The actual FFT size and sampling frequency for the LTE downlink are not specified.

According to Nyquist criteria ,wecan take basband sampling frequency greater then or equal to twice of maximum frequency component( >= 2fm).

for 20MHz

baseband sampling frequency **( fs ) = sub carrier Spacing * 2048= 15 * 2048 = 30.72Mhz**

This is itself chosen for backward compatibility with UMTS, for which the chip rate is 3.84 MHz – exactly one eighth of the assumed LTE sampling frequency.

for 5 Mhz

baseband sampling frequency **( fs ) = sub carrier Spacing * 2048= 15 * 512 = 7.68Mhz**