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If I am passing a QAM signal through Root raised cosine Tx filter and measure the constellation at output, it seems very bad. If this bad output again pass with other another root raised cosine Rx filter, there is a significant improvement in EVM. Can I ask the reason behind this.

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To satisfy the Nyquist criterion to eliminate the inter-symbolic interference (ISI) you need the whole link (TX, Channel and RX) to constitute a raised cosine filter, so you are having the root of the transfer function in the TX and the other square root in the RX. Since the overall response is just multiplication of the TX and RX transfer function, accordingly, you will get a full raised cosine when you do so.

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  • $\begingroup$ Thanks for the answer. I am aware about the points you mentioned. I am also aware that we are having root raise cosine for upsampling in Tx and same for downsampling in Rx. In this way, we can reduce the required bandwidth for transmission of the signal. My question is still there, upsampling is something impacting the EVM distortion in my opinion and when we downsample it at receiver, we recover well and IQ constellation get much better but never like ideal. However, if we keep raised cosine filter directly instead of root raised cosine, can we recover the signal constellation by just downsam $\endgroup$ Oct 12, 2023 at 18:25
  • $\begingroup$ Thanks for the answer. I am aware about the points you mentioned. I am also aware that we are having root raise cosine for upsampling in Tx and same for downsampling in Rx. In this way, we can reduce the required bandwidth for transmission of the signal. My question is still there, upsampling is something impacting the EVM distortion in my opinion and when we downsample it at receiver, we recover well and IQ constellation get much better but never like ideal. in summary, a root raised cosine filter has poor EVM while raised cosine filter is giving better EVM performance. Whats the reason? $\endgroup$ Oct 12, 2023 at 23:11
  • $\begingroup$ "if we keep raised cosine filter directly instead of root raised cosine, can we recover the signal constellation by just downsampling at receiver end." -- yes you can, but with a less-than-optimal probability of bit error. In other words, the receiver will be less robust to noise, timing margin, and all other real-world effects on its performance. $\endgroup$
    – MBaz
    Oct 13, 2023 at 14:52

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