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Satellite Communications in the New Space Era: A Survey and Future Challenges

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, G. Goussetis

arXiv:2002.08811v2eess.SP

TL;DR

Satellite communications are being reshaped by broadband demand, technological advances, and private investment, while the field lacks a consolidated account of current developments and open challenges. This survey organizes the state of the art across applications and five technical axes, concluding with future research topics for SatCom systems.

  • Problem

    Broadband transformation and emerging applications create a need to consolidate SatCom advances, requirements, and unresolved research challenges.

  • Method

    The paper surveys SatCom applications and classifies recent contributions across system aspects, air interface, medium access, networking, and testbeds.

  • Results

    The survey captures recent technical, industrial, and standardization advances, highlights applications and use cases, and identifies future challenges and open research topics.

  • Takeaways & Limitations

    The review provides a structured reference for understanding current SatCom research directions and the issues requiring further investigation.

  • Takeaways & Limitations

    High-dimensional multibeam satellite systems make precoding computationally complex, motivating further research into low-complexity techniques.

Abstract

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Satellite communications have recently entered a period of renewed interest motivated by technological advances and nurtured through private investment and ventures. The present survey aims at capturing the state of the art in SatComs, while highlighting the most promising open research topics. Firstly, the main innovation drivers are motivated, such as new constellation types, on-board processing capabilities, nonterrestrial networks and space-based data collection/processing. Secondly, the most promising applications are described i.e. 5G integration, space communications, Earth observation, aeronautical and maritime tracking and communication. Subsequently, an in-depth literature review is provided across five axes: i) system aspects, ii) air interface, iii) medium access, iv) networking, v) testbeds & prototyping. Finally, a number of future challenges and the respective open research topics are described.

I. INTRODUCTION … D. Earth Observation Data Collection

SatComs are shifting from traditional applications toward broadband data services, driven by media streaming and the need to extend broadband coverage. The survey organizes recent technological advances, applications, contributions, and open research challenges, including emerging constellation types such as LEO systems.

  • I. INTRODUCTION: SatCom system design is refocusing on broadband data services as media streaming replaces linear broadcasting and broadband coverage must be extended.The passage frames these developments as the main motivation for the current transformation of SatComs.
  • I. INTRODUCTION: The New Space initiative has produced numerous innovative broadband and Earth observation missions that require advances in SatCom systems.
  • I. INTRODUCTION: The survey aims to structure the description of SatCom technological advances while highlighting major research challenges and open issues.It also directs readers to developments, applications and use cases, and classified recent SatCom contributions.
  • A. New Constellation Types: GEO satellites have traditionally supported SatCom because they avoid rapid terminal–satellite movement and provide wide coverage from one satellite.Multibeam systems add frequency reuse and high-throughput broadband rates across the coverage area.
  • A. New Constellation Types: Large LEO constellations are attracting strong interest because they can provide high-throughput broadband services with low latency.The passage attributes their renewed feasibility to matured manufacturing and launch processes.
  • D. Earth Observation Data Collection: The supplied excerpt identifies additional coverage areas including satellite use cases for uRLLC, cloud interfaces through ground stations, low-altitude platforms, and Earth observation data collection.

A. Constellation types … B. Network Testbeds

The surveyed topics span satellite applications, communication protocols, air-interface and channel techniques, networking and data collection, and testbeds with future research directions. The block covers both enabling technologies and prototyping areas across these domains.

  • E. Space Communications: Space-communication use cases include ADS-B protocol operation and maritime AIS.These are presented as separate topics under Space Communications.
  • B. MAC Protocols for Satellite IoT; V. Air Interface: Enablers & Topics: The review addresses forward-link and return-link scheduling, alongside new scheduling techniques for satellite systems.The passages identify these as scheduling topics across MAC and air-interface sections.
  • A. Channel Modelling: Channel-modelling topics include passive and focused reflector antennas.This topic is listed under Channel Modelling.
  • D. Data Collection: The networking and data-collection coverage includes antenna trends for LEO/MEO missions and the ground.The supplied passages also identify Networking as an enabler area associated with Data Collection.
  • E. Onboard regeneration / Flying Base Stations: Onboard regeneration and flying base stations are examined through fixed-assignment-based approaches.The passage identifies fixed assignment as a topic in this networking subsection.
  • A. PHY & MAC: SDR Based: PHY and MAC research includes SDR-based approaches and precoding, MIMO, and MU-MIMO.These topics are listed together under the SDR-based PHY/MAC subsection.
  • B. Network Testbeds: Testbeds and prototyping cover network testbeds, satellite-IoT air interfaces, and future and open topics.These topics are listed under the testbeds and future-topics portions of the review.

H. Advanced satellite resource orchestration … A. 5G Non Terrestrial Network

The survey connects New Space developments, enhanced satellite capabilities, and nonterrestrial networking to emerging SatCom applications, including 5G integration and optical satellite communications. It also identifies constellation evolution, onboard processing, and application-driven connectivity as central themes.

