Content for  TS 23.303  Word version:  18.0.0

In non-roaming case, when the ProSe Function in the HPLMN of the announcing UE receives a Monitor Request from a UE which is in the vicinity of the announcing UE and detects the associated Announcing Enabled indication stored in the announcing UE context, the ProSe Function triggers an Announcing Alert procedure towards the announcing UE in HPLMN.

The UE is only allowed to announce in the carrier frequency signalled from serving PLMN. When inter-PLMN discovery transmission is supported, the carrier frequency may be operated by Local PLMN. In roaming case or UE performs inter-PLMN discovery transmission, when the ProSe Function in the HPLMN of the announcing UE receives a Monitor Request from a UE which is in the vicinity of the announcing UE and detects the associated Announcing Enabled indication stored in the UE context, the ProSe Function triggers an Announcing Alert procedures towards the announcing UE in VPLMN or Local PLMN.

The following functions for public safety direct discovery are supported:

• UE-to-Network Relay Discovery.
• Determination is needed regarding within the ProSe Communication which user(s) are in ProSe Communication range at any given time (shortly referred to as "Group Member Discovery").

Group Member Discovery is a form of restricted discovery type in that only users that are affiliated with each other are able to discover each other (e.g. only users sharing the same Discovery Group ID).The parameters for Group Member Discovery, as well as the overall procedure, are different from the restricted discovery for non-Public Safety use. UE-to-Network Relay Discovery involves the use of pre-provisioned parameters to first discover a UE-to-Network Relay, and a subsequent communication link establishment. This makes the overall discovery procedure as restricted type, in that only Remote UEs with valid credentials and some form of pre-affiliation are able to successfully complete the overall procedure. Public Safety discovery for ProSe UE-to-Network Relay Discovery and Group Member Discovery uses the PC5-D protocol stack that is depicted in Figure 5.1.1.5.1-1. In the case of Direct Discovery for Public Safety (i.e. UE-to-Network Relay Discovery and Group Member Discovery), the ProSe Restricted Code is not used, and UEs use pre-configured or provisioned information for the Discovery procedures as defined in clause 4.5.1.1.2.3.2 and clause 4.5.1.1.2.3.4. Additional information not directly used for discovery can be also advertised using the PC5-D protocol stack, like relayed TMGIs, the ECGI of the service cell, in single or separate discovery messages of type "Relay Discovery Additional Information". Both Model A and Model B discovery are supported:

• Model A uses a single discovery protocol message (Announcement).
• Model B uses two discovery protocol messages (Solicitation and Response).

For Relay Discovery Additional Information, only Model A discovery is used. Depicted in Figure 5.3.7.1-1 is the procedure for public safety direct discovery with Model A.

For UE-to-Network Relay Discovery: Step 1.   For Group Member Discovery: Step 1.   For Relay Discovery Additional Information: Step 1. Depicted in Figure 5.3.7.1-2 is the procedure for public safety direct discovery with Model B.

For UE-to-Network Relay Discovery:   For Group Member Discovery: