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On the Path to 6G: Embracing the Next Wave of Low Earth Orbit Satellite Access
Xingqin Lin, Stefan Cioni, Gilles Charbit, Nicolas Chuberre, Sven Hellsten, Jean-Francois Boutillon
TL;DR
LEO satellite access could complement terrestrial networks to connect unserved areas, but its development faces operational and technical challenges. The paper surveys constellation evolution, satellite capabilities, access solutions, standardization, and business considerations, concluding that 6G should pursue tight terrestrial–LEO integration. A 60-second trajectory prediction can limit maximum delay error to 0.08 μs and Doppler-shift error to 4.8 Hz.
Problem
Providing space-based Internet through LEO mega-constellations offers connectivity for unconnected areas, but operational and technical challenges remain.
Method
The article surveys LEO constellation evolution, satellite capabilities, technical access challenges and solutions, standardization, business considerations, and future 6G integration questions.
Results
For 60 s-ahead trajectory prediction, maximum radial satellite–UE delay error is 0.08 μs and Doppler-shift error is 4.8 Hz.
Takeaways & Limitations
6G terrestrial–LEO integration should be designed from day one through a clean-slate approach rather than adapting terrestrial systems afterward.
Abstract
from arXiv · showhide
Offering space-based Internet services with mega-constellations of low Earth orbit (LEO) satellites is a promising solution to connecting the unconnected. It can complement the coverage of terrestrial networks to help bridge the digital divide. However, there are challenges from operational obstacles to technical hurdles facing the development of LEO satellite access. This article provides an overview of state of the art in LEO satellite access, including the evolution of LEO satellite constellations and capabilities, critical technical challenges and solutions, standardization aspects from 5G evolution to 6G, and business considerations. We also identify several areas for future exploration to realize a tight integration of LEO satellite access with terrestrial networks in 6G.
I. THE NEW SPACE RENAISSANCE
LEO mega-constellations are emerging as a complement to terrestrial networks for global connectivity, supported by lower-orbit advantages, falling costs, and advances in satellite technology. The transition from 5G toward 6G is expected to require progressively tighter terrestrial–satellite integration.
- LEO satellite access can complement terrestrial networks to provide connectivity everywhere and help bridge the digital divide.
- LEO orbits span 350–2,000 km; proximity to Earth lowers latency, launch energy, and satellite transmission power compared with MEO and GEO.
- At 600 km, satellites travel around 7.8 km/s and cover limited areas individually, requiring large constellations for service continuity.
- Renewed LEO interest is driven by reusable launchers, COTS components, lean manufacturing, advanced communications technologies, and investment in connecting unconnected populations.
- 3GPP is adapting 5G for satellite access, initially targeting sub-6 GHz handset connectivity and broadband access for advanced terminals.
- Because 5G was designed primarily for terrestrial systems, satellite integration is expected to remain relatively loose and may provide suboptimal performance, whereas 6G is anticipated to integrate more tightly.
A. LEO satellite constellations
Early LEO systems served limited handset-oriented voice and data markets, while renewed mega-constellations are being designed around broader 5G-era use cases and market segments.
- Iridium used 77 satellites at 780 km, while Globalstar used 48 satellites at approximately 1,400 km.
- The first-generation Iridium and Globalstar systems targeted 2G handset services, including calls and short data messaging up to 14.4 kbps.
- Early LEO systems were less successful than expected because they provided voice and data connectivity to only a limited number of user equipments.
- New mega-constellations from companies including OneWeb, Starlink, AST SpaceMobile, Amazon Kuiper, and TeleSat plan hundreds or thousands of satellites.
- Below 6 GHz, LEO systems mainly target IoT services and low-medium-rate handheld connectivity, while Ku/Ka-band systems target broadband connectivity.
B. LEO satellite capabilities
LEO satellite capabilities are shaped by spectrum range, power, traffic diversity, and manufacturing constraints, with active antennas and digital processing enabling increasingly flexible payloads.
- Payload sizing must reflect average target traffic, while limited platform power motivates efficient recharging and selective operation during low-demand periods.
- Inter-satellite links can reduce ground gateways, infrastructure costs, and operational costs while extending service beyond coastal areas.
- Sub-6 GHz and Ku/Ka-band payloads face different constraints, making a single payload covering both ranges impractical in the foreseeable future.
- Relatively large space antennas can provide comparable uplink and downlink received powers for below-6-GHz handheld use cases.
- Payloads must flexibly allocate space resources for nonuniform traffic and moving or rooftop-mounted devices while controlling manufacturing and integration costs.
- Starlink had more than 1,600 satellites in orbit, with active antennas supporting individually shapeable and steerable beams configurable from 8 to 32 beams.
- Next-generation payloads are expected to dynamically modify carrier frequencies, bandwidths, per-beam power, beam layouts, and beam-hopping configurations.
III. KEY TECHNICAL CHALLENGES AND SOLUTIONS OF LEO SATELLITE ACCESS
LEO satellite motion creates time-varying propagation, Doppler, and cell patterns that challenge access procedures and require specialized synchronization and mobility solutions.
- Fast LEO satellite movement produces time-varying propagation delays, large Doppler shifts, and changing quasi-Earth-fixed or Earth-moving cell patterns.
- For 600-km LEO satellites serving handheld UEs, example S-band parameters include relatively high EIRP density and antenna gain-to-noise-temperature figures.
A. Serving satellite cell access
LEO satellite motion creates large Doppler shifts and propagation-delay variation, requiring UE-specific uplink frequency and timing adjustments. GNSS-assisted trajectory prediction can provide sufficiently accurate compensation over short horizons.
