Simple
devices that can link up via Wi-Fi but don’t need batteries could make it
easier to spread computing throughout your home.
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| Air power: This antenna harvests signals from TV, radio, and cellular transmissions so that small Wi-Fi devices can get by without batteries. |
A new breed
of mobile wireless device lacks a battery or other energy storage, but it can
still send data over Wi-Fi. These prototype gadgets, developed by researchers
at the University of Washington, get all the power they need by making use of
the Wi-Fi, TV, radio, and cellular signals that are already in the air.
The
technology could free engineers to extend the tendrils of the Internet and
computers into corners of the world they don’t currently reach. Battery-free
devices that can communicate could make it much cheaper and easier to widely
deploy sensors inside homes to take control of heating and other services.
Smart
thermostats on the market today, such as the Nest, are limited by the fact that
they can sense temperature only in their immediate location. Putting low-cost,
Wi-Fi-capable, and battery-free sensors behind couches and cabinets could
provide the detailed data needed to make such thermostats more effective. “You
could throw these things wherever you want and never have to think about them
again,” says Shyam Gollakota, an assistant professor at the University of
Washington who worked on the project.
The
battery-free Wi-Fi devices are an upgrade to a design the same group
demonstrated last year—those devices could only talk to other devices like
themselves (see “Devices Connect with Borrowed TV Signals and Need No Power Source”). Versions were built that could power LEDs, motion detectors,
accelerometers, and touch-sensitive buttons.
Adding
Wi-Fi capabilities makes the devices more practical. Gollakota hopes to
establish a company to commercialize the technology, which should also be
applicable to other wireless protocols, such as Zigbee or Bluetooth, that are
used in compact devices without access to wired power sources, he says. A paper
on the new devices will be presented at the ACM Sigcomm conference in Chicago
in August.
Engineers
have worked for decades on ways to generate power by harvesting radio signals
from the air, a ubiquitous resource thanks to radio, TV, and cellular network
transmitters. But although enough energy can be collected that way to run
low-powered circuits, the power required to actively transmit data is
significantly higher. Harvesting ambient radio waves can collect on the order
of tens of microwatts of power. But sending data over Wi-Fi requires at least
tens of thousands of times more power—hundreds of milliwatts at best and
typically around one watt of power, says Gollakota.
The
Washington researchers got around that challenge by finding a way to have the
devices communicate without having to actively transmit. Their devices send
messages by scattering signals from other sources—they recycle existing radio
waves instead of expending energy to generate their own.
To send
data to a smartphone, for example, one of the new prototypes switches its
antenna back and forth between modes that absorb and reflect the signal from a
nearby Wi-Fi router. Software installed on the phone allows it to read that
signal by observing the changing strength of the signal it detects from that
same router as the battery-free device soaks some of it up.
The
battery-free Wi-Fi devices can’t harvest enough energy to receive and decode
Wi-Fi signals in the conventional way. But they can detect the presence of the
individual units, or “packets,” that make up a Wi-Fi transmission. To send data
to the battery-free device, a conventional Wi-Fi device sends a specific burst
of packets that lets the receiving device know it should listen for a
transmission. The data is then is encoded in a stream of further packets with
gaps interspersed between them. Each packet signals a 1 and each gap a 0 of the
digital message.
Ranveer Chandra, a senior researcher in mobile computing at Microsoft Research, says
the technology could help accelerate dreams of being able to deploy cheap,
networked devices that have been slow to arrive. “Given the prevalence of
Wi-Fi, this provides a great way to get low-power Internet of things devices to
communicate with a large swath of devices around us,” he says. RFID tags, which
also lack batteries, are the closest technology in use today, says Chandra. But
they can only communicate with specialized reader devices, he says. The
Washington approach fits better with existing infrastructure.
However,
increasing the range of the system will be important for it to be widely
useful, notes Chandra. The upcoming paper on the technology reports a range of
only 65 centimeters, which barely spans a small table, let alone a single room
in a house. Gollakota says that in recent, still unpublished experiments, the
range has been extended to just over two meters, and 10 meters and beyond
should be possible.
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