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Content for  TS 38.300  Word version:  17.2.0

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16.12  Sidelink Relay |R17|p. 166

16.12.1  Generalp. 166

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 NG-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 L2 U2N Relay UE and L2 U2N Remote UE shall be in RRC_CONNECTED to perform transmission/ reception of relayed unicast data; and
  • The L2 U2N Relay UE can be in RRC_IDLE, RRC_INACTIVE or RRC_CONNECTED as long as all the L2 U2N Remote UE(s) that are connected to the L2 U2N Relay UE are either in RRC_INACTIVE or in RRC_IDLE.
A single unicast link is established between one L2 U2N Relay UE and one L2 U2N Remote UE. The traffic to the NG-RAN of L2 U2N Remote UE via a given L2 U2N Relay UE and the traffic of the L2 U2N Relay UE shall be separated in different Uu RLC channels.
For L2 U2N Relay, the L2 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.
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16.12.2  Protocol Architecturep. 166

16.12.2.1  L2 UE-to-Network Relayp. 166

The protocol stacks for the user plane and control plane of L2 U2N Relay architecture are illustrated 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 the 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 header is not present over PC5 hop, but the SRAP header 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 performs 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 Uu Radio Bearers (SRBs or DRBs) of the same L2 U2N Remote UE and/or different L2 U2N Remote UEs can be multiplexed over the same egress 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 end-to-end 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 end-to-end Uu Radio Bearer of the L2 U2N Remote UE;
  • The PC5 SRAP sublayer at the L2 U2N Remote UE supports UL bearer mapping between L2 U2N Remote UE end-to-end Uu Radio Bearers and egress PC5 Relay RLC channels.
For L2 U2N Relay, for downlink:
  • The Uu SRAP sublayer performs DL bearer mapping at gNB to map end-to-end Uu Radio Bearer (SRB, DRB) of L2 U2N Remote UE into Uu Relay RLC channel. The Uu SRAP sublayer performs 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 L2 U2N Relay UE Uu interface;
  • The Uu SRAP sublayer supports L2 U2N Remote UE identification for DL traffic. The identity information of L2 U2N Remote UE end-to-end Uu Radio Bearer and a local Remote UE ID are included into the Uu SRAP header by the gNB at DL for the L2 U2N Relay UE to enable DL bearer mapping between ingress Uu Relay RLC channels and egress PC5 Relay RLC channel;
  • The PC5 SRAP sublayer at the L2 U2N Relay UE performs DL bearer mapping between ingress Uu Relay RLC channels and egress PC5 Relay RLC channels;
  • The PC5 SRAP sublayer at the L2 U2N Remote UE correlates the received packets with the right PDCP entity associated with the given end-to-end Radio Bearer of the L2 U2N Remote UE based on the identity information included in the PC5 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(s) to be used in SRAP header. L2 U2N Remote UE obtains the local Remote ID from the gNB via Uu RRC messages including RRCSetup, RRCReconfiguration, RRCResume and RRCReestablishment.
The end-to-end DRB(s) or end-to-end SRB(s), except SRB0, of L2 U2N Remote UE can be multiplexed to the PC5 Relay RLC channels and Uu Relay RLC channels in both PC5 hop and Uu hop, but an end-to-end DRB and an end-to-end SRB can neither be mapped into the same PC5 Relay RLC channel nor be mapped into the same Uu Relay RLC channel.
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 UE ID via RRCReconfiguration message. 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 Discoveryp. 168

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 illustrated 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 or configure via dedicated RRC signalling a Uu RSRP threshold, which is used by the U2N Remote UE to determine if it can transmit Relay discovery 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 or configure via dedicated RRC signalling 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 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 dedicated resource pool(s) for Relay discovery are 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 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 Relay discovery message transmission.
For in-coverage U2N Relay UE, and for both in-coverage and out of coverage U2N Remote UEs, NR sidelink resource allocation principles are applied for Relay discovery message transmission.
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 U2N Remote UE and U2N Relay UE can determine from SIB12 whether the gNB supports Relay discovery, or Non-Relay discovery, or both.
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16.12.4  Relay Selection/Reselectionp. 169

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 reselection, handover, Uu RLF, or Uu RRC connection establishment/resume failure has been indicated by U2N Relay UE via PC5-RRC signalling;
  • When U2N 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 or RRC_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 the U2N Remote UE implementation to select either a cell or a U2N Relay UE. A L3 U2N Remote UE may select a cell and a L3 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 or RRC_INACTIVE, the PC5-RRC message(s) are used to inform their connected U2N 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 or L3 U2N Relay UE performs handover, detects Uu RLF, or its Uu RRC connection establishment/resume fails. 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 PC5 release procedure and may perform cell or relay reselection.
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16.12.5  Control plane procedures for L2 U2N Relayp. 170

