[HN Gopher] WiFi: "beamforming" only begins to describe it (2014)
___________________________________________________________________
WiFi: "beamforming" only begins to describe it (2014)
Author : Tomte
Score : 181 points
Date : 2023-02-02 10:39 UTC (12 hours ago)
(HTM) web link (apenwarr.ca)
(TXT) w3m dump (apenwarr.ca)
| pgorczak wrote:
| One addition on receiver side amplification: it is done all the
| time. Each piece of equipment has a noise floor that can be
| higher than the one in the signal (think weak signals but with a
| good SNR). A low noise amplifier at the receiver can improve the
| signal in that case.
| dboreham wrote:
| Every receiver (perhaps the exception is a spark-gap receiver)
| has an amplifier. The article is trying to say that you can't
| improve the S/N ratio of a signal with an amplifier. You can
| always make it worse though. The aim is to make it less worse.
| nemacol wrote:
| I would like anti-beamforming so I can create a wifi shadow and
| mess with people on my network.
| labcomputer wrote:
| Already exists, it's called MU-MIMO in the 802.11 world.
|
| Edit: Just to expand on this a bit... typically, the nulls of
| an antenna's radiation pattern are sharper (in terms of dB per
| degree) than the peaks (and this is true for both phased arrays
| and traditional antennas).
|
| In other words, you can shape the beam to put one receiver in a
| very deep null while only slightly reducing the RSS at the
| other receiver. Do that for two beams at once, and each
| receiver gets (almost) as much power (from the intended beam)
| as they would if the beam were steered directly at it, while
| getting very little power from the other beam.
|
| Adding more elements to the TX side enables a greater number of
| and deeper nulls.
| wnkrshm wrote:
| You'd need to be connected to their wifi hardware to do that,
| since you wouldn't get interference between your beams and
| theirs without having two coherent sources.
| mlaang wrote:
| I've made a shadertoy doing essentially the same thing, but
| forming directed beams.
|
| Two beam version: https://www.shadertoy.com/view/NtyXDm
|
| One beam version: https://www.shadertoy.com/view/st3XWX
| readyplayeremma wrote:
| That is really cool! What a fun way to visualize beamforming!
| mlaang wrote:
| Thanks.
| lolc wrote:
| Reads like a good intro into how beamforming works. One detail I
| found dubious is this part about bigger antennas:
|
| > To stretch our non-mathematical metaphor well beyond its
| breaking point, a "bigger sphere" will contain more signal as
| well as more noise, so just expanding the size of your region
| doesn't affect the SNR. That's why, and I'm very sorry about
| this, a bigger antenna actually doesn't improve your reception at
| all.
|
| In my understanding that is incomplete because it ignores amp
| noise. When your antenna is larger, you get more signal (yes
| including more noise), so you need to amplify it less before the
| signal can be sampled. That means less amplification noise and
| the effective SNR is better. Thus antenna gain, not just antenna
| directionality, will improve reception.
| giovannibonetti wrote:
| Larger antennas help even though both signal and noise are
| amplified. The difference is that former is more amplified due
| to coherence: the antenna is shaped in a way to add the signal
| parts in the same phase. On the other hand, the noise is added
| together in a non-coherent way, with different phases, which
| makes the result smaller.
|
| Think about it as the difference between summing two unit
| vectors in the same direction - with a resulting vector of
| magnitude 2 - and doing the same thing with two ortogonal unit
| vectors - the result will have a magnitude sqrt(2).
| themitigating wrote:
| So I'm curious as to why UHF and VHF require larger antennas.
| My parent's house was about 100 miles from the WTC and the
| antenna was massive. People in the city though used smaller
| sized ones.
| sgtnoodle wrote:
| People closer to the transmitter receive enough raw signal
| power that they can get away with a really insensitive
| antenna. Farther away, the antenna needs to be more optimal
| to pick up enough signal to be usable. It's not that the
| giant antenna is magically better, it's that the smaller
| antennas are mediocre.
| Spooky23 wrote:
| You brought back a fun memory. When my family moved upstate,
| about 125 miles north of the city, my dad was determined to
| watch the yankee games on channel 11, and my grandfather
| wanted to see his Mets.
|
| He special ordered a massive antenna at Radio Shack. This
| thing disassembled was the full length of an old full size
| Chevy station wagon, I'm guessing 12 feet in the box and a
| bit longer assembled. Dad and grandpa mounted the thing to a
| pole that was also quite high attached to a device that would
| rotate the antenna.
|
| When it was time for a game, you had to rotate the antenna to
| hit a specific gap in the mountains, which varied for
| different channels. Good times.
| h2odragon wrote:
| VHF requires larger antennas than UHF.
|
| most of the reason bigger TV antennas were a thing was the
| "bigger is better" illusion. They make fancy stacks of
| elements and pt "+28 Dbi" numbers on the box but at any
| specific frequency those could be outperformed by a simple
| single cut wire. (they weren't utter horseshit: considered
| across the _whole band_ they could be better for being
| bigger, but thats a technical and theoretical "win")
|
| There are places where "more metal harvests more signal" in
| antenna design, but they're little dots on a broad map, not
| where you'd expect them to be. Herringbone antennas at WiFi
| freqs are fun.
