Dear viewer, these subtitles were generated by a machine via the service Trint and therefore are (very) buggy. If you are capable, please help us to create good quality subtitles: https://c3subtitles.de/talk/913 Thanks!
For the first time today, we're going
to visualize electromagnetic radiation
in the range of the Wi-Fi frequencies,
we have Friedemann Rhinehart from
the Munich Technical Institute.
He has these
junior research project.
And with one of his students, he pursued
this project. The student is Philip Hall
and the stage is yours.
Thanks.
Well.
So 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, I 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, run times,
protocols and so on.
But we can look at them from a very
different point of view, from
the point of view that
it's actually just light.
This is kind of a bold statement,
but it's actually a fairly exact
statement. So from a physicist's
perspective, a wireless signal
is virtually the same as the beam
of a laser or 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 to visible
light that we can see with our eyes
is that the wavelength of these 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 micrometer, the
wireless signals that we typically employ
in our devices have a
wavelength in the range of a few
centimeters to a few tens of
centimeters.
And as a second very important
difference, we cannot see them.
So.
But from a physicists perspective, this
is very interesting because
that has 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 room as we sit here,
it's actually glowing in various colors.
Only we can't see it because it's glowing
somewhere in the microwave radio
domain.
And this also implies that
there is kind of a potential big
security leak associated with these
wireless devices in the
sense that any device that you use,
no matter how well you encrypt your data,
is going to transmit a
full three dimensional picture of
the world to
your environment no matter what you do.
And so
when we realized that
we got intrigued and we
wondered whether there would be a way
to actually record that radiation
and to reveal that picture
to, in a way, built 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 in this
frequency domain.
And so to cut a long story short,
we managed to do that.
And this is the result
that we got.
This is a picture of our lab.
And the colorful thing that you
see is wireless
radiation coming from a router on
the back side of the room
that we visualize with our method.
And we can actually see it propagate
in space.
And we can even see that objects
that we place in the lab cast a shadow
in that beam.
And in what follows,
I would like to walk you through
how we did that and
what it might be used for, for.
And and
I also would like to discuss whether it's
actually a serious security concern.
But before I do that, I would like
to start from a somewhat
broader overview and tell you a bit
more about what has
happened in the past few years
in this field.
So it turns out that hacking
wireless signals in the sense
that thinking in the sense of thinking
about other ways, other useful
data that we can extract from
that signals that are omnipresent
in our modern world, it
has become quite a vibrant area of
research in the past five years.
And we haven't been the only ones
that 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 to look
at Wi-Fi radiation from a different point
of view. And maybe the
most famous result is this.
This is a result from a group
at MIT in
Boston, Massachusetts,
and they actually
have Wi-Fi routers in the way that
they exploit the fact
that modern routers
have an array of antennas
and they use that to actually
send beams in different directions
and to focus the beams on the devices
that they want to interact with.
So modern routers, they can actually send
two different signals to
cell phones on the right and on the left
end of the room.
And you can check that.
You can use that to scan a beam
and two dimensions and
look at the reflections.
So in a way to build 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 room.
You can look at the reflections and you
get a fairly detailed picture of the
world. You can even vaguely resolve
human beings.
You can also just
pursue a simpler approach, you can only
map attenuation,
so you take a Wi-Fi
router and the Wi-Fi receiver
and you map the attenuation
of the signal between the two of them
and you can then mount them
on a movable platform.
So as a fancy example,
for instance, a drone so you can
fly these two šemeta
and receive a drones around a building
map, the attenuation on
every line of sight that you
covered during that flight
to tomography on that data.
And by doing that, you can
actually extract three dimensional
views of what has been in between
these drones.
So there has been a lot of activity
in that field.
But still, this previous
work did not yet fully
answer this question that we got
interested in, like what would the world
look with wi fi eyes?
And if you
look at that question, the state
of the art had been pretty much
that we had something like the compound
eye of an insect.
There have been many ways to
tamper with wireless radiations
in in the sense that we
look at different directions, either by
directing a beam into some direction or
by looking at attenuation
in a certain direction and
do that for multiple angles to
get a picture of the world.
