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5g nr handover call flow

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5G New Radio (NR) introduces several new concepts and procedures compared to its predecessors, including more sophisticated handover mechanisms. The handover process ensures uninterrupted communication for a user equipment (UE) as it moves across different cells or even between different Radio Access Technologies (RATs). Below is a technical overview of the 5G NR handover call flow.

1. Initial Context Setup:

Before any handover process begins, the UE establishes an initial context with the source base station (gNB, or gNodeB). This context includes necessary parameters, context information, and radio resources.

2. Measurement and Triggering:

The UE continuously monitors neighboring cells to determine if there are better cells for potential handover. This is facilitated by:

  • Measurement Reports: The UE sends periodic measurements and reports to the serving gNB about neighboring cells' quality and other parameters.
  • Event A and Event B: These are predefined criteria (like a certain threshold breach) based on which the UE may trigger the handover process.

3. Handover Decision:

Once the UE detects that the quality of the serving cell is degrading or there's a better cell available, the decision for a handover is made. This decision can be:

  • Network-Controlled Handover (NCHO): Controlled by the network, where the serving gNB instructs the UE to perform a handover.
  • UE-Initiated Handover: Triggered by the UE based on its measurements.

4. Handover Preparation:

After the decision to handover is made, the following preparations occur:

  • Path Switching: The target gNB and source gNB establish a path for data forwarding.
  • Context Transfer: The serving gNB transfers the UE's context (session and mobility) to the target gNB. This ensures that the target gNB can continue the service without disruption.

5. Handover Execution:

The actual handover process occurs in this phase:

  • Radio Bearer Reconfiguration: Existing radio bearers are reconfigured or new bearers are established with the target gNB.
  • Data Forwarding: Once the handover is successful, the data packets are forwarded from the source gNB to the target gNB and then to the UE.

6. Handover Completion:

After successful data forwarding:

  • Release of Old Resources: The source gNB releases the radio resources allocated to the UE.
  • Verification and Optimization: The network verifies the quality of the handover and optimizes resources if necessary.

7. Post-Handover:

The UE continues its communication with the target gNB. Depending on the handover type (e.g., inter-gNB, intra-gNB), the procedures might slightly vary.

Important Aspects:

  • Handover Command and Request: These are messages between the network and the UE, signaling the intent to handover and initiate the process.
  • Data Buffering: To ensure seamless communication, data buffering mechanisms might be in place to hold packets during the transition phase.
  • Mobility Management: Procedures like Registration Area Update (RAU) or Tracking Area Update (TAU) in 5G support the mobility of the UE.

The 5G NR handover call flow is a complex series of signaling and data transfer procedures designed to ensure uninterrupted communication for mobile users as they move across the network.