## 7 Results and Discussion
### Simulation Results with No Sunlight  
Considering the D-LOS results, in the clearest water profile S1 (Figure 10a-d), it is seen that the best wavelengths for links under 30 m seems to be in the blue-green region. This is most prominent in depths closer to the DCM. However, above and below the DCM, it is seen that even the deeper blue wavelengths are preferred for longer horizontal links. However, with decreasing profile clarity, the depths above the DCM seem to shift this preference also to the blue-green region as seen in all results of S4 (Figure 10e-h), and then to the yellower wavelengths as seen in the S8 profile (Figure 10m-p).  
It is visible that for very short D-LOS links, yellow was preferable for all water profiles with increasing turbidity. Yellow is also preferable at slightly longer link distances within the same profile where the geometric loss of signal was greater. This could be attributed to the generally higher photodiode responsivity for longer wavelengths, until such distances where the higher attenuation of the yellow-amber spectrum in clearer waters would cause this spectrum reception to diminish compared to the blue-green wavelengths. The maximum achievable 0 dB distance (the magenta-colored line) seems to decline for all water profiles approaching the DCM. The maximum distance reached below the DCM seems to be greater than above it (S1-S8). This is due to the Chl-\(a\) concentrations reaching a near-zero value, which is much less than above the DCM. Furthermore, the reason why the more turbid profiles seem to achieve greater 0 dB distances at shallower depths could be explained also by the steeper decline of the Chl-\(a\) concentrations (Figure 5) and the dominant contributor to the attenuation is then eventually only seawater.  
Observing the P2P-LOS results (Figure 11), we further see that a bluer spectrum is favored for the clear water profiles, which shifts to greener spectra with increasing turbidity. S8 (Figure 11d,h) is an exception due to its very high turbid conditions in the shallower depths causing very high scattering, and thus there is lesser attenuation of the yellow-amber spectrum there. As expected, the 0 dB link distances (Figure 11a-h) are much greater than they were for the D-LOS profile results. These overall observations could be due to the very low geometric losses caused due to the narrow beam divergence angle of the emitter.  
Figure 9: Downwelling solar spectral irradiance \(E_{t}\) (\(\lambda,z\)) for profiles S1 (**a**) and S8 (**d**), and the resultant background power \(P_{BG}(\lambda,z)\) for corresponding water profiles in the horizontal (**b**,**e**) and in the vertical (**c**,**f**) receiver directions, respectively.  
Figure 11: Wavelength of best O-SNR for P2P-LOS UOWC at nighttime for profiles S1 (**a,e**), S4 (**b,f**), S5 (**c,g**), and S8 (**d,h**) up to first 350 m with emitter powers of 1 W in the first row up and 10 W in the second row. The magenta line shows the 0 dB distance, and the color beneath it is the wavelength that achieves it.  
Figure 10: Wavelength of best O-SNR for D-LOS UOWC at nighttime for profiles S1 (**a–d**), S4 (**e–h**), S5 (**i–l**), and S8 (**m–p**) with beam divergence and power parameters: (\(\theta\) = 60\({}^{\circ}\), 1 W) column 1; (\(\theta\) = 60\({}^{\circ}\), 10 W) column 2; (\(\theta\) = 120\({}^{\circ}\), 1 W) column 3; and (\(\theta\) = 120\({}^{\circ}\), 10 W) column 4. The magenta line shows the 0 dB distance, and the color beneath it is the wavelength that achieves it.