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[Discuss-gnuradio] QPSK demodulator does not operate when the transmissi

From: Damindra Bandara
Subject: [Discuss-gnuradio] QPSK demodulator does not operate when the transmission rate is low
Date: Mon, 21 Mar 2016 15:08:18 -0400


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.

Thank you

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