TECHNICAL FIELD

The present disclosure generally relates to the field of wireless communications. For example, this disclosure presents devices and methods for data retransmission, such as user equipment (UE) for assisting a sidelink-assisted retransmission, a network device, and corresponding methods.

BACKGROUND

Ultra-reliable low-latency communication (URLLC) is of high relevance and the key targeted service when moving toward wireless connectivity for automated control tasks. For instance, in Industry 4.0 and/or industrial Internet-of- Things (loT) scenarios, numerous devices operated in a factory can facilitate several Device-to-Device (D2D) links among each other. A direct communication enabled among these devices (or UEs) is called a D2D communication via sidelink. In this case, devices participating in the sidelink transmission are also called D2D UEs. By exploiting these multiple D2D UEs and the corresponding link diversity made available by those, a sidelink-assisted retransmission scheme has been proposed, where D2D links (or sidelinks (SL)) are exploited to retransmit a packet that was not successfully received in downlink (DL) transmission by a target UE, yielding increased reliability of that packet transmission. The sidelink-assisted retransmission scheme pinpoints significant improvements in reliability, continuously scaling by one order of magnitude with the degree of diversity made available by D2D retransmission.

The sidelink-assisted retransmissions exploit the large number of D2D links available in the vicinity of a target UE to support a DL transmission from the base station to the target UE if necessary. The D2D links between the target UE and the neighbor UEs are assumed to be independently fading, which can be exploited by letting one or more of the neighbor UEs retransmit the original packet cooperatively by applying transmit diversity schemes in a distributed manner, such as distributed space-time block codes (STBC) or cyclic delay diversity (CDD). These diversity schemes facilitate a constructive addition of multiple versions of the same signal propagated via different D2D communication links. A neighbor UE assisting the retransmission for a target UE via sidelink is referred to as an assisting UE. SUMMARY

Since the same packet is retransmitted via D2D links without re-coding, it is desired to have a required signal -to-noise ratio (SNR) or signal-to-interference-plus-noise ratio (SINR) at the intended receiver (i.e., the target UE) for successful decoding according to a modulation and coding scheme (MCS) used for the original packet. This allows for optimized power control for sidelink-assisted retransmissions. The D2D retransmissions with power control can be done with relatively small transmit powers (compared to DL transmissions) enabling power savings, thanks to the close vicinity of the neighbor UEs to the target UE. These small transmit powers, however, pose the problem of whether and how the D2D resources assigned for the retransmissions may be (re-)used for other D2D or uplink (UL) transmissions.

In current radio resource management (RRM) schemes for resource reuse, UL UEs having UL transmissions with a base station are prioritized in resource allocations, then D2D UEs (having D2D links with other UEs) are seeked for reusing the same resources as allocated for UL transmissions by the base station. In this conventional scheme, the base station acts as a central controller, assuming that it has complete knowledge of channel conditions between D2D UEs. This conventional scheme is not suitable for sidelink-assisted retransmissions, because UL transmissions are prioritized and hence the D2D users with URLLC service may be scheduled with a delay.

In view of the above, this disclosure generally aims at facilitating sidelink-assisted (cooperative) retransmissions. For example, one objective is to ensure that the sidelink-assisted (cooperative) retransmissions do not interfere with other transmissions in the same band. This disclosure also aims to propose radio resource management (RRM) for sidelink-assisted retransmissions.

These and other objectives are achieved by this disclosure, for instance, as described in the independent claims. Advantageous implementations are further described in the dependent claims.

A first aspect of this disclosure provides an assisting UE for assisting a retransmission via sidelink for a target UE. The assisting UE is configured to obtain a transmission power required for the retransmission, and send sidelink retransmission information (SLRI) to a network device. The SLRI comprises the transmission power. Optionally, the retransmission via sidelink may be referred to as a sidelink-assisted retransmission, or a sidelink-assisted cooperative retransmission. For instance, one or more assisting UEs may be configured to retransmit missed packets to the target UE via sidelink. In this disclosure, the sidelink may also be referred to as a D2D link.

By informing the network device about the transmission power required for the retransmission, the network device is able to assure that resources scheduled for the retransmission may be reused for the uplink transmission based on the transmission power. In this way, network resources are more efficiently used, and network capacity can be increased. An overall network (or system) throughput can be achieved.

In an implementation form of the first aspect, the SLRI may further comprise a path loss measurement between the assisting UE and the target UE.

In an implementation form of the first aspect, the SLRI may further comprise a retransmission ID.

In this way, different retransmissions can be distinguished according to the retransmission ID, and thus, multiple retransmissions via sidelink can be supported.

