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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