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The current momentum behind the satellite communications and networking area is substantial. The recent ambitious plans for establishing satellite mega-constellations with thousands of low Earth orbit (LEO) satellites clearly demonstrate the rejuvenated interest in satellite communications and networking by the industry ecosystems and standardization organizations. These developments are further fueled by technological innovations in enhanced payload architectures and antenna systems, microwave, digital and optical technologies, solar panels and batteries as well as the ground segment technologies, rendering the true multidisciplinary nature of this topic.

One main difference between the legacy satellite systems and the mega-constellations of the 6G era satellite systems is the networking complexity aspect. These next-generation satellite networks have very high-speed inter-satellite links in addition to highly flexible resource allocation. For efficient operation, the new satellite networks are expected to be autonomous, intelligent, resilient, self-organizing and self-controlling as much as possible to reduce the cost of human intervention in highly complicated settings. These challenges come with a high potential of enabling new services and applications including direct-to-device connections through massive connectivity as well as orbital edge computing and caching. Clearly, the satellite mega-constellations in the 6G era will create unprecedented opportunities once the relevant challenges are fully addressed by the research community.

Our main objective of this Best Readings collection is to suggest a comprehensive list of references that can aid a wide spectrum of researchers, from novices to experts in the field, with a focus on mega constellation satellite communications and networking. We start by presenting the related books, overviews, and tutorials that can serve as comprehensive guides to satellite networks. A list of the special issues on this topic is given. A collection of standards-related articles are presented, mainly from 3GPP's point of view. A list of seminal works is also provided as the foundation for new researchers in the field. Then, we provide a list of recent research works that address the specific challenges and the potential of mega-constellations, clustered into 10 different topics, namely; massive connectivity, constellation design, non-orthogonal multiple access (NOMA), interference analysis and mitigation, space networking, orbital edge computing and caching, security, satellite Internet of things (IoT), positioning, navigation and timing (PNT) and finally enabling technologies and emerging applications. The presence of high-rate intersatellite links and the integration of the satellite networks with the terrestrial counterparts have motivated us to focus on the massive connectivity and the constellation design themes. NOMA  and the interference analysis and mitigation themes are considered due to the complexity of multiple access interference in mega-constellations. The emerging networking aspect in space is selected as the fifth theme. The use of satellites as computation devices are considered in theme six. Possible security vulnerabilities of the LEO mega-constellations are focused in theme seven. The important of the IoT and PTN applications are elaborated in theme eight and nine. Finally, the selection is concluded with a future forward perspective focusing on enabling technologies and emerging applications.  As guest editors, we hope that the researchers in the field find the provided lists valuable.

Issued: July 2023


Güneş Karabulut Kurt, Polytechnique Montréal, Canada
Ángeles Vazquez Castro, Universitat Autonoma de Barcelona, Spain
Riccardo De Gaudenzi, European Space Agency, The Netherlands

Taneli Riihonen, Tampere University, Finland
Juan A. Fraire, Inria, CONICET and Saarland University, France
Halim Yanikomeroglu, Non-Terrestrial Networks (NTN) Lab, Carleton University, Canada

Editorial Staff

Xianbin Wang
Editor-in-Chief, ComSoc Best Readings
Western University
London, ON, Canada

Matthew C. Valenti
Senior Editor, ComSoc Best Readings
West Virginia University
Morgantown, WV, USA


S. Cakaj, Ground Station Design and Analysis for LEO Satellites: Analytical, Experimental and Simulation Approach, Wiley-IEEE Press, 2023.
This book focuses on the design and evaluation of the ground stations in LEO constellations, including channel models, coverage / interference analysis, and the associated challenges with the emerging developments. Detailed discussions about the performance of each block of the ground station are included.

E. Lagunas, S. Chatzinotas, K. An, and B.F. Beidas, Non-Geostationary Satellite Communications Systems, IET, 2022.
This recent book on non-geostationary satellite orbit (NGSO) communications systems provides insight into the key design features and architectures for use-case oriented scenarios.

A.K. Majumdar, Laser Communication with Constellation Satellites, UAVs, HAPs and Balloons: Fundamentals and Systems Analysis for Global Connectivity, Springer International Publishing, 2022.

The role of laser / optical intersatellite links is highlighted in this book for global connectivity in a vertical network architecture that combines satellites with unmanned aerial vehicles (UAVs), high altitude platform stations (HAPS) and stratospheric balloons. 

T.M. Braun and W.R. Braun, Satellite Communications Payload and System, Wiley-IEEE Press, 2021.
This book considers both geostationary satellite and non-geostationary satellites  and provides in depth description of payload units and the end-to-end system context in which the payload operates.

Overviews and Tutorials

R. Ferrús, H. Koumaras, O. Sallent, G. Agapiou, T. Rasheedd, M.-A. Kourtis, C. Boustie, P. Gélard, and T. Ahmed, "SDN/NFV-Enabled Satellite Communications Networks: Opportunities, Scenarios and Challenges," Physical Communication, vol. 18, part 2, pp.  95-112, March 2016.

T. de Cola, A. Ginesi, G. Giambene, G. C. Polyzos, V. A. Siris, N. Fotiou, and Y. Thomas, "Network and Protocol Architectures for Future Satellite Systems", Foundations and Trends® in Networking, vol. 12: No. 1-2, pp 1-161, 2017.

L. Kuang, X. Chen, C. Jiang, H. Zhang, and S. Wu, "Radio Resource Management in Future Terrestrial-Satellite Communication Networks," IEEE Wireless Communications, vol. 24, no. 5, pp. 81-87, Oct. 2017.

B. Di, L. Song, Y. Li, and H. V. Poor, "Ultra-Dense LEO: Integration of Satellite Access Networks into 5G and Beyond," IEEE Wireless Communications, vol. 26, no. 2, pp. 62-69, Apr. 2019.

M. Giordani and M. Zorzi, "Non-Terrestrial Networks in the 6G Era: Challenges and Opportunities," IEEE Network, vol. 35, no. 2, pp. 244-251, March/Apr. 2021.

J.A. Fraire, O. De Jonckère, and S.C Burleigh. “Routing in the Space Internet: A Contact Graph Routing Tutorial,” Journal of Network and Computer Applications, vol. 174, no. 15 Jan. 2021.

O. Kodheli, E. Lagunas, N. Maturo, S. K. Sharma, B. Shankar, J.F. Mendoza Montoya, J.C. Merlano Duncan, D. Spano, S. Chatzinotas, S. Kisseleff, J. Querol, L. Lei, T.X. Vu, and G. Goussetis, "Satellite Communications in the New Space Era: A Survey and Future Challenges," IEEE Communications Surveys & Tutorials, vol. 23, no. 1, pp. 70-109, 2021.

X. Fang, W. Feng, T. Wei, Y. Chen, N. Ge, and C. -X. Wang, "5G Embraces Satellites for 6G Ubiquitous IoT: Basic Models for Integrated Satellite Terrestrial Networks," IEEE Internet of Things Journal, vol. 8, no. 18, pp. 14399-14417, Sept.  2021

A. U. Chaudhry and H. Yanikomeroglu, "Laser Intersatellite Links in a Starlink Constellation: A Classification and Analysis," IEEE Vehicular Technology Magazine, vol. 16, no. 2, pp. 48-56, June 2021.

H. Xie, Y. Zhan, G. Zeng, and X. Pan, "LEO Mega-Constellations for 6G Global Coverage: Challenges and Opportunities," IEEE Access, vol. 9, pp. 164223-164244, 2021.

