Section: 5.11.2 UE Radio Capability Handling
Content: 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 for a UE supporting a small number of frequency bands; or multiple kilo bytes for a UE supporting many frequency bands and a large multiplicity of combinations of these frequency bands) 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". NOTE 1: The UTRAN Radio Capabilities are excluded from the information stored in the MME owing to issues with the handling of dynamic UMTS security parameters. If a UE supports both NB-IoT and WB-E-UTRAN, the UE handles the UE Radio capability information as follows: - When the UE is camping on NB-IoT the UE provides only NB-IoT UE radio capabilities to the network. - When the UE is camping on WB-E-UTRAN, the UE provides UE radio capabilities including WB-E-UTRAN UE radio capabilities but not NB-IoT UE radio capabilities to the network. In order to handle the distinct UE radio capabilities, the MME stores a separate NB-IoT specific UE Radio Capability information when the UE provides the UE Radio Capability information while camping on NB-IoT. When the UE is camping on NB-IoT, the MME sends, if available, the NB-IoT specific UE Radio Capability information to the E-UTRAN. When the UE is camping on WB-E-UTRAN, the MME sends, if available, UE radio capabilities including WB-E-UTRAN UE radio capabilities but not NB-IoT radio capabilities. For a UE that supports NR as a Secondary RAT, the UE's NR radio capabilities are contained within the UE Radio Capability IE. 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 or UE RADIO CAPABILITY MATCH 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 to upload it to the MME in the S1 interface UE CAPABILITY INFO INDICATION message. The size of the UE Radio Capability information may be greater than can be carried in single RRC message but less than the maximum size of messages on the S1 interface. In this case, to obtain the information that it needs the RAN should send multiple requests to the UE for different sub-sets of UE Radio Capability information (e.g. one request per RAT). Then the RAN shall combine these subsets (excluding UTRAN and NB-IoT capabilities) into a single UE Radio Capability IE and upload it to the MME in the S1 interface UE CAPABILITY INFO INDICATION message. The MME stores the UE Radio Capability information, and includes it in further INITIAL CONTEXT SETUP REQUEST or UE RADIO CAPABILITY MATCH REQUEST messages in other cases than Attach procedure, Tracking Area Update procedure for the "first TAU following GERAN/UTRAN Attach" and "UE radio capability update" procedure. 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). When the CIoT EPS Optimisations do not apply, if the MME has not stored the UE Radio Capability information, in order to obtain UE radio capability for paging information, the MME can trigger the retrieval of the UE Radio Capability information by indicating UE Radio Capability request in DOWNLINK NAS TRANSPORT message during Attach or TAU procedure. For the CIoT EPS Optimisations, during the Attach procedure or the Tracking Area Update procedure e.g. for the "first TAU following GERAN/UTRAN Attach", or mobility between a cell that does not broadcast SystemInformationBlockType31(-NB) and an E-UTRA cell that broadcasts SystemInformationBlockType31(-NB)), if the MME does not send an S1 interface INITIAL CONTEXT SETUP REQUEST to the E-UTRAN, the MME should obtain the UE Radio Capability information by sending either the DOWNLINK NAS TRANSPORT message indicating UE Radio Capability request or the CONNECTION ESTABLISHMENT INDICATION message without UE Radio Capability information included to the E-UTRAN. 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, as specified in TS 36.300[ Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 ]  [5]. In subsequent ECM connections, if the MME does not send an S1 interface INITIAL CONTEXT SETUP REQUEST to the E-UTRAN, the MME sends the UE Radio Capability to the E-UTRAN in the CONNECTION ESTABLISHMENT INDICATION message or DOWNLINK NAS TRANSPORT message. 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 and E-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". At handover, 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. However, there may be situations where the size of the UE Radio Capability may be too large for the information on all of the UE's RATs to be carried in a single message on one or more of the network interfaces involved in the handover. Hence, the source RAN shall ensure that the size of the UE Radio Capability information does not cause the size of the "source to target transparent container" to exceed the limits that can be handled by interfaces involved in the handover (e.g. Iu interface (TS 25.413[ UTRAN Iu interface Radio Access Network Application Part (RANAP) signalling ]  [22]) and, following SRVCC, E interface (TS 29.002[ Mobile Application Part (MAP) specification ]  [86])). This may result in some radio capability information being omitted from the "source to target transparent container" at inter-RAT handover. In the case that a source RAN node omits some radio capability information from the "source to target transparent container" at handover, the source RAN node shall ensure that any future target RAN node can detect that that radio capability information has been omitted. Owing to issues with dynamic UTRAN security parameters, special rules apply to the handling of the UTRAN radio capability information at inter-RAT handover (see e.g. the HandoverPreparationInformation message description in TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37] and the usage of the PS Handover Complete Ack message in TS 43.129[ None ]  [8] and TS 48.018[ None ]  [42]) To allow for the addition of future radio technologies, frequency bands, and other enhancements, the MME shall store the UE Radio Capability Information as defined in TS 23.008[ Organization of subscriber data ]  [28]. 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. When the UE is in ECM-IDLE with AS information stored (as defined in clause 4.11 for User Plane CIOT EPS optimisation), NAS shall trigger AS to establish a new RRC connection and not resume the existing one in order to send Tracking Area Update indicating "UE radio capability update". As a result of this, the access stratum in the UE will discard the AS information and establish a new RRC connection as defined in TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37]. The UE shall perform a Tracking Area Update procedure at every change between a cell that does not broadcast SystemInformationBlockType31(-NB) and an E-UTRA cell that broadcasts SystemInformationBlockType31(-NB). This Tracking Area Update shall indicate that the Tracking Area Update is for a "UE radio capability update". The MME may also request for Voice Support Match Information. If requested, the eNodeB then derives and provides an indication to the MME whether the UE radio capabilities are compatible with the network configuration (e.g. whether the UE supports the frequency bands that the network may rely upon for providing "full" PS voice coverage or whether the UE supports the SRVCC configuration of the network e.g. E-UTRAN to GERAN) as defined in clause 5.3.14. The signalling of the UE Radio Access Capabilities described in this clause can be optimised by means of the RACS feature defined in clause 5.11.3a.