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Communications Out of Thin Air

Ambient Backscatter Concept Proven

The piper will be paid.

You can do all kinds of things to reduce currents in your wireless sensor node or other module that will be sending a signal. Heck, you can magically make it draw zero power, and still the piper will be paid.

Because when it comes time to transmit that data, then, by definition, you will expend power. That power is required to send your message from you over the air to wherever. That doesn’t happen for free. And it’s typically the most power-hungry part of a well-defined, optimized wireless module. There may be ways to get that transmission power down (like through envelope tracking), but even if you make it 100% efficient, that simply means that the only power used is the power of the signal. Which means you’ll still need to pony up that power.

Or does it?

There’s a new technique being investigated that challenges that notion. Although… there is no completely free lunch. A signal involves energy. Period. It’s just a question of where that energy comes from; who pays for the lunch? What if you could pull energy that someone else already paid for out of thin air?

The amazing thing about this story is that it’s surprisingly simple and intuitive. When it comes to communications and protocols, especially with cell phones, things are enormously complicated, and the obscure math and countless layers ensure that only the anointed really understand what’s going on. So for something in its infancy to be dead simple was a delightful surprise to me.

That said, I was a bit concerned about my ability to write about it. So much research these days is locked behind paywalls, but some universities have free access policies, so walled-off papers are often available from the university that did the research. That’s the case for this story, but there’s a giant warning on the paper making me wonder if I need permission to be in the same room as a monitor displaying the paper’s contents. Even “abstracting” is protected, they said. I was frankly wondering, based on the language, whether posting a link required permission. (I never did get a response from them clarifying that…)

But, with a little more thought, I realized that copyrights protect written words, not ideas. So if I use all my own words (which is what we do in these pages) and do my own drawings, then I’m not violating anyone else’s copyright, and I can transmit the ideas at a level that will wet your whistles; you can dig further if you wish. So here goes.

There’s a scheme in common use called “backscatter.” It’s employed by RFID readers: they transmit a signal to an RFID tag and the tag responds by reflecting that signal in a modulated way – scattering it back to the reader. This type of system involves the reader as a master, interrogating individual tags and determining which signals to listen to. And it means that the tag doesn’t need to muster enough power to generate a new radio signal for the response.

The idea we’re going to talk about today takes that concept one step further. It’s been developed, with a proof of concept in place, by researchers at the University of Washington (aka You Dub). In huge swaths of the world, the air is full of radio signals. Whether it’s radio or TV or cell phones or ham or whatever, we are awash in a sea of RF. Now… it takes energy to send those signals, so those signals transmit not only information, but energy as well.

The thing about broadcast is that it’s indiscriminate. It sends a signal absolutely everywhere, and, frankly, most of that signal is wasted, dissipating off into a forest or hillside or out into space. Very narrow slivers end up at a terminal somewhere – a TV or phone or radio.

We’ve looked at wireless power and microwave harvesting, but those tend to involve only the transmission and extraction of energy, not information, from a signal. RFID backscatter per se doesn’t harvest energy (although it can be combined with an energy harvester to power the control circuits); it merely uses existing signals, as sent by a reader, as the basis of the return signal. Taking that one step further and removing the reader from the picture, what if we could repurpose all those signals that we’re swimming in?

Let’s take a tropical detour to Gilligan’s island. Let’s say that an airplane flies over, and they’re desperate to communicate with the plane. Now, of course, the professor could just jimmy the radio, but he doesn’t seem to be around. (Funny… Marianne is missing too… O_o). So what would those guys do that’s super fast (faster than lighting a fire) to let the plane know that there’s someone down there? Simple: take a mirror and flash it at the plane.

They can send this message with no added signal energy: all they’re doing is bouncing the sunlight that’s already there in a way that can be interpreted by a human. So… what if you could play that same trick with the ambient RF signals? This is what’s been [you] dubbed “ambient backscatter.” It’s like RFID backscatter, except that you’re using the signals all around us rather than generating a new signal with a reader.

