**>> IFFT size/length** means number of sine waves the IFFT takes as input to construct the time domain signal. In case of 20 MHz the IFFT size is 2048 that means 2048 sub-carriers are used to construct the time domain signal. (Note: not all are occcupied).

the IFFT size is "IDFT(Tx)/DFT(Rx) size" in the table provided above.

So for 3 Mhz it is 256 not 320.

for 2nd Query -

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