Re: [Discuss-gnuradio] QPSK demodulation: Constellation Soft Decoder question

[jmfriedt]

after a few more experiments with synthetic signals, I figured out that
the Constellation Decoder output is already the set of bit pairs
expected from the QPSK constellation, not the families themselves. So
as expecter -1-j->00, -1+j->01, 1+j->11 and 1-j->10, it all makes
sense with an output rate twice the input rate in the case of QPSK.

JM

> I am investigating Meteor M2 decoding. So far I had tackled BPSK
> decoding (RDS, GPS) and it is the first time I analyze QPSK signals.
> Based on github.com/otti-soft/meteor-m2-lrpt.git, decoding works
> fantastically well, but I don't understand why. Costas and MM Clock
> recovery are understood, but it is the first time I meet the
> Constellation Soft Decoder. I am unable to find any information on its
> output and operating principle, but most worrying to me is that I
> believed that the real output would exhibit four levels following the
> decision of which bit pair was the most probable, but when displaying
> on a scope the output of the Constellation Soft Decoder I only see two
> well defined levels.
> Can anyone bring some insight on this processing block, which would
> help me understand the structure of the resulting file and hence start
> tackling the digital decoding part of the protocol. 
> The flowgraph I use in this investigation is visible at
> http://jmfriedt.sequanux.org/airspy_m2_lrpt_rxtest.png with an input
> file containing complex I/Q samples collected during a Meteor M2 pass.
> 
> Thanks, JM
> 
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-- 
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 26 rue de l'Epitaphe,
25000 Besancon, France