10A ProSe direct discovery for public safety use

24.3343GPPProximity-services (ProSe) User Equipment (UE) to ProSe function protocol aspectsRelease 17Stage 3TS

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

10A.1 Overview

This clause describes the procedures for ProSe direct discovery for public safety use at a ProSe-enabled public safety UE over the PC5 reference point.

10A.2 Procedures

10A.2.1 General

The following procedures are defined for the ProSe direct discovery for public safety use:

– announcing UE procedure for UE-to-network relay discovery;

– monitoring UE procedure for UE-to-network relay discovery;

– discoverer UE procedure for UE-to-network relay discovery;

– discoveree UE procedure for UE-to-network relay discovery;

– announcing UE procedure for group member discovery;

– monitoring UE procedure for group member discovery;

– discoverer UE procedure for group member discovery;

– discoveree UE procedure for group member discovery;

– announcing UE procedure for UE-to-network relay discovery additional information; and

– monitoring UE procedure for UE-to-network relay discovery additional information;

Each ProSe-enabled Public Safety UE needs to obtain the security parameters from the ProSe Key Management Function before participating in ProSe direct discovery for public safety use, as specified in 3GPP TS 33.303 [6]. For each given Relay Service Code in UE-to-network relay discovery or Discovery Group ID in group member discovery, the ProSe Key Management Function (PKMF) will provide the following in the security parameters:

– PSDK (Public Safety Discovery Key) and the associated Expiry Time for this PSDK;

– configurations to signal which combination of keys to be used for the discovery process; and

– optionally, if DUCK is to be used, an indication of which PC5_DISCOVERY message fields shall be protected by the DUCK.

After receiving the PSDK from the PKMF for the relay service or discovery group, the UE shall use it to derive specific DUIK, DUCK and DUSK needed to protect the ProSe direct discovery messages for the corresponding public safety use, as specified in 3GPP TS 33.303 [6].

10A.2.1A Radio resource selection

The UE shall select the radio resource parameters to be used for ProSe direct discovery as follows:

– when the UE is served by E-UTRAN and intends to use the ProSe radio resources (i.e. carrier frequency) obtained from the serving cell,

1) the UE shall use the radio resource parameters indicated by the serving cell (same or different from that of the serving cell) as specified in 3GPP TS 36.331 [12], if the corresponding PLMN is authorised for ProSe direct discovery; and

2) according to the radio resources (i.e. carrier frequency) along with PLMN ID which the serving cell indicates as allowed for ProSe direct discovery as defined in 3GPP TS 36.331 [12], if the UE intends to use one of the radio resources not operated by the serving cell for ProSe direct discovery and the corresponding PLMN is authorised for ProSe direct discovery, the UE shall search for a cell with the indicated PLMN operating the indicated radio resources as defined in 3GPP TS 36.331 [12] and 3GPP TS 36.304 [23], and obtain the radio resource parameters for ProSe direct discovery from that cell, without performing PLMN selection; and

– when the UE is not served by E-UTRAN or when the UE is served by E-UTRAN and intends to use the provisioned ProSe radio resources (i.e. carrier frequency):

1) if the UE can determine itself located in a geographical area, and the UE is provisioned with radio parameters for the geographical area, then the UE shall search for a cell with any PLMN operating the selected provisioned radio resources (i.e. carrier frequency) associated with that geographical area, and:

NOTE: It is out of scope of the present specification to define how the UE can locate itself in a specific geographical area. When the UE is in coverage of a 3GPP RAT it can for example use information derived from the serving PLMN. When the UE is not in coverage of a 3GPP RAT it can use other techniques as determined by local regulations.

i) if the UE finds such a cell belonging to a PLMN in which the UE is authorised for ProSe direct discovery and the cell provides radio resource parameters for ProSe direct discovery, then the UE shall use the indicated radio resource parameters for ProSe direct discovery;

ii) if the UE finds such a cell but not in a PLMN authorised for ProSe direct discovery, then the UE shall not use the ProSe direct discovery in that carrier frequency; or

iii) if the UE does not find any such cell in any PLMN, then the UE shall use the provisioned radio resource parameters; or

2) else the UE shall not initiate ProSe direct discovery.

If the UE is performing ProSe direct discovery using radio resource parameters associated with a geographical area and moves out of that geographical area, the UE shall perform ProSe direct discovery using new radio resource parameters selected for ProSe direct discovery as described in this subclause.

10A.2.2 Announcing UE procedure for UE-to-network relay discovery

10A.2.2.1 General

The purpose of the announcing UE procedure for UE-to-network relay discovery is:

– to enable a ProSe-enabled public safety UE to announce availability of a connectivity service provided by a UE-to-network relay of the ProSe-enabled public safety UE to other ProSe-enabled public safety UEs, upon a request from upper layers as defined in 3GPP TS 23.303 [2]; or

– to enable a ProSe-enabled public safety UE to measure the PC5_DISCOVERY message signal strength between the ProSe-enabled public safety UE and the ProSe UE-to-network relay UE(s) for relay selection/reselection.

10A.2.2.2 Announcing UE procedure for UE-to-network relay discovery initiation

The UE is authorised to perform the announcing UE procedure for UE-to-network relay discovery if:

a) the UE is authorised to act as a UE-to-network relay in the PLMN indicated by the serving cell as specified in clause 5, and

– the UE is served by E-UTRAN and the UE is authorised to perform ProSe direct discovery for public safety use announcing in the PLMN as specified in clause 5, and the lower layers indicate that discovery operation of a UE-to-network relay is supported; or

– the UE is authorised to perform ProSe direct discovery for public safety use announcing when not served by E-UTRAN as specified in clause 5 and intends to use the provisioned radio resources for UE-to-network relay discovery; and

b) the UE is configured with the Relay Service Code parameter identifying the connectivity service to be announced and with the User Info ID for the UE-to-network relay discovery parameter, as specified in clause 5;

otherwise the UE is not authorised to perform the announcing UE procedure for UE-to-network relay discovery.

Figure 10A.2.2.2.1 illustrates the interaction of the UEs in the announcing UE procedure for UE-to-network relay discovery.

Figure 10A.2.2.2.1: Announcing UE procedure for UE-to-network relay discovery

When the UE is triggered by an upper layer application to announce availability of a connectivity service provided by a UE-to-network relay, if the UE is authorised to perform the announcing UE procedure for UE-to-network relay discovery, then the UE:

a) if the UE is served by E-UTRAN, and the UE in EMM-IDLE mode needs to request resources for sending PC5_DISCOVERY messages for relay discovery for public safety as specified in 3GPP TS 36.331 [12], shall perform a service request procedure or tracking area update procedure as specified in 3GPP TS 24.301 [11];

b) shall obtain a valid UTC time for the discovery transmission from the lower layers and generate the UTC-based counter corresponding to this UTC time as specified in subclause 12.2.2.18;

c) shall generate a PC5_DISCOVERY message for UE-to-Network Relay Discovery Announcement according to subclause 11.2.5.1. In the PC5_DISCOVERY message for UE-to-Network Relay Discovery Announcement, the UE:

1) shall set the ProSe Relay UE ID to a ProSe Relay UE ID used for ProSe direct communication for the connectivity service to be announced;

2) shall set the Announcer Info parameter to the User Info ID for the UE-to-network relay discovery parameter, configured in clause 5;

3) shall set the Relay Service Code parameter to the Relay Service Code parameter identifying the connectivity service to be announced, configured in clause 5;

4) shall set the UTC-based counter LSB parameter to include the eight least significant bits of the UTC-based counter; and

5) shall set the Resource Status Indicator bit of the Status Indicator parameter to indicate whether or not the UE has resources available to provide a connectivity service for additional ProSe-enabled public safety UEs;

d) shall apply the DUIK, DUSK, or DUCK with the associated Encrypted Bitmask, along with the UTC-based counter to the PC5_DISCOVERY message for whichever security mechanism(s) configured to be applied, e.g. integrity protection, message scrambling or confidentiality protection of one or more above parameters, as specified in 3GPP TS 33.303 [6]; and

e) shall pass the resulting PC5_DISCOVERY message for UE-to-Network Relay Discovery Announcement to the lower layers for transmission over the PC5 interface with an indication that the message is for relay discovery for public safety use.

