5G SA RACH Procedure– A Technical Deep Dive

Introduction

The Random Access CHannel (RACH) procedure is the gateway to the 5G Standalone (SA) network. It allows the User Equipment (UE) to synchronize, identify itself, and request resources for uplink communication.

In 5G SA, the initial attach procedure is launched through a contention-based RACH, triggered when a UE powers on or reselects a cell in RRC_IDLE.

Overview: UE Attach & RACH Flow

There are two types of RACH procedures: Contention-Based and Contention-Free, each suited to different scenarios and configured accordingly by the network. 

Image source: https://www.linkedin.com/pulse/nr-rach-process-syed-mohiuddin/

1. Contention-Based Random Access (CBRA)

2. Contention-Free Random Access (CFRA)

Description: Most commonly used RACH procedure. Multiple UEs may randomly choose the same preamble. If two or more UEs pick the same preamble, a collision may occur — resolved later via contention resolution (RRC level). Procedure: UE randomly selects a preamble from the configured set (e.g., 0–63). Sends MSG1 (RACH preamble). gNB responds with MSG2 (RAR – Random Access Response). UE sends MSG3 (RRC Connection Request). gNB sends MSG4 (RRC Connection Setup or Contention Resolution). If multiple UEs sent the same MSG3, only one receives MSG4 → others retry. Used in: Initial Access after power-on RRC_IDLE → RRC_CONNECTED transition Cell reselection (when no context is retained) Beam failure recovery (CBRA fallback) Network reentry without prior scheduling

Description: A scheduled RACH procedure. gNB assigns a dedicated preamble to a UE ahead of time. Eliminates the risk of contention or collision. Faster and more deterministic. Procedure: UE is assigned a dedicated preamble (signaled via RRC). UE sends MSG1 using the preconfigured preamble. gNB sends MSG2 directly to the specific UE. No contention resolution needed; steps MSG3 and MSG4 are often skipped or minimized. Used in: Handover (especially inter-gNB or inter-RAT) Dual Connectivity (DC) addition (e.g., EN-DC) Beam failure recovery (if preamble is reserved) UE-specific optimization cases Latency-sensitive scenarios (e.g., URLLC)

CBRA MSG1-MSG4 IEs with explanation:

Step 1: MSG1 – PRACH Preamble Transmission

• UE randomly selects a preamble index.

• Transmits PRACH on the PRACH Occasion based on timing and frequency settings.

• Waits for Random Access Response (RAR) within the RA Response Window.

Example UE Log:

Mapping a Preamble Index to an RA-RNTI (Random Access Radio Network Temporary Identifier) is crucial in 5G NR for contention-based random access. The RA-RNTI is not explicitly sent in Msg1 (PRACH preamble); rather, it's calculated by the gNB and UE independently based on the PRACH occasion.

This RA-RNTI is then used in Msg2 (RAR – Random Access Response), allowing the UE to recognize that the Msg2 is intended for it.

RA-RNTI = 1 + s_id + 14 × t_id + 14 × 80 × f_id + 14 × 80 × 8 × ul_carrier_id

where

·       s_id is the index of the first OFDM symbol of the PRACH occasion (0 ≤ s_id < 14),

·       t_id is the index of the first slot of the PRACH occasion in a system frame (0 ≤ t_id < 80),

·       f_id is the index of the PRACH occasion in the frequency domain (0 ≤ f_id < 8),

·       ul_carrier_id is the UL carrier used for Random Access Preamble transmission (0 for NUL carrier, and 1 for SUL carrier).

Example:

Assume PRACH Configuration Index 16, as per 3GPP TS 38.211 Table 6.3.3.2-2 (for FR1, FDD, 15 kHz SCS), maps to:

• s_id = 0 as first OFDM symbol is “0”

• Number of PRACH slots per subframe = 1 (t_id)

• PRACH Occasions per Frame = 10 (one per subframe in specific subframes)

• Symbols = 0 (starts at symbol 0 of the slot)

• only one RA-RB (PRACH frequency resource block) used in this configuration, then fid​=0

• ul-carrier-id=0

RA_RNTI= 1+0+14*1+ 14*80*0+14*80*8*0 =15

Step 2: MSG2 – Random Access Response (RAR)

• gNB responds on PDSCH mapped via RAR-RNTI.

• Contains:

  • Timing Advance Command

  • UL Grant for MSG3

  • Temporary C-RNTI

Example UE Log:

Timing Advance Command

• This instructs the UE to advance its uplink transmissions, compensating for propagation delay to ensure UL time alignment.

• Expressed as a 12-bit field in the MAC RAR.

• Actual adjustment in time domain:

TA (in Ts)=16×(TA_Command)(for SCS = 15kHz)

Where Ts=1/(30.72×10^6)=≈32.55 ns

Example:If TA Command = 62, then time advance = 16 × 62 = 992 Ts ≈ 32.3 μs.

UL Grant for Msg3

This grant tells the UE when and where to send Msg3 (its actual identity or request).

