That is to say, groupcast members who successfully received a given groupcast message, such as first wireless terminal 110, may keep a copy of data of the successfully received groupcast message in their memory, e.g., in the MAC buffer, but not send feedback, such as HARQ feedback, in case said data needs to be retransmitted to group members that have previously NACK'ed the message in question. Instead, said groupcast members who have successfully received a given groupcast message may monitor, similarly as the source transmitter, for the feedback associated with the retransmission of other peer group members.

Said groupcast members who have successfully received a given groupcast message may provide derived feedback of the retransmission(s) to the source transmitter of the groupcast message to allow the source transmitter to exploit the acquired monitoring information to identify which of the successful groupcast receivers may act as a proxy groupcast transmitter, i.e., efficient alternative retransmitter(s). Said derived feedback may be related to said monitoring, e.g., be associated with reception of the retransmission at another member. Said derived feedback may be the same as feedback received from said another member, but it may be different as well and comprise for example additional meta-data.

The source transmitter may select at least one other wireless terminal and request the at least one other wireless terminal to act as a proxy, to perform retransmissions on behalf of the source transmitter. Hence, timely and reliable delivery of groupcast data to all groupcast members can be ensured, including groupcast receivers which are out of a coverage area of the source transmitter.

With reference to FIG. 1 again, second wireless terminal 112 may identify and request first wireless terminal 110 to act as a proxy transmitter. In some embodiments, first wireless terminal 110 may either perform the delegated retransmission(s) on behalf of second wireless terminal 112, i.e., second wireless terminal 112 would not be sending additional retransmissions in such a case. Alternatively, first wireless terminal 110 may supplement retransmissions of second wireless terminal 112, i.e., first wireless terminal 110 and second wireless terminal 112 may send retransmission(s) simultaneously to help delivery of the groupcast message to third wireless terminal 114, if third wireless terminal is suffering from interruption of direct connection to second wireless terminal 112 due to blockage caused by obstacle 118.

Embodiments of the present invention may be exploited in various implementations, such as centralized, distributed, complementary, stand/alone, etc. It should be noted that zoning or range information, which may be available within SL groupcast sessions, may be too ambiguous for an efficient determination of who could become a suitable proxy for a particular groupcast member. For example, the source transmitter may not be able to use Reference Signal Receive Power, RSRP, measurements of HARQ feedback to determine how individual groupcast members are mutually interconnected with out-of-coverage UEs.

FIG. 2 illustrates a signalling graph in accordance with at least some example embodiments. With reference to FIG. 1, on the vertical axes are disposed, from the left to the right, second wireless terminal 112, first wireless terminal 110, fourth wireless terminal 116 and third wireless terminal 114. Time advances from the top towards the bottom.

At step 202, second wireless terminal 112 may transmit a groupcast message to receiving members of a group of wireless terminals, the group comprising first wireless terminal 110, third wireless terminal 114 and fourth wireless terminal 116. In some embodiments, proactive retransmissions may be pre-configured. The groupcast message may be transmitted under the requirement of reliable data delivery to all of said groupcast members. For instance, the requirement of reliable data delivery may be due to configuration of pre-defined logical channel priority level, PC5 Quality of Service, QoS, delay budget and/or maximum number of transmissions. Said receiving members of the group may be configured to provide feedback, such as HARQ feedback, to second wireless terminal 112. At step 202, all of said receiving members of the group may acknowledge reception of the groupcast message as well.

At step 204, third wireless terminal 114 may suffer from blockage. For instance, obstacle 118 may have moved so that it is now in between second wireless terminal 112 and third wireless terminal 114.

At step 206, second wireless terminal 112 may transmit another groupcast message to all wireless terminals. As third wireless terminal 114 suffers from blockage, it may not receive said another groupcast message. First wireless terminal 110 and fourth wireless terminal 116 may acknowledge reception of said another groupcast message but third wireless terminal 114 cannot acknowledge the reception due to the blockage. First wireless terminal 110 and fourth wireless terminal 116 may also forward data of the successfully received message to higher layers of the protocol stack but keep copies of said data in their memories for some time, e.g., until a keeping timer elapses or second wireless terminal 112 transmits a new groupcast transmission.

