5G Standalone Access Registration Explained: gNB and UE RRC Connection Setup

5G Standalone Access Registration: Getting to Know gNB Interactions and RRC Connection Setup

In a 5G Standalone (SA) setup, the communication starts off between the User Equipment (UE) and the 5G NodeB (gNB) with the RRC (Radio Resource Control) Connection Setup process. This step is crucial for creating the signaling link that enables the UE to connect with the 5G Core (5GC).

The image shows the gNB Interactions in 5G Standalone Access Registration, focusing on the initial RRC signaling messages that get exchanged during the Random Access Procedure and the RRC setup phase, ultimately leading to the NAS Registration Request being sent to the core network.

Overview: What Happens When Accessing 5G Standalone?

Before any data can be sent or sessions set up, the UE has to:

Sync up with the network.

Get a temporary identity.

Request and establish an RRC connection with the gNB.

Send its NAS Registration Request to the 5G Core (AMF).

These steps are essential to ensure the UE is authenticated and securely linked to the network, making it possible to access 5G services.

Step-by-Step Breakdown of gNB Interactions

Here’s a look at the 5G-NR RRC Connection Setup process in Standalone mode as illustrated in the image.

Step 1: Msg1 – Preamble Transmission

The action kicks off when the UE sends a preamble as part of the Random Access Procedure (RACH) using a Zadoff-Chu sequence. This preamble allows the gNB to identify the UE's access attempt and gauge timing advance for uplink synchronization.

Purpose:

To make contact with the gNB.

To request uplink resources for initial access.

To signal that it's ready for RRC setup.

This is referred to as the Random Access Preamble Transmission (Msg1).

Step 2: Allocate Temporary C-RNTI

As soon as Msg1 is received, the gNB gives the UE a Temporary Cell Radio Network Temporary Identifier (C-RNTI). This ID serves temporarily to represent the UE during the early access phase until a permanent C-RNTI is assigned.

Step 3: PDCCH DCI Format 1_0 [RA-RNTI]

Next, the gNB transmits Downlink Control Information (DCI) using PDCCH DCI Format 1_0, which is aimed at the RA-RNTI. This indicates that a Random Access Response (Msg2) is set for the UE.

Included parameters:

Frequency domain resource assignment

Time domain resource assignment

Downlink Modulation and Coding Scheme (MCS)

This ensures the UE knows when and where to expect its Random Access Response.

Step 4: Msg2 – Random Access Response

The gNB sends Msg2, the Random Access Response (RAR), that includes key configuration details allowing the UE to move forward.

Msg2 Contents:

Timing Advance Command: Syncs the UE’s uplink timing.

UL Grant: Offers resources for the next uplink message (Msg3).

Temporary C-RNTI: Confirms the temporary ID that was assigned.

PUSCH resource allocation for Msg3 transmission.

TPC command (Transmit Power Control).

CSI request if channel measurements are needed.

This response connects the UE’s initial contact to the start of the RRC connection setup.

Step 5: Msg3 – RRC Setup Request

Once the UE receives Msg2, it sends Msg3 (RRCSetupRequest) using the uplink grant. This is the UE’s official request to create an RRC connection with the gNB.

Key fields in the RRC Setup Request:

ue-Identity: Contains the Random UE ID.

establishment Cause: Shows why the UE is making the connection (like a mobile-originated data request, emergency call, or signaling).

This marks the shift from random access signaling to dedicated RRC-level communication.

Step 6: PDCCH DCI Format 1_0 [C-RNTI]

When Msg3 is received, the gNB gives the UE a final (permanent) C-RNTI and sends out scheduling info for Msg4. This control info once again uses PDCCH DCI Format 1_0, but now it's addressed to the new C-RNTI.

This step means the gNB and UE are now communicating with the permanent radio identifier, marking an important milestone in establishing the control connection.

Step 7: Setup SRB1

Before sending the RRCSetup message, the gNB sets up Signaling Radio Bearer 1 (SRB1). SRB1 is a logical channel used for:

Exchanging RRC messages after the connection is established.

Carrying NAS signaling between the UE and AMF through the gNB.

This bearer is key to ensuring reliable control-plane communication as the connection goes live.

Step 8: Msg4 – RRCSetup

Next, the gNB sends Msg4 (RRCSetup) to the UE. This message is crucial — it outlines the RRC connection parameters and establishes the necessary radio bearers for signaling and future data exchanges.

Contents of the RRCSetup Message:

radio Bearer Config (srb-To Add Mod List): Defines the SRB1 setup.

master Cell Group: Contains configurations for physical and MAC layers.

rlc-Bearer To Add Mod List: For logical channel mappings.

mac-Cell Group Config: MAC layer specifics.

physical Cell Group Config: Physical layer settings.

This sets up the entire control plane groundwork for communication between UE and gNB.

Step 9: PDCCH DCI Format 0_0 [C-RNTI]

The UE uses this step to send uplink data as per the schedule. PDCCH DCI Format 0_0 includes:

Frequency domain assignment

Time domain resource assignment

Uplink MCS (Modulation and Coding Scheme)

This ensures that the UE can send its RRC Setup Complete message (Msg10) reliably.

Step 10: RRC Setup Complete [dedicated NAS-Message: Registration Request]

Lastly, the UE transmits RRC Setup Complete, which carries the dedicated NAS message — Registration Request — to the 5G Core Network (AMF).

NAS Registration Request Includes:

Registration type: Initial registration or mobility update.

5G-GUTI: Temporary mobile ID.

Last TAI (Tracking Area Identity): Shows UE’s last known area.

Requested NSSAI: Info on network slices.

UE Capability Info: Supported bands, features, etc.

List of PDU Sessions: If they were pre-established or requested.

At this stage, the UE is connected at the RRC level and starts the NAS-level registration with the 5G Core.

Significance of gNB Interactions in 5G SA Access Registration

This whole process showcases the tight coordination happening between the UE, gNB, and 5GC components. Here’s why it’s so important:

Seamless Synchronization: Makes sure UE aligns perfectly with network timing.

Identity Management: Assigns unique IDs (Temporary and Permanent C-RNTI) for reliable communication.

Secure Signaling: Sets up SRB1 for protected control-plane signaling.

Efficient Resource Scheduling: Utilizes PDCCH DCIs for optimal uplink/downlink allocation.

NAS Integration: Bridges the RRC layer (gNB) with the NAS layer (AMF), completing end-to-end registration.

This groundwork enables all following processes like security setup, PDU session establishment, and data transfer.

Technical Insight: Why Standalone (SA) Matters

In 5G Standalone, all signaling and user data flow exclusively through the 5G Core (5GC), without LTE playing a part. This architecture brings:

Ultra-low latency thanks to native 5G signaling.

Network slicing support via NSSAI management.

Optimized radio efficiency through flexible numerology and beamforming.

Simplified architecture for pure 5G deployments.

The RRC Connection Setup process outlined here is the first step toward realizing these SA benefits.

Conclusion

The gNB interactions during 5G Standalone Access Registration lay down the vital steps for 5G’s initial connection process. From the Random Access (Msg1–Msg2) through to RRC setup (Msg3–Msg4) and NAS Registration, each stage guarantees that the UE moves smoothly from idle to connected status, ready for secure and efficient 5G communication.

Getting a good grasp of this signaling sequence is essential for telecom professionals who design, optimize, or troubleshoot 5G networks — since every successful data session starts with this precise coordination among the UE, gNB, and the 5G Core.