## 8 Conclusions  
The quality of an underwater mobile optical wireless communication link is determined by several time-varying factors from downwelling sunlight to changing attenuation levels in inhomogeneous vertical channels, making the determination of optimal UOWC link parameters challenging. In this study, we focused on how optimal light wavelengths can enhance the SNR in such environments for horizontal and vertical links. We used MATLAB to conduct the numerical modeling for a holistic and realistic integration of the influence of sunlight on eight different vertically stratified ocean types in increasing order of turbidity, deriving the IOPs from the depth distribution of Chl-\(a\) concentration. Thereafter, UOWC has been simulated under the varied key transmission parameters of wavelength, output power, and beam divergence angle for a horizontal link (general case) and a downlink (worst case) for insights into any co-dependencies. The results showed a significant influence of the full sunlight spectrum in the upper layers of all seawater profiles with a deeper penetration of the blue-green solar spectrum in clearer oceans. Regarding the optimal transmission wavelengths for the SNR, this is further contingent upon the magnitude of geometric loss and node positioning with reference to the deep chlorophyll maximum. It is favorable to use the yellow-amber spectrum in a short range in coastal waters at all depths above the DCM, and for above-DCM links in clear oceanic waters that experience greater geometric loss. This spectrum advantage shifts to the blue-green spectrum with increasing depth for clear oceanic and coastal profiles. However, under solar influence, blue-green links established in the deeper layers of coastal profiles seem to show higher SNRs than clear oceanic profiles of same depth, possibly due to filtering out sunlight in the upper layers that are more abundant with Chl-\(a\). The maximum achievable link range decreased approaching the DCM prior to increasing again.  
We considered perfectly horizontal and vertical links, not accounting for salinity gradients, turbulence, or reflections at the air-water boundary. We assumed the sun was at zenith, vertically above the water surface, acknowledging that this neglects the peaking of light intensity in specific directions at very shallow depths due to the effect from solar zenith and azimuth angles. Additionally, while SPCTRL2 was used for its simplicity, more advanced solar spectrum models may be used for accuracy where certain specific non-public domain information about the locations of interest are available.  
The findings suggest that UOWC-based sensor networks would benefit from adapting the carrier wavelengths to optimize for the node location, link distances, ocean type, and time of day, especially if they are within the photic region. Consequently, we propose wavelength-based modulation and routing strategies to discriminate the influence of sunlight. This study will further benefit from experimental validation. We intend to explore these in our subsequent research work.  
Conceptualization, T.G.W. and B.-C.S.; methodology, T.G.W., B.-C.S. and K.V.; software, T.G.W.; validation, T.G.W.; formal analysis, T.G.W.; investigation, T.G.W.; resources, B.-C.S. and K.V.; data curation, T.G.W.; writing--original draft preparation, T.G.W.; writing--review and editing, B.-C.S. and K.V.; visualization, T.G.W. and B.-C.S.; supervision, B.-C.S. and K.V.; project administration, B.-C.S.; funding acquisition, B.-C.S. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Data Availability Statement: Data are contained within the article.  
The authors declare no conflicts of interest.