spektrum analysis from signals with fft

Hello everybody,

I am currently working on various signal analyzes. Including with transferred signals in the spectral range using the FFT.

First of all, in order to come to my actual question, I have to explain that I already slightly dampen the incoming signal with an I²R filter to remove outliers within the signal.

Now to my actual question: Is it still necessary after the FFT to edit the rectangle window with a Hanning window or is the actual standard window of Matlab sufficient?