![INTELSAT, EUTELSAT, and INMARSAT annual revenues (left). Annual sales of VSAT terminals by type (right) [7].](https://www.researchgate.net/profile/Simone-Morosi/publication/332825285/figure/fig1/AS:1086063120515073@1635949034033/INTELSAT-EUTELSAT-and-INMARSAT-annual-revenues-left-Annual-sales-of-VSAT-terminals_Q320.jpg)
![Specific prices for two launcher categories [10].](https://www.researchgate.net/profile/Simone-Morosi/publication/332825285/figure/fig2/AS:1086063120515074@1635949034085/Specific-prices-for-two-launcher-categories-10_Q320.jpg)
![Number of small satellites in the last 15 years. [10].](https://www.researchgate.net/profile/Simone-Morosi/publication/332825285/figure/fig3/AS:1086063120515077@1635949034119/Number-of-small-satellites-in-the-last-15-years-10_Q320.jpg)
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Review Article
From Connectivity to Advanced Internet Services: A
Comprehensive Review of Small Satellites Communications
and Networks
Scott C. Burleigh,1Tomaso De Cola ,2Simone Morosi ,3Sara Jayousi,3
Ernestina Cianca ,4and Christian Fuchs 2
1Jet Propulsion Laboratory, California Institute of Technology, USA
2German Aerospace Center (DLR), Institute of Communications and Navigation, 82234 Oberpfaenhofen, Germany
3Information Engineering Department, University of Florence, 50139 Florence, Italy
4Department of Electronic Engineering, University of Rome Tor Vergata, Italy
Correspondence should be addressed to Simone Morosi; simone.morosi@uni.it
Received 27 December 2018; Accepted 18 March 2019; Published 2 May 2019
Academic Editor: Pham Tien Dat
Copyright © Scott C. Burleigh et al. is is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Recently, the availability of innovative and aordable COTS (Commercial O-e-Shelf) technological solutions and the ever-
improving results of microelectronics and microsystems technologies have enabled the design of ever smaller yet ever more
powerful satellites. e emergence of very capable small satellites heralds an era of new opportunities in the commercial space
market. Initially applied only to scientic missions, Earth observation and remote sensing, small satellites are now being deployed
to support telecommunications services. is review paper examines the operational features of small satellites that contribute to
their success. An overview of recent advances and development trends in the eld of small satellites is provided, with a special focus
on telecommunication aspects such as the use of higher frequency bands, optical communications, new protocols, and advanced
architectures.
1. Introduction
In the short span of the rst two decades of the new
millennium, a revolution has taken place in the eld of
satellite systems: the availability of innovative and aordable
COTS technological solutions and the ever-improving results
that are produced by microelectronics and microsystems
technologies have paved the way toa process of size reduction
for the satellite components and to the design of smaller and
smaller satellites that have been dened as small satellites
(whose weight is less or equal to kg), microsatellites
(from to kg), nanosatellites (1−10kg), and picosatel-
lites (0.1 − 0.99 kg) [].
ese technological trends have allowed new oppor-
tunities in the space market and the implementation of
long-awaited projects that have been postponed or sup-
pressed for years due to high inherent costs. More impor-
tantly, a new space rush has been originated by these
technological achievements with hundreds of small satellites
being launched in the last few years and even more envisaged
to be commissioned in the near future.
So far, the main drivers of small satellites develop-
ments have been Earth observation and remote sensing,
as they may greatly contribute to lling the gap of data
poverty in many industry verticals (e.g., agriculture, disas-
ter management, forestry, and wildlife). Nevertheless, new
investments in developing mega-constellations (hundreds)
of pico/nanosatellites [] for providing global communica-
tions, the increased role of satellites in Machine-to-Machine
(MM) communications [], and the interest in taking
advantage of one of the main possibilities enabled by small
satellites, which is the development of distributed systems
with interconnected satellites, are moving the attention also
towards the telecommunication aspects. erefore, this paper
provides an overview of recent advances and development
trends in the eld of small satellites, with a special focus on
Hindawi
Wireless Communications and Mobile Computing
Volume 2019, Article ID 6243505, 17 pages
https://doi.org/10.1155/2019/6243505
Wireless Communications and Mobile Computing
4500
4000
3500
3000
2500
2000
1500
1000
500
0
Revenue ($million)
1964
1966
1968
1970
1972
1974
1976
1978
1980
1982
1984
1986
1988
1990
1992
1994
1996
1998
2000
2002
2004
2006
2008
Ye a r
INMARSAT
EUTELSAT
INTELSAT
900,000
800,000
700,000
600,000
500,000
400,000
300,000
200,000
100,000
0
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
Consumer Internet
SME Broadband
enterprise
F : INTELSAT, EUTELSAT, and INMARSAT annual revenues (le). Annual sales of VSAT terminals by type (right) [].
the telecommunication aspects such as the use of higher-
frequency bands and optical communications, protocols, and
architectures.
It is worth outlining that some surveys about small
satellites have been published recently [–]: whereas []
focuses on the evolution of the antennas for small sats, []
concentrates on intersatellite link and related communica-
tion protocols for small sat constellations. On the other
hand, [] reviews the history of small satellite development
and summarizes its capabilities and applications. A rather
comprehensive review is provided in [], which deals with
many aspects, from hardware components and structures, to
network topology and communication protocols; moreover,
[] focuses on Cubesat class of small sats. With respect to pre-
vious surveys, this paper provides a more extensive overview
on telecommunication aspects and aims at describing this
rapid evolving eld, giving more insights into new protocols,
architectures, and technology developments.
e paper is organized as follows: in Section , a brief
history of the evolution of the small satellites is provided,
trying to unveil the commercial reasons of their success.
An overview of the services and applications which are
enabled by the small satellite is given in Section . Sec-
tion is devoted to a description of the evolution of the
payloads, focusing on the used frequencies and the So-
ware Dened Radio (SDR) concept. New telecommunication
architectures and the suitable protocols for small satellites
based systems are described in Sections and , respec-
tively. Finally, the perspectives and the open challenges are
discussed in Section and the conclusions are drawn in
Section .
2. A Brief History of Small Sats Evolution
From the dawn of the space era to their latest developments,
satellite communications have been one of the most reli-
able indicators of the technical and societal evolution: as
a matter of fact, in the last of few decades, amazing and
unexpected progresses and changes have been obtained in
the diverse elds of broadcasting, mobile communications,
Earth observation and remote sensing, interplanetary explo-
ration, transport, and remote monitoring, so encompassing
commercial, civil, and military applications []. However, it
is worth stressing that starting from the postwar times to the
today scenarios, satellite systems have undergone themselves
a radical and systemic evolution which proves the fact that
their abilities perfectly adapt to the ever-changing needs of
both the society and the market; particularly, while in the rst
decades, governments and national agencies were the main
players in the start of space race, in the design of satellite
missions, and in the development of satellite-based systems,
more recently private companies have largely increased their
role in this strategic industry [].
is trend has also been enforced by the privatization
of the main international satellite organizations which has
taken place at the end of the last century and produced
high revenues as shown by Figure []. As far as the
VSAT and broadband satellite systems are concerned, the
same trends of deregulation and stimulation of the market
forces have been experienced from their launch to the
nal successful spreading as reported by the graphs in
Figure [].
