Enhancing LEO satellite uplink performance through spatial mode diversity-based scintillation mitigation

Authors: Clément JACQUARD, Henri LEHEC, Thibault MICHEL, Laurie PAILLIER, Maxime JOOS, Julien SAMAAN, Matthieu MEUNIER, Pu JIAN, Guillaume LABROILLE, and Olivier PINEL

Cailabs, Rennes, FRANCE

Abstract

This paper introduces the novel application of leveraging spatial mode diversity for improved Earth-to-LEO (Low Earth Orbit) satellite uplink communication. Optical satellite communications face the persistent challenge of atmospheric turbulence, which induces scintillation, beam wandering, and ultimately link-fading and bit error rate deterioration, significantly impairing the reliability and efficiency of uplink signals. Conventional techniques to mitigate scintillation effects include pre-compensation of the atmospheric channel, yet this solution often comes with increased complexity, cost, and limited applicability in diverse atmospheric conditions, especially for LEO satellite communications. Another technique to mitigate these effects is to use multiple uncorrelated laser transmitters, resulting in signals transmitted through different fading channels. Conventionally, spatial diversity schemes are based on the emitters being transmitted through multiple telescopes or multiple sub-pupils of the same telescope. However, these schemes present challenges in the implementation, as it may be difficult to co-align a large number of emitters.

Our approach diverges from these traditional methods by utilizing spatial diversity through the incoherent combination of multiple colinear spatial modes. The emitted beam from each telescope is an incoherent combination of 3 to 6 orthogonal spatial modes, achieved through small wavelength offsets, polarization, and spatial beam shaping. Additional spatial diversity is obtained by distributing the spatial modes over two telescopes, each capable of generating up to 6 W in the C band for a total emitted power of 12 W. This allows for up to 12 distinct incoherent modes to add up incoherently in the plane of the on-board optical terminal’s aperture.

Key to our methodology is the use of Commercial Off-The-Shelf (COTS) components and Multi-Plane Light Conversion (MPLC) technology developed by Cailabs, a single device which facilitates the deployment of spatial diversity with no co-alignment required by the user. The effectiveness of this technology in mitigating scintillation and consequently reducing bit error rates (BER) through fading mitigation is demonstrated via an emulated atmospheric turbulence testbed. Results indicate a significant improvement in the stability and quality of the uplink channel, promising enhanced performance for satellite communication networks.