In an implementation form of the first aspect, for obtaining the transmission power, the assisting UE may be further configured to: receive a retransmission request from the target UE, in which the retransmission request comprises the transmission power; and obtain the transmission power from the retransmission request.

In an implementation form of the first aspect, for obtaining the transmission power, the assisting UE may be further configured to: receive a retransmission request from the target UE; and compute the transmission power based on the retransmission request. In an implementation form of the first aspect, the assisting UE may be further configured to: receive resource allocation from the network device, wherein the resource allocation indicates sidelink resources allocated for the retransmission via sidelink; and perform the retransmission via sidelink for the target UE according to the resource allocation.

A second aspect of the present disclosure provides a network device. The network device is configured to receive SLRI from an assisting UE. The assisting UE is configured to assist a retransmission via sidelink for a target UE. The SLRI comprises a transmission power required for the retransmission via sidelink. The network device is further configured to perform resource allocation for uplink transmission performed in parallel with the retransmission via sidelink based on the SLRI.

It is noted that the resource allocation for uplink transmission performed in parallel with the retransmission via sidelink based on the SLRI may be understood that the uplink transmission (e.g. from an uplink UE to the network device) and the retransmission via sidelink (from the assisting UE to the target UE) reuse the same (or at least overlapping) resources, which may be referred to as an underlay mode.

Optionally, for performing the resource allocation, the network device may be configured to adopt an underlay RRM algorithm that is used to determine whether an underlay mode can be used based on the SLRI.

Based on the transmission power required for the retransmission comprised in the SLRI, the network device is able to efficiently allocate network resources. The SLRI in this disclosure contains useful elements for performing RRM for the retransmission via sidelink and the uplink transmission at the network device. The SLRI allows information exchange between the assisting UE and the network device. For example, the information on the D2D link between the assisting UE and the target UE, i.e., the transmission power, may allow the network device to decide whether to use underlay mode for the resource allocation.

In this way, network resources are more efficiently used, and network capacity can be increased. An overall network (or system) throughput can be achieved. In an implementation form of the second aspect, the SLRI may further comprise a path loss measurement between the assisting UE and the target UE.

In an implementation form of the second aspect, the SLRI may further comprise a retransmission ID.

In an implementation form of the second aspect, the network device may be further configured to determine, according to the SLRI, whether the network device can bear an interference caused by the retransmission via sidelink.

The interference is determined based on reusing the same resources for the retransmission via sidelink and one or more uplink transmissions. For instance, the network device may be configured to determine the interference caused by the assisting UE performing the retransmission via sidelink on the same resources where one or more uplink UEs simultaneously transmit one or more uplink transmissions to the network device. If the interference is below a certain threshold, resource reuse is allowed.

In an implementation form of the second aspect, for performing the resource allocation, the network device may be specifically configured to: determine one or more uplink UEs for performing one or more uplink transmissions in parallel with the assisting UE for performing the retransmission via sidelink with the target UE; and allocate sidelink resources for the assisting UE and uplink resources for the one or more uplink UEs.

The sidelink resources may be used by the assisting UE for the retransmission. The uplink resources may be used by the one or more uplink UEs for the one or more uplink transmissions with the network device. Allocating the sidelink resources may take precedence over allocating the uplink resources. That is, the priority of scheduling sidelink resources is higher than that of scheduling uplink resources.

In this way, in URLLC scenarios, the strict latency constraint for the sidelink-assisted DL transmission can be fulfilled. In an implementation form of the second aspect, for a respective uplink UE, the network device may be further configured to: determine an interference caused by the respective uplink UE on the target UE; determine a signal-to-interference-plus-noise ratio (SINR) at the target UE based on the transmission power, the path loss measurement, and the interference caused by the respective uplink UE; determine whether the SINR is above a pre-determined safety level; and in response to determining that the SINR is above the pre-determined safety level, allocate the sidelink resources and the uplink resources in an underlay manner.

In an implementation form of the second aspect, the network device may be configured to determine whether the SINR is above the pre-determined safety level based on a distance of the respective uplink UE to the target UE.

In an implementation form of the second aspect, the network device may be further configured to: select a final uplink UE from the one or more uplink UEs taking account of an uplink intra-cell interference to the target UE; and allocate the sidelink resources for the assisting UE and uplink resources for the final uplink UE in an underlay manner.

Optionally, the final uplink UE may be selected by the network device based on a criterion causing the minimum intra-cell interference to the target UE.

By using the underlay manner for arranging the sidelink resources and the uplink resources, radio resources are reused by different UEs (e.g., the assisting UE and the final uplink UE). In this way, more uplink UEs can be supported and network capacity can be increased.

