1. UE RRC States – The Big Picture:
A UE in NR can be in exactly one of the following RRC states at any given time:
3GPP Reference: "A UE is either in RRC_CONNECTED state or in RRC_INACTIVE state when an RRC connection has been established. If this is not the case, i.e. no RRC connection is established, the UE is in RRC_IDLE state."
2. RRC_IDLE – Detailed Breakdown (with Examples):
DRX Configurations
UE-specific DRX can be configured by upper layers (NAS) – e.g., for power saving during idle paging cycles.
At lower layers (MAC/RRC), the UE may be configured with a DRX for PTM (Point-to-Multipoint) transmission of MBS broadcast – this allows battery-efficient reception of broadcast TV or public safety alerts.
Example: A UE in a stadium receives an MBS broadcast of a live event. Even in IDLE, it wakes up at configured DRX occasions to receive PTM data.
UE Actions in RRC_IDLE:
3. RRC_INACTIVE – Deep Dive:
DRX Configurations
- UE-specific DRX may be configured by upper layers (NAS) or by RRC layer.
At lower layers, the UE may be configured with:
A DRX for PTM transmission of MBS broadcast
AND/OR a DRX for PTM transmission of MBS multicast (separate from broadcast)
UE Actions in RRC_INACTIVE (with Critical Nuances):
SDT-Specific Monitoring Logic (Critical):
While T319a (SDT timer) is running: UE monitors control channels associated with the shared data channel to check if data is scheduled for it.
While SDT ongoing and T319a NOT running, if CG-SDT (Configured Grant-SDT) is selected and extended CG-SDT periodicity is configured (i.e.,
cg-SDT-PeriodicityExtis configured):
UE monitors Paging channel for:
CN paging using 5G-S-TMSI
RAN paging using fullI-RNTI (full Inactive RNTI)
Exception: Not applicable if UE is acting as L2 U2N Remote UE.
While SDT procedure is NOT ongoing:
Monitors paging for CN paging (5G-S-TMSI) and RAN paging (fullI-RNTI).
If configured for MBS multicast reception, monitors paging using TMGI.
Other RRC_INACTIVE Capabilities:
Performs neighbouring cell measurements and cell (re-)selection – UE-controlled.
Performs measurements on L2 U2N Relay UEs and relay (re-)selection – same as IDLE.
Performs RAN-based notification area updates – periodic and mobility-based.
Acquires system information – can send SI request while SDT is not ongoing.
Logged measurements – only while SDT is not ongoing.
Idle/Inactive measurements – only while SDT is not ongoing.
MBS broadcast reception – acquires MCCH change notification + broadcast control info/data.
MBS multicast reception in RRC_INACTIVE – if configured, acquires multicast MCCH change notification + multicast control info/data.
Transmits SRS for Positioning – UE sends Sounding Reference Signals to enable network-based positioning (e.g., for emergency calls).
Example: A UE in Inactive state with a configured multicast subscription monitors multicast MCCH, receives group data, and simultaneously transmits SRS for indoor positioning – all while not being in CONNECTED.
4. RRC_CONNECTED – Enterprise-Grade Features
Key Characteristics
Stores AS context (active).
Transfer of unicast data to/from UE.
Transfer of MBS multicast data to UE.
DRX configurations:
UE-specific DRX at lower layers.
DRX for PTM transmission of MBS broadcast.
DRX for MBS multicast.
Cell-specific DTX/DRX (Discontinuous Tx/Rx) – for network power savings.
Carrier Aggregation & Dual Connectivity
For UEs supporting CA (Carrier Aggregation): Use of one or more SCells (Secondary Cells) aggregated with the SpCell (Special Cell) for increased bandwidth.
For UEs supporting DC (Dual Connectivity): Use of one SCG (Secondary Cell Group) aggregated with the MCG (Master Cell Group).
Network-Controlled Mobility (Crucial Details):
UE Actions in RRC_CONNECTED
Monitors Short Messages (with P-RNTI over DCI) if configured.
Monitors control channels (PDCCH) associated with shared data channel (PDSCH/PUSCH) to detect scheduled data.
Provides channel quality and feedback information – CQI, PMI, RI, CSI-RS reports.
Performs neighbouring cell and/or L2 U2N relay measurements and measurement reporting – event-triggered (A1–A6, B1, B2).
Acquires system information – on-demand or broadcast.
Performs immediate MDT measurement together with available location reporting – for network optimization.
If configured for MBS broadcast: acquires MCCH change notification + broadcast control info/data.
Performs logging of measurements for network-side data collection, if configured – for SON (Self-Organizing Networks).
Example: A UE in CONNECTED, with CA (2 CCs) and DC (EN-DC), while receiving MBS multicast, reports CSI every 20 ms, performs handover to LTE, and logs MDT data for the operator.
5. State Transitions:
NR-Only State Machine
Inter-RAT Mobility Between:
NR/5GC
E-UTRA/EPC (LTE with Evolved Packet Core)
E-UTRA/5GC (LTE connected to 5GC – option 5/7)
Example State Transitions (Inter-RAT):
Connected mode: NR → E-UTRA handover (inter-RAT handover) using measurement reports.
Idle/Inactive mode: Cell reselection between NR and E-UTRA based on priority and thresholds.
Mobility from E-UTRA/EPC to NR/5GC: Handover or redirection.
Mobility Between NR/5GC and UTRA-FDD (3G)
Specifically covers handover (in connected mode) and cell reselection (in idle/inactive).
Used in scenarios where 5G/LTE coverage is unavailable and UE falls back to 3G WCDMA.
Example: UE on a 5G call moves into a tunnel with only 3G coverage – network triggers IRAT handover to UTRA-FDD.
6. Real-World Deployment Scenarios:
Smart City Sensor Network – UEs stay in RRC_INACTIVE, use SDT for periodic reports, and rely on L2 U2N Relay for coverage extension in underground parking lots.
Public Safety Broadcast – UEs in RRC_IDLE receive MBS broadcast alerts via PTM DRX, while first responders in RRC_CONNECTED receive MBS multicast group calls with CA/DC for high reliabilit
High-Speed Train – UE in RRC_CONNECTED performs inter-RAT handover from NR to E-UTRA as it passes through rural areas, with MDT logging enabled for operator QoS analysis.
Reference: TS 38.331