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Content for  TS 38.300  Word version:  17.0.0

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16.12  Sidelink Relay |R17|Word‑p. 161

16.12.1  GeneralWord‑p. 161

Sidelink relay is introduced to support 5G ProSe UE-to-Network Relay (U2N Relay) function (specified in TS 23.304) to provide connectivity to the network for U2N Remote UE(s). Both L2 and L3 U2N Relay architectures are supported. The L3 U2N Relay architecture is transparent to the serving RAN of the U2N Relay UE, except for controlling sidelink resources. The detailed architecture and procedures for L3 U2N Relay can be found in TS 23.304.
A U2N Relay UE shall be in RRC_CONNECTED to perform relaying of unicast data.
For L2 U2N Relay operation, the following RRC state combinations are supported:
  • Both U2N Relay UE and U2N Remote UE shall be in RRC CONNECTED to perform transmission/reception of relayed unicast data; and
  • The U2N Relay UE can be in RRC_IDLE, RRC_INACTIVE or RRC_CONNECTED as long as all the U2N Remote UE(s) that are connected to the U2N Relay UE are either in RRC_INACTIVE or in RRC_IDLE.
For L2 U2N Relay, the U2N Remote UE can only be configured to use resource allocation mode 2 (as specified in clause 5.7.2 and clause 16.9.3.1) for data to be relayed.
A single unicast link is established between one L2 U2N Relay UE and one L2 U2N Remote UE. The traffic of U2N Remote UE via a given U2N Relay UE and the traffic of the U2N Relay UE shall be separated in different Uu RLC channels over Uu.
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16.12.2  Protocol ArchitectureWord‑p. 161

16.12.2.1  L2 UE-to-Network RelayWord‑p. 161

The protocol stacks for the user plane and control plane of L2 U2N Relay architecture are presented in Figure 16.12.2.1-1 and Figure 16.12.2.1-2. The SRAP sublayer is placed above the RLC sublayer for both CP and UP at both PC5 interface and Uu interface. The Uu SDAP, PDCP and RRC are terminated between L2 U2N Remote UE and gNB, while SRAP, RLC, MAC and PHY are terminated in each hop (i.e. the link between L2 U2N Remote UE and L2 U2N Relay UE and the link between L2 U2N Relay UE and the gNB).
For L2 U2N Relay, the SRAP sublayer over PC5 hop is only for the purpose of bearer mapping. The SRAP sublayer is not present over PC5 hop for relaying the L2 U2N Remote UE's message on BCCH and PCCH. For L2 U2N Remote UE's message on SRB0, the SRAP sublayer is not present over PC5 hop, but the SRAP sublayer is present over Uu hop for both DL and UL.
Reproduction of 3GPP TS 38.300, Fig. 16.12.2.1-1: User plane protocol stack for L2 UE-to-Network Relay
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Reproduction of 3GPP TS 38.300, Fig. 16.12.2.1-2: Control plane protocol stack for L2 UE-to-Network Relay
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For L2 U2N Relay, for uplink:
  • The Uu SRAP sublayer supports UL bearer mapping between ingress PC5 Relay RLC channels for relaying and egress Uu Relay RLC channels over the L2 U2N Relay UE Uu interface. For uplink relaying traffic, the different end-to-end RBs (SRBs or DRBs) of the same Remote UE and/or different Remote UEs can be multiplexed over the same Uu Relay RLC channel;
  • The Uu SRAP sublayer supports L2 U2N Remote UE identification for the UL traffic. The identity information of L2 U2N Remote UE Uu Radio Bearer and a local Remote UE ID are included in the Uu SRAP header at UL in order for gNB to correlate the received packets for the specific PDCP entity associated with the right Uu Radio Bearer of a Remote UE;
  • The PC5 SRAP sublayer at the L2 U2N Remote UE supports UL bearer mapping between Remote UE Uu Radio Bearers and egress PC5 Relay RLC channels.
For L2 U2N Relay, for downlink:
  • The Uu SRAP sublayer supports DL bearer mapping at gNB to map end-to-end Radio Bearer (SRB, DRB) of Remote UE into Uu Relay RLC channel over Relay UE Uu interface. The Uu SRAP sublayer supports DL bearer mapping and data multiplexing between multiple end-to-end Radio Bearers (SRBs or DRBs) of a L2 U2N Remote UE and/or different L2 U2N Remote UEs and one Uu Relay RLC channel over the Relay UE Uu interface;
  • The Uu SRAP sublayer supports Remote UE identification for DL traffic. The identity information of Remote UE Uu Radio Bearer and a local Remote UE ID are included into the Uu SRAP header by the gNB at DL in order for Relay UE to map the received packets from Remote UE Uu Radio Bearer to its associated PC5 Relay RLC channel;
  • The PC5 SRAP sublayer at the Relay UE supports DL bearer mapping between ingress Uu Relay RLC channels and egress PC5 Relay RLC channels;
  • The PC5 SRAP sublayer at the Remote UE correlates the received packets for the specific PDCP entity associated with the right Uu Radio Bearer of a Remote UE based on the identity information included in the Uu SRAP header.
A local Remote UE ID is included in both PC5 SRAP header and Uu SRAP header. L2 U2N Relay UE is configured by the gNB with the local Remote UE ID to be used in SRAP header. Remote UE obtains the local Remote ID from the gNB via Uu RRC messages including RRCSetup, RRCReconfiguration, RRCResume and RRCReestablishment. Uu DRB(s) and Uu SRB(s) are mapped to different PC5 Relay RLC channels and Uu Relay RLC channels in both PC5 hop and Uu hop.
It is the gNB responsibility to avoid collision on the usage of local Remote UE ID. The gNB can update the local Remote UE ID by sending the updated local Remote ID via RRCReconfiguration message to the Relay UE. The serving gNB can perform local Remote UE ID update independent of the PC5 unicast link L2 ID update procedure.
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16.12.3  Relay DiscoveryWord‑p. 163