On the other hand, the end of the twentieth century has
also seen the birth and the rst steps of a new paradigm that
is based on the exploitation of the so-called small satellites,
whosesizeandweightaremuchsmallerthanthehuge
geostationary orbit (GEO) or the big medium Earth orbit
(MEO) and low Earth orbit (LEO) ones. ese new systems
are identied as micro-, nano-, and picosatellites according
to their dimensions []. e early missions of small satellites
were mainly organized and performed by research groups
of Universities and Research Organizations with the goals
of enabling a technology demonstration or an application
validation [].
Wireless Communications and Mobile Computing
Specic prices into SSO for two launcher categories
Source : Euroconsult’s 2016 Prospects for the Small Satellite Market
k$/kg
80
70
60
50
40
30
20
10
0Falcon 9
Soyuz Dnepr RockotPSLV Vega
Firefly
Rocket Lab
Virg in
Dedicated smallsat launchesShared smallsat launches
F : Specic prices for two launcher categories [].
Smallsat <500kg demand between 2010 2020
OneWeb
Cubesat constellations
Single cubesat
Single satellite mission >10 kg
Large EO
constellation >20#
Other constellation
Source : Euroconsult’s 2016 Prospects
for the Small Satellite Market
400
350
300
250
200
150
100
50
0
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
F : Number of small satellites in the last years. [].
More interestingly, these early attempts have paved the
way to a new philosophy which has been aimed at implement-
ing and exploiting very-low-cost satellites []. As reported
in [], the estimated production and launch cost for a single
small satellite can be approximately assumed to span from
100,000 to 200, 000 USD: particularly, in the case of shared
small satellite, the launch unitary costs per 𝑘𝑔 can lower
down to few kEuros, as sketched in Figure .
As a result, these unprecedented features of small satellites
have been favourably considered by market forces which have
been largely stimulated in the last thirty years and pushed
to start a new gold rush in space, with original objectives
and well-targeted applications such as Earth observation
and communications, in the civil, military, and commercial
dominions. Since the possible applications will be reviewed
in the following section, it is now important to provide some
rough numbers that can give the idea of how strong the new
race to the space exploitation is.
Overall, satellites (< 400𝑘𝑔) have been launched
between and , while the expectations for the succes-
sive ve years are targeted on other launches []: these
trends are shown in Figure . ese numbers are conrmed
by the launches of nano/microsatellites which have been
recorded in [].
Moreover, three constellations for satcom and Earth
observation accounted for % of the total and this share
should grow up to % in the next years driven by several
large projects. As a result, this analysis unveils the main
booster of the massive development of the small satellites: the
ability to relatively easily build a constellation. is peculiar
aspect will be considered in the following sections with a
specic focus on the intersatellite communications.
Finally, it is worth introducing the main player of the
massive increase of the number of small satellites: the Cube-
Sat. e CubeSat has been designed as the goal of a Stanford
University program which was started in to obtain a very
low-cost/weight satellite which could be quickly developed
andusedforeducationalpurposes[].Togetherwiththe
California Polytechnic State University, Stanford University
developed CubeSat specications with the goal to obtain a
customizable satellite, but with standard shape and weight,
in order to simplify launch and deployment operations. As
it is known, a CubeSat is made by one (U) or more (nU)
cm X cm X 10cm units, with a mass of up to . kg per
unit []. e nature of CubeSats has enabled the standardised
production of subsystems that can even be purchased as a
COTS product from online shops, so keeping the mission cost
very low [].
ese peculiar features of the CubeSat solution have been
very important in the fast increase of the small satellite
missions and in the huge development of companies whose
main core is in the new space market, such as Terra Bella
(formerly Skybox Imaging), Spire, Planet Labs, and OneWeb,
who are developing mega-constellations of small spacecras
in LEO orbit [].
3. Overview of Services and Applications
Aroundtheyear,theSmallSatswereabletoproperly
exploit innovative COTS technological solutions (hardware
and soware), achieving the ability to compete eectively and
to make prot. e successful growth of the modern Small-
Sats services, encompassing a large variety of application
contexts, shall be analysed also based on a new management
approach the small satellite organizations started to adopt: the
agile methodology. is paradigm comes from the IT indus-
try and it is based on a highly iterative design technique: well-
dened objectives, missions and requirements, incremental
changes to the design for a continuous improvement of the
system performance, short timescale, and reduced cost. Agile
approaches and the exploitation of the latest o-the-shelf
technologies represent the two main drivers of the New Space
Age [].
In the following, a brief overview of the main applications
and services of the SmallSats is provided. Since some of
the following acronyms may be unknown to the reader, a
comprehensive list is provided in Table .
(1) Earth Observation and Remote Sensing. So far, the primary
use for nano/microsatellites has been Earth observation (EO)
and remote sensing. e implementation of large satellite
constellations allows performing many simultaneous and
distributed measurements or observations (Earth resources
Wireless Communications and Mobile Computing
T : L i s t o f t h e a c r o nyms .
AcronymDenition Acronym Denition
ACM Adaptive Coding and modulation ADC Analog to Digital Converter
BP Bundle Protocol CCSDS Consultative Committee for Space Data Systems
COTS Commercial O the Shelf DLR German Aerospace Center
DSA Dynamic Spectrum Access DSP Digital Signal Processor
DTN Delay Tolerant Network DVB-S Digital Video Broadcasting - Satellite nd generation
EDRS European Data Relay System ELaNa Educational Launch of Nanosatellites
EO Earth observation ESA European Space Agency
FEC Forward Error Correction FPGA Field Programmable Gate Array
FSK Frequency Shi Keying GEO Geostationary Orbit
GPP General Purpose Processor GRACE Gravity Recovery and Climate Experiment
GSTP General Support Technology Programme HTS High roughput Satellite
HTTP Hypertext Transfer Protocol HW Hardware
ICN Information-centric networking IOT Internet of things
IP Internet Protocol ISS International Space Station
IT Information technology JPL Jet Propulsion Lab
LDPC Low Density Parity Check LEO low Earth Orbit
LTP Licklider Transmission Protocol LUCE LUnar Cubesats for Exploration
LUMIO Lunar Meteoroid Impacts Observer MAC Mean Access Control
MarCO Mars Cube One MEC Multi-Access Edge Computing
MEO medium Earth Orbit MMIC Monolithic Microwave Integrated Circuit
MM Machine-to-Machine NASA National Aeronautics and Space Administration
NC Network Coding NEA Scout Near-Earth Asteroid Scout
NFV Network function Virtualization OPALS Optical Payload for Lasercomm Science
OSIRIS Optical Space Infrared Downlink PICASSO Pico-Satellite for Atmospheric and Space Science Observations
PRETTYPassive REecTomeTrY QARMAN QubeSat for Aerothermodynamic Research and Measurements on Ablation
QKD Quantum Key Distribution (QKD) QoS Quality of Service
QoE Quality of Experience SCAN Space Communications and Navigation
SDN Soware Dened Networking SDLS Space Data Link Security
SDR Soware Dened Radio SOTA Small Optical Transponder
SW Soware TBIRD Terabyte Infrared Delivery
TCP Transmission Control Protocol TTL Time-To-Live
UHF Ultra High Frequency USD US Dollar
USLP Unied Space Link Protocol VHF Very High Frequency
VLC Visible Light Communications VMMO Volatile and Mineralogy Mapping Orbiter
monitoring, weather monitoring, and disaster monitoring)
with an increased temporal resolution of collected data (i.e.,
shorter revisit times) [].
