TECHNIQUE FOR HEANDLING RADIO LINK FAILURE IN RELAYED RADIO COMMUNICATIONS

Technical Field

The present disclosure relates to a technique for handling a radio link failure (RLF) in a relayed radio communication. More specifically, and without limitation, methods and devices are provided for handling an RLF in a relayed radio communication at a remote radio device in radio communication with a radio access network (RAN) or a further remote radio device through a relay radio device.

Background

The Third Generation Partnership Project (3GPP) and the Wi-Fi Alliance specify radio access technologies such as Fourth Generation Long Term Evolution (4G LTE), Fifth Generation New Radio (5G N R) and Wi-Fi, each of which supports device-to-device (D2D) communications. For example, 3GPP has specified a sidelink (SL) for LTE and NR. The SL is also referred to as a proximity service (or PROximity-based Service, ProSe).

The D2D communication may be used to relay a data unit to or from a remote radio device (e.g., a remote UE or RM-UE), e.g., in case a relay radio device (e.g., a relay UE or RL-UE) has coverage to a 3GPP network node such as a gN B, while the remote radio device is out of coverage.

In 3GPP meeting RAN2#lll-e, a layer 2 (L2) relay mechanism was discussed. The L2 relay mechanism comprises an adaptation layer. For example, as has been summarized in the 3GPP document R2-2008266 "Summary of [ATlll-e] [605][Relay] L2 Relay Mechanism" by MediaTek Inc. for RAN2#lll-e, it has been agreed to support an adaptation layer for relaying the relayed radio communication.

However, the state of the art does not guarantee or control service continuity in case of an RLF in sidelink relay scenarios. For example, since in the relay radio communication, i.e. in a relay path, more than two entities are involved such as the remote UE, the relay UE, and the gNB or the further remote UE, each of them has to be in consistent states and/or perform consistent actions once the RLF has been detected.

Summary

Accordingly, there is a need for a technique that handles a RLF in relayed radio communications using at least one sidelink. Particularly, there is a need for a technique that enables consistent action and/or states by the entities involved in the relayed radio communication with minimal signaling overhead.

As to a first method aspect, a method of handling a radio link failure (RLF) at a remote radio device in relayed radio communication with a radio access network (RAN) or a further remote radio device through a relay radio device is provided. The method comprises a step of determining an RLF in a sidelink (SL) between the remote radio device and the relay radio device, which SL is used for the relayed radio communication. The method further comprises a step of performing or initiating, responsive to the determined RLF, at least one of a release, a reconfiguration, a reestablishment, and a reselection of the relayed radio communication.

By performing, responsive the determined RLF, an action, which comprises the release, the reconfiguration, the reestablishment and/or the reselection of the relayed radio communication (e.g., of the SL), embodiments can guarantee and/or control service continuity. For example, the step of performing or initiating the action may define how to handle the RLF in sidelink relay scenarios for consistent actions and/or consistent states (e.g., radio resource control states).

Since in the relayed radio communication (i.e., the relay path) more than two entities (e.g., the relay radio device, the remote radio device, and the RAN or the further remote radio device) are involved, same or further embodiments can perform radio link monitoring (RLM) and/or react by performing the actions responsive to the determined (e.g., detected) RLF.

The technique may be implemented as a method for radio link monitoring (RLM) and/or determining RLF. Alternatively or in addition, the technique may be implemented for handling RLM and/or RLF at or for the relay radio device, e.g., in sidelink relay scenarios. The relay radio device may receive and/or relay a configuration message from the RAN or the further remote radio device (also briefly: further radio device) to the remote radio device and/or in the other direction. The configuration message may be indicative of one or more parameters or instructions for the release, the reconfiguration, the reestablishment and/or the reselection, e.g., in response to a report of the determined RLF, which is optionally forwarded to the RAN.

The remote radio device (e.g. UE) may be in D2D radio connection to the relay radio device (e.g., UE). Alternatively or in addition, the relay radio device may have radio coverage of the RAN (e.g., a network node of the RAN, e.g., a gNB).

The relaying may be implemented at a layer 2 (L2), which may be referred to as an L2 relay mechanism. The L2 may comprise an adaptation layer (e.g., referred to as "adaptation relay") for relaying (e.g., receiving, transmitting and/or forwarding) data unit of the relayed radio communication. The adaptation layer may be implemented on a first hop or a second hop or further hops of the relayed radio communication. For example, the adaption layer may be implemented for the D2D communication between the relay radio device and the RAN (e.g., network node, e.g., gNB), e.g., on the uplink (UL) or downlink (DL), e.g., the Uu link, e.g., for UE to RAN relay. Alternatively or in addition, the adaption layer may be implemented for the D2D communication between the relay radio device and the remote radio device, e.g., on the sidelink (SL, e.g., a PC5 link), for UE to UE relay.

The first method aspect may be implemented alone or in combination with any one of the claims, particularly the claims 1 to 38.

The method may further comprise transmitting or receiving a data unit (DU, e.g., a packet data unit, PDU) through the relay radio device to or from the RAN or the further remote radio device. The relayed radio communication may be between the remote radio device, on the one end, and the RAN (e.g., a network node of the RAN) or the further remote radio device, on the other end. For example, the remote radio device may terminate the radio communication, and/or the RAN (e.g., the network node) or the further remote radio device may terminate the radio communication. The transmitting of the DU and the receiving of the DU may be referred to as a hop of the relayed radio communication. The transmitting and the receiving may comprise a radio transmission and a radio reception, respectively. Furthermore, the DU transmitted from the relay radio device or the DU received at the relay radio device may be respectively received at or transmitted from the remote radio device or the RAN or the further radio device.

The DU may be a packet data unit (PDU), e.g., of an adaptation layer, or a packet data convergence protocol (PDCP) layer, or a radio resource control (RRC) layer.

Alternatively or in addition, the adaptation layer, the PDCP layer, and/or the RRC layer (e.g., at the relay radio device) may be configured to relay the DU.

The further remote radio device may be any further radio device. For example, the remote radio device and the further remote radio device may use different radio access technologies (RATs) for their respective sidelinks with the relay radio device.

The method (e.g., according to the first method aspect) may be performed at and/or by the remote radio device.

At least one or each of the steps of the method may be performed at and/or by the remote radio device. For example, the RLF in the SL or the relayed radio communication may be determined at the remote radio device. Alternatively or in addition, at least one of the release, the reconfiguration, the reestablishment and the reselection of the relayed radio communication may be performed or initiated at the remote radio device.

The method (e.g., according to the first method and/or second aspect) may be performed at and/or by at least one of a packet data convergence protocol (PDCP) layer of a protocol stack of the relayed radio communication; a radio resource control (RRC) layer of a protocol stack of the relayed radio communication; and a medium access control layer (MAC) of a protocol stack of the relayed radio communication.

The relayed radio communication (e.g., according to the first and/or second method aspect) may comprise a sidelink (SL) between the remote radio device and the relay radio device. The SL between the remote radio device and the relay radio device (e.g., according to the first and/or second method aspect) may use a PC5 radio interface or a direct WiFi radio interface.

The relayed radio communication (e.g., according to the first and/or second method aspect) may comprise a further SL between the further remote radio device and the relay radio device.

The further SL between the further remote radio device and the relay radio device (e.g., according to the first and/or second method aspect) may use a PC5 radio interface or a direct Wi-Fi radio interface.

The SL and/or the further SL may encompass any device-to-device (D2D) radio communication. The D2D radio communication may be a direct radio communication or a peer-to-peer radio communication, e.g., a SL according to 3GPP LTE or 3GPP NR or a peer-to-peer radio communication according to Wi-Fi direct.

The D2D communication between the remote radio device and the relay radio device may be terminated at the remote radio device and the relay radio device.

Alternatively or in addition, the D2D communication between the relay radio device and the RAN or the further radio device may be terminated at the relay radio device and the RAN or the further remote radio device.

The SL between the remote radio device and the relay radio device and the further SL between the further remote radio device and the relay radio device (e.g., according to the first and/or second method aspect) may use different radio access technologies (RATs).

The relayed radio communication (e.g., according to the first and/or second method aspect) may comprise an uplink (UL) and/or downlink (DL) between the RAN and the relay radio device.

The UL and/or the DL (e.g., according to the first and/or second method aspect) may use a Uu radio interface. At least one or each of the UL, DL and SL may form or span one segment (e.g., leg) of the relayed radio communication. The relay radio device may correspond to one hop of the relayed radio communication, e.g., between two segments of the relayed radio communication.

The method (e.g., according to the first method aspect) may further comprise performing radio link monitoring (RLM) on the SL. The RLF may be detected as a result of the RLM.

The method (e.g., according to the first method aspect) may further comprise performing the RLM of at least one of the SL between the remote radio device and the relay radio device; another SL between the remote radio device and a candidate relay radio device for the reselection; and a direct UL and/or DL between the remote radio device and the RAN.

The direct UL and/or DL may be independent of the relay radio device.

The RLF (e.g., according to the first and/or second method aspect) may be determined in at least one of the SL between the remote radio device and the relay radio device; and the direct UL and/or the DL between the remote radio device and the RAN.

The radio communication (e.g., according to the first and/or second method aspect) may comprise at least two segments. The release, the reconfiguration, the reestablishment, and/or the reselection of the relayed radio communication (e.g., according to the first method aspect) may be performed or initiated on at least one or each of the segments.

Any of the segments may be a radio link (briefly: link) between the relay radio device and any one of the RAN, the remote radio device, and further remote radio device. For example, the segments may comprise at least one of the SL between the relay radio device and the remote radio device, the further SL between the relay radio device and the further remote radio device, the UL and/or DL between the relay radio device and the RAN.

The release, the reconfiguration, the reestablishment, and/or the reselection (e.g., according to the first method aspect) may be performed for at least one of the SL between the remote radio device and the relay radio device; and the direct UL and/or the DL between the remote radio device and the RAN. The RAN (e.g., according to the first method aspect) may comprise one or more network nodes.

The direct UL and/or DL may be between the remote radio device and another network node of the RAN, the other network node being different from the network node of the RAN providing radio coverage to the relay radio device.

Any network node may be a base station, or may correspond to a cell, e.g., serving the relay radio device.

The determining the RLF (e.g., according to the first method aspect) may comprise at least one of detecting the RLF at the remote radio device; and receiving a control message at the remote radio device, the control message being indicative of the RLF.

The control message may be received from the relay radio device.

The performing or initiating, responsive to the determined RLF, the release of the relayed radio communication (e.g., according to the first and/or second method aspect) may comprise releasing the entire relayed radio communication.

The performing or initiating, responsive to the determined RLF, of the release of the relayed radio communication (e.g., according to the first method aspect) may comprise releasing at least two or each of the segments of the relayed radio communication.

For example, the relayed radio device may release the network radio connection to the RAN (or the further SL to the further remote radio device) and the SL to the remote radio device.

The performing or initiating the release of the relayed radio communication (e.g., according to the first and/or second method aspect) may comprise implicitly indicating the release to at least one of the relay radio device, the RAN, and the further remote radio device. The implicit indication (e.g., according to the first and/or second method aspect) may comprise a radio inactivity of the remote radio device causing expiry of an inactivity timer at the at least one of the relay radio device, the RAN, and the further remote radio device.

The inactivity may comprise the relay radio device refraining from transmitting a keep-alive message to the at least one of the remote radio device, the RAN, and the further remote radio device.

The relayed radio communication (e.g., according to the first and/or second method aspect) may comprise at least two segments relayed by the relay radio device. The RLF (e.g., according to the first method aspect) may be determined for the SL being one of the segments of the relayed radio communication. The at least one of the release, the reconfiguration, the reestablishment, and/or the reselection of the relayed radio communication (e.g., according to the first and/or second method aspect) may be performed or initiated for the SL.

The RLF (e.g., according to the first and/or second method aspect) may be determined for the SL between the relay radio device and the remote radio device. The reconfiguration (e.g., according to the first method aspect) may be performed or initiated for the SL between the relay radio device and the remote radio device.

The reconfiguration of the SL (e.g., according to the first and/or second method aspect) may comprise changing at least one of a bandwidth part (BWP) for the SL; a carrier frequency for the SL; a logical channel (LCH) or signaling radio bearer (SRB) of the SL, or a bearer of the SL; and a data radio bearer (DRB) of the SL.