| sgtnoodle wrote:
| Yagi arrays are a real thing, and they do take up a lot of
| space.
| ryanianian wrote:
| The single cut wire will only be better at its resonant
| frequency. The TV antennas are designed to cover their
| entire bands.
| pcl wrote:
| UHF and VHF (that is, Ultra High Frequency and Very High
| Frequency) are the 3GHz-300MHz range and the 300MHz-30MHz
| range, respectively.
|
| That translates to a 10cm-1m wavelength for UHF and 1m-10m
| for VHF. Minimal effective antenna size for a basic wire
| antenna is 1/4 the wavelength, so you end up with big
| antennas for low frequency / large wavelength.
|
| https://udel.edu/~mm/ham/randomWire/
| derstander wrote:
| Note that different organizations use the same terms for
| slightly different bands. In the IEEE nomenclature for
| radar, UHF is 300MHz to 1GHz. 3GHz is solidly within
| S-Band. And L-Band is in between them.
| xen2xen1 wrote:
| Every frequency has a wavelength, as in a literal length of
| wave, which means a literal length of perfectly sized
| antenna. There's a formula. The right size for FM (100mhz)
| antenna is something like 4 or 5 feet, but for wifi (2.4ghz),
| it's the size of a water molecule. That is not an accident,
| as the FCC did not lease 2.4 out permanently since water
| absorbs it, hence we can just plop an unregulated router/AP
| in our house. This paragraph is probably vague and
| generalized, and possibly wrong-ish towards the end, but
| close enough to aid general understanding.
| lolc wrote:
| Fluid water absorbs it very well, but the wavelength for
| 2.4 GHz is 0.125 m.
| usrusr wrote:
| "length of a water molecule (at an accuracy of 1 ft)"
| galangalalgol wrote:
| Yeah antennas need to resonate, not just absorb. A litre
| of water wouldn't be a very good antenna at 2.4, but it
| would warm up due to absorption if it is a trong signal.
| A vhf antenna might make a terrible antenna at 2.4 as
| well, but it would also warm up in a strong enough
| signal. Don't belive me? Shove one in a microwave (please
| don't really).
| kris_wayton wrote:
| The article does later get into "changing the shape of the
| scoop", meaning a more directional antenna. And the designs for
| directional antennas are generally larger than omnidirectional
| ones.
| mjb wrote:
| "Beamforming" (and the author is right to be skeptical of that
| term) is some really cool magic. My understanding of the history
| is that a lot of the technology comes from military radars.
| Development there was driven by power and compactness, and the
| relatively novel need (at the time) of building a radar that
| could both track targets and scan for new targets at the same
| time. Before that, you'd have a scan radar (antenna turning round
| and round), and a tracking radar (antenna pointed at the target).
| With beam forming tricks you can scan without moving the antenna,
| and then quickly jump back to get some additional tracking data.
| These days track-while-scan is easier because we can use computer
| models to do things like track interpolation, but back when this
| stuff was being developed that was much, much harder.
|
| Twenty years ago when I was doing my PhD work, we were working
| primarily on passive radars (technically something called Passive
| Coherent Location (PCL)). With passive radars you have some
| transmitter of opportunity (a TV tower, or cell tower, or
| somebody else's radar), and want to observe the energy from that
| transmitter bounce off other things. One thing that's hard about
| that is that the transmitter is usually closer than the target,
| and the target doesn't reflect all its energy, so the returns
| from the target are many order of magnitude weaker than the
| direct return from the transmitter. So you need to design your
| antenna or antenna array very carefully to get only a small
| amount of the transmitter power (don't want to saturate your
| analog side), while getting as much of the target power as
| possible. You can do that with static antennas with "manual
| beamforming", but dynamic approaches allow you to correct for
| things like multi-path and changing atmospheric conditions.
|
| Our particular work was in very low cost versions of this, with
| the end goal of spreading transmitters all around ("netted
| radar"). Good for cost, good for sensitivity, good for counter-
| stealth, but meant a very limited budget for any station. With
| the technology available now for building antenna arrays, we
| could have done so much better than we did back then. Amazing how
| the technology has changed in just a couple decades.
|
| This seems to have been driven by consumer products. Wifi, cell
| phones, bluetooth, etc. The miniaturization, cost reduction,
| power reduction, etc. The technology came originally from big
| military radars, but what's driven the revolution seems to be the
| volumes and demands of consumer products. That's definitely going
| back into military and high-end civil technology, which is going
| to drive even more interesting requirements. A fun case study in
| technology.
|
| (I have no non-public information on military radar technology,
| US or otherwise.)
| beambot wrote:
| I worked on applications similar to your PCL: Coherent
| backscatter systems for long-range RFID. It was really fun to
| apply "old school" techniques such as phased array direction of
| arrival and FMCW ranging for passive RFID tags.
