The size of the cyclic prefix can be chosen from three options:
Normal prefix for 15 kHz: This is the standard option. It uses a cyclic prefix of 160 samples for the first symbol in each slot and for the following six symbols a prefix of 144 samples. All in all this makes seven OFDM symbols per slot (14,000 symbols per second per subcarrier). The longer cyclic prefix of the first symbol in the slot is used to improve the performance of time and frequency synchronization algorithms at the receiver side. The standard cyclic prefix size of 144 symbols corresponds to a time of 4.69 ?s or a distance of 1.4 km, which is the limit for the maximum delay spread in multi-path environments.
Extended prefix for 15 kHz: If bigger cells with longer delay spread are required (cell sizes in the range of 100 km) then a longer cyclic prefix is required. EUTRAN thus offers the option to have a cyclic prefix of 512 samples corresponding to 16.67 ?s. This would allow a four times greater delay spread, but now only 6 OFDM symbols fit into one slot, reducing the symbol rate to 12 ksps. This is a capacity reduction of 14% compared to normal prefix.
Extended prefix for 7.5 kHz: For EUTRAN cells that are exclusively used for MBMS with all cells on the same frequency (MBSFN=MBMS Single Frequency Network) there is also the option to have a cyclic prefix of 33.33 ?s. This would allow to place MBSFN cells around 10 km apart from each other.