Patent Description:
Groupcasting in wireless communication networks refers to transmitting messages to a group of wireless terminals. That is, at least one groupcast message may be transmitted to at least two receivers of a group. Groupcasting may be exploited in cellular communication networks, such as in networks standardized by the 3rd generation partnership project, 3GPP. For instance, groupcasting may be used in cellular communication networks operating according to <NUM> radio access technology which is currently being standardized by the 3GPP. <NUM> radio access technology may also be referred to as New Radio, NR, access technology. Groupcasting may naturally be exploited in other cellular communication networks and in non-cellular communication networks as well, such as in Wireless Local Area Networks, WLANs. In general, there is a need to provide improved methods, apparatuses and computer programs which enable reliable groupcasting in wireless communication networks. Documents <CIT>,<CIT>, <CIT> and <CIT> discuss issues related to sidelink.

Some example embodiments are defined in the dependent claims.

The example embodiments and features, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various example embodiments of the invention.

Embodiments of the present invention provide enhancements for groupcasting in cellular communication networks. More specifically, embodiments of the present invention make it possible for a first wireless terminal to receive a retransmission of a groupcast message successfully from a second wireless terminal, but if the first wireless terminal has already acknowledged successful reception of the groupcast message, the first wireless terminal may skip a transmission of feedback, wherein said feedback would be related to reception of the retransmission at the first wireless terminal, such as a positive acknowledgement. Instead of transmitting said feedback, the first wireless terminal may monitor feedback related to reception of the retransmission at a third wireless terminal and transmit derived feedback associated with reception of the retransmission at the third wireless terminal to the second wireless terminal. Said derived feedback related to reception of the retransmission at the third wireless terminal may be derived from said feedback, i.e., said derived feedback may be the same as said feedback, or said derived feedback may be different and comprise for example additional meta-data.

Hence, reliability may be improved, because the first wireless terminal waits for said feedback associated with reception of the retransmission at the third wireless before acknowledging the retransmission. The second wireless terminal is aware that the first wireless terminal performs said monitoring, because the first wireless terminal has already acknowledged reception of the groupcast message.

<FIG> illustrates an example of a communication network in accordance with at least some example embodiments. According to the example shown in <FIG>, there may be a wireless communication network comprising four wireless terminals. Said wireless terminals may be configured to communicate using direct Device-to-Device, D2D, communications, such as Sidelink, SL, communications. The wireless communication network of <FIG> comprises first wireless terminal <NUM>, second wireless terminal <NUM>, third wireless terminal <NUM> and fourth wireless terminal <NUM>. Said wireless terminals may be for example in, or attached to, separate cars. However, embodiments of the present invention may be exploited in any suitable scenario. In some embodiments, the wireless communication network may further comprise wireless network node <NUM> and core network <NUM>.

First wireless terminal <NUM> may be referred to as a proxy wireless terminal, which receives and possibly retransmits groupcast messages. Second wireless terminal <NUM> may be referred to as a source transmitter, which is an original source of said groupcast messages. Third wireless terminal <NUM> may receive groupcast messages from first wireless terminal <NUM> but not from second wireless terminal <NUM>. Fourth wireless terminal <NUM> may receive groupcast messages at least from first wireless terminal <NUM> and second wireless terminal <NUM>. First wireless terminal <NUM>, third wireless terminal <NUM> and fourth wireless terminal <NUM> may be referred to as receiving, destination wireless terminals.

First wireless terminal <NUM> and second wireless terminal <NUM> may be connected to each other via air interface <NUM>. Similarly, first wireless terminal <NUM> and third wireless terminal <NUM> may be connected to each other via air interface <NUM> as well but there may be no direct connection between second wireless terminal <NUM> and third wireless terminal <NUM>. For instance, there may be obstacle <NUM> between second wireless terminal <NUM> and third wireless terminal <NUM>. Obstacle <NUM> may be a truck for example. Third wireless terminal <NUM> may hence experience link blockage and cannot receive groupcast messages transmitted by second wireless terminal <NUM>. Due to channel reciprocity, third wireless terminal <NUM> may not able to reach second wireless terminal <NUM> with feedback, such as a decodable, Hybrid Automatic Repeat reQuest, HARQ, message.

