6.3 5G ProSe Direct Discovery

23.3043GPPProximity based Services (ProSe) in the 5G System (5GS)Release 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

6.3.1 5G ProSe Direct Discovery with 5G DDNMF

6.3.1.1 Overview

5G ProSe Direct Discovery is defined as the process that detects and identifies another UE in proximity using NR radio signals. There are two types of 5G ProSe Direct Discovery supported over PC3a reference point: open and restricted, as defined in TS 23.303 [3]. 5G ProSe Direct Discovery can be a standalone service or can be used for subsequent actions e.g. to initiate 5G ProSe Direct Communication.

ProSe-enabled UEs which have obtained authorization to participate in 5G ProSe Direct Discovery shall not continue in participating in 5G ProSe Direct Discovery procedures over PC3a reference point defined in clause 6.3.1 when they detect loss of NG-RAN coverage in the serving PLMN.

With 5G ProSe Direct Discovery, the UE can use inter-PLMN discovery transmission based on the indication from the serving NG-RAN 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 38.331 [16].

6.3.1.2 Overall procedure for 5G ProSe Direct Discovery (Model A)

Figure 6.3.1.2-1: Overall procedure for Model A 5G ProSe Direct Discovery

This procedure is applied for open and restricted 5G ProSe Direct Discovery when the ProSe enabled UE is served by NG-RAN.

1. Service authorisation for 5G ProSe Direct Discovery services is performed for as defined in clause 6.2.

If the UE is authorised to announce:

2a. When the UE is triggered to announce, then it sends a discovery request for announcing to the 5G DDNMF in HPLMN as defined in clause 6.3.1.4. In addition, for restricted 5G ProSe Direct Discovery, the 5G DDNMF further interacts with the ProSe Application server for the authorization of the discovery request.

3a. If the request is successful and is provided with ProSe Application Code/ProSe Restricted Code, it starts announcing on PC5 interface.

For ProSe restricted discovery and UE requests "on demand" announcing, ProSe Restricted Code may be provided to UE after this procedure. In this case, UE waits for the ProSe Restricted Code allocation and starts to announce the ProSe Restricted Code on PC5 after receiving it in Announcing Alert procedure specified in clause 6.3.1.6.

NOTE 1: More details on the Access Stratum protocol of this step are provided in RAN specifications.

If the UE is authorised to monitor:

2b. When the UE is triggered to monitor, it sends a discovery request for monitoring to the 5G DDNMF as defined in clause 6.3.1.4. In addition, for restricted 5G ProSe Direct Discovery, the 5G DDNMF further interacts with the ProSe Application server for the authorization of the discovery request.

3b. If the request is successful and the UE is provided with a Discovery Filter consisting of ProSe Application Code(s)/ProSe Restricted Code(s) and/or ProSe Application Mask(s), it starts monitoring for these ProSe Application Codes/ProSe Restricted Codes on the PC5 interface.

NOTE 2: More details on the Access Stratum protocol of this step are provided in RAN specifications.

4b. When the UE detects that one or more ProSe Application Code(s)/ProSe Restricted Code(s) that match the filter (see clause 5.8.1), it reports the ProSe Application Code(s)/ProSe Restricted Code(s) to the 5G DDNMF as defined in clause 6.3.1.5.

Non-roaming direct discovery procedures cover the case where both the "announcing UE" and "monitoring UE" are served by their respective HPLMN. Roaming direct discovery procedures cover the other cases.

6.3.1.3 Overall procedure for 5G ProSe Direct Discovery (Model B)

Figure 6.3.1.3-1: Overall procedure for Model B 5G ProSe Direct Discovery

This procedure is applied for restricted 5G ProSe Direct Discovery when the ProSe enabled UE is served by NG-RAN.

1. Service authorisation for 5G ProSe Direct Discovery services is performed as defined in clause 6.2.

If the UE is authorised to perform restricted 5G ProSe Direct Discovery, Model B, as a Discoveree UE, the following steps take place:

2a. When the UE is triggered to perform restricted 5G ProSe Direct Discovery, Model B, it sends a discovery request to the 5G DDNMF in the HPLMN to obtain a ProSe Response Code as defined in clause 6.3.1.4. The 5G DDNMF further interacts with ProSe Application Server for the authorization of the discovery request.

