At a high level, a router running PoWiFi imitates a continuous transmission while minimizing the impact on Wi-Fi performance. To do so, it injects small amounts of superfluous broadcast traffic on multiple Wi-Fi channels (e.g., 1, 6, and 11) such that the cumulative occupancy across the channels is high. We also design a multi-channel harvester that effi- ciently harvests power across multiple Wi-Fi channels. Because the harvester cannot distinguish between transmissions across channels, it effectively sees a continuous transmission from the router and hence can efficiently harvest power.
This is exactly how their PoWiFi router works. The paper is an excellent proof of concept for the technology - as far as it goes.
However, I have some serious issues with the idea of filling up all of the slack airtime with power transmissions.
The power harvester they describe is a key ingredient in the mix. With further refinements, in addition to modifying their AP's energy detection levels, it could prove to be a welcome addition to WLAN technology in general.
It deserves more study by all of us, if for no other reason than to keep a check on the marketting types that would try to get it sold without further refinement.
It obeys the basic rules of the 802.11 standard. It follows the CSMA protocol. It sounds neat... Heck, the RF environment at my desk is so busy I could probably use one of their harvesters right now !
As is, its use would increase co-channel interference and create near-far and hidden node problems for other networks. They make a big point of how it does not affect clients already attached to the PoWiFi router, but don't even mention authentication/association delays for new members. They acknowledge, but pretty much ignore difficulties encountered in neighborhoods with multiple PoWiFi routers.
All in all, an interesting if not dangerous technology.
They deserve a lot of credit for the work they have done so far. But let's keep an eye on them.
Your thoughts? Anyone ?
A well presented paper. I'm glad they mentioned Tesla who went to his grave with the same unrealized dream. The big issue with any RF energy harvesting is the inverse square law. At 2.4GHz there is 40dB attenuation over the first one meter and 20dB for each ten time increase in range. Their temperature sensor requires around -20dBm at near enough to 100% duty cycle to work. They claim around 20 feet range to be energy neutral IF the device is bathed in wifi electro smog permanently. There are already 10 year lifetime battery powered wireless temperature sensors on the market. Now this is not a new idea, spies have been using this in the audio eavesdropping business for over 70 years and video more recently. Not a problem if you can park a truck with a massive concealed microwave antenna aimed at the receiver coupled with a multi kilowatt transmitter with EIRPs in the many Megawatt category. Check out this completely passive RF energy harvesting audio bug.
Please note there is a bug in the link above. Search on The_Thing_(listening_device)
I suggest they market a new type of video surveillance device where the camera is built into the AP's antenna!
Heh, I was hoping that would stir some discussion of the concept.
My 'No' was more a tongue-in-cheek response to the creators with respect to the same concerns you've also mentioned: "I have some serious issues with the idea of filling up all of the slack airtime with power transmissions". That was my immediate thought as well, which is why I joked ... 'No! (as in "No you won't!")' immediately following the paragraph in which they described doing exactly that ;-)
I completely agree with your assessment of "All in all, an interesting if not dangerous technology." and was specifically hoping that both yourself and karlenr would respond as I always seem to enjoy your posts on these forums the most. Your knowledge certainly exceeds my own, and I always pick up something new.
Well, you got what you asked for :-).
I was hoping for more people to join in. This is definitely something that needs to be discussed and implemented correctly. I'd hate for marketeers, with no real technical expertise, to saddle us with Franken-routers.
Clearly, their Harvester is the key ingredient. I am impressed on their choice of substrate. (I've seen the results of poor selection before.) However, the harvester needs to be even more sensitive. We do not need AP's everywhere pumping out 30 dBm, 24/7. I realize the difficulties, but a universal harvester needs to work with 18 or so dBm from the AP's. But this large an improvement could take decades to acheive, and you can tell from the excitement that this idea has generated, that sales are not that far off. The FCC needs to be ahead of the technology on this one - but not too much!
I am a little confused by their Return Loss plot. The fact that it begins, and ends, with such large losses, makes me think that they are not really looking at the antenna. Maybe their sampling follows an amplifier ? I don't know how you quantify or compare that.
Using a little smoke and mirrors, they make a strong point of their AP not affecting it's own clients performance. I'm afraid too many greedy people will run with that idea, and ignore other realities. For an example, I'm not a eco-nut, but the router is definitely NOT a green device.
How does this Franken-router behavior affect authentication & association speeds after its first client is established - both on itself and for neighboring AP's? How will it affect roaming in general ?
I imagine, correction I hope, that PoWiFi would be restricted to the 2.4 GHz band. It seems like the AP should have to implement a policy to provide more airtime for any networks real data, not just its own. Perhaps their CCA algorithms could use lower threshold settings. This approach would require stricter Compliance controls than those currently in place.
I've been looking at other technologies lately, Zigbee in this case.
It would seem to me that POWiFi could be a real pain-in-the-xxx for ZigBee.
Anyone with Zigbee experience out there that could comment on this ?