Cheers, I am demodulating a QPSK signal on GNURadio, the data transmitted is a known HEX file with the "1ACFFC1D" foreword every 124 bytes, to check if the demodulation is a success, it contains ascii code saying "Bonjour :)" indefinitely to spot easily an error.
I also compare the bits in a time sink. The signal is modulated on a separated system and is transmitted to the the USRP via a coax cable. The constellation seems neat to me and the file uses all the symbols to trigger all the points in the constellation. However when I launch the flow-graph, the foreword isn't always recognized and I have to reload the costas loop manually and then everything hooks up and the demod is fine. Everything is fine in the photo above, but the constellation is looking the same when bits are not recovered.
First I thought that the points of the constellation were maybe shifted so I mapped the constellation with four different shifts and tried to grab the foreword for each one but it did not work :
If you have any ideas or explanation of why it does not hook up at each run I would be very grateful because I am kind of lost about it.
Have a nice day !