5G NR Initial Access Procedure Explained

This page describes the 5G NR Initial Access Procedure between UE (User Equipment) and gNB (Next Generation NodeB), including beam management.

All the messages exchanged between the UE and gNB during the 5G NR Initial Access procedure, also known as the Random Access Procedure, are described here. It’s also referred to as the initial cell search procedure.

Introduction: 5G NR (New Radio) is the latest cellular wireless technology, designed to deliver data rates up to 10 times faster than LTE (4G) technology. It adheres to 3GPP specifications release 15 and above.

In this page, we will delve into the initial access procedure, specifically the random access procedure. This procedure is crucial for the UE to obtain an initial uplink grant and synchronize with the gNB (i.e., the network).

It covers:

• Random Access procedure initialization
• Random Access Resource selection
• Random Access Preamble transmission
• Random Access Response reception
• Contention Resolution
• Completion of the Random Access procedure

Beam Management in 5G NR

Beam management is used in 5G NR to acquire and maintain a set of TRxPs (Transmission Reception Points) and/or UE beams. These beams are then used for downlink (DL) and uplink (UL) transmission/reception.

Reference signals play a key role in beam management. In IDLE mode, 5G NR uses PSS (Primary Synchronization Signal), SSS (Secondary Synchronization Signal), and PBCH DMRS (Physical Broadcast Channel Demodulation Reference Signal). In CONNECTED mode, it utilizes CSI-RS (Channel State Information Reference Signal) in the downlink and SRS (Sounding Reference Signal) in the uplink.

The following operations are performed as part of the 5G NR beam management procedure. They apply to both SA (Standalone) and NSA (Non-Standalone) modes:

• Beam Sweeping: This involves covering a spatial area using a set of beams transmitted and received according to pre-specified intervals and directions.
• Beam Measurement: This is the evaluation of the quality of the received signal at the gNB or the UE. Different metrics can be used, such as RSRP (Reference Signal Received Power), RSRQ (Reference Signal Received Quality), and SINR/SNR (Signal-to-Interference-plus-Noise Ratio / Signal-to-Noise Ratio).
• Beam Determination: This refers to the selection of the appropriate beam(s) at either the gNB or the UE, based on the measurements obtained during the beam measurement procedure.
• Beam Reporting: This is the procedure used by the UE to send beam quality and beam decision information to the Radio Access Network (RAN).

Random Access Procedures come in two flavors: contention-based and contention-free. The message exchanges for each are illustrated below:

Contention-based RA is used when the UE is not yet synchronized or has lost synchronization. Contention-free RA is used when the UE was previously synchronized to another gNB.

Both procedures rely on the transmission of a random access preamble from the UE to the gNB. This preamble is transmitted on specific time/frequency resources, indicated by the gNB to the UEs on control channels.

5G NR Initial Access Procedure between UE and gNB

The following steps describe the messages exchanged between the UE and gNB during the initial access (random access) procedure:

1. As shown in the figure above, the gNB periodically transmits SS blocks carrying synchronization signals (PSS, SSS) and broadcast channels (PBCH) using beam sweeping. One SS block contains 1 symbol PSS, 1 symbol SSS, and 2 symbols PBCH. An SS burst carries one or multiple SS blocks. The combination of PSS and SSS helps to identify approximately 1008 physical cell identities.

   

2. The UE performs beam measurements and determines the best beam for synchronization. Subsequently, the UE decodes 5G NR system information, including MIB/SIB, on that beam. Minimum System Information (SI) is carried on the PBCH channel. The Remaining Minimum System Information (RMSI) is carried on the PDSCH. The numerology used for RMSI is indicated in the PBCH payload. A CORESET (Control Resource Set) is dedicated for RMSI scheduling and is not confined within the PBCH Bandwidth. A RMSI PDCCH monitoring window, which recurs periodically, is associated with each SS/PBCH block. Other System Information (OSI) contains on-demand system information and is carried on PDSCH using the same numerology as RMSI.

3. The UE uses the same beam and attempts random access by transmitting a RACH preamble (Message#1) on the configured RACH resource. The gNB responds with a RAR (“RA Response”) message (Message#2).

4. The UE transmits Message#3 (RRC Connection Request).

5. The gNB responds with Message#4 (RRC Connection Setup).

6. The UE receives SS blocks and CSI-RS, which it uses to generate beam/CSI reports.

7. The gNB requests beam/CSI reporting using DCI (Downlink Control Information). The UE responds with the requested beam/CSI report on PUSCH/PUCCH.

8. Once the random access procedure is complete, a dedicated connection is established between the UE and the gNB, using a dedicated connection ID.

9. Either the gNB or the UE switches beams based on the SNR (Signal to Noise Ratio).

References:

• 3GPP TS 38.321 V15.2.0 (2018-06), Medium Access Control (MAC) protocol specification (Release 15)
• 3GPP TS 38.300 V15.2.0 (2018-06), NR; NR and NG-RAN Overall Description; Stage 2 (Release 15)