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Re: [Discuss-gnuradio] A little DQPSK demodulation progress and a quest

From: Joseph DiVerdi
Subject: Re: [Discuss-gnuradio] A little DQPSK demodulation progress and a question
Date: Mon, 31 Mar 2003 08:59:30 -0700

At 8:30 AM +0100 3/31/03, Ian Wraith wrote:
>Hello All
>         I am able to report a little progress with my little spare time
>project of writing a PI/4 DQPSK demodulator for GNU Radio. So far I have
>written a little code for a class I call pi4dqpsk_demod. This takes the
>I/Q data from the soundcard by connecting to GrAudioSource and will
>eventually output the demodulated binary data.
> At the moment though all the class does is take in the I & Q data then
>filter them with a root raised cosine filter. For debugging purposes it
>also writes the raw and filtered I/Q values to a CSV file for later
>analysis. You can see a little of this data in graphical form by looking
>here ..
> My next stage is to recover the symbol timing which isn't as easy as I
>first thought. My original idea was to sum the I squared and Q squared
>through a band pass filter at the symbol rate and the resulting waveform
>peaks would be the symbol timing. However if you look at this document ..
>then go to figure 30 on page 38 you will see the author uses a much more
>complex method of symobol timing recovery using a PLL. Can anyone tell me
>why my original idea for timing recovery wouldn't work ? or suggest
>another method of doing this ?

Dear Ian,

Nice progress on your project. 

Would you mind preparing one more graph image? It would be interesting to see 
the data that you have acquired and graphed as points (not lines) with each 
point plotted as the real value vs. the imaginary. The result should be a 
constellation plot as seen in the paper you referenced. The system phase noise 
will potentially be observable as may a number of other items.

BTW, I found the paper a good reference and intend to share it with others who 
are interested in learning in this area.

A comment on your preliminary ideas on the symbol recovery: the (square root of 
the) sum of the squared I and Q outputs yields the (magnitude) magnitude 
squared of the demodulated signal. The *phase shift* information is contained 
in the phase of the demodulated signal and that's what you will need to analyze 
in order to extract the symbols. Time to work on a robust arctan2 function. ;)

Best regards,
Joseph A. DiVerdi, Ph.D., M.B.A.          
http://xtrsystems.com/           970.980.5868 (voice) 
PGP Key ID: 0xD50A9E33           970.224.3723 (fax)

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