🔹 5G NTN & NB-IoT NTN Attach & Registration Scenarios
1. Scenario: UE fails to attach to the 5G NTN network.
Q: What could be the possible reasons?
A:
- Check PLMN Selection & Cell Barring for satellite connectivity.
- Verify RRC Connection Setup Request/Response for satellite beam assignment.
- Analyze NAS Registration Request & AMF response.
- Ensure correct GPS location synchronization for NTN connectivity.
- Debug Doppler shift correction parameters in gNB.
2. Scenario: NB-IoT device takes too long to attach to the NTN network.
Q: What could be the reason?
A:
- Verify Random Access Procedure (PRACH preamble delay due to long RTT).
- Ensure proper beam acquisition from satellite.
- Check if NB-IoT NTN UE is in power-saving mode (PSM/eDRX affecting attach time).
- Debug Extended Timing Advance (ETA) & HARQ retransmissions.
🔹 RRC & Radio Link Issues in NTN
3. Scenario: UE fails during RRC Connection Setup in NTN.
Q: What are the possible causes?
A:
- Incorrect beam acquisition or SSB (Synchronization Signal Block) detection.
- Timing Advance miscalculation due to long RTT.
- Interference from terrestrial networks affecting NTN signals.
4. Scenario: RRC Connection Reconfiguration fails after beam handover.
Q: How will you debug?
A:
- Check RRC Reconfiguration Failure messages.
- Verify beam handover execution time vs satellite movement speed.
- Analyze SIB19 (NTN-Specific System Information Broadcast) for incorrect satellite ID.
🔹 Mobility & Handover in NTN
5. Scenario: UE experiences frequent call drops during satellite beam handover.
Q: What are the possible reasons?
A:
- High Doppler shift leading to synchronization loss.
- Incorrect beam-switching thresholds set in gNB.
- Delay in handover execution due to long RTT (~500-700ms for GEO satellites).
6. Scenario: UE does not switch to a new satellite beam after entering a new coverage area.
Q: What could be wrong?
A:
- Incorrect beam tracking algorithm in NTN UE.
- SIB19 configuration not updated to indicate available beams.
- Hysteresis thresholds too high, delaying beam reselection.
🔹 QoS & Latency Challenges in NTN
7. Scenario: High latency observed in NTN 5G PDU session.
Q: How will you troubleshoot?
A:
- Analyze RTT delay due to satellite distance (~500ms in GEO, ~50ms in LEO).
- Check TCP retransmissions and optimize congestion control.
- Ensure Edge caching (MEC) is used to reduce latency.
8. Scenario: UE experiences inconsistent data throughput in NTN.
Q: What could be the cause?
A:
- Beam handover interruptions causing data stalls.
- HARQ retransmission failures leading to packet loss.
- Misalignment in Doppler correction impacting PHY/MAC scheduling.
🔹 HARQ & MAC Layer Issues
9. Scenario: HARQ failure rate is high in NTN.
Q: What could be wrong?
A:
- Long RTT causing excessive HARQ retransmissions.
- MCS (Modulation & Coding Scheme) too aggressive for NTN link conditions.
- Error in HARQ feedback timing due to satellite delay variations.
10. Scenario: UE reports poor BLER (Block Error Rate) in NTN mode.
Q: How will you debug?
A:
- Check SINR degradation due to satellite beam misalignment.
- Verify PHY layer FEC (Forward Error Correction) encoding settings.
- Ensure power control algorithms compensate for large path loss.
🔹 Doppler Effect & Synchronization Issues
11. Scenario: UE fails to maintain synchronization in LEO satellite mode.
Q: What could be the reason?
A:
- Fast-moving LEO satellites cause high Doppler shift (~40-100 kHz).
- Incorrect frequency correction in UE & gNB.
- UE failing to apply Doppler compensation in RRC Setup phase.
12. Scenario: Random Access Procedure fails due to timing advance miscalculation.
Q: How will you resolve this?
A:
- Ensure ETA (Extended Timing Advance) is properly configured.
- Adjust PRACH configuration index for satellite distance.
🔹 Power Saving & Coverage Optimization in NTN
13. Scenario: NB-IoT device is draining battery quickly in NTN mode.
Q: What are the possible causes?
A:
- eDRX (Extended Discontinuous Reception) cycle too short.
- PSM (Power Saving Mode) not correctly negotiated with AMF.
- Frequent beam re-selection causing high energy consumption.
14. Scenario: UE fails to transition into PSM (Power Saving Mode).
Q: What could be the issue?
A:
- AMF not sending TAU Accept with PSM parameters.
- Incorrect UE PSM timer values causing early wake-up.
🔹 NTN-Terrestrial Interworking Issues
15. Scenario: UE fails to switch from 5G NTN to a terrestrial 5G network.
Q: What will you check?
A:
- Ensure Inter-RAT handover procedure (NTN to LTE/5G) is defined in AMF.
- Verify SIB19 and SIB1 correctly advertise terrestrial network options.
16. Scenario: UE experiences service interruption when moving from GEO satellite to terrestrial 5G.
Q: What could be the reason?
A:
- Terrestrial cells not broadcasting proper neighbor lists for NTN handover.
- Wrong PLMN selection priorities in UE.
🔹 Beamforming & Antenna Issues
17. Scenario: Beamforming gain is lower than expected in NTN.
Q: What will you investigate?
A:
- Ensure gNB beamforming parameters match UE antenna configuration.
- Check Satellite beam width and EIRP levels.
18. Scenario: UE reports low RSRP in NTN despite being under coverage.
Q: How will you debug?
A:
- Verify SSB power allocation & beam sweeping parameters.
- Check for cloud cover or atmospheric interference.
🔹 NTN-Specific Security & Routing Issues
19. Scenario: Authentication fails in NTN due to SUPI encryption issues.
Q: What could be the root cause?
A:
- Ensure correct SUPI-to-SUCI encryption using home PLMN key.
- Verify AMF and UDM compatibility for NTN security algorithms.
20. Scenario: UE fails to reach a remote server over NTN due to routing issues.
Q: What will you analyze?
A:
- Check UPF route selection in NTN Core Network.
- Ensure correct NTN-DN (Data Network) mapping in SMF.
