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passive WIFI

Discussion in 'Radio and Wireless' started by cjdelphi, Mar 13, 2016.

  1. cjdelphi

    cjdelphi

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    Oct 26, 2011


    I'm not sure about how this works, if it can receive a packet then it's oscillating at the righf frequency?

    What's the analog circuit required for in the first place?
     
  2. Gryd3

    Gryd3

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  3. Merlin3189

    Merlin3189

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    Aug 4, 2011
    It's hard to tell whether this is bunkum or just something so wrapped up in PR and marketing speak as to be unrecognisable.
    What I can't understand about it, is what is consuming less power than what to produce what. I believe in physics, particularly energy conservation. So if someone says he is generating 100mW of radio signal using 100uW of power, then I don't believe it, however obscure and the description and whatever Humpty Dumpty words are used. (Ref. Gryd3's links)

    That is where your analog amplifier comes in. MOS chips generally consume tiny amounts of power and produce low power signals. If you want 10mW of RF power, then you need to amplify this signal, whether by analogue or digital, with a circuit which consumes more than 10mW.
    As far as the receiving device goes, it neither knows nor cares how much power is being sent out by the transmitter. It depends solely on the strength of signal it receives. The amount of RF power it needs is tiny, but most of the transmitted power never reaches it. For the sort of antenna used on most home wifi, the available power falls off somewhere between a square and cube inverse power law. That's if there are no obstructions. Obstructions absorb power and cause reflections which interfere with the direct signals.
    So at home, if I sit in the same room as the wifi source, I get a good signal. In an adjacent room the signal is a couple of bars weaker (whatever that means) but I still get the full 50Mb/s. Downstairs in the back room it is much weaker. On a good day I can still get 36Mb/s, but some days (perhaps a neighbour is transmitting on the same or an adjacent frequency) it can drop to minimum useable strength and give only 1Mb/s. 11Mb/s is what I get most there. I might like to use it in the garden in summer, but the signal is too weak most of the time.
    At work we used wifi transmitters that output somewhat more power (there's a good reason for vagueness) and we get slightly more range and much better data rates.
    The point being, receivers need signal power to give an adequate s/n ratio. So people generally pump out enough power to give an acceptable bit rate for their link.

    Now as we mostly know, sheer power is not the only answer:
    Directive antennae can give effective increase in power to a receiver by sending more of its power in a particular direction - if we know what direction that is. There are wifi systems that have electronically steerable arrays, listen for a client, do DF on it, then align their antenna in the best direction. So can establish a reliable link with less power or longer range or higher bit rates or whatever combination they choose.
    Access points can scan the available frequencies and choose the least noisy one to use for a connection at any given moment. Less noise, better s/n and bit rate for a given power, or same s/n & bit rate for lower power.
    Clever signal coding, processing and error detection & correction can improve the throughput: s/n ratio.
    But all of this is well known and mostly in the IEEE standards.

    I think I understand ways that passive wireless transmitters could work, but until someone comes up with an explanation that sounds more like an RFC or draft standard than an advert for snake oil, my vote is that it is bunkum.
     
    davenn likes this.
  4. Gryd3

    Gryd3

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    I like your write-up, although I think the whole purpose of this tech is making very low power WiFi devices that you can embed into other things. What is skimmed over is that these 'passive' WiFi chips operate in a similar manner to RFID and need a power source, and the output and range of this power source was not covered in detail in the video. I'm sure you noted the video shows two 'passive' WiFi devices on the table, and a device on the right-hand of the screen? Without this device transmitting, the passive devices would not operate. They are simply offloading the power requirement from one device to another.
    We will see what happens if this technology matures...
     
  5. AnalogKid

    AnalogKid

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    Jun 10, 2015
    Actually, no one said that. In fact, this technique does not "generate" anything. That's the point.

    A chronic problem with hotel and motel key cards is the extremely low magnetic coercivity of the magnetic stripe. It is easy to re-record, which makes it easy to erase. So easy that merely being near a standard credit card (with its much higher field strength stripe) can kill it. Ahh, but progress - In my Phoenix trip to watch The Ohio State Buckeyes defeat Noter-Doter in the Fiesta Bowl, my room came with an RFID key card. Just wave it near the door lock and it opens. This is the same as my work ID / building access card, and is the same as passive WiFi. So if it is bunkum, it is bunkum that was theorized in the 40's, developed in the 60's, commercialized in the 70's and finally has come down enough in price to be noticed by the general public.