16.12.5.1  RRC Connection Managementp. 170

The L2 U2N Remote UE needs to establish its own PDU sessions/DRBs with the network before user plane data transmission.
The NR sidelink PC5 unicast link establishment procedures can be used to setup a secure unicast link between L2 U2N Remote UE and L2 U2N Relay UE before L2 U2N Remote UE establishes a Uu RRC connection with the network via L2 U2N Relay UE.
The establishment of Uu SRB1/SRB2 and DRB of the L2 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 a L2 U2N Relay and L2 U2N Remote UE:
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 L2 U2N Remote and L2 U2N Relay UE perform discovery procedure, and establish a PC5-RRC connection using the NR sidelink PC5 unicast link establishment procedure.
Step 2.
The L2 U2N Remote UE sends the first RRC message (i.e., RRCSetupRequest) for its connection establishment with gNB via the L2 U2N Relay UE, using a specified PC5 Relay RLC channel configuration. If the L2 U2N Relay UE is not in RRC_CONNECTED, it needs to do its own Uu RRC connection establishment upon reception of a message on the specified PC5 Relay RLC channel. After L2 U2N Relay UE's RRC connection establishment procedure, gNB configures SRB0 relaying Uu Relay RLC channel to the U2N Relay UE. The gNB responds with an RRCSetup message to L2 U2N Remote UE. The RRCSetup message is sent to the L2 U2N Remote UE using SRB0 relaying Uu Relay RLC channel over Uu and a specified PC5 Relay RLC channel over PC5.
Step 3.
The gNB and L2 U2N Relay UE perform relaying channel setup procedure over Uu. According to the configuration from gNB, the L2 U2N Relay/Remote UE establishes a PC5 Relay RLC channel for relaying of SRB1 towards the L2 U2N Remote/Relay UE over PC5.
Step 4.
The RRCSetupComplete message is sent by the L2 U2N Remote UE to the gNB via the L2 U2N Relay UE using SRB1 relaying channel over PC5 and SRB1 relaying channel configured to the L2 U2N Relay UE over Uu. Then the L2 U2N Remote UE is as in RRC_CONNECTED with the gNB.
Step 5.
The L2 U2N Remote UE and gNB establish security following the Uu security mode procedure and the security messages are forwarded through the L2 U2N Relay UE.
Step 6.
The gNB sends an RRCReconfiguration message to the L2 U2N Remote UE via the L2 U2N Relay UE, to setup the end-to-end SRB2/DRBs of the L2 U2N Remote UE. The L2 U2N Remote UE sends an RRCReconfigurationComplete message to the gNB via the L2 U2N Relay UE as a response. In addition, the gNB may configure additional Uu Relay RLC channels between the gNB and L2 U2N Relay UE, and PC5 Relay RLC channels between L2 U2N Relay UE and L2 U2N Remote UE for the relaying traffic.
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16.12.5.2  Radio Link Failurep. 171

The L2 U2N Remote UE in RRC_CONNECTED suspends Uu RLM (as described in clause 9.2.7) when connected to the gNB via a L2 U2N Relay UE.
The L2 U2N Relay UE declares Uu Radio Link Failure (RLF) following the same criteria as described in clause 9.2.7.
After Uu RLF is declared, the L2 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 L2 U2N Remote UE(s), which may trigger RRC connection re-establishment for L2 U2N Remote UE; or
  • indicating to upper layer to trigger PC5 unicast link release.
Upon detecting PC5 RLF, the L2 U2N Remote UE may trigger RRC connection re-establishment.
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16.12.5.3  RRC Connection Re-establishmentp. 171

The L2 U2N Remote UE may perform the following actions during the RRC connection re-establishment procedure:
  • If only suitable cell(s) are available, the L2 U2N Remote UE initiates RRC re-establishment procedure towards a suitable cell;
  • If only suitable L2 U2N Relay UE(s) are available, the L2 U2N Remote UE initiates RRC re-establishment procedure towards a suitable relay UE's serving cell via selected suitable L2 U2N Relay;
  • If both a suitable cell and a suitable relay are available, the L2 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 Resumep. 171

The RRC connection resume procedure described in clause 9.2.2 is applied to L2 U2N Remote UE.

