## 2 NOMA System Model
### Downlink NOMA  
In the downlink NOMA-VLC system, this paper mainly considers a single-cell downlink scenario with one base station and \(N\) users, \(U_{i},i=1,2,3,\ldots,N\), and all terminals are equipped with a single optical antenna. It is worth noting that there is a similar case for the uplink scenario. For users with weak channel conditions, a higher power is allocated for the sending signal, while the contrary is the case for users with better channel conditions. At the transmitter end, the base station communicates with all users through the same time-frequency resource and different powers. At the receiving end, reception is performed through SIC. When the SIC receiver demodulates the signal of a NOMA user, it detects and removes the signals of other NOMA users with higher power and considers the users with lower power as interference. In particular, a receiving user with optimal channel conditions eliminates the signal of all other NOMA users when demodulating its own signal. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Characteristics** & **Material-1** & **Material-2** \\ \hline Volatile substances (toluene) & 99.65 wt\% & 96.51 wt\% \\ \hline Non-volatile organics & 0.06 wt\% & 0.85 wt\% \\ \hline \multirow{2}{*}{Inorganics} & (QD CdSe/ZnS) & (QD CsPbBr\({}_{3}\)) \\  & 0.29 wt\% & 2.64 wt\% \\ \hline Density & 0.870 g/cm\({}^{3}\) & 0.867 g/cm\({}^{3}\) \\ \hline QD (inorganics + ligands) Con. & 3 mg/mL & 3 mg/mL \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Characteristics of quantum dot Material-1 (CdSe/ZnS) and Material-2 (CsPbBr\({}_{3}\)-50). \begin{table}
\begin{tabular}{c c c} \hline \hline
**Optical Performance** & **Material-1** & **Material-2** \\ \hline PL peak emission wavelength & 526 nm & 514 nm \\ \hline Full wave at half maximum & 26 nm & 18 nm \\ \hline Photoluminescence quantum yield & 86\% & 80\% \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Optical performance of quantum dot Material-1 (CdSe/ZnS) and Material-2 (CsPbBr\({}_{3}\)-50). In practice, interference reconstruction is complicated in the SIC detection for multiple users. The transmitter assigns higher power \(P_{1}\) to the first user's signal \(x_{1}\), and it assigns lower power \(P_{2}\) to the second user's signal \(x_{2}\). In this case, the power allocation factor can be expressed as \(\frac{P_{1}}{P_{1}+P_{2}}\). Then the transmitter superimposes both signals and transmits them simultaneously. \(h_{i}\) is the channel gain for an optical communication system. The superimposed signal \(s\) can be expressed as  
\[s=\sqrt{P_{1}}x_{1}+\sqrt{P_{2}}x_{2} \tag{1}\]  
Figure 3: Material-2: (**a**) normalized excitation spectra of QDs, (**b**) normalized emission spectra of QDs, (**c**) TEM, and (**d**) high-resolution TEM images of the QD material. Figure 2: Material-1: (**a**) normalized excitation spectra of QDs, (**b**) normalized emission spectra of QDs, (**c**) TEM, and (**d**) high-resolution TEM images of the QD material. The total signal \(y_{i}\) (\(i=1,2\)) can be expressed as the sum of the received signal and the channel Gaussian white noise \(w_{i}\):  
\[y_{i}=h_{i}s+w_{i} \tag{2}\]  
Moreover, we calculate the theoretical capacity for the strong and weak users of the NOMA system according to Equation (3). We consider the normalized bandwidth to be 1. As shown in Figure 4, the capacity of the strong user (\(R_{1}\)) is higher than that of the weak user (\(R_{2}\)) for the downlink NOMA system. \[R_{1}=\log_{2}(1+\frac{P_{1}|h_{1}|^{2}}{w_{1}}),\ R_{2}=\log_{2}(1+\frac{P_{2 }|h_{2}|^{2}}{P_{1}|h_{2}|^{2}+w_{2}}) \tag{3}\]  
Considering a NOMA system with two users, the signal with higher power is \(x_{2}\), so the first output is \(x_{2}\) by direct detecting. The signal estimate of \(x_{2}\) is recovered, and it is subtracted from the received signal \(y\). Then, the second output \(x_{1}\) is demodulated. Thus, the user with weak power needs to demodulate the signal twice [63]. Using such a detection sequence, it can realize the multiplexing of two signals in the power domain.