In-orbit experiment of 48-hour error-free FSO communication enabled by an effective DFS compensation in a cross-orbital satellite link

Inter-satellite optical wireless communications (ISOWC) has emerged as an enabling technology for next-generation satellite internet constellations and so has attracted a great of deal interest from the research community. However, establishing stable free-space optical (FSO) links over several thousand kilometers entails challenges due to instabilities and uncertainties in the optical beam pointing, acquisition, and tracking (PAT). Furthermore, compared to co-orbital communications, cross-orbital satellites experience greater relative motion, requiring inter-satellite FSO systems to overcome the frequency difference between transmitter and receiver induced by Doppler frequency shift (DFS). In this work, we report an autonomously operating, highly integrated laser communication payload (LCP) and demonstrate in-orbit, error-free, stable cross-orbital inter-satellite FSO communication at 5 Gbps for 48-hours continuously. Enabled by advanced forward error correction (FEC) processing, high-performance optoelectronic equipment, high-precision PAT, and DFS compensation, this in-orbit demonstration sets an industry benchmark, achieving a cross-orbital communications link at the thousand kilometer altitude level and an inter-satellite distance range of 3400 km to 3700 km. Compared to our prior in-orbit demonstration, the LCP achieves an optical signal receiving sensitivity of -49.2 dBm, representing a 7.6 dB improvement over the direct-detection (DD) system operating at 2.8 Gbps. Establishing long-term and stable cross-orbital inter-satellite FSO communications links is a difficult and complex engineering task, and this work represents a major milestone towards realizing future 6th-generation (6G) satellite optical communications networks.