The performing or initiating of the reconfiguration of the SL (e.g., according to the first and/or second method aspect) may comprise transmitting a reconfiguration message to the relay radio device. The reconfiguration message (e.g., according to the first and/or second method aspect) may be indicative of the reconfiguration of the SL. The RLF (e.g., according to the first and/or second method aspect) may be determined by receiving a control message from the relay radio device. The control message may be indicative of the RLF on the SL and optionally, on the link between the relay radio device and the RAN or the further remote radio device. The reconfiguration may be performed or initiated for the SL in response to receiving a control message from the relay radio device. The control message may be indicative of the reconfiguration of the SL and/or a reconfiguration or reestablishment of the link between the relay radio device and the RAN or the further remote radio device.

The link between the relay radio device (and the RAN) may be a Uu link.

The reestablishment (e.g., according to the first and/or second method aspect) may be a radio resource control (RRC) reestablishment. Alternatively or in addition, the reestablishment or the reselection may be initiated by the relay radio device and performed by the remote radio device. Alternatively or in addition, the reestablishment may be initiated by the remote radio device and performed by the relay radio device. Alternatively or in addition, the reestablishment or reselection may comprise a change of the relay radio device serving the remote radio device.

The relay radio device serving the remote radio device and/or used for the relayed radio communication prior to the reselection may be referred to as the previous relay radio device. The relay radio device serving the remote radio device and/or used for the relayed radio communication after or as a result of the reselection may be referred to as the changed relay radio device.

The performing or initiating of the reestablishment or the reselection (e.g., according to the first method aspect) may comprise transmitting, from the remote radio device to the changed relay radio device, a configuration information indicative of a configuration of the previous SL.

The configuration information may be indicative of the configuration of the previous (e.g., existing) SL (e.g., a PC5 configuration).

The performing or initiating of the reestablishment or the reselection (e.g., according to the first method aspect) may comprise receiving, from the changed relay radio device, optionally in response to the configuration information, a reconfiguration information indicative of a reconfiguration of the SL between the relay radio device and the remote radio device. The reconfiguration information may be indicative of a change of the configuration of the SL (e.g., a PC5 configuration).

The SL between a previous relay radio device and the remote radio device (e.g., according to the first method aspect) may be maintained during the reestablishment or the reselection.

The performing or initiating of at least one of the release, the reconfiguration, the reestablishment, and the reselection (e.g., according to the first method aspect) may comprise transmitting, from the remote radio device to the relay radio device, a report of the RLF, optionally comprising the RLF of the SL between the relay radio device and the remote radio device.

The report may be transmitted to the previous relay radio device or the changed relay radio device.

The determining of the RLF (e.g., according to the first method aspect) may comprise an early stage of the RLF and a late stage of the RLF, which is later than the early stage. The report may be transmitted before the late stage of the RLF of the SL. The report may be transmitted responsive to the early stage of the RLF of the SL.

The method (e.g., according to the first method aspect) may further comprise refraining from performing or initiating the release or the reconfiguration of the relayed radio communication, optionally the SL between the relay radio device and the remote radio device, until reception of a reconfiguration information, optionally in the control message, in response to the transmitted report.

The method (e.g., according to the first method aspect) may further comprise performing RLM of the SL between the relay radio device and the remote radio device for a predefined time period after determining the RLF of the SL between the relay radio device and the remote radio device.

The performing or initiating of at least one of the release, the reconfiguration, the reestablishment and the reselection (e.g., according to the first method aspect) may comprise transmitting, to the relay radio device from the remote radio device, an indication of the RLF or the early RLF, optionally in the report. The report may also be referred to as a report message.

The determining of the RLF (e.g., according to the first method aspect) may comprise an early stage of the RLF and a late stage of the RLF, which is later than the early stage. The indication (e.g., according to the first method aspect) may be transmitted before the late stage of the RLF of the SL between the relay radio device and the remote radio device. The indication (e.g., according to the first method aspect) may be transmitted responsive to the early stage of the RLF of the SL between the relay radio device and the remote radio device.

Herein, the remote radio device may determine the early RLF based on conditions (e.g., of the SL, optionally channel conditions) indicative of an impending RLF without waiting for the conditions to completely deteriorate (e.g., corresponding to the late RLF) and/or without waiting for expiry of a timer (e.g., triggering the RLF or the late RLF).

The method (e.g., according to the first method aspect) may further comprise performing RLM of the direct UL and/or DL between the remote radio device and the RAN for a predefined time period after determining the RLF of the direct UL and/or DL between the relay radio device and the RAN.

The determining of the RLF in the SL (e.g., according to the first method aspect) may comprise at least one of performing a RLM for the SL between the relay radio device and the remote radio device after determining the RLF on the SL between the relay radio device and the remote radio device for a predefined time. The determining of the RLF in the SL (e.g., according to the first method aspect) may further comprise revoking the RLF for the SL between the relay radio device and the remote radio device if the SL is reestablished between the relay radio device and the remote radio device within the predefined time. As to a second method aspect, a method of handling a radio link failure (RLF) at a remote radio device in relayed radio communication with a radio access network (RAN), or a further remote radio device, through a relay radio device is provided. The method comprises a step of receiving, from the remote radio device, a report indicative of an RLF in a sidelink (SL) between the remote radio device and a previous relay radio device, which SL is used for the relayed radio communication. The method further comprises a step of establishing, responsive to the received report, another SL to the remote radio device, which other SL is to be used for the relayed radio communication.

The second method aspect may be implemented alone or in combination with any one of the claims, particularly the claims 39 to 43.

The second method aspect may further comprise any feature, or may comprise or initiate any step, disclosed in the context of the first method aspect or may comprise a feature or step corresponding thereto. For example, the relay radio device may transmit a report indicating that a status of the relayed radio communication at the relay radio device (e.g., a status of the SL) and/or indicating the determined RLF, and the RAN (e.g., a network node of the RAN) may receive said report from the relay radio device. Alternatively or in addition, any indication or message that the relay radio device may receive from the RAN may correspond to transmitting a control message in correspondence with any of the steps or features disclosed in the context of the first method aspect.

The report may be received from the remote radio device at a (e.g., candidate for) the changed relay radio device.

The method (e.g., according to the second method aspect) may be performed at and/or by a changed relay radio device other than the relay radio device previously used, optionally in response to a reselection performed or initiated by the remote radio device. The method (e.g., according to the second method aspect) may further comprises transmitting, optionally on a SL between the previous relay radio device and the changed relay radio device, a relay control message from the changed relay radio device to the previous relay radio device, the relay control message being indicative of at least one of the reported RLF in the previous SL and the reselection of changed relay radio device. The report (e.g., according to the second method aspect) may comprise, or may be comprised in, a message for reselection of the changed relay radio device. The message for reselection (e.g., according to the second method aspect) may be a discovery message for the other SL.

The establishing (e.g., according to the second method aspect) may comprise transmitting a control message, to the remote radio device, optionally from the changed relay radio device.

The control message (e.g., according to the second method aspect) may be configured to initiate at least one of the release, the reconfiguration, and the reestablishment of the relayed radio communication or the previous SL at the relay radio device according to any one of the embodiments according to the first method aspect. The control message (e.g., according to the second method aspect) may be configured to initiate the reselection of the changed relay radio device. The control message (e.g., according to the second method aspect) may be configured to initiate the steps of any one of the embodiments according to the first method aspects.

The method (e.g., according to the second method aspect) may further comprise any feature or step according to the first method aspect, or a feature or a step corresponding thereto.

As to another aspect, a computer program product is provided. The computer program product comprises program code portions for performing any of the steps according to the first and/or second method aspects when the computer program product is executed on one or more computing devices, optionally stored on a computer-readable recording medium.

As to a first device aspect, a remote radio device for handling a radio link failure (RLF) at a remote radio device in relayed radio communication with a radio access network (RAN), or a further remote radio device, through a relay radio device is provided. The remote radio device (e.g., according to the first device aspect) is configured to determine an RLF in a sidelink (SL) between the remote radio device and the relay radio device, which SL is used for the relayed radio communication. The remote radio device (e.g., according to the first device aspect) is further configured to perform or initiate, responsive to the determined RLF, at least one of a release, a reconfiguration, a reestablishment, and a reselection of the relayed radio communication. The remote radio device (e.g., according to the first device aspect) may further comprise the features or being further configured to perform the steps of any one of embodiments according to the first method aspects.

As to a second device aspect, a relay radio device for handling a radio link failure (RLF) at a remote radio device in relayed radio communication with a radio access network (RAN) or a further remote radio device through a relay radio device is provided. The relay radio device is configured to receive, from the remote radio device, a report indicative of an RLF in a sidelink (SL) between the remote radio device and a previous relay radio device, which SL is used for the relayed radio communication. The relay radio device is further configured to establish, responsive to the received report, another SL to the remote radio device, which other SL is to be used for the relayed radio communication.

The relay radio device (e.g., according to the second device aspect) may further comprise, and/or may further be configured to perform, the steps of any one of embodiments according to the second method aspects.

As to a further device aspect, a communication system including a host computer is provided. The host computer comprises processing circuitry configured to provide user data; and a communication interface configured to forward user data to a cellular or ad hoc radio network for transmission to a user equipment (UE). The UE comprises a radio interface and processing circuitry, the processing circuitry of the UE being configured to execute the steps of any one of embodiments according to the first and/or second method aspects.

The communication system (e.g., according to the further device aspect) may further include the UE.

The radio network (e.g., according to the further device aspect) may further comprise a base station or radio device functioning as a gateway configured to communicate with the UE and to perform any of the steps of the first and/or second method aspects. The processing circuitry of the host computer (e.g., according to the further device aspect) may be configured to execute a host application, thereby providing the user data. The processing circuitry of the UE (e.g., according to the further device aspect) may be configured to execute a client application associated with the host application.

Moreover, the first method aspect may be performed at or by a transmitting or receiving station (briefly: transmitter or receiver), e.g., the remote radio device for a direct uplink (UL) and/or direct downlink (DL) connection to the RAN and/or a sidelink (SL) connection to the relay radio device. Alternatively, or in combination, the second method aspect may be performed at or by a transmitting or receiving station (briefly: transmitter or receiver), e.g., the relay radio device or a changed relay radio device (optionally in radio coverage of the RAN) for an uplink or downlink connection to the RAN and/or the SL connection to the remote radio device.

The channel or link used for the data transmission and the radio reception, i.e., the channel between the transmitter and the receiver may comprise multiple subchannels or subcarriers (as a frequency domain). Alternatively, or in addition, the channel or link may comprise one or more slots for a plurality of modulation symbols (as a time domain). Alternatively, or in addition, the channel or link may comprise a directional transmission (also: beamforming transmission) at the transmitter, a directional reception (also: beamforming reception) at the receiver or a multipleinput multiple-output (MIMO) channel with two or more spatial streams (as a spatial domain).

The transmitter and the receiver may be spaced apart. The transmitter and the receiver may be in data or signal communication exclusively by means of the radio communication, e.g., the D2D communication.

The technique may be implemented separately for a hop between a remote UE and a relay UE and a hop between a relay UE and the RAN (e.g., the network node, e.g., gNB) (for U2N relay) or a receiving remote UE (for U2U relay).

The first method aspect may be implemented at the remote radio device. The second method aspect may be implemented at the relay radio device. For a hop in the relayed radio communication (e.g., at the relay radio device), the hop may comprise at least one transmitter (TX or TX node) and one receiver (RX or RX node).

For concreteness and not limitation, any radio device may be referred to as a user equipment (UE). Furthermore, the RAN may be embodied by one or more network nodes, which may be referred to as a Next Generation Node B (gNB).

Embodiments of the technique (e.g., the device and method aspects of the technique) allow handling actions (briefly: actions or action steps or any functionality performed in response to the determined RLF and/or performed by any of the reacting units disclosed herein) of the remote UE when an RLF is determined (e.g., detected). These actions may have the main target to reduce or avoid a connectivity interruption delay and/or to reduce or avoid the signaling overhead.

For example, the remote UE may, upon determining (e.g., detecting) the RLF in the link between remote UE and relay UE (i.e., the SL) perform or initiate at least one of the following actions.

A first action comprises the remote UE releasing the entire relay path (i.e., the relayed radio communication). This action may mean that the relay UE may figure out that the relay path was released if the inactivity timer expires or via keep alive message or via the expiry of the timer T400. Once the RLF is detected, the relay UE may inform also the gNB.

A second action comprises the reselection, e.g., a path selection or path reselection. The reselection may be triggered by the remote UE. The changed relay UE (also referred to as new relay UE) may inform about the RLF the previous relay UE (also referred to as old relay UE), e.g., by establishing a sidelink (e.g., a PC5 connection) between the relay radio devices. This can be faster than waiting for the inactivity timer to expire or to wait for the keep alive message.

