Hi Phil, Of course, you are right, but Trellis is computationally more expensive than a straight change of the modem parameters. The DMR software stack is huge, compared to what a small community of largely open source enthusiasts can come up with. (my problem with DMR as a radio equipment standard is the use of a closed source audio codec, not the modem) It would be even better if someone was to write a pure SDR implementation of DMR and pit that against David's 4FSK modem :) Real performance measurements would convince enverybody of the merits of it.
Cheers, Adrian On 9/12/17, Phil Frost <ind...@bitglue.com> wrote: > Wouldn't the ISI largely (theoretically, completely) be ameliorated by > trellis decoding? And I'm not talking about FEC, but rather incorporating > the ISI into the expected Cartesian coordinates for each symbol. > > I did some experimentation with this technique on BPSK31, and I was able to > get a little more performance in simulations with a matched filter and > trellis decoding than fldigi and PSKCore got with their unmatched filters > designed to minimize the ISI. Though in the case of BPSK31 the ISI isn't > all that bad, so there less than 1 dB of a gain to be had. Perhaps with DMR > there's more potential? > > On Tue, Sep 12, 2017 at 6:50 AM Adrian Musceac <kanto...@gmail.com> wrote: > >> Hi, >> >> In this post: http://www.rowetel.com/?p=4650 David was asking the >> question why the 4FSK modem used by the DMR standard seemed to be >> performing so poorly, especially as compared to his ideal 4FSK modem. >> I was curious myself, so I implemented both and analysed them using my >> modified version of Gqrx, known by some people as gqrx-digital. >> The results can be seen in this video: >> https://www.youtube.com/watch?v=hLeJvqrFrS8 >> >> As you can see, the DMR modem uses a sensitivity of PI/sps, which leads >> to >> a lot of ISI, about 8 dB worth, compared to the optimal 4FSK modem which >> uses a sensitivity of 2*PI/sps. The reason why the DMR modem does this is >> twofold: >> One is bandwidth efficiency. But the most significan one is the fact that >> DMR radios use the same FM modulator for both digital and analog voice. >> Using double sensitivity would lead to analog voice being too wide for >> standard FM channels in commercial bands. >> >> Of course, for me as a developer, I can create two Gnuradio flowgraphs, >> so >> I can use one modulation type for digital, and another one (proper) for >> analog FM transmission. >> >> Hope this is useful for someone. >> >> 73, >> Adrian YO8RZZ >> >> ------------------------------------------------------------------------------ >> Check out the vibrant tech community on one of the world's most >> engaging tech sites, Slashdot.org! http://sdm.link/slashdot >> _______________________________________________ >> Freetel-codec2 mailing list >> freetel-cod...@lists.sourceforge.net >> https://lists.sourceforge.net/lists/listinfo/freetel-codec2 >> > _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