    Passive WiFi is a modern variation of the Great Seal Bug, arguably the greatest-ever inception of the true nature of radio waves.

    https://en.wikipedia.org/wiki/Passive_Wi-Fi

    ak
     
  6. Gryd3

    Gryd3

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    RFID requires a source to energize it though, as does passiv WiFi... what kind of power do you think would be required from this 'source' to allow the passive WiFi to operate?
    It's a neat idea... you could have a WiFi device running off a pair of AA for a few months it would seem...
     
  7. AnalogKid

    AnalogKid

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    If you watch the video, the passive devices are being detected at 30 dB less energy than the standard WiFi signals in the same room. In a passive RFID system the source transmitter does power the tag. The tradeoff is that the higher the transmitter power, the farther away the tag or ID card can be and still respond - but the higher the radiation level flooding anyone walking by. In passive WiFi the passive device has its own energy source; it is not powered in any way by the host WiFi signal, so the hotspot power is at industry standard levels. What it does is create, literally, a disturbance in the force. That disturbance appears to the WiFi hotspot as modulation.

    ak
     
  8. Merlin3189

    Merlin3189

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    Aug 4, 2011
    Pretty much agree with all your comments.
    If this is very short range like RFID, no problem.
    They just made it sound like the answer to all our wifi problems, which that would not be.

    Yes ak, I know they didn't claim any specific output power, but if they want to match the performance of even a home wifi access point, they need to put out a signal at least in the 10's mW range and you don't get that from chips running a few 10's uW. (Unless you use a Tesla coil, of course! Or maybe Graphene. Or whatever is "in" with the free energy nutters at the moment. Oh, perhaps that's passive wifi :rolleyes:)

    Maybe they have a solution for the 'network of everything' in the home. A powerful transmitter in each room, powering all the passive devices with enough juice to send a signal to a receiver in the same or maybe adjacent room could be useful.

    If their signal is 30dB down on a normal wifi signal at the same range, you're talking about 30dB lower s/n ratio, or only 10% of the range of a normal signal.

    Now it would be interesting to know more technical detail here. Just because their chip uses only 50uW does not mean that the reflected (their description) signal is only of that order. Maybe their device receives more power than that from the mother ship, their chip can modulate that larger signal and send out more than 50uW?
    Perhaps one of you younger whiz kids (or analogue kids!) could do the sums to see if this is plausible, but my guess is you'd be lucky to reliably receive significantly more power from the mothership. (I think of radios receiving nW powers or even pW rather than many uW - digicomm resources link )
    Perhaps the device can receive and store up energy from the mothership transmitter to power an occasional single small packet transmission? (Though it doesn't sound like that.)

    It could be something interesting, but they put me off with all the hype. Just give us the facts ma'am. Just the facts.
    I'm a bit of an old cynic at times and that video and accompanying blurb reminded me so much of some of the sales presentations I suffered at work. Particularly the ones labelled, Technical Briefing for Senior Managers - ie. our salesmen can tell you any old guff so long as there's lots of buzz words in it, because we know you don't understand any of it. And it may be just their bad luck that a couple of videos further down on YouTube was a guy telling us how to triple our wifi speed by taping two AA cells to the antenna. Maybe I shouldn't judge them by the company they keep.
     
    Gryd3 likes this.
  9. AnalogKid

    AnalogKid

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    "They just made it sound like the answer to all our wifi problems, which that would not be."

    No, they didn't. I don't know what all our wifi problems are, but I didn't hear anything about them in the video. What I heard was that many short-range applications do not need a full-power transmitter, and that a very low power, short range device (not necessarily a transmitter) that interacts with a standard wifi hot spot in the standard manner could enable putting wifi connectivity into a larger array of (primarily smaller) products and systems.

    The sums are pretty simple, because 0 + 0 = 0.

    Much of your response centers on various aspects of the system receiving and transmitting power. This is not correct. Again, a passive WiFi system does not receive power from the field. It is not powered by the field. The units in the video are power by batteries. Their chip does not "send out more than 50uW". It does not send out 50 fW. It does not transmit energy. They "didn't claim any specific output power" because there is no output power. It is not a re-spin of a conventional WiFi communication system. It does not and cannot replace a standard WiFi access point, because it depends on an external access point to supply the RF field. It is different. It operates on different principles. As stated in the video and/or the website, it does not attempt or claim the range of a traditional WiFi system. Commercial products using this technique have been available for over 40 years, so I think plausible has been covered. What makes this system impressive is the data rate. Everything else about it, while maybe not as well known as other communication methods, was worked out before either or us was born.

    ak
     
    Last edited: Mar 15, 2016
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