You can, with an antenna, reflect or absorb the signals that impinge on the antenna. In many cases, these signals are already arriving from all directions, since the original broadcast waves interact with lots of things and bounce around on their way to your TV (we’ll focus on TV as an example, since it’s what the researchers have used). So your TV, which has been built to deal with these multipath signals, may see a slight change in the amount of signal chaos that it has to filter through. The perturbance you create could just have well come from you walking through the room.

But by alternately reflecting and absorbing the signal, you can modulate your reflected wave to transmit your own signal. Like Gilligan doing Morse code with the mirror. You could then build something that specifically listens for this change.

“But wait,” you object, “those RF waves are already carrying signals. Heck, Gilligan’s Island could be playing on there at this very moment!” Yes they are, and it could. So you take MHz signals and turn them into, say, kHz data. The original signal is just so much noise riding on your slower signal. A simple low pass filter is all that’s needed to whisk everyone off the island, or at least off your signal.

And “simple” is the key here: you’ve got to be able to perform this trick using as little power as possible, or else you completely defeat the purpose. It does take circuitry and some nominal power to do this, but you want to keep that power low, and, critically, you’re not transmitting any new power.

Most RF receivers move quickly into the digital domain to process received signals; that takes too much power for this application. So the researchers went with analog instead. And they had two problems to solve. The first was eliminating the TV signal; the second was discriminating 1s from 0s in the code.

The first was done with a simple RC low-pass filter, averaging out the MHz. The second was a slightly trickier concept: because of power variations in the original signal, there’s no fixed level to use as a reference for “high” and “low.” One particular “low” may actually be higher than a “high” at another time. It all depends on the constructive or destructive interference of the original TV signals, which changes over time.

So what they did was to take the filtered signal and average it again. The first averaging eliminated the TV signal, isolating the lower-speed signal; the second averaging averaged the lower-speed signal. Now you can compare the actual low-speed signal to the average to decide whether it’s above or below and then use that to determine “high” and “low.” As the power level moves around, the average will as well, so it all tracks.

In theory, the two averaging circuits are the same but have different resistor values. They were able to combine them as shown below into a more efficient small circuit.


Modulation is also really simple. They used a dipole antenna and shorted the two sides together to reflect rather than absorb the signal.


The coding scheme they used is called FM0; transitions rather than levels set the values and bit boundaries. A simple square wave is a series of 1s; an inversion mid-bit gives a 0. The code is naturally balanced in that the number of highs and lows will always even out.


Of course, constantly checking for a signal will use power, so instead they use a lower-power circuit to check for a bit change. If one is detected, then the rest of the circuit wakes up. They’ve defined a simple communication frame; it includes a series of transitions at the beginning as a pre-preamble. This gives the receiver time to notice that something’s coming and wake up in time to get the preamble.

The other thing they had to deal with was multiple access: again, unlike RFID, there’s no master reader managing all the tag drudges. This is a wireless democracy, with different nodes communicating at will with each other or with a hub. So different modules might try to talk at the same time.

To manage this, they look at the average transition density in local signals when they want to transmit. This is 1-(#highs – #lows)/(#highs + #lows). (Note that these are the FM0 transitions, which balance, not the actual data, which could be anything.) If someone else is already transmitting, they you should see that the number of highs and lows is roughly equal, so the calculation should approach 1. If there is no signal, then you won’t get that nice balance, and, due to the averaging and comparator, you’ll see mostly highs or lows, and the formula will approach 0. In which case you are free to transmit.

See how simple this system is? No complex frequency-hopping math. Of course, there’s a tradeoff: you can’t send data as fast as, say, cell phones can. By a wide margin. And your range is much more limited. They were able to transmit at 1 kHz up to 2.5’ outdoors (where the signal is stronger) and 1.5’ indoors. Yeah, not that far. But they’re careful to note that this is a first pass proof of concept; much more optimization is possible.

