What are the four steps in 4-Step RACH?

Step 1 (Msg1): UE transmits a PRACH preamble on the configured RACH occasion. The preamble is a Zadoff-Chu sequence selected from the available set. The gNB uses the preamble for timing estimation (determining the propagation delay for timing advance). Step 2 (Msg2): gNB responds with the Random Access Response (RAR) on PDSCH, containing: timing advance command, temporary C-RNTI, and uplink grant for Msg3. RAR must arrive within a configured window (typically 10-40 ms). Step 3 (Msg3): UE transmits its RRC connection request on PUSCH using the granted resources. Includes UE identity for contention resolution. Step 4 (Msg4): gNB sends contention resolution message on PDSCH, confirming the UE's identity. Upon successful reception, the UE considers the random access procedure complete.

What is the difference between contention-based and contention-free RACH?

Contention-based RACH (CBRA): the UE randomly selects a preamble from a shared pool. Multiple UEs may select the same preamble simultaneously (collision). Contention is resolved in Msg4. Used for initial access and scheduling requests. Contention-free RACH (CFRA): the gNB assigns a dedicated preamble to a specific UE via RRC signaling. No collision possible. Only Msg1 and Msg2 are needed (2 steps). Used for handover (the target cell pre-assigns a preamble) and beam failure recovery where latency must be minimized. CFRA is effectively a 2-step process, different from the Release 16 2-Step RACH which is a contention-based simplification.

What PRACH formats does 5G NR support?

5G NR supports long-format preambles (Format 0-3) inherited from LTE and short-format preambles (Format A1-C2) added for 5G. Long formats use 839-length Zadoff-Chu sequences with subcarrier spacing of 1.25 or 5 kHz, designed for large cells with propagation delays up to 100 km. Short formats use 139-length sequences with SCS of 15, 30, 60, or 120 kHz, designed for small cells and mmWave deployments with shorter CP and guard periods. At mmWave (FR2), only short formats with 60 or 120 kHz SCS are used, matching the wider subcarrier spacing needed for higher Doppler tolerance.

mmWave & 5G

4-Step RACH

The standard random access channel procedure in 5G NR, consisting of four message exchanges between the UE and gNB: Msg1 (PRACH preamble), Msg2 (Random Access Response with timing advance), Msg3 (RRC connection request), and Msg4 (contention resolution). 4-Step RACH is used for initial cell access, handover, scheduling request, beam failure recovery, and uplink synchronization. It provides robust operation across all coverage scenarios but takes 10-15 ms, which led to the development of the 2-Step RACH simplification in Release 16.

Category: mmWave & 5G

Messages: Msg1, Msg2, Msg3, Msg4

Latency: 10-15 ms typical

Understanding 4-Step RACH

Before a UE can communicate with a 5G cell, it must establish uplink timing synchronization and obtain a unique identity (C-RNTI). The RACH procedure achieves this. The UE sends a preamble, the gNB estimates the round-trip delay and responds with a timing advance command, the UE sends its connection request with corrected timing, and the gNB confirms the connection.

In 5G NR, the RACH procedure adds beam management: the UE selects the best downlink beam via SSB measurements and transmits the preamble on the RACH occasion associated with that beam. The gNB uses the preamble's beam association to determine the optimal DL and UL beam pair for the connection.

4-Step RACH Message Flow

Msg1 (UE → gNB): PRACH Preamble
Zadoff-Chu sequence (L=839 long, L=139 short)
gNB extracts: timing estimate, preamble index, beam

Msg2 (gNB → UE): Random Access Response
Contains: TA command, TC-RNTI, UL grant for Msg3
RAR window: 10-40 ms configurable

Msg3 (UE → gNB): RRC Request on PUSCH
UE identity (S-TMSI or random value), BSR, PHR
Transmitted with corrected timing advance

Msg4 (gNB → UE): Contention Resolution
Echoes UE identity. TC-RNTI promoted to C-RNTI.

Total: 10-15 ms (4 exchanges + processing delays).

RACH Trigger Events

Trigger	CBRA/CFRA	Typical Scenario	Latency Tolerance
Initial access	CBRA	UE power-on, cell selection	100+ ms
Handover	CFRA (dedicated preamble)	Mobility between cells	10-50 ms
Beam failure recovery	CFRA or CBRA	mmWave beam blockage	<20 ms
Scheduling request	CBRA	UL data after inactivity	5-20 ms
UL timing resync	CBRA	TA timer expiry	10-50 ms

Common Questions

Frequently Asked Questions

What are the four steps?

Msg1: PRACH preamble (Zadoff-Chu sequence). Msg2: RAR (timing advance, UL grant, TC-RNTI). Msg3: RRC request on PUSCH with UE identity. Msg4: Contention resolution confirming connection. Total: 10-15 ms.

Contention-based vs. contention-free?

CBRA: UE picks random preamble, collision possible, resolved in Msg4. Used for initial access. CFRA: gNB pre-assigns a dedicated preamble, no collision, only Msg1+Msg2 needed. Used for handover and beam recovery.

What PRACH formats?

Long (L=839, 1.25/5 kHz SCS, cells up to 100 km) and short (L=139, 15/30/60/120 kHz SCS, small cells and mmWave). FR2 uses short formats with 60/120 kHz SCS for higher Doppler tolerance.

Related Terms

← 3GPP Channel Model 5G Antenna Module →