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DMR hotspot with MMDVM, GNU Radio and a duplex SDR

From: Adrian Musceac
Subject: DMR hotspot with MMDVM, GNU Radio and a duplex SDR
Date: Tue, 13 Jul 2021 08:30:04 +0300

Hello list,

Here's another thing you can use GNU Radio and your SDR for: to create a MMDVM hotspot for digital voice modes.

There is an existing fork of MMDVM called mmdvm-sdr. The author had already done the heavy lifting involved in porting to x86 and communicating with MMDVMHost via a virtual serial port. I just had to add ZeroMQ to it and set up communication with GNU radio flowgraphs and now I have a duplex* hotspot for DMR. MMDVM is set to work in duplex mode and I used an ADALM-Pluto for testing, but any duplex SDR should work the same. I'm not such a big fan of these modes since they involve a proprietary vocoder, nevertheless DMR is pretty interesting and I already have the radios for it but not the hotspot hardware. Other modes were not tested, but should work as well, since they don't have TDMA complications.

 * It transmits both timeslots just fine since the DMR BS sets its own time reference with CACH data. The inbound channel is single slot only though, because of the buffering and delays involved it can not meet the timing required to get both slots, so MS transmissions regardless on which slot they originate will be picked up on slot 2 and routed to the net as such.

mmdvm-sdr code with ZeroMQ changes: https://github.com/kantooon/mmdvm-sdr/tree/zmq 

Example GNU Radio flowgraphs:



The GNU radio flowgraphs are pretty basic, aside from ZeroMQ sinks and sources, just FM modulation/demodulation and resampling.

Thanks to Jonathan G4KLX for creating such useful software, r4d10n for porting it and Johan for prompting me to look into this.

Ideas, improvements welcome.


Adrian YO8RZZ

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