LTE Handover Overview

This tutorial describes LTE Handover types and procedures.

LTE changed the way traditional mobile network were working. Leaving circuit switched technology far behind LTE became the first All-IP mobile network. Ĺike other mobile networks LTE also have big challenges when it comes to mobility areas like handover and reselection. Handover in particular is more complicated in LTE as LTE has to deal with Intra-LTE handovers as well as handovers between LTE and UTRAN, LTE and GERAN and other mobile networks.

The basic objective of handover procedures are:

• QoS should be maintained all the time. Not after handover but during handover as well.
• Handover should not drain UE battery.
• UE should able to continue it’s normal services before and after handover. For example a voice call before handover should be maintained after handover as well.
• Seamless handoff between 2G/3G/CDMA/LTE technologies.

Normally there are two type of handover approach available in mobile networks.

• Network Controlled: In this case network makes handover decisions.
• Mobile Evaluated: The UE will make handover decisions and inform network about it. But still network takes the final decision based on radio resource available in target cell.

In LTE network a hybrid approach is used. UE sends measurement information to network and based on those measurements network asks UE to move to a target cell.

Types of Handover in LTE network

• Intra-LTE Handover: In this case source and target cells are part of the same LTE network.
• Inter-LTE Handover: Handover happens towards other LTE nodes. (Inter-MME and Inter-SGW)
• Inter-RAT: Handover between different radio technologies. For example handover from LTE to WCDMA.

Intra-LTE Handovers

There are different use cases for Intra-LTE handovers. There are primarily three types of Intra-LTE handover can be possible

Intra-MME/SGW: Handover using X2 Interface

X2 is the interface between two eNodeBs, serving eNodeB and target eNodeB in this case. When X2 interface is present then handover is completed without EPC (Evolved Packet Core) involvement. The release of the resources at source eNodeB is triggered by target eNodeB.

Intra-MME/SGW: Handover using S1 Interface

In case when X2 interface is not available and source eNodeB and target eNodeB are part of same MME/SGW then handover is carried out through S1 interface. The S-eNB initiates the handover by sending a Handover required message over the S1-MME reference point. The EPC does not change the decisions taken by the S-eNB.

Inter-LTE Handovers

Inter-MME Handover

In Inter-MME handover two MME are involved in handover, source MME and target MME. The source MME (S-MME) is in charge of the source eNodeB and target MME (T-MME) is in charge of target eNodeB.

Inter-MME handover occurs when UE moves between two different MMEs but connected to same SGW.

Inter-MME/SGW Handover

This is same as Inter-MME but only difference is that here UE need to move from one MME/SGW to another MME/SGW. Source eNodeB is part of one MME/SGW and target eNodeB is in another MME/SGW.

Inter-RAT Handover

Handover from eUTRAN to UTRAN

In case of handover between eUTRAN to UTRAN, the source eNodeB is connected to source MME and SGW and target RNC is connected to Target SGSN and Target SGW.

First the required resources are reserved in UTRAN system and the handover is carried out.