Patent Description:
In Third Generation Partnership Project (3GPP) standards for wireless communication, every individual member of a group may need to give a report of every packet that is sent whether received or lost. This may create a large amount of overhead.

A wireless transmit/receive unit (WTRU) may receive a downlink communication from a network over a first interface transmitted to one or more WTRUs in a group of WTRUs. The WTRU may determine an access class of the WTRU based on a packet loss percentage of the downlink communication. The access class may be associated with a contention window for accessing a second interface. The WTRU may transmit packet loss information to the one or more WTRUs over the second interface in the contention window. The WTRU may receive packet loss feedback from the one or more WTRUs over the second interface. The WTRU may determine that the access class of the WTRU is a highest class of the one or more WTRUs. The WTRU may transmit a single groupcast negative acknowledgement (gNACK) to the network on behalf of the one or more WTRUs over the first interface based on the packet loss feedback. <CIT> discloses a method for compressed hybrid automatic repeat request feedback for device to device cluster communications. <CIT> discloses a system and method for multicast/broadcast reliability enhancement over wireless LANs. <CIT> discloses a method and apparatus for co-operative reception for network controlled device to device.

For example, the communications systems <NUM> may employ one or more channel access methods, such as code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), orthogonal FDMA (OFDMA), single-carrier FDMA (SC-FDMA), zero-tail unique-word discrete Fourier transform Spread OFDM (ZT-UW-DFT-S-OFDM), unique word OFDM (UW-OFDM), resource block-filtered OFDM, filter bank multicarrier (FBMC), and the like.

As shown in <FIG>, the communications system <NUM> may include wireless transmit/receive units (WTRUs) 102a, 102b, 102c, 102d, a radio access network (RAN) <NUM>, a core network (CN) <NUM>, a public switched telephone network (PSTN) <NUM>, the Internet <NUM>, and other networks <NUM>, though it will be appreciated that the disclosed embodiments contemplate any number of WTRUs, base stations, networks, and/or network elements. By way of example, the WTRUs 102a, 102b, 102c, 102d, any of which may be referred to as a station (STA), may be configured to transmit and/or receive wireless signals and may include a user equipment (UE), a mobile station, a fixed or mobile subscriber unit, a subscription-based unit, a pager, a cellular telephone, a personal digital assistant (PDA), a smartphone, a laptop, a netbook, a personal computer, a wireless sensor, a hotspot or Mi-Fi device, an Internet of Things (IoT) device, a watch or other wearable, a head-mounted display (HMD), a vehicle, a drone, a medical device and applications (e.g., remote surgery), an industrial device and applications (e.g., a robot and/or other wireless devices operating in an industrial and/or an automated processing chain contexts), a consumer electronics device, a device operating on commercial and/or industrial wireless networks, and the like.

Each of the base stations 114a, 114b may be any type of device configured to wirelessly interface with at least one of the WTRUs 102a, 102b, 102c, 102d to facilitate access to one or more communication networks, such as the CN <NUM>, the Internet <NUM>, and/or the other networks <NUM>. By way of example, the base stations 114a, 114b may be a base transceiver station (BTS), a NodeB, an eNode B (eNB), a Home Node B, a Home eNode B, a next generation NodeB, such as a gNode B (gNB), a new radio (NR) NodeB, a site controller, an access point (AP), a wireless router, and the like.

In an embodiment, the base station 114a and the WTRUs 102a, 102b, 102c may implement a radio technology such as NR Radio Access , which may establish the air interface <NUM> using NR.

The WTRU <NUM> may include a full duplex radio for which transmission and reception of some or all of the signals (e.g., associated with particular subframes for both the UL (e.g., for transmission) and DL (e.g., for reception) may be concurrent and/or simultaneous. In an embodiment, the WTRU <NUM> may include a half-duplex radio for which transmission and reception of some or all of the signals (e.g., associated with particular subframes for either the UL (e.g., for transmission) or the DL (e.g., for reception)).

