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:
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
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
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
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
very slow timeconstant for my PLL.
No packets get through yet, I think because the PLL only locks after a
of symbols are missed and the constellation is not very stable.
The output constellation of the output of the clock-recovery now can be
The output files (--log) before and after clock-recovery is in
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
find QAM modulator code; I have transmitted this and received it on my
Signature signal analyzer and see the perfect constellations. The code
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,
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
on the MPSK receiver and implementing D8PSK. This work is almost
and will be merged into the branch soon. I would look the
block code from now on. It's decision-aided and does the Costas loop and
(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.