Q1-Q10 · Frame & Numerology
Q1. Define numerology in 5G NR. A: SCS = 2^μ × 15 kHz, μ ∈ {0..6}. Q2. Why is 1 ms subframe invariant? A: Anchors timers (k1, k2, RA-RNTI windows) across mu. Q3. How long is a slot at μ=3? A: 125 μs. Q4. What is T_c? A: 1 / (480k · 4096) ≈ 0.509 ns — basic time unit. Q5. What is κ? A: T_s / T_c = 64, bridges LTE/NR timing. Q6. Difference between Normal and Extended CP? A: Normal = 14 sym/slot, all μ; Extended = 12 sym/slot, only μ=2. Q7. Where does the long CP go in a slot? A: Symbols 0 and 7·2^μ. Q8. How many slots in a 10 ms frame at μ=1? A: 20. Q9. Default FR1 numerology? A: μ=1 (30 kHz). Q10. Default FR2 numerology? A: μ=3 (120 kHz).
Q11-Q20 · Resource Grid & BWP
Q11. RE vs RB? A: RE = 1 SC × 1 sym; RB = 12 SC × 1 sym. Q12. CRB vs PRB? A: CRB cell-wide from Point A; PRB BWP-relative. Q13. What is Point A? A: Absolute frequency anchor — every PRB index counts from it. Q14. How is Point A signaled? A: offsetToPointA in SIB1 + k_SSB in MIB. Q15. How many BWPs can be active? A: 1 DL + 1 UL at any time. Q16. How many configured? A: Up to 4 DL + 4 UL per UE. Q17. Three BWP switching mechanisms? A: RRC reconfig, DCI, inactivity timer. Q18. Why does NR have BWPs? A: UE battery saving, scheduler flexibility, per-BWP numerology. Q19. Max PRBs per BWP? A: 275. Q20. PRB = ? A: CRB − BWPstart.
Q21-Q30 · Channels & Coding
Q21. Which channels use LDPC? A: PDSCH, PUSCH. Q22. Which use Polar? A: PDCCH, PBCH. Q23. BG1 vs BG2? A: BG1 high-rate K≤8448; BG2 low-rate K≤3840. Q24. What is RV? A: Redundancy Version, 4 values, controls rate-matched bit start. Q25. PDSCH mapping Type A vs B? A: A starts sym 0/2; B starts mid-slot for mini-slots. Q26. PUCCH formats? A: 0/1 (≤2 bits), 2 (>2 bits short), 3/4 (large payload). Q27. Long PRACH formats? A: 0,1,2,3 — 1.25 kHz SCS preamble. Q28. Short PRACH formats? A: A1-A3, B1-B4, C0, C2 — 15/30/60/120 kHz aligned with PUSCH. Q29. CORESET? A: Control Resource Set — region where UE searches for PDCCH. Q30. DCI 1_1 vs 1_0? A: 1_1 UE-specific large; 1_0 fallback compact.
Q31-Q40 · MIMO & Beamforming
Q31. MAX MIMO layers in PDSCH? A: 8. Q32. RI / PMI / CQI? A: Rank Indicator / Precoding Matrix Indicator / Channel Quality Indicator. Q33. Type I vs Type II codebook? A: Type I single-beam; Type II multi-beam high-resolution. Q34. eType II? A: Enhanced Type II (Rel-16) — frequency-domain compressed. Q35. How does the gNB pick PMI? A: From UE's CSI report. Q36. P1 / P2 / P3 beam management? A: P1 = SSB beam-sweep coarse; P2 = CSI-RS finer; P3 = UE Rx beam refinement. Q37. BFR? A: Beam Failure Recovery — UE detects degraded TCI, sends BFR PRACH. Q38. What is TCI state? A: Container for QCL info — RRC pool, MAC-CE activates 8, DCI selects 1. Q39. QCL Type A? A: Doppler shift, Doppler spread, average delay, delay spread shared. Q40. QCL Type D? A: Spatial Rx parameter (UE Rx beam direction) — mmWave only.
Q41-Q50 · Procedures & Reference Signals
Q41. RACH 4-step? A: Preamble → RAR → Msg3 → Msg4 (contention resolution). Q42. RACH 2-step? A: MsgA (preamble + Msg3) → MsgB (RAR + contention resolution). Q43. RA-RNTI formula? A: 1 + s_id + 14·t_id + 14·80·f_id + 14·80·8·ul_carrier_id. Q44. T_TA? A: Timing Advance = (N_TA + N_TA,offset) · T_c. Q45. SSB content? A: PSS + SSS + PBCH on 240 SCs × 4 sym. Q46. Number of PCIs? A: 1008 (336 SSS × 3 PSS). Q47. SSB burst L values? A: 4 (sub-3 GHz FR1), 8 (FR1 above 3 GHz), 64 (FR2). Q48. DMRS Type 1 vs 2? A: Type 1 = 6 SC/port/sym, 8 ports; Type 2 = 4 SC/port/sym, 12 ports. Q49. CSI-RS purpose? A: Channel sounding for CSI feedback. Q50. PT-RS purpose? A: Phase noise tracking, especially in mmWave high-order QAM.
Frequently Asked Questions
How should I prepare for a 5G NR PHY interview?
Master TS 38.211 (channels), 38.212 (coding), 38.214 (data procedures). Walk through PDSCH end-to-end (DCI → resource map → DMRS → LDPC → modulation → layer → precode → RE map). Practice numerology arithmetic. Be able to explain QCL Type D in plain English.
What's the most common 5G interview question?
"Walk me through what happens from cell-search to first PDSCH reception." If you can answer this in 5 minutes covering SSB→MIB→SIB1→RACH→Msg3→Msg4→RRC→DCI→PDSCH, you've nailed it.
Which 5G certification carries the most weight?
Vendor: Nokia 5G Champion, Ericsson 5G Specialist, Huawei HCIE-5G all carry weight at operators. Vendor-neutral: 3GPP delegate-level demonstration in interviews matters more than any single cert.