For example, the CN <NUM> may provide the WTRUs 102a, 102b, 102c with access to circuit-switched networks, such as the PSTN <NUM>, to facilitate communications between the WTRUs 102a, 102b, 102c and traditional landline communications devices.

The AP may have access or an interface to a Distribution System (DS) or another type of wired/wireless network that carries traffic in to and/or out of the BSS. 11e DLS or an <NUM>.

The primary channel may be a fixed width (e.g., <NUM> wide bandwidth) or a dynamically set width. In certain representative embodiments, Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) may be implemented, for example in <NUM> systems.

11af and <NUM>. 11af and <NUM>. 11n, and <NUM>. 11af supports <NUM>, <NUM>, and <NUM> bandwidths in the TV White Space (TVWS) spectrum, and <NUM>. 11ah may support Meter Type Control/Machine-Type Communications (MTC), such as MTC devices in a macro coverage area.

11n, <NUM>. 11ac, <NUM>. 11af, and <NUM>. If the primary channel is busy, for example, due to a STA (which supports only a <NUM> operating mode) transmitting to the AP, all available frequency bands may be considered busy even though a majority of the available frequency bands remains idle.

The WTRUs 102a, 102b, 102c may communicate with gNBs 180a, 180b, 180c using subframe or transmission time intervals (TTIs) of various or scalable lengths (e.g., containing a varying number of OFDM symbols and/or lasting varying lengths of absolute time).

The CN <NUM> shown in <FIG> may include at least one AMF 182a, 182b, at least one UPF 184a, 184b, at least one Session Management Function (SMF) 183a, 183b, and possibly a Data Network (DN) 185a, 185b.

For example, the AMF 182a, 182b may be responsible for authenticating users of the WTRUs 102a, 102b, 102c, support for network slicing (e.g., handling of different protocol data unit (PDU) sessions with different requirements), selecting a particular SMF 183a, 183b, management of the registration area, termination of non-access stratum (NAS) signaling, mobility management, and the like. For example, different network slices may be established for different use cases such as services relying on ultra-reliable low latency (URLLC) access, services relying on enhanced massive mobile broadband (eMBB) access, services for MTC access, and the like. The AMF 182a, 182b may provide a control plane function for switching between the RAN <NUM> and other RANs (not shown) that employ other radio technologies, such as LTE, LTE-A, LTE-A Pro, and/or non-3GPP access technologies such as WiFi.

Unmanned aerial vehicles (UAVs) may be deployed in a variety of applications, for example, parcel delivery, agriculture, oil and gas inspection, and cinematography. There have been several technological challenges that have been identified in the recent 3GPP Aerial Study Item from a communications perspective, where the expectation is to have UAVs coexist with terrestrial mobile users. One of the main differences in operational scenarios between UAVs and terrestrial users is that, the former is expected to operate at altitudes between <NUM> and <NUM> feet, while the latter operates anywhere up to <NUM> feet. The operational scenario for drones may have a wider altitude range and may experience heterogeneous channel propagation environments. Problems that have been identified by the Aerial Study Item on the coexistence of aerial and terrestrial users may include one or more of the following: uplink and/or downlink interference caused by and/or affecting aerial vehicles, optimizations of signaling aspects (e.g., measurement report, cell reselection triggers, handover/mobility aspects etc.,), and identification of aerial vehicles. The emphasis of the 3GPP study item thus far has been on the Uu interface between the UAVs and a network. Sidelink communications have been not been addressed. Sidelink communications in 3GPP have considered ProSe Direct Discovery, Direct Communication (since Rel-<NUM>), and V2X communication (since Rel-<NUM>).

One of the use cases that has been considered in the recent feasibility study of new technology markets is UAV to UAV collaboration, which may be done locally without network coordination. A group of UAVs may be deployed to accomplish a common mission. For example, a group of UAVs may search for a common intruder or suspect, continuously monitor of natural disasters, or perform autonomous surveys. In these scenarios, the common assumptions and its variants may be modeled, for example, based on one or more of the following scenarios.

There may be one UAV controller, such as a pilot, that manages all the UAVs in the group. For example, the pilot in control may provide a command & control message common to all members of the group through broadcast messaging, while providing member-specific restricted command & control message through unicast to one UAV.

