# An ANN-Based Adaptive Predistorter for LED Nonlinearity in Indoor Visible Light Communications
## 1 Introduction  
Light-emitting diode (LED) solid-state lighting has received considerable attention recently because of its wide applicability and advantages over conventional lighting sources for monitor backlighting, traffic indicators, and general illumination. Lighting plays a key role in people's daily lives. LEDs offer advantages such as low energy consumption, long lifetimes, small sizes, and easy production, and they are increasingly being used as replacements for incandescent lamps. LEDs are expected to become a dominant part of the next generation of lighting sources. Recently, visible light communication (VLC) has been widely studied and has become a candidate for incorporation in the IEEE 802.15.7r1 standard to achieve lighting and communications simultaneously [1]. Through intensity modulation (IM), i.e., by modulating the light-emitting power in accordance with the transmitted data, and through direct detection (DD), i.e., by detecting the subtle light intensity variation of the transmitted optical light, VLC enables the functionality of wireless communication in a manner that is relatively simple and low in cost. Notably, this communication function does not increase power and arrangement costs, and many studies have shown that VLC systems combined with Ethernet or power line communication offer a promising solution for wireless broadband access in indoor environments [2; 3]. Orthogonal frequency-division multiplexing (OFDM) transmission had been considered for use in VLC systems to achieve a high data transmission rate [4]. Traditionally, the OFDM modulation and demodulation were realized by the inverse fast Fourier transform (IFFT) and the FFT, respectively. However, the traditional OFDM scheme results incomplex-valued time-domain signals and cannot be directly applied to VLC transmission. As such, several studies have investigated optical OFDM schemes aimed at making the traditional OFDM scheme suitable for VLC systems [5; 6; 7]. Basically, the real-valued bipolar signals can be obtained by using Hermitian symmetry when allocating modulated symbols to frequency domain subcarriers, after which the IFFT can be applied. By adding a direct current bias, direct current biased optical OFDM (DCO-OFDM) converts the real-valued bipolar signals to unipolar ones [5; 6]. On the other hand, asymmetric clipping optical OFDM (ACO-OFDM) is used to impose the anti-symmetric property and directly clip the negative parts of the real-valued bipolar signals without losing information [7]. Both DCO-OFDM and ACO-OFDM have attracted considerable attention for use in VLC systems. However, LED nonlinearity severely degrades the performance of OFDM VLC systems due to the high peak-to-average power ratio (PAPR) in OFDM signals [8]. Normally, nonlinear devices are operated with a certain power backoff, which can be defined as the ratio of maximum saturation output to the average output. Increasing the power backoff reduces the nonlinear distortion, but cause lower power efficiency. Reducing power backoff is thus desirable, while the signal nonlinear distortion would seriously sacrifice bit error rate (BER) performance. Thus, LED nonlinearity not only reduces the power efficiency of such systems, but also degrades their BER performance. Although PAPR reduction schemes can decrease the PAPR in OFDM signals, both DCO-OFDM and ACO-OFDM signals still exhibit a considerable dynamic range and would be distorted by LED nonlinearity [9; 10]. Given this knowledge of LED behavior, the most straightforward approach to mitigating the impact of nonlinearity would be to pre-process the DCO-OFDM signal to suppress the nonlinearity in the LED emitted signal, and to possibly enable receiver side with low complexity, which is known as transmitter digital predistortion [11; 12; 13]. Initially, the LED characteristics can be learned by using training sequences. Subsequently, the adaptive predistorter is capable of tracking the LED characteristics due to temperature variation and aging by using the transmitted data sequences.