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

[George Nychis]

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_circle_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