M. M. Azari, S. Solanki, S. Chatzinotas, O. Kodheli, H. Sallouha, A. Colpaert, J. Fabian Mendoza Montoya, S. Pollin, A. Haqiqatnejad, A. Mostaani, E. Lagunas, and B. Ottersten, "Evolution of Non-Terrestrial Networks From 5G to 6G: A Survey," IEEE Communications Surveys & Tutorials, vol. 24, no. 4, pp. 2633-2672, 2022.

T. Darwish, G. Karabulut Kurt, H. Yanikomeroglu, G. Lamontagne, and M. Bellemare, "Location Management in Internet Protocol-Based Future LEO Satellite Networks: A Review," IEEE Open Journal of the Communications Society, vol. 3, pp. 1035-1062, 2022.

H. Al-Hraishawi, H. Chougrani, S. Kisseleff, E. Lagunas, and S. Chatzinotas, “A Survey on Non-Geostationary Satellite Systems: The Communication Perspective,” IEEE Communications Surveys & Tutorials, pp. 101–132, vol. 25, no. 1, 2023.

D. Zhou, M. Sheng, J. Li, and Z. Han, “Aerospace Integrated Networks Innovation for Empowering 6G: A Survey and Future Challenges,” IEEE Communications Surveys & Tutorials, Feb. 2023.

R. De Gaudenzi, O. del Rio Herrero, G. Gallinaro, S. Cioni, and D. P. Arapoglou, “Random Access Schemes for Satellite Networks, From VSAT to M2M: A Survey,” Wiley International Journal of Satellite Communications and Networking, vol. 36, no. 1, pp. 66-107. Jan./Feb. 2018.

M.Y. Abdelsadek, A.U. Chaudhry, T. Darwish, E. Erdogan, G. Karabulut-Kurt, P.G. Madoery, O.B. Yahia, and H. Yanikomeroglu, "Future Space Networks: Toward the Next Giant Leap for Humankind," IEEE Transactions on Communications, vol. 71, no. 2, pp. 949-1007, Feb. 2023.

Special Issues

A. Vanelli-Coralli, T. de Cola, F. Simoens, B. F. Beidas, S. Guo, and A. Ginesi, "Guest Editorial Advances in Satellite Communications - Part 1," IEEE Journal on Selected Areas in Communications, vol. 36, no. 2, pp. 217-220, Feb. 2018.

A. Vanelli-Coralli, T. de Cola, F. Simoens, B. F. Beidas, S. Guo, and A. Ginesi, "Advances in Satellite Communications - Part 2: Guest Editorial," IEEE Journal on Selected Areas in Communications, vol. 36, no. 5, pp. 967-970, May 2018.

K. Xue, T. de Cola, D.S.L. Wei, A. Pérez-Neira, H.S. Hassanein, L. Kuang, and S. Chatzinotas, "Guest Editorial: Space Information Networks: Technological Challenges, Design Issues, and Solutions," IEEE Network, vol. 35, no. 4, pp. 16-18, July/Aug. 2021.

G. Karabulut Kurt, Á. Vázquez-Castro, and E. Bastug, "Guest Editorial: Low Earth Orbit Satellites to Enable Access Equality," IEEE Communications Magazine, vol. 60, no. 4, pp. 16-17, Apr. 2022.

G. Karabulut Kurt, Á. Vázquez-Castro, and E. Bastug, "Guest Editorial: Low Earth Orbit Satellites to Enable Access Equality: Part II," IEEE Communications Magazine, vol. 60, no. 6, pp. 22-22, June 2022.

Z. Xiao, Z. Han, A. Nallanathan, O.A. Dobre, B. Clerckx, J. Choi, C. He, and W. Tong, "Guest Editorial Special Issue on Antenna Array Enabled Space/Air/Ground Communications and Networking," IEEE Journal on Selected Areas in Communications, vol. 40, no. 10, pp. 2767-2772, Oct. 2022.

Standards-related Articles

G. Giambene, S. Kota, and P. Pillai, "Satellite-5G Integration: A Network Perspective," IEEE Network, vol. 32, no. 5, pp. 25-31, Sept./Oct. 2018.

A. Guidotti, A. Vanelli-Coralli, M. Conti, S. Andrenacci, S. Chatzinotas, N. Maturo, B. Evans, A. Awoseyila, A. Ugolini, T. Foggi, L. Gaudio, N. Alagha, and S. Cioni, "Architectures and Key Technical Challenges for 5G Systems Incorporating Satellites," IEEE Transactions on Vehicular Technology, vol. 68, no. 3, pp. 2624-2639, March 2019.

O. Liberg, S.E. Löwenmark, S. Euler, B. Hofström, T. Khan, X. Lin, and J. Sedin, "Narrowband Internet of Things for Non-Terrestrial Networks," IEEE Communications Standards Magazine, vol. 4, no. 4, pp. 49-55, Dec. 2020.

X. Lin, S. Rommer, S. Euler, E. A. Yavuz, and R. S. Karlsson, "5G from Space: An Overview of 3GPP Non-Terrestrial Networks," IEEE Communications Standards Magazine, vol. 5, no. 4, pp. 147-153, Dec. 2021.

T. Darwish, G. Karabulut Kurt, H. Yanikomeroglu, M. Bellemare, and G. Lamontagne, "LEO Satellites in 5G and Beyond Networks: A Review From a Standardization Perspective," IEEE Access, vol. 10, pp. 35040-35060, 2022.

Theoretical Foundations

Foundations of satellite communication (SATCOM) introduced here contain a collection of well-established SATCOM theories that includes channel models, system-level techniques such as MIMO or physical layer mod/cod, multiple access, resource allocation and interference management.

Channel Modeling

E. Lutz, D. Cygan, M. Dippold, F. Dolainsky, and W. Papke, "The Land Mobile Satellite Communication Channel-Recording, Statistics, and Channel Model," IEEE Transactions on Vehicular Technology, vol. 40, no. 2, pp. 375-386, May 1991

G. E. Corazza and F. Vatalaro, "A Statistical Model for Land Mobile Satellite Channels and Its Application to Nongeostationary Orbit Systems," IEEE Transactions on Vehicular Technology, vol. 43, no. 3, pp. 738-742, Aug. 1994.

F. P. Fontan, M. Vazquez-Castro, C. E. Cabado, J. P. Garcia, and E. Kubista, "Statistical Modeling of the LMS Channel," IEEE Transactions on Vehicular Technology, vol. 50, no. 6, pp. 1549-1567, Nov. 2001.

A. Abdi, W. C. Lau, M.-S. Alouini and M. Kaveh, "A New Simple Model for Land Mobile Satellite Channels: First- and Second-Order Statistics," IEEE Transactions on Wireless Communications, vol. 2, no. 3, pp. 519-528, May 2003.  

R. Prieto-Cerdeira, F. Perez-Fontan, P. Burzigotti, A. Bolea-Alamañac, and I. Sanchez-Lago, “Versatile Two-State Land Mobile Satellite Channel Model with First Application to DVB-SH Analysis”, Wiley International Journal on Satellite Communications and Networking, vol. 28, no. 5-6, pp. 291-315, Sept. ‐ Dec. 2010.


L. Wei and C. Schlegel, "Synchronization Requirements for Multi-User OFDM on Satellite Mobile and Two-Path Rayleigh Fading Channels,"  IEEE Transactions on Communications, vol. 43, no. 2/3/4, pp. 887-895, Feb./March/Apr. 1995.

