I'm trying to understand how OFDM works thanks also to the Simulink OFDM block.
Now, I have understood that the OFDM modulation "convert" a faster bit rate using a certain bandwidth into a number of slower bit rate streams using a series of subcarriers each of them large a portion of the original bandwidth. I have also understood that because of having a huge bank of filters that need to exactly filter the bandwidths of the subcarriers a faster and way less expensive solution is to convert the bit streams using the IFTT in TX.
Now, I haven't understood how the IFTT conversion is linked to the bandwidth occupation of each subcarrier and the number of subcarriers.
For instance, if I have a bitstream of 2000 bit/s so, the bit period is 5e-4; the bitstream is modulated with a 4-QAM so at the IFFT input I will have 1000 symbols per second (each one with a period of 1e-3 s). I'm using an IFTT with length = 64 (which is the standard value for the OFDM modulation block in Simulink) how many carriers have I got?
Another thing I can't understand is the cyclic prefix length. I know it is used to avoid ISI in RX but, how long should it be? I guess I have to adapt it to my channel if it has some sort of multipath fading.