A more extensive use of small satellites for EO and remote
sensing calls for higher and higher datarate links to download
the acquired information in a short time.
(2) Science and Technology Demonstration Missions. Micro-
and nanosatellites enable a wider access to space and rep-
resent an aordable test for young engineers and scientists
to prove prototype systems and experience the idea of
a future satellite. To this aim, NASA created the NASA
Educational Launch of Nanosatellites (ELaNa), an initia-
tive oriented to students of several disciplines (science,
technology, engineering, and mathematics). A number of
ESA CubeSat missions have been funded under the In-
Orbit Demonstration part of the General Support Tech-
nology Programme (GSTP): GOMX- and GOMX-B for
demonstrating new capabilities of nanosatellites, QARMAN
(QubeSat for Aerothermodynamic Research and Measure-
ments on Ablation) for demonstrating re-entry technologies,
PICASSO (Pico-Satellite for Atmospheric and Space Science
Observations) for the analysis of the ozone distribution in the
stratosphere, the temperature prole up to the mesosphere
and the electronic plasma characterization in the ionosphere,
RadCube for real-time monitoring of the cosmic radiation
and space weather environment, and PRETTY (Passive
REecTomeTrY), a nanosatellite to measure and register ice
on the glaciers or on the poles and wave movements of the
oceans.
Wireless Communications and Mobile Computing
58%
26%
12%
4%
50%
16%
10%
22%
3%
Historical
(2013 – 2017)
Communications
EO/RS
Scientic
Future
(2018 – 2022)
Technology
Novel Applications
EO/RS
Scientic
Technology
Communications
F : Nano/microsatellite market forecast, th edition, approved for public release, SpaceWorks Enterprises, Inc. (SEI), .
(3) Interplanetary Exploration Missions. Small Satellite Plat-
forms have led to a new era of space exploration espe-
cially thanks to new enabling technologies and new highly
capable launch vehicles, which open many opportunities for
future lunar and planetary exploration. National Aeronautics
and Space Administration (NASA) and European Space
Agency (ESA) have adopted the Interplanetary CubeSat
Model, supporting missions and studies, ranging from Mars
and Lunar observation to the study of meteoroids and
asteroids. Some of them are MarCO (Mars Cube One)
[], NEA Scout (Near-Earth Asteroid Scout) [], LUCE
(LUnar Cubesats for Exploration), LUMIO (Lunar Mete-
oroid Impacts Observer), VMMO (Volatile and Mineralogy
Mapping Orbiter), Lunar Flashlight [], and Arkyd series
[].
(4) Communications Services. Small micro- and nanosatellites
organized in constellations can be used for providing data
distribution (broadcasting applications) and data exchange
(Internet of things and Machine-to-Machine paradigm) and
also for extending the Internet access to the entire Earth [].
According to Space Works Market outlook, in the next years,
communication constellations of micro- and nanosatellites,
which are now in the technology demonstration phase,
will be used to serve and support the rapidly growing
Internet of things (IOT) and Machine-to-Machine (MM)
market.Sky&SpaceGlobal,KeplerCommunications,Hiber
(Magnitude Space), Helios Wire, Astrocast, Blink Astro,
Fleet Space, and Myriota are some of the main commu-
nications operators oering IOT/MM and data relaying
services.
An overview of the nano/microsatellite trends by appli-
cation in the near term is provided in Figure [].
Although the analysis highlights that the primary use
for nano/microsatellites will remain Earth observation and
remote sensing, an increase of communications constella-
tions is expected. SpaceWorks estimates that about com-
munications nano/microsatellites will require launch over the
next years.
(5) Commercial, Civil, and Military Applications. Tran s p or t ,
smart environments (including remote monitoring), quality
of life, safety, and security represent the main application con-
texts of the adoption of small, micro-, and nanosatellites [].
As examples of commercial constellation of nanosatellites,
Aerial & Maritime and Sky & Space Global are two GomSpace
commercial missions: the former is oriented to aircra and
vessel tracking for situational awareness, while the latter
will provide a global communication infrastructure in space.
Moreover, Astrocast has a project for oering global MM
services as remote monitoring, geolocalization, intelligent
data collection, and predictive maintenance [].
Figure shows the SpaceWorks analysis on the Nano/
Microsatellite Operator Trends: Military Operators (aiming
to support national defense activities), Commercial Opera-
tors (whose purpose is prot revenue generating activities),
Wireless Communications and Mobile Computing
0%
25%
50%
75%
100%
Historical (2013 - 2017) Future (2018 - 2022)
Percentage of Satellites
Civil
Military
Commercial
Commercial operators
are expected to
encompass over 70%
of nano/microsatellites
launched in the next 5
years
Despite rapid
commercial market
share growth, civil and
military operator
demand is expected to
remain consistent over
the next 5 years
F : Nano/microsatellite market forecast, th edition, approved for public release, SpaceWorks Enterprises, Inc. (SEI), .
and Civil Operators (nonmilitary or non-prot activities) are
considered
4. Payload Evolution
In the early implementation of small satellites, mainly used
as a platform for university and technology development
projects [], the payload was supposed to perform very
simple operations such as transmission of a beacon, storing
data or transmitting data collected by simple sensors at very
low data rate ( to .kbps). Amateur frequencies at UHF were
mainly used and operated via the standard AX. []. At
such low frequencies, wire antennas (dipoles, monopoles, and
helical) are especially common as the wavelength is long and
achieving good radiation eciency within a small volume is
challenging. A considerable number of the CubeSats that are
currently in space use wire antennas for their simplicity of
implementation. Moreover, the omnidirectionality of dipoles
makes them viable candidates for intersatellite communi-
cations. Emerging applications and the associate need for
transmitting at higher data rate or performing more complex
tasks, keeping low mass and weight, have raised the need for
larger bandwidths and higher-frequency bands, an increasing
request of digital implementation and SW control.
