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A Survey on Non-Geostationary Satellite Systems: The Communication Perspective

Hayder Al-Hraishawi, Houcine Chougrani, Steven Kisseleff, Eva Lagunas, Symeon Chatzinotas

arXiv:2107.05312v4eess.SP

TL;DR

NGSO systems create communication opportunities but face challenges in supporting massive access, uninterrupted connectivity, network management, and coexistence with GSO systems. This survey reviews NGSO communication technologies, networking architectures, deployment challenges, and future research directions.

  • Problem

    NGSO networks must support growing users and satellite nodes while addressing massive-access limitations, uninterrupted connectivity, network-management demands, and interference with GSO systems.

  • Method

    The paper conducts a comprehensive survey spanning physical connectivity, radio access, inter-satellite networking, system architectures, security, caching, and deployment requirements.

  • Results

    The survey synthesizes NGSO communication developments, integration challenges involving GSO and terrestrial networks, and innovative research directions for reliable and efficient satellite communications.

  • Takeaways & Limitations

    NGSO research must address communication, networking, constellation, resource-management, interference, and user-equipment challenges to support broader satellite-network integration.

Abstract

from arXiv · show

The next phase of satellite technology is being characterized by a new evolution in non-geostationary orbit (NGSO) satellites, which conveys exciting new communication capabilities to provide non-terrestrial connectivity solutions and to support a wide range of digital technologies from various industries. NGSO communication systems are known for a number of key features such as lower propagation delay, smaller size, and lower signal losses in comparison to the conventional geostationary orbit (GSO) satellites, which can potentially enable latency-critical applications to be provided through satellites. NGSO promises a substantial boost in communication speed and energy efficiency, and thus, tackling the main inhibiting factors of commercializing GSO satellites for broader utilization. The promised improvements of NGSO systems have motivated this paper to provide a comprehensive survey of the state-of-the-art NGSO research focusing on the communication prospects, including physical layer and radio access technologies along with the networking aspects and the overall system features and architectures. Beyond this, there are still many NGSO deployment challenges to be addressed to ensure seamless integration not only with GSO systems but also with terrestrial networks. These unprecedented challenges are also discussed in this paper, including coexistence with GSO systems in terms of spectrum access and regulatory issues, satellite constellation and architecture designs, resource management problems, and user equipment requirements. Finally, we outline a set of innovative research directions and new opportunities for future NGSO research.

I. INTRODUCTION

NGSO satellites are attracting renewed interest for broadband, low-latency, and globally connected communications, while motivating research on their integration with terrestrial and existing satellite networks. This survey addresses the remaining technical and deployment gaps across multi-orbit NGSO systems.

  • NGSO satellites are being deployed to meet demand for global broadband, high-speed, ultra-reliable, and low-latency communications.
  • NGSO systems can support NTN and 6G requirements involving large throughput and global connectivity across diverse sectors.
  • Compared with GSO, NGSO targets reduced propagation delay, smaller equipment, and lower signal losses for delay-sensitive services.
  • Related surveys: Recent surveys cover satellite architectures, inter-satellite communications, small satellites, 5G NTN integration, and satellite-terrestrial convergence, but not the whole multi-orbit NGSO communication system.
  • The paper therefore surveys NGSO communication aspects, integration challenges, and future research directions for existing and future terrestrial networks.

C. Scope and Contributions

The paper surveys multi-orbit NGSO communication systems from physical-layer technologies through applications, alongside their architectures, deployment challenges, and integration with terrestrial systems. It also organizes NGSO systems by applications and identifies future research opportunities.

  • Scope: It classifies NGSO systems by applications, including broadband mega-constellations and space downstream missions.
  • Scope: The survey covers NGSO systems from the physical layer through applications, including radio access, networking, system characteristics, and architectures.
  • Challenges: The paper reviews deployment, integration, and operational challenges involving GSO coexistence, regulation, constellation design, resource management, and user hardware.
  • Future directions: It studies satellite-terrestrial integrated communication evolution and proposes innovative research directions using NGSO features for communication infrastructure.
  • Future directions: New NGSO application scenarios and future technical advances are presented as a resource for understanding current research and motivating further work.

II. NGSO SYSTEM CHARACTERISTICS AND CLASSIFICATION

NGSO systems span early low-rate services and newer broadband constellations, with architectures and providers designed for global, high-speed connectivity. Their space-based Internet systems combine space, ground, and user segments, while multi-orbit deployments pursue coverage and service diversity.

