The 3rd Generation Partnership Project (3GPP) is a collaborative effort that brings together seven telecommunications standards development organizations, referred to as Organizational Partners. 3GPP provides a stable environment to produce the reports and specifications that define 3GPP technologies, including mobile telecommunications network standards. The advance of 5G and interest in integrating non-terrestrial networks (NTNs), such as satellites, into the broader mobile network ecosystem aims to create a seamless and unified communication system. Satellites can provide connectivity in remote areas where terrestrial networks are economically unfeasible. Satellites add redundancy to communication networks, enhancing their robustness against disasters that may disrupt terrestrial infrastructure. Satellites ensure consistent connectivity for users, such as those on airplanes or ships, who frequently move across different terrestrial network boundaries.

Satellite communication networks utilize spaceborne platforms such as Low Earth Orbit (LEO) satellites, Medium Earth Orbit (MEO) satellites, and Geosynchronous Earth Orbit (GEO) satellites. The LEO satellite network is anticipated to play a crucial role in future integrated networks for universal internet and communication services. Various use cases for satellite access underscore the importance of integrating satellite networks with 5G and beyond 5G+:

• Broadcasting and Multicasting Services: This includes TV broadcasting and video streaming. Satellites can efficiently broadcast alert signals to numerous users during emergencies. Additionally, broadcasting and multicasting services for edge network content delivery via mobile network infrastructure is a promising application in 5G/5G+. Consequently, 3GPP highlights the role of satellite access in 5G/5G+ by enabling network scalability for efficient broadcast and multicast resource utilization.
• Internet of Things (IoT) Services: IoT service providers can leverage satellite mega-constellations for seamless global connectivity for devices with limited battery life and circuitry. Thus, 3GPP underscores the importance of satellites in 5G/5G+ for ensuring reliable service continuity for machine-type communications and IoT devices. Satellites also guarantee service availability for critical communications and transportation sectors such as railways, maritime, and aviation.
• Global Connectivity and Coverage: This ensures global coverage through roaming between terrestrial 5G and satellite networks, useful for tracking shipping containers and drone deliveries. Robust global coverage is vital for public safety services and emergency responders. In disaster scenarios where terrestrial networks may be compromised, satellite access can provide a reliable alternative. 3GPP identifies the role of satellites in 5G/5G+ for delivering services in underserved or unreachable areas, such as remote regions, aircraft, and ships.
• Expansion of 5G Terrestrial Networks: Extending coverage to borders and remote areas where devices may need to switch operators can be facilitated through satellite access. For low-density remote areas, direct satellite access or satellite backhaul offers a cost-effective solution. 3GPP thus acknowledges satellites' role in enhancing terrestrial networks' performance economically.
• Fast and Secure End-to-End Connectivity: Satellites provide critical applications like banking and high-frequency trading with a reliable alternative to optical fiber. In 5G/5G+, satellite access is part of the Non-Terrestrial Network (NTN). Typically, NTN comprises spaceborne platforms (GEO, MEO, LEO satellites) or airborne platforms acting as base stations or relay nodes.

From a 3GPP standardization perspective, a typical satellite access network may include a service link between the wireless device and the NTN platform. The NTN platform, which can have a transparent payload (acting as a relay) or a regenerative payload (offering base station functions). The NTN Gateway connecting the NTN payload to the ground base station or core network. Feeder links between the NTN gateway and the NTN platform. Inter-satellite links (ISLs) for direct communication between NTN payloads, usually operating in RF or optical bands. User Equipment (UE) or specialized terminals for the satellite system when it does not serve wireless devices directly.

Key Features in 3GPP Standardization

• Release 15 and Release 16: These releases provide the groundwork for 5G, focusing primarily on terrestrial networks. However, they set the stage for future inclusion of NTNs by introducing flexible architecture and interface definitions.
• Release 17: This release marked a significant milestone with the formal inclusion of NTN as part of the 5G ecosystem. Defining the architecture for integrating satellite networks with 5G, including both geostationary orbit (GSO) and non-geostationary orbit (NGSO) satellites. Developing propagation models to account for the unique characteristics of satellite communication, such as long transmission delays and Doppler shifts. Specifying how satellite networks can interface with the 5G core network, ensuring seamless handovers and interoperability.
• Release 18 and Beyond: Building on Release 17, Release 18 focuses on enhancing the support for NTNs, addressing challenges such as latency reduction, improved handover mechanisms, and advanced antenna technologies. It also looks at the coexistence of satellite and terrestrial networks to optimize resource utilization.

Release-15/16 NTN Standardization Process

In Releases 15 and 16, 3GPP has made several preliminary standardization efforts to support the integration of satellite access with 5G terrestrial networks. These efforts provided insights into the NTN ecosystem and its unique characteristics, such as propagation delays and cell/beam layout differences due to the high altitude of NTN platforms. Differences in multipath delay and Doppler spectrum models between terrestrial networks and NTN are notable. While satellite orbits can make delay variations predictable, 3GPP ensures that the location information of NTN gateways and wireless devices remains secure. Spectrum allocation for satellites is typically limited to frequencies below the mmWave spectrum, with FDD being the standard operation mode, though TDD is also considered to improve bandwidth efficiency. Efficient beam management is crucial to mitigate pathloss attenuation, making higher frequencies accessible for NTN. Rapid cell/beam changes due to LEO satellites' movement can lead to frequent handovers and inefficiencies. For platforms like HAPS, slight displacements can cause cell border shifts, necessitating robust handover, paging, and tracking area management techniques.