  • I. Quantum Key Distribution through Optical Satcom: Optical satellite communications and satellite swarms with synchronization are identified as advanced topics within quantum key distribution through optical SatCom.The supplied subsection labels specifically identify these two topic areas.
  • D. Internet of Space Things/ Planetary Communications: Hybrid constellations combine different orbits to support seamless handover between MEO and GEO and backhaul LEO satellite data through higher-orbit assets.The passage gives MEO–GEO connectivity and LEO backhauling as examples of hybrid constellation architectures.
  • B. On-board Capabilities: Enhanced onboard power-generation and processing efficiency enables flexible routing, channelization, beamforming, free-space optics, and signal regeneration despite stringent reliability and power constraints.Traditional relay operation, path loss, limited power, launch-cost coupling, and limited repairability constrain onboard processing.
  • C. Non Terrestrial Networks: Nonterrestrial networks integrate satellites, UAVs, and HAPs into 5G by addressing their architectural and air-interface peculiarities while exploiting wide coverage.NTN standardization seeks seamless integration of these assets into 5G systems.
  • D. New Space: New Space combines privatized launches, satellite miniaturization, and novel space-data services, enabling quicker and relatively inexpensive access to orbit.Private companies, multiplexed cube/micro/nanosatellites, and emerging data-collection constellations are identified as defining developments.
  • III. APPLICATIONS & USE CASES: SatCom applications span 5G integration, space and optical communications, Earth observation, tracking, and sensor-data collection.The applications section explicitly frames SatComs as significant across multiple use cases, while New Space enables data collection from ground sensors via satellites.
  • A. 5G Non Terrestrial Network: NTN 5G supports service continuity for moving platforms and mission-critical communications, and extends service ubiquity to otherwise unserved regions.The cited 3GPP use cases include reliability through continuity and coverage across deserts, oceans, and forests.

1) Satellite use cases for eMBB: … 2) High Altitude Platforms:

The paper surveys satellite-enabled 5G use cases across eMBB, mMTC, and uRLLC, then examines VLEO, HAP, and LAP platforms as intermediate communications layers. HAPs offer regional, lower-delay connectivity and flexible deployment, but face autonomy and weather-related deployment challenges.

  • 1) Satellite use cases for eMBB:: Satellite-supported eMBB includes backhauling and tower feed, direct connectivity in remote areas, hybrid terrestrial-satellite broadband, and communications on moving platforms.These uses support network optimization, underserved premises, and connectivity for aircraft, vessels, and trains.
  • 2) Satellite use cases for mMTC:: Satellite support for mMTC can offload terrestrial IoT networks and provide continuity for wide-area applications in energy, transport, agriculture, and local data collection.Examples include pipeline surveillance, fleet management, asset tracking, livestock management, smart-grid metering, and sensors on moving platforms.
  • 3) Satellite use cases for uRLLC:: Satellites cannot directly support uRLLC services requiring latency below 1 ms and reliability of 1 packet loss in 10^5 packets because of communication-link latency.The cited applications include autonomous driving, remote surgery, and factory automation.
  • B. VLEO and SatCom-assisted Aerial Networks:: Technological advances have produced intermediate communications layers—VLEO satellites, HAPs, and LAPs—between terrestrial and traditional satellite segments.The platforms are classified according to their operating altitude.
  • 1) Very Low Earth Orbit:: VLEO platforms are simpler, smaller, and cheaper than LEO satellites, but denser atmospheric conditions increase aerodynamic forces, drag, and orbital-lifetime challenges.The increased drag can also create opportunities for orbit and attitude control, while requiring more frequent replacement of smaller platforms.
  • 2) High Altitude Platforms:: HAPs complement satellite networks through backhauling or trunking, combining regional coverage with shorter ground links, reduced signal degradation, and direct broadcast or multicast access.Satellite–HAP backhauling can use optical links, while HAP–ground links improve the link budget for smaller antennas or wider bandwidth.
  • 2) High Altitude Platforms:: HAPs provide intermediate coverage, rapid deployment, reconfiguration, and propagation delays of approximately 50–85 µs, but global deployment is constrained by limited autonomy and weather.High winds can displace platforms, while low temperatures reduce battery lifetime; HAPs have nevertheless been deployed successfully in emergencies.

3) Low Altitude Platforms: … D. Earth Observation Data Collection

The section surveys UAV-based low-altitude platforms, satellite-supported aeronautical and maritime tracking, and Earth-observation data services. It emphasizes flexible aerial coverage, global tracking benefits, agile space-data analytics, and the coverage and connectivity challenges of LEO systems.

