Effects of cyclic prefix on OFDM systems over time-varying channels

In this paper, effects of cyclic prefix (CP) are investigated for orthogonal frequency division multiplexing (OFDM) systems over time-varying channels. Although employing a CP as the guard interval is a simple way to combat the inter-symbol interference (ISI) and the inter-carrier interference (ICI), it reduces the transmission efficiency of the system, especially for channels with long channel-impulse response (CIR), such as hilly terrain channels. On the other hand, the time-varying channel also gives rise to ICI. We show that to achieve a signal-to-interference ratio of 20 dB, for slow fading channels, half the length of CIR is enough for CP. For fast fading channels, the Doppler frequency become domination factor to affect OFDM systems