Modeling and Mitigating LED Nonlinearity using Nonlinear ARX model with Wavelet Networks

In this paper, a nonlinear autoregressive exogenous (NARX) model with a wavelet network is applied to model and compensate the nonlinearity of the LED in Visible Light Communications (VLC). The NARX model shows the ability to accurately describe the response of the LED. PAM-4 signal with a symbol rate of 5 Msym/s is used to demonstrate the performance of the NARX adaptive compensator. The eyediagrams show that this compensator can substantially improve the distorted signal. The complexity of the NARX adaptive compensator is relatively low, with only 15 units. This also facilitates the adaptive parameters updating process due to the small number of parameters in the NARX adaptive compensator

LED Nonlinearity Post-compensator with Legendre polynomials in Visible Light Communications

The nonlinear effect of Light-Emitting Diodes (LEDs) is one of the important factors that hamper the bit rate of Visible Light Communications (VLC). To mitigate the nonlinearity, we propose a Legendre-polynomials-based post-compensator derived from a post-distorter deduced by a physical-based nonlinear LED model. We represent the formulation of the post-distorter with Legendre series expansion to ease the computation burden for training coefficients. Since only feed-forward structure is embodied in the series representation, the coefficients of the proposed compensator are easy to access with adaptive algorithms of low complexity. To validate the effectiveness of our proposed model, we adopt Recursive Least Square (RLS) and Least Mean Square (LMS) algorithms to train the coefficients of our proposed compensator with the same iteration steps. In particular, RLS achieves lower Mean square Errors (MSEs) with faster convergence speed. Due to the relatively low complexity of these nonlinear algorithms, our proposed compensator is more actual for implementation on VLC hardware platforms like Field-Programmable Gate Array (FPGA)

Achievable Rate of MIMO-OFDM VLC over Low-Pass Channels

Visible light communication (VLC) is a short-range optical wireless communication (OWC) utilizing white light-emitting diode (LED) lighting, so that the VLC systems can provide both illumination and communication. Multiple-input multiple-output (MIMO) is an attractive technology to efficiently improve the achievable rate of VLC with multiple LED luminaries which experience a low-pass effect in practical channels. In this paper, we investigate the performance of MIMO-VLC over three general low-pass channels, including exponential, first-order and Gaussian low-pass channels. Over frequency domain, two power loading strategies for multi-subcarrier orthogonal frequency division multiplexing (OFDM) are considered, namely uniform power loading and water-filling power loading. Expressions on the achievable rate to the corresponding link power budget and bandwidth are derived. Low-pass MIMO-OFDM VLC with a matrix channel decomposition has not extensively been treated theoretically in literature, to the best of our knowledge

Physics-Based LED Modeling and Nonlinear Distortion Mitigating With Real-Time Implementation

In this paper, a nonlinear model for Light Emitting Diodes (LEDs) inspired by semiconductor physics, and a corresponding post-compensator are implemented in a Field Programmable Gate Array (FPGA) for real-time Visible Light Communications (VLC). Our experiments demonstrate that the LED model effectively characterizes the dynamic LED nonlinearity, including the memory effects. The output signal of this nonlinear LED model shows a good resemblance with the measured LED output. In addition, a dedicated nonlinear equalizer, say, a post-compensator, inspired by this LED physical model can mitigate the nonlinear distortion substantially. Thereby it facilitates high data rate over the bandwidth-limited LED. It shows that the nonlinear compensator is attractive for practical real-time digital signal processing systems due to its high performance and low complexity