The Cell Radio Network Temporary Identifier (C-RNTI) is a unique identifier used in cellular networks, including 5G. It is a temporary identifier assigned to a UE by the base station (in the case of 5G, this is the gNodeB) when the UE first successfully establishes a connection to the cellular network. Usually it is assigned during the RACH process (msg 4) .

The C-RNTI is specific to the cell the UE is currently connected to. If the UE moves to a different cell, a new C-RNTI will be assigned by the new cell's gNodeB. This identifier is critical for efficient and accurate communication between the UE and the network infrastructure.

The structure of MAC-CE for C-RNTI is as follows. The C-RNTI MAC CE is identified by MAC subheader with LCID

< 38.321 - Figure 6.1.3.2-1: C-RNTI MAC CE >

C-RNTI Overview

C-RNTI (Cell Radio Network Temporary Identifier) is a unique temporary identifier assigned to a UE by a base station (gNodeB in 5G). It is typically assigned during the Random Access Channel (RACH) procedure (message 4).

The C-RNTI is specific to the cell the UE is currently connected to. If the UE moves to a different cell, a new C-RNTI will be assigned by the new cell's gNodeB. This identifier is crucial for maintaining seamless and efficient communication between the UE and the network infrastructure.

Purposes of C-RNTI

The C-RNTI serves the following purposes:

• Identification: It uniquely identifies the UE within a particular cell, which is essential for the management and delivery of data.
• Communication: It is used in the uplink and downlink to multiplex and demultiplex the data and control information. This means that when a signal is received, the gNodeB can identify which UE sent it, and when the gNodeB sends a signal, the UE knows it is the intended recipient.
• Resource Allocation: It is used for scheduling decisions on the uplink and downlink. When the gNodeB allocates resources for the UE to transmit data, it references the C-RNTI (Each DCI is masked with C-RNTI).

Use in the RACH Process

During the RACH procedure, the C-RNTI is assigned in msg 4 as part of the contention resolution process. Once assigned, the C-RNTI allows the UE to establish a unique communication link with the gNodeB.

Key Features of C-RNTI

• Temporary but Essential: Valid as long as the UE remains connected to a specific cell. Upon handover, a new C-RNTI is issued by the new cell.
• Critical for Uplink and Downlink Scheduling: The gNodeB uses the C-RNTI to determine and allocate resources for data transmission, ensuring efficient network operations.
• Foundation of Secure Communication: Masking of DCI with C-RNTI ensures that communication is directed only to the intended recipient.

Reference

[1]