Optionally, the network device may be a radio access network (RAN) device, a base station, an eNodeB (eNB), a gNodeB (gNB), or the like.

A third aspect of the present disclosure provides a system comprising at least one assisting UE according to the first aspect or any implementation forms thereof, and a network device according to the second aspect or any implementation forms thereof. A fourth aspect of the present disclosure provides a method for assisting a retransmission via sidelink for a target UE. The method comprises the following steps: obtaining, by an assisting UE, a transmission power required for the retransmission, and sending, by the assisting UE, SLRI to a network device, in which the SLRI comprises the transmission power.

In an implementation form of the fourth aspect, the SLRI may further comprise a path loss measurement between the assisting UE and the target UE.

In an implementation form of the fourth aspect, the SLRI may further comprise a retransmission ID.

In an implementation form of the fourth aspect, the step of obtaining the transmission power may comprise: receiving, by the assisting UE, a retransmission request from the target UE, in which the retransmission request comprises the transmission power; and obtaining, by the assisting UE, the transmission power from the retransmission request.

In an implementation form of the fourth aspect, the step of obtaining the transmission power may comprise: receiving, by the assisting UE, a retransmission request from the target UE; and computing, by the assisting UE, the transmission power based on the retransmission request.

In an implementation form of the fourth aspect, the method may further comprise: receiving, by the assisting UE, resource allocation from the network device, wherein the resource allocation indicates sidelink resources allocated for the retransmission via sidelink; and performing, by the assisting UE, the retransmission via sidelink for the target UE according to the resource allocation. A fifth aspect of the present disclosure provides a method for a retransmission via sidelink. The method comprises the following steps: receiving, by a network device, sidelink retransmission information, SLRI, from an assisting UE, wherein the assisting UE is configured to assist a retransmission via sidelink for a target UE, and the SLRI comprises a transmission power required for the retransmission via sidelink; and performing, by the network device, resource allocation for uplink transmission performed in parallel with the retransmission via sidelink based on the SLRI.

In an implementation form of the fifth aspect, the SLRI may further comprise a path loss measurement between the assisting UE and the target UE.

In an implementation form of the fifth aspect, the SLRI may further comprise a retransmission ID.

In an implementation form of the fifth aspect, the method may further comprise determining, by the network device according to the SLRI, whether the network device can bear an interference caused by the retransmission via sidelink.

In an implementation form of the fifth aspect, the step of performing the resource allocation may comprise: determining, by the network device, one or more uplink UEs for performing one or more uplink transmissions in parallel with the assisting UE for performing the retransmission via sidelink with the target UE; and allocating, by the network device, sidelink resources for the assisting UE and uplink resources for the one or more uplink UEs.

The sidelink resources may be used by the assisting UE for the retransmission. The uplink resources may be used by the one or more uplink UEs for the one or more uplink transmissions with the network device. Allocating the sidelink resources may take precedence over allocating the uplink resources. That is, the priority of scheduling sidelink resources is higher than that of scheduling uplink resources. In an implementation form of the fifth aspect, for a respective uplink UE, the method may comprise: determining, by the network device, an interference caused by the respective uplink UE on the target UE; determining, by the network device, an SINR at the target UE based on the transmission power, the path loss measurement, and the interference caused by the respective uplink UE; determine whether the SINR is above a pre-determined safety level; and in response to determining that the SINR is above the pre-determined safety level, allocate the sidelink resources and the uplink resources in an underlay manner.

In an implementation form of the fifth aspect, the method may comprise determining, by the network device, whether the SINR is above the pre-determined safety level based on a distance of the respective uplink UE to the target UE.

In an implementation form of the fifth aspect, the method may further comprise: selecting, by the network device, a final uplink UE from the one or more uplink UEs taking account of an uplink intra-cell interference to the target UE; and allocating, by the network device, the sidelink resources for the assisting UE and uplink resources for the final uplink UE in an underlay manner.

A sixth aspect of the present disclosure provides a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to perform the method according to the fourth aspect or any of its implementation forms.

A seventh aspect of the present disclosure provides a non-transitory storage medium storing executable program code which, when executed by a processor, causes the method according to the fourth aspect or any of its implementation forms to be performed.

An eighth aspect of the present disclosure provides a chipset comprising a memory and a processor, which are configured to store and execute program code to perform the method according to the fourth aspect or any of its implementation forms. A ninth aspect of the present disclosure provides a computer program product comprising instructions which, when the program is executed by a computer, cause the computer to perform the method according to the fifth aspect or any of its implementation forms.

