DLR: New world record in optical free-space data transmission
Eighteen months after achieving their previous record, researchers at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) - this time together with ADVA, a leading provider of open networking solutions - set a new record for data transmission using free-space laser communications - 13.16 terabits per second. At this data rate, the content of all printed the books in the world could be transmitted in approximately 30 seconds. The IP traffic in Germany in 2020, which is expected to grow to 144 petabytes per day, could also be supported. However, the actual goal is to provide broadband Internet via satellite to rural areas that are not connected to the terrestrial network infrastructure. For that purpose, according to the BATS study, an aggregate data rate of three to four terabits per second would be required to serve all of Europe.
Broadband Internet as a key for digitalization
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. "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 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. This is achieved using optical communications, as demonstrated in this test. Optical free-space communications transports the high-rate data streams in a similar manner to optical-fibre communications in ground-based backbone networks.
New world record in laser communications for satellites
The new world record was achieved through a collaboration between DLR and ADVA. DLR provided the concept and the optical systems required for successful atmospheric transmission. In the joint demonstration, the wavefront distortions of the lowest order - the angle of arrival of the wavefront at the receiver - were compensated. These wavefront distortions arise from temperature differences in the atmosphere. Everyone has experienced the shimmering effect above a hot road, often referred to as 'heat haze'. The same phenomenon also occurs during satellite transmission. The signal arriving at the two-centimetre-wide aperture of the receiver must be coupled into a fibre just a few micrometres in diameter (smaller than a human hair) and subsequently be amplified and processed. ADVA provided the optical wavelength-division multiplexing (WDM) terminals, which are commonly used in today's fibre-optic networks. The increase by nearly a factor of eight in data throughput with 40 percent less bandwidth, compared to DLR's 2016 record, was made possible by ADVA's FSP 3000 CloudConnectTM technology. The data was transmitted in 53 WDM channels on a 50 gigahertz channel grid. Each channel was modulated using dual-polarisation 16 QAM (Quadrature Amplitude Modulation) and received with soft-decision forward error correction at a payload data rate of 200 gigabits per second per channel. In addition, a 100 gigahertz channel with 100 gigabit per second QPSK (Quadrature Phase-Shift Keying) was used to obtain more detailed analysis of the distortions caused by atmospheric propagation.
The distance covered during the trials was 10.45 kilometres and is comparable - with respect to turbulence - to the 'worst-case scenario' connection between a ground station and a geostationary satellite.
Goal - high connection availability
High connection availability is required for telecommunications services. For this, it is necessary that the connection have no fades (short-term outages). Due to the very high data rates, even extremely short interruptions can lead to large data losses - an interruption of just one millisecond leads to the loss of gigabits of data. This lost data must either be reconstructed using complex algorithms or be retransmitted. The latter solution not only reduces the link capacity, but also increases the link latency and is therefore undesirable.
To mitigate fades, DLR also compensated for wavefront distortions of higher orders using adaptive optics in a follow-on demonstration. In this way, communication interruptions from the satellite to the ground could be further reduced. The acquired channel estimation was also used to predistort the signal from the ground to the satellite, achieving increased availability at Hohenpeißenberg (where the virtual satellite was located). In contrast to the equipment at Hohenpeißenberg, an actual satellite moves relative to the ground station. The changes resulting from this relative motion were established and the measurements again confirmed the theoretical predictions. It therefore became evident that the selected scenario and the experiments are optimal for testing such approaches. These methods, which are necessary to further increase the stability of the transmission and reduce the system complexity, will be further developed in the coming months.