Optimisations of UE radio capability signalling
Background
With the increase in the size of UE radio capabilities driven by additional supported bands, the size of the UE Radio Capabilities will significantly grow from Rel-15 onwards, therefore an efficient approach to signal UE Radio Capability information was needed.
UE Radio Capability ID
The system optimisations for the 5GS (documented in TS
23.501) and for the EPS (documented in TS 23.401), that apply to both
NR and E-UTRA, but not NB-IoT, consisting of using UE Radio Capability IDs as
an alternative to signaling the UE Radio Capabilities container in system
procedures:
- between the UE and the CN (over
Uu)
- between the CN and the RAN
(impacting N2/S1 interfaces)
- within the RAN in e.g. the
handover procedures (impacting Xn/X2/S1/N2 interfaces)
- within the CN.
The UE
Radio Capability ID format is defined in TS 23.003. The UE Radio Capability
ID is signaled by the UE in NAS as specified in TS 24.501 for the 5GS and
as specified in TS 24.301 for the EPS. Two possible options for the
assignment of UE Radio Capability ID exist:
- Manufacturer-assigned: The UE
Radio Capability ID may be assigned by the UE manufacturer in which case it
includes a UE manufacturer identification (i.e. a Vendor ID). In this case, the
UE Radio Capability ID uniquely identifies a set of UE radio capabilities for a
UE by this manufacturer in any network.
- Network-assigned: If a
manufacturer-assigned UE Radio Capability ID is not used by the UE or the
serving network, or it is not recognised by the serving network’s UE Capability
Management Function (UCMF), the UCMF may allocate UE Radio Capability IDs for
the UE corresponding to each different set of UE radio capabilities which the
network may receive from the UE at different times. In this case, the UE Radio
Capability IDs which the UE receives are applicable to the serving network and
uniquely identify the corresponding sets of UE radio capabilities in this
network. The network-assigned UE Radio Capability ID includes a Version ID in its
format. The value of the Version ID is the one configured in the UCMF, at the
time when the UE Radio Capability ID value is assigned. The Version ID value
makes it possible to detect whether a UE Radio Capability ID is current or
outdated.
UE Radio
Capability IDs and the mapping to the corresponding UE radio capabilities are
stored in a new function called the UE Capability Management Function (UCMF) in
the CN. The UCMF is used for:
- storage of dictionary entries
corresponding to either Network-assigned or Manufacturer-assigned UE Radio
Capability IDs.
- assigning Network-assigned UE
Radio Capability ID values.
- provisioning of
Manufacturer-assigned UE Radio Capability ID entries in the UCMF performed from
an AF that interacts with the UCMF either directly or via the NEF/SCEF (or via
Network Management).
 |
| UCMF architecture and related reference points in 5GS (left) and EPS (right) |
System
procedures are defined for 5GS in TS 23.502 and for EPS in TS 23.401.
UE Capability Segmentation
The RAN
work item [10] calls for specification of a segmentation mechanism, so that in
cases of excessively large UE capability signalling (e.g. capability
information messages exceeding the maximum size of a PDCP SDU), the capability
can be segmented into multiple RRC messages.
Segmentation applies to both NR and E-UTRA.
Segmentation
is performed in the RRC protocol layer, with a separate RRC PDU for each
segment. The UE encodes the capability
information message, then divides the encoded message into segments such that
the size of each segment does not exceed the maximum size of a PDCP SDU (8188
octets in E-UTRA, 9000 octets in NR); the RAN node (eNB or gNB) receives the
segments and reassembles them to reconstruct the original capability
information message. Segmentation is
applied only in case the size of the encoded capability information message
exceeds the maximum size of a PDCP SDU.
The signalling formats support up to 16 segments for a single capability
information message.