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Holography of Wi-Fi radiation

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for the rest of today we're going to visualize electromagnetic radiation in the range of the Wi-Fi frequencies we have a Friedman rain not from the Munich Technical Institute he has a and the z junior research project and we 1 of these 2 then he pursue this project is to is physical and the figures here facts we will pH sold good morning and thanks for having me here on this Congress which I enjoyed very much so far so in the talk that I would like to give you would like to look at wireless signals from a very different perspective when we think of wireless signals we usually think of data packets messages runtimes uh protocols and so on but we can look at them from a very different point of fuel from the point of view that it's actually just liked
this is kind of a bold statement but it's actually a fairly exact statements so from a physicist perspective a wireless signal is virtually the same as the beam of a laser the light coming from the Sun it's electromagnetic radiation which is described by Maxwell's equations and so on so it's an oscillating electric and magnetic field the only difference so visible light that we can see with our eyes is that the wavelength of the signals is much longer so while the visible light that makes up the visible world has a wavelength on the range in the range of a micrometre the wireless signals that we typically employ in our devices have uh wavelength in the range of few centimeters to few tens of centimeters and as a 2nd very important difference we cannot see them so and but from a physicist perspective this is very interesting because uh that has a somewhat surprising implication it has the implication that our modern world is actually brightly lit by all the wireless devices that we use every day so this whole Rome as we sit here and it's actually glowing in various colors on only we can't see it because it's going somewhere in the microwave radio domain and this also implies that the there is kind of a potential big security leak associated with these wireless devices and in the sense that any device that you use no matter how well you encrypt your data is going to transmit the full three-dimensional picture of the world's 2 it your environment no matter what you do and Saul when we realize that we got intrigue and we wondered whether there would be a way to actually we caught that radiation and to review that picture to you in a way to build a camera that would be sensitive to this kind of radiation and that would enable us to see the world in the way that we would see it if our eyes were sensitive it in this frequency domain and so to cut a long story short we
managed to do that and the this is the result that we got this is a picture of our land and the colorful thing that you see is wireless radiation coming from a rotor on the back side of the room and that was a lies with our method and we can actually see it propagate in space and uh we can even see that objects that replacing in the lab cast a shadow and that the and in what follows I would like to walk you through how we did that and what it might be used for for and um and I also like to discuss whether it's actually isn't there is serious security concern and but before I do that I would like to start from a somewhat of a broad overview and tell you a bit more about what has happened in the past few years and in this field so it turns out that packing wireless signals in the sense that thinking of in the sense of thinking about other ways other useful data that we can extract from that the signals that are only present in our modern world it has become quite the vibrant area of research in the past 5 years and we haven't been the only ones that I realized that this is actually a very interesting thing to look at so there have been many other labs that have pursued many different directions still it look at my variation from a different point of view and maybe
the most famous result this this this is the result from a group at MIT and the Boston Massachusetts the and they have them actually pack Wi-Fi wrote in the way that they exploits the fact that modern rotors of have an array of antennas and they use that to actually send beings in different directions and took focus the beams on the devices that they want to interact with the sole modern rotors they can actually sent to different signals to the cell phones on the right and on the left end of the room and you can pick that uh you can use that to scan beam in 2 dimensions and look at the reflections so in a way to builds a phased array radar if you wish and this is the kind of picture that you get with that so you can scan a beam through the wrong you can look at the reflections and you get a fairly detailed picture of the world you can even vaguely resolve the human beings you can
also just pursue a similar approach you can only map attenuation so if you take the Wi-Fi Ruta and the wife a receiver and you know that the attenuation of the signal between the 2 of them and you can then mount them on the movable platform uh as a fancy example for instance of drawn so you can fly these tool the emitter and receiver drones around the building not the attenuation on every line of sight that you know covered during that flight do tomography on that data and so and by doing that you can actually extract three-dimensional views of what has been in between the strong the the so all of there has been a lot of activity in that field but still this
previous work did not yet fully and it well answer this question that we you got interested in like what would the world look with Wi-Fi highs and the if you look at that question the state of the art that have been pretty much that we had something like the compound I often insect there have been many ways to tamper with wireless radiations in the in the sense that we the look at different directions either by directing a beam into some direction or by looking at attenuation in a certain direction and you do that for multiple angles to get a coarse picture of the work and so we got interested in whether we could actually get a full three-dimensional view of the 4 front and of all the light propagating in space pretty much in the same way as we can do it with visible light soul and we thought about that and it turned out that you can do it and you can do it
