WO2018083381A1

WO2018083381A1 - Idle mode mobility enhancement for an out-of-coverage remote user equipment

Info

Publication number: WO2018083381A1
Application number: PCT/FI2017/050751
Authority: WO
Inventors: Ling Yu; Dawid Koziol; Vinh Van Phan; Fernando SANCHEZ MOYA
Original assignee: Nokia Technologies Oy
Priority date: 2016-11-04
Filing date: 2017-10-31
Publication date: 2018-05-11

Abstract

A method including detecting by a first user equipment (UE) an out-of-coverage (OoC) situation of the first user equipment in a network; determining by the first user equipment whether to establish a sidelink (SL) connection with a second user equipment in the network, where the determining comprises at least one of: a pre- configuration of the first user equipment or a network configuration to stay in the idle state when in the out-of-coverage (OoC) situation or stay in the idle state when the out-of-coverage (OoC) situation depending on UE type and/or needs and/or services of the first user equipment, a network configuration, and when the first user equipment does not have the pre-configuration and/or a network configuration, using configuration information received from the second user equipment; and based upon the determination to establish the sidelink (SL) connection, transmitting a message from the first user equipment to the second user equipment.

Description

Idle Mode Mobility Enhancement For An Out-of-Coverage Remote User

Equipment

BACKGROUND

Technical Field

The exemplary and non-limiting embodiments relate generally to an out-of-coverage (OoC) user equipment (UE) and, more particularly, to how to reach an out-of- coverage (OoC) user equipment (UE).

Brief Description of Prior Developments

Device-to-Device (D2D) communication is known. Using a relay UE to relay communications between a remote UE and a network is known.

SUMMARY The following summary is merely intended to be exemplary. The summary is not intended to limit the scope of the claims.

In accordance with one aspect, a method is provided comprising: detecting by a first user equipment (UE), while the first user equipment is in an idle state, an out-of- coverage (OoC) situation of the first user equipment in a network; determining by the first user equipment, based upon the first user equipment having detected that the first user equipment is in the out-of-coverage situation, whether to establish a sidelink (SL) connection with a second user equipment in the network, where the determining comprises at least one of: a pre-configuration of the first user equipment to stay in the idle state when in the out-of-coverage (OoC) situation, or stay in the idle state when the out-of-coverage (OoC) situation depending on UE type and/or needs and/or services of the first user equipment, a combination of the pre- configuration of the first user equipment and a network configuration, and when the first user equipment does not have the pre-configuration, using configuration information received from the second user equipment; and based upon the determination to establish the sidelink (SL) connection, transmitting a message from the first user equipment to the second user equipment. In accordance with another aspect, an example method comprises receiving by a first user equipment (UE) in an idle state a discovery message from a second user equipment in a network, where the first user equipment is in an out-of-coverage situation, and where the discovery message comprises an idle mode mobility configuration; determining by the first user equipment, based at least partially upon the idle mode mobility configuration received from the second user equipment, to establish a sidelink (SL) connection of the first user equipment with the second user equipment; and based upon the determination to establish the sidelink (SL) connection of the first user equipment with the second user equipment, transmitting a message by the first user equipment to the second user equipment comprising paging occasion (PO) related information.

In accordance with another aspect, an example method comprises, while a second user equipment is connected to a first user equipment by a sidelink connection, where the first user equipment is in an idle state and in an out-of-coverage (OoC) situation in a network, monitoring by the second user equipment for a paging message for the first user equipment in non-access stratum (NAS) signaling; and upon receiving the non-access stratum (NAS) signaling by the second user equipment, where the non-access stratum (NAS) signaling comprises the paging message for the first user equipment, transmitting the paging message by the second user equipment to the first user equipment.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and other features are explained in the following description, taken in connection with the accompanying drawings, wherein:

Fig. 1 is a diagram illustrating a system comprising features of an example embodiment;

Fig. 2 is a diagram illustrating some examples of components of the system shown in Fig.1 ;

Fig. 3 is a diagram illustrating some components of the wireless system shown in Figs. 1 and 2; Fig. 4 is a diagram illustrating example methods and options; Fig. 5 is a diagram illustrating example methods and options; Fig. 6 is a diagram illustrating an example method; Fig. 7 is a diagram illustrating an example method; Fig. 8 is a diagram illustrating an example method.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring to Fig. 1 , there is shown a diagram of a system 10 incorporating features of an example embodiment. The system 10 comprises a base station 12, such as an eNB for example, a first user equipment 14 and a second user equipment 16. In other examples, more than one base station may be involved and more than two user equipment may be involved. The base station 12 has a cell 18 for direct communications with a user equipment. First user equipment 14 is in the cell 18 and, thus, can communicate directly with the base station 12 as indicated by link 20. However, second user equipment 16 is outside the cell 18 and, thus, cannot communicate directly with the base station. Instead a device-to-device (D2D) link 22 is provided between the two user equipment 14, 16 to allow the base station 12 and the second user equipment 16 to communicate with each other indirectly. Fig . 2 shows an alternate D2D link 22' between a smart phone as the first user equipment 14 and a smart watch 16' as the second user equipment. Thus, features as used herein may be applicable to a wearable device, and although the device 16' may be within the cell 18 it may not be in direct communication with the base station 12; needing the D2D link 22' to communicate with the network and perhaps the Internet. Referring also to Fig. 3, in the wireless system 230 a wireless network 235 is adapted for communication over a wireless link 20 with the apparatus 14, such as a mobile communication device which may be referred to as a UE 14, via a network access node, such as a Node B (base station), and more specifically an eNB 12. The network 235 may include a network control element (NCE) 240 that may include MME/S-GW functionality, and which provides connectivity with a network, such as a telephone network and/or a data communications network (e.g., the internet 238).