The UE shall ensure that it keeps on passing the same PC5_DISCOVERY message and the indication that the message is for relay discovery for public safety use to the lower layers for transmission until the UE is triggered by an upper layer application to stop announcing availability of a connectivity service provided by a UE-to-network relay, or until the UE stops being authorised to perform the announcing UE procedure for UE-to-network relay discovery. How this is achieved is left up to UE implementation.

10A.2.2.3 Announcing UE procedure for UE-to-network relay discovery completion

When the UE is triggered by an upper layer application to stop announcing availability of a connectivity service provided by a UE-to-network relay, or when the UE stops being authorised to perform the announcing UE procedure for UE-to-network relay discovery, the UE shall instruct the lower layers to stop announcing.

When the UE stops announcing, if the UE is in EMM-CONNECTED mode, the UE shall trigger the corresponding procedure in lower layers as specified in 3GPP TS 36.331 [12].

10A.2.3 Monitoring UE procedure for UE-to-network relay discovery

10A.2.3.1 General

The purpose of the monitoring UE procedure for UE-to-network relay discovery is:

– to enable a ProSe-enabled public safety UE to become aware of proximity of a connectivity service provided by a UE-to-network relay, upon a request from upper layers as defined in 3GPP TS 23.303 [2]; or

– to enable a ProSe-enabled public safety UE to perform measurements of signal strength of PC5_DISCOVERY messages from ProSe UE-to-network relay UE(s) for relay selection/reselection.

10A.2.3.2 Monitoring UE procedure for UE-to-network relay discovery initiation

The UE is authorised to perform the monitoring UE procedure for UE-to-network relay discovery if:

a) the following is true:

1) the UE is not served by E-UTRAN, is authorised to perform ProSe direct discovery for public safety use monitoring when the UE is not served by E-UTRAN as specified in clause 5, is authorised to act as a remote UE towards a UE-to-network relay as specified in clause 5 and is configured with the radio parameters to be used for ProSe direct discovery for public safety use when not served by E-UTRAN;

2) the UE is served by E-UTRAN, is authorised to perform ProSe direct discovery for public safety use monitoring in at least one PLMN as specified in clause 5, is authorised to act as a remote UE towards a UE-to-network relay, and the lower layers indicate that discovery operation of a UE-to-network relay is supported; or

3) the UE is:

– in EMM-IDLE mode, in limited service state as specified in 3GPP TS 23.122 [24], and the reason for the UE being in limited service state is one of the following:

i) the UE is unable to find a suitable cell in the selected PLMN as specified in 3GPP TS 36.304 [23];

ii) the UE received an ATTACH REJECT message or a TRACKING AREA UPDATE REJECT message or a SERVICE REJECT message with the EMM cause #11 "PLMN not allowed" as specified in 3GPP TS 24.301 [11] or a LOCATION UPDATING REJECT message or a GPRS ATTACH REJECT message or ROUTING AREA UPDATE REJECT message or SERVICE REJECT message with cause #11 "PLMN not allowed" as specified in 3GPP TS 24.008 [30]; or

iii) the UE received an ATTACH REJECT message or a TRACKING AREA UPDATE REJECT message or a SERVICE REJECT message with the EMM cause #7 "EPS services not allowed" as specified in 3GPP TS 24.301 [11] or a LOCATION UPDATING REJECT message or a GPRS ATTACH REJECT message or ROUTING AREA UPDATE REJECT message or SERVICE REJECT message with cause #7 "GPRS services not allowed" as specified in 3GPP TS 24.008 [30]; and

– authorised to perform ProSe direct discovery for public safety use monitoring when the UE is not served by E-UTRAN as specified in clause 5, authorised to act as a remote UE towards a UE-to-network relay as specified in clause 5, and:

i) configured with the radio parameters to be used for ProSe direct discovery for public safety use when not served by E-UTRAN; or

ii) the lower layers indicate that discovery operation of a UE-to-network relay is supported; and:

NOTE 1: When the lower layers indicate that discovery operation of a UE-to-network relay is supported, the serving cell broadcasts a common radio resources pool for public safety discovery reception and the UE can use this common radio resources pool while in limited service state.

b) the UE is configured with the Relay Service Code parameter identifying the connectivity service to be monitored and with the IP version(s) to be used for the traffic of the connectivity service to be monitored, as specified in clause 5;

otherwise the UE is not authorised to perform the monitoring UE procedure for UE-to-network relay discovery.

Figure 10A.2.3.2.1 illustrates the interaction of the UEs in the monitoring UE procedure for UE-to-network relay discovery.

Figure 10A.2.3.2.1: Monitoring UE procedure for UE-to-network relay discovery

When the UE is triggered by an upper layer application to monitor proximity of a connectivity service provided by a UE-to-network relay; or when the UE has established a direct link with a ProSe UE-to-network relay UE as specified in subclause 10.4.2, and if the UE is authorised to perform the monitoring UE procedure for UE-to-network relay discovery, then the UE shall instruct the lower layers to start monitoring for PC5_DISCOVERY messages with an indication that the message is for relay discovery for public safety use.

Upon reception of a PC5_DISCOVERY message for UE-to-Network Relay Discovery Announcement according to subclause 11.2.5.1, for the target Relay Service Code of the connectivity service which the UE is authorized to monitor, the UE shall use the associated DUSK, if configured, and the UTC-based counter obtained during the monitoring operation to unscramble the PC5_DISCOVERY message as described in 3GPP TS 33.303 [6]. Then, if a DUCK is configured, the UE shall use the DUCK and the UTC-based counter to decrypt the configured message-specific confidentiality-protected portion, as described in 3GPP TS 33.303 [6]. Finally, if a DUIK is configured, the UE shall use the DUIK and the UTC-based counter to verify the MIC field in the unscrambled PC5_DISCOVERY message for UE-to-Network Relay Discovery Announcement.

NOTE 2: The use of an erroneous UTC-based counter for processing the received PC5_DISCOVERY messages at the ProSe-enabled Public Safety UE can cause MIC check failure after DUIK is used for integrity check, and malformed contents after DUSK is used for unscrambling or DUCK is used for deciphering. How a ProSe-enabled Public Safety UE ensures the accuracy of the UTC-based counter is left to UE implementation.

Then if:

– the Relay Service Code parameter of the PC5_DISCOVERY message for UE-to-Network Relay Discovery Announcement is the same as the Relay Service Code parameter configured as specified in clause 5 for the connectivity service being monitored; and

– the User Info ID of the UE-to-network relay is not configured as specified in clause 5 for the connectivity service being monitored, or the Announcer Info parameter of the PC5_DISCOVERY message for UE-to-Network Relay Discovery Announcement is the same as the User Info ID of the UE-to-network relay configured as specified in clause 5 for the connectivity service being monitored;

then the UE shall consider that the connectivity service the UE seeks to monitor has been discovered. In addition, the UE can measure the signal strength of the PC5_DISCOVERY message for UE-to-Network Relay Discovery Announcement for relay selection or reselection.

10A.2.3.3 Monitoring UE procedure for UE-to-network relay discovery completion

When the UE is triggered by an upper layer application to stop monitoring proximity of a connectivity service provided by a UE-to-network relay, or when the UE stops being authorised to perform the monitoring UE procedure for UE-to-network relay discovery, the UE shall instruct the lower layers to stop monitoring.

When the UE stops monitoring, if the UE is in EMM-CONNECTED mode, the UE shall trigger the corresponding procedure in lower layers as specified in 3GPP TS 36.331 [12].

