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In this paper, new timing synchronization methods for optical wireless systems using asymmetrically clipped optical orthogonal frequency division multiplexing(ACO-OFDM) are described. It is well known that OFDM systems are sensitive to timing synchronization errors. Existing synchronization methods cannot be used directly or do not perform well in ACO-OFDM systems because ACO-OFDM signals are unipolar. Therefore, new synchronization methods using a novel training symbol designed for the characteristics of ACO-OFDM are presented. The sensitivity of the new methods to the phase of the receiver sampling clock is considered and it is shown that if only the lower frequency subcarriers in the training symbol are non zero, the sensitivity can be reduced. Analysis and simulation results show that the new timing synchronization methods are effective in ACO-OFDM systems.