## 1 Introduction  
Visible light communications (VLC) are now envisaged as a mature and promising technology for future wireless communications [1, 2, 3]. It has been proposed for many different scenarios, both indoors and outdoors [4]. For indoor applications, there exists a wide variety of applications, such as extending the capacity and complementing the radio frequency (RF) systems [5], positioning and resource monitoring [6, 7], intruder detection [8] or providing coverage where RF transmission is not allowed [9], among others. Outdoor VLC applications comprise vehicle-to-vehicle communications [10, 11, 12] or some trials and analysis for long distance transmission [13, 14, 15]. In this context, VLC have been standardized as 802.15.7 [16], and considering their integration in the IP network, usually referred to as light fidelity (LiFi), under the standard 802.11 bb [17]. In order to ease the reading of the paper, the list of acronyms has been summarized in Table 1, whereas the list of variables and symbols is in Table 2. \begin{table}
\begin{tabular}{l l} \hline ACO-OFDM & Asymmetrically clipping optical–orthogonal frequency division multiplexing \\ \hline ADO-OFDM & Asymmetrically clipped direct current biased optical OFDM \\ \hline ADR & Angle diversity receiver \\ \hline AP & Access point \\ \hline AWGN & Additive white Gaussian noise \\ \hline BD & Block diagonalization \\ \hline BER & Bit error rate \\ \hline CSI & Channel state information \\ \hline CU & Central unit \\ \hline DCO-OFDM & DC-biased optical-orthogonal frequency division multiplexing \\ \hline DD & Direct detection \\ \hline DPC & Dirty paper coding \\ \hline FDD & Frequency division duplex \\ \hline FFR & Fractional frequency reuse \\ \hline FFT & Fast Fourier transform \\ \hline FoV & Field of view \\ \hline FR & Frequency reuse \\ \hline IFFT & Inverse fast Fourier transform \\ \hline IM & Intensity modulation \\ \hline IR & Infrared \\ \hline JT-CoMP & Joint transmission—Coordinated multipoint \\ \hline LED & Light emitting diodes \\ \hline LoS & Line-of-sight \\ \hline LTE & Long Term Evolution \\ \hline MIMO & Multiple-input multiple-output \\ \hline MMSE & Minimum mean squared error \\ \hline M-PAM & Multi-level pulse amplitude modulation \\ \hline M-PPM & Multi-level pulse position modulation \\ \hline MRC & Maximum ratio combining \\ \hline MSE & Mean squared error \\ \hline NLoS & Non-line-of-sight \\ \hline NOMA & Non-orthogonal multiple access \\ \hline OOK & On-off keying \\ \hline O-OFDM & Optical orthogonal frequency division multiplexing \\ \hline PD & Photodiode \\ \hline PLC & Power line communication \\ \hline PWM & Pulse width modulation \\ \hline QAM & Quadrature amplitude modulation \\ \hline \end{tabular}
\end{table} 
Table 1: List of acronyms in this survey. \begin{table}
\begin{tabular}{l l} \hline RGB & Red-green-blue \\ \hline RF & Radio frequency \\ \hline SDMA & Spatial division multiple access \\ \hline SINR & Signal-to-interference-plus-noise ratio \\ \hline SISO & Single-input-single-output \\ \hline SNR & Signal-to-noise ratio \\ \hline TDD & Time division duplex \\ \hline TDMA & Time division multiple access \\ \hline VLC & Visible light communications \\ \hline ZF & Zero forcing \\ \hline \end{tabular}
\end{table} 
Table 1: _Cont._