Radio Resource Management

G. Cocco, T. de Cola, M. Angelone, Z. Katona, and S. Erl, "Radio Resource Management Optimization of Flexible Satellite Payloads for DVB-S2 Systems," IEEE Transactions on Broadcasting, vol. 64, no. 2, pp. 266-280, June 2018..

P. Angeletti and R. De Gaudenzi, “Heuristic Radio Resource Management for Massive MIMO in Satellite Broadband Communication Networks”, IEEE Access, vol. 9, pp. 147164 - 147190, Oct. 2021.

Multiple Access

R. De Gaudenzi, C. Elia, and R. Viola, “Bandlimited Quasi-Synchronous CDMA: A Novel Satellite Access Technique for Mobile and Personal Communication Systems”, IEEE Journal on Selected Areas in Communications,  vol. 10, no. 2,  pp. 328 – 343, Feb. 1992.

T. Le-Ngoc and S. V. Krishnamurthy, “Performance of Combined Free/Demand Assignment Multiple‐Access Schemes in Satellite Communications”, International Journal of Satellite Communications, vol. 14, no 1, Jan. 1996, pp. 11-21.

E. Casini, R. De Gaudenzi, and O. Del Rio Herrero, "Contention Resolution Diversity Slotted ALOHA (CRDSA): An Enhanced Random Access Scheme for Satellite Access Packet Networks," IEEE Transactions on Wireless Communications, vol. 6, no. 4, pp. 1408-1419, Apr. 2007.

G. Liva, "Graph-Based Analysis and Optimization of Contention Resolution Diversity Slotted ALOHA", IEEE Transactions on Communications, vol. 59, no. 2, pp. 477-487, Feb. 2011.

R. De Gaudenzi, O. del Río Herrero, G. Acar, and E. Garrido Barrabés, “Asynchronous Contention Resolution Diversity ALOHA: Making CRDSA Truly Asynchronous”, IEEE Transactions on Wireless Communications, vol. 13,  no. 11, November 2014, pp. 6193 – 6206.

B. Clerckx, Y. Mao.  E. A. Jorswieck, J. Yuan,  D. J. Love, E. Erkip, and D. Niyato “A Primer on Rate-Splitting Multiple Access: Tutorial, Myths, and Frequently Asked Questions”, IEEE Journal on Selected Areas in Communications, vol. 41, no. 5, May 2023.

Z. Ding, X. Lei, G. K. Karagiannidis, R. Schober, J. Yuan, and V. K. Bhargava, “A Survey on Non-Orthogonal Multiple Access for 5G Networks: Research Challenges and Future Trends”,  IEEE Journal on Selected Areas in Communications, vol. 35, no. 10, pp. 2181 – 2195, Oct. 2017.


P. -D. Arapoglou, K. Liolis, M. Bertinelli, A. Panagopoulos, P. Cottis, and R. De Gaudenzi, "MIMO over Satellite: A Review," IEEE Communications Surveys & Tutorials, vol. 13, no. 1, pp. 27-51, 2011.

P. Angeletti and R. De Gaudenzi,  “A Pragmatic Approach to Massive MIMO for Satellite Broadband Communication Systems”, IEEE Access, vol.  8, pp. 132212 – 132236,  July 2020.

Interference Management

F. Vatalaro, G. E. Corazza, C. Caini, and C. Ferrarelli, “Analysis of LEO, MEO, and GEO Global Mobile Satellite Systems in the Presence of Interference and Fading,” IEEE Journal on Selected Areas in Communications, vol. 13, no. 2, pp. 291–300, Feb. 1995.  

S. K. Sharma, S. Chatzinotas, and B. Ottersten, “Inline Interference Mitigation Techniques for Spectral Coexistence of GEO and NGEO Satellites,” International Journal of Satellite Communications and Networking, vol. 34, no. 1, pp. 11–39, Sep. 2014.

Topic 1: Massive Connectivity

Through the use of high-speed inter-satellite links and the integration with terrestrial networks, the emerging mega-constellations are offering massive connectivity that generates the possibility of offering direct-to-device communications by alleviating the problem of the high path loss between the terminals on the ground and the satellite.

L. You, K.-X. Li, J. Wang, X. Gao, X.-G. Xia, and B. Ottersten, “Massive MIMO Transmission for LEO Satellite Communications,” IEEE Journal on Selected Areas in Communications, vol. 38, no. 8, pp. 1851–1865, Aug. 2020.
This paper proposes a massive MIMO scheme for LEO satellite communication with full frequency reuse and statistical channel state information. Mainly, a MIMO channel model considering Doppler and delay at the users and a low complexity downlink precoder and uplink receiver are developed to increase the data rate of LEO satellite communication systems.

Q. Huang, M. Lin, W.-P. Zhu, J. Cheng, and M.-S. Alouini, “Uplink Massive Access in Mixed RF/FSO Satellite-Aerial-Terrestrial Networks,” IEEE Transactions on Communications, vol. 69, no. 4, pp. 2413–2426, Apr. 2021.
This work presents an SDMA scheme to improve the uplink transmission in a satellite aerial terrestrial network that uses a high-altitude platform (HAP) as a relay. The SDMA scheme maximizes the ergodic sum rate by exploiting the imperfect angular information of each UE, clustering UEs into groups, and implementing a beamforming scheme at HAP.

Q. Chen, G. Giambene, L. Yang, C. Fan, and X. Chen, "Analysis of Inter-Satellite Link Paths for LEO Mega-Constellation Networks," IEEE Transactions on Vehicular Technology, vol. 70, no. 3, pp. 2743-2755, March 2021.
This paper analyzes mega-constellation networks' routing complexity and inter-satellite links (ISLs). A theoretical model is proposed to estimate the ISL hop count between ground users and derive the spatial distribution properties of hop count. Numerical results show that optimizing the constellation phasing factor can effectively reduce the average hop count for the whole network and specific regional users.

C. Liu, W. Feng, Y. Chen, C.-X. Wang, and N. Ge, “Cell-Free Satellite-UAV Networks for 6G Wide-Area Internet of Things,” IEEE Journal on Selected Areas in Communications, vol. 39, no. 4, pp. 1116–1131, Apr. 2021.
This paper investigates the need to serve many IoT devices outside the coverage of terrestrial cellular networks and proposes a cognitive satellite-UAV network (CSUN) to overcome this limitation. A multi-domain resource allocation scheme is proposed for CSUNs consisting of a satellite and a swarm of UAVs, establishing cell-free on-demand coverage and implementing opportunistic spectrum sharing.

K.-X. Li, L. You, J. Wang, X. Gao, C.G. Tsinos, S. Chatzinotas, and B. Ottersten, "Downlink Transmit Design for Massive MIMO LEO Satellite Communications," IEEE Transactions on Communications, vol. 70, no. 2, pp. 1014-1028, Feb. 2022.
This paper focuses on the downlink (DL) transmit design for massive MIMO LEO satellite communication systems. The paper establishes a channel model for DL massive MIMO LEO satellite systems, where only slow-varying statistical channel state information is used at the transmitter. The paper shows that single-stream precoding is the optimal choice to maximize the ergodic sum rate.