10A.2.4 Discoverer UE procedure for UE-to-network relay discovery

10A.2.4.1 General

The purpose of the discoverer UE procedure for UE-to-network relay discovery is:

– to enable a ProSe-enabled public safety UE to solicit proximity of a connectivity service provided by a UE-to-network relay, upon a request from upper layers as defined in 3GPP TS 23.303 [2]; or

10A.2.4.2 Discoverer UE procedure for UE-to-network relay discovery initiation

The UE is authorised to perform the discoverer UE procedure for UE-to-network relay discovery if:

a) the following is true:

1) the UE is not served by E-UTRAN, is authorised to perform ProSe direct discovery for public safety use discoverer operation when the UE is not served by E-UTRAN as specified in clause 5, is authorised to act as a remote UE towards a UE-to-network relay as specified in clause 5 and is configured with the radio parameters to be used for ProSe direct discovery for public safety use when not served by E-UTRAN;

2) the UE is served by E-UTRAN, is authorised to perform ProSe direct discovery for public safety use discoverer operation in the PLMN indicated by the serving cell as specified in clause 5, is authorised to act as a remote UE towards a UE-to-network relay, and the lower layers indicate that discovery operation of a UE-to-network relay is supported; or

3) the UE is:

– in EMM-IDLE mode, in limited service state as specified in 3GPP TS 23.122 [24], and the reason for the UE being in limited service state is one of the following:

i) the UE is unable to find a suitable cell in the selected PLMN as specified in 3GPP TS 36.304 [23];

– authorised to perform ProSe direct discovery for public safety use discoverer operation when the UE is not served by E-UTRAN as specified in clause 5, authorised to act as a remote UE towards a UE-to-network relay as specified in clause 5, and:

i) configured with the radio parameters to be used for ProSe direct discovery for public safety use when not served by E-UTRAN; or

ii) the lower layers indicate that discovery operation of a UE-to-network relay is supported and that the UE does not need to request resources for sending PC5_DISCOVERY messages for public safety as specified in 3GPP TS 36.331 [12]; and

NOTE 1: When the lower layers indicate that discovery operation of a UE-to-network relay is supported and that the UE does not need to request resources for sending PC5_DISCOVERY messages for public safety as specified in 3GPP TS 36.331 [12], the serving cell broadcasts a common radio resources pool for public safety discovery transmission and the UE can use this common radio resources pool while in limited service state.

b) the UE is configured with the Relay Service Code parameter identifying the connectivity service to be solicited and with the User Info ID for the UE-to-network relay discovery parameter, as specified in clause 5;

otherwise the UE is not authorised to perform the Discoverer UE procedure for UE-to-network relay discovery.

Figure 10A.2.4.2.1 illustrates the interaction of the UEs in the Discoverer UE procedure for UE-to-network relay discovery.

Figure 10A.2.4.2.1: Discoverer UE procedure for UE-to-network relay discovery

For PC5_DISCOVERY message signal strength measurement, the UE manages a periodic measurement timer T4110, which is used to trigger the periodic PC5_DISCOVERY message signal strength measurement between the UE and the ProSe UE-to-network relay UE with which the UE has a link established. It is started whenever the UE has established a direct link with a ProSe UE-to-network relay UE as specified in subclause 10.4.2 and restarted whenever the UE receives the PC5_DISCOVERY message for UE-to-Network Relay Discovery Response from the ProSe UE-to-network relay UE with which the UE has a link established.

When the UE is triggered by an upper layer application to solicit proximity of a connectivity service provided by a UE-to-network relay, or when the periodic measurement timer T4110 expires, and if the UE is authorised to perform the discoverer UE procedure for UE-to-network relay discovery, then the UE:

b) shall obtain a valid UTC time for the discovery transmission from the lower layers and generate the UTC-based counter corresponding to this UTC time as specified in subclause 12.2.2.18;

c) shall generate a PC5_DISCOVERY message for UE-to-Network Relay Discovery Solicitation according to subclause 11.2.5.1. In the PC5_DISCOVERY message for UE-to-Network Relay Discovery Solicitation, the UE:

1) shall set the Discoverer Info parameter to the User Info ID for the UE-to-network relay discovery parameter, configured in clause 5;

2) shall set the Relay Service Code parameter to the Relay Service Code parameter identifying the connectivity service to be solicited, configured in clause 5;

3) if the PC5_DISCOVERY message for UE-to-Network Relay Discovery Solicitation is used to trigger the signal strength measurement for the PC5_DISCOVERY message from a specific ProSe UE-to-network relay UE with which the UE has a link established, shall set the ProSe Relay UE ID parameter to the ProSe Relay UE ID of that ProSe UE-to-network relay UE; and

4) shall set the UTC-based counter LSB parameter to include the four least significant bits of the UTC-based counter;

e) shall pass the resulting PC5_DISCOVERY message for UE-to-Network Relay Discovery Solicitation to the lower layers for transmission over the PC5 interface with an indication that the message is for relay discovery for public safety use.

If the PC5_DISCOVERY message for UE-to-Network Relay Discovery Solicitation is used to solicit proximity of a connectivity service provided by a UE-to-network relay, the UE shall ensure that it keeps on passing the PC5_DISCOVERY message for UE-to-Network Relay Discovery Solicitation and the indication that the message is for relay discovery for public safety use to the lower layers for transmission until the UE is triggered by an upper layer application to stop soliciting proximity of a connectivity service provided by a UE-to-network relay, or until the UE stops being authorised to perform the discoverer UE procedure for UE-to-network relay discovery. How this is achieved is left up to UE implementation.

If the PC5_DISCOVERY message for UE-to-Network Relay Discovery Solicitation is used to trigger the PC5_DISCOVERY message signal strength measurement between the UE and the ProSe UE-to-network relay UE with which the UE has a link established, the UE shall start the retransmission timer T4109. If retransmission timer T4109 expires, the UE shall retransmit the PC5_DISCOVERY message for UE-to-Network Relay Discovery Solicitation and restart timer T4109. If no response is received from the ProSe UE-to-network relay UE with which the UE has a link established after reaching the maximum number of allowed retransmissions, the UE shall trigger relay reselection procedure.

NOTE 2: The maximum number of allowed retransmissions is UE implementation specific.

Upon reception of a PC5_DISCOVERY message for UE-to-Network Relay Discovery Response according to subclause 11.2.5.1, for the target Relay Service Code of the connectivity service which the UE is authorized to discover, the UE shall use the associated DUSK, if configured, and the UTC-based counter obtained during the reception operation to unscramble the PC5_DISCOVERY message as described in 3GPP TS 33.303 [6]. Then, if a DUCK is configured, the UE shall use the DUCK and the UTC-based counter to decrypt the configured message-specific confidentiality-protected portion, as described in 3GPP TS 33.303 [6]. Finally, if a DUIK is configured, the UE shall use the DUIK and the UTC-based counter to verify the MIC field in the unscrambled PC5_DISCOVERY message for UE-to-Network Relay Discovery Response.

Then if:

– the Relay Service Code parameter of the PC5_DISCOVERY message for UE-to-Network Relay Discovery Response is the same as the Relay Service Code parameter of the PC5_DISCOVERY message for UE-to-Network Relay Discovery Solicitation; and

– the User Info ID of the UE-to-network relay is not configured as specified in clause 5 for the connectivity service being solicited, or the Discoverer Info parameter of the PC5_DISCOVERY message for UE-to-Network Relay Discovery Response is the same as the User Info ID of the UE-to-network relay configured as specified in clause 5 for the connectivity service being solicited;

then the UE shall consider that the connectivity service the UE seeks to discover has been discovered. In addition, the UE can measure the signal strength of the PC5_DISCOVERY message for UE-to-Network Relay Discovery Response for relay selection or reselection. If the UE has received the PC5_DISCOVERY message for UE-to-Network Relay Discovery Response from the ProSe UE-to-network relay UE with which the UE has a link established, the UE shall stop the retransmission timer T4109, and start the periodic measurement timer T4110.

