In order to answer this question, take a look at the figure below. The figure below outlines the more detailed time-domain structure for LTE downlink transmission for each slot of length.
Tslot = 0.5 ms consists of a number of OFDM symbols.
A sub-carrier spacing f = 15 kHz corresponds to a useful symbol time Tu = 1/ f ≈ 66.7 μs . The overall OFDM symbol time is then the sum of the useful symbol time and the cyclic-prefix length T cyclic prefix.
As illustrated in Figure below, LTE defines two cyclic- prefix lengths:
The exact cyclic-prefix lengths, expressed in multiples of Ts, are given in Figure below.
Normal Cyclic Prefix
It should be noted that, in case of the normal cyclic prefix, the cyclic- prefix length for the first OFDM symbol of a slot is somewhat larger compared to the remaining OFDM symbols. The reason is simply to fill up the entire 0.5 ms slot.
Extended Cyclic Prefix
The main use of the extended cyclic prefix is for MBSFN-based multicast/broadcast transmission.
Reduced Sub-carrier Spacing
As mentioned above, in addition to the 15 KHz sub-carrier spacing, a reduced sub-carrier spacing flow = 7.5 kHz is also defined for LTE and specifically targeting MBSFN transmission. The use of the reduced sub-carrier spacing also scales the OFDM symbol time, including the cyclic-prefix length, by a factor of two, thus providing a twice as long cyclic prefix (≈ 33.3 μs). In the case of the 7.5 kHz sub-carrier spacing, the resource block consists of 24 sub-carriers, i.e., the resource-block “bandwidth” is still 180 KHz.