WO2025095765A1

WO2025095765A1 - Ue-to-ue relay communication

Info

Publication number: WO2025095765A1
Application number: PCT/KR2024/096457
Authority: WO
Inventors: Tingyu XIN
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 2023-11-03
Filing date: 2024-11-04
Publication date: 2025-05-08
Anticipated expiration: 2026-05-03
Also published as: GB2635274A; GB202316926D0; GB202415088D0; GB202317290D0

Abstract

There is disclosed a method, for a second UE providing a relay function between a first UE and a third UE. A first ID of a first type is allocated to the first UE. The method comprises: allocating a first ID of a second type to the first UE; and transmitting, to the first UE, a first message comprising the first ID of the first type and the first ID of the second type. The first UE makes an association between the first ID of the first type and the first ID of the second type based on the first message.

Description

UE-TO-UE RELAY COMMUNICATION

Certain examples of the present disclosure provide one or more techniques for UE-to-UE relay communication. For example, certain examples provide one or more techniques for supporting mapping between ProSe L2 ID and local ID in a 3^rd Generation Partnership Project (3GPP) 5^th Generation (5G) New Radio (NR) network.

Various acronyms, abbreviations and definitions used in the present disclosure are defined at the end of this description.

Overview of 5G ProSe UE-to-UE Relay

5^th generation (5G) proximity serivce (ProSe) user equipment (UE)-to-UE (U2U) Relay scenarios is supported in 3GPP release (Rel)-18 for both public safety services and commercial services (e.g. interactive service). The scenarios for both 5G ProSe Layer-3 UE-to-UE Relay Communication and 5G ProSe Layer-2 UE-to-UE Relay Communication were standardized. The Reference architecture of Layer-2 and Layer-3 5G ProSe UE-to-UE Relay is shown in Figure 1. The 5G ProSe End UEs communicate with each other via a 5G ProSe UE-to-UE Relay. Both 5G ProSe Layer-2 and Layer-3 UE-to-UE Relay provides the relaying functionality to support connectivity between 5G ProSe End UEs. The two End UEs are named as source 5G ProSe End UE and target 5G ProSe End UE between which traffic is transmitted.

The control and user plane protocol stack of Layer-2 UE-to-UE relay are shown in Figures 2a and 2b.

Compared to layer-2 UE-to-UE relay, the control and user plane protocol stack of layer-3 UE-to-UE relay is the same as those of PC5 unicast link between 5G ProSe End UEs (see Figures 3 and 4).

Overview of 5G ProSe UE-to-UE Relay Discovery

Similar to the scenarios of 5G ProSe UE-to-Network relay Discovery and 5G ProSe direct Discovery, both discovery Model A and Model B are applicable in 5G ProSe UE-to-UE Relay Discovery scenarios.

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

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

To perform 5G ProSe UE-to-UE Relay Discovery, the 5G ProSe End UE and the 5G ProSe UE-to-UE Relay are pre-configured or provisioned with the related information.

5G ProSe UE-to-UE Relay Discovery Model A

The procedure for 5G ProSe UE-to-UE Discovery with Model A are shown in Figure 5, as defined in clause　6.3.2.4.2 and 5.8.4.2 of 3GPP TS　23.304.

In step 1, For UE-to-UE Relay discovery, the UE-to-UE Relay has already other UEs in proximity (e.g. via a previous 5G ProSe UE-to-UE Relay Discovery or 5G ProSe UE-to-UE Relay Communication procedures). The 5G ProSe UE-to-UE Relay obtains the User Info ID of other UEs in proximity per RSC.

In step 2, the 5G ProSe UE-to-UE Relay sends a UE-to-UE Relay Discovery Announcement message. The UE-to-UE Relay Discovery Announcement message contains the Type of Discovery Message, User Info ID of the 5G ProSe UE-to-UE Relay, RSC and list of User Info ID of the 5G ProSe End UEs supporting the RSC. The UE-to-UE Relay Discovery Announcement message is sent using the Source Layer-2 ID and Destination Layer-2 ID as described in clause 5.8.4 of TS　23.304.

5G ProSe UE-to-UE Relay Discovery Model B

The procedure for 5G ProSe UE-to-UE Discovery with Model B are shown in Figure 6, as defined in clause　6.3.2.4.3 and 5.8.4.2 of 3GPP TS　23.304.

In step 1, the discoverer 5G ProSe End UE (UE-1) sends a 5G ProSe UE-to-UE Relay Discovery Solicitation message. The 5G ProSe UE-to-UE Relay Discovery Solicitation message contains the Type of Discovery Message, User Info ID of itself, RSC and User Info ID of the discoveree 5G ProSe End UE (UE-2) and is sent using the Source Layer-2 ID (src L2 ID) and Destination Layer-2 ID (dst L2 ID) as described in clause　5.8.4 of 3GPP TS　23.304.

- Source Layer-2 ID is self-selected by UE-1;

- Destination L2 ID is selected based on the configuration as described in clause 5.1.5.1 of 3GPP TS　23.304.

In step 2, a 5G ProSe UE-to-UE Relay that matches the RSC sends a 5G ProSe UE-to-UE Relay Discovery Solicitation message. The 5G ProSe UE-to-UE Relay Discovery Solicitation message contains the Type of Discovery Message, User Info ID of the discoverer 5G ProSe End UE (UE-1), User Info ID of UE-to-UE Relay, RSC and User Info ID of the discoveree 5G ProSe End UE (UE-2) and is sent using the Source Layer-2 ID and Destination Layer-2 ID as described in clause　5.8.4 of 3GPP TS　23.304.

- Source Layer-2 ID is self-selected by UE-1;

In step 3, the discoveree 5G ProSe End UE (UE-2) that matches the RSC and the User Info ID of the discoveree 5G ProSe End UE (UE-2) responds to the 5G ProSe UE-to-UE Relay with a 5G ProSe UE-to-UE Relay Discovery Response message. The 5G ProSe UE-to-UE Relay Discovery Response message contains the Type of Discovery Message, RSC, User Info ID of the discoverer 5G ProSe End UE (UE-1) and User Info ID of discoveree 5G ProSe End UE (UE-2) and is sent using the Source Layer-2 ID and Destination Layer-2 ID as described in clause　5.8.4 of 3GPP TS　23.304. If the discoveree 5G ProSe End UE (UE-2) receives multiple UE-to-UE Relay Discovery Solicitation messages from different 5G ProSe UE-to-UE Relays with the same RSC and the User Info ID of the discoveree 5G ProSe End UE (UE-2), it may choose to respond or not to a 5G ProSe UE-to-UE Relay (e.g. based on the PC5 signal strength of each message received).

- Source Layer-2 ID: the discoveree 5G ProSe End UE (UE-2) self-selects a Source Layer-2 ID for 5G ProSe UE-to-UE Relay Discovery Response message.

- Destination Layer-2 ID: set to the Source Layer-2 ID of the received 5G ProSe UE-to-UE Relay Discovery Solicitation message.

In step 4, the 5G ProSe UE-to-UE Relay sends a 5G ProSe UE-to-UE Relay Discovery Response message. The 5G ProSe UE-to-UE Relay Discovery Response message contains the Type of Discovery Message, User Info ID of UE-to-UE Relay, RSC, User Info ID of the discoverer 5G ProSe End UE (UE-1) and User Info ID of the discoveree 5G ProSe End UE (UE-2) and is sent using the Source Layer-2 ID and Destination Layer-2 ID as described in clause　5.8.4 of 3GPP TS　23.304.

- Source Layer-2 ID: the 5G ProSe UE-to-UE Relay self-selects a Source Layer-2 ID for 5G ProSe UE-to-UE Relay Discovery Response message.

Overview of 5G ProSe UE-to-UE Relay Communication

In Rel-18, the 5G ProSe Communication via 5G ProSe Layer-3 and Layer-2 UE-to-UE Relay were both specified, e.g. in clauses 5.14, 6.4.3.7, 6.7 of 3GPP TS　23.304.

The 5G ProSe UE-to-UE Relay Communication uses direct commutation between two 5G ProSe enabled UEs as baseline with introducing additional features to build two hops between source and relay UEs, and relay and target UEs for an end-to-end solution.

The layer-2 link management related issue are specified in clause　6.4.3.7 of 3GPP TS　23.304. the procedures of 5G prose communication via 5GC prose layer-2 and layer-3 UE-to-UE (U2U) relay are specified in clause　6.7 of 3GPP TS　23.304.

5G ProSe Communication via 5G ProSe Layer-3 UE-to-UE Relay

The 5G ProSe Layer-3 UE-to-UE Relay provides the functionality to support connectivity between 5G ProSe Layer-3 End UEs, and can provide generic function that can relay any IP, Ethernet or Unstructured traffic. 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.

In

step

1 and 2, Service authorization and provisioning are performed for source 5G ProSe Layer-3 End UE, target 5G ProSe Layer-3 End UE and 5G ProSe Layer-3 UE-to-UE Relay, and the source 5G ProSe Layer-3 End UE performs discovery of a 5G ProSe Layer-3 UE-to-UE Relay.

3. The source 5G ProSe Layer-3 End UE sends a Direct Communication Request message to initiate the unicast Layer-2 link establishment procedure with the 5G ProSe Layer-3 UE-to-UE Relay. The parameters included in the Direct Communication Request message are:

- User Info ID of source and target 5G ProSe End UEs, and User Info ID of 5G ProSe UE-to-UE Relay

- optional) Destination Layer-2 ID of target 5G ProSe End UE

- Other info: i.e. ProSe Service Info, RSC, Security Information, etc.

The Source Layer-2 ID of the Direct Communication Request message is self-assigned by the source 5G ProSe Layer-3 End UE and the Destination Layer-2 ID is set to the Source Layer-2 ID of the discovery message of the 5G ProSe Layer-3 UE-to-UE Relay.

4. If the User Info ID of 5G ProSe Layer-3 UE-to-UE Relay in the Direct Communication Request message matches the 5G ProSe UE-to-UE Relay's User Info ID and the RSC in the Direct Communication Request matches one RSC that the relay supports, the 5G ProSe Layer-3 UE-to-UE Relay responds by establishing the security with the source 5G ProSe Layer-3 End UE. When the security protection is enabled, the source 5G ProSe Layer-3 End UE sends the parameters as described in clause 6.4.3.7.3 of 3GPP TS　23.304 to the 5G ProSe Layer-3 UE-to-UE Relay.

The Source Layer-2 ID used for the security establishment procedure is self-assigned by the 5G ProSe Layer-3 UE-to-UE Relay and the Destination Layer-2 ID is set to the Source Layer-2 ID of the received Direct Communication Request message.

Upon receiving the security establishment procedure messages, the source 5G ProSe Layer-3 End UE obtains the 5G ProSe Layer-3 UE-to-UE Relay's Layer-2 ID for future communication, for signalling and data traffic for this unicast link.

5. After the Security Establishment procedure in step 4 is completed, the 5G ProSe Layer-3 UE-to-UE Relay sends a Direct Communication Request message to initiate the unicast Layer-2 link establishment procedure with the target 5G ProSe Layer-3 End UE. The parameters included in the Direct Communication Request message are described in clause 6.4.3.7.1 of 3GPP TS　23.304:

- Other info: i.e. ProSe Service Info, RSC, Security Information, etc.

The Source Layer-2 ID of the Direct Communication Request message is self-assigned by the 5G ProSe Layer-3 UE-to-UE Relay and the Destination Layer-2 ID is the unicast Layer-2 ID of target 5G ProSe Layer-3 End UE associated with the User Info ID of target 5G ProSe Layer-3 End UE.