  • 3) Low Altitude Platforms:: UAVs provide rapidly deployable, flexible aerial base stations with strong LoS links and mobility for restoring or enhancing cellular coverage.They can support high-rate transmissions and serve remote, densely populated, and disaster-affected areas in real time.
  • 3) Low Altitude Platforms:: UAV communication networks remain limited by cost, regulation, public acceptance, operation range, safety, beyond-visual-line-of-sight integration, and 5G capability assessment.Autonomous UAV base stations and relays in multilayer architectures remain major research topics.
  • C. Aeronautical and Maritime Tracking and Communication: Satellite-supported aeronautical and maritime tracking resembles low-rate, sporadic D2D and IoT communication with simple protocols.Satellites can extend tracking beyond the reach of conventional terrestrial systems.
  • 1) Automatic Dependent Surveillance-Broadcast:: LEO satellite ADS-B receivers can provide low-latency, secure global coverage where oceanic and Arctic airspace remains unsupervised and ground stations are congested.Space-based ADS-B forms part of a broader ATM relay network.
  • 1) Automatic Dependent Surveillance-Broadcast:: Frequent, reliable space-based ADS-B can improve airspace efficiency and aircraft security while optimizing climbing trajectories and reducing fuel consumption.The resulting increase in generated data must be routed to control centers.
  • 2) Maritime Automatic Identification System:: Satellite-based AIS extends maritime identification and positioning to global coverage, improving response reliability and enabling unmanned transoceanic transport.Potential benefits include safer hazardous-material transport, longer non-time-critical journeys, lower fuel consumption, and direct electrical or solar power use.
  • D. Earth Observation Data Collection: Agile EO services combine satellite data collection with machine-learning analytics, while LEO systems face short visibility windows that require many satellites, global gateways, or ISLs for continuous connectivity.EO supports monitoring weather, oceans, vegetation, disasters, and logistics such as container movements.

E. Space Communications … (gNB)

The merged sections frame satellite communications as essential to space exploration and organize SatCom design around constellation choices, system architecture, topology, payload processing, and inter-satellite links. They emphasize trade-offs among orbit altitude, coverage, latency, attenuation, connectivity, and onboard cooperation, while situating future exploration within expanding private-sector activity.

  • E. Space Communications: Satellite communications have enabled major space-exploration achievements and are increasingly important as Space 4.0 targets stable human presence beyond Earth and private firms pursue lunar and asteroid resources.Examples include Apollo 11, New Horizons, Rosetta, and Voyager 1; the sector is also attracting young start-ups.
  • IV. SYSTEM ASPECTS: The system-aspects discussion establishes terminology and preliminaries for analyzing satellite communication systems across their principal design components.These preliminaries support the subsequent treatment of constellations, architecture, topology, payloads, and links.
  • A. Constellation types: Orbit altitude strongly affects latency, signal attenuation, and coverage: LEO spans 500–900 km, MEO 5,000–25,000 km, and GEO 36,000 km.GEO can cover about one third of Earth and more than 99% of world population and economic activity, whereas non-GEO systems require more satellites for continuous global coverage.
  • A. Constellation types: Constellation design must evaluate coverage and related trade-offs under practical constraints such as the user terminal’s minimum elevation angle.Coverage is identified as the principal performance parameter because reliable regional coverage is the first requirement for maintaining the communication link.
  • B. Communication architecture: SatCom systems comprise space, ground, and user segments, with gateway, feeder, forward, return, and user links connecting satellite relays to fixed or mobile terminals.Telemetry, Tracking and Command stations monitor, test, configure, and maintain satellites and their orbits.
  • (gNB): Communication topology depends on the application, with star and mesh configurations supporting point-to-point and point-to-multipoint connectivity through satellite relays.In star topology, the satellite relays between nodes and a hub, while mesh topology connects multiple peer nodes.
  • (gNB): Satellites may operate as transparent amplify-and-forward relays or regenerative nodes that perform processing such as decoding and interference cancellation.Transparent payloads mainly amplify, spatially filter, and frequency-convert signals, whereas regenerative payloads add signal processing.
  • (gNB): Inter-satellite links allow satellites to cooperate on complex missions, reduce per-satellite complexity, and offload data using RF antennas or optical wireless technology.Optical links provide narrower laser-generated beams, offering a distinct advantage for these inter-satellite connections.

C. Interface with other systems … D. Spectrum

The paper describes satellite interfaces with 5G/NTN architectures and shared ground-station cloud access, then examines spectrum bands, congestion, coexistence, and interference challenges. Key trade-offs involve payload complexity, infrastructure sharing, bandwidth, weather susceptibility, and spectrum availability.

  • 1) Interface with xG systems through NTN:: NTN architectures vary by payload type—transparent or regenerative—and user access link—direct or non-direct—to interface satellites with 5G networks.Regenerative payloads can perform base-station functions, reduce RTT, and support inter-satellite links for constellation handovers.
  • 2) Interface with the Cloud through a Ground Station Network:: Shared Ground Station Networks collect satellite data into centralized cloud infrastructure, allowing customers to access data without investing in personal ground stations.The infrastructure also provides storage, processing, and routing capabilities for the large data volumes expected from LEO constellations.
  • D. Spectrum: Satellite communications use 1–50 GHz EHF spectrum, divided into lower-frequency L, S, X, and C bands and higher-frequency Ku, K, Ka, and Q/V bands.Different bands suit different climate conditions, services, and users.
  • D. Spectrum: Congestion in C and Ku bands is driving multi-beam satellites with smaller spots and migration toward Ka-band, which offers greater bandwidth but greater weather susceptibility.Higher frequencies also enable smaller antennas and support multi-antenna systems.
  • D. Spectrum: C-band reallocation for 5G must preserve vital satellite services because the band has traditionally been reserved for satellite use.The 3400–3800 MHz C-band is identified as a candidate for initial 5G mobile deployment in Europe.
  • D. Spectrum: Q/V-band feeder-link migration can relieve Ka-band congestion and provide higher feeder-link bandwidth, but severe weather impairments require gateway diversity.The migration also frees Ka-band spectrum for user links.
  • D. Spectrum: NGSO systems face costly, limited spectrum, motivating coexistence among LEO/MEO and GSO satellites, among NGSO systems, and with terrestrial networks.Cognitive radio and integrated satellite-terrestrial backhaul can improve spectrum utilization, capacity, and licensing costs, but coexistence creates interference that requires careful mitigation.