Model A and Model B discovery models as defined in TS 23.304 are supported for U2N Relay discovery. The protocol stack used for discovery is presented in Figure 16.12.3-1.
Reproduction of 3GPP TS 38.300, Fig. 16.12.3-1: Protocol Stack of Discovery Message for UE-to-Network Relay
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The U2N Remote UE can perform Relay discovery message (i.e. as specified in TS 23.304) transmission and may monitor the sidelink for Relay discovery message while in RRC_IDLE, RRC_INACTIVE or RRC_CONNECTED. The network may broadcast a threshold, which is used by the U2N Remote UE to determine if it can transmit Relay discovery solicitation messages to U2N Relay UE(s).
The U2N Relay UE can perform Relay discovery message (i.e. as specified in TS 23.304) transmission and may monitor the sidelink for Relay discovery message while in RRC_IDLE, RRC_INACTIVE or RRC_CONNECTED. The network may broadcast a maximum Uu RSRP threshold, a minimum Uu RSRP threshold, or both, which are used by the U2N Relay UE to determine if it can transmit Relay discovery messages to U2N Remote UE(s).
The network may provide the Relay discovery configuration using broadcast or dedicated signalling for Relay discovery. In addition, the U2N Remote UE and U2N Relay UE may use pre-configuration for Relay discovery.
The resource pool(s) used for NR sidelink communication can be used for Relay discovery or the network may configure a resource pool(s) dedicated for Relay discovery. Resource pool(s) dedicated for Relay discovery can be configured simultaneously with resource pool(s) for NR sidelink communication in system information, dedicated signalling and/or pre-configuration. Whether a dedicated resource pool(s) for Relay discovery is configured is based on network implementation. If resource pool(s) dedicated for Relay discovery are configured, only those resource pool(s) dedicated for Relay discovery shall be used for Relay discovery. If only resource pool(s) for NR sidelink communication are configured, all the configured transmission resource pool(s) can be used for Relay discovery and sidelink communication.
For U2N Remote UE (including both in-coverage and out of coverage cases) that has been connected to the network via a U2N Relay UE, only resource allocation mode 2 is used for discovery message transmission.
The Relay discovery reuses NR sidelink resource allocation principles for in-coverage U2N Relay UE, and for both in-coverage and out of coverage U2N Remote UEs.
The sidelink power control for the transmission of Relay discovery messages is same as for NR sidelink communication.
No ciphering or integrity protection in PDCP layer is applied for the Relay discovery messages.
The UE can determine from SIB12 whether the gNB supports Relay discovery, Non-Relay discovery, or both.
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16.12.4  Relay Selection/ReselectionWord‑p. 164