16.12.5.5  System Informationp. 172

The in-coverage L2 U2N Remote UE is allowed to acquire any necessary SIB(s) over Uu interface irrespective of its PC5 connection to L2 U2N Relay UE. The L2 U2N Remote UE can also receive the system information from the L2 U2N Relay UE after PC5 connection establishment with L2 U2N Relay UE.
The L2 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 L2 U2N Relay UE. The L2 U2N Remote UE in RRC_IDLE or RRC_INACTIVE can inform L2 U2N Relay UE of its requested SIB type(s) via PC5-RRC message. Then, L2 U2N Relay UE triggers on-demand SI/SIB acquisition procedure as specified in TS38.331 [12] according to its own RRC state (if needed) and sends the acquired SI(s)/SIB(s) to L2 U2N Remote UE via PC5-RRC message.
Any SIB that the RRC_IDLE or RRC_INACTIVE L2 U2N Remote UE has a requirement to use (e.g., for relay purpose) can be requested by the L2 U2N Remote UE (from the L2 U2N Relay UE or the network). For SIBs that have been requested by the L2 U2N Remote UE from the L2 U2N Relay UE, the L2 U2N Relay UE forwards them again in case of any update for requested SIB(s). In case of RRC_CONNECTED L2 U2N Remote UE(s), it is the responsibility of the network to send updated SIB(s) to L2 U2N Remote UE(s) when they are updated. The L2 U2N Remote UE de-configures SI request with L2 U2N Relay UE when entering into RRC_CONNECTED state.
For SIB1 forwarding, for L2 U2N Remote UE, both request-based delivery (i.e., SIB1 request by the U2N Remote UE) and unsolicited forwarding are supported by L2 U2N Relay UE, of which the usage is left to L2 U2N Relay UE implementation. If SIB1 changes, for L2 U2N Remote UE in RRC_IDLE or RRC_INACTIVE, the L2 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  Pagingp. 172

When both L2 U2N Relay UE and L2 U2N Remote UE are in RRC IDLE or RRC INACTIVE, the L2 U2N Relay UE monitors paging occasions of its connected L2 U2N Remote UE(s). When a L2 U2N Relay UE needs to monitor paging for a L2 U2N Remote UE, the L2 U2N Relay UE should monitor all POs of the L2 U2N Remote UE.
When L2 U2N Relay UE is in RRC_CONNECTED and L2 U2N Remote UE(s) is in RRC_IDLE or RRC_INACTIVE, there are two options for paging delivery:
  • The L2 U2N Relay UE monitors POs of its connected L2 U2N Remote UE(s) if the active DL BWP of the L2 U2N Relay UE is configured with CORESET and paging search space;
  • The delivery of the L2 U2N Remote UE's paging can be performed through a dedicated RRC message from the gNB to the L2 U2N Relay UE. The dedicated RRC message for delivering L2 U2N Remote UE paging to the RRC_CONNECTED L2 U2N 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 L2 U2N Relay UE in RRC_CONNECTED, if configured with paging search space, can determine whether to monitor POs for a L2 U2N Remote UE based on the indication within the PC5-RRC signalling received from the L2 U2N Remote UE.
The L2 U2N Remote UE in RRC_IDLE provides 5G-S-TMSI and UE specific DRX cycle (configured by upper layer) to the L2 U2N Relay UE for requesting to perform PO monitoring. The L2 U2N Remote UE in RRC_INACTIVE provides the minimum value of two UE specific DRX cycles (configured respectively by upper layer and NG-RAN), 5G-S-TMSI and I-RNTI to the L2 U2N Relay UE for PO monitoring. The L2 U2N Relay UE in RRC_CONNECTED can notify the L2 U2N Remote UE information (i.e. 5G-S-TMSI/I-RNTI) to the gNB via the SidelinkUEInformationNR message for paging delivery purpose. The L2 U2N Relay UE receives paging messages to check the 5G-S-TSMI/I-RNTI and sends relevant paging record to the L2 U2N Remote UE accordingly.
The L2 U2N Relay UE uses unicast signalling to send paging to the L2 U2N Remote UE via PC5.
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16.12.5.7  Access Controlp. 173

The L2 U2N Remote UE performs unified access control as defined in TS 38.331. The L2 U2N Relay UE does not perform UAC for L2 U2N Remote UE's data.

16.12.5.8  Mobility Registration Update and RAN Area Updatep. 173

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 L2 U2N Relay UE) and the new serving cell is outside of the L2 U2N Remote UE's configured RNA/TA.

16.12.6  Service Continuity for L2 U2N relayp. 173

16.12.6.0  Generalp. 173

The service continuity procedure is applicable only for the mobility cases of path switch from indirect to direct path, and from direct to indirect path when the L2 U2N Remote UE and L2 U2N Relay UE belong to the same gNB.

