Hello,
I’ll preface this explanation with the warning that recently there was a large update to the OFDM components and now the parameter Position Array is gone. The OptiSystem team added a lot of functionality to the OFDM components as well.
In the recent past, the component functioned in the following fashion. Setting the Number of FFT points sets the total number of available (not necessarily used) sub-carrier positions centered at DC or 0 Hz. The number of sub-carriers would then set the total number of sub-carriers actually used (for example 4 sub-carriers might be used for a QPSK format). The position array then sets the starting point of the range of the sub-carriers used.
For example, if I set the Number of FFT points to 32 and set the position array to 16 with number of sub carriers set to 4. The sub-carriers used will be 16, 17, 18, 19, which corresponds to 0 Hz, 1.25 GHz, 2.5 GHz, 3.75 GHz (these frequencies are calculated from a Bit rate of 10 Gbps and 4-QAM modulation).
These options were found to be not entirely transparent for users and that’s why it was changed.
Regards