Communications of the ACM

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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