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.
* 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
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.
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.
I'm gonna steal this line.
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.
In practice... I have no idea. RF is too much for me.
It's really wild how a foot or two difference could make all the difference in the world...
The shapes aren't very regular, but that could easily be explained by the shape of the room and its contents.
Anyways, reminds me of this: https://vimeo.com/20412632
The author had a nice summary here: http://www.reddit.com/r/dataisbeautiful/comments/2vuvhf/prec...
It's always nice to see multiple approaches to the same problem.
Imagine all these things, wifi, FM/AM Radio, iBeacons, cell towers etc.. It must be so noisy in the radio spectrum..
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.