NTN

Timing Synchronization

Communiation through Satellites in NTN introduces significant challenges in timing and synchronization due to the large distances involved, which result in much longer propagation delays compared to terrestrial networks. For example, a satellite in geostationary orbit (GEO) at ~36,000 km altitude introduces a round-trip time (RTT) of about 240 ms, whereas terrestrial networks typically have RTTs of a few milliseconds.

In NTNs, the large distances and relative motion between the UE and the satellite introduce significant challenges in timing, synchronization, and frequency alignment. This section of the 3GPP specification ensures that the UE and network can communicate reliably by:

Adjusting for propagation delays using offsets and Timing Advances.
Managing HARQ processes to handle long RTTs.
Pre-compensating for Doppler shifts to maintain frequency alignment.

This is critical for applications like satellite-based internet (e.g., Starlink), remote sensing, and global IoT connectivity, where NTNs are increasingly important.

The note addresses how to manage these delays to ensure proper communication between the UE (e.g., a smartphone or IoT device), the gNB (on the satellite), and the NTN gateway (on Earth), especially when the gNB and NTN gateway are co-located.

Scheduling and Timing

In NTNs, scheduling and timing are critical due to the significant propagation delays introduced by long distances between UE and satellite-based gNBs. To maintain synchronization between downlink (DL) and uplink (UL) transmissions, frame alignment is achieved at the uplink time synchronization reference point (RP), with adjustments made using a configured offset. To address timing discrepancies and ensure accurate scheduling, the system incorporates a Common Timing Advance (Common TA) along with two additional timing offsets: K_offset and K_mac. These parameters collectively compensate for round-trip delays and allow sufficient processing time for the UE between DL reception and UL transmission. Furthermore, they facilitate proper coordination in procedures such as random access and MAC CE command handling, ensuring that frame alignment and resource scheduling remain robust despite the inherent latency of NTN environments.

Timing Advance and additional terminologies related to NTN Timing Adjustment are well summarized in the following diagram.

< 38.300 - Figure 16.14.2.1-1: Illustration of timing relationship (for collocated gNB and NTN Gateway) >

Service Link RTT: The delay between the UE (on Earth) and the satellite.
Feeder Link RTT: The delay between the satellite and the NTN gateway (on Earth).
Common TA: The RTT between the RP and the NTN payload.
T_TA: The total timing advance, which includes the service link RTT and feeder link RTT.
K_mac: The offset applied to account for the RTT between the RP and gNB.

Following is breakdown of each component on the diagram and its definition.

Uplink (UL) Frame Alignment and Timing Reference Point (RP):

The UE aligns its uplink (UL) frame timing with the reference point (RP) on the satellite (gNB), adjusted by an offset called N_TA,offset. This offset accounts for the propagation delay between the UE and the satellite.

Propagation Delay in NTNs:

Due to the large distances in NTNs, the propagation delay is significant. To manage this, the system uses a Common Timing Advance (Common TA) and two offsets: K_offset and K_mac.

Timing Relationships:

Common TA: This is the round-trip time (RTT) between the RP (on the satellite) and the NTN payload (also on the satellite). Since the gNB and NTN gateway are co-located, this RTT represents the delay between the satellite and the ground gateway.
K_offset: A scheduling offset that must be at least equal to the sum of the service link RTT (between UE and satellite) and the Common TA. It ensures the UE has enough time to process downlink receptions and uplink transmissions. (Must be >= RTT of Service Link + Common TA.)
K_mac: Another offset, approximately equal to the RTT between the RP and the gNB (on the satellite). It allows the UE sufficient processing time for downlink and uplink tasks.

Practical Configuration:

The offset K_offset is used to give the UE enough time to process downlink data and prepare uplink transmissions.
K_mac is used to determine the start of the Random Access Response (RAR) window or MsgB window after a Msg1/MsgA transmission (part of the random access procedure).

HARQ (Hybrid Automatic Repeat Request) Operation

HARQ is a mechanism used in wireless communication to ensure reliable data transmission by combining error correction and retransmission. In NTNs, the large RTT complicates HARQ operation, so the document specifies how to configure it:

Downlink HARQ:

HARQ feedback (ACK or NACK) can be enabled or disabled per HARQ process.
Disabling HARQ feedback is allowed if the RTT since the last scheduled HARQ process exceeds a certain time limit

Uplink HARQ:

Two modes are defined: HARQ mode A and HARQ mode B.
These modes can be configured per HARQ process

HARQ mode B allows scheduling a HARQ process before the RTT of the previous HARQ process has elapsed, which helps manage the long delays in NTNs.
Configuration Flexibility:

Whether HARQ feedback is enabled or disabled, the network decides the configuration.

For processes with HARQ feedback enabled, the network can choose between enabling or disabling all HARQ processes.
For processes with a specific HARQ mode, the network can choose between HARQ mode A or HARQ mode B.

Timing Advance and Frequency Pre-compensation

In NTNs, precise timing and frequency control are essential to counteract the large delays and Doppler effects caused by the high-altitude movement of satellites. Timing Advance and Frequency Pre-compensation mechanisms are employed to maintain uplink synchronization and signal integrity. Before establishing a connection to an NTN cell, the UE must possess a valid GNSS position along with ephemeris data and a configured Common Timing Advance. Using this information, the UE calculates the round-trip time between itself and the uplink reference point (RP) and applies a pre-compensation value, T_TA, to align its uplink transmissions. Additionally, the UE autonomously estimates the Doppler shift on the service link and adjusts its transmission frequency accordingly. These procedures ensure that uplink signals arrive at the gNB within the expected timing and frequency windows, which is vital for maintaining reliable communication in the dynamic NTN environment.

Timing Advance (TA) for Synchronization

The UE needs to synchronize with the NTN cell using GNSS (Global Navigation Satellite System) position, ephemeris data (satellite position and velocity), and Common TA parameters.
The UE calculates the RTT between itself and the RP (on the satellite) and pre-compensates the Timing Advance (T_TA) for this RTT.
The UE also computes the frequency Doppler shift (due to the relative motion between the UE and the satellite) and pre-compensates for it in uplink transmissions.
If the UE lacks a valid GNSS position or ephemeris data, it cannot pre-compensate, and the Timing Advance and frequency pre-compensation must be updated in connected mode (i.e., while the UE is actively communicating with the network).

Doppler Shift Management

Doppler shift occurs because the satellite is moving relative to the UE, causing a frequency shift in the received signal.
The UE pre-compensates for the Doppler shift on the service link (UE to satellite).
Doppler shift on the feeder link (satellite to NTN gateway) and any transponder frequency errors are managed by the network, not the UE.

Summary :

Timing in NTNs:

NTNs have large propagation delays due to the distance between the UE, satellite, and NTN gateway.
The system uses offsets like K_offset, K_mac, and Common TA to manage these delays and ensure proper scheduling and synchronization.

HARQ in NTNs:

HARQ feedback can be enabled or disabled to handle long RTTs.
Two HARQ modes (A and B) provide flexibility in scheduling retransmissions.

Timing Advance and Doppler Compensation:

The UE uses GNSS and ephemeris data to calculate the RTT and Doppler shift, pre-compensating for both to ensure accurate uplink timing and frequency alignment.
The network handles Doppler shift on the feeder link.