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 <a...@silverblocksystems.net> 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 > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >
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