## 4 Enabling Technologies for FSO Communication
### Cooperative Relaying
#### 4.8.1 Relay-Assisted Transmission
This approach not only offers a novel method to boost transmission reliability in FSO systems but also provides a viable solution for FSO networking [338; 339; 360]. One benefit of this virtual multiple-aperture system, as opposed to MIMO FSO systems, is that the multiple paths are spatially separated, which guarantees the independence of the associated fading channels [336]. Also, compared to the conventional MIMO FSO system, the cooperative FSO system allows for the flexible selection of relay nodes, which helps to diversify transmission paths and effectively eliminate channel correlation. Thus, in cooperative FSO communication systems, data are transmitted through a direct link \begin{table}
\begin{tabular}{l l l l l l l} \hline \hline \multicolumn{3}{c}{**Link**} & \multicolumn{3}{c}{**Channel Model**} & \\ \hline
**1 st Hop** & **2 nd Hop** & **1 st Hop** & **2 nd Hop** &
\begin{tabular}{l} **Relaying** \\ **Mode** \\ \end{tabular} & **Detection Technique/Modulation** & **Metrics** & **Reference** \\ \hline FSO & RF & DGG & EGK & AF, DF & IM/DD, HD & ABER, OP, EC & [13] \\ RF & FSO & SR & GG & AF & QPSK & SER & [341] \\ FSO & FSO & NE & NE & AF & IM/DD OOK & ABER, OP & [88] \\ RF & FSO & Rayleigh & GG & DF & IM/DD, HD & ABER, OP & [342] \\ RF & FSO & N-\(m\) & GG & DF & IM/DD OOK & OP & [343] \\ RF & FSO & Rayleigh & GG & DF & SIM & IC & [344] \\ RF & FSO & SR & GG & DF, AF & IM/DD OOK & SOP, ASC & [345] \\ FSO & FSO & GG & \(\alpha-\mu\) & DF & IM/DD & EC & [346] \\ FSO & FSO & FG & GG & DF & SIM DBPSK & BER & [347] \\ RF & FSO & Rayleigh & GG & DF & IM/DD, HD & ABER, OP, EC & [348] \\ FSO & RF & GG & GN-\(m\) & AF & IM/DD, HD & ABER, OP, EC & [67] \\ FSO & RF & GG & GK & AF & SIM & OP & [349] \\ FSO & RF & EW & N-\(m\) & DF & IM/DD & OP, ABER & [350] \\ FSO & RF & \(\mathcal{M}\) & \(\kappa-\mu\) & AF & IM/DD & OP, EC & [351] \\ RF & FSO & Rayleigh & GG & AF & IM/DD & BER, EC & [352] \\ FSO & RF & GG & FTR & AF, DF & SIM, IM/DD, HD & OP, BER, EC & [353] \\ RF & FSO & N-\(m\) & GG & AF & IM/DD, HD & OP, BER, EC & [354] \\ RF & FSO & Rayleigh & GG & AF & SIM & OP, BER & [355] \\ RF & FSO & BX & \(\mathcal{M}\) & DF & IM/DD, HD & OP, BER, EC & [356] \\ RF & FSO & Rayleigh & GG & AF & IM/DD & OP, BER, EC & [6] \\ RF & FSO & \(\alpha-\mu\) & GG & AF & IM/DD & OP, BER & [357] \\ RF & FSO & \(\alpha-\mu\) & DGG & AF & IM/DD & OP, BER, EC & [358] \\ FSO & RF & \(\alpha-\mu\) & GG & DF & IM/DD & OP, BER, EC & [359] \\ \hline \hline \end{tabular} SER: Symbol error rate, AF: Amplify-and-forward, DF: Decode-and-forward, QPSK Quadrature Phase-Shift Keying, OOK: on-off keying, DBPSK: differential binary phase-shift keying, NE: negative exponential, GG: Gamma–\(\mu\): Alpha–Mu Fading, SR: shadowed-Rician fading distribution, N-\(m\): Nakagami-\(m\) fading, FTR: Fluctuating two-ray fading, GN-\(m\): Generalized Nakagami–\(m\) fading, GK: Generalized-\(\mathcal{K}\) fading, Malaga-\(\mathcal{M}\) distribution, EW: Exponentiated Weibull fading, BX: Beaulieu-Xie fading, DGG: Double Generalized Gamma Distribution, EGK: Extended Generalized-K fading, OP: Outage probability, SOP: secrecy outage probability, ASC/EC/IC: Average Secrecy/Ergodic/Intercept capacity, BER: bit-error rate, ABER: average bit error rate, IM/DD: intensity-modulation direct detection, HD: heterodyne detection, SIM: subcarrier intensity modulation scheme.  
\end{table} 
Table 5: Literature on cooperative hybrid FSO. from a source node to a destination node, as well as through one or more relay nodes. These relay nodes aid in enhancing signal strength and reliability, particularly in adverse atmospheric conditions [284]. Additionally, several cooperative protocols have been suggested for use in relay-assisted FSO links. The traditional methods include amplify-and-forward (AF) [331; 332; 334; 361; 362; 363; 364; 365] and DF [331; 363] protocols. In AF relaying, optical-to-electrical (OE) conversion is performed at each relay, followed by electrical amplification and modulation of a laser source.