NR-NTN Call Flow — GNSS Fix to PDU Session

Introduction

This is the same registration flow that a terrestrial NR device runs — cell selection, random access, RRC setup, NAS registration, PDU session establishment — with one new stage bolted on the front (GNSS acquisition) and every subsequent stage carrying more round-trip weight than its terrestrial equivalent. Nothing here replaces the terrestrial procedures; it schedules them around a much longer propagation delay.

Before RRC: GNSS and SIB19

1. GNSS fix acquisition. The UE acquires its own position (cold start can take several seconds; a warm/assisted fix is much faster). No RF exchange with the satellite is needed for this step — it’s a receive-only GNSS operation.
2. Cell search & SIB acquisition. The UE synchronises to the NR-NTN cell’s SSB, reads MIB/SIB1, then reads SIB19 for ephemeris, ta-Common, cellSpecificKoffset, kmac and the elevation-angle threshold.
3. Local computation. Combining its GNSS position with the broadcast ephemeris, the UE computes its own timing-advance term and Doppler pre-compensation — see Timing Advance and Doppler & Frequency Pre-compensation. Only once this is done does the UE consider itself ready to attempt access.

Random access: PRACH to Msg4

With TA and Doppler pre-applied, the UE runs 4-step (or 2-step) random access as described in Random Access in NTN and PRACH & Preamble:

1. Msg1 (PRACH preamble) — sent pre-advanced and frequency pre-compensated so it lands inside the gNB’s detection window despite the long path.
2. Msg2 (RAR) — arrives after roughly one round trip; carries the closed-loop TA residual and the uplink grant for Msg3. The grant timing already accounts for cellSpecificKoffset so the scheduled Msg3 slot isn’t in the past by the time it arrives.
3. Msg3 (RRCSetupRequest) — the UE’s first RRC message, carried on the granted PUSCH resource, requesting a connection with an establishment cause.
4. Msg4 (RRCSetup, contention resolution) — the gNB confirms the UE won contention and delivers the RRC connection configuration.

This exchange alone costs roughly two full round trips (Msg1→Msg2, Msg3→Msg4) before the UE has an active RRC connection — on GEO that’s over a second before signalling has even reached NAS.

RRC connection setup

On receiving RRCSetup, the UE moves to RRC_CONNECTED and replies with RRCSetupComplete, which piggybacks the first NAS message (typically a Registration Request) so NAS signalling doesn’t need its own separate round trip to get started. See RRC & Connection Control (NR-NTN) for the NTN-specific IEs (NTN-Config, timer scaling) carried in this exchange.

NAS registration over the new RRC connection

1. Registration Request (inside RRCSetupComplete) reaches the gNB, is forwarded over NGAP to the AMF.
2. Authentication — AMF/AUSF challenge, UE responds with 5G-AKA material. Each authentication round trip pays the full satellite delay again.
3. Security Mode Command / Complete — activates AS/NAS security.
4. Registration Accept — confirms registration, assigns/updates the 5G-GUTI and TAI list. Because NTN tracking areas can map onto beam footprints or fixed ground cells depending on deployment, the TAI list here matters more for mobility than it does terrestrially — see GNSS, Location & TACs.

PDU session establishment and DRB setup

1. PDU Session Establishment Request (NAS, often piggybacked or sent shortly after registration) reaches the SMF via AMF.
2. SMF/UPF path setup — N4 session at the UPF, IP address allocation.
3. RRCReconfiguration — the gNB configures the data radio bearer(s) mapped to the new PDU session’s QoS flows.
4. RRCReconfigurationComplete — UE confirms; user-plane data can now flow, subject to the same TA/Doppler tracking continuing throughout the connection as the satellite moves.

Reading the flow as a log

Representative sequence — illustrative, exact message names/order vary by vendor trace format:

Round-trip budget by cell type

Figures are illustrative order-of-magnitude estimates for the signalling round trips only; they exclude GNSS acquisition time, processing delays, and any retransmissions.

Summary

The NR-NTN call flow is the terrestrial 5G registration and PDU session procedure with one extra step at the very front (GNSS fix) and a round-trip cost at every stage that scales with orbit — a few tens of milliseconds on LEO, several seconds on GEO. Nothing in the message sequence itself changes; what changes is the patience the UE and network timers need, which is exactly what RRC timer scaling exists to provide.