6. If the User Info ID of target 5G ProSe Layer-3 End UE and RSC included in the Direct Communication Request match the target UE's User Info ID and the RSC that the target UE supports, the target 5G ProSe Layer-3 End UE responds by establishing the security with the 5G ProSe Layer-3 UE-to-UE Relay. When the security protection is enabled, the 5G ProSe Layer-3 UE-to-UE Relay sends the parameters as described in clause 6.4.3.7.1 of 3GPP TS　23.304 to the target 5G ProSe Layer-3 End UE.

The Source Layer-2 ID used for the security establishment procedure is self-assigned by the target 5G ProSe Layer-3 End UE and the Destination Layer-2 ID is set to the Source Layer-2 ID of the received Direct Communication Request message.

Upon receiving the security establishment procedure messages, the 5G ProSe Layer-3 UE-to-UE Relay obtains the target 5G ProSe Layer-3 End UE's Layer-2 ID for future communication, for signalling and data traffic for this unicast link.

7. The target 5G ProSe Layer-3 End UE sends a Direct Communication Accept message to the 5G ProSe Layer-3 UE-to-UE Relay that has successfully established security with. The parameters included in the Direct Communication Accept message are described in clause 6.4.3.7.1 of 3GPP TS　23.304:

- User Info ID of target 5G ProSe End UE.

8. For IP traffic, IPv6 prefix or IPv4 address is allocated for the target 5G ProSe Layer-3 End UE as defined in clause 5.5.1.4 of 3GPP TS　23.304.

9. After receiving the Direct Communication Accept message from the target 5G ProSe Layer-3 End UE, the 5G ProSe Layer-3 UE-to-UE Relay sends a Direct Communication Accept message to the source 5G ProSe Layer-3 End UE that has successfully established security with. The parameters included in the Direct Communication Accept message are described in clause 6.4.3.7.1 of 3GPP TS　23.304:

- User Info ID of target 5G ProSe End UE.

- User Info ID of 5G ProSe UE-to-UE Relay.

10. For IP traffic, IPv6 prefix or IPv4 address is allocated for the source 5G ProSe Layer-3 End UE.

11. For IP communication, the 5G ProSe Layer-3 UE-to-UE Relay may store an association of User Info ID and the IP address of target 5G ProSe Layer-3 End UE into its DNS entries and the 5G ProSe Layer-3 UE-to-UE Relay may act as a DNS server to other UEs. The source 5G ProSe Layer-3 End UE may send a DNS query to the 5G ProSe Layer-3 UE-to-UE Relay to request IP address of target 5G ProSe Layer-3 End UE after step 10 if the IP address of target 5G ProSe Layer-3 End UE is not received in step 9 and the 5G ProSe Layer-3 UE-to-UE Relay returns the IP address of the target 5G ProSe Layer-3 End UE to the source 5G ProSe Layer-3 End UE.

For Ethernet communication, the 5G ProSe Layer-3 UE-to-UE Relay maintains the association between PC5 links and Ethernet MAC addresses received from the 5G ProSe Layer-3 End UE.

For Unstructured traffic communication, for each pair of source and target 5G ProSe Layer-3 End UEs, the 5G ProSe Layer-3 UE-to-UE Relay maintains the 1:1 mapping between the PC5 link with source 5G ProSe Layer-3 End UE and the PC5 link with target 5G ProSe Layer-3 End UE.

12. The source 5G ProSe Layer-3 End UE communicates with the target 5G ProSe Layer-3 End UE via the 5G ProSe Layer-3 UE-to-UE Relay.

5G ProSe Communication via 5G ProSe Layer-2 UE-to-UE Relay

The 5G ProSe Layer-2 UE-to-UE Relay provides the functionality to support connectivity between 5G ProSe Layer-2 End UEs, and provide forwarding functionality that can relay any type of traffic over the PC5 link. In clause　6.7.2 of 3GPP TS　23.304, the procedures of to 5G ProSe Layer-2 UE-to-UE Relay are defined. Service authorization and provisioning has been performed for the 5G ProSe Layer-2 UE-to-UE Relay and the 5G ProSe End UEs as described in clause　6.2 of 3GPP TS　23.304 before this procedure.

For now, SA2 has not fully specified the 5G ProSe Communication via 5G ProSe Layer-2 UE-to-UE Relay, i.e. some procedures and information carried by different messages. Further RAN WG agreements and coordination with RAN WG are required.

Step　1-7 and step　9 in Figure 7: Layer-2 link establishment via 5G ProSe Layer-3 UE-to-UE Relay can be reused for 5G ProSe Communication via 5G ProSe Layer-2 UE-to-UE Relay. High level descriptions and procedures of 5G ProSe Communication via 5G ProSe Layer-2 UE-to-UE Relay are shown in Figure 8: 5G ProSe Communication via 5G ProSe Layer-2 UE-to-UE Relay. Step 1-3 in Figure 8 can be replaced by Step　1-7 and step　9 in Figure 7: Layer-2 link establishment via 5G ProSe Layer-3 UE-to-UE Relay.

1. Model A or Model B 5G ProSe UE-to-UE Relay Discovery is performed and a source 5G ProSe End UE selects a suitable 5G ProSe Layer-2 UE-to-UE Relay for the communication with a target 5G ProSe End UE.

2. The source 5G ProSe End UE decides whether to use an existing PC5 link with the 5G ProSe UE-to-UE Relay for the required service. If an existing PC5 link is used then the Layer-2 link modification procedure as specified is used towards a 5G ProSe UE-to-UE Relay, otherwise a Layer-2 link establishment procedure is used towards a 5G ProSe UE-to-UE Relay.

- Similar to Layer-3 U2U relay communication, in order to establish the layer-2 link between source 5G ProSe End UE and 5G ProSe Layer-2 UE-to-UE Relay, the source 5G ProSe End UE send Direct Communication Request message to initiate the unicast layer-2 link establishment procedure. The Direct Communication Request message includes, as specified in clause　6.4.3.1 of 3GPP TS　23.304:

o Source User Info: the initiating UE's Application Layer ID (i.e. UE-1's Application Layer ID).

o If the ProSe application layer provided the target UE's Application Layer ID in step 2, the following information is included:

- Target User Info: the target UE's Application Layer ID (i.e. UE-2's Application Layer ID).

o ProSe Service Info: and Security Information

- The source Layer-2 ID and destination Layer-2 ID used to send the Direct Communication Request message are determined as specified in clauses 5.8.2.1 and 5.8.2.4 of 3GPP TS　23.304. The destination Layer-2 ID may be broadcast or unicast Layer-2 ID. When unicast Layer-2 ID is used, the Target User Info shall be included in the Direct Communication Request message.

- For 5G ProSe direct communication, during the PC5 unicast link establishment procedure, Layer-2 IDs are exchanged and should be used for future communication between the two UEs. This rule should be also applicable for the 1^st hop between source 5G ProSe End UE and 5G ProSe Layer-2 UE-to-UE Relay.

- This procedure is towards the selected 5G ProSe UE-to-UE Relay and for Layer-2 link establishment, the security establishment is performed before step　3 is initiated.

3. The 5G ProSe Layer-2 UE-to-UE Relay decides whether to use an existing PC5 link between the 5G ProSe UE-to-UE Relay and the target 5G ProSe End UE for the required service and initiates Layer-2 link establishment procedure or Layer-2 link modification procedure with the target 5G ProSe End UE.

This procedure is performed towards the target 5G ProSe End UE using the unicast Layer-2 ID.

The 5G ProSe Layer-2 UE-to-UE Relay sends a Direct Communication Accept message or Link Modification Accept message to the source 5G ProSe End UE after step　3 is completed.

4. The source 5G ProSe End UE establishes an end-to-end connection for unicast mode communication with the target 5G ProSe End UE as described in clause　6.4.3.7.

The data is transferred between the source 5G ProSe End UE and the target 5G ProSe End UE via the 5G ProSe Layer-2 UE-to-UE Relay. The 5G ProSe Layer-2 UE-to-UE Relay forwards all the data messages between the source 5G ProSe End UE and the target 5G ProSe End UE, as specified in 3GPP TS　38.300　[12].

Link Identifier Update

Link identifier update for a unicast link in direct communication

Figure 9: Link identifier update procedure in clause 6.4.3.2 of 3GPP TS 23.304 shows the link identifier update procedure for a unicast link. The identifiers used for the unicast mode of 5G ProSe communication over PC5 reference point (e.g. Application Layer ID, Source Layer-2 ID and IP address/prefix) might updated over change due to the configured privacy requirements. A UE (5G prose layer-2/layer-3 source/target end UE) may decide to change the identifiers for other reasons, e.g. application layer requirement. to prevent service interruptions, the new identifiers should be exchanged between the source source and the peer UEs.

0. UE-1 and UE-2 have a unicast link established.

1. UE-1 decides to change its identifier(s), and generates its new Layer-2 ID and sends a Link Identifier Update Request message to UE-2 using the old identifiers.

The Link Identifier Update Request message includes the new identifier(s) to use (including the new Layer-2 ID, Security Information, optionally the new Application Layer ID and optionally new IP address/prefix if IP communication is used).

After sending the Link Identifier Update Request message, if the UE-1 has data to send, UE-1 keeps sending data traffic to UE-2 with the old identifiers until UE-1 sends the Link Identifier Update Ack message to UE-2.

2. Upon reception of the Link Identifier Update Request message, UE-2 changes its identifier(s).

UE-2 responds with a Link Identifier Update Response message which includes the new identifier(s) to use (including the new Layer-2 ID, Security Information, optionally the new Application Layer ID and optionally a new IP address/prefix if IP communication is used).

UE-2 continues to receive traffic with the old Layer-2 ID from UE-1 until UE-2 receives traffic with the new Layer-2 ID from UE-1. After sending the Link Identifier Update Response message, UE-2 keeps sending data traffic to UE-1 with the old identifier, if UE-2 has data to send, until UE-2 receives the Link Identifier Update Ack message from UE-1.

3. Upon reception of the Link Identifier Update Response message, UE-1 responds with a Link Identifier Update Ack message.

The Link Identifier Update Ack message includes the new identifier(s) from UE-2, as received on the Link Identifier Update Response message.

The Link Identifier Update Ack message is sent using the old identifiers. UE-1 continues to receive traffic with the old Layer-2 ID from UE-2 until UE-1 receives traffic with the new Layer-2 ID from UE 2.

4. The ProSe layer of UE-1 passes the PC5 Link Identifier for the unicast link and the updated Layer-2 IDs (i.e. new Layer-2 ID for UE-1 for the source and new Layer-2 ID of UE-2 for the destination) down to the AS layer. This enables the AS layer to update the provided Layer-2 IDs for the unicast link.

UE-1 starts using its new identifiers and UE-2's new identifiers for this unicast link.

5. Upon reception of the Link Identifier Update Ack message, the ProSe layer of UE-2 passes the PC5 Link Identifier for the unicast link and the updated Layer-2 IDs (i.e. new Layer-2 ID of UE-2 for the source and new Layer-2 ID for UE-1 for the destination) down to the AS layer. This enables the AS layer to update the provided Layer-2 IDs for the unicast link.

UE-2 starts using its new identifiers and UE-1's new identifiers for this unicast link.

Link identifier update for PC5 communication via 5G ProSe UE-to-UE Relay

Some of the Link Identifier Update procedure for a unicast link in direct communication described above under the preceding heading is reused for link identifier update for PC5 communication via 5G ProSe UE-to-UE Relay. Procedures of link identifier update for PC5 communication via 5G ProSe UE-to-UE Relay are shown in Figure 10.