The U2N Remote UE performs radio measurements at PC5 interface and uses them for U2N Relay selection and reselection along with higher layer criteria, as specified in TS 23.304. When there is no unicast PC5 connection between the U2N Relay UE and the U2N Remote UE, the U2N Remote UE uses SD-RSRP measurements to evaluate whether PC5 link quality towards a U2N Relay UE satisfies relay selection criterion.
For relay reselection, U2N Remote UE uses SL-RSRP measurements towards the serving U2N Relay UE for relay reselection trigger evaluation when there is data transmission from U2N Relay UE to U2N Remote UE, and it is left to UE implementation whether to use SL-RSRP or SD-RSRP for relay reselection trigger evaluation in case of no data transmission from U2N Relay UE to U2N Remote UE.
A U2N Relay UE is considered suitable by a U2N Remote UE in terms of radio criteria if the PC5 link quality measured by U2N Remote UE towards the U2N Relay UE exceeds configured threshold (pre-configured or provided by gNB). The U2N Remote UE searches for suitable U2N Relay UE candidates that meet all AS layer and higher layer criteria (see TS 23.304). If there are multiple such suitable U2N Relay UEs, it is up to U2N Remote UE implementation to choose one U2N Relay UE among them. For L2 U2N Relay (re)selection, the PLMN ID and cell ID can be used as additional AS criteria.
The U2N Remote UE triggers U2N Relay selection in following cases:
  • Direct Uu signal strength of current serving cell of the U2N Remote UE is below a configured signal strength threshold;
  • Indicated by upper layer of the U2N Remote UE.
The U2N Remote UE may trigger U2N Relay reselection in following cases:
  • PC5 signal strength of current U2N Relay UE is below a (pre)configured signal strength threshold;
  • Cell (re)selection, handover or Uu RLF has been indicated by U2N Relay UE via PC5-RRC signalling;
  • When Remote UE receives a PC5-S link release message from U2N Relay UE;
  • When U2N Remote UE detects PC5 RLF;
  • Indicated by upper layer.
For L2 U2N Remote UEs in RRC_IDLE/INACTIVE and L3 U2N Remote UEs, the cell (re)selection procedure and relay (re)selection procedure run independently. If both suitable cells and suitable U2N Relay UEs are available, it is up to UE implementation to select either a cell or a U2N Relay UE. A L3 U2N Remote UE may select a cell and a U2N Relay UE simultaneously and this is up to implementation of L3 U2N Remote UE.
For both L2 and L3 U2N Relay UEs in RRC_IDLE/INACTIVE, the PC5-RRC message(s) are used to inform their connected Remote UE(s) when U2N Relay UEs select a new cell. The PC5-RRC message(s) are also used to inform their connected L2 or L3 U2N Remote UE(s) when L2/L3 U2N Relay UE performs handover or detects Uu RLF. Upon reception of the PC5 RRC message for notification, it is up to U2N Remote UE implementation whether to release or keep the unicast PC5 link. If U2N Remote UE decides to release the unicast PC5 link, it triggers the L2 release procedure and may perform relay reselection.
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16.12.5  Control plane procedures for L2 U2N RelayWord‑p. 165

16.12.5.1  RRC Connection ManagementWord‑p. 165

The U2N Remote UE needs to establish its own PDU sessions/DRBs with the network before user plane data transmission.
The NR V2X PC5 unicast link establishment procedures can be reused to setup a secure unicast link between U2N Remote UE and U2N Relay UE before U2N Remote UE establishes a Uu RRC connection with the network via U2N Relay UE.
The establishment of Uu SRB1/SRB2 and DRB of the U2N Remote UE is subject to Uu configuration procedures for L2 UE-to-Network Relay.
The following high level connection establishment procedure in Figure 16.12.5.1-1 applies to L2 U2N Relay:
Reproduction of 3GPP TS 38.300, Fig. 16.12.5.1-1: Procedure for L2 U2N Remote UE connection establishment
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Step 1.
The U2N Remote and U2N Relay UE perform discovery procedure, and establish PC5-RRC connection using NR V2X procedure.
Step 2.
The U2N Remote UE sends the first RRC message (i.e., RRCSetupRequest) for its connection establishment with gNB via the Relay UE, using a specified PC5 Relay RLC channel configuration. If the U2N Relay UE is not in RRC_CONNECTED, it needs to do its own connection establishment upon reception of a message on the specified PC5 Relay RLC channel. During Relay UE's RRC connection establishment procedure, gNB may configure SRB0 relaying Uu Relay RLC channel to the U2N Relay UE. The gNB responds with an RRCSetup message to U2N Remote UE. The RRCSetup message is sent to the U2N Remote UE using SRB0 relaying channel over Uu and a specified PC5 Relay RLC channel over PC5.
Step 3.
The gNB and U2N Relay UE perform relaying channel setup procedure over Uu. According to the configuration from gNB, the U2N Relay/Remote UE establishes an PC5 Relay RLC channel for relaying of SRB1 towards the U2N Remote/Relay UE over PC5.
Step 4.
The RRCSetupComplete message is sent by the U2N Remote UE to the gNB via the U2N Relay UE using SRB1 relaying channel over PC5 and SRB1 relaying channel configured to the U2N Relay UE over Uu. Then the U2N Remote UE is RRC connected over Uu.
Step 5.
The U2N Remote UE and gNB establish security following Uu procedure and the security messages are forwarded through the U2N Relay UE.
Step 6.
The gNB sends an RRCReconfiguration message to the U2N Remote UE via the U2N Relay UE, to setup the SRB2/DRBs for relaying purpose. The U2N Remote UE sends an RRCReconfigurationComplete message to the gNB via the U2N Relay UE as a response. In addition, the gNB configures additional Uu Relay RLC channels between the gNB and U2N Relay UE, and PC5 Relay RLC channels between U2N Relay UE and U2N Remote UE for the relay traffic.
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16.12.5.2  Radio Link FailureWord‑p. 166