Wireless terminals <NUM>, <NUM>, <NUM> and <NUM> may comprise, for example, a smartphone, a cellular phone, a Machine-to-Machine, M2M, node, Machine-Type Communications, MTC, node, an Internet of Things, IoT, node, a car telemetry unit, a laptop computer, a tablet computer or, indeed, any kind of suitable wireless terminal, like a User Equipment, UE. In some example embodiments, wireless network node <NUM> may be considered as a serving node for at least one wireless terminal.

Air interface <NUM> between wireless terminals <NUM>, <NUM>, <NUM> and <NUM> may be configured in accordance with a Radio Access Technology, RAT, which wireless terminals <NUM>, <NUM>, <NUM> and <NUM> are configured to support. Similarly, air interface <NUM> between for example second wireless terminal <NUM> and wireless network node <NUM> may be configured in accordance with a RAT which second wireless terminal <NUM> and wireless network node <NUM> are configured to support.

Examples of cellular RATs include Long Term Evolution, LTE, New Radio, NR, which may also be known as fifth generation, <NUM>, radio access technology and MulteFire. A cellular RAT may be standardized by the <NUM>rd Generation Partnership Project, 3GPP, for example. Hence, wireless terminals <NUM>, <NUM>, <NUM> and <NUM> may be configured to operate according to the 3GPP standards. Similarly, wireless network node <NUM> may be configured to operate according to the 3GPP standards as well. On the other hand, examples of non-cellular RATs include Wireless Local Area Network, WLAN, and Worldwide Interoperability for Microwave Access, WiMAX.

For instance, embodiments of the present invention may be applied in LTE Vehicular-to-everything, V2X, communications. In addition, embodiments of the present invention may be applied in NR SL communications.

In case of RATs, wireless terminals may be referred to as UEs while wireless network node <NUM> may be referred to as a BS. In the context of LTE, wireless network node <NUM> may be referred to as eNB while in the context of NR, wireless network node <NUM> may be referred to as gNB. In the context of WLAN, wireless network node <NUM> may be referred to as an access point while wireless terminals may be referred to as mobile stations. In any case, example embodiments are not restricted to any particular wireless technology. Instead, example embodiments may be exploited in any wireless communication network wherein reliable groupcasting is desirable.

Wireless network node <NUM> may be connected, directly or via at least one intermediate node, with core network <NUM> via wired interface <NUM>. Core network <NUM> may be, in turn, coupled via interface <NUM> with another network (not shown in <FIG>), via which connectivity to further networks may be obtained, for example via a worldwide interconnection network. Wireless network node <NUM> may be connected, directly or via at least one intermediate node, with core network <NUM> or with another core network.

Embodiments of the present invention are related to using direct D2D communications, such as SL based UE-to-UE communications like V2X and NR SL, wherein wireless terminals may transmit messages to each other without communicating via wireless network node <NUM>. A SL refers to a direct communication link between wireless terminals <NUM>, <NUM> and <NUM> in a cellular communication network. SL communications may be performed over PC5 for example, as described for example in 3GPP TS <NUM> for LTE and 3GPP TS <NUM> for NR, based on a principle of a transmitter oriented one-to-many broadcast/groupcast communication and/or one-to-one unicast communication.

In case of groupcasting, a source transmitter, such as second wireless terminal <NUM>, may transmit data simultaneously to multiple destination wireless terminals, such as first wireless terminal <NUM>, third wireless terminal <NUM> and fourth wireless terminal <NUM>. First wireless terminal <NUM>, third wireless terminal <NUM> and fourth wireless terminal <NUM> may be referred to as groupcast members. In some embodiments, the source transmitter may have minimal control over said groupcast members.

Embodiments of the present invention enable reliable groupcasting by guaranteeing delivery of all groupcast retransmissions to all group members in a timely and resource-efficient manner. Reliable delivery is particularly challenging in dense, urban-area scenarios, wherein millimetre-waves are used. At least in such scenarios groupcast members may experience blockage and/or interruption of wireless connections to the source transmitter. Also, in high mobility scenarios groupcast data delivery may fail when one or more of said receiving, destination group members move temporarily or permanently out of a coverage area of the source transmitter.

