We propose a non-redundant pulse amplitude modulation (PAM) mapping scheme for unipolar orthogonal frequency division multiplexing (U-OFDM) optical wireless communication (OWC) systems, robust against light-emitting diode (LED) non-linearity. To the best of our knowledge, this is the first work to utilize the inherent null signals in U-OFDM blocks to reduce signal distortion caused by LED nonlinearity. A large-amplitude signal is mapped into a PAM-dependent signal that consists of a PAM signal, an exceeding margin and a polarity margin. The PAM signal that represents a large part of the original large-amplitude signal replaces the null signal in the adjacent block. At the receiver, the PAM signal and the PAM-dependent signal are combined to recover the amplitude of the original signal, and the polarity margin is used to recognize the polarity of the original signal. The proposed PAM mapping scheme is spectrally efficient, requiring no side information for signal detection. Simulation results verify the proposed scheme.

We propose a non-redundant pulse amplitude modulation (PAM)-coupled unipolar orthogonal frequency division multiplexing (PU-OFDM) optical wireless communication (OWC) system, robust against light-emitting diode (LED) nonlinearity. To the best of our knowledge, this is the first work to utilize the inherent null signals in U-OFDM blocks to reduce signal distortion caused by LED nonlinearity. A large-amplitude signal is mapped into a PAM-dependent signal that consists of a PAM signal, an exceeding margin and a polarity margin. The PAM signal that represents a large part of the original large-amplitude signal replaces the null signal in the adjacent block. At the receiver, the PAM signal and the PAM-dependent signal are combined to recover the amplitude of the original signal, and the polarity margin is used to recognize the polarity of the original signal. The proposed PU-OFDM system is spectrally efficient, requiring no side information for signal detection. In order to map the large-amplitude signals into small-amplitude signals, we design three PU-OFDM signal schemes: i) PAM mapping (PM) for low complexity; ii) enhanced PAM mapping (ePM), enhancing BER performance via time diversity for relative large LED linear range; iii) accumulated PAM mapping (aPM), with strong signal compression ability for severe LED nonlinearity. Simulation results verify the proposed system and schemes.