16.12.6.1  Switching from indirect to direct pathp. 173

For service continuity of L2 U2N Relay, the following procedure is used, in case of L2 U2N Remote UE switching to direct path:
Reproduction of 3GPP TS 38.300, Fig. 16.12.6.1-1: Procedure for L2 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 L2 U2N Remote UE are reported when configured measurement reporting criteria are met. The sidelink relay measurement report shall include at least L2 U2N Relay UE's source L2 ID, serving cell ID (i.e., NCGI/NCI), and sidelink measurement quantity result. The sidelink measurement quantity can be SL-RSRP of the serving L2 U2N Relay UE, and if SL-RSRP is not available, SD-RSRP is used.
Step 2.
The gNB decides to switch the L2 U2N Remote UE onto direct Uu path.
Step 3.
The gNB sends the RRCReconfiguration message to the L2 U2N Remote UE. The L2 U2N Remote UE stops UP and CP transmission via the L2 U2N Relay UE after reception of the RRCReconfiguration message with the path switch configuration.
Step 4.
The L2 U2N Remote UE synchronizes with the gNB and performs Random Access.
Step 5.
The UE (i.e., L2 U2N Remote UE in previous steps) sends the RRCReconfigurationComplete message to the gNB via the direct path, using the configuration provided in the RRCReconfiguration message. From this step, the UE (i.e., L2 U2N Remote UE in previous steps) uses the RRC connection via the direct path to the gNB.
Step 6.
The gNB sends the RRCReconfiguration message to the L2 U2N Relay UE to reconfigure the connection between the L2 U2N Relay UE and the gNB. The RRCReconfiguration message to the L2 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 related to the L2 U2N Remote UE).
Step 7.
Either L2 U2N Relay UE or L2 U2N Remote UE's AS layer can release PC5-RRC connection and indicates upper layers to release PC5 unicast link after receiving the RRCReconfiguration message from the gNB. The timing to execute link release is up to UE implementation.
Step 8.
The data path is switched from indirect path to direct path between the UE (i.e., previous L2 U2N Remote UE) and the gNB. The PDCP re-establishment or PDCP data recovery in uplink is performed by the UE (i.e., previous L2 U2N Remote UE) for lossless delivery during path switch if gNB configures it.
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16.12.6.2  Switching from direct to indirect pathp. 174

The gNB can select a L2 U2N Relay UE in any RRC state i.e., RRC_IDLE, RRC_INACTIVE, or RRC_CONNECTED, as a target L2 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 L2 U2N Relay UE in RRC_CONNECTED:
Reproduction of 3GPP TS 38.300, Fig. 16.12.6.2-1: Procedure for L2 U2N Remote UE switching to indirect path via a L2 U2N Relay UE in RRC_CONNECTED
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Step 1.
The L2 U2N Remote UE reports one or multiple candidate L2 U2N Relay UE(s) and Uu measurements, after it measures/discovers the candidate L2 U2N Relay UE(s):
  • The L2 U2N Remote UE filters the appropriate L2 U2N Relay UE(s) according to relay selection criteria before reporting. The L2 U2N Remote UE shall report only the L2 U2N Relay UE candidate(s) that fulfil the higher layer criteria;
  • The reporting includes at least a L2 U2N Relay UE ID, a L2 U2N Relay UE' s serving cell ID, and a sidelink measurement quantity information. SD-RSRP is used as sidelink measurement quantity.
Step 2.
The gNB decides to switch the L2 U2N Remote UE to a target L2 U2N Relay UE. Then the gNB sends an RRCReconfiguration message to the target L2 U2N Relay UE, which includes at least the L2 U2N 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 L2 U2N Remote UE. The RRCReconfiguration message includes at least the L2 U2N Relay UE ID, Remote UE's local ID, PC5 Relay RLC channel configuration for relay traffic and the associated end-to-end radio bearer(s). The L2 U2N Remote UE stops UP and CP transmission over the direct path after reception of the RRCReconfiguration message from the gNB.
Step 4.
The L2 U2N Remote UE establishes PC5 RRC connection with target L2 U2N Relay UE.
Step 5.
The L2 U2N Remote UE completes the path switch procedure by sending the RRCReconfigurationComplete message to the gNB via the L2 U2N Relay UE.
Step 6.
The data path is switched from direct path to indirect path between the L2 U2N Remote UE and the gNB.
In case the selected L2 U2N Relay UE for direct to indirect path switch is in RRC_IDLE or RRC_INACTIVE, after receiving the path switch command, the L2 U2N Remote UE establishes a PC5 link with the L2 U2N Relay UE and sends the RRCReconfigurationComplete message via the L2 U2N Relay UE, which triggers the L2 U2N Relay UE to enter RRC_CONNECTED state. The procedure for L2 U2N Remote UE switching to indirect path in Figure 16.12.6.2-1 can be also applied for the case that the selected L2 U2N Relay UE for direct to indirect path switch is in RRC_IDLE or RRC_INACTIVE with the exception that the RRCReconfiguration message is sent from the gNB to the L2 U2N Relay UE after the L2 U2N Relay UE enters RRC_CONNECTED state, which happens between step 4 and step 5.
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