A third action comprises the remote UE transmitting an indication to the RAN (e.g., a gNB) or the further remote radio device (e.g., a destination UE) that a RLF (briefly: failure) on the relay path (e.g., the SL) has been determined (e.g., received or detected). This may be performed if there is (i.e., can be establish) a direct Uu connection between the remote UE and the gNB or the destination UE. In this case the remote UE may not release the relay path (e.g., the SL) but instead waiting for a reconfiguration (e.g., a new configuration) from the gNB and/or destination UE.

A fourth action comprises the remote UE transmitting an indication to the relay UE that an RLF has been determined (e.g., detect), which may optionally be valid only for early-RLF detection, i.e., if the RLF is an early RLF. This means that the relay UE may be notified about the reconfiguration via PC5-RRC signaling, or via a control PDU of the adaptation layer or via a MAC CE. Alternatively or in addition, the remote UE may instruct (e.g., by means of the report) the relay UE to perform one or more recovery actions (e.g., via the PC5-RRC signaling, or via a control PDU of the adaptation layer or via a MAC CE).

In any aspect, any radio device (e.g., the remote radio device or the relay radio device or the further remote radio device) and/or any network node may form, or may be part of, a radio network, e.g., according to the Third Generation Partnership Project (3GPP) or according to the standard family IEEE 802.11 (Wi-Fi). The radio network may be or may comprise the radio access network (RAN).

The RAN may comprise one or more network nodes (e.g., base stations). Alternatively, or in addition, the radio network may be a vehicular, ad hoc and/or mesh network. Any of the radio devices may be embodied by a vehicle of the vehicular network.

The first method aspect may be performed by one or more embodiments of the remote radio device in the radio network. The second method aspect may be performed by one or more embodiments of the relay radio device.

Any of the radio devices may be a mobile or wireless device, e.g., a 3GPP user equipment (UE) or a Wi-Fi station (STA). The radio device may be a mobile or portable station, a device for machine-type communication (MTC), a device for narrowband Internet of Things (NB-loT) or a combination thereof. Examples for the UE and the mobile station include a mobile phone, a tablet computer and a self-driving vehicle. Examples for the portable station include a laptop computer and a television set. Examples for the MTC device or the NB-loT device include robots, sensors and/or actuators, e.g., in manufacturing, automotive communication and home automation. The MTC device or the NB-loT device may be implemented in a manufacturing plant, household appliances and consumer electronics. Any of the radio devices may be wirelessly connected or connectable (e.g., according to a radio resource control, RRC, state or active mode) with any of the base stations. Herein, the base station may encompass any station that is configured to provide radio access to any of the radio devices. The base stations may also be referred to as transmission and reception point (TRP), radio access node or access point (AP). The base station or one of the radio devices functioning as a gateway (e.g., between the radio network and the RAN and/or the Internet) may provide a data link to a host computer providing the data. Examples for the base stations may include a 3G base station or Node B, 4G base station or eNodeB, a 5G base station or gNodeB, a Wi-Fi AP and a network controller (e.g., according to Bluetooth, ZigBee or Z-Wave).

The RAN may be implemented according to the Global System for Mobile Communications (GSM), the Universal Mobile Telecommunications System (UMTS), 3GPP Long Term Evolution (LTE) and/or 3GPP New Radio (NR).

Any aspect of the technique may be implemented on a Physical Layer (PHY), a Medium Access Control (MAC) layer, a Radio Link Control (RLC) layer and/or a Radio Resource Control (RRC) layer of a protocol stack for the radio communication.

As to another aspect, a computer program product is provided. The computer program product comprises program code portions for performing any one of the steps of the method aspect disclosed herein when the computer program product is executed by one or more computing devices. The computer program product may be stored on a computer-readable recording medium. The computer program product may also be provided for download, e.g., via the radio network, the RAN, the Internet and/or the host computer. Alternatively, or in addition, the method may be encoded in a Field-Programmable Gate Array (FPGA) and/or an Application-Specific Integrated Circuit (ASIC), or the functionality may be provided for download by means of a hardware description language.

First device aspects may be provided or implemented alone or in combination with any one of the claims, particularly the claims 46, 47, 50 and 51. Furthermore, any of the first device aspects may be provided or implemented alone or in combination with any one of the embodiments described hereinbelow.

The device may be configured to perform any one of the steps of the first method aspect. Second device aspects may be provided or implemented alone or in combination with any one of the claims, particularly the claims 48, 49, 52, 53, 54, and 55. Furthermore, each of the second device aspects may be provided or implemented alone or in combination with any one of the embodiments described hereinbelow.

The device may be configured to perform any one of the steps of the second method aspect.

As to a still further aspect a communication system including a host computer is provided. The host computer may comprise a processing circuitry configured to provide user data, e.g., depending on the location of the UE determined in the locating step. The host computer may further comprise a communication interface configured to forward user data to a cellular network for transmission to a user equipment (UE), wherein the UE comprises a radio interface and processing circuitry, a processing circuitry of the cellular network being configured to execute any one of the steps of the first and/or second method aspect.

The communication system may further include the UE. Alternatively, or in addition, the cellular network may further include one or more base stations and/or gateways configured to communicate with the UE and/or to provide a data link between the UE and the host computer using the first method aspect and/or the second method aspect.

The processing circuitry of the host computer may be configured to execute a host application, thereby providing the user data and/or any host computer functionality described herein. Alternatively, or in addition, the processing circuitry of the UE may be configured to execute a client application associated with the host application.

Any one of the devices, the UE, the base station, the system or any node or station for embodying the technique may further include any feature disclosed in the context of the method aspects, and vice versa. Particularly, any one of the units and modules, or a dedicated unit or module, may be configured to perform or initiate one or more of the steps of the method aspect.

Brief Description of the Drawings

Further details of embodiments of the technique are described with reference to the enclosed drawings, wherein: Fig. 1A shows an example schematic block diagram of a remote radio device embodiment of device for handling an RLF in a relayed radio communication;

Fig. IB shows an example schematic block diagram of a relay radio device embodiment of device for handling an RLF in a relayed radio communication;

Fig. 2A shows an example flowchart for a method of handling an RLF in a relayed radio communication, which method may be implementable by the device of Fig. 1A;

Fig. 2B shows an example flowchart for a method of handling an RLF in a relayed radio communication, which method may be implementable by the device of Fig. IB;

Fig. 3 shows an example deployment scenario for a relayed radio communication;

Fig. 4 schematically shows a physical resource grid of a 3GPP NR implementation;

Fig. 5 schematically illustrates an architecture of a relayed radio communication using a relay radio device, e.g. according to the device of Fig. 2;

Fig. 6 schematically illustrates examples of protocol stacks for a L3 remote radio device-to-network relay;

Fig. 7 schematically illustrates an example of a RM radio device to network relay;

Fig. 8 schematically illustrates a user plane stack for an L2 RL radio device, the RAN and/or a further remote radio device and an RM radio device embodying the devices of Fig. 1A and IB;

Fig. 9 schematically illustrates a control plane stack for an L2 RL radio device, the RAN and/or a further radio device and an RM radio device embodying the devices of Fig. 1A and IB;

Fig. 10 schematically illustrates a connection establishment for a relayed radio connection for an RL radio device, the RAN and/or a further radio device and an RM radio device embodying the devices of Fig. 1A and IB; Fig. 11 schematically illustrates an example of a relayed radio communication, for which an RLF is detected, involving embodiments of the devices of any one of Figs. 1A and IB;

Fig. 12 schematically illustrates a reference example of a relayed radio communication, for which an RLF is detected, involving embodiments of the devices of any one of Figs. 1A and IB;

Fig. 13A shows an example schematic block diagram of a RL radio device embodying the device of Fig. 1A;

Fig. 13B shows an example schematic block diagram of a network node embodying the device of Fig. IB;

Fig. 14 schematically illustrates an example telecommunication network connected via an intermediate network to a host computer;

Fig. 15 shows a generalized block diagram of a host computer communicating via a base station or radio device functioning as a gateway with a user equipment over a partially wireless connection; and

Figs. 16 and 17 show flowcharts for methods implemented in a communication system including a host computer, a base station or radio device functioning as a gateway and a user equipment.

Detailed Description

In the following description, for purposes of explanation and not limitation, specific details are set forth, such as a specific network environment in order to provide a thorough understanding of the technique disclosed herein. It will be apparent to one skilled in the art that the technique may be practiced in other embodiments that depart from these specific details. Moreover, while the following embodiments are primarily described for a New Radio (NR) or 5G implementation, it is readily apparent that the technique described herein may also be implemented for any other radio communication technique, including 3GPP LTE (e.g., LTE-Advanced or a related radio access technique such as MulteFire), in a Wireless Local Area Network (WLAN) according to the standard family IEEE 802.11, for Bluetooth according to the Bluetooth Special Interest Group (SIG), particularly Bluetooth Low Energy, Bluetooth Mesh Networking and Bluetooth broadcasting, for Z-Wave according to the Z-Wave Alliance or for ZigBee based on IEEE 802.15.4.

Moreover, those skilled in the art will appreciate that the functions, steps, units and modules explained herein may be implemented using software functioning in conjunction with a programmed microprocessor, an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), a Digital Signal Processor (DSP) or a general purpose computer, e.g., including an Advanced RISC Machine (ARM). It will also be appreciated that, while the following embodiments are primarily described in context with methods and devices, the invention may also be embodied in a computer program product as well as in a system comprising at least one computer processor and memory coupled to the at least one processor, wherein the memory is encoded with one or more programs that may perform the functions and steps or implement the units and modules disclosed herein.

Fig. 1A schematically illustrates an example block diagram of a device according to the first device aspect, i.e., a device for handling a RLF at a remote radio device in relayed radio communication with a RAN or a further remote radio device through a relay radio device. The device is generically referred to by reference sign 100-RM.

The device 100-RM may comprise any one of the units 102-RM and 104-RM for performing the steps labelled 202-RM and 204-RM, respectively, preferably according to the list of claims or any embodiment disclosed herein.

The device 100-RM may comprise a determining unit 102-RM that determines an RLF in a SL between the remote radio device and the relay radio device, which SL is used for the relayed radio communication. The device 100-RM may further comprise a reacting unit 104-RM that performs or initiates, responsive to the determined RLF, at least one of a release, a reconfiguration, a reestablishment, and a reselection of the relayed radio communication.

Any of the units of the device 100-RM may be implemented by modules configured to provide the corresponding functionality.

The device 100-RM may also be referred to as, or may be embodied by, the remote radio device (i.e., RM radio device, e.g., RM-UE or labelled 100-RM). The device 100- RM and the relay radio device as well as relay radio device and the RAN (e.g. a network node of the RAN) or the further remote radio device are in a radio communication (preferably D2D communication or Uu) at least for relaying the relayed radio communication.

The relay radio device (i.e., RL radio device) may be a RL-UE (labelled 100-RL). The network node may be labelled 100-NN.

Fig. IB schematically illustrates an example block diagram of a device according to the second device aspect, i.e., a device for handling a RLF at a remote radio device in relayed radio communication with a RAN or a further remote radio device through a relay radio device. The device is generically referred to by reference sign 100-RL.

The device 100-RL may comprise any one of the units 102-RL and 104-RL for performing the steps labelled 202-RL and 204-RL, respectively, preferably according to the claims or any embodiment disclosed herein.

The device 100-RL may comprise a receiving unit 102-RL that receives, from the remote radio device, a report indicative of an RLF in a SL between the remote radio device and a previous relay radio device, which SL is used for the relayed radio communication. The device 100-RL may further comprise a reacting unit 104-RL that establishes, responsive to the received report, another SL to the remote radio device, which other SL is to be used for the relayed radio communication.

Any of the units of the device 100-RL may be implemented by modules configured to provide the corresponding functionality.

The device 100-RL may also be referred to as, or may be embodied by, the relay radio device (i.e., RL radio device, e.g., RL-UE or labelled 100-RL). The device 100-RL and the remote radio device as well as device 100-RL and the RAN (e.g. a network node of the RAN) or the further remote radio device are in a radio communication (preferably D2D communication or Uu) at least for relaying the relayed radio communication.

The remote radio device (i.e., RM radio device) may be a RM-UE (labelled 100-RM). The network node may be labelled 100-NN.

