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).
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