A coverage hole does not always justify a new full base station. A repeater can extend the radio path, but an uncontrolled amplifier can also forward noise and interference. A Network-Controlled Repeater adds a control connection so the network can coordinate the forwarding beams and transmission opportunities.

This note focuses on the Rel-18 normative procedures and the preceding study. The earlier study, TR 38.867, is itself a Rel-18 report; it should not be labelled a Rel-17 NCR specification. Compare this mechanism with the related IAB (Integrated Access / Backhaul) note, which describes a different way to extend coverage through wireless backhaul.

• Executive Summary
• Why add network control to an RF repeater ?
  • Earlier study and Rel-18 normative work
  • What benefit can forwarding provide ?
• NCR-MT, NCR-Fwd and the three links
  • Control link, backhaul link and access link
  • How is this different from IAB ?
• Rel-18 forwarding control: RRC, MAC CE and DCI
  • Periodic and semi-persistent resources
  • Aperiodic forwarding and DCI format 2_8
• Beam direction and UL/DL timing
  • Forwarding windows and TDD configuration
  • Access beam index versus backhaul TCI
• Connection, recovery and forwarding stop conditions
  • Identification, authorization and normal operation
  • RRC state and failure handling
• RRC parameters and selected ASN.1
  • Forwarding resource configuration
  • Aperiodic fields and DCI interpretation
• Reference

Executive Summary

NCR combines a control endpoint with an RF forwarding function. These two roles must remain separate when interpreting the architecture, the signalling and the behavior after control-link failure.

Why add network control to an RF repeater ?

A conventional RF repeater amplifies the signal presented to it, including unwanted energy. In NR, useful forwarding also depends on the beam direction and the UL/DL timing. The network has scheduling information that an uncontrolled RF amplifier does not have.

Earlier study and Rel-18 normative work

TR 38.867 examined the NCR model and the usefulness of side control information. Its motivation included beamforming, UL/DL coordination and ON/OFF behavior. These study topics explain the design direction but do not replace the final protocol procedures.

Rel-17 work had already specified RF and EMC requirements for conventional NR RF repeaters. Rel-18 NCR adds network control to the forwarding function. Keeping these milestones separate avoids claiming that every Rel-17 NR repeater supports NCR signalling or that every option discussed in the study became mandatory.

What benefit can forwarding provide ?

A repeater can provide a useful radio path around an obstruction or into a weak-coverage area. Its placement must still provide a usable donor-side signal, sufficient antenna isolation and an access-side path that reaches the intended users.

Network control can limit forwarding to useful times and directions, reducing unnecessary amplification and interference. It cannot reconstruct information already lost in noise before the repeater input. The gain, noise figure, output-power limit and linearity still constrain performance. Better received power does not automatically imply better SINR or more cell capacity.

The IAB note provides a useful comparison when deciding whether the site needs RF forwarding or a protocol-processing radio node. NCR is a coverage tool within the serving network, not a substitute for all the scheduling and capacity functions of an additional gNB.

• The network coordinates when and where forwarding occurs; RF quality and interference still determine whether that forwarding helps.

NCR-MT, NCR-Fwd and the three links

The NCR node contains two logical functions with different jobs. The NCR-MT maintains the control relationship with the gNB. The NCR-Fwd carries out the RF forwarding behavior selected through that relationship. The served UE traffic is not routed through the NCR-MT control bearer.

Control link, backhaul link and access link

The control link connects gNB and NCR-MT through NR Uu. The NCR-Fwd backhaul link faces the gNB, while its access link faces the served UEs. Here the word backhaul identifies an RF forwarding link, not an IAB BAP transport path.

Figure 1 separates these links. The blue upper path carries control signalling, and the lower path forwards radio signals. The internal arrow represents control of NCR-Fwd; it is not a standardized packet interface between two independently deployed network functions.

Figure 1. Logical NCR architecture. UE traffic follows the RF forwarding path, not the NCR-MT control path.

