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Signal Processing

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    $\begingroup$ I didn't read through your entire code processing but where do you do timing offset correction? Phase and Delay are not the same thing: phase is the rotation of the symbols while delay is a time offset; so I am suspecting that you may not have implemented yet an STO correction? (Or you have an issue with your correction approach?) $\endgroup$ Commented May 10, 2022 at 12:12
  • $\begingroup$ @DanBoschen, applied timing offset (STO = 0.25 samples) is less than the length of cyclic prefix, so it does not cause ISI or ICI. It just causes phase offset (rotation of the received constellation). My problem is that I see some noise (even before correcting the timing offset-induced phase offset) at high-frequency subcarriers as can be seen in Fig.4 compared with low-frequency subcarriers as shown in Fig.3 $\endgroup$ Commented May 10, 2022 at 12:28
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    $\begingroup$ that is a different phase rotation for every subcarrier (proportional to each subcarrier rather than a constant rotation for each). If you are doing that correction then you are correcting for STO. I suspect the issue may be in doing that- I suggest confirming your correction is a linear phase versus subcarrier index. For a 1/4 sample offset the phase will go linearly from 0 to pi/2 over each of the subcarriers $\endgroup$ Commented May 10, 2022 at 13:42
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    $\begingroup$ @DanBoschen I have double-checked, yes the phase rotation is linear and proportional to the subcarrier index, $\endgroup$ Commented May 11, 2022 at 8:46
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    $\begingroup$ It's convolved with a Dirichlet Kernel which is the aliased Sinc function but I don't see how it would treat any of the bins differently. What I am suspicious of if taking the FFT of the time domain sequence with the CP added which has now condensed that whole vector in the time domain into the frequency range given by the original waveform without CP in the frequency range (since there is no change of sampling rate or your used of time for the 1/4 sample delay). Can you try introducing your quarter sample delay using the original (frequency domain) waveform without CP? then add CP to that $\endgroup$ Commented May 11, 2022 at 13:09