3a. If the request is successful and the UE is provided with a ProSe Response Code and an associated Discovery Query Filter(s), then the UE starts monitoring for the ProSe Query Code on PC5 interface.

4a. If a received ProSe Query Code matches any of the Discovery Query Filter(s), the UE announces the associated ProSe Response Code on the PC5 interface.

NOTE 1: More details on the Access Stratum protocol of this step are provided in RAN specifications.

If the UE is authorised to perform restricted 5G ProSe Direct Discovery, Model B, as a Discoverer UE, the following steps take place:

2b. When the UE is triggered to perform restricted 5G ProSe Direct Discovery, Model B, it sends a discovery request to the 5G DDNMF in the HPLMN for a ProSe Query Code as defined in clause 6.3.1.4. The 5G DDNMF further interacts with ProSe Application Server for the authorization of the discovery request.

3b. If the request is successful and the UE is provided with a ProSe Query Code and the Discovery Response Filter(s) consisting of ProSe Response Code(s) and ProSe Application Mask(s), the UE announces the ProSe Query Code on the PC5 interface.

4b. The UE starts to monitor on PC5 interface for any ProSe Response Code(s) that might match the Discovery Response Filter(s).

NOTE 2: More details on the Access Stratum protocol of this step are provided in RAN specifications.

5b. When the UE detects a match for one or more ProSe Response Code(s), it reports the ProSe Response Code to the 5G DDNMF as defined in clause 6.3.1.5.

Non-roaming direct discovery procedures cover the case where both the Discoveree UE and Discoverer UE are served by their respective HPLMN. Roaming direct discovery procedures cover the other cases.

6.3.1.4 Discovery Request procedures

The Discovery Request procedure can be used by the "announcing UE" or "monitoring UE" in order to be authorised to access the discovery resources and perform 5G ProSe Direct Discovery. The exact signalling procedures involving the UE, the 5G DDNMFs, and the ProSe Application Server are specified in TS 23.303 [3] clause 5.3.3, with the following modifications:

– the 5G DDNMF takes the role of "ProSe Function" in the procedure;

– Upon receiving a Discovery Request for restricted discovery from a UE, if the 5G DDNMF does not have a valid PDUID for that UE, the 5G DDNMF searches the PCF for the UE using Nbsf_Management_Subscribe Request and BSF provides the address of the PCF for the UE in Nbsf_Management_Subscribe Response as defined in the clause 5.2.13.2.6 in TS 23.502 [5]. The 5G DDNMF gets the PDUID and subscribes to notifications on Change of PDUID using Npcf_AMPolicyAuthorization_Subscribe from the PCF for the UE as defined in the clause 5.2.5.8.6 in TS 23.502 [5], including the SUPI, Event ID set to "Change of PDUID", and immediate reporting flag to indicate that the current PDUID value should be provided to the consumer. The PCF provides the PDUID and its validity timer.

NOTE: If the address of the PCF for the UE is changed, the BSF notifies the 5G DDNMF of the changed PCF address in Nbsf_Management_Notify as defined in the clause 5.2.13.2.8 in TS 23.502 [5].

– At the time the PCF generates a new PDUID, if the subscription to "Change of PDUID" is active it sends Npcf_AMPolicyAuthorization_Notify to the 5G DDNMF to report a new PDUID and its validity timer.

– the HSS is replaced by UDM;

– the E-UTRAN is replaced by NG-RAN and E-UTRA is replaced with NR;

– corresponding 5GS identifiers replace the EPS identifiers, e.g. use SUPI instead of IMSI, and use GPSI instead of MSISDN;

– PC5_tech parameter is omitted and the intended PC5 radio technology is NR.

The Discovery Request procedure can also be used by the Discoveree UE or the Discoverer UE in order to be authorised to access the discovery resources and perform 5G ProSe Direct Discovery, Model B. The exact signalling procedures are defined in TS 23.303 [3] clause 5.3.3A, with the same modifications as in the above list apply.

The events reported by the PCF, described in clause 6.1.3.18 in TS 23.503 [9] are extended to report "Change of PDUID" to the 5G DDNMF.

