16.12.4 Relay Selection/Reselection

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 [48]. 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 candidate parent 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 [48]). 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, NID, CAG 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, Uu/PC5 RLF, relay reselection, PC5 link release with parent UE 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 UE in RRC_IDLE or RRC_INACTIVE, the PC5-RRC message is used to inform their connected U2N Remote UE(s) or child UE(s) when U2N Relay UE selects a new cell. If the PC5-RRC message for informing is triggered by Uu/PC5 RLF, the intermediate U2N Relay UE in RRC_IDLE or RRC_INACTIVE can send the PC5-RRC message after it performs a recovery action successfully. If the recovery action is failed, it is the intermediate U2N Relay UE implementation whether to release or send the PC5-RRC message to the U2N Remote UE(s) or child UE(s). The intermediate U2N Relay UE may omit sending the PC5-RRC message if the relay reselection or cell selection does not cause the change of the serving cell. The PC5-RRC message(s) are also used to inform their connected L2 or L3 child UE(s) when L2 or L3 U2N Relay UE performs handover, detects Uu/PC5 RLF, detects its Uu RRC connection establishment/resume fails, or is released PC5 unicast link with its parent UE. Upon reception of the PC5 RRC message for notification, it is up to U2N Remote UE/intermediate U2N Relay UE implementation whether to release or keep the unicast PC5 link with its parent UE. If U2N Remote UE/intermediate U2N Relay UE decides to release the unicast PC5 link, it triggers the PC5 release procedure and may perform cell or relay reselection. Upon reception of the PC5 RRC message for notification, it is up to intermediate U2N Relay UE implementation whether to trigger PC5 link release or send the notification message to the child UE(s).

For the discovery model A, the intermediate U2N Relay UE should send the discovery announcement message after PC5 connection establishes with its parent UE. There are no additional SD/SL-RSRP threshold criteria when forwarding discovery messages.

For the discovery model B, if the intermediate U2N Relay UE already has been established a PC5 connection with its parent UE, the intermediate U2N Relay UE may send a response message without forwarding the received discovery solicitation message to the parent UE. Otherwise, the intermediate Relay UE should forward the received discovery solicitation message to the candidate parent UE only when the PC5 RSRP between the child UE and itself is above a SD-RSRP threshold. Upon receiving the discovery solicitation message, a last U2N Relay UE without having a PC5 link between itself and an intermediate U2N Relay UE sends the discovery response message only when the PC5 RSRP between itself and child UE is above the SD-RSRP threshold. Upon receiving the discovery response message, the intermediate U2N Relay UE does not check the PC5 AS condition to forward the response message towards the U2N Remote UE since it assumed that the PC5 links have already been checked when the solicitation messages were forwarded. Upon receiving the discovery response message, the U2N Remote UE considers an intermediate Relay UE(s) as a candidate first U2N Relay UE(s) along the path to the last U2N Relay UE if the PC5 RSRP towards the first U2N Relay UE is above a SD-RSRP threshold.

The U2U Remote UE performs radio measurements (i.e., SD-RSRP and/or SL-RSRP) at PC5 interface and uses them for U2U Relay selection and reselection along with higher layer criteria, as specified in TS 23.304 [48].

For relay selection, U2U Remote UE uses SL-RSRP measurements towards the peer U2U Remote UE for relay selection trigger evaluation when valid SL-RSRP measurements are available. For relay reselection, U2U Remote UE uses SL-RSRP measurement towards the U2U Relay UE for relay reselection trigger evaluation when there is data transmission from U2U Relay UE to U2U Remote UE. It is left to U2U Remote UE implementation whether to use SL-RSRP or SD-RSRP for relay selection or reselection trigger evaluation in case of no data transmission. The thresholds for SD-RSRP and SL-RSRP can be configured separately for the trigger evaluation of U2U relay selection or reselection. The same value(s) of the SD-RSRP and SL-RSRP thresholds, which is used for relay selection or reselection, are applied for all the discovery models including DCR with integrated discovery.

The U2U Remote UE may trigger U2U Relay selection in the following cases:

• When the SL-RSRP or SD-RSRP between U2U Remote UEs is below a (pre)configured signal strength threshold;
• When U2U Remote UE receives an indication to trigger U2U relay selection from the upper layer of the UE.

The U2U Remote UE may trigger U2U Relay reselection in the following cases:

• When the SL-RSRP or SD-RSRP of the current U2U Relay UE is below a (pre)configured signal strength threshold;
• When U2U Remote UE receives an indication from the upper layer due to detecting PC5 RLF;
• When L2 U2U Remote UE receives an indication from the upper layer due to receiving the PC5 RLF indication from the L2 U2U Relay UE;
• When U2U Remote UE receives a PC5-S link release message from U2U Relay UE;
• When U2U Remote UE receives an indication to trigger U2U relay reselection from the upper layer of the UE.

For the discovery model A, the U2U Relay UE should announce via discovery announcement message only the neighbour U2U Remote UE(s) for which the SD-RSRP/SL-RSRP between the U2U Relay and the neighbour U2U Remote UE(s) is above a configured threshold. Upon discovery message reception, U2U Remote UE considers a U2U Relay UE as a candidate U2U Relay UE if the SD-RSRP towards the U2U Relay UE is above a configured threshold and the upper layer criteria are met.

For the discovery model B, when the U2U Relay UE receives the discovery solicitation message from U2U Remote UE, the U2U Relay UE forwards the discovery solicitation message only if the SD-RSRP between the U2U Relay UE and the U2U Remote UE is above a threshold. After the peer U2U remote UE receives a discovery solicitation message from the U2U Relay UE, the peer U2U Remote UE transmits the discovery response message only if the SD-RSRP between the peer U2U Remote UE and the U2U Relay UE is above a configured threshold. Upon discovery response message reception forwarded by the U2U Relay UE, the U2U Remote UE considers a U2U Relay UE as a candidate U2U Relay UE if the SD-RSRP towards the U2U Relay UE is above a configured threshold and the upper layer criteria are met.

For the DCR message with integrated discovery, when the U2U Relay UE receives the DCR message with integrated discovery from U2U Remote UE, the U2U Relay UE forwards the DCR message with integrated discovery only if the SL-RSRP between the U2U Relay UE and the U2U Remote UE is above a configured SD-RSRP threshold (not the SL-RSRP, as broadcast is used). Upon receiving DCR message with integrated discovery from one or multiple U2U Relay UEs, the peer U2U Remote UE should consider to which received DCR message to respond amongst candidate U2U Relay UEs towards which the SL-RSRP is above a configured SD-RSRP threshold (not the SL-RSRP, as broadcast is used) and that satisfy upper-layer criteria, and select a U2U Relay UE among them.