RE: [Discuss-gnuradio] M&M impl, its parameters, and why does itwork for GMSK?

[Long, Jeffrey P.]

While we are on the topic of clock recovery and someone more
knowledgeable than me is reading :) I would like to pose a question to
Bob. How might one do clock recovery on a M-ary CPM signal? It sounds
like a ML sequence technique might be an option but is there a simpler
ad-hoc technique that works as well? My "superiors" tell me to just do
clock recovery on a 2-ary modulated preamble and then hold it for the
rest of the packet when it switches to your M-ary mode. I suppose this
would work but I think it might be a little too much work in GNU right
now until the meta data extra channel thing is ready, right? For
simplicity I would prefer a technique that could run on streaming data
like the M&M.

thanks
Jeff


-----Original Message-----
From: address@hidden
[mailto:address@hidden On Behalf Of
Bob McGwier
Sent: Friday, April 11, 2008 1:45 PM
To: George Nychis; gnuradio mailing list
Subject: Re: [Discuss-gnuradio] M&M impl, its parameters, and why does
itwork for GMSK?

George Nychis wrote:
> Thanks for the response.
>
> You're correct, it can work with complex or real input.  GMSK and the

> MSK implementation are both using the real input, passed on from 
> gr_quadrature_demod_cf.  Here is the MSK impl:
>
https://moo.cmcl.cs.cmu.edu/trac/cmu_sdrg/browser/phys/802.15.4/trunk/s
rc/python/ieee802_15_4.py#L84 
>
>
> I'm not sure if I and Q are meant to be de-staggered before hand or
not.
>
> - George
>

George, Jeffrey:

You are discovering by inference in the M&M research work what most 
communications professionals know because someone told them  (usually 
Matlab) ;-).

Staggered QAM and MSK minimize envelope variation and as such, the 
nonlinearity in M&M generates a much weaker clock tone. At the risk of 
bringing up sore topics,  Matlab uses M&M where it is appropriate.  For

MSK and GMSK, they use a fourth order nonlinearity to generate the
clock 
tone.

I use them and their large R&D shop as "proof by intimidation" rather 
than going through all of the arguments why this should be so but they 
start from the small envelope variation I mentioned.

google search Matlab MSK-type signal timing recovery.  They have my 
favorite timing author ever (Umberto Mengali) referenced.

Long before M&M wrote,  Umberto Mengali wrote a paper on the asymptotic

maximum likelihood clock recovery method.  After you approximate the 
hyperbolic tangent with the simplest first order approximation and use 
two tap differentiator, VOILA,  M&M pops out.

-- 
AMSAT Director and VP Engineering. Member: ARRL, AMSAT-DL,
TAPR, Packrats, NJQRP, QRP ARCI, QCWA, FRC. ARRL SDR WG Chair
"Debugging is twice as hard as writing the code in the first place.
Therefore, if you write the code as cleverly as possible, you are, by
definition, not smart enough to debug it." - Brian W. Kernighan 



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