Source-linked AI summary
LEO Small-Satellite Constellations for 5G and Beyond-5G Communications
Israel Leyva-Mayorga, Beatriz Soret, Maik Röper, Dirk Wübben, Bho Matthiesen, Armin Dekorsy, Petar Popovski
TL;DR
LEO small-satellite constellations offer a way to support ubiquitous 5G and B5G connectivity across heterogeneous services, while their constrained platforms, mobility, and link variability pose design challenges. This paper surveys constellation architecture and logical links, characterizes physical links, and evaluates techniques including adaptive coding and modulation, MIMO, and radio-resource allocation. Its results show short LEO delays, substantial Doppler variation, and the potential to support eMBB, mMTC, and URC when links are properly designed.
Problem
Integrating dense LEO small-satellite constellations into 5G and B5G requires addressing constrained satellites, rapidly changing links, and heterogeneous service requirements.
Method
The paper surveys physical and logical links, compares their delays, Doppler shifts, and achievable data rates, and evaluates enabling physical-layer and radio-access techniques.
Results
The characterized links exhibit propagation delays below 4 ms in typical GSLs, nearly 600 kHz Doppler shifts in GSLs, and rates in inter-plane ISLs comparable to intra-plane ISLs.
Takeaways & Limitations
Properly designed LEO physical and logical links can support eMBB, mMTC, and URC, including latency requirements of a few tens of milliseconds.
Abstract
from arXiv · showhide
The next frontier towards truly ubiquitous connectivity is the use of Low Earth Orbit (LEO) small-satellite constellations to support 5G and Beyond-5G (B5G) networks. Besides enhanced mobile broadband (eMBB) and massive machine-type communications (mMTC), LEO constellations can support ultra-reliable communications (URC) with relaxed latency requirements of a few tens of milliseconds. Small-satellite impairments and the use of low orbits pose major challenges to the design and performance of these networks, but also open new innovation opportunities. This paper provides a comprehensive overview of the physical and logical links, along with the essential architectural and technological components that enable the full integration of LEO constellations into 5G and B5G systems. Furthermore, we characterize and compare each physical link category and explore novel techniques to maximize the achievable data rates.
I. INTRODUCTION
LEO small-satellite constellations can complement 5G and B5G by supporting eMBB, mMTC, and URC, but their dense deployment and constrained platforms create distinctive communication challenges. The paper surveys these characteristics, classifies constellation links, and evaluates enabling physical-layer and radio-access techniques.
- LEO characteristics and 5G/B5G role: LEO constellations deployed between 500 and 2000 km can provide nearly-global coverage for eMBB, mMTC, and URC services.URC support includes one-way latency guarantees on the order of 30 ms, with typical 2 ms ground-to-LEO propagation delays.
- Challenges: Small satellites reduce manufacturing, size, weight, and launch costs but face stringent connectivity, processing, and energy constraints, alongside potentially large Doppler shifts.These constraints are exacerbated by long transmission distances and rapid movement of the LEO space segment.
- Paper scope and contributions: The paper describes LEO constellation characteristics and challenges, compares achievable data rates, propagation delays, and Doppler shifts across physical links, and identifies their role in 5G and B5G.The comparisons use parameters from 3GPP technical reports.
- Paper scope and contributions: Its contributions include taxonomies of logical links and relevant data types, identification of enabling technologies, and evaluations of adaptive coding, modulation, MIMO, and radio-resource allocation.The physical-layer analysis specifically explores distributed MIMO using transmitting small satellites flying in close formation.
II. LEO SMALL-SATELLITE CONSTELLATIONS: CHARACTERISTICS AND CHALLENGES
LEO constellations combine short propagation paths and broad communication applicability with demanding mobility, coverage, and small-satellite resource constraints. Their rapidly changing geometry requires dynamic link mechanisms and creates substantial Doppler challenges.
- Constellation organization: A satellite constellation consists of orbital planes, communication passes, and space, ground, and user segments.A pass typically lasts a few minutes, depending on elevation angle and terminal positions.
- Latency: LEO propagation can outperform terrestrial long-distance links because space propagation is faster than propagation through optical fiber.Depending on packet lengths, data rates, and queueing delays, this can reduce total latency over long distances.
- Mobility challenges: LEO satellites move rapidly, requiring dynamic link creation and maintenance while producing larger Doppler shifts than terrestrial systems.At 500 km altitude, satellite speed can reach 7.6 km/s.
III. CONNECTIVITY
LEO constellations carry user, control, and telemetry/telecommand traffic, with TMTC supporting satellite monitoring and mission control.
