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In this paper, we investigate the problem of carrier frequency-offset (CFO) synchronization and channel estimation in orthogonal frequency-division multiple access (OFDMA) systems operating over unknown frequency-selective fading channels. We first devise a novel iterative method for joint CFO and channel estimation, based on the time-domain training blocks. The proposed joint estimator consists of two recursive least-square (RLS) algorithms. We then propose a more precise pilot-aided RLS algorithm to estimate the residual frequency synchronization errors or track small CFO changes for each user. With this, the accuracy of channel estimation is also enhanced. The analysis and simulation results show that, the proposed estimation and tracking scheme which is fully compatible with the existing standards is able to attain fast convergence, high stability, and ideal performances in all ranges of signal-to-noise ratio (SNR). Moreover, it can work well for wide tracking range up to ±0.5 of the subcarrier spacing. [ABSTRACT FROM PUBLISHER]