## 4 Results and Discussion
### Computation Data Visualisation
Figure 5b with ISI indicates that \(h_{f}\) of Equation (18) has a distinctive undershoot at both ends of \(h_{f}\) curve while the optimum \(h_{f}\) shows a very small undershoot. Their corresponding outputs \(r_{f}\) mirror the same difference too. By observing the frequency bandwidth (FBW) profiles of \(h_{f}\) (general and optimum) in Figure 4b,e and Figure 5b,e of use cases I and II, respectively, we can conclude that the FBWs are nearly the same for each scenario. It is worth stating that the FBW narrowing in Figure 4c (without ISI and AGTN) and Figure 5c (without ISI and with white AGTN) are due to seawater channel \(h_{c}\). At the same time, FBW narrowing in CB and MCB wouldn't happen because [10; 11; 12; 13; 14; 15; 16] assumed delta impulse response for the fiber channel. In Figures 4d and 5d the FBW narrowing is due to the signal ISI (the second term in Equation (6c)) and seawater channel \(h_{c}\), while the FBW narrowing in [10; 11; 12; 13; 14; 15; 16] is due to signal ISI only. The eye diagrams in Figure 4e,f and Figure 5e,f of use cases I and II, respectively, are associated with the filter impulse function \(h_{f}\) (general and optimum). The vertical and horizontal opening of the eye diagram for the electronic signal in use case I (without ISI and WTGN) in Figure 4e is relatively equal to that for use case II (without ISI and with WTGN) in Figure 5e. This is a clear indication that the optimum filter for use case II plays the role of equalization to maintain the signal tail boundaries and peak at their designated points. Figure 5e (use case II) shows a slight reduction rather than a considerable reduction in the vertical and horizontal eye-opening when we compare it to Figure 4f (without WTGN). The work in [10; 11; 12; 13; 14; 15; 16] had not addressed this aspect of the filter, so we couldn't compare it. The first observation we can conclude from Figures 6a and 7a is that the undershoot of the optimum \(h_{f}\) for Harbor water is more profound than that of Coastal water. The optimum \(h_{f}\) protects against the more dispersive channel. Also, it is easy to observe from Figures 6d and 7d that the\(-\)3 dB FBW for the Coastal and Harbor waters are practically equal. This can be attributed to the undershoot depth being deeper for Harbor water than for Coastal water. Such a conclusion cannot be drawn in the CB and MCB filter case because [10; 11; 12; 13; 14; 15; 16] assumed the delta function for the fiber channel. Figure 6 shows the numerical computation outcome of use case III for Coastal water, which only includes the ISI scenario alongside the colored AGTN. Figure 6a shows a set of three optimum solutions of \(h_{f}\) of Equation (22) for different \(\{N_{0}\), \(a_{0}\), \(a_{1}\}\) value sets. The undershoot decreases with \(N_{0}\), because the AGTN is an additive element to the ISI signal component. However, this does not impact the filter bandwidth, as shown in (6c). This is one of the indications that the filter \(h_{f}\) is doing equalization. The corresponding outputs \(\tau_{f}\) in Figure 6b mirror the exact behavior of \(h_{f}\). Figure 6f shows the limited impact on the eye-opening compared to Figure 6e, which exhibits the eye diagram of the transmitted optical pulse \(h_{t}\). Hence, Figure 6f shows the filter's compelling performance in combating the ISI, which is partly a footprint of the scattering and scintillation impairments. The electronic filter \(h_{f}\) can reshape the pulses (i.e., equalization) when its bandwidth is smaller than the transmission bit rate [25] and relatively smaller than the water channel \(h_{C}\). This is depicted in Figure 6c,d for Coastal water and Figure 7c,d for Harbor water. Such a conclusion cannot be made for CB and MCB due to the nullification of the propagation channel in the design steps in [10; 11; 12; 13; 14; 15; 16]. Additionally, the optimum filter \(h_{f}\) for Harbor water in Figure 7d has a bandwidth narrower than the channel impulse response \(h_{c}\), making \(h_{f}\) a bandpass filter for the ISI signal component.