M. Abdelsadek, G. Karabulut-Kurt, H. Yanikomeroglu, P. Hu, G. Lamontagne, and K. Ahmed, “Broadband Connectivity for Handheld Devices via LEO Satellites: Is Distributed Massive MIMO the Answer?,” IEEE Open Journal of the Communications Society , March 2023.
This paper proposes using distributed massive MIMO (DM-MIMO) techniques to improve the data rates of handheld devices for supporting broadband connectivity through LEO satellite networks. The paper discusses DM-MIMO-based satellite networks from different perspectives, including the channel model, network management, and architecture. It theoretically evaluates such networks' performance by deriving closed-form expressions for spectral efficiency and using extensive simulations based on actual data from a Starlink constellation.

T. Ma, B. Qian, X. Qin, X. Liu, H. Zhou, and L. Zhao, "Satellite-Terrestrial Integrated 6G: An Ultra-Dense LEO Networking Management Architecture," IEEE Wireless Communications, 2023, Early Access.
This article proposes a multi-layered management architecture for an ultra-dense LEO satellite-terrestrial integrated network. The architecture includes global and local controllers, introduced to reduce network management complexity and a grouping and clustering method to assign MEO and LEO satellites as controllers. The proposed architecture enables efficient implementation of network status control, mobility management, resource management, and service management.

Topic 2: Constellation Design

The emerging LEO constellations also offer design possibilities at the constellation level. The primary works in the design of constellations and the fully integrated terrestrial satellite networks are listed below.

A. Kak and I. F. Akyildiz, "Designing Large-Scale Constellations for the Internet of Space Things with CubeSats," IEEE Internet of Things Journal, vol. 8, no. 3, pp. 1749-1768, 1 Feb.1, 2021.
This article discusses the emergence of CubeSats and their use in creating low-cost satellite networks, which has led to the Internet of Space Things (IoST) encompassing air, ground, and space. Additionally, a modular and customizable large-scale constellation design framework is presented to optimize CubeSat density, coverage, and connectivity parameters for IoST use cases. The framework's efficacy is validated through extensive performance comparisons with state-of-the-art constellations.

C. -Q. Dai, M. Zhang, C. Li, J. Zhao, and Q. Chen, "QoE-Aware Intelligent Satellite Constellation Design in Satellite Internet of Things," IEEE Internet of Things Journal, vol. 8, no. 6, pp. 4855-4867, 15 March15, 2021.
This article proposes a quality of experience (QoE)-aware satellite constellation design scheme to enhance user satisfaction by constructing QoE factors. A network model is established with LEO and ground IoT devices, and an intelligent optimization algorithm named multilayer tabu search (MLTS) is designed to obtain reasonable satellite orbits with the best QoE. The proposed scheme assesses QoE factors by considering coverage performance, communication fluency, regional demand capacity, and profitability.

P. Wang, B. Di, and L. Song, "Mega-Constellation Design for Integrated Satellite-Terrestrial Networks for Global Seamless Connectivity," IEEE Wireless Communications Letters, vol. 11, no. 8, pp. 1669-1673, Aug. 2022.
This letter investigates the design of a mega-constellation for an integrated satellite-terrestrial network, where users can access the core network via backhaul links from multi-layer satellites in space and terrestrial access points on the ground. The objective is to achieve seamless global connectivity and provide high-rate backhaul transmission. The designed constellation requires more satellites as the terrestrial infrastructures are distributed more evenly in latitude and remain unchanged regardless of the terrestrial transmission capacity of high-latitude user terminals.

G. M. Capez, S. Henn, J. A. Fraire, and R. Garello, "Sparse Satellite Constellation Design for Global and Regional Direct-to-Satellite IoT Services," IEEE Transactions on Aerospace and Electronic Systems, vol. 58, no. 5, pp. 3786-3801, Oct. 2022.
This article introduces and designs sparse constellations for the direct-to-satellite Internet of things (DtS-IoT), which do not require ground infrastructure. The goal is to reduce the number of in-orbit satellites by properly dimensioning the delivery delay and optimal positioning of the orbiting gateways. The authors derive realistic constraints on the maximum gap time between two consecutive passing-by satellites, introduce and optimize an algorithm to design quasi-optimal topologies for sparse IoT constellations and apply their design to global and regional coverage.

Topic 3: Non-Orthogonal Multiple Access

The data traffic in satellite networks is expected to be non uniformly distributed in space and time, and satellite constellations have to to cope with this challenge. Effective radio resource management (RRM) techniques including non-orgononal multiple-access are expected to play a key role.

E. Lagunas, S. K. Sharma, S. Maleki, S. Chatzinotas, and B. Ottersten, "Resource Allocation for Cognitive Satellite Communications with Incumbent Terrestrial Networks," IEEE Transactions on Cognitive Communications and Networking, vol. 1, no. 3, pp. 305-317, Sept. 2015.
This article discusses the issue of spectrum scarcity affecting both satellite communications and terrestrial systems due to the increasing demand for multimedia applications and the lack of available unlicensed spectrum. The paper proposes resource allocation techniques for a cognitive spectrum utilization scenario in which the satellite system exploits the spectrum allocated to terrestrial networks as incumbent users without causing harmful interference.

X. Zhu, C. Jiang, L. Kuang, N. Ge, and J. Lu, "Non-Orthogonal Multiple Access Based Integrated Terrestrial-Satellite Networks," IEEE Journal on Selected Areas in Communications, vol. 35, no. 10, pp. 2253-2267, Oct. 2017.
This paper investigates the downlink transmission of a non-orthogonal multiple access (NOMA)-based integrated terrestrial-satellite network, where terrestrial and satellite networks are equipped with multiple antennas and use beamforming techniques to serve multiple users. The paper first examines the capacity performance of the terrestrial and satellite networks separately. Then it proposes a joint iteration algorithm to maximize the total system capacity while adhering to an interference temperature limit for the satellite.

J. Jiao, Y. Sun, S. Wu, Y. Wang, and Q. Zhang, "Network Utility Maximization Resource Allocation for NOMA in Satellite-Based Internet of Things," IEEE Internet of Things Journal, vol. 7, no. 4, pp. 3230-3242, Apr. 2020.
This work focuses on optimizing the long-term network utility of non-orthogonal multiple access satellite-based Internet of Things downlink system with limited communication resources. The authors formulated a joint network stability and resource allocation optimization problem, establishing two virtual queues for data queueing and power expenditure and proposing a solution under the Karush–Kuhn–Tucker (KKT) conditions using the particle swarm optimization algorithm.

X. Liu, X. B. Zhai, W. Lu, and C. Wu, "QoS-Guarantee Resource Allocation for Multibeam Satellite Industrial Internet of Things with NOMA," IEEE Transactions on Industrial Informatics, vol. 17, no. 3, pp. 2052-2061, March 2021.
This paper proposes a multi-beam satellite industrial internet of things (IIoT) in Ka-band to achieve wide-area coverage and long-distance transmissions, which uses non-orthogonal multiple access for each beam to improve transmission rate. Importantly, it presents a satellite-ground integrated IIoT, which uses the ground cellular network to supplement the satellite coverage in blocked areas, and the power allocation and network selection for the integrated IIoT to decrease the transmission cost.

Q. Hu, J. Jiao, Y. Wang, S. Wu, R. Lu, and Q. Zhang, "Multitype Services Coexistence in Uplink NOMA for Dual-Layer LEO Satellite Constellation," IEEE Internet of Things Journal, vol. 10, no. 3, pp. 2693-2707, 1 Feb.1, 2023.
This article proposes a multitype services coexistence handover non-orthogonal multiple access (NOMA) schemes for a dual-layer mega LEO satellite constellation that can provide uplink NOMA for mission-critical communications (CUs), massive machine type communications (MUs), and enhanced mobile broadband (EUs). EUs are served in the higher layer satellites and may handover to the lower layer satellites to coexist with CUs or MUs. CUs and MUs perform grant-based and grant-free NOMA on resource blocks in the lower-layer satellites.

