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Radio Resource Control (RRC) State Machine in 5G New Radio (NR)

Radio Resource Control (RRC) State Machine in 5G New Radio (NR)
Radio Resource Control (RRC) State Machine in 5G New Radio (NR)

In the context of 5G New Radio (NR), the Radio Resource Control (RRC) state machine is critical for managing the connectivity and signaling between User Equipment (UE) and the network. This article delves into the RRC states and transitions, highlighting their roles and functionalities within the 5G ecosystem.


Overview of RRC States In 5G

The RRC states in NR include:

  • RRC Idle: The starting state for a UE when it first connects to a 5G cell, such as upon device startup or after inter-system cell reselection from LTE.

  • RRC Connected: Achieved by completing the RRC Setup procedure, establishing a logical connection between the UE and the Base Station.

  • RRC Inactive: A state that provides a balance between RRC Idle and RRC Connected, allowing quick resumption of data transfer with reduced signaling load and battery consumption.


RRC Idle State

When a UE first camps on a 5G cell, it enters the RRC Idle state. This can occur immediately after the device powers on or following an inter-system cell reselection from LTE. In this state:

  • The UE monitors the Physical Downlink Control Channel (PDCCH) for Paging DCI using the P-RNTI.

  • The UE reads System Information and performs Registration Area updates with the Core Network.

  • Mobility is managed through cell reselection based on signal strength and quality.


Transition to RRC Connected State

To transition from RRC Idle to RRC Connected, the UE undergoes the RRC Setup procedure. In RRC Connected mode:

  • The UE is assigned one or two Cell Radio Network Temporary Identifiers (C-RNTI), used for resource allocation.

  • The UE is configured with Signaling Radio Bearers (SRB) for signaling messages and Data Radio Bearers (DRB) for application data.

  • Uplink and downlink data transfer is enabled, with the UE reporting Channel State Information (CSI) and the Base Station managing resource allocations.


RRC Connected State

In the RRC Connected state, the UE is actively communicating with the network:

  • The UE monitors control channels for resource allocations and maintains its Access Stratum context.

  • The Base Station maintains NG signaling connections with the Access and Mobility Management Function (AMF) and GTP-U tunnels with the User Plane Function (UPF) for data transfer.

  • Mobility is handled through handovers, ensuring seamless connectivity as the UE moves between cells.


RRC Inactive State

The RRC Inactive state offers a compromise between the high signaling load of RRC Connected and the latency of RRC Idle:

  • The UE can quickly resume data transfer with minimal latency, thanks to an already established context.

  • Battery consumption is reduced by using longer Discontinuous Reception (DRX) cycles and minimizing channel quality reporting requirements.

  • The UE monitors Paging DCI on the PDCCH and performs RAN Notification Area (RNA) updates, maintaining its location with a resolution to an RNA.


Transition to RRC Inactive State

Transitioning from RRC Connected to RRC Inactive involves the RRC Release procedure with a SuspendConfig parameter, indicating the move to RRC Inactive rather than RRC Idle:

  • The NG signaling connection between the Base Station and AMF, along with GTP-U tunnels between the Base Station and UPF, are preserved.

  • The UE's context is maintained by both the network and the UE, allowing for rapid resumption of connectivity when needed.


Network Behavior and RRC State Monitoring

The AMF can request notifications from the Base Station regarding the UE's transitions between RRC Connected and RRC Inactive. This information is critical for adjusting supervision timers:

  • When the UE is in RRC Connected, the AMF applies shorter supervision timers due to the expectation of a rapid response to downlink transactions.

  • In RRC Inactive, longer supervision timers are applied as the UE must be paged before transactions can be forwarded.


Transition to RRC Idle State

The UE transitions to RRC Idle by completing the RRC Connection Release procedure:

  • This involves releasing the C-RNTI, all radio bearers, and radio resources.

  • The UE may be provided with Redirected Carrier Information, allowing it to move to another NR carrier or LTE carrier based on coverage.

  • The RRC Release message can include Cell Reselection Priorities and Deprioritization information, influencing the UE's cell reselection behavior.


RRC State Transitions Between NR and LTE

Transitions between NR and LTE RRC states are defined to ensure seamless inter-system connectivity:

  • Handover: UE can transition between LTE and NR RRC Connected states.

  • Cell Reselection: UE can move between LTE and NR RRC Idle states, with respective system information blocks providing parameters for cell reselection.

  • RRC Inactive: The UE can reselect from NR RRC Inactive to LTE RRC Idle, leveraging the flexibility introduced in Release 15 specifications.


Conclusion

The RRC state machine in 5G NR is integral to managing UE connectivity, balancing performance, signaling efficiency, and battery consumption. Understanding these states and their transitions enables effective management of 5G networks, ensuring robust and seamless connectivity for users.

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