Comment by nickodell
7 years ago
>Volk One devices can connect to each other, from several miles away.
Assuming that a voice call requires 8kbit/s of bandwidth, and assuming that you have about a watt of power, how feasible is it to connect through a phone that is a mile away?
Serious question - I don't know how to do the math here.
If you want to learn how to do the math, this is old but a good starting point: http://www.ti.com/lit/ml/slap127/slap127.pdf
(In general, radio engineering is taught at college-level and requires some basic math/physica if you are going with “rules of thumb” and fairly advanced math if you want to understand/derive exact equations).
One watt wouldn't be a problem at that distance, but any frequency this can operate on requires line-of-sight or close to it. "Several miles away" might be realistic in the middle of a desert, but in areas with hills or lots of large buildings? Not likely.
Generally speaking, line-of-sight is almost always what limits point-to-point comm range for handhelds, rather than raw power. On higher frequencies, even vegetation can be a big deal.
Trivial with the correct infrastructure - you can easily get 8kbps in 12.5kHz of bandwidth (a reasonably common channel size in the sub-GHz bands).
Trick is, you ideally need line of site (in which case your effective range is effectively the horizon), and directional antennas for distances more than a relatively short distance (a few miles maybe).
Some math follows. 30dBm is 1 W, and -100dBm is a low, but probably workable signal. We'll assume 0 antenna gain or loss to make the math easier.
Free Space Path Loss (in dB): 20×log10(d) + 20×log10(f) + 92.45
d=distance in km, f=frequency in GHz
So at 915MHz, we can get about 85km in free space @ 915MHz (this is actually past the horizon, so not really possible except from high elevations).
30dBm − (20×log10(85) + 20×log10(.915) +92.45) = -100dBm
Several miles? Sure, maybe, if conditions are good and you're line-of-sight. Good luck turning every phone into a cellphone tower.
This seems like an overly optimistic scam.