Is multi channel transceiver capable of receiving simultaneously from multiple transmitters?
A Rf multichannel transceivers ? I believe they are called cell phone towers. Probably a similar approach might work. If you have 2 or more transceivers linked in a cellular configuration with overlapping coverage area will give you the 15km range.

I believe digital modes are less susceptible to Doppler affects
than their analog predecessors. You may have signal absorption  issues with VHF frequencies with 100 percent humidity . Other issues have to be considered as well such as terrain and vegetation in the path of the signal. 

Digital modes require a good signal strength to avoid data loss.  If you are using simply a wire antenna with nonexistent  gain then you need a miracle.