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 communicates with a ProSe-enabled UE through a Public Land Mobile Network (PLMN) and assists the ProSe discovery and the ProSe direct communication. The ProSe function is a logical function that is used for PLMN-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”.
The ProSe direct communication in 3GPP Release 12 is one example of the inter-terminal direct communication. Similarly to the ProSe in the 3GPP Release 12, inter-terminal direct communication in a public land mobile network (PLMN) includes the discovery phase and direct communication phase that are assisted by a function or a node (e.g., ProSe function) located 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”.
The term “public land mobile network” in this specification indicates a wide-area radio infrastructure network, and means a multiple-access type mobile communication system. The multiple-access mobile communication system enables mobile terminals to perform radio communication substantially simultaneously by sharing radio resources including at least one of time resources, frequency resources, and transmission power resources among the mobile terminals. Typical examples of multiple-access technology include Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA), Code Division Multiple Access (CDMA), Orthogonal Frequency Division Multiple Access (OFDMA), and any combination thereof. The public land mobile network includes a radio access network and a core network. Examples of the public land mobile network include a 3GPP Universal Mobile Telecommunications System (UMTS), a 3GPP Evolved Packet System (EPS), a 3GPP2 CDMA2000 system, a Global System for Mobile communications (GSM (Registered Trademark))/General packet radio service (GPRS) system, a WiMAX system, and a mobile WiMAX system. The EPS includes a Long Term Evolution (LTE) system and an LTE-Advanced system.
Patent Literature 1 discloses that in response to detecting congestion in a network or a failure in a network node (e.g., base station), a server located in the network selects a terminal group to switch to inter-terminal direct communication and transmits, to radio terminals belonging to this terminal group, information about permission for direct communication and configuration information for a communication environment for the direct communication. The information about permission for direct communication indicates, for example, an identifier of radio terminals belonging to the terminal group. The configuration information for the communication environment indicates, for example, a frequency band, transmission power, and an identifier of a receiving terminal, which are used for the inter-terminal direct communication.