- Traffic types: LEO constellation traffic comprises user data, network control data, and telemetry and telecommand data.TMTC is exchanged between ground stations and satellites to report subsystem status and issue mission-operation commands.
A. PHYSICAL LINKS AND PERFORMANCE CHARACTERIZATION
The paper characterizes LEO ground-to-satellite and inter-satellite links under Walker-star assumptions using propagation delay, Doppler shift, and achievable data rates. The links offer short delays but differ substantially in Doppler behavior, variability, and handover demands.
- Link dynamics: Ground-to-satellite links have short passes and require frequent satellite handovers, while inter-plane links undergo rapid changes and short contact times.Inter-plane handovers require neighbor discovery, matching, and connection-setup signaling.
- Evaluation framework: The physical-link evaluation compares propagation delay, Doppler shift, and achievable data rates using simulations based on 3GPP technical-report parameters.Satellite transmitters use EIRP density as the design parameter, while ground terminals use fixed transmission power.
- System model: The analysis uses a Walker-star constellation with P polar planes, 600 km minimum altitude, 10 km altitude increments, pre-established links, and perfect beam steering.Inter-plane links are formed using a greedy matching algorithm, with users modeled by a homogeneous Poisson point process.
- Propagation delay: Propagation delays below 4 ms are typical in GSLs, and intra-plane and inter-plane ISLs achieve similar delays in a 480-satellite constellation.These delays enable multi-hop transmissions that are physically unattainable in GEO systems.
- Doppler shift: Intra-plane ISLs avoid Doppler shift because their inter-satellite distances remain relatively stable.This contrasts with the larger Doppler variation in GSLs and inter-plane links.
- Achievable data rates: In interference-free conditions, GSL and intra-plane-link 95th-percentile rates resemble their medians, whereas inter-plane-link 95th-percentile rates are much higher.The difference follows from lower distance variability in GSLs and intra-plane ISLs than in inter-plane ISLs.
B. LOGICAL LINKS
Logical links describe end-to-end paths between ground terminals and satellites, using one or more physical GSL and ISL links. Four endpoint combinations support terrestrial, operational, application, and space-segment functions.
- A logical link connects a source transmitter to an end receiver and may traverse physical GSLs and ISLs unknown to the endpoints.
- The four logical link types are ground-to-ground, ground-to-satellite, satellite-to-ground, and satellite-to-satellite.
- Ground-to-ground links relay information between distant ground points and support handover, routing, and relay coordination.
- Ground-to-satellite links support maintenance and control, including ISL establishment, routing instructions, caching, and telecontrol.
- Satellite-to-ground links retrieve application data and support handover, link establishment, radio resource management, fault detection, and telemetry.
- Satellite-to-satellite links support distributed processing, sensing, routing, topology management, neighbor discovery, and autonomous space-segment operations.
- A multi-hop constellation relay can extend IoT coverage in remote areas, with status updates forwarded through the constellation using the ground-to-ground logical link.
- Earth and space observation use satellite-to-ground links for data retrieval and satellite-to-satellite links for cooperation among satellites.
IV. INTEGRATION OF LEO INTO 5G AND BEYOND
This section reviews standardization progress and radio access technologies for supporting 5G services through LEO constellations.
- The section surveys the standardization process and relevant radio access technologies for 5G services in LEO constellations.
A. 3GPP: ONGOING WORK
3GPP work integrates non-terrestrial networks into future 5G NR releases, with alternative payload, connectivity, and UE-access architectures for LEO systems.
- 3GPP is integrating LEO, MEO, GEO, and airborne platforms into future 5G NR non-terrestrial-network releases to support ubiquitous coverage.
- 5G satellite architectures use transparent payloads as relays or regenerative payloads with partially or fully functional gNB capabilities.
- Regenerative payloads can use the gNB Xn interface across the constellation, while multi-connectivity may connect a UE to terrestrial and satellite networks or two satellites.
- UEs can access the constellation through gateways for backhaul, preserving legacy UE support, or communicate directly with satellites or HAPs to maximize coverage.
- 5QI values identify services through parameters such as latency budget, maximum error rate, and priority, with dynamic values available for services outside predefined categories.
B. PHYSICAL LAYER
The physical-layer discussion addresses Doppler sensitivity, modulation and coding choices, adaptive transmission, and cooperative MIMO for LEO links. It reports trade-offs involving terrestrial compatibility, pass geometry, and distributed transmission.
- Waveforms: 5G NR’s OFDM waveform is sensitive to Doppler, requiring accurate compensation and subcarrier spacing for GSL shifts up to 600 kHz.