And so we got interested in whether we
could actually get a full, three
dimensional view of the full
wavefront and all the light propagating
in space, pretty much in the same way
as we can do it with visible light.
So we thought about that
and it turned out that you can do it and
you can do it in a surprisingly simple
way. So there is a technique
that essentially solves the problem
and it's holography.
So holography holograms
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.
They look much more like
a window into a virtual world
that has been frozen by taking this
hologram.
And you can make them actually
in a fairly simple way or you don't
even need a lens.
All you need to do to record such a
hologram is illuminate
your object with what we call coherent
light and get to that in a second
and record it on
a face sensitive camera.
So these are very technical terms.
Let me try to explain that in a
more visual way.
So in a way, a hologram
is a photograph to zero.
It captures more information than
a two dimensional photograph, which
we usually take,
which where information is restricted
in the sense that the usual photograph
only captures intensity
of light so
it can record in a two dimensional plane
where there was much light and
where there was very little light.
And the resulting photo 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,
a hologram can do more.
So a hologram is a face coherent
recording.
And that, in a visual way means
that not only records
brightness of the light, it also
records the direction of
the light beams hitting this hologram
during exposure.
So in a hologram, in a way, you
freeze all the light rates that enter
this specific plane.
And if you develop
that hologram, you in a way revive
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 have a three dimensional
view of it.
So how is
that done?
So this is done by
a phase coherent recording
and that is this this
technical term essentially captures this
idea of recording
direction along
with intensity.
And to see how that works.
I have to well, get back to the point
that light actually
is electromagnetic waves
and waves, pretty much like waves
on a lake, have valleys
and hills.
So areas where electric field is very
strong and areas where electric field is
very weak and these
valleys and peaks propagate in space.
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 a way encoded
in the difference between these peaks and
valleys.
So if you can take a photograph
where you also
register whether the peak came first
or the valley came first, you
also register direction
and that's
what it takes. So it takes a light source
where these peaks and valleys are very
well defined.
This is what we call a coherent light
source. And it takes a very sensitive
camera, a camera that can
record whether the peaks of the valley
came first.
Now, it turns out that this is a
fairly difficult thing to do in the
optical domain, people
managed to do it, but it was
such a big breakthrough that it actually
got awarded a Nobel Prize
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 record
every peak and every value
simply by registering
the wave with a good oscilloscope
and.
That, in particular means that
it should be possible to do holography
of this wireless radiation, so to come
up with an experimental setup
where we take a picture
in some two dimensional plane of space,
we register the brightness
and phase of the
microwave light at every point in space,
and we can then later on use that to
reconstruct a virtual view of the world.
And this is pretty much how we did
it. So this is our experimental setup.
We explicitly
wanted to capture light from arbitrary
devices. So the
light sources that we used always were
commercial off the shelf
Wi-Fi routers that we
asked to download a big video from
YouTube 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 sometimes play some objects
in the beam to get a more interesting
picture, and we finally
record this
beam of microwave light
in a scanning aperture approach.
So unfortunately, it's very, very
expensive to to build microwave
cameras. You would need a huge array of
antennas. So in a first
step, it's easier to just take one
antenna and scan it across every pixel
that you want to emit.
This is what we did.
So we have an antenna that we mounted
on a scanning platform and
we reference that signal to
the signal of a stationary antenna.
This is what it looked like in real life,
so it's you see, it's not
very fancy, it's actually it's probably
the cheapest experiment I've ever been
involved in.
And we used plywood
and fishtank and tools like
that to do it essentially
to solve this problem of scanning the
antenna through our lab.
But at some point we were quite well
and so we could record these pictures.
So the signal that we get from
the set up is
a wave, a wave that we can record
with an oscilloscope and
that we actually record twice, once
coming from the scanning antenna.
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 here, but it turns
out that if you Fourier transform
these signals and you divide them by each
other, you can, in a way,
normalize that signal to that reference
and get
a virtual second wave
where the
bit pattern actually cancels.
And you only record
the, um, the
the face delay and the attenuation
of that wave as it travels to the
scanning antenna.
So for every pixel we record a set
of data, like on the upper left,
we do some processing to
remove the bit pattern and only
get the coherent wave.
