Low-Power Devices Use Backscatter to Transmit Data Several Kilometers

A special modulation scheme and LoRa combine to boost backscatter for the Internet of Things

3 min read
A photo of a transmitter, a receiver, and the backscatter sensor built by the University of Washington.
Photo: Dennis Wise/University of Washington

As the Internet of Things grows, sensors and other devices must collect and transmit data while consuming as little power as possible. One way to do this is to take advantage of backscatter by having IoT devices reflect radiofrequency signals transmitted to them. Tuned properly, these waves can deliver information over short distances.

A team from the University of Washington, with the Internet of Things in mind, has expanded the range of backscatter to several kilometers. Last week, the group presented research at Ubicomp 2017.  They showed that small sensors, transmitting signals using a special modulation technique, can backscatter data over greater distances than ever before.

That group plans to commercialize its technology through a startup called Jeeva Wireless, and expects to have a commercial backscatter system for sale within six months.

If backscatter can be used over long distances, it would be easier to build huge networks of sensors that could periodically send data to administrators. In theory, such a network could allow you to collect basic data from anything within range that you wished to stick a tag on—including yourself, other people, or pets.

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.

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.

The group tested the setup on a vegetable farm, in a large house, and in an office building. In all three places, they found that they could achieve reliable communications over hundreds of meters or several kilometers by backscattering. The team says a single receiver and transmitter could provide sufficient coverage for a bunch of tags on a one-acre farm, which could be used to monitor water levels or soil temperature.

An image of a backscatter device sitting in a vegeable patch. A backscatter device built by the University of Washington undergoes testing on a nearby vegetable farm.Photo: Dennis Wise/University of Washington

“We’re not going to stream a video across half a kilometer; we’re talking about transmitting a few bytes every few minutes,” says Vamsi Talla, CTO of Jeeva, who worked on the project with Shyam Gollakota while completing postdoctoral research at the University of Washington.

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.

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.

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.

The team also integrated a version of their technology into a contact lens and a flexible skin patch designed to administer a local anesthetic. Both prototypes successfully used backscatter to transmit data across a large atrium, whereas the group says “smart” contact lenses in the past could only send data about one meter.

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.

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