5G NR RRC Decoder. Decode NR Signaling Messages Online

5G NR RRC (Radio Resource Control) is the Layer 3 signaling protocol defined in 3GPP TS 38.331 that manages the radio connection between the UE and the gNodeB in the NG-RAN (Next Generation Radio Access Network). NR RRC builds on the LTE RRC framework but introduces significant changes to support the wider bandwidth, higher frequencies, beam-based operation, and new deployment architectures of 5G. NR RRC handles connection setup, measurement configuration, handover, security activation, carrier aggregation, dual connectivity (NR-DC, EN-DC), bandwidth part (BWP) management, beam reporting, and conditional handover procedures.

Compared to LTE, NR RRC introduces the RRC_INACTIVE state for power-efficient IoT and mobile broadband, replaces RadioResourceConfigDedicated with the more modular CellGroupConfig structure, and adds extensive beam management parameters for FR2 (mmWave) operation. NR RRC messages can be substantially larger than their LTE counterparts due to the complexity of BWP configurations, CSI-RS resources, and multi-beam MIMO parameters. The HiCellTek decoder parses these complex NR structures using a C++ ASN.1 engine compiled from the official TS 38.331 R17 schema, rendering the full tree including nested CellGroupConfig, SpCellConfig, and MeasObjectNR.

CellGroupConfig is the central NR RRC structure that defines the complete radio configuration of a cell group. In NR, a cell group consists of the SpCell (Special Cell, either PCell or PSCell) and optional SCells for carrier aggregation. CellGroupConfig is carried inside RRCReconfiguration and contains the following key sub-structures:

• SpCellConfig: Primary cell configuration including ServingCellConfigCommon, initialDownlinkBWP, initialUplinkBWP, PDSCH-Config, PUSCH-Config, CSI-MeasConfig, and beam failure recovery parameters.
• sCellToAddModList: List of secondary cells for NR carrier aggregation with per-SCell ServingCellConfig and BWP configurations.
• MAC-CellGroupConfig: MAC layer parameters including DRX configuration (on-duration, inactivity timers), BSR (Buffer Status Report) config, PHR (Power Headroom Report) config, and scheduling request resources.
• PhysicalCellGroupConfig: Physical layer group parameters: HARQ-ACK codebook, p-NR-FR1/FR2, SRS switching, and configured grant configurations.
• RLC-BearerToAddModList: RLC bearer configurations mapping logical channels to RLC entities (AM, UM, TM modes).

In EN-DC (LTE anchor + NR secondary), the entire NR CellGroupConfig is encapsulated as an OCTET STRING inside the LTE nr-SecondaryCellGroupConfig-r15 IE within RRCConnectionReconfiguration. In NR-DC (NR anchor + NR secondary), CellGroupConfig appears directly in the NR RRCReconfiguration. The HiCellTek decoder handles both scenarios, automatically detecting tunneled payloads and rendering the full CellGroupConfig R17 tree.

In NSA (Non-Standalone) 5G deployments using EN-DC (Option 3x), the LTE eNodeB acts as the master node and the NR gNodeB as the secondary node. The NR radio configuration is not sent directly to the UE over the NR air interface during initial setup. Instead, it is tunneled inside the LTE RRC layer: the eNodeB sends an LTE RRCConnectionReconfiguration message containing the nr-SecondaryCellGroupConfig-r15 IE, which wraps the NR CellGroupConfig as a raw ASN.1 OCTET STRING.

Decoding this tunnel manually is complex: you need to first decode the LTE RRC message per TS 36.331, locate the OCTET STRING, extract the raw bytes, and then decode them as NR CellGroupConfig per TS 38.331. The HiCellTek decoder automates this entire process. When it detects nr-SecondaryCellGroupConfig-r15 in a decoded LTE RRC frame, it automatically extracts the NR payload (handling both raw hex and Base64 encoding) and renders the full NR CellGroupConfig tree inline. This makes it easy to verify the NR configuration that the UE received via the LTE tunnel, critical for EN-DC troubleshooting, SCell activation validation, and FR2 beam configuration analysis.