E. Standardization … 1) Fixed Satellite:

The paper presents standardization as essential for interoperability and an open, competitive market, covering DVB, integrated 5G-terrestrial satellite efforts, and CCSDS. It then introduces air-interface and channel-modeling considerations, including frequency-dependent atmospheric effects in fixed-satellite systems.

  • E. Standardization: Common open standards enable interoperability between manufacturers, supporting competition, technological development, and consumer benefits.
  • 1) DVB:: DVB provides widely adopted international open standards spanning satellite, cable, and terrestrial television across the physical and data-link layers.
  • 2) 5G NTN standardization:: Satellite-industry participation in 3GPP 5G standardization is increasing because integrated satellite-terrestrial networks offer market potential.
  • 3) CCSDS:: CCSDS develops freely available recommended standards and practices to promote interoperability, cross support, and lower operating and communications costs among space agencies.
  • V. AIR INTERFACE: ENABLERS & TOPICS: Air-interface design in satellite communications depends on channel and propagation characteristics shaped by operating frequency and system configuration.
  • A. Channel Modelling: Satellite systems encounter diverse channels, commonly involving dynamic channel components and an absence of scatterers near satellite transmit antennas.
  • 1) Fixed Satellite:: Fixed-satellite channels above 10 GHz are primarily line-of-sight AWGN channels, with Ku- and Ka-band propagation additionally affected by atmospheric fading.Long-term effects include precipitation attenuation, gaseous absorption, clouds, tropospheric scintillation, and signal depolarization; dynamic models capture fade slope and fade duration.

B. Antennas … 1) Digital Payloads:

The survey describes antenna and digital-payload architectures that improve spectral efficiency, flexibility, reliability, and coverage while exposing trade-offs in complexity, efficiency, interference, and testing. It highlights steerable antennas for evolving traffic patterns and hybrid on-board processing as important directions for future satellite networks.

  • 2) Mobile Satellites:: Multibeam antennas use narrower beams to increase gain, enable frequency reuse, and maximize spectral efficiency compared with contour-beam coverage.GEO passive architectures have evolved toward SFPB and MFPB multibeam antennas.
  • 1) Passive and focused reflector antennas:: SFPB antennas provide high gain, low sidelobes, and favorable carrier-to-interference ratios, but typically require three or four reflectors for contiguous coverage.Large feed-horn apertures improve efficiency and reduce sidelobes but constrain feed spacing and beam separation.
  • 1) Passive and focused reflector antennas:: MFPB antennas achieve contiguous coverage with one or two main reflectors by overlapping feed clusters, but require more complex beamforming networks and may reduce bandwidth or aperture efficiency.SFPB and MFPB architectures remain widely used for GEO high-throughput systems, with multiport amplifiers adding power-allocation flexibility.
  • 2) Active antenna arrays:: Active antennas distribute amplification across radiating elements, improving reliability and graceful degradation through lower peak RF power, while DRA and AFR choices depend on system requirements.DRA solutions suit LEO/MEO systems with large fields of view and lower gain demands, whereas reflector-based geometries suit GEO HTS gain targets.
  • 2) Active antenna arrays:: Digital processing enables analogue, direct-digital, or hybrid beamforming, while sparse arrays and quasi-optical networks reduce element counts or support lightweight reconfigurable beam steering with trade-offs in cost, sidelobes, and complexity.Array thinning trades sidelobe levels against element count, whereas stacked Rotman lenses can steer pencil beams along two principal axes.
  • 3) Trends in antennas for LEO/MEO missions and the ground segment:: Reconfigurable and steerable antennas are needed for LEO/MEO satellites whose traffic distribution evolves along the orbit, while ground terminals increasingly target flat-panel beam steering for satellite-on-the-move and non-GEO connectivity.Mechanically steerable designs can suffer broad beamwidth and interference problems, motivating alternative approaches that preserve performance while reducing cost.
  • 1) Digital Payloads:: Digital payloads comprise digital transparent and regenerative processing paradigms, with hybrid processing able to regenerate packet headers for on-board routing and potentially transform satellite-network services.DTPs sample waveforms without demodulation or decoding and remain agnostic to air-interface evolution, whereas regenerative processing supports deeper on-board operations.
  • 1) Digital Payloads:: On-board digital payloads can reduce latency and inefficient resource use while adding flexibility, but non-interfering monitoring and accurate wideband amplifier testing remain open challenges.In-orbit testing measures payload parameters such as filter responses, high-power-amplifier response, and G/T; wideband methods must reduce testing time and improve accuracy.

