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There is a nice PDF here regarding the operation of passive WiFi > http://passivewifi.cs.washington.edu/
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?
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.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.
So if someone says he is generating 100mW of radio signal using 100uW of power
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?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
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