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Ambient Backscatter: Wireless Communication Out of Thin Air [pdf] (washington.edu)
120 points by interconnector 1290 days ago | hide | past | web | 42 comments | favorite

Whoa -- We present the design of a communication system that enables two devices to communicate using ambient RF as the only source of power. Our approach leverages existing TV and cellular transmissions to eliminate the need for wires and batteries, thus enabling ubiquitous communication where devices can communicate among themselves at unprecedented scales and in locations that were previously inaccessible.

p.s. just pasting a couple lines of text from the article, for those who are wondering what this is after looking at the comments, my typical "use case".

Whoa -- not to be a buzzkill but there is some massive overhype on this. Not to even begin to mention what happens when the FCC gets involved. Spectrum is bought and owned, you know. And zero unnatural interference is tolerated. Sure, you can argue it won't interfere with any towers but lawyers will argue some other points I'm sure.

Fortunately the authors have addressed this in the paper:

"Legality: In general, it is illegal to broadcast random signals on spectrum reserved for TV (or cellular) channels. However, battery-free backscattering devices (e.g. RFID tags) are unregulated and not tested by FCC because the emission levels from such devices is very low [7] and because they are only modulating their reflection of a pre-existing signal rather than actively emitting a signal in re-served spectrum. Ambient backscatter also falls into this category, and would therefore be legal under current policies."

And the tolerance levels are not set at _zero_, but at a specific decibel level on a specific measuring setup. (I've been researching this professionally lately, especially ETSI EN 389-401). Pretty much all high-speed digital electronics in plastic cases will be radiating on the TV band, but at an extremely low level that passes the regulations.

This system modulates by switching its antenna between grounded and ungrounded. At no point does it inject transmission power into the antenna. Your house is already full of small metal objects that are slightly resonant in the TV band - such as cutlery. You could probably replicate this experiment by attaching a switchable ground to a fork and holding it _right next_ to a TV antenna. Thanks to the inverse square law the effect is so weak that it really does have to be right next to the antenna; in the paper they report no detectable effects at a distance of more than a few inches.

Agreed, StandardFuture. This is impressive, but TV channels and signals are owned by TV broadcasters. So:

* Not only the FCC, but the NAB [National Association of Broadcasters] will be on this pronto.

* So, consider either:

___o Allowing the FCC to retain a channel for a few years of ambient backscatter experimentation and evaluation (funding a set of transmitters for a while, experimenting with what transmitter signal structure might allow wider ambient backscatter bandwidth and node-to-node distances), then offer it as a tax-supported service ( as with GPS ), or sell it, else

___o Having the FCC allow an interested private owner of TV channels privately experiment with ambient backscatter engineering as proposed (locally, multipoint-piggybacked over 8VSB TV), develop the technology, then license it.

* In no way am I intending to cast aspersions on the authors' achievements here; instead I am very, very impressed! It shows indefinitely durable (battery-less) RF networking having practical applications drawing only half a microwatt for 'transmission,' carrying a general purpose programmable microcontroller on board, using a passive method to modulate an ambient RF signal with a man-made signal below the by the intended (TV) signal. And using analog components where they give significant power advantages, while retaining programmability sufficient to support a networking stack. Wow.

Disclosure: I work in support of the Broadcast TV Industry

Edits: (1) Tax-supported is not 'free' (2) Clarify that ambient backscatter is multipoint, 12 orders-of-magnitude lower power, and distributed, in contrast to the single point of origin of the 8VSB TV transmission

Reading the paper, what do you think is 'massive overhype'? They demonstrate working devices on the 1-2 foot scale.

The regulatory question is certainly an interesting one. Is it 'interference' to reflect very modest amounts of an existing signal? Multipath is already a very real phenomenon; if these devices do not appreciably affect transmission, then are they truly interfering?

Put another way, are the regulations written from the point of view of effects (degraded transmission for the spectrum licensee), or in absolute terms (no communication at a given frequency?)

I would say that the regulatory question is the only question left. It's obvious that this works; but how well will it 'cooperate' in an environment?

>Reading the paper, what do you think is 'massive overhype'? They demonstrate working devices on the 1-2 foot scale.

To be a more effective network device the one thing you will have to do is (at least) try to increase this signal efficiency. That spells future regulatory problems.

>Multipath is already a very real phenomenon.

Again, interference management is already handled within devices. But now you want to add more interference into the environment? See what I mean by: good luck with that in a court.

