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Content for  TS 23.304  Word version:  18.4.0

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5.9 Support for 5G ProSe for UEs in limited service state  5.10 PC5 operation in EPS for Public Safety UE  5.11 Communication path selection between PC5 and Uu reference points  5.12 NAS level congestion control for 5G ProSe UE-to-Network Relay  5.13 Support for PC5 DRX operations  5.13.1 General  5.13.2 PC5 DRX operations for 5G ProSe Direct Discovery and 5G ProSe UE-to-Network Relay Discovery  5.13.3 PC5 DRX operations for 5G ProSe Direct Communication and 5G ProSe UE-to-Network Relay Communication  5.14 5G ProSe UE-to-UE Relay Communication  5.14.1 5G ProSe Layer-3 UE-to-UE Relay Communication  5.14.2 5G ProSe Layer-2 UE-to-UE Relay Communication  5.15 Path switching between two UE-to-Network Relays  5.16 Communication path switching between PC5 and Uu reference points  5.17 Multi-path communication via Uu and via 5G ProSe UE-to-Network Relay  5.17.1 General  5.17.2 Multi-path communication via direct Uu path and via 5G ProSe Layer-3 UE-to-Network Relay  5.18 Support of Public Warning Notification Relaying 
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5.9  Support for 5G ProSe for UEs in limited service state  p. 64

For UE in limited service state, as defined in TS 23.122, 5G ProSe can be used over PC5 reference point with the following considerations.
UEs that are authorized to use 5G ProSe over PC5 reference point according to clause 5.1 shall be able to use the corresponding services following the principles defined in clause 5.1.2.2 for 5G ProSe Direct Discovery, clause 5.1.3.2 for 5G ProSe Direct Communication, clause 5.1.4.2 for 5G ProSe UE-to-Network Relay and clause 5.1.5.2 for 5G ProSe UE-to-UE Relay when the UE enters in limited service state in 5GS:
  • because UE cannot find a suitable cell of the selected PLMN as described in TS 23.122; or
  • as the result of receiving one of the following reject reasons defined in TS 23.122:
    • a "PLMN not allowed" response to a registration request or;
    • a "5GS services not allowed" response to a registration request or service request.
A UE in limited service state shall only use the radio resources and procedure available in CM-IDLE mode for ProSe over PC5 reference point, for details see TS 36.300 and TS 38.300.
UEs shall not use 5G ProSe over PC5 reference point using the "operator-managed" radio resources, as specified in clauses 5.1.2.1, 5.1.3.1, 5.1.4.1 and 5.1.5.1, if the UE has entered in limited service state due to all other situations (e.g. no SIM in the MS, an "illegal MS" or "illegal ME" response to a registration request, or an "IMSI unknown in HLR" response to a registration request) defined in TS 23.122, where the UE is unable to obtain normal service from a PLMN. The UEs may use ProSe over PC5 reference point using the "non-operator-managed" radio resources, as specified in clauses 5.1.2.1, 5.1.3.1, 5.1.4.1 and 5.1.5.1, according to the corresponding principles defined in clauses 5.1.2.2, 5.1.3.2, 5.1.4.2 and 5.1.5.2.
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5.10  PC5 operation in EPS for Public Safety UE  p. 65

When the UE is in EPS, the UE shall use the valid ProSe policy and parameters provisioned by the ProSe Function in EPC for ProSe Direct Discovery and Prose Direct Communication. If the UE does not have valid ProSe policy and parameters, the UE shall request the network to provision the ProSe policy and parameters.
The UE that is authorized to perform ProSe Direct Discovery and/or ProSe Direct Communication in EPS can perform the authorized PC5 operation in EPS as specified in TS 23.303.
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5.11  Communication path selection between PC5 and Uu reference points  p. 65