The U2N Remote UE in RRC_CONNECTED suspends Uu RLM (as described in clause 9.2.7) when U2N Remote UE is connected to gNB via U2N Relay UE.
The U2N Relay UE declares Radio Link Failure (RLF) following the same criteria as described in clause 9.2.7.
After RLF is declared, the U2N Relay UE takes the following action on top of the actions described in clause 9.2.7:
  • a PC5-RRC message can be used for sending an indication to its connected U2N Remote UE(s), which may trigger RRC connection re-establishment for U2N Remote UE.
Upon detecting PC5 RLF, the U2N Remote UE may trigger connection re-establishment.
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16.12.5.3  RRC Connection Re-establishmentWord‑p. 166

The U2N Remote UE may perform the following actions during the RRC connection re-establishment procedure:
  • If only suitable cell(s) are available, the U2N Remote UE initiates RRC re-establishment procedure towards a suitable cell;
  • If only suitable U2N Relay UE(s) are available, the U2N Remote UE initiates RRC re-establishment procedure towards a suitable relay UE's serving cell;
  • If both a suitable cell and a suitable relay are available, the U2N Remote UE can select either one to initiate RRC re-establishment procedure based on implementation.
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16.12.5.4  RRC Connection ResumeWord‑p. 166

The RRC connection resume mechanism described in clause 9.2.2 is applied to U2N Remote UE.

16.12.5.5  System InformationWord‑p. 166

The in-coverage U2N Remote UE is allowed to acquire any necessary SIB(s) over Uu interface irrespective of its PC5 connection to Relay UE. The U2N Remote UE can also receive the system information from the Relay UE after PC5 connection establishment with U2N Relay UE.
The U2N Remote UE in RRC_CONNECTED can use the on-demand SIB framework as specified in TS 38.331 to request the SIB(s) via U2N Relay UE. The U2N Remote UE in RRC_IDLE or RRC_INACTIVE can inform U2N Relay UE of its requested SIB type(s) via PC5-RRC message. Then, U2N Relay UE triggers on-demand SI/SIB acquisition procedure as specified in TS 38.331 according to its own RRC state (if needed) and sends the acquired SI(s)/SIB(s) to U2N Remote UE via PC5-RRC.
Any SIB that the RRC_IDLE or RRC_INACTIVE U2N Remote UE has a requirement to use (e.g., for relay purpose) can be requested by the U2N Remote UE (from the U2N Relay UE or the network). For SIBs that have been requested by the U2N Remote UE from the U2N Relay UE, the U2N Relay UE forwards them again in case of any update for requested SIB(s). In case of RRC_CONNECTED U2N Remote UE(s), it is the responsibility of the network to send updated SIB(s) to U2N Remote UE(s) when they are updated. The U2N Remote UE de-configures SI request with U2N Relay UE when entering into RRC_CONNECTED state.
For SIB1 forwarding, for U2N Remote UE, both request-based delivery (i.e., SIB1 request by the U2N Remote UE) and unsolicited forwarding are supported by U2N Relay UE, of which the usage is left to U2N Relay UE implementation. If SIB1 changes, for U2N Remote UE in RRC_IDLE or RRC_INACTIVE, the U2N Relay UE always forwards SIB1.
For the L2 U2N Remote UE in RRC_IDLE or RRC_INACTIVE, the short message over Uu interface is not forwarded by the L2 U2N Relay UE to the L2 U2N Remote UE. The L2 U2N Relay UE can forward PWS SIBs to its connected L2 U2N Remote UE(s).
RAN sharing is supported for L2 U2N Relay UE. In particular, the L2 U2N Relay UE may forward, via discovery message, cell access related information before the establishment of a PC5-RRC connection.
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16.12.5.6  PagingWord‑p. 167

