Evaluation of daylight background noise for satellite-to-ground free-space optical communication during daytime operation

There is growing global interest in establishing free-space optical (FSO) communication links, such as ground-satellite links (GSLs) of at least hundreds of kilometers, intersatellite links of thousands of kilometers, and future deep space links of much greater dimensions. Enabling outdoor wireless FSO communication systems to be utilized during daylight hours can increase their availability in space-air-ground networks; however, this is usually accompanied by incoherent background radiation that impairs the signal-to-noise ratio (SNR) and bit error rate (BER). Therefore, a preliminary review of the background noise is required before constructing ground terminals with a suitable SNR in a harsh environment with high levels of solar noise. Herein, we evaluated the background noise that sunlight provides to ground terminals and quantitatively examined its impact on the SNR, communication performance, and beacon detection accuracy of the developed all-free-space ground terminal. Furthermore, we present the results of a daytime demonstration of a 7-km terrestrial free-space optical communication link by employing our ground terminal that was designed based on these analyses. The results verified that 2.5-Gbps data transmission up to 7 km is feasible, even in expected daytime satellite tracking scenarios with high background noise, by the developed system with spectral and spatial filtering to achieve an acceptable SNR. The background noise results of our research are anticipated to further the research on quantum communication networks, light detection and ranging (LiDAR), and green energy technologies.