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In orthogonal frequency division multiple access (OFDMA), closely spaced multiple subcarriers are assigned to different users for parallel data transmissions. The subcarriers are overlapping but orthogonal to each other such that there is no inter-carrier interference. Carrier frequency offsets between the transmitter and the uplink receiver will cause the loss of the orthogonality among subcarriers, hence introduce inter-carrier interference resulting in multiple access interference. This paper proposes a carrier frequency offset estimation algorithm for the uplink of interleaved OFDMA systems. This algorithm requires only one OFDMA block and utilizes the inner structure of the signals without any aid from training symbols or the knowledge of channels. Simulation results illustrate the high accuracy and efficiency of the proposed estimation algorithm.