need help in understanding constellation diagrame???

[nauman2558]

can anyone plz tell why i am getting these two different constellation diagrames as shown in attatched png, however their BER=0. i am basically doing qpsk in labview using modulation toolkit then upconverting my complex siganl to real passband equivalent. then downconverting it and then demodulating it. at transmitter i am getting constellation diagrame with straight lines and at receiver i am getting ronuded sided constellation diagrame. can anyone plz tell me why there is a diffeerence in both of these diagrames.

[A_Donfigulation_Error_Has]

Hi nauman2558.

I suspect that this discrepancy is a result of the demodulating the signal and from the signal going through filters. Please take a chance to download and explore the Broadband Developer's Executable  https://lumen.ni.com/nicif/us/ekitbasedev/content.xhtml ,as it will help shed some light on what is going on and may answer some other questions you may have. After you have the executable installed, please click on "Constellation Plot vs. Baseband Waveforms". At the Transmitter end, I believe the signals appear to be perfect digital sigals, please see transmitter.png. After you route the signal to your receiver, the signal looks a bit distorted due to demodulation, please see receiver.png. In other words, it's not possible to recreate the modulated signal exactly, 

Regards,

Jason L.