Internet access has become essential for most economic activities. Furthermore, people have become more dependent on broadband Internet access in their personal life. In most urban areas, decent broadband access is available. Many initiatives aim at improving the situation in rural areas, but costs significantly increase with the distance from urban centers. The economic disadvantage in areas without broadband access – the so-called ‘digital divide’ – is expected to reinforce the migration towards urban centers. Preventing such migration on a global scale is thus an important task for society.
Satellites deliver promising solutions. They provide communications to entire continents with a minimal infrastructure and thus are an excellent complement to ground-based systems. Today, developments are evolving along two paths: on the one hand, towards constellations of several hundreds to several thousands of satellites in Low Earth Orbits (LEO) and, on the other hand, towards constellations of a handful of high-performance satellites located in Geostationary Earth Orbits (GEO). The institute contributes to both concepts.
Communications satellites need to connect the user to the satellite (user link) and the satellite to the Internet (feeder link). In the design of the user link one uses so-called spot beams that are as focussed on the users as much as feasible. This allows to reuse frequencies and to save power. With such technologies, large satellites can support aggregate data rates of several terabits per second. The same data rate must be transported through the feeder link. Today, feeder links are implemented
that use radio technologies similar to those of the user links. This requires a large number of ground stations, even at much lower data rates. The number of ground stations grows beyond reasonable limits in view of the large throughputs considered in the future. This is the motivation for the institute to work on the development of optical communication technologies, comparable to the fibre optics used in terrestrial transport networks.
Optical technologies open up a new dimension in data rates
The frequency windows in which the atmosphere is transparent is a thousand times wider in the infrared domain than at radio frequencies (RF). In addition, optical transmit and receive terminals are significantly smaller and lighter than their RF counterparts. Unfortunately, optical signals are absorbed by clouds and are strongly distorted by the atmosphere. An extreme case of distortion is seen in the heat shimmer above hot roads. Clouds may inhibit the use of optical free-space communications. In satellite communications, there is much freedom in the placement of ground stations. Thus one may select well-suited areas in order to take advantage of the benefits of optical communications, provided that the impact of atmospheric disturbances can be overcome. This is one of the tasks that the institute aims at solving.
Coding and information theory are key to efficient communications
This idea has gained acceptance since the 1960s. The institute’s non-binary LDPC codes are currently the best-performing codes for medium packet lengths. This type of packets is dominant in Internet traffic. Since the performance of these codes is getting very close to the theoretical limit, the focus of further developments is increasingly shifted towards reduced-complexity decoding.
Satellite connections are continuously subject to fading. With
data rates of one terabit per second and more, this implies that large quantities of data are lost in each fade. At the institute, appropriate packet-level codes have thus been developed. They mitigate this problem and were designed to ensure an extremely efficient decoding.
In general, many users aim at accessing the radio channels
simultaneously. This is called random access and was addressed by a number of protocols, which were all associated with a significant loss in throughput. These losses could gradually be eliminated in a series of significant contributions by ESA and DLR.
Today’s Internet was developed for remote access to computers and networks in an essentially static environment. Current applications
and infrastructures are no longer operated under such conditions. The institute is therefore working on new approaches that are better suited for current scenarios – including in particular applications in aeronautics, vessel control, rail and road transport, or disaster management.