introduction.tex 7.68 KB
 Tran Huy Vu committed Apr 04, 2018 1 2 \section{Introduction} \label{sec:intro}  Archan MISRA committed Apr 06, 2018 3   U-RAJESH-SIS\rajesh committed Apr 08, 2018 4 5 6 7 8 9 10 11 12 %Embedded sensors, deployed on small form-factor devices, have transformed %our ability to pervasively observe the state of the physical world, %including the monitoring of human activities, ambient context and machinery %state. As obvious examples, (a) inertial or physiological sensors in %wearable devices (such as smartwatches and smart-necklaces) have been used %to monitor an individual's eating behavior~\cite{XXX}, smoking~\cite{XXX} or %stress levels~\cite{XXX}; (b) vibration, audio or light sensors have been %used on innovative IoT platforms to detect operating conditions and %anomalies in factories, city neighborhoods and critical infrastructure.  Archan MISRA committed Apr 06, 2018 13   U-RAJESH-SIS\rajesh committed Apr 08, 2018 14 \emph{Energy} remains perhaps the greatest challenge in the pervasive  Archan MISRA committed Apr 08, 2018 15 16 deployment of sensing systems, such as wearable devices for human activities (e.g. eating behaviour~\cite{thomaz2015}, smoking~\cite{parate2014}, or stress  Tran Huy Vu committed Apr 09, 2018 17 levels~\cite{ertin2011}) or embedded devices used for environmental sensing~(e.g.,~\cite{campbell2014}). In particular,  U-RAJESH-SIS\rajesh committed Apr 08, 2018 18 19 20 21 22 23 24 25 26 27 28 sensors such as accelerometers or gyroscopes simply consume too much energy to operate continuously without either a dedicated power source or a large battery. However, using battery power introduces two distinct disadvantages: (i) frequent recharging may simply be cumbersome or impractical--e.g., wearable-based health monitoring may be much more palatable if an embedded device may be worn for months without needing to be taken off and recharged; (ii) perhaps not as widely appreciated, high-density storage batteries can \emph{leak}, causing corrosion and other serious hazards, especially when the sensors are deployed in volume and out of sight (e.g., embedded inside factory equipment in industrial IoT settings).  Archan MISRA committed Apr 06, 2018 29   U-RAJESH-SIS\rajesh committed Apr 08, 2018 30 To overcome this disadvantages, many solutions using renewable energy  Archan MISRA committed Apr 08, 2018 31 32 harvesting capabilities have been proposed--such as ambient light~\cite{hande2007}, temperature gradients~\cite{campbell2014b} and kinetic energy~\cite{ryokai2014}. Each such technique is innovative, but has its own limitations--e.g.,  U-RAJESH-SIS\rajesh committed Apr 08, 2018 33 34 ambient light cannot be used for sensors mounted in poorly lit or occluded locations (e.g., in a dark warehouse or on occluded body locations).  Archan MISRA committed Apr 06, 2018 35   U-RAJESH-SIS\rajesh committed Apr 08, 2018 36 37 38 39 40 41 In this paper, we demonstrate the practical feasibility of using WiFi packets from a commodity WiFi AP (access point) to power a wearable sensor device. The widespread coverage of WiFi deployments makes this an intriguing, orthogonal alternative for pervasive energy harvesting. Wireless charging, itself, is not novel, but current solutions require either close proximity (3-5cm) to the transmitting power source (e.g., the  Archan MISRA committed Apr 08, 2018 42 Qi~\cite{liu2015} standard used by modern high-end phones), or can only charge  U-RAJESH-SIS\rajesh committed Apr 08, 2018 43 ultra-low power passive RFID tags~\cite{yeager2008} at longer ranges. More  Archan MISRA committed Apr 08, 2018 44 45 recently, PoWiFi~\cite{talla2015powering} demonstrated the use of WiFi, using multiple channels simultaneously, to power an ultra-low power wearable (with a temperature or camera sensor), with low duty cycles.  Archan MISRA committed Apr 06, 2018 46   Archan MISRA committed Apr 08, 2018 47 48 \emph{Our key scientific contributions are two-fold}: we show (a) how to increase the harvested WiFi power (via directional WiFi transmissions) to much higher levels (O(100$\mu$W)), even on a single channel, on an embedded  U-RAJESH-SIS\rajesh committed Apr 08, 2018 49 50 device, at a much greater distance ($\sim$3-4meters from the transmitter) than had been previously possible. This enables many more use cases, in  Archan MISRA committed Apr 08, 2018 51 industrial IoT, smart homes, etc.; and (b) that, with novel triggered-sensing  Tran Huy Vu committed Apr 09, 2018 52 techniques (that further extend the energy-driven intermittent sensing paradigm articulated in~\cite{hester2017b}), our solution can be used to collect useful gesture-related data from a batteryless, wearable sensing device, using an embedded accelerometer.  