[Discuss-gnuradio] extending gr-trellis to perform Viterbi MLSD on GMSK

[Nick Foster]

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/address@hidden/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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