10A.2.4.3 Discoverer UE procedure for UE-to-network relay discovery completion

When the UE is triggered by an upper layer application to stop soliciting for proximity of a connectivity service provided by a UE-to-network relay, or when the UE stops being authorised to perform the Discoverer UE procedure for UE-to-network relay discovery, the UE shall instruct the lower layers to stop the discoverer operation.

When the UE stops discoverer operation, if the UE is in EMM-CONNECTED mode, the UE shall trigger the corresponding procedure in lower layers as specified in 3GPP TS 36.331 [12].

10A.2.5 Discoveree UE procedure for UE-to-network relay discovery

10A.2.5.1 General

The purpose of the discoveree UE procedure for UE-to-network relay discovery is to enable a ProSe-enabled public safety UE with a UE-to-network relay to respond to solicitation from other ProSe-enabled public safety UEs on proximity of a connectivity service provided by the UE-to-network relay, upon a request from upper layers as defined in 3GPP TS 23.303 [2].

10A.2.5.2 Discoveree UE procedure for UE-to-network relay discovery initiation

The UE is authorised to perform the discoveree UE procedure for UE-to-network relay discovery if:

a) the UE is authorised to act as a UE-to-network relay in the PLMN indicated by the serving cell as specified in clause 5, and

– the UE is served by E-UTRAN and the UE is authorised to perform ProSe direct discovery for public safety use discoveree operation in the PLMN as specified in clause 5, and the lower layers indicate that discovery operation of a UE-to-network relay is supported; or- the UE is authorised to perform ProSe direct discovery for public safety use discoveree operation when not served by E-UTRAN as specified in clause 5 and intends to use the provisioned radio resources for UE-to-network relay discovery; and

b) the UE is configured with the Relay Service Code parameter identifying the connectivity service to be responded to and with the User Info ID for the UE-to-network relay discovery parameter, as specified in clause 5;

otherwise the UE is not authorised to perform the discoveree UE procedure for UE-to-network relay discovery.

Figure 10A.2.5.2.1 illustrates the interaction of the UEs in the discoveree UE procedure for UE-to-network relay discovery.

Figure 10A.2.5.2.1: Discoveree UE procedure for UE-to-network relay discovery

When the UE is triggered by an upper layer application to start responding to solicitation on proximity of a connectivity service provided by the UE-to-network relay, and if the UE is authorised to perform the discoveree UE procedure for UE-to-network relay discovery, then the UE:

a) if the UE is served by E-UTRAN, and the UE in EMM-IDLE mode needs to request resources for sending PC5_DISCOVERY messages for public safety as specified in 3GPP TS 36.331 [12], shall perform a service request procedure or tracking area update procedure as specified in 3GPP TS 24.301 [11]; and

b) shall instruct the lower layers to start monitoring for PC5_DISCOVERY messages with an indication that the message is for relay discovery for public safety use.

Upon reception of a PC5_DISCOVERY message for UE-to-Network Relay Discovery Solicitation according to subclause 11.2.5.1, for the Relay Service Code of the connectivity service which the UE is authorized to respond, the UE shall use the associated DUSK, if configured, and the UTC-based counter obtained during the reception operation to unscramble the PC5_DISCOVERY message as described in 3GPP TS 33.303 [6]. Then, if a DUCK is configured, the UE shall use the DUCK and the UTC-based counter to decrypt the configured message-specific confidentiality-protected portion, as described in 3GPP TS 33.303 [6]. Finally, if a DUIK is configured, the UE shall use the DUIK and the UTC-based counter to verify the MIC field in the unscrambled PC5_DISCOVERY message for UE-to-Network Relay Discovery Solicitation.

Then, if the Relay Service Code parameter of the PC5_DISCOVERY message for UE-to-Network Relay Discovery Solicitation is the same as the Relay Service Code parameter configured as specified in clause 5 for the connectivity service and either the ProSe Relay UE ID parameter is not included or the included ProSe Relay UE ID parameter is the same as the ProSe Relay UE ID associated with the Relay Service Code parameter configured as specified in clause 5, the UE:

a) shall obtain a valid UTC time for the discovery transmission from the lower layers and generate the UTC-based counter corresponding to this UTC time as specified in subclause 12.2.2.18;

b) shall generate a PC5_DISCOVERY message for UE-to-Network Relay Discovery Response according to subclause 11.2.5.1. In the PC5_DISCOVERY message for UE-to-Network Relay Discovery Response, the UE:

1) shall set the ProSe Relay UE ID to a ProSe Relay UE ID used for ProSe direct communication for the connectivity service;

2) shall set the Discoveree Info parameter to the User Info ID for the UE-to-network relay discovery parameter, configured in clause 5;

3) shall set the Relay Service Code parameter to the Relay Service Code parameter of the PC5_DISCOVERY message for UE-to-Network Relay Discovery Solicitation;

4) shall set the UTC-based counter LSB parameter to include the eight least significant bits of the UTC-based counter; and

c) shall apply the DUIK, DUSK, or DUCK with the associated Encrypted Bitmask, along with the UTC-based counter to the PC5_DISCOVERY message for whichever security mechanism(s) configured to be applied, e.g. integrity protection, message scrambling or confidentiality protection of one or more above parameters, as specified in 3GPP TS 33.303 [6]; and

d) shall pass the resulting PC5_DISCOVERY message for UE-to-Network Relay Discovery Response with an indication that the message is for relay discovery for public safety use to the lower layers for transmission over the PC5 interface.

10A.2.5.3 Discoveree UE procedure for UE-to-network relay discovery completion

When the UE is triggered by an upper layer application to stop responding to solicitation on proximity of a connectivity service provided by a UE-to-network relay, or when the UE stops being authorised to perform the discoveree UE procedure for UE-to-network relay discovery, the UE shall instruct the lower layers to stop monitoring.

When the UE stops monitoring, if the UE is in EMM-CONNECTED mode, the UE shall trigger the corresponding procedure in lower layers as specified in 3GPP TS 36.331 [12].

10A.2.6 Announcing UE procedure for group member discovery

10A.2.6.1 General

The purpose of the announcing UE procedure for group member discovery is to enable a ProSe-enabled public safety UE to announce availability in a discovery group to other ProSe-enabled public safety UEs, upon a request from upper layers as defined in 3GPP TS 23.303 [2].

10A.2.6.2 Announcing UE procedure for group member discovery initiation

The UE is authorised to perform the announcing UE procedure for group member discovery if:

a) the following is true:

1) the UE is not served by E-UTRAN, is authorised to perform ProSe direct discovery for public safety use announcing when the UE is not served by E-UTRAN as specified in clause 5, and is configured with the radio parameters to be used for ProSe direct discovery for public safety use when not served by E-UTRAN;

2) the UE is served by E-UTRAN, and is authorised to perform ProSe direct discovery for public safety use announcing in the PLMN indicated by the serving cell as specified in clause 5; or

3) the UE is:

– in EMM-IDLE mode, in limited service state as specified in 3GPP TS 23.122 [24], and the reason for the UE being in limited service state is one of the following:

i) the UE is unable to find a suitable cell in the selected PLMN as specified in 3GPP TS 36.304 [23];

– authorised to perform ProSe direct discovery for public safety use announcing when the UE is not served by E-UTRAN as specified in clause 5 and:

i) configured with the radio parameters to be used for ProSe direct discovery for public safety use when not served by E-UTRAN; or

ii) the lower layers indicate that the UE does not need to request resources for sending PC5_DISCOVERY messages for public safety as specified in 3GPP TS 36.331 [12]; and

NOTE: When the lower layers indicate that the UE does not need to request resources for sending PC5_DISCOVERY messages for public safety as specified in 3GPP TS 36.331 [12], the serving cell broadcasts a common radio resources pool for public safety discovery transmission and the UE can use this common radio resources pool while in limited service state.

b) the UE is configured with the Discovery Group ID parameter identifying the discovery group to be announced and with the User Info ID for the group member discovery parameter, as specified in clause 5;

otherwise the UE is not authorised to perform the announcing UE procedure for group member discovery.