>if these devices do not appreciably affect transmission, then are they truly interfering?

Yes, they are still interfering. And the problem only grows as you scale this "new" network.


And to answer the final question: Again, you would have to see what lawyers will try to argue.

Interestingly, the authors claim in the paper that batteryless backscattering devices are not regulated by the FCC, citing FCC regs.

"Legality: In general, it is illegal to broadcast random signals on spectrum reserved for TV (or cellular) channels. However, battery free backscattering devices (e.g. RFID tags) are unregulated and not tested by FCC because the emission levels from such devices [7] ... Ambient backscatter also falls into this category, and would therefore be legal under current policies.

[7] New policies for part 15 devices, FCC, TCBC workshop, 2005."

Well, I hope you'd agree that it's OK to leave the lawyers out of the discussion at this very early stage. (And technical workarounds might well exist: e.g. a TV channel's worth of spectrum in a major metro might be given over just to providing remote power to devices.)

A little relevant data from the paper: They do a somewhat crude experiment in the paper looking for corruption of TV signal, and find no effects except when the TV antenna is within a few inches of the device.

Which of course shows that these devices do indeed interfere with the normal reception of the signals they are parasitizing.

The legal statement about the FCC clause is there to try to cover their asses on the research that went into this and to try to market the possibility of a wide adoption of these types of devices. I'm guessing they did not bother testing inside a screened room then if they believe that? Maybe? Maybe not?

EDIT: I am guessing not. I imagine they used real-world backscatter to test this out.

The basic thought here is that there's a ton of energy in our environment, specifically in the form of high-amplitude RF waves from TV broadcasts, and we should be able to use that energy to do work.

This project is trying to figure out what the equivalent of an rfid tag looks like with that power model. They come up with a scheme of being able to absorb versus reflect the ambient signal to communicate between unpowered tags. Maybe the most productive way to think about this is as a step into the larger research area of how to effectively harvest energy and do useful stuff with devices that don't need batteries.

When I was at university, there was a final year undergraduate project that used similar technology. In this case, the student was trying to build a passive radar jammer, which modulated the reflected radar signal in a way that the driver of a car could select what speed they wanted to appear on a traffic radar.

The method was to build a corner reflector out of three orthogonal conducting sheets, but to connect one of the sheets to ground via a PIN diode switch. By turning the diode on and off, the reflection coefficient of the corner reflector could be modulated, in turn modulating the reflected signal.

Do you have a link? That is fascinating! Did it end up working?

I don't have a link. It got written up as an undergraduate thesis in about 1991, but as far as I know the University of Sydney doesn't keep copies of such documents.

I'm not sure what the outcome was, though it must have gotten to a certain point of success, since I've memories of a car driving around with a corner reflector on it.

The same student was experimenting with using a plasma as the modulated reflector, but that one ended when he connected the wrong end of the vacuum pump to his gear, accidentally pressurised it, and blew it up!

There isn't much useful "work" one can do with a microwatt at "human" scale in a day. Except, as this paper shows, build a networkable wireless system camping on those microwatts.

Not meaning to be critical, rather just want to clarify that our modern-day gigawatt-scale energy grid yields an enormous 15 orders of magnitude more electrical power than the microwatt-scale "harvesting" of TV signals can. That's an enormous power spectrum, shown to have a newly useful application (batteryless mesh networking) at microwatt scale.

Too bad Nikolai Tesla's not around to help us figure out how to harness that energy.

His coil works in reverse too.


link please?

So it's not really some new uber-complex 'recycled communications network' as much as it is purely "Radio Solar Powered" ...

Pretty cool, although, not entirely a new concept:

http://en.wikipedia.org/wiki/Crystal_radio (no battery needed for wireless reception)

Using that concept to also produce transmissions is the kicker here.

Now, "the walls have ears" takes on an entirely new meaning in "The Internet of Things"...

Tesla was interested in this, eh? I did this sort of thing in 1967 when I put a capacitor on the output of my crystal set, and was able to (very weakly) light a standard panel lamp. Nothing is new, only the details change.....

Unwarranted grumpiness. Harvesting power from radio transmisssion is indeed not new.

What they demonstrate, however, is transmission from the 'receiver' to additional devices by modulating how much power is absorbed or reflected.

This is fundamentally different from just using the harvested power to run a second transmitter, because modulating backscatter is 'orders of magnitude' more efficient.

