Backscattering is a form of wireless transmission based on modulated reflection of external RF signals. Whereas conventional radios generate their own signals, backscatter radios transmit data by making tiny changes to reflected signals. This approach requires a minimal number of active components, promising simple low-cost battery-free operations.
Since the source of the RF signal is external, such transmission does not require an ‘active’ radio transceiver, allowing devices to function in an extremely low power regime (under 10 𝜇𝑊). The power needed to operate the transmitter can be harvested from the external RF signal itself, and thus it is possible for such devices to be batteryless.
The technique of radio backscatter is commonly used in RFID (Radio Frequency IDentification) systems. Radio backscatter communication is an attractive solution in such systems because various involved tags do not need to actively transmit any radio signal. Instead, they simply reflect a radio signal transmitted by an interrogator and modulate the reflection by controlling their own reflection coefficient.
However, the low frequencies that backscatter radios often employ and the strategies they use to encode data in reflected signals typically limit their data rates. For example, radio-frequency identification (RFID) tags, which often employ backscatter radios at sub-gigahertz frequencies, transmit data at only kilobits per second rates. At the 2.4 gigahertz frequency often used by WiFi and Bluetooth, backscatter is generally limited to hundreds of megabits per second.
The principle of backscattering has the potential to enable a full realization of the Internet of Things. As the Internet of Things grows, sensors and other devices must collect and transmit data while consuming as little power as possible. But today’s flexible electronics and other sensors that can’t employ bulky batteries and need to operate with very low power typically can’t communicate with other devices more than a few feet or meters away. This limits their practical use in applications ranging from medical monitoring and home sensing to smart cities and precision agriculture.
One way to do this is to take advantage of backscatter by having IoT devices reflect radio frequency signals transmitted to them. The advantage to using reflected, or “backscattered,” radio signals to convey information is a sensor can run on extremely low power that can be provided by thin cheap flexible printed batteries or can be harvested from ambient sources — eliminating the need for bulky batteries. The disadvantage is that it’s difficult for a receiver to distinguish these extremely weak reflections from the original signal and other noise.
mmWave Backscatter Radio for Gigabit Speeds
Traditionally, mmWave communications, called the extremely high-frequency band, is considered “the last mile” for broadband, with directive point-to-point and point-to-multipoint wireless links. This spectrum band offers many advantages, including wide available GHz bandwidth, which enables very large communication rates, and the ability to implement electrically large antenna arrays, enabling on-demand beamforming capabilities. However, such mmWave systems depend on high-cost components and systems.
Now scientists have developed a backscatter radio operating at millimeter-wave frequencies of 24 to 28 gigahertz, the kind used in upcoming 5G cell phones. Our breakthrough is being able to communicate over 5G/millimetre-wave (mmWave) frequencies without actually having a full mmWave radio transmitter—only a single mmWave transistor is needed along much lower frequency electronics, such as the ones found in cell phones or WiFi devices. Lower operating frequency keeps the electronics’ power consumption and silicon cost low. Our work is scalable for any type of digital modulation and can be applied to any fixed or mobile device.
The new device consists of an antenna array and a single high-frequency transistor. The transistor can apply a voltage or not to make the antenna respectively either receive or reflect incoming signals. The high-frequency radio waves the new device works with allow it to transmit data at similarly high rates. The researchers also implemented more complex data-encoding strategies than often used with backscatter radio to help support high data rates.
The new device is capable of data rates of 2 gigabits per second over distances of 0.5 meters, consuming just 0.17 picojoules per bit. This means it requires thousands of times less power than standard radios—whereas commonly used millimeter-wave radio components consume 600 to 700 milliwatts of power, the new device uses roughly 0.5 milliwatts.
However, the millimeter-wavelength radio signals this new device works with are easily absorbed by the air, walls and many other obstacles. The scientists tuned the composition and geometry of their antenna and transistor to help account for and minimize these losses. More complex data-encoding schemes can increase data rates, “although the distance of communication will decrease,” says study co-author Spyridon Daskalakis, an electrical and computer engineer at Herriot-Watt University in Edinburgh, Scotland. “So we can get 10 gigabits per second, but the distance will be centimeters, or you can go for lower data rates but greater distances.”
Potential applications for the new device include those for high data rates over short distances and low power. “For example, you can imagine transferring lots of photos from your phone by placing it close to a reader and transmitting gigabytes of data in just seconds,” Daskalakis says. “You might also imagine this helping to transmit data within cars.”
The single transistor at the core of the backscatter radio is simple enough to manufacture using inkjet printing of silver nanoparticle inks on flexible polymer backings, drastically reducing fabrication costs and the amount of time needed to prototype new devices, the researchers say. The scientists detailed their findings in the journal Nature Electronics in June 2021.
HD Video streaming
Researchers in the US claim they have created a way to stream HD video that uses 10,000 times less power than current technologies. A team of engineers at the University of Washington have developed a technique that enables wearable cameras such as smart glasses or Snapchat’s Spectacles to send video to a connected smartphone for processing. The result is a camera system that requires far less power to run, and could be a breakthrough for smart glasses.
Currently, streaming cameras process and compress videos before transmitting them to a connected device, but the engineers used a technique called backscatter, which reflects data from the camera lens directly to a smartphone via an antenna. The smartphone then processes the video instead, saving power previously used up on the camera.
