# Modeling of Satellite-to-Underwater Integrated FSO-PON System Using NOMA-VLC
## 4 Results and Discussion
The VLC range changes from 5 to 15 m for different underwater devices. The performance of the DN and UP transmission diminishes in terms of an increase in log (BER) values. Additionally, the number of underwater devices increases from 10 to 290. The system performance again decreases for all varied VLC ranges. However, VLC transmission in the UP direction shows better performance than the DN direction for all varied VLC ranges. The maximum supportable underwater devices for UP and DN transmission over 5, 10 and 15 m VLC ranges are 280, 250 and 201. For DN, more than 290 for UP transmission, respectively, resulting in a minimum log (BER) of \(-\)9. Therefore, the maximum supportable underwater devices for UP and DN links are more than 290 and 280, respectively, over a 5 m VLC range. The system performance decreases primarily due to the presence of fiber impairments, such as ISI, FWM etc., that increase the fiber nonlinearities and noise. Figure 4: log (BER) performance of bidirectional 10/2.5 Gbps per wavelength hybrid PON-FSO system utilizing NOMA-VLC over varied VLC rang in open ocean for DN and UP. Figure 6 presents the BER performance analysis for the suggested system utilizing distinct values of FOV angles, considering 10 km FSO, 100 km fiber and 5 m VLC ranges in the clear ocean for both DN and UP transmissions. The BER of the considered system increases from \(-35\) to 0, with an increase in FOV from 300 to 700. At the minimum acceptable log (BER) of \(-9\), the ROP for 300, 500 and 700 is 2, 4 and 5 dBm in DN, while ROP for 300, 500 and 700 is 9, 8 and 5 dBm in UP. Thus, UP shows better performance w.r.t. DN, out of which 300 FOV with high ROP shows the best performance followed by 500 and 700. Figure 5: BER performance of integrated PON-FSO system incorporating NOMA-VLC over varied VLC range in Open Ocean for different no. of active undersea devices. Figure 6: log (BER) analysis of bidirectional 10/2.5 Gbps per wavelength hybrid PON-FSO system under clear ocean for varied field of view. Figure 7 presents the OSNR (dB) versus ROP (dBm) of the proposed hybrid FSO-PON system incorporating NOMA-VLC over 100 km fiber range, 10 km FSO range and 5 m VLC range in the clear ocean, coastal ocean and turbid harbor water types for both DN and UP transmission. It is clearly measured that, as the water type changes from clear ocean to coastal ocean and then to turbid harbor, the performance of the FSO-fiber-VLC links in DN and UP direction degrades in terms of decrease in OSNR for different ROPs values. Furthermore, the ROP increase from \(-10\) to 14 dBm and the OSNR values for different water types improves from 4 to 38 dB. Moreover, UP transmission shows better performance than DN for all water types, having superior performance under clear ocean water, followed by the coastal ocean and turbid harbor, respectively. Thus, the maximum achieved OSNR for the clear ocean, coastal ocean and turbid harbor is 39, 36 and 35 dB, respectively, at an ROP of 14 dBm in the UP direction. While in the DN direction, the maximum achieved OSNR for the clear ocean, coastal ocean and turbid harbor is 30, 27 and 25 dB, respectively, at 14 dBm ROP. It signifies that the maximum obtained OSNR in UP direction clear ocean underwater condition is 39 dB. In addition, the major cause of the decrease in system performance for ROP and OSNR is, again, the presence of fiber impairments as well as FSO link losses. Figure 8a,b depicts the performance of the suggested system over variable fiber ranges in terms of output optical time domain (OTD), along with corresponding eye patterns for DN and UP channels, correspondingly. It is clear that the timing diagrams of signals at ONUs become distorted with the increase in SMF distance. This owes to the presence of fiber impairments and noise in SMF. Again, the clear timing diagram is obtained for back-to-back transmission and it illustrates the best performance out of the other diagrams. Out of all the channels, uplink channels show better performance than downlink channels.