Optical feeders for geostationary High Throughput Satellites (HTS) systems based on 1.55µm wavelength technology are expected to enable to transmit up to several terabits over one active link. A desirable option of transmission architecture is an optical feeder link transparent with respect to the user air interface. This can be implemented using either a digital or an analog modulation of the optical carrier. The digital option increases the optical bandwidth to be transmitted, however it benefits from error correcting codes, interleaving and framing which are efficient against atmospheric turbulence impairments. The analog option is more efficient concerning the optical bandwidth; however the atmospheric turbulence impairments can only be mitigated by a more complex optical ground terminal. Both analog and digital options could be feasible in the 2025-2030 time-frames but the digital option is more mature with respect to the atmosphere impairments mitigation techniques.

This paper reports on the assessment of the communication performance of DPSK-and OOK-based free-space optical (FSO) links in satellite-based applications, including LEO downlinks, LEO inter-satellite links, and feeder links for GEO Satellites. The performance is assessed by means of simulations as well as transmission experiments at 10 Gbps. The impact of optical filter bandwidth, optical delay-line-interferometer accuracy, frequency offset between transmitter laser and receiver, is quantified. OOK performance critically depends on transmitter extinction ratio, and proves robust to Doppler-induced frequency offsets. DPSK is sensitive to any laser wavelength drift and offset with the DLI response. Nevertheless, compensation of this drift is considered manageable, making it possible to maintain the 3dB advantage of DPSK vs. OOK in practical use conditions. INTRODUCTIONWith their potential for much higher capacity, smaller beam divergence, and lower terminal mass, volume and power, free-space optical (FSO) communication links are anticipated to dramatically exceed the performance of traditional radio-frequency-based solutions in an increasing number of satellite-based applications. In-orbit demonstrations so far were based either on simple On-Off Keying (OOK) with direct detection, or on Binary Phase-Shift Keying (BPSK) with complex homodyne detection 1 . Differential Phase-Shift Keying (DPSK) is compatible with optical demodulation and direct detection, thus offering the advantage of about 3dB higher noise tolerance at affordable added complexity 2 . Even though it has been considered for a long time 3 , only a few forthcoming FSO link demonstrations in space, namely LCRD 4 and JDRS 5 , will operate DPSK, and its effective suitability for other space applications is still under assessment. The present paper aims at evaluating the impact of critical parameters on DPSK transmission performance in satellitebased FSO communications, with a special attention paid to direct-to-the-Earth LEO down-links, inter-satellite links (ISL) in LEO constellations, and feeder links to high-throughput GEO satellites. The specific requirements deriving from the Doppler effect are taken into account. The theoretical performance of 10 Gbps DPSK-and OOK-based optical communication links, established by means of a commercial simulation software tool, are reported with Non-Return-to-Zero (NRZ) coding. Then, NRZ DPSK and OOK transmission experiments at 10 Gbps channel rate are reported and the results are compared. An experimental assessment of the critical link elements has been carried out and their influence on the Bit Error Rate (BER) performance has been quantified. Such parameters include optical filter bandwidth, optical delay-line-interferometer accuracy, frequency offset between transmitter laser and receiver. SATELLITE-BASED FSO COMMUNICATION APPLICATIONSFSO communications hold the potential to offer unrivalled advantages over RF counterparts, mostly stemming from the large difference in wavelength. Moving to higher carrier freque...

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To concurrently cope with the scarcity of RF frequency bands, the growing capacity demand and the required lower cost of the ground segment, Very High Throughput Satellites systems must rely on new technical solutions. Optical feeder links are considered as a promising alternative to surpass classical RF technology, offering assets inherent to optical technologies (large bandwidth, no frequency regulation, low beam divergence, components availability). Nevertheless the potential of this technology shall not conceal the remaining challenges to be overcome to make it relevant for operational missions: clouds, turbulence, power generation and high efficiency modulations. VERTIGO (Very High Throughput Satellite Ground Optical Link) is a 3-year H2020 project funded by the European commission and started mid-2019 focusing on the optical link itself regardless of site diversity aspect and aiming at demonstrating in ground demonstrations required technologies to implement very high capacity optical feeder links. In this paper we present the current status and perspectives of the project.

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