5.3.7 Direct Discovery for Public Safety use

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

5.3.7.1 General

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.

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.

Figure 5.3.7.1-1: Public safety direct discovery with Model A

For UE-to-Network Relay Discovery:

1. The ProSe UE-to-Network Relay sends a UE-to-Network Relay Discovery Announcement message. The parameters contained in this message are described in clause 4.6.4.3.

For Group Member Discovery:

1. The announcing UE sends a Group Member Discovery Announcement message. The parameters contained in this message are described in clause 4.6.4.9.

NOTE: A UE may send multiple Group Member Discovery Announcement messages (Model A) if the UE belongs to more than one discovery group.

For Relay Discovery Additional Information:

1. The ProSe UE-to-Network Relay sends a Relay Discovery Additional Information message. The parameters contained in this message are described in clause 4.6.4.10.

Depicted in Figure 5.3.7.1-2 is the procedure for public safety direct discovery with Model B.

Figure 5.3.7.1-2: Public safety direct discovery with Model B

For UE-to-Network Relay Discovery:

1. The Remote UE sends a UE-to-Network Relay Discovery Solicitation message. The parameters contained in this message are described in clause 4.6.4.3.

2. The ProSe UE-to-Network Relays that match the values of the Relay Service Code contained in the solicitation message respond to the Remote UE with a UE-to-Network Relay Discovery Response message. The parameters contained in this message are described in clause 4.6.4.3.

For Group Member Discovery:

1. The discoverer UE sends a Group Member Discovery Solicitation message. The parameters contained in this message are described in clause 4.6.4.9.

2. The discoveree UEs that match the values of the parameters contained in the solicitation message based on the Discovery Group ID, respond to the discoverer UE with a Group Member Discovery Response message. The parameters contained in this message are described in clause 4.6.4.9.