  • NGSO systems comprise early constellations for voice and low-rate data and newer constellations for global broadband services.
  • NGSO constellations can use inter-satellite links to route data in space and non-equatorial orbits to cover higher latitudes than GSO satellites.
  • A. Space-based Internet Providers: NGSO Internet providers target high-speed, low-latency access for underserved and high-latitude populations beyond practical terrestrial broadband deployment.
  • A. Space-based Internet Providers: Space-based Internet systems consist of space, ground, and user segments, supported by network management and control centres.The ground segment relays Internet data through gateways, while management and control centres handle system management and real-time signalling.
  • A. Space-based Internet Providers: Representative systems differ substantially in scale and architecture, including Starlink, OneWeb, O3b, and planned Project Kuiper constellations.The cited examples range from O3b’s 20 satellites to Starlink’s planned 12,000-satellite initial phase and Amazon’s planned constellation of over 3,000 LEO satellites.

B. NGSO Space Missions

NGSO space missions use small satellites across tracking and Earth-observation applications. Their lower cost and accessibility support operational services such as fleet, logistics, maritime, and aircraft tracking.

  • Small-satellite developments have made space more affordable and accessible to countries, universities, startups, and schools.
  • Small satellites can capture high-resolution Earth and space imagery and improve cartography across remote and populated regions.
  • Asset Tracking: NGSO asset-tracking missions support fleet, logistics, maritime, and aircraft monitoring, including in areas with limited terrestrial coverage.
  • Asset Tracking: Spire Global operates a multi-purpose nanosatellite constellation collecting AIS, ADS-B, and real-time weather datasets.

3) Scientific and Environmental Missions:

NGSO scientific and environmental missions apply small satellites to meteorology, agriculture, education, environmental protection, and government programs. These missions use payloads tailored to each objective and are supported by continuing technical advances.

  • NGSO missions use objective-specific payloads across broad scientific, environmental, and experimental disciplines.
  • Meteorology: Meteorological missions support storm detection and weather-model development, including NASA’s RainCube testing for rain and snowstorm analysis.
  • Agriculture: Agricultural applications include crop monitoring, harvest control, product-quality improvement, disease detection, and drought-impact analysis.
  • Education and Government: Small satellites support educational space experiments and government programs ranging from national security to emergency response.
  • Environmental Protection: Environmental missions monitor forest fires, melting ice, ocean pollution, oil spills, marine life, and desertification.
  • Artificial intelligence is identified as a future advance for automatic constellation management and intelligent ground-station operation.

III. NGSO COMMUNICATION PROSPECTS

NGSO communication prospects depend on physical connectivity, advanced antennas, link-budget advantages, and networking techniques. The survey also identifies mobility-related impairments and unresolved research needs for massive-MIMO integration and protocol adaptation.

  • NGSO physical-layer research covers antennas, link budgets, inter-satellite connectivity, waveform design, link diversity, multiplexing, and access procedures.
  • 1) Antenna Systems: Multi-beam, phased-array, direct-radiating, and active antennas provide higher-gain beams, wider scanning, lower scan loss, and flexible payload opportunities.
  • 1) Antenna Systems: Low-cost flat-antenna arrays can rapidly steer beams and switch between satellites and constellations, supporting beam steering and interference nulling.
  • 1) Antenna Systems: Massive MIMO can increase degrees of freedom, spectral efficiency, and data rates, but nonzero sidelobes create inter-beam interference requiring suppression.
  • 1) Antenna Systems: Further work is needed on NGSO massive-MIMO architecture, channel estimation, precoding, inter-user interference, and related capabilities.
  • 2) Link Budget: LEO links outperform GSO links in the cited link-budget comparison, including up to 16 dB higher uplink CNR for VSAT terminals.
  • 2) Link Budget: NGSO’s lower-orbit links offer lower signal losses and propagation delays, supporting smaller equipment, lower terminal power, higher spectral efficiency, and latency-critical applications.
  • Satellite mobility causes variable receive power and Doppler frequency offsets, complicating channel estimation and increasing channel-estimation requirements.

3) Inter-satellite Links:

The survey reviews inter-satellite communication technologies, emphasizing RF, optical, and emerging THz links alongside waveform developments for NGSO systems. It identifies high-speed space links and advanced optical techniques as important research opportunities, while noting implementation challenges.