And then we end up with something on
the like on the
lower part of this plot.
So we do that for every pixel,
for every pixel we get.
The brightness, the amplitude
and the face, the direction of the beam,
we actually get that for a wide range of
frequencies because Wi-Fi
is
multi frequency scheme.
So you can have different channels.
And we end up with a dataset like below.
So this is a hologram.
It's unfortunately not something
that you would easily be able to
make sense of just by looking at it.
And so
we we since we don't have eyes for this
radiation, we cannot see the radiation by
itself. And even if we record it like
that, the information is not very
meaningful.
But fortunately, there
exists a number of reconstruction
algorithms.
So this problem has been solved and
coherent optics where people
actually can.
Render three dimensional views of
what a hologram would look like simply
by knowing the pattern
that you produce on a photographic plate,
so by scanning a photographic plate and
feeding that in the right algorithm, you
can get a three dimensional view of the
hologram without ever
looking at it in real light.
And this you can also do with
microwave radiation.
So from that hologram,
we can reconstruct three
dimensional views.
We did that and this is a
while. This is the test setup that in
the end worked best.
So we have a commercial Wi-Fi router
sitting on the back of the lab.
It's labeled embitter.
In the upper picture.
We play some absorbing objects in
the beam that we made from
aluminum tin foil.
And then we record a hologram in a
plane that you cannot see here.
That is essentially the front
plane of that image.
We feed the data into a reconstruction
algorithm.
And in that way, we can now
render a three dimensional view,
as it would appear if we had
eyes for Wi-Fi radiation and we looked
through that plane of the
above picture.
So we can actually,
since it's a three dimensional approach,
we can actually not
only reconstruct this view, we can
actually focus on different planes.
And so this is what we do here.
So obviously, a first interesting thing
to do is to focus back
into the emitter plane.
If you focus into the plane of the
Wi-Fi router,
you actually nicely see a bright
spot at one 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 one frequency,
with one wavelength of light, you
become very sensitive to something that
is known as speckles.
So this microwave 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
clouds and
we can interestingly enhance that quite
a bit if we repeat the experiment for
different frequencies.
So four different frequency bands within
a Wi-Fi channel and we superimpose
the images.
We get a much clearer picture where
it is very clear that the router is the
brightest spot.
This is actually something that is very
difficult to do with real light people
struggling to make that work
for microwave light, it's
essentially straightforward.
And we can then go on and focus
in different planes, so, for instance,
in the plane of this absorbing object in
the beam, and if we do that,
we nicely see that this so.
So as we move move from
the emitter to the object, we nicely see
how the beam expands and
that when once we get to the object
plane, there is a big shadow appearing
in the light wave.
Nicely, you can actually
see that if you focus slightly
below, slightly below or slightly
above that plane,
the image blurs.
So it really is a three dimensional
image. It's like
focusing with a photo camera.
You can blur it if you for defocus
into the wrong plane.
So
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 incoherent,
visible light. You can take just plain
dumb photographs as we do that most of
the time.
But you can also play clever tricks
to enhance, for instance, absorbing
objects in the light.
It's called dark field microscopy.
You usually do it with microscopy.
You can image polarization
of light or you could enhance
only weakly
refractive objects.
And all these schemes we could actually
emulate by just numerically
post-processing the hologram data.
So there's a lot of things to be looked
at like
what does the world look like if we
enhance absorbing objects or
do chairs have an index of
refraction that is different
from air and questions
like that?
So in a way, to
summarize that, we
managed to record pictures and we
actually managed to establish
kind of an analogy between wireless
communication and coherent optics.
So I made up a little dictionary
what that how
terms translate into each other.
So wireless signals that
we usually think of as packets of data,
it's actually light.
It can have different colors.
We this is a degree of freedom
that we haven't used yet, but we could
do it, for instance, bye
bye, bye bye.
Splitting signals, according to their
ID that they transmit.
So in a way,
once we look at the big Pappert pattern
signals with different bit patterns in a
way correspond to different colors.
And so we could have multiple
emitters in the picture,
we could actually tell them apart and
would get a multicolored picture of where
every router in
our room illuminates the room
with a different color.