and is surprisingly simple way so there is a technique that essentially solves this problem and it's holography so pornography although GM is something that you probably all have seen it's these amazing pictures that appear three-dimensional if you look at them and that you can tilt and you can actually look at objects from different angles they do not look like a photograph the uh look much more like a window into a virtual worlds that has been frozen by taking this photograph and you can make them as actually in a fairly simple way you so you don't even need lens all you need to build to report such a hologram is illuminate your object with what we call a coherent light I get to that in a 2nd and recorded on a phase-sensitive camera so all these are very technical terms from let me try to explain that in a more visual
way so in a way a hologram is the photograph to 0 uh it captures more information than the two-dimensional photograph which we usually take on it which where information is restricted in the sense that the usual photograph only captures intensity of light soul it can record in a two-dimensional plane where there was a much light and where there was very little light and the resulting folder will be very bright and very dark but it's going to be restricted to a two-dimensional plane it will appear as a flat piece of paper if you look at it hologram can do more so all hollow-gram is a phase-coherent recording and that's in a visual way means that it not only recourse brightness of the light it also recalls the direction of the light beams hitting this hologram during exposure so in a hologram in a way you freeze all the light rays that enter the specific plane and on if you develop that
hollow-gram you in a way revise all these light beams again and so when we look at the hologram we really look at the very same light rays at the very same light field that came from the object during exposure and that in particular implies that we can look at it from various angles and we can and we have a three-dimensional view of it the so how is that's done so uh this is done by a phase-coherent recording and and you that is this this technical term essentially captures this ideal of recording direction along along with intensity and and to see how that works will I have to know what get back to the point that and light actually is electromagnetic waves and waves pretty much like waves on the lake have valleys and hills so areas where electric field is very strong and areas where electric feud is very weak and these valleys and peaks propagate in space the so if you throw a stone into a lake and you watch the resulting waves you will notice that these waves always move perpendicular to both the peaks and valleys so the direction of the wave is in no way encoded In the difference between this peak and values so if you can take a photograph where you also register whether the PKN 1st of the valley came 1st you also register directory and and that's uh what it takes so it takes a light source where these peaks and valleys out very well defined this is what we call a coherent light source and it takes a face-sensitive camera a camera that can record whether the peaks of the values came 1st no it turns out that this is a fairly difficult thing to do in the optical domain some people managed to do it but it was such a big breakthrough that it actually got awarded the nobel prize for 40 years ago fortunately it's actually much simpler in the radio frequency domain because the
frequency here is much lower than for visible light and so we can actually recorded every peak and every value simply by registering the wave with the good or service calls called the and that in particular means that a whole lot with me it should be possible to do holography off this wireless radiation so to come up with an experimental setup where we take a picture in some two-dimensional plane of space we were just there brightness and phase of the in microwave light at every point in space and we can then later on and use that to reconstruct the virtual view of the worked and this is pretty much uh how we did it so this is our experimental setup we mean explicitly wanted to capture light from an arbitrary devices sold the light sources that we used always were conventional off-the-shelf Wi-Fi that we asked to download a big video from you due to generate a lot of traffic and data that we could image but we make no assumption whatsoever on the signal that they actually emit it could be anything it could even be encrypted we um sometimes play some objects in the beam to get a more interesting picture and we finally we this be more of microwave light In a scanning approach approach so unfortunately it's very high very expensive to to build microwave cameras you would need a huge area of antenna soul In the 1st step it's easier to just take 1 antenna and scattered across every pixel that you want to image this is what we did so we have an antenna that we mounted on a scanning platform and we reference that signal to them the signal of a stationary antenna this is what it looked
like in real life so it's uh you see it's not very fancy it's actually that's probably the cheapest experiment I've ever been involved in and a solid we we used plywood and fish at a snake and tools like that to do it and they essentially to solve this problem of scanning the antenna through our and but at some point maybe were quite well and so we could recall these pictures so the signal that we
get from this set up is the wave the wave that we can record with an oscilloscope and that we actually report twice once coming from the scanning antenna and this is what is going to make up the image data later on and once from this reference antenna so we make no assumption whatsoever on the signal that is transmitted so we cannot rely on knowing what kind of bits and bytes are transmitted to but it turns out that if you Fourier transform the signals sent to divide them by each other you can in a way we normalize that signal to that reference and get the virtual 2nd wave where the and that pattern actually cancels and you only recall or at the the phase delay and the attenuation of all of that wave as it travels to the scanning antenna sold for every pixel we recall a set of data like on the upper left we do some processing to remove the the bit pattern and only get the coherent and then we end up with something on there like on there some law part of this plot so we do that for every