The UE 14 includes a controller, such as a computer or a data processor (DP) 214, a computer-readable memory medium embodied as a memory (MEM) 216 that stores a program of computer instructions (PROG) 218, and a suitable wireless interface, such as radio frequency (RF) transceiver 212, for bidirectional wireless communications with the eNB 12 via one or more antennas.

The eNB 12 also includes a controller, such as a computer or a data processor (DP) 224, a computer-readable memory medium embodied as a memory (MEM) 226 that stores a program of computer instructions (PROG) 228, and a suitable wireless interface, such as RF transceiver 222, for communication with the UE 14 via one or more antennas. The eNB 12 is coupled via a data/control path 234 to the NCE 240. The path 234 may be implemented as an interface. The eNB 12 may also be coupled to another eNB via data/control path 236, which may be implemented as an interface. The NCE 240 includes a controller, such as a computer or a data processor (DP) 244, a computer-readable memory medium embodied as a memory (MEM) 246 that stores a program of computer instructions (PROG) 248.

At least one of the PROGs 218, 228 and 248 is assumed to include program instructions that, when executed by the associated DP, enable the device to operate in accordance with exemplary embodiments of this invention, as will be discussed below in greater detail. That is, various exemplary embodiments of this invention may be implemented at least in part by computer software executable by the DP 214 of the UE 14; by the DP 224 of the eNB 12; and/or by the DP 244 of the NCE 240, or by hardware, or by a combination of software and hardware (and firmware). Base station 15 may have the same type of components as the base station 12. The second UE 16, 16' comprises components such as shown of the UE 14 in Fig. 3 including an antenna, at least one processor 214', at least one memory 216' and software or code 218' to perform operations.

For the purposes of describing various exemplary embodiments in accordance with this invention the UE 14 and the eNB 12 may also include dedicated processors, for example RRC module 215 and a corresponding RRC module 225. RRC module 215 and RRC module 225 may be constructed so as to operate in accordance with various exemplary embodiments in accordance with this invention.

The computer readable MEMs 216, 226 and 246 may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, flash memory, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The DPs 214, 224 and 244 may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on a multicore processor architecture, as non-limiting examples. The wireless interfaces (e.g., RF transceivers 212 and 222) may be of any type suitable to the local technical environment and may be implemented using any suitable communication technology such as individual transmitters, receivers, transceivers or a combination of such components. Features as described herein may be used to help facilitate the reachability of an Out-of-Coverage (OoC) remote user equipment (UE) by a network. This may be accomplished with an idle mode mobility enhancement for an OoC remote UE using capabilities of device-to-device (D2D) based UE-to-Network relay. Referring also to Fig. 4, for a remote UE which is in an idle state and which discovers or determines it is Out-Of-Coverage (OoC), the OoC remote UE then determines whether it should try to keep in the idle state (such as EMM-idle for example). This may be done based on a pre-configu ration 24 and/or a network configuration. For example, if the remote UE 16 is in an EMM-idle state before OoC is detected, upon detecting OoC, the remote UE 16 may try to stay in the EMM-idle state if a pre- configuration 24 requires the remote UE 16 to do so. The pre-configu ration 24 may either explicitly indicate that staying in the EMM-idle state is preferred for OoC, or it may be linked to the services that the remote UE 16 is intended to use for OoC such as, for example, if only group communication type of service is allowed for the remote UE. The pre-configu ration may also depend of the type of the UE. Thus, the remote UE 16 may not need to be in the EMM-idle state for OoC.

In addition to or instead of the pre-configuration 24, as indicated by 26 and 28, a network configuration may be also taken into account by the remote UE 16 to determine whether the EMM-idle state should try to be kept for OoC. The network configuration may be either received by the remote UE before it detects OoC such as shown by 26 or received via the relay UE 14 as illustrated by 28. If received via the relay UE 14, this may require the remote UE 16 to discover the relay UE 14 and listen to the relayed system information or a NAS signaling. The remote UE 16 supporting the UE-to-network (UE2NW) relay capability may try to discover the relay UE 14 when the remote UE 16 is OoC if the UE2NW relay feature is configured to the remote UE 16 regardless of whether the EMM-idle state is configured or not. Therefore, the remote UE 16 may try to monitor D2D discovery messages from the relay UE(s) 14 when OoC is identified/detected if configured to do so.

How the network selects through which relay UE 14 to configure the remote UE 16 may depend on whether the network knows the relationship between the remote UE 16 and the relay UE 14 beforehand or not. For a "wearable" device use case (e.g. Peter's watch will use Peter's mobile phone to access a network via UE2NW relay as illustrated by Fig. 2), such relationship between the wearable device/remote UE 16' and mobile phone/relay UE 14 may be known by the network beforehand. The network may then configure the relay UE 14 (i.e. user's mobile phone) to the specific remote UE 16' (i.e. user's wearable devices). For some public safety use cases (e.g. one fire-fighter's phone in OoC uses another fire-fighter's phone in a coverage-to- access network via a UE2NW relay), such one-to-one relationship may not be known beforehand by the network. In this case, the network may configure every UE 14 in a coverage edge to relay the network configuration to allow the OoC remote UE 16 in proximity to receive such configuration information. As noted above, after the remote UE 16 determined to keep an EMM-idle state, it identifies a proper relay 14. If the configuration is done when out-of-Coverage, then the remote UE 16 might already know the proper relay UE 14.

