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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