Terabit-throughput satellite system technology
Digitalisation is a new revolution, which is transforming society, improving citizens’ quality of life and enhancing the efficiency of economic processes. Industry 4.0 and the Internet of Things are examples of this, and they both require broadband Internet connections. However, as latest statistics show, the global population as well as Europe are highly underserved with broadband connection. It is estimated that by 2020, these areas in Europe only will create an aggregated throughput demand of 3.8 Tbit/s. “Satellites play a key role in enabling global Internet access at high data rates everywhere,” explains Christoph Günther, Director of the DLR Institute of Communications and Navigation. The coverage area will be illuminated by numerous beams from the geostationary orbit (GEO) satellites, re-using radio frequencies and hence enabling a very high link capacity between users and the spacecraft. In order to fully exploit this capacity, the satellites must be connected to the Internet via so-called feeder link. This can be achieved using optical communications. Optical free-space communications transports the high-rate data streams in a similar manner to optical-fibre communications in ground-based backbone networks. The enabling technology in this respect is dense wavelength-division multiplex (DWDM), where the vast optical spectrum is split into numerous channels and in each of the channels multi-Gbit/s transmission is realised. All channels are then transmitted through atmosphere to the satellite, where they are again split and forwarded to users via standard RF links.
In frame of the project THRUST, the DLR's Institute of Communications and Navigation set itself a goal to demonstrate feasibility of technology capable of achieving such very high data rates in GEO-feeder link scenario. An optical terminal capable of reception of the optical wave and its coupling into the optical fibre had been developed. This is especially challenging with the atmospheric channel between the terminals and the thickness of the fibre of 10 micrometres (a fraction of a human hair).
In order to verify the system performance under realistic conditions, we scaled the demonstrator to a slant link between valley (where ground station was positioned) and mountain top (with satellite). The link was realised over 10km distance in region of Bavarian Alps between DLR location in Weilheim and DWD at Hohenpeißenberg. The atmospheric conditions in such link emulated an actual GEO satellite link with a very low elevation below 10 degrees.
During the project THRUST, not only DLR demonstrated the feasibility of the DWDM technology over worst-case atmospheric channel, but also two world-records were set: 1,72 Tbit/s transmission in 2016 [1, 2] and 13,16 Tbit/s in 2017  using commercial fibre-based communication systems. Parallel to these demonstrations, own coherent optical communications breadboard was developed . DLR employs phase modulation of the signal, which can be then detected coherently with much higher sensitivity and robustness against atmospheric distortions.
In the last stage of the project, adaptive optics (AO) system, similar to that used in astronomy was adopted to improve system performance by means of compensating the atmospheric effects. This is usually performed in the link from the satellite to the ground, when the distortion is larger than the telescope. DLR designed and integrated the AO system within the terminal so that the same correction can be used also to the transmitted beam from the ground, pre-compensating the atmospheric distortions and so to effectively improve the reception conditions at the satellite.
The project THRUST so shows the feasibility of optical communications as key enabling technology in future satellite communication systems for global connectivity. In frame of project THRUST, DLR has proven the technology in worst-case link conditions. The researchers have set the world-record for highest data throughput in free-space optical communications link twice. Moreover, we demonstrated high-fidelity optical coherent communications system performance and shown how the pre-distortion adaptive optics could be employed. DLR will further focus to demonstrate end-to-end performance targeting prototype of future system.
 J. Poliak, R. M. Calvo and F. Rein, "Demonstration of 1.72 Tbit/s Optical Data Transmission Under Worst-Case Turbulence Conditions for Ground-to-Geostationary Satellite Communications," in IEEE Communications Letters, vol. 22, no. 9, pp. 1818-1821, Sept. 2018. doi: 10.1109/LCOMM.2018.2847628
 P. Conroy, J. Surof, J. Poliak, and R. M. Calvo, "Demonstration of 40 GBaud intradyne transmission through worst-case atmospheric turbulence conditions for geostationary satellite uplink," in Applied Optics, vol. 57, no. 18 pp. 5095-5101, 2018.
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