## 6 Results and Discussion
### Discussion  
As a directly visible observation, the F-type arrays of both three and five PDs show a complete blind region whenever \((\alpha+\beta)\geq 90^{\circ}\), which can be seen in the subfigures (a) and (d) of Figures 6-8 and in Figure 9a,d,g. Comparisons between the corresponding 5 PD, C55, C25, and F arrays showed that this blind region is more significant for flatter arrays. At \(\beta=90^{\circ}\), the C55 array performed the best, whereas at \(\beta=-90^{\circ}\), the C25 and C55 arrays seemed to compete for performance based on their location. This is expected as the channel model given by Equation (16) dictates that the AOI should be less than \(90^{\circ}\), where the more curved C55 array benefits from a greater spread of FOVs. This is a disadvantage for flat arrays as it shows that complete link loss could result if the Rx was suddenly disoriented due to UUV collision with fish or objects in the underwater environment. Having all PDs oriented in the same direction may also cause a drastic, unreliable BER response as shown in Figure 10a. Nonetheless, the F-type array showed the maximum received power when \(\beta\leq 0^{\circ}\), in the regions at which \(\alpha\approx~{}\beta\) when compared to the other array types at the same \(\beta\) and in the same water condition. This is due to the AOI reaching \(0^{\circ}\) to planar PD surfaces in the F-type array. However, it is observed that the locations where maximum power is attained by the F-type array are not along the \(\alpha=\beta\) angle but along a direction where the combined effects of the resolved AOI component and LED intensity distribution have achieved an optimal level. This is visible in subfigures (j) and (m) of Figures 6-8. Although this may also be true for the C25 and C55 configurations, it cannot be determined  
Figure 9: \(P_{r_{tot}}\) for respective 3 PD arrays at \(\beta=60^{\circ}\) rotation in: (**a–c**) Jerlov I (\(\lambda=450\) nm); (**d–f**) Jerlov III (\(\lambda=550\) nm) and (**g–i**) Jerlov 3C (\(\lambda=550\) nm). Red and green lines show 10- and 0-dB contours, respectively. so easily due to greater individual PD FOV spreads. However, when \(\beta=0^{\circ}\), the \(\alpha=0^{\circ}\) direction for the F-type array produced the greatest overall power across all water types and in both 3 PD and 5 PD configurations due to also observing the greatest light intensity along the horizontal axis. This variability factor may be better addressed from the Tx side by designing a Tx that emits uniform intensity distribution. However, more spread-out configurations fared much better for \(\beta>0^{\circ}\) across all arrays than their flatter counterparts, regardless of the number of PDs in the array, as seen when comparing 3 PD C55 results in Figure 9c,f,i with their corresponding 5 PD F and C25 results in Figures 6-8. Figure 10: BER analysis for respective 5 PD arrays in: (**a–c**) Jerlov I; (**d–f**) Jerlov III; and (**g–l**) Jerlov 3C. A wavelength of 450 nm was used for Jerlov I and 550 nm was used for Jerlov III and 3C, respectively, due to them being the least attenuated light wavelengths in the respective water types, as per Table 3. Jerlov I was modeled for a 15 m vertical displacement, and Jerlov III and 3C at a 1 m vertical displacement. Analysis of the individual received powers by the PDs in each array configuration showed that the separation distance between PDs played a lesser role as the distance from Tx increased. There is no noticeable difference between C- and D-type arrays of equal vertex depth. This may be due to their equivalent overall FOVs. However, convex arrays are recommended overall due to the minimized effect of shadowing by the solid housing. The effect of having more PDs in the array becomes less significant at greater distances, possibly owing to the much greater geometric loss compared to the collective photodetector surface area. It is more important for PDs to maintain their orthogonality to the incoming ray at any location.