David Rowe is creating fully open digital voice hardware, including a TDMA repeater system. He presented his latest work at linux.conf.au 2016 and it is pretty amazing. Worth learning about even if you aren't into radio:
Open Source Two Way Radio: https://www.youtube.com/watch?v=xDeMT7eUf4c
BTW, anyone likely to be reading this in the US can earn an FCC Technician license granting full privileges to the VHF/UHF bands David discusses here in maybe 5-6 hours of study.