# Performance Analysis of FSO-UWOC Mixed Dual-Hop Relay System with Decode-and-Forward Protocol
## 5 Simulation Results and Analysis
### Outage Probability Simulation Results and Analysis  
Figure 2 illustrates the theoretical and MC simulation results for the system's OP as a function of the average SNR in the mixed dual-hop FSO-UWOC relay communication system. The analysis encompasses weak and strong atmospheric turbulence scenarios, parameterized by Rytov variances \(\sigma_{R}^{2}=0.6\) and \(\sigma_{R}^{2}=2\), which correspond to fading parameters of the GG distribution \((\alpha,\beta)=(5.41,3.78)\) and \((\alpha,\beta)=(3.99,1.70)\). Setting the strong pointing error for the first hop FSO link \(\xi_{1}=1.14\) (\(\sigma_{s}=10\) cm, \(r_{a}=20\) cm, \(w_{z}=40\) cm), while the second hop UWOC link has a weak pointing error \(\xi_{2}=4.0\) (\(\sigma_{s}=10\)cm, \(r_{a}=30\) cm, \(w_{z}=20\) cm). The atmospheric link spans a transmission distance of \(Z=200\) m, while the underwater link extends over \(d=20\) m, and the oceanic turbulence scintillation index is set to \(\sigma_{I}^{2}=0.1074\). Solid lines represent the theoretical values of IM/DD detection, dashed lines represent the theoretical values of HD detection, circles represent the simulated values of IM/DD detection, and triangles represent the simulated values of HD detection. From the simulation results shown in Figure 2, it can be observed that, under fixed UWOC link parameters such as underwater transmission distance, oceanic turbulence intensity, and pointing error conditions, the system experiences a rapid deterioration in OP performance due to the increase in atmospheric turbulence strength in the FSO link, regardless of whether IM/DD or HD is employed. This indicates that atmospheric turbulence has a significant impact on the OP performance. As for the detection schemes, since HD is a coherent detection method, HD achieves a lower system OP value than the incoherent IM/DD for the same average SNR. Moreover, the theoretical calculation results of the system align well with the MC simulation results, and the asymptotic OP values at high SNR closely match the corresponding theoretical values, further confirming the correctness of the closed-form derivation for the theoretical and asymptotic values of the OP, as described above. Figure 3 demonstrates the impact of two different pointing error conditions, \(\xi_{1}=1.14\) and \(\xi_{1}=1.52\), on the system's OP. According to the definition \(\xi_{1}^{2}=w_{z_{eq}}^{2}/4\sigma_{s}^{2}\), smaller \(\xi_{1}\) values correspond to stronger pointing errors. The simulation curves from Figure 3 indicate that, under the same atmospheric turbulence conditions, the smaller the value of pointing error \(\xi_{1}\) (i.e., as pointing errors become more severe), the worse the performance of the OP is. When the atmospheric turbulence Rytov variance \(\sigma_{R}^{2}\) changes from 0.6 to 2, with \(\xi_{1}=1.52\), the corresponding OP curve shows a more pronounced difference compared  
Figure 2: Outage probability under different detection schemes of a mixed FSO-UWOC system (\(\xi_{1}=1.14\), \(\xi_{2}=4.00\)). to the curve corresponding to \(\xi_{1}=1.14\). This indicates that pointing errors exert a more dominant influence on the OP than atmospheric turbulence effects. The asymptotic curves at high SNR are in good agreement with the theoretical curves of OP, which also verifies the correctness of the theoretical derivation of the asymptotic expression. The solid line represents the theoretical value at \(\xi_{1}=1.14\), and the dotted line represents the theoretical value at \(\xi_{1}=1.52\). Figure 4 demonstrates the impact of different link transmission distances on the OP of the FSO-UWOC system, with the atmospheric turbulence Rytov variance set to \(\sigma_{R}^{2}=2\) and the oceanic turbulence scintillation index \(\sigma_{I}^{2}=0.1074\). In Figure 4a, pointing error parameters for the FSO and UWOC links are configured as \(\xi_{1}=1.14\) and \(\xi_{2}~{}=~{}4.0\), respectively.