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In this paper, we propose an efficient scheme using a new single-symbol preamble structure for OFDM data-aided synchronization. The preamble symbol is constituted of two successive equal-duration sub-symbols, the second one of which is a differentially encoded version of the first one, using an adequately designed precoding sequence. This structure leads to almost perfect channel autocorrelation profile with sharp in-phase peak and zero out-of-phase correlation. For comparison, we review some previous methods also based on data-aided synchronization for OFDM systems. We here explore the performance of the new method in the case of the AWGN channel. The performance is presented in terms of correct frame start detection rate and estimator's variance for time estimation. For fractional frequency offset estimation, the performance is evaluated in terms of mean squared error. The obtained results prove that the accuracy of the frame start detection and the fractional frequency offset estimation are greatly enhanced, even at very low SNRs. We show that compared to the best benchmark, gains of about 5 dB and 20 dB are achieved for timing precision of one sample and of one tenth of a sample, respectively. We also show that for a target correct detection rate of 95%, our approach provides more than 17 dB gain with respect to the best benchmark. Although we concentrate here on OFDM, the proposed approach can be applied to other systems such as TDMA or CDMA.