WO2023088574A1

WO2023088574A1 - Device and method for assisted retransmissions

Info

Publication number: WO2023088574A1
Application number: PCT/EP2021/082535
Authority: WO
Inventors: Mario Hernán CASTAÑEDA GARCIA; Richard Stirling-Gallacher; Malte Schellmann
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2021-11-22
Filing date: 2021-11-22
Publication date: 2023-05-25

Abstract

The present disclosure relates to entities and methods for assisted retransmission in groupcast transmissions. The disclosure proposes a first UE that is configured to: transmit a TB to a group of second UEs; receive a first indication from at least one second UE of the group, wherein the first indication indicates that the at least one second UE can perform a retransmission of the TB; and omit an own retransmission of the TB based on the first indication. Further, the disclosure proposes a second UE being configured to receive a TB from a first UE; send a first indication to the first UE when the second UE can perform a retransmission of the TB; and transmit the received TB to other second UEs of the group of second UEs.

Description

DEVICE AND METHOD FOR ASSISTED RETRANSMISSIONS

TECHNICAL FIELD

The present disclosure relates to communication networks, in particular, to a retransmission scheme in wireless communication networks. The disclosure introduces devices and methods for supporting an assisted retransmission in a groupcast transmission. The assisted retransmission refers to a retransmission performed by a device other than the device that originally transmitted the data.

BACKGROUND

A groupcast transmission consists of a transmission by a transmit (Tx) device to a group of receive (Rx) devices. The Tx device can be a Tx user equipment (UE). Each Rx device can be an Rx UE. In the following, a groupcast transmission from a Tx UE to Rx UEs in 5G New Radio (NR) Vehicle-to-Everything (V2X) communications is considered. Typically, the Tx UE transmits a transport block (TB) in a Physical Sidelink Shared Channel (PSSCH).

V2X communications refer to wireless communications between a vehicle and another device, including uplink and/or downlink transmissions between a vehicle and a Base Station (BS) as well as vehi cl e-to- vehicle (V2V) communications between two or more vehicles.

A transmission may be unsuccessful due to several reasons, for example, the hidden node issue, the half-duplex constraint, blockage, consecutive packet loss, low SNR, etc. Unsuccessful transmissions impact the reliability of the transmission of a TB. The reliability of transmissions can be improved with enhanced resource allocation schemes, e.g., when a UE assists in the resource selection of the Tx UE.

However, reception can still be unsuccessful even with enhanced resource allocation, as other resources selected by the Tx UE may face similar issues: e.g., persistent failure due to blockage between the Tx UE and an Rx UE (in a V2V link the loss in Rx power due to a vehicle blockage can be up to 10 dB), or low SNR at an Rx UE (i.e., due to large path loss and/or limited Tx power from the Tx UE). In NR V2X, a Tx UE can retransmit a TB up to 32 times. In the case of the previous issues, however, the reliability of a transmission may not improve with further retransmissions from the Tx UE. Therefore, an improved retransmission scheme is desired.

SUMMARY

An objective of this disclosure it thus to introduce a retransmission approach that allows the Tx UE to be aware of a retransmission that can be performed by a UE other than the Tx UE. Thereby, the Tx UE can avoid retransmitting the TB. A retransmission performed by a device or UE other than the device that originally transmitted the data, i.e. Tx UE, is referred to as an assisted retransmission. Another objective is to distinguish the assisted retransmission from a normal retransmission performed by the Tx UE. Another objective is to enable the assisted retransmission, while considering the communication range requirement of the Tx UE.

These and other objectives are achieved by the solution of the present disclosure as provided in the independent claims. Advantageous implementations are further defined in the dependent claims.

A first aspect of the disclosure provides a first UE for groupcast transmission, wherein the first UE is configured to: transmit a TB to a group of second UEs; receive a first indication from at least one second UE of the group, wherein the first indication indicates that the at least one second UE can perform a retransmission of the TB; and omit an own retransmission of the TB based on the first indication.

This disclosure accordingly proposes a retransmission approach, wherein a Tx UE, i.e., the first UE, receives an indication from an Rx UE, i.e., a second UE, if it can assist with retransmission, for instance, if the Rx UE can perform retransmission of a TB. Such an Rx UE may be referred to as an “assisting UE” in this disclosure. A retransmission of a TB performed by the assisting UE can be referred to as an “assisted retransmission”. This indication sent by the assisting UE allows the Tx UE to be aware that another UE can assist with a retransmission. This enables the Tx UE to avoid retransmitting the TB when there is an assisting UE that can perform the retransmission.

In an implementation form of the first aspect, the first UE is further configured to receive a second indication as a negative acknowledgment (NACK) for the TB, from at least one other second UE of the group, wherein the NACK indicates that the at least other second UE did not successfully receive the TB.

An Rx UE that was not successful in the reception of the TB, i.e., the at least one other second UE, sends a NACK. Notably, the at least one other second UE may have received the TB, but it may have not been able to successfully decode the TB. In such a case, it also sends the NACK. With the second indication, the Tx UE is aware that a retransmission of the TB is needed.

In an implementation form of the first aspect, the first indication comprises an acknowledgment (ACK) that is indicative that the at least one second UE successfully received the TB.

Notably, the Rx UE that sends the first indication to the Tx UE, should have successfully received the TB from the Tx UE.

In an implementation form of the first aspect, the first indication is further indicative of a reception at the at least one second UE of a NACK from at least one other second UE.

Notably, the at least one second UE is the assisting UE that can perform the retransmission of the TB. The at least one other second UE is the Rx UE that fails to successfully receive the TB. If the assisting UE receives the NACK from the at least one other second UE (where the NACK indicates that the at least one other second UE did not successfully receive the TB), it indicates that the at least one other second UE may close to the assisting UE. In other words, the chance of the at least one other second UE receiving the TB retransmitted by the assisting UE will be higher.

In an implementation form of the first aspect, the first UE is further configured to transmit a third indication to the group of second UEs, wherein the third indication indicates that a retransmission performed by a second UE of the group is supported for the TB.

Possibly, without receiving the third indication, the Rx UE which is able to successfully receive the TB may not start listening for the NACK from other Rx UEs. Therefore, the Rx UE may not perform the retransmission of the TB. In an implementation form of the first aspect, the first UE is further configured to transmit a first threshold to the group of second UEs, wherein the first threshold is to be used by one or more second UEs for determining whether the one or more second UEs can send the first indication.

For instance, if an Rx UE successfully receives the TB, and also receives a NACK from other Rx UE, the Rx UE may decide to send the first indication to the Tx UE when a receive power of the NACK is above the first threshold.

In an implementation form of the first aspect, the first UE is further configured to transmit first control information to the group of second UEs, wherein the first control information indicates one or more reserved resources for a retransmission of the TB.

In NR V2X, a Tx UE may send Sidelink Control Information (SCI) along with a TB. SCI in NR V2X is typically transmitted in two stages. Optionally, the Tx UE can indicate in the first stage SCI one or more reserved resources for a retransmission of the TB. Based on this, the Rx UEs are aware of reserved resources for the retransmission of the TB.

In an implementation form of the first aspect, the first UE is further configured to transmit second control information to the group of second UEs, wherein the second control information comprises one or more of the following:

- a source identifier of the TB,

- one or more destination identifiers of the TB,

- a coverage range associated with the first UE.

- a zone identifier associated with the first UE.

Optionally, the second stage SCI indicates the source ID and destination ID of a transmission. The source ID and the destination ID represent identifiers of the Tx UE and intended recipients of the TB (Rx UEs). The source ID allows an Rx UE to know the identity of the Tx UE. The second stage SCI can also indicate the minimum communication range and the Tx UE’s zone ID. The minimum communication range specifies the required distance for a transmission between a Tx UE and an intended Rx UE. The Tx UE’s zone ID indicates the zone in which the Tx UE is located and thus provides an indication of the Tx UE’s location. In an implementation form of the first aspect, the second control information further comprises the third indication, and/or the first threshold.

Optionally, the Tx UE indicates that whether assisted retransmissions are supported for this TB, and the threshold which is used to determine whether an Rx UE can become an assisting UE, in the second stage SCI.

In an implementation form of the first aspect, the first UE is further configured to provide configuration information to the group of second UEs, wherein the configuration information is used by one or more second UEs for determining whether the one or more second UEs can send the first indication.

Assisted retransmissions performed by an Rx UE can be enabled or disabled via configuration by the Tx UE or by the gNB. For example, an Rx UE can be (pre-)configured to decide whether it becomes an assisting UE.

