7 ms·

Wifi: “beamforming” only begins to describe it

153 points by soundsop 12y ago

mmastrac 12y ago
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 12y ago
  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/scripting.html#dom-context-2d-ellipse http://www.whatwg.org/specs/web-apps/current-work/multipage/... 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); }
deleted 12y ago
[deleted]
Geee 12y ago
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.jpg http://www.mpdigest.com/issue/Articles/2013/Mar/agilent/fig2...
- hosay123 12y ago
  Pretty sure the two diagrams are equivalent, its just the one you linked has been greatly simplified
StuffMaster 12y ago
Is this anything like how phased-array radar works?
- lpmay 12y ago
  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 12y ago
    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 12y ago
      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.
abstrakraft 12y ago
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_ 12y ago
  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.
  - madengr 12y ago
    Well to start, the antenna generates it's own noise since it is not 100% efficient; i.e. it has a real resistance in addition to it's radiation resistance. If you put a real antenna in a box made of perfect electrical conductor, it would still generate noise, so you really don't halve the noise by increasing the gain 3 dB, you double the signal. Here is a good tutorial on phased arrays. It's actually a lot easier to think of it when receiving a skewed wavefront, and the phase shifters (delay elements) compensating for the delay. http://www.radartutorial.eu/06.antennas/an14.en.html http://www.radartutorial.eu/06.antennas/an14.en.html
    - jerf 12y ago
      OK, while that seems like a valid tidbit if one has to implement this stuff, it doesn't seem like that's a helpful complication to introduce when trying to explain the basic concept.
  - abstrakraft 12y ago
    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).
p1mrx 12y ago
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 12y ago
  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 12y ago
    > 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
- foobarqux 12y ago
  That's a bad analogy. Just adding a transmitter doubles your power usage. MIMO is more efficient at the same power.
  - jrockway 12y ago
    It's a good way of understanding spatial streams. Each pixel is a highly-directional transmit antenna, and each of your rods and cones is a highly-directional receiver. The result is 1920x1080 streams that you can interpret all at once! (Of course, given the extremely wide bandwidth of visible light and the high SNR between your TV and other light sources, you don't need that many spatial streams to beam the information representing HDTV across space. But humans.)
mikemoka 12y ago
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
akiselev 12y ago
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-antennas/JTI_Antenna-2450AT18A100_10-03.pdf http://www.johansontechnology.com/images/stories/ip/rf-anten... [2] ftp://ftp2.nearson.com/Drawings/Antenna/SG102N-2450V2.pdf [3] http://upload.wikimedia.org/wikipedia/commons/d/dc/Large_Drive-In_EMC_Test_Chamber.png http://upload.wikimedia.org/wikipedia/commons/d/dc/Large_Dri...
- cnvogel 12y ago
  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.
dghughes 12y ago
Resonance is witchcraft. That's all anyone needs to know. Move along.
thisjepisje 12y ago
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 12y ago
  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/
- judk 12y ago
  It's _QED_ (Quantum Electrodynamics), which is both a book (compiled lecture notes) and a video series. It's on YouTube. It was a non-science-major informal lecture series. Very little math.
quarterwave 12y ago
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.
wglb 12y ago
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 12y ago
  "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 12y ago
    "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.

153 points by soundsop 12y ago

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 12y ago
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/scripting.html#dom-context-2d-ellipse http://www.whatwg.org/specs/web-apps/current-work/multipage/... 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); }

deleted 12y ago

[deleted]

Geee 12y ago

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.jpg http://www.mpdigest.com/issue/Articles/2013/Mar/agilent/fig2...

hosay123 12y ago
Pretty sure the two diagrams are equivalent, its just the one you linked has been greatly simplified

StuffMaster 12y ago

Is this anything like how phased-array radar works?

lpmay 12y ago
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 12y ago
  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 12y ago
    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.

abstrakraft 12y ago

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_ 12y ago
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.
- madengr 12y ago
  Well to start, the antenna generates it's own noise since it is not 100% efficient; i.e. it has a real resistance in addition to it's radiation resistance. If you put a real antenna in a box made of perfect electrical conductor, it would still generate noise, so you really don't halve the noise by increasing the gain 3 dB, you double the signal. Here is a good tutorial on phased arrays. It's actually a lot easier to think of it when receiving a skewed wavefront, and the phase shifters (delay elements) compensating for the delay. http://www.radartutorial.eu/06.antennas/an14.en.html http://www.radartutorial.eu/06.antennas/an14.en.html
  - jerf 12y ago
    OK, while that seems like a valid tidbit if one has to implement this stuff, it doesn't seem like that's a helpful complication to introduce when trying to explain the basic concept.
- abstrakraft 12y ago
  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).

p1mrx 12y ago

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 12y ago
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 12y ago
  > 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
foobarqux 12y ago
That's a bad analogy. Just adding a transmitter doubles your power usage. MIMO is more efficient at the same power.
- jrockway 12y ago
  It's a good way of understanding spatial streams. Each pixel is a highly-directional transmit antenna, and each of your rods and cones is a highly-directional receiver. The result is 1920x1080 streams that you can interpret all at once! (Of course, given the extremely wide bandwidth of visible light and the high SNR between your TV and other light sources, you don't need that many spatial streams to beam the information representing HDTV across space. But humans.)

mikemoka 12y ago

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

akiselev 12y ago

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-antennas/JTI_Antenna-2450AT18A100_10-03.pdf http://www.johansontechnology.com/images/stories/ip/rf-anten... [2] ftp://ftp2.nearson.com/Drawings/Antenna/SG102N-2450V2.pdf [3] http://upload.wikimedia.org/wikipedia/commons/d/dc/Large_Drive-In_EMC_Test_Chamber.png http://upload.wikimedia.org/wikipedia/commons/d/dc/Large_Dri...

cnvogel 12y ago
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.

dghughes 12y ago

Resonance is witchcraft. That's all anyone needs to know. Move along.

thisjepisje 12y ago

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 12y ago
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/
judk 12y ago
It's _QED_ (Quantum Electrodynamics), which is both a book (compiled lecture notes) and a video series. It's on YouTube. It was a non-science-major informal lecture series. Very little math.

quarterwave 12y ago

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.

wglb 12y ago

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 12y ago
"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 12y ago
  "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.