The technique may be applied to uplink (UL), downlink (DL) or direct communications between radio devices, e.g., device-to-device (D2D) communications or sidelink communications. Each of the device 100-RL, the device 100-NN, and the device 100-RM may be a radio device and/or a network node (e.g., a base station). Herein, any radio device may be a mobile or portable station and/or any radio device wirelessly connectable to the network node (e.g., a base station) and/or the RAN, or to another radio device. A radio device may be a user equipment (UE), a device for machine-type communication (MTC) or a device for (e.g., narrowband) Internet of Things (loT). Two or more radio devices may be configured to wirelessly connect to each other, e.g., in an ad hoc radio network or via a 3GPP sidelink connection. Furthermore, any base station may be a station providing radio access, may be part of a radio access network (RAN) and/or may be a node connected to the RAN for controlling radio access. Further a base station may be an access point, for example a Wi-Fi access point.

Fig. 2A shows an example flowchart for a method 200-RM according to the first method aspect in the claims.

The method 200-RM may be performed by the device 100-RM. For example, the units 102-RM and 104-RM may perform the steps 202-RM and 204-RM, respectively.

Fig. 2B shows an example flowchart for a method 200-RL according to the second method aspect in the claims.

The method 200-RL may be performed by the device 100-RL. For example, the units 102-RL and 104-RL may perform the steps 202-RL and 204-RL, respectively.

Fig. 3 shows an example deployment scenario for a relayed radio communication 300. The deployment scenario comprises a network node 100-NN of a RAN with coverage area 302. A RL radio device 100-RL is in the coverage area 302 of the network node 100-NN. A RM radio device 100-RM is outside of the coverage area 302 of the network node 100-NN, but in proximity to the RL radio deice 100-RL. By being in the proximity, the RM radio device 100-RM and the RL radio device 100-RL may be in a D2D communication.

Any embodiment may be implemented using a frame structure for the relayed radio communication and/or the D2D communication, e.g., according to 3GPP NR. Similar to LTE, NR uses OFDM (Orthogonal Frequency Division Multiplexing) in the DL (e.g., from a network node, gNB, eNB, or base station, to a user equipment or UE).

Fig. 4 schematically illustrates a physical resource grid 400 for a 3GPP NR implementation of the technique.

The basic NR physical resource over an antenna port can be seen as a time-frequency grid as illustrated in Fig. 4, where a resource block (RB) 402 in a 14-symbol slot 408 is shown. A RB 402 corresponds to 12 contiguous subcarriers 404 in the frequency domain. RBs 402 are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. Each resource element (RE) 406 corresponds to one OFDM subcarrier during one OFDM symbol 410 interval. A slot 408 comprises 14 OFDM symbols 410.

Different subcarrier spacing values are supported in NR. The supported subcarrier spacing values (also referred to as different numerologies) are given by Af=(15x2 ^A p) kHz where p E (0,1, 2, 3, 4). Af=15 kHz is the basic (or reference) subcarrier spacing that is also used in LTE.

In the time domain, DL and UL transmissions in NR are organized into equally-sized subframes of 1ms each similar to LTE. A subframe is further divided into multiple slots 408 of equal duration. The slot length for subcarrier spacing Af=(15x2 ^A p) kHz is (l/2) ^A p ms. There is only one slot 408 per subframe for Af=15kHz, and a slot 408 consists of 14 OFDM symbols 410.

DL transmissions are dynamically scheduled, e.g., in each slot the gNB transmits DL control information (DCI) about which radio device (e.g., UE) data is to be transmitted to and which RBs in the current DL slot the data is transmitted on. This control information is conventionally transmitted in the first one or two OFDM symbols in each slot in NR. The control information is carried on the Physical Control Channel (PDCCH), and data is carried on the Physical Downlink Shared Channel (PDSCH). A radio device (e.g., a UE) first detects and decodes PDCCH and, if a PDCCH is decoded successfully, it (e.g., the UE) then decodes the corresponding PDSCH based on the DL assignment provided by decoded control information in the PDCCH.

In addition to PDCCH and PDSCH, there are also other channels and reference signals transmitted in the downlink, including synchronization signal blocks (SSBs), channel state information reference signals (CSI-RS), etc. UL data transmissions, carried on Physical Uplink Shared Channel (PUSCH), can also be dynamically scheduled by the gNB by transmitting a DCI. The DCI (which is transmitted in the DL region) indicates a scheduling time offset so that the PUSCH is transmitted in a slot in the UL region.

Any embodiment may be implemented using a sidelink (SL) in NR for the D2D communication.

SL transmissions over NR are specified for Rel. 16. These are enhancements of the ProSe (PROximity-based SErvices) specified for LTE. Four new enhancements are particularly introduced to NR sidelink transmissions as follows:

• Support for unicast and groupcast transmissions are added in NR SL. For unicast and groupcast, the physical sidelink feedback channel (PSFCH) is introduced for a receiver radio device (e.g., a receiver UE) to reply the decoding status to a transmitter radio device (e.g., a transmitter UE).

• Grant-free transmissions, which are adopted in NR UL transmissions, are also provided in NR SL transmissions, to improve the latency performance.

• To alleviate resource collisions among different SL transmissions launched by different radio devices (e.g., different UEs), it enhances channel sensing and resource selection procedures, which also lead to a new design of PSCCH.

• To achieve a high connection density, congestion control and thus the quality of service (QoS) management is supported in NR SL transmissions.

To enable the above enhancements, new physical channels and reference signals (RSs) are introduced in NR (available in LTE before.):

• PSSCH (Physical Sidelink Shared Channel, SL version of PDSCH): The PSSCH is transmitted by a SL transmitter radio device (e.g., SL transmitter UE), which conveys SL transmission data, system information blocks (SIBs) for radio resource control (RRC) configuration, and a part of the sidelink control information (SCI). • PSFCH (Physical Sidelink, SL version of PUCCH): The PSFCH is transmitted by a SL receiver radio device (e.g., a SL receiver UE) for unicast and groupcast, which conveys 1 bit information over 1 RB for the HARQ. acknowledgement (ACK) and the negative ACK (NACK). In addition, channel state information (CSI) is carried in the medium access control (MAC) control element (CE) over the PSSCH instead of the PSFCH.

• PSCCH (Physical Sidelink Common Control Channel, SL version of PDCCH): When the traffic to be sent to a receiver radio device (e.g., a receiver UE) arrives at a transmitter radio device (e.g., a transmitter UE), a transmitter radio device (e.g., transmitter UE) should first send the PSCCH, which conveys a part of SCI (Sidelink Control information, SL version of DCI) to be decoded by any radio device (e.g., UE) for the channel sensing purpose, including the reserved time-frequency resources for transmissions, demodulation reference signal (DMRS) pattern and antenna port, etc.

• Sidelink Primary/Secondary Synchronization Signal (S-PSS/S-SSS): Similar DL transmissions in NR, in SL transmissions, primary and secondary synchronization signals (called S-PSS and S-SSS, respectively) are supported. Through detecting the S-PSS and S-SSS, a radio device (e.g., a UE) is able to identify the SL synchronization identity (SSID) from the radio device (e.g., UE) sending the S-PSS/S-SSS. Through detecting the S-PSS/S-SSS, a radio device (e.g., UE) is therefore able to know the characteristics of the radio device (e.g., UE) transmitting the S-PSS/S-SSS. A series of processes of acquiring timing and frequency synchronization together with SSIDs of radio devices (e.g., UEs) is called initial cell search. Note that the radio device (e.g., UE) sending the S- PSS/S-SSS may not be necessarily involved in SL transmissions, and a node (e.g., a UE and/or eNB and/or gNB) sending the S-PSS/S-SSS is called a synchronization source. There are 2 S-PSS sequences and 336 S-SSS sequences forming a total of 672 SSIDs in a cell.

• Physical Sidelink Broadcast Channel (PSBCH): The PSBCH is transmitted along with the S-PSS/S-SSS as a synchronization signal/PSBCH block (SSB). The SSB has the same numerology as PSCCH/PSSCH on that carrier, and an SSB should be transmitted within the bandwidth of the configured BWP. The PSBCH conveys information related to synchronization, such as the direct frame number (DFN), indication of the slot and symbol level time resources for sidelink transmissions, in-coverage indicator, etc. The SSB is transmitted periodically at every 160 ms.

• DMRS, phase tracking reference signal (PT-RS), channel state information reference signal (CSI-RS): These physical reference signals supported by NR DL/UL transmissions are also adopted by SL transmissions. Similarly, the PT-RS is only applicable for FR2 transmission.

Another new feature is the two-stage SL control information (SCI). This a version of the DCI for SL. Unlike the DCI, only part (first stage) of the SCI is sent on the PSCCH. This part is used for channel sensing purposes (including the reserved time-frequency resources for transmissions, demodulation reference signal (DMRS) pattern and antenna port, etc.) and can be read by all radio devices (e.g., UEs) while the remaining (second stage) scheduling and control information such as a 8-bits source identity (ID) and a 16-bits destination ID, NDI, RV and HARQ. process ID is sent on the PSSCH to be decoded by the receiver radio device (e.g., UE).

Similar as for PRoSE in LTE, NR SL transmissions have the following two modes of resource allocations:

• Mode 1: SL resources are scheduled by a network node (e.g., gNB).

• Mode 2: The radio device (e.g., UE) autonomously selects SL resources from a configured or preconfigured SL resource pool(s) based on the channel sensing mechanism.

For the in-coverage radio device (e.g., UE), a network node (e.g., gNB) can be configured to adopt Mode 1 or Mode 2. For the out-of-coverage radio device (e.g., UE), only Mode 2 can be adopted.

As in LTE, scheduling over the SL in NR is done in different ways for Mode 1 and Mode 2.

Mode 1 supports the following two kinds of grants, namely dynamic grants and configured grants.

Dynamic grant: When the traffic to be sent over SL arrives at a transmitter radio device (e.g., UE), this radio device (e.g., UE) should launch the four-message exchange procedure to request SL resources from a network node, e.g. gNB (SR on UL, grant, BSR on UL, grant for data on SL sent to the radio device, e.g., UE). During the resource request procedure, a network node (e.g., gNB) may allocate a SL radio network temporary identifier (SL-RNTI) to the transmitter radio device (e.g., UE). If this SL resource request is granted by a network node (e.g., gNB), then a network node (e.g., gNB) indicates the resource allocation for the PSCCH and the PSSCH in the downlink control information (DCI) conveyed by PDCCH with cyclic redundancy check (CRC) scrambled with the SL-RNTI. When a transmitter radio device (e.g., UE) receives such a DCI, a transmitter radio device (e.g., UE) can obtain the grant only if the scrambled CRC of DCI can be successfully solved by the assigned SL-RNTI. A transmitter radio device (e.g., UE) then indicates the time-frequency resources and the transmission scheme of the allocated PSSCH in the PSCCH, and launches the PSCCH and the PSSCH on the allocated resources for SL transmissions. When a grant is obtained from a network node (e.g., gNB), a transmitter radio device (e.g., UE) can only transmit a single transport block (TB). As a result, this kind of grant is suitable for traffic with a loose latency requirement.

Configured grant: For the traffic with a strict latency requirement, performing the four-message exchange procedure to request SL resources may induce unacceptable latency. In this case, prior to the traffic arrival, a transmitter radio device (e.g., UE) may perform the four-message exchange procedure and request a set of resources. If a grant can be obtained from a network node (e.g., gNB), then the requested resources are reserved in a periodic manner. Upon traffic arriving at a transmitter radio device (e.g., UE), this radio device (e.g., UE) can launch the PSCCH and the PSSCH on the upcoming resource occasion. This kind of grant is also known as grant- free transmissions.

In both dynamic grant and configured grant, a SL receiver radio device (e.g., UE) cannot receive the DCI since it is addressed to the transmitter radio device (e.g., UE), and therefore a receiver radio device (e.g., UE) should perform blind decoding to identify the presence of PSCCH and find the resources for the PSSCH through the SCI.

When a transmitter radio device (e.g., UE) launches the PSCCH, CRC is also inserted in the SCI without any scrambling.

In the Mode 2 resource allocation, when traffic arrives at a transmitter radio device (e.g., UE), this transmitter radio device (e.g., UE) should autonomously select resources for the PSCCH and the PSSCH. To further minimize the latency of the feedback HARQ. ACK/NACK transmissions and subsequently retransmissions, a transmitter radio device (e.g., UE) may also reserve resources for PSCCH/PSSCH for retransmissions. To further enhance the probability of successful TB decoding at one shot and thus suppress the probability to perform retransmissions, a transmitter radio device (e.g., UE) may repeat the TB transmission along with the initial TB transmission. This mechanism is also known as blind retransmission. As a result, when traffic arrives at a transmitter radio device (e.g., UE), then this transmitter radio device (e.g., UE) should select resources for the following transmissions:

1) The PSSCH associated with the PSCCH for initial transmission and blind retransmissions.

