Advanced digital waveforms for low-Earth-orbit (LEO) FSO links

For low-Earth-orbit (LEO) satellite communication networks, free space optical (FSO) communication offers high data capacity and security without the spectrum limitations of more conventional RF approaches. However, receive signal power in FSO-LEO links can be highly variable based on multiple dynamic loss mechanisms occurring at different time scales. As a LEO pass moves from higher to lower elevation angles, propagation and scattering losses can vary by more than 10dB over a timescale of minutes. Separately, signal fading caused by atmospheric turbulence can also contribute greater than 10dB variation except at a much faster timescale on the order of milliseconds. Rather than implementing a modulation scheme based on the worst case link margin for a given FSO-LEO link, here we consider intensity modulated, direct detection (IM/DD) digital waveforms that can be dynamically adapted to the changing link conditions to provide increased bandwidth efficiency. In this work, we describe the development of IM/DD waveform modems and a waveform characterization test-bed which incorporates a scintillation playback system. BPSK, QPSK, 8PSK, and 16- QAM waveform performance will be presented under varying scintillation profiles.