RAT/Frequency Selection Priority Index - RFSP Index

Radio resource management functions are concerned with the allocation and maintenance of radio communication paths, and are performed by the radio access network. The RRM strategy in E-UTRAN may be based on user specific information.

To support radio resource management in E-UTRAN the MME provides the parameter 'Index to RAT/Frequency Selection Priority' (RFSP Index) to an eNodeB across S1. The RFSP Index is mapped by the eNodeB to locally defined configuration in order to apply specific RRM strategies. The RFSP Index is UE specific and applies to all the Radio Bearers.

Examples of how this parameter may be used by the E-UTRAN:
- to derive UE specific cell reselection priorities to control idle mode camping.
- to decide on redirecting active mode UEs to different frequency layers or RATs.
The MME receives the subscribed RFSP Index from the HSS (e.g., during the Attach procedure). For non-roaming subscribers the MME chooses the RFSP Index in use according to one of the following procedures, depending on operator's configuration:
- the RFSP Index in use is identical to the subscribed RFSP Index, or
- the MME chooses the RFSP Index in use based on the subscribed RFSP Index, the locally configured operator's policies and the UE related context information available at the MME, including UE's usage setting and voice domain preference for E-UTRAN, if received during Attach and Tracking Area Update procedures.

One example of how the MME can use the "UE voice capabilities and settings" is to select an RFSP value that enforces idle mode camping on 2G/3G for a UE acting in a "Voice centric" way and provisioned with "CS Voice preferred, IMS Voice as secondary", in order to minimize the occurrancy of RAT changes. Another example is the selection of an RFSP value that prevents idle mode camping on 2G for a UE provisioned with "IMS PS voice preferred, CS Voice as secondary" if other RATs supporting IMS Voice are available, as the UE would in such case always select the CS domain for its voice calls.

For roaming subscribers the MME may alternatively choose the RFSP Index in use based on the visited network policy, but can take input from the HPLMN into account (e.g., an RFSP Index value pre-configured per HPLMN, or a single RFSP Index value to be used for all roamers independent of the HPLMN).

The MME forwards the RFSP Index in use to the eNodeB across S1. The RFSP Index in use is also forwarded from source eNodeB to target eNodeB when X2 is used for intra-E UTRAN handover.

The MME stores the subscribed RFSP Index value received from the HSS and the RFSP Index value in use. During the Tracking Area Update procedure, the MME may update the RFSP Index value in use (e.g. the MME may need to update the RFSP Index value in use if the UE related context information in the MME has changed). When the RFSP Index value in use is changed, the MME immediately provides the updated RFSP Index value in use to eNodeB by modifying an existing UE context or by establishing an new UE context in the eNodeB. During inter-MME mobility procedures, the source MME forwards both RFSP Index values to the target MME. The target MME may replace the received RFSP Index value in use with a new RFSP Index value in use that is based on the operator's policies and the UE related context information available at the target MME.

The S1 messages that transfer the RFSP Index to the eNodeB are specified in TS 36.413 [36].