- Due to the signal propagation phenomena, like reflection or diffraction, a receiver can receive several delayed versions of the same signal.
- This creates Inter-Symbol Interference (ISI).
The modulation is based on the amplitude and on the phase, so in case of overlapping there are 2 different
The receiver is not able to decode the state of the symbol.
- The guard time is called the Cyclic Prefix (CP). It permits to facilitate demodulation.
- The cyclic prefix transforms the classical channel convolution into a cyclic convolution which permits easy demodulation after FFT.