# NOMA-Based VLC Systems: A Comprehensive Review
## 2 Related Work  
In this section, we review the studies on NOMA-VLC systems previously reported in the literature. We highlight several works which have been performed on the NOMA-VLC. Recently, NOMA has emerged as a promising candidate to support 5G technologies. All NOMA users can get support for frequency and time resources which are advantageous to enhance the sum rate of NOMA-based VLC systems [47]. NOMA has proven to be more appropriate for downlink VLC systems [48]. Similarly, some studies have developed a downlink NOMA-VLC system using LED source and multiple users [49]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Characteristics** & **1G** & **2G** & **3G** & **4G** & **5G** \\ \hline Time span & 1970–1980 & 1990–2004 & 2004–2010 & 2010–Now & Around 2020 \\ \hline MA technique & FDMA & CDMA/TDMA & CDMA & OFDMA & NOMA \\ \hline Physical resource & Frequency & Time & Time/PN codes & Orthogonal & Power domain/ \\  & & & & frequency & Code domain \\ \hline Network’s core & PSTN & PSTN & Packet network & Internet & Internet \\ \hline Duplex mode & FDD & FDD & FDD/TDD & FDD/TDD & FDD/TDD \\ \hline Technologies & NMT, AMPS & IS-54, GSM & EDGE, UMTS & LTE, LTE-A, Wimax & Mm Waves, MIMO \\ \hline Frequency & 30 kHz & 1.8 GHz & 1.6–2 GHz & 2–8 GHz & 3–30 GHz \\ \hline Data rate & 2 Kbps & 64 Kbps & 2 Mbps & 1 Gbps & \textgreater{}1 Gbps \\ \hline Hand off & Horizontal & Horizontal & Horizontal & Horizontal/Vertical & Horizontal/Vertical \\ \hline Services & Analog voice & Digital voice, SMS, MMS & Audio/Video & Mobile multimedia, wearable devices & IoT, video streaming, interactive multimedia, 3D games \\ \hline \hline \end{tabular}
\end{table} 
Table 2: A comparison between the MA techniques in successive generations of cellular networks. Figure 3: From OMA to NOMA through power domain multiplexing. Previous publications [5; 49] suggested and demonstrated studies on NOMA implementation in VLC. In [5], a PA approach termed \"gain ratio power allocation\" (GRPA) that takes into account channel characteristics in NOMA-VLC systems was presented, which improves system performance over static power allocation (SPA). The researchers also discovered that by altering the field of view (FOV) of photodiodes (PDs) and the semi-angle of transmitting LEDs, the total rate may be increased even further. Based on actual indoor VLC channel models, the investigators in [34] demonstrated the advantages of NOMA in VLC over OFDMA. Ref. [50] researched and assessed the sum rate of NOMA-based VLC system considering one LED and several users. In parallel, an alternative power allocation approach that enhances system capacity while considering illumination intensity and user fairness was developed in [51], and a low-complexity power control algorithm was also provided and assessed. The utilization of NOMA paired with OFDMA in VLC systems was proven in [49] to reach increased capacity via trials. Nevertheless, the aforesaid research into system performance upgrades is insufficiently thorough since it only includes a single or partial component that influences performance of the system. In reality, while prior studies have increased the sum rate, there is a notable gap of tradeoff between sum-rate efficiency and fairness, user quality of service (QoS), and optical intensity at the same time. Furthermore, most recent researchers emphasize on the situation of just two users, ignoring the scenario of inadequate SIC, wherein non-negligible leftover interference occurs owing to user movement and feedback delays at the receiver, resulting in poor detection accuracy and a fall in the attainable sum rate. In a recent work [47], the authors set up a single-LED, N-user NOMA-VLC framework within. In addition, this work represents the first effort to create an optimum power allocation technique (also known as MFOPA) by taking into account several elements that govern the system's performance concurrently.