For example, in <FIG> obstacle <NUM> may be a truck which may temporarily obstruct a connection between second wireless terminal <NUM> and third wireless terminal <NUM>. For example, if second wireless terminal <NUM> is attached to a car and third wireless terminal <NUM> is attached to a pedestrian, obstacle <NUM> in between second wireless terminal <NUM> and third wireless terminal <NUM> may create_a potential safety hazard as a position of the pedestrian cannot be directly transmitted to the most relevant destination, the car. At least safety-critical vehicular applications are generally characterized by stringent expiry deadlines on groupcast data, e.g., units or tens of milliseconds, which implies the need for an efficient solution to this problem.

Groupcasting may be thus unreliable if the source transmitter, such as second wireless terminal <NUM>, is the only source of transmitted data as well as the only recipient of feedback on reception status of a group member, such as third wireless terminal <NUM>. In such a case, the source transmitter would be the only groupcast member that can solve data delivery problems and if its retransmissions are based on HARQ feedback for example, the only available repair mechanism could fail.

Embodiments of the present invention therefore make it possible for other groupcast members, such as first wireless terminal <NUM>, to assist the source transmitter with delegated data delivery to peer groupcast members that have previously Negatively Acknowledged, NACK'ed, an initial transmission from the source transmitter. More specifically, the source transmitter may request other groupcast members who have positively Acknowledged, ACK'ed, the initial transmission to keep the successfully received initial groupcast transmission in their buffer and skip their own feedback, such as Physical Sidelink Feedback Channel, PSFCH, for the first retransmission. The source transmitter may also request other groupcast members to monitor the PSFCH transmission of the at least one UE that have NACKed (or DTXed) the initial transmission and perform a groupcast retransmission on behalf of the original source transmitter. In some embodiments, DTX may mean that the intended groupcast receiving wireless terminal is configured to provide feedback, but the source transmitting wireless terminal does not get any feedback (neither ACK nor NACK) from the receiving wireless terminal. That is, DTX may mean the absence of any feedback if the receiving wireless terminal(s) are configured to provide feedback.

That is to say, groupcast members who successfully received a given groupcast message, such as first wireless terminal <NUM>, 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> again, second wireless terminal <NUM> may identify and request first wireless terminal <NUM> to act as a proxy transmitter. In some embodiments, first wireless terminal <NUM> may either perform the delegated retransmission(s) on behalf of second wireless terminal <NUM>, i.e., second wireless terminal <NUM> would not be sending additional retransmissions in such a case. Alternatively, first wireless terminal <NUM> may supplement retransmissions of second wireless terminal <NUM>, i.e., first wireless terminal <NUM> and second wireless terminal <NUM> may send retransmission(s) simultaneously to help delivery of the groupcast message to third wireless terminal <NUM>, if third wireless terminal is suffering from interruption of direct connection to second wireless terminal <NUM> due to blockage caused by obstacle <NUM>.

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

At step <NUM>, second wireless terminal <NUM> may transmit a groupcast message to receiving members of a group of wireless terminals, the group comprising first wireless terminal <NUM>, third wireless terminal <NUM> and fourth wireless terminal <NUM>. 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 <NUM>. At step <NUM>, all of said receiving members of the group may acknowledge reception of the groupcast message as well.

At step <NUM>, third wireless terminal <NUM> may suffer from blockage. For instance, obstacle <NUM> may have moved so that it is now in between second wireless terminal <NUM> and third wireless terminal <NUM>.

At step <NUM>, second wireless terminal <NUM> may transmit another groupcast message to all wireless terminals. As third wireless terminal <NUM> suffers from blockage, it may not receive said another groupcast message. First wireless terminal <NUM> and fourth wireless terminal <NUM> may acknowledge reception of said another groupcast message but third wireless terminal <NUM> cannot acknowledge the reception due to the blockage. First wireless terminal <NUM> and fourth wireless terminal <NUM> 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 <NUM> transmits a new groupcast transmission.

At step <NUM>, second wireless terminal <NUM> may transmit a first retransmisson of said another groupcast message, because it did not receive an ACK from third wireless terminal <NUM> 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 <NUM>, proxy candidates, such as first wireless terminal <NUM> and fourth wireless terminal <NUM>, may monitor a feedback channel, like PSFCH, associated with the first retransmission of second wireless terminal <NUM>. If second wireless terminal <NUM> 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 <NUM> and fourth wireless terminal <NUM> 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 <NUM> 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 <NUM>, second wireless terminal <NUM> may ignore a missing positive acknowledgement concerning successful reception of the first retransmission of the groupcast transmission at first wireless terminal <NUM>. That is, second wireless terminal <NUM> 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 <NUM>. 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 <NUM> may be aware that first wireless terminal <NUM> will not provide the positive acknowledgement because first wireless terminal <NUM> has already acknowledged the initial transmission.

