Section: 5.11.3a UE Radio Capability Signalling optimization
Content: With the increase of the size of UE radio capabilities driven e.g. by additional frequency bands and combinations thereof for E-UTRA and NR, an efficient approach to signal UE Radio Access Capability information over the radio interface and other network interfaces is defined with RACS. In this Release of the specification, RACS does not apply to NB-IOT (terrestrial or satellite). RACS works by assigning an identifier to represent a set of UE radio capabilities. This identifier is called UE Radio Capability ID. A UE Radio Capability ID can be either UE manufacturer-assigned or PLMN-assigned, as specified in clause 5.2.7. The UE Radio Capability ID is an alternative to the signalling of the UE Radio Capability information over the radio interface, within E-UTRAN, from E-UTRAN to NG-RAN, from MME to E-UTRAN and between CN nodes supporting RACS. The UCMF (UE radio Capability Management Function) stores all UE Radio Capability ID mappings in a PLMN and is responsible for assigning every PLMN-assigned UE Radio Capability ID in this PLMN, see clause 4.4.13. The UCMF shall be configured with a Version ID for PLMN assigned UE Radio Capability IDs, defined in clause 4.4.13. The UCMF stores the UE Radio Capability IDs alongside the UE Radio Capability information and the UE Radio Capability for Paging they map to. Each UE Radio Capability ID stored in the UCMF can be associated to one or both UE radio capabilities formats specified in TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [89] and TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37]. The two UE radio capabilities formats shall be identifiable by the MME and UCMF and the MME shall store the TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37] format only. An E-UTRAN which supports RACS can be configured to operate with one of two modes of operation when providing the UE radio capabilities to the MME when the E-UTRAN executes a UE Radio Capability Enquiry procedure (see TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37]) to retrieve UE radio capabilities from the UE. - Mode of operation A): The E-UTRAN provides to the MME both UE Radio Capability formats (i.e. the TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37] format and TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [89] format). The E-UTRAN derives one of the formats using local transcoding of the other format it receives from the UE and extracts the E-UTRAN UE Radio Capability for Paging and NR UE Radio Capability for Paging from the UE Radio capabilities. - Mode of operation B): The E-UTRAN provides to the MME the TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37] format only. In a PLMN supporting RACS only in EPS, Mode of Operation B shall be configured. If the PLMN supports RACS in both EPS and 5GS: - If RAN nodes in the EPS and 5GS are configured in Mode of operation B, then the UCMF shall be capable to transcode between TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [89] and TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37] formats. The UCMF shall be able to generate the RAT-specific UE Radio Capability for Paging information from the UE Radio capabilities. - If E-UTRAN is configured to operate according to Mode A, then also the NG-RAN shall be configured to operate according to mode A and the UMCF is not required to transcode between TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [89] and TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37] formats. The MME shall provide the UCMF with the UE Radio Capability for Paging information. When the E-UTRAN updates the MME with new UE radio capabilities information, the MME provides the information obtained from the E-UTRAN to the UCMF even if the MME already stores a UE Radio Capability ID for the UE. The UCMF then returns a value of UE Radio Capability ID. If the value is different from the one stored in the MME, the MME updates the UE Radio Capability ID it stores and provides this new value to the E-UTRAN (if applicable) and to the UE. PLMN-assigned UE Radio Capability ID is assigned to the UE using the GUTI Reallocation procedure, Attach Accept or TAU Accept as defined in present specification. In order to be able to interpret the UE Radio Capability ID a network entity or node may store a local copy of the mapping between the UE Radio Capability ID and its corresponding UE Radio Capability information i.e. a dictionary entry. When no mapping is available between a UE Radio Capability ID and the corresponding UE Radio Capability information in a network entity or node, this network entity or node shall