5G Xn Handover Procedure Explained

Introduction

In 5G networks, Xn Handover is a mobility procedure that allows a User Equipment (UE) to move from one gNB to another using the Xn interface between the base stations.

The key advantage of Xn handover is that the handover is handled directly between gNBs without heavy involvement from the core network.

This results in:

• faster mobility handling
• reduced signaling load on the core network
• lower handover latency

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 Xn Handover Happens

Xn handover occurs when:

• the UE moves to a neighboring gNB
• both gNBs have an Xn interface connection
• the target cell provides better radio conditions

Typical triggers include:

• decreasing RSRP
• measurement event A3
• network load balancing

Network Elements Involved

UE (User Equipment)

Measures radio signals from neighboring cells and reports measurements.

Source gNB

The base station currently serving the UE.

Target gNB

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

AMF

In Xn handover, the AMF involvement is minimal, since the gNBs communicate directly.

Interfaces Used

Xn Handover Call Flow

Below is the simplified signaling sequence.

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

During this procedure, the UE transitions from the source gNB to the target gNB.

Step-by-Step Explanation

Step 1: Measurement Reporting

The UE continuously measures the signal quality of neighboring cells.

Measurements include:

• RSRP
• RSRQ
• SINR

The UE sends a Measurement Report to the source gNB.

Important parameters to check

Engineers should verify:

• measurement event (A3, A5)
• measurement thresholds
• neighbor cell signal strength

Step 2: Handover Decision

The source gNB analyzes the measurement report.

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

Important parameters to check

Check:

• handover offset values
• cell load conditions
• neighbor relations

Step 3: Handover Preparation

The source gNB sends an Xn Handover Request to the target gNB.

The target gNB:

• allocates radio resources
• prepares UE context
• confirms readiness

Important parameters to check

Verify:

• target cell capacity
• 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 includes:

• target cell identity
• radio configuration
• synchronization parameters

Important parameters to check

Check:

• target cell ID
• mobility control information
• RRC configuration parameters

Step 5: Handover Execution

The UE disconnects from the source gNB and synchronizes with the target gNB.

The UE then sends RRC Reconfiguration Complete.

Step 6: Path Update

After the UE connects to the target gNB, the user plane path is updated.

Data traffic now flows through the target gNB.

Data Path After Handover

After handover completion, the data path becomes:

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

User applications continue running without interruption.

Xn Handover vs N2 Handover

Troubleshooting Xn Handover

Handover Failure

Possible causes:

• Xn interface connectivity issue
• insufficient resources in target cell
• neighbor configuration errors

Radio Link Failure

Possible reasons:

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

Ping-Pong Handover

Possible causes:

• aggressive handover thresholds
• overlapping coverage
• mobility parameter misconfiguration

Key Messages in Xn Handover

Relevant 3GPP Specifications

The Xn Handover 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

Summary

The Xn Handover procedure allows a UE to move between base stations using the direct Xn interface between gNBs.

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 target cell

This approach reduces handover latency and signaling overhead, making it an efficient mobility mechanism in 5G NR networks.