Topic 4: Interference Analysis and Mitigation

In addition to multiple access techniques, infra / intra system interference analysis and mitigation will be pivotal to obtain high spectral efficiency in satellite networks to serve many users. The integration with the terrestrial networks makes this topic especially challenging and the related research is ongoing.

B. Di, H. Zhang, L. Song, Y. Li, and G. Y. Li, “Ultra-Dense LEO: Integrating Terrestrial-Satellite Networks Into 5G and Beyond for Data Offloading,” IEEE Transactions on Wireless Communications, vol. 18, no. 1, pp. 47–62, Jan. 2019.
In this paper, the integration of ultra-dense LEO networks and terrestrial networks are considered for data offloading. User assignments are considered according to the data rate and delay requirements, highlighting the promise of integrated networks to accommodate multi-user scenarios.

Z. Lin, Z. Ni, L. Kuang, C. Jiang, and Z. Huang, “Dynamic Beam Pattern and Bandwidth Allocation Based on Multi-Agent Deep Reinforcement Learning for Beam Hopping Satellite Systems,” IEEE Transactions on Vehicular Technology, vol. 71, no. 4, pp. 3917–3930, Apr. 2022.
The authors consider deep reinforcement learning (DLR)-based dynamic beam pattern and bandwidth allocation scheme that aims to jointly exploit the time, space and frequency of the beam.

A. Talgat, M. A. Kishk and M.-S. Alouini, "Stochastic Geometry-Based Analysis of LEO Satellite Communication Systems," IEEE Communications Letters, vol. 25, no. 8, pp. 2458-2462, Aug. 2021.
The authors investigate the  user coverage probability for a scenario where satellite gateways  are deployed on the ground to act as a relay between the users and the LEO satellites. The locations of the LEO satellites are modeled as a binomial point process on a spherical surface, and the required number of satellite gateways for a target performance is evaluated.

N. Okati, T. Riihonen, D. Korpi, I. Angervuori, and R. Wichman, “Downlink Coverage and Rate Analysis of Low Earth Orbit Satellite Constellations Using Stochastic Geometry,” IEEE Transactions on Communications, vol. 68, no. 8, pp. 5120–5134, Aug. 2020.
The authors introduce a foundational framework based on stochastic geometry and introduce the analytical expressions for the downlink coverage probability and average data rate of generic LEO networks. Guidelines for selecting the design parameters for future massive LEO constellations are also presented.

N. Okati and T. Riihonen, “Nonhomogeneous Stochastic Geometry Analysis of Massive LEO Communication Constellations,” IEEE Transactions on Communications, vol. 70, no. 3, pp. 1848–1860, Mar. 2022.
The authors generalize the stochastic geometry-based framework analysis for massive LEO constellations by taking into account the inherent non-uniform distribution of satellites across different latitudes. Modeling a LEO network as a nonhomogeneous Poisson point process, they present analytical expressions for the downlink coverage probability and average data rate and observe optimum points for both the constellation altitude and the number of orthogonal frequency channels.

L. Cottatellucci, M. Debbah, G. Gallinaro, R. Mueller, M. Neri, and R. Rinaldo, “Interference Mitigation Techniques for Broadband Satellite Systems,'' in Proc. 24th AIAA International Communications Satellite Systems Conference, San Diego, CA, USA, June 2006.
This paper presents an overview of interference mitigation techniques aiming at increasing the system capacity of broadband multimedia systems. A multi-star network topology using a bent-pipe transparent transponder is assumed. The focus is on assessing potential performance gain using linear precoding techniques.

P.-D. Arapoglou, A. Ginesi, S. Cioni, S. Erl, F. Clazzer, S. Andrenacci, and A. Vanelli-Coralli, “DVB-S2X-Enabled Precoding for High Throughput Satellite Systems”, Wiley International Journal of Satellite Communications and Networking, vol. 34, no. 3, pp. 439-455, May 2016.
This paper presents results concerning the practical issues that arise when precoding is applied over an aggressive frequency re-use HTS network. In addressing these concerns, the paper also proposes pragmatic solutions that have been developed to overcome these limitations. The potential gains using precoding are derived through system-level simulations.

V. Joroughi, M. Ángel Vázquez, and A. I. Pérez-Neira, “Precoding in Multigateway Multibeam Satellite Systems,'' IEEE Transactions on Wireless Communications, vol. 15, no. 7, pp. 4944-4956, July 2016.
This paper considers a multi gateway multibeam satellite system with multiple feeds per beam. Each gateway serves a set of beams (cluster) in these systems so that the overall data traffic is generated in different geographical areas. Full frequency reuse among beams is considered so that interference mitigation techniques are mandatory. Precisely, this paper aims to design the precoding scheme, which, in contrast to single gateway schemes, entails two main challenges related to the distributed feeder link. 

B. Devillers, A. Perez-Neira, and C. Mosquera, “Joint Linear Precoding and Beamforming for The Forward Link of Multi-Beam Broadband Satellite Systems”, in Proc. IEEE Global Telecommunications Conference (GLOBECOM), Houston, TX, USA, pp. 1-6, Dec. 2011.
This paper builds on the combination of advanced interference mitigation techniques and ground-based beamforming to cope with this increased level of user interference. Focussing on the forward link of a multi-beam broadband satellite system, the authors consider the joint design of linear precoding and ground-based beamforming at the gateway. The provided simulation results quantify the performance gain generated by this joint design concerning precoding for fixed on-board beamforming. The robustness of the considered scheme to channel estimation errors is also analyzed.

C. Braun, A. M. Voicu, L. Simic, and P. Mahonen, “Should We Worry About Interference in Emerging Dense NGSO Satellite Constellations?”, in Proc. IEEE International Symposium on Dynamic Spectrum Access Networks (DySPAN), Newark, NJ, USA, 11-14 Nov. 2019.
The authors extensively study inter-satellite coexistence in the Ku-band by considering NGSO and GSO systems. The degrading effects of the interference are noted, and the need for new spectrum regulation is highlighted. Band-splitting solutions are considered for interference mitigation.

Topic 5: Space Networking

Mega-constellations introduce networking aspects and flow and congestion control, management/operations, and topology planning. Some leading works are listed below.

L. Bertaux, S. Medjiah, P. Berthou, S. Abdellatif, A. Hakiri, P. Gelard, F. Planchou, and M. Bruyere, "Software Defined Networking and Virtualization for Broadband Satellite Networks," IEEE Communications Magazine, vol. 53, no. 3, pp. 54-60, March 2015.
The potential benefits of the software-defined networking (SDN), network virtualization and network functions virtualization (NFV) concepts are considered in this work for seamless terrestrial network integration.

J. Liu, Y. Shi, L. Zhao, Y. Cao, W. Sun, and N. Kato, "Joint Placement of Controllers and Gateways in SDN-Enabled 5G-Satellite Integrated Network," IEEE Journal on Selected Areas in Communications, vol. 36, no. 2, pp. 221-232, Feb. 2018.
The paper makes use of the SDN paradigm for integrated terrestrial satellite networks. The placement problem of the controllers and the gateways is considered with the target of improving network reliability and transmission latency. A low complexity approach is considered for a practical solution. 

