Obtain Time Domain Signal From Frequency Domain Signal

I have a respiration signal which was curve fitted using fourier8. Then the psd of the curve fitted signal was calculated using welch method.

I have changed the frequency of the signal in frequency domain. Now, I am trying to plot the signal with new frequency, but can't figure out how to proceed.

Here is my code,

clc clear all

load 'resp.mat';

t = [1:1:18000]./300;

Fs = 300;

resp_signal = resp(1:18000,:); % Respiration Signal for 60 sec

[resp_f,gof,output] = fit(t',resp_signal,'fourier8'); % Curve fitted respiration signal

y_resp = resp_f(t);

psig1 = y_resp - mean(y_resp);

[pxx1,w1] = pwelch(psig1,[],[],[],Fs);

figure

plot(w1,pxx1);

axis([0 10, 0 100]);

[~,loc1] = max(pxx1);

freq1 = w1(loc1);

resp_rate1 = freq1 * 60;

bp = input('Enter Respiration Rate: ');

bps = bp * 17.068;

Fss = bps;

psig2 = y_resp - mean(y_resp);

[pxx2,w2] = pwelch(psig2,[],[],[],Fss);

figure

plot(w2,pxx2);

axis([0 10, 0 200]);

[~,loc2] = max(pxx2);

freq2 = w2(loc2);

resp_rate2 = freq2 * 60;

I am trying to convert 'pxx2' variable into time domain signal. I would be really grateful, if anyone could help me out with this.

[Adam]

doc ifft

would normally be used for this, though I have never used pwelch so I'm not really familiar with what it gives you.