Thus, three different configuration options may be:

• Pre-configuration to maintain EMM-idle in OoC or depending on UE type/needs/services; · Network configuration to maintain EMM-idle in OoC in addition to/instead of pre-configuration, if this is done while in coverage. The in-coverage configuration may be done via either dedicated or broadcast signaling. For in-coverage a configuration may also be done through relay.; and

• If the configuration is performed when out of coverage, the network may configure the remote UE through relay. The in-coverage UE with relay capability may relay configuration information related to an OoC UE idle mode mobility configuration (e.g. whether OoC UE should keep EMM-idle state or not and corresponding conditions/rules) based on a network configuration. If the OoC remote UE 16 determines to keep the EMM-idle state, such as as long as possible, the OoC remote UE 16 may determine whether to maintain a sidelink (SL) connection with at least one relay UE 14 all the time or establish a sidelink (SL) connection with at least one relay UE 14 "on demand" or using broadcast based SL communication (for example, SL connection establishment may not be triggered for paging relay) based on the pre-configuration and/or the network configuration. The term "as long as possible" may mean if there is proper relay UE 14 in proximity of the remote UE 16 to provide UE2NW relay service for the remote UE 16.

When the remote UE 16 determines to keep the EMM-idle state for OoC, the remote UE may try to use the proper relay UE 14 to perform a required idle mode task such as, for example, location/tracking area update, monitoring and answering paging, and performing service request. Options, in terms of identifying a proper relay UE 14 for performing the idle mode tasks, may include for example: a. Option 1 : the proper relay UE 14 is identified by a relay discovery on sidelink (SL), and the sidelink (SL) connection between the remote UE 16 and discovered proper relay UE is only established "on demand"; for example when location update, paging monitoring or service request is triggered. b. Option 2: the proper relay UE 14 is always identified by the sidelink (SL) connection establishment or pairing between the remote UE 16 and the relay UE so that the sidelink (SL) connection or pairing is maintained for the remote UE as long as EMM-idle state needs to be kept based on the pre-configuration and/or the network configuration. Pairing in this case means that the paired UEs (remote and relay) are confirmed to be in the distance allowing for them to communicate directly using sidelink communications. This can, but does not necessarily need to be synonymous to being connected. For example, it could also be achieved by measuring signal level of discovery messages.

If both options are supported by the system, the determination of which option to use may be based on the pre-configuration and/or network configuration. If based upon the pre-configuration, this may be either based upon an explicit pre-configuration or upon a pre-configu red criteria on an option selection such as, for example, based on the status of the discovered relay UE. For example, this may be based upon the number of discovered relay UEs and/or the stability of the discovered relay UE group, etc. If based upon the network configuration, this may be based, for example, on a network preference on visibility or invisibility of the SL connection establishment to the network, etc.

Option 1 - On Demand

With reference particularly to option 1 shown in Fig. 4, the remote UE 16 may start looking for a suitable relay UE 14 when SL connection establishment is triggered "on demand". However, this does not prevent the remote UE from monitoring/discovering the relay UE at any other time. As indicated by 30, to facilitate sidelink (SL) connection establishment on demand for the OoC remote UE location/tracking area update and paging monitoring, the relay UE 14 may relay relevant system information (e.g. location/tracking area ID, DRX related configuration etc.) in an SL discovery message so that the OoC remote UE may trigger the SL connection establishment on demand based on the received relevant system information as indicated by 32 and 34. For example the OoC remote UE may trigger the SL connection establishment on demand based upon when the location/tracking area update needs to be initiated due to a change of the location/tracking area as indicated by 36 or the calculated paging occasion (PO) is coming within pre-defined time period as indicated by 37. Paging of remote UEs (either in-coverage or OoC) via a relay UE can be either transparent or non-transparent to the network (e.g. MME) based on network configuration. With a non-transparent option, this means that the network has some prior knowledge about the association between the remote UE and the relay UE, or about the remote UE being OoC. In this option the network is aware that the paging message destined for the remote UE will be relayed to the remote UE via the relay UE. With a transparent option, this means that the network does not need to have such prior knowledge, and that the network is not aware whether a paging message sent by the network, and destined for a remote UE, will be read by the remote UE directly or will be relayed to the remote UE via a relay UE. For transparent option the network pages the OoC remote UE using the remote UE's PO. The relay UE 14 may be configured to monitor the remote UE's POs using two different ways, which result in two different relay UE behaviours:

• The remote UE provides the relay UE with its paging occasion (PO) or the relevant configuration information required by the relay UE to calculate the POs of the remote UE. However, the remote UE does not provide its identifier

(International Mobile Subscriber Identity (IMSI) or SAE-Temporary Mobile Subscriber Identity (S-TMSI) for example), which is sent in the paging message by the network. Therefore, it is necessary for the relay UE to relay all paging messages received in the remote UE's POs to the remote UE. The remote UE may then evaluate whether any of them is destined for the remote

UE. In an alternative embodiment the remote UE may disclose at least some identifier information e.g. S-TMSI to the relay UE.

• The remote UE requests the relay UE to monitor for its paging and, in addition to its PO configuration information required by the relay UE to calculate remote the UE's POs, provides the relay UE with its higher layer identifier(s), for example S-TMSI and/or IMSI. The relay UE monitors the remote UE's POs and checks whether they contain the remote UE's identifier. If the relay UE identifies that a paging message contains the remote UE's identifier, the relay UE triggers a connection establishment between the remote UE and the network either by relaying this paging message or by informing the remote UE that it should establish the connection with the network in some other way such as, for example, by including a certain flag in a discovery message or inform via the broadcast based SL communication or by one-to-one SL communication. In order to allow the remote UE to trigger the location/tracking area update and also derive the paging occasion from the system frame number (SFN), the relay UE may relay the relevant system information such as master information block (MIB) (for remote UE to derive SFN for paging occasion calculation) and also paging and location/tracking area update related configuration (e.g. location/tracking area ID, Discontinuous Reception (DRX) related configuration, etc.) This information may be relayed in SL discovery message in option 1 as the remote UE establishes the SL connection with the relay UE only on demand.