be able to retrieve it and store it. - An MME which supports RACS shall store such UE Radio Capability ID mapping at least for all the UEs that it serves that have a UE Radio Capability ID assigned. - The E-UTRAN performs local caching of the UE Radio Capability information for the UE Radio Capability IDs for the UEs it is serving, and potentially for other UE Radio Capability IDs according to suitable local policies. - When the E-UTRAN needs to retrieve the mapping of a UE Radio Capability ID to the corresponding UE Radio Capability information, it queries the MME using S1 signalling defined in TS 36.413[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) ]  [36]. - When the MME needs to get the UE Radio Capability Information and the UE Radio Capability for Paging associated to a UE Radio Capability ID it provides the UE Radio capability ID to the UCMF with an indication that it is requesting the TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37] format, and the UCMF returns a mapping of the UE Radio Capability ID to the corresponding UE Radio Capability information in TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37] format to the MME along with the E-UTRAN UE Radio Capability for Paging. - When the MME needs to obtain a PLMN assigned UE Radio Capability ID for a UE from the UCMF, it provides the UE Capability information it has for the current radio configuration of the UE and the IMEI/TAC for the UE. The MME shall provide to the UCMF the UE Radio Capability information (and at least in Mode A, the UE Radio Capability for Paging) obtained from the E-UTRAN in one or both the TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [89] and TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37] formats depending on how the RAN is configured. The UCMF stores the association of IMEI/TAC with this UE Radio Capability ID and the UE Radio Capability information and the UE Radio Capability for Paging in all the formats it receives. The UE Radio Capability information formats the MME provides shall be identifiable at the UCMF. - UEs, MMEs and RAN nodes which support RACS learn the current value of the Version ID when a new PLMN assigned UE Radio Capability ID is received from the UCMF and the Version ID it contains is different from the ones in their PLMN assigned UE Radio Capability ID cache. For a PLMN, PLMN assigned UE Radio Capability IDs related to old values (i.e. not current value) of the Version ID can be removed from cache but, if so, prior to removing any cached PLMN-assigned UE radio Capability IDs with the current value of the Version ID. The MME, RAN and UE may still continue to use the stored PLMN assigned UE Radio Capability IDs with old values of the Version ID, until these are purged from cache. If an out of date PLMN assigned UE Radio Capability ID is removed from an MME cache, the MME shall proceed to assign a new PLMN assigned UE Radio Capability ID to all the UEs for which the UE context includes the removed PLMN-assigned UE Radio Capability ID, using the GUTI Reallocation procedure, or when these UEs perform a Tracking Area Update. If the MME attempts to resolve a PLMN assigned UE Radio capability ID with an old Version ID, the UCMF shall return an error code indicating that this Version ID is no longer current. - If at any time the MME has neither a valid UE Radio Capability ID nor any stored UE radio capabilities for the UE, the MME may trigger the RAN to provide the UE Radio Capability information and subsequently request the UCMF to allocate a UE Radio Capability ID. - The RAN, in order to support MOCN network sharing scenarios, shall be capable to cache PLMN assigned UE Radio Capability IDs per PLMN ID. A network may utilise the PLMN-assigned UE Radio Capability ID, without involving the UE, e.g. for use with legacy UEs. Mutual detection of the support of the RACS feature happens between directly connected E-UTRAN nodes and between E-UTRAN and MME using protocol means as defined in TS 36.413[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) ]  [36] and TS 36.423[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 Application Protocol (X2AP) ]  [76]. To allow for a mix of RACS-supporting and non-RACS-supporting RAN nodes over the X2 interfaces, the UE Radio Capability ID should be included in the Path Switch signalling during X2 based handover and Handover Request during S1 based handover between MME and E-UTRAN. In addition, RACS-supporting RAN nodes can be discovered across inter-CN node boundaries during S1 handover using the "RACS Indication" in "Target eNB to Source eNB Transparent Container" within the HANDOVER REQUEST ACKNOWLEDGE message to