pixel for every pixel we get the brightness the amplitude and the phase the direction of being uh we actually get that for while a wide range of frequencies because a Wi-Fi is a the frequency schemes so you can have different channels and we end up with a dataset like the so this is a hologram it's unfortunately not something that you would easily be able to go makes sense off just by looking at it and so all the we we was since we don't have eyes for this radiation we cannot see the radiation by itself and even if recorded like that the information is not very meaningful the but fortunately there exist a number of reconstruction algorithms so this problem has been solved incoherent optics where people actually can render three-dimensional views of uh what a hologram would look like simply by knowing the pattern that you produce on a photographic plates so by scanning a photographic plate and uh feeling that in the right algorithm you can get a three-dimensional view of the whole program without ever looking at it in real light and this you can also do with some microarray Freddy H soul from that hollow-gram we can
reconstruct the a three-dimensional views but we did that and this is where this is the test set about it in the and work best so we have a commercial life just sitting on the back of the lab it's labeled in better the other picture we play some absorbing objects in the beam that we made from my aluminum and tin foil and then we we recorded hologram in a plane that you cannot see here that is essentially the the front plane of that the image we see the day time to reconstruction algorithm and in that way we can now render three-dimensional view as it would appear if we had eyes for why the radiation and we looked through that plane of the above picture so all we can actually use as as a three-dimensional approach we can actually not only construct this view we can actually focus on different planes and so all this is what we do here so obviously of 1st interesting thing to Bill is to focus back in into the emitter plane uh if you focus into the plane of the wife Arruda you actually is nicely seeing a bright spot at 1 point in the image which is the image of this glowing light bulb in the microwave domain it turns out interestingly that the picture is not very pretty if you just do it like that because if you only do it with 1 frequency with 1 wavelength of light you become very sensitive such as something that is known as speckles so this my grave light bounces back and forth between all the walls in the lab and these waves interfere with each other in a very erratic way so we end up with something that looks like the clouds and and we can interestingly enhance that quite a bit if we repeat the experiment for different frequencies so for different frequency bands within the Wi-Fi channel and we superimpose the images we get a much clearer picture where it is very clear that the root is the brightest spot this is actually something that is very difficult to do with the real life people struggle to make that work and for all microarray flight it's essentially straightforward uh and we can then go
on and focus on different planes so for instance in the plane of this absorbing object in the and if we do that we nicely see that this is so so as we move back and more from the emitter to the object be nicely see the whole the BMX ponds and that added when once we get to the object plane there's a big shadow appearing in uh the light waves nicely here uh you can actually see that if you focus slightly below the study the Law slightly above the plane the image plots so it really is a three-dimensional image it's like focusing with a photo camera you would converge if you focus do focus into the wrong place soul that's essentially the data that we took there is potential for a
lot more data processing so it turns out that there are numerous very powerful imaging schemes that work with coherent visible light so in coherent visible light you can take just plain dumb photographs as we do that most of the time but you can also play clever tricks at to enhance for instance absorbing objects in the light of the scott . future my trust QP usually do it with microscopy you can image polarization of light or you could enhance only weakly 11 refractive objects and all these schemes we could actually emulate by just numerically processing the whole room data so there's a lot of things to be looked at uh like what does the world look like if we enhance absorbing objects or or do chairs have and in the index of refraction of that is different from air and the questions like that so in a way to
summarize that we managed to record pictures and we actually managed to establish a kind of an analogy between wireless communication and coherent optics so I made up so the little dictionary what that's uh a whole whole um that terms translate into each other soul wireless signals but we usually think of as packets of data is actually liked it can have different colors and we could but this is a a degree of freedom that we haven't used yet but we could do it for instance by by by by splitting signals according to their society that they transmit so in a way is once we look at the big pet bit-pattern signals with different bit patterns in a way correspond to different colors and uh so we could have multiple images in in in a picture we could actually tell them apart and we would get a multicolored picture where every motor in our Roman and illuminates the room with a different color and this would be a very interesting thing to do in the future the it turns out that the pictures can be a lot better if you do white light orography if you use more than 1 frequency band this is something that has an analogy in the wireless domain where increasing the bandwidth usually improves performance of radon if you want to learn something about the environment rather than the signal itself usually do time-domain ranging in the classical wireless domain but in optics it's just imaging so you form an image and you look at it a and what used to be the run time delay of the signal but is now facing and direction of light fields so all of that was the the work that we did we were hit very happy that made it work we actually got it accepted in this journal and that was kind of interesting it's a fairly prestigious journal in our field and so when you submit a paper you have to fill a questionnaire on why you believe this paper is worthy of being published there and there are categories like you know it it it it's a big advance of an established technique it's an unexpected breakthrough it's this and that and we decided to opt for category number 4 which is it is of singular appeal to all physicists have sold at the so all of so by that argument we actually made it accepted there and then it was met with a lot of interests to them not only in science but even more so in the uh in the real evolves some of media and of companies and so that's what I would