A tenth aspect of the present disclosure provides a non-transitory storage medium storing executable program code which, when executed by a processor, causes the method according to the fifth aspect or any of its implementation forms to be performed.

An eleventh aspect of the present disclosure provides a chipset comprising a memory and a processor, which are configured to store and execute program code to perform the method according to the fifth aspect or any of its implementation forms.

It has to be noted that all devices, elements, units and means described in the present disclosure could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof.

BRIEF DESCRIPTION OF DRAWINGS

The above-described aspects and implementation forms will be explained in the following description in relation to the enclosed drawings, in which

FIG. 1 shows an example of an assisting UE and a network device according to this disclosure;

FIG. 2 shows an example of SLRI signal flow according to this disclosure;

FIG. 3 shows a further example of SLRI signal flow according to this disclosure;

FIG. 4 shows a flow chart of an underlay RRM algorithm with SINR-based validation;

FIG. 5 shows a flow chart of an underlay RRM algorithm with distance-based validation;

FIG. 6 shows a diagram of a method according to the present disclosure; FIG. 7 shows a diagram of a method according to the present disclosure;

FIG. 8 shows a performance comparison between underlay mode and orthogonal mode;

FIG. 9 shows a scenario of data retransmission via sidelink; and

FIG. 10 shows examples of interferences in a communications system.

DETAILED DESCRIPTION OF EMBODIMENTS

This disclosure provides an RRM scheme tailored for sidelink-assisted retransmissions considering the URLLC Quality-of-Service (QoS) requirements. The UL underlay spectrum sharing between the D2D UEs and UL UEs is designed specifically for the reuse of radio resources allocated to sidelink-assisted retransmissions. The radio resources in this disclosure may be referred to as time-frequency radio resources. The time-frequency radio resources may span over a certain period of time slots in the time domain and over one or more sub-channels (or sub-carriers) in the frequency domain. For example, the radio resource may comprise a plurality of resource blocks. A resource block may be a smallest unit of resources that can be allocated by a base station. For instance, a resource block may take up several sub-carriers in the frequency domain and one or more slots in the time domain.

FIG. 9 shows a scenario of data retransmission via sidelink. As illustrated in FIG. 9, a target UE may experience a failed downlink transmission from a base station (e.g. an eNodeB (eNB) or gNodeB (gNB)). Neighbor node(s) that has a D2D link with the target UE may retransmit missed data packets via the D2D link to the target UE. Radio resources need to be coordinated for cellular transmission (e.g. uplink transmission) and the retransmission via sidelink (also referred to as D2D transmissions). There may be two modes for resource allocation between the cellular transmission and the D2D transmission: orthogonal mode and underlay mode. In the orthogonal mode, cellular and D2D transmissions use orthogonal time/frequency resources. In the underlay mode according to this disclosure, cellular transmission (e.g. uplink transmission) accesses the same time/frequency resources allocated for the D2D transmission (e.g. the retransmission via sidelink). That is, in the underlay mode, D2D and cellular transmissions share common radio resources (e.g., share common resource blocks).

It is noted that in this disclosure, the retransmission via sidelink has a higher priority than any other cellular transmission (e.g. uplink transmission). That is, the base station may be configured to allocate resources for the retransmission via sidelink first (i.e., prior to allocating resources for any uplink transmission). The resources allocated for the retransmission via sidelink may also be referred to as D2D resources. Then, the base station is configured to determine whether the D2D resources can be reused for uplink transmission based on the SLRI. If the D2D resources can be reused, then these D2D resources are reused for the uplink transmission. This case is referred to as the underlay mode. If the D2D resources cannot be reused (e.g., due to high interference, which is above a threshold), then the base station is configured to allocate orthogonal resources for the uplink transmission, which are orthogonal to the D2D resources. This case is referred to as the orthogonal mode.

Reuse of the D2D resources for cellular transmission (e.g. uplink transmission) in the underlay mode is only allowed if the additional interference at the base station and the target UE, respectively, remains below a certain threshold. FIG. 10 shows examples of interferences in a communications system. As recited in FIG. 10, there may be multiple interferences that need to be considered, such as intra-cell interference caused by the transmission of neighbor nodes (being in the same cell as the target UE) and intercell interference (ICI).

For facilitating the sidelink-assisted retransmission, one or more assisting UEs are selected from a group of neighbor UEs with respect to the target UE. For facilitating the sidelink-assisted retransmission, one or more assisting UEs may be selected from UEs in the vicinity of the target UE. The selected one or more UEs may also be referred to as one or more D2D UEs that have one or more D2D links with the target UE.

