16.12.1 General

Sidelink relay supports 5G ProSe UE-to-Network Relay (U2N Relay) function (specified in TS 23.304 [48]) to provide single/multi-hop 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 [48].

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 and, in case of multi-hop L2 U2N relay, between L2 U2N Relay UEs. 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.

In multi-hop U2N Relay, U2N Remote UE refers to both the actual U2N Remote UE and the intermediate U2N Relay UE that also functions as a U2N Remote UE. The intermediate U2N Relay UE can have its own traffic acting as a U2N Remote UE simultaneously.

For L2 U2N Relay, the L2 U2N Remote UE can only be configured to use resource allocation mode 2 (as specified in 5.7.2 and 16.9.3.1) for data to be relayed.

Sidelink relay additionally supports 5G ProSe UE-to-UE Relay (U2U Relay) function (specified in TS 23.304 [48]) to provide connectivity between U2U Remote UEs. Both L2 and L3 U2U Relay architectures are supported. The L3 U2U Relay architecture is transparent to the AS layer of the U2U Relay UE. The detailed architecture and procedures for L3 U2U Relay can be found in TS 23.304 [48].

A U2U Relay UE is used to provide coverage extension of the sidelink transmissions between two U2U Remote UEs. For the coverage extension, the U2U Remote UE can communicate with a peer U2U Remote UE(s), which are not reachable within the sidelink coverage, via the U2U Relay UE.

The U2U Relay UE and U2U Remote UE can be in any RRC state. The U2U Relay UE and the U2U Remote UEs can be in the coverage of the same or different cells or out-of-coverage. Both sidelink resource allocation modes, i.e., mode 1 and mode 2 are supported for the U2U Relay UE and U2U Remote UEs. For U2U Relay, a single PC5 unicast link is established between U2U Relay UE and each of the U2U Remote UEs. After PC5 unicast link establishment between U2U Relay UE and U2U Remote UEs, end-to-end PC5 unicast link connection establishment is performed between U2U Remote UEs. Only unicast is supported between U2U Relay UE and U2U Remote UEs.