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This paper aims to apply the superimposed pilot sequence technique to semi-blind channel estimation for OFDM systems. We propose two kinds of joint semi-blind channel estimation and data detection structure which are based on the Viterbi algorithm and the maximum a posteriori (MAP) algorithm, respectively. Further, two kind iterative structures of joint semi-blind channel estimation, data detection and convolutional decoding, which based on soft out Viterbi algorithm (SOVA) and soft in soft out (SISO)-MAP algorithm respectively are proposed. We also present a reduced complexity scheme based on the principles of reduced-state sequence estimation. Numerical simulations demonstrate the performance of the proposed methods, comparing with the classical pilot-symbol-aided modulation (PSAM) estimation method [JK Moon et al, 2000] previously described and the subspace blind estimation method [Muquet, B et al., 2002].