pwelch finds two-sided PSD at carrier frequency

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I have the signal rt=xt+it, where x and i are BPSK-modulated rectangular pulse train transmitted at carrier frequency fx and fi. When I try to find the PSD of it alone using the pwelch function like so
[psdi,fri] = pwelch(it,[],[],(fx-5*W:10^6:fx+5*W),fs);
where W and fs are the signal bandwidth and sampling rate of xt, I get a mirrored PSD around fx, which is not what I expect. I expect the mirroring to happen at -fi. Why is this happening?

Accepted Answer

dpb on 19 Sep 2022
"...I get a mirrored PSD around fx, ... Why...?"
Because in
[psdi,fri] = pwelch(it,[],[],(fx-5*W:10^6:fx+5*W),fs);
you specifically asked for the two-sided estimates at the frequencies fx +/- 5*W; that's centered about fx, not fi
Change fx to fi and joy shall ensue...

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