In an implementation form of the first aspect, the configuration information comprises a distance threshold, wherein when a distance between a second UE and the first UE is above the distance threshold, the second UE can send the first indication.

Optionally, an Rx UE can be (pre-)configured to decide whether it becomes an assisting UE by considering the distance between the Rx UE and the Tx UE, e.g., if the distance between the Tx UE and the Rx UE is above the distance threshold.

In an implementation form of the first aspect, the configuration information further comprises a power threshold, wherein when a receive power of a reference signal from the first UE to a second UE is below the power threshold, the second UE can send the first indication.

Optionally, an Rx UE can be (pre-)configured to decide whether it becomes an assisting UE based on a condition on the Reference Signal Receive Power (RSRP) of the Tx UE, e.g., if the RSRP is below an RSRP threshold.

In an implementation form of the first aspect, the second control information further comprises the configuration information. Optionally, the configuration information is included in the second stage SCI.

A second aspect of the disclosure provides a second UE for groupcast transmission, the second UE being configured to: receive a TB from a first UE, wherein the TB is transmitted to a group of second UEs including the second UE; send a first indication to the first UE when the second UE can perform a retransmission of the TB; and transmit the received TB to other second UEs of the group of second UEs.

This disclosure proposes a retransmission approach where an Rx UE, i.e., the second UE, sends an indication if it can assist with a retransmission, i.e., if the Rx UE can perform a retransmission of a TB. Such an Rx UE may be referred to as an “assisting UE” in this disclosure. A retransmission of a TB performed by the assisting UE can be referred to as an “assisted retransmission”.

In an implementation form of the second aspect, the second UE is further configured to receive a NACK for the TB, from at least one other UE of the group of second UEs, wherein the NACK indicates that the at least one other UE did not successfully receive the TB.

Notably, an Rx UE that was not successful in the reception of the TB, i.e., the at least one other UE, sends a NACK. Notably, the at least one other UE may have received the TB, but it may have not been able to successfully decode the TB. In such a case, it also sends the NACK. When the second UE receives such a NACK, it is aware that a retransmission of the TB is needed.

In an implementation form of the second aspect, the first indication comprises an ACK that is indicative of a successful reception at the second UE of the TB.

Sending the first indication to the Tx UE indicates that the second UE can perform a retransmission of the TB. Notably, the assisting UE should have received successfully the TB from the Tx UE before retransmitting the TB.

In an implementation form of the second aspect, the first indication is indicative of a reception at the second UE of a NACK from the at least one other UE of the group of second UEs. Notably, the at least one other UE is an Rx UE that fails to successfully receive the TB. If the assisting UE, i.e., the second UE, receives the NACK from the at least one other UE (where the NACK indicates that the at least one other UE did not successfully receive the TB), it indicates that the at least one other UE may be close to the assisting UE. In other words, the chance of the at least one other second UE receiving the TB retransmitted by the assisting UE will be higher.

In an implementation form of the second aspect, the second UE is further configured to send the first indication, when a receive power of the NACK is above a first threshold.

Optionally, the assisting UE can send the first indication if it has received the NACK with a receive power above a threshold. This may further increase the reliability of the TB retransmission.

In an implementation form of the second aspect, the second UE is further configured to receive the first threshold from the first UE.

Optionally, the first threshold can be configured by the Tx UE. When the first threshold is configured by the Tx UE it can be sent in the second stage SCI that is sent with the TB in a transmission or retransmission of the TB. The first threshold can also be pre-configured.

In an implementation form of the second aspect, the second UE is further configured to receive a third indication from the first UE, wherein the third indication indicates that a retransmission performed by one second UE of the group is supported for the TB.

Possibly, without receiving the third indication, the Rx UE which is able to successfully receive the TB may not start listening for the NACK from other Rx UEs. Therefore, the Rx UE may not perform the retransmission of the TB.

In an implementation form of the second aspect, the second UE is further configured to receive first control information from the first UE, wherein the first control information indicates one or more reserved resources for a retransmission of the TB. In NR V2X, a Tx UE may send SCI along with a TB. SCI in NR V2X is typically transmitted in two stages. Optionally, the Tx UE can indicate in the first stage SCI one or more reserved resources for a retransmission of the TB. Based on this, the Rx UEs are aware of reserved resources for the retransmission of the TB.

In an implementation form of the second aspect, the second UE is further configured to transmit the received TB using the one or more reserved resources.

In this example, the assisting UE sends the assisted retransmissions in a resource reserved by the Tx UE for a retransmission of the TB. In this way, the assisting UE does not need to select a resource for the assisted retransmission.

In an implementation form of the second aspect, the second UE is further configured to send the one or more reserved resources as a part of control information when transmitting the received TB.

For the assisted retransmission, the assisting UE may indicate the resources reserved by Tx UE for the retransmissions of the TB, for instance including the reserved resources in the control information that is sent along with the received TB. In this way, other Rx UEs can be aware of resources where further retransmissions of the TB could take place.

In an implementation form of the second aspect, the second UE is further configured to receive second control information from the first UE, wherein the second control information comprises one or more of the following:

- a source identifier of the TB,

- one or more destination identifiers of the TB,

- a coverage range associated with the first UE.

- a zone identifier associated with the first UE.

Optionally, the second stage SCI indicates the source ID and destination ID of a transmission. The source ID and the destination ID represent identifiers of the Tx UE and intended recipients of the TB (Rx UEs). The source ID allows an Rx UE to know the identity of the Tx UE. The second stage SCI can also indicate the minimum communication range and the Tx UE’s zone ID. The minimum communication range specifies the required distance for a transmission between a Tx UE and an intended Rx UE. The Tx UE’s zone ID indicates the zone in which the Tx UE is located and thus provides an indication of the Tx UE’s location.

In an implementation form of the second aspect, the second control information further comprises the third indication, and/or the first threshold.

In an implementation form of the second aspect, the second UE is further configured to transmit the received TB to the other UEs of the group of second UEs together with the second control information.

For instance, in NR V2X, the second stage SCI is multiplexed with the TB in the PSSCH.

In an implementation form of the second aspect, the second UE is further configured to obtain configuration information from the first UE or from a base station, and send the first indication according to the configuration information.

Possibly, assisted retransmissions performed by an Rx UE can be enabled or disabled via configuration by the Tx UE or by the base station (e.g., gNB), when the UEs are in coverage. Further, configuring an Rx UE to be able to become or not become an assisting UE can also be done by the Tx UE or by the gNB.

In an implementation form of the second aspect, the configuration information comprises a distance threshold, wherein when a distance between the second UE and the first UE is above the distance threshold, the second UE can send the first indication.

Optionally, an Rx UE can be (pre-)configured to decide whether it becomes an assisting UE by considering the distance between the Rx UE and the Tx UE, e.g., if the distance between the Tx UE and the Rx UE is above the distance threshold. In an implementation form of the second aspect, the configuration information further comprises a power threshold, wherein when a receive power of a reference signal from the first UE to the second UE is below the power threshold, the second UE can send the first indication.

Optionally, an Rx UE can be (pre-)configured to decide whether it becomes an assisting UE based on a condition on the RSRP of the Tx UE, e.g., if the RSRP is below an RSRP threshold.

In an implementation form of the second aspect, the second control information further comprises the configuration information.

Optionally, the configuration information is included in the second stage SCI.

In an implementation form of the second aspect, the second UE is further configured to send a fourth indication indicating that the TB is transmitted by a UE different than the first UE, when performing the retransmission of the received TB.

For the assisted retransmission, this disclosure further proposes that the assisting UE sends also an indicator specifying whether the retransmission is performed by a UE different than the Tx UE, i.e., the original source of the TB. This allows other Rx UEs to be aware that it is a retransmission not performed by the Tx UE. This enables Rx UEs to have a different behavior regarding ACK feedback, i.e., Rx UEs which have already successfully received the TB do not need to send the ACK again.

In an implementation form of the second aspect, the second control information further comprises the fourth indication.

Optionally, the indicator specifying whether the retransmission is performed by a UE different than the Tx UE, may be sent in the second stage SCI, e.g., with at least one additional bit.

A third aspect of the disclosure provides a method performed by a first UE for groupcast transmission, comprising: transmitting a TB to a group of second UEs; receiving a first indication from at least one second UE of the group, wherein the first indication indicates that the at least one second UEs can perform a retransmission of the TB; and omitting an own retransmission of the TB based on the first indication. Implementation forms of the method of the third aspect may correspond to the implementation forms of the first UE of the first aspect described above. The method of the third aspect and its implementation forms achieve the same advantages and effects as described above for the first UE of the first aspect and its implementation forms.