When both U2N Relay UE and U2N Remote UE are in RRC IDLE or RRC INACTIVE, the U2N Relay UE monitors paging occasions of its connected U2N Remote UE(s). When a U2N Relay UE needs to monitor paging for a U2N Remote UE, the U2N Relay UE should monitor all POs of the U2N Remote UE.
When U2N Relay UE is in RRC CONNECTED and U2N Remote UE(s) is in RRC_IDLE or RRC_INACTIVE, there are two options for paging delivery:
  • The U2N Relay UE monitors POs of its connected U2N Remote UE(s) if the active DL BWP of U2N Relay UE is configured with CORESET and paging search space;
  • The delivery of the U2N Remote UE's paging can be performed through dedicated RRC message from the gNB to the U2N Relay UE. The dedicated RRC message for delivering Remote UE paging to the RRC_CONNECTED Relay UE may contain one or more Remote UE IDs (5G-S-TMSI or I-RNTI).
It is up to network implementation to decide which of the above two options to use. The U2N Relay UE in RRC CONNECTED, if configured with paging search space, can determine whether to monitor POs for a U2N Remote UE based on PC5-RRC signalling received from the U2N Remote UE.
The U2N Remote UE in RRC_IDLE provides 5G-S-TMSI and UE specific DRX cycle (configured by upper layer) to the U2N Relay UE to request it to perform PO monitoring. The U2N Remote UE in RRC_INACTIVE provides minimum value of two UE specific DRX cycles (configured by upper layer and configured by RAN), 5G-S-TMSI and I-RNTI to the U2N Relay UE for PO monitoring. The L2 U2N Relay UE can notify Remote UE information (i.e. 5G-S-TMSI/I-RNTI) to the gNB via SidelinkUEInformationNR message for paging delivery purpose. The U2N Relay UE receives paging messages to check the 5G-S-TSMI/I-RNTI and sends relevant paging record to the Remote UE accordingly.
The U2N Relay UE can use unicast signalling to send paging to the U2N Remote UE via PC5.
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16.12.5.7  Access ControlWord‑p. 167

The U2N Remote UE performs unified access control as defined in TS 38.331. The U2N Relay UE in RRC-CONNECTED does not perform UAC for U2N Remote UE's data.

16.12.5.8  Mobility Registration Update and RAN Area UpdateWord‑p. 167

The L2 U2N Remote UE performs Mobility Registration Update/RNAU based on the L2 U2N Relay UE's serving cell when it is connected with the L2 U2N Relay UE. A L2 U2N Remote UE in RRC_IDLE or RRC_INACTIVE initiates Mobility Registration Update/RNAU procedure if the serving cell changes (due to cell change by the U2N Relay UE) and the new serving cell is outside of the U2N Remote UE's configured RNA/TA.

