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Why is the signal sent by VST inconsistent with the signal sent by USRP?

Hi everyone. I have two VST-5644 and four USRP-2922 devices. I want to build a 2x2 MIMO system. I have used the example "6x6 MIMO-OFDM System with NI USRP and LabVIEW Communications" and modified it to 2x2 by four USRP-2922, the system works normally. It can demod and generate constellations well (Fig.1 ).


Then I want to transfer this system to two VSTs(transmit) and two USRPs(receive).But the receiver(two USRPs) can't demod the signal which transmits from VSTs..  In order to verify, I fed the same signal to VSTs and USRPs to transmit, just called Sig1 transmitted by VST and Sig2 transmitted by USRP. Ideally, the Sig1 should equal to Sig2. The receiver can still only demodulate Sig2, not sig1.

 

I am confusing about this situation. I can find some differences between Sig1 and Sig2. The Sig1 looks like mix some low frequency with the transmitted signal, and Sig2 looks like not. But I think demod.vi is fully capable of handling such low-frequency mixing that is visible to my eye. 

 

Does anyone know the difference between the signals sent by VST and USRP?

Thanks a lot.

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usrp_constellation.png

usrp_sig.png

 These are the result of received signal that is transmitted by two USRPs, and the part of the received signal.

vst_constellation.png

vst_sig.png

  These are the result of received signal that is transmitted by two VSTs, and the part of the received signal. It looks like mix some low frequency signal compared with above signal.

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