• UL Grant includes:

  • Frequency allocation (Resource Blocks for PUSCH)

  • Time domain resource (e.g., slot/subslot offset)

  • MCS index (modulation and coding scheme)

  • HARQ process info

  • TBS size (Transport Block Size)

Example:UL Grant might instruct the UE to transmit Msg3 on PUSCH starting 6 slots later, using RBs 10–20 with QPSK.

Temporary C-RNTI

• A 16-bit identifier temporarily assigned to the UE.

• This is used by the gNB to:

  • Track the UE in subsequent messages.

  • Identify the UE in Msg3, Msg4, and beyond, before final RRC connection setup.

• If contention resolution is successful, this Temp C-RNTI becomes the final C-RNTI.

  
  

 Step 3: MSG3 – RRC Connection Request

• UE sends RRC Connection Request on PUSCH using the allocated UL resources.

Example UE Log:

 value UL-CCCH-Message ::=

{

  message c1 : rrcSetupRequest :

      {

        rrcSetupRequest

        {

          ue-Identity ng-5G-S-TMSI-Part1 : '00011010 00000100 00001000 01011011 0000001'B,

          establishmentCause mo-Data,

          spare '0'B

        }

      }

}

Step 4: MSG4 – RRC Connection Setup

• gNB replies with:

  • RRC Connection Setup

  • Final C-RNTI assignment

  • SRB configuration

value DL-CCCH-Message ::=

{

  message c1 : rrcSetup :

      {

        rrc-TransactionIdentifier 0,

        criticalExtensions rrcSetup :

          {

            radioBearerConfig

            {

              srb-ToAddModList

              {

                {

                  srb-Identity 1

                }

………………….

………………….

………………….

………………….

Benefits/Accomplishments of RACH procedure

1. Establishes Uplink Synchronization

• RACH allows the UE to align its timing with the gNB.

• The gNB provides a Timing Advance (TA) in the RAR (MSG2) to correct the UE’s transmission timing.

• Essential for TDD systems and massive MIMO setups, where precise timing is critical.

  
  

2. UE Identification and Temporary C-RNTI Assignment

• During the RAR step, the UE receives a temporary C-RNTI, enabling the gNB to distinguish it from others.

• This temporary ID is used in subsequent messages (e.g., MSG3 and MSG4) until a final C-RNTI is assigned.

3. Dynamic Resource Allocation

• RACH allows the gNB to allocate uplink resources dynamically.

• This enables the UE to send its RRC Connection Request even without pre-configured grants.

• Supports random access from unknown or new UEs.

  
  

4. Supports Mobility and Reselection

• After reselection (e.g., in RRC_IDLE → RRC_CONNECTED transition), RACH helps re-establish the uplink path.

• Ensures fast reconnection or reattachment during mobility or after losing coverage.

  
  

5. Collision Management with Contention Resolution

• Contention-based RACH allows many UEs to initiate access simultaneously.

• RACH uses a robust contention resolution mechanism (MSG3/MSG4) to manage simultaneous accesses without system crash.

  
  

6. Trigger for RRC Connection Establishment

• The RACH MSG3 typically carries the RRC Connection Request.

• This starts the transition from RRC_IDLE → RRC_CONNECTED, leading to session setup, authentication, etc.

PRACH Parameters Explained (from SIB1)

Let’s break down the key PRACH parameters found in SIB1 → rach-ConfigCommon.

Parameter	Purpose	Typical Value	Example
prach-ConfigurationIndex	Determines PRACH format, subcarrier spacing, slots	84	Format A1 (15 kHz), periodic
preambleReceivedTargetPower	Starting power for PRACH	-104 dBm	UE uses this for initial Tx
preambleTransMax	Max number of preamble attempts	10	Retry limit
ra-ResponseWindow	Time to wait for RAR	5 slots	Must match gNB timing
ssb-perRACH-Occasion	SSBs linked to PRACH	1 or 8	Configures how many beams are mapped
msg1-FDM	FDM options for MSG1	2 or 4	Determines PRACH frequency domain
zeroCorrelationZoneConfig	Reduces interference between preambles	10	ZCZ config index
numRA-Preambles	Total available preambles	64	0–63 (standard full set)

How PRACH Preambles Work in 5G SA

What is a PRACH Preamble?

A PRACH preamble is a specially formatted signal transmitted by the UE on the PRACH occasion. It helps the gNB identify and estimate the uplink timing of the UE.

Each PRACH preamble consists of:

• Cyclic Prefix (CP)

• Zadoff-Chu (ZC) Sequence — a root sequence,

• Format-specific extensions (depending on PRACH Format: A1/A2/B1/C2 etc.)

5G NR introduces enhanced flexibility, broader use cases (e.g., mmWave, beamforming, ultra-low latency), and the need to scale across small to very large cells, which demands a more adaptable PRACH design. 

