# What is the relationship between FFT size and demodulation in OFDM

As I mentioned in the title of the question, how can we demodulate the data in order to get the same number of subcarriers with different FFT size.

For example, 10 MHz LTE can be sampled with 15.36 Msps and modulated/demodulated with a 1024-point FFT for 15 kHz subcarrier spacing and the 600 subcarriers around DC give the 9 MHz of utilized BW. Alternatively, we can use sampling rate 30.72 Msps (which is 2 * 15.36 Msps) and 2048-point FFT (which is 2*1024-point) to mod/demod and get the same 600 subcarriers.

So, I'm wondering, when we use 1024, we can demodulate the data and get the 600 subcarriers correctly, now when using 2048, how can we demodulate the length and get the same number of subcarriers? assume we are using QPSK modulation.

her is the formula of the DFT:

So, when having 2N instead of N, how can we get the same number of subcarriers? that will be different as below equations for 1048 and 2048 respectively:

In first equation I will k = 1024 and in the second it will be 2048 .. so, again, how can I get the same number of subcarriers from those two different lengths ?

thank you

• Comments are not for extended discussion; this conversation has been moved to chat. – Peter K. Aug 12 '18 at 19:06