2) The PSSCH associated with the PSCCH for retransmissions.

Since each transmitter radio device (e.g., UE) in SL transmissions should autonomously select resources for above transmissions, how to prevent different transmitter radio devices (e.g., UEs) from selecting the same resources turns out to be a critical issue in Mode 2. A particular resource selection procedure is therefore imposed to Mode 2 based on channel sensing. The channel sensing algorithm involves measuring reference signal received power (RSRP) on different subchannels and requires knowledge of the different radio devices (e.g., UEs) power levels of DMRS on the PSSCH or the DMRS on the PSCCH depending on the configuration. This information is known only after receiver SCI launched by (all) other radio devices (e.g., UEs). The sensing and selection algorithm is rather complex.

The D2D communication may be based on or initiated by a discovery procedure.

There are D2D discovery procedures for detection of services and applications offered by other radio devices (e.g., UEs) in close proximity. This is part of LTE Rel 12 and Rel 13. The discovery procedure has two modes, mode A based on open announcements (broadcasts) and mode B, which is request/response. The discovery mechanism is controlled by the application layer (ProSe). The discovery message is sent on the Physical Sidelink Discovery Channel (PSDCH) which is not available in NR. Also, there is a specific resource pool for announcement and monitoring of discovery messages. The discovery procedure can be used to detect radio devices (e.g., UEs) supporting certain services or applications before initiating direct communication.

The relayed radio communication through the relay radio device, e.g. device 100-RL, may be implemented as a Layer 3 (L3) UE-to-Network relay. In the TR 23.752 v0.3.0 clause 6.6, the layer-3 based UE-to-Network relay is described as further discussed in connection to Fig. 5.

As shown in Fig. 5, the ProSe 5G UE-to-Network Relay entity 100-RL provides the functionality to support connectivity to the network 100-NN, 508 for Remote UEs 100-RM. It can be used for both public safety services and commercial services (e.g. interactive service).

A UE is considered to be a Remote UE 100-RM for a certain ProSe UE-to-Network relay 100-RL if it has successfully established a PC5 link 502 to this ProSe 5G UE-to- Network Relay 100-RL. A Remote UE 100-RM can be located within NG-RAN 100-NN coverage or outside of NG-RAN clOO-NN overage.

The ProSe 5G UE-to-Network Relay 100-RL shall relay unicast traffic (UL and DL) between the Remote UE 100-RM and the network 100-NN, 508, e.g. using the Uu interface 504. The ProSe UE-to-Network Relay 100-RL shall provide generic function that can relay any IP traffic.

The network may comprise an NG-RAN 100-NN, a 5G Core Network (5GC) 508 and an N6 link 506 to Access Stratum (AS) 510.

One-to-one Direct Communication is used between Remote UEs 100-RM and ProSe 5G UE-to-Network Relays 100-RL for unicast traffic as specified in solutions for Key Issue #2 in the TR 23.752 v0.3.0.

Fig. 6 schematically illustrates examples of protocol stacks for a L3 UE-to-Network Relay, e.g., according to ProSe 5G UE-to-Network Relay specified in the 3GPP document TR 23.752 v0.3.0.

Hop-by-hop security is supported in the PC5 link 502 and Uu link 504. If there are requirements beyond hop-by-hop security for protection of RM radio device traffic, security over IP layer 602, 606, 612 needs to be applied.

Further security details (integrity and privacy protection for RM radio device to network communication) will be specified in SA WG3. A ProSe 5G UE-to-Network Relay capable radio device (e.g., UE) 100- RL may register to the network (if not already registered) and establish a PDU session enabling the necessary relay traffic, or it may need to connect to additional PDU session(s) or modify the existing PDU session in order to provide relay traffic towards RM radio device(s) 100-RM (e.g., UE(s)). At least in some embodiments, PDU session(s) supporting UE-to-Network Relay shall only be used for Remote ProSe UE(s) relay traffic.

In Fig. 6, the network comprises a user plane function (UPF) at reference sign 614 with N3 link 610 to the network node 100-NN. The application layer 604 is an example of a transparent layer. Layers 606, 608 comprise an adaptation layer fort he relayed radio communication.

Fig. 7 schematically illustrates an example of a ProSe 5G UE-to-Network Relay according to the 3GGP document TR 23.752 v0.3.0.

The RM radio device (e.g., UE) to network relayed radio communication in Fig. 7 comprises the following steps:

Step 0. During the Registration procedure, Authorization and provisioning is performed at reference signs 706 and 708 for the RL radio device (e.g., ProSe UE-to- NW relay) 100-RL and the RM radio device (e.g., remote UE) 100-RM, respectively. Authorization and provisioning procedure may be any solution for key issue #1 and #3 in the TR 23.752 v0.3.0.

Step 1. The ProSe 5G UE-to-Network Relay may, at reference sign 710, establish a PDU session for relaying with default PDU session parameters received in step 0 (at reference signs 706, 708) or pre-configured in the RL radio device (e.g., UE-to-NW relay) 100-RL, e.g. S-NSSAI, DNN, SSC mode. In case of IPv6, the RL radio device (e.g., ProSe UE-to-Network Relay) 100-RL obtains the IPv6 prefix via prefix delegation function from the network as defined in TS 23.501 V16.5.0.

Step 2. Based on the Authorization and provisioning in step 0 (at reference sign 706, 708), at reference sing 712 the RM radio device (e.g., Remote UE) 100-RM performs discovery of a RL radio device (e.g., ProSe 5G UE-to-Network Relay) 100-RL using any solution for key issue #1 and #3 in the TR 23.752 v0.3.0. As part of the discovery procedure the RM radio device (e.g., Remote UE) 100-RM learns about the connectivity service the RL radio device (e.g., ProSe UE-to-Network Relay) 100-RL provides.

Step 3. The RM radio device (e.g., Remote UE) 100-RM selects at reference sign 714 a RL radio device (e.g., ProSe 5G UE-to-Network Relay) 100-RL and establishes a connection for One-to-one ProSe Direct Communication as described in

TS 23.287 V16.3.0 and/or modifies an existing communication as shown at reference sing 716.

If there is no PDU session satisfying the requirements of the PC5 connection with the RM radio device (e.g., remote UE) 100-RM, e.g. S-NSSAI, DNN, QoS, the RL radio device (e.g., ProSe 5G UE-to-Network Relay) 100-RL initiates a new PDU session establishment or modification procedure for relaying.

Step 4. At reference sing 718, IPv6 prefix or IPv4 address is allocated for the RM radio device (e.g., remote UE) 100-RM as it is defined in TS 23.303 V16.0.0 clauses 5.4.4.2 and 5.4.4.3. From this point the uplink and downlink relaying can start.

Step 5. The RL radio device (e.g., ProSe 5G UE-to-Network Relay) 100-RL sends a RM radio device (e.g., Remote UE) Report (e.g., comprising Remote User ID and/or IP info) message (e.g., through the access and mobility management function, AMF, 702) to the session management function (SMF) 704 for the PDU session associated with the relay. The Remote User ID is an identity of the RM radio device (e.g., Remote UE) user (provided via User Info) that was successfully connected in step 3 at reference signs 714, 716. The SMF 704 stores the Remote User IDs and the related IP info in the RL radio device (e.g., ProSe 5G UE-to-Network Relay) for the PDU connection associated with the relay.

For IP info the following principles apply: for IPv4, the UE-to-network Relay (e.g., comprising legs 722, 724) shall report TCP/UDP port ranges assigned to individual RM radio devices (e.g., Remote UE(s)) 100-RM (along with the Remote User ID); for IPv6, the UE-to-network Relay(e.g., comprising legs 722, 724) shall report IPv6 prefix(es) assigned to individual RM radio devices (e.g., Remote UE(s)) 100-RM (along with the Remote User ID). The RM radio device (e.g., Remote UE) Report message at reference sign 720 shall be sent when the RM radio device (e.g., Remote UE) disconnects from the ProSe 5G UE- to-Network Relay (e.g. upon explicit layer-2 link release and/or based on the absence of keep alive messages over PC5) to inform the SMF 704 that the RM radio device(s) (e.g., Remote UE(s)) 100-RM has/have left.

In the case of Registration Update procedure involving SMF 704 change the Remote User I D(s) and/or related IP info corresponding to the connected RM radio device(s) (e.g., Remote UE(s)) are transferred to the new SMF 704 as part of SM context transfer for the relayed radio communication (e.g., ProSe 5G UE-to-Network Relay 100-RL).

It is noted that in order for the SMF 704 to have the RM radio device(s) (e.g., Remote UE(s)) 100-RM information, the Home Public Land Mobile Network (HPLMN) and the Visited PLMN (VPLMN), in which the RL radio device (e.g., ProSe 5G UE-to-Network Relay) 100-RL is authorized to operate, needs to support the transfer of parameters related to the RM radio device(s) (e.g., Remote UE(s)) 100-RM in case the SMF 704 is in the HPLMN.

It is further noted that when RM radio device(s) (e.g., Remote UE(s)) 100-RM disconnect from the RL radio device (e.g., ProSe UE-to-Network Relay) 100-RL, it is up to implementation how relaying PDU sessions are cleared and/or disconnected by the RL radio device (e.g., ProSe 5G UE-to-Network Relay) 100-RL.

After being connected to the RL radio device (e.g., ProSe 5G UE-to-Network Relay) 100-RL, the RM radio device (e.g., Remote UE) 100-RL keeps performing the measurement of the signal strength of the discovery message sent by the RL radio device (e.g., ProSe 5G UE-to-Network Relay 100-RLfor relay reselection.

The technique may also work when the RM and/or RL radio device (e.g., ProSe 5G UE- to-Network Relay UE) 100-RM and/or 100-RL connects in EPS using LTE. In this case for the RM radio device (e.g., Remote UE) report the procedures defined in TS 23.303 V16.0.0 can be used.

The relayed radio communication through the RL radio device 100-RL may be implemented as a Layer 2 (L2) UE-to-Network relay. In the TR 23.752 v0.3.0 clause 6.7, the layer-2 based RL radio device (e.g., UE-to- Network relay 100-RL) is described.

Hereinbelow, an example of the protocol architecture supporting a L2 RL radio device (e.g., UE-to-Network Relay UE) 100-RL is provided in connection with Fig. 8.

The L2 RL radio device (e.g., UE-to-Network Relay UE) 100-RL provides forwarding functionality that can relay any type of traffic over the PC5 link 502.

The L2 RL radio device (e.g., UE-to-Network Relay UE) 100-RL provides the functionality to support connectivity to the 5GS (e.g., NG-RAN 100-NN) for RM radio devices (e.g., Remote UEs) 100-RM. A radio device (e.g., UE) is considered to be a RM radio device (e.g., Remote UE) 100-RM if it has successfully established a PC5 link 502 to the L2 RL radio device (e.g., UE-to-Network Relay UE) 100-RL. A RM radio device (e.g., Remote UE) 100-RM can be located within NG-RAN 100-NN coverage or outside of NG-RAN 100-NN coverage.

Fig. 8 illustrates the protocol stack for the user plane transport according to the 3GPP document TR 23.752 v0.3.0, related to a PDU session, including a Layer 2 RL radio device (e.g., UE-to-Network Relay UE) 100-RL. The PDU layer 802 corresponds to the PDU carried between the RM radio device (e.g., Remote UE) 100-RM and the Data Network (DN), e.g. represented by the UPF 614 in Fig. 8, over the PDU session. It is important to note that the two endpoints of the PDCP link are the RM radio device (e.g., Remote UE) and the network node (e.g., gNB) 100-NN. The relay function is performed below PDCP, e.g., a schematically depicted at reference sign 606. This means that data security is ensured between the RM radio device (e.g., Remote UE) 100-RM and the RAN and/or network node (e.g., gNB) 100-NN without exposing raw data at the RL radio device (e.g., UE-to-Network Relay UE) 100-RL.

The adaptation relay layer 606 within the RL radio device (e.g., UE-to-Network Relay UE) 100-RL can differentiate between signaling radio bearers (SRBs) and data radio bearers (DRBs) for a particular RM radio device (e.g., Remote UE) 100-RM. The adaption relay layer 606 is also responsible for mapping PC5 traffic (at reference sing 502) to one or more DRBs of the Uu interface at reference sing 504. The definition of the adaptation relay layer 606 is under the responsibility of RAN WG2.

