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In OFDM based communication a guard interval needs to be inserted in order to avoid intersymbol interference. The duration of the guard interval should be equal to the maximum delay spread of the channel.
If the guard interval is transmitted with zeros. Then the carriers within the same OFDM symbol will start interfering with each other which may lead to Intercarrier Interference. This can be avoided by using cyclic prefixing.
Guard interval is used to avoid ISI.
Using CP in the guard Interval ICI is avoided.
I agree with Kamal and Trying to make it more clear
No channel Equalizer is used in OFDM and CP is used instead to avoid ISI
See the attached Pic
The OFDM theory requires that a Cyclic Prefix (CP) must be added at the beginning of the OFDM symbol.
the CP allows the receiver to absorb much more efficiently the delay spread due to the multipath and to maintain frequency orthogonality.
The CP that occupies a duration called the Guard Time (GT), often denoted TG, is a temporal redundancy that must be taken into account in data rate computations.
The ratio TG/Td is very often denoted G in WiMAX/802.16 documents.
if the multipath effect is important (a bad radio channel), a high value of G is needed, which increases the redundancy and then decreases the useful data rate; if the multipath effect is lighter (a good radio channel), a relatively smaller value of G can be used.