5.4.4.1 UE radio capability information storage in the AMF

This clause applies when no radio capability signalling optimisation is used between a UE and the network.

The UE Radio Capability information is defined in TS 38.300 [27] and contains information on RATs that the UE supports (e.g. power class, frequency bands, etc). Consequently, this information can be sufficiently large that it is undesirable to send it across the radio interface at every transition of UE CM state in the AMF from CM-IDLE to CM-CONNECTED. To avoid this radio overhead, the AMF shall store the UE Radio Capability information during CM-IDLE state for the UE and RM-REGISTERED state for the UE and the AMF shall if it is available, send its most up to date UE Radio Capability information to the RAN in the N2 REQUEST message, i.e. INITIAL CONTEXT SETUP REQUEST or UE RADIO CAPABILITY CHECK REQUEST.

NOTE 1: Due to issues with the handling of dynamic UMTS security parameters, the UTRA UE Radio Capability information is excluded from the information that is uploaded and stored in the AMF (see TS 38.300 [27]).

The AMF deletes the UE radio capability when the UE RM state in the AMF transitions to RM-DEREGISTERED. When the AMF receives Registration Request with the Registration type set to Initial Registration or when it receives the first Registration Request after E-UTRA/EPC Attach with Registration type set to Mobility Registration Update, the AMF deletes the UE radio capability.

The UE Radio Capability is maintained in the core network, even during AMF reselection.

NOTE 2: The UE Radio Capability is not transferred to EPC during the inter-system mobility.

If the UE's NG-RAN or E-UTRAN UE Radio Capability information changes while in CM-IDLE state, the UE shall perform the Registration procedure with the Registration type set to Mobility Registration Update and it also includes "UE Radio Capability Update". (For specific requirements for a UE operating in dual-registration mode see clause 5.17.2.1). When the AMF receives Mobility Registration Update Request with "UE Radio Capability Update" requested by the UE, it shall delete any UE Radio Capability information that it has stored for the UE. If the UE's NG-RAN UE Radio Capability information changes when the UE is in CM-IDLE with Suspend, NAS shall trigger AS to establish a new RRC connection and not resume the existing one in order to perform the Registration procedure with the Registration type set to Mobility Registration Update including "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 [51].

If the trigger to change the UE's NG-RAN or E-UTRAN UE Radio Capability information happens when the UE is in CM-CONNECTED state, the UE shall first enter CM-IDLE state and then perform the Registration procedure with the Registration type set to Mobility Registration Update and it also includes "UE Radio Capability Update".

NOTE 3: It is up to UE implementation to immediately release the NAS signalling connection locally or wait for certain services to complete for e.g. emergency services, before entering CM-IDLE state.

The RAN stores the UE Radio Capability information, received in the N2 message or obtained from the UE, for the duration of the UE staying in RRC_CONNECTED or RRC_INACTIVE state. Before any 5G SRVCC handover attempt from NG-RAN to UTRAN, the RAN retrieves the UE's UTRA UE Radio Capabilities from the UE. (For specific requirements for a UE operating in dual-registration mode see clause 5.17.2.1).

If the AMF sends N2 REQUEST (i.e. INITIAL CONTEXT SETUP REQUEST or UE RADIO CAPABILITY CHECK REQUEST) message to NG-RAN without UE Radio Capability information in that message and there is no UE Radio Capability information available in RAN, this triggers the RAN to request the UE Radio Capability from the UE and to upload it to the AMF in the N2 UE RADIO CAPABILITY INFO INDICATION message.

If a UE supports both NB-IoT and other RATs 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 not camping on NB-IoT, the UE provides UE radio capabilities for the RAT but not NB-IoT UE radio capabilities to the network.

In order to handle the distinct UE radio capabilities, the AMF 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 AMF sends, if available, the NB-IoT RAT specific UE Radio Capability information to the E-UTRAN.

When the UE is not camping on NB-IoT, the AMF sends, if available, UE radio capabilities for the RAT but not NB-IoT radio capabilities.