Hacker News Reader

Comment by tux1968 | Hacker News Reader

tux1968

The low-power devices don't actually generate any signal at all thus don't have to pay that expensive power cost. Instead a passive antenna is modulated to reflect or absorb ambient signal, the reflected signal conforms to traditional 802.11 packets. The ambient signals are created by a relatively high-powered device that is plugged into wall power.

Here is a 2 minute video showing an early prototype of their work from 2014:

https://www.youtube.com/watch?v=snZWgAZqo2c

View on HN

I've looked at the paper [1], but I can't figure out how this is different from backscattered RFID with 802.11 on top. The paper says RFID requires a full-duplex radio while the WiFi chipset is orders of magnitude less expensive. Are they modulating differently or is there some RF magic?

[1] http://passivewifi.cs.washington.edu/files/passive_wifi.pdf

The technology is ~50 years old, and was originally invented by the Soviets to eavesdrop on the US embassy in Moscow.

https://youtu.be/mAai6dRAtFo?t=9m35s

https://en.wikipedia.org/wiki/The_Thing_(listening_device)

TL;DR Using a passive antenna to modulate a directed microwave emission.

When will they reevaluate those twelve wing aeroplane designs

Wow that's interesting, how did they find "The Thing"?

https://en.wikipedia.org/wiki/The_Thing_(listening_device)#D...

Passive RFID is powered by the incoming radio waves that is 1000x stronger than the signal transmission energy from the RFID tag.

There is no statement to the power requirement for passive WIFI from an active WIFI power source. The active WIFI transmitter may require outputting radio waves many times more powerful in order to receive a signal transmission from the passive wifi device.

edit: For that reason, I'm not that excited about this, and this is likely why it wasn't pursued earlier due to the power requirements of active <-> passive signalling. If it pollutes the EM spectrum with powerful radio waves that are magnitudes more powerful than what we're outputting today, then I hope it gets abandoned. If it's the same power consideration, then it's worth pursuing imo.

Come to think of it, rather than following the usual inverse square law, power should drop off with the fourth power of distance (just like with radar) - there'd be one inverse-square dropoff on the path from active transmitter to transponder, multiplied by a second inverse square loss on the return path. That's got to hurt performance.

The maximum transmitting power is regulated. In the article they state that it can use some kind of crafted wifi packet to be compatible with any existing devices.

Edit: After looking at the pater, it seems that I may have misunderstood the article, it needs a plugged RF carrier tone generator at at fixed frequency.

Imagine coming up with the frequency scheme in an apartment situation; if the building were running the network it would be one thing, but I would expect individual units to foster their own connection.

Is it the sort of tech used by some devices in the NSA ANT catalog? It is useful for spying because they are undetectable if they are not subjected to a carrier frequency. There was also The Thing[1]

[1]https://en.wikipedia.org/wiki/The_Thing_(listening_device)

Did they just say enabling internet for billions of battery free devices? Is it like a push method? So I guess I could imagine state updates, or task pushes to autonomous agents.

Their video from 2013 shows some pretty cool applications of an earlier version of this tech that looks essentially like passive NFC using ambient RF rather than using 802.11.

Very cool stuff!

As far as I can tell, it seems to work the same way that passive RFID tags work. Pretty neat, I guess.

Almost. Technically it's the same as active (not passive) RFID tags: both reflect RF signals, and both need a source of battery power is needed to open and close the antenna circuit.

Passive RFIDs harvest power from the RF signals.

Reply on news.ycombinator.com