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

Overview

Whether idle (RRC_IDLE) or connected (RRC_CONNECTED), a UE's (readiness to) connectivity must be maintained while powered on. Mobility management while IDLE is user-controlled, meaning that it is the UE that seeks the best PLMN and cell to camp on. UE initial selection and ongoing reselection procedures (described in Chapter 11) enable the UE to constantly identify the most appropriate (whether suitable or acceptable) cell or technology to camp on. It is the network, however, that takes over this decision when the UE is connected. Once connected, management becomes network controlled but assisted by the UE's measurement reports in selecting best cell, frequency, or RAT that best satisfies the user's requirements and capabilities. Figure 13.1. shows the states for inter-RAT mobility across 3GPP technologies.

Figure 13.1 States for inter-RAT mobility across 3GPP technologies. Reproduced by permission of © 2010 3GPP. Further use is strictly prohibited.

As will be described below, 3GPP specifies drivers and limitation for various handoff types that dictate the best action in both states. It also describes the relevant signaling for RRC, X2 (for RAT handovers), and S1 (for core-involved handovers). At times, the network may require additional measurements from the UE for improved decisions. Certain optimizations have also been introduced in Release 9 that enable UE initiated ...