Dear Alistu,
The setting of these parameters is usually done by the designer. They affect the system performance and accuracy. The default values in the component is what is typically published. However, users can change them as needed in their system.
The BER Test set needs to know the number of subcarrier to determine number of bits per subcarrier and to synchronized the received bits. This includes knowing if there are prefix, training and pilot symbols.
There is no FFT points in BER Test Set.
I recommend you refer to text book on OFDM for more information.
regards,
Ahmad