AD-HOC: Advanced DSP HTS Optical Communications

Overview

Context:
Driven by the demand for high-capacity telecommunications services, the deployment of GEO Very High Throughput Satellites (VHTS) faces constraints in conventional RF technologies. Free-Space Optical Communications (FSOC) is explored for high-speed satellite data transmission, with challenges from atmospheric turbulence, nonlinearities, low signal-to-noise ratios, and limited onboard resources. To achieve feeder link capacities approaching 1 Tbps, coherent modulation formats are considered indispensable, though they introduce additional challenges in space environments.

The realization of robust, reliable, and ultra-high-capacity optical feeder links for GEO VHTS requires innovative designs and tailored solutions beyond current technologies.

This PhD research project addresses the development of innovative techniques to significantly enhance spectral efficiency of optical feeder links through advanced modulation schemes and digital signal processing algorithms.

The first step is to rigorously model the satellite FSO channel for uplink and downlink feeder links under realistic operational conditions. This includes static impairments (noise, offsets, skews, imbalances) and dynamic effects (atmospheric turbulence, noise), as well as architecture-induced impairments (group delays, polarization rotations, nonlinearities, filtering, etc.).

The second research axis targets the design and optimization of high-order modulation formats specifically adapted to satellite FSOC. Beyond dual-polarization QPSK, the work will explore advanced M-ary formats such as M-PAM, M-QAM, and M-PSK. The objective is to develop signal processing algorithms that account for satellite FSO impairments. Because the channel is highly time-varying due to turbulence, adaptive modulation and coding (AMC) strategies will be investigated to dynamically adjust transmission parameters based on real-time estimation.

This includes cross-layer optimization, prediction metrics, and low-latency tracking algorithms to ensure robust operation under sub-millisecond variations.

The third research axis addresses amplifier-induced nonlinearities, a key issue in long GEO FSO links. Unlike terrestrial fiber systems where nonlinearities stem from the Kerr effect, satellite FSO links are dominated by high-power amplifier effects. The thesis will explore advanced digital and analog mitigation strategies, including reduced-complexity digital back propagation. Trade-offs between complexity and performance will be analyzed to ensure real-time implementation within satellite processing and power budgets.

The final axis involves the development of an FPGA-based test bench to validate the proposed algorithms under realistic conditions. This platform will include a full DSP chain supporting advanced modulation formats and nonlinearity compensation mechanisms, enabling offline and real-time evaluation. Special attention will be given to resource efficiency, latency, and numerical precision management.

Atmospheric turbulence models will be integrated into the bench to emulate realistic satellite FSO link effects and assess system performance under varying impairment intensities.

For more information about the topics and the co-financial partner:
contact Directeur de thèse - robin.gerzaguet