Thank you very much Alitsu for your reply.. I am new to optisystem and am having a bit of the issues with designing this direct detection ofdm. I have designed it on matlab and i want to integrate it to optisystem. But i want the normal direct detection to work before adding my matlab which works perfectly well. i am using subcarriers of 256, sumbol size of 200 and cyclic prefix point of 64 with a QPSK modulation. I am having errors on my OFDM demodulator when i try to simulate it, can you pls help as i have attached the file below.