The transmitter of an OFDMA system uses IFFT block to create the signal. Each input for the IFFT block corresponds to the input representing a particular sub-carrier and can be modulated independently of other sub-carriers. The IFFT block is followed by adding the cyclic extension(cyclic prefix, CP). The motivation for adding the cyclic extension is to avoid inter-symbol interference (ISI). When the transmitter adds a cyclic extension longer than the channel impulse response, the effect of the previous symbol can be avoided by removing the cyclic extension at the receiver.
Now coming to what are types of CP and why they are used. In uplink, the cyclic prefix can have two possible values:
Short cyclic prefix - the 0.5 ms slot can accommodate 6 symbols, typically used in small cells
Extended cyclic prefix - the 0.5 ms slot can accommodate 7 symbols , thus data payload is reduced, typically used in area with extreme time dispersion
In practical sense, the advantages in having 7 symbols far exceeds the possible degradation from inter-symbol interference caused when the channel delay spread is longer than the cyclic prefix. Bear in mind there are limit where after a certain point or range or size in the cell, the signal corruption due to the residual time dispersion cannot be covered by the cyclic prefix anymore, and also this will not justify the corresponding additional power loss.
If you want more detail on how CP works including math behind CP check this http://www.dsplog.com/2008/02/17/cylcic-prefix-in-orthogonal-frequency-division-multiplexing/
