Constellation diagram is not as expected

Please view the image for reference.

I wanted to simulate the transceiver in simulink. First, I tried to simulate the transmitter. After the Baseband modulation, I could observe the qpsk constellation without any distortion. But, Constellation is distorted after the Raised cosine filter.

Please let me know how to solve this.

Answer 1

[Pooja Kumari]

Hello,

I understand you are facing issues with a distorted QPSK constellation after applying a raised cosine filter in your Simulink simulation. You can follow the troubleshoot steps to resolve the issues:

1. Check Filter Parameters: Ensure the roll-off factor, filter span, and samples per symbol are correctly set for your raised cosine filter.
2. Sampling Rate Consistency: Verify that the sampling rate is the same before and after the filter to avoid timing errors. The sampling rate must be high enough to accurately represent the signal after filtering.
3. Timing Recovery: Implement a timing recovery block if there's a symbol timing offset.
4. Gain Normalization: Adjust the gain if the filter has altered the signal amplitude.
5. Phase Offset: Correct any phase offset introduced by the filter. The Raised Cosine filter introduces group delay, which can cause phase shifts in the signal.
6. Review the step size and simulation time to ensure accurate results.

You can refer to the below documentation for more information on Root Raised Cosine Filter:

https://www.mathworks.com/help/comm/ug/filter-using-simulink-raised-cosine-filter-blocks.html

[Harish Kumaran Dhandapani]

Thanks for your answers.

In my case, I imported S-parameter touchstone data into simulink which act as communication channel.

Now , it works fine. I simulated for different modulation schemes as well.

Thanks a lot Pooja!