Fig. 9 illustrates the protocol stack of the non-access stratum (NAS) connection according to the 3GPP document TR 23.752 v0.3.0 for the RM radio device (e.g., Remote UE) 100-RM to the NAS-MM and NAS-SM components at reference sings 702 and 704, respectively. The NAS messages are transparently transferred between the RM radio device (e.g., Remote UE) 100-RM and 5G-RAN 100-NN over the Layer 2 RL radio device (e.g., UE-to-Network Relay UE) 100-RL using the following:

PDCP end-to-end connection where the role of the RL radio device (e.g., UE-to- Network Relay UE) 100-RL is to relay the PDUs over the signaling radio bear without any modifications.

N2 connection between the 5G-RAN 100-NN and AMF 702 over N2 at reference sign 902.

N3 connection between AMF 702 and SMF 704 over Nil at reference sign 904.

The role of the RL radio device (e.g., UE-to-Network Relay UE) 100-RL is to relay the PDUs from the signaling radio bearer without any modifications.

A connection establishment for the RM radio device 100-RM may comprise at least one of the steps in below described procedures in connection with Fig. 10, which schematically illustrates a connection establishment for a relayed (e.g., indirect) radio communication via a RL radio device (e.g., UE-to-Network Relay UE) 100-RL as described in the 3GPP document TR 23.752 v0.3.0.

Step 0. If in coverage, the RM radio device (e.g., Remote UE) 100-RM and RL radio device (e.g., UE-to-Network Relay UE) 100-RL may independently perform the initial registration to the network according to registration procedures in TS 23.502 V16.5.0 at reference sign 1004. The allocated 5G global unique temporary identifier (GUTI) of the RM radio device (e.g., Remote UE) 100-RM is maintained when later NAS signaling between RM radio device (e.g., Remote UE) 100-RM and Network 100- NN is exchanged via the RL radio device (e.g., UE-to-Network Relay UE) 100-RL.

It is noted that the current procedures shown here assume a single hop relay. The technique disclosed herewith may be extended to multi-hop relay.

In a step 1 of the procedure at reference sign 1006, if in coverage, the RM radio device (e.g., Remote UE) 100-RM and RL radio device (e.g., UE-to-Network Relay UE) 100-RL independently get the service authorization for indirect communication from the network, e.g., from a Policy Charging Function (PCF) 1002. Steps 2-3. The RM radio device (e.g., Remote UE) 100-RM and RL radio device (e.g., UE-to-Network Relay UE) 100-RL perform RL radio device (e.g., UE-to-Network Relay UE) discovery and selection at reference sign 1008.

Step 4. The RM radio device (e.g., Remote UE) 100-RM initiates a one-to-one communication connection with the selected RL radio device (e.g., UE-to-Network Relay UE) 100-RL over PC5, by sending an indirect communication request message to the RL radio device (e.g., UE-to-Network Relay) 100-RL at reference sing 1010.

Step 5. If the RL radio device (e.g., UE-to-Network Relay UE) 100-RL is in CM_IDLE state, triggered by the communication request received from the RM radio device (e.g., Remote UE) 100-RM, the RL radio device (e.g., UE-to-Network Relay UE) 100-RL sends a Service Request message over PC5 to its serving AMF 702' at reference sign 1012.

The Relay's AMF 702' may perform authentication of the RL radio device (e.g., UE-to- Network Relay UE) 100-RL based on NAS message validation and, if needed, the AMF 702'will check the subscription data.

If the RL radio device (e.g., UE-to-Network Relay UE) 100-RL is already in CM_CONNECTED state and is authorized to perform Relay service, the step 5 at reference sign 1012 is omitted.

Step 6. The RL radio device (e.g., UE-to-Network Relay UE) 100-RL sends the indirect communication response message to the RM radio device (e.g., Remote UE) 100-RM at reference sign 1014.

Step 7. The RM radio device (e.g., Remote UE) 100-RM sends a NAS message to the serving AMF 702" at reference sign 1016. The NAS message is encapsulated in an RRC message that is sent over PC5 to the RL radio device (e.g., UE-to-Network Relay UE) 100-RL, and the RL radio device (e.g., UE-to-Network Relay UE) 100-RL forwards the message to the NG-RAN 100-NN. The NG-RAN 100-NN derives the RM radio device's (e.g., Remote UE's) serving AMF 702" and forwards the NAS message to this AMF 702".

It is noted that here it is assumed that the RM radio device's (e.g., Remote UE's) PLMN is accessible by the RL radio device (e.g., UE-to-Network Relay's) PLMN and that the RL radio device (e.g., UE-to-Network Relay UE) AMF 702' supports all S- NSSAIs (e.g., network slice selection assistance information) the RM radio device (e.g., Remote UE) 100-RM may want to connect to.

If the RM radio device (e.g., Remote UE) 100-RM has not performed the initial registration to the network in step 0 at reference sign 1004, the NAS message is the initial registration message. Otherwise, the NAS message is a service request message.

If the RM radio device (e.g., Remote UE) 100-RM performs initial registration via the RL radio device (e.g., UE-to-Network relay) 100-RL, the RM radio device's (e.g., Remote UE's) 100-RM serving AMF 702" may perform authentication of the RM radio device (e.g., Remote UE) 100-RM based on NAS message validation and, if needed, the RM radio device's (e.g., Remote UE's) AMF 702" checks the subscription data.

For service request case, User Plane connection for PDU Sessions can also be activated. The other steps follow the clause 4.2.3.2 in TS 23.502 V16.5.0.

Step 8. The RM radio device (e.g., Remote UE) 100-RM may trigger the PDU Session Establishment procedure as defined in clause 4.3.2.2 of TS 23.502 V16.5.0 at reference sign 1018.

Step 9. The data is transmitted between RM radio device (e.g., Remote UE) 100-RM and UPF 614 via RL radio device (e.g., UE-to-Network Relay UE) 100-RL and NG-RAN 100-NN on the legs 722, 724' and 724". The RL radio device (e.g., UE-to- Network Relay UE) 100-RL forwards all the data messages between the RM radio device (e.g., Remote UE) 100-RM and NG-RAN 100-NN using the RAN specified L2 relay method.

Alternatively or in addition, in any embodiment disclosed herein, the relay radio device and/or the remote radio device and/or the further remote radio device may be configured to perform at least one or each of the following steps (e.g., in the step of determining the RLF and/or in the step of performing or initiating the release, the reconfiguration or the reestablishment).

Upon indication from sidelink RLC entity that the maximum number of retransmissions for a specific destination has been reached; or upon T400 expiry; or upon indication from sidelink MAC entity that the maximum number of consecutive HARQ. discontinuous transmission (DTX) for a specific destination has been reached; or upon integrity check failure indication from sidelink PDCP entity concerning sidelink signaling radio bearer type 2 (SL-SRB2) or type 3 (SL-SRB3), the respective radio device may perform or initiate at least one of the following action steps.

A first action step comprises considering (e.g., determining) the sidelink radio link failure (i.e., the RLF of the SL) to be detected for this destination.

A second action step comprises releasing the DRBs of this destination, e.g., according to sub-clause 5.8.9. la.1 of 3GPP document TS 38.331, e.g., version 16.2.0.

A third action step comprises releasing the SRBs of this destination, e.g., according to sub-clause 5.8.9. la.3 of 3GPP document TS 38.331, e.g., version 16.2.0.

A fourth action step comprises discarding the NR sidelink communication related configuration of this destination.

A fifth action step comprises resetting the sidelink specific MAC of this destination.

A sixth action step comprises considering (e.g., determining) the PC5-RRC connection is released for the destination.

A seventh action step comprises indicating the release of the PC5-RRC connection to the upper layers for this destination. The indication may be indicative of PC5 being unavailable.

An eighth action step may comprise performing, if UE is in RRC_CONNECTED, the sidelink UE information for NR sidelink communication procedure, e.g., as specified in section 5.8.3.3 or sub-clause 5.10.X in the 3GPP document TS 36.331 e.g., version 16.2.0, or the 3GPP document TS 38.331, e.g., version 16.2.0.

Furthermore, the respective radio device may maintain or fulfill a keep-alive procedure, e.g., by indicating the determined RLF to upper layers. Whether and/or how to indicate the determined RLF may depend on an UE implementation.

Any embodiment disclosed herewith may meet one or more objective defined for 3GPP Rel-17 SI on NR sidelink relay in the 3GPP contribution RP-193253 and/or the below objectives and/or objective studied during 3GPP Rel-17 time frame. Embodiments of the technique may fulfil at least one of the following requirements for a relay mechanism, particularly as to service continuity, e.g., in a single-hop NR sidelink-based relayed radio communication.

1. A mechanism with minimum specification impact to support the System Architecture requirements for sidelink-based UE-to-network and UE-to-UE relay, focusing on the following aspects (if applicable) for layer-3 relay and layer-2 relay;

A. Relay (re-)selection criterion and procedure;

B. Relay/Remote UE authorization;

C. QoS for relaying functionality;

D. Service continuity;

E. Security of relayed connection after SA3 has provided its conclusions;

F. Impact on user plane protocol stack and control plane procedure, e.g., connection management of relayed connection.

2. A mechanism to support upper layer operations of discovery model or procedure for sidelink relaying, assuming no new physical layer channel or signal.

Embodiments of the technique may take into account further input from 3GPP SA WGs, e.g., SA2 and SA3, for the bullets above (if applicable).

Any embodiments described herein for a UE-to-network relay also discloses corresponding embodiments for a UE-to-UE relay, e.g., using the same or corresponding features and steps.

Embodiments may be forward compatibility for multi-hop relay support in a future release needs to be taken into account.

According to the above study objectives, SL based radio device-to-network and/or UE-to-network (U2N) relay and radio device-to- radio device and/or UE to UE (U2U) relay is envisaged to be studied. The study will also consider forward compatibility, e.g., the solution may be easily extended to be applicable for multi-hop relay.

Any transmitter and receiver may be implemented by a network node (e.g., a base station) 100-NN of the RAN or by the relay radio device (e.g., RL-UE) 100-RL or by the remote radio device (e.g., RM-UE) 100-RM. Embodiments are described in the context of NR, e.g., the RM radio device (e.g., Remote UE) 100-RM and the RL radio device (e.g., relay UE) 100-RL are deployed in a same or different NR cell, e.g. cell 302 in Fig. 3. The embodiments are also applicable to other relay scenarios including radio device (e.g., UE) to network relay or radio device (e.g., UE) to radio device (e.g., UE) relay where the RM radio device (e.g., Remote UE) 100-RM and the RL radio device (e.g., relay UE) 100-RL may be based on LTE SL and/or NR SL, the Uu connection between the RL radio device (e.g. relay UE) and the RAN (e.g., base station) 100-NN may be LTE Uu or NR Uu. A relay scenario containing multiple relay hops is also covered. The connection between a RM radio device (e.g. Remote UE) 100-RM and a RL radio device (e.g. relay UE) 100-RL is also not limited to a SL. Any short range communication technology, such as Wi-Fi, is equally applicable.

In the below embodiments, for illustrative purposes any D2D communication may be (as an example without limitation) a SL between two radio devices (e.g. UEs).

The embodiments are also applicable to a relay scenario where the RL radio device (e.g., relay UE) 100-RL is configured with multiple connections (e.g., the number of connections is equal to or larger than two) to the RAN 100-NN (e.g., dual connectivity and/or carrier aggregation, etc.).

The embodiments are described in the context of NR, i.e., remote UE and relay UE are deployed in a same or different NR cell. The embodiments are also applicable to other relay scenarios including UE to network relay or UE to UE relay where the link between remote UE and relay UE may be based on LTE sidelink or NR sidelink, the Uu connection between relay UE and base station may be LTE Uu or NR Uu. A relay scenario containing multiple relay hops is also covered. The connection between remote UE and relay UE is also not limited to sidelink. Any short range communication technology such as Wi-Fi is equally applicable. In the below embodiments, any grant issued by the gNB is for a sidelink transmission between two UEs.

The embodiments are applicable to both L2 relay and L3 relay based relay scenarios.

For concreteness, and without limitation thereto, the relay radio device is referred to as a relay UE (or RL-UE), the remote radio device is referred to as a remote UE (or RM-UE), and the RAN or the network node of the RAN is referred to as a gNB. The further remote radio device is referred to as a further radio device, another UE or another destination UE.

Furthermore, the term "direct path" may stand for a direct radio connection from a remote UE to a gNB or a UE. The term "indirect path" may stand for an indirect radio connection between a remote UE and a gNB or another destination UE via an intermediate node, e.g., the relay radio device (which may be a relay UE or a relay network node, e.g., a relay gNB).

The technique may be implemented for a RLM procedure and/or a RLF procedure performed at the remote UE 100-RM. The RLM procedure and/or the determining of the RLF may be performed on the link between the remote UE and relay UE (e.g., the SL). In the following, embodiments considering that the destination node is the gNB are described, but the same embodiments may also be realized in case the destination node (e.g., the gNB) is another UE (e.g., the further remote radio device).