NCR-MT establishes SRBs and can optionally establish DRBs for OAM traffic. That DRB support does not turn NCR-MT into the data relay for every served UE. The forwarded signal is associated with the cell used by the NCR-MT control link; forwarding other signals is left to implementation in the architecture description.

How is this different from IAB ?

IAB includes an IAB-MT and IAB-DU and supports protocol handling for access and wireless backhaul. NCR has an RF forwarding function instead of an IAB-DU. Both can extend coverage, but their processing, deployment and control requirements differ.

• Do not infer IAB-DU, BAP routing or a new serving-cell protocol endpoint from the presence of an NCR backhaul link.

Rel-18 forwarding control: RRC, MAC CE and DCI

The gNB does not need to send an entirely new RRC configuration for every forwarding opportunity. RRC establishes the resource descriptions, while MAC and physical-layer indications can select or activate behavior on shorter timescales. The applicable timing and capability rules still govern when changes take effect.

Periodic and semi-persistent resources

A periodic forwarding resource associates a time resource with a beam index. Its configuration includes timing information such as periodicity, offset and duration. The resource set also establishes the reference subcarrier spacing used to interpret the timing.

Semi-persistent forwarding resources are also configured through RRC, but an NCR Access Link Beam Indication MAC CE activates or deactivates the selected resource set. The MAC CE can update beam indices for that operation. This differs from a permanently active RF amplifier and from an independent scheduler located inside NCR-Fwd.

Aperiodic forwarding and DCI format 2_8

NCR-MT can monitor a UE-specific search space for DCI format 2_8 with CRC scrambled by NCR-RNTI. The indication selects time resources and beam indices for NCR-Fwd access-link operation. It does not assign a transport block for NCR-Fwd to decode.

The configured resource describes a slot offset, symbol offset and duration. Interpretation also depends on the reference SCS and the supported aperiodic beam-indication timing. Therefore, a DCI reception timestamp alone is insufficient to locate the forwarding window. TS 38.213 section 20 gives the reference-slot and application rules.

When resource indications overlap, priority matters. A periodic or semi-persistent resource carrying the applicable priority flag can take precedence over an aperiodic indication. Without that flag, the DCI-indicated beam takes precedence in the specified overlap case. Do not assume that the most dynamic indication always wins. The procedure also constrains overlapping resources with conflicting beam indices.

• Decode the RRC resource description first, then apply MAC/DCI activation, timing and priority rules to find the actual forwarding behavior.

Beam direction and UL/DL timing

The repeater must apply a useful beam in the correct direction and at the correct time. Forwarding a valid signal in an inappropriate symbol can create interference. NCR therefore combines resource indications with the applicable UL/DL configuration and control-link constraints.

Forwarding windows and TDD configuration

For downlink symbols, NCR-Fwd receives on its backhaul link and transmits on its access link. For uplink symbols, the direction reverses. TS 38.213 relates these operations to the common and, when provided, dedicated UL/DL configuration.

Figure 2 shows an illustrative timeline, not a prescribed slot pattern. The selected beam-resource windows coexist with a symbol direction. A resource indication is not permission to disregard the UL/DL direction, a link-recovery stop condition or the NCR capability limits.

Figure 2. Selected forwarding windows must agree with direction, beam control and the permitted operating state.

Access beam index versus backhaul TCI

An access-link beam index identifies a configured physical beam of the NCR-Fwd. Backhaul receive and transmit behavior can instead use TCI, QCL or spatial-filter information under the specified rules. These identifiers are not interchangeable coordinates in one universal beam table.

OAM can provide information about the physical access beams to the gNB and NCR. How those beams are characterized and provided is implementation dependent. The QCL note and Beam Management note explain the underlying NR reference-signal concepts.