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

At step <NUM>, first wireless terminal <NUM> 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 <NUM>, and transmitted to second wireless terminal <NUM> by first wireless terminal <NUM>. First wireless terminal <NUM> may transmit to second wireless terminal <NUM>, e.g., unsolicited HARQ feedback associated with said ongoing retransmission(s). The unsolicited HARQ feedback may comprise an identity of first wireless terminal <NUM> (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 <NUM>, maximum number of retransmissions by second wireless terminal <NUM> is reached and/or less than N NACK reports from other transmitting peer wireless terminals, such as fourth wireless terminal <NUM>, is detected (if detectable).

The unsolicited feedback may concern wireless terminals that have NACKed the transmitted initial groupcast from second wireless terminal <NUM>. Alternatively, or in addition, the unsolicited feedback may be delivered to second wireless terminal <NUM> by said wireless terminals that have ACKed the transmitted initial groupcast message. For instance, third wireless terminal <NUM> 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 <NUM>, 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 <NUM>, second wireless terminal <NUM> may determine a proxy wireless terminal, such as first wireless terminal <NUM>. Second wireless terminal <NUM> may use the unsolicited feedback provided by monitoring wireless terminals, such as first wireless terminal <NUM> and fourth wireless terminal <NUM>, 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 <NUM>, second wireless terminal <NUM> 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 <NUM> fails to successfully deliver data to third wireless terminal <NUM>. Unsuccessful reception may be indicated by a NACK or determined if no expected HARQ feedback is received from third wireless terminal <NUM>. Unsuccessful reception may be indicated by a report on an overheard NACK event, which first wireless terminal <NUM> 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 <NUM> which has reported, e.g., unsolicited HARQ indicating good signal strength to third wireless terminal <NUM>, 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 <NUM> and third wireless terminal <NUM>, which has NACKed the initial groupcast transmission, or an SCI grant transmitted by second wireless terminal <NUM>. The retransmission parameters may indicate for example the timing and format to be used for retransmission by first wireless terminal <NUM>. 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 <NUM> 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 <NUM> (e.g., accept if unicast connection to third wireless terminal <NUM> is already established, reject if connectivity to third wireless terminal <NUM> is unknown or insufficient as derived from zoning or HARQ monitoring). It should also be noted that unknown RSRP to third wireless terminal <NUM> may be harmless if first wireless terminal <NUM> 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 <NUM> may consider own activity cycle (e.g., if first wireless terminal <NUM> 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 <NUM> (e.g., the observed HARQ status may be provided when third wireless terminal <NUM> was observed to ACK past retransmissions of second wireless terminal <NUM> (e.g., with a spoofed L2 ID or not) to assist first wireless terminal <NUM> 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 <NUM> to assist second wireless terminal <NUM> with the selection of the complementary/replacement proxy wireless terminal.

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

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

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

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

In some embodiments, second wireless terminal <NUM> 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 <NUM>.

At step <NUM>, second wireless terminal <NUM> may monitor feedback. At step <NUM>, first wireless terminal <NUM> may stop delegated groupcast retransmission(s).

In the example shown in <FIG>, the initial groupcast message may be retransmitted by first wireless terminal <NUM> only when the retransmission task is explicitly delegated by second wireless terminal <NUM> to first wireless terminal <NUM>. Second wireless terminal <NUM> may delegate the same retransmission event(s) (e.g., associated with delivering lost data to third wireless terminal <NUM>) 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 <NUM> and second wireless terminal <NUM> may be performed in orthogonal (i.e., non-overlapping) time-frequency resources. Alternatively, the retransmissions of first wireless terminal <NUM> and second wireless terminal <NUM> may be performed simultaneously in the same resources as indicated, e.g., by the SCI grant of second wireless terminal <NUM>. Second wireless terminal <NUM> may indicate a redundancy version and other relevant transport block paratemeters that allow first wireless terminal <NUM> to apply an identical transmission format. This ensures that both retransmissions, of first wireless terminal <NUM> and second wireless terminal <NUM>, add constructively add at a receiver (if both are received within the cyclic prefix duration).

