i try to recreate my co-ofdm using sample from optisystem13 named OFDM Coherent Detection Single Port Dual Polarization 16QAM. however, i was unable to change the value at the OFDM demodulator and BER test set to the value i want. however, i was able to change it on the OFDM modulator. the maximum subcarrier in the BER test set is stuck at 128. plus i would like to ask about the position of the pilot symbol. will it affect the performance if i change their position? were their position fixed? how do i decide how many pilot symbol i need?