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[Discuss-gnuradio] A little DQPSK demodulation progress and a question

From: Ian Wraith
Subject: [Discuss-gnuradio] A little DQPSK demodulation progress and a question
Date: Mon, 31 Mar 2003 08:30:11 +0100

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 ?




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