LTTS_Pre-assessment for layering

1. A 5G NR UE is connected on a 100 MHz n78 carrier at 3.5 GHz with 30 kHz SCS. The UE enters a high-speed train section. The gNB observes that RSRP is almost unchanged, but SINR varies quickly, CQI drops, and PDSCH HARQ retransmissions increase. DM-RS is still detected and transmit power is stable. PDSCH had been using 256-QAM with a high code rate.

Which physical-layer mechanism most likely explains the BLER increase?

2. A real-time gaming flow is carried on an NR bearer for which occasional packet loss is preferable to late retransmissions. The operator compares RLC AM and RLC UM. Under bursty radio loss, AM reduces packet loss but produces occasional large latency spikes.

Which RLC choice better matches the stated service objective?

3. An RLC AM entity has a large SDU ready, but the MAC grants only a small number of bytes in the next transmission opportunity. The RLC layer sends part of the information and later continues it when more grant becomes available.

Which function is being exercised?

4. An NR UE has a large uplink application backlog. The gNB continues to schedule only small grants because the last Buffer Status Report indicated little data. The UE's buffer grew significantly after that report.

Which MAC mechanism should update the scheduler's view?

5. A UE has two uplink logical channels: a delay-sensitive signaling flow with high priority and a large best-effort data flow with lower priority. Both are backlogged, and the gNB gives a small uplink grant.

Which MAC behavior should normally occur?

6. A 5G NR UE uses PDCP duplication over two radio legs for a latency-critical bearer. Because one leg is temporarily congested, the two copies of the same PDCP PDU arrive at very different times. Upper layers receive each packet only once, and no application duplicate is observed.

Which PDCP behavior is primarily responsible?

7. A cell-edge NR UE is already transmitting near its maximum total power. The scheduler suddenly doubles the number of PUSCH PRBs while keeping a demanding MCS. The UE's total transmit power cannot increase further, and PUSCH BLER rises even though path loss is unchanged.

Which physical-layer effect is most likely?

8. An LTE cell becomes heavily loaded and applies access barring to ordinary mobile-originated data. Two idle UEs try to access at the same time: one is starting a background data session and the other is making an emergency call. The network wants to suppress nonessential access without blocking emergency service.

Which RRC/NAS access behavior is most appropriate?

9. A connected LTE UE is moving from Cell A toward Cell B. Cell B becomes a few dB stronger than Cell A and remains better for the configured time-to-trigger. The network receives the expected measurement report and initiates handover.

Which event most closely matches this behavior?

10. A 5G NR FR2 UE is served by a narrow beam. While the UE turns a corner, the serving-beam RSRP drops sharply for a few tens of milliseconds, but another beam from the same cell becomes strong. The UE initiates beam-failure recovery and service resumes without an RRC re-establishment.

Which interpretation is most accurate?

11. A low-latency NR bearer is configured with a short PDCP discard timer. During a brief radio outage, packets accumulate above RLC. When radio conditions recover, some older packets are never transmitted even though newer packets are delivered.

What is the most likely explanation?

12. An NR DRB uses RLC AM. A PDSCH transmission fails at PHY and is retransmitted by HARQ. After several HARQ attempts, the MAC gives up and the RLC PDU is still missing. Later, RLC retransmits the missing data successfully.

Why are both mechanisms present?

13. A UE detects a very strong LTE cell, but system information marks the cell as barred. A weaker neighboring cell is not barred and otherwise meets selection criteria.

What should the UE do?

14. An idle NR UE is known to be registered and reachable, but it repeatedly misses paging. RF coverage is good. Tracing shows that the UE and network disagree on the UE's paging occasion/DRX-related configuration, so the UE is asleep when the page is transmitted.

What is the most likely RRC-level problem?

15. A gNB wants to configure 4-layer downlink MIMO and 256-QAM for a newly connected UE. The UE capability information indicates support for only two layers and no 256-QAM on the serving band.

What should the network do?

