[Discuss-gnuradio] Problems in Costas loop for QPSK

[Minsuk Kang]

Dear all,

I use mpsk_receiver_cc() to synchronze DQPSK symbols at a receiver.

Since I get too many packet losses when running the benchmark(in the digital folder) with DQPSK, I have closely looked at the codes.

I finally found that the output of costas_loop is not well synchronized from time to time. If I use higher rate like 500k, it happens more often.

Here's the constellation of the output of the Costas loop (http://bit.ly/ij4ytv) when it is not well synchronized. Sometimes the constellation becomes worse than this. 

I used the default parameters of dqpsk.py except fmin and fmax.

(When I set fmin/fmax=-0.1/0.1 instead of -0.25/0.25, I get better constellation.)

Is there anyone who has encountered the same problem?

Or, is there any other code that I can use to synchronize QPSK symbols at the receiver?

PS. I'am using USRP1 / gnuradio 3.3.0 / Ubuntu 10.04

Thanks in advance.

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Minsuk Kang