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5G NR RACH Procedure Overview – CBRA and CFRA

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The Random Access Channel (RACH) procedure in 5G NR (New Radio) is a mechanism by which User Equipments (UEs) initiate communication with the network, particularly when they need to establish a connection or request resources. There are two primary RACH procedures in 5G NR: Contention-Based Random Access (CBRA) and Contention-Free Random Access (CFRA). Let's explore these procedures in technical detail:

1. Conten​tion-Based Random Access (CBRA):

1.1 Objective:

  • CBRA is used when multiple UEs attempt to access the network simultaneously, contending for the available resources.

1.2 Process Overview:

  • Random Access Preamble Transmission:
    • The UE selects a random access preamble and transmits it on the Physical Random Access Channel (PRACH).
  • The preamble serves as a unique identifier for the UE, allowing the network to identify and respond to the specific UE.
  • Contention Resolution:
    • The network acknowledges the reception of the preamble.
    • The UE receives a contention resolution message (CR) indicating whether it should proceed to the next step or wait for further instructions.
  • Msg1 (Initial Access Request):
    • If contention resolution is successful, the UE sends Message 1 (Msg1), which includes essential information such as the identity of the UE and the reason for access.
  • Network Response:
    • The network processes Msg1 and responds accordingly, providing the necessary resources or requesting additional information from the UE.

1.3 Parameters:

  • Random Access Preamble Format:
    • Preambles are defined with different formats, and the UE randomly selects one.
  • Preamble Transmission Timing:
    • The UE follows a specific timing for transmitting the preamble on the PRACH.

2. Conten​tion-Free Random Access (CFRA):

2.1 Objective:

  • CFRA is used when the network assigns specific time resources to UEs, minimizing contention and ensuring a deterministic access scheme.

2.2 Process Overview:

  • Grant-Based Access:
    • UEs receive a grant from the network specifying when and how to access the PRACH.
    • The UE uses the allocated resources to transmit Msg1 without contention.
  • Msg1 (Initial Access Request):
    • Similar to CBRA, the UE sends Message 1 (Msg1) on the PRACH, including essential information.
  • Network Response:
    • The network processes Msg1 and responds accordingly, either granting additional resources or providing the necessary services.

2.3 Parameters:

  • Resource Allocation:
    • The network allocates specific time resources for the UE to perform the random access procedure.
  • Grant Format:
    • The format of the grant specifies the resources allocated to the UE, including timing and frequency information.

3. Key Considerations:

3.1 Collision Handling (CBRA):

  • In CBRA, if multiple UEs select the same preamble, collisions may occur. Random selection of preambles helps mitigate collisions.

3.2 Deterministic Access (CFRA):

  • CFRA is used for scenarios where deterministic access is crucial, such as for critical machine-type communication.

3.3 Efficiency and Scalability:

  • CBRA is more suitable for scenarios with a lower number of simultaneous access attempts, while CFRA provides more efficient access in scenarios with higher contention.

3.4 Resource Allocation Flexibility:

  • CBRA is more flexible as UEs contend for resources, while CFRA provides more deterministic resource allocation.

In summary, the 5G NR RACH procedure encompasses both CBRA and CFRA to address different scenarios and network requirements. CBRA is suitable for scenarios with higher contention, while CFRA is designed for deterministic and efficient access in specific use cases, such as URLLC. The key is to balance contention and resource allocation to optimize the overall performance of the random access process.