From fibers to satellites: Lessons to learn and pitfalls to avoid when optical communications move to long distance free space

The paper summarizes the recent investigation on feasibility of adapting state-of-the-art coherent fiber-optics (FO) systems for Free Space Optical (FSO) scenarios. This investigation is critically dependent on the intertwined aspects of architecture, as well as device and propagation impairments (including the channel) appearing in the aforementioned systems. Towards this, the work identified the key system differences between the two systems. Particularly, the FSO channel model was investigated, impact of atmospheric turbulence on FSO was discussed and a channel series was generated. Subsequently, relevant FO techniques including coherent detection, wavelength division multiplexing and Time-Frequency packing (TFP) were reviewed. Another departure from FSO works was the emphasis on coherent reception; receiver architectures and diversity schemes were first investigated. The former strived to make fair comparison amongst the receivers considering the diverse nature of perturbation added, while the latter indicated gain in performance through increase of diversity order (2-4 dB gain). An immediate conclusion is a suggestion on adaptation of wavelength diversity when coherent receivers. The investigation also evaluated the capacity and outage of fast and slow fading channels with parameters motivated by the channel modelling work. The shaping gain was evaluated and an LDPC code design example was provided for FSO downlinks. Finally, TFP enabled a remarkable performance gain when applied to coherent detection schemes, but only marginal with direct detection. The paper concludes by pointing to the next steps that build on this investigation and the need to corroborate with measurements.