84 COMMUNICATIONS OF THE ACM | MARCH 2017 | VOL. 60 | NO. 3
on nonoverlapping Wi-Fi channels to maximize the
cumulative occupancy across the channels. To harvest this energy,
we introduce the first multichannel harvester that efficiently
harvests power across multiple Wi-Fi channels and generates
the 1. 8–2.4V necessary to run microcontrollers and sensor
To be practical, Po WiFi must not significantly degrade
network performance. So our second component is a transmission mechanism that minimizes the impact on Wi-Fi
performance while effectively providing continuous power to
harvesters. Specifically, to minimize the impact on associated
Wi-Fi clients, Po WiFi injects power packets on a channel only
when the number of data packets queued at the Wi-Fi interface is below a threshold. Further, the router transmits power
packets at the highest Wi-Fi bit rates to minimize their duration, maximizing fairness to other Wi-Fi transmitters.
To further minimize its impact on neighboring Wi-Fi
networks, Po WiFi uses two key techniques.
• Rectifier-aware transmissions. The key intuition is that
when there are packets on the air, a harvester’s temporary energy supply charges exponentially, but it also discharges exponentially during silent periods. To balance
power delivery and channel occupancy, Po WiFi must
minimize energy loss due to leakage. We achieve this by
designing an occupancy modulation scheme that
jointly optimizes the rectifier’s voltage behavior and the
Wi-Fi network’s throughput to ensure that harvesting
sensors can meet their duty-cycling requirements (see
Rectifier-aware Po WiFi transmissions section).
• Scalable concurrent transmissions. A key goal is to maintain good network performance when there are multiple
Po WiFi routers in an area. Our insight is that Po WiFi’s
power packets do not contain useful data, and so the
transmissions from multiple Po WiFi routers can safely
collide. Further, by making each Po WiFi router transmit random power packets, we ensure that concurrent
packet transmissions do not destructively interfere to
reduce available power at sensors.
We build prototype Po WiFi routers using Atheros chipsets
and harvesters using off-the-shelf components. Our experi-
ments demonstrate the following:
• The power packets at the Po WiFi router do not noticeably
affect TCP or UDP throughput or webpage load times1 at
an associated client. Meanwhile, Po WiFi achieves an
average cumulative occupancy of 95.4% across the three
nonoverlapping 2. 4 GHz Wi-Fi channels.
• PoWiFi’s unintrusive transmission strategy allows
neighboring Wi-Fi networks to achieve better-than-equal-share fairness, because a Po WiFi router transmits
power packets at the highest bit rate to minimize its
• Using a rectifier-aware transmission scheme that can
adapt to a harvester’s energy needs, Po WiFi’s per-channel
occupancy is as low as 4.4% while delivering power to a
sensor 16 ft away that reads temperature values once every
• We perform a proof-of-concept evaluation of our concurrent transmission mechanism with one, three, and six
Po WiFi routers. While the variance of neighboring Wi-Fi
networks’ throughput slightly increases, their mean
throughput does not statistically differ. This shows the
feasibility of scaling our design with multiple Po WiFi
To demonstrate the potential of our design, we build two
battery-free, Wi-Fi powered sensing systems shown in Figure 2:
a temperature sensor and a camera. The devices use Wi-Fi
power to run their sensors and a programmable microcontroller that collects the data and sends it over a UART interface. The camera and temperature-sensor prototypes can
operate battery-free at distances of up to 17 and 20 ft, respectively, from a PoWiFi router. As expected, the duty cycle at
which these sensors can operate decreases with distance.
Further, the sensors can operate in through-the-wall scenarios when separated from the router by various wall materials.
We also integrate our harvester with 2.4V nickel–metal
hydride (NiMH) and 3.0 V lithium-ion (Li-Ion) coin-cell batteries. We build battery-recharging versions of the above
sensors wherein Po WiFi trickle charges the batteries. The
battery-recharging sensors can run energy-neutral operations at distances of up to 28 ft.
Finally, we deploy Po WiFi routers in six homes in a metropolitan area. Each home’s occupants used the PoWiFi
router for their Internet access for 24h. Even under real-world network conditions, Po WiFi efficiently delivers power
while having a minimal impact on user experience.
Figure 2. Prototype hardware demonstrating Po WiFi’s potential. The prototypes harvest energy from Wi-Fi signals through a standard 2 dBi Wi-Fi
antenna (not shown). The low gain antenna ensures that the device is agnostic to the antenna orientation and placement. We developed (a) a battery
free camera to capture images, (b) a temperature sensor to measure temperature, (c) a Li-ion battery charger, and (d) a NiMH battery charger.
(a) Battery free camera (b) Temperature sensor (c) Li-ion battery charger (d) NiMH battery charger