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

From: Martin Dvh
Subject: Re: [Discuss-gnuradio] QAM, carrier tracking and clock recovery
Date: Mon, 29 Jan 2007 22:36:31 +0100
User-agent: Debian Thunderbird 1.0.2 (X11/20061113)

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
>>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-
>>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
>>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:
>>The output files (--log) before and after clock-recovery is in
> 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.

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 

> Tom
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