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

[Ian Wraith]

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 ..

http://borg.shef.ac.uk/~ianw/dqpsk.html

 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 ..

http://www-s.ti.com/sc/psheets/spra341/spra341.pdf

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 ?

Regards

Ian

address@hidden