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

[Achilleas Anastasopoulos]

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
http://www.eecs.umich.edu/~anastas/docs/cpm.pdf
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.

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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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Achilleas