UAVs may have the ability to autonomously form groups using sidelink communications. Alternately, the UAVs may belong to a particular group from a service level perspective, for example, nominated by a central controller. UAVs may have direct links to each other and also to the central controller.

One member of the UAV group may be deemed a head UAV, while other UAVs may be nominated as follower UAVs. The assignment of the head and follower UAVs may be determined jointly by the UAVs instantaneously, for example dynamically, based on a set of pre-specified rules and/or encountered conditions. Alternately, the head and followers may be statically chosen for a mission by a central controller. The route that a UAV group may need to follow for a mission may be pre-specified and deterministic.

Evolved multimedia broadcast and multicast service (eMBMS) was introduced in Release <NUM> in 3GPP and may be used for broadcast/multicast service. The eMBMS communications may be a unidirectional service. Packets may be delivered from a source to a group of receivers and there may be no feedback sent to the source from any of the receivers.

One reason for the lack of feedback may be a high probability that there will be a member of the broadcast/multicast group to have not received a packet. This may particularly occur if the size of the broadcast/multicast group is reasonably large and each member of the group experiences independent channel conditions. Hence, for every packet transmitted by a source (e.g., eNB), the probability of re-transmitting the packet increases with increase in broadcast/multicast group size. Although the coding rate and modulation coding scheme (MCS) may be designed based on the weakest user in the group, there are no guarantees on packet reception by every member of the group. Further, retransmissions and conservative coding may lead to decreased spectral efficiency, especially for the stronger users in the group. In order to strike a balance between complexity in re-transmission that can be triggered due to a member of the group requesting for retransmission of the packet and reliability, there may be fixed number of re-transmissions performed by the source, irrespective of packet reception by the members of the group. This may lead to a decreased efficiency.

If there are multiple members in the group that have lost the same packets, there may be a flooding of negative acknowledgements (NACKs) that the source may have to handle. This may be due to the fact that a WTRU may not be aware of the packet loss scenario of the other WTRUs and may individually send feedback.

The File Delivery over Unidirectional Transport (FLUTE) protocol may be used to deliver a file using a unidirectional bearer. It may not be possible to guarantee error free reception over FLUTE. Accordingly, unicast schemes may be used to recover lost packets.

After a broadcast session is complete, the source may initiate a unicast session with each of the members of the broadcast/multicast group. Each member may provide a FLUTE level such as, for example, the application level and/or packet loss information. The source may retransmit only the lost packets to each member until successful delivery. Alternatively, if the packets are fountain coded, each member may provide only the number of packets lost, and may not have to let the source know which packets have been lost. The source may transmit a higher number of packets than the member requires, which may improve decodability of the file.

The physical downlink shared channel (PDSCH) may be used for unicast data transmission and the physical multicast channel (PMCH) may be used for evolved multimedia broadcast multicast service (eMBMS) transmission. The PDSCH may be multiplexed, both across time and frequency, while the PMCH may be multiplexed only across time. That is, the PDSCH may be allocated to users both across time and frequency, while this may not be possible with the PMCH. The PMCH may be switched only on a subframe level. The PMCH may be coordinated across multiple cells that constitute a multicast-broadcast single-frequency network (MBSFN) area. It may be difficult to assure uniformity in resource allocation from a total resource block (PRB) perspective of all the cells in the MBSFN area due to independent unicast resource allocation that each eNB is likely to have. The granularity of resource allocation on the PMCH may be coarse at a sub-frame level, which may lead to poor resource utilization. For example, if the broadcast rate is low, or if users are moving across cells in which multicast transmission may be performed, invoking eMBMS service may not be efficient due to the requirement of coordinating all the eNBs at the sub-frame level. Single cell-point to multicast (SC-PTM) may address this, whereby each cell may independently use its PDSCH to service both broadcast/multicast as well as unicast.

Schemes for broadcast/multicast may use conservative coding rates, MCS, and fixed re-transmissions to enhance packet decodability in broadcast. Further, unicast may be used to re-transmit lost packets to members of the broadcast/multicast group. There may be a need in finding optimal transmission schemes for packet losses using network/index-coding.

