building a 900Mhz long-range data communications network
I assume you are in north america, so, for a 15km radio link at the 900MHz band, you will need  A modem and antenna that will give you a link budget of ~110dB, this assumes you use the maximum allowed by the FCC (1W). For a USB port that can deliver 0.5W at best, you will need another power source, or limit your transmissions to very short bursts.

You might also need directional antennas (No point to multipoint) and a high (~30m) Tower.
There are modems with such link budget, I have not used them personally.

A link to the XTend from the previous poster.