2) Precoding/MU-MIMO: · 3) Non Orthogonal Multiple Access:

Precoding/MU-MIMO enables aggressive spectrum reuse in multibeam SatCom through spatial processing, but faces complexity, nonlinearities, multicast constraints, and implementation challenges. NOMA improves spectrum utilization through simultaneous resource sharing and interference cancellation, yet satellite-specific propagation and grouping conditions limit direct transfer from terrestrial designs.

  • 2) Precoding/MU-MIMO:: Precoding exploits antenna-array spatial degrees of freedom to support SDMA and full frequency reuse across multibeam satellite downlinks.The gateway computes a precoding matrix, while the payload routes precoded signals through an antenna array serving multiple beams.
  • 2) Precoding/MU-MIMO:: Hundreds of beams can make precoding matrices extremely large, motivating low-complexity linear techniques for practical implementation.The computational burden grows with multibeam system dimensions and can complicate implementation.
  • 2) Precoding/MU-MIMO:: Symbol-level precoding uses channel-state and data information to control interference constructively rather than simply eliminate it.Its design incorporates both CSI and the symbols intended for users.
  • 2) Precoding/MU-MIMO:: Practical precoding must address multicast framing and nonlinear per-antenna amplifier distortion, whose multi-beam predistortion remains challenging.Multicast frames prevent straightforward user-by-user matrix calculation, while TWTAs introduce waveform distortion and correlated effects.
  • 2) Precoding/MU-MIMO:: Precoding research increasingly targets practical full-frequency-reuse systems, including laboratory testbeds for implementation and validation.The survey reports considerable progress in developing ad hoc testbeds alongside theoretical research.
  • 3) Non Orthogonal Multiple Access:: NOMA lets multiple terminals share one time-frequency resource, using multiuser detection and successive interference cancellation to manage co-channel interference.This breaks conventional OMA orthogonality and improves spectrum utilization.
  • 3) Non Orthogonal Multiple Access:: Satellite NOMA studies combine user grouping, scheduling, precoding, beamforming, and power allocation, with high-correlation channel pairing reported as capacity-oriented.Satellite-terrestrial studies also report NOMA performance improvements over OMA in outage probability and ergodic capacity.
  • 3) Non Orthogonal Multiple Access:: Terrestrial NOMA solutions cannot be transferred directly because satellite propagation differs and users within a beam often experience similar path loss.These channel characteristics alter suitable user-grouping strategies compared with terrestrial systems.

D. Data Collection … 1) Optical Communications:

The section surveys satellite IoT and wideband downlink technologies, alongside optical feeder links, emphasizing coverage, Doppler, throughput, processing, and atmospheric impairments. It highlights specialized air interfaces, high-rate modem architectures, gateway diversity, and transparent or regenerative payload trade-offs.

  • 1) Satellite IoT Air Interface:: Satellite LPWANs use NB-IoT, LoRa, and Sigfox to provide global connectivity and service continuity for long-range IoT.These technologies have different PHY layers shaped by extended coverage and low-power requirements.
  • 1) Satellite IoT Air Interface:: Satellite IoT air interfaces address LEO Doppler through Turbo-FSK for NB-IoT, improved LoRa acquisition, and FCrSK with strong Doppler immunity.For GEO systems, longer round-trip times motivate new air interfaces and UCSS waveforms for ultra-narrowband communications.
  • 2) Wideband Downlinks:: Wideband satellite downlinks require very-high-symbol-rate terminal modems because observation satellites and LEO sensors transmit large volumes during short satellite passages.Using the full Ka-band can produce signal spectra up to 1.5 GHz.
  • 2) Wideband Downlinks:: Wideband modem design must balance performance, latency, and complexity, while parallelism can cause access conflicts and hardware limitations can create frequency selectivity.Higher processing complexity may introduce delays that reduce algorithmic performance, and selectivity generally increases with bandwidth.
  • 2) Wideband Downlinks:: Up to 1.4 Gsps is achievable with a modem architecture targeting signal bandwidths up to 1.5 GHz, despite substantial frequency-offset and drift impairments.These impairments arise from wide frequency spans and Doppler effects in wideband scenarios.
  • 1) Optical Communications:: Optical feeder links can provide very high throughput with fewer gateways and lower ground-segment cost, but atmospheric attenuation from rain and clouds remains challenging.Q/V-, W-band, RF, and optical approaches are considered as bandwidth demand continues to grow.
  • 1) Optical Communications:: Cloud blockage and turbulence impair optical links, motivating geographically separated optical ground stations for macro-diversity and multiple apertures for turbulence mitigation.Ground-station placement seeks high cloud-free line-of-sight probability, while transmitter diversity combats turbulence over the optical feeder uplink.
  • 1) Optical Communications:: Regenerative optical payloads offer stronger FEC-based protection but higher complexity, whereas transparent payloads simplify implementation with analog or digital processing and corresponding performance trade-offs.Digital transparent processing can include FEC but introduces bandwidth expansion, noise, and complexity; analog transparent processing offers no optical-link protection.

