Digitally Mitigating Doppler Shift in High-Capacity Coherent FSO LEO-to-Earth Links

The range of applications enabled by the low-Earth orbit (LEO) constellations continues to increase, going from providing worldwide broadband internet access, to the transmission of enormous quantities of data gathered from space sensing and exploration. Therefore, it is imperative to find a technology for Earth-satellite and inter-satellite links that is able to provide ultra-high capacity and reliability. The alliance of coherent optical systems with free-space optics (FSO) can be considered a solution for these links, bringing the ultra-high capacity of fiber optic transmission systems to the wireless domain. However, the free-space channel brings some challenges to the telecommunication system, namely the atmospheric turbulence that can hinder data transmission, mainly when associated with typical satellite communication features such as the Doppler effect. Driven by this challenge, in this work, we propose a technique for mitigation of the impact of Doppler effect by dynamically adapting the transmitted signal symbol-rate while keeping the system bit-rate fixed through probabilistic constellation shaping (PCS). This methodology significantly increases the range of supported frequency shifts imposed by the Doppler effect in an experimental 600 Gbps transmission. Moreover, we address the reliability problem imposed by atmospheric turbulence, considering two different scenarios: i) a continuous transmission system with guaranteed reliability and ii) a bursty transmission without reliability constraints that solely aims at the maximization of the overall throughput of the system. In both scenarios, we show that the proposed methodology of adapting the system symbol-rate together with the transmitted constellation shaping can increase the supported bit-rate by more than 70 Gbps.