Ambient backscatter: Free energy harvesting from TV signals, to power a ubiquitous internet of things

Researchers at the University of Washington have created a new method of wireless networking that allows devices to communicate with each other without batteries or their own source of power. Dubbed “ambient backscatter,” this method sucks power out of the air from TV and cellular transmissions, and then modulates this signal with its own data. These modulated signals are then picked up by other devices in the same ambient backscatter network. Requiring a battery or other source of power is probably the most prohibitive factor when it comes to the ubiquity of electronic technology. In theory, this could be the breakthrough to finally allow for a ubiquitous internet of things, where every location is blanketed with devices and sensors that communicate with each other, creating truly smart homes, offices, and cities.
You may or may not realize this, but much of the Earth is blanketed in high-power radio waves, mostly from television and cellular towers. These towers pump out broadcast RF radiation that is picked up by your TV or mobile phone. There’s no “intelligence” or sophistication in such a method — the radio waves aren’t targeted directly at your television set or phone. The waves are just floating around in the atmosphere, waiting to be plucked out of thin air by a receiver that’s tuned to the right frequency. In most cases, the receiver then processes these radio waves into useful data, which is then displayed on a screen or played through an earpiece.
What the University of Washington researchers have done, however, is quite different. They have created a device that has an antenna tuned to UHF TV signals (539MHz in this case), but instead of displaying a picture, the signal’s energy is harvested to power a small device. The device contains logic chip that controls the impedance (resistance) of the antenna. By altering the impedance of the antenna, the TV signal is reflected (backscattered) back into the atmosphere in a specific and controllable way. In other words, the device can generate a signal of its own — a signal that can be picked up by other local devices — without its own source of power, simply by reflecting TV signals in a clever way.

So far, the researchers have created multiple prototype devices, each around the size of a credit card, that use ambient backscatter to communicate with each other, at 1Kbps over 2.5 feet (76cm) outdoors and 1.5 feet (46cm) indoors. This might not sound very good, but 1 kilobyte per second is more than enough for sensor networks, or smart devices in your home/office — and for a prototype, 2.5 feet is pretty good. You only need to get it up to six or 10 feet to make a ubiquitous network, an internet of things, feasible. It goes without saying, too, that these ambient backscatter devices are cheap and easy to produce, too; as you can see in the picture above, it’s essentially just an antenna and a simple microcontroller.
At this point, you’re probably wondering whether it’s actually legal to leech energy from TV signals, and more importantly whether you’re actually allowed to transmit your own signals on a licensed UHF TV band. According to the researchers, their ambient backscatter devices fall under the same governance as another very popular battery-free device: RFID tags. Because these devices emit RF radiation at incredibly low levels (microwatts), and because they’re not actively generating a signal in a reserved block of spectrum, they’re not regulated by the FCC. In the research paper [PDF], they also test whether these ambient backscatter devices interfere with TV signals — and as long as it’s at least 7.2 inches (18cm) away from the TV antenna, no interference is seen. This is partly due to the low output power of the transponder, but also because broadcast TV protocols already account for multipath distortion — that is, signals arriving at different times, depending on atmospheric obstructions (buildings, clouds, mountains, etc.)
Moving forward, it’s possible to make the ambient backscattering devices smaller by using different antenna topologies (meandered, folded dipole), and increased bandwidth could be obtained by using a larger frequency band (again by modifying the antenna). Relying on TV signals and other sources of RF radiation is ingenious, and in large parts of the world can provide almost-guaranteed sources of power. This really could be the technology that finally enables us to put sensors on everything, and to turn every normal object into a smart object that can interact with every other object and computer on the internet. You’ve seen what happens when billions of people are persistently online thanks to mobile phones; now imagine what would happen if you added billions of traffic signals, cars, supermarket shelves, refrigerators, and air conditioning systems to the mix.