4.1. Evolution in the Frequency Bands and Antennas. In
recent years, the use of higher frequencies than the common
VHF/UHF bands such as S-Band (mainly for telemetry) and
X-Band (for data transmission) has become more widely
available thanks to the advent of commercially available
Monolithic Microwave Integrated Circuits (MMICs). e
shi towards higher-frequency bands implies other require-
ments on the spacecra design, mainly on the power system
and the antennas. For instance, already at frequencies higher
than S-band, the eciency of solid-state high-power ampli-
ers drops from % (at UHF) to %. At such frequencies,
most common antennas are still wire antennas or planar
antennas, such as patch and slot antennas. Patch antennas
have gained special attention for CubeSats, as they are
relatively easy to fabricate. A variety of patch antenna designs
have been investigated at S bands. Downlinks on S-Band
would be expected to be able to implement data rates from
kbpstoMbps.Largerdataratesrequiretheuseofhigher-
frequency bands such as Ku-, K-, and Ka-band, which are the
state of the art for large spacecras, but they are still young
technologies in the small satellite world. A Ka-band trans-
mitter on a CubeSat began orbital operations in []. At
such higher frequencies, it is p ossible to implement also high-
gain reector antennas which can meet the strict size and
weight requirements of a small satellite. Reectarray antennas
are also very suitable, as they provide high gain and can be
easily integrated with the CubeSat structure. eir structure
consists of at panels, which can be folded and stowed on
the CubeSat []. In [], the development of a reectarray for
the Mars Cube One (MarCO) is described. MarCO is the rst
CubeSat mission designed for Mars operation. e frequency
of operation in this case was . GHz, with a measured gain
of . dB.
As a matter of fact, small satellites also represent a viable
and cost-eective way to test new frequency bands for satellite
communications (both in terms of HW components and
propagation channel), such as W-band []. e investigation
of such high-frequency bands is mainly motivated by the
need of bandwidths in High roughput Satellites []. On
the other hand, those frequencies could be an interesting
option for intersatellite links of small sats []. At Q/V and
W-band, horn antennas can be a viable option for small sats
as they provide good gain and could be fabricated also for uni-
versity experiments []. A potential horn design that could
Wireless Communications and Mobile Computing
F : Le: CAD-model of OSIRISCubesat terminal. Right: Artist’s impression of terminal integration in U Cubesat [].
be considered for future Ka-band CubeSat communication is
discussed in [, ].
e need of higher data rates, low cost, and small size
has also moved the attention towards FSO communications,
especially for intersatellite links as presented in the next
section.
4.2. Laser Communication Terminals for Small Satellites and
CubeSats. In recent years, free-space optical communica-
tionshavebecomeamaturealternativetotraditionalRFcom-
munication systems. With the use of laser communication
terminals in systems like the European Data Relay System
(EDRS) [] for intersatellite links, the technology has passed
the barrier from research to the operational application.
Concerning downlinks from satellite to Earth, a number of
demonstrations have been performed in recent years, like
the Small Optical Transponder (SOTA) experiment [] of
the National Institute of Information and Communications
Technology, which used a dedicated satellite, or the Optical
Payload for Lasercomm Science (OPALS) experiment [] of
the NASA’s Jet Propulsion Lab (NASA-JPL), which demon-
strated optical downlinks from the International Space Sta-
tion (ISS). e Aerospace Corporation demonstrated an
optical downlink from a .U-CubeSat []. Even optical links
from the Moon to Earth have been demonstrated [].
A number of further demonstration missions are cur-
rently planned, such as NASA’s Terabyte Infrared Delivery
(TBIRD) mission, aiming at demonstrating a 100 Gbps link
from a CubeSat to ground [], or within the Optical Space
Infrared Downlink (OSIRIS) programme of the German
Aerospace Center (DLR), which aims at demonstrating opti-
cal downlinks from small satellites and CubeSats to Earth
[].
Practical implementations of current optical commu-
nication systems for small satellite applications may reach
data rates of about 10 Gbps with a terminal weight in the
order of 5kg and a power consumption of about 50 W. For
applications on CubeSats, Figure shows OSIRIS CubeSat
implementation as an example. e terminal weighs in
the order of 300g, consumes 8Wofelectrical power,and
requires only .U of space within the CubeSat. It reaches a
data rate of 100Mbps. OSIRISCubeSat will be oered on the
market by DLR’s commercialization partner Tesat Spacecom
under the market name CubeL.
An important challenge in optical satellite-to-ground
communications is the limited availability due to clouds. is
can be overcome by employing a world-wide network of
optical ground stations. By using a sucient buer memory
onboard the satellites, this enables overcoming the issues
due to limited availability of the space-to-ground link [].
Although most optical ground stations available to date have
been developed mainly for research purposes, both new and
established ground segment operators have expressed strong
interest in building up the required infrastructure. us, it
is only a matter of time until optical links can be used in an
operational manner, even in small-satellite applications.
4.3. Towards SDR Payloads. Since the early development
of small satellites, one trend in the payload design can
be identied: privileging the use of low-cost COTS and
in general HW components and moving towards a digi-
tal implementation. anks to the availability of modern
high-speed and low-power digital signal processors and
high speed memories, the trade-o between the HW/SW
implementation is moving more and more towards the SW
implementation and the concept of SDR. SDR is an evolution
of exible and recongurable payloads. An early adopter
of recongurable technology for space applications was the
Australian FedSat microsatellite communications payload
launched in . e FedSat communications payload uti-
lized Field Programmable Gate Array (FPGA) components
for baseband digital signal processing and included a code
upload mode allowing it to be reprogrammed while in orbit
[]. e evolution from recongurable and reprogrammable
devices to SDR has been driven by the demand for exible
and recongurable radio communications in support of
military and public safety operations and it has been pushed
by advances in the enabling technologies such as Analog
Wireless Communications and Mobile Computing
to Digital Converters (ADCs), General Purpose Processors
(GPPs), Digital Signal Processors (DSPs), and FPGAs. SDR
payloads are considered as a needed technological step in
traditional satellite systems for assuring a longer lifetime and
a more ecient resource utilization [], even if so far, few
SDR payloads have own on big satellites. For small satellites,
which are designed with few years of lifetime in mind, the
reason for moving towards SDR payloads is mainly related to
the oered exibility to adapt to new science opportunities
and potentially reducing development cost and risk through
reuse of common space platforms to meet specic mission
requirements. SDR can be used to support multiple signals,
increase data rates over reliable intersatellite and ground links
to Earth, and also help in facing the shortage of available
frequencies for communications in the more crowded bands.
As a matter of fact, the use of an SDR approach also allows
implementing Dynamic Spectrum Access (DSA) techniques
and hence a more ecient spectrum utilization. To date, no
satellite application of DSA is in use, although companies
such as Tethers Unlimited (U.S.), with funding from NASA,
are looking at upgrading SDR platforms with advanced
cognitive radio.
e challenge of this more digital approach is related
to one of the strong limitations of a small satellite: power
consumption. For this reason, FPGA has been preferred so
far,especiallyforhigherdataratesintheX-andKa-band,
as they allow performing compute intensive tasks in parallel
and use more eciently every clock cycle []. Additionally,
modern FPGAs have embedded processing systems, such
as ARM cores, integrated inside the FPGA. Few SDRs
have already own in small satellites and other are under
development, e.g., AstroSDR, NanoDock SDR, GAMALINK,
and STI-PRX-. It is denitely a hot topic of research and
development, and there is growing interest in developing and
testing new solutions. Pinto et al. [] exploited SDR in small
satellite systems to design an intersatellite communication
model which could be easily recongured to support any
encoding/decoding, modulation, and other signal processing
schemes. In [], a novel SDR architecture on an embedded
system is proposed, whose potential applications are the
ground station for multisatellite communications, deployable
mobile ground station network, and can be further extended
to distributed satellite system.