The RLM (i.e., an RLM procedure) and/or the determining of the RLF may be performed at the remote UE 100-RM. Fig. 11 schematically illustrates an example implementation of the technique in case the RLF is detected at the SL (e.g., a PC5 link).

The assumption is that the remote UE 100-RM performs radio link monitoring (RLM) on the link between the remote UE 100-RM and the relay UE 100-RL, i.e., the SL. Further, if a direct link is available between the remote UE 100-RM and the destination node (e.g., the RAN and/or the further remote radio device), the remote UE 100-RM may perform also RLM on this direct link (e.g., an Uu link or UL or DL).

Fig. 12 illustrates a reference example of a RLF on the Uu link between the relay UE and the RAN.

The remote radio device 100-RM may be embodied alternatively or in combination with any of the features disclosed for the first method aspect and first device aspect, particularly those in the list of claims, according to at least one of the following embodiments.

In a first embodiment, the remote UE performs RLM on the SL (e.g., a PC5 link) between the remote UE and a relay UE. An RLF event is determined (e.g., declared) if at least one of the below RLF conditions is met. A first RLF condition comprises that a maximum number of out of sync on the link instances has been reached.

The remote UE may monitor the PC5 radio channel quality based on a specific reference symbol. The remote UE compares the measured channel quality with the out-of-sync and in-sync thresholds, Qout and Qin respectively. The SL physical channel evaluates the (e.g., PC5) channel quality periodically and sends indication on out-of-sync or in-sync, e.g., to layer 3. The remote UE layer 3 then evaluates, if the radio link failure based on the in-sync and out-of-sync indications, that output from the layer 3 filter. For RLM on the SL (e.g., PC5 link), a counter and/or a timer may be defined. In an example, when the consecutively received out-of-sync indications are beyond a configured counter, a timer is started. While the timer is running, the radio link considered to be recovered if the UE consecutively receives a configured number of in-sync indications from the physical layer.

A second RLF condition comprises that a maximum number of RLC retransmissions has been reached.

A third RLF condition comprises that a configuration or reconfiguration error occurs upon reception of a RRC configuration/reconfiguration signaling message.

A fourth RLF condition comprises a maximum number of HARQ retransmissions or discontinuous transmission (DTX) has been reached.

Meanwhile, the remote UE may be also configured with RLM and RLF in Uu RRC, which actually monitors the Uu connection between the relay UE and the gNB. In a L2 UE to network relay scenario, remote UE's Uu RRC entity and other lower layer Uu protocol entities (such as RLC, and MAC) are located in different places.

Herein, it may be optionally assumed that the remote UE's RRC entity can receive LI and L3 measurement indicators of the Uu link.

In this case, the remote UE may keep the two RLM/RLF procedures running in parallel.

In a second embodiment, the remote UE may keep the two RLM/RLF procedures running in parallel, in this case, there may be RLF triggered on both links. Upon declaration (i.e., determining) of an RLF event firstly on the PC5 link, the remote UE will further check the other link to see if RLF is also being triggered.

After that, the remote UE may take any one of the below options to determine whether to release the relay path.

Option 1: releases the relay path, and releases both the PC5-RRC connection and the Uu RRC connection regardless if the other link has also failed.

Option 2: the remote UE waits for a configured time period. A timer may be configured accordingly. The timer is intended to allow the UE to wait for a time period to further check if the link in failure can be recovered. The timer may be a new timer, or reuse an existing timer (e.g., a timer in Uu RLM/RLF procedure). During the timer is running, the UE keeps the relay path unchanged, meanwhile, the remote UE stops data transmission or reception on the relay path.

The remote UE may take at least one of the below actions.

• While the timer is running, if the remote UE declares an RLF event for the Uu link, the remote UE performs actions specified in Option 1. The remote UE stops the timer.

• While the timer is running, if the remote UE doesn't declare an RLF event for the Uu link, the UE will further check if the PC5 link is recovered. If the PC5 link is recovered, the remote UE cancels the triggered RLF event on the PC5 link and resumes data transmission and reception on the relay path. The remote UE stops the timer.

• While the timer is expired, if the remote UE doesn't declare an RLF event for the Uu link however the PC5 link is not recovered either, the remote UE performs actions specified in Option 1. The remote UE stops the timer.

The gNB can configure which option that the remote UE can apply. Alternatively, which option is applied by the remote UE is captured in a spec in a hard-coded fashion.

In a third embodiment, the remote UE may keep the two RLM/RLF procedures running in parallel, in this case, there may be RLF triggered on both links. Upon declaration of an RLF event firstly on the Uu link, the remote UE will further check the other link to see if RLF is also being triggered.

After that, the remote UE may apply at least one of the below options (which may be similar as for the second embodiment) to determine whether to release the relay path (e.g., the SL or the relayed radio communication).

Option 1: releases the relay path, and releases both the PC5-RRC connection and the Uu RRC connection regardless if the other link has also failed.

Option 2: the remote UE waits for a configured time period. A timer may be configured accordingly. The timer is intended to allow the UE to wait for a time period to further check if the link in failure can be recovered. The timer may be a new timer, or reuse an existing timer. During the timer is running, the UE keeps the relay path unchanged, meanwhile, the remote UE stops data transmission or reception on the relay path.

The remote UE may take at least one of the below actions.

• While the timer is running, if the remote UE declares an RLF event for the PC5 link, the remote UE performs actions specified in Option 1. The remote UE stops the timer.

• While the timer is running, if the remote UE doesn't declare an RLF event for the PC5 link, the UE will further check if the Uu link is recovered. If the Uu link is recovered, the remote UE cancels the triggered RLF event on the Uu link and resumes data transmission and reception on the relay path. The remote UE stops the timer.

• While the timer is expired, if the remote UE doesn't declare an RLF event for the PC5 link however the Uu link is not recovered either, the remote UE performs actions specified in Option 1. The remote UE stops the timer.

The gNB can configure which option that the remote UE can apply. Alternatively, which option is applied by the remote UE is captured in a spec in a hard coded fashion.

In a fourth embodiment, upon declaration (i.e., determining) of the RLF on one or both links (e.g., on the SL), if the relay path has been released (e.g., according to the second embodiment and/or the third embodiment), the remote UE 100-RM may apply at least one of the below recovery options to re-establish the connection towards the destination node.

Option 1: trigger relay UE reselection, and select a target relay UE. The connection to the old relay UE may be released before or after the reselection procedure finishes.

Option 2: select a cell and perform RACH to establish a direct Uu connection meaning in this case that there will be only a direct connection and not a relay one.

In Option 2, the remote UE may have already stored a target cell or a list of target cells for potential direct connections when the remote UE was in coverage. Alternatively, the stored cell list was obtained from the old relay UE. In yet another alternative, the remote UE is allowed to trigger cell selection and reselection procedure to search potential target cells.

Option 3: the remote UE is allowed to reselect either a target relay UE or a target cell. During the reselection procedure, the remote UE may measure radio quality for both target relay UEs and target cells. Based on measurements, the remote UE selects the one (either a relay UE or a target cell) with strongest radio channel quality. The measurement may be performed in terms of metrics such as RSRP, RSRQ, RSSI, SINR, SNR etc. In order to compare measurements between PC5 link and Uu link, an offset may be configured. For a target relay UE, the remote UE may be also required to consider measurements on links including both PC5 link between the remote UE and the target relay UE, and Uu link between the target relay UE and gNB.

The gNB can configure which option that the remote UE can apply. Alternatively, which option is applied by the remote UE is captured in a spec in a hard coded fashion.

After the remote UE selects either a target relay UE or a target cell, the remote UE can establish a new connection towards the selected UE or the target cell. In this way, the remote UE can recover from the RLF failure.

In a fifth embodiment, after recovery from the RLF event, the remote UE transmits a report message including failure related information to the RAN node (e.g., the new serving gNB, and/or the old serving gNB). The report may be further sent to the CN nodes (e.g., the new serving AMF and/or the old serving AMF). The report message may be also forwarded to the old relay UE about the RLF detected on the old relay path and that the remote UE has released that old relay path.

In a sixth embodiment, at least one of the below information may be carried or indicated by the report message (briefly: report):

• Remote ID

• Destination L2 ID

• Old Relay ID

• Cell ID

• New Relay ID

• Relay path ID

• Failure cause/ Indicators on events that RLF has been declared on the links (PC5 and/or Uu link).

• Available measurements on the links (e.g., the SL, such as PC5, and/or Uu link) and/or radio quality indicators of the links (e.g., the SL, such as PC5 and/or Uu link) on the SL for which the RLF has been determined. Such measures or indicators may comprise at least one of reference signal received power (RSRP), reference signal received quality (RSRQ), Received Signal Strength Indicator (RSSI), signal-to-noise ratio (SNR), and signal to noise-and- interference ratio (SINR).

• QoS indicators such as latency, packet loss, priority, jitter etc. of services which are affected by RLF.

• Buffer status report.

• Power headroom report.

• The indices for cells/BWPs/carriers//PLMNs that suffer from RLF.

In a seventh embodiment, the remote UE can perform RLM in order to detect an "early RLF". Here with "early RLF" we mean that the link where the RLM is performed is not totally failed, but the probability that is going to fail is high. In this case, the remote UE it has still the chance to transmit messages with the current relay UE even if is not guaranteed that the link will be stable. When the remote UE has detected an early RLF on the link where the RLM was performed, the remote UE sends an indication to the relay UE for indicating the detected early RLF. Once receiving the early RLF indication, the relay UE may inform the gNB (that can then reconfigure the relay path) or reconfigure the PC5 link between the remote UE and relay UE. If during the early RLF, is not possible to recover the relay path, the remote UE can perform any of the actions described in the previous embodiments. In an eighth embodiment, which actions the remote UE should perform, according to what is described in any of the above embodiment, is configured by the gNB via dedicated Uu RRC signaling or SIBs or can also be pre-configured. Alternatively, which actions the remote UE should perform may also be configured by the relay UE via direct PC5 RRC signaling or sidelink broadcast/groupcast.

In a ninth embodiment, the remote UE when sending a report indicating RLF to the gNB or relay UE, it may send such indication via RRC, via control PDU of the adaptation layer, or via MAC CE, or via DCI (if is over Uu link) or SCI (if is over PC5 link).

Any embodiment of the RL UE may perform at least one of the following actions. Embodiments can target to handle UE actions when an RLF is detected. These actions may have the main target to lower the connectivity interruption delay and reduce the signaling overhead. In particular, the methods proposed for the remote UE comprises that, upon detecting RLF in the link between remote UE and relay UE (i.e., the SL):

• Remote UE releases the entire relay path.

This means the relay UE may figure out that the relay path was released if the inactivity timer expires or via keep alive message or via the expiry of the timer T400. Once the RLF is detected, the relay UE informs also the gNB.

• Path (re)selection is triggered by the remote UE.

Here, the new relay UE may inform about the failure the old relay UE (if possible, to establish a PC5 connection between them. This is probably faster than waiting for the inactivity timer to expire or to wait for the keep alive message.

• Remote UE send an indication to the gNB/destination UE that a failure on the relay path has been detected.

This only works if there is (can be establish) a direct Uu connection between the remote UE and the gNB/destination UE. Furthermore, the remote UE may not release the path but instead waiting for a new configuration from the gNB/destination UE.

• Remote UE send an indication to the relay UE that an RLF has been detect (this is valid only for early-RLF detection).

This means that the relay UE may be notified about the reconfiguration via the PC5- RRC signaling, or via a control PDU of the adaptation layer or via a MAC CE. As a further alternative, the remote UE may also instruct the relay UE to perform some recovery actions (via the PC5-RRC signaling, or via a control PDU of the adaptation layer or via a MAC CE).

Embodiments can target to handle UE actions when an RLF is detected. These actions have the main target to lower the connectivity interruption delay and reduce the signaling overhead. This it is still valid whether a L3- or a L2-based relay solution is used

Fig. 13A shows a schematic block diagram for an embodiment of a relay radio device 100-RM. The device 100-RM comprises one or more processors 1304-RM for performing the method 200-RM and memory 1306-RM coupled to the processors 1304-RM. For example, the memory 1306-RM may be encoded with instructions that implement at least one of the units labeled lxy-RM (e.g., 102-RM and 104-RM).

