Powering the Next Billion Devices
By Vamsi Talla, Bryce Kellogg, Benjamin Ransford, Saman Naderiparizi, Joshua R. Smith, and Shyamnath Gollakota
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.
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
• Wi-Fi uses OFDM, an efficient waveform for power delivery
because of its high peak-to-average power ratio.
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
0 0.5 1 1. 5 2 2. 5
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.