T. Li, H. Zhou, H. Luo, and S. Yu, "SERvICE: A Software Defined Framework for Integrated Space-Terrestrial Satellite Communication," IEEE Transactions on Mobile Computing, vol. 17, no. 3, pp. 703-716, 1 March 2018.
A three-plance architecture composed o the management plane, control plane, and forwarding plane is introduced. Also, an optimization framework is proposed to achieve flexible satellite network traffic engineering jointly with fine-grained QoS guarantees.

A. Papa, T. de Cola, P. Vizarreta, M. He, C. Mas-Machuca, and W. Kellerer, "Design and Evaluation of Reconfigurable SDN LEO Constellations," IEEE Transactions on Network and Service Management, vol. 17, no. 3, pp. 1432-1445, Sept. 2020.
The benefits of the software-defined network’s fundamental principle, the separation of the data and the control planes are considered in space networks, and the controller placement problem is investigated to minimize the average flow setup time in the presence of varying traffic demands.

X. Qin, T. Ma, Z. Tang, X. Zhang, H. Zhou, and L. Zhao, "Service-Aware Resource Orchestration in Ultra-Dense LEO Satellite-Terrestrial Integrated 6G: A Service Function Chain Approach," IEEE Transactions on Wireless Communications, 2023, Early Access.
The large scale, heterogeneity, and high mobility of the integrated satellite-terrestrial network are used for efficient resource orchestration for service delivery. The notion of a service function chain is introduced, and an efficient multiple-service delivery scheme is proposed.

J.A. Fraire and E.L. Gasparini, “Centralized and Decentralized Routing Solutions for Present and Future Space Information NetworksIEEE Network, vol. 35, no. 4, 110-117.
The authors propose two centralized routing schemes and compare the performances of the proposed approaches with that of the distributed techniques while considering the computational complexity, memory utilization, and energy consumption perspectives.

Topic 6: Orbital Edge Computing and Caching

The functionality of the satellites in mega-constellations extends beyond communication to cover computing where computation tasks are performed on the LEO satellites.

Z. Zhang, W. Zhang, and F. -H. Tseng, "Satellite Mobile Edge Computing: Improving QoS of High-Speed Satellite-Terrestrial Networks Using Edge Computing Techniques," IEEE Network, vol. 33, no. 1, pp. 70-76, Jan./Feb. 2019.
The authors propose a dynamic network virtualization technique to design cooperative computation offloading for parallel computation in integrated satellite-terrestrial networks to improve the QoS levels. 

B. Di, H. Zhang, L. Song, Y. Li, and G. Y. Li, "Ultra-Dense LEO: Integrating Terrestrial-Satellite Networks Into 5G and Beyond for Data Offloading," IEEE Transactions on Wireless Communications, vol. 18, no. 1, pp. 47-62, Jan. 2019.
The paper considers an ultra-dense LEO-based integrated terrestrial-satellite network architecture for data offloading. An optimization is formulated to maximize the sum rate, the number of accessed users, and a solution strategy is given.

X. Zhu, C. Jiang, L. Kuang, and Z. Zhao, "Cooperative Multilayer Edge Caching in Integrated Satellite-Terrestrial Networks," IEEE Transactions on Wireless Communications, vol. 21, no. 5, pp. 2924-2937, May 2022.
The authors present cooperative multilayer edge caching in integrated satellite-terrestrial networks, where the base station cache, the satellite cache, and the gateway cache can cooperatively provide content services with the goal of reducing the communication delay.

Y. Hao, Z. Song, Z. Zheng, Q. Zhang, and Z. Miao, "Joint Communication, Computing, and Caching Resource Allocation in LEO Satellite MEC Networks," IEEE Access, vol. 11, pp. 6708-6716, 2023.
The authors investigate a joint communication computing and caching resource allocation problem for multi-access edge computing in LEO satellites. Low complexity approaches are considered for practical relevancy.

Topic 7: Security

While mega-constellations bring numerous benefits regarding global connectivity or improved data services, they also certainly increase the number of security threats at different space system design domains, potentially affecting critical services. In consequence, mega-constellations require by-design security measures against potential attacks at space and ground segments and network, access, or signal levels. While the latter is a well-explored area, as shown in the following readings, further work is required towards providing global security guarantees for mega-constellations.

J. Lei, Z. Han, M. Á. Vazquez-Castro, and A. Hjorungnes, "Secure Satellite Communication Systems Design with Individual Secrecy Rate Constraints," IEEE Transactions on Information Forensics and Security, vol. 6, no. 3, pp. 661-671, Sept. 2011.
In this paper, the authors addressed for the first time the security of multi-beam satellite communication using physical layer security techniques. In particular, the authors apply joint power control and beamforming optimization. The resulting system design can achieve optimized power allocation to ensure the individual intended users' secrecy rate requirements.

G. Zheng, P. -D. Arapoglou, and B. Ottersten, "Physical Layer Security in Multibeam Satellite Systems," IEEE Transactions on Wireless Communications, vol. 11, no. 2, pp. 852-863, Feb. 2012.
This paper sets the analytical framework for physical layer security techniques for multibeam satellite systems with multiple (passive) eavesdroppers. The design objective is to minimize via transmit beamforming the total transmit power on board the satellite while satisfying individual intended users' secrecy rate constraints assuming perfect channel state information about the eavesdroppers at the satellite.

M. Lin, Z. Lin, W. -P. Zhu, and J. -B. Wang, "Joint Beamforming for Secure Communication in Cognitive Satellite Terrestrial Networks," IEEE Journal on Selected Areas in Communications, vol. 36, no. 5, pp. 1017-1029, May 2018. 
This paper investigates secure communication for cognitive satellite-terrestrial networks with software-defined architectures. Specifically, the authors propose beamforming align using the interference from the terrestrial network as a green source to enhance the physical-layer security for the satellite network.

Á. Vázquez-Castro and M. Hayashi, "Physical Layer Security for RF Satellite Channels in the Finite-Length Regime," IEEE Transactions on Information Forensics and Security, vol. 14, no. 4, pp. 981-993, Apr. 2019.
This paper proposes the design of a secure satellite physical layer based on the wiretap information theoretical framework for radiofrequency satellite channels. The authors also perform a finite-length performance evaluation using practical error-correcting codes included in current satellite communication standards.

D.-H. Jung, J.-G. Ryu, and J. Choi, "When Satellites Work as Eavesdroppers," IEEE Transactions on Information Forensics and Security, vol. 17, pp. 2784-2799, July 2022.
In this paper, satellite non-colluded non-cooperative eavesdroppers are assumed to be randomly distributed at arbitrary altitudes. The distributions of the signal-to-noise ratios at both the serving satellite and the most detrimental eavesdropping satellite are derived to obtain ergodic and outage secrecy capacities. The results are helpful for the design of new constellations.

O. Ben Yahia, E. Erdogan, G. Karabulut Kurt, I. Altunbas, and H. Yanikomeroglu, “Optical Satellite Eavesdropping,” IEEE Transactions on Vehicular Technology, vol. 71, no. 9, pp. 10126-10131, Sept. 2022.
In this paper, the authors investigate the case of an eavesdropping spacecraft intercepting  optical  communications between  a low-Earth orbit  satellite and a  high altitude platform station. The authors identify two scenarios for quantifying the average secrecy capacity and the secrecy outage probability. It is found that turbulence-induced fading  significantly  impacts  the  secrecy  performance  of free-space  optical  communication

Topic 8: Satellite IoT

The Internet of things (IoT) is a fundamental feature of 6G, as communication extends from persons to objects such as sensors or actuators. Satellites bring a unique opportunity in 6G IoT to achieve coverage in regions with limited terrestrial infrastructures (e.g., oceans, mountains, deserts, etc.). The following readings cover the taxonomy of satellite IoT, including indirect satellite IoT (where satellites communicate with a gateway on the ground) and direct satellite IoT (where satellites directly communicate with the IoT ground end-devices). Other papers also cover constellation design and network architecture. Candidate technologies are 3GPP NB-IoT and LoRa / LoRaWAN.

