Re: [Discuss-gnuradio] Proper synchronization of f0T correction to use gr-trellis for a receiver (Re: gr-trellis test_cpm.py question)

[Nick Foster]

Unrelated to the question at hand, but since you're on the subject: how are you resetting the Viterbi decoder state between packets? I tried the same thing some years ago and it worked well except for that problem.

--n

On Thu, Apr 6, 2017 at 12:38 PM Andy Walls <address@hidden> wrote:

On Mon, 2017-04-03 at 09:06 -0400, Achilleas Anastasopoulos wrote:
> sure, feel free to look into the gr-trellis documentation and provide
> some feedback.
> If you have further questions please let us know.
>
> best,
> Achilleas

Hi Achilleas:

My objective is to implement an AIS (GMSK, BT=0.4, L=3) receiver, using
the Viterbi algorithm for optimal demodulation of the CPM symbols.

In examining gr-trellis/examples/python/test_cpm.py, I see that
everything is perfectly synchronized, for the purposes of
demonstration. (The addition of a 0.0 to the end of the 99% energy
orthonormal basis vectors for the matched filter correlators, to have
the taps completely fall into the initial all-0 history of the fir
filter blocks, was a nice trick BTW).

In my design concept for a receiver, I believe I have worked out
carrier frequency offset correction, phase offset correction, symbol
timing recovery at either 4 or 5 samples per symbol, and injecting
samples to properly realign the symbols entering the decimating matched
filter correlators when a new burst is received.

What I can't quite figure out is how to properly synchronize the
correction of f0T carrier frequency shift introduced by the CPM
decomposition, without unintentionally adding an arbitrary phase shift
to the symbol's signal in the CPM decomposition.

Do I restart the complex exponential frequency shift sequence with a
phase of 0 at the start of each symbol?  I think that works for Q=4.
But what about for Q=5?

The reason I ask is that it appears the phase of the complex
correlation output by the matched filters will affect the metric for
which CPM decomposition signal gets selected as the best match.

Looking at the 16, 99% energy CPM decomposition signals generated by
the test_cpm.py script:

>>> print abs(Sf.transpose())
[[ 1.81592306  0.83465307]
 [ 1.81592306  0.83465307]
 [ 1.90550352  0.600571  ]
 [ 1.90550352  0.600571  ]
 [ 1.96823385  0.34970555]
 [ 1.96823385  0.34970555]
 [ 1.90550352  0.600571  ]
 [ 1.90550352  0.600571  ]
 [ 1.90550352  0.600571  ]
 [ 1.90550352  0.600571  ]
 [ 1.96823385  0.34970555]
 [ 1.96823385  0.34970555]
 [ 1.90550352  0.600571  ]
 [ 1.90550352  0.600571  ]
 [ 1.81592306  0.83465307]
 [ 1.81592306  0.83465307]]

Many of the signals can only be distinguished from each other properly
when the correlator outputs have proper phase.

Thank you for any advice you can provide.

Regards,
Andy

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