Considerations for Inter RAT Mobility

The standards LTE and LTE-Advanced provided detailed descriptions for inter-RAT mobility, both to and from other 3GPP and non-3GPP technologies (respectively, GERAN/UTRAN and cdma2000/HRPD). A brief overview of cell reselection and handover procedures for 3GPP is provided below.

Cell Reselection

A UE can only search and measure for neighboring GERAN cells if their details are provided in the serving cell's NCL. For UTRAN, however, the serving cell can provide a list of carrier frequencies. A UE, in its continuing search for a better cell (frequency, technology) to camp on, selects the RAT with the highest priority. Priorities are set by registered PLMN (i.e., a PLMN that the UE has successfully registered on), and are valid only within that PRLMN.

If the UE is camped on another RAT, the UE need to acquire the carrier frequencies of the neighboring EUTRAN cells in order to be able to search and measure. There is no need to indicate cell-specific reselection parameters since such parameters are common to all neighboring cells on an E-UTRA frequency.

Handover

A basic principle in LTE and LTE-Advanced is that handovers to GERAN and UTRAN are minimized. Such handovers are controlled through the source access system, which decides whether or not to initiate handover and, when a handover is initiated, provides sufficient information to the target system. They are described as being “backward handovers”, meaning that the target system must acknowledge that sufficient resources are available and ready for the incoming UE prior to handover execution. To facilitate these backward handovers, interfaces are defined between the corresponding MME/S-GW and the 2G/3G SGSN. Moreover, it is responsibility of the target access system to provide specific guidance us on how to make the radio access there. The relevant information are sent by the target system are transparently relayed by the source system. At the same time, the handover should not require any UE to core (CN) communication to redirect flows to the target system.