# Problem with QPSK receiver

**URL:** <https://www.ruby-forum.com/t/problem-with-qpsk-receiver/243045>\
**Category:** GNU Radio\
**Created:** [August 17, 2015, 11:48pm UTC](https://www.ruby-forum.com/t/problem-with-qpsk-receiver/243045 "2015-08-17T23:48:10Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Sadaf\_N](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/s/858c86/32.png) [@Sadaf\_N](https://www.ruby-forum.com/u/Sadaf_N)\
**Post date:** [August 17, 2015, 11:48pm UTC](https://www.ruby-forum.com/t/problem-with-qpsk-receiver/243045/1 "2015-08-17T23:48:10Z")

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Hi all,

First time posting here, so hello everyone:)

I am trying to build this pair of GNUradio flow graph for QPSK signal  
transmitting and receiving over the air through USRP. Now the  
transmitter side is straight forward, only a random number generator and  
QPSK modulator plus a multiplier. The receiver side is as attached.

The problem is, I can get well separated constellation points after  
synchronizations, however the they are not correct. For example, if I  
set the transmitter side to modulate and send symbol ‘333333…’, the  
symbols after demodulation at receiver side is suppose to be  
‘333333…’. instead, I get ‘01320132…’ repeatedly. It seems that my  
constellation is constantly rotated with some frequency. I am not using  
differential modulation or demodulation, so it is some thing else  
rotating my constellation.

My guess is that my synchronization is not good enough, there is a  
slight frequency offset which move the phase backward(forward?) from  
time to time.

Any comments or suggestion are really appreciated.

Howe