like to look at in the 2nd part of my talk what can we actually do without with it ever be useful so if you think about that question what could use it for uh there's a very obvious answer and if you don't have it right now all been them you can actually read on Russian
today where they get picked up our story and the headline was that this could be used to match your home so you may wonder whether it again know some people say 0 this is only fake news but I can tell you that it is for this story this is not fake news that's based on on the real thing soul uh it's this idea can we use it to spy on other people are in the in the in a more optimistic of fashion can we use it for security enforcement and security applications and this is something that
many media jumped on so all of these many of the headlines that we got were related to that can we use that can can my neighbors see me in my bathroom when I I have my cellphone lying there and things like that but so it's a question to which we somehow need to respond and it
would that be feasible and at least in principle it looks like that so if you have a Wi-Fi root and you're wrong and you walk around in your body will scatter some reflections and you can pick up these reflections run them through our algorithm and get an image of the world and I'm somewhat skeptical whether this is a really serious use case form because 1st of all remember what it looked
like so it's a huge device and it's always going to remain a huge device because you need to have information on a very large area this is what the whole scheme is based on so even if you mounted that on a drone you still would need to fly from uh around many points uh around a building which would probably not go unnoticed and and maybe even more importantly it's something
that actually is already being done right now soul um for many of these applications you actually don't need imaging imaging is kind of a bonus but it's a complicated and expensive bonus and you can learn a lot about the world just by looking at the signal at 1 point in space and this is something that is going commercial so there are companies that says systems where you essentially plot something like a 2nd rotor into your room uh this rotor is not emitting signals that are used for communication it's actually looking at signals that are scattered from the from the environment it it's on analyzing them in a very detailed way and by doing that you can actually get a fairly good idea of whether there are people moving in the room how many of them are there are in the bathroom or in some kitchen or whatever else so it's something where our scheme probably is too complicated which is good news because it's not a security concerns the bad news if you wish is that Wi-Fi is a very leaky thing and and this general idea that we can use this radiation to spy on other people it's actually a valid security concerns OK if we wanted to to
to do it if you wanted to sell that as a security application I think we would run into a 2nd very important problem and the improbable Bodin problem here is that uh if you will have an application where people actually are willing to buy expensive equipment to get a lot of information then it's going to be some kind of dedicated device and so all of them it's not so much more expensive to actually combine that with the dedicated tailor-made emitter that is emitting a specific signal and once you can do that uh you can play a trick which is called
wideband and this is a very powerful trick that is spoiling the game for many of these applications for us so I'm going to explain explain it in a little bit more so the idea that you might want to use is that if you want to build something like an X-ray forms you amid some signal from a dedicated emitter you detect the reflection in a dedicated detector you measure the run time delay and from that to infer whether there are people moving around and at which distance they so if you go that's the resolution that you get is to the to the bit rate of your signal if you which is essentially the inverse of the bandwidth soul if these data packets are very brought the resolution would drop the and from the point of view of radar Wi-Fi actually is not a very good signal it because it has a very narrow bandwidth so for 2 point 4 gigahertz my physically only 20 megahertz and this corresponds to a resolution of 10 meters and if you build a dedicated to Mr. uh obviously you can drop that restriction you can build an emitter
with a much higher bitrates and this is what people do also you can push that after bit the rate of gigahertz to tens of gigahertz and in nature that is sending some garbage signal that is spread across them all the frequencies up to 10 gigahertz and it by doing that you actually get down to centimeter resolution in radar so if you will invest into a dedicated device this is a very powerful trick to play and this is actually being done so
there is I I did some research on the internet and there are devices for security forces that the look a bit like these devices that you used to search for pipes before bringing into the wall you place them on a wall it takes a few 2nd and it's going to tell you whether there are people in that room that you can't see and of which distance they are so it gives you an image on the display which looks pretty much like the so to sum it
up security applications well are an interesting thing to think about but it's probably not of really realistic use case for this key let's look at something else let's look at civil engineering I already mentioned there of these devices that you used to search for what a positive for making it and these devices well they work but um and I did it myself and it's the it's not a very reliable thing to work with solar and there is room for improvement and uh it's interesting to ask whether we could fill a gap in that market and so all the way that would work
