7.5.1 Inter-Symbol Interference Removal Using A Guard Interval

In a radio frequency communications link, there is often more than one path that allows transmitted signals to arrive at the intended receiver. As well as the direct path between the transmitter and receiver, longer paths often exist when the transmitted signal reflects off nearby obstacles. These longer paths allow a delayed and attenuated version of the signal to arrive at the receiver. The delayed and attenuated signals will add to the direct-path signal and cause the total received signal to be distorted.

In addition to this longer symbol time, in OFDM systems, a guard interval is added to the start of each symbol. At the receiver, a portion of the end of the current symbol is copied into the guard interval to replace the received signal. The end of the current symbol will be free of the distortion caused by the symbol transitions so a symbol without this type of distortion is produced. The inclusion of a guard interval at the beginning of each symbol increases the time taken to transmit the symbol. The additional time taken to transmit the guard interval will therefore reduce the symbol rate of the system. However, this reduction in symbol rate is typically smaller than that required to accommodate the guard slots in a TDM system.

The resulting symbol will now only have distortion caused by adding the direct signal to the delayed and attenuated signals from the indirect paths. When these signals are added together, they produce a sine wave with a different phase and amplitude from the transmitted signal. Fortunately, this change in phase and amplitude is only related to the channel conditions and can be removed using channel equalization at the decoder.