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Orthogonal frequency division multiplexing (OFDM) can effectively improve the information transfer speed and has the robustness for multipath dispersion. OFDM is a promising technology for optical communications. An efficient timing synchronization scheme for asymmetrically clipped optical OFDM (ACO-OFDM)-based optical wireless communications systems with implicit training is proposed. The pseudo-noise (PN) sequence modulated by ACO-OFDM as training sequence is sent synchronously with a data symbol in the same symbol cycle. By using local sequence and the received signal correlation operation, receiver accumulates the energy of the implicit training to achieve timing synchronization. Via simulation, both in line of sight (LOS) and diffuse channel, the proposed scheme also has better synchronization accuracy, less timing offset estimator variance. And it has the flexibility of time and power allocation, and it is able to improve the bandwidth efficiency.