Commit 729b49e3 authored by Archan MISRA's avatar Archan MISRA

broke up system into 2 sections: wearable+ testbed

put proper references in batteryless.bib
added reference (including our anon paper) in introduction.tex
parent 68855817
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......@@ -15,9 +15,9 @@ anomalies in factories, city neighborhoods and critical infrastructure.
There is, naturally, great interest in building practical sensor wearable and/or IoT platforms with renewable energy harvesting capabilities. Energy harvesting on wearables is not a new idea. Popular forms of such energy harvesting include ambient light (via photovoltaic cells), temperature gradients (for wearables with skin-contact) and kinetic energy~\cite{XXX}. Each such technique is innovative, but also has certain limitations--e.g., ambient light is not useful for sensors mounted in poorly lit or occluded locations (e.g., in the racks of a dark warehouse or on body locations that are occluded by clothing).
In this paper, we explore and demonstrate the practical feasibility of building a WiFi energy harvesting based wearable platform, whereby the RF power in WiFi packet transmissions from a commodity WiFi AP (access point) is used to recharge and operate a wearable sensor device. Given the widespread deployment and coverage of WiFi, WiFi-based recharging offers an intriguing, orthogonal alternative for pervasive energy harvesting. Wireless recharging itself is not novel (e.g., it is now supported on high-end smartphones), but the current paradigm requires the wearable to be in close proximity (3-5cm) to the transmitting power source. More recently, WiFi-based energy harvesting has been demonstrated in PoWiFi~\cite{XXX}, to power an ultra-low power wearable with an embedded temperature sensor. Our key scientific contribution is two-fold: (a) we show how to increase the harvested WiFi power to much higher levels (O(100$\mu$W)) on an embedded device, at a much greater distance ($\sim$3-4meter from the WiFi AP) than has been previously possible, thereby making such charging feasible in real-world WiFi deployments, and (b) we show that, with appropriate triggered-sensing techniques, such harvested WiFi power may be used to sense and retrieve useful data from an accelerometer, a much higher-power sensor than demonstrated in prior studies.
In this paper, we explore and demonstrate the practical feasibility of building a WiFi energy harvesting based wearable platform, whereby the RF power in WiFi packet transmissions from a commodity WiFi AP (access point) is used to recharge and operate a wearable sensor device. Given the widespread deployment and coverage of WiFi, WiFi-based recharging offers an intriguing, orthogonal alternative for pervasive energy harvesting. Wireless recharging itself is not novel (e.g., it is now supported on high-end smartphones), but the current paradigm requires the wearable to be in close proximity (3-5cm) to the transmitting power source, or is used to charge ultra-low power passive RFID tags~\cite{yeager2008}. More recently, WiFi-based energy harvesting has been demonstrated in PoWiFi~\cite{talla2015powering}, to power an ultra-low power wearable with an embedded temperature sensor. Our key scientific contribution is two-fold: (a) we show how to increase the harvested WiFi power to much higher levels (O(100$\mu$W)) on an embedded device, at a much greater distance ($\sim$3-4meter from the WiFi AP) than has been previously possible, thereby making such charging feasible in real-world WiFi deployments, and (b) we show that, with appropriate triggered-sensing techniques, such harvested WiFi power may be used to sense and retrieve useful gesture-related data from an accelerometer (a much higher-power sensor than demonstrated in prior studies).
Our overall solution, called \names, utilizes some of the key recently-developed capabilities of WiFi networks. At a high-level, \name employs 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 to the client device. To determine the client device's direction, \name utilizes recent AoA (angle-of-arrival) estimation capabilities demonstrated on commodity AP devices. This AP-side innovations are paired with novel energy-conserving features on the wearable device, which activates its various communication and sensing components intelligently and selectively, to help capture only key events.
Our overall solution, called \names, utilizes some of the key recently-developed capabilities of WiFi networks. At a high-level, \name employs 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 to the client device. To determine the client device's direction, \name utilizes recent AoA (angle-of-arrival) estimation capabilities demonstrated on commodity AP devices. This AP-side innovations are paired with novel energy-conserving features on the wearable device, which activates its various 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 \name prototype, which shows that it is possible to build and operate a batteryless, wrist-mounted wearable sensor device, which can collect and transmit significant accelerometer data, while being charged by RF transmissions from a \emph{realistically-distant} WiFi AP. To achieve this goal, we make the following key contributions:
\begin{itemize}
......@@ -25,7 +25,7 @@ Our overall solution, called \names, utilizes some of the key recently-developed
\item \emph{Design \& Implementation of an Intermittently-Triggered Wearable Sensor:} We build 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 transmissions from the wearable, and (ii) the power overhead of wearable system, including the accelerometer, is XXX mW--while low, this is still 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. In addition, the wearable utilizes a supercapacitor to store the harvested RF energy, and smoothen out transient fluctuations in power supply and drainage.
\item \emph{Demonstrate Feasibility of XXX 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. \am{Need to add some details on the studies and the results}
\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. \am{Need to add some details on the studies and the results}
\end{itemize}
We believe that our work lays the foundational principles of a practical WiFi-based energy harvesting mechanism for future embedded sensing devices, which can find applications beyond such wearables to other scenarios (such as static sensors deployed in commercial buildings or industrial warehouses).
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