6.3.1.5 Discovery Reporting procedures

The Discovery Reporting procedure can be used by the "monitoring UE" (in Model A) and Discoverer UE (in Model B) to request the 5G DDNMF to resolve a matched ProSe Discovery Code(s) (ProSe Application Code for open discovery, and ProSe Restricted Code for restricted discovery) and obtain the corresponding ProSe Application ID(s) or RPAUID, and additional information, e.g. metadata.

The signalling procedures for the "monitoring UE" (in Model A) is specified in TS 23.303 [3] clause 5.3.4, and the signalling procedures for the Discoverer UE (in Model B) is specified in TS 23.303 [3] clause 5.3.4A, with the following modifications:

– the 5G DDNMF takes the role of "ProSe Function" in the procedure;

– the HSS is replaced by UDM;

– corresponding 5GS identifiers replace the EPS identifiers, e.g. use SUPI instead of IMSI, and use GPSI instead of MSISDN;

– PC5_tech parameter is omitted and the intended PC5 radio technology is NR.

6.3.1.6 Announcing Alert Procedures for restricted discovery

When supported by the 5G DDNMF and the UE, the Announcing Alert procedure allows the 5G DDNMF to postpone the ProSe Restricted Code allocation, so that the announcing UE would be only triggered by this procedure to announce when the 5G DDNMF receives a Monitor Request from a UE in the vicinity of the announcing UE. This procedure is an optional step of the Discovery Request procedure defined in clause 6.3.1.4.

The signalling procedure of Announcing Alert Procedure is specified in TS 23.303 [3] clause 5.3.5, with the same modifications listed in clause 6.3.1.4.

6.3.1.7 Direct Discovery Update Procedures

The 5G DDNMF can at any time update/revoke a previously allocated ProSe Application Code, or Discovery Filters. The UE can decide at any time to stop announcing a ProSe Application Code or monitoring set of Discovery Filter(s). The Direct Discovery Update procedure as specified in TS 23.303 [3] clause 5.3.6A.1 allows both the 5G DDNMF and the UE to update or revoke the previously authorized discovery. In the defined signalling procedures, the 5G DDNMF(s) takes the role of the "ProSe Function".

A user may decide at any time to change the discovery permissions relating to other users in a ProSe Application Server; then the corresponding ProSe Application Server triggers the procedure as specified in clause 5.3.6A.2 of TS 23.303 [3] towards the affected 5G DDNMF(s) to update/revoke the discovery permissions. In the defined signalling procedures, the 5G DDNMF(s) takes the role of the "ProSe Function".

6.3.2 5G ProSe Direct Discovery procedures over PC5 reference point

6.3.2.1 General

A PC5 communication channel is used to carry the discovery message over PC5 and the discovery message over PC5 is differentiated from other PC5 messages by AS layer.

Both Model A and Model B discovery as defined in TS 23.303 [3] are supported:

– Model A uses a single discovery protocol message (Announcement).

– Model B uses two discovery protocol messages (Solicitation and Response).

Depicted in figure 6.3.2.1-1 is the procedure for 5G ProSe Direct Discovery with Model A.

Figure 6.3.2.1-1: 5G ProSe direct discovery with Model A

1. The Announcing UE sends an Announcement message. The Announcement message may include the Type of Discovery Message, ProSe Application Code or ProSe Restricted Code, security protection element, [metadata information]. The Application layer metadata information may be included as metadata in the Announcement message.

The Destination Layer-2 ID and Source Layer-2 ID used to send the Announcement message are specified in clause 5.8.1.2 and clause 5.8.1.3.

The Monitoring UE determines the Destination Layer-2 ID for signalling reception. The Destination Layer-2 ID is configured with the UE(s) as specified in clause 5.8.1.2.

Depicted in figure 6.3.2.1-2 is the procedure for 5G ProSe Direct Discovery with Model B.

Figure 6.3.2.1-2: 5G ProSe direct discovery with Model B

1. The Discoverer UE sends a Solicitation message. The Solicitation message may include Type of Discovery Message, ProSe Query Code, security protection element.

The Destination Layer-2 ID and Source Layer-2 ID used to send the Solicitation message are specified in clause 5.8.1.2 and clause 5.8.1.3.

How the Discoveree UE determines the Destination Layer-2 ID for signalling reception is specified in clause 5.8.1.2.