A new generation of SDR technologies have been inte-
grated in the SCAN Testbed (Space Communications and
Navigation Testbed), which is an advanced integrated com-
munications system and laboratory facility to be installed on
the International Space Station (ISS), to develop, test, and
demonstrate new communications, networking, and naviga-
tion capabilities in the space environment []. e SCAN
Testbed consists of recongurable and reprogrammable SDR
transceivers/transponders operating at S-band, Ka-band, and
L-band, along with the required RF/antenna systems neces-
sary for communications.
5. New Telecommunication Architectures
Small satellites are playing an increasingly important role in
telecommunication architectures in two main ways:
(i) ey are increasingly used to form application-
focused segments of the infrastructure supporting
existing communication architectures, notably the
Internet.
(ii) ey also form and/or utilize altogether new, distinct
communication architectures.
5.1. As Supporting Infrastructure. e use of Earth-orbiting
satellites to conduct Internet trac is of course not new.
From TELSTAR in through Iridium, Globalstar, ViaSat,
and EchoStar, the market for relaying data via radio links to
satellites has grown rapidly. But historically, those satellites
have been large and expensive, whether operating in LEO
orGEOorbits.Whatisnewistheuseoflargenumbersof
small satellites for this purpose. e eld has grown rapidly
in recent years as new concepts are proposed; many of them
highly ambitious:
(i) e OneWeb constellation is initially expected to
comprise small Internet service delivery satellites
in LEO orbit, potentially growing to satellites
[].
(ii) Samsung has proposed a -satellite constellation,
projected to be able to carry one billion terabytes of
Internet data per month [].
(iii) e SpaceX corporation’s Starlink constellation is
envisioned to comprise up to , small satellites in
LEO orbit, with the capacity to carry up to % of local
Internet trac in densely populated areas [].
5.2. As Participants in New Architectures. In addition to
supporting the propagation of trac within the Internet,
however, small satellites require new, increasingly capable
telecommunication architectures to sustain their own oper-
ations. Coordination among satellites in LEO orbit relies
on cross-links between satellites, relay services provided
by ground stations (typically via the terrestrial Internet),
or a combination of both. is capability is critical for
constellations such as the GRACE (Gravity Recovery and
Climate Experiment) mission and the QB- project.
Moving farther, the twin MARCO spacecras (each a U
CubeSat) accompanying the InSight spacecra on its mission
to Mars will primarily serve to relay information from the
InSight lander to its mission operations center on Earth, while
the lander is engaged in entering the atmosphere of Mars,
descending to the surface, and landing. As shown in Figure ,
the link from InSight to each MARCO orbiter will be in
the UHF band, while the MARCO vehicles communication
with Earth will be by X-band radio transmission. Each
MARCO can use only one of these links at a time, so
the communication architecture will be very dierent from
the continuous end-to-end connectivity that characterizes
Internet trac.
Projecting that deviation from the Internet trac model
back to high-volume terrestrial communications, a satellite
communications architecture that is designed to tolerate the
associated delays in end-to-end communication on a large
scale has been proposed. e “Ring Road” architecture []
Wireless Communications and Mobile Computing
F : e MARCO communication architecture. Image credit:
NASA/JPL-CalTech.
D
B
E
A
2
3
4
1
“cold spot” (no Internet connectivity)
“hot spot” (connected to Internet)
“courier” (nanosatellite, a “data mule”)
C
non-RingRoad node on Internet
Y
non-RingRoad node on local area net
X
F : e Ring Road network architecture [].
is based on the use of delay-tolerant networking (DTN)
protocols [], discussed later. e basic idea is to deploy,
gradually, one satellite at a time, a constellation of DTN
Bundle Protocol (BP) [] routers in LEO orbit. As shown
in Figure , the network encompasses three classes of DTN
nodes:
(i) Router satellites, called “courier” nodes, in polar orbit
(ii) Nodes residing in computers that are attached to the
Internet, called “hot spots”
(iii) Nodes residing in computers that are highly isolated,
with no electronic connectivity, called “cold spots.”
e constellation operates as follows:
(i) A user at a cold spot node issues data in a bundle (such
as an email message or an HTTP proxy query). e
node queues the bundle up for transmission to the
next courier that ies overhead [].
(ii) Eventually, a courier ies over the cold spot. e
courier’s orbit is well known, so the contact between
the courier and the cold spot can be scheduled far in
advance. e courier and cold spot begin communi-
cations using BP over LTP (Licklider Transmission
Protocol []; see more details in the next section)
over whatever radio frequencies are available. Bundles
from elsewhere that are destined for this cold spot
called “forward trac” are transmitted from the
courier to the cold spot node for forwarding within
the local network, if any. Bundles issued from the
cold spot called “return trac” are transmitted to the
courier and queued on-board forfuture transmission.
(iii) e courier computes a route for each bundle it
receives from the cold spot. It knows about its own
future scheduled contacts, so any bundle that is
destined for some other cold spot that the courier will
reach before the bundle’s TTL (Time-To-Live) expires
is queued for future transmission to that cold spot. All
other bundles are queued for transmission to the next
hotspotthecourierwillyover.
(iv) When the courier ies over a hot spot, the queued
bundles are transmitted to the hot spot and the
courier receives bundles that the hot spot node has
queued for transmission to that courier.
(v) When a hot spot node receives bundles from a courier,
it computes a route for each bundle. If the bundle’s
destination endpoint is directly reachable via the
Internet (e.g., a database server in Montreal), then
the hot spot uses BP over TCP/IP to send the bundle
immediately to that endpoint. Otherwise, the hot spot
consults the contact schedule to determine which
courier has the earliest scheduled contact with the
destination cold spot and then reconsults the contact
schedule to determine which hot spot has the earliest
scheduled contact with that courier. If the rst hot
spotthatwillseethatcourieristhelocalhotspotitself,
then the hot spot simply queues the bundle locally for
future transmission to that courier; otherwise, it uses
BP over TCP/IP to send the bundle immediately to
that computed best-way-forward hot spot.