would be that them you rely on the radiation that is emitted from wireless devices in other rooms across the wall that you want to Britain and that will illuminates this small with the microwave light so that pipes and power lines in the world would cast a shadow and then you would we caught a hologram across the wall that you want to drill reconstruct that image and see everything apart but still we have this challenge of technical complexity but I think in this case it would be surmountable because uh we could actually scale up this system from was single-antenna device that scanning the plane to one-dimensional array of antennas like a magic wand that would waive across the wall and that would give you a picture on your smartphone of what's inside so this is a fairly realistic thing thing
unfortunately here tool and and you have to compete with ultra-wideband soul if you build a complicated magic wand it's not so much more expensive to include an ultra-wideband emitter and to do very precise radar and there are companies doing that you may have read about 1 about its kind of a rate of that you can plug on the back of your smart phone and you can uh x-ray your wall and it's going to do some signal processing and give you actually a picture of where the pipe is Saul there would be strong competition from still our scheme would would have some advantages so we could use of radiation that is actually coming from things behind the wall that could be an advantage and and so in principle that could be something to think about so could
work that and there's
1 other thing that we initially thought would be a good idea uh that is tracking emitters inside buildings so this is actually a very important thing that is becoming more and more important as we move towards Internet of Things to actually tell where in the building in three-dimensional space you have some RF tack and our scheme in principle could do that so we could think of installing the two-dimensional huge array of many many antennas in the ceiling office-building doing holography and then see actually the position of each and every smartphone and wife for root and moving around in real time we didn't do the experiment but we did a numerical simulation to see whether this could work and this is what you see here so we built a 3 D model of virtual storage hall with steel bars in the uh Flores and with some steel shelves and 1 of the floor and the image that we get is encouraging soul with that scheme we probably would be able to tracked down emitters with CM care precision at video rates and what is more than we probably would
even be able to get a course information about the the objects in that building so this is a movie of the simulation data and it's it essentially the hologram reconstruction as we focus on 2 successively law planes moving all the way from the top of the building to the ground floor and so this is what we get as we move down and we nicely see these shelves and we nicely see these bars casting shadows in the been before we converge on the emitter which is appearing as a sport with CM can resolution so this could work but obviously it would be a
very expensive thing to do so you would need a large array of antennas M is some probably not not something that you could do right away them but when we talk to companies it turned out that there is a solution that is somewhere in between where it could get really interesting and it's
uh it in a way that reduced implementation of key soul rather than going to the full two-dimensional array of antennas we could actually go to a one-dimensional array like this magic wand and that's still would speed up acquisition by a factor of 100 to poles and compared to ours single-antenna proof of principle implementation so you could have you you could actually take pictures of large-scale attended a scale structures and probably on a minute to our scale if you are scanned this device around the structure by say a drone or a car and once you have that data and you could actually look into the building with Wi-Fi eyes and understand how radiation propagates in there and this could actually be
relevant for this whole field of indoor tracking so uh maybe to be a bit more specific on that this is actually a very important unsolved challenge at the moment so many companies get more and more interested in locating RF tags with centimeter-scale precision to track inventories under or devices and it's a market that is and actually predicted to grow to a billion dollars size in a few years however at
the press and things OK so 1 very strange straightforward application we we could enter here is that we could do site survey for existing solutions so if you will bought some system for indoor tracking and you found that it for some reason it didn't work in your factory hall then you could come with this a one-dimensional array of antennas get this full picture and you could see that there is actually a nasty reflection on this metallic wall and if you replace that by some concrete coating it will make things much better in this kind of thing so this could be actually interesting to to get the full picture only for that reason and but it could also be interesting as an R&D tool to actually understand better what the signals look like and how they propagate to make these schemes better and there's work to do
because at the moment of these schemes are are well very successful but still not fully satisfactory the so people have tried many things you can move around with a camera and scan barcodes you can use RFID is but then you can only read them from a very close distance so it's not very convenient for large-scale factories the you can use all kinds of beacons you can take them labels with ultrasound rule to Wi-Fi whatever and but uh with that you run into this bandwidth issue and you only get meters care resolution of it here again you can play the ultra-wideband trick so you can have dedicated with a very wide frequency band and that will give you centimeter can resolution and this is on this already is it's incredibly successful today but it comes at a large prize and that's the physical prize these chips are expensive soul it the 3rd the tags would cost to something like 10 euros or more and and it's very hungry in terms of power so you need to also that with a battery and you need to replace the battery every few months or years and this is not very convenient so there is a quest opener to make these schemes work with passive