3GPP NTN DL/UL Timing Synchronization
5G NR Non-Terrestrial Network (NTN), downlink and uplink timing synchronization between the UE and gNB has introduces several unique challenges while comparing with the traditional 5G NR networks (TN). The primarily reason for this is the larger and high variable propagation delays and significant doppler effects resulted because of moving satellites. The procedures, parameter considerations, and technical enhancements are markedly distinct.
Downlink Synchronization is NTN (gNodeB —>UE)
In both TN and NTN, the downlink synchronization (gNodeB —>UE ) process starts with the UE detecting the broadcast information PSS/SSS by the gNodeB. This allows the UE to determine cell identity, frame timing, and frequency alignment.
For NTN, the delay between the satellite based gNodeB and the UE can be in the range from tens to hundreds of milliseconds (e.g., ~240 ms for GEO orbits), this makes precise frame and frequency alignment more critical.
In NTN, to take case these long delays, the gNodB broadcasts additional information in SIB19 called NTN assistance broadcast information, 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 in NTN – TA and Pre-Compensation
In TN, uplink timing synchronization (UE —>gNodeB) mainly relies on the Timing Advance (TA) procedure, where the gNodeB 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 following
- GNSS-derived UE position (if available)
- Satellite’s ephemeris and velocity
- gNodeB signaled common Timing Advance (Common TA)
The UE estimates the expected round-trip time from its location to the satellite/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.
Call Flow for NTN DL-UL Synchronization
Following call flow illustrates, how 5G NR Non-Terrestrial Network (NTN) DL Sync and UL Synch work through a transparent satellite payload.
NTN UE Synch call flow can be understood as a sequence of assistance information delivery → UE synchronization → pre-compensated random access → timing adjustment.
Step 1 – SIB19 is provided by the ground gNB
The ground gNB prepares the NTN-specific system information. This is carried in system information SIB19. Depending on the NTN configuration, SIB19 provides information such as: Satellite ephemeris — satellite position and velocity information, Feeder-link and service-link delay information, Common TA information and Reference-point information required for timing calculations. Other NTN parameters required by the UE for synchronization and mobility.
Step 2- SIB19 is transmitted through the transparent payload
The satellite acts as a transparent payload, meaning it primarily forwards the NR signal rather than terminating the RAN protocol stack. The gNB sends the system information toward the satellite. The transparent payload forwards the information over the service link to the UE. The UE therefore receives SIB19 as part of the normal NR system-information procedure.
Step 3 – UE obtains its GNSS position
UE determines its geographical location using GNSS. This information is important because the satellite-to-UE distance changes with satellite movement and UE location. The UE can use its location together with the NTN assistance information received through SIB19.
Step 4 – UE calculates timing and frequency pre-compensation
Before transmitting on the uplink, the UE estimates the expected propagation delay and Doppler shift.
It combines: GNSS position, satellite ephemeris, timing information from SIB19, and other configured NTN parameters. The UE then applies timing pre-compensation so that its uplink transmission reaches the gNB at approximately the expected time. Frequency pre-compensation is also applied to reduce the impact of Doppler caused by satellite movement.
Step 5 – UE transmits a pre-compensated PRACH preamble
The UE initiates Random Access by transmitting a PRACH preamble. Unlike terrestrial NR, the UE may need significant NTN-specific pre-compensation before transmitting PRACH. This is important because the large satellite propagation delay can otherwise cause the PRACH transmission to arrive outside the expected timing window at the gNB.
Step 6 – Transparent payload forwards the PRACH
The satellite receives the UE’s PRACH signal. Because the payload is transparent, it forwards the PRACH toward the ground gNB. The gNB receives the PRACH after the satellite and feeder/service-link propagation delays.
Step 7 – gNB processes the PRACH and determines uplink timing
The gNB detects the PRACH preamble. It determines whether additional Timing Advance (TA) correction is required. The NTN pre-compensation performed by the UE reduces the amount of timing correction that must be provided by the gNB.
Step 8 – gNB sends the Timing Advance information
If additional adjustment is required, the gNB provides a Timing Advance command. The command is transported through the satellite path toward the UE. This provides fine uplink timing adjustment after the UE’s initial pre-compensation.
Step 9 – UE applies the Timing Advance
The UE receives the TA information and adjusts its uplink transmission timing. This compensates for the remaining timing error after the initial NTN pre-compensation. The objective is to keep the UE’s uplink transmissions aligned with the gNB’s expected reception timing.
Step 10 – RRC connection establishment continues
Once random access and uplink timing are sufficiently aligned, the normal RRC connection procedure can continue. The flow may involve messages such as: RRCSetupRequest, RRCSetup and RRCSetupComplete. Timing Advance MAC control information can also be used for fine uplink timing adjustment during the connection.
Key Timing Parameters for NTN DL/UL Synchronization
Following table show the key timing parameters important for DL/UL synchronization. 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.
| Parameter | Role in NTN | Typical / Example Value | Protocol-Test Relevance |
|---|---|---|---|
| Common TA | Offset for RTT between Ref. Point & payload | ~~120 ms for a GEO one-way path of ~36,000 km | Determines when UL transmissions are expected at the network side |
| K_offset | Scheduling offset for DL-to-UL gap | RTT + common TA, | Critical when validating DL-to-UL scheduling and HARQ timing |
| K_MAC | Offset for MAC layer scheduling | Related to NTN RTT and configured timing parameters | Affects PDSCH/PUSCH scheduling, HARQ, and MAC procedure validation |
| Doppler Precomp | Frequency shift compensation | Up to several kHz (LEO) | Important for validating frequency synchronization, PRACH access, PUSCH/PDSCH reception, and link stability |
Comparison DL/UL Synchronization for NTN and TN
Following table provide a comparison on DL/UL synchronization for NTN and TN
| Feature | Terrestrial Network (TN) | Non-Terrestrial Network (NTN) |
|---|---|---|
| DL Sync | PSS/SSS detection, usually sub-ms delay | PSS/SSS + SIB19/ephemeris info, long delay |
| Uplink Timing | gNB-triggered TA (µs-ms) | Pre-compensation by UE (100s ms) |
| Doppler | Minimal (unless High SpeedTrain) | 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 |
Testing NTN DL/UL Synchronization in Lab
To validate downlink and uplink synchronization for 5G NR NTN networks considering the feeder-link delay 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.
Validation 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.
Conclusion
The basic DL/UL synchronization procedure remains similar in NTN to the terrestrial 5G NR. The NTN still uses PSS/SSS for downlink synchronization and Timing Advance (TA) for uplink timing. However, NTN is more challenging because signals travel much longer distances, experience Doppler shifts, and the satellite is constantly moving.
To handle these challenges, NTN uses additional parameters such as Common TA, K_offset, K_mac, and SIB19, along with regular position and satellite ephemeris updates. These parameters help the UE maintain accurate timing and frequency synchronization as the satellite moves.
