Superposition Based Nonlinearity Mitigation for ACO-OFDM Optical Wireless Communications

We propose a superposition based light emitting diode (LED) nonlinearity mitigation scheme for asymmetrically clipped optical-orthogonal frequency division multiplexing (ACO-OFDM) optical wireless communications (OWC) systems, where a non-redundant signal stream is superimposed with an ACO-OFDM source signal stream, requiring no pilot and no side information. Based on the anti-symmetry property that a non-redundant signal and its symmetric signal is equal to zero with the same absolute value, a non-redundant signal is used to reduce the large amplitude of the ACO-OFDM signal and enhance its symmetric ACO-OFDM signal that is clipped to zero due to negative signal clipping, allowing LED nonlinearity mitigation. The proposed scheme is also energy-efficient, as the large-amplitude signal reduction is equal to the symmetric signal enhancement, requiring no extra power. However, the designed non-redundant signals fall on odd subcarriers and interfere with source signals. The even subcarriers include the absolute value of source signals, which can be utilized to recover source signals. The polarity of source signals on even subcarriers can be determined by ensuring that the remaining part is higher than the clipping part of the large-amplitude signal in the process of LED nonlinearity mitigation. Simulation results show that the proposed scheme outperforms a number of state-of-the-art methods in the literature as well as ACO-OFDM systems with no LED nonlinearity mitigation in terms of bit error rate (BER).