Handover in LTE

3GPP Long Term Evolution (LTE) is a mobile communication standard designed to offer higher data rates, higher system throughout and lower latency for delay sensitive applications and services. To offer these enhancements a change in the radio interface was important. LTE uses Orthogonal Frequency Dvision Multiplexing (OFDM) for downlink and Single Carrier Frequency Domain Multiple Access (SC-FDMA) for uplink hence abandoning the complex soft-handover of W-CDMA used in 3G mobile networks.

The goal of any wireless communication system is to provide fast and seamless handover from one cell to another. The procedure of handover in LTE depends very much on the type of application being used, the interruption of a large file download may be tolerable but not soo much for a VoIP call.


There are three types of Handovers in LTE:

• Intra-LTE Handover: Handover within the LTE nodes i.e. MME and SGW.
• Inter-LTE Handover: Handover with the other LTE nodes i.e. Inter-MME and Inter-SGW
• Inter-RAT Handover: Handover with different Radio access technologies e.g. UMTS and WLAN.

We will take these types one by one.

Intra-LTE Handover

Intra-LTE handover relates to the handover within the LTE nodes i.e. MME and SGW. There are two main types of interfaces used for intra-LTE handover; the X2 interface and the S1 interface. X2 interface uses the X2 Application Protocol (X2AP) signalling and the S1 interface uses the S1 Application Protocol (S1AP) signalling.

Handover using the X2 interface

This procedure is in use when the UE moves from a source eNodeB (SeNB) to the target eNodeB (TeNB) using the X2 interface where the Mobility Management Entity (MME) and Serving Gateway (sGW) are unchanged meaning that the same MME and SGW are serving both the eNodeB's. The handover is only possible if there is a direct connectivity between the SeNB and the TeNB through the X2 interface. For handover preparation the SeNB passes all the necessary information required to the TeNB (e.g. the RRC context and E-RAB attributes). The UE accesses the TeNB cell using the non-contention based Random Access Channel preamble. In this method the handover is performed without the involvement of Evolved Packet Core (EPC) i.e. the preparation messages are directly exchanged between the SeNB and the TeNB.

(Diagrams soon to follow)


...to be continued