  • Inter-satellite links support command, control, communication, and information processing while reducing NGSO networks’ dependence on ground stations.
  • THz communications can provide high-speed, long-distance satellite links because space avoids the atmospheric attenuation affecting terrestrial THz transmission.
  • THz satellite communications remain constrained by difficult high-power transmitter production and noisy receivers, motivating research on transmitters, receivers, and adaptive antenna arrays.
  • FSO links offer higher data rates, smaller size, and lower power than alternatives, but require more complex acquisition and tracking.
  • FSO technology has progressed toward operational use through projects such as EDRS, while CCSDS has developed specifications for coding and synchronization.
  • Future space-based FSO research can adapt coherent modulation, multiplexing, coherent receivers, and advanced digital signal processing from fiber-optic systems.
  • DVB-S2(X) supports adaptive modulation and coding based on link SNR, while NGSO waveform design must address variable delay, Doppler shift, and nonlinear degradation for 5G NR integration.

5) Access Design and Multiplexing:

The survey organizes NGSO access design around random and coordinated schemes, then examines NOMA, massive access, MIMO, and carrier aggregation for heterogeneous and growing user populations. It highlights scalability, interference, mobility, and resource-coordination challenges while identifying multi-satellite signal combination as an opportunity.

  • Access classification: Radio access is divided into random access, which is opportunistic and independent, and coordinated access, which centrally assigns dedicated non-interfering resources.
  • Access classification: Coordinated access suits high-throughput, high-QoS communications, whereas uncoordinated access suits power-limited terminals with limited visibility and lower rate requirements.
  • Random access: Random-access methods include slotted and unslotted Aloha variants, including spread-spectrum Aloha, LoRa, and Sigfox for satellite and IoT scenarios.
  • Coordinated access: Coordinated access assigns resources systematically through paradigms including FDMA, TDMA, CDMA, and SDMA.
  • Advanced access: NOMA can improve NGSO access flexibility and capacity by serving multiple users non-orthogonally, while addressing interference, terminal count, and spectral-efficiency objectives.
  • Massive access: Existing access methods face scalability limits for massive user populations because random access can collide and coordination overhead can overwhelm the system.
  • Multi-satellite access: Combining signals from multiple visible NGSO satellites can improve aggregate data rate, beam-load balancing, and link robustness through path diversity.
  • System implications: NGSO can provide continuous, ubiquitous, and cost-effective coverage, but scalable deployment requires further physical- and radio-link-layer development.

B. Networking Aspects

Networking aspects of NGSO systems center on space information networks, in-space backhauling, and routing architectures that reduce ground dependence while coping with dynamic, heterogeneous satellite environments.

  • 1) Space Information Networks:: ISLs and IOLs can reduce reliance on gateways and make connectivity more economically suitable where gateway sites are difficult to acquire.The architecture remains constrained by restricted spectrum, energy, and orbital resources.
  • 1) Space Information Networks:: Space information networks integrate GSO and NGSO satellites with HAPS to support real-time communications, data processing, traffic routing, and seamless coverage.They also enable cooperation between satellites for throughput maximization and latency minimization.
  • 2) In-space Backhauling:: In-space backhauling requires routing mechanisms that account for topology variation, bandwidth, link delay, traffic profiles, and node computation and storage.These parameters change across heterogeneous multi-orbit networks.
  • 2) In-space Backhauling:: Traditional routing requires each satellite to store the entire network topology and routing tables, which is difficult and costly in complex SINs.The resulting burden consumes power and bandwidth.
  • 3) Software-defined Networking:: SDN architectures for multi-layer SINs still need development to handle node dynamics and heterogeneous traffic flexibly.Existing small-satellite platforms remain largely terrestrial-dependent and focus mainly on monitoring or remote-area Internet provisioning.

4) Network Slicing:

NGSO network slicing and softwarization address growing network complexity by supporting heterogeneous services through shared radio access, while control, mobility, interference, and resource constraints complicate deployment.

