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The accuracy of channel state information (CSI) is critical for demodulation of multi-level modulated OFDM signals in multipath environments where the received signal experiences fading both in amplitude and phase. CSI estimates are commonly obtained from a priori known signal either appended as the training symbols in the beginning of a stream of OFDM symbols or inserted as the pilot tones in certain subcarriers of an individual OFDM symbol. In both cases, conventional channel estimation techniques process signal samples from the useful part of OFDM symbols. However, in excessively time-dispersive multipath channels where the receiver might be synchronized to a path in the middle of the overall channel impulse response, conventional channel estimation techniques may not capture some of the path components or its accuracy is degraded due to ISI. In this paper, we propose an improved channel estimation technique based on the use of a white preamble and the cyclic time-shifting property of the Fourier transform. Our computer simulation results show that the proposed scheme using a white preamble can improve the BER performance by more than 4 dB in a scenario considered in this paper.