Figure 10A.2.6.2.1 illustrates the interaction of the UEs in the announcing UE procedure for group member discovery.

Figure 10A.2.6.2.1: Announcing UE procedure for group member discovery

When the UE is triggered by an upper layer application to announce availability in a discovery group, if the UE is authorised to perform the announcing UE procedure for group member discovery, then the UE:

b) shall obtain a valid UTC time for the discovery transmission from the lower layers and generate the UTC-based counter corresponding to this UTC time as specified in subclause 12.2.2.18;

c) shall generate a PC5_DISCOVERY message for Group Member Discovery Announcement according to subclause 11.2.5.1. In the PC5_DISCOVERY message for Group Member Discovery Announcement, the UE:

1) shall set the ProSe UE ID to the Layer 2 ID used for unicast communication configured in clause 5;

2) shall set the Announcer Info parameter to the User Info ID for the group member discovery parameter, configured in clause 5;

3) shall set the Discovery Group ID parameter to the Discovery Group ID parameter identifying the discovery group to be announced, configured in clause 5; and

4) shall set the UTC-based counter LSB parameter to include the eight least significant bits of the UTC-based counter;

d) shall apply the DUIK, DUSK, or DUCK with the associated Encrypted Bitmask, along with the UTC-based counter to the PC5_DISCOVERY message for whichever security mechanism(s) configured to be applied, e.g., integrity protection, message scrambling or confidentiality protection of one or more above parameters, as specified in 3GPP TS 33.303 [6]; and

e) shall pass the resulting PC5_DISCOVERY message for Group Member Discovery Announcement to the lower layers for transmission over the PC5 interface with an indication that the message is for public safety use.

The UE shall ensure that it keeps on passing the same PC5_DISCOVERY message and the indication that the message is for public safety use to the lower layers for transmission until the UE is triggered by an upper layer application to stop announcing availability in a discovery group, or until the UE stops being authorised to perform the announcing UE procedure for group member discovery. How this is achieved is left up to UE implementation.

10A.2.6.3 Announcing UE procedure for group member discovery completion

When the UE is triggered by an upper layer application to stop announcing availability in a discovery group, or when the UE stops being authorised to perform the announcing UE procedure for group member discovery, the UE shall instruct the lower layers to stop announcing.

When the UE stops announcing, if the UE is in EMM-CONNECTED mode, the UE shall trigger the corresponding procedure in lower layers as specified in 3GPP TS 36.331 [12].

10A.2.7 Monitoring UE procedure for group member discovery

10A.2.7.1 General

The purpose of the monitoring UE procedure for group member discovery is to enable a ProSe-enabled public safety UE to become aware of proximity of other ProSe-enabled public safety UEs in a discovery group, upon a request from upper layers as defined in 3GPP TS 23.303 [2].

10A.2.7.2 Monitoring UE procedure for group member discovery initiation

The UE is authorised to perform the monitoring UE procedure for group member discovery if:

a) the following is true:

1) the UE is not served by E-UTRAN, is authorised to perform ProSe direct discovery for public safety use monitoring when the UE is not served by E-UTRAN as specified in clause 5, and is configured with the radio parameters to be used for ProSe direct discovery for public safety use when not served by E-UTRAN;

2) the UE is served by E-UTRAN, and is authorised to perform ProSe direct discovery for public safety use monitoring in at least one PLMN as specified in clause 5; or

3) the UE is:

– in EMM-IDLE mode, in limited service state as specified in 3GPP TS 23.122 [24, and the reason for the UE being in limited service state is one of the following:

i) the UE is unable to find a suitable cell in the selected PLMN as specified in 3GPP TS 36.304 [23];

– authorised to perform ProSe direct discovery for public safety use monitoring when the UE is not served by E-UTRAN as specified in clause 5, and:

i) configured with the radio parameters to be used for ProSe direct discovery for public safety use when not served by E-UTRAN; or

ii) the lower layers indicate that public safety discovery operation is supported; and:

NOTE 1: When the lower layers indicate that public safety discovery operation is supported, the serving cell broadcasts a common radio resources pool for public safety discovery reception and the UE can use this common radio resources pool while in limited service state.

b) the UE is configured with the Discovery Group ID parameter identifying the discovery group to be monitored, as specified in clause 5;

otherwise the UE is not authorised to perform the monitoring UE procedure for group member discovery.

Figure 10A.2.7.2.1 illustrates the interaction of the UEs in the monitoring UE procedure for group member discovery.

Figure 10A.2.7.2.1: Monitoring UE procedure for group member discovery;

When the UE is triggered by an upper layer application to monitor proximity of other UEs in a discovery group, and if the UE is authorised to perform the monitoring UE procedure for group member discovery, then the UE shall instruct the lower layers to start monitoring for PC5_DISCOVERY messages with an indication that the message is for public safety use.

Upon reception of a PC5_DISCOVERY message for Group Member Discovery Announcement according to subclause 11.2.5.1, for the target Discovery Group ID of the discovery group to be monitored, the UE shall use the associated DUSK, if configured, and the UTC-based counter obtained during the monitoring operation to unscramble the PC5_DISCOVERY message as described in 3GPP TS 33.303 [6]. Then, if a DUCK is configured, the UE shall use the DUCK and the UTC-based counter to decrypt the configured message-specific confidentiality-protected portion, as described in 3GPP TS 33.303 [6]. Finally, if a DUIK is configured, the UE shall use the DUIK and UTC-based counter to verify the MIC field in the unscrambled PC5_DISCOVERY message for Group Member Discovery Announcement.

NOTE 2: The use of an erroneous UTC-based counter for processing received PC5_DISCOVERY messages at the ProSe-enabled Public Safety UE can cause MIC check failure after DUIK is used for integrity check, and malformed contents after DUSK is used for unscrambling or DUCK is used for deciphering. How a ProSe-enabled Public Safety UE ensures the accuracy of the UTC-based counter is left to UE implementation.

Then if the Discovery Group ID parameter of the PC5_DISCOVERY message for Group Member Discovery Announcement is the same as the configured Discovery Group ID parameter as specified in clause 5, the UE shall consider that other UE in the discovery group the UE seeks to monitor has been discovered.

10A.2.7.3 Monitoring UE procedure for group member discovery completion

When the UE is triggered by an upper layer application to stop monitoring proximity of other UEs in a discovery group, or when the UE stops being authorised to perform the monitoring UE procedure for group member discovery, the UE shall instruct the lower layers to stop monitoring.

When the UE stops monitoring, if the UE is in EMM-CONNECTED mode, the UE shall trigger the corresponding procedure in lower layers as specified in 3GPP TS 36.331 [12].

10A.2.8 Discoverer UE procedure for group member discovery

10A.2.8.1 General

The purpose of the discoverer UE procedure for group member discovery is to enable a ProSe-enabled public safety UE to solicit proximity of other ProSe-enabled public safety UEs in a discovery group, upon a request from upper layers as defined in 3GPP TS 23.303 [2].

