# Performance Analysis of FSO-UWOC Mixed Dual-Hop Relay System with Decode-and-Forward Protocol
## 5 Simulation Results and Analysis
### Average Channel Capacity Simulation Results and Analysis  
Figure 7 presents the theoretical and MC simulation curves of the average channel capacity for a dual-hop FSO-UWOC system under an atmospheric turbulence Rytov variance \(\sigma_{R}^{2}=0.6\) and a fixed marine turbulence scintillation index \(\sigma_{l}^{2}=0.1074\). Pointing error parameters for both hops are set to the same value (\(\xi_{1}=\xi_{2}=1.14\) or \(\xi_{1}=\xi_{2}=4.00\)), depicted by pink/blue curves, respectively. The atmospheric and underwater transmission distances are fixed at \(Z=200\) m, and \(d=20\) m, respectively. Simulation results demonstrate strong alignment between theoretical average channel capacity values and MC simulations, validating the accuracy of the derived closed-form expressions. The expressions formulated in Section 4.3 indicate that channel capacity depends on the detection method, pointing errors of the two hops, atmospheric turbulence coefficients, and oceanic turbulence parameters. Therefore, under identical detection, turbulence, and distance conditions, dual-hop pointing errors become the primary factor influencing channel capacity: weaker pointing errors yield superior average channel capacity performance. Additionally, since HD is coherent detection, for the same average SNR, HD will yield a higher system average channel capacity value than non-coherent IM/DD detection.  
Figure 8 illustrates the impact of varying link transmission distances on the average channel capacity of the FSO-UWOC system under weak atmospheric turbulence (Rytov variance \(\sigma_{R}^{2}=0.6\)) and moderate oceanic turbulence (scintillation index \(\sigma_{l}^{2}=0.1074\)). Both hops are configured with weak pointing errors (\(\xi_{1}=\xi_{2}=4.00\)). Figure 8a presents theoretical and simulation results for average channel capacity as a function of the average  
Figure 7: Average channel capacity of mixed FSO-UWOC system under different detection methods (\(\xi_{1}=1.14\), \(\xi_{2}=4.00\)).  
SNR 7, with atmospheric transmission distances set to 200 m and 500 m and the underwater distance fixed at 20 m. Simulation results show that increasing the FSO link's transmission distance degrades average channel capacity under identical parameters. This degradation arises from increased atmospheric path loss in longer FSO links, which lowers the system's average SNR. Notably, however, the impact of atmospheric transmission distance on channel capacity remains relatively moderate compared to other factors like turbulence intensity or pointing error severity.  
Figure 8b displays the average channel capacity curves as a function of the average SNR, with the atmospheric transmission distance fixed at 200 m and underwater distances set to 20 m and 30 m, respectively. The results show that increasing the underwater transmission distance induces significant degradation in average channel capacity, particularly under the IM/DD scheme. Specifically, the system exhibits notably higher channel capacity at 20 m compared to 30 m across all tested SNR values.