Powering the Next Billion Devices
with Wi-Fi

By Vamsi Talla, Bryce Kellogg, Benjamin Ransford, Saman Naderiparizi, Joshua R. Smith, and Shyamnath Gollakota

DOI: 10.1145/3041059

Abstract

We present the first power over Wi-Fi system that delivers power to low-power sensors and devices and works with existing Wi-Fi chipsets. We show that a ubiquitous part of wireless communication infrastructure, the Wi-Fi router, can provide far field wireless power without significantly compromising the network’s communication performance. Building on our design, we prototype battery-free temperature and camera sensors that we power with Wi-Fi at ranges of 20 and 17 ft, respectively. We also demonstrate the ability to wirelessly trickle-charge nickel–metal hydride and lithium-ion coin-cell batteries at distances of up to 28 ft. We deploy our system in six homes in a metropolitan area and show that it can successfully deliver power via Wi-Fi under real-world network conditions without significantly degrading network performance.

1. INTRODUCTION

In the late 19th century, Nikola Tesla dreamed of eliminating wires for both power and communication. 16 As of the early 21st century, wireless communication is extremely well established—billions of people rely on it every day. Wireless power, however, has not been as successful. In recent years, near-field, short range schemes have gained traction for certain range-limited applications, like powering implanted medical devices20 and recharging cars3 and phones from power delivery mats. 8 More recently, researchers have demonstrated the feasibility of powering sensors and devices in the far field using RF signals from TV7 and cellular19 base stations. This is exciting, because in addition to enabling power delivery at farther distances, RF signals can simultaneously charge multiple sensors and devices because of their broadcast nature.

In this work, we show that a ubiquitous part of wireless
infrastructure, the Wi-Fi router, can be used to provide far-field
wireless power without significantly compromising network
performance. This is attractive for three key reasons:
• Unlike TV and cellular transmissions, Wi-Fi is ubiquitous
in indoor environments and operates in unlicensed
spectrum (the “ISM” band) where transmissions can
legally be optimized for power delivery. Repurposing
Wi-Fi networks for power delivery can ease the deploy-
ment of RF-powered devices without additional power
infrastructure.

• Wi-Fi uses OFDM, an efficient waveform for power delivery because of its high peak-to-average power ratio. 17 Given Wi-Fi’s economies of scale, Wi-Fi chipsets provide a cheap platform for sending these power-optimized waveforms, enabling efficient power delivery.

• Sensors and mobile devices are increasingly equipped with 2.4GHz antennas for communication via Wi-Fi, Bluetooth, or ZigBee. We can, in principle, use the same antenna for both communication and Wi-Fi power harvesting with a negligible effect on device size.

 

The key challenge for power delivery over Wi-Fi is the fundamental mismatch between the requirements for power delivery and the Wi-Fi protocol. To illustrate this, Figure 1 plots the voltage at a tuned harvester in the presence of Wi-Fi transmissions. While the harvester can gather energy during Wi-Fi transmissions, the energy leaks during silent periods. In this case, the Wi-Fi transmissions cannot meet the platform’s minimum voltage requirement. Unfortunately for power delivery, silent periods are inherent to a distributed medium access protocol such as Wi-Fi, in which multiple devices share the same wireless medium. Continuous transmission from the router would be optimal for power delivery but would significantly degrade the performance of Wi-Fi clients and other nearby Wi-Fi networks.

This paper introduces PoWiFi, the first power over Wi-Fi system that delivers power to energy-harvesting sensors and devices while preserving network performance. We achieve this by codesigning harvesting hardware circuits and Wi-Fi router transmissions. At a high level, a router running Po WiFi imitates a continuous transmission from a harvester’s perspective while minimizing the impact on Wi-Fi clients and neighboring Wi-Fi networks. The key intuition is that it is unlikely that all the Wi-Fi channels are simultaneously occupied at the same instant. PoWiFi opportunistically injects superfluous broadcast traffic (which we call power packets)

The original version of this paper was published in ACM CoNext 2015.

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0 0.5 1 1. 5 2 2. 5 V olt ag e ( V )

Time (in ms)

Minimum threshold voltage

Figure 1. Key challenge with Wi-Fi power delivery. While the harvester can gather power during Wi-Fi transmissions, the power leaks during silent periods, limiting Wi-Fi’s ability to meet the minimum voltage requirements of the hardware.