Context

Driven by the ever-growing demand for high-capacity telecommunications services such as HDTV and broadband Internet, the deployment of Geostationary Orbit (GEO) Very High Throughput Satellites (VHTS) has become a critical priority. Conventional radio-frequency (RF) technologies, however, are increasingly constrained by spectrum scarcity, regulatory limits, intrinsic performance and SWaP-C (size,weight, power and cost) ceilings. As a result, Free-Space Optical Communications (FSOC) has gained significant attention as a promising solution for high-speed satellite data transmission. Yet, the implementation of FSOC in space systems presents major challenges especially cross-atmospheric links. These links are highly vulnerable to turbulence effects, resulting in severe signal degradation and outage events. Additional impairments—including nonlinearities, low signal to noise ratios, low received optical power, very high throughput targets, and limited onboard resources—further complicate their deployment.  

To achieve the goal of feeder link capacities approaching 1 Tbps, coherent modulation formats are indispensable. However, applying coherent techniques in FSOC introduces challenges absent from terrestrial optical networks, such as turbulence-induced fading, ultra-low received powers, and the immaturity of key components like optical amplifiers, stable laser sources, and front-ends. While some few successful experimental demonstrations of coherent ground-to-satellite transmissions have been reported using FPGA or Commercial Off-The-Shelf (COTS) technology for in-orbit tests, these efforts have been limited to Low Earth Orbit (LEO) or inter-satellite links—contexts with significantly less stringent constraints on radiation hardness, reliability, and data rates compared to GEO feeder links.  
Therefore, the realization of robust, reliable, and ultra-high-capacity optical feeder links for GEO VHTS requires innovative designs and tailored solutions that go beyond the capabilities of current technologies.

Objectives 

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.

Impairments Modeling : The first step of this PhD research will be 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.).

Advanced Modulation Design : 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.

Nonlinearity Compensation: 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 (SPM, XPM, FWM), satellite FSO links are dominated by high-power amplifier effects. The thesis will explore advanced digital and analog mitigation strategies, including reduced-complexity digital backpropagation. Trade-offs between complexity and performance will be analyzed to ensure real-time implementation within satellite processing and power budgets.

Test Bench Implementation: 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.

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For more Information about the topics and the co-financial partner (found by the lab!); contact Directeur de thèse - robin.gerzaguet@irisa.fr

Then, prepare a resume, a recent transcript and a reference letter from your M2 supervisor/ engineering school director and you will be ready to apply online  before March 13th, 2026 Midnight Paris time!