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

[George Nychis]

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/src/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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