Mobile Gadgets That Connect to Wi-Fi without a Battery

Simple devices that can link up via Wi-Fi but don’t need batteries could make it easier to spread computing throughout your home.

MIT Technology Review, TomSimonite, August 1, 2014

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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