Overview of RRC States in 5G NR: Idle, Connected, and Inactive

Overview of RRC States in 5G NR: Idle, Connected, and Inactive

Introduction To RRC States in 5G NR

The 5G New Radio (NR) technology offers enhanced connectivity, higher data rates, and more efficient resource management compared to previous generations. A critical aspect of this efficiency is the Radio Resource Control (RRC) protocol, which manages the radio resources and states of User Equipment (UE). This article explores the characteristics and functions of the three primary RRC states: RRC_IDLE, RRC_CONNECTED, and RRC_INACTIVE, providing insights into their roles in 5G networks.

1. RRC_IDLE

1.1 Overview

RRC_IDLE is a low-activity state designed to conserve battery life and manage UE mobility without active communication. In this state, the UE is not actively engaged in data transfer but can still receive system information and paging messages.

1.2 Key Functions

• System Information Reading: The UE reads system information from the Broadcast Control Channel (BCCH). This includes details required for cell selection and reselection, which are crucial for network access and mobility management.

• Cell Reselection: The UE independently manages mobility based on cell reselection parameters broadcast by the network. This process involves measuring serving and neighboring cells and selecting the best cell or beam to camp on.

• Registration Area Updates: The UE performs both triggered and periodic Registration Area updates to ensure the Access and Mobility Management Function (AMF) can reach it. These updates help in maintaining the UE’s reachability and tracking within the network.

• No RAN Tracking: In RRC_IDLE, the Radio Access Network (RAN) does not actively track the UE's location. The AMF handles tracking with a resolution up to the Registration Area, informed by updates from the UE.

• Paging: Paging messages are broadcast across the Registration Area by the base station to notify the UE of incoming connections or data. A hierarchical paging approach can be employed to reduce paging load and delay.

1.3 Characteristics

• Battery Efficiency: RRC_IDLE is designed to minimize power consumption while ensuring that the UE remains reachable for paging.

• No Active Data Transfer: The UE cannot transfer application data in RRC_IDLE, and there are no established GTP-U tunnels or N2 signaling connections.

2. RRC_CONNECTED

2.1 Overview

RRC_CONNECTED is an active state where the UE can communicate directly with the network for data transfer and signaling. This state supports application data exchange and network control tasks such as handovers.

2.2 Key Functions

• Data and Signaling Transfer: The UE can send and receive application data through Data Radio Bearers (DRBs) and signaling through Signaling Radio Bearers (SRBs). For instance, NAS signaling for SMS can be transmitted over an SRB.

• Resource Allocation: The base station assigns the UE a Cell Radio Network Temporary Identifier (C-RNTI) used for resource allocations on the Physical Downlink Control Channel (PDCCH). This may include other identifiers like MCS-C-RNTI or CS-RNTI for specific purposes.

• NG-C Interface: There is a dedicated signaling connection between the AMF and the base station for NG Application Protocol (NGAP) messages, facilitating UE-specific signaling and NAS message encapsulation.

• GTP-U Tunnels: The User Plane Function (UPF) and the base station maintain GTP-U tunnels for each PDU session, supporting the transfer of packets with different QoS requirements.

• Channel State Information (CSI): The UE provides CSI, including indicators like CQI, RI, and LI, which the network uses for link adaptation, MIMO, and beam management.

• Connected Mode DRX: To conserve battery power, the UE may use Connected Mode Discontinuous Reception (DRX) with short sleep cycles following data transfers, transitioning to longer cycles over time.

• Network-Controlled Mobility: Mobility management involves cell-level and beam-level handovers, with cell-level handovers using RRC signaling and beam-level handovers managed by lower layers.

2.3 Characteristics

• Active Data Transfer: The UE actively exchanges data and signaling, with the network maintaining GTP-U tunnels and NGAP connections.

• Handover Support: The UE supports both intra-system and inter-system handovers, enhancing mobility and connection stability.

3. RRC_INACTIVE

3.1 Overview

RRC_INACTIVE is an intermediate state between RRC_CONNECTED and RRC_IDLE, introduced to balance battery efficiency and quick resumption of activity. This state enables the UE to suspend its connection while remaining registered with the network, allowing for rapid reactivation.

3.2 Key Functions

• Suspend Configuration: The UE moves to RRC_INACTIVE after receiving an RRC Release message containing the suspendConfig parameter, which includes the Inactive Radio Network Temporary Identifier (I-RNTI) and paging cycle details.

• RAN Notification Area: The ran-NotificationAreaInfo specifies the cells within the RAN Notification Area (RNA). The UE can check if the target cell during cell reselection is within the RNA, allowing it to remain in RRC_INACTIVE or re-enter RRC_CONNECTED.

• Periodic Updates: The UE performs periodic RAN Notification Area updates based on the t380 timer to confirm its presence within the RNA.

• Resumption: The UE can resume its connection by sending an RRC Resume Request to a new serving base station, which uses the I-RNTI to identify the original serving base station and resume the RRC connection.

3.3 Characteristics

• Quick Reactivation: The state allows for rapid resumption of activity without full re-establishment of connections, balancing battery efficiency and responsiveness.

• Maintained Connections: The NG signaling and GTP-U tunnels are maintained but may need to be transferred to a new serving base station if the UE moves while inactive.

Conclusion

The RRC protocol in 5G NR offers a sophisticated mechanism to manage UE states, balancing battery efficiency, connectivity, and mobility. Understanding the intricacies of RRC_IDLE, RRC_CONNECTED, and RRC_INACTIVE states is crucial for optimizing network performance and enhancing the user experience in 5G environments.

References

• 3GPP TS 38.331: "Radio Resource Control (RRC) protocol specification."

• 3GPP TS 33.501: "Security architecture and procedures for 5G system."

This guide provides a detailed examination of RRC states, essential for engineers and researchers working on 5G technology.