Protocols in which members of a broadcast/multicast group may perform group cooperation leveraging the presence of sidelink/WiFi-direct communications to improve packet repair efficiency of a broadcast/multicast Uu communication and spectral efficiency are described herein.

<FIG> illustrates a concept of UAV grouping. An example UAV group may include a first UAV <NUM>, a second UAV <NUM>, a third UAV <NUM>, and a fourth UAV <NUM>, which may be deployed to accomplish a common mission. The members of the group may receive a common command & control (C2) message that may be provided by a UAV traffic management (UTM) controller, or pilot, and may be transmitted via multicast by an eNB <NUM>. The eNB <NUM> may communicate with one or more of the first UAV <NUM>, the second UAV <NUM>, the third UAV <NUM>, and the fourth UAV <NUM> over a Uu interface. The eNB <NUM> may configure a group radio network temporary identifier (G-RNTI) for the group of UAVs in order to transmit a common group-specific C2. The group-specific C2 may be a single transmission on the downlink that every member of the group sharing the G-RNTI may decode. Due to independent channel conditions experienced by each member of the group, the packets lost and received may be different for each other member of the group. The difference in packets lost and received by different group members may be locally exploited. For example, one member of a group may send its received packet to another member that lost this packet using device to device or vehicle to vehicle (D2D/V2V) sidelink communication.

For example, the variable Bi may represent the packets lost by member i of the group, the variable S may represent the packets transmitted by the eNB, and the variable u may represent the number of members in the group. The set of lost packets, L, by the group may be <MAT>.

The set of lost packets, L, by the group may go to zero (i.e., null-set) as u increases because there may be at least one member of the group that likely receives a packet transmitted by the eNB.

In this example, the packet {<NUM>}, may not be received by any member of the UAV group, while the other packets {<NUM>,<NUM>,<NUM>}, may be received by one or more members of the UAV group. Packets {<NUM>,<NUM>,<NUM>} may be locally recovered among the members of the group, thereby avoiding members of the group having to rely on the eNB to retransmit the packets by unicast or multicast. However, because packet {<NUM>} was not received by any member of the group, it may not be recovered locally.

In order to recover lost Uu packets through sidelink, each member may need to be aware of the packet lost/received information of the other group members. A protocol may be used so that the packet loss information of every member is shared with other group members efficiently. Once every member is aware of the packets lost or received by the other members, a protocol may be used for sidelink based packet repair in which packets may be efficiently exchanged among members so that every member may obtain its lost packets directly from another member. Sidelink channels may be implemented using specific protocols such as IEEE <NUM> or V2V.

The protocols used for Sidelink Assisted Downlink Broadcast (SADB), may include groupcast NACK (gNACK) transmission, efficient protocol for packet loss dissemination among group members, and efficient protocol for packet repair among group members.

In SADB, a group member may obtain lost packets from another member of the group, instead of having network infrastructure (e.g., the eNB) re-transmit the packets. In some instances, re-transmissions in a broadcast/multicast may not be permitted, for example in a layer of the 3GPP and/or IEEE <NUM> protocol stack. However, errors that occur in broadcast transmissions may be corrected by unicast mechanisms such as FLUTE.

In SADB, downlink broadcast packets lost by a group member may be available from another member of the group. The packet losses experienced by group members may be independent, such that a lost packet from one member may be available from at least one other group member. The SADB protocol may be built at the application layer over user datagram protocol/internet protocol (UDP/IP) and may be independent of lower layers. The SADB protocol may also be built on lower layers such as the packet data convergence protocol (PDCP), the radio link control (RLC), and the media access control/physical (MAC/PHY) layer. The PDCP implementation for SADB may be similar to application level implementation. The RLC implementation of SADB may require the RLC layer to transmit in acknowledged mode, which may incur overhead, as packets may need to be re-transmitted even if only one of the group members loses a packet. Application level protocol for SADB may be considered below.

