5G-NTN: DL/UL Timing Synchronization

Venkateshu
Sep 26
5 min read

Downlink and uplink timing synchronization between the UE and gNB in a 5G NR Non-Terrestrial Network (NTN) introduces several unique challenges compared to traditional (terrestrial) 5G NR networks (TN), primarily due to the much greater and more variable propagation delays and significant Doppler effects caused by satellite movement. The procedures, parameter considerations, and technical enhancements are markedly distinct.

Downlink Synchronization (UE to gNB)

In both TN and NTN, the downlink synchronization process starts with the UE detecting the Primary and Secondary Synchronization Signals (PSS/SSS) broadcast by the gNB. This allows the UE to determine cell identity, frame timing, and frequency alignment.
For NTN, the delay between the satellite-based gNB and the UE can range from tens to hundreds of milliseconds (e.g., ~240 ms for GEO orbits), making precise frame and frequency alignment essential.
To address these long delays, the gNB in NTN broadcasts additional information in SIB19, conveying satellite ephemeris, current satellite position, and timing advance parameters so that the UE can estimate and compensate for delay and Doppler shift before uplink initiation.

Uplink Synchronization – Timing Advance and Pre-Compensation

In TN, uplink timing synchronization mainly relies on the Timing Advance (TA) mechanism, where the gNB adjusts the UE’s transmission timing based on the measured round-trip travel time. TA values are relatively small (in the order of a few microseconds to milliseconds).
In NTN, before random access, the UE must autonomously pre-compensate the TA and frequency offset based on:
- GNSS-derived UE position (if available)
- Satellite’s ephemeris and velocity
- gNB signaled common Timing Advance (Common TA)
The UE estimates the expected round-trip time from its location to the satellite (and potentially to a co-located gateway), then adjusts its uplink transmission timing accordingly. If GNSS or ephemeris information is missing, the UE may not transmit until these are restored.

Key Timing Parameters for NTN

Parameter	Role in NTN	Example Value (GEO)
Common TA	Offset for RTT between Ref. Point & payload	~120 ms (for one-way 36,000km)
K_offset	Scheduling offset for DL-to-UL gap	RTT + common TA
k_mac	Offset for MAC layer scheduling	RTT to/from gNB
Doppler Precomp	Frequency shift compensation	Up to several kHz (LEO)

These parameters are configured and signaled to the UE to enable reliable scheduling and HARQ operations.
The UE may also periodically report its TA in connected mode, with "triggered" reporting in NTN to handle the satellite’s movement-induced changes.

Feature	5G NR Terrestrial (TN)	5G NR Non-Terrestrial (NTN)
Downlink Sync	PSS/SSS detection, usually sub-ms delay	PSS/SSS + SIB19/ephemeris info, long delay
Uplink Timing	gNB-triggered TA (µs-ms range)	Pre-compensation by UE (ms-100s ms range)
Doppler Mgmt	Minimal (unless HST)	Essential; UE calculates Doppler/frequency shifts
Signaling	Standard RRC messages, TA cmd	SIB19, Common TA, satellite ephemeris data
Random Access	Standard PRACH, fast feedback	PRACH pre-delay, slow HARQ, GNSS aided
Practical challenges	Site densification, sector calibrations	GNSS dependency, ephemeris, orbit dynamics

Practical Aspects and Considerations

GNSS Dependency: For NTN, reliable GNSS reception is fundamental for the UE to determine its location and pre-compensate for delay and Doppler. Loss of GNSS means the UE cannot access the network until a fix is restored.
SIB19 and Ephemeris: System Information Block 19 provides the satellite’s ephemeris. Accurate, real-time ephemeris data is required for precise synchronization.
HARQ Design: HARQ round-trip timing in NTN may require new timing information elements and greater buffers for extremely long response windows.
Configuration Offsets: Operators must set appropriate values for Common TA, K_offset, and k_mac in coverage areas, balancing scheduling efficiency and UE processing time.
Ping Time and QoS: Even with synchronization, the inherent RTT (especially GEO) limits voice and URLLC services. Applications must be designed with this delay in mind.

Example

For a UE connecting via a GEO satellite at 36,000 km, the downlink frame arrives with a delay of ~120 ms; the UE uses SIB19 and GNSS data to estimate propagation, applies a TA pre-compensation (e.g., 240 ms), and shifts its transmit window for accurate PRACH during random access. Doppler shift is also pre-calculated, especially in LEO scenarios where satellite velocity is significant.
In TN, the same procedure involves negligible pre-compensation (microseconds), and Doppler is only addressed in high-speed train or vehicular scenarios.

Test Procedure Overview

To validate downlink (DL) and uplink (UL) synchronization in 5G NR NTN networks considering the feeder-link delay, the test procedure generally involves the following key steps:

Test Setup:
- Configure the NTN test environment with a transparent payload satellite or HAPS relay.
- Set up the ground gNB (or gateway) and ensure that it broadcasts NTN-specific system information including SIB19 with satellite ephemeris and feeder-link delay parameters.
- Ensure UE supports NTN features including GNSS-based positioning for timing advance pre-compensation.
Downlink Synchronization Validation:
- Power on the UE and allow it to acquire synchronization signals (PSS/SSS) from the gNB via the transparent payload.
- Confirm that UE decodes SIB19 and receives accurate satellite ephemeris and feeder-link delay info.
- Measure the UE's downlink frame timing relative to the expected arrival time calculated from feeder-link delay, satellite position, and gNB frame timing.
Uplink Timing Advance Validation with Feeder-Link Delay:
- UE uses GNSS location and broadcast ephemeris/feeder-link delay info for initial uplink timing advance pre-compensation.
- UE sends PRACH preamble with pre-compensated timing to ground gNB.
- Ground gNB measures actual reception timing then sends a Timing Advance Command (RRC MAC CE) if further TA adjustment is needed.
- Confirm that uplink transmissions (PRACH and subsequent UL data) align within the expected timing window when accounting for feeder-link delay and round trip propagation time.
Dynamic Delay and Drift Testing:
- Test feeder-link delay variations and drift rate (which can be up to ±24 µs/sec or ppm scale).
- Measure synchronization maintenance by monitoring TA updates and drift compensation via network signaling (e.g., Timing Advance Command or updated SIB info).
- Evaluate performance under satellite mobility scenarios (e.g., LEO satellites) to ensure synchronization robustness.
Performance Metrics:
- Timing offset between expected and actual symbol/frame arrival at UE for DL.
- TA error margin on the uplink between UE transmission and gNB reception.
- Impact of feeder-link delay variations on HARQ timing and round trip time.
- Validating extended timers (e.g., T300, T319) to support NTN long delay operations.

Summary

While the fundamental synchronization procedures (PSS/SSS for DL, TA for UL) are conceptually similar, practical implementation in NTN must compensate for massive propagation delays, Doppler shifts, and dynamic satellite movement. Additional parameters (Common TA, K_offset, k_mac, SIB19) and periodic position/ephemeris updates are crucial, making NTN synchronization significantly more complex than standard terrestrial 5G NR deployments.

References