Release-17 NTN Standardization

Compared to terrestrial networks, satellite communication experiences longer propagation delays. In a transparent LEO satellite scenario, round-trip transmission time (RTT) can reach 50 milliseconds, while for GEO satellites, it can be up to 600 milliseconds. These delays necessitate adjustments in the NR to accommodate large RTTs. Key solutions adopted in Release 17 include:

• Uplink Timing Synchronization: Uplink synchronization in NR is achieved via timing advance (TA) commands. The structure of TA commands needs to be updated for larger RTTs in NTN. The 3GPP suggests using broadcast system information to deliver assistance information, like satellite ephemeris data and feeder link delay, enabling wireless devices to measure RTT and compensate for delays.
• Random Access Response Timer: The current random access response timer, designed for terrestrial networks, may not suffice for NTN's long RTTs. The suggested solution is to delay the timer start by the RTT, ensuring the wireless device monitors the PDCCH efficiently without unnecessary power consumption.
• Hybrid Automatic Repeat Request (HARQ): HARQ operation requires adjustments to handle large propagation delays. Increasing the number of HARQ processes is one solution, though it imposes significant memory and processing demands. Instead, the number of HARQ processes is doubled, and some HARQ feedback and retransmission processes are disabled to reduce the impact of HARQ stalling and enhance spectral efficiency.

These updates aim to balance complexity, performance, and efficiency, ensuring seamless integration of satellite and terrestrial networks in the evolving 5G/5G+ landscape. Several critical areas are under active development within 3GPP to enhance satellite access network integration:

• Enhanced User Equipment (UE) Capabilities: Development of user equipment that can operate efficiently with both terrestrial and satellite networks, including multi-mode devices that switch seamlessly between different types of networks based on availability and performance.
• Network Slicing: Implementing network slicing in NTNs to offer customized services for different use cases, such as IoT, broadband, and emergency communications. This involves dynamic resource allocation and management across terrestrial and satellite segments.
• Interference Management: Addressing the challenges of interference between terrestrial and satellite networks, especially in shared frequency bands. This includes advanced interference detection and mitigation techniques.
• Handover and Mobility Management: Improving algorithms for handover and mobility management to ensure uninterrupted connectivity for users moving between terrestrial and satellite coverage areas.
• Security Enhancements: Strengthening security protocols to protect data integrity and privacy in a heterogeneous network environment that includes satellite links.

Link Budget in NTNs

A critical aspect of NTN standardization involves addressing the link budget, which is impacted by the extensive propagation distances between User Equipment and aerial platforms. These distances range from approximately 600 kilometers to 36,000 kilometers for satellite-based NTNs, vastly exceeding the typical cell radius in terrestrial networks. Despite the potential line-of-sight paths, the pathloss due to large propagation distances significantly attenuates the electromagnetic signals. The reliance on a line-of-sight path limits the usability of NTNs, as it requires devices to maintain a clear view of the sky, which is impractical for indoor environments or devices carried in pockets or bags. As part of the effort to enhance NTN coverage, 3GPP studied the link budget and coverage performance for various data and control channels in different NTN environments in Rel-18. The channels evaluated included the physical uplink shared channel (PUSCH), physical uplink control channel (PUCCH), physical random-access channel (PRACH), and several others. One common approach to enhance coverage is through repeated message transmissions until a sufficiently high effective signal-to-noise ratio is achieved at the receiver. However, optimizing this solution requires careful consideration of several design factors, such as the entity responsible for determining repetitions, the triggering conditions for repetitions, the methodology for repeating transport blocks, and the coordination between UEs and base stations.

Future of Satellite Access Network Standardization in 3GPP

• Integration with Emerging Technologies: Exploring the synergy between NTNs and emerging technologies such as artificial intelligence (AI) and machine learning (ML) to optimize network performance and resource allocation.
• Advanced Antenna Systems: Development of advanced antenna systems, including phased array and beamforming technologies, to enhance the efficiency and capacity of satellite communication.
• Higher Frequency Bands: Investigating the use of higher frequency bands (e.g., Ka-band, V-band) for satellite communication to provide higher data rates and lower latency.
• Quantum Communications: Researching the potential of quantum communications for satellites, which could revolutionize secure communication by utilizing quantum cryptography.
• Sustainability and Space Debris Management: Addressing the environmental impact of satellite networks, including strategies for mitigating space debris and ensuring sustainable use of space resources.

The standardization of satellite access networks within 3GPP is a significant step towards the integration of satellite and terrestrial communication systems and ongoing developments aimed at further enhancing the integration of NTNs into the global telecommunications infrastructure. The adoption of 3GPP standards by new satellite operators will drive down costs, increase competition, and enhance interoperability, benefiting end-users. Open 3GPP specifications for satellite connectivity present the best opportunity to establish a large NTN ecosystem, linking terrestrial and satellite systems on a unified mobile platform. Satellite systems should complement rather than compete with terrestrial systems, supporting cooperation between satellite operators and terrestrial communication service providers (CSPs) for mutual benefits. Specific satellite spectrum use is preferable to avoid interference, ensuring seamless integration and global coverage.

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