0. A PC5 link is established between UE1 and a UE-to-UE Relay and another PC5 link is established between the UE-to-UE Relay and UE2 for communication between UE-1 and UE-2.

1. According to the privacy requirement, UE1 may trigger link identifier update procedure. UE1 sends a Link Identifier Update Request message to the UE-to-UE Relay including the following parameters:

- its new Layer-2 ID, new security information, new Application layer ID (if provided by the upper layer).

- its peer UEs information (e.g. UE2's IP address, UE2's Application layer ID).

2. UE-to-UE Relay self-assigns a new L2 ID, new security information and possibly new IP address/prefix for PC5 link with UE1.

- UE-to-UE Relay then sends a PC5 Relay Update Request message to each peer UE (e.g. UE2), including: UE1's old IP address/prefix, UE1's old and new Application layer ID, UE1's new IP address/prefix.

3. UE2 receives the PC5 Relay Update Request message and saves UE1's new IP address/prefix. UE2 sends a PC5 Relay Update Response message to the UE-to-UE Relay including all parameters received on the PC5 Relay Update Request message.

4. UE-to-UE Relay sends a Link Identifier Update Response message to UE1 including UE1's new IP address/prefix, UE-to-UE Relay's new Layer-2 ID, new security information and possibly new IP address/prefix and/or new Application layer ID.

5. UE1 saves its new IP address/prefix and UE-to-UE Relay's new parameters and sends a Link Identifier Update ACK message to the UE-to-UE Relay, including its new IP address received on the Link Identifier Update Response message.

UE1 and UE-to-UE Relay start using the new Layer-2 IDs and new security information for PC5 communication. UE1 starts using its new IP address for IP data exchange with UE2.

Overview of RAN2 requirements on ID mapping for traffic routing

RAN2 sent LS to SA2 in S2-2311980 / R2-2311566 to inform SA2 the requirements on mapping of different IDs for layer-2 U2U relay traffic routing between source and target end UEs via U2U relay.

In the LS, RAN　2 informed that:

In RAN2 #123, RAN2 agreed that, instead of a L2 ID, a Local ID is used as a UE ID in a SRAP header for L2 based U2U relay and two Local IDs are included in the SRAP header to identify source and target Remote UE respectively. In RAN2 # 123bis , RAN2 also agreed that the RRCReconfigurationSidelink message is used to indicate the Local ID pair from relay UE to source and target Remote UEs respectively.

RAN2 further discussed how to link the User Info ID with the Local ID at the remote UE , based on two options that are both feasible from RAN2 perspective, and then RAN2 made a working assumption that the RRCReconfigurationSidelink message carries both the L2 ID and the Local ID of the peer Remote UE and assumes that the association between the User Info ID and the L2 ID of the peer Remote UE is done at the Remote UE ProSe layer.

Therefore, RAN2 would ask SA2 if the above working assumption is feasible from SA2 perspective. RAN2 will appreciate timely response from SA2 before TSG -RAN#102. RAN2 intend to implement this working assumption if SA2 indicate it is feasible.

For now, the RAN2 agreements and working assumption cannot be supported by SA2 specifications.

The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.

The principal object of the embodiments herein is to provide methods to support Layer-2 user equipment (UE)-to-UE (U2U) relay traffic transmission between the source remote UE and target/peer remote UE via U2U relay by associating the Local ID and Layer-2 ID of the source end/remote UE and target/peer end UE.

Yet another object of the embodiments herein is to provide one or more techniques for associating the User Info ID, Layer-2 ID pair and Local ID, for 5G ProSe Layer-3 and layer-2 Relay discovery and communication.

It is an aim of certain examples of the present disclosure to address, solve and/or mitigate, at least partly, at least one of the problems and/or disadvantages associated with the related art, for example at least one of the problems and/or disadvantages described herein. It is an aim of certain examples of the present disclosure to provide at least one advantage over the related art, for example at least one of the advantages described herein.

The present invention is defined in the independent claims. Advantageous features are defined in the dependent claims. Embodiments or examples disclosed in the description and/or figures falling outside the scope of the claims are to be understood as examples useful for understanding the present invention.

Other aspects, advantages and salient features of the invention will become apparent to those skilled in the art from the following detailed description taken in conjunction with the accompanying drawings.

According to an example of the present disclosure, by establishing association with Layer-2 ID and Local ID, the layer-2 U2U relay UE can route the traffic correctly from source end UE to the target end UE or another way around.

Figure 1 illustrates an example of Reference architecture for 5^th generation (5G) proximity service (ProSe) user equipment (UE)-to-UE (U2U) Relay;

Figure 2a illustrates example of End-to-End control plane protocol stacks using a 5G ProSe Layer-2 UE-to-UE Relay;

Figure 2b illustrates example of End-to-End user plane protocol stacks using a 5G ProSe Layer-2 UE-to-UE Relay;

Figure 3a illustrates example of Discovery Plane PC5 Interface;

Figure 3b illustrates example of Control Plane for NR PC5 reference point;

Figure 4 illustrates an example of User Plane for NR PC5 reference point, between two 5G ProSe End UEs;

Figure 5 illustrates an example of 5G ProSe UE-to-UE Relay Discovery with Model A;

Figure 6 illustrates an example of 5G ProSe UE-to-UE Relay Discovery with Model B;

Figure 7 illustrates an example of Layer-2 link establishment via 5G ProSe Layer-3 UE-to-UE Relay;

Figure 8 illustrates an example of 5G ProSe Communication via 5G ProSe Layer-2 UE-to-UE Relay;

Figure 9 illustrates the Link Identifier Update procedure in clause 6.4.3.2 of 3GPP TS 23.304;

Figure 10 illustrates the Link Identifier Update procedure and IP address/prefix change with Layer-3 UE-to-UE Relay in clause 6.7.1.2 of 3GPP TS　23.304;

Figure 11 illustrates an example of 5G ProSe UE-to-UE Relay Discovery with Model B with IDs Numbered;

Figure 12 illustrates an example of 1^stand 2^nd pair L2 ID;

Figure 13 illustrates an example of data packets for U2U relay communication;

Figure 14 illustrates an example of the relationship between the layer-2 ID pairs and local ID pair;

Figure 15 illustrates an exemplary method for a second UE providing a relay function between a first UE and a third UE;

Figure 16 illustrates an exemplary method for a first UE, wherein a second UE provides a relay function between the first UE and a third UE; and

Figure 17 is a block diagram of an exemplary network entity that may be used in certain examples of the present disclosure.

The following description of examples of the present disclosure, with reference to the accompanying drawings, is provided to assist in a comprehensive understanding of the present invention, as defined by the claims. The description includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the examples described herein can be made without departing from the scope of the invention.

The same or similar components may be designated by the same or similar reference numerals, although they may be illustrated in different drawings.

Detailed descriptions of techniques, structures, functions, operations or processes known in the art may be omitted for clarity and conciseness, and to avoid obscuring the subject matter of the present invention.

The terms and words used herein are not limited to the bibliographical or standard meanings, but, are merely used to enable a clear and consistent understanding of the invention.

Throughout the description and claims of this specification, the words "comprise", "include" and "contain" and variations of the words, for example "comprising" and "comprises", means "including but not limited to", and is not intended to (and does not) exclude other features, elements, components, integers, steps, processes, operations, functions, characteristics, properties and/or groups thereof.

Throughout the description and claims of this specification, the singular form, for example "a", "an" and "the", encompasses the plural unless the context otherwise requires. For example, reference to "an object" includes reference to one or more of such objects.

Throughout the description and claims of this specification, language in the general form of "X for Y" (where Y is some action, process, operation, function, activity or step and X is some means for carrying out that action, process, operation, function, activity or step) encompasses means X adapted, configured or arranged specifically, but not necessarily exclusively, to do Y.

Features, elements, components, integers, steps, processes, operations, functions, characteristics, properties and/or groups thereof described or disclosed in conjunction with a particular aspect, embodiment, example or claim are to be understood to be applicable to any other aspect, embodiment, example or claim described herein unless incompatible therewith.

The skilled person will appreciate that the techniques described herein may be used in any suitable combination.

Certain examples of the present disclosure provide one or more techniques for UE-to-UE relay communication. For example, certain examples provide one or more techniques for supporting mapping between ProSe L2 ID and local ID in a 3^rd Generation Partnership Project (3GPP) 5^th Generation (5G) New Radio (NR) network. However, the skilled person will appreciate that the present invention is not limited to these examples, and may be applied in any suitable system or standard, for example one or more existing and/or future generation wireless communication systems or standards, including any existing or future releases of the same standards specification, for

example 3GPP

5G, 5G-advanced or 6^th Generation (6G).

The functionality of the various network entities and other features disclosed herein may be applied to corresponding or equivalent entities or features in the same or any other suitable communication systems or standards. Corresponding or equivalent entities or features may be regarded as entities or features that perform the same or similar role, function or purpose within the network.

A particular network entity may be implemented as a network element on a dedicated hardware, as a software instance running on a dedicated hardware, and/or as a virtualised function instantiated on an appropriate platform, e.g. on a cloud infrastructure.

The skilled person will appreciate that the present invention is not limited to the specific examples disclosed herein. For example:

- The techniques disclosed herein are not limited to 3GPP 5G.

- One or more entities in the examples disclosed herein may be replaced with one or more alternative entities performing equivalent or corresponding functions, processes or operations.

- One or more of the messages in the examples disclosed herein may be replaced with one or more alternative messages, signals or other type of information carriers that communicate equivalent or corresponding information.

- One or more further elements or entities may be added to the examples disclosed herein.

- One or more non-essential elements or entities may be omitted in certain examples.

- The functions, processes or operations of a particular entity in one example may be divided between two or more separate entities in an alternative example.

- The functions, processes or operations of two or more separate entities in one example may be performed by a single entity in an alternative example.

- Information carried by a particular message in one example may be carried by two or more separate messages in an alternative example.

- Information carried by two or more separate messages in one example may be carried by a single message in an alternative example.

- The order in which operations are performed and/or the order in which messages are transmitted may be modified, if possible, in alternative examples.

Certain examples of the present disclosure may be provided in the form of an apparatus/device/network entity configured to perform one or more defined network functions and/or a method therefor. Certain examples of the present disclosure may be provided in the form of a system (e.g. network or wireless communication system) comprising one or more such apparatuses/devices/network entities, and/or a method therefor.

3GPP RAN2 WG has agreed to use a newly introduced Local ID, instead of a L2 ID, as a UE ID in a SRAP header for layer-2 (L2) based U2U relay. The Local ID pair for source and target end UEs can be indicated via PC5 RRC messages from relay UE to source and target Remote UEs, respectively. Therefore, the link between User Info ID, Layer-2 ID and Local ID should be established and cleared in the specification.

For now, there is no specified solutions on how and who establish the association of User Info ID, Layer-2 ID and Local ID. If the association of User Info ID, Layer-2 ID and Local ID is not clarified, the layer-2 U2U relay would not be able to route the traffic from source end UE to the target end UE or another way around. As a result, the traffic cannot be transmitted correctly between 5G ProSe source end UE and target end UE via 5G ProSe UE-to-UE Relay. The summary of the 3 different IDs are in Table 1. The different layers of UE, i.e. the AS layer (Access Stratum layer) or ProSe layer, have different knowledge of the IDs. Therefore, to associate the IDs for traffic routing of layer-2 U2U relay, one or more layers of the end UEs and relay should have knowledge of all the IDs. And the association of IDs could be built on top of the UEs knowledge of the IDs.