  • 4) Network Slicing:: Network slicing is proposed for multi-layer NGSO networks because it can support heterogeneous services sharing the same radio access network.SDN and NFV provide the programming and software-based network-function foundations for this approach.
  • 4) Network Slicing:: SDN enables dynamic programming through centralized control points, while NFV deploys computing and storage functions as software.Together, they increase flexibility in managing growing NGSO networks.
  • System Operation:: NGSO operation must jointly configure multiple satellites as constellations move relative to users and experience geographically uneven traffic demand.Deploying ISLs can partially mitigate the capital expenditure associated with large gateway networks.
  • System Operation:: Centralized architectures can provide efficient network management but increase complexity and operating expenditures.This creates a trade-off between management efficiency and operational cost.
  • 1) Resources Management and Optimization:: Resource allocation must jointly consider power, bandwidth, time, beams, and antennas, although NGSO has fewer payload resources and rapidly outdated optimization parameters.Non-convex, parameter-heavy problems may require iterative convexification, low-complexity metaheuristics, or machine learning.
  • 1) Resources Management and Optimization:: Overlapping coverage can cause adjacent NGSO satellites to serve the same demand, producing interference and resource waste while requiring more complex joint optimization.This effect is especially relevant for asynchronous satellites in different orbital planes.

2) Interference Management:

NGSO interference management must address aggregate emissions, heterogeneous constellation interactions, and coexistence with GSO systems, while spectrum-sharing schemes offer potential efficiency and reliability benefits.

  • 2) Interference Management:: Interference analysis must account for aggregate interference from many multi-beam satellites using frequency reuse.Traditional mitigation methods may be ineffective because emerging NGSO deployment parameters are heterogeneous and ambiguous.
  • 2) Interference Management:: GSO–NGSO coexistence studies examine exclusion angles, power control, inter-site distance, throughput effects, and band splitting across Ka and V bands.The relevant interference interactions depend on both frequency band and constellation properties.
  • 2) Interference Management:: Interference between ISLs can occur wherever inter-satellite communication takes place, not only where satellite coverage areas overlap.Relative motion and constellation dynamics make this scenario especially complex.
  • 2) Interference Management:: Optical ISLs offer greater robustness against interference and signal jamming than RF ISLs.This advantage is relevant to interference affecting GSO satellites and ground stations.
  • 2) Interference Management:: Interweave spectrum sharing senses licensed spectrum before transmission, whereas underlay sharing constrains NGSO power to a primary GSO interference threshold.These paradigms are considered for coexistence between satellite systems.
  • 2) Interference Management:: Integrating spectrum-sharing paradigms into NGSO communications can provide benefits in spectral efficiency and transmission reliability.

4) Secure Communications:

Secure communications and deployment regulation are central NGSO concerns: satellite links face attacks and computational constraints, while large constellations require coordinated spectrum, infrastructure, and coexistence rules.

  • 4) Secure Communications:: NGSO security must address eavesdropping, jamming, spoofing, and unauthorized commands across satellite and ground-related communications.Limited onboard computational power can make satellites vulnerable to processing overload during short visibility windows.
  • 4) Secure Communications:: Upper-layer cryptography has high computational complexity, motivating research into QKD, blockchain, and physical-layer security.QKD may be unsuitable for fast-passing LEO satellites because key exchange can require time.
  • 4) Secure Communications:: Blockchain technology is proposed to authenticate space transactions between NGSO constellations and ground stations while supporting decentralized tracking and monitoring.
  • 4) Secure Communications:: Physical-layer security can provide protection without adding encryption and decryption complexity, but satellite–terrestrial links impose distinct design constraints.
  • A. Regulatory and coexistence issues: The paper organizes NGSO deployment challenges around constellation and architecture design, coexistence, operational issues, and user equipment requirements.These challenges are framed in the context of integrating NGSO systems with GSO and terrestrial networks.
  • A. Regulatory and coexistence issues: EPFD limits aggregate emissions from all NGSO satellites to protect GSO receivers, replacing the insufficient single-entry PFD approach for rapidly growing constellations.

B. Satellite Constellation Design

NGSO constellation design must adapt coverage and capacity to non-uniform, uncertain demand while balancing flexibility, cost, energy use, and user-equipment complexity. Lower orbit improves latency but increases satellite-tracking and handover requirements.

