NB-IoT RACH Procedure: Network Entry for Data Call

This page describes the NB-IoT RACH Procedure, also known as the LTE NB-IoT network entry procedure. It outlines the various NB-IoT messages exchanged between the UE (User Equipment) and the LTE-NB network during the RACH procedure (i.e., the network entry procedure).

Introduction:

The NB-IoT random access procedure is similar to the LTE random access procedure. We’ll highlight some of the unique aspects of this process within the NB-IoT standard.

LTE NB Access Network Architecture

As illustrated in the NB-IoT architecture, there are two primary systems in NB-IoT: the UE and the eNB (evolved NodeB or base station).

Let’s understand the data transfer procedure, or data call, in NB-IoT. The messages exchanged between the UE and the eNB are categorized into the following phases:

• 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’s break down the RACH procedure (network entry procedure or data transfer) in NB-IoT technology step by step:

** Step-1:** The UE receives NPSS (Narrowband Primary Synchronization Signal) and NSSS (Narrowband Secondary Synchronization Signal) preamble signals from the eNB in the downlink. This allows the UE to synchronize with the eNB.

** Step-2:** After synchronization and confirming that access is not barred, the UE sends a random access preamble using the NPRACH (Narrowband Physical Random Access Channel) channel. The UE utilizes the appropriate NPRACH configuration based on its coverage class estimation.

After the eNB detects the NPRACH preamble, it sends back a RAR (Random Access Response), designated as “msg2”. The RAR contains TA (Timing Advance) parameters as well as scheduling information that the UE will use to transmit “msg3” (the Connection Resume Request).

** Step-3:** In “msg4,” which is the “RRC connection resume,” the network resolves any contention issues that arise due to multiple UEs transmitting the same random access preamble. The eNB then transmits a connection setup or resume message. The UE responds with an RRC connection setup or resume complete message to complete the transition to the “Connected State.”

** Step-4:** Once the connection is established, data transfer between the UE and the eNB takes place as depicted by the various messages in the figure.

** Step-5:** When the data transfer is complete, the eNB can release the connection, as shown in the diagram.

Reference: 3GPP 36 series LTE