## 5 Results and Discussion  
This section presents and discusses the results obtained from the numerical simulation of polarization division multiplexing (PDM)-enabled underwater optical wireless communication (UWOC) using the on-off keying (OOK) modulation scheme. First, the proposed system is tested under freshwater conditions, assuming a salinity level of 0.2 ppt. Figure 4 shows the measurement of bit error rate (BER) at different distance of UWOC link for both channels by evaluating the performance of NRZ and RZ encoding schemes. It clearly demonstrates that the NRZ modulation scheme outperforms the RZ scheme in terms of BER. As shown in Figure 3a, at a distance of 8 m, the NRZ scheme achieves an impressively low BER of approximately \(10^{-8}\). However, as the distance extends to 10.5 m, the BER rises to approximately \(10^{-4}\). On the other hand, the RZ curve follows a similar trend but with generally higher BER values compared to NRZ. At a distance of 8 m, the RZ scheme demonstrates a BER of around \(10^{-6}\) which increases to approximately \(10^{-3}\) at a distance of 10.5 m. Similarly, for channel 2, for NRZ and RZ encoding scheme, the BER is also reported as \(10^{-4}\) and \(10^{-3}\), respectively, at the distance of 10.5 m. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Component Name** & **Parameters** & **Value** \\ \hline Laser & Power & 100 mW \\ \hline  & Wavelength & 1550 nm \\ \hline  & Linewidth & 10 MHz \\ \hline PRBS & Data rate & 1 Gbps \\ \hline  & Sequence length & 1024 \\ \hline  & Samples & 32,768 \\ \hline UWOC Link & Receiver aperture diameter & 10 cm \\ \hline  & Alkalinity & 7 pH \\ \hline  & Transmission loss & 80\% \\ \hline  & Target reflectivity & 10\% \\ \hline APD & Dark current & 10 nA \\ \hline  & Gain & 3 \\ \hline  & Responsivity & 1 A/W \\ \hline  & Thermal noise & \(100\times 10^{-24}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Numerical simulation parameters. This indicates that the NRZ modulation scheme provides better communication performance, with lower errors, over longer distances. Figure 5 demonstrates that for both the RZ and NRZ encoding schemes, the eye diagrams show clear openings. This further indicates successful transmission of high-speed data at the distance of 9.5 m underwater. Figure 4: Measurement of BER under freshwater conditions: (**a**) channel 1; (**b**) channel 2. Figure 5: Measured eye diagrams at 9.5 m distance: (**a**) channel 1 with RZ encoding; (**b**) channel 2 with RZ encoding; (**c**) channel 1 with NRZ encoding; (**d**) channel 2 with NRZ encoding. Moreover, as discussed earlier, attenuation increases with the salinity of the water in different oceans. We have tested the proposed system for salinity levels ranging from 32 ppt to 35 ppt. Figure 6 presents the results of testing the proposed system using NRZ encoding at different salinity levels in UWOC links. As shown in Figure 6, at a salinity level of 32 ppt, the BER for channel 1 remains relatively low across the distances tested. At a distance of 1.4 m, the BER is approximately \(10^{-15}\), and it gradually increases to around \(~{}10^{-6}\) at a distance of 1.8 m, whereas at a salinity level of 33 ppt the BER values are slightly higher compared to the previous salinity level. At a distance of 1.4 m, the BER is approximately \(10^{-10}\) increasing to around \(~{}10^{-6}\) at a distance of 1.8 m. When the salinity level is 34 ppt, we observe a further increase in the BER values. The BER starts at approximately \(~{}10^{-10}\) at a distance of 1.4 m and rises to about \(~{}10^{-5}\) at a distance of 1.8 m. Lastly, the salinity level of 35 ppt exhibits the highest BER values among all the curves.