UE RACH Procedure
The Random Access Channel (RACH) Procedure is the foundational "handshake" that a User Equipment (UE) performs to establish an initial connection with a cellular network (LTE or 5G NR).
#Overview
When a UE powers on, enters a new cell, or needs to re-establish a connection, it must "knock on the door" of the base station (eNB in LTE, gNB in 5G NR). This knock is the RACH procedure — a structured sequence of messages exchanged between the UE and the network to achieve uplink synchronization, request resources, and resolve potential collisions.
Why RACH Matters
Initial AccessUE powers on and camps on a cell
RRC Re-establishmentRadio link failure occurs
HandoverUE moves to a new cell
Uplink Data ArrivalUE has data to send but no UL sync
Downlink Data ArrivalNetwork has data but UE is not synchronized
Beam Failure Recovery5G NR beamforming link is lost
#Types of RACH
1. Contention-Based RACH (CBRA)
- UE randomly selects a preamble from a pool
- Multiple UEs may select the same preamble → collision possible
- Requires a 4-step message exchange with contention resolution
- Most commonly used for initial access and RRC connection requests
2. Contention-Free RACH (CFRA)
- Network assigns a dedicated preamble to a specific UE
- No collision is possible since the preamble is unique
- Only requires 2 steps (Msg1 + Msg2) — no contention resolution
- Used during handover, DL data arrival, and beam failure recovery
#Contention-Based RACH — The 4-Step Procedure
Animated 4-step RACH — messages fly between UE and gNB with step-by-step highlighting
#Detailed Step-by-Step Breakdown
Step 0 Pre-Step — Downlink Synchronization & System Info
Before the UE can transmit anything, it must:
- Synchronize with the cell using:
- PSS (Primary Synchronization Signal) — identifies the physical layer identity within the cell group
- SSS (Secondary Synchronization Signal) — identifies the cell group identity
- Read the MIB (Master Information Block) via PBCH — contains essential cell configuration
- Read SIB1 / SIB2 (System Information Blocks) — contains:
- Available RACH preambles (64 per cell)
- PRACH time/frequency resources
- Power control parameters
- RACH response window size
- Maximum number of preamble retransmissions
Step 1 Msg1 — Random Access Preamble Transmission
| Parameter | Detail |
|---|---|
| Channel | PRACH (Physical Random Access Channel) |
| Preamble Pool | 64 Zadoff-Chu sequence-based preambles per cell |
| Selection | UE randomly picks one preamble from the available pool |
| Preamble Format | Determines sequence length & cyclic prefix (affects cell coverage range) |
| Power | Calculated based on path loss estimate + target received power |
Key Points
- The preamble serves two purposes: signals the network that a UE wants to connect, and allows the gNB to estimate the UE's timing offset (propagation delay).
- Some preambles are reserved for contention-free access (handover, etc.).
- Preambles are grouped into Group A and Group B — Group B is used when the UE has a larger Msg3 to send (based on data volume threshold).
Step 2 Msg2 — Random Access Response (RAR)
| Parameter | Detail |
|---|---|
| Channel | PDSCH (scheduled via PDCCH addressed to RA-RNTI) |
| Response Window | UE monitors PDCCH for a configurable window (ra-ResponseWindow) |
| Contents | Timing Advance, UL Grant, Temporary C-RNTI |
RAR Contents Breakdown
| Field | Size | Purpose |
|---|---|---|
| Timing Advance Command | 12 bits | Adjusts UE uplink transmission timing to compensate for propagation delay |
| Uplink Grant | 20 bits | Allocates PUSCH resources (RB assignment, MCS, power control) for Msg3 |
| Temporary C-RNTI | 16 bits | Temporary identity for the remainder of the RACH procedure |
| Backoff Indicator (optional) | 4 bits | Tells UE to wait 0–960 ms before retrying (used under congestion) |
If No RAR Is Received
- UE performs power ramping (increases transmit power by
powerRampingStep, typically 2 dB) - UE retransmits Msg1 with the same or a different preamble
- Maximum retransmissions controlled by
preambleTransMax
Step 3 Msg3 — RRC Connection Request (First Scheduled Uplink)
| Parameter | Detail |
|---|---|
| Channel | PUSCH (using resources granted in Msg2) |
| Content | UE Identity + RRC Setup Request (or RRC Re-establishment Request) |
| UE Identity | S-TMSI (if already registered) or a random 40-bit value |
Key Points
- This is the first scheduled uplink transmission — the UE now has proper timing alignment and allocated resources.
- The UE identity is critical for contention resolution in Step 4.
- The message also includes the establishment cause (e.g., mobile-originated data, emergency, high-priority access).