Example applications are for use as a bus pass, where you can transfer funds from one card to another, and for grocery store tagging, where an item flashes if it’s on the wrong shelf. Both of which can be done with close proximity.

They didn’t notice any TV interference unless they got closer than 7.2” from the antenna. Crucially, because they’re not creating a signal, just bouncing one that’s already there, they don’t need to go through transmitter approval from regulatory bodies like the FCC.

They built a small module that included a flashing low-power LED and a touch sensor as an interface. In addition to using reflected signals, they also harvested the power from the RF signals. They were able to operate battery-free up to 6.5 miles from the transmitting tower.

Of course, not everyone lives right by a tower, and rural folks may find much less energy in the air. (Which may be why they moved out there.) So this won’t work everywhere. But, at least as a proof of concept, it’s pretty cool. And it kind of feels like you’re getting away without paying the piper.


More info:

Google “University of Washington ambient backscatter” and you should find a PDF link.

18 thoughts on “Communications Out of Thin Air”

  1. Great concept! Thanks for writing it up. Your observation about radio signals being disturbed by someone walking across a room reminds me of thoughts I had a while back about using ambient radio fields as a basis for sensing presence of a person or people in a room.

  2. I grant that there is plenty of “free energy” in RF, the sun, power lines, coal, oil, wood, nuke, and other stored energy sources, but let’s be precise about energy costs to use it (access it, harvest it, transport it, process it, distribute it, re-sell it, and clean up after it).

    As engineers our goals are to find the right cost efficient trade-offs … where a dime sized solar panel may harvest more energy than a several meter RF collector. Sure there are places where there isn’t any direct solar for weeks, but they are also places where there are not 100KW commercial transmitters either.

    The suggestion “They used a dipole antenna and shorted the two sides together to reflect rather than absorb the signal.” probably isn’t the correct engineering analysis, in that reflection is a significantly different physical action than the more likely reradiation of absorbed energy that occurs at specific resonate frequencies for an antenna (and supporting structures).

    The math for additional RF free space path loss is pretty straight forward for anything that is a reradiator, passive or otherwise. At 6dBm per doubling in distance, requires the antenna area to grow by a factor of 4 on each doubling of distance. This just isn’t practical scaling to increase range past a few meters, as it starts to get difficult to compensate for the directionality (both to the transmitter being harvested, and to the receiver being targeted) built into antennas longer than a wave length or two. Add to that multipath plus dense building and folage attenuation, and reliability is a strong problem, where it might only work in the additive parts of the interference grid.

    After a few hundred meters from a commercial high power transmitter, the painted RF energy per square meter is insignificant as compared with solar radiation.

    This topic is however interesting …. but as engineers we are responsible NOT to become the next generation of uneducated free power advocates, simply by failing to do the math and accurately present the total system level power levels (both in and out) and responsible alternatives.

    In your example of using the sun “no added signal energy: all they’re doing is bouncing the sunlight that’s already there in a way that can be interpreted by a human.” That is simply false, as it requires energy to modulate the position of the mirror, energy lost to reflect it with the mirror, and energy lost in the path to the plane. As engineers we look at the entire system … was the plane position such that the energy reflected by the mirror, was also masked by similar light reflecting off the water behind/beside the mirror. As engineers the system is important … including signal to noise ratios and the margins needed to recognize the signal information. Do we use a silvered mirror? A black mirror? Clear or colored glass for a particular pass sprectrum? Is 1mm, 10mm, 10cm, 10m square large enough? Is the mirror significantly more expensive than the energy being modulated over the life of the application?

    Let’s also be precise about copyrights, your statement “But, with a little more thought, I realized that copyrights protect written words, not ideas. So if I use all my own words” is also false. Using all your own words, rewriting a novel, changing the names, places, and dates, but following all the same ideas embedded in the story line, does violate the copyright. So copyright protects the ideas behind the story’s creation, not just the precise words used, in a very particular way … which may include derivative works. As an author, that is something you really need to understand, and respect, because at the bottom of this page, EEJournal clearly makes similar strong claims “All material on this site copyright © 2003 – 2014 techfocus media, inc. All rights reserved. EEJournal”

    Again, all energy starts out free … it’s the total costs to use it, and clean up after it, that matter in the final cost/benefit competitive analysis.

