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An improved channel estimator with inter-symbol interference (ISI) free window measurement for OFDM systems is proposed by exploiting the inherent correlation between the cyclic prefix and portion of the received OFDM signal. This scheme make use of real time measured size of ISI free window to estimate the maximum delay spread of channel, then a transform domain channel estimator is adopted to further reduce the noise variance. Due to the ISI-free window can be estimated before the application of FFT to the received signal, it is a parallel processing, and very suitable for practical systems. Simulation result demonstrated that the proposed method can track the variation of delay spread of wireless channel, and improve the MSE of channel estimation significantly, thus give at least 1-db improvement in TU channel and 0.5-db in HT channel.