The ability to power remote sensors and devices using Wi-Fi signals could be the enabling technology behind the Internet of things, say electrical engineers. University of Washington researchers have developed a way to broadcast power to remote devices using an existing technology that many people already have in their living rooms: ordinary Wi-Fi. They call their new approach power over Wi-Fi or PoWi-Fi.
PoWiFi is the first power over Wi-Fi system that works with existing Wi-Fi chipsets and minimizes its impact on Wi-Fi performance. The work is also related to efforts from startups such as Ossia and Wattup. These efforts claim to deliver around 1 W of power at ranges of 15 feet and charge a mobile phone. Analysis shows that this requires continuous transmissions with an EIRP (equivalent isotropic radiated power) of 83.3 dBm (213 kW). This not only jams the Wi-Fi channel but also is 50,000 times 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 multi-channel design can be used to deliver high power while having minimal effect on Wi-Fi performance.
Recent work on Wi-Fi backscatter enables low-power connectivity with existing Wi-Fi devices. Backscatter communication is order of magnitude more power-efficient than traditional radio communication and hence enables Wi-Fi connectivity without incurring Wi-Fi’s power consumption. However, is focused on the communication mechanism and to the best of our knowledge, does not evaluate the feasibility of delivering power using Wi-Fi. Our work is complementary to and can in principle be combined to achieve both power delivery and lowpower connectivity using Wi-Fi devices
The idea is simple in concept. Wi-Fi radio broadcasts are a form of energy that a simple antenna can pick up. Until now, Wi-Fi receivers have all been designed to harvest the information that these broadcasts carry.
The University of Washington team’s approach to this is refreshingly straightforward. They simply connect an antenna to a temperature sensor, place it close to a Wi-Fi router and measure the resulting voltages in the device and for how long it can operate on this remote power source alone.
The simple answer is that the voltage across the sensor is never high enough to cross the operating threshold of around 300 millivolts. However, it often comes close.
But a closer examination of the data makes for interesting reading. The problem is that Wi-Fi broadcasts are not continuous. Routers tend to broadcast on a single channel in bursts. This provides enough power for the sensor but as soon as the broadcast stops, the voltages drop. The result is that, on average, the sensor does not have enough juice to work.
That gave Talla and pals an idea. Why not program the router to broadcast noise when it is not broadcasting information and employ adjacent Wi-Fi channels to carry it so that it doesn’t interfere with data rates.
Wi-Fi broadcasts can be on any of 11 overlapping channels within a 72 MHz band centered on the 2.4 GHz frequency. This allows for three non-overlapping channels to be broadcast simultaneously.
Arxiv - Powering the Next Billion Devices with Wi-Fi (15 pages)
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