Free Space Optical Communication for Inter-Satellite Link: Architecture, Potentials and Trends

The sixth-generation network is expected to achieve global coverage based on the space-air-ground integrated network, and the latest satellite network will play an important role in it. The introduction of inter-satellite links (ISLs) can significantly improve the throughput of the satellite network, and has recently received a lot of attention from both academia and industry. In this article, we illustrate the advantages of using the laser for ISLs due to its longer communication distance, higher data speed, and stronger security. Specifically, space-borne laser terminals - with the acquisition, pointing, and tracking mechanism, which realizes long-distance communication - are illustrated. Advanced modulation and multiplexing modes that make high communication rates possible are introduced. The security of ISLs ensured by the characteristics of both laser and the optical channel is also analyzed. Moreover, some open issues, such as advanced optical beam steering, routing and scheduling algorithm, and integrated sensing and communication are discussed to direct future research.