Based on relevant information received in the SL discovery message from relay UE, the remote UE 16 may trigger the SL connection establishment to the selected relay UE 14 if location/tracking area update should be triggered or the calculated paging occasion is coming within a pre-configured time period such as, for example, a pre- configured time period is configured taking into account the time the remote UE needs to spend to find a suitable relay UE and establish a connection with that relay UE for a possible page as well as expectable response to the page. In an alternative embodiment, to avoid an SL connection establishment with the relay UE before each upcoming paging occasion, a broadcast based SL communication may be used instead of a one-to-one SL communication for the relay UE to forward the paging message to the remote UE. For Idle-mobility related information relayed by a relay UE discovery message, the network may configure either one specific relay UE to relay the information if a relationship is known beforehand. Otherwise, the network may configure all or a chosen subset of the relevant UEs in a coverage edge to relay the information. As the SL connection between the relay UE and the remote UE is established on demand, the relayed information is, therefore, proposed via the D2D discovery message instead of a communication message because the D2D discovery message can be sent without SL connection establishment between the relay UE and the remote UE.

If the SL connection establishment is triggered due to location/tracking area update, the relayed location/tracking area update may be made hidden from the network, e.g. Mobility Management Entity (MME):

• The remote UE may initiate the location/tracking area update procedure to the network via the relay UE by sending an uplink (UL) information transfer Radio Resource Control (RRC) message (such as containing a location/tracking area update request Non Access Stratum (NAS) message) to the base station or E-UTRAN Node B (eNB). • The eNB may forward the location/tracking area update request NAS message to the MME.

• The MME may accept the location/tracking update request and send back the location/tracking update accept NAS message to the remote UE relayed by the relay UE.

• The SL communication link may then be released if there is no further communication from remote UE to the network.

If SL connection establishment is triggered due to the need of a monitoring paging message: · The remote UE may indicate to the relay UE to monitor paging messages of the remote UE. The remote UE may either indicate its paging occasion (PO) or relevant information for calculating the PO with or without full remote UE ID (e.g. S-TMSI and/or IMSI) to the relay UE.

• The relay UE may monitor paging messages received at the PO related to the remote UE regardless of whether or not the relay UE is in an idle state or a connected state. Upon receiving the paging message at the remote UE's PO, the relay UE may send the received paging message to the remote UE via the SL communication. Additionally, if the remote UE provides the full UE ID to the relay UE, the relay UE may check the received paging message and request the remote UE to access to the network only if the relay UE detects the paging message of the remote UE. In this way, SL connection establishment is not necessary to be triggered for every PO. The SL connection establishment may be only triggered for the remote UE to provide its PO related information to the relay UE. After the relay UE has the valid remote UE's PO related information, the SL connection may not be triggered at every PO of remote UE, but only when the relay UE detects the paging message of the remote UE.

• If the remote UE detects its own paging message in the forwarded paging message or request of network access from the relay UE, the remote UE may access to the network through the connected relay UE or directly to the network if the remote UE is in coverage. Otherwise, the SL connection may be released if no further communication to the network is needed. The benefit of providing full remote UE's ID to the relay UE is that the number of paging messages which need to be relayed by the relay UE is limited; resulting in less signalling overhead and power consumption by the remote UE and the relay UE. The disadvantage of providing full remote UE's ID to the relay UE is that it requires disclosing a remote UE's identifier(s) to the relay UE. Option 1 does not have this disadvantage. Normally it might be unacceptable to disclose a remote UE's IMSI to another UE, but it might be acceptable for S-TMSI. It should be noted that LTE system paging using IMSI is performed only in abnormal situations where S-TMSI is not available in the MME. If broadcast SL communications is used for paging relaying after the relevant information on paging relay has been exchanged from the remote UE to the relay UE via established SL connection:

• The SL connection may be established between the remote UE and the relay UE to allow the remote UE to indicate to the relay UE for monitoring paging message of the remote UE. The remote UE may either indicate its paging occasion (PO) or relevant information for calculating PO with or without full remote UE ID (e.g. S-TMSI and/or IMSI) to the relay UE. The relay UE, the remote UE and optionally the network negotiate and agree on the sidelink resources where the remote UE should be monitoring for relayed paging messages. After this, the SL connection may be released and a new SL connection does not need to be established again for paging monitoring as long as the remote UE can discover the same relay UE.

• The relay UE may monitor the paging messages received at the PO related to the remote UE regardless of whether the relay UE is in an idle state or a connected state. Upon receiving the paging message at the remote UE's PO, the relay UE may send the received paging message to the remote UE via the SL broadcast communication without establishing a one-to-one SL connection. Additionally, if the remote UE provides the full UE ID to the relay UE, the relay UE may check the received paging message and request the remote UE to access to the network only if relay UE detects the paging message of remote UE. In this way, only relevant paging messages are relayed using sidelink.

• If the remote UE detects its own paging message in the forwarded paging message the remote UE may access to the network by establishing an indirect connection via the relay UE or directly with the network if the remote

UE is in-coverage. Option 2 - Maintained SL Connection or Pairing

Referring particularly to Fig. 5 in regard to option 2, a SL connection exists between the relay UE 14 and the remote UE 16 as illustrated by 35. Transparent and non- transparent options are available in this option. For the transparent option, the remote UE's PO can be used in the same way as option 1 . For the non-transparent option, the network pages the remote UE using the relay UE's PO, which requires "S1 " paging message enhancement as further discussed below. As another sub- option, a common PO for all or only OoC remote UEs may be introduced so that the relay UE, that has the SL connected or paired remote UE, will monitor the common PO for remote UE paging message. These are illustrated by 38. As illustrated by 40 another sub-option, the idle relay UE 14 may be paged for triggering the relay UE move to the RRC-connected state and, then, use dedicated NAS signaling 42 as proposed below to deliver the paging message of the remote UE to the relay UE that forwards the paging message to the remote UE 44 over SL. Herein NAS means dedicated NAS signaling from the MME to the relay UE in a connected mode. For the non-transparent option, if SL connected relay UE is in a RRC connected state, a new dedicated NAS signaling for transmitting remote UE's paging message from the network (e.g. MME) to the relay UE may be introduced so that the relay UE can forward the paging message to the remote UE via the SL communication or the SL discovery upon receiving the dedicated NAS message of remote UE's paging message.