The "communication path selection between PC5 and Uu reference points" refers to the procedure on how a UE selects a communication path between PC5 reference point and Uu reference point before it communicates with another UE. The communication path over PC5 reference point means that the communication with another UE is performed by using 5G ProSe Direct Communication only. The communication path over Uu reference point means that the communication with another UE is performed via the network.
Path selection policy is provided to the UE to indicate which path(s) is preferred for all or specific ProSe services (i.e. PC5 preferred, Uu preferred or no preference indicated) as specified in clause 5.1.3.1.
The ProSe Application Server can provide a path preference for ProSe Services to UDR as specified in clause 6.2.5 and this may be used by PCF for path selection policy generation and update as specified in clause 6.2.2.
The UE may use the provisioned path selection policy to select the appropriate communication path for all or specific ProSe services.
UE operation related to the path selection for ProSe service is as follows:
  • The UE evaluates the path selection policy in the policy and parameters for ProSe Direct Communication applicable to the ProSe service and selects the communication path as below:
    • If PC5 preferred is indicated, the UE should prefer to use the PC5 for communication path for the ProSe service.
    • If Uu preferred is indicated, the UE should prefer to use the Uu for communication path for the ProSe service.
    • If no preference is indicated or no path selection policy is provisioned, the UE selects either a Uu or PC5 communication path based on its pre-configuration or implementation for the ProSe service.
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5.12  NAS level congestion control for 5G ProSe UE-to-Network Relay  p. 66

The 5G ProSe UE-to-Network Relay may be subject to NAS level congestion control, as specified in clause 5.19.7 of TS 23.501.
Both 5G ProSe Layer-2 UE-to-Network Relays and Layer-3 UE-to-Network Relays, when NAS Mobility Management congestion control as specified in clause 5.19.7.2 of TS 23.501 is activated, i.e. the 5G ProSe UE-to-Network Relay receives a Mobility Management back-off timer from the AMF, the 5G ProSe UE-to-Network Relay is not able to serve the 5G ProSe Remote UE after the 5G ProSe UE-to-Network Relay enters CM_IDLE state. If the 5G ProSe UE-to-Network Relay has a Mobility Management back-off timer when it enters CM_IDLE state the 5G ProSe UE-to-Network Relay releases the PC5 connections with its 5G ProSe Remote UEs indicating it is temporarily not available, so the Remote UE can (re)select another 5G ProSe UE-to-Network Relay. The 5G ProSe UE-to-Network Relay does not perform UE-to-Network Relay Discovery as described in clause 6.3.2.3 and does not accept any PC5 connections for relaying until the back-off timer expires if the 5G ProSe UE-to-Network Relay is in CM_IDLE state.
A Remote UE may also be subject to NAS level congestion control as specified in TS 23.501.
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5.13  Support for PC5 DRX operations  p. 66

5.13.1  General  p. 66

PC5 DRX operations are supported to enable 5G ProSe-enabled UE power saving for the following functions:
  • 5G ProSe Direct Discovery;
  • Unicast, groupcast and broadcast mode 5G ProSe Direct Communication;
  • 5G ProSe UE-to-Network Relay Discovery and 5G ProSe UE-to-Network Relay Communication.
Support for PC5 DRX operations in the AS layer is specified in TS 38.300.

5.13.2  PC5 DRX operations for 5G ProSe Direct Discovery and 5G ProSe UE-to-Network Relay Discovery  p. 66

For 5G ProSe Direct Discovery and 5G ProSe UE-to-Network Relay Discovery when the UE is "not served by NG-RAN", the UE uses the provisioned default PC5 DRX configuration for PC5 DRX operation as specified in clause 5.1.2.1 and clause 5.1.4.1, respectively.