A fourth aspect of the disclosure provides a method performed by a second UE for groupcast transmission, comprising: receiving a TB from a first UE, wherein the TB is transmitted to a group of second UEs including the second UE; sending a first indication to the first UE when the second UE can perform a retransmission of the TB; and transmitting the received TB to other second UEs of the group of second UEs.

Implementation forms of the method of the fourth aspect may correspond to the implementation forms of the second UE of the second aspect described above. The method of the fourth aspect and its implementation forms achieve the same advantages and effects as described above for the second UE of the second aspect and its implementation forms.

A fifth aspect of the disclosure provides a computer program or computer program product comprising a program code for carrying out, when implemented on a processor, the method according to the third aspect and any implementation forms of the third aspect, or the fourth aspect and any implementation forms of the fourth aspect.

It has to be noted that all devices, elements, units and means described in the present application could be implemented in 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 of specific embodiments, 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 of the present disclosure will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which:

FIG. 1 shows a groupcast transmission with an assisted retransmission;

FIG. 2 shows a first UE according to an embodiment of the disclosure;

FIG. 3 shows a second UE according to an embodiment of the disclosure;

FIG. 4 shows a groupcast transmission according to an embodiment of the disclosure;

FIG. 5 shows a groupcast transmission according to an embodiment of the disclosure;

FIG. 6 shows a groupcast transmission according to an embodiment of the disclosure;

FIG. 7 shows a UE behavior according to an embodiment of the disclosure;

FIG. 8 shows signaling exchanges between UEs according to an embodiment of the disclosure;

FIG. 9 shows a groupcast transmission according to an embodiment of the disclosure;

FIG. 10 shows a groupcast transmission according to an embodiment of the disclosure;

FIG. 11 shows a groupcast transmission according to an embodiment of the disclosure;

FIG. 12 shows a groupcast transmission according to an embodiment of the disclosure;

FIG. 13 shows a groupcast transmission according to an embodiment of the disclosure;

FIG. 14 shows a groupcast transmission according to an embodiment of the disclosure;

FIG. 15 shows a method according to an embodiment of the disclosure; and

FIG. 16 shows a method according to an embodiment of the disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

Illustrative embodiments of a first UE, a second UE and corresponding methods for supporting groupcast transmissions, particularly assisted retransmissions in groupcast transmissions, are described with reference to the figures. Although this description provides a detailed example of possible implementations, it should be noted that the details are intended to be exemplary and in no way limit the scope of the application.

Moreover, an embodiment or example may refer to other embodiments or examples. For example, any description including but not limited to terminology, element, process, explanation, and/or technical advantage mentioned in one embodiment/example is applicative to the other embodiments or examples. Notably, for groupcast transmission, the following two types of Hybrid Automatic Repeat Request (HARQ) feedback are supported for a PSSCH reception.

Acknowledgment/Negative Acknowledgement (ACK/NACK) feedback: Each Rx UE has a dedicated Physical Resource Block (PRB) for its HARQ feedback. On its dedicated PRB, each Rx UE sends NACK or ACK depending on whether the PSSCH reception was successful or not. NACK and ACK are distinguished with a different cyclic shift of a code base sequence, i.e., one code sequence for NACK and another orthogonal code sequence for ACK. Each Rx UE has a cyclic shift pair available for its ACK/NACK feedback: one cyclic shift for NACK and the other cyclic shift for ACK.

NACK-only feedback: One PRB is shared for the HARQ feedback of all Rx UEs. Rx UEs with unsuccessful reception and which are within the minimum communication range of the Tx UE, send NACK feedback on the shared PRB. Only the cyclic shift (code sequence) for NACK is used. The cyclic shift (code sequence) for ACK is not used.

The HARQ feedback is sent in a Physical Sidelink Feedback Channel (PSFCH) symbol following an offset rule depending on the PSSCH slot. Within a PSFCH symbol, the PRB and the cyclic shift pair used for the HARQ feedback of an Rx UE (for ACK/NACK and NACK- only feedback) are determined based on a Tx UE identifier. In the case of ACK/NACK feedback, the PRB and the cyclic shift pair also depend on an Rx UE identifier.

For ACK/NACK feedback, a groupcast transmission of a TB is successful if the TX UE receives ACK from all the Rx UEs. For NACK-only feedback, a groupcast transmission of a TB is successful if the TX UE does not receive a NACK from any Rx UE. Although an Rx UE may not decode successfully a TB sent by the Tx UE, a NACK sent by the Rx UE as HARQ feedback can be successfully received by the Tx UE, as the HARQ feedback is more robust.

To improve the reliability of a groupcast transmission, a retransmission performed by another UE can be considered, i.e., a retransmission not performed by the Tx UE. An Rx UE, which has successfully received a TB, can perform a retransmission. Such a retransmission may be referred to as an assisted retransmission. FIG. 1 shows a Tx UE and a number of Rx UEs (UE A, UE B, UE C, and UE D), where UE B works as an “assisting UE” to assist with the retransmission UE D.

Therefore, this disclosure proposes a retransmission approach that allows the Tx UE to be aware of an assisted retransmission that can be performed by a UE other than the Tx UE, i.e., an assisting UE, thus to avoid the TX UE retransmitting the TB. In addition, this disclosure also proposes to distinguish the assisted retransmission from normal retransmission performed by the Tx UE. Furthermore, this disclosure also enables the assisted retransmission while considering the communication range requirement of NACK-only feedback in NR V2X.

FIG. 2 shows a first UE 200 adapted for groupcast transmission according to an embodiment of the disclosure.

The first UE 200 may comprise processing circuitry (not shown) configured to perform, conduct or initiate the various operations of the first UE 200 described herein. 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 first UE 200 may further comprise memory circuitry, which stores one or more instruction(s) that can be executed by the processor or by the processing circuitry, in particular under the control of the software. For instance, the memory circuitry may comprise a non-transitory storage medium storing executable software code which, when executed by the processor or the processing circuitry, causes the various operations of the first UE 200 to be performed. In one embodiment, the processing circuitry comprises 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 first UE 200 to perform, conduct or initiate the operations or methods described herein.

The first UE 200 is configured to transmit a TB 201 to a group of second UEs 301, 302. The groupcast transmission may include a transmission from the first UE 200, i.e., a Tx UE, to a group of second UEs 301, 302, i.e., Rx UEs. The first UE 200 is configured to receive a first indication 202 from at least one second UE 301 of the group. In particular, the first indication 202 indicates that the at least one second UE 301 can perform a retransmission of the TB 201. Such a retransmission of the TB performed by a UE different than the Tx UE can be referred to as an “assisted retransmission”. Accordingly, such an Rx UE, i.e., the at least one second UE 301 that can perform the retransmission, may be referred to as an “assisting UE” in this disclosure. The first UE 200 is configured to omit an own retransmission of the TB 201 based on the first indication 202.

This disclosure proposes a retransmission approach where a Tx UE, i.e., the first second UE 200, receives an indication, i.e., the first indication 202, from an Rx UE, i.e., the at least one second UE 301, if it can become an assisting UE and assist with retransmission (if the Rx UE can perform retransmission of the TB). This indication sent by the assisting UE allows the Tx UE to be aware that another UE can assist with a retransmission. This enables the Tx UE to avoid retransmitting the TB, when there is an assisting UE that can perform the retransmission.

Optionally, the first UE 200 may be further configured to receive a second indication as a NACK for the TB 201, from at least one other second UE 302 of the group. The NACK indicates that the at least one other second UE 302 did not successfully receive the TB 201.

Notably, the at least one other second UE 302 may have received the TB, but it may have not been able to successfully decode the TB. In such a case, it also sends the NACK. With the second indication, the Tx UE is aware that a retransmission of the TB is needed.

Optionally, the first indication 202 comprises an ACK that is indicative that the at least one second UE 301 successfully received the TB 201. Notably, if the at least one second UE 301 can become an assisting UE, it should have received the TB 201 from the first UE 200.

