which there are two copies of the received waveform–one on time and the other delayed by some time. Inter-symbol interference (ISI) is induced because the tail part of symbol 1 will interfere with the processing of symbol 2. To eliminate ISI, a guard interval of Ng samples is usually inserted at the beginning of each OFDM symbol
Fig:Guard Interval
The length of the guard interval is made longer than the delay spread of the wireless channel. As a result, the degree of delay spread in the operating environments must be obtained beforehand. Note that the guard interval actually wastes transmission resources, hence the ratio of the guard interval length to the effective OFDM symbol duration is usually kept below 1/4. During the guard interval, the transmitter can send null waveform. This scheme is called zero padding (ZP) transmission .. A ZP–OFDM system has lower transmission power and a simpler transmitter structure. Unfortunately, the ZP–OFDM scheme introduces ICI, as the orthogonality among subcarriers is destroyed when multiple copies of the time-shifted ZP–OFDM waveform are received. To remove ICI, cyclic prefixing (CP) transmission is preferred. To generate the CP signal, an additional buffer is required in an OFDM transmitter
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