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.
