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I have two SDRs and I want to do a simple test: I want to send data values using QPSK modulation with coordinates: (1,1), (-1, 1), (1, -1), (-1, -1), but on Rx site I got all possible values on the circle created by rotation of the square. Why phases of received signal are changed?

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  • \$\begingroup\$ Could you edit your post to elucidate the uninformed as to what this question is about? What is SDR and what is QPSK? Part of the site purpose is to educate as well as answer your specific question. \$\endgroup\$ – Transistor Sep 17 '16 at 8:04
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Your receiver has to track the reference phase of the transmitter. It can extract this from the data, or it can use embedded reference sequences. Somewhere in the Rx QPSK setup, there are some parameters that choose what method it uses. It sounds like these are set by default to something incompatible with the present transmission format.

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  • \$\begingroup\$ Thanks, I have AD9361 R2, now i'm looking for any ways to do it \$\endgroup\$ – sluge Aug 18 '16 at 12:06
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You will need a software PLL (phase locked loop) at the receiving side to properly synchronize to the symbols, or otherwise they would drift.

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  • \$\begingroup\$ Hello Mark, thanks a lot for your answer. I work with I/Q samples, not the raw signal \$\endgroup\$ – sluge Aug 18 '16 at 12:05
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    \$\begingroup\$ It is also applicable to I/Q symbols. Please see "MPSK receiver" in Gnuradio. \$\endgroup\$ – Mark Aug 18 '16 at 13:48

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