According to an embodiment of the disclosure, the assisting UE may sends an ACK as the first indication 202. Groupcast transmissions with NACK-only feedback have less reliability compared to ACK/NACK groupcast feedback. It is considered in particularly assisted retransmissions for groupcast transmission with NACK-only feedback. For groupcast transmissions with NACK-only feedback, the assisting UE can send the ACK with the unused cyclic shift (i.e., corresponding to the ACK) from the cyclic shift pair available for the NACK- only groupcast feedback. In this way, no additional signal is required for the first indication 202. The ACK can be sent in the PSFCH (i.e., PRB) for the HARQ feedback corresponding to a groupcast transmission of the TB. Therefore, no additional resources are required for sending the first indication 202.

The first indication 202 sent by the assisting UE is received by the Tx UE. This allows the Tx UE to be aware that another UE can assist with a retransmission. This approach thus enables the Tx UE to avoid retransmitting the TB, when the assisting UE performs the retransmission.

For the assisted retransmission, the assisting UE sends a received TB, that the UE has received successfully. This has the advantage that the assisted retransmission can increase the reliability of the TB transmission.

Optionally, the first indication 202 is further indicative of a reception at the at least one second UE 301 of a NACK from at least one other second UE 302.

The at least one other second UE is the Rx UE that fails to successfully receive the TB. If the at least one second UE 301, i.e., the assisting UE, receives the NACK from the at least one other second UE 302 (where the NACK indicates that the at least one other second UE did not successfully receive the TB), it may be considered that the at least one other second UE 302 may be close to the assisting UE. In other words, the chance of the at least one other second UE 302 receiving the TB retransmitted by the the assisting UE will be higher.

Possibly, the assisting UE can send the first indication 202 if it has also received a NACK in a previous HARQ feedback for a groupcast transmission of the TB. The assisting UE can send the first indication 202 if it has received the NACK with a receive power above a threshold. This may indicate that an assisted retransmission by this assisting UE can be beneficial for an Rx UE requiring a retransmission. In the following, NACK being received with a receive power above a threshold may be referred to simply as NACK being received above a threshold or NACK above a threshold.

In NR V2X, a Tx UE sends SCI along with a TB. The SCI is sent in two stages. The first stage SCI is sent in the Physical Sidelink Control Channel (PSCCH), while the second stage SCI is multiplexed with the TB in the PSSCH. In NR V2X, the Tx UE can indicate in the first stage SCI up to 2 reserved resources for a retransmission of a TB. A Tx UE may also indicate in the first stage SCI the resource reservation interval (RRI), which indicates the interval period for a transmission of a TB or a retransmission. Based on this, the Rx UEs are aware of reserved resources for a retransmission of a TB.

Notably, the assisting UE may send the assisted retransmissions in a resource reserved by the Tx UE for a retransmission of the TB. In this way, the assisting UE does not need to select a resource for the assisted retransmission.

Optionally, the first UE 200 may be further configured to transmit a third indication to the group of second UEs 301, 302, wherein the third indication indicates that a retransmission performed by a second UE of the group is supported for the TB 201.

Possibly, the third indication enables an Rx UE which is able to successfully receive the TB to start listening for the NACK from other Rx UEs. In the case that the assisted retransmission, i.e., the retransmission performed by a UE other than the Tx RE, is not supported, the Rx UE may not perform the retransmission of the TB.

Optionally, the first UE 200 may be further configured to transmit first control information to the group of second UEs 301, 302, wherein the first control information indicates one or more reserved resources for a retransmission of the TB 201.

Optionally, the first UE 200 may be further configured to transmit second control information to the group of second UEs 301, 302, wherein the second control information comprises one or more of the following:

- a source identifier of the TB 201,

- one or more destination identifiers of the TB 201,

- a coverage range associated with the first UE 200.

- a zone identifier associated with the first UE 200.

In NR V2X, the second stage SCI indicates the source ID and destination ID of a transmission. The source ID and the destination ID represent identifiers of the Tx UE and intended recipients of the TB. The source ID allows an Rx UE to know the identity of the Tx UE, which can be used for determining the PSFCH used for the HARQ feedback. The second stage SCI can also indicate the minimum communication range and the TX UE’s zone ID. The minimum communication range specifies the required distance for a transmission between a Tx UE and an intended Rx UE. The minimum communication range depends on the service or use case. In NR V2X, the zone may comprise a square area, where the sides of the zones are configurable per minimum communication range and can be equal to for example 5, 10, 20, 30, 40, or 50 m, such that the appropriate granularity can be selected. A region is divided into zones, where each zone has a zone ID. The zone ID associated with the Tx UE indicates the zone in which the Tx UE is located and provides an indication of the Tx UE’s location.

FIG. 3 shows a second UE 301 adapted for groupcast transmission according to an embodiment of the disclosure.

The second UE 301 may comprise processing circuitry (not shown) configured to perform, conduct or initiate the various operations of the second UE 301 described herein. 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 second UE 301 may further comprise memory circuitry, which stores one or more instruction(s) that can be executed by the processor or by the processing circuitry, in particular under the control of the software. For instance, the memory circuitry may comprise a non- transitory storage medium storing executable software code which, when executed by the processor or the processing circuitry, causes the various operations of the second UE 301 to be performed. In one embodiment, the processing circuitry comprises 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 second UE 301 to perform, conduct or initiate the operations or methods described herein.

The second UE 301 is configured to receive a TB 201 from a first UE 200. Possibly, the first UE 200 may be the first UE shown in FIG. 2. The groupcast transmission may include a transmission from the first UE 200, i.e., a Tx UE, to a group of second UEs 301, 302, i.e., Rx UEs. The TB 201 is transmitted to the group of second UEs 301, 302 including the second UE 301. The second UE 301 is configured to send a first indication 202 to the first UE 200 when the second UE 301 can perform a retransmission of the TB 201. Such a retransmission of the TB performed by a UE different than the Tx UE can be referred to as an “assisted retransmission”. In other words, the first indication 202 indicates the first UE 200 that the second UE 301 can perform an assisted retransmission. Accordingly, the second UE 301 that can perform the assisted retransmission, may be referred to as an “assisting UE” in this disclosure.

The second UE 301 is further configured to transmit the received TB 201 to other second UEs 302 of the group of second UEs 301, 302.

This disclosure proposes a retransmission approach where an Rx UE, i.e., the second UE 301, sends an indication if it can assist with retransmission, i.e., if the Rx UE can perform retransmission of a TB. This indication sent by the second UE 301 allows a Tx UE, i.e., the first UE 200, to be aware that another UE can assist with a retransmission. This enables the Tx UE to avoid retransmitting the TB, when there is an assisting UE that can perform the retransmission.

Optionally, the second UE 301 may be further configured to receive a NACK for the TB 201, from at least one other UE 302 of the group of second UEs 301, 302, wherein the NACK indicates that the at least one other UE 302 did not successfully receive the TB 201.

Notably, an Rx UE that was not successful in the reception of the TB, i.e., the at least one other UE 302, sends a NACK. Notably, the at least one other UE may have received the TB, but it may have not been able to successfully decode the TB. In such a case, it also sends the NACK. When the second UE 301 receives such a NACK, it is aware that a retransmission of the TB is needed.

Optionally, the first indication 202 comprises an ACK that is indicative that the second UE 301 successfully received the TB 201. Notably, if the second UE 301 can become an assisting UE to assist the retransmission of the TB 201, it should have received successfully the TB 201 from the first UE 200.

Optionally, the first indication 202 is further indicative of a reception at the second UE 301 of a NACK from at least one other second UE 302. Possibly, the second UE 301 may be further configured to send the first indication 202, when a receive power of the NACK is above a first threshold.

Possibly, the second UE 301 can send the first indication 202 if it has also received a NACK in a previous HARQ feedback for a groupcast transmission of the TB. The second UE 301 may send the first indication 202 if it has received the NACK with a receive power above the first threshold. This may ensure the quality and reliability of the retransmission from the second UE 301 to the at least one other second UE 302. In the following, NACK being received with a receive power above a threshold may be referred to simply as NACK being received above a threshold or NACK above a threshold.

Optionally, the second UE 301 may be further configured to receive a third indication from the first UE 200, wherein the third indication indicates that a retransmission performed by one second UE of the group is supported for the TB 201.

Possibly, the third indication enables the second UE 301 which is able to successfully receive the TB to start listening for the NACK from other Rx UEs. In the case that the assisted retransmission, i.e., the retransmission performed by a UE other than the Tx RE, is not supported, an Rx UE may not perform the retransmission of the TB.