  • Demand-aware deployment: Constellation parameters include orbit type, altitude, number of orbital planes, satellites per plane, and inter-plane phase factor.
  • Demand-aware deployment: Fixed constellation designs can be inefficient because they ignore non-uniform and uncertain demand across Earth.
  • Demand-aware deployment: Staged flexible deployment adapts constellation growth to demand evolution, initially covering regions with high anticipated demand.
  • Flexible architectures: Reconfigurable constellations adjust satellite orbital characteristics for different regional or global areas of interest, but repeated reconfiguration increases maneuvering demands and energy consumption.
  • User equipment: Lower NGSO altitude reduces latency but makes satellite motion faster from Earth, requiring user terminals to track satellites and perform handovers.
  • User equipment: Electronic beam steering and multi-satellite tracking can replace costly mechanical steering and support lower-cost, multi-orbit user terminals.
  • User equipment: 3GPP integration supports handheld access to LEO and GSO systems in S-band, while higher-gain terminals can use S-band and Ka-band.

D. Operational Issues

NGSO deployment introduces operational, regulatory, environmental, and architectural challenges alongside its communication potential. The survey highlights coexistence, orbital sustainability, adaptable network architectures, and standards for responsible operation.

  • Environmental and regulatory issues: Mega-constellations raise concerns about light pollution, space debris, satellite collisions, and effects on astronomical observation.
  • Environmental and regulatory issues: Information sharing among regulators, astronomers, and industry could support best practices and standards protecting astronomy and constellation sustainability.
  • System integration: NGSO success depends on regulatory coexistence, optimized constellation patterns, and suitable user equipment within heterogeneous terrestrial and non-terrestrial networks.
  • Architectural flexibility: ORAN can improve interoperability and openness by splitting the RAN across vendor-independent hardware and standardized software and control interfaces.
  • Architectural flexibility: Conventional satellite networks are often hard-wired to a single manufacturer, limiting flexibility and adaptability during missions exceeding 10 years.
  • Architectural flexibility: ORAN reconfiguration and vendor independence could support constellation expansion and non-proprietary hardware or software replacement, but diverse hardware requires careful compatibility design.

B. Broadband Connectivity for Space Missions

Space-based Internet providers can connect small downstream satellites through higher-orbit networks, reducing dependence on distributed ground stations and enabling real-time space communications. These benefits introduce routing, load-balancing, processing, and energy-scheduling challenges.

  • Connectivity architecture: Space-based Internet systems can provide global coverage, low-latency communication, and high-speed Internet access for small-satellite missions.
  • Connectivity architecture: Direct Internet access through a higher-orbit provider can keep downstream small satellites connected without private or shared distributed ground-station networks.
  • Connectivity architecture: Reducing required ground stations can make space systems more inexpensive and sustainable while enabling real-time and reliable space communications.
  • Network operations: Space-based Internet systems can coordinate multiple constellations and transmit TT&C data between small satellites and ground network control centers.
  • Network operations: Achieving these connectivity benefits requires complex load balancing, shortest-delay path selection, and handling of high relative speeds and frequent handovers.
  • Onboard processing: Limited onboard processing makes complex tasks such as resource optimization, Earth-observation processing, and IoT aggregation difficult for a single satellite processor.
  • Space-based cloud: Space-based cloud systems offer distributed secure storage and faster transfer than traditional terrestrial clouds for delay-sensitive services.
  • Space-based cloud: Space-based data centers face sporadic ground-station contact, limited satellite power, time-varying channels, and scheduling requirements not addressed by terrestrial-cloud algorithms.

F. Caching Over NGSO Satellites

NGSO networks can extend connectivity and computing across caching, aerial, IoT, and terrestrial systems. Their benefits depend on managing time-varying topology, limited onboard resources, and coordination across heterogeneous platforms.

  • Caching architecture: NGSO satellites can cache content near users, multicast data, and rapidly update cached copies across locations.
  • Caching architecture: Hybrid federated satellite-terrestrial content delivery can serve repeated requests without multiple transmissions, saving spectrum and reducing delay.
  • Caching architecture: Combining caching with edge computing can integrate data processing, content analysis, and storage across NGSO networks.
  • Caching architecture: Caching placement must account for time-varying topology, limited onboard resources, fast convergence, and low algorithmic complexity.
  • Aerial integration: UAVs and HAPS offer broad coverage, strong line-of-sight links, flexible deployment, and mobility for integrated satellite-air-ground systems.
  • Aerial integration: Lower NGSO latency is described as necessary for proper autonomous operation of UAVs.
  • Aerial integration: Massive networks combining multiple satellite orbits, UAVs, HAPS, and terrestrial infrastructure create coordination, navigation, and synchronization challenges.
  • Overall prospects: The survey anticipates seamless global broadband connectivity, high service density, space-based clouds, and faster, safer, more flexible storage and processing.
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