Once the layer-2 ID of the source end UE, relay and target end UE are updated, the link between User Info ID, Layer-2 ID and Local ID should be also updated accordingly. Otherwise, the sending end UEs are not able to include the correct IDs into the data packet header, and the relay is not able to routing the data packets to the correct receiving peer end UE, and the receiving peer end UE is not able to determine the data is from which source end UE and whether the data is sent to itself or not.

Certain examples of the present disclosure provide one or more techniques for associating the User Info ID, Layer-2 ID pair and Local ID, for 5G ProSe Layer-3 and layer-2 Relay discovery and communication, i.e. for traffic routing by the relay for layer-2 U2U relay communication scenario.

In certain examples, the source 5G prose end UE, the target 5G prose end UE, and 5G prose layer-3/layer-2 relay are enabled to associate the user info ID and layer-2 ID based on interaction (via PC-5 signaling, i.e. discovery related messages, direct communication request and response, security establishment related messages). The association can be stored and maintained at source 5G prose end UE, the target 5G prose end UE, and 5G prose layer-3/layer-2 relay for further information association and matching. The 5G prose layer-3/layer-2 relay may indicate the association to the source end UE and target end UE. As result, the source end UE is aware of the association between layer-2 ID and user info of the target end UE (peer end/remote UE); the target end UE is aware of the association between layer-2 ID and user info of the source end UE (peer end/remote UE); the relay is aware of the associations between layer-2 ID and user info of the target end UE and source end UE, and optionally the associations between layer-2 ID and user info of the relay with paired with bot source end UE and target end UE.

Upon the two hops between the source end UE and relay, and between relay and target end UE are established, for U2U relay communication traffic routing purposes (for the relay to understand/ determine the data packet is going the specific source end UE or target end UE), the local ID will be linked/associated to user info ID. Upon receiving the Local ID and layer-2 ID of the peer end/ remote UE, the UE (source end UE and target end UE) match the layer-2 ID to the association with user info ID. Once the association is found, the local ID that links to the layer-2 ID can be associated to the user info ID for further data and message transmission. The association could be done in either prose layer or AS layer/ RRC layer.

When the UE (source end UE or target end UE) would like to send data/ traffic to the specific peer end/remote UE via the U2U relay, the UE will associate the user info ID of the peer end UE to the local ID (via layer-2 ID). The sending UE will include the layer-2 ID (pair) of relay and the UE and local ID (pair) of the peer end UE and the UE, optional layer-2 ID (pair) of the UE and peer end UE into the header of the data packet to the peer end UE.

Once the relay receives the data packet from the sending UE, based on the layer-2 ID (pair) of relay and the sending UE, local ID (pair) of the sending UE and peer end UE, relay determines the data packet is going to the specific peer end/remote UE by matching the received layer-2 ID and local ID to the existing association. The relay adds layer-2 ID (pair) of relay and the peer end UE and local ID (pair) of the peer end UE and the sending UE, optional layer-2 ID (pair) of relay and sending UE into the data packet and sent it to the peer end UE.

Once the peer end UE receives the data packets, based on the local ID (pair), layer-2 ID (pair) and the existing association, the peer UE can match the received information to the association and can recognize the data packet is from the sending UE via the relay.

If a link identifier update (LIU) is needed, e.g. for security purposes, the new layer-2 ID of the peer end/remote UEs should be informed to each other by relay to update the associations between any of Layer-2 ID, user info ID, and local ID. The new layer-2 ID of the end remote UE that triggers the LIU (UE-1) should be informed to the peer end UE (UE-2), i.e. by the relay. The LIU triggering end UE (UE-1) includes its new layer-2 ID in Link Identifier Update Request message to the relay.

Upon receiving the Link Identifier Update Request message, the relay updates the associations of any of Layer-2 ID, user info ID, and local ID of the LIU triggering end UE (UE-1), stores the new association, and sends the new layer-2 ID of LIU triggering end UE (UE-1) to the peer end UE (UE-2) via relay update request.

Upon receiving the relay update request, the peer end UE (UE-2) self-selects a new layer-2 ID of itself, stores the new layer-2 ID LIU triggering end UE (UE-1), updates the association of any of Layer-2 ID, user info ID, and local ID of the LIU triggering end UE (UE-1), stores the new association, and sends its new layer-2 ID to the relay via relay update response.

Upon receiving the relay update response, the relay updates the associations of any of Layer-2 ID, user info ID, and local ID of the LIU triggering end UE (UE-1) and peer end UE (UE-2), stores the new association, and sends the new layer-2 ID of peer end UE (UE-2) to the LIU triggering end UE (UE-1) via link identifier update response.

Upon receiving the link identifier update response, the LIU triggering end UE (UE-1) updates the association of association of any of Layer-2 ID, user info ID, and local ID of the peer end UE (UE-2), stores the new association, and include one or more of: its new layer-2 ID, new layer-2 ID of relay paired with the LIU triggering end UE (UE-1), new layer-2 ID of the peer end UE (UE-2) and new layer-2 ID of relay paired with the peer end UE (UE-2) in the Link Identifier Update Ack message.

Upon receiving the Link Identifier Update Ack message, the relay will use the new IDs and associations for further traffic routing between the two end UEs.

Association between User Info ID and Layer-2 ID

Common mechanism for U2U relay discovery and communication

For the layer-2 U2U relay, as it has been agreed in RAN2, a newly introduced SRAP layer 'local ID' will be attached into the header of the data packets for layer-2 U2U relay communication. And RAN2 also made a working assumption that the RRCReconfigurationSidelink message carries both the L2 ID and the Local ID of the peer Remote UE and assumes that the association between the User Info ID and the L2 ID of the peer Remote UE is done at the Remote UE ProSe layer.

As in has been described above under the headings "Overview of 5G ProSe UE-to-UE Relay Discovery" and "Overview of 5G ProSe UE-to-UE Relay Communication", during discovery and communication (layer-2 link) establishment procedures different combinations of the User Info ID source 5G ProSe End UE, target 5G ProSe End UE and 5G ProSe UE-to-UE Relay might be carried by discovery and communication related messages which are sent using source and destination L2 IDs.

The source layer-2 ID can be considered as the layer-2 ID of the msg sender. The destination L2 ID is the layer-2 ID of the msg receiver. By exchanging the message during U2U relay discovery and/or communication (layer-2 link) establishment or modification, layer-2 IDs of the source end UE, relay, target end UE are also exchanged.

In certain examples, the layer-2 ID of a UE is the Source Layer-2 ID of a message sent by this UE or the destination layer-2 ID of the message received by this UE.

In the current specification, the source L2 ID is always self-selected by UEs based on implementation. Therefore the UE may select the same source L2 ID for the same RSC, relay, and target UE during the life of the U2U relay communication.

The destination (dst) L2 IDs could be selected based on policies and configuration of the msg sender; or based on the knowledge of the UEs, i.e. in Figure 6, step　4, the destination L2 ID of the U2U relay discovery response message (msg) is the source L2 ID of the U2U relay discovery solicitation msg in step　1. In this way, upon receiving the msg in step　4, the MAC layer of end UE-1 can use the dst L2 ID of the U2U relay discovery response msg to determine whether the msg to send to UE-1 or not.

When implementing the procedures of U2U relay discovery and communication (for example as described above under the headings "Overview of 5G ProSe UE-to-UE Relay Discovery" and "Overview of 5G ProSe UE-to-UE Relay Communication"), i.e. sending and receiving messages carrying user info IDs, corresponding source and destination L2 IDs are chosen and used by source and target end UEs and relay. However, in the current specification, it is not clear whether the source and target end UEs and relay will store, keep, and maintain the relationship of the source and destination L2 IDs, corresponding discovery and direct communication request and response (DCR) messages, and UEs (including the source and target end UEs and the relay).

The U2U relay discovery and communication was developed based on direct communication (DC) between two peer UEs. For the DC case with only two peer UEs, both to them all fully acknowledged of the layer-2 ID and user info of each other. However, for the U2U relay discovery and communication, it is not supported to exchange the layer-2 ID between source end UE and target end UE (via the relay). Therefore, in the current spec,

- the source end UE does not know the layer-2 ID of the target end UE. The source end UE cannot associate the layer-2 ID of the target end UE to the user info ID of the target end UE.

- the target end UE does not know the layer-2 ID of the source end UE. The target end UE cannot associate the layer-2 ID of the source end UE to the user info ID of the source end UE.

- the relay knows the layer-2 ID of the source and target end UEs. The relay can associate the layer-2 IDs of the source and target end UEs to their user info IDs.

In order to support the above RAN2 assumption, 'association between the User Info ID and the L2 ID of the peer Remote UE is done at the Remote UE ProSe layer', the behaviors of target and source end UEs and relay should be defined. The source 5G ProSe (Layer-3 and/or Layer-2) End UE, the target 5G ProSe (Layer-3 and/or Layer-2) End UE, the 5G ProSe (Layer-3 and/or Layer-2) UE-to-UE Relay should derive, store and maintain the association between the user info IDs and the L2 IDs used for the messages that carry the corresponding user info IDs; or, in another word, should derive, store and maintain the association between the user info IDs and the L2 IDs of the source end UE, target end UE and/or rely.

The 5G ProSe end UEs and 5G ProSe U2U relay may release and update the association between the user info IDs and the L2 IDs:

- The association might be updated when the L2 IDs or Layer-2 link are updated.

- The association might be released when the associated L2 link between the UEs associated to the user info IDs in the association between the user info IDs and the L2 IDs is released.

There are several options on which UE(s) will store, keep and maintain the association of L2 IDs and user info IDs.

- Option k1: only the relay UE establishes, keeps/stores, and maintains the association.

o This option can avoid any misalignments of the association stored in different UEs (i.e. relay, source end UE, target end UE). The relay can send the association to end UEs or other relays for future use.

o Or, the relay UE link other information (i.e. local ID) for the end UEs and sends the association to end UEs for further use.

- Option k2: both the relay UE and end UEs establish, keep/store, and maintain the association.

- Option k3: only the end UEs, source end UE and target end UE, establish, keep/store, and maintain the association.

There are several options on the association of the L2 IDs of UEs and user info IDs to be stored, kept and maintained:

- Option a1: in the association contains the L2 ID of relay and the corresponding user info ID of relay.

- Option a2: i.e. for discovery model A, any combination of the following options:

o at the relay, (source and destination) L2 ID of relay is associated to the user info ID of list of user info IDs in the U2U relay discovery announcement msg, or L2 ID of relay is associated to the user info ID of list of user info IDs in the U2U relay discovery announcement msg and user info ID of relay.

o at the monitoring UE, the (source and destination) L2 ID of relay is associated to user info ID of relay, user info ID of the monitoring UE, optional list of user info IDs in the U2U relay discovery announcement msg.

- Option a3: for discovery model with B and DCR (direct communication request and/or response)/ communication establishment /layer-2 link establishment, the source end UE/ discoverer UE, target end UE/ discoverer UE, i.e. UE-2, and the relay associates one or more of the following:

o the L2 ID of target end UE/ discoveree to the user info ID of target end UE/ discoveree ;

o the L2 ID of source end UE/ discoverer to the user info ID of source end UE/ discoverer;

o the L2 ID of relay to the user info ID of relay;

o user info ID of relay, user info ID of target end UE/discoveree UE, optional user info of itself (source end UE/ discoverer UE) to the user info ID of them.