The modulation and coding scheme (MCS) used for the sidelink-assisted retransmission remains the same as in the original DL transmission. Therefore, the base station is capable of determining a respective D2D target SINR required at the target UE for successful decoding of the (retransmitted) packet. The base station is also aware of the positions of all active UEs or at least can infer the positions of all active UEs using conventional positioning techniques. The base station can therefore determine the link distances for the active UEs. The active UE may be referred to as a UE that has active transmission queues. However, it is noted that the assisting UE is not necessarily active at the time of retransmission request from the target UE.

FIG. 1 shows an example of an assisting UE 110 and a network device 120 according to this disclosure. The network device 120 in FIG. 1 is in connection with the assisting UE 110, and the target UE 130. The network device 120 may also be in connection with one or more uplink UEs 150.

The network device 120 may multicast downlink transmissions (that share the same content) 101 A, 101B to the target UE 130 and the assisting UE 110, respectively. At the target UE 130, the downlink transmission 101B may fail. In this case, the target UE 130 may send a retransmission request 102 to the assisting UE 110.

Optionally, the target UE 130 and the assisting UE 110 may be pre-defined in a group of UEs. In the group, there may be other UEs. The UEs in the group may be adapted to receive multicast downlink transmissions that share the same content from the network device 120. If the target UE experiences a failure in receiving its multicast downlink transmission, it may ask any other UE in the group to serve as assisting UE 110 and retransmit the packets of the failed multicast downlink transmission via a D2D link. The assisting UE 110 may be chosen by the target UE 130 based on distance or signal strength.

In this disclosure, one assisting UE 110 is discussed as an example. It is noted that more than one assisting UE 110 can be involved in the retransmission. The assisting UE 110 in FIG. 1 is for assisting a retransmission via sidelink for the target UE 130. The assisting UE 110 is configured to obtain a transmission power required for the retransmission, and send SLRI 103 comprising the transmission power to the network device 120. Correspondingly, the network device 120 is configured to receive the SLRI comprising the transmission power from the assisting UE 110, and perform resource allocation for uplink transmission in parallel with the retransmission via sidelink based on the SLRI. That is, the network device 120 is configured to perform RRM for the retransmission and uplink transmission, taking account of the required transmission power comprised in the SLRI.

For performing the resource allocation for uplink transmission in parallel with the retransmission via sidelink based on the SLRI, the network device 120 may be configured to: allocate resources for the retransmission via sidelink; determine, based on the SLRI, whether the resources for the retransmission via sidelink can be reused for the uplink transmission; and in response to determining that the resources for the retransmission via sidelink can be reused, reuse the same resources for the uplink transmission. If the network device 120 determines that the resources for the retransmission via sidelink cannot be reused based on the SLRI, the network device 120 may be configured to allocate orthogonal resources for the uplink transmission, wherein the orthogonal resources are orthogonal to the resources allocated for the retransmission via sidelink. That is, the resources for the retransmission via sidelink are not reused in this case.

Optionally, the SLRI may further comprise a retransmission ID and a path loss measurement between the assisting UE 110 and the target UE 130. The path loss measurement may relate to the distance between the assisting UE 110 and the target UE 130.

Optionally, for obtaining the transmission power, the assisting UE 110 may be configured to receive a retransmission request from the target UE. The retransmission request comprises the transmission power. Alternatively, the assisting UE 110 may be configured to compute the transmission power based on the retransmission request. For instance, the assisting UE 110 may determine the MCS of the to-be-retransmitted packet (which may be the same as the one used in the corresponding failed downlink transmission), and may calculate a target SINR based on the MCS. Then, the assisting UE 110 may calculate the transmission power based on the target SINR.

The SLRI in this disclosure contains useful elements for performing RRM for the retransmission via sidelink at the network device 120. The SLRI allows information exchange between the assisting UE 110 and the network device 120. For example, the information on the D2D link between the assisting UE 110 and the target UE 130, i.e., the transmission power, may allow the network device 120 to decide whether to use underlay mode for the resource allocation.

The assisting UE 110 may be further configured to receive resource allocation from the network device 120. The resource allocation may indicate sidelink resources allocated for the retransmission 104 via sidelink. The assisting UE 110 may be configured to perform the retransmission 104 via sidelink 104 for the target UE 130 according to the resource allocation. The retransmission 104 via sidelink may be seen as a sidelink transmission. Therefore, the assisting UE 110 may be seen as a D2D UE. Optionally, more than one assisting UE 110 may be configured to assist the retransmission 104 via sidelink. When there are two or more assisting UEs 110, the two or more assisting UEs 110 may cooperatively transmit missed packets via sidelink to the target UE 130.

