Tom Rondeau wrote: >>-----Original Message----- >>From: [EMAIL PROTECTED] [mailto:discuss- >>[EMAIL PROTECTED] On Behalf Of Martin Dvh >>Sent: Sunday, January 28, 2007 6:15 PM >>To: gnuradio mailing list >>Subject: [Discuss-gnuradio] QAM, carrier tracking and clock recovery >> >>Hi All, >>I am trying to build a working QAM modulator/demodulator, based on the >>existing BPSK/QPSK code. >> >>Is the Mueller and Muller clock-recovery as used in the current QPSK >>examples also usable for QAM? >> >>I couldn't find a good description of the algorithm or the parameters. >> >>For clock recovery the only options I found on the net which are supposed >>to work for QAM are modified early/late tracking and modified >>zero-crossing. >>Has anybody any info on this? >> >>I am also looking for the best option for carrier-tracking. >>For QAM there are special requirements for carrier-tracking and clock- >>recovery. >>A simple costas loop will not do for carrier tracking. >> >>This is what I have so far: >> >>For now I implemented a to-the fourth-power PLL for carrier tracking. >>(PLL locks to the fourth power of the input, output is divided by four) >>This is supposed to work for all QAM versions and should NOT suffer from >>catastrophic collaps when the BER gets too high. >>(The other option. Decision based carrier-tracking does suffer from >>catastrophic collaps.) >>The disadvantage is the long time before lock. >> >>For clock-recovery I am trying to use the existing clock-recovery. >> >>I am fiddling with the parameters and I do get a non-very-stable but >>recognisable QAM constellation as output of the clock-recovery when I use >>a >>very slow timeconstant for my PLL. >>No packets get through yet, I think because the PLL only locks after a lot >>of symbols are missed and the constellation is not very stable. >> >>The output constellation of the output of the clock-recovery now can be >>seen in: >>http://www.olifantasia.com/projects/gnuradio/QAM/screenshots/ >>The output files (--log) before and after clock-recovery is in >>http://www.olifantasia.com/projects/gnuradio/QAM/ >> >>greetings, >>Martin > > > Martin, > > I've been doing a bit of work on this problem and have two developer > branches open for this. In branches/developers/trondeau/digital-wip you will > find QAM modulator code; I have transmitted this and received it on my > Signature signal analyzer and see the perfect constellations. The code also > performs Gray coding on the constellation. This code only has the QAM > modulator side and no demodulator. This should be very quick to work up, and > it sounds like you might already have it. Yes, I already have similar working code. > > In branches/developers/trondeau/digital-wip2, I've been doing modifications > on the MPSK receiver and implementing D8PSK. This work is almost complete > and will be merged into the branch soon. I would look the 'gr_mpsk_receiver' > block code from now on. It's decision-aided and does the Costas loop and M&M > (a modified version) at the same time and improves the DQPSK reception. Do you have any links for a desciption of the MM algorithm and its parameters and/or performance.
> > The mpsk_receiver should be fairly easily extended to work for QAM, too, > with improved decision making on the quadrant and "shell" the symbols are > in. It's similar to what you said, except it does joint estimation of the > timing. > > I'm swamped with other responsibilities right now, so I won't have time to > hack on the QAM work, although I really want to get the 8PSK code into the > trunk, so expect that soon. If this code helps you in working on your QAM > solutions, please use it, and we can hopefully work together to get this > done and part of the release. I will have a look at it. Thanks. I am also swamped with other work but I would really like to get QAM working to get DVB-C working. (It will eventually need a lot of tuning to get >40 Mbit/sec working in realtime) Martin > > Tom > > > > > _______________________________________________ > Discuss-gnuradio mailing list > [email protected] > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > _______________________________________________ Discuss-gnuradio mailing list [email protected] http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
