Robust Non-redundant PAM-coupled U-OFDM OWC Systems with LED Nonlinearity

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.