# Modeling of Satellite-to-Underwater Integrated FSO-PON System Using NOMA-VLC
## 4 Results and Discussion  
This section shows the derived BER expressions for three users of MNZCC code employed in a hybrid FSO-PON system with NOMA-VLC evaluated in OptiSystem 15.0 and MATLAB(tm) r2022a software. Table 2 tabulates the system configuration parameters for the result analysis. The various hypotheses considered for the simulation work of the proposed systems are given below:  
* For performance analysis, the effect of fiber, FSO and VLC channels deteriorations, such as ISI, CD, FWM, noise (thermal, shot etc. ), beam divergence as well as geometric loss, are considered;
* To measure the performance in PON, FSO and VLC, various different DN and UP wavelengths are selected, i.e., 1596 nm (DN) and 1527.2 nm (UP) for TWDM-PON; 1575 nm (DN) and 1260 nm (UP) for WDM-OCDMA; 1550 nm (both DN and UP) for FSO; and 625 nm, 525 nm and 455 nm (both DN and UP) for NOMA-VLC channels with 0.2 power allocation coefficient;
* The environmental conditions for satellite-to-land communication using FSO links and underwater VLC links are considered as clear, except otherwise stated;
* PD's FOV in NOMA-VLC is considered as 50\({}^{0}\), except when otherwise stated. Figure 3 shows the SNR distribution of the VLC system in length, breadth and width (15 m, 15 m, 15 m) dimension of the receiver section in UOWC. Figure 3: SNR distribution in VLC receiver range (15 m, 15 m, 15 m). \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline
**Transmission** & **Wavelength** & **G** & **NF** & **P\({}_{1}\)** & **OSNR\({}_{1}\)** & **Po** & **OSNR\({}_{\bullet}\)** \\
**Channel** & **(nm)** & **(dB)** & **(dB)** & **(dB)** & **(dB)** & **(dB)** & **(dB)** \\ \hline \multirow{6}{*}{VLC system} & Red (DN) & \(-3\) & \(2.5\) & \(3\) & \(103\) & \(2\) & \(101\) \\  & Green (DN) & \(-2.5\) & \(2.5\) & \(7.5\) & \(110\) & \(7\) & \(106\) \\ \cline{1-1}  & Blue (DN) & \(-3.5\) & \(2.5\) & \(3\) & \(99\) & \(-2\) & \(92\) \\ \cline{1-1}  & Red (UP) & \(-2.5\) & \(2\) & \(5.5\) & \(111\) & \(4\) & \(107\) \\ \cline{1-1}  & Green (UP) & \(-2.5\) & \(2\) & \(5.5\) & \(108\) & \(5\) & \(105\) \\ \cline{1-1}  & Blue (UP) & \(-2.5\) & \(2\) & \(2.5\) & \(102\) & \(2\) & \(100\) \\ \cline{1-1}  & 1550 (DN) & \(-1.5\) & \(3\) & \(3.5\) & \(98\) & \(-1\) & \(93\) \\ \cline{1-1}  & 1550 (UP) & \(-1.5\) & \(2\) & \(4\) & \(110\) & \(3\) & \(106\) \\ \hline \hline \end{tabular}
\end{table} 
Table 9: _Cont._The log (BER) curves for the satellite-to-underwater proposed system are plotted against the SNR for varied DN and UP VLC ranges (5-15 m) in Figure 4. The fiber and FSO ranges are considered 100 km and 10 km, respectively, with varied VLC ranges under the impact of Clear Ocean. It shows that the BER curves improve as the SNR decreases. The BER performance of the UP VLC channel is superior to DN transmission. This is due to the fact that power (P\({}_{\text{in}}\) = 0 dBm per channel) and data rate (=2.5 Gbps per channel) allocated to upstream channels are lesser than those of the downstream channels. The figure also presents that better performance of the 5 m VLC range followed by 10 m and 15 m for both DN and UP channels. The obtained SNR values for DN transmission are \(-\)1, 1 and 4 dB for DN. For UP transmission, these are 10, 8 and 6 dB over for 5, 10, and 15 m VLC ranges, respectively, considering an acceptable log (BER) of \(-\)9. This is due to the propagation of light underwater effects diminishes by scattering, absorption and turbulence effects. Thus, the incoming optical signals underwater undergo scattering and absorption losses as it strikes the water molecules as well as particles, owing to which there is a loss in signal intensity and, hence, degrades the system performance [1]. Figure 5 presents the log (BER) performance with different numbers of underwater connected devices of the bidirectional 10/2.5 Gbps per wavelength hybrid FSO-PON system utilizing NOMA-VLC over varied VLC underwater range from 5 to 15 m, fixed 100 kmfiber range and 10 km FSO range in clear ocean water.