pwelch finds two-sided PSD at carrier frequency

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?

Answer 1

[dpb]

"...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...