# NOMA-Based VLC Systems: A Comprehensive Review
## 3 Integration of NOMA-Based VLC with Emerging Technologies
### MISO/MIMO Techniques in NOMA-Based VLC Systems  
By utilizing illuminating LED arrays, MIMO has been widely used in VLC systems as a natural and effective technique to widen system coverage and boost system capacity [68,69]. By installing multiple sources at the transmitter or receiver, additional spatial DoFs can be enabled. Massive MIMO (mMIMO) can also substantially boost the spectral efficiency due to the large number of spatial DoFs introduced by the multiple sources [70]. However, \begin{table}
\begin{tabular}{c c c} \hline \hline
**Reference** & **Objective** & **Research Findings** \\ \hline
[6] & Evaluation of error vector magnitude, BER, and spectral efficiency & Given technique is more robust and outperforms OFDM-based NOMA \\ \hline
[17] & To maximize the sum rate & NGDPA enhances the sum rate performance as compared to GRPA \\ \hline
[34] & Ergodic sum rate and coverage probability analysis & NOMA outperforms traditional OMA technique \\ \hline
[53] & To maximize the sum rate & The performance of NOMA-OFDM is better than OMA-OFDM in the context of achievable data rate \\ \hline
[56] & BER analysis & Closed-loop expressions for BER validate simulation results \\ \hline
[58] & Rate splitting & Offers an overview of MA techniques in VLC systems. It proposes rate-splitting multiple access (RSMA) and highlights its potentials and capabilities in VLC systems. \\ \hline
[59] & To maximize the sum rate & WDM-NOMA outperforms NOMA in the context of sum rate \\ \hline
[62] & Outage probability analysis & Rate splitting trade-off permits outage performance balancing among users \\ \hline
[63] & To maximize the sum rate & Game theory based optimal power allocation and user grouping \\ \hline
[64] & Evaluation of user fairness, outage probability, and sum rate & Dynamically choosing the appropriate MA technique that attains better performance \\ \hline
[65] & To maximize the sum rate and max-min rate criteria & Optimized power allocation and user grouping to achieve high sum rate than OMA \\ \hline
[66] & BER analysis & Enhanced BER performance compared to NOMA considering SIC for various power levels \\ \hline
[67] & SER analysis & Users at various locations attain identical SER through adequate power allocation \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Summary of recent works on NOMA-based VLC. considering the future demand of coverage for an excessive number of users, the spatial DoFs provided by MIMO or mMIMO are not sufficient. In this case, NOMA can be incorporated to serve a higher number of users. The use of NOMA in MIMO-VLC systems has not received much research. The experimental verification of a MIMO-NOMA-aided VLC system in [71] did not take power allocation into account. In MIMO-NOMA-aided VLC systems, the PA strategies of single-LED NOMA-VLC systems cannot be immediately applied. For MIMO-NOMA radio-frequency (RF) systems, a number of PA techniques, including signal alignment [72], hybrid precoding, and post-detection [73] have been discussed in the literature thus far. These techniques, however, have a significant computational cost. For the possible widespread use of the MIMO-NOMA methodology in real-world VLC systems, effective PA techniques with low computing complexity are crucial. In a recent study [17], the authors extend NOMA to MIMO-VLC systems and provide a cutting-edge power distribution technique, called normalized gain difference power allocation (NGDPA), for effective and simple power distribution in MIMO-NOMA-VLC systems. Numerical simulations are used to assess the total rate performance of a 2 \(\times\) 2 MIMO-NOMAVLC system installed inside. It is demonstrated that, as compared to NOMA with GRPA, the achievable sum rate of the 2 \(\times\) 2 MIMO-VLC system may be substantially increased by using NOMA with the suggested NGDPA approach. A 2 \(\times\) 2 MIMO-VLC system for k number of users is illustrated in Figure 4.  
NOMA is used in a MIMO VLC system with many users in [74].