[Discuss-gnuradio] "Error rate" block with USRP

[Azza]

Hi All,

I am working with GNU Radio Companion 3.6 on CentOS. I want to calculate the
BER, so I have been using the "Error rate" block. 
First, I send a random QPSK-modulated signal through a simulated noisy
channel (AWGN). I calculate the BER using the function error:
BER=0.5*erfc(sqrt(snr)) and compare it to the output of the "Error rate"
block. I find almost same results.
Second, I send a random QPSK-modulated signal through one USRP and receive
it with another USRP. Here is the problem: contrary to expections, the
output of "Error rate" block is always 0.5, and it does not vary with the
variation of SNR.
You find attached the flowgraph of this second scenario. 
<http://gnuradio.4.n7.nabble.com/file/n47625/gnu-ber.png> 
I am wondering if this block should not be used with USRPs to calculate the
BER. If not, any suggestions?
Any help is appreciated. Thank you.

Regards,
Azza



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