasks Several research works have addressed the use of UAVs for data collection from IoT devices and for wireless energy transfer purposes 13 however UAVenabled combined RF energy harvesting and wakeup is still a new research direc tion with plenty of open problems that need to be addressed For instance the UAVs trajectory and height should be optimized to enable combined charging and wakeup This must be achieved while maximizing the power received by individual IoT devic es and while minimizing the energy consumption of both the UAV and IoT devices The flight trajectory can also be optimized based on the design objectives RF energy harvesting only RF wakeup only or mixed flight while accounting for the size weight and power constraints of the UAV IntegrAtIon In b5g6g netWorks Although 3GPP and IEEE 80211 have introduced RF wakeup in their recent standardization efforts 14 15 to extend the lifespan of batterypowered IoT devices many challenges still persist especially in terms of integration with existing transceiver designs and alignment with the controlplane and dataplane protocol stack Advanced technologies in B5G6G systems such as ultramassive MIMO multiuser energy beamforming and reconfigurable intelligent surfaces can be exploited to enhance the efficiency of the proposed combined RF harvest ing and wakeup architecture These technologies allow for the smart reconfiguration of the propagation medium and for focusing the transmitted RF energy in narrow beams toward the IoT devices This requires however efficient channel estimation techniques to acquire the channel state information and opti mize the beamforming accordingly which is quite challenging with batterylimited IoT devices references 1 S Verma et al Toward Green Communication in 6GEnabled Massive Inter net of Things IEEE Internet of Things J vol 8 no 7 2020 pp 540815 2 O L López et al Massive Wireless Energy Transfer Enabling Sustainable IoT toward 6G Era IEEE Internet of Things J vol 8 no 11 2021 pp 881635 3 R Piyare et al Ultra Low Power WakeUp Radios A Hardware and Net working Survey IEEE Commun Surveys Tutorials vol 19 no 4 2017 pp 211757 4 Y Mafi et al UltraLowPower IoT Communications A Novel Address Decoding Approach for WakeUp Receivers IEEE Trans Green Commun Net vol 6 no 2 2021 pp 110721 5 A A Benbuk et al Tunable Asynchronous and Nanopower Baseband Receiver for Charging and Wakeup of IoT Devices IEEE Internet of Things J vol 9 no 4 2021 pp 302336 6 Z Xu et al Analysis and Design Methodology of RF Energy Harvesting Rectifier Circuit for UltraLow Power Applications IEEE Open J Circuits and Systems 2022 7 K Kaushik et al LowCost WakeUp Receiver for RF Energy Harvesting Wire less Sensor Networks IEEE Sensors J vol 16 no 16 2016 pp 627078 8 A Eid et al A Compact SourceLoad Agnostic Flexible Rectenna Topolo gy for IoT Devices IEEE Trans Antennas Propag vol 68 no 4 2020 pp 262129 9 Y Huang N Shinohara and T Mitani Impedance Matching in Wireless Power Transfer IEEE Trans Microwave Theory and Techniques vol 65 no 2 2016 pp 58290 10 A A Benbuk N Kouzayha A Eid J Costantine Z Dawy F Paonessa and G Virone Leveraging UAVs for passive RF charging and ultralowpower wakeup of ground sensors IEEE Sensors Lett vol 4 no 5 2020 pp 14 11 A A Benbuk et al A NanoWatt DualMode Address Detector for a WiFi Enabled RF WakeUp Receiver 2019 IEEE Sensors 2019 pp 14 12 Analog Devices ADXL346 Accelerometer httpswww analogcommedia entechnicaldocumentationdatasheetsadxl346pdf accessed Nov 2022 13 O Cetinkaya D Balsamo and G V Merrett Internet of MIMO Things UAVAssisted WirelessPowered Networks for Future Smart Cities IEEE Inter net of Things Mag vol 3 no 1 2020 pp 813 14 IEEE 80211ba2021 IEEE Standard for Information Technology Telecom munications and Information Exchange Between Systems Local snd Metro politan Area NetworksSpecific Requirements Part 11 Wireless lan medium Access Control MAC and Physical Layer PHY Specifications Amendment 3 WakeUp Radio Operation IEEE Std 2021 15 D E RuizGuirola et al EnergyEfficient WakeUp Signalling for Machine Type Devices Based on TrafficAware LongShort Term Memory Prediction IEEE Internet of Things J 2022 bIogrAphIes Ahmed Abed benbuk ajb09mailaubedu received a MEng from the depart ment of electrical and computer engineering at the American University of Beirut AUB His research interests include RF energy harvesting RF wakeup and ultra lowpower electronics nour kouzAyhA nourkouzayhakaustedusa is a postdoctoral fellow with the Information Theory Lab King Abdullah University of Science and Technology KAUST Her research interests are in the area of wireless communications inter net of things and aerial networks Joseph CostAntine jcostantineieeeorg is an associate professor of electri cal and computer engineering at the American University of Beirut AUB His research and teaching interests are in the fields of applied electromagnetics RF energy harvesting and electromagnetic sensors zAher dAwy zaherdawyaubedulb is a professor of electrical and computer engineering at the American University of Beirut AUB His research and teach ing interests are in the fields of wireless networks internet of things and mobile health systems Authorized licensed use limited to Instituto Nacional De Telecomunicações INATEL