2) Satellite swarms and synchronization: … 1) Forward Link Scheduling:

The section surveys emerging satellite swarms, deep-space communication constraints, MAC-layer resource access, and forward-link scheduling. It emphasizes synchronization and propagation challenges, adaptive resource management, and cross-layer scheduling that jointly considers channel conditions and service requirements.

  • 2) Satellite swarms and synchronization:: Satellite swarms may comprise tens to thousands of nano- or femto-satellites, but stringent synchronization requirements have limited their adoption.Absolute phase, frequency, and time synchronization are essential for distributed and collaborative beamforming.
  • 3) Deep Space Comms:: Deep-space communications face very low received signal levels because spacecraft are at least 2 million km from Earth, with downlinks further constrained by spacecraft power generation.Power generation becomes difficult far from the Sun, making the downlink especially challenging.
  • 3) Deep Space Comms:: Deep-space links address low-SNR and coverage limitations through globally separated Deep Space Network sites equipped with 35- and 70-meter antennas and cryogenic feeds.Sites are separated by approximately 120 degrees to provide continuous coverage while limiting interference and rain fading.
  • 3) Deep Space Comms:: Large deep-space transmission delays prevent real-time spacecraft operation, so spacecraft are generally controlled through pre-sequenced command programs.The supplied table reports additional free-space loss and transmission delay for different Solar System locations relative to a GEO satellite.
  • VI. MEDIUM ACCESS CONTROL: ENABLERS & TOPICS: The MAC layer controls channel access so multiple terminals or network nodes can communicate, while packet scheduling distributes satellite resources across beams and receivers.DVB-S2 scheduling uses adaptive coding and modulation based on measured SINR and channel conditions.
  • 1) Forward Link Scheduling:: Forward-link schedulers must consider channel status, packet priority, QoS requirements, buffer occupation, demand satisfaction, and interference avoidance.Frequency reuse creates interference, motivating simultaneous scheduling of users with ideally orthogonal channel vectors across beams.
  • 1) Forward Link Scheduling:: Forward-link scheduling should jointly account for PHY-layer throughput, NET-layer service requirements, QoS classes, and channel conditions in a cross-layer design.This integration is motivated by the need to queue packets according to both QoS class and channel state.
  • 1) Forward Link Scheduling:: Precoding and non-orthogonal access further intertwine PHY, MAC, and NET functions because user rates depend on packet scheduling, while prior methods often fix group sizes across beams.The cited prior approaches also use two-step user grouping procedures, including random or predetermined reference-user selection.

2) Return Link Scheduling: · 3) Resource allocation: · 4) Beamhopping:

The surveyed techniques address satellite return-link access, flexible multi-beam resource allocation, and demand-aware beam illumination. They improve resource utilization but face limitations from bursty traffic, interference, and the complexity of matching resources to demand.

  • 2) Return Link Scheduling:: MF-TDMA centrally distributes return-link resources among terminals and provides bandwidth-efficient multi-user access.The Network Control Center collects terminal traffic demands and allocates available resources under MF-TDMA.
  • 2) Return Link Scheduling:: Fixed MF-TDMA assignments can waste resources under bursty traffic, motivating uncoordinated random-access alternatives despite possible packet collisions.DVB-RCS2 optionally supports random access on the return link.
  • 3) Resource allocation:: Multi-beam resource management seeks to optimize and time-share expensive satellite resources.The reviewed solutions focus on resource management in multi-beam satellite systems.
  • 3) Resource allocation:: Power assignment allocates limited on-board power according to traffic demands and channel conditions, but some approaches neglect interbeam interference.Interbeam interference depends on the power assigned to each beam and affects total system performance.
  • 3) Resource allocation:: Digital channelization enables flexible power movement between beams, while adaptive bandwidth and frequency assignment provide an additional flexibility level.Power assignment is described as the first and easiest flexibility level; channelization adds another degree of flexibility.
  • 3) Resource allocation:: Dynamic bandwidth allocation spans orthogonal, semi-orthogonal, and full-frequency-reuse schemes, with semi-orthogonal and full reuse identified as most promising.The reviewed algorithms address demand matching, ranging from simple sub-optimal iterations to more computationally expensive methods.
  • 4) Beamhopping:: Beam hopping responds to uneven and time-varying beam demand by flexibly illuminating beams, enabling higher frequency reuse and fewer active power amplifiers.It also introduces illumination-pattern design, synchronization, and burst-acquisition challenges.
  • 4) Beamhopping:: Beam hopping performance can degrade severely in high-throughput full-frequency-reuse scenarios because simultaneously activated neighboring co-channel beams generate self-interference.The degradation is particularly pronounced when neighboring co-channel beams are active at the same time.

5) Carrier Aggregation: … 1) Coordinated:

The surveyed technologies address satellite broadband flexibility, satellite-IoT access under challenging channel and traffic conditions, and spectrum sharing among heterogeneous systems. Carrier aggregation improves bandwidth assignment, while MAC and coexistence designs balance efficiency, scalability, interference, and complexity.