As mentioned in the previous embodiments, in NR V2X, a Tx UE sends SCI along with a TB, in two stages. The first stage SCI is sent in the PSCCH, while the second stage SCI is multiplexed with the TB in the PSSCH. In NR V2X, the Tx UE can indicate in the first stage SCI up to 2 reserved resources for a retransmission of a TB. A Tx UE may also indicate in the first stage SCI the resource reservation interval (RRI), which indicates the interval period for a transmission of a TB or for a retransmission. Based on this, the Rx UEs are aware of reserved resources for a retransmission of a TB.

Optionally, the second UE 301 may be further configured to receive first control information from the first UE 200, wherein the first control information indicates one or more reserved resources for a retransmission of the TB 201.

Optionally, the second UE 301 may be further configured to transmit the received TB 201 using the one or more reserved resources. For the assisted retransmission, this disclosure also proposes that the assisting UE indicates the resources reserved by Tx UE for the retransmissions of the TB. The assisting UE can be aware of the reserved resources based on the reserved resources indicated by the Tx UE in the first stage SCI of a previous transmission of the TB. This has the advantage that other Rx UEs are aware of resources where further retransmissions of the TB could take place.

Optionally, the second UE 301 may be further configured to send the one or more reserved resources as a part of control information when transmitting the received TB 201.

Optionally, the second UE 301 may be further configured to receive second control information from the first UE 200, wherein the second control information comprises one or more of the following:

- a source identifier of the TB 201,

- one or more destination identifiers of the TB 201,

- a coverage range associated with the first UE 200.

- a zone identifier associated with the first UE 200.

Optionally, the second UE 301 may send the assisted retransmission along with the source ID, destination ID, minimum communication range and zone ID associated with the successfully received TB. The source ID, destination ID, minimum communication range and zone ID can be taken from the second stage SCI which was sent with the successfully received TB at the second UE 301, the assisting UE. The assisting UE sending the same source ID and destination ID, allows other Rx UEs to be able to identify the sender and target of the original groupcast transmission sent by the Tx UE. As the assisting UE sends the same minimum communication range and zone ID, the assisted retransmission has the same communication range concerning the Tx UE as the TB transmission from the Tx UE. For groupcast NACK-only feedback, this means that no UEs outside the original communication range would send HARQ feedback, i.e. NACK.

Optionally, the second UE may be further configured to send a fourth indication indicating that the TB 201 is transmitted by a UE different than the first UE 200, when performing the retransmission of the received TB 201. That is, it is proposed that the assisting UE also sends an indicator specifying whether the retransmission is performed by a UE different than the Tx UE, i.e., the original source of the TB 201. This indication allows Rx UEs to be aware that it is a retransmission of a TB not performed by the Tx UE. This thus enables Rx UEs to have a different behavior regarding HARQ feedback, e.g., Rx UEs which have already successfully received the TB do not need to send HARQ feedback.

Optionally, the fourth indication, i.e., the indicator specifying whether the retransmission is performed by a UE different than the Tx UE, may be sent in the second stage SCI, e.g., with at least one additional bit. In this way, the identifier can be included in the second stage SCI which can have variable length.

FIG. 4 shows a groupcast transmission involving a Tx UE and a number of Rx UEs, i.e., UE A, UE B, UE C, UE D and UE E, according to an embodiment of the disclosure. Possibly, the Tx UE may be the first UE 200 shown in FIG. 2 or FIG. 3.

Notably, the Rx UEs in a groupcast transmission with NACK-only feedback can be classified into two sets:

- Set #1: Rx UEs which did not successfully receive the TB and which are within the minimum communication range with respect to the Tx UE,

- Set #2: Rx UEs which successfully decoded the TB.

As the example shown in FIG. 4, Set #1 includes UE D and UE E, while Set #2 includes UE A, UE B and UE C. Notably, UE D or UE E may be the at least one other UE 302 that fails to successfully receive the TB 201, as shown in FIG. 2 or FIG. 3.

In the following, a UE, which has not successfully received the TB and which is within the minimum communication range of the Tx UE, is simply referred to as a UE which has not successfully received the TB.

FIG. 5 shows an example of the proposed retransmission approach based on the groupcast transmission as shown in FIG. 4. In a first resource, i.e., the initial Tx resource, the Tx UE sends a TB in a groupcast transmission to the Rx UEs. Considering NACK-only feedback, only those Rx UEs which have not successfully decoded the TB and are within the minimum communication range send NACK. As shown in FIG. 4, UE D and UE E are in Set #1, and UE A, UE B, and UE C are in Set #2. Thus, the UEs in set #1 send NACK in the PSFCH used for the HARQ feedback of the initial transmission, while the Tx UE listens for the NACK feedback. In addition, the Rx UEs which have successfully received the TB listen for the NACK, i.e., the UEs in Set #2 listen for the NACK feedback.

Based on the NACK sent by the UEs in Set #1, the Tx UE may send a retransmission of the TB in a first reserved resource. If an Rx UE has not received successfully the retransmission of the TB (and it is within the minimum communication range of the Tx UE), it sends NACK in the PSFCH associated with the retransmission. It is proposed in this disclosure that a UE in Set #2 sends an ACK if it received a NACK (in the PSFCH associated with the initial transmission) with a receive power above a threshold. In the depicted example, it is assumed that UE B receives a NACK with a receive power above the threshold, while UE A and UE C did not receive the NACK above the threshold. UE B may be the second UE 301 as shown in FIG. 2 or FIG. 3. This means that UE B can be an assisting UE, i.e., it can perform an assisted retransmission. In addition, it is proposed that the Tx UE not only listens for NACK but also for ACK. If the Tx UE receives an ACK, it is aware that assisted retransmission can take place. In addition, the ACK reception indicates that there was an unsuccessful reception, as the ACK transmission was triggered by an Rx UE receiving a NACK. Notably, no NACK reception and no ACK reception provide doubled assurance of a successful reception.

The assisted retransmission can take place in the next resource which has been reserved by the Tx UE for a retransmission. In such a next resource, the Tx UE dose not retransmit the TB as it is aware that an assisted retransmission will take place. In addition, the assisting UE may indicate, e.g., with a 1 -bit indicator in the second stage SCI, that the retransmission of the TB is an assisted retransmission. This enables other Rx UEs, e.g., other UEs in Set #2, to be aware that it is an assisted retransmission and so they do not need to send HARQ feedback. After the assisted retransmission, the assisting UE can listen for NACK in the associated PSFCH, i.e., to enable further assisted retransmissions. This proposed procedure can be repeated, as with each retransmission occasion there may be more UEs that have successfully received the TB, which can assist with a retransmission. After the assisted retransmission performed by UE B, UE D has been successfully received the TB. Thus, UE D would now be able assist with a retransmission to UE E. Further retransmission can be performed by the assisting UE depending on the number of reserved resources which have been announced by the Tx UE. In the given example, one can have two retransmissions from the Tx UE followed by two assisted retransmissions and so on, depending on (pre-)configuration.

FIG. 6 shows an example of an assisted retransmission based on this disclosure. This example is also based on the groupcast transmission as shown in FIG. 4, while UE B acts as an assisting UE.

It should be noted that the assisting UE retransmits the TB along with the minimum communication range and the zone ID of the Tx UE. The Tx UE’s zone ID is an indication of the Tx UE’s location. As shown in FIG. 6, the region is divided into square zones, each zone having a zone ID. As the assisting UE sending the Tx UE’s zone ID, the minimum communication range with respect to the Tx UE does not change. Thus, even if other UEs which are outside the minimum communication range (with respect to the Tx UE) are able to receive the assisted retransmission, they will not send HARQ feedback, i.e., in case of NACK. In addition, the assisting UE indicates, e.g., with a 1-bit indicator, that the retransmission is an assisted retransmission. This enables that Rx UEs which previously successfully received the TB (e.g., UE A and UE C) do not send HARQ feedback for the assisted retransmission, while Rx UEs which have previously sent NACK (e.g., UE D and UE E), send HARQ feedback for the assisted retransmission, e.g., in case of NACK.

According to this disclosure, a UE can be (pre-)configured to be able to become an assisting UE. In this case, assisted retransmissions by the UE are enabled. A UE can be (pre-)configured not to become an assisting UE. In this case, assisted retransmissions by the UE are disabled. Assisted retransmissions by a UE can be enabled or disabled via configuration by the Tx UE, or by a base station such as a gNB, when the UEs are in coverage. Assisted retransmissions can also be enabled via pre-configuration. Assisted retransmission can also be configured per resource pool, e.g., some resource pools may support assisted retransmissions.

