# Free Space Optical Communication: An Enabling Backhaul Technology for 6G Non-Terrestrial Networks
## 3 FSO-Based Backhauling for NTNs
### Physical Layer Design
[83] investigated the performance of several HAPS/UAV-based FSO communication systems in various weather conditions. In [105], [PERSON] et al. focused on optimizing the trajectory of a fixed-wing UAV-based FSO system with IM/DD. This optimization task aimed to maximize the operational duration of FSO-based backhaul networks, while adhering to energy and data rate limitations. [PERSON] et al. [106] investigated the capacity of UAV-based FSO links. They derived the PDFs to analyze the ergodic capacity of FSO links with both IM/DD and heterodyne detection. [PERSON] et al. [107] considered the use of FSO-based UAV access networks as an additional means to enhance backhaul link capacity. [PERSON] et al. [108] examined a trajectory optimization of a low-altitude fixed-wing UAV to maximize the flight duration of an IM/DD FSO system. [PERSON] et al., in [109], investigated various aspects of coherent optical communications for inter-satellite links in LEO satellite constellations. They assessed different modulation formats and coding schemes for intra- and interorbital connections, determining the feasibility and Signal-to-Noise Ratio (SNR) margins. Their study found that high-order modulation formats with high symbol rates can be unfeasible for some connections. They also examined Doppler shifts for different connection scenarios and proposed a new method for all-digital Doppler shift compensation, when the traditional methods are insufficient. [PERSON] et al. in [110] addressed challenges in Earth-satellite and inter-satellite coherent FSO links for LEO constellations, aiming to provide ultra-high capacity and reliability. They proposed a technique for mitigating the Doppler effect using Probabilistic Constellation Shaping (PCS) and adapted the system symbol-rate, while keeping the bit-rate fixed. Their experimental 600 Gbps transmission showed a significant increase in the range of supported frequency shifts imposed by the Doppler effect. Additionally, they tackled the reliability problem caused by atmospheric turbulence in continuous and bursty transmission scenarios, demonstrating that their methodology increased the supported bit-rate by more than 70 Gbps. [PERSON] et al. [111] investigated pointing errors and considered an extreme situation where a UAV moves beyond the receiver's Field-of-View (FoV), leading to a disruption in the signal. Subsequently, they proposed closed-form expressions for both outage probability and Bit Error Rate (BER) of IM/DD FSO links over both lognormal and gamma-gamma atmospheric turbulence channels. [PERSON] et al., in [112], considered a HAPS-based FSO link and derived outage probability expressions for different turbulence levels, highlighting the importance of optimizing the transmitter beam and receiver FoV to minimize the outage probability in varying conditions. [PERSON] et al. [113] developed an iterative algorithm to position an aerial base station optimally. This algorithm also handled resource allocation, with the goal of maximizing the throughput for the access link. The optimization process was subject to meeting the Quality of Service (QoS) requirements for individual users within a hotspot region. A similar issue of associating UAVs and access points was explored by [PERSON] et al. [114]. In their study, [PERSON] et al. addressed the challenge of optimizing the deployment of Networked Flying Platforms (NFPs), while ensuring balanced traffic loads and considering the reliability of FSO links and the number of FSO transceivers, assuming IM/DD. They proposed two algorithms to tackle this issue. The authors conductedsimulations to assess the algorithms' performances, focusing on the number of deployed NFPs and maximum link utilization. [PERSON] et al. [PERSON] et al. (2015) focused on assessing the signal losses within FSO links between a rotary-wing UAVs in a hovering state and a central control unit.