Depicted in Figure 9.1.1-1 is an access specific architecture providing support for optimised E-UTRAN-HRPD handovers. in the non-roaming case. Depicted in Figure 9.1.1-2 is an access specific architecture providing support for optimised E-UTRAN-HRPD handovers in the roaming case with Home routed traffic.

The S101 reference point, and E-UTRAN and HRPD access is used for transparent transfer of pre-registration and handover signalling between the UE and the target access system The purpose of the procedures is to minimise the total service interruption time experienced at the UE, by allowing the UE to attach and perform service activation (in the case of E-UTRAN) or to perform a session configuration or traffic allocation request (in the case of HRPD) in the target access system before leaving the source access system. In case where the UE is connected to the E-UTRAN and conditions are such that a handover to HRPD may be required, the source system provides the UE with sufficient information to perform pre-registration with the target HRPD access and core network, over the S101 tunnelling interface. If conditions subsequently warrant that a handover should occur, the handover signalling will also be performed over the S101 tunnelling interface. Once the UE is ready to connect to the target system, it switches to the HRPD access. Alternatively, the E-UTRAN may redirect the UE to HRPD using RRC Connection Release with Redirection Information set as specified in clause 5.3.8 of TS 36.331. If pre-registration has not been performed successfully, upon receiving the redirection message, the UE acquires the HRPD channel and performs the non-optimized handover according to clause 8.2.2 from step 3 onwards. If pre-registration is successful, upon receiving the redirection message, the UE follows the RRC Connection Release with Redirection procedure to reselect the HRPD cell according to TS 36.331 and then performs the idle-mode optimized handover procedure as specified in clause 9.4 from step 3 onwards. In case where the UE is connected to the HRPD and conditions are such that a handover to E-UTRAN may be required, the source system provides the UE with sufficient information to perform pre-registration with the target EPS. The pre-registration may be performed over the S101 tunnelling interface. If conditions subsequently warrant that a handover should occur, the handover signalling may also be performed over the S101 tunnelling interface. Once the UE is ready to connect to the target system, it switches to the E-UTRAN access.

In order to support handover from E-UTRAN to HRPD of limited service state UEs, the following additional clarifications apply:

• When a limited service state UE without a UICC or with an unauthenticated IMSI, initiates emergency attach procedures for handover or initial attach to HRPD access to setup emergency sessions, the UE provides its IMEI during the attach procedure. The IMSI, if available, is also provided to the HRPD access by the UE during the attach procedure. If IMSI is not available or IMSI is not authenticated, an IMEI based NAI is used on the S2a interface in this case. Also, authentication with HSS is either skipped or if performed and fails, the HRPD attach procedure is continued.
• In the above scenario, for UEs that supports both E-UTRAN and HRPD access and handover is supported in the network between these accesses, the HRPD access selects a statically configured PDN-GW for the UE. This statically configured PDN-GW is the same as the statically-configured PDN-GW selected for the UE in E-UTRAN access.
• For optimized handover between E-UTRAN and HRPD for Emergency attached UEs in E-UTRAN without an UICC or with an unauthenticated IMSI, IMEI shall be used as the S101 Session ID to identify the UE within the MME and HRPD access nodes. If the IMSI is unauthenticated, the IMSI is provided on the S101 tunnel to the HRPD access with an indication that it is unauthenticated.

The procedures for both optimized and non-optimized handovers specified in the other sections in this specification support, without modifications, handover of emergency sessions from E-UTRAN to HRPD access for normal mode UE's, i.e. UEs with IMSI that authenticated successfully. In this Release of the specification, handover of IMS Emergency Sessions from HRPD access to E-UTRAN access is not supported.