LTE, LTE-Advanced and WiMAX: Towards IMT-Advanced Networks

13

Mobility Management

The UE states and state transitions (described in Chapter 11) also dictate the UE and network behavior when it comes to maintaining connectivity. 3GPP have ensured sufficient mechanisms to minimize handover delay and disruption through a simplified RAT and core and management architecture. This enhancement is mainly realized through a signaling hierarchy where the core's involvement in the user's mobility is only made when moving between different management structures (e.g., from one MME to another, or to or from a HeNB) or between RATs. Towards LTE-Advanced, 3GPP has also introduced further handover optimizations that facilitate, for example, soft (zero interruption) handovers.

This chapter is organized as follows. Section 13.1 describes the relationship between the different UE states in 3GPP, and describes transitions between LTE (EUTRAN), UTRAN and GSM. Section 13.2, on the other hand, describes mobility drivers in LTE, identifying the different triggers that would initiate intra- or inter-frequency handovers in LTE, in addition to triggers for inter-RAT handovers. Mobility management for LTE UE is explained in Section 13.3, describing mobility management for both the IDLE and the CONNECTED states. Meanwhile, considerations for Inter-RAT mobility, including procedures for cell reselection and handover, are detailed in Section 13.4. Femtocell or HeNB mobility is reviewed in Section 13.5. Finally, Section 13.6 describes X2 and S1 signaling required for ...