Step 4 Msg4 — Contention Resolution
| Parameter | Detail |
|---|---|
| Channel | PDSCH (addressed to Temporary C-RNTI or C-RNTI) |
| Content | Echo of the UE identity from Msg3 |
| Purpose | Resolves collisions when multiple UEs selected the same preamble |
Resolution Logic
Two UEs select preambles — watch contention resolution decide the winner
#Power Ramping & Retransmission
When the RACH attempt fails (no RAR, contention resolution failure, or maximum retransmissions reached), the UE follows a recovery procedure:
Power ramping — transmit power increases with each retransmission until success or max reached
#5G NR Enhancement — 2-Step RACH
4-step vs 2-step RACH race — watch which completes first with real-time latency counters
5G NR introduced a 2-step RACH procedure to reduce latency for time-critical applications.
4-Step vs 2-Step Comparison
| Aspect | 4-Step RACH | 2-Step RACH |
|---|---|---|
| Messages | Msg1 → Msg2 → Msg3 → Msg4 | MsgA → MsgB |
| MsgA | — | Preamble (PRACH) + Payload (PUSCH) combined |
| MsgB | — | Response + Contention Resolution combined |
| Latency | Higher (~13–16 ms) | Lower (~7–9 ms) |
| Use Case | General access | URLLC, low-latency scenarios |
| Fallback | — | Falls back to 4-step if MsgA not decoded |
4-Step RACH
UE ───Msg1──► gNB UE ◄──Msg2─── gNB UE ───Msg3──► gNB UE ◄──Msg4─── gNB
2-Step RACH
UE ───MsgA (Preamble + Payload)──► gNB UE ◄──MsgB (Response + Resolution)─ gNB
2-Step RACH Behavior
- If the gNB successfully decodes both parts of MsgA → responds with MsgB (success)
- If the gNB decodes the preamble but not the payload → sends a fallback RAR (equivalent to Msg2), and the UE continues with Msg3/Msg4 (falls back to 4-step)
- If nothing is decoded → UE retries MsgA with power ramping
#Key Parameters Reference
| Parameter | Description | Typical Value |
|---|---|---|
prach-ConfigIndex | Defines PRACH time/frequency resources | Cell-specific |
numRA-Preambles | Number of available contention-based preambles | 52–64 |
preambleTransMax | Max preamble retransmissions | 3–10 |
powerRampingStep | Power increase per retransmission | 0, 2, 4, 6 dB |
ra-ResponseWindow | Window to monitor for RAR | 2–10 subframes |
mac-ContentionResolutionTimer | Timer for Msg4 reception | 8–64 subframes |
preambleReceivedTargetPower | Target received power at gNB | -120 to -90 dBm |
ra-PreambleIndex (CFRA) | Dedicated preamble for contention-free RACH | Assigned by network |
#RACH in LTE vs 5G NR
| Feature | LTE | 5G NR |
|---|---|---|
| Preamble Sequence | Zadoff-Chu (length 839 for FDD) | Zadoff-Chu (length 839 or 139 depending on SCS) |
| RACH Types | 4-step only | 4-step and 2-step |
| Subcarrier Spacing | Fixed 1.25 kHz / 5 kHz for PRACH | Flexible (15, 30, 60, 120 kHz) |
| Beam-based RACH | No | Yes — SSB-associated RACH occasions |
| Numerology | Fixed | Flexible numerology support |
| Supplementary Uplink | No | SUL (Supplementary Uplink) supported |
#RACH Occasions & SSB Association (5G NR)
In 5G NR, the network uses beamforming, and RACH resources are associated with specific SSBs (Synchronization Signal Blocks):
SSB beams map to RACH occasions — UE selects the best beam and its associated time-frequency slot
- Each SSB represents a different beam direction from the gNB
- The UE selects the RACH occasion associated with the best SSB (strongest signal)
- This allows the gNB to know which beam the UE is in, enabling beam-aligned random access
- The parameter
ssb-perRACH-Occasioncontrols how many SSBs map to each RACH occasion
#Common Failure Scenarios & Recovery
| Failure | Cause | Recovery |
|---|---|---|
| No RAR received | Preamble not detected, poor coverage | Power ramping + retransmission |
| RAR received but Msg3 fails | UL grant issue, HARQ failure | HARQ retransmission of Msg3 |
| Contention resolution failure | Collision (same preamble by multiple UEs) | Backoff + restart from Msg1 |
| Max retransmissions reached | Persistent failure | Radio Link Failure → cell reselection |
| Beam failure (5G NR) | Beam misalignment | Beam Failure Recovery via CFRA |
#Summary Flowchart
#Further Reading
- 3GPP TS 38.321 — NR MAC Protocol Specification (RACH procedure details)
- 3GPP TS 38.331 — NR RRC Protocol Specification (RRC Connection Request)
- 3GPP TS 36.321 — LTE MAC Protocol Specification
- 3GPP TS 38.211 — NR Physical Channels (PRACH waveform details)