issue in DFT of OFDM by blocks and N-by-N
Try using something like the following. You want to map the rows this way for all columns, hence the colon to denote all columns. Double-check that you get what you want from these. I didn't run it to verify it gives exactly what you want, but it should fully populate all columns, I believe.

D_IFFT_fore(1:(N/2),:) = D_map(((N/2)+1):N,:);
D_IFFT_fore((N_FFT-((N/2)-1)):N_FFT,:) = D_map(1:(N/2),:);