In this paper, we consider different types of guard intervals for OFDM systems, i.e. cyclic prefix (CP), zero padding (ZP) and known symbol padding (KSP). We compare the different OFDM systems with respect to their SNR performance. We show that CP-OFDM and ZP-OFDM have exactly the same performance, whereas KSP-OFDM has a slightly worse performance. Further, we consider data aided channel estimation for the three OFDM systems; the MSE of the estimators is compared to the corresponding Cramer-Rao bound. It turns out that in practice, channel estimation for CP-OFDM slightly outperforms the one for ZP-OFDM. The practical channel estimation techniques for KSP-OFDM perform worst.