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The greatest threat to the OFDM system is the loss of orthogonality between subcarriers due to the carrier frequency offset (CFO) which results in significant performance degradation. In addition to this last problem was the effect of propagation over multipath fading channel. Hence, both the channel and the CFO need to be estimated and corrected at the receiver for subsequent symbol detection. A new joint semi-blind CFO and channel estimation methods for OFDM systems based on maximum-likelihood (ML) is investigated in this paper. The proposed algorithm is based on virtual subcarriers (VSCs) and on a new design of pilot patterns in OFDM blocks. The simulation results are given to verify the validity of the proposed methods. Keywords-Orthogonal Frequency Division Multiplexing (OFDM); carrier frequency offset (CFO); virtual subcarriers (VSCs) ; null subcarriers (NSC); no zero pilot symbols.

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