NR RRC (Rel-18 baseline)

# 3GPP TS 38.331 — NR Radio Resource Control (RRC) ## What It Specifies The complete RRC protocol for NR: message syntax (ASN.1), signaling procedures, and state machine governing UE-gNB control-plane interaction over the Uu interface. Covers both SA and NSA (EN-DC, NE-DC, NGEN-DC) configurations. ## Key Procedures Defined - **RRC state transitions**: RRC_IDLE → RRC_INACTIVE → RRC_CONNECTED, including suspend/resume - **Connection establishment/reconfiguration**: `RRCSetup`, `RRCReconfiguration`, `RRCReestablishment` including MCG/SCG failure handling - **Measurement configuration and reporting**: Event-triggered (A1–A6, B1–B2) and periodic; RSRP/RSRQ/SINR report configs, layer-3 filtering - **Handover**: intra/inter-frequency, inter-RAT, CHO (Conditional HO), DAPS HO - **Beamforming and BM**: SSB/CSI-RS based beam reporting, L1/L3 measurement linkage - **RAN-based paging and notification areas** (RNA/RNAU) for RRC_INACTIVE - **Secondary cell/node addition and modification**: SgNB/SCG addition via `RRCReconfiguration` carrying `SCG-Config` - **SIB broadcast**: SIB1–SIB20+ scheduling and content definitions - **NTN-specific extensions** (Rel-17/18): TA pre-compensation, ephemeris delivery ## Key Interfaces Primarily **Uu** (UE↔gNB). RRC containers also carried transparently over **Xn** (XnAP) and **X2** (X2AP) during HO/SCG procedures, and over **N2** (NGAP) via NAS transport. ## When a RAN Engineer Uses It - Decoding drive-test or QXDM/

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