I am trying to transmit messages from a message source modulated using QPSK. A message is transmitted in every second. At the receiving end I am writing the messages to a message sink. I am using Etuss USRP N200 to test the setup.
The problem is I do not receive the message at the other end. When I observe the constellations at the receiver it shows that the QPSK demodulator does not remain synchronized.
When I tried this in a simulation environment (using a channel model) the experiment works fine. If I transmit a file using this setup it works fine too. But when I transmit a message every 1 second it is not transmitting.
The transmitter end: msg source -->packet encoder --> QPSK modulator --> USRP sink
Receiver end: USRP source --> Poly phase clock sync --> CMA equilizer --> Costas loop --> QPSK demodulator --> Packet decoder --> msg sink
I really appreciate if you could help me to solve this issue.