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 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).

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 >

Reference

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