Free Space Optical Communication: An Enabling Backhaul Technology for 6G Non-Terrestrial Networks

Abstract

The deployment of non-terrestrial networks (NTNs) is envisioned to achieve global coverage for 6G and beyond. In addition to space nodes, aerial NTN nodes such as high-altitude platform stations (HAPSs) and rotary-wing unmanned aerial vehicles (UAVs) could be deployed, based on the intended coverage and operational altitude requirements. NTN nodes have the potential to support both wireless access and backhauling. While the onboard base station provides wireless access for the end users, the backhauling link connects the airborne/space-borne base station to the core network. With its high data transmission capability comparable to fiber optics and its ability to operate in the interference-free optical spectrum, free space optical (FSO) communication is ideally suited to backhauling requirements in NTNs. In this paper, we present a comprehensive tutorial on airborne FSO backhauling. We first delve into the fundamentals of FSO signal transmission and discuss aspects such as geometrical loss, atmospheric attenuation, turbulence-induced fading, and pointing errors, all of which are critical for determining received signal levels and related link budget calculations. Then, we discuss the requirements of airborne backhaul system architectures, based on use cases. While single-layer backhaul systems are sufficient for providing coverage in rural areas, multi-layer designs are typically required to establish connectivity in urban areas, where line of sight (LoS) links are harder to maintain. We review physical layer design principles for FSO-based airborne links, discussing both intensity modulation/direct detection (IM/DD) and coherent modulation/coherent demodulation (CM/CD). Another critical design criteria for airborne backhauling is self-sustainability, which is further discussed in our paper. We conclude the paper by discussing current challenges and future research directions. In this context, we discuss reconfigurable intelligent surfaces (RIS) and spatial division multiplexing (SDM), for improved performance and an extended transmission range. We emphasize the importance of advanced handover techniques and scalability issues for practical implementation. We also highlight the growing role of artificial intelligence/machine learning (AI/ML) and their potential applications in the design and optimization of future FSO-based NTNs.

A Comprehensive Review of UAV-Assisted FSO Relay Systems

“…However, in the case of operational networks where a variable number of UAVs work with other communication systems, such as terrestrial networks or satellite links, the integration of handover mechanisms into the network may require security considerations such as robust encryption and authentication to protect data during handovers [77]. Furthermore, as more platforms are deployed into the next-generation 3D network, safety becomes more important and demands strict adherence to necessary flight regulations [78], with the possible need to acquire licenses and/or certificates that authorize flight operations within specified air spaces [16]. If not proactively addressed, such regulatory and compliance issues can pose a hurdle to prompt deployment logistics.…”

Exploring Universal Filtered Multi Carrier Waveform for Last Meter Connectivity in 6G: A Street-Lighting-Driven Approach with Enhanced Simulator for IoT Application Dimensioning

“…The smart industry sector, including factory automation and logistics, witnesses the potential of VLC in optimizing wireless connectivity within industrial warehouses [11]. Deploying free space optical (FSO) communication as a backhauling system in cities characterized by towering skyscrapers offers a high-capacity and wireless solution for data transmission across urban landscapes [12,13]. The military sector explores the secure, non wall-penetrating nature of VLC, potentially replacing wired communication means for local applications [14].…”