10A.2.8.2 Discoverer UE procedure for group member discovery initiation

The UE is authorised to perform the discoverer UE procedure for group member discovery if:

a) the following is true:

1) the UE is not served by E-UTRAN, is authorised to perform ProSe direct discovery for public safety use discoverer operation when the UE is not served by E-UTRAN as specified in clause 5, and is configured with the radio parameters to be used for ProSe direct discovery for public safety use when not served by E-UTRAN;

2) the UE is served by E-UTRAN, and is authorised to perform ProSe direct discovery for public safety use discoverer operation in the PLMN indicated by the serving cell as specified in clause 5; or

3) the UE is:

– in EMM-IDLE mode, in limited service state as specified in 3GPP TS 23.122 [24, and the reason for the UE being in limited service state is one of the following:

i) the UE is unable to find a suitable cell in the selected PLMN as specified in 3GPP TS 36.304 [23];

– authorised to perform ProSe direct discovery for public safety use discoverer operation when the UE is not served by E-UTRAN as specified in clause 5, and:

i) configured with the radio parameters to be used for ProSe direct discovery for public safety use when not served by E-UTRAN; or

ii) the lower layers indicate that the UE does not need to request resources for sending PC5_DISCOVERY messages for public safety as specified in 3GPP TS 36.331 [12]; and:

b) the UE is configured with the Discovery Group ID parameter identifying the discovery group to be solicited and with the User Info ID for the group member discovery parameter, as specified in clause 5;

otherwise the UE is not authorised to perform the discoverer UE procedure for group member discovery.

Figure 10A.2.8.2.1 illustrates the interaction of the UEs in the discoverer UE procedure for group member discovery.

Figure 10A.2.8.2.1: Discoverer UE procedure for group member discovery

When the UE is triggered by an upper layer application to solicit proximity of other UEs in a discovery group, and if the UE is authorised to perform the discoverer UE procedure for group member discovery, then the UE:

b) shall obtain a valid UTC time for the discovery transmission from the lower layers and generate the UTC-based counter corresponding to this UTC time as specified in subclause 12.2.2.18;

c) shall generate a PC5_DISCOVERY message for Group Member Discovery Solicitation according to subclause 11.2.5.1. In the PC5_DISCOVERY message for Group Member Discovery Solicitation, the UE:

1) shall set the Discoverer Info parameter to the User Info ID for the group member discovery parameter, configured in clause 5;

2) shall set the Discovery Group ID parameter to the Discovery Group ID parameter identifying the discovery group to be solicited, configured in clause 5;

3) shall set either the Target User Info parameter or the Target Group Info parameter according to the target information provided by the upper layer application; and

4) shall set the UTC-based counter LSB parameter to include the eight least significant bits of the UTC-based counter;

e) shall pass the resulting PC5_DISCOVERY message for Group Member Discovery Solicitation to the lower layers for transmission over the PC5 interface with an indication that the message is for public safety use.

The UE shall ensure that it keeps on passing the same PC5_DISCOVERY message to the lower layers for transmission with an indication that the message is for public safety use until the UE is triggered by an upper layer application to stop soliciting proximity of other UEs in a discovery group, or until the UE stops being authorised to perform the discoverer UE procedure for group member discovery. How this is achieved is left up to UE implementation.

Upon reception of a PC5_DISCOVERY message for Group Member Discovery Response according to subclause 11.2.5.1, for the target Discovery Group ID of the discovery group to be discovered, the UE shall use the associated DUSK, if configured, and the UTC-based counter obtained during the monitoring operation to unscramble the PC5_DISCOVERY message as described in 3GPP TS 33.303 [6]. Then, if a DUCK is configured, the UE shall use the DUCK and the UTC-based counter to decrypt the configured message-specific confidentiality-protected portion, as described in 3GPP TS 33.303 [6]. Finally, if a DUIK is configured, the UE shall use the DUIK and UTC-based counter to verify the MIC field in the unscrambled PC5_DISCOVERY message for Group Member Discovery Response.

Then if the Discovery Group ID parameter of the PC5_DISCOVERY message for Group Member Discovery Response is the same as the Discovery Group ID parameter of the PC5_DISCOVERY message for Group Member Discovery Solicitation, the UE shall consider that other UE in the discovery group the UE seeks to discover has been discovered.

10A.2.8.3 Discoverer UE procedure for group member discovery completion

When the UE is triggered by an upper layer application to stop soliciting proximity of other UEs in a discovery group, or when the UE stops being authorised to perform the discoverer UE procedure for group member discovery, the UE shall instruct the lower layers to stop discoverer operation.

When the UE stops discoverer operation, if the UE is in EMM-CONNECTED mode, the UE shall trigger the corresponding procedure in lower layers as specified in 3GPP TS 36.331 [12].

10A.2.9 Discoveree UE procedure for group member discovery

10A.2.9.1 General

The purpose of the discoveree UE procedure for group member discovery is to enable a ProSe-enabled public safety UE to respond to solicitation from other ProSe-enabled public safety UEs on proximity in a discovery group, upon a request from upper layers as defined in 3GPP TS 23.303 [2].

10A.2.9.2 Discoveree UE procedure for group member discovery initiation

The UE is authorised to perform the discoveree UE procedure for group member discovery if:

a) the following is true:

1) the UE is not served by E-UTRAN, is authorised to perform ProSe direct discovery for public safety use discoveree operation when the UE is not served by E-UTRAN as specified in clause 5, and is configured with the radio parameters to be used for ProSe direct discovery for public safety use when not served by E-UTRAN;

2) the UE is served by E-UTRAN, and is authorised to perform ProSe direct discovery for public safety use discoveree operation in the PLMN(s) indicated by the serving cell as specified in clause 5; or

3) the UE is:

– in EMM-IDLE mode, in limited service state as specified in 3GPP TS 23.122 [24], and the reason for the UE being in limited service state is one of the following:

i) the UE is unable to find a suitable cell in the selected PLMN as specified in 3GPP TS 36.304 [23];

– authorised to perform ProSe direct discovery for public safety use discoveree operation when the UE is not served by E-UTRAN as specified in clause 5, and:

i) configured with the radio parameters to be used for ProSe direct discovery for public safety use when not served by E-UTRAN; or

ii) the lower layers indicate that the UE does not need to request resources for sending PC5_DISCOVERY messages for public safety as specified in 3GPP TS 36.331 [12]; and

b) the UE is configured with the Discovery Group ID parameter identifying the discovery group to be responded to and with the User Info ID for the group member discovery parameter, as specified in clause 5;

otherwise the UE is not authorised to perform the Discoveree UE procedure for group member discovery.

Figure 10A.2.9.2.1 illustrates the interaction of the UEs in the Discoveree UE procedure for group member discovery.

Figure 10A.2.9.2.1: Discoveree UE procedure for group member discovery

When the UE is triggered by an upper layer application to start responding to solicitation on proximity of a UE in a discovery group, and if the UE is authorised to perform the discoveree UE procedure for group member discovery, then the UE:

b) shall instruct the lower layers to start monitoring for PC5_DISCOVERY messages with an indication that the message is for public safety use.

Upon reception of a PC5_DISCOVERY message for Group Member Discovery Solicitation according to subclause 11.2.5.1, for the Discovery Group ID of the discovery group which the UE is configured to respond for, the UE shall use the associated DUSK, if configured, and the UTC-based counter obtained during the monitoring operation to unscramble the PC5_DISCOVERY message as described in 3GPP TS 33.303 [6]. Then, if a DUCK is configured, the UE shall use the DUCK and the UTC-based counter to decrypt the configured message-specific confidentiality protected portion, as described in 3GPP TS 33.303 [6]. Finally, if a DUIK is configured, the UE shall use the DUIK and UTC-based counter to verify the MIC field in the unscrambled PC5_DISCOVERY message for Group Member Discovery Solicitation.

