Re: [Discuss-gnuradio] QAM, carrier tracking and clock recovery

[Tom Rondeau]

Martin Dvh wrote:
> Tom Rondeau wrote:
>   
> > > -----Original Message-----
> > > From: Martin Dvh [mailto:address@hidden
> > > Sent: Monday, January 29, 2007 4:37 PM
> > > To: Tom Rondeau; gnuradio mailing list
> > > Subject: Re: [Discuss-gnuradio] QAM, carrier tracking and clock recovery
> > >
> > > Tom Rondeau wrote:
> > >
> > >       
> > > > > -----Original Message-----
> > > > > From: address@hidden [mailto:discuss-
> > > > > address@hidden 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.
> > >       
> > Yep, and it's in the code ;)  If you look at gr_clock_recovery_mm_cc.h,
> > there's a reference to a good receiver book that goes over the basic M&M
> > loop. In my developer branch, gr_mpsk_receiver_cc.h has a link to a paper
> > with a modified M&M loop implemented here.
> >     
> Thanks, but I was hoping for some info which doesn't require an IEEE 
> subscription or buying a book.
> (Free as in Beer)
>
> But I will just try to grasp it by staring at the code a little longer.
> That usually has on of two results:
> I get it
> I don't get it. I start writing my own algorithm. After much tweaking I end up 
> with the original algorithm, but now I know why.
>
> Greetings,
> Martin

Ah, sorry, life a the university spoils me sometimes. I don't have a 
good one to recommend off hand, but here's Mathwork's man page:
http://www.mathworks.com/access/helpdesk/help/toolbox/commblks/ref/index.html?/access/helpdesk/help/toolbox/commblks/ref/muellermullertimingrecovery.html&http://www.google.com/search?q=mueller+and+muller&start=0&ie=utf-8&oe=utf-8&client=firefox-a&rls=org.mozilla:en-US:official

Hopefully, it's a place to start, though it gives you very little insight.

Tom