Title: 		Target Id for Handover to UMTS 

Source: 		Ericsson, 

Agenda Item: 	5

Purpose: 		Discussion

In order to facilitate a handover to UTRAN the source BSS must among other things identify to the target RNC the identity of the target cell for the handover for resource allocation in UTRAN. After having evaluated the measurement reports from the UE in GSM mode and based on evaluation criteria,  the BSS proposes an handover to a UTRAN by sending a unique identification of the target UTRAN cell in BSSMAP Handover Required Message to the core network.

Two alternative solution exists, either the MSC or BSS may generate the target RNC id for routing purpose. These options have been shortly discussed in SMG2.

Discussion - Where should Target RNC id for routing be generated ?

One alternative is to allocate CGI as target address within BSS, which BSS sends to the MSC. The CGI is used by the network to route to the target MSC. The target RNC id calculation is then made in the target UMTS MSC.
The other alternative is to allocate target RNC id in BSS to route to the target RNC via the network.

There are several reasons to allocate target CGI in BSS instead of allocate target RNC id in BSS.

One important reason is that GSM is structured in that way that the BSS’s has no knowledge about other BSS identities. If RNC identities are allocated in BSS it would unnecessary change the GSM architecture.

With the solution to use the CGI for routing purposes to the target network a “soft” connection between GSM and UMTS is achieved. If a reconfiguration of RNC’s is made in the UMTS network only the translation table in the UMTS MSC has to be updated, if the CGI is kept unchanged. I.e. the configuration of the GSM network and UMTS network is kept more independent.

Today the current MSC implementations already supports routing analysis based on CGI for target BSS calculation at inter BSC handover.  Adding handling of RNC id in BSS would imply handling of new data in BSS.

Discussion – Transportation of  Target Cell Info in Handover signalling
For release 99 of Handover to UMTS the minimum requirements on what to send in Target Cell Info to UTRAN is one single UTRAN target cell id.

The container solution to transport transparent information between GSM and UMTS should be applied for UTRAN and BSS specific information. Currently two containers are discussed the Source RNC to Target RNC container and the existing GSM container to be generated by BSS. The concept could be such that the Source to Target RNC shall be used at Intersystem handover and Old BSS to New BSS at inter BSC handover . The Source RNC to Target RNC container is sent in BSSMAP Handover Required message on the A-interface and in Relocation Request on the Iu interface. 
The Target Cell Id to be sent transparent is generated in the format defined in TS 25.413.

For routing purposes in UMTS core network the CGI needs to be mapped by the BSS into the Cell Identifier list in the BSSMAP Handover Required message.
The Cell Identifier Discriminator encoded with 1000 is used to indicate that it is a cell to be used at GSM to UMTS handover. The structure of the cell is defined in TS 25.401 ( chapter Service Area Identifier). 
For routing purposes from GSM to UMTS only one cell is allocted.