- Synchronization challenges: 24 ppm Doppler shift, or ±48 kHz at 2 GHz, can exceed oscillator inaccuracies and misalign uplink transmissions across UEs.Different UEs observe different Doppler shifts, threatening uplink orthogonality in OFDMA systems.
- Synchronization challenges: UEs need individual frequency adjustments and timing advances to compensate for Doppler differences and propagation delays ranging from milliseconds to tens of milliseconds.The central design question is how each UE determines its required compensation values.
- Compensation method: A GNSS-equipped UE can combine its position and velocity with broadcast satellite ephemeris to calculate Doppler shifts and propagation delay.The approach is being standardized in 3GPP and requires prediction away from the ephemeris reference time.
- Prediction accuracy: At 60 s ahead, maximum radial delay and Doppler-shift errors are 0.08 μs and 4.8 Hz, respectively.These errors fit within an example 4.7 μs OFDM cyclic prefix and the ±0.1 ppm uplink requirement, illustrated for 2 GHz as ±200 Hz.
B. Mobility management
LEO mobility management differs fundamentally from terrestrial mobility because satellite cells move predictably across users. This produces frequent serving-cell changes and requires adapted measurement, tracking, and handover procedures.
- Mobility characteristics: LEO serving-cell changes occur over several seconds for Earth-moving cells and several minutes for quasi-Earth-fixed cells.Because cells move rather than remaining fixed, terrestrial mobility procedures require reconsideration.
- Mobility procedures: UEs can use satellite ephemeris to predict cell trajectories and schedule measurements at appropriate times.Measurement results support cell selection and reselection in idle mode and mobility procedures in connected mode.
- Idle-mode mobility: Earth-moving tracking areas can force stationary UEs to perform repeated tracking-area updates as the satellite-defined areas move.The network must maintain UE tracking so it can page idle devices promptly.
- Connected-mode mobility: Conditional handover can reduce signaling and improve robustness when connected-mode UEs require frequent handovers.The UE stores a network-supplied command and executes it when the specified condition is met.
IV. STANDARDIZATION ASPECTS
3GPP standardization is positioning satellite access within a common mobile ecosystem, addressing interoperability, device access, and multiple non-terrestrial-network use cases. Continued 5G evolution toward 6G is expected to tighten this integration.
- Rationale for standardization: Legacy satellite systems combined ETSI standards with proprietary protocols, which did not ensure vendor interoperability or a sustainable ecosystem.The paper contrasts this with the prosperous ecosystem produced by long-running 3GPP standardization in terrestrial mobile networks.
- 5G integration: 3GPP-based satellite standards can support seamless satellite-cellular access, multi-vendor deployment, lower costs, and native 5G features.The stated benefits include mobility, multi-connectivity, slicing, energy saving, and third-party network management.
- Non-terrestrial-network scope: 3GPP work spans satellite access and backhaul, including IoT NTN based on NB-IoT and LTE-M for low-cost, low-rate IoT devices.The work covers system aspects as well as radio-access-network, core, and terminal areas.
- Radio-interface evolution: 5G NR satellite access is evolving for sub-6 GHz handheld connectivity and higher-frequency connectivity to VSAT or ESIM terminals.These frequency ranges correspond to different terminal and service targets.
V. SATELLITE SERVICES AND BUSINESS ASPECTS
LEO satellite services span fixed broadband for areas lacking fiber or cable and mobile broadband that complements terrestrial coverage. Their capabilities, device requirements, and market segments differ across connectivity use cases.
- Service segments: 3GPP-based LEO fixed broadband can provide an alternative for local-area networks where fiber or high-speed cable is unavailable.
- Service segments: Direct-to-device LEO mobile broadband is intended to complement terrestrial networks by extending ubiquitous mobile service coverage.
- Capabilities and devices: Several Mbps over 15 MHz of S-band spectrum can support voice, messaging, and basic data connectivity for smartphones.
- Capabilities and devices: Higher-gain or higher-power devices such as ESIMs and VSATs can achieve higher speeds than smartphones.
- Market segments: The direct-connectivity market includes paid anywhere connectivity in developed markets and basic communications in uncovered developing areas.
VI. CONCLUSIONS AND 6G OUTLOOK
The paper argues that globally integrated LEO and terrestrial access will be important for 6G, but achieving it requires addressing technical, architectural, spectrum, and deployment challenges. It proposes clean-slate 6G design and early trials to guide tighter integration.
- VI. CONCLUSIONS AND 6G OUTLOOK: 6G will need LEO satellite access alongside terrestrial access to achieve truly global coverage and seamless mobility between the two networks.
- VI. CONCLUSIONS AND 6G OUTLOOK: A clean-slate 6G design should consider LEO satellite access from day one rather than extending 5G NTN incrementally.
- VI. CONCLUSIONS AND 6G OUTLOOK: New waveforms should address LEO’s time-varying Doppler shift and satellite payload power constraints while improving robustness to timing and frequency errors.
- VI. CONCLUSIONS AND 6G OUTLOOK: Integrated network architecture should support load balancing, seamless mobility, dynamic resource allocation, constellation management, and diverse deployment scenarios.
- VI. CONCLUSIONS AND 6G OUTLOOK: More flexible spectrum usage could improve interference coordination and spectrum efficiency as terrestrial and LEO systems become tightly integrated.
- VI. CONCLUSIONS AND 6G OUTLOOK: Early trials before 6G standardization around 2025 can provide practical knowledge to guide the design of tightly integrated terrestrial and LEO access.