2. The Discoveree UE that matches the solicitation message responds to the Discoverer UE with the Response message. The Response message may include Type of Discovery Message, ProSe Response Code, security protection element, [metadata information]. The Application layer metadata information may be included as metadata in the Response message.

The Source Layer-2 ID used to send the Response message is specified in clause 5.8.1.3. The Destination Layer-2 ID is set to the Source Layer-2 ID of the received Solicitation message.

NOTE: Details of security protection element will be defined by SA WG3.

6.3.2.2 Group Member Discovery

6.3.2.2.1 General

Group Member Discovery is applicable to public safety use and commercial services. To perform Group Member Discovery, the UE is configured with the related information as described in clause 5.2.

Group Member Discovery is a form of restricted discovery in that only users that are affiliated with each other can discover each other (e.g. only users sharing the same Application Layer Group ID).

In the case of Public Safety use, the ProSe Restricted Code is not used for Group Member Discovery, and pre-configured or provisioned information for the Discovery procedures as defined in clause 5.2 is used.

NOTE: The Group Member Discovery performed by Application Layer in coordination with Application Server is out of scope of this specification.

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).

6.3.2.2.2 Procedure for Group Member Discovery with Model A

Depicted in Figure 6.3.2.2.2-1 is the procedure for Group Member Discovery with Model A.

Figure 6.3.2.2.2-1: Group Member Discovery with Model A

1. The announcing UE sends a Group Member Discovery Announcement message. The Group Member Discovery Announcement message includes the Type of Discovery Message, Announcer Info and Application Layer Group ID (See clause 5.8.1).

The Destination Layer-2 ID and Source Layer-2 ID used to send the Group Member Discovery Announcement message are specified in clause 5.8.1.2 and clause 5.8.1.3.

The Monitoring UE determines the Destination Layer-2 ID for signalling reception as specified in clause 5.8.1.2.

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

6.3.2.2.3 Procedure for Group Member Discovery with Model B

Depicted in Figure 6.3.2.2.3-1 is the procedure for Group Member Discovery with Model B.

Figure 6.3.2.2.3-1: Group Member Discovery with Model B

1. The discoverer UE sends a Group Member Discovery Solicitation message. The Group Member Discovery Solicitation message includes the Type of Discovery Message, Discoverer Info, Application Layer Group ID and optionally Target Info (see clause 5.8.1).

The Destination Layer-2 ID and Source Layer-2 ID used to send the Group Member Discovery Solicitation message are specified in clause 5.8.1.2 and clause 5.8.1.3.

How the Discoveree UE determines the Destination Layer-2 ID for signalling reception is specified in clause 5.8.1.2.

2. The discoveree UEs that match the values of the parameters (including Application Layer Group ID and Target Info) contained in the solicitation message, responds to the discoverer UE with a Group Member Discovery Response message. The Group Member Discovery Response message includes the Type of Discovery Message, Discoveree Info and Application Layer Group ID (see clause 5.8.1).

The Source Layer-2 ID used to send the Group Member Discovery Response message is specified in clause 5.8.1.3. The Destination Layer-2 ID is set to the Source Layer-2 ID of the received Group Member Discovery Solicitation message.

6.3.2.3 5G ProSe UE-to-Network Relay Discovery

6.3.2.3.1 General

5G ProSe UE-to-Network Relay Discovery is applicable to both 5G ProSe Layer-3 and Layer-2 UE-to-Network relay discovery for public safety use and commercial services. To perform 5G ProSe UE-to-Network Relay Discovery, the 5G ProSe Remote UE and the 5G ProSe UE-to-Network Relay are pre-configured or provisioned with the related information as described in clause 5.1.

In 5G ProSe UE-to-Network Relay Discovery, the UEs use pre-configured or provisioned information for the relay discovery procedures as defined in clause 5.1.4.1.

The Relay Service Code (RSC) is used in the 5G ProSe UE-to-Network Relay discovery, to indicate the connectivity service the 5G ProSe UE-to-Network Relay provides to the 5G ProSe Remote UE. The RSCs are configured on the 5G ProSe UE-to-Network Relay and the 5G ProSe Remote UE as defined in clause 5.1.4. The 5G ProSe UE-to-Network Relay and the 5G ProSe Remote UE are aware of whether a RSC is offering 5G ProSe Layer-2 or Layer-3 UE-to-Network Relay service based the policy as specified in clause 5.1.4. A 5G ProSe UE-to-Network Relay supporting multiple RSCs can advertise the RSCs using multiple discovery messages, with one RSC per discovery message.