(vi) When a hot spot node receives bundles from some
node in the Internet (possibly another hot spot), it
computes a route for each bundle as above. When a
courier ies overhead, it exchanges bundles with the
Wireless Communications and Mobile Computing
courier. When the courier subsequently ies over a
cold spot, it exchanges bundles with it in the same
way, and so on.
e concept oers a number of advantages:
(i) Unlike a crosslink-based routing-fabric constellation,
there is no need to orbit the whole constellation all
at once in order to get data moving. e network
could begin with one hot spot, one cold spot, and
one courier. In that case, the round-trip time for
the cold spot would be very long as there would
be only one contact per N orbits of the satellite,
where N is however many orbits would be needed
to bring the cold spot back into the satellites ground
track. Nonetheless, bidirectional data ow between
the cold spot and any point on the Internet would
be reliably supported, albeit at very low eective data
rates. As more satellites are added, the frequency
of coverage of any given cold spot increases and N
drops, which increases the carrying capacity of the
network as a whole (the aggregate storage capacity
of all the couriers), so that the number of cold spots
supported can increase. Adding more hot spots on
the ground would also incrementally increase the
carrying capacity of the network, by enabling earlier
drainage of the return-trac bundles in couriers’ on-
board storage and thereby making room for more
bundles, which would further increase the number of
supportable cold spots you could support.
(ii) While the routing is somewhat complex, it takes place
in potentially powerful ground-based computers at
hot spots, not in the courier satellites. is means
that small, mass-produced satellites can be suitable as
couriers.
(iii) All elements of the architecture are, therefore, rela-
tively inexpensive.
As a conclusion, this SmallSat-based architecture could
enable very widely available network data service at low cost,
starting with a very modest initial investment.
5.3. Integration with Terrestrial Architectures. e potentials
oered by Small- and CubeSats constellations from a service
point of view have to be analysed from a wider angle
in order to consider the data availability from dierent
stakeholders. In the case of processing centres placed nearby
control centres or in any case directly connected to them via
dedicated terrestrial infrastructure, the architecture design
may essentially consist in the extension of the exemplary one
illustrated in the previous subsections. is can be achieved
by terminating the proposed DTN architecture directly at the
processing centres or by making use of specialised gateways
capable of interfacing native DTN architectures with non-
DTN aware counterpart (i.e., in the case of legacy networks
building on pure TCP/IP protocol architectures).
On the other hand, the increasing interest towards the
service provided by small satellite constellations may result in
distributing data to enterprises, universities, schools, public
authorities, and single users for dierent applications (e.g.,
space data exploitation, education purposes, surveillance and
monitoring, etc.). In this context, data retrieval will likely
happen over Internet terrestrial infrastructure, hence calling
for proper integration strategies to be deployed between
the ground segment of the small satellite system and the
core terrestrial network. is integration task can be easily
considered in the broader plan of converging satellite and
G networks [] (and papers included in that special issue),
which has recently become a hot topic for the satellite
industry. Without entering the details of the architecture
proposals [] elaborated to meet this goal, it is of pivotal
importance to provide a exible integrated architecture.
Network exibility is indeed recommended in order to
ensure proper coexistence of existing Internet ows and
small satellite data retrieval, which may be regarded in terms
of dierent network slices, each characterised by diverse
QoS/QoE characteristics. To this end, the implementation
of proper SDN (Soware Dened Networking) and NFV
(Network Function Virtualization) solutions is desirable, so
as to achieve also the “sowarisation” of the satellite network,
whose understanding is however still not complete and will
deserve additional studies for the case of small satellite
constellations.
Still related to the objective of distributing small satellite
data across the Internet is providing the network architecture
with content-oriented functions in order to dierentiate
QoS management and routing functions applied to the data
objects obtained from the small satellite systems. is may
suggest the application of the existing Information Cen-
tric Networking architectures [], whose baseline concept
should be however adapted in order to meet the content
characteristics of the data objects retrieved from the satellite
systems and to interface with the network architecture (e.g.,
DTN-based) proposed for the satellite network (as illustrated
in the previous subsection).
In more detail, ICN-based architectures build on publish-
subscribe (pub-sub) paradigms, so that users subscribe to
content distributions services and accordingly contents are
distributed upon request reception. One of the main peculiar-
ities of ICN networks is in that contents are explicitly mapped
to object names, which enable more advanced content-aware
routing and security schemes. Moreover, this approach helps
implement a content-centric networking approach, hence
superseding the typically employed host-centric approach
(i.e., as implemented in IP-based systems), where locations
and content descriptions are mapped into a unique iden-
tier (e.g., IP address), hence posing some limitations on
implementing content-based networking functions. Another
intrinsic key advantage of ICN networks is to implement
distributed caching functionalities throughout the entire
network, hence possibly simplifying the integration of MEC
(Multi-Access Edge Computing) and Cloud Computing func-
tionalities, which are pivotal building blocks in the modern
communication networks.
ICN functionalities are typically supported by spe-
cialised networking elements, i.e., ICN routers, which can
be deployed not only in terrestrial networks, but also in the
space counterpart, provided that satellites oer the necessary
Wireless Communications and Mobile Computing
End-Users
MEC/Cloud Computing
Satellite constellation
Message Formats
Message Formats Message Formats
Message Formats
Message Formats
Content
request
Content
retrieval 5G Network
ICN/DTN
terrestrial
router
ICN/DTN
space router
F : Integrated satellite-G network based on ICN/DTN concepts for content delivery [].
storage and computing capabilities. As a matter of fact, the
coexistence of DTN- and ICN-based protocol architectures in
the same network deployment is possible in order to exploit
the main advantage oered by the two with respect to disrup-
tion resilience and caching, although specic adaptations of
the protocol interfaces are necessary (not treated in this paper
as beyond the scope).
In general, the overall network architecture encompass-
ing G and satellite segments, building on ICN/DTN archi-
tectures, and interacting with MEC and cloud computing
elements is exemplied in Figure , where the case of a
satellite constellation complementing a G access network to
boost the content delivery is sketched.
6. Advances in Communications and
Network Protocols
New protocols for communication with and among small
satellites have emerged rapidly in the past decade. e new
capabilities are provided at multiple layers of the protocol
stack.
6.1. Physical Layer. Originally, the only communication links
supported for CubeSat satellites were UHF links operated
via AX.. Given the low requirements in terms of data rate
of most of the original, mainly scientic, missions, simple
modulation schemes have been used, such as binary-FSK [,
]. It is also worth outlining that the AX. protocol allows
detecting errors but not correcting them. e emerging
need for transmitting at higher data rate and keeping low
mass and weight is pushing to use larger bandwidths and
higher-frequency bands, as reported in Section , but also
to use more eciently the available bandwidths through
more advanced modulations schemes. Moreover, the shi
towards SDR payload and ground stations, made possible by
the rapid evolution of digital electronics, opens the opportu-
nity to implement more advanced communication protocols
and modulation schemes, including error correction capa-
bilities and dynamic adaptation of modulation parameters
Wireless Communications and Mobile Computing
depending on the current link conditions []. is has
motivated some theoretical studies on the choice of the most
appropriate modulations [, ]. However, there are already
some innovative transceiver designed for CubeSat and Small
Satellites using higher-frequency bands such as X-band up to
Ka-band, implementing Variable and Adaptive Coding and
Modulation(VCM,ACM)capabilities[,].Forinstance,
RADIOSAT is an innovative transceiver developed by ESA,
working at a Ka-band and integrated with a DVB-S modem,
overall characterized by low power consumption. On the
design of intersatellite link, it is worth mentioning the recent
studies on the use of Visible Light Communications (VLCs),
which can provide higher data rates with smaller, light-
weight nodes, while avoiding the usual interference problems
associated with RF, as well as the apparent radio spectrum
scarcity below the GHz band. Furthermore, the electronics
required for achieving precision pointing accuracy for laser
communication systems will be avoided. With approximately
THz of free bandwidth available for VLC, high capacity
data transmission rates could be provided over short dis-
tances using arrays of LEDs [].