M. De Sanctis, E. Cianca, G. Araniti, I. Bisio, and R. Prasad, "Satellite Communications Supporting Internet of Remote Things," IEEE Internet of Things Journal, vol. 3, no. 1, pp. 113-123, Feb. 2016.
This paper provides an overview of research on using satellite communication in the Internet of remote things (IoRT) applications, highlighting important issues such as interoperability, quality of service, and group-based communications. However, the authors note that there is currently limited research on the specific application of satellites to IoRT scenarios and suggest that more effort should be devoted to exploring the potential benefits of satellite constellations, particularly in cost-benefit analysis.

Z. Qu, G. Zhang, H. Cao, and J. Xie, "LEO Satellite Constellation for Internet of Things," IEEE Access, vol. 5, pp. 18391-18401, 2017.
This paper provides an overview of an IoT system based on a low earth orbit satellite constellation, discussing issues such as constellation design, interference mitigation, network architecture, routing, and higher layer design. The authors note that there is growing interest in the potential use of satellite constellations for IoT applications and suggest that further research is needed to address issues such as transmission scheme, system security, and low power consumption design in order to make this technology a reliable cost-benefit solution.

D. Zhou, M. Sheng, J. Wu, J. Li, and Z. Han, "Gateway Placement in Integrated Satellite–Terrestrial Networks: Supporting Communications and Internet of Remote Things," IEEE Internet of Things Journal, vol. 9, no. 6, pp. 4421-4434, March, 2022.
This article presents a multi-objective optimization problem formulation for gateway placement in IoRT networks that considers both communication and service data demands, including channel conditions and hybrid service data distribution. The proposed algorithm is designed to optimize the total revenue of service data demand within coverage while ensuring local information confidentiality. Simulation results demonstrate that the proposed algorithm can effectively enhance resource utilization and network service coverage performances, and the service data demand distribution is found to have a major influence on gateway placement.

D. Han , Q. Ye,   H. Peng, W. Wu, H. Wu, W. Liao, and X. Shen, "Two-Timescale Learning-Based Task Offloading for Remote IoT in Integrated Satellite-Terrestrial Networks," IEEE Internet of Things Journal, vol. 10, no. 12, pp. 10131 – 10145, June 2023.
In this paper, the authors propose an architecture for satellite IoT to support task offloading for remote IoT users. They introduce an algorithm for making real-time decisions on offloading link selection and bandwidth allocation. The goal is to minimize overall task offloading delay while considering dynamic task arrivals and channel conditions. The proposed algorithm shows effectiveness in reducing delay. The architecture is suitable for serving remote IoT users with heavy traffic loads. In future work, the authors plan to focus on task offloading scheduling for mobile users, which can help cope with spatial-temporal variations in user density.

J.A. Fraire, O. Iova, and F. Valois, "Space-Terrestrial Integrated Internet of Things: Challenges and Opportunities," IEEE Communications Magazine, vol. 60, no. 12, Dec. 2022.  
This article discusses the potential of using near-Earth orbit satellites to provide IoT connectivity in regions with limited terrestrial network connections. The proposed architecture allows direct and indirect satellite access from IoT devices on the ground. The article analyzes the suitability of NB-IoT and LoRa/LoRaWAN as potential candidates for satellite IoT deployments. The authors also highlight the key research challenges that need to be addressed to achieve successful space-terrestrial IoT integration, such as adapting existing IoT infrastructure to cope with the specifics of orbital dynamics. The authors argue that the opportunities from a global IoT vision are significant and can impact both traditional and unserved business sectors.

O. del Rio Herrero and R. De Gaudenzi, “High Efficiency Satellite Multiple Access Scheme for Machine-to-Machine Communications”, IEEE Transactions on Aerospace Systems, vol. 48, no. 4, pp. 2961-2989, Oct. 2012.
This article reports the key design drivers and performance of a high-efficiency satellite mobile messaging system well adapted to the machine-to-machine communication services targeting, in particular, the vehicular market. It is shown that the proposed return link multiple access solution provides an unprecedented random access channel aggregated spectral efficiency of around 2 bit/s/Hz in the presence of power unbalance with reliable packet delivery over typical land mobile satellite (LMS) channels.

S. Cioni, R. De Gaudenzi, O. Del Rio Herrero, and N. Girault, “On the Satellite Role in the Era of 5G Massive Machine Type of Communications”, IEEE Network, vol. 32, no. 5, Sept./Oct. pp. 54-61, 2018.
The article addresses the question of the possible role of satellite systems in massive Machine mMTC services. Key satellite mMTC system design trade-offs are outlined jointly with some examples of network sizing for Low Earth Orbiting and Geostationary satellites.

Topic 9: Positioning, Navigation and Timing (PNT)

Densely populated LEO mega-constellations can offer additional services other than conventional communication and networking aspects. One promising application is their use for localization and positioning, navigation, and timing (PNT) services either as stand-alone or jointly with the conventional GNSS systems.

A. Nardin, F. Dovis, and J. A. Fraire, "Empowering the Tracking Performance of LEO-Based Positioning by Means of Meta-Signals," IEEE Journal of Radio Frequency Identification, vol. 5, no. 3, pp. 244-253, Sept. 2021.
This paper investigates a piggyback PNT service. A Doppler resilient meta-signal approach is considered against the high mobility of the LEO satellites.

F. S. Prol, R. Morales Ferre, Z. Saleem, P. Välisuo, C. Pinell, E. S. Lohan, M. Elsanhoury, M. Elmusrati, S. Islam, K. Çelikbilek, K. Selvan, J. Yliaho, K. Rutledge, A. Ojala, L. Ferranti, J. Praks, M. Z. H. Bhuiyan, S. Kaasalainen, and H. Kuusniemi, "Position, Navigation, and Timing (PNT) Through Low Earth Orbit (LEO) Satellites: A Survey on Current Status, Challenges, and Opportunities," IEEE Access, vol. 10, pp. 83971-84002, 2022.
This paper presents a comprehensive survey of PNT services through LEO constellations, highlighting the necessary architectural requirements to build a new LEO-PNT system.

R. Morales Ferre, J. Praks, G. Seco-Granados, and E. S. Lohan, "A Feasibility Study for Signal-in-Space Design for LEO-PNT Solutions with Miniaturized Satellites," IEEE Journal on Miniaturization for Air and Space Systems, vol. 3, no. 4, pp. 171-183, Dec. 2022.
This work provides an LEO-PNT solution, considering the signal-in-space (SIS), the ground segment, and the user / receiver segment. The impact of constellation types, achievable coverage limits, geometric dilution of precision (GDOP) bounds,  and achievable carrier-to-noise ratios (CNRs) are investigated through the use of realistic channel models.

J. Khalife and Z. M. Kassas, "Performance-Driven Design of Carrier Phase Differential Navigation Frameworks with Megaconstellation LEO Satellites," IEEE Transactions on Aerospace and Electronic Systems, vol. 59, no. 3, pp. 2947 - 2966, June 2023.
The authors develop a navigation framework with carrier phase differential measurements from mega-constellation LEO satellite signals, where the measurement errors due to ephemeral errors and ionospheric / tropospheric delays are derived, as well as the statistics of the dilution of precision are characterized. The Starlink constellation is used as a specific LEO mega-constellation example to demonstrate the developed framework.

