5G NR Initial Access Procedure | 5G NR Random Access Procedure

This page describes 5G NR Initial Access Procedure between UE and gNB including beam management. All the messages exchanged between UE and gNB in 5G NR Initial Access procedure i.e. Random Access Procedure are described. It is also known as initial cell search procedure.

Introduction:
5G NR (New Radio) is the latest cellular wireless technology developed to deliver 10 times fast data rate compare to LTE (i.e. 4G) technology. It follows 3GPP specifications release 15 and above. In this page we will understand initial access procedure i.e. random access procedure. It helps to get the initial uplink grant for UE and helps in performing synchronization with the gNB (i.e. network). It covers Random Access procedure initialization, Random Access Resource selection, Random Access Preamble transmission, Random Access Response reception, Contention Resolution and Completion of the Random Access procedure.

Beam Management in 5G NR

Beam management procedure is used in 5G NR in order to acquire and maintain a set of TRxP (s) and/or UE beams which can be used for DL and UL transmission/reception. TRxP stands for Transmission Reception Point. Reference signals are used for beam management. In IDLE mode, 5G NR uses PSS, SSS and PBCH DMRS where as in CONNECTED mode it uses CSI-RS (in the downlink) and SRS (in the uplink).

Following operations are performed in 5G NR beam management procedure. They are applied to both the modes viz. SA (StandAlone) and NSA (Non StandAlone).
• Beam Sweeping: It refers to covering a spatial area with a set of beams transmitted and received according to pre-specified intervals and directions.
• Beam measurement: It refers to evaluation of the quality of the received signal at the gNB or at the UE. Different metrics could be used such as RSRP, RSRQ and SINR or SNR for this purpose.
• Beam determination: It refers to the selection of the suitable beam or beams either at the gNB or at the UE, according to the measurements obtained with the beam measurement procedure.
• Beam reporting: It refers to the procedure used by the UE to send beam quality and beam decision information to the Radio Access Network (RAN).

Random Access Procedures are of two types viz. contention based and contention free. The messages exchanged in these procedures are depicted in the following figure-1.

Contention based RA is applied when UE is not yet synchronized or lost its synchronizaton. Contention free is appled when UE was perviously synchronized to another gNB. Both the precudures rely on transmission of random access preamble from UE to gNB. The preamble is transmitted on specific time/frequency resources which are indicated by gNB to UEs on control channels.

5G NR Initial Access Procedure between UE and gNB

Following steps describe messages exchanged between UE and gNB during initial access procedure or random access procedure.
➤As shown in the figure-2, gNB (i.e. 5G NR Base Station) 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 SS and 2 symbols PBCH. SS burst carry one or multiple SS blocks. Both PSS and SSS combination help to identify about 1008 physical cell identities.
➤UE does beam measurements and determines best beam during sychronization. Consecutively UE decodes 5G NR system informations viz. MIB/SIB on that beam. Minimum SI (System Informations) is carried onto PBCH channel. The rest of the Remaining Minimum System Information (RMSI) is carried on PDSCH. The numerology used for RMSI is indicated in PBCH payload. CORESET is dedicated for RMSI scheduling. CORESET is not confined within PBCH Bandwidth. There is an RMSI PDCCH monitoring window associated with SS/PBCH block, which recurs periodically. OSI (Other System Information) contains on-demand system information delivery. OSI is carried on PDSCH using same numerology as used for RMSI.
➤UE uses same beam and attempts random access by transmitting RACH preamble (i.e. Message#1) on the configured RACH resource. The gNB responds with RAR ("RA Response") message. This is Message#2.
➤UE transmits Message#3 (i.e. RRC Connection Request). The gNB responds with Message#4 (i.e. RRC Connection Setup).
➤As shown UE receives SS block and CSI-RS which it uses for generation of beam/CSI report.
➤The gNB requests beam/CSI reporting with the help of DCI (Downlink Control Information). UE responds with beam/CSI report as requested by gNB on PUSCH/PUCCH.
➤Once random access procedure is completed, dedicated connection is established between UE and gNB with dedicated connection ID.
➤Either gNB or UE switches beam based on 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)

5G NR Numerology | 5G NR Terminology

5G NR Control channels | 5G NR Traffic Channels | 5G NR Reference Signals and sequences

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