This would be a very interesting thing to
do in the future.
It turns out that the pictures can be a
lot better if you do white light
holography, if you use more than one
frequency band.
This is something that has an analogy in
the wireless domain where increasing
the bandwidth usually improves
performance of radar.
If you want to learn something about the
environment rather than the signal
itself, you 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.
And what used to be the runtime delay
of a signal is now
facing direction of a light filled.
So that was
the work that we did.
We were very happy that we 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'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
four, which is it is of singular
appeal to all physicists.
So.
So so by that argument, we
actually made it accept it there and
it was met with a lot of interest,
not only in science, but even
more so in the in
the real world of media
and of companies.
And so that's what I would like to look
at in the second part of my talk.
What can we actually do with that?
Will it ever be useful?
So if you think about that question,
what could you use it for?
There is a very obvious answer,
if you don't have
it right now, then
you can actually read it on Russia today
where they
picked up our story.
And the headline was that this could be
used to map your home.
So you may wonder whether I
know some people say this is only fake
news, but I can tell you that at least
for this story, this is not fake news
that's based on a on the real thing.
So it's
this idea. Can we use it to spy
on other people or in
a in a more optimistic fashion?
Can we use it for security, enforcement
and security applications?
And this is something that many
media jumped on so
many of the headlines that we thought
were related to that.
Can we use that?
Can can my neighbor see me in my bathroom
when I have my cell phone lying there
and things like that?
So it's a question to which we somehow
need to respond.
Would that be feasible?
And at least in principle, it
looks like that.
So if you have a Wi-Fi router in your
room and you walk around,
then your buddy 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,
though, because, first 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 around
many points around a building
which would probably not go unnoticed.
And maybe even more importantly,
it's something that actually is already
being done right now.
So 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 one point in space.
And this is something that is going
commercial. So there are companies that
sell systems where you
essentially plug something like
a second router into your room.
This router is not emitting
signals that are used for communication.
It's actually looking at signals that
are scattered from the environment,
that it's 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 they in the bathroom or in the
kitchen or whatever else.
So it's something where our probably is
too complicated, which is good
news because it's not a security concern.
The bad news, if you wish, is that
Wi-Fi is a very leaky thing.
And this general idea
that we can use the stray radiation to
spy on other people, it's actually a
valid security concern.
OK, if we wanted to
to to do it, if we wanted to
sell that as a security application, I
think we would run into a second very
important problem.
And the important problem here is that
if you 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 it's
not so much more expensive to actually
combine that with a dedicated
TaylorMade emitter that
is emitting a specific signal.
And once you can do that,
you can play a trick, which is called
Ultra Bend.
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 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 forums,
you emit some signal from a dedicated
emitter.
You detect the reflection in a dedicated
detector, you measure the runtime
delay. And from that you infer whether
there are people moving around and at
which distance they are.
So if you do that, the resolution
that you get is related
to the bitrate of your
signal, if you which it's essentially
the inverse of the bandwidth.
So if these data packets are very
broad, the resolution
will drop.
And from the point of view of radar,
wi fi actually is not a very
good signal because it has a very
narrow bandwidth.
So for two point four gigahertz Wi-Fi,
it's actually only 20 megahertz.
And this corresponds to
a resolution of 10 meters.
And if you build a dedicated emitter,
obviously, you can drop
that restriction. You can build an
emitter with a much higher bitrate.
And this is what people do.
So you can push that
up to a bit rate of gigahertz to
tens of gigahertz and emitter.
That is sending some garbage signal
that is spread across
all the frequencies up to 10 gigahertz.
And by doing that, you actually get
down to a centimeter resolution in radar.
So if you invest into a dedicated
device, this is a very powerful trick to
play and this is actually being
done. So there is I did some research
on the Internet and there are devices for
security forces that look
a bit like these devices that you use
to search for pipes before drilling into
the wall.
You place them on a wall.
It takes a few seconds and it's going to
tell you whether there are people in
that room that you can't see and
at which distance they are.
So it gives you an image on the display,
which looks pretty much like that.
So to sum that up,
security applications,
well, are an interesting thing to think
about, but it's probably not a really
realistic use case for this scheme.