16. Several UEs attempt initial access to the same 5G NR cell after a temporary outage. The gNB detects many PRACH transmissions with adequate received power, but a subset of UEs repeatedly restart random access. For those UEs, Msg2 is sometimes received, yet contention resolution later fails. Increasing UE PRACH transmit power does not materially improve success.

What is the most likely radio-access reason?

17. An NR UE reliably transmits small HARQ-ACK payloads on PUCCH near the cell edge. After the UCI payload grows because CSI is added, the configured PUCCH transmission begins to fail frequently. PDSCH remains healthy and the UE is not moving.

Which design change is most appropriate?

18. An LTE UE loses the serving radio link while in RRC_CONNECTED. It still has a valid security/context state and quickly finds a suitable cell belonging to the same network. The UE wants to restore the connection faster than starting a completely new session.

Which RRC procedure is most appropriate?

19. An RLC receiver gets PDUs with sequence numbers 100, 101, 103, 104; SN 102 is missing. It waits for reordering/reassembly, but 102 does not arrive before the relevant timer expires. Data above the gap is then handled according to the RLC mode and state.

Which problem is the timer designed to prevent?

20. An LTE UE near a cell border repeatedly hands over between two cells as fast fading makes their measured levels alternate by 1-2 dB. There is no coverage hole, but signaling load and packet interruption increase.

Which tuning change is most likely to reduce the ping-pong behavior?

21. A battery-powered NR UE uses connected-mode DRX. Downlink packets arriving just after the UE leaves its on-duration are often delivered tens of milliseconds later, although radio quality and scheduler capacity are good.

What is the most likely reason for the added delay?

22. An NR UE has new uplink data but no configured grant and no suitable PUSCH opportunity. It attempts to send a Scheduling Request on PUCCH, but the SR is repeatedly missed because the configured PUCCH resource is in deep fade. The gNB therefore does not provide a dynamic grant.

What is the direct consequence?

23. A 5G NR UE was released from RRC_CONNECTED into RRC_INACTIVE with its context retained. A few seconds later, new uplink data arrives while the UE is still within the area where the inactive context is valid.

Which procedure best exploits RRC_INACTIVE?

24. An NR UE in RRC_IDLE sends an RRCSetupRequest and starts the relevant establishment timer. Due to downlink interference, it never decodes an RRCSetup or RRCReject. The timer expires.

What is the expected interpretation of this event?

25. An NR UE reports CQI corresponding to a high MCS during a quiet interval. Immediately afterward, an external interferer turns on and remains active for several slots. The gNB continues to use the old MCS until a newer CQI arrives, producing a burst of HARQ NACKs.

Which mechanism best describes the problem?

26. A 5G NR PDSCH transport block is first transmitted with a high MCS and fails. The gNB retransmits the same HARQ process using a different redundancy version, and the UE successfully decodes after combining the two receptions even though neither reception alone had enough quality.

What enabled the successful decode?

27. A gNB schedules a UE over a wide 100 MHz NR carrier. Wideband CQI is moderate and stable, yet BLER is high whenever PDSCH is allocated in one particular portion of the band. Spectrum measurements show a narrowband interferer affecting only that region, while the rest of the band is clean.

Which action is most directly supported by the physical-layer evidence?

28. Two neighboring 5G NR TDD cells use the same carrier but have independently configured DL/UL slot patterns. UEs near the border experience severe uplink BLER only during certain slots. Their PUSCH power is sufficient, but the serving gNB measures bursts of very strong interference exactly when the neighboring cell transmits downlink at high power.

What is the most likely cause?

29. An LTE UE at the cell edge has acceptable downlink quality but begins to show intermittent PUSCH decoding failures. The eNB sees that the UE's uplink arrival time is drifting toward the edge of the expected timing window. The downlink RSRP is stable, and the UE is not power limited. After a timing-advance update, uplink BLER returns to normal.

What best explains the failure before the update?

30. An NR UE powers on and detects synchronization signals from a cell with strong RSRP. It reads system information and finds that the desired PLMN is not offered by the cell, while another nearby cell broadcasts the desired PLMN.

What is the correct interpretation?