indicate that that target eNodeB is able to acquire the UE radio capabilities through reception of the UE Radio Capability ID in future mobility actions, as defined in TS 36.413[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) ]  [36]. The support of RACS by peer MMEs or AMFs is based on configuration in a PLMN or across PLMNs. A UE that supports WB-EUTRA and/or NR indicates its support for RACS to MME using UE Core Network Capability as defined in clause 5.11.3. A UE that supports RACS and is already assigned with an applicable UE Radio Capability ID in the PLMN, shall signal the UE Radio Capability ID in Attach procedure, as defined in clause 5.3.2, and Tracking Area Update procedure, as defined in clause 5.3.3 and based on triggers defined in TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [46]. If both PLMN-assigned and UE manufacturer-assigned UE Radio Capability IDs are available in the UE and applicable in the PLMN, the UE shall signal the PLMN-assigned UE Radio Capability ID. The UE shall delete the PLMN-assigned UE Radio Capability ID(s) for the related PLMN upon receiving an indication from this PLMN. When a PLMN decides to request a particular type of UE to use UE manufacturer-assigned UE Radio Capability ID(s): - The UCMF sends either a Nucmf_UECapabilityManagement_Notify or URCMP Event Notification Request message defined in TS 29.674[ Interface between the UE radio Capability Management Function (UCMF) and the Mobility Management Entity (MME); Stage 3 ]  [91] to the MME including either a list of UE Radio Capability IDs (if the UE was previously using any PLMN assigned IDs) or the IMEI/TAC values corresponding to UE types that are requested to use UE manufacturer-assigned UE Radio Capability ID. These values are stored in a "UE Manufacturer Assigned operation requested list" in the MME. - The MME uses the GUTI reallocation command message, Attach Accept message or Tracking Area Update Accept message to request the UE to delete all the PLMN-assigned UE Radio Capability ID(s) for this PLMN if the UE is, respectively, registering or is registered with PLMN assigned UE Radio Capability ID or IMEI/TAC values matching one value in the UE manufacturer-assigned operation requested list. NOTE 1: It is expected that in a given PLMN the UCMF and MMEs will be configured to either use a UE manufacturer-assigned operation requested list based on a list of PLMN assigned UE Radio Capability IDs or a list of TACs, but not both. NOTE 2: The strategy for triggering of the deletion of PLMN-assigned UE Radio Capability ID(s) in the UE by the MME is implementation-specific (e.g. can be used only towards UEs in ECM-Connected state). - a UE that receives indication to delete the all the PLMN-assigned UE Radio Capability IDs in the Attach Accept message, Tracking Area Update Accept message or GUTI reallocation command message, deletes any PLMN-assigned UE Radio Capability IDs for this PLMN. The UE proceeds to register with a UE manufacturer-assigned UE Radio Capability ID that is applicable to the current UE Radio configuration. - When the "UE Manufacturer Assigned operation requested list" contains PLMN assigned UE Radio Capability IDs, the UCMF shall avoid re-assigning PLMN assigned UE Radio Capability IDs that were added to the "UE Manufacturer Assigned operation requested list" in the MMEs to any UE. - The MME stores a PLMN assigned ID in the UE manufacturer-assigned operation requested list for a time duration that is implementation specific, but TACs are stored until the UCMF require to remove certain TACs from the list (i.e. the list of TACs which are requested to use UE manufacturer-assigned UE Radio Capability IDs in the MME and UCMF is synchronised at all times). - The UCMF can request at any time the MME to remove PLMN assigned ID(s) or TAC(s) values form the UE manufacturer-assigned operation requested list. NOTE 3: The MME can decide to remove a UE Radio Capability ID related to selected PLMN from the "UE Manufacturer Assigned operation requested list" list e.g. because no UE with that UE Radio Capability ID has connected to the network for long time. If later a UE with such UE Radio Capability ID connects to the network, the MME contacts the UCMF to resolve the UE Radio Capability ID, and at this point the UCMF can trigger again the deletion of the UE Radio Capability ID by including this in the UE manufacturer-assigned operation requested list of the MME. The serving MME stores the UE Radio Capability ID for a UE in the UE context and provides this UE Radio Capability ID to E-UTRAN as part of the UE context information using S1 signalling. During inter PLMN mobility, the new MME