U-RAJESH-SIS\rajesh committed Apr 08, 2018 53 54 55 56  Our solution, called \names, uses beam-formed transmissions, by a multi-antenna AP, of WiFi power packets'' (transmissions performed explicitly to transfer RF energy) to deliver bursts of directed WiFi energy  Tran Huy Vu committed Apr 09, 2018 57 58 59 to a client device. To point the beam towards the client, \name utilizes AoA (angle-of-arrival) estimation techniques~\cite{xiong2013arraytrack}. These AP-side techniques are paired with novel energy-conserving features on the wearable  U-RAJESH-SIS\rajesh committed Apr 08, 2018 60 61 62 63 64 65 66 67 device, which activates its communication and sensing components intelligently and selectively, to help capture only key events. While the core ideas were articulated in our preliminary work~\cite{tran2017}, this paper presents a detailed design, implementation and empirical validation of \names. \noindent \textbf{Key Contributions:} To our knowledge, we are the first to design and empirically demonstrate a working prototype (called \names) that  Archan MISRA committed Apr 08, 2018 68 uses RF transmissions, on a single channel, from a \emph{realistically-distant} WiFi AP to power a  U-RAJESH-SIS\rajesh committed Apr 08, 2018 69 batteryless, wrist-mounted wearable sensor device (which is collecting and  Archan MISRA committed Apr 08, 2018 70 transmitting significant accelerometer data generated by regular human movement). To achieve this goal, we make  U-RAJESH-SIS\rajesh committed Apr 08, 2018 71 the following key contributions:  Archan MISRA committed Apr 06, 2018 72 73 74  \begin{itemize}  U-RAJESH-SIS\rajesh committed Apr 08, 2018 75 76 77 78 \item \emph{Use of Beamformed WiFi Transmissions for Power Delivery:} Through empirical experiments, it is clear that the harvested power, from a conventional omni-directionally transmitting WiFi AP, is too low for practical use: around $1-3\mu$W at distances of 3-4 meter. To tackle this  Archan MISRA committed Apr 08, 2018 79 problem, we extend prior work to beamform the WiFi transmissions  U-RAJESH-SIS\rajesh committed Apr 08, 2018 80 81 82 83 84 85 86 87 88 89 90 91 to spatially concentrate the transmitted power. Via experimental studies, we show that even with real-world errors in direction estimation and beamforming, directional transmissions with an 8-antenna array result in dramatically higher levels of harvested energy ($\sim 700-800 \mu$W), even when the embedded device is 3-4 meters distant from the WiFi AP. \item \emph{Design \& Implementation of an Intermittently-Triggered Wearable Sensor:} We built a wrist-worn, \names-compatible wearable device, which utilizes WiFi harvesting to power an inertial sensor used in various gesture-tracking applications. Such a wearable device, worn by a mobile user, gives rise to two challenges: (i) the WiFi AP must be able to track the wearable's changing location, without requiring constant active  Archan MISRA committed Apr 08, 2018 92 93 transmissions from the wearable, and (ii) the peak power overhead of the wearable system, including the accelerometer and the RF frontend, is over 40 mW-- while low, this is much  U-RAJESH-SIS\rajesh committed Apr 08, 2018 94 95 96 97 98 99 100 higher than the harvested power of 700-800$\mu$W to permit continuous sensing. To tackle both these challenges, the wearable employs a simple kinetic energy harvester-based trigger to first detect \emph{significant motion} of the wearable device. Such significant motion triggers both (i) the transmission of ping'' packets by the wearable, which allows the AP to determine the wearable's new AoA, and (ii) the activation of the accelerometer sensor, during the likely occurrence of meaningful gestures.  U-RAJESH-SIS\rajesh committed Apr 08, 2018 101 In addition, the wearable utilizes a super capacitor to store the harvested  U-RAJESH-SIS\rajesh committed Apr 08, 2018 102 103 104 105 106 107 RF energy, and smoothen out transient fluctuations in power supply and drainage. \item \emph{Demonstrate Feasibility of \name in a Real Office Environment:} We utilize a series of quasi-controlled studies, in an office environment, to demonstrate the overall effectiveness of the proposed \name approach.  Tran Huy Vu committed Apr 09, 2018 108 Our microbenchmarks shows a huge potential use of the \name architecture for battery-less device. Though our user study shows that the application of \name to wearable devices is much more challenging, we do notice one case that the system provides sufficient energy for the wearable to work. In other cases, we do notice that our AP can transmit tens of \micro W to the device.  Archan MISRA committed Apr 06, 2018 109 110  \end{itemize}  U-RAJESH-SIS\rajesh committed Apr 08, 2018 111 112 113  We believe that our work lays the foundational principles of a practical WiFi-based energy harvesting mechanism for future embedded sensing devices  Archan MISRA committed Apr 08, 2018 114 115 that have application beyond just static sensors (e.g., in commercial or industrial sites) to include wearable devices.  Archan MISRA committed Apr 06, 2018 116 117 118 119