## 5 Results
In this respect, it is straightforward to modify the x-axes corresponding to Figures 3-6 to include the effect of absorption in water. For example, in Figure 3, the new x-axis will range from \(28.25\) to \(73.25\) dBs (instead of from \(5\) to \(50\)) if either \(d=17.73\) m and coastal water are considered, increasing the required SNR into \(23.25\) dBs to obtain the same values of BER once \(L\) is included. Let us start with the uncoded OOK scheme, for which the results, shown in Figure 3 from the evaluation of Equation (21), will offer a reference for the other coded transmission techniques analyzed in this paper. As expected, for the lower intensity of turbulence, ABER tends to dramatically increase. These results show that in order to achieve an ABER of \(10^{-4}\) in the presence of atmospheric and oceanic turbulence, it will be necessary to accomplish an SNR ranging from \(11.5\) dB (corresponding to a turbulence strength of \(\sigma_{h_{2}}^{2}=0.0496\) for the underwater link and the absence of turbulence for the atmospheric one) to \(50\) dB (both terrestrial  
Figure 3: Analytical average bit error rate (ABER) and Monte Carlo simulation results (circles) vs. SINR \(\gamma_{0}\) for a conventional OOK format under weak (\(\sigma_{h_{2}}^{2}=0.0496\)), weak-to-moderate (\(\sigma_{h_{2}}^{2}=0.2453\)) and strong (\(\sigma_{h_{2}}^{2}=1.0652\)) salinity induced turbulence and different Malaga turbulence intensities in the terrestrial FSO link. Scenarios and values of irradiance variances in the underwater medium are taken from acquired data presented in [68]. In solid lines, the performance of the system when only oceanic turbulence is considered is represented. As a reference, the ideal AWGN channel is depicted with a solid line. and underwater FSO paths characterized by a strong turbulence of \(\sigma_{h_{1}}^{2}=2.699\) and \(\sigma_{h_{2}}^{2}=1.0652\), respectively). In contrast, for the ideal case of the absence of turbulence in both terrestrial and the underwater media, only 8.6 dB is required for SNR to satisfy an ABER of \(10^{-4}\). Apart from that, from Figure 3, we may anticipate the preponderance of one stretch over the other according to the change presented in the slope of some ABER curves. In this respect, there may exist a higher vulnerability to one concrete segment (terrestrial or underwater) that can be evaluated from Equation (6). Hence, for high SNRs, \(F_{\gamma_{1}}(\gamma)F_{\gamma_{2}}(\gamma)\approx 0\), and the CDF (and also the PDF) for the CSI-assisted AF-based TU-FSO system would be reduced to the sum of the CDFs (or PDFs) associated with terrestrial and underwater FSO hops. In this assumption, it is straightforward to determine which segment (terrestrial or submarine) contributes the most to the cooperative optical link. For example, in such Figure 3, the blue solid line represents the performance of the system when only oceanic turbulence is taken into account (let us remember that the scattering was introduced in the system as an average variance obtained after averaging the inter-symbol interference over the oceanic turbulence). On its part, the dashed-dotted purple line shows the behavior of the AF-based TU-FSO system for the ideal case of no oceanic turbulence and an atmospheric turbulence featured by \(\alpha=50\), \(\beta=10\) and \(\rho=0.9\). The inclusion of those two latter phenomena (dashed-dotted blue line) implies that the resulting performance is, approximately, the sum of the two single aforementioned PDF curves (only oceanic turbulence and only atmospheric turbulence). The same verification can be carried out with the solid red line (system with only oceanic turbulence of variance \(\sigma_{h_{2}}^{2}=1.0652\)) and the purple line with a '+' marker (system with only Malaga atmospheric turbulence with \(\alpha=1\), \(\beta=2\) and \(\rho=0.1\)).