The ISI interpretation really only makes sense if you are using an
analog FM demodulator for both waveforms. This is not the best way to
demodulate FSK - it's a rough method - a kludge - that has somehow found
it's way into a standard!
BTW it's not really "David's modem" - Brady and I just implemented a
textbook FSK demodulator and took a little care to ensure it met the
theoretical performance. Then compared it to other modems being used
for digital voice.
8dB is pretty significant ... 6.3 in linear power. So a 6.3W radio with
the DMR modem works as well as a 1W modem with an ideal FSK modem. I
think that's about 15%, so every DMR (and C4FM) radio wastes 85% of
their tx power (and battery when transmitting). Still, maybe I've made
an error somewhere, would be good to get independent verification.
Cheers,
David
On 13/09/17 05:11, 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
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