RF Technologies for Low-Power Wireless Communications: Ambient Backscatter
I love family reunions, though, with forty people in my extended family they can get a bit hectic. Whether we’re playing cards, swimming, or at the dinner table there’s always someone telling jokes and stories. In fact, most people are trying to talk so sometimes you have to yell to be heard. It can also be difficult to communicate in the electromagnetic spectrum. Your devices often have to “shout” their own signals into the void in order to transmit data. That transmission takes electronics and energy that some devices don’t have the space or battery life for. A research group at the University of Washington is attempting to address those issues by communicating via ambient backscatter. This method could reduce the circuitry and power needed to transmit data by several orders of magnitude. If ambient backscatter can truly be used for wireless networks then it would be extremely useful for large Internet of Things sensor networks.
As engineers, we know that the world around us is full of electromagnetic signals, many of them generated by devices we design.These transmissions can be detected, and even harvested, by other devices. They can also interfere with unintended receivers, which is why the FCC gets so finicky about radiated emissions testing. However, these researchers have found a way to piggyback on ambient signals in the air and use them to communicate information between devices.
There has been a lot of talk recently about replacing batteries in IoT sensors and devices with energy harvesting systems. Some people have even postulated that high energy RF signals, like TV station transmissions, can be harvested to power devices. This idea is not wholly practical for several reasons. However, one of the main ones is there’s just not enough power. You would need to be able to power a microprocessor, a sensor, and a wireless circuit at least, and possibly even memory. There are low power processors, sensors, and memory, but wireless connections come with a price. That is, until ambient backscatter comes into the equation.
Ambient backscatter can be used to transmit signals using significantly less energy than conventional wireless modules. It works by modulating the RF signals that are already in the air to communicate data. So instead of receiving a signal and then creating a reply, an ambient backscatter PCB could just modulate and redirect a nearby signal without having to create its own. This solves the problem of needing power hungry wireless chips and opens up the possibility of operating IoT devices using harvested RF energy.
Ambient backscatter acts similarly to a RFID tag, but without a dedicated transmitter.
Proof of Concept
The University of Washington’s research group hasn’t just theorized about this system, they’ve actually built it. In order to do this, they designed a fairly simple PCB that can harvest energy, receive, and transmit signals.
A primary factor in using ambient backscatter is picking which RF signals you want to use. You want a signal with high power, for the energy harvester, and that’s also constant, which will enable communication.Thus the researchers decided to piggyback on TV stations. They’re always on, have great coverage, and use a very strong signal. Another important point is that TV transmitters and receivers account for multipath distortion. When backscattering a signal you’re purposefully interfering with that signal. Apparently, this is not technically illegal because these devices only modulate and reflect existing signals but do not actually “transmit.” Legal or not, you shouldn’t try this with a signal that can’t handle the interference.
Now let’s look at their device. It uses a single antenna to harvest energy, transmit, and receive, an efficient design. For transmission, the board modulates the impedance of the antenna to distort and reflect the TV signals. The group wanted to receive signals without using power hungry components like ADCs or oscillators. Instead, they used a traditional digital receiver and theorized an ultra low power analog one. They differentiate their reflected signals from the original transmissions by checking data rates. Their modulated signals transmit much more slowly than the TV stations, thus they can hone in on their signal and decode it. Using these components the researchers were able to transmit data between ambient backscattering devices. That means that this technology could enable extremely low power machine to machine communication.
Now you can help your device do its job just by watching TV.
There are more applications for this technology than I have family members. Some of the most interesting ones are wearables, the IoT, and even payment processing.
Wearables - Currently if you want to compare step counts with someone your device will need to upload those counts to a server and then your device will have to download them. With this kind of technology, your wearables could communicate directly with each other, enabling a more organic approach to data transmission.
The IoT - The Internet of Things will have vast networks of low power sensors that will need to communicate with each other. By utilizing this kind of system these sensors could use much less power, leading to a more efficient and reliable system.
Payment Processing - This last example is from the study itself. They created “bus passes” that could transmit money directly from card to card. Imagine being able to pay someone electronically without needing a cell signal or WiFi.
This technology is going to be incredibly useful in the near future. Not only does it allow device-to-device communication without the use of wireless modules. It also could enable a generation of battery-less devices that can harvest energy from the air. The University of Washington has already shown us how this can be don and has provided a few insights into even more energy efficient ways to use ambient backscatter. Everything from the IoT to our credit cards could eventually make use of this communication system.
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