# Free Space Optical Communication: An Enabling Backhaul Technology for 6G Non-Terrestrial Networks
## 3 FSO-Based Backhauling for NTNs
### Physical Layer Design
They conducted a detailed quantification of various losses, aiming to provide a comprehensive understanding of the challenges and limitations associated with FSO links when considering the dynamic nature of a hovering rotary-wing UAV. [PERSON] et al. [PERSON] et al. (2016) extended this work by considering various factors such as pointing errors, UAV orientation fluctuations, and atmospheric turbulence. In their subsequent research [PERSON] et al. (2016), [PERSON] et al. assessed the performance of FSO links involving UAVs equipped with modulating retro-reflectors. They derived expressions for the PDF of SNR, outage probability, and BER under different atmospheric turbulence conditions. In order to enhance the transmission range and link stability, researchers further explored the concept of using FSO-based airborne nodes as relays [PERSON] et al. (2016); [PERSON] et al. (2015); [PERSON] et al. (2015); [PERSON] et al. (2015). For example, [PERSON] et al. [PERSON] et al. (2016) explored the concept of utilizing UAVs as moving relays equipped with buffers to enhance the performance of terrestrial links. [PERSON] et al. [PERSON] et al. (2016) investigated a serial relaying scheme employing hovering UAVs as relay nodes in a triple-hop configuration. Their study included an analysis of system performance over varying atmospheric turbulence channels. In [PERSON] et al. (2015), [PERSON] et al. considered a dual-hop system that combined PLL access links and FSO backhaul links to connect mobile users with a fixed ground station. They took into account the stability of hovering UAVs, which influenced the quality of both FSO and RF connections. In a related investigation [PERSON] et al. (2016), [PERSON] et al. considered a dual-hop setup involving a single UAV relay and analyzed end-to-end SNR at the destination. Recently, a joint parallel-serial multi-hop system was considered in [PERSON] et al. (2015). In particular, the authors studied airborne parallel multi-hop FSO with IM/DD communication over lognormal fading channels. Utilizing the fact that the channel coefficient acts quasi-statically in the airborne FSO link due to the UAV's semi-fixed hovering, the relay selection might rely on outdated coefficients from the previous node. They first derived a statistical model for the outdated lognormal channel. Then, in order to maintain the end-to-end BER within acceptable limits, they identified suitable pre-amplification factors for each hop. These factors ensured that, despite outdated channel conditions, the received power remained above a specified threshold. Several works have explored relay-assisted FSO links in the context of satellite communications. For example, [PERSON] et al. [PERSON] et al. (2016) investigated establishing ground-to-satellite/satellite-to-ground FSO links. They identified limitations in FSO uplink communication due to beam-wander, scintillation, and pointing errors, and proposed combining FSO with RF links, to create more reliable hybrid systems. Their research also explored the integration of FSO and RF technologies in space-air-ground networks using UAVs as relay stations, ultimately demonstrating that hybrid FSO/RF systems can outperform FSO alone in uplink scenarios. [PERSON] et al. [PERSON] et al. (2017) analyzed the outage probability, average BER, and average transmission rate of a dual-hop link between deep space and terrestrial in a weather-dependent satellite-terrestrial network with a rate adaptation hybrid RF/FSO link. [PERSON] et al., in [PERSON] et al. (2017), investigated the performance of cooperative transmission systems in FSO communication. Their study focused on dual-hop systems, combining RF and FSO transmission. They used decode and forward relay with error-correcting codes, including quasicyclic-Low-Density Parity Check (LDPC) codes for FSO links, and the Additive White Gaussian Noise (AWGN) model for RF links.