A Network-Controlled Repeater node, referred to as NCR-node, is an RF repeater that enables wireless amplifying-and-forwarding functionality in NG-RAN. The NCR-node is capable of receiving and applying side control information from a gNB with additional functionality to support Network-Controlled Repeater.
The NCR-node comprises an NCR-MT and an NCR-Fwd. The NCR-MT is an entity supporting a subset of the UE functionality that communicates with the gNB to receive side control information via a control link based on the NR Uu interface. The NCR-Fwd is the function performing amplifying-and-forwarding of signals between gNB and UE via the NCR-Fwd backhaul link and NCR-Fwd access link, respectively. The NCR-Fwd can support multiple beams towards the UE. The behaviour of the NCR-Fwd is controlled according to the side control information received from the gNB. The NCR-node is modelled as depicted in Figure 4.9.1-1
An NCR-MT establishes SRBs and, optionally, DRB(s) with a gNB. The establishment of DRB(s) can be used to transport OAM traffic.
The signal that NCR-Fwd forwards is associated to the cell that the NCR-MT is connected to via the control link. Whether the NCR-Fwd can forward other signals is up to implementation.
Carrier Aggregation (CA), Multi-Radio Dual Connectivity (MR-DC), handover and its related features (e.g., CHO, DAPS, CPAC, etc.) are not supported by NCR-MT, as defined together with other limitations in TS 38.306
NR RRC signalling is utilized to configure the NCR-MT to receive side control information from a gNB, which is used by the NCR-Fwd to determine whether and how to amplify-and-forward RF signals. If the side control configuration is removed, the NCR-Fwd ceases its amplifying-and-forwarding function.
When the NCR-MT is in RRC_CONNECTED state, the NCR-Fwd may amplify-and-forward RF signals based on the side control information received from the gNB. The NCR-MT does not support RRM measurements in RRC_CONNECTED.
When the NCR-MT transitions from RRC_CONNECTED state to RRC_INACTIVE state, the NCR-Fwd may continue to amplify-and-forward RF signals in accordance with the last side control information received from the gNB. When the NCR-MT is in RRC_INACTIVE state, the NCR-Fwd ceases to amplify-and-forward RF signals if no suitable cell is detected, or if the NCR-MT selects a different cell than the last serving cell on which side control configuration was received.
When an NCR-MT in RRC_INACTIVE state determines degradation of the NCR-Fwd backhaul link beam, then the NCR-Fwd should cease amplifying-and-forwarding RF signals, and the NCR-MT should attempt to resume its RRC connection (with cause value mo-Signalling). The criteria to evaluate backhaul beam degradation are left to the NCR-node implementation.
When the NCR-MT transitions from RRC_CONNECTED state to RRC_IDLE, the NCR-Fwd ceases any amplifying-and-forwarding of RF signals. How an NCR-MT transitions back from RRC_IDLE state to RRC_CONNECTED state is left to NCR-node or network implementation.
An NCR-MT can detect RLF on the control link as specified in clause 5.3.10 of TS 38.331
. When RLF is detected, the NCR-MT performs the RRC re-establishment procedure as specified in TS 38.331
. During the RRC re-establishment procedure, the NCR-Fwd ceases to amplify-and-forward RF signals.
After successfully performing the RRC re-establishment procedure, the NCR-MT waits for the new side control configuration for the NCR-Fwd to resume the amplifying-and-forwarding of RF signals.
An NCR-MT can also perform Beam Failure Detection (BFD) and Beam Failure Recovery (BFR) as described in clause 9.2.8
. Once the NCR-MT detects beam failure in the control link, the NCR-Fwd should cease amplifying-and-forwarding RF signals until BFR is completed.
The transport connection between the NCR-node and its OAM may be provided by the NCR-MT's PDU session. A Network-Controlled Repeater may be configured with a list of allowed gNB cell(s) that the NCR-MT is allowed to connect with, and/or a list of forbidden gNB cell(s) that the NCR-MT is not allowed to connect with.
The information on the physical beam(s) used by NCR-Fwd for access link may be provided by OAM to the gNB and the Network-Controlled Repeater for operation. How to characterize and provide the physical beams of NCR-Fwd is up to implementation.
Network-Controlled Repeater identification is performed in RAN, and Network-Control Repeater authorization is performed in 5GC. The general procedure of the Network-Controlled Repeater management is illustrated in Figure 4.9.5-1
The gNB broadcasts the Network-Controlled Repeater supported information via system information.
When a Network-Controlled Repeater is trying to access the network as a Network-Controlled Repeater, the Network-Controlled Repeater indication is sent to the serving gNB.
The serving gNB selects an appropriate AMF for the Network-Controlled Repeater.
AMF provides Network-Controlled Repeater authorization information to the gNB.
Other steps refer to the signalling flow as defined in clause 184.108.40.206