No-Power Wi-Fi Uses Backscatter to Exchange Data
Engineers at the University of Washington create new communication system
August 8, 2014
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
Wi-Fi connectivity could fuel Internet of Things reality
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