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[Discuss-gnuradio] extending gr-trellis for general coherent CPM demodul

From: Achilleas Anastasopoulos
Subject: [Discuss-gnuradio] extending gr-trellis for general coherent CPM demodulation
Date: Wed, 25 Feb 2009 19:57:31 -0500
User-agent: Thunderbird (Windows/20081209)

Hi all,

I just committed an update to gr-trellis that can handle coherent demodulation of an arbitrary CPM signal (CPFSK, RC-CPFSK, GMSK, etc)

It consists of an additional fsm constructor that provides the FSM description and a couple of helper utilities (in fsm_utilities.py)
that can help construct the appropriate matched filters etc.
The remaining functions can be used verbatim (ie, all the viterbi/siso blocks in gr-trellis)

I have also added a test_cpm.py file with an example case.

The theory behind the decomposition is all detailed in a
classic paper by Bixio Rimoldi "A decomposition approach to CPM"
IEEE Trans. Information Theory, March 1988.
A summary of this can be found in
I didn't have the time to make an xml file out of my latex file, so for the time being this summary is not committed to the trunk.

Careful: this is a "coherent" demodulator, so it will not
work with real data from the USRP. It does neither
symbol timing tracking nor phase tracking.
The focus was to demonstrate the generality of the decomposition.

If you want to make a practical CPM demod based on this approach, you need to either (i) do timing/phase estimation OUTSIDE the Viterbi decoding (either using a non-data aided scheme or a coded-data aided scheme) and use the rest of the demodulator as provided, or (better) (ii) modify the viterbi block in gr-trellis to add timing/phase estimation based on tentative decisions (with some delay "d") from the decoder. If anyone wants to take up this task I can help. I'll probably won't have time to do it myself before the summer...


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