As shown in <FIG>, packet {<NUM>} may be lost by all of the group members. It may not be possible to repair the packet through sidelink based local cooperation. A gNACK may be transmitted by any member of the group to indicate that a packet has not been received. The gNACK message may indicate that packet {<NUM>} was not received not only by the transmitting member, but also by all of the group members.

One way to determine the common packets lost by all group members may be for each member to broadcast its packet lost/received information locally to its group members. A pre-specified ordering of users that every user has to follow to access the channel at a particular time slot (e.g., a time slot/frequency resource that a user should use for broadcasting its loss/received information) may be used. One advantage of using pre-specified channel access mechanisms may be collision avoidance. Packet status information (e.g., received and lost information) may be known to members of the group.

There may be N group members and each member may transmit status information on each of n packets. A common frequency resource, for example, a packet status transmission resource pool may be available. This scheme may use N time resources with a total number of Nn bits to obtain global information on each of the packets of each member in the group.

Table <NUM> shows packet status information for six packets (P1-P6) received by a downlink broadcast for three users (U1, U2, U3). In Table <NUM>, a value of <NUM> represents packets successfully received and a value of <NUM> represents lost packets.

Each user may transmit the status of each packet that has not been reported in a pre-specified packet order (e.g., increasing packet sequence number). The first user U1 may have to transmit six bits to indicate the status of six packets. The second user U2 may only have to transmit the status of the packets that were reported to be lost by the first user because only the lost packets may be of interest. There may be a lead time (e.g., <NUM> subframes) after each user transmits so that other members may receive the message and prepare their own packet status information accordingly. For example, if U2 transmits second, then it may report the status of packets not received by U1 in a pre-specified order (e.g., increasing packet sequence number) which ,may be {P1, P2}, incurring <NUM> bits. The third user U3 may need to report the status of only the packets reported to have been lost by both U1 and U2. If U3 transmits third it may provide the status of {P2}, which incurs one bit. After U3 transmits, it may become clear which packet was not received by all the users. Accordingly,, the number of bits required for transmission may be: <NUM>+<NUM>+<NUM> = <NUM> bits.

Table <NUM> illustrates the number of bits required for gNACK determination for different user transmission orders.

The order of transmission may play an important role in optimizing the total number of bits required to disseminate the gNACK information. It may be advantageous for the user with the maximum number of correctly received packets to transmit first and have a higher transmission priority. The user that transmits first may have to provide status information of all packets, irrespective of how many it has correctly received. Users that transmit subsequently may provide only the difference information. It may be desirable to provide maximum information on the reception of previously unreported received packets at every transmission instant.

Due to the distributed framework, it may not be possible for a group member to know whether it has the most number of received packets. Hence there may need to be a mechanism where the group members, implicitly, may be able to determine the number of received packets as compared to other members of the group. This concept may be the basis of the gNACK protocol which may be a priority based probabilistic schema that leverages the aforementioned observations.

The gNACK protocol may proceed in blocks of n packets, P=({<NUM>,<NUM>. n}) received through downlink broadcast. For a group of size N, the variable Ai may denote the packets received by user-i, and the variable <MAT> may denote the loss percentage of user-i. The users may be categorized into different access classes, based on loss percentage. An example categorization of users into access class is shown in Table <NUM>.

Users that have the least packet losses may be given the highest priority to access the channel. For example, users that have <MAT> may be assigned to Class-<NUM>. Class-<NUM> may have the minimum access delay to a channel, as seen by the contention window range.

The contention window range for an access class may be defined to be <MAT>
and may be non-overlapping across different classes, for example CWi ⊈ CWi+<NUM>, such that <MAT>.

The back off range for access class-i can be less than class-(i + <NUM>) so as to provide access advantage for higher priority classes. If multiple users fall into the same access class, then the users may randomly choose their back off time according to the access class's specific contention window range.