At step 208, second wireless terminal 112 may transmit a first retransmission of said another groupcast message, because it did not receive an ACK from third wireless terminal 114 or did not receive any feedback at all from third wireless terminal. The first retransmission may comprise, or be transmitted along with, an indication for feedback channel monitoring, such as PSFCH monitoring.

At step 210, proxy candidates, such as first wireless terminal 110 and fourth wireless terminal 116, may monitor a feedback channel, like PSFCH, associated with the first retransmission of second wireless terminal 112. If second wireless terminal 112 retransmits said another groupcast message upon detecting NACK or Discontinuous Transmission, DTX, feedback reception from at least one receiving wireless terminal of the group, wireless terminals that have already successfully received the initial groupcast message may monitor the feedback channel associated with the retransmission(s), i.e., do not send their own HARQ feedback on the same data anymore. That is, for example first wireless terminal 110 and fourth wireless terminal 116 may monitor the feedback channel associated with the retransmission. In some embodiments, PSFCH monitoring may start after a configurable number of counted retransmissions “Num_ReTX_entering_PSFCH_monitoring”, to determine the data reception status of peer groupcast UEs from their HARQ feedback.

In some embodiments, second wireless terminal 112 may control which receiving wireless terminals engage in the proposed retransmission scheme, e.g., to exclude incompatible wireless terminals. Wireless terminals eligible for monitoring may be determined/announced for example during a setup of a groupcast session. A flag may be used to signal that all wireless terminals must, or must not, engage in said monitoring. The flag may be an explicit command contained in a Sidelink Control Information, SCI, or a Medium Access Control, MAC, Control Element, CE.

At step 212, second wireless terminal 112 may ignore a missing positive acknowledgement concerning successful reception of the first retransmission of the groupcast transmission at first wireless terminal 110. That is, second wireless terminal 112 may disregard the missing positive acknowledgement, i.e., not to pay attention to the fact that it has not received any positive acknowledgement from first wireless terminal 110. In some embodiments, the source transmitting wireless terminal may ignore a missing HARQ feedback for the retransmission(s) from those receiving terminals which have acknowledged already the initial transmission and now perform feedback channel monitoring. Second wireless terminal 112 may be aware that first wireless terminal 110 will not provide the positive acknowledgement because first wireless terminal 110 has already acknowledged the initial transmission.

For instance, first wireless terminal 110 may refrain from acknowledging successful reception of the first retransmission of said another groupcast message to second wireless terminal 112 and skip a transmission of feedback related to reception of the first retransmission at first wireless terminal 112, because it has already ACK'ed the initial transmission.

At step 214, first wireless terminal 110 may transmit, upon receiving the first retransmission of the groupcast message, derived feedback, such as unsolicited feedback related to said monitoring. Said derived feedback may be related to reception of the first retransmission at third wireless terminal 114, and transmitted to second wireless terminal 112 by first wireless terminal 110. First wireless terminal 110 may transmit to second wireless terminal 112, e.g., unsolicited HARQ feedback associated with said ongoing retransmission(s). The unsolicited HARQ feedback may comprise an identity of first wireless terminal 110 (UE_ID), an identity of the first retransmission (re-TX_data_ID), RSRP of the first retransmission (RSRP_TX) and/or RSRP of the feedback channel (RSRP_PSFCH_RX_UE). The unsolicited HARQ feedback may be conditioned and/or delayed by a timer, such as expiry of a resolve timer which may be started when a first NACK is detected, no ACK is provided by affected third wireless terminal 114, maximum number of retransmissions by second wireless terminal 112 is reached and/or less than N NACK reports from other transmitting peer wireless terminals, such as fourth wireless terminal 116, is detected (if detectable).

