Why Is Tx Constellation Displayed as a Cloud in QAM Example of MT?

Updated Jul 18, 2019

Reported In

Hardware

  • USRP Software Defined Radio Device

Software

  • Modulation Toolkit
  • LabVIEW
  • LabVIEW NXG
  • LabVIEW Communications System Design Suite

Issue Details

When I run the QAM Tx example of the modulation toolkit for USRP devices, the constellation on the front panel of VI looks like a cloud. Why does this happen?

Solution

In niUSRP EX QAM.vi, the pulse shaping filter can be applied to modulated symbols to minimize the inter-symbol interference.
On the cluster control of Pulse Shaping Filter, you can select one of possible pulse shaping filters amongĀ Raised Cosine, Root Raised Cosine, and Gaussian.
According to the value of Filter Length on Pulse Shaping Filter in niUSRP EX QAM.vi, each modulated symbol produces some samples which are not identical to the actual constellation point.
All of the filtered samples to be transmitted from USRP are displayed on Tx Signal Constellation. You can check this by changing the value of Tx Filter on Pulse Shaping Filter to none.