After the association between L2 IDs and user info IDs are established/ derived, there are several options of which layer (i.e. ProSe or AS layer) to store and use the association at source end UE, target end UE , and relay.

- The 5G ProSe end UEs and 5G ProSe U2U relay may keep the association between user info ID and L2 ID and user info ID in ProSe layer for further use.

- The 5G ProSe end UEs and 5G ProSe U2U relay may send/pass the association between user info ID and L2 ID and user info ID to the AS or lower layers from the ProSe layer for further traffic and data transmission.

- The 5G ProSe end UEs and 5G ProSe U2U relay can send/pass the association between user info ID and L2 ID and user info ID and the user info IDs to the AS layer from the ProSe layer further traffic and data transmission.

During 5G ProSe UE -to- UE Relay Discovery using Model A

During 5G ProSe UE-to-UE Relay Discovery using Model A as described in relation to Figure 5, in step 2, the 5G ProSe UE-to-UE Relay sends a UE-to-UE Relay Discovery Announcement message. The UE-to-UE Relay Discovery Announcement message contains User Info ID of the 5G ProSe UE-to-UE Relay and list of User Info ID of the 5G ProSe End UEs supporting a specific RSC in the same message. The UE-to-UE Relay Discovery Announcement message is sent using the Source Layer-2 ID and Destination Layer-2 ID. The source layer-2 ID could be considered as the layer-2 ID of the relay at this stage.

The 5G ProSe UE-to-UE Relay and the 5G ProSe End UEs which received the U2U relay discovery announcement msg should derive, store, keep and maintain the association of the Source Layer-2 ID and Destination Layer-2 ID of the Discovery Announcement message and the User Info ID of the 5G ProSe UE-to-UE Relay and/or list of User Info ID of the 5G ProSe End UEs in the msg, as mentioned in option a2.

Option a2: for discovery model A, any combination of the following options:

- at the relay: after/ upon sending the Discovery Announcement message, the (source and destination) L2 ID of relay is associated to one or more of the following:

o the user info ID of list of user info IDs in the U2U relay discovery announcement msg

o user info ID of relay.

- at the monitoring UE: upon/ after receiving the Discovery Announcement message, the L2 ID of relay / the source layer-2 ID of Relay Discovery Announcement message, and/or the L2 ID of the monitoring UE is associated to one or more of the following:

o user info ID of relay

o user info ID of the monitoring UE

o optional list of user info IDs in the U2U relay discovery announcement msg.

For the ProSe relay scenarios, i.e. Layer-2 U2U relay, more than one U2U relay discovery announcement messages might be announced by relay, i.e. for different RSC or to different end UEs. For the end UEs, more than one U2U relay discovery announcement messages might be received from the same relay with different RSCs or user info ID, or from other different relays. In certain examples, the association of Source Layer-2 ID and Destination Layer-2 ID and the User Info ID of the 5G ProSe UE-to-UE Relay and/or list of User Info ID of the 5G ProSe End UEs in the msg should be always maintained by the relay and end UEs, until:

- The association might be updated when the L2 IDs or Layer-2 link are updated.

- The association might be released when the associated L2 link is released.

Examples are given in Table 2.

During 5G ProSe UE -to- UE Relay Discovery using Model B and During 5G ProSe UE-to-UE Relay Communication establishment

During the 5G ProSe UE-to-UE Relay Discovery using Model B, as disclosed in relation to Figure 6, the destination L2 ID of the U2U relay discovery response msg might be set to the Source Layer-2 ID of the received 5G ProSe UE-to-UE Relay Discovery Solicitation message, i.e. from discoverer UE to relay, as detailed in

step

3 and 4 in Figure 6. In short, the U2U relay discovery response sent by the relay to the discoverer UE uses the L2-ID of discoverer UE as destination layer-2 id and uses L2 ID of relay as source layer-2 ID.

Based on Figure 6, the L2 IDs in Figure 11 are numbers in different steps. The source L2 ID of the U2U relay discovery response msg is self-selected by the UE that initiates the message, and can be considered as the layer-2 ID of the UE. For sending the U2U relay discovery response msg, the sender uses the source L2 ID of the received corresponding U2U relay discovery solicitation msg as the destination L2 ID, i.e. in step　3, the end UE-2 use the source L2 ID of the UE-to-UE Relay Discovery Solicitation message in step　2 as the destination L2 ID of the U2U relay discovery response msg.

During and after the 5G ProSe UE-to-UE Relay Discovery using Model B, at least until the layer-2 link between UE-1 and relay and link between relay and UE-2 are updated, the end UE-1 and UE-2 and relay UE-1 (the relay which UE-2 responds to) should keep and store the L2 IDs of the UEs involved or the (source and destination) L2 IDs associated to the messages in Figure 11 and associated to the user info IDs carried in the message. When to release the source and destination L2 IDs and/or the association can up to UE implementation, or up to a configured timer.

The association of the (source and destination) L2 IDs and user info IDs (of source end UE, target end UE, and relay) can be stored, kept and maintained at different UEs/ entities, i.e. at the source end UE/ discoverer UE, at the target end UE/ discoveree UE, and/or relay.

For the U2U relay discovery Model, also U2U relay communication, the relay is paired with source end UE and target UE to establish the end-to-end connection/ from source end UE to target end UE connection. The relay UE may use the same different layer-2 ID to pair with the source end UE and target end UE. Therefore in some cases, the Layer-2 ID of relay paired with source end UE and Layer-2 ID of relay paired with target end UE can be interpreted as layer-2 ID of relay.

The following clarification of layer-2 ID of discoverer UE/ source end UE, layer-2 ID of relay paired with discoverer UE/ source end UE, layer-2 ID of discoveree UE/ target end UE, layer-2 ID of relay paired with discoveree UE/ target end UE applied to both U2U relay discovery with B and U2U relay communication.

The skilled person will appreciate that the definition of the layer-IDs may be applied to any or all of the examples disclosed herein.

- layer-2 ID of discoverer UE/ source end UE: the source L2 ID-1 of 5G ProSe UE-to-UE Relay Discovery Solicitation message sent by discoverer 5G ProSe End UE (UE-1) in step　1 of Figure 11, L2 ID-4 of 5G ProSe UE-to-UE Relay Discovery Response message sent by relay in step　4 of Figure 11. The source L2 ID-1 of direct communication msg in step　3 of Figure 7, destination layer-ID of direct communication response in step　9 of Figure 7, and any other source layer-2 ID of the msg initiated by source end UE in Figure 7, and any other destination layer-ID of the msg sent to source end UE in Figure 7.

- Layer-2 ID of relay that the discoveree UE responses to (i.e. relay-1) paired with discoverer UE/ Layer-2 ID of relay paired with source end UE: the Destination ID-1 of 5G ProSe UE-to-UE Relay Discovery Solicitation message sent by discoverer 5G ProSe End UE (UE-1) in step　1, and/or source L2 ID-4 of 5G ProSe UE-to-UE Relay Discovery Response message sent by relay in step　4 of Figure 11. Or the source layer-2 ID of the messages initiated by relay sending to the source end UE in Figure 7, and the destination layer-2 id of the messages sent by the source end UE to the relay in Figure 7.

- Layer-2 ID of relay that the discoveree UE responses to (i.e. relay-1) paired with discoveree UE/ Layer-2 ID of relay paired with target end UE: of the source L2 ID-2 of 5G ProSe UE-to-UE Relay Discovery Solicitation message sent by rely-1 in step　2, and/or destination L2 ID-3 of 5G ProSe UE-to-UE Relay Discovery Response message sent by discoveree 5G ProSe End UE (UE-2) in step　3. Those 2 ID are the same. The source L2 ID of direct communication msg in step　5 in Figure 7, and/or source L2 ID of any other messages initiated by the relay sending the target UE in Figure 7, and any other destination layer-ID of the msg sent to relay by the target end UE in Figure 7.

- Layer-2 ID of discoveree/ Layer-2 ID of target end UE: The Destination ID-2 of 5G ProSe UE-to-UE Relay Discovery Solicitation message sent by relay in step　2, and source L2 ID-3 of 5G ProSe UE-to-UE Relay Discovery Response message sent by discoveree 5G ProSe End UE (UE-2) in step　3 in Figure 11. The source layer-2 ID of the msg initiated by target end UE in Figure 7, and any other destination layer-ID of the msg sent to source end UE in Figure 7.

For both 5G ProSe Communication via 5G ProSe Layer-3 and Layer-2 UE-to-UE Relay, the layer-2 link are established per hop: 1^st hop is between source end UE to relay, the 2^nd hop is between relay and target end UE. After step　9 in Figure 7 is completed, the end-to-end connection could be established. The (source and destination) L2 IDs can be grouped into two pairs: (for UE-to-UE Relay Communication as shown in Figure 12)

- 1st pair: the layer-2 ID of discoverer UE and Layer-2 ID of relay that the discoveree UE responses to (i.e. relay-1) paired with discoverer UE. Or the layer-2 ID of source end UE and Layer-2 ID of relay paired with source end UE. This pair of layer-2 ID is used for traffic and message transmission between relay and discoverer UE/ source end UE over the 1^st hop.

o When source end UE transmit message or data to relay, the source ID of the msg or data packet is the L2 ID of source end UE, and the destination L2 ID is the L2 ID of relay paired with source end UE, Step a in Figure 12.

o When the relay transmits msg or data to source end UE, the source and destination L2 ID are swapped; the source ID of the msg or data packet is the L2 ID of the relay paired with source end UE, and the destination L2 ID is the L2 ID of the source end UE. Step b in Figure 12.

- 2nd pair: the Layer-2 ID of relay that the discoveree UE responses to (i.e. relay-1) paired with discoveree UE, or the layer-2 ID of target end UE and Layer-2 ID of relay paired with target end UE. This pair of layer-2 ID is used for traffic and message transmission between relay and discoveree UE/target end UE over the 2^nd hop.

o When target end UE transmits message or data to relay, the source ID of the msg or data packet is the L2 ID of target end UE, and the destination L2 ID is the L2 ID of relay paired with target end UE. Step d in Figure 12.

o When the relay transmits msg or data to target end UE, the source and destination L2 ID are swapped; the source ID of the msg or data packet is the L2 ID of the relay paired with target end UE, and the destination L2 ID is the L2 ID of the target end UE. Step c in Figure 12.

The Layer-2 ID of the relay paired with discoverer UE/ source end UE may be same as or different from The Layer-2 ID of the relay paired with discoveree UE/ target end UE.

During 5G ProSe UE-to-UE Relay Discovery using Model B and U2U relay communication, more details are below. The association of the (source and destination) L2 IDs and user info IDs (of source end UE, target end UE, and relay) to be stored, kept and maintained with different options, option k1, k2 and k3.

- Option k1: only the relay UE establishes, keeps/stores, and maintains the association.

The relay may associate the layer-2 ID of an end UE to the user info ID of this end UE. i.e., the relay associates:

- Association A: the layer-2 ID of source end UE/ discoverer to the user info ID of this source end UE/ discoverer, optional associating the L2 ID of relay paired with source end UE/ discoverer and its user info ID.

- Association B: the layer-2 ID of target end UE/ discoveree to the user info ID of this target end UE/ discoveree, optional associating the L2 ID of relay paired with source end UE/ discoverer and its user info ID.

