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This letter deals with the frequency synchronization problem in the uplink of OFDMA communications systems with interleaved or generalized subcarrier allocation. An algorithm to estimate the carrier frequency offsets (CFOs) of all the active users is presented. The estimator relies upon finding the zeros of a suitable designed filter. The design of the filter boils down to the solution of a least squares (LS) problem and thus entails relatively low complexity. While under low subcarrier load conditions the estimation root mean squared error (RMSE) is higher than that of other existing methods, under heavy or full load achieves comparable and even superior performance.