Commit 43513104 authored by U-RAJESH-SIS\rajesh's avatar U-RAJESH-SIS\rajesh

edited the intro

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\section{Introduction}
\label{sec:intro}
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.
\emph{Energy} remains perhaps the greatest challenge in the pervasive
deployment of such sensing nodes: sensors such as accelerometers or
gyroscopes simply consume too much energy to be operated continuously,
without needing periodic recharge. Battery-powered sensing devices have 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).
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, 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. 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}
\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 problem, we adopt the idea, from~\cite{XXX}, of beamforming the WiFi transmissions 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 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 \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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