5.3.1 General

23.3033GPPProximity-based services (ProSe)Release 17Stage 2TS

5.3.1.1 Overview

ProSe Direct Discovery is defined as the process that detects and identifies another UE in proximity using E-UTRA or WLAN direct radio signals.

There are two types of ProSe Direct Discovery: open and restricted. Open is the case where there is no explicit permission that is needed from the UE being discovered, whereas restricted discovery only takes place with explicit permission from the UE that is being discovered.

ProSe Direct Discovery can be a standalone service enabler that could for example use information from the discovered UE for certain applications in the UE that are permitted to use this information e.g. "find a taxi nearby", "find me a coffee shop". Additionally depending on the information obtained ProSe Direct Discovery can be used for subsequent actions e.g. to initiate ProSe Direct Communication.

ProSe-enabled non-Public Safety UEs which have obtained authorization to participate in ProSe Direct Discovery procedures shall not continue in participating in ProSe Direct Discovery procedures as soon as they detect loss of E-UTRA coverage in the serving PLMN.

With E-UTRA-based ProSe Direct Discovery the UE can use inter-PLMN discovery transmission based on the indication from the serving eNodeB or the provisioned radio resource on the UE. How the serving cell authorizes the UE to use inter-PLMN radio resource is specified in TS 36.331 [33].

5.3.1.2 ProSe Direct Discovery Models

The following models for ProSe Direct Discovery exist:

Model A ("I am here")

This model defines two roles for the ProSe-enabled UEs that are participating in ProSe Direct Discovery.

– Announcing UE: The UE announces certain information that could be used by UEs in proximity that have permission to discover.

– Monitoring UE: The UE that monitors certain information of interest in proximity of announcing UEs.

In this model the announcing UE broadcasts discovery messages at pre-defined discovery intervals and the monitoring UEs that are interested in these messages read them and process them.

NOTE: This model is equivalent to "I am here" since the announcing UE would broadcast information about itself e.g. its ProSe Application Code in the discovery message.

With E-UTRA-based ProSe Direct Discovery the UE can act as "announcing UE" only in the carrier frequency signalled by the serving PLMN but may act as a "monitoring" UE also in the resources of the serving PLMN and Local PLMNs, when using Model A mode. When inter-PLMN discovery transmission is supported, the carrier frequency may be operated by a PLMN other than the serving PLMN, as described in TS 36.331 [33].

Both open and restricted discovery types are supported by Model A.

Model B ("who is there?" / "are you there?")

This model when restricted discovery type is used, defines two roles for the ProSe-enabled UEs that are participating in ProSe Direct Discovery.

– Discoverer UE: The UE transmits a request containing certain information about what it is interested to discover.

– Discoveree UE: The UE that receives the request message can respond with some information related to the discoverer’s request.

It is equivalent to " who is there/are you there" since the discoverer UE sends information about other UEs that would like to receive responses from, e.g. the information can be about a ProSe Application Identity corresponding to a group and the members of the group can respond.

With E-UTRA-based ProSe Direct Discovery when using Model B discovery, the discoverer UE and discoveree UE can announce in the carrier frequency signalled by the serving PLMN. When inter-PLMN discovery transmission is supported, the carrier frequency may be operated by a PLMN other than the serving PLMN, as described in TS 36.331 [33]. The discoverer UE and discoveree UE are allowed to monitor or announce in the serving PLMN and Local PLMNs when authorized.

Only restricted discovery type is supported by Model B.

The Public Safety discovery is considered restricted. The monitoring UE/discoverer UE needs to have authorization (such as through pre-provisioned parameters) to perform discovery of the appropriate service(s).

5.3.1.3 WLAN-based ProSe Direct Discovery

This clause summarises the specifics of WLAN-based ProSe Direct Discovery in reference to the call flows for Discovery Request (clause 5.3.3) and Discovery Reporting (clause 5.3.4), without modifying the call flows themselves:

– The notion of "Local PLMN" in the sense of "PLMN operating the carrier frequency for inter-PLMN transmission" does not apply. The "Announcing PLMN ID" parameter in the Discovery Request for open discovery is therefore not used.

– In the Match Report the UE indicates that the report is for WLAN-based ProSe Direct Discovery for charging purposes.