shall delete the UE Radio Capability ID received from the old MME, unless the operator policy indicates that all UE Radio Capability IDs used in the old PLMN are also valid in the new PLMN. NOTE 4: If MME decides to allocate TAIs of multiple PLMN IDs as part of Tracking Area to the UE then MME provides the UE Radio Capability ID of the new selected PLMN to the eNodeB during UE mobility, whether the UE Radio Capability ID is taken from stored UE context previously assigned by the same new selected PLMN or generated freshly each time a new PLMN is selected is up to MME implementation. The UE stores the PLMN-assigned UE Radio Capability ID in non-volatile memory when in EMM-DEREGISTERED state and can use it again when it registers in the same PLMN. NOTE 5: It is assumed that UE does not need to store the access stratum information (i.e. UE-EUTRA-Capability and UE-NR-Capability specified in TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37] and TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [89], respectively) that was indicated by the UE to the network when the PLMN-assigned UE Radio Capability ID was assigned by the network. However, it is assumed that the UE does store the related UE configuration (e.g. whether or not GERAN or UTRAN or MBMS is enabled/disabled). At any given time at most one UE Radio Capability ID is stored in the UE context in CN and RAN. The number of PLMN-specific UE Radio Capability IDs that the UE stores in non-volatile memory is left up to UE implementation. However, to minimise the load (e.g. from radio signalling) on the Uu interface and to provide smoother inter-PLMN mobility (e.g. at land borders) the UE shall be able to store at least the latest 16 PLMN-assigned UE Radio Capability IDs (along with the PLMN that assigned them). This number is independent of the UE manufacturer-assigned UE Radio Capability ID(s) the UE may store. It shall be possible for a UE to change, e.g. upon change in its usage settings, the set of UE radio capabilities in time and signal the associated UE Radio Capability ID, if available. The UE stores the mapping between the UE Radio Capability ID and the corresponding UE Radio Capability information for every UE Radio Capability ID it stores. If the UE's Radio Capability information changes and there is no associated UE Radio Capability ID for the updated UE Radio Capability information, the UE shall perform the Tracking Area Update procedure for "UE radio capability update" as defined in clause 5.11.2. 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 either include the UE Radio Capability ID applicable to the new area, or, shall indicate that the Tracking Area Update is for a "UE radio capability update". The E-UTRAN may apply RRC filtering of UE radio capabilities when it retrieves the UE Radio Capability information from the UE as defined in TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37]. NOTE 6: In a RACS supporting PLMN, the filter of UE radio capabilities configured in E-UTRAN is preferably as wide in scope as possible (e.g PLMN-wide). In this case, it corresponds e.g. to the super-set of bands, band-combinations and RATs the PLMN deploys and not only to the specific E-UTRAN node or region. NOTE 7: If the filter, included in the UE Radio Capability information, of UE radio capabilities configured in two E-UTRAN nodes is different, during handover between these two nodes, it is possible that the target E-UTRAN node might need to enquire the UE for its UE Radio Capability information again and trigger re-allocation of a PLMN-assigned UE Radio Capability ID leading to extra signalling. Additionally, a narrow filter might reduce the list of candidate target nodes. If a UE supports both NB-IoT and possibly other RATs the UE handles the RACS procedures as follows: - Since there is no support for RACS in NB-IoT, if the UE supports RACS in non-NB-IoT RATs (i.e. for WB-EUTRA and/or NR): - NB-IoT specific UE Radio Capability information is handled in UE, RAN and MME according to clause 5.11.2. - when the UE is not camping on NB-IoT, the UE provides UE radio capabilities for other RATs but not NB-IoT UE radio capabilities, according to TS 36.331[ Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification ]  [37]. As a result the UE Radio Capability ID that is assigned by the network corresponds only to the UE radio capabilities of the non-NB-IoT RATs. The UE uses the UE Radio Capability IDs assigned only in Attach and TAU procedures performed over non-NB-IoT RATs. Support for RACS in 5GS is defined in TS 23.501[ System architecture for the 5G System (5GS) ]  [83] and TS 23.502[ Procedures for the 5G System (5GS) ]  [84].