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Constellation gets distorted after LPF

(joined August 2015)
Participant

Hi Everyone,
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.

Regards,
Dhiman

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Responses (2):

    • #28479

      Hi Everyone,
      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.

      Regards,
      Dhiman

      Attachments:
    • #28481
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      alistu
      Participant

      Hi Dhiman,

      Getting to see a clear constellation diagram after the OFDM modulator block at the transmitter has no theoretical basis. Constellation diagrams before OFDM modulator and after OFDM demodulator have information. Otherwise I don’t think there is anything wrong with filter cut-off here.

      Regards

      • #28482

        Thanks Alistu for the reply. I also think so that there is no such a relation. But still there should be some reason Why the constellation gets distorted.

        Regards,
        Dhiman

        +2

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