> Did anyone use it to receive pi/4 DQPSK signal before? In the op25-dev project <http://sedition.org.au/op25/> we see a very common variant of PI/4 DQPSK, so-called CQPSK/LSM.
Experimentation has shown that the gr_mpsk_receiver_cc() is not optimal for CQPSK. There seem to be multiple causes, e.g.,: 1) PI/4 (D)QPSK apparently looks enough like 8-PSK that the standard GR QPSK phase detector has difficulty achieving phase lock. There's a hack to rotate the second of every two symbols by 45 degress which seems to give a large improvement [see Fig 10. of my LSM page at http://www.lightlink.com/mhp/lsm/ ] 2) At least for the specific case of LSM/CQPSK, it was found that a Gardner symbol timing loop was superior to the M&M loop (and to the GR PFB clock sync block) as used in the GR receivers. The op25 site has example code for using both the gr_mpsk_receiver_cc as well as a complete working example using Gardner's method, including the PI/4 hack mentioned above. Finally, you should also take a look at the new "magic" PFB receiver (dqpsk2.py in GR) for your application... Max _______________________________________________ Discuss-gnuradio mailing list [email protected] https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