The remote UE may try to keep the SL connection or pairing with at least one relay UE all the time if EMM-idle state should be kept for the OoC remote UE so that the required information exchange (e.g. relevant location/tracking area update or paging related information as discussed below) between the relay UE and the remote UE may be performed via the established SL connection or broadcast based SL communication. Based on the network configuration, the SL connection establishment or pairing between the remote UE and the relay UE may be either visible/non-transparent or invisible/transparent to the core network (e.g. MME in LTE). So, the paging of the remote UE via the relay UE may be made either transparent or non-transparent to the network correspondingly.

For invisible/transparent option: the procedures for remote UE location/tracking area update and paging may be the same as option 1 noted above, except that the SL connection establishment is not triggered on demand and the location/tracking area update and paging related configuration information is forwarded from the relay UE to the remote UE via SL communication instead of SL discovery message. This is true when the SL connection is maintained, but not necessarily for pairing. For visible/non-transparent option: the relay UE may inform the MME when the SL connection or pairing with the remote UE is established so that the MME knows which relay UE the remote UE has an SL connection or pairing established with. The remote UE may skip the location/tracking update procedure with the network because the network (e.g. MME in LTE) can identify the remote UE's location based on the relay UE's location and the indication of the SL connection or pairing between them.

If the relay UE is in a RRC-idle state:

• the paging message for the remote UE may be sent at the relay UE's PO. In order to allow the eNB to calculate the relay UE's PO also for the remote UE's paging message, in the Paging message delivered from core network (e.g. MME in LTE) to the radio access network (e.g. eNB in LTE), it may include the remote UE's ID in the UE paging identity IE, but set the UE Identity Index Value IE based on the relay UE's ID. As another option, a common OoC remote UE's PO may be introduced in each cell and the relay UEs in the cell may be configured to monitor the common remote UE's PO if the SL connection is established between the relay UE and any remote UE. As another option, the network pages the relay UE to trigger the UE to move to a RRC-connected state and then use the procedures proposed as below for RRC-connected state relay UE to deliver the paging message to the remote

UE.

• The relay UE, upon receiving the paging message at its own PO or common remote UE's PO, may forward the paging message to the remote UE via the SL communication link. If the relay UE is in a RRC-connected state:

• The paging message for the remote UE may be sent to the relay UE via a new dedicated NAS message, which carries the remote UE paging message from the core network (e.g. MME in LTE) to the relay UE.

• The relay UE, upon receiving the NAS message of remote UE paging message, may forward the paging message to the remote UE via the SL communication link. Another alternative may be that the NAS message is not relayed, but instead the NAS message is destined for the Relay UE and indicates that a certain Remote UE (identified by some ID as described above for example) should trigger connection to the network. Then, the Relay UE may indicate that remote UE should establish a connection using some other message sent over sidelink discovery or sidelink communications. It should be noted that terms "remote UE" and "relay UE" are used above to distinguish the potential roles of the UEs in the indirect communications to the network, i.e. remote UE is a UE, which will have its data, which is to be sent to or from the network relayed over the relay UE once the indirect communication connection is established. The described procedures happen before such connection is established for the most part.

Features as described herein may be used for 3GPP Rel-14 and beyond systems (e.g. 5G), but may also be applicable to other radio technologies such as WLAN for example. Features may be related to enhancements of UE-to-Network relay based on Device-to-Device (D2D) communications, also known as Proximity Services (ProSe), direct communications or sidelink operation/communications. Features may be used for enhancement of LTE D2D (FeD2D) for Internet-of-Things (loT) and wearables that are characterized by small form-factor, but at the same time are expected to provide long operation without a need to recharge their battery. In addition, it is also preferable to have a common solution for both commercial and public safety use cases in order to gain the economic scale of public safety business. With features as described herein, a network may be able to reach an OoC (Out of Coverage) remote UE (such as a loT/wearable low-power device) through a relay UE.

The requirements regarding indirect communication are described for example in 3GPP TS 22.278 V14.1 .0. The work on an Evolved UE to Network Relaying framework is ongoing in 3GPP Rel-14. According to its objectives an Evolved ProSe Remote UE should be able to use Indirect 3GPP Communication either in E-UTRAN coverage or out of E-UTRAN coverage. The 3GPP network should be able to identify, authenticate, address and reach an Evolved ProSe Remote UE when it accesses the 3GPP network via an Indirect 3GPP Communication. How to reach an OoC (Out-of-Coverage) UE by a network in an efficient way may be addressed with features as described herein. In order for a network to reach the OoC remote UE, the network is able to deliver paging message to the remote UE through the relay UE. This is addressed in two scenarios:

• keeping the relay UE and the remote UE association/connection/pairing all the time for reachability/paging message delivery. This may be either transparent or non-transparent to the network, and · an association/connection between the relay UE and the remote UE is established on demand. For the above scenarios, paging details are proposed as described above.

