
Wifi: “beamforming” only begins to describe it - soundsop
http://apenwarr.ca/log/?m=201408#01
======
akiselev
Interesting article but man the first section could have used some more
graphics. Drawings of the sphere/circle he talks about is included with almost
any antenna datasheet and looking at the pattern for various types of antennas
is helpful.

For example, the last page of a datasheet [1] for a ceramic chip antenna (size
of a capacitor or resistor commonly used on small electronics like
smartwatches or glasses with -0.5 dBi gain) shows the sphere in various cross
sections. The last page of a datasheet [2] for a "whip" antenna shows an E
plane pattern (looking at the side of the antenna as it stands up) that looks
like a pattern from a Rorschach test but the H-field (looking down at the
antenna) is almost a perfect sphere.

Most (all?) of these diagrams come from RF testing rooms like the one in this
shot [3]. You can't see it in the photo but usually in the floor is a rotating
platform with a coax cable carrying an RF control signal coming from the
testing instrumentation.

[1] [http://www.johansontechnology.com/images/stories/ip/rf-
anten...](http://www.johansontechnology.com/images/stories/ip/rf-
antennas/JTI_Antenna-2450AT18A100_10-03.pdf)

[2] ftp://ftp2.nearson.com/Drawings/Antenna/SG102N-2450V2.pdf

[3]
[http://upload.wikimedia.org/wikipedia/commons/d/dc/Large_Dri...](http://upload.wikimedia.org/wikipedia/commons/d/dc/Large_Drive-
In_EMC_Test_Chamber.png)

~~~
cnvogel
I have no idea about the amount of diagrams in data sheets... But you
certainly can generate all of these diagrams by simulation, which is
frequently done when researching/designing new antenna types.

[http://www.nec2.org](http://www.nec2.org) is a very popular program.

------
abstrakraft
I discourage anyone wanting to understand the physics of beamforming antennae
from reading this article. The author has a very poor grasp of the concepts.

~~~
akerl_
Can you elaborate? Without some kind of analysis of the mistakes in the
article, we don't really have a way to judge if we should belief your
statement about the author, nor can we improve our understanding.

A more descriptive listing of some of the issues with the article, or links to
better resources, would rock.

~~~
abstrakraft
Any time someone appeals to quantum physics to offer a handwaving explanation
of something that classical physics covers just fine, watch out. Using path
integrals to introduce interferometry is a waste of time, and in this case,
just plain wrong.

For a better source, I'd start with the wikipedia articles on beamforming,
phased arrays, and interferometry
([http://en.wikipedia.org/wiki/Beamforming](http://en.wikipedia.org/wiki/Beamforming),
[http://en.wikipedia.org/wiki/Phased_array](http://en.wikipedia.org/wiki/Phased_array),
[http://en.wikipedia.org/wiki/Interferometry](http://en.wikipedia.org/wiki/Interferometry)).

~~~
MarkSweep
Both the beamforming Wikipedia article and the OP basically say "by having
several out of phase antennas you can create constructive interference at the
intended receiver of the signal". What am I missing?

~~~
judk
OP (and also Richard Feybnan) believes that QM provides a more intuitive
explanation for the elusive "why" waves (interference) exist. Commenters here
don't like that.

------
p1mrx
A traditional radio would be like an LED and a photocell; you can blink the
LED and detect it with the photocell, but the maximum data rate is fairly
small.

MIMO is more like vision. With a 2D array of pixels and a 2D array of
photocells, you can transmit a vast amount of information through the same
volume of space, and nearby equipment can reuse the same colors without much
interference.

With smarter radios and more antennas, radio still has a long way to go before
we hit the universe's data cap.

~~~
Geee
Expect everything bounces around, so it's like trying to decipher TV by
watching the flickering on the wall. :) Actually, MIMO makes use of this (it
works better in 'bouncy' non-line-of-sight multipath environments). By just a
small antenna separation, the spatial channel characteristics can vary a lot
as the signals might take a completely different paths, and the receiver is
able to separate them from each other.

~~~
IvyMike
> decipher TV by watching the flickering on the wall.

Off topic, but... this has been done. Amazes me to this day.

"Optical Time-Domain Eavesdropping Risks of CRT Displays"
[http://www.cl.cam.ac.uk/~mgk25/ieee02-optical.pdf](http://www.cl.cam.ac.uk/~mgk25/ieee02-optical.pdf)

------
mmastrac
Broken in Firefox due to missing Canvas2D ellipse method. I filed a bug in
Bugzilla
([https://bugzilla.mozilla.org/show_bug.cgi?id=1067039](https://bugzilla.mozilla.org/show_bug.cgi?id=1067039)),
but a quick fix would be replacing the ellipse with a rectangle.