The one or more processors 1304-RM may be a combination of one or more of a microprocessor, controller, microcontroller, central processing unit, digital signal processor, application specific integrated circuit, field programmable gate array, or any other suitable computing device, resource, or combination of hardware, microcode and/or encoded logic operable to provide, either alone or in conjunction with other components of the device 100-RM, such as the memory 1306-RM, remote radio device functionality. For example, the one or more processors 1304-RM may execute instructions stored in the memory 1306-RM. Such functionality may include providing various features and steps discussed herein, including any of the benefits disclosed herein. The expression "the device being operative to perform an action" may denote the device 100-RM being configured to perform the action.

As schematically illustrated in Fig. 13A, the device 100-RM may be embodied by a RM radio device 1300-RM, e.g., functioning as a UE. The RM radio device 1300-RM comprises a radio interface 1302-RM coupled to the device 100-RM for radio communication with one or more base stations or UEs.

Fig. 13B shows a schematic block diagram for an embodiment of a relay radio device 100-RL. The device 100-RL comprises one or more processors 1304-RL for performing the method 200-RL and memory 1306-RL coupled to the processors 1304-RL. For example, the memory 1306-RL may be encoded with instructions that implement at least one of the units labeled lxy-RL (e.g., 102-RL and 104-RL). The one or more processors 1304-RL may be a combination of one or more of a microprocessor, controller, microcontroller, central processing unit, digital signal processor, application specific integrated circuit, field programmable gate array, or any other suitable computing device, resource, or combination of hardware, microcode and/or encoded logic operable to provide, either alone or in conjunction with other components of the device 100-RL, such as the memory 1306-RL, relay radio device functionality. For example, the one or more processors 1304-RL may execute instructions stored in the memory 1306-RL. Such functionality may include providing various features and steps discussed herein, including any of the benefits disclosed herein. The expression "the device being operative to perform an action" may denote the device 100-RL being configured to perform the action.

As schematically illustrated in Fig. 13B, the device 100-RL may be embodied by a RL radio device 1300-RL, e.g., functioning as a relay radio device. The RL radio device 1300-RL comprises a radio interface 1302-RL coupled to the device 100-RL for radio communication with one or more base stations or UEs.

With reference to Fig. 14, in accordance with an embodiment, a communication system 1400 includes a telecommunication network 1410, such as a 3GPP-type cellular network, which comprises an access network 1411, such as a radio access network, and a core network 1414. The access network 1411 comprises a plurality of base stations 1412a, 1412b, 1412c, such as NBs, eNBs, gNBs or other types of wireless access points, each defining a corresponding coverage area 1413a, 1413b, 1413c. Each base station 1412a, 1412b, 1412c is connectable to the core network 1414 over a wired or wireless connection 1415. A first user equipment (UE) 1491 located in coverage area 1413c is configured to wirelessly connect to, or be paged by, the corresponding base station 1412c. A second UE 1492 in coverage area 1413a is wirelessly connectable to the corresponding base station 1412a. While a plurality of UEs 1491, 1492 are illustrated in this example, the disclosed embodiments are equally applicable to a situation where a sole UE is in the coverage area or where a sole UE is connecting to the corresponding base station 1412.

The telecommunication network 1410 is itself connected to a host computer 1430, which may be embodied in the hardware and/or software of a standalone server, a cloud-implemented server, a distributed server or as processing resources in a server farm. The host computer 1430 may be under the ownership or control of a service provider, or may be operated by the service provider or on behalf of the service provider. The connections 1421, 1422 between the telecommunication network 1410 and the host computer 1430 may extend directly from the core network 1414 to the host computer 1430 or may go via an optional intermediate network 1420. The intermediate network 1420 may be one of, or a combination of more than one of, a public, private or hosted network; the intermediate network 1420, if any, may be a backbone network or the Internet; in particular, the intermediate network 1420 may comprise two or more sub-networks (not shown).

The communication system 1400 of Fig. 14 as a whole enables connectivity between one of the connected UEs 1491, 1492 and the host computer 1430. The connectivity may be described as an over-the-top (OTT) connection 1450. The host computer 1430 and the connected UEs 1491, 1492 are configured to communicate data and/or signaling via the OTT connection 1450, using the access network 1411, the core network 1414, any intermediate network 1420 and possible further infrastructure (not shown) as intermediaries. The OTT connection 1450 may be transparent in the sense that the participating communication devices through which the OTT connection 1450 passes are unaware of routing of uplink and downlink communications. For example, a base station 1412 need not be informed about the past routing of an incoming downlink communication with data originating from a host computer 1430 to be forwarded (e.g., handed over) to a connected UE 1491. Similarly, the base station 1412 need not be aware of the future routing of an outgoing uplink communication originating from the UE 1491 towards the host computer 1430.

By virtue of the methods 200-RM and 200-RL being performed by any one of the UEs 1491 or 1492 and/or any one of the base stations 1412, the performance of the OTT connection 1450 can be improved, e.g., in terms of increased throughput and/or reduced latency.

Example implementations, in accordance with an embodiment, of the UE, base station and host computer discussed in the preceding paragraphs will now be described with reference to Fig. 15. In a communication system 1500, a host computer 1510 comprises hardware 1515 including a communication interface 1516 configured to set up and maintain a wired or wireless connection with an interface of a different communication device of the communication system 1500. The host computer 1510 further comprises processing circuitry 1518, which may have storage and/or processing capabilities. In particular, the processing circuitry 1518 may comprise one or more programmable processors, application-specific integrated circuits, field programmable gate arrays or combinations of these (not shown) adapted to execute instructions. The host computer 1510 further comprises software 1511, which is stored in or accessible by the host computer 1510 and executable by the processing circuitry 1518. The software 1511 includes a host application 1512. The host application 1512 may be operable to provide a service to a remote user, such as a UE 1530 connecting via an OTT connection 1550 terminating at the UE 1530 and the host computer 1510. In providing the service to the remote user, the host application 1512 may provide user data, which is transmitted using the OTT connection 1550. The user data may depend on the location of the UE 1530. The user data may comprise auxiliary information or precision advertisements (also: ads) delivered to the UE 1530. The location may be reported by the UE 1530 to the host computer, e.g., using the OTT connection 1550, and/or by the base station 1520, e.g., using a connection 1560.

The communication system 1500 further includes a base station 1520 provided in a telecommunication system and comprising hardware 1525 enabling it to communicate with the host computer 1510 and with the UE 1530. The hardware 1525 may include a communication interface 1526 for setting up and maintaining a wired or wireless connection with an interface of a different communication device of the communication system 1500, as well as a radio interface 1527 for setting up and maintaining at least a wireless connection 1570 with a UE 1530 located in a coverage area (not shown in Fig. 15) served by the base station 1520. The communication interface 1526 may be configured to facilitate a connection 1560 to the host computer 1510. The connection 1560 may be direct or it may pass through a core network (not shown in Fig. 15) of the telecommunication system and/or through one or more intermediate networks outside the telecommunication system. In the embodiment shown, the hardware 1525 of the base station 1520 further includes processing circuitry 1528, which may comprise one or more programmable processors, application-specific integrated circuits, field programmable gate arrays or combinations of these (not shown) adapted to execute instructions. The base station 1520 further has software 1521 stored internally or accessible via an external connection.

The communication system 1500 further includes the UE 1530 already referred to. Its hardware 1535 may include a radio interface 1537 configured to set up and maintain a wireless connection 1570 with a base station serving a coverage area in which the UE 1530 is currently located. The hardware 1535 of the UE 1530 further includes processing circuitry 1538, which may comprise one or more programmable processors, application-specific integrated circuits, field programmable gate arrays or combinations of these (not shown) adapted to execute instructions. The UE 1530 further comprises software 1531, which is stored in or accessible by the UE 1530 and executable by the processing circuitry 1538. The software 1531 includes a client application 1532. The client application 1532 may be operable to provide a service to a human or non-human user via the UE 1530, with the support of the host computer 1510. In the host computer 1510, an executing host application 1512 may communicate with the executing client application 1532 via the OTT connection 1550 terminating at the UE 1530 and the host computer 1510. In providing the service to the user, the client application 1532 may receive request data from the host application 1512 and provide user data in response to the request data. The OTT connection 1550 may transfer both the request data and the user data. The client application 1532 may interact with the user to generate the user data that it provides.

It is noted that the host computer 1510, base station 1520 and UE 1530 illustrated in Fig. 15 may be identical to the host computer 1430, one of the base stations 1412a, 1412b, 1412c and one of the UEs 1491, 1492 of Fig. 14, respectively. This is to say, the inner workings of these entities may be as shown in Fig. 15 and independently, the surrounding network topology may be that of Fig. 14.

In Fig. 15, the OTT connection 1550 has been drawn abstractly to illustrate the communication between the host computer 1510 and the use equipment 1530 via the base station 1520, without explicit reference to any intermediary devices and the precise routing of messages via these devices. Network infrastructure may determine the routing, which it may be configured to hide from the UE 1530 or from the service provider operating the host computer 1510, or both. While the OTT connection 1550 is active, the network infrastructure may further take decisions by which it dynamically changes the routing (e.g., on the basis of load balancing consideration or reconfiguration of the network).

The wireless connection 1570 between the UE 1530 and the base station 1520 is in accordance with the teachings of the embodiments described throughout this disclosure. One or more of the various embodiments improve the performance of OTT services provided to the UE 1530 using the OTT connection 1550, in which the wireless connection 1570 forms the last segment. More precisely, the teachings of these embodiments may reduce the latency and improve the data rate and thereby provide benefits such as better responsiveness. A measurement procedure may be provided for the purpose of monitoring data rate, latency and other factors on which the one or more embodiments improve. There may further be an optional network functionality for reconfiguring the OTT connection 1550 between the host computer 1510 and UE 1530, in response to variations in the measurement results. The measurement procedure and/or the network functionality for reconfiguring the OTT connection 1550 may be implemented in the software 1511 of the host computer 1510 or in the software 1531 of the UE 1530, or both. In embodiments, sensors (not shown) may be deployed in or in association with communication devices through which the OTT connection 1550 passes; the sensors may participate in the measurement procedure by supplying values of the monitored quantities exemplified above, or supplying values of other physical quantities from which software 1511, 1531 may compute or estimate the monitored quantities. The reconfiguring of the OTT connection 1550 may include message format, retransmission settings, preferred routing etc.; the reconfiguring need not affect the base station 1520, and it may be unknown or imperceptible to the base station 1520. Such procedures and functionalities may be known and practiced in the art. In certain embodiments, measurements may involve proprietary UE signaling facilitating the host computer's 1510 measurements of throughput, propagation times, latency and the like. The measurements may be implemented in that the software 1511, 1531 causes messages to be transmitted, in particular empty or "dummy" messages, using the OTT connection 1550 while it monitors propagation times, errors etc.

Fig. 16 is a flowchart illustrating a method implemented in a communication system, in accordance with one embodiment. The communication system includes a host computer, a base station and a UE which may be those described with reference to Fig. 14 and 15. For simplicity of the present disclosure, only drawing references to Fig. 16 will be included in this section. In a first step 1610 of the method, the host computer provides user data. In an optional substep 1611 of the first step 1610, the host computer provides the user data by executing a host application. In a second step 1620, the host computer initiates a transmission carrying the user data to the UE. In an optional third step 1630, the base station transmits to the UE the user data which was carried in the transmission that the host computer initiated, in accordance with the teachings of the embodiments described throughout this disclosure. In an optional fourth step 1640, the UE executes a client application associated with the host application executed by the host computer. Fig. 17 is a flowchart illustrating a method implemented in a communication system, in accordance with one embodiment. The communication system includes a host computer, a base station and a UE which may be those described with reference to Figs. 14 and 15. For simplicity of the present disclosure, only drawing references to Fig. 16 will be included in this section. In a first step 1710 of the method, the host computer provides user data. In an optional substep (not shown) the host computer provides the user data by executing a host application. In a second step 1720, the host computer initiates a transmission carrying the user data to the UE. The transmission may pass via the base station, in accordance with the teachings of the embodiments described throughout this disclosure. In an optional third step 1730, the UE receives the user data carried in the transmission.

As has become apparent from above description, embodiments of the technique can handle actions of the UE (e.g., the remote UE) when an RLF is determined (e.g., detected). In same or further embodiments, the actions have the main target to reduce a connectivity interruption delay and/or to reduce a signaling overhead.

The technique may be applied and (at least some of) the advantages may be achieved in combination with a L3-based or a L2-based relay mechanism.

Many advantages of the present invention will be fully understood from the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the units and devices without departing from the scope of the invention and/or without sacrificing all of its advantages. Since the invention can be varied in many ways, it will be recognized that the invention should be limited only by the scope of the following claims.