Accordingly, the second UE 301 shown in FIG. 2 or FIG. 3 may be further configured to obtain configuration information from the first UE 200 or from a base station, and send the first indication 202 according to the configuration information. The configuration to enable or disable assisted retransmissions at a UE can be done with dedicated signaling or with a groupcast signaling. The dedicated signaling involves configuring a UE to be able to become or not become an assisting UE. The groupcast signaling involves configuring a group of UEs to be able to become or not become assisting UEs. The Tx UE or gNB can indicate which UE in a group is allowed to become an assisting UE, i.e., which UEs can perform assisted retransmissions.

The configuration to enable or disable assisted retransmissions can be done per TB. A Tx UE can indicate whether the Rx UEs can perform assisted retransmissions for a given TB, i.e., for the retransmissions of the TB. The indication to enable or disable assisted retransmissions for a given TB can be included in the second stage SCI that is sent with the TB in a transmission or retransmission of the TB.

If assisted retransmissions at a UE are enabled, the UE may further decide on its own if it becomes an assisting UE or not. As disclosed in previous embodiments, an Rx UE can decide to become an assisting UE based on whether it receives a NACK above a threshold. The threshold can be configured by the Tx UE or by the gNB, when in coverage. When the threshold is configured by the Tx UE it can be sent in the second stage SCI that is sent with the TB in a transmission or retransmission of the TB. The threshold can also be pre-configured. The threshold can be based on the density of the UEs within the group.

Optionally, the threshold can also be determined at the UE based on the distance between the UE and the Tx UE. The threshold can also be determined at the UE based on the RSRP of the Tx UE, e.g., the RSRP measured with the DMRS sent by the Tx UE in PSCCH or PSCCH of a transmission or retransmission of the TB.

Furthermore, the threshold may also be adapted with each retransmission, i.e., the threshold can be decreased with each retransmission. This allows that assisted retransmissions to Rx UEs which are closer to an assisting UE are first considered.

Possibly, a UE can also be (pre-)configured to decide whether it becomes an assisting UE by considering the distance between the UE and the Tx UE, e.g., if the distance between the Tx UE and the UE is above a first distance threshold. A UE can also be (pre-)configured to decide whether it becomes an assisting UE based on a condition on the RSRP of the Tx UE, e.g., if the RSRP (of the DMRS sent in PSCCH or PSCCH) is below a first RSRP threshold.

Assisting UEs can also be (pre-)configured to first listen for ACK, before performing an assisted retransmission. Whether a UE listens for ACK can also be configured depending on the distance between the UE and the Tx UE or on the RSRP of the Tx UE, e.g., the RSRP measured with the DMRS sent by the Tx UE in PSCCH or PSCCH of a transmission or retransmission of the TB. UEs which are closer to the Tx UE, i.e., have a distance to the Tx UE below a second distance threshold or have an RSRP that is higher than a second RSRP threshold, can be configured to first listen for ACK before performing an assisted retransmission.

The first and second distance thresholds and the first and second RSRP thresholds can be configured by the Tx UE or by the gNB, when in coverage. When the first and second distance thresholds and the first and second RSRP thresholds are configured by the Tx UE, it can be sent in the second stage SCI that is sent with the TB in a transmission or retransmission of the TB. The first and second distance thresholds and the first and second RSRP thresholds can also be preconfigured.

Optionally, whether a UE listens for ACK can also be configured depending on the receive power of the received NACK.

In addition, a UE can be (pre-)configured to send twice the indication that it can perform an assisted retransmission. Another UE can be (pre-)configured to send the indication once and to additionally listen for the indication sent by another UE. For instance, considering that this indication comprises an ACK, in this case, a UE can be (pre-)configured to send the ACK twice, i.e., after two retransmissions of the TB, depending on whether the NACK is received above a second threshold. UEs which receive the NACK above the second threshold send the ACK twice, where the second threshold is larger than the threshold used to determine whether a UE becomes an assisting UE, i.e., when the NACK is above the threshold. UEs which receive the NACK below or equal to the second threshold but above the threshold, send the ACK only once and listen for the ACK of other assisting UEs before performing an assisted retransmission.

The second threshold may also be configured by the Tx UE or by the gNB, when in coverage. The second threshold can also be pre-configured. The second threshold can also be determined at the UE based on the distance between the UE and the Tx UE or based on the RSRP of the Tx UE.

The transmit power of the assisted retransmission can be determined with power control at the assisting UE. The Tx power of the assisted retransmission can be determined based on the receive power of the NACK. The power at the assisting UE for the assisted retransmission can also be determined based on the distance to the Tx UE or on the RSRP of the Tx UE. The Tx power can also be increased with each assisted retransmission.

FIG. 7 shows a flow chart of how an Rx UE in a groupcast transmission acts according to an embodiment of the disclosure. In this example, a UE first checks if it successfully received a TB, i.e., if the UE is able to decode the TB. If this is not the case, the UE sends a NACK. If the UE is able to decode the TB, it then determines whether it is allowed to send assisted retransmissions, i.e., whether it has been (pre-)configured to become an assisting UE. If the assisted retransmissions by the UE are enabled, then the UE checks whether it receives a NACK for the TB with a receive power above a threshold. If this is the case, the UE sends ACK as an indication that it can assist with a retransmission. In addition, the UE sends an assisted retransmission of the TB.

FIG. 8 shows signaling exchanges between UEs in a groupcast transmission. Possibly, the Tx UE may be the first UE 200 shown in FIG. 2 or FIG. 3. The Tx UE sends a TB in a groupcast transmission. The Tx UE also indicates that assisted retransmissions are supported for this TB. This indication can be sent in the second stage SCI sent with the TB. In addition, the Tx UE indicates the threshold which is used to determine whether a UE becomes an assisting UE or not. The threshold can be sent in the second stage SCI sent with the TB.

Afterward, the UE in Set #1, which was not able to successfully decode the TB, sends a NACK, where UE in Set #1 may be the at least one other second UE 302 shown in FIG. 2 or FIG. 3. The Tx UE and the UE in Set #2 which is able to successfully receive the TB listen for the NACK. Possibly, the UE in Set #2 may be the second UE 301 shown in FIG. 2 or FIG. 3 which can become an assisting UE. If the UE which successfully received the TB, receives the NACK above the threshold, the UE sends an ACK to the Tx UE. The UE which successfully received the TB then performs the assisted retransmission of the TB, while the Tx UE does not perform the retransmission.

FIG. 9 shows an example of how UEs perform in the groupcast transmission according to an embodiment of this disclosure. A UE in Set #2 sends ACK (indicating it can be an assisting UE) after it has received a NACK in the PSFCH of a previous transmission of a TB. The assisting UE sends the ACK in the PSFCH of a retransmission of the TB. When the assisting UE sends the ACK, the Tx UE listens for the ACK. If the Tx UE receives an ACK, it knows that an assisting retransmission can take place in the next reserved resource for a retransmission of the TB (e.g., on the second reserved resource shown in FIG. 9). For that next resource, the Tx UE does not transmit the TB and the assisting UE transmits the TB. In the PSFCH of the assisted retransmission of the TB, the assisting UE listens for NACK, i.e., to determine if the assisted retransmission was successful. For example, this can be detected if the receive power of the NACK for the assisted retransmission is less than the receive power of the NACK which triggered the assisting UE to send the ACK. Depending on (pre-)configuration, further assisted retransmissions from the assisting UE can be performed. In addition, the assisted retransmissions can be alternated with retransmissions from the Tx UE, where further reserved resources for retransmissions of the TB can be indicated.

Based on this embodiment shown in FIG. 9, the Tx UE is aware that an Rx UE can assist in a retransmission and hence, the Tx UE can avoid retransmitting the TB. However, the assisted retransmission by the assisting UE may no longer be needed, if no UE sends NACK while the assisting UE sends ACK. This can happen due to the half-duplex issue during the PSFCH, where the assisting UE is not able to listen for NACK when it sends ACK.

FIG. 10 shows another example of how UEs perform in the groupcast transmission according to an embodiment of this disclosure. In this example, a UE in Set #2 sends ACK (indicating it can be an assisting UE), after it has received a NACK in the PSFCH of a previous transmission of a TB. The assisting UE sends the ACK in the PSFCH of a retransmission of the TB. In addition, the assisting UE listens again for NACK before performing the assisted retransmission (e.g., in the PSFCH associated with the second reserved resource). When the assisting UE sends the ACK, the Tx UE listens for the ACK. If the Tx UE receives an ACK, it knows that an assisting retransmission can take place in the second next reserved resource for a retransmission of the TB (e.g., on the third reserved resource). If no NACK is received by the assisting UE, when the assisting UE listens again for the NACK before performing the retransmission, the assisting UE does not perform the assisted retransmission.