Scenario	Recommended Format	Why?
Rural, large cells	Long (Format 0–3)	Longer delay spread, large timing uncertainty
Urban, small cells	Short (A1–B4)	Tighter timing, lower delay
mmWave, TDD, URLLC	Short (C0–C2)	Ultra-low latency, beam mgmt

Long Sequence: length 839, four preamble formats(Format 0, 1, 2, 3) that originated from the LTE preambles are supported, mainly targeting large cell deployment scenarios. These formats can only be used in FR1 and have a subcarrier spacing of 1.25 or 5 kHz.

Short sequence: length 139, nine different preamble formats (A1, A2, A3, B1, B2, B3, B4, C0, C2) are introduced in NR, mainly targeting the small/normal cell and indoor deployment scenarios.

How gNB Generates PRACH Preambles

gNB-side Configuration:

1. PRACH Config Index

   
   

   Defines slot/frame timing, subcarrier spacing, and PRACH format (via TS 38.211, Table 6.3.3.2-2).

2. ZC Root Sequences

   
   

   gNB configures one or more Zadoff-Chu root sequences:

rach-ConfigCommon:

  - prach-RootSequenceIndex: 44

  - zeroCorrelationZoneConfig: 10

  - numRA-Preambles: 64

3. Correlation Zone and Mapping

   gNB uses:

   • prach-RootSequenceIndex as base

   • zeroCorrelationZoneConfig to generate up to 64 unique orthogonal preambles

     
     

Each UE preamble corresponds to a cyclic shift of the root ZC sequence, ensuring minimal collision.

• These determine how often PRACH is available — e.g., once every 10ms, 20ms, etc.

• The PRACH occasion must align with the SSB beams via ssb-perRACH-Occasion. 3GPP defines parameters “ssb-perRACH-OccasionAndCB-PreamblesPerSSB”, “msg1-FDM” to align SSB & RACH occasions as shown below.  

  
  

Example:

prach-ConfigurationIndex = 7

→ PRACH is available every 9th subframe with starting symbol 0.

Frequency Domain Allocation for PRACH:

• msg1-FDM: Number of frequency-domain partitions (1, 2, or 4)

• frequencyStart: PRB index for PRACH allocation

How Frequency Domain Allocation Works:

• Total PRACH bandwidth is split across up to 4 frequency domain locations (FDM).

• Each PRACH occasion can be divided into multiple RACH resources (based on FDM) to support more UEs.

• UE randomly selects one of these frequency-domain chunks during PRACH.

Example:

msg1-FDM = 2 → PRACH spans 2 frequency regions

frequencyStart = 10 → PRACH starts at PRB 10

If the PRACH occasion is split into 2 FDMs, each domain may occupy a 6 PRB span (for example), allowing two parallel preamble transmissions in a single time slot.

How UE Selects a PRACH Preamble

UE-side Selection Logic:

When RACH is triggered (due to Power ON or Cell Reselection), UE reads rach-ConfigCommon from SIB1, and proceeds as follows:

1. Determine PRACH Occasion:

   Based on:

   • Slot timing

   • SSB beam-to-PRACH mapping (via ssb-perRACH-Occasion)

   • msg1-FDM (1, 2, or 4 frequency-domain multiplexed occasions)

1. Random Preamble Index Selection:

   
   

   UE randomly picks one index from the allowed range (e.g., 0–63).

   
   

   Example:

numRA-Preambles: 64

Random Selection: Index 41

2. Transmit with Configured Format & Power:

   Based on:

   • PRACH Format (A1, A2, etc.)

   • SCS: 15 kHz, 30 kHz

   • Power: preambleReceivedTargetPower (e.g., -104 dBm)

   • Beam Index (based on best SSB RSRP)

Example UE Log:

5GNR MAC RACH Attempt - SUCCESS   

   Symbol_Start : 0

    Preamble_Format : FORMAT_0 (0)

    Prach_Config : 18

    Uroot : 401

    RAID : 29

    FDM : 0

    Cyclic_Shift_v : 119

    N_CS : 119

             Ra_RNTI : 99

Tuning & Troubleshooting

Symptom	Possible Cause	Resolution
No MSG2 (RAR)	RAR missed or outside response window	Check ra-ResponseWindow, SSB beam alignment
MSG3 Collision	Same preamble used by multiple UEs	Use more preambles (numRA-Preambles)
Excessive Retries	Wrong power config or wrong SSB	Adjust preambleReceivedTargetPower or beam mapping
UE stuck after MSG3	MSG3 received, but RRC fails	Review PUSCH timing and UL grant

Summary

The RACH procedure is the handshake that initiates communication between UE and gNB. It combines physical, MAC, and RRC layer coordination. Understanding PRACH configuration and analyzing UE logs is vital for engineers working on modem validation, network optimization, and gNB development.

References

• TS 38.211 – PRACH Format and Mapping

• TS 38.213 – PRACH Configuration Parameters

• TS 38.321 – MAC Layer RACH Procedure

• TS 38.331 – RRC Messages and States

• https://www.sharetechnote.com/html/5G/5G_RACH.html#RACH_Occasion

• IEEE Access, ARVIND CHAKRAPANI “On the Design Details of SS/PBCH, Signal Generation and PRACH in 5G-NR”