In this chapter we shall have an introduction of 5G NR RRC Layer.
RRC Layer is a control plane protocol stack.
RRC layer is used for Access Layer Signalling AS.
RRC is a L3 layer protocol.
Functions of RRC Layer
-> AS and NAS related system message broadcast
-> RRC connection control: Paging
-> Establish, maintain and release RRC connection between UE and NG-RAN
-> Establishment, configuration, maintenance and release of SRBs (Signal Radio Bearers) and DRBs (Data Radio Bearers)
-> Measurement configuration and reporting:
–> Establishment/modification/release of measurement configuration
–> Setup and release of measurement gaps;
–> Measurement reporting.
UE states and state transitions
5G NR RRC supports 3 states:
1. RRC_IDLE
2. RRC_CONNECTED
3. RRC_INACTIVE
– RRC_IDLE:
– A UE specific DRX may be configured by upper layers;
– UE controlled mobility based on network configuration;
– The UE:
– Monitors Short Messages transmitted with P-RNTI over DCI (see clause 6.5);
– Monitors a Paging channel for CN paging using 5G-S-TMSI;
– Performs neighbouring cell measurements and cell (re-)selection;
– Acquires system information and can send SI request (if configured).
– RRC_INACTIVE:
– A UE specific DRX may be configured by upper layers or by RRC layer;
– UE controlled mobility based on network configuration;
– The UE stores the UE Inactive AS context;
– A RAN-based notification area is configured by RRC layer;
The UE:
– Monitors Short Messages transmitted with P-RNTI over DCI (see clause 6.5);
– Monitors a Paging channel for CN paging using 5G-S-TMSI and RAN paging using fullI-RNTI;
– Performs neighbouring cell measurements and cell (re-)selection;
– Performs RAN-based notification area updates periodically and when moving outside the configured RAN-based notification area;
– Acquires system information and can send SI request (if configured).
– RRC_CONNECTED:
– The UE stores the AS context;
– Transfer of unicast data to/from UE;
– At lower layers, the UE may be configured with a UE specific DRX;
– For UEs supporting CA, use of one or more SCells, aggregated with the SpCell, for increased bandwidth;
– For UEs supporting DC, use of one SCG, aggregated with the MCG, for increased bandwidth;
– Network controlled mobility within NR and to/from E-UTRA;
– The UE:
– Monitors Short Messages transmitted with P-RNTI over DCI (see clause 6.5), if configured;
– Monitors control channels associated with the shared data channel to determine if data is scheduled for it;
– Provides channel quality and feedback information;
– Performs neighbouring cell measurements and measurement reporting;
– Acquires system information.
Source: 3GPP TS 38.331