'POWER from AIR' backscatter tech now juices up Internet of Stuff Wi-Fi gizmos
Batteries not included required
Researchers who last year demonstrated they could harvest stray RF signals to power RFID tags have scaled-up their technology to power Wi-Fi devices.
The University of Washington team is pitching their ultra low-power “backscatter harvesting” technology with the inevitable Internet of Things (IoT) tag, since if it could be commercialised the technology would help remove the battery from the battalions of sensors that IoT advocates imagine will one day rule our world.
In previous research, the group got RFID tags to represent binary 1 and 0 states by either absorbing or reflecting localised TV signals – something that required only minimal conversion of RF to electricity to power the device.
The latest demonstration still works within the limitations of the power that can be harvested from ambient RF signals. To get devices that could connect to the Internet, the researchers worked on creating ultra-low-power Wi-Fi-enabled tags.
Apart from their own devices, the boffins say everything they used in their test was off-the-shelf: Intel Wi-Fi cards, Linksys routers, and the university's existing Wi-Fi infrastructure. Their radio-gobbling tags were able to communicate over distances of 2.1m at 1 Kbps – far enough for a sensor to chat to a nearby gateway, and fast enough to transmit basic information like temperature or whether a light is switched on.
In their paper (PDF), the researchers say the communications medium is very similar to the TV-frequency-based RFID tags they showed off last year. “At a high level, a Wi-Fi Backscatter tag communicates with a Wi-Fi device by modulating its Wi-Fi channel”, they write. “Since channel information including CSI and RSSI is widely available on commodity Wi-Fi devices, the Wi-Fi receiver can extract the modulated information by measuring the changes in its channel”.
As previously, their ultra-low-power devices signal a 1 or 0 by either absorbing or reflecting Wi-Fi signals. Commodity Wi-Fi devices like smartphones and routers act as “readers” and “helpers” to interpret the change in channel state as data.
To send data to the tag the transmitter sends short Wi-Fi packets: the presence or absence of the special packets will be interpreted by the tag as a 1 or 0.
In the university's press release, co-author of the paper Joshua Smith (UW associate professor of computer science and electrical engineering) says: “You might think, how could this possibly work when you have a low-power device making such a tiny change in the wireless signal? But the point is, if you’re looking for specific patterns, you can find it among all the other Wi-Fi reflections in an environment”. ®