5G-NR RACH trigger reasons with details

The Random Access Channel (RACH) in 5G New Radio (NR) is used by User Equipments (UEs) to initiate communication with the network. The RACH procedure is triggered by various events or conditions, known as trigger reasons. Let's explore the technical details of 5G-NR RACH trigger reasons:

1. Initial Access:

• Description:
  • A UE triggers the RACH procedure when it initially powers on or when it enters a new cell and needs to establish a connection with the network.
• Procedure:
  • The UE sends a Random Access Preamble to the gNB (Next Generation NodeB) to request access.

2. Cell Change Order:

• Description:
  • The network instructs a UE to change its serving cell, triggering the RACH procedure for the UE to connect to the new cell.
• Procedure:
  • The UE follows the network's instructions to perform a cell change and initiates the RACH procedure in the new cell.

3. Handover:

• Description:
  • A UE initiates the RACH procedure when instructed by the network to perform a handover to a new cell.
• Procedure:
  • The UE sends a Random Access Preamble in the target cell to complete the handover process.

4. RRC Inactive State:

• Description:
  • When a UE is in RRC (Radio Resource Control) inactive state and needs to communicate with the network, it triggers the RACH procedure.
• Procedure:
  • The UE sends a Random Access Preamble to request access and transition to RRC active state.

5. RRC Connection Re-establishment:

• Description:
  • In scenarios where the RRC connection between the UE and the network is lost or disrupted, the UE triggers the RACH procedure to re-establish the connection.
• Procedure:
  • The UE sends a Random Access Preamble to request access and re-establish the RRC connection.

6. UE Inactivity Timer Expiry:

• Description:
  • If a UE remains inactive for an extended period, the network may release resources and instruct the UE to initiate the RACH procedure when it needs to resume communication.
• Procedure:
  • The UE sends a Random Access Preamble to request access and resume communication with the network.

7. UE-Requested Scheduling:

• Description:
  • The UE may request scheduling resources for uplink transmission and trigger the RACH procedure when the requested resources are granted.
• Procedure:
  • The UE sends a Random Access Preamble to request access and utilize the granted resources for uplink transmission.

8. Contention Resolution Failure:

• Description:
  • In cases where the contention resolution fails during a previous RACH attempt, the UE may retry the RACH procedure.
• Procedure:
  • The UE sends a new Random Access Preamble to retry the RACH procedure for access.

9. Other Trigger Events:

• Description:
  • Various other events, such as specific timers or conditions defined by the network, may trigger the RACH procedure.
• Procedure:
  • The UE sends a Random Access Preamble based on the specific trigger event to request access.

10. Access Class Barring:

• Description:
  • The network may impose access restrictions (access class barring) on a UE, limiting its access attempts. When the barring timer expires, the UE can trigger the RACH procedure again.
• Procedure:
  • The UE sends a Random Access Preamble after the access class barring timer expires to request access.

11. Power-Up Related RACH Trigger:

• Description:
  • The UE may trigger the RACH procedure after a power-up event.
• Procedure:
  • The UE sends a Random Access Preamble to request access after powering up.

Note:

• Random Access Preamble:
  • The Random Access Preamble is a short sequence transmitted by the UE to initiate the RACH procedure. It includes information that helps the network identify the UE and allocate resources for further communication.

Understanding these trigger reasons is crucial for designing efficient and reliable access procedures in 5G NR networks, ensuring that UEs can successfully establish connections under various circumstances.