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5G Connected Mode Handover Explained

call-flow 5G NR | NG-RAN | RRC | Handover

Introduction

In 5G networks, Connected Mode Handover allows a User Equipment (UE) to move between cells while maintaining an active RRC connection.

Unlike Idle Mode Mobility, where the UE performs mobility decisions itself, handover decisions in connected mode are controlled by the network.

The purpose of connected mode handover is to:

  • maintain uninterrupted data sessions
  • prevent radio link failures
  • optimize network performance

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

  • 3GPP TS 38.300 - NR Overall Description
  • 3GPP TS 38.331 - RRC Protocol
  • 3GPP TS 23.502 - 5G System Procedures
5G Connected Mode Handover call flow diagram

When Connected Mode Handover Happens

A handover occurs when:

  • the UE moves away from the serving cell
  • a neighboring cell offers better signal quality
  • the network needs to balance load

Typical triggers include:

  • RSRP degradation
  • interference conditions
  • cell congestion

Types of Handover in 5G

Handover Type Description
Xn HandoverDirect handover between gNBs
N2 HandoverHandover controlled by the core network
Intra-gNB HandoverBetween cells within the same gNB

Network Elements Involved

UE (User Equipment)

Measures signal quality and reports measurements to the network.

Source gNB

The currently serving base station controlling the UE.

Target gNB

The base station to which the UE will be transferred.

AMF

Participates in N2 handovers when core network coordination is required.

Interfaces Used

Interface Description
NR-UuRadio interface between UE and gNB
XnInterface between neighboring gNBs
N2Interface between gNB and AMF
N3User plane tunnel between gNB and UPF

Connected Mode Handover Call Flow

Below is the simplified signaling sequence.

UE          Source gNB         Target gNB
 |              |                 |
 |--Measurement Report---------->|
 |              |                 |
 |              |--Handover Req-->|
 |              |                 |
 |              |<--Handover Ack--|
 |<--RRC Handover Command--------|
 |                               |
 |===== UE switches to target cell =====|
 |                               |
 |----RRC Reconfiguration Complete---->|

During this procedure, the UE switches from the source cell to the target cell without interrupting the data session.

Step-by-Step Explanation

Step 1: Measurement Reporting

The UE continuously measures signal quality of neighboring cells.

These measurements include:

  • RSRP
  • RSRQ
  • SINR

The UE sends a Measurement Report to the serving gNB.

Important parameters to check

Engineers should verify:

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

Step 2: Handover Decision

The source gNB analyzes the measurement report and decides whether a handover is required.

The source gNB selects the target gNB.

Important parameters to check

Check:

  • neighbor cell configuration
  • handover thresholds
  • load balancing policies

Step 3: Handover Preparation

The source gNB sends a Handover Request to the target gNB via the Xn interface.

The target gNB prepares resources for the UE.

Important parameters to check

Verify:

  • target cell resources
  • radio bearer configuration
  • security context transfer

Step 4: Handover Command

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

The UE then:

  • disconnects from the source cell
  • synchronizes with the target cell

Important parameters to check

Check:

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

Step 5: Handover Completion

The UE connects to the target gNB and sends RRC Reconfiguration Complete.

User plane traffic continues through the target cell.

Data Path After Handover

Once handover completes, the data path becomes:

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

The user session remains active, so applications do not experience interruption.

Handover Measurement Events

Common measurement events used in 5G handovers:

Event Description
A1Serving cell becomes better
A2Serving cell becomes worse
A3Neighbor cell becomes better than serving cell
A5Serving cell below threshold and neighbor above threshold

Event A3 is the most commonly used trigger for handovers.

Troubleshooting Handover Problems

Handover Failure

Possible causes:

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

Radio Link Failure

Possible reasons:

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

Ping-Pong Handover

Possible causes:

  • incorrect threshold settings
  • aggressive mobility parameters
  • overlapping coverage

Key Messages in Connected Mode Handover

Message Purpose
Measurement ReportUE reports neighbor measurements
Handover RequestSource gNB requests target resources
Handover CommandUE instructed to move to target cell
RRC Reconfiguration CompleteUE confirms handover

Relevant 3GPP Specifications

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

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

Summary

The Connected Mode Handover procedure enables a UE to move between cells while maintaining an active connection.

The process involves:

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

This procedure ensures seamless mobility and uninterrupted data sessions in 5G networks.