What is TC-RNTI (Temporary C-RNTI)

TC-RNTI: A Temporary Identifier in 5G Random Access

In the world of 5G New Radio (NR) mobile communication, Temporary C-RNTI (TC-RNTI) plays a critical role during the Random Access Procedure (RAP). It acts as a temporary identifier used to establish communication between a User Equipment (UE) and the network (gNB - base station).

Here's a deeper dive into the technicalities of TC-RNTI:

Function:

• The RAP allows UEs to request access to the network for data transmission.
• As part of the RAP process, the UE transmits a Random Access Preamble, a unique signal that identifies the UE to the gNB.
• In response, the gNB sends a Random Access Response (RAR) message containing the TC-RNTI.
• This TC-RNTI serves as a temporary identifier for the UE during the ongoing RAP.
• The UE uses the received TC-RNTI for subsequent messages exchanged during the RAP completion process.

Benefits of TC-RNTI:

• Reduced Collision Risk: In a scenario with multiple UEs attempting access simultaneously, TC-RNTI helps differentiate between them during the initial response phase.
• Efficient Resource Allocation: By assigning a temporary identifier, the gNB can efficiently allocate resources for further communication with the UE during RAP completion.

TC-RNTI vs. C-RNTI (Cell RNTI):

• C-RNTI: After successful RAP completion, the UE establishes a dedicated connection with the network. This connection is identified by a unique identifier called C-RNTI (Cell RNTI). C-RNTI serves as the primary identifier for the UE throughout its communication session with the gNB.
• TC-RNTI: It's a temporary identifier used only during the initial RAP exchange. Once the connection is established, the UE transitions to using C-RNTI for all communication.

Additional Points:

• TC-RNTI Format: It's typically a 16-bit value, offering a range of possible identifiers for UEs.
• TC-RNTI Lifetime: The TC-RNTI remains valid only during the ongoing RAP procedure. If the RAP is unsuccessful, the TC-RNTI is discarded by the UE.
• Contention-based vs. Non-contention-based RAP:
  • Contention-based RAP: Multiple UEs might contend for access simultaneously. In this scenario, the UE successfully receives and uses the TC-RNTI for further communication during RAP completion.
  • Non-contention-based RAP: When the network pre-allocates resources for a specific UE, no contention is involved. The UE might not receive a TC-RNTI in the RAR message, and it would continue using its existing C-RNTI (if any) for communication.

In Conclusion:

TC-RNTI serves as a vital temporary identifier during the initial phase of 5G NR Random Access. It facilitates communication between the UE and the network, enabling efficient resource allocation and connection establishment. Understanding TC-RNTI is crucial for grasping the intricacies of the 5G NR access procedures.