I have a bit different take on the whole sampling thing. It goes like this.
If you observe carefully LTE 20MHz is not one single carrier modulated with 20 MHz (18MHz actually) of spreaded bandwidth.
If I just consider one subcarrier out of 2048, the information bandwidth is 15kHz only.
This 15kHz bandwidth is same for 1st (at-9 MHz) middle (0) or last (+9MHz) subcarrier.
Now if I have a 15kHz carrier at baseband all I need is sampling of 30kHz for real signal or 15kHz for complex signal.
We shall assume that max signal which we want to faithfully sample is of 15kHz Bandwidth but at 18 MHz carrier.
To sample that I need at least 18MHz sample rate as signal is complex.
There may be many undersampling rate (in different nyquist zone) possible as the bandwidth is too small compared to carrier but sampling rate must be chosen so that it satisfy sampling criterion for all the sub carrier.
Hence something more than 18MHz shall satisfy Nyqusit (first zone) for last subcarrier as well as all subcarriers.
Now to answer the question of why 30.72 and not 18 or 20Mhz sampling rate, we shall understand the system evaluation.
LTE is evaluation from WCDMA, where base rate is 3.84MCps (Mega Chips per second). All the circuitory and clocks are ready build and designed with this rate.
So to chose a rate more than 18 MHz we shall use a good multultiple of 3.84, which if you see is 8 here (30.72/3.84).
And this is how everything converge to this number.
Comments are welcome on my take.