Constellation gets distorted after LPF
I am attaching here with a portion of my design file. it is basically a OFDM modulation, Where I have assumed a bit rate of 10 Gbps, and the modulation scheme used here is 16-QAM, after the 16-QAM sequence decoder and the OFDM modulator the constellation seen are as per the ideal one. But when we connect a low pass filter after it the constellation gets distorted I am not getting why its happening so. I have tried with cut off frequency of “0.75 * bit rate” and “0.5 * bitrate” and “bit rate”, for all the cases the constellation becomes very much distorted looks similar like 64-QAM. I have taken a higher sequence length (8192) also. What may be the possible reasons for this..? and How to overcome this…?
In some of the previous forum topics I was suggested to use a trial and error method which is very much time consuming. So if there is any mathematical relation or formulation for cut-off frequency please let me know. Thanks in advance.
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