depends on the applied data/modulator filter prior to modulation. the trellis coding doesnt actually remove the isi.... it's all about pre filtering and post equalisation, directed rather than static. Though the DMR modem is super simple.
Be aware, some modulation types are designed to have "closed eye" (when observed in the baseband) , and the ISI is undone by a equaliser in software. The software equaliser can undo whatever linear process happens in the baseband. It takes time though and given the burst nature of DMR, it comes in hell or high water and is decoded as it is. (yes the trellis coding assists error control) . Things that are HARD to compensate/equalise are non linear effects such as asymmetrical IF responses (also can be caused by being off frequency) and assymetrical group delays in IF chains. the direct IQ baseband radios afford some advantages in these cases. g On 13/09/2017 5:41 AM, Phil Frost 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 > <mailto: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-codec2@lists.sourceforge.net > <mailto:Freetel-codec2@lists.sourceforge.net> > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 > > > > ------------------------------------------------------------------------------ > 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-codec2@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/freetel-codec2 ------------------------------------------------------------------------------ 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-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
