[Paper Review] Powering the Next Billion Devices with Wi-Fi

Vamsi Talla, Bryce Kellogg+4 more|arXiv (Cornell University)|May 26, 2015

Energy Harvesting in Wireless Networks24 references18 citations

TL;DR

This paper introduces PoWiFi, the first system that enables far-field wireless power delivery using existing Wi-Fi routers and chipsets without degrading network performance. By co-designing multi-channel transmission with superfluous broadcast traffic and a multi-band energy harvester, PoWiFi powers battery-free sensors and recharges coin-cell batteries up to 28 feet away, validated in real homes with 20–17 ft ranges for sensors and 41% charge in 2.5 hours for wearable devices.

ABSTRACT

We present the first power over Wi-Fi system that delivers power and works with existing Wi-Fi chipsets. Specifically, we show that a ubiquitous piece of wireless communication infrastructure, the Wi-Fi router, can provide far field wireless power without compromising the network's communication performance. Building on our design we prototype, for the first time, battery-free temperature and camera sensors that are powered using Wi-Fi chipsets with ranges of 20 and 17 feet respectively. We also demonstrate the ability to wirelessly recharge nickel-metal hydride and lithium-ion coin-cell batteries at distances of up to 28 feet. Finally, we deploy our system in six homes in a metropolitan area and show that our design can successfully deliver power via Wi-Fi in real-world network conditions.

Motivation & Objective

• To enable far-field wireless power delivery using existing Wi-Fi infrastructure without compromising communication performance.
• To address the challenge that Wi-Fi's silent periods cause energy leakage, preventing harvesters from reaching operational voltage.
• To co-design router transmission and energy harvester hardware to create a continuous effective power signal across multiple Wi-Fi channels.
• To demonstrate practical deployment of energy-harvesting sensors and battery chargers in real-world indoor environments.
• To enable scalable, low-cost, and ubiquitous wireless power delivery using the widespread 2.4 GHz ISM band.

Proposed method

• The PoWiFi router transmits superfluous UDP broadcast packets across multiple Wi-Fi channels (1, 6, 11) to maintain high cumulative channel occupancy, simulating continuous transmission.
• A multi-channel energy harvester is designed to collect RF energy across 2.4 GHz bands, with impedance matching optimized across frequencies to reduce signal reflections.
• The system uses OFDM waveforms from standard Wi-Fi chipsets, leveraging their high peak-to-average power ratio for efficient power delivery.
• Harvested energy powers battery-free sensors (temperature and camera) and recharges NiMH and Li-ion coin-cell batteries wirelessly.
• The harvester is integrated with a 2 dBi omnidirectional antenna to ensure orientation independence and maximize reception range.
• A general-purpose USB charger prototype is built to validate charging of consumer wearables like Jawbone UP24 devices.

Experimental results

Research questions

• RQ1 Can existing Wi-Fi routers be modified to deliver usable far-field wireless power without degrading network performance?
• RQ2 Can a multi-channel transmission strategy overcome the energy leakage caused by Wi-Fi’s silent periods in a distributed medium access protocol?
• RQ3 Can standard Wi-Fi chipsets and antennas be used to power battery-free sensors and recharge small batteries at practical distances?
• RQ4 Is it feasible to deploy such a system in real-world residential Wi-Fi environments with typical interference and channel conditions?
• RQ5 Can the system be extended to support charging of commercial wearable devices using standard USB interfaces?

Key findings

• PoWiFi successfully powers a battery-free temperature sensor at a range of 20 feet and a battery-free camera sensor at 17 feet in real-world environments.
• The system recharges nickel–metal hydride and lithium-ion coin-cell batteries at distances up to 28 feet using standard Wi-Fi signals.
• A prototype USB charger based on PoWiFi charged a Jawbone UP24 wearable device from 0% to 41% in 2.5 hours at a distance of 5–7 cm from the router.
• Deployment in six metropolitan homes confirmed reliable power delivery under real-world Wi-Fi conditions, including interference and multi-user contention.
• The multi-channel transmission strategy maintains high cumulative channel occupancy, enabling continuous energy harvesting despite protocol-scheduled silent periods.
• The system achieves efficient power delivery with minimal impact on Wi-Fi performance, as the added traffic is low-rate, broadcast UDP packets that do not require decoding.