The exponential growth of the needs in data transfer, at the global scale, leads to consider high data rate optical links between the ground and telecommunication satellites used as relays. The current most appealing configuration relies on bidirectional ground / geostationary satellite optical links at high data rate (of the order of one Tbit/s), so-called GEO-Feeder links [VédrenneICSOS2017]. The extreme requirements in performance of such systems imply innovative Developments on all the segments of the chain: high power sources, modulation/demodulation format and handling of the propagation channel. The latter segment concerns mainly the management of atmospheric turbulence effects that induce not only a mean loss on the optical signal but also deep fadings (>15 dB) on long durations (few millisecond) compared to the symbol time. The mitigation of these effects is therefore essential to reach the required data rates. It relies both on optical (adaptive optics correction) and numerical (coding/interleaving) strategies that are at the heart of the present research subject.
Adaptive optics is used on the ground side to sense and correct for atmospheric turbulence effects on the downlink beam, before injection in a single mode fiber. The reciprocity principle [ShapiroOptCommNetw2012] allows showing that this correction is also relevant to pre-compensate the uplink beam. In the case of coaligned up/downlink beams one can even demonstrate, with some symmetry hypotheses on the system, that ground and space optical signals are actually identical, the link is said “reciprocal”. This perfect symmetry is however broken by the point-ahead angle: angle between down and uplink beams imposed by the movement of the satellite on its orbit (ShapiroOptCommNetw2012]. The repercussions of pointing ahead are of different nature: it limits the efficiency of the uplink pre-compensation (anisoplanatism effect) and therefore the uplink performance, besides the ground and space optical signals are no more strictly identical, the link is then said “partially reciprocal” [RobertPhysRev2016]. The statistical properties of the channel are of course essential for the optimization of the numerical part of the link and for the evaluation of the overall telecom performance (data rate, latency).
At this stage it is therefore essential to obtain an experimental demonstration of adaptive optics precompensation
and to improve our knowledge of the propagation channel statistics. In the framework of the FEEDELIO project, ONERA performs for ESA the first experimental demonstration of a pre-compensation by adaptive optics on a near ground link that is shown to be representative of the operational ground – geostationary satellite (VédrenneICSOS2017]. The experimental tests will occur first semester of 2019 on the ESA Optical Ground Station site (Canary Islands, Spain).
The PhD work is in line with these developments and aims at mastering and optimizing the ground-GEO optical links corrected by adaptive optics, from the propagation channel to the telecom transmission modalities (coding, interleaving...). This objective relies on the following research topics:
- · study of adaptive optics for ground-GEO links (chiefly for uplink pre-compensation),
- · statistical characterization of up and downlink channels and construction of a channel model for such bidirectional links,
- · study of the theoretical performance limits (channel capacity...) for the channel model considered,
- · performance optimization (data rate, latency, availability) based on the channel model: modulation format and code rate adaptation (link adaptation) and/or compensation of channel effects through numerical pre-coding.
The PhD student will benefit from FEEDELIO data so as study of the adaptive optics correction quality as a function of the number of corrected modes and of the point-ahead angle. He/she will also have access to our simulation tools (semi-analytical model SAOST and end-to-end code TURANDOT-AOST). Experimental results and simulations will indeed be compared so as to still improve the codes and the performance evaluation reliability.
Channel characterization will rely on our recent work [CanuetJOSAA2018] that concentrates on the characterization of the downlink propagation channel via telecom metrics (in particular the distribution law of fade durations). An extension is therefore needed so as to model the uplink channel and to characterize the partial reciprocity of up and down links.
Such channel models pave the way towards an optimization of numerical strategies (coding/interleaving) dedicated to ground-GEO, and even a joint optimization of optical means (adaptive optics design) and numerical means. The PhD student will in particular analyze the interest of accounting for partial link reciprocity in the transmission strategy. The kind of approach is mentioned in the literature [PuryearOptCommNetw2013] but has however not been studied yet in the context of adaptive optics corrected ground-space links. Note still that, in the related domain of optical links for ground-space clock comparison, we have effectively demonstrated that the account of the link partial reciprocity allows to drastically reduce the turbulence impact [RobertPhysRev2016]. This work allows envisioning very high precision ground-space frequency transfer and opens the path to new perspectives in fundamental physics.
This work will provide innovative concepts for the compensation and reduction of turbulence effects on ground-GEO optical links, essential element for the development of future GEO-Feeder systems. These concepts can eventually be extended to other free space optics configurations (ground – LEO satellites or terrestrial links).
Engineering schools in Physics / Optics and / or Telecoms, or Master on these topics.
Compétences souhaitées : Optique/Physique, statistiques, télécoms, théorie de l’information
To apply, we invite you to contact the PhD/research supervisor and fill, with him/her, the co-financing part of the online application form (Reply to the offer) by April 1st, 2019.