The challenge isn't so much the codec. The challenge is the narrow bandwidth of
the radios. The radios use a highly filtered waveform that is designed for
short range operation, through high powered repeaters at altitude.
By designing a wider bandwidth radio (4FSK at carriers separated by the symbol
rate, for example), you can improve the link by 10 dB.
So, step one would be a narrow-band (5 kHz) SSB radio for VHF. Step two would
be a < 100 usec RX/TX delay (fast power up, as the TDMA slot times are in
milli-seconds). Step 3 would be a multiplex waveform like OFDM or 4FSK.
Using FM would defeat the purpose, as it doubles the bandwidth. It is like
using AM on HF - two side-bands instead of one, power in the carrier, instead
of none.
73 de k5okc
In the DMR camp, in particular the TYT MD-380 and clones group,we have a
possible hand-held with downloadable firmware thatcould support another codec.
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