Let's look at something else.
Let's look at civil engineering, I
already mentioned it. There are these
devices that you use to search
for water pipes before making a drill.
And these devices, well, they
work. But
I did it myself.
And it's it's not a very reliable
thing to work with.
So there is room for improvement.
And it's interesting to ask whether
we could fill a gap in that market.
So the way that would work
would be that
you rely on the radiation
that is emitted from wireless
devices in other rooms across
the wall that you want to drill.
And that will illuminate this wall
with microwave light so
that pipes and power lines in the wall
would cast a shadow and then
you would record a hologram across
the wall that you want to drill,
reconstruct that image and see every
single pipe.
Still, we have this challenge of
technical complexity.
But I think in this case it would be
surmountable because
we could actually scale up this
system from a single
antenna device that is scanning the plane
to a one dimensional array of antennas,
like a magic wand that would wave across
the wall.
And that would give you a picture on your
smartphone of what's inside.
So this is a fairly realistic
thing, unfortunately,
here, to, um, you
have to compete with ultraright bent.
So if you built a complicated
magic wand, it's not so much more
expensive to include an ultra wide
bent emitter and to do very
precise radar.
And there are companies doing that you
may have read about all about.
It's a kind of a radar that you can
plug on the back of your smartphone and
you can X-ray your wall
and it's going to do some signal
processing and give you actually a
picture of where the pipe is.
So there would be strong competition.
Still, our scheme would have some
advantages so we could use radiation
that is actually coming from things
behind the wall. That could be an
advantage.
And so in principle, that could be
something to think about.
So it could work.
There's one thing that we initially
thought would be a good idea
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 tag.
And our scheme in principle could
do that. So we could think of installing
a two dimensional, huge array of many,
many antennas in the ceiling of this
building, doing holography
and then see actually the
position of each and every smartphone and
WiFi Briffa router 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 a virtual storage hall
with steel bars in the
floors and with some steel
shelves in one of the floor.
And the image that we get
is encouraging.
So with that scheme, we probably would be
able to track down embitters
with centimeter scale precision
at video rates and what
is more, and we probably would even
be able to get information about
the objects in that building.
So this is a movie of the simulation
data and it's essentially
the hologram reconstruction as we focus
onto successively lower 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 spires casting
shadows in the beam before we converge
on the emitter, which is appearing
as a spot with centimeter 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.
It's probably not not
something that you could do right away.
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 in a way
a reduced implementation of that scheme.
So rather than going
to a full two dimensional array
of antennas, we could actually
go to a one dimensional array
like this magic wand, and
that still would speed up acquisition
by a factor of 100000 compared
to our single antenna proof of principle
implementation.
So you could have you could actually
take pictures of large
scale, 10 meter scale structures
and probably on a minute to our
scale, if you
can scanned this device around the
structure by, say, a
drone or a car.
And once you have that data,
you could actually look into the building
with wi fi eyes and understand how
radiation propagates in their.
And this could actually be
relevant for this whole field
of indoor tracking.
So maybe it would 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 or devices.
And it's a market that is
actually predicted to grow to a billion
dollar size in a few years.
However, at the press.
So, OK, so one very strange,
straightforward application where we
could enter here is that we could do
site survey for existing solutions.
So if you bought
some system for tracking and
you found that for
some reason it didn't work in your
factory hall, then you could come
with this 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. 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
these signals look like and how they
propagate to make these schemes better.
And there's work to do because
at the moment these
schemes are, well, very successful,
but still not fully satisfactory.
So people have tried many things.
You can move around with a camera
and scan barcodes, you can use
RFID, but then you
can only read them from a very close
distance. So it's not very convenient for
large scale factories.
You can use all kinds of beacons.
You can check
labels with ultrasound, Bluetooth,
Wi-Fi, whatever.
But with that, you run
into this bandwidth issue and you only
get meter scale resolution.
OK, here again, you
can play the ultra wideband trick so you
can have dedicated amateurs with
a very wide frequency band
and that will give you a centimeter scale
resolution.
And or this already is incredibly
successful today, but it comes
at a large price and it's
a physical price.
