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In this letter, we propose a decision-directed channel estimation scheme for single-carrier frequency-domain equalization in amplify-and-forward (AF) relaying networks. For coherent combining of the received signals in AF-based cooperation, conventional channel estimation schemes have focused on estimation of source-destination and source-relay-destination channels. However, if the individual channels in the source-relay-destination link are not considered, the relaying gain is limited because the noise propagation from the relay to the destination affects the effective signal-to-noise ratio at the destination. The proposed scheme estimates the effective channel parameters for maximal-ratio combining, in the absence of channel-state information (CSI) from the relay. Simulation results show that the proposed scheme outperforms the conventional scheme and approaches the performance with perfect CSI.