6.2. Link Layer. While operators of Earth-orbiting CubeSats
initially had few options beyond AX., new and more
capable protocols that are suited for space ight operations
at Earth and beyond Earth orbit are becoming available.
e new CCSDS Unied Space Link Protocol (USLP) []
is designed to be adaptable to a very wide range of space
data transmission conditions. It includes a “virtual channels”
concept that enables a single physical link to be transparently
shared among multiple data streams at higher layer, together
with further multiplexing accommodation multiplexer access
points that enable multiple data services to share a single
virtual channel. It also provides mechanisms for aggregating
small service data units and segmenting those aggregations,
for extensive control over the sizes of protocol data units.
CCSDS has also dened a security service at the link
layer, called Space Data Link Security (SDLS). Security is
rapidly becoming an urgent concern of space ight mission
designers, as security breaches at ground stations and mission
operations centers served by the Internet grow ever more
troublesome. SDLS provides a security standard for simple
space ight missions, where a single spacecra is in contact
with its control center through a ground station. It includes
data origin authentication, connection and connectionless
condentiality, connection integrity with and without recov-
ery, and connectionless integrity.
6.3. Network Layer. DTN concepts date back to the early
days of the Interplanetary Networking Research Group of the
Internet Research Task Force. DTN is a network architecture
that is aimed at eking as much data communication as
possible out of inhospitable networks—in particular, those
where link interruptions (whether anticipated or not) are
frequent and signicant and/or where signal propagation
latency is high. e eects of high delay and of disconnection
are in fact similar in many ways, and the network architecture
features developed for DTN serve to mitigate both. e
central fact in both circumstances is the potential inability
of each network node to request timely assistance from
any other, for any purpose, and at any given moment. e
unifying principle in the design of the features of DTN, then,
is recognition of this fact. Nodes must be able to make their
own operational decisions locally, on their own, with global
information that may well be stale or incomplete, and the
network must be able to continue to operate at some useful
level even when these decisions are awed. e core protocol
of DTN is BP [], a network-layer protocol that functions
as the DTN analog to the Internet Protocol. BP is similar to
IP in that a BP node receives data issued by an application
entity, stores the data in some medium, and forwards the data
through the network toward the node serving the application
entity that is the destination of the data. It principally diers
from IP in that a forwarding node does not immediately
discard data items (called “bundles”) for which no onward
communication link is currently available; instead, it may
store bundles for a lengthy period of time, waiting for a
link to become available. e DTN analog to the Internet’s
TCP is the LTP []. An LTP “engine” divides an outbound
bundle into small “segments” and transmits the segments to
the LTP engine serving the BP node that BP has determined
to be the best next proximate destination for the bundle
the next step on the bundle’s end-to-end path. Both LTP
and TCP account for transmitted data, detecting data loss
and automatically recovering from that loss by retransmitting
segments as necessary. e principal dierence between LTP
and TCP is this:
(i) In TCP, the entity that discovers and reports data
loss is the TCP instance serving the application entity
that is the destination of the data, and the data
loss is reported to the TCP instance that serves the
application entity that was the source of the data. at
is, retransmission is “end-to-end” and TCP is situated
above IP in the Internet protocol stack.
(ii) In a space ight mission scenario, end-to-end retrans-
missions could result in extremely lengthy delays in
data delivery because the source and destination of
data might be on dierent planets separated by many
light minutes of propagation latency. In LTP, data loss
is instead reported to the LTP instance at the prox-
imatesourceofthedata(theimmediatelypriorBP
node on the end-to-end path), which retransmits the
lost segments as early as possible. LTP retransmission
is “point-to-point” within the network, and LTP is
situated below BP in the DTN protocol stack.
Complementary to the use of DTN protocol solution is
the exploitation of network coding (NC) [] for improving
the robustness of data transmission as well as optimised use
of the available network resources (i.e., bandwidth). Taking
as reference the case of Ring Road network model for small
satellite constellations, network coding can be applied on all
the network nodes (i.e., on the space and ground segments)
[]. In this case, network coding functionalities would
actually consist in online (on y) encoding and decoding
functions. In more detail, each NC-enabled node will be in
Wireless Communications and Mobile Computing
charge of collecting a given number of information packets
andtoencodethemsoastogenerateacertainnumber
of redundancy packets, where the overall network coding
conguration plays an important role in what concerns
both the specic number of input information and output
redundancy packets as well as the adopted coding strategy
[]. In this respect, the use of random linear network coding
has gained quite some popularity in the last two decades,
so that it is nowadays considered on the most appealing
approach to implement NC in real network deployments. In
particular, the application on random linear network coding
of data chunks to be dumped to ground stations would
help increase the reliability of data exchange against sporadic
uctuations of the transmission channel quality. Moreover,
the network coding can be also exploited to transmit a
reduced number of data packets, hence improving the actual
bandwidth utilisation. is advantage can be particularly
relevant if multicast data communications are exploited
[, ], so that the performance advantages recognised for
network coding can be fully exploited.
On the other hand, in spite of the aforementioned
advantages, it is also worth considering the complexity
implications arising from the implementation of network
coding on the space segment []. As a matter of fact,
network coding implementation requires some dedicated
computation capability for online coding functions as well as
specic on-board storage to keep temporary copies of the data
chunks being subject to encoding or decoding procedures.
Moreover, some attention has to be also paid to the protocol
layer wherein network coding is being applied, so that oen
either (i) layered or (ii) integrated approaches are considered
[]. In the former, NC is implemented as a dedicated shim
layer placed in between existing protocol layers in order to
have a limited increase in the overall system implementa-
tion. In the latter, instead, NC functionalities have to be
incorporated in an existing protocol, hence increasing the
overall implementation complexity. Another point relates
to the actual position of NC functionalities in a protocol
stack, for which no specic consensus has been reached
yet. On the one hand, it would be desirable to keep NC
implementation as much closer as possible to the lower layers
of the protocol stack (i.e., datalink) in order to have a more
ecient recovery of possible packet losses. On the other hand,
implementing NC in the upper layers of the protocol stack
would help matching more precisely the characteristics of
data servicesand eventually also meet the corresponding QoS
requirements. In this respect, a good compromise could be
to implement NC functionalities directly within the bundle
protocol or immediately beneath it as part of any of the
convergence layers (i.e., UDP or LTP) considered for that
specic mission design. As such, it is immediate to see that
all these requirements have to be properly taken into account
in the full system design, with respect to the capabilities
oered by existing satellite payloads and the actual service
requirements to be targeted by the considered system.
