Interoperable UE Handovers in LTE

With the fast-changing mobile landscape and convergence in all aspects of telecommunications, “seamless handover” is important for any technology to succeed. Operators and consumers both benefit from seamless handover in terms of cost effectiveness, enhanced features, location independence and ease of use, not only within a Long Term Evolution (LTE) network but also between networks including UMTS, GSM and CDMA.

In this paper we briefly touch upon the procedures executed by the user equipment (UE) and the various network elements to provide the handover services requested by the UE. We cover Intra-LTE and LTE to/from UMTS handovers.

Objectives of Handover Procedures

1. It is important that QoS is maintained, not just before and after a handover, but during the handover as well.
2. Handover shall not drain the UE battery power.
3. Service continuity shall be maintained (i.e., minimal handover latency).
4. Seamless handoff to 3G / 2G / CDMA technology.

There are two ways a handoff can be decided:

1. Network Evaluated: the network makes the handover decision
2. Mobile Evaluated: the UE makes the handoff decision and informs the network about it. In this instance, the final decision will be made by the network based upon on the Radio Resource Management.

In 3G and LTE networks, a hybrid approach is used to decide on the handover. In this case, the UE will assist in the handoff decision by measuring the neighboring cells and reporting the measurements to the network, which in turn decides upon the handoff timing and the target cell/node. The parameters to measure and the thresholds for reporting are decided by the network.

In LTE there are three types of handovers:

1. Intra-LTE: Handover happens within the current LTE nodes (intra-MME and Intra-SGW)
2. Inter-LTE: Handover happens toward the other LTE nodes (inter-MME and Inter-SGW)
3. Inter-RAT: Handover between different radio technology networks, for example GSM/UMTS and UMTS

We’ll look at Intra-LTE handovers with X2AP signaling and S1AP signaling first, then Inter-RAT handovers in LTE (i.e., handover between LTE and UMTS).

There is much more!

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