Hi all,
I've spent a few days familiarizing myself with gr-trellis as best as I can,
and I'm interested in extending the gr-trellis module to handle Viterbi
equalization of ISI channels for GMSK demodulation. I saw Toby Oliver's thread
in Sept. '06
(http://www.mail-archive.com/discuss-gnuradio@gnu.org/msg05615.html) discussing
a possible modification with Achilleas Anastasopoulos but never saw anything
checked in as a result. I'm just looking to use the trellis code to demodulate
low-BT GMSK in a more optimal way than the current Gnuradio implementation, and
I have similar questions to Toby's:
* How should I go about modifying make_isi_lookup() to add support for
two-dimensional modulations? What format is trellis expecting?
* I see the test_viterbi_equalization1.py file, which appears to do MLSD on an
ISI channel for 4-PAM (and other one-dimensional modulations). Am I correct
that if make_isi_lookup() is modified to support quadrature modulations, simply
changing the modulation type in this example would be enough to make it work? I
guess I'm asking more specifically if trellis.viterbi_combined_X will support
an ISI lookup table for PSK modulations without modification.
* Is there a good reason I should avoid tackling this problem? I'd hate to be
duplicating someone else's work in this area, or barking up the wrong tree.
For further information, I've written a packet-based AIS decoder for Gnuradio,
and I'm disappointed at the quality of the data coming out of the GMSK
demodulator. It's 9600 symbols per second @ BT=0.3, so there's enough ISI that
I think MLSD would provide significant reduction in observed BER. Besides, it
seems like it would be a useful capability to have added to Gnuradio. Any other
tips anyone has that might help me in doing this would certainly be welcome!
Nick
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