From the Transmitter comes a block of size N+L (where N is the number of subcarriers and L is the cyclic prefix length). Then the block goes through a Channel which adds delay and AWGN, so this process can be described as a convolution of a transmitted block with a channel (the channel length H).
The question: The output of the convolution increases the size of transmitted block (the output from a channel is of lenth [N+L+H]) so how can FFT be performed (of course after removing cyclic prefix)? (because the number of subcarriers must be the size of IFFT/FFT)