A method to use UE as a mobile device relay using PC5 link or sidelink to relay data traffic to/from end UEs for coverage extension is proposed. A relay UE can be the group head UE of a UE relay group including one or more remote UEs. A remote UE can be in-coverage or out-of-coverage of the serving cell served by a serving base station. In a first novel aspect, the network is aware of remote UE existence without direct Uu link. In addition, methods for resource allocation and interference management are proposed. In a second novel aspect, solutions for synchronization of remote UEs on PC5 link, and solutions for connection setup between relay UE and remote UEs on PC5 link are proposed.