5G NTN Radio Protocol Enhancements

Introduction

The integration of Non-Terrestrial Networks (NTN) into the 5G ecosystem represents a significant evolution, aiming to provide ubiquitous connectivity by leveraging satellites and other airborne platforms. While both traditional 5G Terrestrial Networks (TN) and 5G NTN utilize the 5G New Radio (NR) interface, the radio protocols for NTN have been significantly adapted to address the unique challenges posed by satellite communications.

The fundamental differences stem from the vast distances, high-speed mobility of satellites, and the resulting impact on signal propagation.

Core Differences in Radio Environment

The primary physical challenges that differentiate NTN from TN environments are:

• Large Propagation Delay: The round-trip time (RTT) for a signal in a terrestrial network is typically less than 1 millisecond. In an NTN, this delay ranges from tens of milliseconds for Low-Earth Orbit (LEO) satellites to several hundred milliseconds for Geostationary-Earth Orbit (GEO) satellites (e.g., up to ~600ms).

• High Doppler Shift: LEO and Medium-Earth Orbit (MEO) satellites move at very high velocities relative to a user on the ground. This movement induces significant and rapidly changing Doppler shifts in the radio signal, a factor that is negligible in TNs where base stations are stationary.

• Satellite and Cell Mobility: Unlike the fixed cells of a terrestrial network, LEO and MEO satellites create moving cells on the Earth's surface. This necessitates advanced mobility and beam management procedures to ensure service continuity as satellites move in and out of view.

  
  

5G NTN Radio Protocol and Interface Design

To accommodate these challenges, 3GPP has introduced specific enhancements across the NR protocol stack for NTN operations. The fundamental protocol layers (PHY, MAC, RLC, PDCP, RRC) remain, but their functions are adapted.

Architectural Design: Transparent vs. Regenerative

The NTN architecture dictates how the radio protocol interface is handled. There are two main approaches :

• Transparent Payload ("Bent-Pipe"): In this model, the satellite acts as a simple analogue repeater in space. It receives the signal from the ground, amplifies it, and relays it back to the user device without any processing. The 5G base station (gNB) remains on the ground, and the standard NR-Uu radio interface is essentially "bent" through the satellite. This architecture is simpler but is subject to longer end-to-end delays because all signals must travel to the ground gNB and back.

• Regenerative Payload: Here, the satellite has on-board processing capabilities and incorporates some or all of the gNB functions. The NR-Uu radio interface is terminated on the satellite itself, which can demodulate, decode, and process the signal before sending it on. This architecture enables lower latency, on-board switching, and the use of Inter-Satellite Links (ISLs) to route traffic directly between satellites without returning to an Earth station. A split architecture, with the gNB-Distributed Unit (DU) on the satellite and the gNB-Central Unit (CU) on the ground, is also possible.

  
  

Control & User plane Protocol Stack

In the 5G RAN, including for Non-Terrestrial Networks (NTN), the radio protocols are split into two distinct planes: the Control Plane and the User Plane. This separation is a fundamental design principle that allows the network to handle signaling (control) and user data (user) traffic independently and efficiently.

While the fundamental layers of the protocol stacks are the same for both 5G TN and NTN, the functions within each layer are adapted for NTN to handle the unique challenges of satellite communication, as detailed in the previous response.

User Plane Protocol Stack

The User Plane is responsible for the transmission, processing, and transfer of all user data, such as video streams, web browsing, or voice calls. Its primary focus is on high throughput and efficient data handling.

User Plane Protocol stack of 5G NTN RAN

The protocol stack for the User Plane involves the following layers, from top to bottom :

1. SDAP (Service Data Adaptation Protocol): This is the highest layer in the User Plane stack, introduced in 5G. Its key function is to map Quality of Service (QoS) Flows from the 5G Core network to the appropriate Data Radio Bearers (DRBs). This ensures that different types of user data (e.g., streaming video vs. email) receive the correct priority and handling over the radio interface.

2. PDCP (Packet Data Convergence Protocol): Below SDAP, the PDCP layer performs several critical functions for user data :

   • Sequence Numbering: Assigns sequence numbers to packets to ensure they are delivered in the correct order.

   • IP Header Compression: Compresses the headers of IP packets to reduce overhead and improve spectral efficiency.

   • Ciphering and Integrity Protection: Encrypts the user data to ensure privacy and protects it from tampering.

3. RLC (Radio Link Control): The RLC layer's main job is to ensure reliable data transfer over the error-prone radio link. Its functions include:

   • Segmentation and Reassembly: It breaks down large packets from the PDCP layer into smaller segments that are suitable for radio transmission and reassembles them at the receiving end.

   • ARQ (Automatic Repeat Request): In Acknowledged Mode (AM), it detects lost segments and requests their retransmission.

