# Free Space Optical Communication: An Enabling Backhaul Technology for 6G Non-Terrestrial Networks
## 4 Challenges and the Road Ahead
### RIS for FSO-Based NTN  
Reconfigurable Intelligent Surface (RIS), also known as Intelligent Reflecting Surface (IRS), involves the use of specially engineered materials, surfaces, or structures that can modify the characteristics of electromagnetic waves incident upon them. The primary goal of an RIS is to enhance the performance and efficiency of wireless communication systems by intelligently controlling the propagation of electromagnetic waves. In a wireless communication system, signals are transmitted from a source to a destination through the surrounding environment (i.e., a channel). This channel includes various objects, obstacles, and surfaces that the signals interact with. RIS technology adds an additional layer to this environment, consisting of surfaces embedded with a large number of individual elements or units that can adjust the phase, amplitude, and direction of incoming electromagnetic waves. By carefully adjusting the properties of these reconfigurable surfaces, it becomes possible to manipulate the effect of the channel on the wireless signals and improve the received signal [148; 149]. Additionally, RISs can be particularly useful in circumventing LoS restrictions, such as cloud blockage. For example, when positioned on the ground, an RIS could effectively redirect the FSO link between two airborne nodes that cannot see each other, facilitating communication despite an obstructed path. IRSs were first explored in the context of radio communication, see e.g., comprehensive surveys in [150; 151]. They were later applied to optical communication [152; 153; 154; 155; 156; 157; 158; 159]. RF and optical-based RIS differ significantly in terms of their materials, operating range, and structural design. RF-based RISs typically employ conductive and dielectric materials, operating within the radio frequency spectrum, and focus on elements like antennas and metamaterials for radio wave manipulation. Conversely, optical RISs utilize materials specialized for controlling light transmission, such as nanostructures and photonic crystals, within the visible, infrared, or ultraviolet spectrum. Understanding these distinctions is crucial when designing an RIS tailored to specific signal types, ensuring efficient manipulation and control of either radio or optical waves. In addition to the initial works that focused on indoor and terrestrial IRSs-based wireless systems [160; 161; 162; 163; 164; 165; 166], the benefits of IRSs were further explored in the context of airborne applications [167; 168; 169; 170; 171; 172; 173; 174; 175; 176; 177]. For example, [PERSON] et al. [173] considered UAVs integrated with IRS to facilitate a hybrid RF/FSO link. They investigated various challenges, including phase shift errors caused by IRS reflecting elements, atmospheric turbulence, and pointing errors arising from fluctuations in an UAV-mounted IRS's position and orientation. They derived closed-form expressions for the average symbol error rate and spectral efficiency, considering the combined impact of these factors. In the study by [PERSON] et al. [174], the authors considered a satellite-aerial-ground integrated network. To mitigate cloud blockages and maintain a high-speed FSO connection from the HAPS to the ground station, they deployed an UAV equipped with a RIS mirror array to reflect signals from the HAPS. [PERSON] et al. [175] employed an UAV as a relay and integrated an IRS to enhance coverage. They investigated the impact of jamming from a malicious UAV on the performance of a FSO communication system. [PERSON] et al. [176] investigated the secrecy performance of a multi-user communication system involving HAPS and UAV. They employed a RIS to assist in optimizing the communication links. The numerical results confirmed the effectiveness of their proposed design in achieving superior secrecy performance compared to the existing benchmarks. [PERSON] et al. [177] considered a communication network involving non-terrestrial vehicles, specifically satellites, UAVs, and a RISs.