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5g to 4g handover call flow

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The handover process in mobile communication refers to the seamless transfer of a user's ongoing communication session from one cell to another. In the case of a handover from 5G to 4G (LTE), it involves several steps to ensure uninterrupted service. Here's a detailed technical explanation of the 5G to 4G handover call flow:

1. Measurement and Decision Making:

  1. Measurement Report Triggering:
    • The mobile device continuously monitors the signal quality of neighboring cells in both 5G and 4G.
    • When certain criteria are met, such as a stronger signal in the 4G network, the mobile device triggers a measurement report.
  2. Measurement Report Transmission:
    • The mobile device sends the measurement report to the 5G base station (gNB) indicating the desirability of handover to a 4G cell.

2. Handover Request:

  1. Handover Request by 5G gNB:
    • The 5G gNB evaluates the measurement report and decides that a handover is necessary.
    • It sends a handover request to the 5G core network (5GC), which includes the target 4G eNB (evolved NodeB).

3. Connection Reconfiguration:

  1. Connection Reconfiguration by 5GC:
    • The 5GC communicates with the 4G core network (EPC - Evolved Packet Core) to prepare for the handover.
    • Parameters for the new 4G connection are configured, and a message is sent back to the 5G gNB.
  2. Connection Reconfiguration by 5G gNB:
    • The 5G gNB informs the mobile device about the upcoming handover and configures it to prepare for the transition to the 4G network.

4. Handover Execution:

  1. Context Transfer to 4G eNB:
    • The 5G gNB establishes communication with the target 4G eNB and transfers the user's context (session information, security keys, etc.).
  2. Handover Command to Mobile Device:
    • The 5G gNB sends a handover command to the mobile device, instructing it to connect to the target 4G cell.

5. 4G Network Entry:

  1. Radio Link Addition by Mobile Device:
    • The mobile device establishes a radio link with the target 4G eNB.
  2. Handover Confirmation by 4G eNB:
    • The 4G eNB confirms the successful handover to the 5G gNB.

6. Path Switch:

  1. Data Forwarding via 4G Network:
    • Data transmission is switched from the 5G to the 4G network, ensuring continuity of the communication session.

7. Handover Completion:

  1. Handover Confirmation by 5G gNB:
    • The 5G gNB receives confirmation from the 4G eNB that the handover is complete.
  2. Release of Resources:
    • Resources allocated for the 5G connection are released, and the 5G gNB updates its status accordingly.

The handover process ensures a smooth transition from 5G to 4G, allowing users to maintain their ongoing communication sessions without interruption. Each step involves communication between the mobile device, the 5G gNB, the 5GC, and the 4G eNB, as well as the coordination of network elements in both the 5G and 4G networks.