As a quick note, I'm working on the channel coding / decoding for GSM L1 (since we'll need those for osmocom-bb-bts), basically generic routines to go from L2 messages to bursts bits to be modulated. (GSM 05.03 essentially).
So if you work on this, I'd suggest starting with getting the part below this working first (tx and rx of bursts)..
Cheers,
Sylvain