OFDM PSD spectrum plot

Hi,

could any of you guys out there, explain to me, what's the reason OFDM PSD plot has this strange pit on DC frequency?

I can't find any information about this, yet I am interested in this topic so I'm excited to hear the answer to my question.

Answer 1

[Hasan Can Yildirim]

In a typical modulation (whether it is OFDM or something else) you have a local oscillator (LO) that upconverts the baseband signal to the desired carrier frequency. This LO has something called DC leakage. As the name suggests, there is a DC energy leaking to your baseband signal when it is upconverted. Thus, you have an unknown and undesired distortion on one subcarrier at DC. If your subcarrier spacing is small, then this distortion will affect not only the subcarrier at DC, but also the neighboring subcarriers. Since you want to perform a reliable channel estimation and recover the symbols on the subcarriers, you want to avoid these kind of distortion. Hence, subcarrier(s) at/around DC are left empty in OFDM based communication systems.