MARCH2017 | VOL. 60 | NO. 3 | COMMUNICATIONS OF THE ACM 91
for power delivery system, we believe that with subsequent
iterations of the harvester we can significantly increase the
capabilities of our system.
Acknowledgments
This research is funded in part by NSF grants CNS-1452494
and CNS-1407583, a Qualcomm Innovation Fellowship, a
Intel Fellowship, and University of Washington.
occupancy, including both client and power traffic,
dipped during this duration. This points to external
causes including interference from other devices in the
environment.
7. RELATED WORK
Early RF power delivery systems were developed as part of
RFID systems to harvest small amounts of power from a
dedicated 900 MHz UHF RFID readers.
13 The power harvested
from RFID signals has been used to operate accelerometers,
13
temperature sensors,
13 and recently cameras.
9 Our efforts
on power delivery over Wi-Fi are complimentary to RFID
systems. In principle, one can combine multiple ISM bands
including 900 MHz, 2. 4 GHz, and 5 GHz to design an optimal
power delivery system. This paper takes a significant step
toward this goal.
Recently, researchers have demonstrated the feasibility
of harvesting small amounts of power from ambient TV7 and
cellular base station signals19 in the environment. While TV
and cellular signals are stronger in outdoor environments,
they are significantly attenuated indoors limiting the corresponding harvesting opportunities. The ability to power
devices using Wi-Fi can augment the above capabilities and
enable power harvesting indoors.
Researchers have explored the feasibility of harvesting
power in the 2. 4 GHz ISM band.
4, 10, 15, 18 These efforts have
demonstrated power harvesting from continuous wave (CW)
transmissions and none have powered devices with existing
Wi-Fi chipsets. In contrast, Po WiFi is the first power over
Wi-Fi system that works with existing Wi-Fi chipsets and
minimizes its impact on Wi-Fi performance. Furthermore,
none of the systems power sensors and microcontrollers or
recharge batteries and operate at distances demonstrated in
this work.
Our work is also related to efforts from startups such
as Ossia2 and Wattup.
21 These claim to deliver around 1W
of power at 15ft and charge a mobile phone.
5 Back-of-the-envelope calculations however show that this requires continuous transmissions with an EIRP (equivalent isotropic
radiated power) of 83. 3 dBm (213k W). This not only jams
the Wi-Fi channel but also is 50,000× higher power than
that allowed by FCC regulations part 15 for point to multi-point links. In contrast, our system is designed to operate
within the FCC limits and has minimal impact on Wi-Fi
traffic. We note that in the event of an FCC exception to
these startups, our multichannel design can be used to
deliver such high power without having significant impact
on Wi-Fi performance.
8. CONCLUSION
There is increasing interest in the Internet of Things where
small computing sensors and mobile devices are embedded in everyday objects and environments. A key issue is
how to power these devices as they become smaller and
more numerous; plugging them in to provide power is
inconvenient and is difficult at large scale. We introduce a
novel far-field power delivery system that uses existing Wi-Fi
chipsets while minimizing the impact on Wi-Fi network
performance. While this is a first step toward using Wi-Fi
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Vamsi Talla, Bryce Kellogg, Benjamin
Ransford, Saman Naderiparizi, Joshua
R. Smith, and Shyamnath Gollakota
({vamsit, samanp, kellogg, jrsjrs,
ransford, gshyam}@ uw.edu) Department
of Computer Science and Engineering,
University of Washington, Seattle, WA.
© 2017 ACM 0001-0782/17/3 $15.00