Abstract—During the New Space era small countries are also
becoming more important players in the space business. While the
space activities are rapidly increasing, it is important to make
operations in a sustainable and safe way in order to preserve
satellite services for future generations. In this survey paper, we
discuss the multi-layer networking approaches in the 6G era
specifically from the sustainability perspective. We review the
most important regulations and international guidelines and
revisit a three-dimensional architecture vision to support the
sustainability target for a variety of application areas. We then
classify and discuss space safety paradigms that are important
sustainability enablers of future satellite communications. These
include space traffic management, debris detection,
environmental impacts, spectrum sharing, and cyber security
aspects. The paper discusses also advances towards a planned
European connectivity constellation that could become a third
flagship infrastructure along with Galileo and Copernicus
systems. Finally, we define potential research directions towards
the 2030s.
Index terms - Low earth orbit satellites, Radio spectrum
management, Aerospace engineering, Spaceborne radar
I. INTRODUCTION
HE two main disruptions driving the development and
rapid growth of satellite communications (SatCom) are
increasing satellite constellations sizes and integration of
satellite and terrestrial networks. The former also aims to
provide broadband services to currently underserved areas with
improved performance. The latter is related to the evolution of
mobile networks where different wireless and wired
technologies converge together. This creates vast amount of
new opportunities in different application fields such as public
safety, digital health, logistics and Internet services in
developing countries. The annual space business related to 5
th
generation (5G) and 6
th
generation (6G) of communication
systems is expected to grow to more than €500B during the next
two decades [1]–[3]. This is more than the whole space business
currently including scientific missions, earth observation (EO)
and navigations.
At the same time the whole space sector is in the
transformation phase due to so called New Space Economy.
Significant reduction of launch costs and easy and affordable
access to space have attracted new innovative players to space
business [4], [5]. Especially Low Earth Orbit (LEO) systems
Manuscript received Jan 05 2022; revised yy.yy, accepted zz.zz. This work
was supported in part by Business Finland through the 6G_Sat project.
Corresponding author: M. Höyhtyä.
M. Höyhtyä, S. Boumard, A. Yastrebova, P. Järvensivu, M. Kiviranta and
A. Anttonen are with the VTT Technical Research Centre of Finland Ltd, Oulu,
Finland (e-mail: firstname.surname@vtt.fi).
and small satellites are increasing rapidly. The most typical
orbit heights are above 500 km but there are significant efforts
to use also very low Earth orbits (vLEO) to provide sensing and
communications services. The so called Karman line, defining
where atmosphere ends and space begins, is above 80 km and
orbiting objects can survive multiple perigees passages at
altitudes around 80–90 km [6]. Small satellites in the range of
80-220 kg can be seen as a sweet spot [5] since they are large
enough for payloads to support e.g. broadband communications
[7]–[9] or synthetic aperture radar (SAR) imaging [10], [11].
A. Multi-Layer Networks
6G systems will be used to provide pervasive services
worldwide in order to support both dense and less dense areas.
To achieve this goal, 6G systems will need to integrate
terrestrial, airborne (drones, high-altitude platforms (HAPs))
and satellite communications at different orbits [12], [13]. This
means that in contrast to traditional research and development
(R&D) work, network analysis, planning and optimization will
be updated from two dimensions to three dimensions (3D),
where also the heights of communications nodes are taken into
consideration [12]–[15]. In this way, 6G networks will be able
to provide drastically higher performance to support e.g.,
passengers in ships and airplanes.
The initiatives spawned recently range from very high
throughput geostationary orbit (GEO) systems to unmanned
aerial vehicles (UAVs) [16]–[18] and small satellite systems
dedicated to machine-to-machine (M2M) and Internet-of-
things (IoT) services [19]–[21]. Especially interesting are
mega-constellations consisting of hundreds to thousands of
small and medium size satellites like those proprietary ones
envisaged by OneWeb, Starlink, Orbcomm and Telesat to
mention but a few. There is also ongoing active work in the 3
rd
Generation Partnership Project (3GPP) standardization to
define non-terrestrial networks (NTN) with interoperable
interfaces in order to have truly seamless connectivity in the
future, described in detail in Section V.B.
B. Space Safety and Sustainability
There are not only technical drivers in the development of
the multi-layer 6G networks. It is essential to develop services
and technologies in a sustainable way in order to ensure high
quality services also to coming generations. To mention a few
examples: 1) According to International Telecommunication
Union (ITU) only half of the world’s population has access to
broadband services above 256 kbits/s currently [22]. 2) The
COVID-19 pandemic has shown that video communications
provide means for people and businesses, including medical