16.12.6  Service Continuity for L2 U2N relayWord‑p. 168

16.12.6.1  Switching from indirect to direct pathWord‑p. 168

For service continuity of L2 U2N Relay, the following procedure is used, in case of U2N Remote UE switching to direct path:
Reproduction of 3GPP TS 38.300, Fig. 16.12.6.1-1: Procedure for U2N Remote UE switching to direct Uu cell
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Step 1.
The Uu measurement configuration and measurement report signalling procedures are performed to evaluate both relay link measurement and Uu link measurement. The measurement results from U2N Remote UE are reported when configured measurement reporting criteria are met. The sidelink relay measurement report shall include at least U2N Relay UE's source L2 ID, serving cell ID (i.e., NCGI), and sidelink measurement quantity information. The sidelink measurement quantity can be SL-RSRP of the serving U2N Relay UE, and if SL-RSRP is not available, SD-RSRP is used.
Step 2.
The gNB decides to switch the U2N Remote UE onto direct Uu path.
Step 3.
The gNB sends RRCReconfiguration message to the U2N Remote UE. The U2N Remote UE stops UP and CP transmission via U2N Relay UE after reception of RRCReconfiguration message from the gNB.
Step 4.
The U2N Remote UE synchronizes with the gNB and performs Random Access.
Step 5.
The UE (i.e., U2N Remote UE in previous steps) sends the RRCReconfigurationComplete to the gNB via direct path, using the configuration provided in the RRCReconfiguration message. From this step, the UE (i.e., U2N Remote UE in previous steps) uses the RRC connection via the direct path to the gNB.
Step 6.
The gNB sends RRCReconfiguration message to the U2N Relay UE to reconfigure the connection between the U2N Relay UE and the gNB. The RRCReconfiguration message to the U2N Relay UE can be sent any time after step 3 based on gNB implementation (e.g., to release Uu and PC5 Relay RLC channel configuration for relaying, and bearer mapping configuration between PC5 RLC and Uu RLC).
Step 7.
Either U2N Relay UE or U2N Remote UE can initiate the PC5 unicast link release (PC5-S). The timing to execute link release is up to UE implementation. The U2N Relay UE can execute PC5 connection reconfiguration to release PC5 Relay RLC channel for relaying upon reception of RRC Reconfiguration by gNB in Step 6, or the UE (i.e., previous U2N Remote UE) can execute PC5 connection reconfiguration to release PC5 Relay RLC channel for relaying upon reception of RRCReconfiguration by gNB in Step 3.
Step 8.
The data path is switched from indirect path to direct path between the UE (i.e., previous U2N Remote UE) and the gNB. The DL/UL lossless delivery during the path switch is done according to PDCP data recovery procedure.
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16.12.6.2  Switching from direct to indirect pathWord‑p. 169

The gNB can select a U2N Relay UE in any RRC state i.e., RRC_IDLE, RRC_INACTIVE, or RRC_CONNECTED, as a target U2N Relay UE for direct to indirect path switch.
For service continuity of L2 U2N Remote UE, the following procedure is used, in case of the L2 U2N Remote UE switching to indirect path via a U2N Relay UE in RRC_CONNECTED:
Reproduction of 3GPP TS 38.300, Fig. 16.12.6.2-1: Procedure for U2N Remote UE switching to indirect path
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Step 1.
The U2N Remote UE reports one or multiple candidate U2N Relay UE(s) and Uu measurements, after it measures/discovers the candidate U2N Relay UE(s):
  • The UE may filter the appropriate U2N Relay UE(s) according to Relay selection criteria before reporting. The UE shall report only the U2N Relay UE candidate(s) that fulfil the higher layer criteria;
  • The reporting can include at least U2N Relay UE ID, U2N Relay UE' s serving cell ID, and sidelink measurement quantity information. The sidelink measurement quantity can be SL-RSRP of the candidate U2N Relay UE, and if SL-RSRP is not available, SD-RSRP is used.
Step 2.
The gNB decides to switch the U2N Remote UE to a target U2N Relay UE. Then the gNB sends an RRCReconfiguration message to the target U2N Relay UE, which can include at least Remote UE's local ID and L2 ID, Uu and PC5 Relay RLC channel configuration for relaying, and bearer mapping configuration.
Step 3.
The gNB sends the RRCReconfiguration message to the U2N Remote UE. The contents in the RRCReconfiguration message can include at least U2N Relay UE ID, PC5 Relay RLC channel configuration for relay traffic and the associated end-to-end radio bearer(s). The U2N Remote UE stops UP and CP transmission over Uu after reception of RRCReconfiguration message from the gNB.
Step 4.
The U2N Remote UE establishes PC5 connection with target U2N Relay UE
Step 5.
The U2N Remote UE completes the path switch procedure by sending the RRCReconfigurationComplete message to the gNB via the Relay UE.
Step 6.
The data path is switched from direct path to indirect path between the U2N Remote UE and the gNB.
In case the selected U2N Relay UE for direct to indirect path switch is in RRC_IDLE or RRC_INACTIVE, after receiving the path switch command, the U2N Remote UE establishes a PC5 link with the U2N Relay UE and sends the RRCReconfigurationComplete message via the U2N Relay UE, which will trigger the U2N Relay UE to enter RRC_CONNECTED state. The procedure for U2N Remote UE switching to indirect path in Figure 16.12.6.2-1 can be also applied for the case that the selected U2N Relay UE for direct to indirect path switch is in RRC_IDLE or RRC_INACTIVE with the exception that step 4 is performed before step 2.
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