Verification of Channel Reciprocity in Long-Range Turbulent FSO Links

High-Altitude Platforms / High-Altitude Pseudo Satellites (HAPs) will extend Internet access to currently uncovered regions. Free-Space Optical (FSO) links will interconnect HAPs over long distances through the stratosphere. Their Forward Error Correction has to be carefully adapted to the atmospheric scintillation channel. Long delay impairs the exploitation of receiver-based channel state information (CSI). Only inherent CSI provided by channel reciprocity allows in time control. Investigation results of this fading-signal correlation phenomenon over long distances however have not yet been reported. Therefore, we experimentally study this effect in a long-range (63 km) monostatic bidirectional atmospheric FSO link for a wide range of parameters. Numerical phase-screen simulations of the transmission scenario confirm the results.