Homemade DIY guys playing with the idea.


Yes, this is being very over hyped, RF backscatter is the same basic mechanism that RFID uses, there are a ton of good tutorials online. There are a number of ways to increase the range of backscatter systems like RFID such as spread spectrum. One of the other problems is multiple access, how can you tell if back scatter is coming from source 1 or 2 or 3, that has also been worked out in RFID using a coding scheme (eg. PN codes). Basically these guys rediscovered RFID technology and got kind of excited. This is not to say that there are not interesting applications, two passive devices can communicate between each other given a strong enough source signal. But the applications are limited, usually, if you have 2 devices that need to communicate between each other, you want to do something with the information being exchanged, maybe light an LED or something, that would take way more energy than you can harvest from ambient RF. Once you realize you need a battery, then it is just easier to put a simple 802.11 or 802.15 transceiver in.

This is a great step towards the internet of things. The main roadblock for attaching sensors to everything right now is power. Even with month or even year-long battery life the maintenance burden and cost makes it much less attractive.

If you could just stick a rice-sized sensor wherever you want and not worry about battery life we'd all have 'smart houses' by now.

This could not be used for 'smart houses' .. willscott had it right when he mentioned RFID stuff. You cannot have 'ultra-low power' and have a transmitter powerful enough on a WLAN.

I'm not sure what you mean. This thing is not a transmitter, it piggybacks on existing waves by modulating their reflections - that's the whole point of the paper. It won't have to compete with a WLAN.

Presumably range can be increased, specially if the principle is adapted to use that WLAN's 2.4ghz stronger signal instead of VHF.

The key feature of this approach that many other commenters seem to be missing:

They are not harvesting power from ambient RF and using it to power a second transmitter. Instead, they are using that harvested power to communicate with other nodes by modulating their device's reflection of ambient RF, a process that they claim is ~100x more power efficient.

Not an RF engineer, can't judge novelty, but this seems really, really clever.

EDIT: added text in italics above to make my post clearer.

Sorry, but that is wrong. Figure 3 explains everything.

EDIT: I should say that yes the technique is using analog conditioning circuitry to get the job done without storing power in a battery.

There is indeed a block in that diagram labeled 'Transmitter', but it is not a transmitter in the colloquial sense of converting onboard power into RF energy. Instead, the 'transmitter' modulates the amount of ambient RF energy that is backscattered, by modulating the antenna impedance.

This appears to be the major innovation in the paper. As lots of commenters have pointed out, harvesting power from ambient RF is nothing new (whether or not a battery is involved).

(Section 3.2): "By modulating the electrical impedance at the port of the antenna one can modulate the amount of incident RF energy that is scattered, hence enabling information to be transmitted."

(Section 8a): "backscatter communication is two orders of magnitude more power-ef´Čücient than state-of-the-art radio communication")

Yeah, it's a cool technique. But it still has no room to scale as far as I can tell. At least not for transmitting inside licensed frequencies ... maybe they should try it out on 2.4 GHz or 3.5 GHz??

If you do not want ugly borders around links in latex:


Much of this is over my head, but 1kb/s across 2.5 feet seems like a very small step.

Is the significant value in this it's ability to function at low power?

I think it's significant because it's a huge increase over '0'.

I think so, yes. No battery means smaller, cheaper devices that can all interconnect which opens up all sorts of possibilities for making everyday objects 'smart'. Your router could tell you where your keys are. Your fridge could tell you when you are out of milk.

On a distributed level, there's a lot of other cool stuff that could be done as well.

1kb/s is an absolute surplus in a lot of machine-to-machine applications. With a lot of embedded sensors and such that run over cellular networks, carriage is billed by the kilobyte, to illustrate that point.

1kb/s is fine, that's enough to tweet. 2.5 feet is not very far but I suspect that's easy to improve, and it would not need much improvement to become really useful - eg if you improve it by a factor of 3 it's good enough to function in a typical corridor, and so on.

1kb/s isn't much compared to what we're used to, but I can see it being sufficient for some communications.

One thing stands out, how do they address noise? Is Backscatter somehow different from an unintentional noise or interference? If this is all low power it would mean the impact of noise or interference will be high and it will only multiply with the number of devices using it nearby. Is there a risk of it becoming something similar to tuning to an AM radio station while traveling in a remote area?

They need to talk to these guys.


Ultra-cheap mesh network, anyone?

This changes everything.

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