## 7 Results and Discussion
### Simulation Results with Sunlight
Figure 12: Wavelength of best O-SNR for D-LOS (top) UOWC in daytime 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. P2P-LOS results are at the bottom for profiles S1, S4, S5, and S8 (left-to-right) with a beam divergence of 5\({}^{\circ}\), and power parameters: 1 W (top row, **q–t**) and 10 W (bottom row, **u–x**). The magenta line shows the 0 dB distance, and the color beneath it is the wavelength that achieves it for both sets. In addition to the wavelength of preference for the maximum O-SNR, it is also essential to estimate the wavelength of preference for link establishment. This is the wavelength at each layer in the stratified water column that achieves the greatest horizontal distance (0 dB distance), where the signal and noise are indistinguishable (the spectral distributions for D-LOS links are given in Figure 13a-p and for P2P-LOS in Figure 13q-x). This corroborates directly to the magenta-colored lines in Figure 12 which show the 0 dB distance at the given depth, where the color beneath is the carrier wavelength that achieves it. The determination of the best O-SNR wavelength helps to determine what wavelength is necessary to achieve the best signal quality at a given depth and distance, but determining the wavelength for maximum distance shows the condition for link establishment from a range perspective. These two considerations are distinct due to the link length (Figure 13) predominantly showing the finite limits possible, while the O-SNR (Figure 12) is determined by the ratio of signal power to noise present at any given location within this limit, where a different light wavelength may be favorable for communication due to experiencing lesser attenuation or having less downwelling light of that spectrum from sunlight. This can yield insights for modulation strategies, where for example, an OOK-based link that needs a SNR of 10-20 dB would prefer a much more drastic wavelength variation with depth. It is clearly seen in the clear water profiles S1-S4 in Figure 12 that for the high-power systems (\(\theta\) = 120\({}^{\circ}\), 10 W, Figure 12d,h,l,p), (\(\theta\) = 60\({}^{\circ}\), 10 W, Figure 12b,f,j,n), and (\(\theta\) = 5\({}^{\circ}\), 10 W, Figure 12u-x), at shallow depths before the DCM, the best wavelengths for link length shift from amber-green to green-blue with increasing transmission power. This is due to the signal overcoming the significance of solar influence. This is confirmed as we look at their low power counterparts: (\(\theta\) = 5\({}^{\circ}\), 1 W, Figure 12q-t), (\(\theta\) = 60\({}^{\circ}\), 1 W, Figure 12a,e,i,m), and (\(\theta\) = 120\({}^{\circ}\), 1 W, Figure 12c,g,k,o); in order of decreasing power, the wavelength for peak distance shifts from blue-green to amber. Therefore, a general suggestion is that using emissions of approximately 10 W with narrow beam divergences in the blue-green wavelengths may be optimal for clear water profiles. However, this is specifically under the sunlight profile shown in Figure 4. Nevertheless, the coastal profiles S5-8 seem to show a greater preference for the green-amber spectrum across all transmission powers at depths above the DCM. Also, as a general trend, the range limit at depths approaching the DCM decreases with increasing turbidity. All horizontal link profiles for S1-8 seem to prefer the yellow-amber spectrum at the DCM depth under sunlight, with the clearer waters achieving the greatest link lengths above it (at those spectra previously mentioned), steeply declining toward the DCM before increasing again. Furthermore, the level of this decline reduces with increasing turbidity from profiles S1-8, respectively. Below the DCM, the favorable spectra for S1-8 shifts toward blue with increasing depth and achieves greater link lengths than above the DCM.