|
| What used to consume entire tabletops is now condensed into
| exceedingly capable software defined radios. It's _insane_ what
| you can get for a couple-$k from places like Ettus:
| https://www.ettus.com/
| mjb wrote:
| Related to tracking radars, conical scan and monopulse tracking
| has got to rank among the best brilliantly simple ideas:
| https://en.wikipedia.org/wiki/Monopulse_radar
|
| A monopulse tracking radar can track a target within hundredths
| of a degree, with no more than a handful of components. You can
| build a monopulse sonar with two opamps.
| derstander wrote:
| The rule of thumb that seems to be common among my peers is
| that monopulse offers an angle accuracy improvement of
| roughly 10 to 1 for objects with nominal SNR (13ish dB).
|
| So a radar with a real beam width of 1deg employing monopulse
| would result in an accuracy of 0.1deg if the object can be
| reliably detected.
| mjb wrote:
| Interesting. I've been out of this field for 20 years, and
| the hundredths of a degree came from my fuzzy memory. Happy
| to defer to your more recent knowledge.
| halz wrote:
| I am reminded of an article some years ago about a group that
| claimed to detect/track the F-35 via this sort of passive radar
| technique. https://archive.is/1t5eT and/or
| https://www.c4isrnet.com/intel-geoint/sensors/2019/09/30/ste...
| H8crilA wrote:
| AESA radars are the apex predator of the electromagnetic
| spectrum, with often over a thousand individual tx/rx modules. If
| you want to see a funny take on it:
| https://www.reddit.com/r/NonCredibleDefense/comments/zvt9ao/...
|
| PS. Do not ever approach a radar, it can boil you alive!
| Arrath wrote:
| Oh that's good.
| mjb wrote:
| > PS. Do not ever approach a radar, it can boil you alive!
|
| "Fun" related story. For a year or so in grad school I was in
| charge of the microwave and radar lab. The safety folks came
| for an annual inspection, and I toured them around the lab.
| Lots of "this can cook you from across the room", "this makes
| 50kV", "this seemingly normal 220V 50Hz outlet is actually 115V
| 400Hz". That kind of thing.
|
| A week later we got the inspection report. It recommended we
| upgrade our requirement for closed shoes to include steel toe
| caps. No other safety measures or signage needed.
| KRAKRISMOTT wrote:
| > _It recommended we upgrade our requirement for closed shoes
| to include steel toe caps._
|
| Are you sure that's a good idea when you work near high
| powered microwave equipment?
| [deleted]
| 5960154000 wrote:
| [flagged]
| qikInNdOutReply wrote:
| We experimented with decentralized networks during our university
| classes and one of the professors described this, basically you
| can form a phased array with loads of small senders in perfect
| timewise sync, sending slightly delayed. Its very energy
| intensive though. The sending and the high resolution clockworks.
| (Similar to gps recieving).
|
| Funny sidenode: We found out that almost every building has a
| huge central antenna, called elevator ropes & shaft. Attach a
| wifi router to that and everyone in the building can overcrowd
| that one central wifi.
|
| Good times.
| yellow_lead wrote:
| > We found out that almost every building has a huge central
| antenna, called elevator ropes & shaft.
|
| Very fun idea :)
| RicoElectrico wrote:
| > high resolution clockworks. (Similar to gps recieving).
|
| There has been a paper about access arbitration to a Wi-Fi
| network using FM RDS data for timing
|
| https://hackaday.com/2015/11/11/improving-wifi-throughput-wi...
| h2odragon wrote:
| > very energy intensive
|
| utterly unrelated (but fun) anecdote: Had a "phased array" CB
| antenna setup consisting of 3 antenna elements, carefully
| placed and spaced on a mobile ground plane, and very carefully
| cut and coiled lengths of Big Chonky cable too hook them all up
| to the radio. It worked well; did Chicago to Colorado once on
| legal 4watt power.
|
| But then someone gave us a "1000w linear amplifier" to try.
| This turned out to be a _bad idea_. the third antenna element
| glowed red a few seconds after key up and then the harness
| joints de-soldered. Best we could figure the SWR _changes_
| induced by the antenna whips moving around weren 't big enough,
| at 4w input power, to even notice. but when stood up to larger
| potential the tiny misalignment caused by the tips of the whips
| waving were enough to result in significant direct power loss.
| derekdahmer wrote:
| Can multiple APs work together to do beam forming or do all there
| antennas have to be in a single AP?
| misterdata wrote:
| Not sure about WiFi but the idea exists in the 5G standard as
| 'coordinated multipoint' (CoMP)
| mmis1000 wrote:
| Wifi APs always operate independently. Even it has the same
| ssid and same local network. Each ap still talk to client by
| themselves. The illusion you moves between AP is done by
| 'disconnect to one and connect to other'
| lxgr wrote:
| You'd have to synchronize transmission times to a very high
| degree of precision.
|
| I'm not sure if that's within the realm of something like PTP,
| even if the APs are connected via wired Ethernet.
___________________________________________________________________
(page generated 2023-02-02 23:01 UTC)