Additional information not directly used for discovery can also be advertised using the PC5-D protocol stack in single or separate discovery messages of type "Relay Discovery Additional Information" as defined in clause 5.8.3.1.

6.3.2.3.2 Procedure for 5G ProSe UE-to-Network Relay Discovery with Model A

Depicted in Figure 6.3.2.3.2-1 is the procedure for 5G ProSe UE-to-Network Discovery with Model A.

Figure 6.3.2.3.2-1: 5G ProSe UE-to-Network Relay Discovery with Model A

1. The 5G ProSe UE-to-Network Relay sends a UE-to-Network Relay Discovery Announcement message. The UE-to-Network Relay Discovery Announcement message contains the Type of Discovery Message, Announcer Info and RSC, and is sent using the Source Layer-2 ID and Destination Layer-2 ID as described in clause 5.8.3.

For 5G ProSe Layer-3 UE-to-Network Relay, the 5G ProSe Layer-3 UE-to-Network Relay shall only include a RSC in the UE-to-Network Relay Discovery Announcement when the S-NSSAI associated with that RSC belongs to the Allowed NSSAI of the UE-to-Network Relay.

The 5G ProSe Remote UE (1 to 3) determines the Destination Layer-2 ID for signalling reception. The Destination Layer-2 ID is configured with the UE(s) as specified in clause 5.1.4.1.

5G ProSe Remote UE (1 to 3) monitors announcement messages with the 5G ProSe UE-to-Network RSC corresponding to the desired services.

Optionally, the 5G ProSe UE-to-Network Relay may also send Relay Discovery Additional Information messages as defined in clause 6.5.1.3. The parameters contained in this message and the Source Layer-2 ID and Destination Layer-2 ID used for sending and receiving the message are described in clause 5.8.3.

The 5G ProSe Remote UE selects the 5G ProSe UE-to-Network Relay based on the information received in step 1.

NOTE: Access Stratum layer information used for 5G ProSe UE-to-Network Relay selection is specified in RAN specifications.

6.3.2.3.3 Procedure for 5G ProSe UE-to-Network Relay Discovery with Model B

Depicted in Figure 6.3.2.3.3-1 is the procedure for 5G ProSe UE-to-Network Relay Discovery with Model B.

Figure 6.3.2.3.3-1: 5G ProSe UE-to-Network Relay Discovery with Model B

1. The 5G ProSe Remote UE sends a 5G ProSe UE-to-Network Relay Discovery Solicitation message. The 5G ProSe UE-to-Network Discovery Solicitation message contains the Type of Discovery Message, Discoverer Info, RSC and optionally Target Info, and is send using the Source Layer-2 ID and Destination Layer-2 ID as described in clause 5.8.3. The 5G ProSe Remote UE discovering a 5G ProSe UE-to-Network Relay sends a solicitation message with the RSC which is associated to the desired connectivity service. The RSC is based on the Policy/Parameters specified in clause 5.1.4.1.

How the 5G ProSe UE-to-Network Relays (1 to 3) determine the Destination Layer-2 ID for signalling reception is specified in clause 5.8.3. The Destination Layer-2 ID is configured with the UE(s) as specified in clause 5.1.4.1.

2. The 5G ProSe UE-to-Network Relays (1 and 2) that match the values of the RSC contained and the Target Info, if any, in the solicitation message respond to the 5G ProSe Remote UE with a UE-to-Network Relay Discovery Response message. The 5G ProSe UE-to-Network Relay Discovery Response message contains the Type of Discovery Message, Discoveree Info and RSC, and is sent using the Source Layer-2 ID and Destination Layer-2 ID as described in clause 5.8.3.

For 5G ProSe Layer-3 UE-to-Network Relay, the 5G ProSe UE-to-Network Relay shall only respond to a matching RSC in the UE-to-Network Relay Discovery Solicitation message when the S-NSSAI associated with that RSC belongs to the Allowed NSSAI of the 5G ProSe UE-to-Network Relay.

The 5G ProSe Remote UE selects the 5G ProSe UE-to-Network Relay based on the information received in step 2.