5.13.3  PC5 DRX operations for 5G ProSe Direct Communication and 5G ProSe UE-to-Network Relay Communication  p. 66

The ProSe layer determines the respective ProSe services (i.e. ProSe identifiers) and derives the corresponding PC5 QoS parameters based on either the mapping of ProSe services (i.e. ProSe identifiers) to PC5 QoS parameters, or the ProSe Application Requirements for the ProSe services (i.e. ProSe identifiers) provided by the application layer. For broadcast and groupcast, the ProSe layer also determines the NR Tx Profile based on the mapping of ProSe services (i.e. ProSe identifiers) to NR Tx Profiles as described in clause 5.1.3.1. The ProSe layer passes the PC5 QoS parameters and destination Layer-2 ID to the AS layer as specified in clauses 6.4.1, 6.4.2 and 6.4.3. The ProSe layer also passes the corresponding NR Tx Profile to the AS layer, if the ProSe layer has determined the corresponding NR Tx Profile.
For broadcast or groupcast, the ProSe layer maintains a list of all ProSe services (i.e. ProSe identifiers), i.e. activated ProSe services and/or ProSe services that the UE is interested for reception, for a given destination Layer-2 ID and determines the NR Tx Profile to be mapped for the respective ProSe service based on the configuration described in clause 5.1.3.1. Whenever the list of the ProSe services for a given destination Layer-2 ID changes, the ProSe layer updates the AS layer for the NR Tx Profiles information by providing all the mapped NR Tx Profiles to the AS layer for the given destination Layer-2 ID, e.g. when providing other information such as the destination Layer-2 ID, PC5 QoS parameters.
For broadcast, the mapping from destination Layer-2 ID to NR Tx Profile is configured in the NG-RAN. The NG-RAN may derive the NR Tx Profile from the destination Layer-2 ID to perform the network scheduled operation mode, alignment of Uu DRX and PC5 DRX, etc.
When the PC5 DRX operation is needed, the AS layer determines the PC5 DRX parameter values for 5G ProSe Direct Communication or 5G ProSe UE-to-Network Relay Communication over PC5 reference point, taking into account, e.g., PC5 QoS parameters and/or destination Layer-2 ID provided by the ProSe layer.
For broadcast and groupcast, the UE enables the PC5 DRX based on the NR Tx Profile.
For unicast mode 5G ProSe Direct Communication and 5G ProSe UE-to-Network Relay Communication, two UEs may negotiate the PC5 DRX configuration in the AS layer and the PC5 DRX parameter values can be configured per pair of source and destination Layer-2 IDs in the AS layer. For transmitting and receiving Direct Communication Request messages, a default PC5 DRX configuration may be used (see TS 38.300).
For broadcast and groupcast when the UE is "not served by NG-RAN", the UE uses the provisioned PC5 DRX configuration for PC5 DRX operation as specified in clause 5.1.3.1.
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5.14  5G ProSe UE-to-UE Relay Communication |R18|  p. 67

5.14.1  5G ProSe Layer-3 UE-to-UE Relay Communication  p. 67

The 5G ProSe Layer-3 UE-to-UE Relay shall provide generic function that can relay any IP, Ethernet or Unstructured traffic.
The 5G ProSe Layer-3 UE-to-UE Relay provides the functionality to support connectivity between 5G ProSe Layer-3 End UEs. A 5G ProSe Layer-3 UE-to-UE Relay and 5G ProSe Layer-3 End UEs can be located within NG-RAN coverage or outside of NG-RAN coverage.
The type of traffic supported over PC5 reference point is indicated by the 5G ProSe Layer-3 UE-to-UE Relay e.g. using the corresponding RSC.
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5.14.2  5G ProSe Layer-2 UE-to-UE Relay Communication  p. 67

The 5G ProSe Layer-2 UE-to-UE Relay provides forwarding functionality that can relay any type of traffic over the PC5 link.
The 5G ProSe Layer-2 UE-to-UE Relay provides the functionality to support connectivity between 5G ProSe Layer-2 End UEs. A 5G ProSe Layer-2 UE-to-UE Relay and 5G ProSe Layer-2 End UEs can be located within NG-RAN coverage or outside of NG-RAN coverage.