In general, a contribution of second wireless terminal <NUM> 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 <NUM> may be achieved without any explicit signalling by second wireless terminal <NUM>. Transmitting proxy wireless terminals, such as first wireless terminal <NUM> and fourth wireless terminal <NUM>, may deem themselves as suitably reaching other wireless terminals in poor coverage conditions compared to second wireless terminal <NUM> autonomously and without any unnecessary delay engage in a retransmission on behalf of second wireless terminal <NUM>. In such a case, second wireless terminal <NUM> 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 <NUM>.

Suitable candidate proxies may engage in autonomous data retransmissions based on a prior configuration by second wireless terminal <NUM>, or at least with a subsequent report to it. For example, second wireless terminal <NUM> may pre-configure that certain range of signal strength to both, second wireless terminal <NUM> and affected third wireless terminal <NUM>, 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 <NUM> with respect to other receiving wireless terminals may be known.

<FIG> illustrates an example apparatus capable of supporting at least some example embodiments. Illustrated is device <NUM>, which may comprise, for example, first wireless terminal <NUM> or second wireless terminal <NUM>, or a device controlling functioning thereof, possibly when installed therein. Comprised in device <NUM> is processor <NUM>, 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 <NUM> may comprise, in general, a control device. Processor <NUM> may comprise more than one processor. Processor <NUM> may be a control device. Processor <NUM> may comprise at least one Application-Specific Integrated Circuit, ASIC. Processor <NUM> may comprise at least one Field-Programmable Gate Array, FPGA. Processor <NUM> may comprise at least one Qualcomm Snapdragon and/or Intel Atom processor. Processor <NUM> 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 <NUM> may be means for performing method steps in device <NUM>, such as determining, causing transmitting and causing receiving. Processor <NUM> 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.

Device <NUM> may comprise a transmitter <NUM>. Device <NUM> may comprise a receiver <NUM>. Transmitter <NUM> and receiver <NUM> may be configured to transmit and receive, respectively, information in accordance with at least one cellular or non-cellular standard. Transmitter <NUM> may comprise more than one transmitter. Receiver <NUM> may comprise more than one receiver. Transmitter <NUM> and/or receiver <NUM> 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 <NUM>/NR standards, for example.

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

Device <NUM> may comprise User Interface, UI, <NUM>. UI <NUM> may comprise at least one of a display, a keyboard, a touchscreen, a vibrator arranged to signal to a user by causing device <NUM> to vibrate, a speaker and a microphone. A user may be able to operate device <NUM> via UI <NUM>, 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 <NUM> or on a cloud accessible via transmitter <NUM> and receiver <NUM>, or via NFC transceiver <NUM>, and/or to play games.

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

Device <NUM> may comprise further devices not illustrated in <FIG>. For example, where device <NUM> comprises a smartphone, it may comprise at least one digital camera. Some devices <NUM> may comprise a back-facing camera and a front-facing camera, wherein the back-facing camera may be intended for digital photography and the front-facing camera for video telephony. Device <NUM> may comprise a fingerprint sensor arranged to authenticate, at least in part, a user of device <NUM>. In some embodiments, device <NUM> lacks at least one device described above. For example, some devices <NUM> may lack a NFC transceiver <NUM> and/or user identity module <NUM>.

Processor <NUM>, memory <NUM>, transmitter <NUM>, receiver <NUM>, NFC transceiver <NUM>, UI <NUM> and/or user identity module <NUM> may be interconnected by electrical leads internal to device <NUM> in a multitude of different ways. For example, each of the aforementioned devices may be separately connected to a master bus internal to device <NUM>, to allow for the devices to exchange information. However, as the skilled person will appreciate, this is only one example and depending on the embodiment various ways of interconnecting at least two of the aforementioned devices may be selected without departing from the scope of the embodiments.

<FIG> is a flow graph of a first method in accordance with at least some example embodiments. The phases of the illustrated first method may be performed by first wireless terminal <NUM> or a device controlling functioning thereof, possibly when installed therein.

