# Free Space Optical Communication: An Enabling Backhaul Technology for 6G Non-Terrestrial Networks
### Space-Borne Nodes  
In 1945, the British science fiction writer and inventor [PERSON] published an article titled \"Extra-Terrestrial Relays\" in the magazine _Wireless World_, where he proposed the concept of GEO satellites [23]. He described how these satellites, positioned in a specific  
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orbit above the Earth's equator, could remain fixed relative to a specific location on the Earth's surface, providing continuous coverage for communication purposes. The realization of [PERSON]'s vision took a significant step forward with the launch of the first artificial satellite, Sputnik, by the Soviet Union in 1957 [24]. Although Sputnik did not have direct telecommunication capabilities, its launch demonstrated the feasibility of placing objects in orbit around the Earth and opened the door for future satellite-based communication systems. Following the launch of Sputnik, the development of satellite communication systems progressed rapidly. In 1962, the first active communication satellite, Telstar 1, was launched into space. Telstar 1 enabled the transmission of television signals, telephone calls, and other data across the Atlantic Ocean, demonstrating the potential of satellite communication for global connectivity [25]. Since then, numerous satellites have been launched into various orbits, including GEO, MEO, and LEO, to support telecommunication services [26]. For example, the International Maritime Satellite Organization (INMARSAT) operates four geostationary satellites at different orbital positions, to provide global coverage to around 97% of the Earth's surface [27] [Section 1.1.3]. Other examples of GEO satellite systems include International Telecommunications Satellite Organization (Intelsat), Societe Europeenne des Satellites (SES), Communications Satellite Corporation (Comsat), European Telecommunications Satellite Organization (Eutelsat), and Canadian Telecommunications Satellite (Telesat). These satellite operators deploy GEO satellites to provide a wide range of communication services, including television broadcasting, broadband internet access, and telecommunication services. Despite offering worldwide continuous coverage by maintaining a stationary position over a particular area in a synchronized orbit, GEO satellites face restricted data rates. Furthermore, the significant delay in signal propagation resulting from their high altitude introduces noticeable latency and hence adversely affects real-time applications. This prompted the idea of providing global wireless coverage through LEO satellites, which was first considered in the 1990s. In 1993, Motorola launched the Iridium constellation, which consists of 66 LEO satellites that formed a mesh network [27; 28]. In this mesh network, signals were relayed between satellites and eventually reached their intended destinations on Earth. The Iridium system was designed to provide mainly voice services, as well as low-rate data services up to 10 kbps [27] [Section 1.3.3.6]. Iridium, however, experienced financial and technical challenges due to the high cost of infrastructure development. The high cost of service made it unattractive and expensive for users [29]. Another early LEO constellation launched to provide global satellite communications coverage was Globalstar, which initially comprised 48 satellites [30]. The company began offering customers mobile voice and data services worldwide in 2002. The data rate provided by the first Globalstar system was around 9.6 kbps and allowed voice calls, short messaging, and limited data transfer. Similar to Iridium, its success was limited. Another early LEO constellation example is Teledesic, which ceased operations in 2002 [31] [Section 1.13]. The first-generation LEO satellites mainly failed, due to their inability to meet expectations. First of all, there was a limitation on the capacity of the early LEO satellites, both in terms of simultaneous users and the amount of data they were capable of handling.