Based on this embodiment shown in FIG. 10, the Tx UE is aware that an Rx UE can assist in a retransmission and hence, the Tx UE can avoid retransmitting the TB. In addition, this embodiment ensures that the assisting UE can avoid unnecessary retransmission. However, the assisted retransmission is delayed by one transmission opportunity. If no NACK is received by the Tx UE, when the assisting UE listens again for the NACK before performing the assisted retransmission, the Tx UE is also aware that a retransmission is not necessary.

FIG. 11 shows another example of how UEs perform in the groupcast transmission according to an embodiment of this disclosure. In this example, it is assumed that there are two different types of UEs in Set #2, i.e. a UE in Set #2A and a UE in Set #2B, where the sets will be detailed in the following. The UEs in Set #2A and Set #2B send ACK, after they have received a NACK in the PSFCH of a previous transmission of a TB. The UEs in Set #2A and Set #2B send the ACK in the PSFCH of a retransmission of the TB, while the Tx UE listens for the ACK. If the Tx UE receives an ACK, it knows that an assisting retransmission can take place. For that next resource, the Tx UE retransmits the TB. In the PSFCH associated with that retransmission, the UEs which have successfully received the TB will send again ACK or listen for ACK as follows. If a UE received the NACK (e.g., in the PSFCH associated with the initial transmission) above the threshold and also above a second threshold, the UE sends again ACK. Such a UE is considered to be in Set #2A. If a UE received the NACK (e.g., in the PSFCH associated with the initial transmission) above the threshold but below or equal to a second threshold, the UE listens for ACK. Such a UE is considered to be in Set #2B. If the UE in Set #2B receives the ACK with a receive power above a third threshold, the UE in Set #2B does not perform an assisted retransmission, as it is aware that a UE in Set #2A has received a NACK with a higher receive power. In this case, the assisted retransmission is performed by the UE in Set #2A, while the Tx UE or the UE in Set #2B dos does not send the retransmission.

In this embodiment, it can be avoided that nearby UEs in Set #2 send an assisted retransmission simultaneously. With the proposed approach, only the UEs which have received a stronger NACK would perform the assisted retransmission. FIG. 12 shows another example of how UEs perform in the groupcast transmission according to an embodiment of this disclosure. In this example, a UE in Set #2 sends ACK (indicating it can be an assisting UE), without having received a NACK, e.g. in the PSFCH of a previous transmission of a TB. The assisting UE sends the ACK in the PSFCH of a transmission of the TB (or of a retransmission of the TB, depending on (pre-)configuration). When the assisting UE sends the ACK, the Tx UE listens for the ACK. If the Tx UE receives an ACK, it knows that an assisting retransmission can take place in the next reserved resource (e.g. on the first reserved resource).

Based on this embodiment, the Tx UE is aware that an Rx UE can assist in a retransmission and hence, the Tx UE can avoid retransmitting the TB. In addition, the assisted retransmission can take place earlier (i.e. with less delay). However, the assisted retransmission may not be received by a UE in Set #1, i.e. in case, the assisting UE is not near a UE that needs a retransmission. Furthermore, the next reserved resource may not be able to be released in case the Tx UE receives ACK and no NACK.

FIG. 13 shows another example of how UEs perform in the groupcast transmission according to an embodiment of this disclosure. In this example, a UE in Set #2 sends ACK (indicating it can be an assisting UE), without having received a NACK, e.g. in the PSFCH of a previous transmission of a TB. The assisting UE sends the ACK in the PSFCH of a transmission of the TB (or of a retransmission of the TB, depending on (pre-)configuration). In addition, the assisting UE listens again for NACK before performing the assisted retransmission (e.g. in the PSFCH associated with the first reserved resource). When the assisting UE sends the ACK, the Tx UE listens for the ACK. If the Tx UE receives an ACK, it knows that an assisting retransmission can take place in the second next reserved resource for a retransmission of the TB (e.g. on the second reserved resource). If no NACK is received by the assisting UE, when the assisting UE listens again for the NACK before performing the retransmission, the assisting UE does not perform the assisted retransmission.

Based on this embodiment, the Tx UE is aware that an Rx UE can assist in a retransmission and hence, the Tx UE can avoid retransmitting the TB. In addition, the assisting UE can avoid unnecessary retransmission. If no NACK is received by the Tx UE, when the assisting UE listens for the NACK before performing the assisted retransmission, the Tx UE is also aware that a retransmission is not necessary. FIG. 14 shows another example of how UEs perform in the groupcast transmission according to an embodiment of this disclosure. In this example, two different types of UEs in Set #2 are considered. For example, Set #2A and Set #2B are determined depending on the distance of a UE to the Tx UE or depending on the RSRP of the Tx UE. It is considered that UEs in Set #2A correspond to UEs with an RSRP of the Tx UE above an RSRP threshold, while UEs in Set #2B correspond to UEs with an RSRP of the Tx UE below or equal to the RSRP threshold. UEs in Set #2A are configured to first send ACK if they have successfully received the TB and they can assist with a retransmission. UEs in Set #2B are configured to first listen for NACK if they have successfully received the TB and they can assist with a retransmission. For the PSFCH of the first retransmission, UEs in Set #2B are configured to send ACK if they received the NACK above a threshold. On the other hand, for the PSFCH of the first retransmission, UEs in Set #2A are configured to listen for ACK and NACK. If the UE in Set #2A receives an ACK, e.g. above a second threshold, it does not perform an assisted retransmission, as it is aware that another UE can perform an assisted retransmission which can be received by a UE which sent NACK. The UE in Set #2B is configured to perform the assisted retransmission in the next reserved resource for a retransmission of the TB. If the UE in Set #2 A does not receive an ACK, then it can perform an assisted retransmission. If the UE in Set #2A receives a NACK and receives an ACK, it could be configured to perform the assisted retransmission, depending on the receive power of the NACK and the receive power of the ACK.

In this embodiment, it can be avoided that UEs in Set #2 send an assisted retransmission simultaneously.

FIG. 15 shows a method 1500 according to an embodiment of the disclosure, particularly for groupcast transmission. In a particular embodiment, the method 1500 is performed by a first UE 200 shown in FIG. 2 or FIG. 3. The method 1500 comprises a step 1501 of transmitting a TB 201 to a group of second UEs 301, 302. The method 1800 further comprises a step 1802 of receiving a first indication 202 from at least one second UE 301 of the group. In particular, the first indication 202 indicates that the at least one second UE 301 can perform a retransmission of the TB 201. Further, the method step 1500 comprises a step 1503 of omitting an own retransmission of the TB 201 based on the first indication 202. FIG. 16 shows a method 1600 according to an embodiment of the disclosure, particularly for groupcast transmission. In a particular embodiment, the method 1600 is performed by a second UE 301 shown in FIG. 2 or FIG. 3. The method 1600 comprises a step 1601 of receiving a TB 201 from a first UE 200. Notably, the TB 201 is transmitted to a group of second UEs 301, 302 including the second UE. Possibly, the first UE may be the first UE 200 shown in FIG. 2 or FIG. 3.

The method 1600 comprises a step 1602 of sending a first indication 202 to the first UE 200 when the second UE can perform a retransmission of the TB 201. The method 1600 further comprises a step 1603 of transmitting the received TB 201 to other second UEs 302 of the group of second UEs 301, 302.

The present disclosure has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed embodiments of the disclosure, 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.

Furthermore, any method according to embodiments of the disclosure may be implemented in a computer program, having code means, which when run by processing means causes the processing means to execute the steps of the method. The computer program is included in a computer-readable medium of a computer program product. The computer-readable medium may comprise essentially any memory, such as a ROM (Read-Only Memory), a PROM (Programmable Read-Only Memory), an EPROM (Erasable PROM), a Flash memory, an EEPROM (Electrically Erasable PROM), or a hard disk drive.