Optionally, the resource allocation may be based on underlay mode. That is, the radio resources allocated by the network device 120 are shared among the retransmission 104 via sidelink (from the assisting UE 110 to the target UE 130) and uplink transmission 105 (from the one or more uplink UEs 150 to the network device 120). In this way, more uplink UEs can be supported in parallel to the retransmission via sidelink, since the retransmission via sidelink does not exclusively take up dedicated radio resources. The assisting UE 110 and the one or more uplink UEs 150 are configured to transmit respective packets using the same resources (allocated by the network device 120). For instance, on at least one particular resource block, the assisting UE 110 is configured to retransmit packets to the target UE 130, while the one or more uplink UE 150 are configured to transmit uplink packets to the network device 120 on the same particular resource block. Thus, network capacity can be increased and overall system throughput can be improved.

In this disclosure, in the underlay mode, the resource allocation (or scheduling) for the retransmission via sidelink may take precedence over the resource allocation for the uplink transmission. That is, the network device 120 may be configured to prioritize the retransmission 104 via sidelink over any other uplink transmission 105 during resource allocation.

FIG. 2 shows an example of SLRI signal flow according to this disclosure. The signal flow is between a base station (BS) 220, an assisting UE 210, a target UE 230, and one or more uplink UEs 250. In FIG 1 and FIG. 2, corresponding elements may share the same features and function likewise. The BS 220 may correspond to the network device 120 of FIG. 1; the assisting UE 210 may correspond to the assisting UE 110 of FIG. 1; the target UE 230 may correspond to the target UE 130 of FIG. 1; and the one or more uplink UEs 250 may correspond to the one or more uplink UEs 150 of FIG. 1.

In this example, the target UE 230 may be configured to compute the transmit power required for the transmission. Then, the target UE 230 may be configured to provide the transmission power to the assisting UE 210, e.g., by sending a retransmission request comprising the transmission power. After obtaining the transmission power from the target UE 230, the assisting UE 210 is configured to send SLRI comprising the transmission power to the base station 220.

The base station 220 may act as a central controller for resource allocation and performing RRM. The base station 220 may be configured to allocate resources for the retransmission via sidelink based on a scheduling algorithm that prioritizes the retransmission via sidelink. Since the MCS for the retransmission remains the same as in the corresponding downlink transmission, the base station 220 may be configured to compute the target SINR at the target UE by mapping the MCS value (or the pre-selected MCS). Then, the base station may be further configured to execute an underlay RRM algorithm. The underlay RRM algorithm is to determine, based on the SLRI received from the assisting UE, whether there is any uplink UE eligible for sharing resources that have been allocated for the retransmission via sidelink. Upon determining there is one or more uplink UE eligible for the underlay mode, the base station may be configured to allocate resources for uplink transmission of the one or more uplink UE in the underlay mode by reusing resources allocated for the retransmission via sidelink. Upon determining that there is no uplink UE eligible for the underlay mode, the base station may be configured to adopt the orthogonal resource allocation mode. That is, orthogonal resources are allocated respectively for the uplink transmission of the one or more uplink UEs and for the retransmission via sidelink.

FIG. 3 shows a further example of SLRI signal flow according to this disclosure. The signal flow is between a BS 320, an assisting UE 310, a target UE 330, and one or more uplink UEs 350. In FIG. 1 and FIG. 3, corresponding elements may share the same features and function likewise. The BS 320 may correspond to the network device 120 of FIG. 1; the assisting UE 310 may correspond to the assisting UE 110 of FIG. 1; the target UE 330 may correspond to the target UE 130 of FIG. 1; and the one or more uplink UEs 350 may correspond to the one or more further UEs 150 of FIG. 1.

In this example, followed by an unsuccessful reception of a downlink multicast transmission, the target UE 330 may be configured to send a retransmission request to an assisting UE 310. The assisting UE 310 may be configured to compute the transmit power required for the transmission and send SLRI comprising the transmission power to the base station 320. Then, the base station 320 may be configured to perform the same features as described in FIG. 2. FIG. 4 shows a flow chart of an underlay RRM algorithm with SINR-based validation. The RRM algorithm is used by the network device for determining whether underlay mode can be used for resource allocation. Therefore, the RRM algorithm in this disclosure may also be referred to as an underlay RRM algorithm.

