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This paper considers the problem of sampling clock synchronization and channel estimation for orthogonal-frequency-division multiplex (OFDM) systems. In such systems, when the number of subcarriers is large, a sampling clock frequency mismatch between the transmitter and the receiver dramatically degrades the performance. So far, the literature proposes ad hoc estimation algorithms. However, a complete performance analysis, especially the Crame´r-Rao bound (CRB) derivation, remains to be done. Obviously, the channel-impulse response is unknown at the receiver and also needs to be estimated. Therefore, the CRB associated with this joint estimation is theoretically evaluated. When the number of subcarriers and the channel degree are large, very compact closed-form expressions for the CRB are obtained. Furthermore, along with the maximum-likelihood (ML) estimator, suboptimal estimation algorithms are introduced and compared with some existing approaches and with the CRB.