The relay may indicate the association to source end UE/ discoverer and target end UE/ discoveree, if the end UEs do not have the information of the association. The relay may indicate the Association A to target end UE/ discoveree via U2U relay response message or direct communication response message or any other new PC-5/ prose layer msg. The relay may indicate the Association B to source end UE/ discoverer, i.e. via U2U relay discovery solicitation msg or direct communication request msg or any other new PC-5/ prose layer msg.

- For Option k2: both the relay UE and source and target end UEs establish, keep/store, and maintain the association, and option k3: only the source end UE and target end UE, establish, keep/store, and maintain the association: the source end UE and target end UE should establish Association A and Association B.

The source end UE / discoverer UE establishes Association A and Association B. In order to established Association B at source end UE / discoverer UE, the Layer-2 ID of the target end UE / discoveree, optional the layer-2 ID of relay paired with this target end UE / discoverer should be indicated, i.e. by the relay via U2U relay discovery response msg (in step 2 of Figure 11), or direct communication accept (DCA) msg (in step 4 or step 9 of Figure 7: Layer-2 link establishment via 5G ProSe Layer-3 UE-to-UE Relay or step 2/4 in Figure 8: 5G ProSe Communication via 5G ProSe Layer-2 UE-to-UE Relay).

In short, the relay may include the Layer-2 ID of the target end UE into the direct communication accept message over the 1^st hop to the source end UE.

Upon receiving the Layer-2 ID of the target end UE from the relay (i.e. via direct communication accept message), the source end UE pass the layer-2 ID of the target end UE from its prose layer to AS layer for further association use and communication use.

The target end UE / discoveree UE establishes Association A and Association B. in order to established Association A at target end UE / discoveree UE, the Layer-2 ID of the source end UE / discoverer, optional the layer-2 ID of relay paired with this source end UE / discoveree should be indicated, i.e. by the relay via U2U relay discovery solicitation msg (in step 3 or after step 4 of Figure 11), or direct communication request (DCR) msg (in step 5, step 6 or after step 9 of Figure 7: Layer-2 link establishment via 5G ProSe Layer-3 UE-to-UE Relay or step 3/4 in Figure 8: 5G ProSe Communication via 5G ProSe Layer-2 UE-to-UE Relay).

In short, the relay may include the Layer-2 ID of the source end UE / discoverer into direct communication request that is sent to the target end UE over the 2^nd hop.

Upon receiving the Layer-2 ID of the source end UE from the relay (i.e. via direct communication request message), the target end UE pass the layer-2 ID of the source end UE from its prose layer to AS layer for further association use and communication use.

- For Option k2 only:

The relay still need associate the layer-2 ID of an end UE to the user info ID of this end UE to establish Association A and Association B. the source end UE/ discoverer UE and the at the target end UE/ discoveree UE also associate the user info ID and layer-2 ID for use. But the relay does not need to send the association to the source end UE or target end UE.

More details for 5G ProSe UE -to- UE Relay Communication / layer-2 link establishment

In step 3 of Figure 7, In order to establish the layer-2 link for the 1^st hop, the source target UE (UE-1) sends Direct Communication Request (DCR) message to initiate the unicast layer-2 link establishment procedure. The Direct Communication Request message includes:

- (optional) Destination Layer-2 ID of target 5G ProSe End UE

- Other info: i.e. ProSe Service Info, RSC, Security Information, etc.

The source Layer-2 ID and destination Layer-2 ID used to send the Direct Communication Request message. The Source Layer-2 ID of the Direct Communication Request message is self-assigned by the source 5G ProSe End UE and the Destination Layer-2 ID is set to the Source Layer-2 ID of the discovery message of the 5G ProSe UE-to-UE Relay, i.e. the Destination Layer-2 ID is same as source L2 ID-4 in step　4 of Figure 11, or source L2 ID in step 2 of Figure 5.

The Source Layer-2 ID of the discovery message of the 5G ProSe UE-to-UE Relay is considered as the layer-2 ID of source 5G ProSe End UE.

Source Layer-2 ID of the discovery message of the 5G ProSe UE-to-UE Relay is considered as the layer-2 ID of relay paired to the source end UE.

For the existing 5G ProSe direct communication, the UE maintains a mapping between the Application Layer IDs and the source Layer-2 IDs used for the PC5 unicast links. In some cases, the user info IDs could replace the Application Layer IDs to present a UE as a type of UE ID. In order to support the assumption made by RAN2, for the 5G ProSe direct communication and the communication via U2U relay over the 1^st hop (between source end UE and relay), the UE should also maintain a mapping between the user info IDs and the (source and destination) Layer-2 IDs used for the PC5 unicast links. i.e. the source end UE maintain the mapping between source and destination L2 IDs Direct Communication Request and the User Info ID of source and target 5G ProSe End UEs, and User Info ID of 5G ProSe UE-to-UE Relay carried by the msg. in short, the source end UE maintain the association between the user info IDs (of relay and/or source end UE) and the Layer-2 ID (of relay and/or source end UE).

In step　4 of Figure 7, to establish the security, the relay send msg to source end UE, for the security establishment msg:

- The Source Layer-2 ID used for the security establishment procedure is self-assigned by the 5G ProSe Layer-3 UE-to-UE Relay and the Destination Layer-2 ID is set to the Source Layer-2 ID of the received Direct Communication Request message, step　4.1 of Figure 7.

o Source Layer-2 ID of the received Direct Communication Request message can be considered as the Layer-2 ID of the source end UE.

- As a response to enable security, the source end UE will send response to the relay, Source Layer-2 ID is self-assigned by the source end UE (i.e. could be the same one as sending Communication Request message) and the source Layer-2 ID is set to the destination Layer-2 ID of the received security establishment request message, step　4.2 of Figure 7.

After step 4, the security over the 1^st hop is established. And the source end UE is aware of the layer-2 ID of the relay paired with source end UE, and the relay is also aware of the layer-2 ID of the source end UE. The relay can link the layer-2 ID of the source end UE and relay to the User Info ID of source and target 5G ProSe End UEs, and User Info ID of 5G ProSe UE-to-UE Relay (and the RSC) in the direct communication msg in step　3. The association can be considered as the association between Layer-2 ID and user info ID for the 1^st hop.

In step　5, the 5G ProSe Layer-3 UE-to-UE Relay sends a Direct Communication Request message to initiate the unicast Layer-2 link establishment procedure for the 2^nd hop with the target 5G ProSe End UE. The parameters included in the Direct Communication Request message are:

- Other info: i.e. ProSe Service Info, RSC, Security Information, etc.

The Source Layer-2 ID of the Direct Communication Request message is self-assigned by the 5G prose UE-to-UE Relay (layer-2 ID of relay paired with target end UE) and the Destination Layer-2 ID is the unicast Layer-2 ID of target 5G ProSe End UE associated with the User Info ID of target 5G ProSe End UE, based on the previous knowledge stored by relay.

In step　6, to establish the security, the target end UE sends msg to relay. for the security establishment msg:

- The Source Layer-2 ID used for the security establishment procedure is self-assigned by the target 5G ProSe End UE (layer-2 ID of target end UE) and the Destination Layer-2 ID is set to the Source Layer-2 ID of the received Direct Communication Request message (layer-2 ID of relay paired with target end UE), step　6.1 of Figure 7.

- As a response to enable security, the relay will send response to the relay, Source Layer-2 ID is self-assigned by the Relay (i.e. could be the same one as sending Communication Request message) and the source Layer-2 ID is set to the destination Layer-2 ID of the received security establishment request message, step　4.2 of Figure 7.

After step 6, the security over the 2^nd hop is established. And the target end UE is aware of the layer-2 id of the relay paired with target end UE, and the relay is also aware of the layer-2 ID of the target end UE. The relay can link the layer-2 ID of the target end UE and relay to the User Info ID of source and target 5G ProSe End UEs, and User Info ID of 5G ProSe UE-to-UE Relay (and the RSC) in the direct communication msg in step　4. The association can be considered as the association between Layer-2 ID and user info ID for the 2^nd hop.

The layer-2 ID of the relay paired with source end UE and layer-2 ID of the relay paired with target end UE maybe the same or different.

In step　7, the target 5G ProSe End UE sends a Direct Communication Accept message to the 5G ProSe Layer-3 UE-to-UE Relay that has successfully established security with. The parameters included in the Direct Communication Accept message are:

- User Info ID of target 5G ProSe End UE in the current specification.

In order to fully acknowledge the relay of the 2^nd hop is establishment for the specific source end, target end UEs and relay, the User Info ID of source 5G ProSe End UE and the User Info ID of 5G ProSe UE-to-UE relay should be also included into the Direct Communication Accept message over the 2^nd hop.

In step　9, after receiving the Direct Communication Accept message from the target 5G ProSe End UE, the 5G ProSe UE-to-UE Relay sends a Direct Communication Accept message to the source 5G ProSe End UE that has successfully established security with. The parameters included in the Direct Communication Accept message are:

- User Info ID of target 5G ProSe End UE and User Info ID of 5G ProSe UE-to-UE Relay in the current specification.

In order to fully acknowledge the source UE of the 1^st hop is establishment for the specific source end, target end UEs and relay, the User Info ID of source 5G ProSe End UE should be also included into the Direct Communication Accept message over the 1^st hop from relay to the source end UE. The relay may also need to include the layer-2 ID of the relay paired with target end UE (if this ID is different from that paired with source end UE) and layer-ID of target source UE.

In any stage after step　7, the Relay is aware of the layer-2 IDs of the 1^st pair and 2^nd pair, including the layer-2 IDs of relay paired with target end UE, layer-2 ID of the relay paired with source end UE, layer-2 ID of target end UE and layer-2 ID of source end UE. The relay also knows the user info IDs of the target and source end UE and the user info ID relay. The relay can associate the 1^st pair and 2^nd pair L2 IDs to the user info IDs for an end-to-end association. Maybe also include the RSC in the association. For another simple solution, the relay may associate the layer-2 ID of target end UE and layer-2 ID of source end UE and the user info IDs of target and source end UE.

In any stage after step　9, the source end UE is aware of the layer-2 ID of the 1^st pair: the layer-2 ID of the relay paired with source end UE, layer-2 ID of source end UE; and also the user info ID of the target and source end UE, and the user info ID relay. The source end UE can link/ associate the layer-2 IDs to the user info of source end UE and relay. The source end UE maybe also aware of the layer-2 ID of the 2^nd pair: layer-2 ID of the relay paired with source end UE, layer-2 ID of source end UE. Then source end UE can link/ associate the layer-2 IDs to the user info of source end UE, target end UE and relay (and the RSC).

After step　9, the target end UE is not aware of the 1^st pair. In order to allow the target end UE establish the association including the layer-2 IDs and user info IDs of the relay, source end UE and target end UE, the relay may send the 1^st pair -layer-2 ID of the source end UE (i.e. the layer-2 ID of the source end UE) to the target end UE - to the target end UE via direct communication request over the 2^nd hop in step　5. Or the relay may send the association between 1^st pair, 2^nd pair layer-2 IDs (of source end UE, target end UE and relay) and the user info IDs (of source end UE, target end UE and relay). Therefore, the target end UE is aware of the end-to-end association.

Associate the User Info ID to Local ID

Common mechanism for association between user info ID and local ID

Based on the techniques described above under the heading "Association between User Info ID and Layer-2 ID", the mapping/ association between the L2 IDs and user info IDs based on U2U relay discovery and U2U relay communication is already established. The next question is how to associate the User Info ID with the Local ID.

As it summarized in Table 1 IDs related to Layer-2 U2U relay scenarios:

- User Info ID is a ProSe layer information, For now, the relay or end UEs do not pass the user info IDs to AS/RRC/ layer or layer-2. The AS layer does not know/ recognize the user info ID.