The unsolicited feedback may concern wireless terminals that have NACKed the transmitted initial groupcast from second wireless terminal 112. Alternatively, or in addition, the unsolicited feedback may be delivered to second wireless terminal 112 by said wireless terminals that have ACKed the transmitted initial groupcast message. For instance, third wireless terminal 114 may be consistently in NACK state due to systemic link failure, have transitioned from ACK to NACK state (e.g., ACK may get lost), be a previously active wireless terminal which cannot be detected anymore (update on inter-connections relationships) and/or be a newly active wireless terminal which could not be observed previously (update on inter-connections relationships).

The unsolicited feedback may be transmitted using a dedicated stand-alone transmission to second wireless terminal 112, e.g., over parallel unicast, Random Access Channel, RACH, -based small data transmission, and/or appending the unsolicited feedback using code-division multiplex) to its own HARQ feedback in one of the subsequent PSFCH occasions associated with that groupcast transmission.

At step 216, second wireless terminal 112 may determine a proxy wireless terminal, such as first wireless terminal 110. Second wireless terminal 112 may use the unsolicited feedback provided by monitoring wireless terminals, such as first wireless terminal 110 and fourth wireless terminal 116, to modify the configuration of groupcast transmission and/or retransmissions. For instance, NACK-triggered retransmissions caused by a receiving wireless terminal may be stopped if the unsolicited feedback indicated an ACK by the same receiving wireless terminal, the number of retransmissions may be increased if multiple receiving wireless terminals in bad radio conditions are reported, the retransmission coding & modulation scheme may be adapted, for example lower constellation and/or lower coding may be chosen to improve Signal-to-Interference and Noise Ratio, SINR, margin if poor connectivity persists, and vice versa.

At step 218, second wireless terminal 112 may transmit a groupcast retransmission delegation request, wherein the groupcast retransmission delegation request is a request for retransmitting said data, as complementary to, or instead of, its own retransmission whenever second wireless terminal 112 fails to successfully deliver data to third wireless terminal 114. Unsuccessful reception may be indicated by a NACK or determined if no expected HARQ feedback is received from third wireless terminal 114. Unsuccessful reception may be indicated by a report on an overheard NACK event, which first wireless terminal 110 may, or may not, have received directly itself, or alternatively from another groupcast member which already ACKed the same data.

The groupcast retransmission delegation request may be transmitted to at least one proxy wireless terminal, such as first wireless terminal 110 which has reported, e.g., unsolicited HARQ indicating good signal strength to third wireless terminal 114, which has NACKed the initial transmission. Alternatively, the groupcast retransmission delegation request may be transmitted to a randomly selected receiving wireless terminal. For instance, there may be receiving wireless terminals which may have recently transitioned to ACK-state but their unsolicited feedback has not been transmitted yet, e.g., because of no or only very recent monitoring opportunity, transmitter configured reporting delay or restriction.

The groupcast retransmission delegation request may be signalled via a GroupcastDelegationRequest message comprising retransmission parameters and an identity of at least one of first wireless terminal 110 and third wireless terminal 114, which has NACKed the initial groupcast transmission, or an SCI grant transmitted by second wireless terminal 112. The retransmission parameters may indicate for example the timing and format to be used for retransmission by first wireless terminal 110. The delegation may occur independently of whether the unsolicited feedback, such as HARQ feedback, was received from a selected wireless terminal. Delegated activity may be constrained by a timer, such as a delegationValidityTimer of pre-configured duration.

When deciding to accept or reject the groupcast retransmission delegation request, first wireless terminal 110 may consider availability of data to be retransmitted (e.g., reject if no copy of data is available due to failed reception or copy deletion), connectivity to third wireless terminal 114 (e.g., accept if unicast connection to third wireless terminal 114 is already established, reject if connectivity to third wireless terminal 114 is unknown or insufficient as derived from zoning or HARQ monitoring). It should also be noted that unknown RSRP to third wireless terminal 114 may be harmless if first wireless terminal 110 only recently ACKed the initial data transmission and did not have opportunity to monitor HARQ.