These chips are expensive, so
the tags will cost you something like 10
years or more.
And it's very hungry
in terms of power. So you need to
ship it with a battery and you need to
replace that battery every few months
or years. And this is not very
convenient.
So there is a quest out there
to make these schemes work with passive
tax to where we
actually rather than having an
active emitter on a label,
we just have some absorbing little
thing. And we illuminated by some
other source and we imaged the shadow
that it casts in the beam.
At the moment, this is not doable.
You in principle could solve
that problem by many ways.
You could buy 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 one by one.
One interesting idea could be
to actually record this entire wavefront.
Look at the picture, understand
how radiation propagates in typical
buildings and
what kind of what part of the signal
you actually need to see this shadow
best.
And here are the key
make believe could come in.
We could record this for WaveFront
and we could simulate any kind
of reduced scheme, like a scheme with
only a few antennas or looking at only
part of the frequency band
with our algorithms.
So summing that up,
it's an interesting technique.
I don't think it's a serious security
concern, but reduced implementations
actually might be one.
Civil engineering might be an application
where we could move into
indoor tracking probably is too
difficult, but R&D for
simpler tracking schemes could
be a very viable way to go.
With that, I'm at the end, I would
like to
acknowledge the person that did all the
work, Philip Hall here
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.
So.
So 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 by just
using this stray radiation that
you get.
And if you do it right, you can actually
take real pictures.
This could be useful for maybe tracking,
maybe civil engineering,
probably most as an R&D tool
to implement to to,
um, develop future
wireless applications.
So thank you very much.
We have 15 minutes for Q&A.
And.
Hello, thank you for your time.
It was really interesting, I was
wondering if you could build Wi-Fi
lenses.
That's a good question.
So it
probably would be difficult
if it if it should be a physical lens,
because that so you not only
would need to find the right material,
which you probably could, you also
would need to make it larger to to,
uh, because the wavefront are large.
But again,
somewhat paradoxically, there are
actually much simpler schemes to do that.
So if you if
you take a train somewhere
in the country, you may see that nowadays
these satellite receivers, they are no
longer like a parabolic mirror, although
sometimes they tend to be flat, like
just a flat plate.
And this is in a way, due to a virtual
lens for this radiation
in that frequency band.
So it's a phased array where you
have many receivers and you delay the
phase electronically, which is
essentially what lens does in
the optical domain.
And in doing
that, you can simulate any kind of lens,
including a parabolic reflector looking
at the antenna.
So for our scheme,
it's actually even better we don't need
a lens because we can simulate any
kind of lens in the computer.
And this is something that we
actually thought about doing, like trying
different building different kinds of
virtual objective lenses to
look at different parts of the radiation.
But I think electronic solutions
will probably be the easier way to go
than a physical lens.
We have one question from the Internet.
What is your prognosis?
When can we expect a practical camera for
a different spectrum of ranges like
TV or radio?
So, um.
Well, I mean, technically, I don't see an
obstacle for transferring
this technique to other frequency domains
like radio or so.
I don't think we will see it in
the near future as a commercial device
because applications are so limited.
But maybe some lab will pick up the
idea and do it.
So it might be something that we might
read about in a few years.
Microphone one.
Thank you for the talk.
Do you know how much of this technology
is present in a typical quarter so
it could affect to gain some
physical information about its
surroundings?
That's yeah, this is an interesting
question.
So obviously, you couldn't use it for our
scheme because you would need to scan it.
You could probably, to some
extent, use it for these other schemes of
the other groups by using this memo
capability of scanning the beam.
I don't know how technically easy
it is to to do that.
You probably could do it in some way.
But the other groups
actually use development kits for
routers. So it doesn't seem to be very
straightforward to at least or
at least a few would think
about coming up with some project to
do it. It's probably easier to
buy a development kit for a
router which has a somewhat more
dedicated hardware than the router
itself.
And yeah, microphone
for, yes,
the technology that you showed,
it looks a bit like a static radar
system, but the power levels are
much lower here.
What's about the ratio between a
commercial radar system and actually
a great deal of power between a
commercial radar system and just
Wi-Fi?
Oh, I probably should be frank
in saying that. I don't know.
