How to decide filter length in filtered OFDM system?

Hello,

I am trying to understand how to decide filter length in filtered OFDM system, the system code is also can be seen from the following link.

Filtered OFDM

   % numFFT = 1024;           % Number of FFT points
   % numRBs = 50;             % Number of resource blocks
   % rbSize = 12;             % Number of subcarriers per resource block
   % cpLen = 72;              % Cyclic prefix length in samples

   % bitsPerSubCarrier = 6;   % 2: QPSK, 4: 16QAM, 6: 64QAM, 8: 256QAM
   % snrdB = 18;              % SNR in dB

   % toneOffset = 2.5;        % Tone offset or excess bandwidth (in subcarriers)
   % L = 513;                 % Filter length (=filterOrder+1), odd

   % numDataCarriers = numRBs*rbSize;    % number of data subcarriers in subband

In this code block, we are using 1024 point FFT and in total 600 data subcarriers. The remainings are unused carriers. There are 1024 subcarriers and 1024-600=424 are unused subcarriers. 212 on either side with zero padding into the system.

Now, if we send this 1024 point IFFT output to the subband filter (only one subband for all ofdm symbol in this example) the filter length is assigned as 513. I couldn't understand why it is not as IFFT/FFT size(1024) and also it's length is less than the total number of data subcarriers which is 600 in this case.

Could you please explain how to decide filter length in F-OFDM system system? If we change the FFT size to 256 (100 data subcarriers),512 (300 data subcarriers) or 2048 (1000 data subcarriers); how we decide the new filter length in each case?

Many thanks in advance!!