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Now, I can see wifi signals (imgur.com)
695 points by bane 947 days ago | hide | past | web | 56 comments | favorite

Neat. The phenomenon is called Rayleigh fading, and is well known, but I've never seen it mapped as clearly. Rayleigh fading is often viewed statistically, and some article even call it "random". As shown here, it's not random at all; it's deterministic and repeatable. It's determined by the position of transmitter, receiver, and reflective and absorbent objects in the environment.

This is why most WiFi devices have two antennas and a diversity receiver.

Spread-spectrum systems are resistant to Rayleigh fading, because multipath nulls are frequency-specific; the two paths have to be half a wavelength different in length to get cancellation. So it seems surprising at first to see this with WiFi, which is a spread-spectrum system.

There's a reason for that. Cancellation becomes less of a problem as you get further from a spread spectrum transmitter, because the phase difference is related to the product of the frequency change times the number of wavelengths along the path. (This really needs pictures.) Once you're far enough away that the spreading moves the nulls at least a half wavelength, the effect is that a fixed percentage of data is lost, and error correction deals with that. In this demo, transmitter and receiver are very close, so the nulls are strong.

That is slick. The wavelength of 2.4Ghz radio being 125 mm and 5Ghz being 62.5mm it looks like he is imaging standing waves in the 5Ghz spectrum. (if the cube is 360mm x 360mm it would have ~6 waves of 5Ghz and ~3 waves of 2.4Ghz RF energy. There is an experiment to see the standing waves in a microwave using chocolate - http://morningcoffeephysics.com/measuring-the-speed-of-light... which demonstrates the same sort of thing, but frankly I think this is much cooler.

He does mention that explanation for explaining the distance between "features" in his visualization, however this does not explain their particular shape, nor their general irregularity. It would be nice to see this repeated outdoors in the desert to reduce reflections and possibly interference.

It would also be nice to see the chocolate experiment repeated in other desserts.

only if you extract the Theobromine. (hi don)

Reminds me of another project visualizing wifi signals: > This project explores the invisible terrain of WiFi networks in urban spaces by light painting signal strength in long-exposure photographs. > A four-metre long measuring rod with 80 points of light reveals cross-sections through WiFi networks using a photographic technique called light-painting.

* More Info: nearfield.org/2011/02/wifi-light-painting * Video: https://vimeo.com/20412632 * Thread on Hacker News: https://news.ycombinator.com/item?id=2270878

That is interesting also, but it is 1D only, the light stick's total illuminated length is signal strength. If they re-did that project using RGB LEDs and each LED colored from Red to Blue based on the signal strength at the LED then you would get a 2D plot, And of course if did as the author and photograph an LED at intervals in a 3D space you could do the 3D plotting.

The whole thing has my head buzzing with ideas. Imagine a drone which has a string of RGB LEDs hanging from it making such plot using GPS/6DOF IMU data to allow a computer to reconstruct the scene. Could be very cool indeed.

I was thinking the same about drones. Take a drone and let it automatically cover an area you're interested in. Maybe following a certain pattern or just random movement.

I tried that experiment once with marshmallows. Zero results and a lot of cleaning. :(.

I feel like I just stepped onto the set of Big Bang Theory.

This is a beautiful hack.

Because terrific work causes us to think of additional questions, I'd add: The only thing I found missing is that there is more than one polarization. So to accurately map the field, you'd need to perform multiple scans, with the receiving antenna rotated 90 degrees for each one.

> Because terrific work causes us to think of additional questions,

I'm gonna steal this line.

Another question: when he uses the CNC to move the sensor around, isn't he also changing the reflective surfaces in the environment? Would this not produce inaccuracies?

Really cool, but I wonder how much his big metal testing rig is interfering with the signal.

Not only the metal testing rig, but also the 60kg RF-absorbing sack of water holding the device in the earlier experiments.

I was thinking the same thing. The next iteration should have a remote antenna with as small a footprint as possible.

Awesome. I'm not sure how to make this argument precise, but I think you could sample the intensity function only every 12.5cm (wavelength at 2.4 Ghz), since waves of a given wavelength are restricted to resonate and interfere at small multiples of it's wavelength.

For reconstruction there should be some ideal interpolation, maybe a 3d sync filter?

Another random interesting fact: if you capture not only the intensity but also phase of only a 2D slice, and have some environmental information (i.e. know how the signal propagates in your room), you can reconstruct the full 3D intensity.

A naive application of Nyquist sampling theorem will say you have to sample at twice the frequency of whatever you're trying to reconstruct - so for 2.4Ghz you need to sample at at least every 6.25cm. The reason why you can't just use the base frequency is cause... well, for example, imagine you have the worst luck ever, and only ever sampled the nodes of the standing wave - it would just look like no signal.

In practice... I have no idea. RF is too much for me.

Unlike heat, E&M is a vector field, so the author should have rotated his antenna at every point :-)

This was done in a pretty spectacular way by Luis Herman a little while back:


Why is there so much variation in strength from such a small change of position?

