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NB-IoT RACH Procedure | LTE NB-IoT Network Entry Procedure for data call


This page describes NB-IoT RACH Procedure also called as LTE NB-IoT network entry procedure. It mentions various NB-IoT messages exchanged between UE and LTE-NB network during RACH procedure (i.e. network entry procedure).

Introduction:
NB-IoT random access procedure is same as LTE random access procedure. We will go through some of the unique aspects of the same with respect to NB-IoT standard.

LTE NB Access Network Architecture

There are two major systems in NB-IoT viz. UE and eNB as shown in the NB-IoT architecture. Let us understand data transfer procedure or data call in NB-IoT. Following are the phases in which the messages exchanged between UE and eNB are categorized.
• Synchronization Phase
• Connection Setup phase or random access procedure
• Data transfer phase
• Disconnection phase

NB-IoT RACH Procedure | LTE NB-IoT Network Entry Procedure

NB-IoT Data Transfer using RACH Procedure or RRC Resume Procedure

Let us understand the RACH procedure or network entry procedure or data transfer in NB-IoT technology in steps.
➤Step-1:UE receives NPSS and NSSS preamble signals from eNB in the downlink and synchronizes with eNB.
➤Step-2:After synchronization and confirmation of access is not barred, UE sends random access preamble using NPRACH channel. UE uses appropriate NPRACH configuration as per coverage class estimation. After eNB detects NPRACH preamble, it sends back RAR (i.e. Random Access Response) designated as "msg2". RAR contains TA parameters as well as scheduling information to be used by UE to transmit "msg3" (i.e. Connection Resume Request).
➤Step-3:In "msg4" i.e. "RRC connection resume", the network resolves any contention due to multiple UEs transmitting same random access preamble and transmits connection setup or resume message. The UE replies with RRC connection setup or resume, complete message in order to complete the transition to the "Connected State".
➤Step-4:After connection is established, data transfer between UE and eNB takes place as depicted by various messages in the figure.
➤Step-5:Once the data transfer is complete, the connection can be released by eNB as shown.

Reference: 3GPP 36 series



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