How can we demodulate the data in OFDM with different FFT size in OFDM system.

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Zeyad Qasem 2018-8-11

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https://ww2.mathworks.cn/matlabcentral/answers/414498-how-can-we-demodulate-the-data-in-ofdm-with-different-fft-size-in-ofdm-system

Suppose we are using QPSK demodulation, so when using double frequency sampling, we can get double the FFT-points in order to get the same number of subcarriers. So, after the FFT demodulation, in first case we will get 1024-FFT-point and in second case we will get 2048-FFT-points, so how can we demodulate them in order to get the same number of subcarriers ? what will be different in order to get the same data.

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.

how will that happen? thank you.