  • 5) Carrier Aggregation:: Carrier aggregation flexibly combines contiguous or non-contiguous carriers across spectrum bands and supports broadband traffic with adaptive coding and modulation.Unlike channel bonding, CA is compatible with ACM and is tailored to emerging broadband services.
  • 5) Carrier Aggregation:: Carrier aggregation improves capacity-demand matching across coverage, accommodates more satellite users, and offers higher revenue potential for broadband operators.Its gateway-side MAAC designs carrier allocation, while receiver-side traffic merging converts aggregated PDU streams into a single stream.
  • B. MAC protocols for Satellite IoT: Satellite-IoT MAC design must combine low complexity with support for enormous device populations generating sporadic traffic.The literature is organized into fixed-assignment and random-access protocol groups.
  • 1) Fixed assignment based:: Fixed-assignment protocols avoid collisions by assigning separate time-frequency resources, but satellite RTT and Doppler effects challenge resource allocation, especially in LEO.NB-IoT uses OFDMA downlink and SC-FDMA uplink; proposed approaches address Doppler without modifying the fixed-assignment MAC protocol.
  • 2) Random access based:: Random access suits sporadic, low-duty-cycle satellite-IoT traffic, whereas conventional DAMA performs poorly with short packets.This mismatch makes random access a natural alternative for satellite return links.
  • C. System Coexistence: Dynamic spectrum sharing enables coordinated or uncoordinated coexistence among satellite and terrestrial networks or multiple satellite networks despite interference and access conflicts.Coordination exchanges spectrum-usage information, while uncoordinated operation can rely on spectrum sensing.
  • 1) Coordinated:: Coordinated dual-satellite architectures reduce complexity by exchanging CSI without symbol-level synchronization, but adjacent-satellite interference limits performance.Intrasatellite multiuser interference can be completely mitigated through precoding.

2) Uncoordinated: … A. PHY & MAC: SDR Based

The surveyed work spans uncoordinated satellite coexistence, SDN/NFV-enabled networking, satellite-assisted caching, and SDR-based communication testbeds. It emphasizes adaptive interference mitigation, flexible 5G integration, wide-area content delivery, and experimentally validated physical-layer techniques.

  • 2) Uncoordinated:: Uncoordinated coexistence uses gateway signaling to support cognitive interference alignment and adaptive beamhopping while protecting primary satellite operations.Secondary satellites can use shared primary beamhopping patterns and timing information to adapt their own patterns without impacting incumbents.
  • 2) Uncoordinated:: Inline interference between NGSO and GSO systems is addressed through satellite diversity, transmission cessation, orbital avoidance, satellite selection, and frequency channelization.These measures respectively reroute traffic, tolerate outages, avoid beam coupling, maximize angular discrimination, or divide spectrum among beams.
  • A. Software Defined Networking and Network Function Virtualization: SDN/NFV is presented as a key enabler for more agile, flexible, and cost-effective satellite networking and integration of satellite components into 5G ecosystems.The survey also notes that SDN remains an emerging technology whose maturity is still developing in satellite communications.
  • B. Caching over Satellite: Satellite backhaul can improve caching by broadcasting content to all base stations or multicasting it to groups, reducing repeated terrestrial transmissions across multiple links.Its high-throughput, wide-coverage beams make satellite backhauling suitable for cache placement and network offloading.
  • B. Caching over Satellite: Two-layer caching and hybrid terrestrial-satellite service models are among the surveyed approaches for optimizing content placement and converged network deployment.The two-layer design places caches on both the satellite and ground station, with cache optimization formulated through a mixed integer linear programming approach.
  • B. Caching over Satellite: Proper network virtualization enables satellite MEC to offload tasks from users beyond terrestrial server coverage while reducing latency and improving energy efficiency versus standalone terrestrial 5G.The cited works identify these benefits for satellite-assisted mobile edge computing.
  • A. PHY & MAC: SDR Based: SDR platforms provide flexible, programmable implementations for multi-standard adaptive communications, channel coding, waveform design, synchronization, interference mitigation, and MU-MIMO experimentation.Implementations use RF front ends with DSP hardware such as processors, FPGAs, or GPUs, and have been applied especially in small-satellite research.
  • A. PHY & MAC: SDR Based: Satellite SDR testbeds remain limited, but existing emulators demonstrate real-time channel modeling, interference-mitigation infrastructure, and end-to-end multibeam forward-link evaluation from LEO to GEO.The SERENADE testbed uses NI USRP infrastructure and models payload impairments including frequency responses, phase noise, amplifier nonlinearities, fading, delays, and noise.

B. Network Testbeds

Satellite simulators, emulators, and proof-of-concept testbeds are increasingly important for developing and validating satellite systems because real-system access is difficult and costly. Effective tools must reproduce propagation, delay, network-access, and radio-resource-management behaviors while supporting configurable real-time constellation networks and 5G integration.