Signal variance is nothing new. I remember personally noticing it in late 90's that with my GSM (900MHz) phone, moving it on table just for about 5 centimeters could bring it down from full signal to no reception at all. Of course with analog radios you also notice how easily signal changes with location. If you've been ever playing with TV antenna in in bad reception and so on. Moving your hand in other room might block TV signal or make it crystal clear even if you would make there's no connection what so ever. With 2.4GHz people often forget that interference from other sources can significantly contribute. So signal quality and signal strength aren't same thing at all. Getting to the root all these things require professional, which I'm not. So one type of measurement defined as "signal strength" probably misleads you badly. Is it a good idea to select a wifi channel that doesn't have any other wifi boxes? Well, the reason might be that the channel is totally overpowered by local wireless CCT or phones. That's the reason why nobody's using it for WiFi and then you think it's a great idea to select a free Wifi channel? Radio stuff is (truly) really tricky. With higher frequencies it's just like light. Why some things are in shadows and some things are well lit?

I'm in a relatively large appartment building.. I can see about 20 other wifi options in my area on my laptop... fortunately, my signal quality is significantly better (higher end asus, with shibby tomato) spent a fair amount of time tweaking the settings a bit that seem to work very well for me... I can keep signal on my phone across the parking lot (I'm facing the pool, which is adjacent to outside parking) ... no wifi really gets into the garage though, but cell phone signal does...

It's really wild how a foot or two difference could make all the difference in the world...

I'd guess that it's due to signal reflections causing points of constructive and destructive interference.[0]

The shapes aren't very regular, but that could easily be explained by the shape of the room and its contents.

[0] http://en.wikipedia.org/wiki/Interference_%28wave_propagatio...

Is it correct that this is a quantum mechanical effect, i.e. that the wave function of each photon interferes with itself?

Well, yes to both questions, but on a macroscopic scale the wave equation can explain the collective effect of zillions of photons to more accuracy than you'll ever need.

I found a great explanation from the reddit link that rawnlq posted.


I'm curious if combining this with Google's Project Tango for accurate 3D pose estimation would be useful for doing a handheld 3D mapping of an area. Add an additional camera to the tablet, have it point at an array of these sensors, and use it to sweep out regions of space quickly.

Pretty cool! Would be cool to add some sort of position sensor on, I guess GPS wouldn't be ideal though. Maybe hack a roomba to map signal strength in your house!

Anyways, reminds me of this: https://vimeo.com/20412632

What is this measuring? RSSI, MCS level, observed throughput?

I'd assume RSSI

What about beamforming? Wouldn't signal strength depend on locations of connected wifi clients?

Strap it onto a quadcopter and you have a portable solution to map every room you could imagine.

There was a project several years ago to map a University's wifi using a self driving RC car with a wifi detector on it. The goal was to identify weak spots in the signal. I can't remember the name of the project and its not popping up on google, anyone else remember this?

Or stick it to the back of a Roomba, for fully automatic mapping of a house

Here's a cool animation (supposed showing the "propogation" of wifi signal):


This is brilliant, I wish it'd get more upvotes and attention. Wireless waves are such nebulous, hard to understand things. It's great to see a visual representation!

Awesome. A while ago somebody did something similar but with really hard math: http://arstechnica.com/gadgets/2014/08/mapping-wi-fi-dead-zo...

It's always nice to see multiple approaches to the same problem.

I have often wondered what Jordi's[1] vision would really be like.. Maybe this is a peek into his world.

Imagine all these things, wifi, FM/AM Radio, iBeacons, cell towers etc.. It must be so noisy in the radio spectrum..

[1] http://en.wikipedia.org/wiki/Geordi_La_Forge

There was a Next Gen episode, I think, where Jordi's visor was displayed on the main viewer on the bridge of the Enterprise with Picard commenting.

Argh why are you both spelling his name wrong!? It's right there in the URL!

True. Oops. Wasn't thinking and just copied the other guy.

Very directional antenna, fast scanning servo gimble, oculus rift... hmmm.

This with the hololens could be rather nice to debug.

That is really cool. It would be cool to put it on an indoor UAV, and have it fly around, mapping wifi signals around a building

Write a paper!

I was really hoping this was going to be an AR hack for seeing WiFi, but this is cool too!

Nice, I didn't know about this board. It looks pretty interesting.

It's a real nice tinker toy for only a few pounds/dollars.

The default firmware uses modem-style 'AT' commands for control, but there is an Open Source LUA implementation, and you can cross-compile in C.




The most top ranked comment says; "PATENT THIS MOTHERFUCKER NOW, DO IT BEFORE APPLE FIND OUT AND STEAL YOUR IDEA." Hope neither this will get patented or Apple will steal it.

Can I come over to your house and play?

Yeah, I went there. And no, I'm not sorry :) http://imgur.com/9bnvWRm

Now do it with my piece of shit Shaw router.

Neat hack but aren't there free tools to do this with a laptop and its wifi adapter? http://www.ekahau.com/wifidesign/ekahau-heatmapper

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