Because the phone relies on those signals for its energy harvesting, it has a range of just 15 meters from the basestation.
This is neither a cellphone nor solar-powered. It's an RF-powered cordless phone. Interesting nonetheless.
Like non-traditional solar panels; a company called Ixys has some interesting small "solar bits"[1] which are what they sound like. Or old-school amorphous solar cells, like the kind that used to power calculators. They're not very efficient, but they perform okay in indoor lighting and they're cheap.
Also, PIN photodiodes[2] are intended for detecting whether an object is in light or not, but they can actually deliver around 25 microAmps @ 5V in less than 5 sq. mm. Not much, but it could charge something ambiently for short bursts of usage. (Edit: and I'd be remiss if I didn't point to the project that made me aware of this: https://hackaday.io/project/21342-bpw34-solar-powered-led-wa...)
So, you know. Hop to, everyone.
[1]: http://ixapps.ixys.com/DataSheet/KXOB22-04X3F_Nov16.pdf
EDIT:
I looked up the spectrum PV cells tend to respond to and some sources on the usual meaning of the word "light", and I admit my original comment was me being pedantic about the more obscure usage of the word. Sorry :(.
Also consider what of the sun's radiation makes it to the surface of the Earth.
"The prototype basestation uses an unlicensed frequency, limited to low-power transmissions. ... Real cell towers have a hundred times as much power, and would increase the range to perhaps a kilometer,"
To be fair, we used more sites closer together instead of a few larger ones, but the point stands. Cell sites biggest advantage is elevation and line of site, not massive transmit power.
Elevation and line of sight are important but power is too!
Ctrl-f "watts" on this page to see what I'm talking about:
https://www.ericsson.com/en/networks/offerings/connecting-th...
However because the base has multiple (hundreds) of low-power transmitters, it must cater for a very large instantaneous peak power when they momentarily add up.
And more importantly, excess Transmit power reduces the re-use ability of that frequency in nearby cells.
The what? The only thing I can find about reciprocity is that antennas have the same characteristics in either direction. That doesn't mean your noise levels are the same, and you very often have different bandwidth requirements in each direction.
Late response, but that assumes that the mobile RX is as good as the tower's RX, and I can assure you that's not the case.
The gear up on a tower has a way better low noise amplifier than a mobile phone, so it's able to "hear" the phone at a greater distance than you'd expect. That's why it's a-okay to have a base transmitter with a quite a bit more power than a mobile TX.
Any excess power prevents re-use of the frequency in adjacent cells.