- The local ID is a SRAP layer information. For now, the ProSe layer does not know/ recognize the information.

- Layer-2 ID: both ProSe layer and AS layer know/ recognize the information.

In certain examples of the present disclosure, it is assumed that the Local ID pair for source end UE, and target end UE and/or relay are already allocated. Similar to the layer-2 IDs shown in Figure 12, the two local IDs of source end UE and target end UE are also paired to identify each other. And the Local ID pair is transmitted to source and target end UE respectively via RRCReconfigurationSidelink message. As mentioned in 3GPP R2-2311566:

In RAN2 #123, RAN2 agreed that, instead of a L2 ID, a Local ID is used as a UE ID in a SRAP header for L2 based U2U relay and two Local IDs are included in the SRAP header to identify source and target Remote UE respectively. In RAN2 # 123bis , RAN2 also agreed that the RRCReconfigurationSidelink message is used to indicate the Local ID pair from relay UE to source and target Remote UEs respectively.

Based on the RAN2 text, it is assumed in certain examples, the local IDs of source and target end UEs (and the local ID of relay)/ the Local ID pair is allocated by relay. Based on the association A and B already stored at relay, the relay understands the mapping of local ID, layer-2 ID and user info ID of relay, source end UE and target end UE.

The simplified format of data packets for U2U relay communication is shown in Figure 13. The local ID pair carries the local IDs of source and target end UEs; and the layer-2 ID pair carries either the 1^st or 2^nd pair layer-2 IDs. The end-to-end packet between source end UE and target end UE can be broken down to the packet between source end UE and relay, and between relay and target end UE, If the packet is between the source end UE and relay, the layer-2 ID pair carries the 1^st layer-ID; if the packet is between the target end UE and relay, the layer-2 id pair carries the 2^nd layer-ID.

The relationship between the layer-2 ID pairs (including 1^st and 2^nd pair) and local ID pairs are shown in Figure 14. The layer-2 ID pairs are per hop for the specific source and target end UEs (UE-1 and UE-2) and relay for the U2U relay communication. The local ID pair is the end-to-end IDs for the source and target end UEs and relay in this U2U relay communication. Therefore, the local ID pair links to 1^st and 2^nd pair of the layer-2 ID. Or the local ID pair links to layer-2 IDs of source end UE, target end UE, and optionally relay. Or the local ID of the source end UE links to the layer-2 ID of source end UE, and the local of target end UE links to the layer-2 ID of target end UE. The above link could be done by relay. Then the relay sends the (in RAN2 LS) RRCReconfigurationSidelink message carries both the L2 ID and the Local ID of the peer Remote UE to both source and target end UE. i.e. for a source end UE the peer remote/end UE is the target end UE; for a target end UE the peer end UE is the source end UE.

Based on the association between L2 ID and user info ID (for example as described above under the heading "Association between User Info ID and Layer-2 ID", i.e. Association A and B) and association established between layer-2 ID and local ID as described under the present heading, the association between the user info ID and local ID could be implemented, a so-called 2-step association in certain examples.

To implement the 2-step association, i.e. by the relay, therefore two main solutions:

- Option p1: Link the Local ID to the user info IDs in ProSe layer: local ID of the peer end UE may be passed to ProSe layer to complete the association.

- Option p2: Link the Local ID to the user info IDs in PC-5 RRC layer: user info ID of the peer end UE may be passed to AS/RRC layer within a UE to complete the association.

Associate the User Info ID, Layer-2 ID pair and Local ID in ProSe layer

To support associating between user info ID and local ID in ProSe layer, upon receiving the RRCReconfigurationSidelink message carries both the L2 ID and the Local ID pair, the source and/or target end UE passes the received L2 ID-1, optionally the received Local ID-1 pair to its ProSe layer. The Prose layer of the end UE find the association for the L2 ID-1 that is already established during U2U discovery and layer-2 link management (establishment, modification etc.) and stored, i.e. association A and B. Once the association of the L2 ID-1 is found out, the user info ID-1 linked to the L2 ID-1 is recognized. And then the prose layer can link the user info ID-1 to local ID-1 for further communication. The ProSe layer may pass the link between user info ID-1 to local ID-1 to its AS layer for further communication. The prose layer may pass the user info ID-1 to local ID-1 to its AS layer for further communication.

The user info ID-1, L2 ID-1 and local ID-1 might be the IDs of the peer end UE, i.e. for a source end UE the peer end UE is the target end UE; for a target end UE the peer end UE is the source end UE.

Associate the User Info ID, Layer-2 ID pair and Local ID in RRC layer

To support associating between user info ID and local ID in RRC layer, Upon receiving the RRCReconfigurationSidelink message carries both the L2 ID and the Local ID pair, the source and/or target end UE starts to match the association for the L2 ID-2 in RRC layer to find out the user info ID-2 that links to L2 ID-2.

- The source and target end UE may request the association A and/or association B between user info ID and L2 ID from its ProSe upon/ after receiving the RRCReconfigurationSidelink msg, or,

- its ProSe already passed the association between user info ID and L2 ID to the RRC layer/ AS layer of the end UE when the association was established, or based on UE implementation.

Once the association of the L2 ID-1 is found in the RRC/ AS layer of the end UE, the user info ID-2 linked to the L2 ID-2 is recognized. And then the RRC/ AS layer of the end UE can link the user info ID-2 to local ID-2 for further communication.

The user info ID-2, L2 ID-2 and local ID-2 might be the IDs of the peer end UE. i.e. for a source end UE the peer end UE is the target end UE; for a target end UE the peer end UE is the source end UE.

Traffic transmission and routing via U2U relay communication

Association update

Based on existing procedures for link identifier update for PC5 communication via 5G ProSe UE-to-UE Relay described above under the heading "link identifier update for PC5 communication via 5G ProSe UE-to-UE Relay", more enhancements may be needed to exchange the newly updated layer-2 ID between source end UE and relay, and relay and target end UE. The established and stored associations of any of Layer-2 ID, user info ID, and local ID should be also updated accordingly during the link identifier update procedure.

Based on the procedures in Figure 10, more information and clarification on how to update the established and stored associations will be added.

- its peer UEs information (e.g. UE2's IP address, UE2's Application layer ID).

In the current 3GPP specification, UE-to-UE Relay then sends a PC5 Relay Update Request message to each peer UE (e.g. UE2), including: UE1's old IP address/prefix, UE1's old and new Application layer ID, UE1's new IP address/prefix.

In addition to the above information, the UE-to-UE Relay should also include the new layer-2 ID of UE1 received in the Link Identifier Update Request message into the PC5 Relay Update Request message, to inform the UE2 of the new layer-2 ID of UE1.

Upon receiving the Link Identifier Update Request message, The UE-to-UE relay Prose layer passes the new layer-2 ID of UE to the AS layer, and the UE-to-UE relay updates the associations of any of Layer-2 ID, user info ID, and local ID of the LIU triggering end UE (UE1), stores the new associations.

The UE2 also stores the new layer-2 ID of UE1 and the new layer-2 ID of relay.

Upon receiving the PC5 Relay Update Request message, the UE2 Prose layer passes the new layer-2 IDs of UE1 and U2U relay to the AS layer. The UE2 updates the associations of any of Layer-2 ID, user info ID, and local ID of the UE1 and relay, and stores the new associations.

The UE2 may generate a new layer-2 ID for itself. The UE2 include any of new layer-2 ID, the new layer-2 ID of the relay and optionally new layer-2 ID of UE1 and new layer-2 ID of relay into Relay update response message to the U2U relay.

Upon receiving the Relay update response message from UE2, The relay's PROSE layer pass the layer-2 ID of UE2 to its AS layer; the relay updates the associations of any of Layer-2 ID, user info ID, and local ID of UE1and associations of any of Layer-2 ID, user info ID, and local ID of UE2, stores the new associations.

The relay also includes any of the following into the Link Identifier Update Response message to UE1: the new layer-2 ID of UE2 received in Relay Update Response in step 3, the new layer-2 ID of UE1 received in step 1 Link Identifier Update Request message or received in Relay Update Response in step 3.

Upon receiving Link Identifier Update Response message from relay, The UE1 stores the new layer-2 ID of UE2 and the new layer-2 ID of relay. The UE1 Prose layer passes the new layer-2 ID of UE2 and/or new layer-2 ID of relay to its AS layer.

The UE1 updates the associations of any of Layer-2 ID, user info ID, and local ID of the UE2 and relay, and stores the new associations.

The UE1 saves one or more of the following: the new layer-2 ID of UE2, the new layer-2 ID of relay, associations of any of Layer-2 ID, user info ID, and local ID of the UE2, associations of any of Layer-2 ID, user info ID, and local ID of the relay.

Certain examples of the present disclosure provide a method, for a second UE providing a relay function between a first UE and a third UE, wherein a first ID of a first type is allocated to the first UE, the method comprising: allocating a first ID of a second type to the first UE; and transmitting, to the first UE, a first message comprising the first ID of the first type and the first ID of the second type, whereby the first UE makes an association between the first ID of the first type and the first ID of the second type based on the first message.

In certain examples, a second ID of the first type may be allocated to the third UE, and the method may further comprise: allocating a second ID of the second type to the third UE; and transmitting, to the third UE, a second message comprising the second ID of the first type and the second ID of the second type, whereby the third UE makes an association between the second ID of the first type and the second ID of the second type based on the second message.

In certain examples, the method may further comprise: receiving from one of the first UE and the third UE, a third message including two IDs of the second type and data; and forwarding the third message to the other of the first UE and the third UE based on the two IDs of the second type included in the third message.

Certain examples of the present disclosure provide a method, for a first UE, wherein a second UE provides a relay function between the first UE and a third UE, and wherein a first ID of a first type is allocated to the first UE, the method comprising: receiving, from the second UE, a first message comprising the first ID of the first type and a first ID of a second type; and making an association between the first ID of the first type and the first ID of the second type based on the first message.

In certain examples, the two IDs of the second type may be provided in a header of the third message.

In certain examples, the first and second IDs of the first type may be IDs associated with a first network layer (e.g. ProSe layer), and the first and second IDs of the second type may be IDs associated with a second network layer (e.g. SRAP layer or AS layer).

In certain examples, the first and second IDs of the first type may be ProSe Layer-2 IDs.

In certain examples, the first and second IDs of the second type may be SRAP Local IDs.

In certain examples, the first and/or second message may comprises an RRC message (e.g. RRCReconfigurationSideLink message).

In certain examples, the third message may comprise a U2U SRAP data packet (e.g. PDU).

In certain examples, the first and third UEs may be ProSe U2U peer UEs (e.g. source and target remote UEs), and the second UE may be a ProSe U2U Relay UE.

In certain examples, the first ID of the first type may be paired with an ID of the first type allocated to the second UE, and the second ID of the first type may be paired with the ID of the first type allocated to the second UE.

In certain examples, the first and second IDs of the second type may be paired with each other.

In certain examples, the method may further comprise storing, by the second UE: information defining one or more mappings between IDs of the first and second types; and/or information defining one or more pairings between IDs of the first type and/or between IDs of the second type.

Certain examples of the present disclosure provide a UE configured to perform a method according to any example, embodiment, aspect and/or claim disclosed herein.

Certain examples of the present disclosure provide a network (or wireless communication system) comprising first, second and third UEs configured to operate according to any examples, embodiments, aspects and/or claims disclosed herein.

