A low-complexity and high-performance clipping-free multilayer optical orthogonal frequency division multiplexing scheme is proposed for optical wireless communication (OWC). At the transmitter, the symbols of multi-layer quadrature amplitude modulation (QAM) are first arranged according to a specific rule in the frequency domain to generate a bipolar time-domain signal using a single inverse fast Fourier transform (IFFT). Then the bipolar signal is superimposed with an adaptive periodic DC bias to meet the real and positive requirements in intensity modulation / direct detection (IM/DD) systems. At the receiver, a zero-value regression (Re0) -based demodulation is proposed to optimize the bit error rate (BER). As the signal generation and demodulation are realized by single IFFT and fast Fourier transform (FFT) respectively, the proposed scheme has nearly the same system complexity as the single-layer modulations and similar spectral efficiency with those advanced multilayer modulations. In addition, the proposed periodic DC improves the power efficiency and avoids the interlayer interference. Numerical simulation results show that, compared with LACO-OFDM with 3 layers, the proposed scheme can attain a PAPR gain of 2.4 dB and BER gain of 1.4 dB and 3 dB for the linear and nonlinear channel, respectively.

Abstract The no‐reference image quality assessment (NR‐IQA) method can evaluate the distortions in an image without the reference image. However, due to the diversity of the image contents and distortion types, it is hard for the existing NR‐IQA algorithms to obtain competitive performance on both synthetically and authentically distorted images. To address the problem, a multiscale feature representation‐based NR‐IQA method that performs well for both synthetic and authentic distortions is proposed. This model consists of two parts: The feature extraction part and the feature fusion part. First, part of the Res2Net‐50 network is chosen as the feature extraction part due to its high ability in increasing the range of receptive fields. Then, the feature fusion part consisting of a novel residual block and two fully connected layers is designed to fuse the extracted features and realize the quality score mapping. After a series of stepwise optimization experiments, the most competitive network architecture consisting of the feature extraction part and the feature fusion part is obtained. Comprehensive experiments on the LIVE, TID2013, CSIQ, KADID‐10k, KonIQ‐10K, and LIVE challenge databases demonstrate that the proposed method can work powerfully on both the synthetic and authentic distortions and also has a strong generalization ability.

Layered or hybrid optical orthogonal frequency division multiplexing (OFDM) has been proposed for use in optical communications due to its excellent spectral and power efficiencies, especially in visible light communications (VLC). However, most of the current works concentrate on transmitter and receiver design as well as the quality of service in communication networks. In this paper, we propose a spectrum-efficient dimmable triple-layer hybrid optical OFDM (DTH-OFDM) scheme to tackle the illumination requirements, considering different practical indoor VLC scenarios from low illumination to high illumination intensities. In the proposed DTH-OFDM scheme, the required dimming level is achieved by jointly adjusting the dimming factors and direct current bias. We investigate the comprehensive performance analysis of the proposed DTH-OFDM in detail, including probability density function, bit error rate (BER), spectral and energy efficiencies. In addition, a joint dimming control and optimal power allocation problem for DTH-OFDM is formulated and solved using convex optimization under the constraints of light emitting diode (LED) nonlinearity, dimming target and communications reliability. Numerical results show that, the proposed DTH-OFDM can offer continuous and arbitrary dimming target with higher spectral efficiency and lower BER compared with its counterparts, as well as an enhanced tolerance to the LED nonlinearity.

In this study, a novel non‐linear companding transform (NCT) is proposed for the DC‐biased optical orthogonal frequency division multiplexing (DCO‐OFDM) based visible light communications (VLC). The authors lower the peak‐to‐average power ratio (PAPR) of the original DCO‐OFDM signals to mitigate the transmission performance degradation induced by the non‐linearity of the light‐emitting diodes (LEDs) transmitter. Firstly, the positive and negative amplitudes of the original DCO‐OFDM signals are separated into two groups. Then, the group with the maximum peak is first processed with a given compress coefficient u 1 and power compensation coefficient c . After that, another coefficient u 2 is calculated by the emission power conservation criteria to further compress the rest group. Finally, these two groups of amplitude samples are combined together before adding a suitable DC bias for driving LEDs. As the mean power is not changed, a PAPR reduction can be obviously realised by the proposed NCT scheme.

In this study, the authors propose an adaptive channel estimation algorithm for asymmetrically clipped optical orthogonal frequency‐division multiplexing visible light communication (VLC) systems and analyse its optimal signal‐to‐noise ratio (SNR) threshold through the simulation experiments. The novel estimation scheme combines the advantages of orthogonal matching pursuit algorithm and least‐square discrete Fourier transform algorithm. It is robust to the changes in channel distribution and SNR range. The performance for a VLC system using 16‐quadrature amplitude modulation is presented in terms of mean square error (MSE) and uncoded bit error rate. In the high SNR range, the proposed scheme improves the MSE up to more than 10 dB.