As a result, the researchers said they had found between 1,000 and 10,000 times less power used on the wearable device compared current streaming technologies. Shayam Gollakota, co-author of the research and an associate professor at the university’s Paul G. Allen School of Computer Science and Engineering, said the backscatter technology had until now been considered too low-power to work with video.
“The fundamental assumption people have made so far is that backscatter can be used only for low-data rate sensors such as temperature sensors,” he said. “This work breaks that assumption and shows that backscatter can indeed support even full HD video. In terms of what it could mean going forward, the team said the next step is to make wireless video cameras that are completely battery-free, opening up wider use cases.
Earlier UW researchers developed long-range backscatter system, which uses reflected radio signals to transmit data at extremely low power and low cost, achieved reliable coverage throughout 4800-square-foot house, an office area covering 41 rooms and a one-acre vegetable farm.
In 2017, UW team shattered long-range communication barrier for devices that consume almost no power
A team from the University of Washington, with the Internet of Things in mind, has expanded the range of backscatter to several kilometers. Researchers have demonstrated for the first time that devices that run on almost zero power can transmit data across distances of up to 2.8 kilometers — breaking a long-held barrier and potentially enabling a vast array of interconnected devices. The group demonstrated that sensors using a special modulation technique, can backscatter data over greater distances than ever before at Ubicomp 2017.
In the past, sending signals via backscatter was possible only over distances of a few meters. The most common use of backscatter today is in radiofrequency identification tags, which are often used to track boxes during shipping. But those tags receive a signal and harness power from a scanner held just a few centimeters away.
The team’s latest long-range backscatter system provides reliable long-range communication with sensors that consume 1000 times less power than existing technologies capable of transmitting data over similar distances. It’s an important and necessary breakthrough toward embedding connectivity into billions of everyday objects.
Until now, devices that can communicate over long distances have consumed a lot of power. The tradeoff in a low-power device that consumes microwatts of power is that its communication range is short,” said Shyam Gollakota, lead faculty and associate professor in the Paul G. Allen School of Computer Science & Engineering. “Now we’ve shown that we can offer both, which will be pretty game-changing for a lot of different industries and applications.”
The sensors are so cheap — with an expected bulk cost of 10 to 20 cents each — that farmers looking to measure soil temperature or moisture could affordably blanket an entire field to determine how to efficiently plant seeds or water. Other potential applications range from sensor arrays that could monitor pollution, noise or traffic in “smart” cities or medical devices that could wirelessly transmit information about a heart patient’s condition around the clock.
The long-range backscatter system will be commercialized by Jeeva Wireless, a spin-out company founded by the UW team of computer scientists and electrical engineers, which expects to begin selling it within six months.
Long-range backscatter technology
In 2017, University of Washington researchers demonstrated for the first time that devices that run on almost zero power can transmit data across distances of up to 2.8 kilometers — breaking a long-held barrier and potentially enabling a vast array of interconnected devices.
The system has three components: a source that emits a radio signal, sensors that encode information in reflections of that signal and an inexpensive off-the-shelf receiver that decodes the information.
In their research, the UW group tested a custom-built backscatter device, and paired it with an off-the-shelf transmitter and receiver. The transmitter sent a single tone in the 900-megahertz band to the tag, which the device then modulated and reflected onto the receiver.
With their design, the group had to overcome the fact that the signal transmitted from the backscatter device is a million times weaker than the signal transmitted to it. And the transmitter itself can cause interference if the receiver picks up that stronger signal at the same time the tag or some other such device is trying to send its weaker one.
To get the highest possible bit rate and to reduce interference, the group used a signal modulation scheme called chirp spread spectrum (also known as CSS) and applied it to LoRa, a specialized wireless communications protocol. LoRa relies on unlicensed spectrum to create massive networks of low power devices that can deliver data over large areas.
Spreading the reflected signals across multiple frequencies using chirp spread spectrum allowed the team to achieve much greater sensitivities and decode backscattered signals across greater distances even when it’s below the noise. Before their work, no one had ever tried to use LoRa for backscatter. Previous backscatter research has mostly leveraged Wi-Fi or Bluetooth. “The secret sauce is basically in the code we write to do the modulation,” says Talla.
The research team, for instance, built a contact lens prototype and a flexible epidermal patch that attaches to human skin, which successfully used long-range backscatter to transmit information across a 3300-square-foot atrium. That’s orders of magnitude larger than the 3-foot range achieved by prior smart contact lens designs.
With CSS, the intermediate device very slightly shifts the frequency of the of the original signal, choosing a slightly different path for the bits that it sends on to the receiver. This strategy reduces the aforementioned interference from the signal broadcast from the transmitter.
The group tested two configurations of their system: one with the backscatter device placed equidistant from the receiver and transmitter, and the other with the device placed right next to the transmitter. With the first arrangement, they could reliably transmit over distances up to 475 meters. With the device placed next to the transmitter, they could broadcast as far as 2.8 kilometers.
Talla says Jeeva’s next step will likely be to sell a commercial system that includes a transmitter, receiver, and backscatter devices. He says potential customers have expressed a particular interest in using this system to track inventory in hospitals. He estimates the devices will cost only between 10 to 20 cents apiece to produce in bulk.
The research was funded by the National Science Foundation. Co-authors include Joshua Smith, professor in the Allen School and the UW Department of Electrical Engineering, and UW electrical engineering doctoral student Ali