The first method may comprise, at step <NUM>, receiving, by a first wireless terminal, a first retransmission of a groupcast message successfully from a second wireless terminal. The first method may also comprise, at step <NUM>, skipping, by the first wireless terminal, a transmission of feedback related to reception of the first retransmission at the first wireless terminal. Moreover, the first method may comprise, at step <NUM>, monitoring, by the first wireless terminal, feedback from a third wireless terminal, wherein said feedback is related to reception of the first retransmission at the third wireless terminal. Finally, the first method may comprise, at step <NUM>, transmitting, by the first wireless terminal, derived feedback related to reception of the first retransmission at the third wireless terminal to the second wireless terminal.

<FIG> is a flow graph of a second method in accordance with at least some example embodiments. The phases of the illustrated second method may be performed by second wireless terminal <NUM> or a device controlling functioning thereof, possibly when installed therein.

The second method may comprise, at step <NUM>, transmitting, by a second wireless terminal, a first retransmission of a groupcast message at least to a first wireless terminal and a third wireless terminal. The second method may also comprise, at step <NUM>, disregarding by the second wireless terminal, a missing positive acknowledgement concerning successful reception of the first retransmission at the first wireless terminal. Finally, the second method may comprise, at step <NUM>, receiving, by the second wireless terminal, derived feedback from the first wireless terminal, wherein said derived feedback is related to reception of the first retransmission at the third wireless terminal.

It should also be understood that terminology employed herein is used for the purpose of describing particular example embodiments only and is not intended to be limiting.

Reference throughout this specification to one example embodiment or an example embodiment means that a particular feature, structure, or characteristic described in connection with the example embodiment is included in at least one example embodiment. Thus, appearances of the phrases "in one example embodiment" or "in an example embodiment" in various places throughout this specification are not necessarily all referring to the same example embodiment.

In addition, various example embodiments and examples may be referred to herein along with alternatives for the various components thereof. It is understood that such example embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations.

In an example embodiment, an apparatus, comprising for example first wireless terminal <NUM> or second wireless terminal <NUM>, may further comprise means for carrying out the example embodiments described above and any combination thereof. The apparatus may be an apparatus of a cellular communication network, such as a <NUM> network, and comprise means for operating in the cellular communication network.

In an example embodiment, a computer program comprising instructions which, when the program is executed by a computer, cause the computer to carry out a method in accordance with the example embodiments described above and any combination thereof. In an example embodiment, a computer program product, embodied on a non-transitory computer readable medium, may be configured to control a processor to perform a process comprising the example embodiments described above and any combination thereof.

In an example embodiment, an apparatus, comprising for example first wireless terminal <NUM> or second wireless terminal <NUM>, may further comprise at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured to, with the at least one processor, cause the apparatus at least to perform the example embodiments described above and any combination thereof. The apparatus may be an apparatus of a cellular communication network, such as a <NUM> network, and configured to operate in the cellular communication network.

Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more example embodiments. In the preceding description, numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of example embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention may be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

While the forgoing examples are illustrative of the principles of the example embodiments in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation may be made without the exercise of inventive faculty.

The expression "at least one of A or B" in this document means A, or B, or both A and B.

At least some example embodiments find industrial application in wireless communication networks, such as in cellular communication networks, like <NUM> networks, and possibly in other wireless communication networks, like WLAN, as well.

Claim 1:
A first wireless terminal, comprising:
- means for receiving, by the first wireless terminal (<NUM>), a transmission of a groupcast message successfully from a second wireless terminal (<NUM>);
- means for acknowledging, by the first wireless terminal (<NUM>), the reception of the groupcast message to the second wireless terminal (<NUM>);
- means for receiving (<NUM>), by the first wireless terminal (<NUM>), a first retransmission of the groupcast message successfully from the second wireless terminal (<NUM>);
- means for skipping (<NUM>), by the first wireless terminal (<NUM>) and based on the first wireless terminal (<NUM>) having successfully received the groupcast message, a transmission of feedback related to reception of the first retransmission at the first wireless terminal (<NUM>);
- means for monitoring (<NUM>), by the first wireless terminal (<NUM>) and based on the first wireless terminal (<NUM>) having successfully received the groupcast message, feedback from a third wireless terminal (<NUM>), wherein said feedback is related to reception of the first retransmission at the third wireless terminal (<NUM>); and
- means for transmitting (<NUM>), by the first wireless terminal (<NUM>), said feedback related to reception of the first retransmission at the third wireless terminal (<NUM>) to the second wireless terminal (<NUM>).