Alternatively, or in addition, when deciding to accept or reject the groupcast retransmission delegation request, first wireless terminal 110 may consider own activity cycle (e.g., if first wireless terminal 110 plans to undergo SL Discontinuous Reception, DRX, inactive period), other information related to delegation validity (e.g., minimum residual value of the delegationValidityTimer) and provide feedback on relevant decision-making inputs to second wireless terminal 110 (e.g., the observed HARQ status may be provided when third wireless terminal 114 was observed to ACK past retransmissions of second wireless terminal 112 (e.g., with a spoofed L2 ID or not) to assist first wireless terminal 110 with the termination of its own re-transmissions, the measured RSRP of the observed HARQ is provided if there exist other wireless terminals with good and/or better coverage to third wireless terminal 114 to assist second wireless terminal 112 with the selection of the complementary/replacement proxy wireless terminal.

At step, 220 a response of first wireless terminal 110 to the groupcast retransmission delegation request may be signalled via a dedicated groupcast delegation accept message if first wireless terminal 110 accepts the delegation request or via a dedicated groupcast reject message if first wireless terminal 110 cannot execute the delegation request or NACKs to the SCI grant by second wireless terminal 112.

At step 222, first wireless terminal 110 may transmit a second retransmission of the groupcast message to third wireless terminal 114. In some embodiments, second wireless terminal 112 may transmit an indication to first wireless terminal 110, wherein the indication indicates a fully transparent mode. The fully transparent mode may be a mode wherein an identity of second wireless terminal 112 should be used by first wireless terminal 110 in the second retransmission of the initial groupcast message. That is, in the fully transparent mode, first wireless terminal 110 may replace its own source identity with a source identity of second wireless terminal 112 to mimic, or spoof, the groupcast transmissions of second wireless terminal 112. In such a case, other group members would not be aware of the retransmission delegation and the retransmission(s) performed by first wireless terminal 110 on behalf of second wireless terminal 112.

Second wireless terminal 112 may not provide feedback related to delegated retransmission(s) of first wireless terminal 110. Instead, second wireless terminal 112 may monitor the associated feedback channel, such as PSFCH resources, in addition to first wireless terminal 110. Similarly, first wireless terminal 110 may monitor the feedback associated with any potential retransmission of second wireless terminal 112.

The delegated retransmission task may be terminated by both, first wireless terminal 110 and second wireless terminal 112, when an ACK is received from third wireless terminal 114, maximum number of retransmission is reached, data timer expired (e.g., delegationValidityTimer) and/or explicit command from second wireless terminal 112 (applies only to first wireless terminal 110) if indicated by higher layers. First wireless terminal 110 may be configured to provide explicit feedback on the task outcome to second wireless terminal 112, such as (N)ACK received, counter exceeded, etc. Second wireless terminal 112 may use implicit feedback to first wireless terminal 110 to terminate the task (e.g., a new data transmission may be indicated).

In some embodiments, a wireless terminal does not need to report its own termination status if a termination notice is received from another wireless terminal. Second wireless terminal 112 may adopt the same criteria to terminate the task.

In some embodiments, second wireless terminal 112 may report unreachable receiving wireless terminals to higher layers (e.g., after N data transmissions are marked as failed). Vice versa, upper layers may report on groupcast exit and stop retransmissions of second wireless terminal 112.

At step 224, second wireless terminal 112 may monitor feedback. At step 226, first wireless terminal 110 may stop delegated groupcast retransmission(s).

In the example shown in FIG. 2, the initial groupcast message may be retransmitted by first wireless terminal 110 only when the retransmission task is explicitly delegated by second wireless terminal 112 to first wireless terminal 110. Second wireless terminal 112 may delegate the same retransmission event(s) (e.g., associated with delivering lost data to third wireless terminal 114) to one or more groupcast receiving wireless terminals (once-off assignment, semi-persistent/persistent schemes) while using any arbitrary multiplexing in time (random, sequential, alternating).

