5G Terminal RACS (Radio Capability Signaling Optimization)

I. RACS Background: With the expansion of terminal (UE) radio capabilities (due to new features, frequency bands, and combinations in E-UTRA and NR, etc.), the number of bytes carrying capability information is increasing. RACS (Radio Capability Signaling Optimization) defines an efficient method for transmitting UE capability information through the radio interface and other network interfaces—RACS is not applicable to NB-IoT.

II.Working Principle: RACS assigns an identifier to a set of UE radio capabilities; this identifier is called the UE Radio Capability ID. This ID can be assigned by the manufacturer or by the PLMN (see 5.9.10 for specific regulations). The UE Radio Capability ID is an alternative signaling method for UE radio capability information, transmitted through the radio interface within NG-RAN, from NG-RAN to E-UTRAN, from AMF to NG-RAN, and between CN nodes;

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III.RACS Support: In 5G (NR) systems, PLMN-assigned UE Radio Capability ID configurations are reassigned to the UE through update commands or registration reception (as defined in TS 23.502[3]). The specific configuration of PLMN-assigned UE Radio Capability ID versions by UCMF is defined in section 5.9.10.

• The UCMF (UE Radio Capability Management Function) stores the mapping relationship of all UE radio capability IDs in the PLMN and is responsible for assigning a UE radio capability ID to each UE in that PLMN (see Section 6.2.21); the UCMF stores UE radio capability ID information and corresponding radio paging capabilities.
• Each UE radio capability ID stored in the UCMF can be associated with one or two UE radio capability formats specified in TS 36.331 [51] and TS 38.331 [28].

---These two UE radio capability formats should be recognizable by the AMF and UCMF, and the AMF should only store the TS 38.331 [28] format.

IV.NG-RAN supporting RACS: When providing UE radio capabilities to the AMF, the NG-RAN can be configured in one of two operating modes. When the NG-RAN performs the UE radio capability query procedure (see TS 38.331 [28]) to obtain radio capabilities from the UE, the NG-RAN performs the following operations:

• Operating Mode A: The NG-RAN provides the AMF with both formats (i.e., TS 38.331 [28] format and TS 36.331 [51] format); the NG-RAN uses local transcoding to extract E-UTRAN UE paging radio capabilities and NR UE paging radio capabilities from the other format received from the UE.
• Operating Mode B: The NG-RAN only provides the AMF with the TS 38.331 [28] format.

----In a PLMN that only supports 5GS, Mode B should be configured.

V.4G+5G: If the PLMN supports RACS for both EPS and 5GS, then:

• If the RAN nodes in EPS and 5GS are configured in Mode B, the UCMF should be able to transcode between TS 36.331 [51] and TS 38.331 [28] formats, and the UCMF should be able to generate RAT-specific UE radio capabilities for paging information from the UE radio capabilities.​​
• If the NG-RAN is configured to operate in Mode A, E-UTRA should also be configured to operate in Mode A, and the UCMF does not need to transcode between TS 36.331 [51] and TS 38.331 [28] formats. The system performs transcoding between the 36.331[51] and TS 38.331[28] formats, and the AMF should provide UE radio capability information for paging.