4. MAC (Medium Access Control): The MAC layer manages access to the shared radio resources. It schedules when and how a UE can transmit or receive data and handles the real-time HARQ (Hybrid ARQ) process for rapid retransmissions of corrupted packets.

5. PHY (Physical Layer): This is the lowest layer, responsible for the actual transmission and reception of radio waves over the air interface. It handles modulation, coding, beamforming, and converting digital data into radio signals.

The data path in the User Plane flows from the UE through these layers to the gNB (5G base station), and then it is tunneled to the User Plane Function (UPF) in the 5G Core network using the GTP-U protocol.

Control Plane Protocol Stack

The Control Plane is responsible for managing the radio connection and all signaling messages between the UE and the network. This includes tasks like establishing and releasing connections, managing mobility (handovers), and configuring radio bearers. Its focus is on reliability and secure message handling.

Control plane stack of 5G NTN RAN

The Control Plane protocol stack shares several layers with the User Plane but has one key additional layer :

1. RRC (Radio Resource Control): This is the main control layer in the RAN. It resides only in the Control Plane and manages all signaling between the UE and the gNB. Its key functions include:

   • Broadcasting System Information: Transmits essential network information that UEs need to connect.

   • Connection Management: Handles the establishment, maintenance, and release of the RRC connection.

   • Mobility Management: Manages cell selection, reselection, and handovers.

   • Configuration: Configures all the lower layers (PDCP, RLC, MAC, PHY) for both User and Control Plane data transfer.

2. PDCP (Packet Data Convergence Protocol): In the Control Plane, the PDCP layer's role is similar to the User Plane but is focused on signaling messages. It provides ciphering and integrity protection for RRC messages to prevent them from being read or altered.

3. RLC (Radio Link Control): The RLC layer ensures that signaling messages are transmitted reliably between the UE and gNB, using its Acknowledged Mode to retransmit any lost messages.

4. MAC (Medium Access Control): The MAC layer schedules the transmission of signaling messages alongside user data, typically giving priority to control messages.

5. PHY (Physical Layer): The PHY layer transmits the signaling messages over the air, just as it does for user data.

Control Plane messages originate in the UE's RRC layer and flow down the stack to the gNB. From the gNB, the signaling is forwarded to the Access and Mobility Management Function (AMF) in the 5G Core network via the NGAP protocol.

Here is a detailed elaboration of the key enhancements for each protocol layer:

1. Physical Layer (PHY) Enhancements

The PHY layer, which manages the raw radio transmission, required the most significant changes.

• Timing and Frequency Pre-Compensation: As previously discussed, the UE proactively calculates and applies corrections for delay and Doppler shift using its GNSS location and the satellite's ephemeris data. This is the cornerstone of the NTN physical layer design.

• Time-Division Duplexing (TDD) is Impractical: In TDD, uplink and downlink transmissions occur on the same frequency but in different time slots. This requires precise, short guard periods between slots. The massive and variable propagation delay in NTN makes it impossible to synchronize these time slots effectively. Therefore, 5G NTN exclusively uses Frequency-Division Duplexing (FDD), where uplink and downlink operate on separate, dedicated frequency bands.

• PRACH Enhancements: The Physical Random Access Channel (PRACH) is used by the UE to initiate contact with the network. For NTN, the PRACH sequences and formats have been enhanced. This ensures that even with the large frequency offsets, the initial access attempt can be successfully detected by the gNB (whether on the ground or in space).

2. Medium Access Control (MAC) Layer Adaptations

The MAC layer is responsible for scheduling data and managing the Hybrid ARQ (HARQ) process for rapid retransmissions of corrupted data packets. Standard HARQ is a "stop-and-wait" protocol that is highly sensitive to delay.

• The HARQ Problem (Stalling): A standard HARQ process works by sending a packet and then waiting for an acknowledgment (ACK) or negative acknowledgment (NACK) before sending the next packet for that process. In terrestrial networks with a Round-Trip Time (RTT) of <1ms, this is efficient. In NTN, the RTT can be hundreds of milliseconds. During this long wait, the HARQ process is stalled, unable to transmit new data, which severely degrades throughput.

• Solution 1: Increasing HARQ Processes (for LEO): To mitigate stalling, the number of parallel HARQ processes has been doubled from 16 to 32 for Non-Geostationary Orbit (NGSO) systems like LEO. This allows the MAC layer to transmit new data using other available processes while it waits for the ACK/NACK from a previous transmission, effectively keeping the data pipe full.

• Solution 2: Disabling HARQ (for GEO): For Geostationary Orbit (GEO) satellites, the RTT is so long (~600ms) that even 32 HARQ processes are not enough to prevent stalling. In these scenarios, 3GPP allows for HARQ feedback to be completely disabled at the MAC layer. This turns the MAC into a simple "fire-and-forget" scheduler. The responsibility for ensuring data reliability is then pushed up to the RLC layer.

