Hello Everyone please refer to the reply by Alan Robinson of this question on research gate. may get some clue about DQPSK. I am also trying to follow what he suggested.
https://www.researchgate.net/post/Eye_Diagram_not_proper#56fce080217e2013ec58db0f
if unable to see from researchgate please let me know.
Thanking you.
Regards,
Dhiman