Reliable Terabits Feeder Link for Very High-Throughput Satellite Systems with SAG-FSO Transmission

Very high-throughput satellite (VHTS) systems are attractive solutions for providing global connectivity and bridging the digital divide. These systems require feeder links with Terabit/s throughputs, which are challenging to achieve with conventional radio frequency (RF) transmissions. Free-space optical (FSO) transmissions, with huge license-free bandwidth, have emerged as a candidate solution for Terabit/s feeder links. Nevertheless, FSO transmissions are susceptible to the adverse impacts of atmospheric turbulence and weather conditions. To mitigate both atmospheric and weather effects, we first propose to combine space-air-ground (SAG) FSO transmission with site diversity. A high-altitude platform (HAP) FSO relay hovering directly above a ground station can successfully mitigate atmospheric turbulence impacts, while site diversity is an effective solution for addressing weather effects. To achieve a similar diversity benefit with reduced cost and lower complexity, we also propose a hybrid SAG-FSO/RF transmission system with multiple HAP relays. The proposed solution switches between HAPs depending on the ground-HAP channel quality. Numerical examples show the proposed feeder link solutions for VHTS systems can achieve a spectral efficiency of more than 3 bps/Hz while maintaining a bit error rate of 10
⁻⁹
at a signal-to-noise ratio of 15 dB.