whty zero padded subcarriers inserted in the middle of the rows and before IFFT, and what is the relation between the number of added null subcarriers and the sampling frequency?

I have a matlab code for OFDM system, in this code the auther added a zero padded subcarriers before the IFFT stage, in the middle of the raws, and added zero padded guard interval after IFFT, my question is that why i need to add zero padding before IFFT, and what is the relation between the number of null subcarriers and sampling frequency?? the code some thing like this A = size(Data); % Data : complex data FS=192e3; %Sampling freq T = 1/FS; %Elementary period fc = 38e3; %Carrier freq Tu = 200e-3 %symbollength; N=512; M=20; tt = 0:T:Tu; %Time vector info = zeros(length(tt), M+1); for zz = 1:M+1 info(1:(N/2),zz) = [Data(1:N/2,zz)]; info(length(tt)-((N/2)-1):length(tt),zz) =[Data((N/2+1):A(1),zz)]; %Zeros placed in the middle of the data end for symbol = 1:M+1; %OFDM carriers = FS.*ifft(info(1:length(info),symbol));

[David Goodmanson]

Hi asmaa, When posting, please highlight and then use the code{ } button to format your code.

For fft and ifft, there are corresponding time and frequency arrays on input and output. These have constant spacing delt or delf between points. For an N-point fft or ifft, in either direction always

delt*delf = 1/N.

If you start with certain valid definitions on delt and delf, and subsequently pad an array with zeros, that changes the value of N, and one of delt and delf must change. Usually you are sure of the corresponding input array and have to make the appropriate assumption about the corresponding output array. For example if you have an input frequency array with spacing delf, and double the number of points by doing zero padding, then delt in your new corresponding output time array is half of what it was.