- Waveforms: UFMC, GFDM, and FBMC offer greater Doppler robustness and flexible resource allocation, but require higher equalization complexity than conventional OFDM.
- Waveforms: FBMC with factor-graph equalization can outperform OFDM in complexity and performance under severe Doppler, while OFDM preserves compatibility with terrestrial UEs.
- Modulation and coding: Commercial LEO missions favor robust, low-PAPR modulation, while 5G NR supports QPSK and 16-order QAM for satellite communications.
- Modulation and coding: Fixed robust modulation and coding schemes suit short GSL packets because minimizing outages and errors avoids long feedback round-trip times.
- Modulation and coding: Adaptive coding and modulation suits long GSL packets because predictable path loss allows transmission rates to follow changing channel conditions.
- Modulation and coding: 31% and 14% higher peak achievable rates than the minimum occur at 30 dBm and 50 dBm, respectively, during the ideal pass.
- MIMO: Cooperative transmission from multiple satellites increases achievable rate without increasing total transmit power or antenna count, while forming narrower beams for higher spectral efficiency.
C. RADIO ACCESS
Radio access in LEO constellations must accommodate uncertain terminal activity, changing inter-satellite geometry, and interference while exploiting predictable orbital structure. The paper considers access and resource-allocation techniques for both ground-to-satellite and inter-plane links.
- Ground-to-satellite access: Grant-based random access can suffer capacity and delay limitations when many terminals directly access satellites, while grant-free access suits short, infrequent massive-IoT packets.The large satellite coverage can exceed 5G grant-based random-access capacity; long endpoint distances also prevent traditional channel sensing for grant-free access.
- Ground-to-satellite access: Non-orthogonal medium access with successive interference cancellation may better support grant-free access for short and infrequent data packets.
- Inter-satellite access: Fixed FDMA or CDMA schemes are attractive for intra-plane ISLs because transmitter-receiver identities and relative distances remain constant.Small-satellite weight, space, and budget limitations may require wide-beam antennas, making interference mitigation relevant.
- Inter-satellite access: Inter-plane ISLs require frequent handovers because satellite pairs and distances change, although predictable constellation geometry can support centralized orthogonal-resource allocation.A centralized entity can mitigate interference among inter-plane links and between inter-plane and intra-plane links.
- Inter-satellite access: The inter-plane data-rate evaluation uses a greedy global resource-allocation algorithm, omnidirectional antennas, and zero-outage calculations under maximum feasible interference.
- Inter-satellite access: 3 orthogonal resources with OFDMA and 4 with CDMA achieve peak rates, while both methods produce closely similar rates across feasible K values.With P = 12, rates are usually more than 2× greater than with P = 7.
D. RADIO SLICING
Radio slicing must support heterogeneous eMBB, URC, mMTC, user, control, and telemetry traffic in a general-purpose satellite constellation. The section contrasts orthogonal and non-orthogonal resource sharing and considers hybrid orbital architectures.
- Service heterogeneity: Network slicing supports heterogeneous services and performance guarantees by preventing degradation caused by other services.
- Service heterogeneity: Orthogonal radio slicing improves QoS predictability but reduces network efficiency, whereas non-orthogonal slicing improves resource utilization at the expense of predictability.
- Service heterogeneity: NOMA-based non-orthogonal slicing may offer better performance trade-offs than orthogonal slicing in terrestrial communications, but LEO-specific models are still needed.
- Multiplexing: Space multiplexing benefits from line-of-sight conditions, link diversity, and multiple antennas, complementing priority-aware mechanisms for time- and frequency-multiplexed traffic.
- Hybrid architectures: Hybrid architectures can combine LEO latency advantages with GEO coverage and communication or computation capabilities, while MEO navigation constellations add flexibility and capacity for heterogeneous requirements.
V. CONCLUSIONS
The paper characterizes LEO constellation opportunities, physical and logical links, and enabling PHY/MAC and radio-slicing technologies for 5G connectivity. It concludes that LEO systems can support ubiquitous connectivity, but their differing link characteristics require careful design.
- Supported services: LEO constellations can support eMBB, mMTC, and URC use cases with latency requirements of a few tens of milliseconds.
- Paper scope: The paper characterizes physical links by propagation delay, Doppler shift, and achievable data rates, and provides a taxonomy connecting logical links, physical links, and use cases.
- Enabling techniques: Adaptive coding and modulation in the GSL and proper multiple-access resource allocation in the ISL are identified as beneficial enabling techniques.