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.
The results are impressive. It turns out that the temperature sensor can operate at distances of up around six meters from the router and by adding a rechargeable battery to the mix, Talla and co were able to increase that to about nine meters.
Even more ambitiously, they also connected a camera to their antenna. This was a low-power Omnivision VGA sensor capable of producing 174 x 144 pixel black and white images, which requires 10.4 milliJoules of energy per picture.
To store energy, they attached a low leakage capacitor to the camera, which activates when the capacitor is charged to 3.1V and continues operating until the voltage drops to 2.4 Volts. The images were stored in a 64 KB non-volatile ferroelectric random access memory.
In the subsequent tests, the camera performed remarkably well. “The battery-free camera can operate up to [about five meters] from the router, with an image capture every 35 minutes,” say Talla and co. By adding a rechargeable battery they increased that to seven meters. The router could even power the camera through a brick wall, demonstrating that it would be possible to attach the device outside while keeping the power supply inside.
Discussions and Future Directions
Wi-Fi router as a charging hotspot.
In addition to powering custom temperature and camera sensors, PoWiFi can transform the vicinity of a Wi-Fi router into a wireless charging hotspot for devices such as FitBit and Jawbone activity trackers.
They designed and built a general-purpose USB charger. It consists of a 2 dBi Wi-Fi antenna attached to a custom harvester that we optimize for higher input power values. We then connect our USB charger to a Jawbone UP24 device and place it 5-7 cm away from the PoWiFi router. We observe that the charger could supply an average current of 2.3 mA and charge the Jawbone UP24 battery from a no-charge state to 41% charged-state in 2.5 hours
PoWiFi with MIMO.
The current implementation uses multiple antennas to transmit concurrently on different WiFi channels. They could use MIMO techniques for transmitting to Wi-Fi clients on these antennas and use them for PoWiFi during the silent durations.
Multiple PoWiFi routers.
In principle, multiple PoWiFi routers would have to time-multiplex their power traffic, thus reducing their cumulative channel occupancy and resulting in inefficient power delivery. Their solution is to allow PoWiFi routers to concurrently transmit their power packets. While this creates collisions between the power traffic, it is acceptable since our UDP broadcast packets do not need to be decoded by any specific client. As a result, the cumulative channel occupancy at each of the routers remains high. Implementing and evaluating this solution, however, is not in the scope of this paper.
Security implications of PoWiFi.
As networks capable of delivering both power and data become prevalent, one can imagine a “power denial-of-service” (PDoS) attack in which a rogue device causes power starvation for other members of the network by generating signals designed to cause carrier sense events at the PoWiFi router. This opens up interesting research opportunities for understanding the tradeoffs for security mechanisms that protect against such attacks in an efficient manner.
Future clean-slate designs and PoWiFi.
They believe that their system is a general design for power delivery in the ISM bands. As Wi-Fi access and densities continue to grow in the ISM band, solutions that deteriorate Wi-Fi performance by jamming any specific frequency are not desirable. Their power delivery solution is integrated with the Wi-Fi protocol and hence can deliver power while having minimal impact on Wi-Fi traffic. Future designs would generalize their multichannel approach to operate across multiple ISM bands (e.g.,900 MHz, 2.4 GHz and 5 GHz). They believe that this paper takes a significant step towards that goal.
We present the first power over Wi-Fi system that delivers power and works with existing Wi-Fi chipsets. Specifically, we show that a ubiquitous piece of wireless communication infrastructure, the Wi-Fi router, can provide far field wireless power without compromising the network’s communication performance. Building on our design we prototype, for the first time, battery-free temperature and camera sensors that are powered using Wi-Fi chipsets with ranges of 20 and 17 feet respectively. We also demonstrate the ability to wirelessly recharge nickel-metal hydride and lithium-ion coin-cell batteries at distances of up to 28 feet. Finally, we deploy our system in six homes in a metropolitan area and show that our design can successfully deliver power via Wi-Fi in real-world network conditions.
SOURCES – Technology Review, Arxiv