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I'm trying to build an RX channel for QPSK modulated signals on GNU Radio. I'm on the first stage of my project so the signal I'm sending in the USRP N210 is a carrier signal centered at 2.206 GHz.

However when I'm trying to observe the signal on GNU Radio using a constellation sink bloc I see all the points of the constellation rotating in a circle (when there should only be one point since I'm sending a carrier signal).

Here is my GNU Radio Flowgraph:

enter image description here

Maybe there is something wrong with the parameters of my blocs of with my configuration of the USRP.

eyllanesc
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Ncaseau
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1 Answers1

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When using QPSK modulation, square constellation in receiver is achieved only after proper symbol timing and carrier phase and frequency compensation. In order to achieve single dot, when single harmonic signal is transmited, in receiver you need a decimator, that picks a single point. If there is phase difference between local oscilators of Tx and Rx the dot will be displaced, and if there is frequency difference the dot will rotate.

eyllanesc
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Ivan Penchev Ivanov
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