Downloaded on March 152023 at 134046 UTC from IEEE Xplore Restrictions apply CCharging and WakeUp of IoT devices Using Harvested RF Energy with NearZero Power Consumption A Figura 2 apresenta um diagrama de blocos do sistema proposto O sistema é composto por cinco blocos principais um circuito de retificação de RF um circuito de gerenciamento de energia PWM um circuito de detecção de energia PWD um microcontrolador e um dispositivo IoT O circuito de retificação de RF é responsável por converter a energia RF captada em energia DC utilizável pelo sistema O circuito de gerenciamento de energia PWM fornece a energia para o dispositivo IoT e é controlado pelo microcontrolador O circuito de detecção de energia PWD detecta a presença de energia RF e informa o microcontrolador sobre a disponibilidade de energia O microcontrolador é o componente central do sistema e controla o PWM e o PWD O dispositivo IoT é o dispositivo a ser alimentado pelo sistema O objetivo do PWM é gerenciar a energia disponível e fornecer energia suficiente para operar o dispositivo IoT Ele regula a energia DC disponível para o dispositivo IoT mantendo o consumo de energia no mínimo possível O objetivo do PWD é detectar a presença de energia RF e informar o microcontrolador sobre a disponibilidade de energia O PWD é usado para evitar que o sistema tente fornecer energia para o dispositivo IoT quando não houver energia RF suficiente disponível 1 Circuitos de retificação de RF responsáveis por converter a energia RF captada em energia DC utilizável pelo sistema Esses circuitos incluem um amplificador de baixo ruído LNA para aumentar a amplitude do sinal RF captado um circuito de filtragem para remover ruídos e um circuito retificador para converter a energia RF em energia DC 2 Circuito de gerenciamento de energia PWM fornece a energia para o dispositivo IoT e é controlado pelo microcontrolador Ele regula a energia DC disponível para o dispositivo IoT mantendo o consumo de energia no mínimo possível 3 Circuito de detecção de energia PWD detecta a presença de energia RF e informa o microcontrolador sobre a disponibilidade de energia Ele é usado para evitar que o sistema tente fornecer energia para o dispositivo IoT quando não houver energia RF suficiente disponível 4 Microcontrolador é o componente central do sistema e controla o PWM e o PWD Ele gerencia a energia disponível e fornece energia suficiente para operar o dispositivo IoT 5 Dispositivo IoT é o dispositivo a ser alimentado pelo sistema O objetivo do sistema é permitir o carregamento e a ativação do dispositivo IoT usando energia RF com um consumo de energia quase nulo O PWM é responsável por gerenciar a energia disponível e fornecer energia suficiente para operar o dispositivo IoT enquanto o PWD é usado para detectar a presença de energia RF e informar o microcontrolador sobre a disponibilidade de energia A Figura 3 ilustra o contexto do sistema proposto Ele mostra a relação entre o dispositivo IoT e a fonte de energia RF A fonte de energia RF transmite energia RF para o ambiente que é capturada pelo dispositivo IoT e convertida em energia DC através do circuito de retificação de RF Em seguida a energia é gerenciada pelo PWM e usada para alimentar o dispositivo IoT O objetivo do sistema é permitir o carregamento e a ativação do dispositivo IoT usando energia RF captada Isso é útil em situações em que a troca frequente de baterias em dispositivos IoT é difícil ou impossível como em locais remotos ou de difícil acesso Além disso o sistema proposto tem um consumo de energia quase nulo o que o torna adequado para dispositivos IoT de baixa potência e prolonga sua vida útil No contexto do sistema proposto a fonte de energia RF é qualquer transmissor de RF no ambiente como uma torre de celular ou um roteador WiFi Essa fonte de energia RF transmite energia RF para o ambiente e o dispositivo IoT capta essa energia usando uma antena A energia RF captada é então convertida em energia DC pelo circuito de retificação de RF que é composto por um amplificador de baixo ruído LNA um circuito de filtragem e um circuito retificador O amplificador de baixo ruído amplifica o sinal RF captado para que ele possa ser processado pelo circuito de filtragem que remove os ruídos e interferências Em seguida o circuito retificador converte a energia RF em energia DC A energia DC resultante é então gerenciada pelo circuito de gerenciamento de energia PWM e usada para alimentar o dispositivo IoT O PWM fornece a energia necessária para o dispositivo IoT enquanto mantém o consumo de energia o mais baixo possível prolongando a vida útil do dispositivo Em resumo a Figura 2 e a Figura 3 mostram o sistema proposto para carregar e ativar dispositivos IoT usando energia RF captada O sistema usa um circuito de retificação de RF um circuito de gerenciamento de energia PWM um circuito de detecção de energia PWD um microcontrolador e um dispositivo IoT O PWM gerencia a energia disponível e fornece energia suficiente para operar o dispositivo IoT enquanto o PWD detecta a presença de energia RF e informa o microcontrolador sobre a disponibilidade de energia A Figura 3 ilustra o contexto do sistema mostrando como o dispositivo IoT capta a energia RF e a transforma em energia DC para alimentar o dispositivo IoT