  3. Just to clarify something that comes up frequently in your comments, I never intended to assert that this was specifically energy-free. The point is that the system doesn’t have to generate the radiated signal – it bounces the existing one. It, of course, still needs energy to do the control. That may be harvested or not, but that’s a separate consideration.

  4. Having addressed the one issue above right after reading your comments, I wanted to come back to address the copyright question, after giving it some more thought. Yes, gray areas exist, as you suggest. This piece, and our approach in general at EE Journal, however, come nowhere near that gray zone.

    My comments regarding the copyright issue were not about trying to be clever and pushing the gray zone. They were more about what seemed like overreach in terms of what could be protected, in a time where various groups try to protect information simply so that they can charge for it. (Note that the copyright notice isn’t on behalf of the author or researcher – it’s the “publishing” organization.)

    This is particularly poignant in a time when much of the engineering press has been reduced to “articles” created by doing some minor edits on press releases. Major chunks of such articles are verbatim from the press release. No one complains because it benefits the press release writer – they get better control of the message.

    There are a very few of us who still focus on original, independent content. But that’s EE Journal’s specific thing. This and every other article or blog post you see here has been freshly minted using our own words and intermixing the facts of the topic with our own ideas, and doing so with a particular style that we hope (and feedback tells us) makes the information more interesting to read. No one would ever confuse this piece and the original paper as being from the same source; they just deal with the same topic and the same facts.

    So I didn’t want to leave that question dangling. We don’t trample the writings of others. Even when I publish a graphic from someone else, I get explicit permission first. The “Courtesy of” attribution means that I really did check. We value copyright protections, both for us and for the sources of information. But I will also squawk occasionally when I think someone’s overreaching.

  5. I didn’t object to what you wrote about the technical subject matter, what I objected to was a less than precise statement about may or may not be protected by copyrights, as you rewrite, condense, summarize, or extract from a copyrighted work. You spent two paragraphs on copyright that didn’t really belong in the article, unless you were prepared to write accurately about the fine points of copyright law, which you punted on.

    A lot of young engineers, and even some older ones have never explored what is actually protected in copyright law. The most important parts to learn are the concepts behind what is, and isn’t “fair use” or “derivative works”, where the “ideas” that form the work may actually be protected.

    Where the litigation starts is defining what is and isn’t a derivative work. Especially derivative works for a profit …. and selling advertising to pay salaries and investors, is for profit.

    From the US copyright office ….

    A “derivative work” is a work based upon one or more preexisting works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications, which, as a whole, represent an original work of authorship, is a “derivative work”.

    § 106 . Exclusive rights in copyrighted works38

    Subject to sections 107 through 122, the owner of copyright under this title has the exclusive rights to do and to authorize any of the following:

    (1) to reproduce the copyrighted work in copies or phonorecords;

    (2) to prepare derivative works based upon the copyrighted work;

    (3) to distribute copies or phonorecords of the copyrighted work to the public by sale or other transfer of ownership, or by rental, lease, or lending;

    (4) in the case of literary, musical, dramatic, and choreographic works, pantomimes, and motion pictures and other audiovisual works, to perform the copyrighted work publicly;

    (5) in the case of literary, musical, dramatic, and choreographic works, pantomimes, and pictorial, graphic, or sculptural works, including the individual images of a motion picture or other audiovisual work, to display the copyrighted work publicly; and

    (6) in the case of sound recordings, to perform the copyrighted work publicly by means of a digital audio transmission.

  6. I recently looked at a research paper which I believe to be the same tech. It only worked near field, the application being something like payment transfer between phones. My understanding was it used some kind of binary field nullification oscillation.

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