Simultaneous control-link and backhaul operation also imposes constraints. For example, if NCR cannot transmit simultaneously on both links, NCR-Fwd does not transmit in a time resource used by NCR-MT transmission. The simultaneous receive/transmit cases have corresponding TCI or spatial-filter relationships. Separate drawings of the two links do not establish hardware independence.

• Resolve the access beam, backhaul assumption, direction and hardware capability together before deciding that a forwarding window is usable.

Connection, recovery and forwarding stop conditions

The repeater is controlled infrastructure, so its forwarding behavior cannot ignore the state of the control connection. Rel-18 defines circumstances where forwarding may continue and others where it stops. These rules matter when a repeater suddenly disappears from a coverage measurement.

Identification, authorization and normal operation

The gNB advertises NCR support through system information. NCR identifies itself during access, and the serving network obtains the relevant authorization from the core. Detection of a strong cell alone does not authorize a device to operate as an NCR.

NCR-MT uses familiar NR procedures for synchronization, random access and control signalling, but supports only the defined subset of UE functions. Rel-18 excludes CA, MR-DC and handover-related features for NCR-MT in this architecture. Do not assume it behaves like a fully mobile commercial UE simply because its control interface uses NR Uu.

RRC state and failure handling

An NCR-MT transition to RRC_INACTIVE does not necessarily stop forwarding immediately. NCR-Fwd may continue using the last side control information, subject to the defined cell and link conditions. This differs from the transition to RRC_IDLE, where forwarding ceases.

A recovered control connection and a resumed forwarding function are therefore separate observations. Correlate RRC recovery with the later side control configuration and activation. The Beam Failure Recovery note explains the control-link recovery background; the NCR-specific forwarding rule adds another step to the analysis.

• After recovery, verify the forwarding configuration as well as the control connection; one does not automatically prove the other.

RRC parameters and selected ASN.1

Forwarding configuration describes resource sets and the information needed to interpret them. The following selected definitions were checked against current TS 38.331 v19.4.0. Their r18 suffix identifies introduction in Rel-18, while the current listing avoids reproducing an obsolete syntax.

Forwarding resource configuration

NCR-FwdConfig groups periodic, semi-persistent and aperiodic configuration. Add/modify and release lists change the stored configuration; they do not all imply immediate activation. The associated MAC and physical-layer procedures determine how those resources are used.

Following is based on 38.331 v19.4.0 (Release 19): NCR-FwdConfig-r18

NCR-FwdConfig-r18 ::=                SEQUENCE {
    periodicFwdRsrcSetToAddModList-r18   SEQUENCE (SIZE (1..maxNrofPeriodicFwdResourceSet-r18)) OF NCR-PeriodicFwdResourceSet-r18
                                                                                                            OPTIONAL,  -- Need N
    periodicFwdRsrcSetToReleaseList-r18  SEQUENCE (SIZE (1..maxNrofPeriodicFwdResourceSet-r18)) OF NCR-PeriodicFwdResourceSetId-r18
                                                                                                            OPTIONAL,  -- Need N
    aperiodicFwdConfig-r18                      SetupRelease {NCR-AperiodicFwdConfig-r18}                   OPTIONAL,   -- Need M
    semiPersistentFwdRsrcSetToAddModList-r18    SEQUENCE (SIZE (1..maxNrofSemiPersistentFwdResourceSet-r18)) OF
                                                                    NCR-SemiPersistentFwdResourceSet-r18    OPTIONAL,  -- Need N
    semiPersistentFwdRsrcSetToReleaseList-r18   SEQUENCE (SIZE (1..maxNrofSemiPersistentFwdResourceSet-r18)) OF
                                                                    NCR-SemiPersistentFwdResourceSetId-r18  OPTIONAL,  -- Need N
    ...
}

