## 7 Results and Discussion
### Simulation Results with Sunlight
The more coastal profiles seem to achieve greater link lengths at shallower depths from the depth position of the DCM. In the downlink for D-LOS (Figure 14) and P2P-LOS (Figure 15) links, the preferred wavelength at close-range links has shifted to the yellow-orange spectrum, and even to red in low-power D-LOS (1 W at \(\theta\) = 60\({}^{\circ}\), Figure 14a,e,i,m and \(\theta\) = 120\({}^{\circ}\) at both power settings, Figure 14c,g,k,l,o,p) approaching the DCM. This can be due to the direct view of downwelling sunlight on the receiver when it is facing up. The amber range is about 20-50 m for the clearer water S1, and grows as the depth approaches the DCM, maintaining this trend for depths beyond the DCM due to more sunlight penetration. The more coastal profiles show a much greater amber region toward the surface like their horizontal link profiles. Figure 13: The 0 dB link distances at each wavelength 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**). Figure 14: Wavelength of best O-SNR for downlink D-LOS UOWC for profiles S1 (**a-d**), S4 (**e-h**), S5 (**i–l**), and S8 (**m-p**) for full depth and link distance with emitter 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. Figure 15: Wavelength of best O-SNR for downlink P2P-LOS UOWC for profiles S1, S4, S5, and S8 (left-to-right) for full depth and link distance with emitter beam divergence of \(5^{\circ}\) and power parameters: 1 W (top row, **a–d**) and 10 W (bottom row, **e–h**). These observations for both downlinks and horizontal links under Chl-\(a\) concentration profiles show that the transmission wavelength has a significant role in the determination of the signal quality and range in the presence of sunlight, which dynamically changes based on the position and orientation of the Tx and Rx within a given stratified environment, in addition to the properties of the environment itself being determined by the surface Chl-\(a\) concentration. It shows that for the maximum performance for a moving link, a broader wavelength spectrum should be considered for the carrier determined based on the water profile, time of day, depth, and link distance. This raises a problem with narrowband wavelength transmissions from emitters limited to a few tens of nm for LEDs and <10 nm for lasers, suggesting that a more dynamic scheme of signaling may be necessary that is responsive to the channel state information (CSI) parameters that include localization information. Nevertheless, to some extent, using the green spectrum at transmission powers over 10 W and narrow beam divergence angles may seem like an overall solution. However, this could be demanding on the power budget of the autonomous UOWC nodes that are better reserved for other tasks. Such narrow links may also suffer misalignment. Additionally, for example, 10 W lasers are highly hazardous and may harm marine animals. They may also not be a viable solution for mobile links where large beam divergences are preferred, such as in wireless networks and swarms, where the results show persisting disturbance from sunlight. In such cases, it seems gradually shifting the transmission wavelengths to the yellow-amber spectrum as the day approaches could be a strategy. However, this is contingent upon many factors, such as geographic location, season, cloud cover, etc.