5.15  Path switching between two UE-to-Network Relays |R18|  p. 67

If relay reselection has been triggered (see clause 6.5.3), the 5G ProSe Remote UE performs the path switching between two UE-to-Network Relays based on the URSP rules of the 5G ProSe Remote UE traffic handling described in clause 6.5.4, 5G ProSe Policy in clause 5.1.4, or information from application layer (if available) with following considerations:
  • The 5G ProSe Remote UE shall first select the target path which has same type with the original path (i.e. 5G ProSe Layer-2 UE-to-Network Relay, 5G ProSe Layer-3 UE-to-Network Relay with N3IWF and 5G ProSe Layer-3 UE-to-Network Relay without N3IWF).
  • If the same type target path is not available, then this 5G ProSe Remote UE re-evaluates the URSP as specified in clause 6.5.4, or 5G ProSe Policy in clause 5.1.4 for the target path selection.
The principles from the 5G ProSe Policy in clause 5.1.4 and URSP rules of the 5G ProSe Remote UE traffic handling described in clause 6.5.4 take the higher priority than the information from the application layer.
The path switching procedures between two indirect network communication paths are as described in clause 6.5.5.
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5.16  Communication path switching between PC5 and Uu reference points |R18|  p. 68

The "communication path switching between PC5 and Uu reference points" refers to the procedure on how a UE switches between Uu communication path and direct PC5 communication path when it is communicating with another UE as illustrated in Figure 5.16-1. The direct communication path over PC5 reference point means that the communication with another UE is performed by using 5G ProSe Direct Communication only. The communication path over Uu reference point means that the communication with another UE is performed via the network.
Copy of original 3GPP image for 3GPP TS 23.304, Fig. 5.16-1: Example scenario of communication path switching between PC5 and Uu reference points (i.e. switching between Figure a and Figure b)
Figure 5.16-1: Example scenario of communication path switching between PC5 and Uu reference points (i.e. switching between Figure a and Figure b)
(⇒ copy of original 3GPP image)
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Communication path switching between PC5 and Uu reference points is performed at ProSe service level, e.g. some of ProSe services of PC5 unicast link or PDU Session, all ProSe services of PC5 unicast link or PDU Session.
The UE takes the path selection policy into account to determine whether and for which ProSe service(s) to switch the communication path. UE operation related to the path switch for ProSe service is as follows:
  • The UE evaluates the path selection policy in the policy and parameters for ProSe Direct Communication applicable to the ProSe service and switches the communication path as below:
    • If the Uu path is used and PC5 preferred is indicated, the UE should prefer to switch to the PC5 communication path for the ProSe service.
    • If the PC5 path is used and Uu preferred is indicated, the UE should prefer to switch to the Uu communication path for the ProSe service.
    • If no preference is indicated or no path selection policy is provisioned, the UE switches between Uu and PC5 communication path based on its pre-configuration or implementation for the ProSe service.
The UE may decide to perform path switching from Uu path to PC5 path, e.g. because the peer UE is in proximity or for offloading some traffic from the network. The UE may decide to perform path switching from PC5 path to Uu path, e.g. when the PC5 signal strength of the unicast link with the peer UE becomes weak.
At path switching, the same traffic type between for PDU Session and for PC5 unicast link is maintained, i.e. IP type, Ethernet type and Unstructured type.
For communication path switching from PC5 to Uu reference point, the Uu QoS parameters of each UE are decided based on PC5 QoS parameters and negotiated via the existing PC5 connection between two UEs. Each UE may perform the UE requested PDU Session Modification as defined in clause 4.3.3 of TS 23.502 to request the Uu QoS parameters. For IP type traffic, the IP addresses/prefixes of the peer UEs over the Uu reference point can be shared via the existing PC5 connection between the UEs by using application layer signalling to assist them to establish communication with each other via the communication path over the Uu reference point.
When both Uu path and PC5 path are available, a make-before-break mechanism is used when switching between PC5 path and Uu path. That is, the UEs first prepare the target path to communicate with each other and then perform the path switching and communicate over the target path and after this the former path may be released.
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5.17  Multi-path communication via Uu and via 5G ProSe UE-to-Network Relay |R18|  p. 69