Moreover, it is realized by the skilled person that embodiments of the first UE 200, the second core network entity 302, the third core network entity 303, or the access network entity 401, comprise the necessary communication capabilities in the form of e.g., functions, means, units, elements, etc., for performing the solution. Examples of other such means, units, elements, and functions are: processors, memory, buffers, control logic, encoders, decoders, rate matchers, de-rate matchers, mapping units, multipliers, decision units, selecting units, switches, interleavers, de-interleavers, modulators, demodulators, inputs, outputs, antennas, amplifiers, receiver units, transmitter units, DSPs, trellis-coded modulation (TCM) encoder, TCM decoder, power supply units, power feeders, communication interfaces, communication protocols, etc. which are suitably arranged together for performing the solution.

Especially, the processor(s) of the first UE 200, the second core network entity 302, the third core network entity 303, or the access network entity 401 may comprise, e.g., one or more instances of a Central Processing Unit (CPU), a processing unit, a processing circuit, a processor, an Application Specific Integrated Circuit (ASIC), a microprocessor, or other processing logic that may interpret and execute instructions. The expression “processor” may thus represent a processing circuitry comprising a plurality of processing circuits, such as, e.g., any, some or all of the ones mentioned above. The processing circuitry may further perform data processing functions for inputting, outputting, and processing of data comprising data buffering and device control functions, such as call processing control, user interface control, or the like.

Claims

1. A first user equipment, UE (200), for groupcast transmission, the first UE (200) being configured to: transmit a transport block (201) to a group of second UEs (301, 302); receive a first indication (202) from at least one second UE (301) of the group, wherein the first indication (202) indicates that the at least one second UE (301) can perform a retransmission of the transport block (201); and omit an own retransmission of the transport block (201) based on the first indication (202).

2. The first UE (200) according to claim 1, further configured to: receive a second indication as a negative acknowledgment, NACK, for the transport block (201), from at least one other second UE (302) of the group, wherein the NACK indicates that the at least one other second UE (302) did not successfully receive the transport block

(201).

3. The first UE (200) according to claim 1 or 2, wherein the first indication (202) comprises an acknowledgment, ACK, that is indicative that the at least one second UE (301) successfully received the transport block (201).

4. The first UE (200) according to one of the claims 1 to 3, wherein the first indication

(202) is further indicative of a reception at the at least one second UE (301) of a NACK from at least one other second UE (302).

5. The first UE (200) according to one of the claims 1 to 4, further configured to: transmit a third indication to the group of second UEs (301, 302), wherein the third indication indicates that a retransmission performed by a second UE of the group is supported for the transport block (201).

34

6. The first UE (200) according to one of the claims 1 to 5, configured to: transmit a first threshold to the group of second UEs (301, 302), wherein the first threshold is to be used by one or more second UEs for determining whether the one or more second UEs can send the first indication (202).

7. The first UE (200) according to one of the claims 1 to 6, further configured to: transmit first control information to the group of second UEs (301, 302), wherein the first control information indicates one or more reserved resources for a retransmission of the transport block (201).

8. The first UE (200) according to one of the claims 1 to 7, further configured to: transmit second control information to the group of second UEs (301, 302), wherein the second control information comprises one or more of the following: a source identifier of the transport block (201), one or more destination identifiers of the transport block (201), a coverage range associated with the first UE (200). a zone identifier associated with the first UE (200).

9. The first UE (200) according to one of the claim 5 or 6, and claim 8, wherein the second control information further comprises the third indication, and/or the first threshold.

10. The first UE (200) according to one of the claims 1 to 9, further configured to: provide configuration information to the group of second UEs (301, 302), wherein the configuration information is used by one or more second UEs for determining whether the one or more second UEs can send the first indication (202).

11. The first UE (200) according to claim 10, wherein the configuration information comprises a distance threshold, wherein when a distance between a second UE and the first UE (200) is above the distance threshold, the second UE can send the first indication (202).

12. The first UE (200) according to claim 10 or 11, wherein the configuration information further comprises a power threshold, wherein when a receive power of a reference signal from the first UE (200) to a second UE is below the power threshold, the second UE can send the first indication (202).

35

13. The first UE (200) according to claim 8 or 9, and one of the claims 10 to 12, wherein the second control information further comprises the configuration information.

14. A second user equipment, UE (301), for groupcast transmission, the second UE (301) being configured to: receive a transport block (201) from a first UE (200), wherein the transport block (201) is transmitted to a group of second UEs (301, 302) including the second UE (301); send a first indication (202) to the first UE (200) when the second UE (301) can perform a retransmission of the transport block (201); and transmit the received transport block (201) to other second UEs (302) of the group of second UEs (301, 302).

15. The second UE (301) according to claim 14, further configured to: receive a negative acknowledgement, NACK, for the transport block (201), from at least one other UE (302) of the group of second UEs (301, 302), wherein the NACK indicates that the at least one other UE (302) did not successfully receive the transport block (201).

16. The second UE (301) according to one of the claims 14 or 15, wherein the first indication (202) comprises an acknowledgement, ACK, that is indicative of a successful reception at the second UE (301) of the transport block (201).

17. The second UE (301) according to one of the claims 14 to 16, wherein the first indication (202) is indicative of a reception at the second UE (301) of a NACK from the at least one other UE (302) of the group of second UEs (301, 302).

18. The second UE (301) according to one of the claims 15 to 17, further configured to: send the first indication (202), when a receive power of the NACK is above a first threshold.

19. The second UE (301) according to claim 18, further configured to: receive the first threshold from the first UE (200).

20. The second UE (301) according to one of the claims 14 to 19, further configured to: receive a third indication from the first UE (200), wherein the third indication indicates that a retransmission performed by one second UE of the group is supported for the transport block (201).

21. The second UE (301) according to one of the claims 14 to 20, configured to: receive first control information from the first UE (200), wherein the first control information indicates one or more reserved resources for a retransmission of the transport block (201).

22. The second UE (301) according to claim 21, further configured to: transmit the received transport block (201) using the one or more reserved resources.

23. The second UE (301) according to claim 21 or 22, further configured to: send the one or more reserved resources as a part of control information when transmitting the received transport block (201).

24. The second UE (301) according to one of the claims 14 to 23, further configured to: receive second control information from the first UE (200), wherein the second control information comprises one or more of the following: a source identifier of the transport block (201), one or more destination identifiers of the transport block (201), a coverage range associated with the first UE (200). a zone identifier associated with the first UE (200).

25. The second UE (301) according to one of the claim 19 or 20, and claim 24, wherein the second control information further comprises the third indication, and/or the first threshold.

26. The second UE (301) according to claim 24 or 25, further configured to: transmit the received transport block (201) to the other UEs (302) of the group of second UEs (301, 302) together with the second control information.

27. The second UE (301) according to one of the claims 14 to 26, further configured to: obtain configuration information from the first UE (200) or from a base station, and send the first indication (202) according to the configuration information.

28. The second UE (301) according to claim 27, wherein the configuration information comprises a distance threshold, wherein when a distance between the second UE (301) and the first UE (200) is above the distance threshold, the second UE (301) can send the first indication (202).

29. The second UE (301) according to claim 27 or 28, wherein the configuration information further comprises a power threshold, wherein when a receive power of a reference signal from the first UE (200) to the second UE (301) is below the power threshold, the second UE (301) can send the first indication (202).

30. The second UE (301) according to one of the claims 24 to 26, and one of the claims 27 to 29, wherein the second control information further comprises the configuration information.

31. The second UE according to one of the claims 14 to 30, further configured to: send a fourth indication indicating that the transport block (201) is transmitted by a UE different than the first UE (200), when performing the retransmission of the received transport block (201).

32. The second UE (301) according to claim 31 and one of the claims 24 to 26, wherein the second control information further comprises the fourth indication.

33. A method (1500) performed by a first user equipment, UE (200), for groupcast transmission, the method (1500) comprising: transmitting (1501) a transport block (201) to a group of second UEs (301, 302); receiving (1502) a first indication (202) from at least one second UE (301) of the group, wherein the first indication (202) indicates that the at least one second UE (301) can perform a retransmission of the transport block (201); and omitting (1503) an own retransmission of the transport block (201) based on the first indication (202).

38

34. A method (1600) performed by a second user equipment, UE (301), for groupcast transmission, the method comprising: receiving (1601) a transport block (201) from a first UE (200), wherein the transport block (201) is transmitted to a group of second UEs (301, 302) including the second UE; sending (1602) a first indication (202) to the first UE (200) when the second UE can perform a retransmission of the transport block (201); and transmitting (1603) the received transport block (201) to other second UEs (302) of the group of second UEs (301, 302). 35. A computer program product comprising computer readable code instructions which, when run in a computer will cause the computer to perform the method (1500, 1600) according to claim 33 or 34.

39