# NOMA-Based VLC Systems: A Comprehensive Review
## 1 Introduction
### Background  
VLC is not intended to substitute for RF, but instead to supplement it; VLC networks may be incorporated into 5G wireless communication systems by connecting to existing optical fiber networks [26]. Whether you have been trying to get a message indoors, outdoors, underwater, or in underground settings, VLC can be an integral part of it. In order to offer both light and communication, VLC proves to be an energy-efficient technology that makes use of the widespread use of light-emitting diodes (LEDs) [27]. The traditional VLC system utilizes basic, commercially available sources, such as a light-emitting diode (LED) for transmission and a photodetector (PD) for reception [26]. Intensity modulation (IM) allows LEDs to send data by subtly adjusting the brightness of the light at extremely  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Reference** & **Communication Domain** & **Research Contribution** \\ \hline
[8] & 5G networks & Review of challenges and potentials of PD-NOMA for 5G systems \\ \hline
[15] & 5G networks & It surveys NOMA techniques for 5G networks \\ \hline
[16] & 6G networks & It provides future vision and research opportunities for next generation NOMA \\ \hline
[5] & VLC & GRPA scheme to enhance performance of NOMA systems \\ \hline
[14] & VLC & EPA method to enhance energy efficiency of NOMA-aided IoT sensor networks \\ \hline
[17] & VLC & NGDPA scheme to improve capacity of NIMO-NOMA \\ \hline
[18] & VLC & Power allocation and user pairing methods for downlink NOMA-VLC \\ \hline
[19] & VLC & Survey of research challenges and future trends for VLC-NOMA \\ \hline
[20] & RF communication & DDPA technique for mMIMO-aided NOMA \\ \hline
[21] & RF communication & Energy-efficient PA for multiuser MIMO-NOMA \\ \hline
[22] & RF communication & PA to ensure QoS requirements in NOMA systems \\ \hline
[23] & RF communication & PA to ensure individual QoS requirements in downlink NOMA systems \\ \hline
[24] & Hybrid RF/VLC & Link selection and user pairing in Co-NOMA systems \\ \hline
[25] & Hybrid RF/VLC & Improvement of reliability and outage performance of co-NOMA systems \\ \hline \hline \end{tabular}
\end{table}
Table 1: Summary of recent works [14].  
Figure 1: An overview of downlink NOMA-VLC.  
fast rates [28]. Direct detection (DD) involves generating an electric current at the receiver end that is proportional to the fluctuation in optical power of the receiving end [29]. This is done with photodetectors or image sensors. The development of VLC technology is still in its infancy, and it will take significant work to see it extensively used in real-world settings. From undersea communication to military reasons, as well as broadband access, interior localization, power line communication (PLC), and intelligent transportation systems (ITS) [30], many different applications are expected to be widely implemented in the next few years.  
Despite these advantages, however, VLC systems still have several drawbacks that must be fixed before their full potential can be realized [28]. Technically, the low modulation bandwidths of LEDs (a few MHz) are the primary obstacle to constructing VLC systems with large attainable data speeds. Blue LEDs containing yellow phosphorous and RGB LEDs with a modulating bandwidth of 2-5 MHz and 10-20 MHz are frequently utilized for VLC due to their white light [31]. When compared to RGB LED, devices that rely on blue LEDs offer more cost-effectiveness and energy efficiency; hence, they are gaining popularity in lighting systems.  
However, RGB's greater modulator bandwidth empowers it highly suitably for transmission. Nevertheless, to solve the modulation bandwidth challenge and employ VLC systems to their maximum potential, economic improvement of high-order modulation methods, MIMO, frequency reuse, and sophisticated channel access techniques are recommended [32].