5.17.1  General  p. 69

Multi-path communication uses one direct network communication path and one indirect network communication path with UE-to-Network Relay as illustrated in Figure 5.17.1-1, where path #1 shows the direct communication path via Uu and path #2 is shows the indirect communication path via 5G ProSe UE-to-Network Relay.
Copy of original 3GPP image for 3GPP TS 23.304, Fig. 5.17.1-1: Multi-path communication via Uu and via 5G ProSe UE-to-Network Relay
Figure 5.17.1-1: Multi-path communication via Uu and via 5G ProSe UE-to-Network Relay
(⇒ copy of original 3GPP image)
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The indirect communication path may be via a 5G ProSe Layer-2 UE-to-Network Relay, or via a 5G ProSe Layer-3 UE-to-Network Relay with or without N3IWF.

5.17.2  Multi-path communication via direct Uu path and via 5G ProSe Layer-3 UE-to-Network Relay  p. 69

A 5G ProSe Layer-3 Remote UE may access the network via a 5G ProSe Layer-3 UE-to-Network Relay with or without N3IWF.
When a 5G ProSe Layer-3 Remote UE accesses the network via a 5G ProSe Layer-3 UE-to-Network Relay without N3IWF, the multi-path communication via direct Uu path and via the UE-to-Network Relay may be applied to the application traffic.
When a 5G ProSe Layer-3 Remote UE accesses the network via a 5G ProSe Layer-3 UE-to-Network Relay with N3IWF, the Layer-3 Remote UE connection via Layer-3 UE-to-Network Relay with N3IWF is considered as "untrusted non-3GPP access to 5GC via N3IWF", therefore the multi-path communication can be achieved using MA PDU Session in ATSSS features specified in clause 5.32 of TS 23.501.
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5.18  Support of Public Warning Notification Relaying |R18|  p. 70

As specified in TS 22.268, if UE that is authorised to act as 5G ProSe UE-to-Network Relay receives Public Warning Notification from the network and the UE is configured to perform Public Warning relaying, it shall handle the Public Warning message acting in the role of 5G ProSe UE-to-Network Relay.
The 5G ProSe UE-to-Network Relay broadcasts the warning message i.e. SIB 6/7/8 received from the network to the 5G ProSe Remote UE(s) by using Broadcast mode of 5G ProSe direct communication as specified in clause 5.3.2. The 5G ProSe UE-to-Network Relay uses a configured Destination Layer-2 ID(s) as specified in clause 5.1.4.1 when broadcasting the warning message and the 5G ProSe Remote UE receives warning messages broadcasted over PC5 reference point by using a configured Destination Layer-2 ID(s) as specified in clause 5.1.4.1. The PC5 QoS parameters as specified in clause 5.1.4.1 are used to broadcast and receive the warning message for the 5G ProSe UE-to-Network Relay and the 5G ProSe Remote UE, respectively.
A 5G ProSe Remote UE can receive the broadcasted warning message without establishing a connection to the 5G ProSe UE-to-Network Relay.
The 5G ProSe UE-to-Network Relay performs the duplication detection function as specified in TS 23.041 to suppress the received duplicated warning messages over Uu. The 5G ProSe UE-to-Network Remote UE performs the duplication detection function as specified in TS 23.041 to detect duplicated warning messages received over PC5 and/or Uu.
If the 5G ProSe Layer-2 UE-to-Network Relay does not have Destination Layer-2 ID to broadcast Public Warning messages, the 5G ProSe Layer-2 UE-to-Network Relay forwards the PWS SIBs (i.e. SIB 6/7/8) to a connected 5G ProSe Layer-2 Remote UE over the unicast link as specified in TS 38.300.
The Public Warning System architecture for 5G System is specified in TS 23.041.
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