The underlay RRM algorithm in this disclosure may be adopted by the network device as a decision maker for resource allocation mode selection between orthogonal (or non-underlay) and underlay mode. The network device may be configured to maintain or determine a list of D2D UEs D to be scheduled in a scheduling interval and a list of UL users U awaiting resources for UL transmissions in a same scheduling interval. D and U are positive integers. The list of D D2D users and the list of U UL users are provided as input of the RRM algorithm. The output of the RRM algorithm is the resource allocation of all UL UEs, where underlay resource allocation is selected if eligibility criteria are met, while otherwise conventional orthogonal resource allocation for UL users is applied. At first, the list of D D2D UEs retransmitting in the current scheduling interval may be sorted by the network device in descending order of their channel quality indicator (CQI) for the link to the target UE 130, as the highest CQI means least transmit power and thus the highest probability for the power to fulfill eligibility criteria, such as falling below the noise floor at the BS. This CQLbased ordering allows to select an assisting UE with the best D2D channel quality for evaluation of a possible underlay resource allocation with a potential UL user. A concept of the RRM algorithm is to check if a UL UE can be found for underlay for each D2D resource allocated for retransmission via sidelink. Any remaining UL users are allocated in orthogonal mode at the end.

The RRM algorithm may be mainly structured into two stages. The first stage is to identify which resources can be reused, which is determined if the received power level at the network device falls below a pre-defined threshold derived from the noise floor. The second stage is to check if a UL UE for underlay can be found which does not impose meaningful interference on the target UE 130. These two stages of the algorithm are performed while iterating through the list of U UL UEs that are to be scheduled in the scheduling interval.

In the first stage, the network device is configured to check whether an additional UL user based on the minimum SINR threshold at the network device can be afforded. The minimum SINR threshold is denoted by T] _minBS , which is defined in order to guarantee a minimum rate for a UL UE sharing the same resources as the D2D UE. Referring to FIG. 4 in combination with FIG. 10, the SINR at the BS is denoted by T _BS and is computed considering the impact of the intracell interference imposed by the underlay D2D UE in the same cell along with the inter-cell interference (ICI) induced by UEs in the neighbor cells. An example of computing r/ _BS may be as follows: 7 Y

Intra-cell interference Inter-cell interference + noise

(Eq.l)

As recited in Eq. 1, the SINR at the BS is constituted of transmit power P _u used by the selected UL user and its UL channel gain ^ _uBS , impacted by the intra-cell interference caused by the D2D UE dl (acting as the assisting UE 110) with transmit power P _dl and channel gain and the inter-cell interference caused by the UL users from the neighbor cells, Sy=i Py^yBs-

Using the SLRI signaled from the assisting UE 110 to the network device 120, the BS can be configured to calculate the intra-cell interference caused by the D2D UE as assisting UE 110, which is denoted by Pai^aiBS, based on the transmit power P _dl required for retransmission via sidelink, and the optional distance/path-loss measure between the assisting UE 110 and the target UE 130 comprised in the SLRI. If an SINR criteria (j _BS > rj _minBS ) is fulfilled, the RRM algorithm proceeds to the next stage; otherwise orthogonal resource allocation mode is selected for the selected UL UE as the BS cannot support the respective UL UE in underlay mode (note that due to the ordering of the D2D users in list D, all other D2D UEs will yield a worse SINR

In the second stage, eligibility of the UL UE 150 for underlay is validated from the target UE perspective, where the network device checks whether the target UE can tolerate the interference induced by a potential UL UE 150 with a given transmission power. For this purpose, two selection criteria are proposed in this disclosure. The first criterion is SINR-based validation, which has been explained above with respect to FIG. 4. The second criterion is distance-based validation.

It is noted that though the list of D D2D UEs is mentioned with respect to FIG. 4, this disclosure is also suitable for a case where one assisting UE is involved. In this case, the CQLbased ordering is not essential. Thus, the step of CQLbased ordering is optional. FIG. 5 shows a flow chart of an underlay RRM algorithm with distance-based validation. As an initial criterion, a dynamic minimum distance threshold denoted by d _mln given below is introduced, which mainly depends on the UL UE transmit power and ensures a minimum distance between the UL UE 150 and the target UE 130:

As recited in Eq. 2, referring to FIG. 9, the minimum distance threshold is computed by taking into account the UL transmit power P _u , the target SINR at the target = Ti _Target ) mapped from the MCS, path-loss (e.g., average channel gain) of the D2D link using the D2D path-loss and the D2D transmit power P _dl , noise power N _o , the ICI from M number of cellular UL UEs and N number of D2D UEs in the neighbor cells, and the path loss exponent a valid for the specific propagation environment (e.g., taken from the indoor factory channel model for industry 4.0 scenarios from 3GPP). M and N are positive integers.

