5G Inter-gNB Handover Procedure Explained

Introduction

In 5G networks, Inter-gNB Handover occurs when a User Equipment (UE) moves from one gNB (Next Generation NodeB) to another while maintaining an active connection.

This type of handover happens when the serving base station and the target base station belong to different gNB nodes.

The goal of this procedure is to:

• maintain uninterrupted data sessions
• preserve radio link quality
• ensure seamless user mobility

The procedure is defined by the 3rd Generation Partnership Project in:

• 3GPP TS 23.502 - 5G System Procedures
• 3GPP TS 38.300 - NR Architecture
• 3GPP TS 38.331 - RRC Protocol

When Inter-gNB Handover Happens

Inter-gNB handover typically occurs when:

• the UE moves out of the coverage area of the serving gNB
• a neighboring gNB provides better signal quality
• network load balancing requires UE relocation

Typical triggers include:

• decreasing RSRP
• increased interference
• network optimization policies

Network Elements Involved

UE (User Equipment)

Measures neighboring cell signals and reports them to the network.

Source gNB

The current serving base station managing the UE connection.

Target gNB

The neighboring base station that will take over the UE connection.

AMF (Access and Mobility Management Function)

Participates in the procedure if core network coordination is required.

Interfaces Used

Inter-gNB Handover Call Flow

Below is the simplified signaling sequence.

UE           Source gNB        Target gNB        AMF
 |               |                 |              |
 |--Measurement Report----------->|
 |               |                 |              |
 |               |--Handover Request-------------->|
 |               |                 |              |
 |               |<--Handover Acknowledge---------|
 |<--RRC Handover Command----------|
 |                                 |
 |====== UE synchronizes to target gNB ======|
 |                                 |
 |----RRC Reconfiguration Complete---------->|

During this procedure, the UE moves from the source gNB to the target gNB while maintaining its data session.

Step-by-Step Explanation

Step 1: Measurement Reporting

The UE continuously measures neighboring cells.

Key measurements include:

• RSRP (Reference Signal Received Power)
• RSRQ (Reference Signal Received Quality)
• SINR

The UE sends a Measurement Report to the source gNB.

Important parameters to check

Engineers should verify:

• measurement event type (A3, A5)
• RSRP levels
• measurement thresholds

Step 2: Handover Decision

The source gNB analyzes the measurement report.

If a neighboring cell offers better signal quality, the source gNB decides to initiate handover.

Important parameters to check

Check:

• neighbor cell configuration
• handover offset values
• load balancing policies

Step 3: Handover Preparation

The source gNB sends a Handover Request to the target gNB.

The target gNB:

• allocates radio resources
• prepares context for the UE

Important parameters to check

Verify:

• radio bearer configuration
• UE context transfer
• security key forwarding

Step 4: Handover Command

The source gNB sends an RRC Reconfiguration message (Handover Command) to the UE.

This message instructs the UE to switch to the target cell.

Important parameters to check

Look for:

• target cell identity
• radio configuration parameters
• mobility control information

Step 5: Handover Completion

The UE synchronizes with the target gNB and sends RRC Reconfiguration Complete.

User plane traffic continues through the new base station.

Data Path After Handover

After handover completes, the data path becomes:

UE -> Target gNB -> UPF -> Data Network

Applications continue running without interruption.

Differences Between Inter-gNB and Intra-gNB Handover

Troubleshooting Inter-gNB Handover

Handover Failure

Possible causes:

• insufficient resources in target cell
• incorrect neighbor configuration
• radio interference

Radio Link Failure

Possible reasons:

• UE cannot synchronize with target cell
• handover command lost
• poor signal conditions

Ping-Pong Handover

Possible causes:

• incorrect handover thresholds
• overlapping cell coverage
• aggressive mobility parameters

Key Messages in Inter-gNB Handover

Relevant 3GPP Specifications

The Inter-gNB Handover procedure is defined by the 3rd Generation Partnership Project in:

• 3GPP TS 23.502 - 5G System Procedures
• 3GPP TS 38.300 - NR Overall Description
• 3GPP TS 38.331 - RRC Protocol

Summary

The Inter-gNB Handover procedure allows a UE to move between different 5G base stations while maintaining an active connection.

The process includes:

1. measurement reporting by the UE
2. handover decision by the source gNB
3. resource preparation at the target gNB
4. UE synchronization with the new cell

This ensures seamless mobility and uninterrupted user experience in 5G networks.