Topic 10: Enabling Technologies and Emerging Applications

Achieving a successful integration of 6G with mega-constellations is conditioned on the development of adequate technologies that lower the cost / risk barrier. To this end, a series of selected readings cover topics such as software defined radios (SDRs), allowing in-orbit radio interface reconfiguration. This is crucial to quickly and safely evolve and optimize the communication protocols in the space environment. Of course, artificial intelligent (AI) is at the spot. In the mega-constellation case, AI is a perfect tool for network management and operations. Papers tackle issues like network slicing, antenna control, topology design, and energy efficiency. These concepts altogether enable novel space-based applications like the so-called Ring-Road Network.

P. I. Theoharis, R. Raad, F. Tubbal, M. U. Ali Khan, and S. Liu, "Software-Defined Radios for CubeSat Applications: A Brief Review and Methodology," IEEE Journal on Miniaturization for Air and Space Systems, vol. 2, no. 1, pp. 10-16, March 2021.
The authors briefly review space software-defined radios (SDRs) with a particular emphasis on CubeSat-related designs. They propose two approaches for designing SDRs for highly integrated and miniaturized platforms such as CubeSats. Additionally, a High Data Rate SDR for CubeSat applications is proposed, equipped with an FPGA SoC that approximates an ideal SDR architecture.

T. K. Rodrigues and N. Kato, "Network Slicing with Centralized and Distributed Reinforcement Learning for Combined Satellite/Ground Networks in a 6G Environment," IEEE Wireless Communications, vol. 29, no. 1, pp. 104-110, Feb. 2022.
This paper discusses the advantages of using network slicing for a resource management system in a network combining satellite and ground systems. The authors propose a framework that utilizes a machine learning technique to understand which links in the network are more desired. The framework uses this algorithm for ranking possible network slices for new user requests and assigns the slice with the lowest cost to reserve more desired paths for requests that truly need them to be satisfied.

L. Lei, Y. Yuan, T. X. Vu, S. Chatzinotas, M. Minardi, and J. F. M. Montoya, "Dynamic-Adaptive AI Solutions for Network Slicing Management in Satellite-Integrated B5G Systems," IEEE Network, vol. 35, no. 6, pp. 91-97, Nov./Dec. 2021.
In this article, the authors have focused on adapting AI solutions to dynamic beyond 5G non-geostationary network slicing. They have highlighted the major challenge of applying AI to network slicing management and proposed tailored solutions for two categories of dynamic scenarios with predictable or unpredictable variations.

K. Tekbıyık, G. Karabulut Kurt, A. R. Ekti, and H. Yanikomeroglu, "Reconfigurable Intelligent Surfaces in Action for Nonterrestrial Networks," IEEE Vehicular Technology Magazine, vol. 17, no. 3, pp. 45-53, Sept. 2022.
In this article, the authors propose the use of reconfigurable intelligent surfaces (RISs) to improve coordination between terrestrial and nonterrestrial networks (NTNs) composed of high-altitude platform stations (HAPSs) and satellites. The paper presents a comprehensive RIS-assisted nonterrestrial and interplanetary communications framework, including challenges, use cases, and open issues.

J. Shi, Z. Li, J. Hu, Z. Tie, S. Li, W. Liang, and Z. Ding, "OTFS enabled LEO Satellite Communications: A Promising Solution to Severe Doppler Effects," IEEE Network, 2023, Early Access.
In this article, the challenges of LEO satellite communications were overviewed regarding coping with the severe Doppler effects, and the drawbacks of current methods were summarized. The basic principles of the orthogonal time frequency space (OTFS) scheme were introduced, and the feasibilities and advantages of applying OTFS to LEO communications were discussed.

Q. Fang, Z. Zhai, S. Yu, Q. Wu, X. Gong, and X. Chen, "Olive Branch Learning: A Topology-Aware Federated Learning Framework for Space-Air-Ground Integrated Network," IEEE Transactions on Wireless Communications, 2023, Early Access.
This paper proposes a novel approach for addressing the challenges of dynamic network slicing management in the context of next-generation satellite networks. The proposed approach leverages dynamic-adaptive artificial intelligence solutions and is evaluated through numerical simulations. The results suggest that the approach effectively manages network slicing in dynamic scenarios and could enhance the performance and efficiency of future satellite networks.

D. Tuzi, T. Delamotte, and A. Knopp, "Satellite Swarm-Based Antenna Arrays for 6G Direct-to-Cell Connectivity,"  IEEE Access, vol. 11, pp. 36907-36928, 2023.
This work proposes using satellite swarms for direct-to-cell connectivity, creating a virtual aperture in space with a smaller number of swarm elements to reduce costs and improve performance. The paper addresses the grating lobe problem and presents the logarithmic spiral array and the enhanced solutions. It also discusses research directions, system design aspects, and opportunities for innovation in deployable structures and mathematical properties.

G. Stock, J.A. Fraire, T. Mömke, H. Hermanns, F. Babayev, and E. Cruz, "Managing Fleets of LEO Satellites: Nonlinear, Optimal, Efficient, Scalable, Usable, and Robust," IEEE Transactions on Computer-Aided Design of Integrated Circuits and Systems, vol. 39, no. 11, pp. 3762-3773, 2020.
This article proposes a power-aware scheduling framework for automated satellite constellation management. It is highly flexible and scalable and can be configured to optimize various networked mission components. The approach has been integrated into the mission operations and control center at GomSpace and is set to be validated in a two-week experiment using the GOMX-4B satellite. Successful validation is a crucial step towards making this technology available to the space sector.

M. Feldmann, J.A. Fraire, F. Walter, and S.C. Burleigh. "Ring Road Networks: Access for Anyone," IEEE Communications Magazine, vol. 60, no. 4, pp. 38-44, 2022.
This article proposes a communication approach combining CubeSat platforms and delay-tolerant networking (DTN) solutions to provide asynchronous connectivity to underserved populations. The resulting ring road networks (RRNs) enable low-cost deployment of network access in communities that Internet satellite constellations cannot serve. The article presents the RRN architecture and the uD3TN protocol stack, which was flight-tested in low Earth orbit.

P. Angeletti, R. De Gaudenzi, D. Petrolati, and E. Re, “Future Technologies for Very High Throughput Satellite Systems”, Wiley International Journal on Satellite Communications and Networks, Oct. 2019.
This paper investigates the possible satellite role and technical solutions required for providing broadband services complementing the terrestrial fifth-generation wireless standards. First, the satellite networks' use cases, and services are reviewed along with system requirements. Then the satellite's potential purpose for broadband service provision and the associated challenges are illustrated with a focus on the system and space/ground technological aspects.

D. Jiang, F. Wang, Z. Lv, S. Mumtaz, S. Al-Rubaye, A. Tsourdos, and O. Dobre,  "QoE-Aware Efficient Content Distribution Scheme For Satellite-Terrestrial Networks," IEEE Transactions on Mobile Computing, vol. 22, no. 1, pp. 443-458, 1 Jan. 2023.
Satellite mega-constellations emerge as a key enabler for content distribution network, for the target QoE requirements. The authors in this paper focus on the integrated networks with varying topology and show that  user-oriented content distribution schemes can reduce the average propagation delay and network load under diverse set of conditions.

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