3. Radio Link Control (RLC) Layer Enhancements

The RLC layer handles segmentation, reassembly, and, crucially, its own reliable retransmission mechanism in Acknowledged Mode (AM).

• Elevated Importance of RLC Acknowledged Mode (AM): When MAC-layer HARQ is disabled (as in GEO systems), the ARQ mechanism of RLC-AM becomes the primary tool for error correction. It detects gaps in the sequence numbers of received packets and requests retransmissions from its peer RLC entity. While its retransmission loop is slower than MAC HARQ, it is essential for reliability over long-latency links.

• Timer Adaptations: The RLC layer uses several timers to manage its operations. For example, a timer checks for the successful acknowledgment of transmitted data. In NTN, these timers must be significantly extended. Without this adaptation, the RLC entity would prematurely assume a packet is lost and trigger an unnecessary retransmission, when in fact the acknowledgment is simply delayed due to the long RTT.

4. Packet Data Convergence Protocol (PDCP) Layer Modifications

The PDCP layer is responsible for functions like IP header compression, ciphering, and maintaining data integrity.

• Discard Timer Extension: PDCP features a discardTimer that defines how long a packet can be buffered for transmission before it is dropped. This is crucial for latency-sensitive services. For NTN, this timer must be configured with a much larger value (or even set to infinite) to prevent the PDCP layer from discarding data that is merely experiencing the long but expected satellite link delay.

• Header Compression: While header compression is a key feature of PDCP, its effectiveness can be reduced over links with high error rates. In some NTN scenarios, it might be disabled to improve robustness at the cost of slightly lower efficiency.

5. Radio Resource Control (RRC) Layer Enhancements

The RRC layer is the brain of the radio interface, managing connection setup, mobility, and the broadcasting of system information.

• New System Information: The RRC is responsible for broadcasting the new NTN-specific system information blocks (SIBs). The most critical new information is the satellite ephemeris data, which is essential for UE pre-compensation.

• Enhanced Mobility Management: Cell handovers in terrestrial networks are typically triggered by signal strength. In NTN, where cells (satellite beams) are constantly moving across the Earth, mobility management is far more complex. RRC introduces new, NTN-specific handover triggers:

  • Time-Based Trigger: The network can command a UE to perform a handover at a specific time. This is useful for predictable satellite passes where service from one satellite will end at a known time.

  • Location-Based Trigger: The network can configure a handover to be executed when the UE enters or leaves a specific geographic area. This ensures a seamless transition as the UE moves across satellite beam footprints on the ground.

• Inter-Network Mobility: The RRC layer also manages mobility between the NTN and terrestrial networks, allowing a UE to hand over from a satellite connection to a ground-based cell tower (hand-in) or vice-versa (hand-out).

Comparison: 5G TN vs. 5G NTN Radio Protocols

The table below summarizes the key distinctions between the radio protocol designs for terrestrial and non-terrestrial networks.

Feature	5G Terrestrial Network (TN)	5G Non-Terrestrial Network (NTN)
Propagation Delay (RTT)	Typically <1 ms	Tens to hundreds of milliseconds (e.g., up to ~600 ms for GEO)
Doppler Shift	Negligible; handled by network	Significant and variable; pre-compensated by the UE
Cell Mobility	Cells are fixed and stationary	Cells can be fixed (GEO) or move at high speed (LEO/MEO)
gNB Location	On the ground	On the ground (transparent) or on the satellite (regenerative)
Timing Advance	Network-controlled; UE adjusts based on gNB commands	UE-controlled; pre-compensated based on GNSS and ephemeris data
HARQ Procedure	Standard operation with up to 16 processes	Modified; up to 32 processes for LEO or disabled for GEO
Key RRC Information	Broadcasts standard system information	Broadcasts ephemeris data, location-based mobility info, and common TA
Duplex Scheme	Supports TDD and FDD	FDD only

References

1. 3GPP TR 38.811: “Study on New Radio (NR) to support non-terrestrial networks”, v. 15.4.0.

2. 3GPP TR 38.821: “Solutions for NR to support Non-Terrestrial Networks (NTN)”, v. 16.1.0.

3. 3GPP TR 36.763: “Study on Narrow-Band Internet of Things (NB-IoT)/enhanced Machine Type Communications (eMTC) support for Non-Terrestrial Networks (NTN)”, v. 17.0.0.

4. 3GPP TS 38.300: “NR; Overall description; Stage-2”, v. 17.0.0.

5. 3GPP TS 38.410: “NG-RAN; NG general aspects and principles”, v. 17.0.0.

6. 3GPP TS 38.420: “NG-RAN; Xn general aspects and principles”, v. 17.0.0.

7. "5G Non-Terrestrial Networks Technologies, Standards, and System Design" By

   Alessandro Vanelli-Coralli, Nicolas Chuberre, Gino Masini, Alessandro Guidotti,

   Mohamed El Jaafari