3GPP Release 12 specifies Proximity-based services (ProSe) (see, for example, Non-patent Literature 1 and 2). The ProSe includes ProSe discovery and ProSe direct communication. The ProSe discovery identifies that radio terminals capable of performing ProSe direct communication (i.e., ProSe-enabled UEs) are in proximity of each other. In an example, the ProSe discovery can be performed through a procedure in which a ProSe-enabled UE detects another ProSe-enabled UE by using only capability of a radio communication technology (e.g., Evolved Universal Terrestrial Radio Access (E-UTRA) technology) possessed by these two UEs. In another example, the ProSe discovery can be performed by a radio access network (E-UTRA Network (E-UTRAN)) or a core network (Evolved Packet Core (EPC)).
The ProSe direct communication enables establishment of communication paths between two or more ProSe-enabled UEs that are in direct communication range after the ProSe discovery procedure is performed. In other words, the ProSe direct communication enables a ProSe-enabled UE to communicate with another ProSe-enabled UE directly without communicating through a base station (eNodeB). The ProSe direct communication may be performed by using a radio communication technology that is also used to access a base station (eNodeB) (i.e., E-UTRA technology) or by using a wireless local area network (WLAN) radio technology (i.e., IEEE 802.11 radio technology).
In 3GPP Release 12, a ProSe function in a core network (EPC) assists the ProSe discovery and the ProSe direct communication. The ProSe function is a logical function that is used for network-related operations required for the ProSe. The ProSe function provides functionality including, for example, (a) communication with third-party applications (ProSe Application Server), (b) authentication of UEs for ProSe discovery and ProSe direct communication, (c) transmission of configuration information (e.g., designation of radio resources and transmission power) for ProSe discovery and ProSe direct communication to UEs, and (d) provision of EPC-level ProSe discovery. The ProSe function may be implemented in one or more network nodes or entities. In this specification, one or more network nodes or entities that implement the ProSe function are referred to as a “ProSe function entity” or a “ProSe function server”.
3GPP Release 12 also defines ProSe UE-to-Network Relaying (see, for example, Sections 4.4.3, 4.5.4, and 5.4.4 of Non-patent Literature 2). The ProSe UE-to-Network Relay relays unicast traffic (downlink and uplink) between a remote UE and a network (E-UTRAN and EPC). More specifically, the ProSe UE-to-Network Relay attaches to a network as a UE, establishes a PDN connection for communication with a ProSe function entity or another Packet Data Network (PDN), and communicates with the ProSe function entity to start ProSe direct communication. Further, the ProSe UE-to-Network Relay executes a discovery procedure with a remote UE, communicates with the remote UE through the inter-terminal direct communication path, and relays unicast traffic (downlink and uplink) between the remote UE and the network. When Internet Protocol version 4 (IPv4) is used, the ProSe UE-to-Network Relay operates as a Dynamic Host Configuration Protocol Version 4 (DHCPv4) Server and Network Address Translation (NAT). When IPv6 is used, the ProSe UE-to-Network Relay operates as a stateless DHCPv6 Relay Agent. In this specification, a UE having the ProSe function and the relay function, like the ProSe UE-to-Network Relay, is referred to herein as a “relay UE”, and a UE to which the relay UE provides a relay service is referred to herein as a “remote UE”.
The ProSe direct communication in 3GPP Release 12 is one example of the inter-terminal direct communication. Similarly to the ProSe in 3GPP Release 12, inter-terminal direct communication in a mobile communication network includes the discovery phase and direct communication phase that are assisted by a function or a node (e.g., ProSe function) in the network. The inter-terminal direct communication is performed between two or more radio terminals in proximity of each other without communicating through any network node (e.g., a base station). The inter-terminal direct communication is also referred to as “device-to-device (D2D) communication” or “peer-to-peer communication”. The ProSe direct communication is an example of the inter-terminal direct communication and is also referred to as “ProSe communication”.