Following is based on 38.331 v19.4.0 (Release 19): NCR-PeriodicFwdResource-r18

NCR-PeriodicFwdResource-r18 ::=  SEQUENCE {
    periodicFwdRsrcId-r18            NCR-PeriodicFwdResourceId-r18,
    beamIndex-r18                    INTEGER (0..63),
    periodicTimeRsrc-r18             SEQUENCE {
        periodicityAndOffset-r18     NCR-PeriodicityAndOffset-r18,
        symbolOffset-r18             INTEGER (0..maxNrofSymbols-1),
        durationInSymbols-r18        INTEGER (1..112)
    },
    ...
}

The periodic resource combines a beam index with a timing description. Its duration is measured in symbols, not milliseconds. Reference SCS is supplied through the resource-set configuration, whose full definition is not reproduced here. Read these selected IEs together with their containing configuration; they are not a complete RRCReconfiguration message.

Aperiodic fields and DCI interpretation

Aperiodic configuration supplies the candidate time resources and the field sizes used for beam indication. A decoder must know that configuration before it can reliably interpret the DCI fields. Reading the DCI payload with assumed widths can produce plausible but incorrect resource indices.

Following is based on 38.331 v19.4.0 (Release 19): NCR-AperiodicFwdConfig-r18

NCR-AperiodicFwdConfig-r18 ::= SEQUENCE {
    aperiodicFwdTimeRsrcToAddModList-r18  SEQUENCE (SIZE (1..maxNrofAperiodicFwdTimeResource-r18)) OF NCR-AperiodicFwdTimeResource-r18
                                                                                                        OPTIONAL, -- Need N
    aperiodicFwdTimeRsrcToReleaseList-r18 SEQUENCE (SIZE (1..maxNrofAperiodicFwdTimeResource-r18)) OF NCR-AperiodicFwdTimeResourceId-r18
                                                                                                        OPTIONAL, -- Need N
    referenceSCS-r18                      SubcarrierSpacing                                             OPTIONAL, -- Need M
    aperiodicBeamFieldWidth-r18           INTEGER (1..6)                                                OPTIONAL, -- Need M
    numberOfFields-r18                    INTEGER (1..32)                                               OPTIONAL, -- Need M
    ...
}

Following is based on 38.331 v19.4.0 (Release 19): NCR-AperiodicFwdTimeResource-r18

NCR-AperiodicFwdTimeResource-r18 ::= SEQUENCE {
    aperiodicFwdTimeRsrcId-r18           NCR-AperiodicFwdTimeResourceId-r18,
    slotOffsetAperiodic-r18              INTEGER (0..14),
    symbolOffset-r18                     INTEGER (0..maxNrofSymbols-1),
    durationInSymbols-r18                INTEGER (1..28),
    ...
}

The slot offset is relative to the reference slot defined by the NCR procedure, not an arbitrary absolute frame position. The symbol offset and duration then locate the window at the configured reference SCS. UE capability information for aperiodic beam-indication timing also participates in that reference-slot calculation.

For deployment checks, retain the NCR authorization result, RRC resource lists, MAC activation state, NCR-RNTI/DCI decoding and observed beam behavior. RF gain and output quality still need separate measurement. These RRC definitions describe control resources; they are not a complete RF gain-control algorithm or an assurance of coverage improvement.

• Check the containing resource set, timing reference and activation state alongside each resource definition.

Reference

The study motivation and model, NCR architecture and lifecycle clauses, forwarding-control procedures and selected RRC definitions were consulted. These are targeted sections rather than a claim that every clause of each specification was reviewed.

• TR 38.867 v18.0.0: Study on NR network-controlled repeaters; study motivation and model.
• TS 38.300 v18.6.0: Section 4.9: NCR architecture, capabilities, signalling, OAM and authorization.
• TS 38.213 v18.6.0: Section 20: forwarding timing, beams, DCI format 2_8 and overlapping-resource rules.
• TS 38.321 v19.4.0: NCR beam-indication MAC CE procedures and formats, sections 6.1.3.66 to 6.1.3.68.
• TS 38.331 v19.4.0: Selected NCR forwarding configuration information elements.