~~~
rwg
It's partially broken in Safari 7.0.6 for the same reason — the ellipse method
isn't a WebKit-ism, it's a WHATWG-ism that Google's implemented:
[http://www.whatwg.org/specs/web-apps/current-
work/multipage/...](http://www.whatwg.org/specs/web-apps/current-
work/multipage/scripting.html#dom-context-2d-ellipse)

A quick workaround is to run the following in the error console:

    
    
        ctx.ellipse = function (x, y, radiusX, radiusY, rotation, startAngle, endAngle, anticlockwise) { ctx.arc(x, y, Math.max(radiusX, radiusY), rotation, startAngle, endAngle, anticlockwise); }

------
wglb
There is a lot about this article that is not to like. With tutorial articles,
I am of the opinion that all analogies are false and therefore bad. And
resorting to quantum physics to describe the impact of Maxwell's equations is
another bad signal.

 _The thing to notice about SNR is that you can increase it by increasing
amplification at the sender (where the background noise is fixed but you have
a clear copy of the signal) but not at the receiver._

This presumes that the signal that you are looking for is above the
detectability threshold at the receiver. It may well not be, if you are trying
to capture a weak signal.

Let's put aside the concept of a MASER used in radio astronomy and the size of
their antennas. Or the fact that your wifi router or dongle has an amplifier
inside of it. Let's do an experiment.

I have two instances of kismet running in my lab here in the leafy suburbs.
One has a 3.5 inch antenna attached to a fit pc with internal wifi, the other
has a 14 inch antenna attached to an Alfa with kismet running on kali in a vm
on my mac laptop. The 14 inch antenna is successfully decoding 167 wifi
networks and the 3.5 inch antenna sees 35 networks. I don't know what the
relative gain rating of these antennas are, or what the relative quality of
the radios is, but the dramatic difference on the receive side is pretty
telling.

~~~
ssivark
_" all analogies are false and therefore bad. And resorting to quantum physics
to describe the impact of Maxwell's equations is another bad signal."_

That (EDIT: The analogy used in the article) is not just a good analogy, but
summarizes our best understanding of both light and quantum physics. (Feynman
knew exactly what he was talking about -- he invented/discovered some of the
fundamentals). Whether you'd use quantum physics as an analogy to explain
light, or light as an analogy to explain quantum physics is a matter of taste
-- which one you find more intuitive and which less intuitive -- which depends
on what you've been told by others previously.

~~~
abstrakraft
"all analogies are false and therefore bad"

I disagree with this statement. If you try to learn everything from first
principles, you'll never get anywhere. Students need analogies to understand
the big picture while learning the details.

"That is not just a good analogy, but summarizes our best understanding of
both light and quantum physics"

And the Lesbeque integral is a more rigourously defined operator that the
Riemann integral, but would you use it to introduce calculus to first year
students? Of course not - you teach the basics, and in grad school, you let
the students that need to worry about the more complicated stuff take Real
Analysis.

There are certainly connections between interferometry and quantum physics.
However, I don't see how the author's explanation of the subject is enhanced
by using quantum physics. For the purpose of this article, classical physics
explains the phenomenology just fine. As I said in another comment below, and
wglb said above, appealing to quantum physics when it is entirely unnecessary
is a bad smell, and is usually nothing more than a distraction to make the
author sound more sophisticated.

~~~
ssivark
The Lebesgue integral can be explained as intuitively as the Riemann integral
([https://en.wikipedia.org/wiki/Lebesgue_integration#mediaview...](https://en.wikipedia.org/wiki/Lebesgue_integration#mediaviewer/File:RandLintegrals.png)).
One can certainly have the intuition for both without too much knowledge of
real analysis. And in general, I believe that analogies are very useful.

The author explains in the article how to understand the behaviour of light.
It turns out quantum particles behave in _exactly the same way_. Feynman
(building on Dirac's observations) built up the theory for quantum particles.
With regards to light, this understanding comes from a couple of centuries ago
(Ref: [Huygens
principle]([https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_princi...](https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle))
and [relations to quantum
mechanics]([https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_princi...](https://en.wikipedia.org/wiki/Huygens%E2%80%93Fresnel_principle#Huygens.27_principle_and_quantum_electrodynamics\)))

~~~
abstrakraft
"The author explains in the article how to understand the behaviour of light."