If the distance between the UL UE 150 and the target UE 130 meets the initial minimum distance criteria (j _d > r] _min ) , a final SINR-based validation is performed to confirm the eligibility of the UL UE 150 for underlay mode. To this aim, the target SINR at the target UE (derived from the pre-defined MCS of the original DL transmission) is used as an SINR threshold (Tj _Target ) for the target UE. The D2D SINR at the target UE is computed as follows: Intra-cell interference Inter-cell interference + noise

(Eq.3)

As recited in Eq.3, the D2D SINR i? _d is computed by including the intra-cell interference from a potential UL UE (P _u <^ _udl ). Additionally, the inter-cell interference caused by the UL UE and D2D UE in the neighbor cell is illustrated by the terms respectively. If the D2D SINR is larger than the D2D target SINR, the UL UE 150 is inserted into the eligible user list U' of eligible UEs, which contains all the UL UEs 150 eligible for underlay mode with the selected D2D UE.

Referring to either FIG. 4 or FIG. 5, each UL UE is iterated by validating either through one of the two selection criteria stated above until the list of U UL UEs is empty (U = { }). The network device 120 may be further configured to select the most suitable UE for underlay mode from the available list U' of eligible UEs. A useful criterion may be to select UE u' from the list U’ yielding the maximum D2D SINR at the target UE for underlay mode in the current resources, i.e., the one causing the least UL intra-cell interference on the target UE 130. In case none of the UEs from list U can fulfill the distance and D2D target SINR criteria, no underlay can be supported and correspondingly the user list U’ will be left empty. Hence, the D2D UE is exclusively assigned the resources in orthogonal mode.

FIG. 6 shows a diagram of a method 600 according to the present disclosure.

The method 600 is for assisting a retransmission via sidelink for a target UE 130 and comprises the following steps: step 601 : obtaining, by an assisting UE 110, a transmission power required for the retransmission; and step 602: sending, by the assisting UE 110, SLRI to a network device, in which the SLRI comprises the transmission power.

The steps of the method 600 may share the same functions and details from the perspective of the UEs shown in the FIGs. 1-5 described above. Therefore, the corresponding method implementations are not described again at this point.

FIG. 7 shows a diagram of a method 700 according to the present disclosure.

The method 700 is for a retransmission via sidelink and comprises the following steps: step 701 : receiving, by a network device, SLRI from an assisting UE 110, in which the assisting UE 110 is configured to assist a retransmission via sidelink for a target UE 130, and the SLRI comprises a transmission power required for the retransmission via sidelink; and step 702: performing, by the network device 120, resource allocation for uplink transmission performed in parallel with the retransmission via sidelink based on the SLRI. The steps of the method 700 may share the same functions and details from the perspective of the UEs shown in the FIGs. 1-5 described above. Therefore, the corresponding method implementations are not described again at this point.

In summary, the aspects and implementation forms of this disclosure are based on SLRI provided by an assisting UE 110 to a network device for facilitating retransmission via sidelink for a target UE 130. The SLRI comprises a transmission power required for the retransmission, which facilitates the network device to determine whether an underlay mode can be used for resource allocation for the retransmission via sidelink and uplink transmission. In this way, radio resources can be utilized more efficiently.

FIG. 8 shows a performance comparison between underlay mode and orthogonal mode (nonunderlay mode). As shown in FIG. 8, using underlay mode may lead to substantial resource savings of 25% on average compared to non-underlay mode.

Moreover, since the D2D (or sidelink) resources are shared with UL UE 150, more UL UEs can be supported. In this way, the network capacity can be increased, thanks to the reuse of resources for UL and sidelink. In general, an improvement in system throughput can be achieved.

This disclosure can be applied to scenarios where ULLRC is required, such as, but not limited to, industrial internet of things (IIoT) and vehicle-to-everything (V2X) communications.

It is noted that the UE (including the assisting UE 110, target UE 130, and UL UE 150) and network device 120 of the present disclosure (as described above) may comprise processing circuitry configured to perform, conduct, or initiate the various corresponding operations described herein, respectively. The processing circuitry may comprise hardware and software. The hardware may comprise analog circuitry or digital circuitry, or both analog and digital circuitry. The digital circuitry may comprise components such as application-specific integrated circuits (ASICs), field-programmable arrays (FPGAs), digital signal processors (DSPs), or multi-purpose processors. The processing circuitry may comprise one or more processors and a non-transitory memory connected to the one or more processors. The non-transitory memory may carry executable program code which, when executed by the one or more processors, causes the device to perform, conduct or initiate the operations or methods described herein, respectively.

The present disclosure has been described in conjunction with various examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed matter, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.