A novel hybrid time-frequency domain equalization scheme is proposed and experimentally demonstrated to mitigate the white light emitting diode (LED) nonlinearity in visible light communication (VLC) systems based on orthogonal frequency division multiplexing (OFDM). We handle the linear and nonlinear distortion separately in a nonlinear OFDM system. The linear part is equalized in frequency domain and the nonlinear part is compensated by an adaptive nonlinear time domain equalizer (N-TDE). The experimental results show that with only a small number of parameters the nonlinear equalizer can efficiently mitigate the LED nonlinearity. With the N-TDE the modulation index (MI) and BER performance can be significantly enhanced.

Layered asymmetrically clipped optical orthogonal frequency division multiplexing (LACO-OFDM) has a high spectral efficiency. However, an iterative detection with a pair of fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) is required for LACO-OFDM, which increases the complexity for the receiver of LACO-OFDM. In this paper, the performance of the receiver with a single FFT is analyzed for LACO-OFDM in visible light communications. In the single-FFT receiver, the different layers of ACO-OFDM signals are distinguished in time domain without the FFT/IFFT pair. Because only one FFT is employed in the proposed receiver, the complexity of O(Nlog2N) can be achieved, which is much lower than the complexity of the conventional LACO-OFDM receiver. Simulation results illustrate that the proposed scheme has a signal-to-noise ratio (SNR) penalty of 1dB ~ 3dB compared with the conventional solution in additive white Gaussian noise (AWGN) channel. However, the proposed receiver outperforms conventional LACO-OFDM solution in light emitting diode (LED) nonlinearity channel.

A single Fourier transform (FFT) receiver with pairwise maximum likelihood (ML) is proposed for layered asymmetrically clipped optical orthogonal frequency division multiplexing (LACO-OFDM). To improve the single-FFT receiver, the pairwise ML detection is made in time domain after a layer diversion in LACO-OFDM. With only one FFT block, the proposed receiver has a low complexity. Simulation results show that the proposed receiver has the better bit error ratio (BER) performance than the conventional iterative scheme for the LACO-OFDM system with 2 layers in additive white Gaussian noise (AWGN) channel. Furthermore, it is proved that compared with the conventional iterative receiver, the proposed receiver is less sensitive to the clipping distortion. As the nonlinearity increased, the proposed scheme can outperform the conventional iterative solution in nonlinearity channel for LACO-OFDM in visible light communications.

An iterative pairwise maximum likelihood (ML) receiver is proposed for asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) in visible light communications (VLC). With the aid of the pairwise ML detection, half of the received ACO-OFDM samples are forced to zero in time domain. However, when the signal-to-noise ratio (SNR) is low, the pairwise ML detection may cause misjudgment. With the iterative pairwise ML detection, the ACO-OFDM symbols are detected in frequency domain and the SNR gain of 3 dB could be available. Simulation results show that the pairwise ML receiver without iteration provides the SNR improvement of 1.5~2.7 dB for 7% forward error correction (FEC) limit. The proposed iterative receiver gives a SNR gain of 2.6~2.9 dB with 7% FEC limit.

A modulation format, polarity separating optical orthogonal frequency division multiplexing (PSO-OFDM), is proposed to mitigate the light-emitting diode (LED) nonlinearity for visible light communication systems. A polarity separator is used to divide the OFDM signal in time domain x(t) into two parts: x+(t) and x−(t), which will be transmitted parallelly from the different LEDs and overlap linearly in free space to realize PSO-OFDM. The experimental results demonstrate that PSO-OFDM has high spectral efficiency and suffers less nonlinear distortions than other methods. Employing PSO-OFDM, the modulation index and bit error rate performance can be significantly enhanced.