Referring also to Fig. 6, an example method may comprise detecting by a first user equipment (UE), while the first user equipment is in an idle state, an out-of-coverage (OoC) situation of the first user equipment in a network as indicated by block 50; determining by the first user equipment, based upon the first user equipment having detected that the first user equipment is in the out-of-coverage situation, whether to establish a sidelink (SL) connection with a second user equipment in the network, where the determining comprises at least one of: a pre-configuration of the first user equipment or a network configuration to: stay in the idle state when in the out-of- coverage (OoC) situation, or stay in the idle state when the out-of-coverage (OoC) situation depending on UE type and/or needs and/or services of the first user equipment, a combination of the pre-configuration of the first user equipment and a network configuration, and when the first user equipment does not have the pre- configuration and/or a network configuration, using configuration information received from the second user equipment as indicated by block 52; and based upon the determination to establish the sidelink (SL) connection, transmitting a message from the first user equipment to the second user equipment as indicated by block 54.

The method may further comprise receiving by the first user equipment (UE) a discovery message from the second user equipment, where the discovery message comprises an idle mode mobility configuration; and determining by the first user equipment, based at least partially upon the idle mode mobility configuration received from the second user equipment, to establish the sidelink (SL) connection of the first user equipment with the second user equipment. The method may further comprise, based upon the determination to establish the sidelink (SL) connection of the first user equipment with the second user equipment, transmitting a message by the first user equipment to the second user equipment comprising paging occasion (PO) related information. The paging occasion (PO) related information may not comprise information identifying the first user equipment. The sidelink (SL) connection between the first and second user equipment may be established when the first user equipment triggers conditions for location/tracking area and paging monitoring. A non-transitory program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine for performing operations, the operations comprising the method steps noted above.

An example apparatus may comprise means for detecting by a first user equipment (UE), while the first user equipment is in an idle state, an out-of-coverage (OoC) situation of the first user equipment in a network; means for determining by the first user equipment, based upon the first user equipment having detected that the first user equipment is in the out-of-coverage situation, whether to establish a sidelink (SL) connection with a second user equipment in the network, where the determining comprises at least one of: a pre-configuration of the first user equipment to: stay in the idle state when in the out-of-coverage (OoC) situation, or stay in the idle state when the out-of-coverage (OoC) situation depending on UE type and/or needs and/or services of the first user equipment, a combination of the pre-configuration of the first user equipment and a network configuration, and when the first user equipment does not have the pre-configuration, using configuration information received from the second user equipment; and based upon the determination to establish the sidelink (SL) connection, means for transmitting a message from the first user equipment to the second user equipment.

An example apparatus may be provided comprising at least one processor; and at least one non-transitory memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to: detect by the apparatus, where the apparatus is a first user equipment (UE), while the first user equipment is in an idle state, an out- of-coverage (OoC) situation of the first user equipment in a network; determine by the first user equipment, based upon the first user equipment having detected that the first user equipment is in the out-of-coverage situation, whether to establish a sidelink (SL) connection with a second user equipment in the network, where the determining comprises at least one of: a pre-configuration of the first user equipment to: stay in the idle state when in the out-of-coverage (OoC) situation, or stay in the idle state when the out-of-coverage (OoC) situation depending on UE type and/or needs and/or services of the first user equipment, a combination of the pre- configuration of the first user equipment and a network configuration, and when the first user equipment does not have the pre-configuration, using configuration information received from the second user equipment; and based upon the determination, establish the sidelink (SL) connection, transmitting a message from the first user equipment to the second user equipment.

Referring also to Fig. 7, an example method may comprise receiving by a first user equipment (UE) in an idle state a discovery message from a second user equipment in a network, where the first user equipment is in an out-of-coverage situation, and where the discovery message comprises an idle mode mobility configuration as indicated by block 60; determining by the first user equipment, based at least partially upon the idle mode mobility configuration received from the second user equipment, to establish a sidelink (SL) connection of the first user equipment with the second user equipment as indicated by block 62; and based upon the determination to establish the sidelink (SL) connection of the first user equipment with the second user equipment, transmitting a message by the first user equipment to the second user equipment comprising paging occasion (PO) related information as indicated by block 64.

The paging occasion (PO) related information may not comprise information identifying the first user equipment. The sidelink (SL) connection between the first and second user equipment may be established when the first user equipment triggers conditions for location/tracking area and paging monitoring. The method may further comprise detecting by the first user equipment, the out-of-coverage (OoC) situation of the first user equipment in the network; determining by the first user equipment, based upon the first user equipment having detected that the first user equipment is in the out-of-coverage situation, whether to establish the sidelink (SL) connection with the second user equipment in the network, where the determining comprises at least one of: a pre-configuration of the first user equipment to: stay in the idle state when in the out-of-coverage (OoC) situation, or stay in the idle state when the out-of-coverage (OoC) situation depending on UE type and/or needs and/or services of the first user equipment, a combination of the pre- configuration of the first user equipment and a network configuration, and when the first user equipment does not have the pre-configuration, using configuration information received from the second user equipment. An example embodiment may be provided in a non-transitory program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine for performing operations, the operations comprising the method as described above.

An example apparatus may be provided comprising means for receiving by a first user equipment (UE) in an idle state a discovery message from a second user equipment in a network, where the first user equipment is in an out-of-coverage situation, and where the discovery message comprises an idle mode mobility configuration; means for determining by the first user equipment, based at least partially upon the idle mode mobility configuration received from the second user equipment, to establish a sidelink (SL) connection of the first user equipment with the second user equipment; and based upon the determination to establish the sidelink (SL) connection of the first user equipment with the second user equipment, means for transmitting a message by the first user equipment to the second user equipment comprising paging occasion (PO) related information.

