Due to multi-path channel effect, multiple copies of the signal reaches
the receiver with different delayes which causes ISI.
Normaly the subcarriers in OFDM symbol are orthogonal or ( they have
exactly an integer number of cycles in the OFDM symbol period T).

Due to multi-path the orthogonality is lost--> ISI..
Using CP would allow us to always have an integer number of cycles in
the FFT interval even when multiple received symbols are delayed.


Thanks
Hany