Re: [Discuss-gnuradio] [Freetel-codec2] 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 <address@hidden> 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 <address@hidden> 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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