An example embodiment may be provided in an apparatus comprising at least one processor; and at least one non-transitory memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to: detect by the apparatus, where the apparatus is a first user equipment, the out-of-coverage (OoC) situation of the first user equipment in the network; determine by the first user equipment, based upon the first user equipment having detected that the first user equipment is in the out-of-coverage situation, whether to establish the sidelink (SL) connection with the second user equipment in the network, where the determining comprises at least one of: a pre-configuration of the first user equipment to: stay in the idle state when in the out-of-coverage (OoC) situation, or stay in the idle state when the out-of- coverage (OoC) situation depending on UE type and/or needs and/or services of the first user equipment, a combination of the pre-configuration of the first user equipment and a network configuration, and when the first user equipment does not have the pre-configuration, using configuration information received from the second user equipment.

Referring also to Fig. 8, an example method may comprise, while a second user equipment is connected to a first user equipment by a sidelink connection, where the first user equipment is in an idle state and in an out-of-coverage (OoC) situation in a network, monitoring by the second user equipment for a paging message for the first user equipment in non-access stratum (NAS) signaling as indicated by block 70; and upon receiving the non-access stratum (NAS) signaling by the second user equipment, where the non-access stratum (NAS) signaling comprises the paging message for the first user equipment, transmitting the paging message by the second user equipment to the first user equipment as indicated by block 72. The method may further comprise monitoring by the second user equipment for a common paging occasion message for the first user equipment. The method may further comprise forwarding the common paging occasion message by the second user equipment to the first user equipment. An example embodiment may be provided in non-transitory program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine for performing operations, the operations comprising the method as described above.

An example apparatus may comprise means for, while a second user equipment is connected to a first user equipment by a sidelink connection, where the first user equipment is in an idle state and in an out-of-coverage (OoC) situation in a network, monitoring by the second user equipment for a paging message for the first user equipment in non-access stratum (NAS) signaling; and means for, upon receiving the non-access stratum (NAS) signaling by the second user equipment, where the non-access stratum (NAS) signaling comprises the paging message for the first user equipment, transmitting the paging message by the second user equipment to the first user equipment. An example embodiment may be provided in an apparatus comprising at least one processor; and at least one non-transitory memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to: while a second user equipment is connected to a first user equipment by a sidelink connection, where the first user equipment is in an idle state and in an out-of-coverage (OoC) situation in a network, monitor by the second user equipment for a paging message for the first user equipment in non-access stratum (NAS) signaling; and upon receiving the non- access stratum (NAS) signaling by the second user equipment, where the non- access stratum (NAS) signaling comprises the paging message for the first user equipment, transmit the paging message by the second user equipment to the first user equipment.

Any combination of one or more computer readable medium(s) may be utilized as the memory. The computer readable medium may be a computer readable signal medium or a non-transitory computer readable storage medium. A non-transitory computer readable storage medium does not include propagating signals and may be, for example, but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any suitable combination of the foregoing. More specific examples (a non-exhaustive list) of the computer readable storage medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing.

It should be understood that the foregoing description is only illustrative. Various alternatives and modifications can be devised by those skilled in the art. For example, features recited in the various dependent claims could be combined with each other in any suitable combination(s). In addition, features from different embodiments described above could be selectively combined into a new embodiment. Accordingly, the description is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.

Claims

1 . A method comprising: detecting by a first user equipment (UE), while the first user equipment is in an idle state, an out-of-coverage (OoC) situation of the first user equipment in a network; determining by the first user equipment, based upon the first user equipment having detected that the first user equipment is in the out-of-coverage situation, whether to establish a sidelink (SL) connection with a second user equipment in the network, where the determining comprises at least one of: a pre-configuration or a network configuration of the first user equipment to: stay in the idle state when in the out-of-coverage (OoC) situation, or stay in the idle state when the out-of-coverage (OoC) situation depending on UE type and/or needs and/or services of the first user equipment, a combination of the pre-configuration of the first user equipment and a network configuration, and when the first user equipment does not have the pre-configuration and/or a network configuration, using configuration information received from the second user equipment; and based upon the determination to establish the sidelink (SL) connection, transmitting a message from the first user equipment to the second user equipment.

2. A method as in claim 1 further comprising: receiving by the first user equipment (UE) a discovery message from the second user equipment, where the discovery message comprises an idle mode mobility configuration; and determining by the first user equipment, based at least partially upon the idle mode mobility configuration received from the second user equipment, to establish the sidelink (SL) connection of the first user equipment with the second user equipment.

3. A method as in claim 2 further comprising, based upon the determination to establish the sidelink (SL) connection of the first user equipment with the second user equipment, transmitting a message by the first user equipment to the second user equipment comprising paging occasion (PO) related information.

4. A method as in claim 3 where the paging occasion (PO) related information does not comprise information identifying the first user equipment.

5. A method as in any one of claims 1 -4 where the sidelink (SL) connection between the first and second user equipment is established when the first user equipment triggers conditions for location/tracking area and paging monitoring.

6. A non-transitory program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine for performing operations, the operations comprising the method as claimed in claim 1 .

7. An apparatus comprising at least one processor; and at least one non-transitory memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to: detect by the apparatus, where the apparatus is a first user equipment (UE), while the first user equipment is in an idle state, an out-of- coverage (OoC) situation of the first user equipment in a network; determine by the first user equipment, based upon the first user equipment having detected that the first user equipment is in the out-of- coverage situation, whether to establish a sidelink (SL) connection with a second user equipment in the network, where the determining comprises at least one of: a pre-configuration or a network configuration of the first user equipment to: stay in the idle state when in the out-of-coverage (OoC) situation, or stay in the idle state when the out-of-coverage (OoC) situation depending on UE type and/or needs and/or services of the first user equipment, a combination of the pre-configuration of the first user equipment and a network configuration, and when the first user equipment does not have the pre- configuration and/or a network configuration, using configuration information received from the second user equipment; and based upon the determination, establish the sidelink (SL) connection, transmitting a message from the first user equipment to the second user equipment.

