# Performance Analysis of FSO-UWOC Mixed Dual-Hop Relay System with Decode-and-Forward Protocol
## 1 Introduction
However, the analysis did not account for the impact of atmospheric and underwater path losses on system performance. The authors of [12] examined the OP and BER of a dual-hop communication system composed of FSO-UWOC links under the RIS-assisted mechanism. The FSO link considered GG atmospheric turbulence fading and pointing errors, while the UWOC link merely employed the Lognormal distribution to model oceanic turbulence, overlooking the influence of pointing errors. Furthermore, path loss effects were neglected for both links in the analysis. The authors of [13] investigated the outage probability performance of a hybrid FSO-UWOC two-hop transmission system operating under the AF protocol. The FSO link used the complicated Fisher-Snedecor F-distribution to model atmospheric turbulence fading, while the UWOC link employed the EGG distribution to simulate oceanic turbulence. Both the links considered pointing errors but did not address the issue of path loss. The authors of [14] proposed a multi-hop FSO-UWOC relay communication system for AUV tracking, analyzing and deriving the end-to-end average BER and OP expressions under fading environments. The FSO link incorporated the effects of GG turbulence fading and pointing errors, while the UWOC link used the Lognormal distribution to model oceanic turbulence. Both the links accounted for atmospheric and underwater path losses by using the Beer-Lambert exponential law. However, the study did not consider the performance differences when different detection schemes were used at the receiver side, such as the heterodyne detection (HD) and intensity modulation with direct detection (IM/DD). In fact, as in the findings in [15] and the studies it cited, the Lognormal distribution is commonly utilized to describe weak atmospheric turbulence effects in the FSO environment, whereas the GG distribution is employed to model moderate-to-strong atmospheric turbulence fading. Both of these turbulence statistical models have been experimentally validated. Regarding the UWOC link, extensive laboratory test data presented in [16; 17; 18] revealed that, among the existing probability density functions (PDFs) previously used by scholars to describe the statistical characteristics of oceanic turbulence, both the GGD and EGG models could achieve an excellent fit between the measured turbulence channel data and their corresponding theoretical curves. Specifically, the GGD model is suitable for simulating the weak turbulence induced by temperature and/or salinity gradients in seawater [16; 17]. Moreover, when the laser source is equipped with a beam expander-and-collimator (BEC) and the receiver is equipped with an aperture averaging lens (AAL), the GGD model can also provide a highly accurate fitting effect for weak, moderate, and strong turbulence. Conversely, the mixture EGG model presented in [18] is more suitable for simulating medium-to-strong turbulence channels in seawater with a high concentration of bubbles [17]. Additionally, regarding the path-loss model for UWOC links, the findings in [19] indicate that the traditional Beer-Lambert (BL) model fails to accurately fit the path loss in marine scenarios. In these environments, the absorption and scattering effects of seawater, along with geometric losses between transmitters and receivers, play a significant role. In such cases, the Elamassie path-loss model presented in [19] is proven to be the optimal choice. From the analysis above, it is clear that there is still much space for us to improve the research results in current RF-UWOC or FSO-UWOC cooperative communications. This is precisely the focus of this study. Table 1 summarizes the aforementioned studies on a cross-medium mixed relay communication system for a more clear comparison, demonstrating the differences in their link channel modeling, relay type, detection methods, derived performance metrics, etc.