16.12.3 Relay Discovery

Model A and Model B discovery models as defined in TS 23.304 [48] are supported for U2N/U2U Relay discovery. The protocol stack used for discovery is illustrated in Figure 16.12.3-1.

The U2N Remote UE can perform Relay discovery message (i.e., as specified in TS 23.304 [48]) 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 [48]) 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).

In multi-hop U2N relay, one U2N Relay UE establishes a PC5 connection with each of its child UE(s). One intermediate U2N Relay UE or U2N Remote UE maintains a single PC5 connection with its parent UE.

The U2U Remote UE and U2U Relay UE can perform Relay discovery message transmission or DCR/DCA message with integrated discovery transmission and may monitor for Relay discovery message or DCR/DCA message with integrated discovery while in coverage (i.e. RRC_IDLE, RRC_INACTIVE, or RRC_CONNECTED) or out-of-coverage.

The network may provide the Relay discovery configuration using broadcast or dedicated signalling. In addition, the U2N/U2U Remote UE, L3 U2N Relay UE and U2U Relay UE may use pre-configuration for Relay discovery. If the candidate intermediate U2N Relay UE is out of coverage without a PC5 connection to a parent UE, it can forward discovery messages based on pre-configuration. If the candidate intermediate U2N Relay UE is in coverage without a PC5 connection to a parent UE or out of coverage with a PC5 connection to a parent UE, it can forward discovery message based on SIB12 or dedicated configuration.

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 NR sidelink communication. Only the resource pool for NR sidelink communication is used for the DCR/DCA message with integrated discovery.

For U2N Remote UE (including both in-coverage and out of coverage cases) that has been connected to the network via single/multi-hop U2N Relay UE(s), 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.

For U2U Remote UE and U2U Relay UE, NR sidelink resource allocation principles, both mode 1 and mode 2, can be 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/U2U Remote UE and U2N/U2U Relay UE can determine from SIB12 whether the gNB supports Relay discovery, or Non-Relay discovery, or both.