8. A method comprising: receiving by a first user equipment (UE) in an idle state a discovery message from a second user equipment in a network, where the first user equipment is in an out-of-coverage situation, and where the discovery message comprises an idle mode mobility configuration; determining by the first user equipment, based at least partially upon the idle mode mobility configuration received from the second user equipment, to establish a sidelink (SL) connection of the first user equipment with the second user equipment; and based upon the determination to establish the sidelink (SL) connection of the first user equipment with the second user equipment, transmitting a message by the first user equipment to the second user equipment comprising paging occasion (PO) related information.

9. A method as in claim 8 where the paging occasion (PO) related information does not comprise information identifying the first user equipment.

10. A method as in any one of claims 8-9 where the sidelink (SL) connection between the first and second user equipment is established when the first user equipment triggers conditions for location/tracking area and paging monitoring.

1 1 . A method as in any one of claims 8-10 further comprising: detecting by the first user equipment, the out-of-coverage (OoC) situation of the first user equipment in the network; determining by the first user equipment, based upon the first user equipment having detected that the first user equipment is in the out-of-coverage situation, whether to establish the sidelink (SL) connection with the second user equipment in the network, where the determining comprises at least one of: a pre-configuration of the first user equipment to: stay in the idle state when in the out-of-coverage (OoC) situation, or stay in the idle state when the out-of-coverage (OoC) situation depending on UE type and/or needs and/or services of the first user equipment, a combination of the pre-configuration of the first user equipment and a network configuration, and when the first user equipment does not have the pre-configuration, using configuration information received from the second user equipment.

12. A non-transitory program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine for performing operations, the operations comprising the method as claimed in claim 8.

13. An apparatus comprising: at least one processor; and at least one non-transitory memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to: detect by the apparatus, where the apparatus is a first user equipment, the out-of-coverage (OoC) situation of the first user equipment in the network; determine by the first user equipment, based upon the first user equipment having detected that the first user equipment is in the out-of- coverage situation, whether to establish the sidelink (SL) connection with the second user equipment in the network, where the determining comprises at least one of: a pre-configuration of the first user equipment to: stay in the idle state when in the out-of-coverage (OoC) situation, or stay in the idle state when the out-of-coverage (OoC) situation depending on UE type and/or needs and/or services of the first user equipment, a combination of the pre-configuration of the first user equipment and a network configuration, and when the first user equipment does not have the pre- configuration, using configuration information received from the second user equipment.

14. A method comprising: while a second user equipment is connected to a first user equipment by a sidelink connection, where the first user equipment is in an idle state and in an out-of-coverage (OoC) situation in a network, monitoring by the second user equipment for a paging message for the first user equipment in non-access stratum (NAS) signaling; and upon receiving the non-access stratum (NAS) signaling by the second user equipment, where the non-access stratum (NAS) signaling comprises the paging message for the first user equipment, transmitting the paging message by the second user equipment to the first user equipment.

15. A method as in claim 14 further comprising monitoring by the second user equipment for a common paging occasion message for the first user equipment.

16. A method as in claim 15 further comprising forwarding the common paging occasion message by the second user equipment to the first user equipment.

17. A non-transitory program storage device readable by a machine, tangibly embodying a program of instructions executable by the machine for performing operations, the operations comprising the method as claimed in claim 14.

18. An apparatus comprising: at least one processor; and at least one non-transitory memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to: while a second user equipment is connected to a first user equipment by a sidelink connection, where the first user equipment is in an idle state and in an out-of-coverage (OoC) situation in a network, monitor by the second user equipment for a paging message for the first user equipment in non-access stratum (NAS) signaling; and upon receiving the non-access stratum (NAS) signaling by the second user equipment, where the non-access stratum (NAS) signaling comprises the paging message for the first user equipment, transmit the paging message by the second user equipment to the first user equipment.

19. An apparatus comprising: means for detecting by a first user equipment (UE), while the first user equipment is in an idle state, an out-of-coverage (OoC) situation of the first user equipment in a network; means for determining by the first user equipment, based upon the first user equipment having detected that the first user equipment is in the out-of- coverage situation, whether to establish a sidelink (SL) connection with a second user equipment in the network, where the determining comprises at least one of: a pre-configuration of the first user equipment to: stay in the idle state when in the out-of-coverage (OoC) situation, or stay in the idle state when the out-of-coverage (OoC) situation depending on UE type and/or needs and/or services of the first user equipment, a combination of the pre-configuration of the first user equipment and a network configuration, and when the first user equipment does not have the pre-configuration, using configuration information received from the second user equipment; and based upon the determination to establish the sidelink (SL) connection, means for transmitting a message from the first user equipment to the second user equipment.

20. An apparatus comprising: means for receiving by a first user equipment (UE) in an idle state a discovery message from a second user equipment in a network, where the first user equipment is in an out-of-coverage situation, and where the discovery message comprises an idle mode mobility configuration; means for determining by the first user equipment, based at least partially upon the idle mode mobility configuration received from the second user equipment, to establish a sidelink (SL) connection of the first user equipment with the second user equipment; and based upon the determination to establish the sidelink (SL) connection of the first user equipment with the second user equipment, means for transmitting a message by the first user equipment to the second user equipment comprising paging occasion (PO) related information.

21 . An apparatus comprising: means for, while a second user equipment is connected to a first user equipment by a sidelink connection, where the first user equipment is in an idle state and in an out-of-coverage (OoC) situation in a network, monitoring by the second user equipment for a paging message for the first user equipment in non-access stratum (NAS) signaling; and means for, upon receiving the non-access stratum (NAS) signaling by the second user equipment, where the non-access stratum (NAS) signaling comprises the paging message for the first user equipment, transmitting the paging message by the second user equipment to the first user equipment.