Another interesting point related to the use of network
coding in the proposed network architecture is about their
use in the form of [] for mitigating packet losses. In this
case, network coding is not implemented throughout the
entire network, but only limited to the network legs exhibiting
more challenges from a communication reliability point of
view. As such, no re-encoding functionalities are necessary
(as those made possible by random linear network coding)
and on the contrary classical packet layer FEC solutions
can be considered, i.e., based on LDPC or Reed-Solomon
codes. In this respect, some proposals have been already
worked out by CCSDS with reference to the case of erasure
codes applied space downlinks [], where the potential of
LDPC-based erasure codes was exploited especially for the
case of free-space optical link communications. Although in
this case, network coding is implemented only on specic
links, the node capability to implement encoding/decoding
functionalities as well as to store data prior to processing
functions is certainly an important requirement to be taken
into account in the system design phase in the light of the
typically resources-constrained implementations of nodes in
space. Other activities looking into implementation of net-
work coding for intersatellite links have been also considered,
although the aforementioned constraints coming from the
space segments were not completely taken into account,
hence requiring additional study for a deeper understanding
of all underlying implications and requirements.
7. Perspectives and Open Challenges
e paper has reviewed the state of the art of small satel-
lite systems, highlighting the distinctive features enabling
novel applications and focusing on telecommunication
services.
e provision of advanced Internet services through
mega-constellations of pico/nanosatellites is going to become
reality in the near future. However, several challenges must be
faced yet, which are summarized in the following.
(i) Physical Layer.
(a) e use of frequency bands higher than Ka-
bandandtheuseoffreespaceoptical(FSO)
communications for Earth-satellite links (i.e.,
not only for intersatellite links), as reported in
Section , raise one important challenge: the
propagation channel can be strongly attenuated.
Both for high frequency RF transmission and
for FSO, this issue could be overcome by pro-
vidingagroundnetworkwithahighnumber
of ground stations at highly diverse sites. e
concept of site diversity has been extensively
studied in the eld of High roughput Satel-
lite (HTS), and recent works have highlighted
the fact that SDN paradigm could provide the
gateways implementing the concept of Smart
Diversity, a high level of recongurability that
could allow ecient resources allocation during
trac switching events [].
(b) Besides the few theoretical studies mentioned in
Section ., and some transceiver implementing
ACM techniques, much more work is needed
to design optimized modulation and coding
Wireless Communications and Mobile Computing
schemes able to satisfy strict requirements in
terms of mass, weight, size, and power con-
sumption.
(ii) MAC Layer.
In view of emerging system constraints, the imple-
mentation in small satellites of the scheduled and
random-access MAC protocols adopted in existing
satellite networks needs further investigation.
(iii) Upper Layers.
Denition is needed for interoperable application-
layer protocols to be employed on top of the lower
layer satellite protocols, addressing a wide range of
application scenarios and trac data congurations.
(iv) Routing over Time.
Due to the frequent topology changes in a CubeSat
network, successful data delivery will require ample
long-term storage at intermediate nodes to deal with
satellite link disruptions.
(v) Security Issues in LEO Satellite Networks.
Telemetry, command and control messages, and mis-
sion specic data are sent through radio links. ere-
fore, security concerns arise. CubeSats are susceptible
to Denial of Service (DoS) attacks as well as eaves-
dropping and data can be accessed by unauthorized
user. e attacker could send spurious commands
causing excessive resourcesconsumption, data loss, or
mission failure. Security challenges are exasperated by
the use of SDR payload which opens the possibility of
placing new soware on the SDR unit through unau-
thorized and potentially malicious soware installed
on the platform []. Another security concern that
has been raised recently is related to the use of small
satellites that have propulsion systems and they could
be hacked and endangering other satellites []. As
also reported in Section , communication protocols
currently implemented for CubeSat have almost no
security features. Security mechanisms developed for
conventional terrestrial networks, characterized by
lengthy handshake exchanges and substantial com-
putational eort, can hardly be directly applied to
networks of small satellites. Power, space, and weight
constraints related to CubeSat pose challenges in
implementing complicated encryption schemes and
computational expensive mechanisms []. Scientists
are already working on them [, ].
e challenge is still open. Interesting works are
ongoing on the use of physical layer approaches
to security in satellite communications [, ]. No
specic work on application of physical layer security
to CubeSat can be found; even this could open novel
solutions to overcome the challenges of security in the
small satellite framework.
Interesting research is ongoing on the use of quantum
cryptography. Some missions have been designed
and developed, using nanosatellites and CubeSats,
which show the feasibility of ground-to-space quan-
tum key distribution (QKD) [–]. QKD uses
individual light quanta in quantum superposition
states to guarantee unconditional communication
security between distant parties. Satellite-based QKD
promises to establish a global-scale quantum network
by exploiting the negligible photon loss and decoher-
ence in the empty outer space. No eavesdropping can
take place as the distribution of entangled photons
between the ground and the satellite is used to
certify the quantum nature of the link. By placing the
entangled photon source on the ground, the space
segments contain “only” the less complex detection
system, enabling its implementation in a compact
enclosure, compatible with the U CubeSat standard
[]. In [], a LEO satellite has been developed
and launched to implement decoy-state QKD with
over kHz key rate from the satellite to ground over
a distance up to km, which is up to orders of
magnitudes more ecient than that expected using an
optical ber (with . dB/km loss) of the same length.
In [], it was demonstrated that a kg CubeSat can
generate a quantum-secure key, which has so far only
been shown by a much larger kg satellite mission.
(vi) Adoption of the SDN/NFV.
It is clear that SDN/NFV paradigms will play a key
role in the integration of satellite systems with G.
However, the use of SDN/NFV in a network of small
satellites has yet to be investigated; as discussed in
Section , it could be important. Indeed, small satellite
network deployments could accelerate the infusion of
SDN concepts into satellite systems. For instance, on-
board SDN-compatible routers could be developed
and operated on small satellites as router functions
migrate into soware.
It is worth mentioning that GPP Service and system
Aspects (SA) activities have identied satellite systems both
as a possible solution for stand-alone infrastructure and as
complements to terrestrial networks []. In this framework,
HTSsystemscouldplayakeyroleinsomeoftheG
application scenarios once they will be able to provide
extremely high data rate.
However, in many other G applications scenarios that
focus on MM communications or require extremely low
latency, only small satellite constellations can really provide
an eective complement to terrestrial systems. It is crucial to
eectively face the challenges discussed above in order not to
miss the opportunities oered by the G ecosystem.
8. Conclusions
An up-to-date review of the operational features of the
small satellite has been provided in this paper, aiming at
highlighting the reasons of their recent attention from the
industries, universities, and stakeholders and describing the
main trends of development. A special emphasis has been
given to the telecommunication aspects such as the use
Wireless Communications and Mobile Computing
of higher-frequency bands, optical communications, new
protocols, and advanced architectures.
Conflicts of Interest
e authors declare that they have no conicts of interest.
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