Certain examples of the present disclosure provide a computer program comprising instructions which, when the program is executed by a computer or processor, cause the computer or processor to carry out a method according to any example, embodiment, aspect and/or claim disclosed herein.

Certain examples of the present disclosure provide a computer or processor-readable data carrier having stored thereon a computer program according to any example, embodiment, aspect and/or claim disclosed herein.

Figure 15 illustrates an exemplary method for a second UE (e.g. a ProSe U2U Relay UE) providing a relay function between a first UE and a third UE (e.g. ProSe U2U peer UEs). A first ID of a first type (e.g. a ProSe Layer-2 ID) is allocated to the first UE.

Referring to Figure 15, in a first operation 1501, the second UE allocates a first ID of a second type (e.g. a SRAP Local ID) to the first UE. In a second operation 1502, the second UE transmits, to the first UE, a first message (e.g. an RRCReconfigurationSideLink message) comprising the first ID of the first type and the first ID of the second type. The first UE makes an association between the first ID of the first type and the first ID of the second type based on the first message.

Figure 16 illustrates an exemplary method for a first UE, wherein a second UE (e.g. a ProSe U2U Relay UE) provides a relay function between the first UE and a third UE (e.g. ProSe U2U peer UEs). A first ID of a first type (e.g. a ProSe Layer-2 ID) is allocated to the first UE.

Referring to Figure 16, in a first operation 1601, the first UE receives, from the second UE, a first message (e.g. an RRCReconfigurationSideLink message) comprising the first ID of the first type and a first ID of a second type (e.g. a SRAP Local ID). In a second operation 1602, the first UE makes an association between the first ID of the first type and the first ID of the second type based on the first message.

Figure 17 is a block diagram of an exemplary network entity that may be used in examples of the present disclosure. For example, the network entities in the examples of Figures 1-16 may comprise an entity of Figure 17. The skilled person will appreciate that a network entity may be implemented, for example, as a network element on a dedicated hardware, as a software instance running on a dedicated hardware, and/or as a virtualised function instantiated on an appropriate platform, e.g. on a cloud infrastructure.

The entity 1700 comprises a processor (or controller) 1701, a transmitter 1703 and a receiver 1705. The receiver 1705 is configured for receiving one or more messages from one or more other network entities, for example as described above. The transmitter 1703 is configured for transmitting one or more messages to one or more other network entities, for example as described above. The processor 1701 is configured for performing one or more operations, for example according to the operations as described above.

The techniques described herein may be implemented using any suitably configured apparatus and/or system. Such an apparatus and/or system may be configured to perform a method according to any aspect, embodiment, example or claim disclosed herein. Such an apparatus may comprise one or more elements, for example one or more of receivers, transmitters, transceivers, processors, controllers, modules, units, and the like, each element configured to perform one or more corresponding processes, operations and/or method steps for implementing the techniques described herein. For example, an operation/function of X may be performed by a module configured to perform X (or an X-module). The one or more elements may be implemented in the form of hardware, software, or any combination of hardware and software.

It will be appreciated that examples of the present disclosure may be implemented in the form of hardware, software or any combination of hardware and software. Any such software may be stored in the form of volatile or non-volatile storage, for example a storage device like a ROM, whether erasable or rewritable or not, or in the form of memory such as, for example, RAM, memory chips, device or integrated circuits or on an optically or magnetically readable medium such as, for example, a CD, DVD, magnetic disk or magnetic tape or the like.

It will be appreciated that the storage devices and storage media are embodiments of machine-readable storage that are suitable for storing a program or programs comprising instructions that, when executed, implement certain examples of the present disclosure. Accordingly, certain examples provide a program comprising code for implementing a method, apparatus or system according to any example, embodiment, aspect and/or claim disclosed herein, and/or a machine-readable storage storing such a program. Still further, such programs may be conveyed electronically via any medium, for example a communication signal carried over a wired or wireless connection.

While the invention has been shown and described with reference to certain examples, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope of the invention, as defined by the appended claims.

Acronyms

In the present disclosure, the following acronyms/definitions may be used. In certain examples, various acronyms and other terms may be interpreted in accordance with the relevant 3GPP specifications (e.g. 5G).

3GPP 3^rd Generation Partnership Project

5G 5^th Generation

6G 6^th Generation

ACK Acknowledgement

AS Access Stratum

DC Direct Communication

DCA Direct Communication Accept

DCR Direct Communication Request and/or Response

DNS Domain name System

ID Identity/Identifier

IP Internet Protocol

IPv4 Internet Protocol version 4

IPv6 Internet Protocol version 6

L2 Layer 2

LIU Link Identifier Update

LS Liaison Statement

MAC Medium Access Control

MSG Message

NR New Radio

PC5/PC-5 Direct communication link between capable ProSe UEs

PDU Protocol Data Unit

ProSe Proximity Services

RAN Radio Access Network

RAN2 3GPP Radio Access Network Working Group 2

Rel Release

RRC Radio Resource Control

RSC Relay Service Code

SA2 3GPP Systems Architecture Working Group 2

SRAP Sidelink Relay Adaption Protocol

U2U UE-to-UE

UE User Equipment

TS Technical Specification

WG Working Group

According to clause　3.1 in 3GPP TS　23.304 and 3GPP TS　23.303, terms related to 5G ProSe UE-to-UE relay are defined. In certain examples of the present disclosure, in order to clarify the entities involved, some of the terms may be simplified and/or abbreviated.

- ProSe: Proximity-based Services

- 5G ProSe -enabled UE : A UE that supports 5G ProSe requirements and associated procedures.

- 5G ProSe UE -to- UE Relay: A 5G ProSe-enabled UE that provides functionality to support connectivity between 5G ProSe End UEs. In certain examples, relay may indicate 5G ProSe UE-to-UE Relay, including both Layer-3 and Layer-2 5G ProSe UE-to-UE Relay.

o In certain examples, U2U relay indicates 5G ProSe UE-to-UE Relay.

- 5G ProSe End UE : A 5G ProSe-enabled UE that connects with another 5G ProSe-enabled UE(s) via a 5G ProSe UE-to-UE Relay. In certain examples, End UE may indicate 5G ProSe End UE.

- Layer-2 ID: L2 ID.

- Source Layer-2 ID: Sou L2 ID. A link-layer identity that identifies a device that originates ProSe communication frames. In certain examples, the layer-2 ID of a UE is the Source Layer-2 ID of a message sent by this UE.

- Destination Layer-2 ID: Des L2 ID. A link-layer identity that identifies a device or a group of devices that are recipients of ProSe communication frames.

- Source 5G ProSe End UE: the UE initiate the 5G ProSe communication. In certain examples, the source UE or source end UE indicate the source 5G ProSe End UE.

- Target 5G ProSe End UE: the UE the source UE would like to connect to for the 5G ProSe communication. In certain examples, the target UE or target end UE indicate the target 5G ProSe End UE.

- Local ID: a Sidelink Relay Adaptation Protocol (SRAP) header for L2 based U2U relay traffic between source and target end UEs.

Claims

A method performed by a relay user equipment (UE) in a wireless communication system, the method comprising:

identifying, a first identification (ID) of remote UE and a second ID of a peer remote UE, wherein the first ID is used to identify the remote UE in a communication between the remote UE and the relay UE and the second ID is used to identify the peer remote UE in a communication between the relay UE and the peer remote UE;

allocating a first local ID of the remote UE and a second local ID of the peer remote UE, wherein the first local ID is used to identify the remote UE and the second local ID is used to identify the peer remote UE in an end-to-end communication between the remote UE and the peer remote UE via the relay UE; and

transmitting, to the remote UE, a radio resource control (RRC) message including the first local ID, the second local ID and information on the an association between the second local ID and the second ID,

wherein a data packet is transmitted from the remote UE to the peer remote UE via the relay UE based on the association.
The method of claim 1, further comprises:

receiving, from the remote UE, the data packet including information on a destination local ID indicating a destination of the data packet;

based on the association, identifying whether the destination local ID matches with the second local ID; and

transmitting, to the peer remote UE, the data packet, in case that the destination local ID is identified to match with the second local ID.
The method of claim 1, wherein the RRC message further includes information on an association between the first ID and the first local ID.
A method performed by a remote user equipment (UE) in a wireless communication system, the method comprising:

identifying, a first identification (ID) of the remote UE, wherein the first ID is used to identify the remote UE in a communication between the remote UE and a relay UE; and

receiving, from the remote UE, a radio resource control (RRC) message including a first local ID, a second local ID and information on an association between the second local ID and a second ID of a peer remote UE, wherein the first local ID is used to identify the remote UE and the second local ID is used to identify the peer remote UE in an end-to-end communication between the remote UE and the peer remote UE via the relay UE, and the second ID is used to identify the peer remote UE in a communication between the relay UE and a peer remote UE,

wherein a data packet is transmitted from the remote UE to the peer remote UE via the relay UE based on the association
The method of claim 4, further comprising:

transmitting, to the relay UE, the data packet including a destination local ID indicating a destination of the data packet,

wherein the data packet is transmitted to the peer remote UE via the relay UE, in case that the destination local ID matches with the second local ID.
The method of claim 4, wherein the RRC message further includes information on an association between the first ID and the first local ID.
A relay user equipment (UE) in a wireless communication system, the relay UE comprising:

a transceiver; and

a controller coupled with the transceiver and configured to:

identify, a first identification (ID) of remote UE and a second ID of a peer remote UE, wherein the first ID is used to identify the remote UE in a communication between the remote UE and the relay UE and the second ID is used to identify the peer remote UE in a communication between the relay UE and the peer remote UE,

allocate a first local ID of the remote UE and a second local ID of the peer remote UE, wherein the first local ID is used to identify the remote UE and the second local ID is used to identify the peer remote UE in an end-to-end communication between the remote UE and the peer remote UE via the relay UE, and

transmit, to the remote UE, a radio resource control (RRC) message including the first local ID, the second local ID and information on an association between the second local ID and the second ID,

wherein a data packet is transmitted from the remote UE to the peer remote UE via the relay UE based on the association.
The relay UE of claim 7, wherein the controller is further configured to:

receive, from the remote UE, the data packet including information on a destination local ID indicating the destination of the data packet,

based on the association, identify whether the destination local ID matches with the second local ID, and

transmit, to the peer remote UE, the data packet, in case that the destination local ID is identified to match with the second local ID.
The relay UE of claim 7, wherein the RRC message further includes information on an association between the first ID and the first local ID.
A remote user equipment (UE) in a wireless communication system, the remote UE comprising:

a transceiver; and

a controller coupled with the transceiver and configured to:

identify, a first identification (ID) of the remote UE, wherein the first ID is used to identify the remote UE in a communication between the remote UE and a relay UE, and

receive, from the remote UE, a radio resource control (RRC) message including a first local ID, a second local ID and information on an association between the second local ID and a second ID of a peer remote UE, wherein the first local ID is used to identify the remote UE and the second local ID is used to identify the peer remote UE in an end-to-end communication between the remote UE and the peer remote UE via the relay UE, and the second ID is used to identify the peer remote UE in a communication between the relay UE and a peer remote UE,

wherein a data packet is transmitted from the remote UE to the peer remote UE via the relay UE based on the association.
The remote UE of claim 10,

wherein the controller is further configured to transmit, to the relay UE, the data packet including a destination local ID indicating a destination of the data packet, and

wherein the data packet is transmitted to the peer remote UE via the relay UE, in case that the destination local ID matches with the second local ID.
The remote UE of claim 10, wherein the RRC message further includes information on an association between the first ID and the first local ID.