How to check signal waveform.

use numeric_std library.

use singed type in your vhdl code (and use std_logic at io), you can easily cast or convert between these two types. Do your internal computation all in signed.

use 16 bit real and 16 bit imag. In your case you must have adequate storage for your fft points per computation. I don't know details of the fast fourier computation technology but I know it splits up the full set of points into smaller sets, unlike traditional DFT which used to deal with the whole vector. You will need loads of multipliers and adders in all cases.

Therefore, you don't need to think of virtual pins. I wouldn't care about it for now. It is used occasionally as explained by the pancake.