So my
take on it is that at least military
radar is employing huge powers like
Kilowatt maybe.
And we are definitely way below that
because we work with commercial devices
that are in the watt range
at most.
So that would be a factor of a thousand,
but there might be advanced radars which
actually use much less power.
Microphone three.
So how long did it take to take
one of the hologram pictures and how
large is the picture?
So it took us a
night. So we typically programed the
device when we left the lab and when we
came in the morning, if we were lucky,
the picture was there.
So it took something like
seconds of acquisition time per pixel,
and the pixels were something like
a few hundred by a few hundred pixels in
size.
So that took a night.
And this, of course, is a big drawback
as it stands right now.
But as I said, I think it would be very
easy to get better in that respect.
So even if we only scale
it in one dimension, you take an array
of antennas in one dimension, like
a magic wand with many, many antennas.
And you can it only in the second one,
potentially the third one, then you
could speed up acquisition by a factor of
100 2000.
So it would be in something like a minute
to arrange to get a full picture
of a building.
Right, microphone one.
So in that case, wouldn't
be performing help
because, well, it works
both ways.
And this way you could also.
Scan the signal, you eliminate
the space with
wouldn't that decrease the
critical dimensions you need to have
on the acquisition antenna
array a bit?
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 cover some
space, but you take more than
one antenna, so you take a set
of antennas. That is like the inverse
of the memo area in a
commercial router.
So it could record
where the light is coming from, a long
haul two-dimensional set of directions,
and then you might be able
to actually accelerate
that scheme quite a bit.
And I think that if we wanted
to go 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 at strategically placed
points in a room
that might be sufficient to get all the
information you need to at least see the
emitters.
I can think of a smaller
array with a lot of antennas
like eight by eight,
but this would basically end up being
a base array
radar, but on steroids.
Right, right.
So this is but this is a very realistic
thing to do. I believe.
So, um, still, if you
so if you ran holographic, you were
processing on that data, it would be a
very coarse resolution image because
it's only about eight pixels.
But if you combine that with scanning
it to some only some strategic
points, then that might be sufficient
to actually get good pictures
even without scanning a whole two
dimensional plane.
We have another question from the
Internet, aside from
the paper, did you publish source code to
hardware schematics to make it easier for
others to reproduce the results?
We didn't publish it, so
we wrote a very extensive
supplementary material where we describe
every well, every detail of how we
actually did the acquisition, including
references to all the hardware components
and plans.
But we already got requests from
hackers and students that wanted to
recreate that.
And of course, if people
just write us an e-mail, we will be very
willing to to give
away all the software and information.
Microphone three, yeah, hello.
Yeah, it's a simulation of
the attendant right in the ceiling of a
building, but you said it would be
too, you know, expensive to actually
refurbish a building that way.
What happens if you, you know,
construct a new building and plan
to build a scene from from the get go?
Would that be more reasonable?
Yeah.
So it's probably not so much refurbishing
the building, which would be so
expensive. It's just the physical array
of antennas itself.
Unfortunately,
if things grow with a square of pixels,
it quickly gets very expensive.
So we did some estimates of
it. Even if these if each of these
antennas would cost you only ten cents
and you wanted to have a thousand by a
thousand pixel area, then you would end
up with 100000 euros of investment
only for the electronics
to acquire the signals.
So that would be a very high,
um. Well, Barria, I
think to a practical implementation.
Microphone for yeah.
Given that it's relatively easy to
a wireless fire into
continuous wave mode,
do you think this would improve the
signal quality and this is something
you've tried so setting
the router to some continuous mode rather
than downloading a video?
We tried both.
And for our scheme it doesn't matter
because we throw away all this bit
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 downloading a big file.
But it actually does
for us. Doesn't matter for future
experiments. Probably one very, very
powerful thing would be to move to
dedicated emitters, obviously.
So if we could have an ultra wide bent
emitter, these pictures would look
way better right from the start.
And this is the follow
up project would definitely be a thing to
do.
Sorry, one quick follow up question.
Couldn't you just maybe print antennas
and a cable and then just lay
cables on the top off like a big
warehouse or something like this?
