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I simulated OFDM on MATLAB,though,the spectrum of it is not white(flat shape). Why? IFFT circuit of actual transmitter uses higher IFFT order than the count of subcarriers? simulated ofdm spectrum


P.S.

I simulated with the following MATLAB code.

MATLAB code

Generate a random number sequence of size 500 → Generate a waveform with IFFT → Apply step-like interpolation → Multiply the carrier wave → Perform FFT

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. Any conclusions reached should be edited back into the question and/or any answer(s). \$\endgroup\$ – Dave Tweed Mar 19 '19 at 12:22
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It's hard to say whether something is wrong with your transmitter without knowing exactly what this picture depicts, but yes, the spectrum of a single subcarrier is sinc-shaped.

the spectrum of it is not white(flat shape). Why?

First of all:

OFDM isn't a flat top spectrum – it only looks like that from "far away", or if you're sampling it at the same rate as the transmitter generated it (which requires synchronization!).

In general, you'll see the "wells" between the sinc carriers, relatively well. It looks the flatter the more subcarriers you have.

In your case:

Your picture looks like you're only looking at a single subcarrier, which means you're effectively doing 1-carrier OFDM. Which is nothing but a box-car pulse-shaped single-carrier transmission. Hence, the sinc shape.

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  • \$\begingroup\$ I added postscript. \$\endgroup\$ – Sano Mar 16 '19 at 13:27

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