[Discuss-gnuradio] Synchronization with additional noise issues

Hi,

I have experienced some problem using the Symbol Synchronizer from
gr-nwr-master (OOT). In the flowgraph attached, I'm doing QPSK with
the DVBS2x modulator, the QPSK demodulator comes from the tutorial
however I switched the different returns in
my_qpsk_demod_cb_impl::get_minimum_distances :


if (sample.imag() >= 0 and sample.real() >= 0) {
return 0x00;
}
else if (sample.imag() >= 0 and sample.real() < 0) {
return 0x02;
}
else if (sample.imag() < 0 and sample.real() < 0) {
return 0x03;
}
else if (sample.imag() < 0 and sample.real() >= 0) {
return 0x01;
}


In my flowgraph, I'm comparing the input of the modulator with the
output of the demodulator. In order to see errors between the 2
streams, the input is subtracted to the output. For a fixed noise
voltage, high enough to get few errors, I will see a normal amount of
errors at first and pretty quickly most of the received samples gets
"wrong", as if the Symbol Synchronizer had lost tracking and the block
delay changed at some point. It might be due to a frequency rotation
too but I expect the Costas Loop working well.

I used the mpsk_stage6.grc to get started with synchronization stuff
on gnu-radio, and the Polyphase Clock Sync is handling well noise
voltage from 0V to 1V. However, when I tried to launch the
mpsk_stage6.grc without differential encoding and decoding the
behavior of the Polyphase Clock Sync turns out to be the same as the
Symbol Synchronizer in my flowgraph.

The point is I want to start working on BER curves, so I need the
synchronization not to be randomly relevant. What am I doing wrong not
to get my Sync block performing with noise addition ?


Regards,

Victor