Then, if:

– the Discovery Group ID parameter of the received PC5_DISCOVERY message is the same as a Discovery Group ID parameter configured as specified in clause 5 for the discovery group;

– the Target User Info parameter is not included in the received PC5_DISCOVERY message or the Target User Info parameter of the received PC5_DISCOVERY message is the same as the User Info ID for the group member discovery parameter specified in clause 5; and

– the Target Group Info parameter is not included in the received PC5_DISCOVERY message or the Target Group Info parameter of the received PC5_DISCOVERY message is the same as the identifier of the targeted group provided by the upper layer application (e.g. ProSe Layer-2 Group ID of the ProSe direct communication service authorisation specified in clause 5);

the UE:

a) shall obtain a valid UTC time for the discovery transmission from the lower layers and generate the UTC-based counter corresponding to this UTC time as specified in subclause 12.2.2.18;

b) shall generate a PC5_DISCOVERY message for Group Member Discovery Response according to subclause 11.2.5.1. In the PC5_DISCOVERY message for Group Member Discovery Response, the UE:

1) shall set the ProSe UE ID to the Layer 2 ID used for unicast communication, configured in clause 5;

2) shall set the Discoveree Info parameter to the User Info ID for the group member discovery parameter, configured in clause 5;

3) shall set the Discovery Group ID parameter to the Discovery Group ID parameter of the PC5_DISCOVERY message for Group Member Discovery Solicitation; and

4) shall set the UTC-based counter LSB parameter to include the eight least significant bits of the UTC-based counter;

d) shall pass the resulting PC5_DISCOVERY message for Group Member Discovery Response with an indication that the message is for public safety use to the lower layers for transmission over the PC5 interface.

10A.2.9.3 Discoveree UE procedure for group member discovery completion

When the UE is triggered by an upper layer application to stop responding to solicitation on proximity of other UEs in a discovery group, or when the UE stops being authorised to perform the discoveree UE procedure for group member discovery, the UE shall instruct the lower layers to stop monitoring.

When the UE stops monitoring, if the UE is in EMM-CONNECTED mode, the UE shall trigger the corresponding procedure in lower layers as specified in 3GPP TS 36.331 [12].

10A.2.10 Announcing UE procedure for Relay Discovery Additional Information

10A.2.10.1 General

The purpose of the announcing UE procedure for Relay Discovery Additional Information is to announce to the remote UEs additional information about:

– the MBMS traffic the ProSe UE-to-network relay is relaying; or

– the E-UTRAN Cell serving the ProSe UE-to-network relay

as defined in 3GPP TS 23.303 [2].

10A.2.10.2 Announcing procedure for Relay Discovery Additional Information

The ProSe UE-to-network relay announces the Relay Discovery Additional Information:

a) if the remote UE requests the ProSe UE-to-network relay to start monitoring a specific TMGI availability by the PC5-S TMGI Monitoring Request message, and as a response the ProSe UE-to-network relay acknowledges with the PC5-S TMGI Monitoring Response message and the TMGI is detected in the serving E-UTRAN cell, then the ProSe UE-to-network relay includes a pair of the TMGI and its corresponding ProSe Layer 2 Group ID in the PC5_DISCOVERY message for Relay Discovery Additional Information until the timer T4105 expires (see the subclause 10.5); or

b) if the remote UE requests the ProSe UE-to-network relay to announce the E-UTRAN Cell Global ID (ECGI) of the cell serving the ProSe UE-to-network relay, and as a response the ProSe UE-to-network relay acknowledges with the PC5-S Cell ID Announcement Response message, then the ProSe UE-to-network relay includes the ECGI of the serving cell in the PC5_DISCOVERY message for Relay Discovery Additional Information until the timer T4107 expires (see the subclause 10.6).

NOTE 1: ProSe UE-to-network relay announces the Relay Discovery Additional Information only when it is in E-UTRAN coverage.

Figure 10A.2.10.2.1 illustrates the interaction of the ProSe UE-to-network relay and the remote UE in the announcing UE procedure for Relay Discovery Additional Information.

Figure 10A.2.10.2.1: Announcing procedure for Relay Discovery Additional Information

The ProSe UE-to-network relay UE may start announcing Relay Discovery Additional Information if:

a) the ProSe UE-to-network relay UE is currently authorised to perform ProSe direct discovery Model A announcing in the serving PLMN if the UE is served by E-UTRAN; and

1) TMGI monitoring has been requested and responded to remote UEs, the ProSe UE-to-network relay UE detects the corresponding TMGI in the serving cell and the timer T4105 has not expired; or

2) ECGI announcement for the serving cell of the ProSe UE-to-network relay UE has been requested and responded to remote UEs, the timer T4107 has not expired.

When the ProSe UE-to-network relay has some additional information to broadcast (i.e. either a pair of TMGI and its corresponding ProSe Layer 2 Group ID or ECGI), then the ProSe UE-to-network relay:

a) shall request the parameters from the lower layers for ProSe direct discovery announcing for public safety use (see 3GPP TS 36.331 [12]). The ProSe UE-to-network relay performs the announcing UE procedure for Relay Discovery Additional Information only if the lower layers indicate that ProSe direct discovery is supported by the network. If the ProSe UE-to-network relay in EMM-IDLE mode needs to request resources for sending PC5_DISCOVERY messages as specified in 3GPP TS 36.331 [12], the ProSe UE-to-network relay shall perform a service request procedure or tracking area update procedure as specified in 3GPP TS 24.301 [11];

b) shall obtain a valid UTC time for the discovery transmission from the lower layers and generate the UTC-based counter corresponding to this UTC time as specified in subclause 12.2.2.18;

c) shall generate PC5_DISCOVERY message(s) for Relay Discovery Additional Information according to subclause 11.2.5.1. In the PC5_DISCOVERY message for Relay Discovery Additional Information, the ProSe UE-to-network relay shall:

1) include the Relay Service Code and the ProSe Relay UE ID used for ProSe direct communication which the remote UE used to request for the Relay Discovery Additional Information;

2) set the Announcer Info parameter to the User Info ID parameter, configured in subclause 5;

3) set the Relay Discovery Additional Information contents by the additional information to broadcast; and

4) shall set the UTC-based counter LSB parameter to include the eight least significant bits of the UTC-based counter;

e) shall pass the resulting PC5_DISCOVERY message for Relay Discovery Additional Information to the lower layers for transmission over the PC5 interface with an indication that the message is for public safety use.

The ProSe UE-to-network relay shall ensure that it keeps on passing the PC5_DISCOVERY messages to the lower layers for transmission until the corresponding timer (i.e. timer T4105 when the additional information is a pair of TMGI and its corresponding ProSe Layer 2 Group ID and timer T4107 when the additional information is ECGI) expires.

During the announcing operation, if one of the above conditions is no longer met, the ProSe UE-to-network relay may instruct the lower layers to stop announcing. When the ProSe UE-to-network relay stops announcing, if the lower layers indicate that the ProSe UE-to-network relay is required to send a discovery indication to the eNodeB and the ProSe UE-to-network relay is in EMM-CONNECTED mode, the ProSe UE-to-network relay shall trigger the corresponding procedure in lower layers as specified in 3GPP TS 36.331 [12].

10A.2.11 Monitoring UE procedure for Relay Discovery Additional Information

10A.2.11.1 General

The purpose of the monitoring UE procedure for Relay Discovery Additional Information is to enable a remote UE to become aware of MBMS traffics the ProSe UE-to-network relay is relaying or the E-UTRAN Cell serving the ProSe UE-to-network relay as defined in 3GPP TS 23.303 [2].

10A.2.11.2 Monitoring procedure for Relay Discovery Additional Information

The remote UE monitors Relay Discovery Additional Information:

a) until the TMGI monitoring refresh timer T4104 expires if the remote UE has requested the ProSe UE-to-network relay to start monitoring a specific TMGI availability by the PC5-S TMGI Monitoring Request message and received the PC5-S TMGI Monitoring Response message from the ProSe UE-to-network relay; or

b) until the ECGI announcement request refresh timer T4106 expires if the remote UE has requested the ProSe UE-to-network relay to announce the ECGI of the cell serving the ProSe UE-to-network relay and received the PC5-S Cell ID Announcement Response message from the ProSe UE-to-network relay.