The retransmissions of first wireless terminal 110 and second wireless terminal 112 may be performed in orthogonal (i.e., non-overlapping) time-frequency resources. Alternatively, the retransmissions of first wireless terminal 110 and second wireless terminal 112 may be performed simultaneously in the same resources as indicated, e.g., by the SCI grant of second wireless terminal 112. Second wireless terminal 112 may indicate a redundancy version and other relevant transport block parameters that allow first wireless terminal 110 to apply an identical transmission format. This ensures that both retransmissions, of first wireless terminal 110 and second wireless terminal 112, add constructively add at a receiver (if both are received within the cyclic prefix duration).

In general, a contribution of second wireless terminal 112 to retransmission on shared resources may be expected to be small due to its poor signal strength at the affected receiver but resource sharing may be considered more resource efficient, especially when a large number of transmitting proxy wireless terminals are active.

In some embodiments, a fully autonomous retransmission by first wireless terminal 110 may be achieved without any explicit signalling by second wireless terminal 112. Transmitting proxy wireless terminals, such as first wireless terminal 110 and fourth wireless terminal 116, may deem themselves as suitably reaching other wireless terminals in poor coverage conditions compared to second wireless terminal 112 autonomously and without any unnecessary delay engage in a retransmission on behalf of second wireless terminal 112. In such a case, second wireless terminal 112 may not be aware that it is supported by an autonomous transmitting proxy wireless terminal, but reliability is further enhanced as there is no need separately request retransmissions.

The transmitting proxy wireless terminal may engage either in a stand-alone transmission on orthogonal resources or a transparent retransmission on shared resources. For retransmission in shared resources, there may be a convention around how, e.g., redundancy versions are selected so that each transmitting proxy wireless terminal may choose the right format of a retransmission without resorting to explicit signalling with second wireless terminal 112.

Suitable candidate proxies may engage in autonomous data retransmissions based on a prior configuration by second wireless terminal 112, or at least with a subsequent report to it. For example, second wireless terminal 112 may pre-configure that certain range of signal strength to both, second wireless terminal 112 and affected third wireless terminal 114, may trigger a retransmission event. This may be an efficient strategy in particular in linear topologies such as in platooning where often also the position of second wireless terminal 112 with respect to other receiving wireless terminals may be known.

FIG. 3 illustrates an example apparatus capable of supporting at least some example embodiments. Illustrated is device 300, which may comprise, for example, first wireless terminal 110 or second wireless terminal 112, or a device controlling functioning thereof, possibly when installed therein. Comprised in device 300 is processor 310, which may comprise, for example, a single- or multi-core processor wherein a single-core processor comprises one processing core and a multi-core processor comprises more than one processing core. Processor 310 may comprise, in general, a control device. Processor 310 may comprise more than one processor. Processor 310 may be a control device. Processor 310 may comprise at least one Application-Specific Integrated Circuit, ASIC. Processor 310 may comprise at least one Field-Programmable Gate Array, FPGA. Processor 310 may comprise at least one Qualcomm Snapdragon and/or Intel Atom processor. Processor 310 may comprise, for example, a Cortex-A8 processing core manufactured by ARM Holdings or a Steamroller processing core produced by Advanced Micro Devices Corporation. Processor 310 may be means for performing method steps in device 300, such as determining, causing transmitting and causing receiving. Processor 310 may be configured, at least in part by computer instructions, to perform actions.

A processor may comprise circuitry, or be constituted as circuitry or circuitries, the circuitry or circuitries being configured to perform phases of methods in accordance with example embodiments described herein. As used in this application, the term “circuitry” may refer to one or more or all of the following: (a) hardware-only circuit implementations, such as implementations in only analog and/or digital circuitry, and (b) combinations of hardware circuits and software, such as, as applicable: (i) a combination of analog and/or digital hardware circuit(s) with software/firmware and (ii) any portions of hardware processor(s) with software (including digital signal processor(s)), software, and memory(ies) that work together to cause an apparatus, such as a network function, to perform various functions) and (c) hardware circuit(s) and or processor(s), such as a microprocessor(s) or a portion of a microprocessor(s), that requires software (e.g., firmware) for operation, but the software may not be present when it is not needed for operation.

