## 4 Enabling Technologies for FSO Communication
### Cooperative Relaying
#### 4.8.1 Relay-Assisted Transmission  
In contrast to the localized diversity techniques used in MIMO systems, relay-assisted solutions provide distributed spatial diversity methods, where neighboring nodes (or relays) support communication between a source node and a destination node. Moreover, cooperative communication is a dynamic area of research because of its potential to improve reliability and broaden the reach of wireless networks without the need for a new infrastructure [323]. It has also become a viable method for achieving spatial diversity benefits [324; 325; 326]. The core concept of cooperative diversity (parallel relaying) is founded on the observation that, in a wireless RF channel, the signal sent by the source node is received by other nodes known as partners or relays. These partners, along with the source, can collaboratively manage and convey their information, forming a virtual antenna array despite each node having only one antenna. Another relay-assisted transmission scheme is multihop transmission (serial relaying), which employs relays arranged in a sequential configuration [327; 328; 329; 330; 331]. This approach is typically employed to extend the coverage area for Txs with limited power. It does not enhance performance against fading influences in wireless RF environments, which means it does not improve the diversity order [332]. Also, cooperative techniques are extensively researched in FSO communication systems to counteract the adverse effects of turbulence-induced atmospheric scintillation [332; 333; 334; 335; 336]. Table 5 presents some of the related work in this area. Current research in cooperative FSO communications primarily focuses on two approaches: all-active relaying, which is used when CSI is unavailable, and selective relaying, which is applied when CSI is present. All-active relaying is a straightforward and efficient two-slot scheme. In the first slot, the source (S) sends the information packet to the relays (Rs). The relays then decode (or amplify) the received symbols and, in the second slot, forward the processed signals to the destination (D). In this setup, no priority is given to any relay, irrespective of the strengths of the source-relay and relay-destination links [332; 333; 334; 335]. In selective relaying, either direct transmissions occur, or one relay is selected from the pool of available relays to forward the information symbols to the destination based on the channel state [336; 337; 338]. This approach prioritizes the transmission along the strongest e2e link. Instead of distributing power across multiple relays, the power is concentrated and directed to the _best relay_ to improve signal reconstruction quality at this relay. The criteria for selecting the optimal path involve minimizing the conditional error probability for quantum-limited systems without background noise [337], and minimizing the OP for systems with Gaussian noise [336]. While all-active relaying is simple and does not require any CSI, selective relaying provides better performance but comes with increased system complexity. This complexity arises from the need to acquire complete CSI for all intermediate channels and to establish feedback links between the communicating nodes [241; 323; 339]. Furthermore, it has been observed that the strength of the two-hop source-relay-destination (S-R-D) link depends on its weakest hop. This protocol shows that both all-active and selective relaying methods can achieve full diversity gains, with the selective method offering an improved coding gain. Recently, inter-relay cooperation has been introduced to enhance the reliability of FSO networks, especially in scenarios where FSO links are established between the relays [340]. In multi-node scenarios, cooperative communication acts as a unique variant of MIMO by utilizing relay nodes to improve the link quality between the source and destination nodes.