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

[Long, Jeffrey P.]

In the case of MSK or GMSK they don't need to be destaggered although
you could if you wanted to recover the MSK like it was QPSK. If you did
this you would need to build a sine pulse match filter for MSK and for
GMSK I believe you are out of luck for matched filtering. 

Just recovering OQPSK (whether or not it was RRC filtered) you could
destagger and then treat it like regular QPSK. I think if you used
differential encoding on the TX side I believe you could use a
differentially coherent technique like Lindsey's book talks about. Not
sure how the matched filtering works out in that case. You probably do
it first and then do the phase math. 

It is interesting how by adding a little memory to the
modulation(flavors of CPM) you have all kinds of interesting techniques
at you disposal. 

-----Original Message-----
From: George Nychis [mailto:address@hidden 
Sent: Friday, April 11, 2008 1:15 PM
To: Long, Jeffrey P.
Cc: address@hidden; Thomas Schmid
Subject: Re: [Discuss-gnuradio] M&M impl, its parameters,and why does
it work for GMSK?

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


Long, Jeffrey P. wrote:
> I could be wrong but I believe that block can work with a complex
input
> or a real input. For the case of GMSK/MSK it should work on a real
> signal derived from the change in phase of the IQ(use
> gr_quadrature_demod_cf) not the IQ itself as is the case in regular
> QPSK. I believe the GMSK demo shows this. Filtering should not really
> matter in either case. If you want to recover OQPSK you might need to
> destagger the IQ stream for M&M to work, right?? MSK is really just
> OQPSK with sine pulse shaping in the IQ domain but since it is really
a
> special case of FSK you can recover using FM detector or differential
> phase techniques.
> 
> 
> 
> 
> 
> -----Original Message-----
> From: address@hidden
> [mailto:address@hidden On Behalf
Of
> George Nychis
> Sent: Friday, April 11, 2008 12:18 PM
> To: address@hidden
> Cc: Thomas Schmid
> Subject: [Discuss-gnuradio] M&M impl, its parameters,and why does it
> work for GMSK?
> 
> Hi all,
> 
> I've attempted to follow the M&M optimization algorithm paper, which
> GNU 
> Radio implements, to try and select proper parameters for a O-QPSK /
> MSK 
> implementation floating around that a UCLA group wrote:
> http://acert.ir.bbn.com/projects/gr-ucla/
> 
> I ask because it seems as though the PHY is not recovering from clock

> drift properly, which would be the goal of the M&M block.  The blue
in 
> this graph is not important, but note the smearing on the right:
>
http://cyprus.cmcl.cs.cmu.edu/tmp/graphs/zigbee_sample_dump9_unit_circl
> e_19v20.jpg
> 
> That smearing is not recovered from properly by M&M and causes loss,
as
> 
> an example.  This causes approximately anywhere from 5-15% frame loss

> over _coax_.
> 
> So I was pointed to the M&M paper, to try and understand why its 
> recovery is not working properly, looking to set proper parameters.
> The 
> paper is here:
> http://ieeexplore.ieee.org/iel1/2220/8932/00392689.pdf
> 
> But, if you read the first paragraph, it states that this algorithm
is 
> NOT for MSK, or GMSK for that matter:
>            "We are assuming a system whose output comprises nom-
> inally synchronised data symbols subject to additive white Gaus-
> sian noise, where the transmitter and the receiver use Nyquist root
> raised-cosine filters."
> 
> So, before I go any further... I was hoping someone could shed light
on
> 
> how this ends up working out for GMSK which does not use an RRC
filter.
> 
> Then, why would it not work with the MSK implementation? ;)  I 
> understand that GMSK is band-limited by the Gaussian filter, is this
> the 
> significant difference?
> 
> I suppose the overall question is the implications of using a CPM
> signal 
> when the GR clock recovery block was written using an RRC filter.
> 
> Thanks!
> George
> 
> 
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