That isn't the argument. What I'm arguing is the (lack of) utility of the
explanation. In what way does it enhance, support, or in any way contribute to
the article? His argument is essentially:

A. We think of light as traveling in straight lines.

B. But because quantum physics, they don't! They travel along infinitely many
paths.

C. But all of these paths cancel out except the straight line path.

D. And that's how beamforming works. Except it's beam-un-forming.

So he brings up a quantum phenomenon to complicate the scenario, then
immediately reduces it back to the original scenario, without ever explaining
how that original scenario works (i.e. how waves add together to create
constructive and destructive interference: note that the words "wave" and
"phase" never occur in same paragraph), then claims that this is somehow
elucidating. So what's the benefit?

~~~
judk
As stated in the article, the point is to give an intuition for how energy
appears to "move" from troughs to peaks: by going everywhere.

This coincides with the Pilot Wave model, where particles travel along paths
modeled by the peaks of mathematical waves.

------
Geee
I don't think that's quite correct. You can definitely create more defined
'beams' when configuring antenna placement and phase shifts correctly. Like
this:
[http://www.mpdigest.com/issue/Articles/2013/Mar/agilent/fig2...](http://www.mpdigest.com/issue/Articles/2013/Mar/agilent/fig2.jpg)

~~~
hosay123
Pretty sure the two diagrams are equivalent, its just the one you linked has
been greatly simplified

------
quarterwave
Anyone dealing with antennas and noise should read the classic 1946 paper by
Dicke:
[http://www.eng.yale.edu/rslab/internal/Papers/dickepaper.pdf](http://www.eng.yale.edu/rslab/internal/Papers/dickepaper.pdf)

tl, dr: One puzzler is that antenna radiation is frequency dependent, while
that of Johnson noise in the microwave regime is flat with frequency. Then,
considering detailed balance, how does an antenna matched to a terminated coax
cable establish thermal equilibrium with space? What happens is that the 1/f^2
of the antenna pattern is cancelled by the f^2 (in the long-wavelength regime)
of the Rayleigh-Jeans blackbody radiation formula.

------
mikemoka
Steve Perlman's take on MIMO:

[http://www.rearden.com/DIDO/DIDO_White_Paper_110727.pdf](http://www.rearden.com/DIDO/DIDO_White_Paper_110727.pdf)

as cited also in this talk:

[https://www.youtube.com/watch?v=1QxrQnJCXKo](https://www.youtube.com/watch?v=1QxrQnJCXKo)

------
thisjepisje
Anyone know where I could find the Feynman lecture the author is talking about
here:

 _The best explanation I 've found relates to quantum mechanics, in a lecture
I read by Richard Feynman at some point._

?

~~~
cnvogel
As people have already mentioned, quantum mechanics isn't needed for
understanding wave interference, but as you've asked:

The Feynman Lectures on Physics can be found online here, you probably want to
read part Ⅲ.

[http://www.feynmanlectures.caltech.edu/](http://www.feynmanlectures.caltech.edu/)

------
StuffMaster
Is this anything like how phased-array radar works?

~~~
lpmay
I think the "phased-array radar" you're referring to would be more precisely
called "active-phased-array radar". Yes, the concepts are similar. By
providing a calibrated time delay between elements of the array, the direction
of focusing can be controlled. I'm not sure how this is done in practice for
actual radars. If it is done in hardware, the array would have a single
steerable pattern. If instead each array element has a clock-synchronized
direct conversion digitizer, the beamforming can be done in the digital domain
and the number of logical beams would be limited by the DSP capacity
available.

~~~
madengr
That is a good summary. Some radars use phase shifters, and some true time
delay elements. They are equivalent for narrow bandwidths, but the true time
delay is needed for wide bandwidths.

~~~
lpmay
Thanks!

In case anyone else is interested: the distinction between phase delay and
time delay comes into play if bandwidth is large as you say, but in one more
case as well. If the array is large enough that the time delay across the
array is on the order of a symbol period you must use a true time delay. If
you use a phase delay instead of a true time delay, energy from separate
symbols will be added together to make the final bit decision and the
resulting ISI will increase BER.

------
dghughes
Resonance is witchcraft.

That's all anyone needs to know. Move along.

