5G NR Standalone Access Registration: Step-by-Step UE and gNB Interaction Explained
Understanding 5G Standalone Access Registration: A Look at UE and gNB Interaction
In the realm of 5G, the Standalone (SA) architecture represents a fully independent 5G system, where both the radio network (gNB) and the core network (5GC) don't depend on any 4G LTE infrastructure.
A key element of this setup is the initial access registration process, which is when the User Equipment (UE) connects to the 5G network for the first time. This entails establishing a Radio Resource Control (RRC) connection and going through a Random Access (RA) procedure to sync up with the gNB.
The image above provides a visual of this process, highlighting how the UE and gNB communicate during the 5G NR RRC Connection Setup. Let’s dive into the steps in a clear, tech-friendly way.
Overview: What Is 5G Standalone Access Registration?
5G Standalone (SA) operation refers to a mode of deployment where both the radio and core networks are entirely 5G, without any reliance on LTE (4G) components. The Access Registration procedure lets the UE:
Sync up with the gNB.
Get its initial radio resources.
Register with the 5G Core Network (5GC).
This journey kicks off when the UE is in the RRCIDLE state and makes its move toward the gNB using the Random Access Channel (RACH) procedure.
Step-by-Step Breakdown of the 5G NR RRC Connection Setup
The connection setup involves a four-step Random Access (RA) procedure, where the UE and gNB work to sync timing, assign identifiers, and set the stage for data exchange.
Step 1: RRCIDLE (Precondition)
The UE is just waiting for a chance to connect to the network. Once it realizes it needs to access, say, to register or send data, it kicks off the Random Access Procedure.
Step 2: Msg1 – Preamble Transmission
Message: Msg1: Preamble
Direction: UE → gNB
Purpose: The UE sends out a preamble on the PRACH (Physical Random Access Channel) to start the access process.
This preamble is based on a Zadoff-Chu sequence, a special type of sequence that has great correlation properties, allowing the gNB to spot and tell apart multiple UEs trying to access the network simultaneously.
The preamble also helps the gNB gauge the timing offset between itself and the UE, which is crucial for uplink synchronization.
Step 3: Start T300 Timer
Event: T300 Timer Started
After sending the preamble, the UE kicks off Timer T300, waiting for a Random Access Response (RAR) from the gNB. If the timer runs out before it gets a response, the UE will try again with a new preamble.
This timer ensures the UE isn’t left hanging forever for a reply and helps manage access collisions.
Step 4: Start Decoding the PDCCH for RA-RNTI
Once the gNB picks up the preamble, it gets ready to send a Random Access Response (RAR). Meanwhile, the UE starts to decode the PDCCH (Physical Downlink Control Channel) using the RA-RNTI (Random Access Radio Network Temporary Identifier).
This lets the UE spot messages meant for it among various potential responses.
Step 5: PDCCH DCI Format 1_0 [RA-RNTI]
Message: PDCCH DCI Format 1_0 [RA-RNTI]
Purpose: This Downlink Control Information (DCI) message contains resource allocation details for the upcoming Msg2 (Random Access Response).
It includes:
Frequency domain resource assignment
Time domain resource assignment
Downlink Modulation and Coding Scheme (MCS)
This enables the UE to gear up for receiving the RAR with the right parameters.
Step 6: Msg2 – Random Access Response
Message: Msg2: Random Access Response
Direction: gNB → UE
Purpose: The gNB sends a Random Access Response (RAR) message to the UE via PDSCH (Physical Downlink Shared Channel).
The RAR is packed with important parameters:
Timing Advance Command: Adjusts the UE’s uplink timing for proper synchronization.
Uplink Grant: Provides resources for Msg3 (uplink transmission).
Frequency hopping flag
PUSCH frequency and time allocation
Uplink MCS (Modulation and Coding Scheme)
TPC (Transmit Power Control) command
CSI (Channel State Information) request
Temporary C-RNTI (Cell Radio Network Temporary Identifier)
This temporary C-RNTI uniquely identifies the UE during this connection setup phase.
Step 7: UE Identity Assignment
After handling Msg2, the UE generates a unique identity, which is a random number between 0 and 2^39 – 1.
This identity comes into play for contention resolution in later steps (Msg3 and Msg4) and makes sure that even if multiple UEs use the same preamble, the network can keep them separate.
The Importance of Zadoff-Chu Sequences in 5G Access
The Zadoff-Chu sequence is crucial in the preamble design of 5G NR.
Key properties:
Constant amplitude and zero autocorrelation.
Excellent correlation between sequences, minimizing interference.
Allows multiple UEs to send different preambles at the same time without collisions.
This mathematical clarity ensures quick and accurate detection of random access attempts, especially in crowded network settings.
Timing and Synchronization Mechanisms
Getting the UE and gNB in sync is super important for 5G NR’s high data rates and low latency.
The Timing Advance Command in Msg2 fine-tunes the UE’s transmission timing, ensuring that uplink signals from multiple UEs reach the gNB at just the right time.
Benefits:
Cuts down on inter-symbol interference.
Optimizes power efficiency.
Keeps uplink scheduling accurate.
Temporary Identifiers and RA-RNTI
During the random access phase, the UE uses temporary identifiers for communication before full registration:
RA-RNTI: Assigned based on when and where the preamble was sent; used to decode RAR messages.
Temporary C-RNTI: Given in Msg2 to temporarily identify the UE prior to permanently assigning an RNTI.
This layered identification approach helps minimize collisions and allows UEs to be uniquely identified throughout the process.
Conclusion
The 5G Standalone Access Registration process is the backbone of 5G connectivity. It ensures the UE and gNB achieve tight synchronization and reliable resource allocation before they start exchanging data.
From the Msg1 preamble transmission using Zadoff-Chu sequences to the Random Access Response (Msg2) containing timing and uplink grants, every step is crafted for efficiency and reliability.
For those in the telecom field, grasping these early interactions is key—not just for optimizing networks but also for troubleshooting and creating solid 5G infrastructures.