In the OFDM block position array arranges the position of the central frequency of the OFDM spectra.For example: put “0” and after that put the value “(Nifft-Nsub)/2” (i.e (128-64)/2=32) and compare. In first case you almost see nothing because your OFDM spectra has been removed to “negative” frequencies. In second case you see the half of the OFDM spectrum and the central frequency of it, is found in the frequency 0 of the diagram. If you put position array 64 with the same, as before, parameters, you will the whole spectra.
Mind the BW of the spectra for each (I or Q) stream, must be the BitRate/2.
cyclic prefix will confuse you. Without cyclic prefix, using symbol extension, will keep orthogoanlity without concern!
About BER, i use BER test set. You will find it: Default/ Test set/ Binary
About its action in your block diagram, seek help topic of the block.
Good luck!