No-Power Wi-Fi Uses Backscatter to Exchange Data

Regular readers of RF Report will know that Wi-Fi signals have been used to locate people in a house and that devices have been built that can harvest RF energy in the air from high-power broadcast transmitters. It can also send data, albeit at a very slow data rate, by changing whether an antenna on the device absorbs or reflects signals.

Engineers at the University of Washington have combined and refined both approaches to create a new communication system that uses RF signals as a power source and reuses existing Wi-Fi infrastructure to provide connectivity to compact devices without batteries. Such devices could be used for applications ranging from determining the structural safety of bridges to the health of your heart.

Shyam Gollakota, a UW assistant professor of computer science and engineer, explains, “If Internet of Things devices are going to take off, we must provide connectivity to the potentially billions of battery-free devices that will be embedded in everyday objects. We now have the ability to enable Wi-Fi connectivity for devices while consuming orders of magnitude less power than what Wi-Fi typically requires.”

A battery-free tag could encode data by reflecting or not reflecting a router's Wi-Fi signals. Wi-Fi devices would detect these minuscule changes, the backscatter from the device. and decode data from the tag. Joshua Smith, a UW associate professor of computer science and engineering and electrical engineering, said, “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.”

UW's Wi-Fi backscatter tag has communicated with a Wi-Fi device at data rates up to 1 kbps with about 2 meters between devices. The UW group plans to extend the range to about 20 meters and have patents filed on the technology. If 1 kbps seems low, remember the modem that you used back in the 1980's delivered data rates less than 1/3 that amount. Going back a bit further, Teletype data rates maxed out at 110 baud (roughly the same as bits per second) or less, and were still useful.

Pictures and additional information are available in the UW news release No-power Wi-Fi connectivity could fuel Internet of Things reality by Michelle Ma.

Doug Lung is one of America's foremost authorities on broadcast RF technology. He has been with NBC since 1985 and is currently vice president of broadcast technology for NBC/Telemundo stations.