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
**normal cyclic prefix**(corresponds to seven OFDM symbols per slot (0.5 msec) - An
**extended cyclic prefix**, (corresponds to six OFDM symbols per slot (0.5 msec)

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.