PRACH = First Uplink Transmission
Before RRC, before any data, the UE sends a Random Access preamble on PRACH. The gNB detects it, derives the timing offset, and replies with a Random Access Response (RAR) on PDSCH containing TA command, UL grant, temp-CRNTI, and the matched preamble ID.
Long Preamble Formats · 0, 1, 2, 3
Long formats use 1.25 kHz SCS for the preamble. Format 0: 1-symbol preamble (0.8 ms), N_CP = 0.1 ms, used for normal cells. Format 1: 2-symbol preamble (1.6 ms), longer reach. Format 2: 4-symbol preamble (2.7 ms), very long reach (rural / macro). Format 3: extended-CP variant.
Short Preamble Formats · A1-A3, B1-B4, C0, C2
Short formats use 15/30/60/120 kHz SCS aligned with PUSCH numerology. A-formats: short CP, no guard. B-formats: short CP + guard. C-formats: extra-long CP for long delay spread. C0 and C2 cover the very-large-cell case in FR2.
4-Step RACH Procedure
1) Msg1 = preamble on PRACH. 2) Msg2 = RAR on PDSCH (RA-RNTI scrambled). 3) Msg3 = RRC Connection Request on PUSCH (using grant from RAR). 4) Msg4 = RRC Connection Setup on PDSCH with contention resolution.
2-Step RACH Procedure (Rel-16)
1) MsgA = preamble + Msg3 PUSCH bundled together (small payload). 2) MsgB = RAR + contention resolution combined. Half the round-trips, half the latency. Used when UE has SDT (small data transfer) or fast-mobility scenarios.
Contention-Based vs Contention-Free
Contention-based: UE picks any preamble from a pool, multiple UEs may collide on same preamble — collisions resolved via Msg4. Contention-free: gNB pre-assigns a unique preamble to a specific UE (used for handover, BFR) — no collision possible.
RA-RNTI Calculation
RA-RNTI = 1 + s_id + 14·t_id + 14·80·f_id + 14·80·8·ul_carrier_id, where s_id is the first symbol index of the PRACH occasion, t_id is the slot index, f_id is the frequency-domain occasion. The gNB scrambles RAR's PDCCH with this RNTI; UE searches for RA-RNTI = its own index.
Timing Advance · The Whole Point
The gNB measures the round-trip delay from preamble detection and computes T_TA = N_TA · T_c. UE then transmits all subsequent uplink T_TA earlier — keeping UL frames aligned at the gNB despite propagation delay. T_TA range covers cells up to 100 km in long-format mode.
Frequently Asked Questions
What is PRACH in 5G NR?
PRACH (Physical Random Access Channel) is the first uplink transmission a UE makes — the entry point for cell access, handover, and beam-failure recovery. It carries a Zadoff-Chu preamble that the gNB uses for detection and timing-advance estimation.
What is the difference between 4-step and 2-step RACH?
4-step RACH is the legacy procedure: preamble → RAR → Msg3 → Msg4. 2-step RACH (Rel-16) combines preamble with Msg3 in MsgA, and combines RAR with contention resolution in MsgB — halving the round-trips and reducing latency.
How are PRACH preamble formats chosen?
Long formats (0-3) use 1.25 kHz SCS for very large cells (rural / macro). Short formats (A1-A3, B1-B4, C0, C2) use 15/30/60/120 kHz SCS aligned with PUSCH numerology — used in dense urban, indoor, and mmWave deployments.
What is RA-RNTI?
Random Access RNTI is the temporary radio network identifier the gNB uses to scramble the PDCCH carrying the RAR (Random Access Response). It is computed from the time-frequency-domain location of the PRACH occasion the UE used.