spectral density of Modulated ofdm signal

Question

I have formed my OFDM symbol his length is 88 (Nsubcarrier=72,Ncp=16) with Fs=1080000 Hz

we call this symbol as Signal_from_baseband

I want to modulate the OFDM symbol with Fc carrier=2.1 GHz and plot the frequency spectrum of the modulated signal with x-axis as the frequency axis in Matlab.

Here is my tentative:

Ts=1/Fs;
N=88;
t=(0:N-1)*Ts;
Modulated_sig=Signal_from_baseband.*exp(j*2*pi*Fc*t)';
f=Fs*(-0.5:1/N:0.5-1/N);
Y=(1/N)*fftshift(abs(fft(y)));
stem(f,Y)

But the plot doesn't show a spectral density centered in Fc

Answer 1

In terms of code issues, you are probably forgetting to divide by Fs in the exponent of the complex exponential. However, Fs does not appear sufficiently high compared to Fc and will alias. If your original signal is sampled at Fs, to properly display the spectrum, you will need to resample your baseband signal to a new sample rate, Fs_new, then divide Fc by Fs_new in the exponent of the complex exponential.