text to where we actually there rather than having an active emitter on the label and we just have some absorbing that thing and we illuminated by some other source and we image this had that it costs in the B and at the moment this is not doable and and men you prince in principle could solve the problem by many ways you could by more receivers you could do more signal processing whatever but it's not clear which route would be the most successful and so rather than trying them by trial and error 1 by 1 1 interesting idea could be to actually recall at this entire wavefront look at the picture understand uh Warhol radiation propagates in typical buildings and you what kind of what part of the signal you actually need to see this had best and here it is our scheme I believe could come in we could recall this phone wavefront and we could simulate any kind of reduced scheme like a scheme with only few 10 or looking at only part of the frequency band and with our algorithms so uh something
that up it's an interesting technique I don't think it's a serious security concern but reduced implementations actually might be 1 civil engineering might be an application where we could move into In the tracking probably is too difficult but are indeed for a simple irinotecan schemes could be a very viable way to go with that I'm at the end
I would like to talk to and acknowledge the person that did all the work FIL upon you actually was brave enough to to start this project as a Bachelor thesis when at the time when everyone else was frowning upon the idea and he was very successful in making all this work
sold at thank sold and that's the end of my talk I would like to draw your attention to the fact that you can look at wireless signal from a very different perspective it's just light and you can hack them but by just using the stray radiation that you get and if you do it right you can actually take real pictures this could be useful for maybe drinking maybe civil engineering the probably most as an R&D tool to uh implemented to to and developed the future wireless applications so thank you very much and and mn how we have used in this work here in a thank this and no thank you for your time was really interesting I was wondering if you could build Wi-Fi lenses the and that's it good question so all uh it probably would be difficult invented what is it should be a physical because that so you not only would need to find the right material which you probably could you also would need to make it large total bit because the wavefronts are launched them but it it into again somewhat paradoxically they're actually much simpler schemes to do that so only if you uh if you could take a train somewhere in the country you may see that nowadays these satellite receivers they are no longer like a parabolic mirror those sometimes they tend to be flat like just a flat plate and this is in a way due to a virtual lands for this radiation that frequency band so it's a phased array where you have many receivers and to delay the phase electronically which is essentially what lent stars in the optical domain and then but in doing that you can simulate any kind of lens including a parabolic reflector looking at the antenna the so far our scheme but it's actually even better we don't Nederlands because we can simulate any kind of lands in the computer and this is something that we uh actually thought about doing like trying different that building different kinds of virtual objective lenses so look at different parts of the radiation but I think erm electronic solutions group in probably be the easier way to go than a physical events well needed and what is the prognosis when can we expect a practical camera for different spectral ranges like to be a radio so on and well I mean technically I don't see an obstacle for transferring this technique to other frequency domains like a radio or so the I don't think we will see it in the near future as a commercial device because applications are so limited but and maybe some that would pick up the idea and to and to which so that it might be something that we might see read about in a few years in because on it it's thank you for the talk of the now how much of this technology is present in into the rotor so could a heck about being summoned the physical information about its warnings the events in the year this is an interesting question so obviously you couldn't use it for our scheme because you would need to scan it you will put probably to some extent use it for these other schemes of the other groups by using this me more capability of scanning the being uh I don't know how technically easy it is to have to do that you probably could do it in some way by its uh the other groups actually use development kits for redress so it doesn't seem to be very straightforward to do at least or at least a few would good thing about coming up with some project to do it it's probably easier to buy a development kit for are them which has a somewhat more dedicated hardware than the road itself and it yeah the microphone In this of this technology you showed an looks a bit like a bystander greater system and that the power levels are much lower here in which about the ratio between commercial raters system initially uh all range of power between commercial radio radar systems and just Wi-Fi all I'm probably should be frank in saying that I don't know so have had my layman take on it is that at is military raid employing huge powers like the kilowatt maybe a and we are definitely way below that because we work with commercial devices that are in the what range of most the uh so that would be a factor of thousand but they might be advanced grados which actually use much less power and right so um how long the thing to take 1 of the column pictures and how large is the picture so it took us the nite so we typically programmed the device when we left the lab and when we came in the morning if we were lucky that picture was that the soul it took something like seconds of acquisition time per pixel and the pixels were something like a few hundred by few hundred pixels in size soul um that to the nite and this of course is a big drawback as it stands right now but as I said I think it it it would be very easy to get better and in that respect so all you and if we only scale it in 1 dimension you take an an array of antennas in 1 dimension like a magic wand with many many antennas and use Canada only and the 