OFDM Guard Interval: Analysis and Observations

Orthogonal frequency division multiplexing (OFDM) is a modulation technique that is used in many modern digital communication systems and has been explored in numerous publications. Specifically, the tradeoff between throughput and ICI/ISI (intercarrier interference/intersymbol interference), which affects the length of the guard interval (GI) is well known. Many equalization methods that decrease the interference for a given (short) GI have been proposed. However, it seems that the exact expressions for the ICI and ISI have never been derived. In this paper these expressions are derived for a general (per tone equalization) setup. Often, the matched filter bound (MFB) for single carrier systems is used for OFDM systems. We make two new observations. First, we prove that under mild conditions, met in common scenarios, the ICI power and the ISI power are approximately equal. Second, we show by example that the MFB does not necessarily hold for OFDM. Both cyclic prefix (CP) and zeros prefix (ZP) are examined.