5G N2 Handover Procedure Explained

Introduction

In 5G networks, N2 Handover is a mobility procedure where the handover process is coordinated by the 5G Core Network through the AMF.

Unlike Xn handover, where gNBs communicate directly, N2 handover is used when the Xn interface is not available or cannot be used.

This procedure ensures that the UE can move between base stations while maintaining active sessions, even when the gNBs cannot directly communicate.

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 N2 Handover Happens

N2 handover typically occurs when:

• the Xn interface between gNBs is unavailable
• the target gNB belongs to a different network domain
• core network coordination is required

Typical scenarios include:

• inter-vendor base stations
• inter-region mobility
• network topology limitations

Network Elements Involved

UE (User Equipment)

Measures neighboring cells and reports measurements to the serving gNB.

Source gNB

The currently serving base station.

Target gNB

The base station that will take over the UE connection.

AMF (Access and Mobility Management Function)

Coordinates the handover procedure between gNBs.

UPF (User Plane Function)

Handles the user-plane traffic path update after handover.

Interfaces Used

N2 Handover Call Flow

Below is the simplified signaling sequence.

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

In this procedure, the AMF acts as the coordinator between the source and target gNB.

Step-by-Step Explanation

Step 1: Measurement Reporting

The UE continuously measures neighboring cells.

Key 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)
• signal thresholds
• neighbor cell configuration

Step 2: Handover Decision

The source gNB decides that a handover is required.

Since the Xn interface cannot be used, the source gNB sends a Handover Required message to the AMF.

Important parameters to check

Check:

• target cell selection
• neighbor relations
• load balancing policies

Step 3: Handover Preparation

The AMF sends a Handover Request to the target gNB.

The target gNB:

• allocates radio resources
• prepares the UE context

Important parameters to check

Verify:

• resource allocation
• UE context transfer
• security context

Step 4: Handover Command

Once preparation is complete, the source gNB sends an RRC Reconfiguration message (handover command) to the UE.

This message includes:

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

Important parameters to check

Look for:

• target gNB cell ID
• handover configuration parameters
• synchronization 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

The user-plane path is updated through the core network.

After this step, data traffic flows through the target gNB.

Data Path After Handover

After handover completion, the data path becomes:

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

Applications continue running without interruption.

N2 Handover vs Xn Handover

Troubleshooting N2 Handover

Handover Failure

Possible causes:

• target gNB resource shortage
• incorrect neighbor configuration
• AMF signaling issues

Radio Link Failure

Possible reasons:

• UE unable to synchronize with target cell
• handover command lost
• radio interference

Session Interruption

Possible causes:

• user plane path update failure
• UPF routing issues
• N3 tunnel configuration problems

Key Messages in N2 Handover

Relevant 3GPP Specifications

The N2 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 N2 Handover procedure allows a UE to move between base stations when the Xn interface cannot be used.

The process includes:

1. measurement reporting by the UE
2. handover decision by the source gNB
3. AMF coordination between gNBs
4. UE synchronization with the target cell

This procedure ensures seamless mobility even in complex network topologies where direct gNB communication is not possible.