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In this paper, a new method for pilot-tone design is presented for multiuser orthogonal frequency division multiplexing(OFDM) systems. We first compute the mean square error (MSE) of the ML channel estimate, then we derive optimal pilot sequences and optimal placement of the pilot tones with respect to this MSE. It is shown that the optimal pilot sequences are equipowered and phase shift orthogonal. Through simulations, it is shown that the optimal pilot sequences derived in this paper outperform the random pilot sequences for joint estimation of channel response and frequency offset in multiuser OFDM systems. Simultaneously, by using the modified maximum likelihood (ML) algorithm can reduce the complex of computation. Simulations indicate that this scheme is especially useful for the system has small frequency offset.