Yeah, you might be able to find some
way to make that work,
but as soon as you need to at least
switch the antennas to select a
particular one, you end
up with a switch and that will be a
semiconductor device and that will be
expensive.
So I'm not an electrical engineer.
I don't see a straightforward way
to do it in a completely
passive scheme where you only have
antennas and nothing else, it might be
doable. And then that array 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.
Microphone three,
yes.
What do you think it is feasible to turn
this around and use a transmitter
array like I want
with a hundred ISPs, uh,
which sent rapidly in succession
of frame and receivers through one
receiver to get
the hologram from Jack?
Sought to invert that, to have something
like, uh, a holographic
array that could create any wireless
waveform that you want
to transmit, array like transmit.
OK.
Mm hmm. And then
have one receiver and.
Yes. 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.
Yes. If you if you if you could switch
them one by one, probably, I think that
would work. But of course, it would be
way more expensive because emitters are
more expensive than receivers.
I know, at least in principle, I think
it should be feasible and
you get for three years,
put 100 percent on the want and
you have one.
I think so.
Right. Yeah, that's true.
I mean, this is maybe maybe in a way,
this is what our colleagues did in
the other schemes where they actually had
a raise of Amita's
and they could face them in a way that
you could scan the beam
they looked at reflexion for rather than
receiving the signal at one point.
But this is something that you probably
could do the other way to.
OK, you're
not to modify your
setup for a different lifestyle and it's
like a triple eight or two, 11 AC
or and or any of the others.
No, we wouldn't.
And we actually did these experiments
both with two point four gigahertz wi fi
and five gigahertz wi fi.
The five gigahertz wi fi looks a bit
nicer and the resulting images because
the wavelength is smaller.
So you can see more details.
But the scheme doesn't make any
assumption 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 future implementations.
So people talk about even moving up
to 60 gigahertz also.
And that would with that, you would be
able to see pictures of nearly optical
quality. So with millimeter scale
resolution.
And if the bandwidth grows along with
that, with much less speckles than we
had, so then
the whole security thing might
be worth another thought to.
Could you, for example, play a small
array of antennas at every
corner of a building and then
record different frequencies
or waves which enter this
race and calculate the position
of a source in the building
from the different signal strengths
of different arrays,
receivers?
So without a race, this is actually
being done.
This is how these many of these indoor
positioning systems work that
you measure signal strength to specific
routers in the building.
Um, so
we could do that. And but I think that
actually blowing up these
antennas, antenna arrays is
a very interesting thing to do.
So, as I said, a 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 go to to make
it really viable scheme
for commercial applications.
Still, microphone one, this is
going to be the last one.
Sorry, it just one quick question.
What you do sounds very similar to a
radio telescopes, too.
So did you actually talk
to people from Lofa or České
or something like that?
Uh, not with respect to that
project.
It's true. So it's very similar.
They essentially do the same thing.
They actually do it on a global scale.
They link together telescopes on
different continents to get a
very sharp picture and they think about
building kilometer scale arrays of little
antennas to do that.
Um, we have been talking
with these people yet
because they probably will be doing
precisely that.
So they,
in a way, will
use it in very much the same way, maybe
in a slightly more restricted way,
because they look at very focused sources
and they, in a way, end up with beam
scanning.
But it's a it's indeed a similar
approach.
We're now moving into tomography
and like twenty one centimeter signal and
so on, and that sounds very similar to
what you were trying and I guess they
have very advanced algorithms because
their signal to noise ratio is abysmal.
Right. And so you might be actually
that. Yeah, this is definitely a good
idea. There is one different one
important difference, though, that they
only look at signals that are infinitely
far from the receiver.
So it's a good
idea and probably we should look at that
a second time.
So as when we did it, it looked
like looking at the coherent
optics papers, um,
would be more useful once it gets
to reconstructing three
dimensional things that are very close to
the receiver.
But they probably have very good they are
probably very good in terms of
spa sampling and, um,
and well, estimation
schemes if you have very noisy data.
So, yes, definitely.
Right. So the next week will be here in
15 minutes.
It's about machine check the proofs in
everyday software and hardware
development. Let's give him a hand.