2nd 1 and potentially the 3rd 1 then you could speed of acquisition by a factor of 100 to thousand so we would be in something like a minute to our range to get a full picture of the building that a little 1 of so in that case sold and the beamforming could help because well it works both ways uh and that this way you could also slow of this kind of signal you eliminate this space with a wooden that's the decrease the and is critical the is the of all the uh acquisition and an array of it yeah this is a very good point and we actually thought about that so you could come up with kind of a hybrid scheme where you do scanning to recover some space but you take more than 1 and 10 I did you say you take a set of antennas that is that the inverse of the me molarity and commercial water so it could 3 cards where the light is coming from along the whole two-dimensional set of directions and then them you might be able to actually get you accelerate that's can quite a bit and I think that if we wanted to go to goal for passive localization and all these things probably the solution in the end would look like that having a set of some enhanced antennas with a good direction sensitivity and having a discrete set of them and strategically placed points in Rome at the and that might be sufficient to get all the information you need to at least see them at us the so I can think of smaller array in with all lot content of like it might be a bit about this this would basically and of the cave based on radio uh ReaderMeter bottom steroids right right so this is but this is a really really very realistic thing to do I believe soul and still if you is so if you ran on a graphic you processing on that date I would be a very coarser resolution image because it's only about 10 it pixels but if you combine that with the scanning it to some only some strategic points then that might be sufficient to uh actually get good pictures even without scanning the whole two-dimensional plane we have another motion from the internet on a set from the data did you publish source code or hardware schematics to make it easier for others to reproduce results uh we didn't publish it for the we wrote a very extensive supplementary material will be described every well every detail of how we actually did the acquisition including references to all the hardware components and plans of the parts that we already got requests from hectares and students that
wanted to recreate batch and of course some people just writers and he may we would be very willing to them to give away all the software and information microphone 3 of idea assimilation all of the antenna array in the ceiling of the building and he said would be to you know expensive to actually refurbish a building that way what happens if you you know of constructing new buildings ends plan to build a scene from the from the get go we're writing more reasonable you have until pro it's probably not so much refurbishing the building which would be so expensive it's just the physical array of antennas itself unfortunately but uh things grow with the square of pixels it's quickly gets very expensive so I we did we did some estimates of it even if these if each of these antennas would cost you only 10 cents and you wanted to have a thousand by thousand pixel array then you would end up with a hundred thousand euros of investment only for the electronics still to acquire the signals so that would be a very high in while barrier I think total practical implementation and microphone for the given that this relatively easy start uh of uh wireless fighting the continuous map the marriage and the thing is within the truth the signal quality and this is something you tried so setting the rotor to some continuous mode rather than downloading a video it we tried both and it for our schema doesn't matter because we throw away all this it pattern-information anyway so we started with that then we bought a model where we couldn't do it for some reason and then we switched to a downloading a big 5 but it actually doesn't frost doesn't matter for future experiments probably 1 very very powerful thing would be to have moved to dedicated images obviously so if we could have an ultra-wideband emitter these pictures would look the way better right from the start and that this in a follow-up project would definitely be a thing to do in i sorry 1 would follow question that could you just may be printed antennas in a cable and then just like cables on the top of like the warehouse or something like this yeah we might be able to find some way to make that work but as soon as you need to if you switch the antenna ask to select a particular 1 you end up with a switch and that would be a semiconductor device and that would be expensive so I'm I'm not an electrical engineer I don't see a straightforward way to do it in a completely passive scheme we only have antennas and nothing else it might be doable and then that are a could be a viable thing to do but as soon as you need only a little switch at every point it's probably too expensive the microphone only would think this is feasible to transistor around and use the transmitter array and I and but want was a 100 years B is the worst centripetally and succession of frame and receive 1 receiver on to get this is a ground from so low to invert to have something like a holographic array that could create any wireless away from that you want of the consider array languages and so and it again with whom and then use can have 1 receiver and this and look at it well yes I think that in principle should be doable since as you say it's just the inverse of our approach this if you if you if you could switch them 1 by 1 from the I think that should work but of course it would be way more expensive because it has a more expensive than the US I 1 of that is in principle I think it should be feasible in an amazed you get firstly euros put the 100 on the wants and you have to wonder where was I think those rights you know I mean this is maybe maybe in a way this is what our colleagues did in the other schemes for they actually had a race off in this house and they could face them in a way that you know they could have been they looked at reflection rather than receiving the signal at 1 point but this is something that you probably would do the other way to the unit found to modify a set up for a different on the slide Tripoli data to 11 AC or and or any of the others but no we