Saturday, April 16, 2011

UE Radio Capability Handling

The UE Radio Capability information contains information on RATs that the UE supports (e.g. power class, frequency bands, etc). Consequently, this information can be sufficiently large (e.g. >50 octets) that it is undesirable to send it across the radio interface at every transition from ECM IDLE to ECM CONNECTED. To avoid this radio overhead, the MME stores the UE Capability information during ECM IDLE state and the MME shall, if it is available, send its most up to date UE Radio Capability information to the E UTRAN in the S1 interface INITIAL CONTEXT SETUP REQUEST message unless the UE is performing an Attach procedure or a Tracking Area Update procedure for the "first TAU following GERAN/UTRAN Attach" or for a "UE radio capability update".

If the UE is performing an Attach procedure or a Tracking Area Update procedure for the "first TAU following GERAN/UTRAN Attach" or for "UE radio capability update", the MME shall delete (or mark as deleted) any UE Radio Capability information that it has stored, and, if the MME sends an S1 interface INITIAL CONTEXT SETUP REQUEST message during that procedure, the MME shall not send any UE Radio Capability information to the E UTRAN in that message. This triggers the E UTRAN to request the UE Radio Capability from the UE and upload it to the MME in the S1 interface UE CAPABILITY INFO INDICATION message.

If the UE is performing a Service Request (or other) procedure and the MME does not have UE Radio Capability information available (or it is available, but marked as "deleted"), then the MME sends an S1 interface INITIAL CONTEXT SETUP REQUEST message to the E UTRAN without any UE Radio Capability information in it. This triggers the E UTRAN to request the UE Radio Capability from the UE and upload it to the MME in the S1 interface UE CAPABILITY INFO INDICATION message.

NOTE 2: This use of the INITIAL CONTEXT SETUP REQUEST message means that for a signalling only procedure such as a periodic Tracking Area Update, the UE Radio Capability would not be sent to the E UTRAN.

NOTE 3: If a "first TAU following GERAN/UTRAN Attach" Tracking Area Update is performed during ECM-CONNECTED mode, e.g. after an inter RAT handover, no INITIAL CONTEXT SETUP REQUEST is sent and the UE Radio Capability information in the MME will remain deleted until the next ECM-IDLE to ECM-CONNECTED transition (or later, e.g. if the next activity from the UE is another Tracking Area Update).

The UE Radio Capability is not provided directly from one CN node to another. It will be uploaded to the MME when the E-UTRAN requests the UE Radio Capability information from the UE. During handover via the MME (both intra RAT and inter RAT), the radio capability information for the source and target 3GPP RATs (with the possible exception of UTRAN) are transferred in the "source to target transparent container". Information on additional 3GPP RATs is optionally transferred in the "source to target transparent container". Transfer of the radio capability information related to the source and/or additional RATs is beneficial as it avoids the need for the target RAT to retrieve the information from the UE prior to a subsequent inter-RAT handover.

To allow for the addition of future radio technologies, frequency bands, and other enhancements, the MME stores the UE Radio Capability Information even if it is larger than specified in TS 36.331 [37], up to a maximum size of 510 octets.

NOTE 4: The 510 octet value comes from the information element encoding rules described in TS 24.007 [45] and the assumption that the information contained within this UE Radio Capability Information Element stored by the MME is the equivalent of information signalled in two information elements in the GERAN NAS signalling for the case of GERAN to E UTRAN PS handover.

The E UTRAN stores the UE Radio Capability information, received in the S1 interface INITIAL CONTEXT SETUP REQUEST message or obtained from the UE, for the duration of the RRC connection for that UE. Before any handover attempt from E UTRAN to UTRAN, the E UTRAN retrieves the UE's UTRAN Radio Capabilities from the UE.

If the UE's non-UTRAN UE Radio Capability information changes while in ECM-IDLE state (including cases of being in GERAN/UTRAN coverage), the UE shall perform a Tracking Area Update indicating "UE radio capability update" when it next returns to E UTRAN coverage.

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