# Free Space Optical Communication: An Enabling Backhaul Technology for 6G Non-Terrestrial Networks
## 4 Challenges and the Road Ahead
### SDM/OAM for FSO-Based NTN  
Spatial Division Multiplexing (SDM) and Orbital Angular Momentum (OAM) are advanced techniques in optical communication, to enhance data transmission capacity [178; 179; 180; 181; 182; 183; 184; 185]. SDM encompasses various methods used to transmit multiple data streams simultaneously using separate spatial paths or modes. One specific case within SDM is Mode-Division Multiplexing (MDM), where orthogonal spatial modes like OAM are employed to carry independent data channels, ensuring efficient multiplexing with low inter-modal crosstalk. SDM has applications in optical fibers, including multicore and multimode fibers [179; 180; 181]. Furthermore, several research works have been conducted on the application of SDM and OAM to terrestrial FSO links. For example, one study by [PERSON] et al. [186] demonstrated the use of a Fabry-Perot cavity to implement the OAM mode in FSO links. The researchers showed that, even with a high blocking percentage, the proposed method could effectively mitigate disturbances and improve the performance of OAM-based FSO links. Another study by [PERSON] et al. [187] focused on the experimental study of atmospheric turbulence detection using an OAM beam. The researchers used a Convolutional Neural Network (CNN) to detect the strength of atmospheric turbulence and proposed techniques to mitigate its effects on OAM-based FSO systems. The study highlighted the importance of accurately detecting turbulence strength, to guide the selection of appropriate mitigation techniques and modulation formats. [PERSON] et al. [188] proposed a unified statistical model for atmospheric turbulence-induced fading in OAM multiplexed FSO systems. They used the generalized gamma distribution to model the propagation of OAM modes through atmospheric turbulent FSO channels and derived closed-form expressions for various performance metrics. [PERSON] et al. [189] focused on relay-assisted transmission in SDM wireless optical communication systems. They developed a dual-hop SDM FSO communication model and demonstrated that the BER of the dual-hop SDM system with a few-mode EDFA-based relay outperformed the single-hop SDM system. [PERSON] et al. conducted simulations in [190] to examine the practicality of utilizing OAM-multiplexing in a FSO link, while considering atmospheric turbulence. In [191], [PERSON] et al. demonstrated the simultaneous correction of pre- and post-turbulence effects in a two-way FSO connection using Adaptive Optics (AO) technology. [PERSON] et al., as described in [PERSON] et al. (2019), assessed the data transmission capabilities of OAM-based FSO links when dealing with severe atmospheric turbulence conditions, with the aid of AO for wavefront phase distortion correction. In [PERSON] et al. (2019), [PERSON] et al. investigated the power loss and signal interference in an FSO link with a constant lateral displacement between the transmitter and receiver. This fixed displacement implies that both the transmitter and receiver are situated within a stable infrastructure, such as buildings, and that these structures do not experience independent oscillations or swaying. In addition to terrestrial links, the use of OAM for FSO communication in the context of NTNs has received attention [PERSON] et al. (2019); [PERSON] et al. (2019); [PERSON] et al. (2019). For example, [PERSON] et al. [PERSON] et al. (2019) conducted experiments to investigate the utilization of OAM-multiplexing to enhance the data transmission capacity in FSO communication for mobile platforms, such as UAVs. They successfully established an OAM-multiplexed FSO link between a ground station and a moving UAV, achieving a total data transmission capacity of 80 Gbps over a 100-meter roundtrip link using 2 OAM beams, each carrying a 40 Gbps Quadrature-Phase-Shift-Keying (QPSK) signal. [PERSON] et al., in [PERSON] et al. (2019), explored the application of OAM in FSO communications, discussing OAM modulation, multiplexing, multicasting, and overcoming turbulence.