wouldn't and we actually did these experiments both with 2 point 4 gigahertz Wi-Fi and 5 gigahertz Wi-Fi the 5 gigahertz Wi-Fi looks a bit nicer and the resulting images because the wavelength is smaller so you can see more details at but this scheme doesn't make any assumption on on the standard employed and we can transfer to any standard we like and this is actually an interesting prospect for the future so both the bandwidth and the frequency of these wireless communication systems is probably going to be increased in in future implementations so people talk about even moving up to 60 gigahertz also and that would that would that you would be able to see pictures of nearly optical qualities of a millimeter scale resolution of things and is the bandwidth grows along with that and with much less speckles than we had to so uh then the whole security thing might be worth another thought to I could you for example plays a smaller weight of antennas at every corner of the building and then we caught the friend frequencies are waves which ended is a radius and calculate the position of form of a source in building formed the different signals ends of different arrays uh receivers soul without erase this is actually being done this is how these and many of these indoor positioning systems work that you measure signal strength to our specific rotors in the building and Saul we could do that and but I think that actually blowing up to the user and 10 hours to antenna arrays is a very interesting thing to do so as I said a high kind of a hybrid scheme where you have some strategically placed points but on each of them you have a small antenna array that could be the way to goal to to make it really viable scheme for commercial applications assume that everyone is going be lost it is some quick question and what you do sounds very similar to a radio telescopes to so did you actually talk to people from low escape or something like that uh not with respect to that project on that's truly so it's very similar and they essentially do the same thing they actually do it on a global scale they link together telescopes on different continents together uh very sharp picture and they think about building kilometres scalar arrays of little antennas to do that and we have been talking with these people yet because they probably will be doing precisely that so all of them they are in a way you would use it in in very much the same way maybe in a slightly more restrictive way because they look at very focused sources something In a wind up with being scanning but it's a it's indeed a similar approach when I was asking moving movement tomography in like 21 centimetre signals and so on and it sounds very similar to what you're trying and to guess they have very advanced algorithms because the signal to noise ratio is that is not right and so you might be actually that you have this that and the idea is wonderful 1 important difference fault that they only look at signals that are infinitely far from from the receiver so uh it's is a good idea and probably we should look at that's a 2nd time and this also as when we did and it looked like looking at their coherent optics papers and would be more useful for once it gets to reconstructing three-dimensional things that are very close to the receiver but they probably have very good so they're probably very good in terms of and sparse sampling and mean and uh well estimation schemes if you have very noisy data so it's different in right so that it will be here in the minutes it's about much interest groups in the release of the inner development this given moment few was to was that
if the if I have a and feeling the kind that at
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Hoax
Digitale Photographie
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Theoretische Physik
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Maschinelles Sehen
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Metadaten

Formale Metadaten

Titel Holography of Wi-Fi radiation
Untertitel Can we see the stray radiation of wireless devices? And what would the world look like if we could?
Serientitel 34th Chaos Communication Congress
Autor Reinhard, Friedemann
Lizenz CC-Namensnennung 4.0 International:
Sie dürfen das Werk bzw. den Inhalt zu jedem legalen Zweck nutzen, verändern und in unveränderter oder veränderter Form vervielfältigen, verbreiten und öffentlich zugänglich machen, sofern Sie den Namen des Autors/Rechteinhabers in der von ihm festgelegten Weise nennen.
DOI 10.5446/34839
Herausgeber Chaos Computer Club e.V.
Erscheinungsjahr 2017
Sprache Englisch

Inhaltliche Metadaten

Fachgebiet Informatik
Abstract Holography of Wi-Fi radiation Philipp Holl [1,2] and Friedemann Reinhard [2] [1] Max Planck Institute for Physics [2] Walter Schottky Institut and Physik-Department, Technical University of Munich When we think of wireless signals such as Wi-Fi or Bluetooth, we usually think of bits and bytes, packets of data and runtimes. Interestingly, there is a second way to look at them. From a physicist's perspective, wireless radiation is just light, more precisely: coherent electromagnetic radiation. It is virtually the same as the beam of a laser, except that its wavelength is much longer (cm vs µm). We have developed a way to visualize this radiation, providing a view of the world as it would look like if our eyes could see wireless radiation. Our scheme is based on holography, a technique to record three-dimensional pictures by a phase-coherent recording of radiation in a two-dimensional plane. This technique is traditionally implemented using laser light. We have adapted it to work with wireless radiation, and recorded holograms of building interiors illuminated by the omnipresent stray field of wireless devices. In the resulting three-dimensional images we can see both emitters (appearing as bright spots) and absorbing objects (appearing as shadows in the beam). Our scheme does not require any knowledge of the data transmitted and works with arbitrary signals, including encrypted communication. This result has several implications: it could provide a way to track wireless emitters in buildings, it could provide a new way for through-wall imaging of building infrastructure like water and power lines. As these applications are available even with encrypted communication, it opens up new questions about privacy.
Schlagwörter Science

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