Re: [Freetel-codec2] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

[glen english]

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  > 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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Re: [Freetel-codec2] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

[David Rowe]

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 > 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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Re: [Freetel-codec2] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

[David Rowe]

Hi Adrian,

Nice to see some one else working in this area.

I imagine another reason is that the people setting the DMR standard 
were constrained by having a black box, closed source speech codec which 
constrained the bit rate they had to deal with.


Have you performed any BER tests to conform the BER versus Eb/No 
performance of the DMR waveform over ideal FSK?


Cheers,

David

On 12/09/17 20:20, Adrian Musceac 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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Re: [Freetel-codec2] [Discuss-gnuradio] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

[Adrian Musceac]

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  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  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
>> https://lists.sourceforge.net/lists/listinfo/freetel-codec2
>>
>

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Re: [Freetel-codec2] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

[Phil Frost]

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  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
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> Freetel-codec2@lists.sourceforge.net
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[Freetel-codec2] Why the DMR 4FSK modem is less optimal than David Rowe's 4FSK modem

[Adrian Musceac]

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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