  • B. Network Testbeds: Simulators, emulators, and PoCs/testbeds enable satellite-system development and validation despite difficult, costly access to operational satellite systems.They support research into RF propagation and performance factors such as attenuation, bit error rate, availability, and channel capacity.
  • B. Network Testbeds: Testbed tools must model orbit- and inter-satellite-link delays and reproduce DVB-S2/RCS network-access and RRM features, including adaptive modulation and coding.The required delay coverage spans GEO, MEO, HEO/LEO, and ISLs, while channel conditions should drive modulation and coding adaptation.
  • B. Network Testbeds: Experimental PoCs apply software-defined networking and 5G technologies to satellite integration, including traffic engineering and cellular–satellite access coordination.The illustrated testbeds include an SDN-based traffic-engineering solution and SATis5; broader requirements include harmonized key management and authentication.
  • B. Network Testbeds: Future simulators and emulators should provide highly configurable real-time networks with time-varying topology and link characteristics for satellite constellations.These capabilities are identified as necessary improvements for tools used by academia and industry.

IX. FUTURE & OPEN TOPICS … F. Aggressive frequency reuse and dynamic spectrum management for both GSO and NGSO

The paper identifies open topics spanning digital-twin-enabled satellite systems, cooperative and hierarchical space architectures, planetary communications, flying base stations, and dynamic spectrum sharing. These directions address satellite reliability, distributed processing, flexible coverage, onboard regeneration, and interference management challenges.

  • A. Digital twins for satellite systems: Digital twins are proposed to replace heuristic design, physical testing, and statistical methods by integrating fleet, maintenance, historical, and onboard sensor data to improve satellite safety and reliability.They also support remotely configurable space services and autonomous satellite swarms, while privacy, information misuse, and nanosatellite tracking, control, and decommissioning remain concerns.
  • B. Cooperative satellite swarms and clusters enabled by intersatellite links: Cooperative satellite swarms and clusters can improve reconfigurability, flexibility, upgradeability, responsiveness, and adaptability, but synchronization and intersatellite links remain difficult for small satellites.Dynamic channels and limited time and frequency references complicate synchronization, while additional ISL transceivers increase payload cost, mass, power consumption, and system complexity.
  • C. Hierarchical Aerial Networks: Hierarchical aerial networks using UAVs, HAPS, and higher flying layers are proposed to extend coverage and improve secure communications, space-to-ground reliability, and capacity.UAVs serve ground users at low and medium layers, while HAPS serve both UAVs and ground users.
  • D. Internet of Space Things/ Planetary Communications: Space communications are progressing toward standardized protocols and a Space Wide Web network supporting expanding astronomy and space-exploration activity.The Consultative Committee for Space Data Systems has coordinated standard protocol efforts since 1982.
  • E. Onboard regeneration / Flying Base Stations: Flying base stations mounted on UAVs can provide an efficient alternative to ultra-dense small-cell deployment, particularly for users moving in crowds.Low-latency LEO satellites with terrestrial waveforms could extend this concept through onboard base-station capabilities enabled by extensive onboard processing and regeneration.
  • F. Aggressive frequency reuse and dynamic spectrum management for both GSO and NGSO: Dynamic spectrum management must address severe co-channel and inline interference as aggressive frequency reuse and GSO–NGSO coexistence increase spectrum-sharing demands.Static planning can avoid predictable inline interference but may limit system dynamism and fail to guarantee secondary NGSO quality of service; proposed approaches include real-time beamhopping, sensing, and terminal-side beamforming.

G. Satellite Network Automation … X. CONCLUSION

The paper identifies automation, software-defined orchestration, satellite QKD, and machine learning as key directions for addressing increasingly dynamic and demanding satellite-terrestrial networks. It concludes by synthesizing recent SatCom advances, applications, and research across system, air-interface, access, and networking aspects.

  • G. Satellite Network Automation: SDN, NFV, and network slicing can automate heterogeneous 5G-satellite networks, but scalable management must handle uncertainty, dynamic topologies, resource isolation, and satellite-specific constraints.Open issues include fast heuristics, online slice management, and extending OpenFlow for satellite motion, energy, storage, and computation limits.
  • H. Advanced Satellite Resource Orchestration: Software-defined radio increases payload adaptability, enabling flexible, cost-competitive connectivity and more dynamic, efficient orchestration of satellite resources.The approach replaces custom hardware with software and supports automatic responses to evolving consumer demand and price expectations.
  • I. Quantum Key Distribution through Optical Satcom: Satellite QKD can distribute private encryption keys over optical links, but reliable operation requires stable links, low QBER, suitable optical components, and precise pointing, acquisition, and tracking.Satellite links address long-distance transmission-loss limitations associated with terrestrial optical fibre, while imposing stringent link-fidelity requirements.
  • J. Machine Learning Applications: Machine learning is being explored for weather monitoring, Earth observation, satellite operations, navigation, and communications resource management.Promising uses include carrier and power allocation, adaptive beamforming, interference-aware scheduling and precoding, beamhopping, heterogeneous-traffic scheduling, and spectrum-event detection.
  • X. CONCLUSION: Satellite communications are positioned to help meet growing demand for broadband, high-speed, heterogeneous, ultra-reliable, and low-latency connectivity as either standalone or integrated systems.The conclusion attributes this renewed importance to the growth of Internet-based applications and services and to continuing satellite technical advances.
  • X. CONCLUSION: The survey consolidates recent scientific, industrial, and standardization advances, highlighting important applications and reviewing SatCom research across systems, air interfaces, medium access, and networking.The conclusion frames the paper as an overview of current research contributions and use cases across the SatCom domain.
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