Optical feeder links (OFLs) benefit from the vast amount of bandwidth available in the THz-regime of the electromagnetic spectrum, and can be considered as enablers for future terabit-per-second satellite systems. A particular challenge for OFLs is to mitigate the effects of fading, caused by a combination of turbulence-induced scintillation, beam wander and pointing errors. The conventional solution is to exploit temporal diversity by a combination of interleaving and forward error correction (FEC). In this study we present an overview of fading mitigation techniques for latency-constrained coherent ground-to-satellite OFL and contribute a generic model which combines various diversity schemes including temporal, spatial, frequency and site diversity. To unlock spatial diversity, multi-beam space-time block coding and multi-beam, multi-λ are proposed and simulated. Though space-time block coding (STBC) provides more diversity gain, it requires accurate timing synchronization at the transmitter and channel state information at the receiver. Temporal, frequency and site diversity all rely on some form of interleaving and the potential diversity, pros and cons of each of these diversity techniques are covered in the presented study. In general, with a strict latency constraint and a tight link budget, frequency diversity, spatial diversity - either by STBC or multi-beam multi-λ - and site diversity can be effective methods to mitigate the effects of fading and close the link budget.

A 10 km ground-To-ground field test of a Terabit/s Optical Feeder Link demonstrator has been carried out. The demonstrator aimed for end-To-end communication performance with the use of Adaptive Optics pre-correction to mitigate the atmospheric turbulence disturbances of the FSO channel. It further includes the multiplexing of multiple uplink channels and RF end-To-end modems to prove the technical feasibility of supporting a Terabit/s communication link. The field test-performed in July 2022-demonstrates error free communication for over 10 minutes. This achievement proves the feasibility of a digital transparent communication architecture for the Terabit/s Feeder link application. The field test results are presented jointly with a companion paper [1] which focusses on the communication aspects. The Adaptive Optics (AO) pre-correction performance is analyzed against the atmospheric turbulence strength and the number of corrected AO modes. The encountered turbulence conditions during the field test caused irradiance fluctuations in the strong fluctuation regime. The field test results show a significant AO wavefront error correction on the downlink. For the uplink irradiance levels, a modest advantage of pre-correcting higher wavefront modes is found against the pre-correction of only the tip-Tilt modes. Finally, with respect to the uplink fading characteristics, the AO pre-correction leads to a reduction of the mean-fade time of approximately 20-30%, compared to AO tip-Tilt pre-correction only.