This definition of circuitry applies to all uses of this term in this application, including in any claims. As a further example, as used in this application, the term circuitry also covers an implementation of merely a hardware circuit or processor (or multiple processors) or portion of a hardware circuit or processor and its (or their) accompanying software and/or firmware. The term circuitry also covers, for example and if applicable to the particular claim element, a baseband integrated circuit or processor integrated circuit for a mobile device or a similar integrated circuit in server, a cellular network device, or other computing or network device.

Device 300 may comprise memory 320. Memory 320 may comprise random-access memory and/or permanent memory. Memory 320 may comprise at least one RAM chip. Memory 320 may comprise solid-state, magnetic, optical and/or holographic memory, for example. Memory 320 may be at least in part accessible to processor 310. Memory 320 may be at least in part comprised in processor 310. Memory 320 may be means for storing information. Memory 320 may comprise computer instructions that processor 310 is configured to execute. When computer instructions configured to cause processor 310 to perform certain actions are stored in memory 320, and device 300 overall is configured to run under the direction of processor 310 using computer instructions from memory 320, processor 310 and/or its at least one processing core may be considered to be configured to perform said certain actions. Memory 320 may be at least in part comprised in processor 310. Memory 320 may be at least in part external to device 300 but accessible to device 300.

Device 300 may comprise a transmitter 330. Device 300 may comprise a receiver 340. Transmitter 330 and receiver 340 may be configured to transmit and receive, respectively, information in accordance with at least one cellular or non-cellular standard. Transmitter 330 may comprise more than one transmitter. Receiver 340 may comprise more than one receiver. Transmitter 330 and/or receiver 340 may be configured to operate in accordance with Global System for Mobile communication, GSM, Wideband Code Division Multiple Access, WCDMA, Long Term Evolution, LTE, and/or 5G/NR standards, for example.

Device 300 may comprise a Near-Field Communication, NFC, transceiver 350. NFC transceiver 350 may support at least one NFC technology, such as Bluetooth, Wibree or similar technologies.

Device 300 may comprise User Interface, UI, 360. UI 360 may comprise at least one of a display, a keyboard, a touchscreen, a vibrator arranged to signal to a user by causing device 300 to vibrate, a speaker and a microphone. A user may be able to operate device 300 via UI 360, for example to accept incoming telephone calls, to originate telephone calls or video calls, to browse the Internet, to manage digital files stored in memory 320 or on a cloud accessible via transmitter 330 and receiver 340, or via NFC transceiver 350, and/or to play games.

Device 300 may comprise or be arranged to accept a user identity module 370. User identity module 370 may comprise, for example, a Subscriber Identity Module, SIM, card installable in device 300. A user identity module 370 may comprise information identifying a subscription of a user of device 300. A user identity module 370 may comprise cryptographic information usable to verify the identity of a user of device 300 and/or to facilitate encryption of communicated information and billing of the user of device 300 for communication effected via device 300.

Processor 310 may be furnished with a transmitter arranged to output information from processor 310, via electrical leads internal to device 300, to other devices comprised in device 300. Such a transmitter may comprise a serial bus transmitter arranged to, for example, output information via at least one electrical lead to memory 320 for storage therein. Alternatively to a serial bus, the transmitter may comprise a parallel bus transmitter. Likewise processor 310 may comprise a receiver arranged to receive information in processor 310, via electrical leads internal to device 300, from other devices comprised in device 300. Such a receiver may comprise a serial bus receiver arranged to, for example, receive information via at least one electrical lead from receiver 340 for processing in processor 310. Alternatively to a serial bus, the receiver may comprise a parallel bus receiver.

Device 300 may comprise further devices not illustrated in