The UE may instruct the lower layers to start monitoring with an indication that the message is for relay discovery for public safety use if:

a) a request from upper layers to monitor for Relay Discovery Additional Information is still in place; and either:

1) the UE is currently authorised to perform ProSe direct discovery Model A monitoring in at least one PLMN if the UE is served by E-UTRAN;

2) the UE is currently authorised to perform ProSe direct discovery Model A monitoring if the UE is not served by E-UTRAN; or

3) the UE is:

– in EMM-IDLE mode, in limited service state as specified in 3GPP TS 23.122 [24], and the reason for the UE being in limited service state is one of the following:

i) the UE is unable to find a suitable cell in the selected PLMN as specified in 3GPP TS 36.304 [23];

– authorised to perform ProSe direct discovery Model A monitoring when the UE is not served by E-UTRAN as specified in clause 5, and:

i) configured with the radio parameters to be used for ProSe direct discovery for public safety use when not served by E-UTRAN; or

ii) the lower layers indicate that public safety discovery operation is supported; and

If the UE is in EMM-CONNECTED mode, the monitoring UE shall also trigger the corresponding procedure in lower layers as specified in 3GPP TS 36.331 [12].

During the monitoring operation, if one of the above conditions is no longer met, the UE may instruct the lower layers to stop monitoring. When the UE stops monitoring, if the UE is in EMM-CONNECTED mode, the UE shall trigger the corresponding procedure in lower layers as specified in 3GPP TS 36.331 [12].

Upon reception of a PC5_DISCOVERY message for Relay Discovery Additional Information according to subclause 11.2.5.1, for the target Relay Service Code to be monitored, the UE shall use the associated DUSK, if configured, and the UTC-based counter obtained during the monitoring operation to unscramble the PC5_DISCOVERY message as described in 3GPP TS 33.303 [6]. Then, if a DUCK is configured, the UE shall use the DUCK and the UTC-based counter to decrypt the configured message-specific confidentiality protected portion, as described in 3GPP TS 33.303 [6]. Finally, if a DUIK is configured, the UE shall use the DUIK and UTC-based counter to verify the MIC field in the unscrambled PC5_DISCOVERY message for Relay Discovery Additional Information.

Then, if:

– the Relay Service Code parameter of the PC5_DISCOVERY message for Relay Discovery Additional Information is the same as the Relay Service Code parameter configured as specified in clause 5 for the connectivity service being monitored; and

– the ProSe Relay UE ID parameter of the PC5_DISCOVERY message for Relay Discovery Additional Information is the same as the ProSe Relay UE ID parameter identifying the relay the remote UE intends to communicate with;

then the UE shall consider that the Relay Discovery Additional Information it intends to monitor has been discovered. In addition, the UE can measure the signal strength of the PC5_DISCOVERY message for Relay Discovery Additional Information for relay selection or reselection.

If the remote UE detects Relay Discovery Additional Information matched with the requested TMGI, then the remote UE starts to receive the MBMS traffic via the corresponding ProSe Layer-2 Group ID. When the ProSe-enabled public safety UE receives a PC5_DISCOVERY message for Relay Discovery Additional Information containing its interested TMGI, the UE may start to receive the MBMS traffic via the corresponding ProSe Layer-2 Group ID even if it does not have a PC5 link established with the ProSe UE-to-network relay or is not authorized to use a ProSe UE-to-network relay.

10A.2.12 UE-to-network relay selection procedure

10A.2.12.1 General

The purpose of the UE-to-network relay selection procedure is to enable a remote UE to select a suitable ProSe UE-to-network relay UE to obtain a connectivity service to EPC.

10A.2.12.2 UE-to-network relay selection procedure initiation

The remote UE shall trigger the UE-to-network relay selection procedure if the following conditions are met:

– the UE is authorised to act as a remote UE towards a ProSe UE-to-network relay UE as specified in clause 5;

– the UE has obtained a list of ProSe UE-to-network relay UE candidate(s) fulfilling ProSe layer criteria with the monitoring procedure for UE-to-network relay discovery as specified in subclause 10A.2.3 or the discoverer procedure for UE-to-network relay discovery as specified in subclause 10A.2.4; and

– the UE has obtained a list of ProSe UE-to-network relay UE candidate(s) fulfilling lower layers criteria as specified in 3GPP TS 36.331 [12].

10A.2.12.3 UE-to-network relay selection procedure completion

If there exists only one ProSe UE-to-network relay candidate satisfying the conditions in subclause 10A.2.12.2, then that ProSe UE-to-network relay UE is selected. If there exist more than one ProSe UE-to-network relay candidate satisfying the conditions in subclause 10A.2.12.2, any relay candidates not satisfying the non-radio related ProSe layer criteria shall be discarded, and out of the remaining relay candidates, the relay candidate with the highest ranking of the lower layer criteria shall be selected. The UE may take the value of the Resource Status Indicator bit of the Status Indicator parameter of the PC5_DISCOVERY message for UE-to-Network Relay Discovery Announcement or PC5_DISCOVERY message for UE-to-Network Relay Discovery Response into account when deciding which ProSe UE-to-network relay to select. It is up to the UE implementation whether the ProSe layer or the lower layers takes the final selection on which ProSe UE-to-network relay UE to select.

10A.2.13 UE-to-network relay reselection procedure

10A.2.13.1 General

The purpose of the UE-to-network relay reselection procedure is to enable a remote UE to reselect a ProSe UE-to-network relay UE to obtain a connectivity service to EPC when the serving ProSe UE-to-network relay UE is no longer suitable.

10A.2.13.2 UE-to-network relay reselection procedure initiation

The remote UE shall trigger the UE-to-network relay reselection procedure if one of the following conditions is met:

a) the UE has received a lower layers indication that the serving ProSe UE-to-network relay UE no longer fulfills the lower layers criteria as specified in 3GPP TS 36.331 [12];

b) the parameters related to ProSe UE-to-network relay in the ProSe direct discovery for public safety use service authorisation (e.g., Relay Service Code, User Info ID, etc.) have been updated and the serving ProSe UE-to-network relay UE no longer fulfills the conditions specified in subclause 10A.2.3;

c) the UE has received a DIRECT_COMMUNICATION_REJECT message from the ProSe UE-to-network relay UE with the cause value "Direct communication to target UE not allowed";

d) the UE has received a DIRECT_COMMUNICATION_RELEASE message from the ProSe UE-to-network relay UE with the cause value "Direct communication with the peer UE is no longer allowed";

e) the UE has received a DIRECT_COMMUNICATION_RELEASE message from the ProSe UE-to-network relay UE with the cause value "Direct connection is not available any more";

f) the UE has not received any response from the ProSe UE-to-network relay UE after M consecutive retransmissions of DIRECT_COMMUNICATION_REQUEST or DIRECT_COMMUNICATION_KEEPALIVE messages; or

g) the UE has not received any response from the ProSe UE-to-network relay UE after M consecutive retransmissions of PC5_DISCOVERY message for UE-to-Network Relay Discovery Solicitation used to trigger the PC5_DISCOVERY message signal strength measurement between the UE and the ProSe UE-to-network relay UE with which the UE has a link established.

NOTE: The value of M is implementation specific and is less than or equal to the maximum number of retransmissions allowed for PC5 Signalling protocol.

In cases c) and d), the remote UE shall exclude the ProSe UE-to-network relay UE which sent the message specified in cases c) or d) from the UE-to-network relay reselection process described below.

To conduct UE-to-network relay reselection process, the UE shall first initiate one of the following procedures or both depending on UE’s service authorisation for ProSe direct discovery for public safety use:

– monitoring procedure for UE-to-network relay discovery as specified in subclause 10A.2.3; or

– discoverer procedure for UE-to-network relay discovery as specified in subclause 10A.2.4.

After the execution of the above discovery procedure(s), the remote UE performs the UE-to-network relay selection procedure as specified in subclause 10A.2.3.