Initialization of the gNACK protocol may include setting Y = { Ø} to be the set of packets that has been reported to be correctly received by the group. In a first step, <MAT> may be calculated from Ai, ∀i = <NUM>,<NUM>,. Every member may be assigned to an access class based on the procedure described above in Error! Reference source not found. In step <NUM>, group member-k, that has chosen the least back off time based on its access class assignment may access the channel and broadcasts its list of received packets Ak to all members. In step <NUM>, Y = Y ∪ Ak may be computed and An = Ø may be set. In step <NUM>, i = <NUM>,<NUM>,. N, Ai = ( Ai ∩ (P\Y) ) may be updated for all members. In step <NUM>, steps <NUM> through <NUM> may be repeated until Ai = Ø, ∀ i = <NUM>,<NUM>. In step <NUM>, gNACK = P\Y may be computed.

An application level use case of this protocol may be in video broadcasting, where group RTCP feedback may be sent by any of the clients indicating RTP packets that none of the clients have received.

Referring to the packet loss scenario of Table <NUM>, users may be assigned to class-<NUM>, if (packet error < <NUM>); class-<NUM>, if (<NUM> < packet error < <NUM>); class-<NUM>, if ( <NUM> < packet error < <NUM>). A gNACK protocol may be initialized by setting = { Ø}, P = {<NUM>,<NUM>,<NUM>,<NUM>,<NUM>,<NUM>}, A<NUM> = {<NUM>,<NUM>,<NUM>,<NUM>}, A<NUM> = {<NUM>,<NUM>}, A<NUM> = {<NUM>}.

In step <NUM> of a first iteration, <MAT>; <MAT>. User-<NUM> may be assigned to class-<NUM>, user-<NUM> may be assigned to class-<NUM>, and user-<NUM> may be assigned to class-<NUM>. User-<NUM> may access the channel. In step <NUM> of the first iteration, A<NUM> = {<NUM>,<NUM>,<NUM>,<NUM>}. In step <NUM> of the first iteration, : Y = { <NUM>,<NUM>,<NUM>,<NUM>}; A<NUM> = { Ø}. In step <NUM> of the first iteration, P\Y = { <NUM>,<NUM>}; A<NUM> = { <NUM>}; A<NUM> = {<NUM>}.

In step <NUM> of a second iteration, user-<NUM> and user-<NUM> may have new error rates of <NUM>/<NUM> as both have only packet-<NUM> (see step <NUM> from the first iteration). Hence, they may be assigned to class-<NUM>. User-<NUM> may select a lower backoff and it may access the channel before user-<NUM>. In step <NUM> of the second iteration, A<NUM> = { <NUM>}. In step <NUM> of the second iteration, Y = { <NUM>,<NUM>,<NUM>,<NUM>,<NUM>}; A<NUM> = { Ø}. In step <NUM> of the second iteration, P\Y = { <NUM>}; A<NUM> = {Ø}. Hence, P\Y = { <NUM>}.

In a MAC/PHY layer approach to gNACK, a broadcast/multicast transmission may be associated with a group-specific G-RNTI that the users of a broadcast/multicast group may use to decode the broadcast packets sent by the eNB. A user specific cell radio network temporary identifier (C-RNTI) may also be used to decode unicast packets. The following description includes frame/sub-frame timing, how gNACK feedback differs from unicast feedback, and parameters that may influence its design.

<FIG> shows downlink and uplink subframe timings for gNACK feedback. The gNACK protocol may proceed in blocks of R subframes. The gNACK feedback for downlink sub-frames of length R may be sent K sub-frames later in the uplink. In this example, it is assumed that K > R. As shown in <FIG>, gNACK feedback for downlink sub-frames [n, (n + R - <NUM>)] may be sent out at uplink frame (n + <NUM>K), while gNACK feedback for downlink sub-frames [(n + R), (n + 2R - <NUM>)] may be sent out at uplink frame (n + <NUM>K).

The parameter K may denote the delay that may be required for sending out a gNACK and may depend on the number of members, N, in the group. If a larger number of members participate in group communications, the time required to determine gNACK may become higher. To ensure gNACK feedback is not delayed excessively, the eNB may limit the group size N to an acceptable number. If the group size exceeds the acceptable and dimensioned number, the eNB may choose to form more than one group. In this situation, two or more G-RNTIs may be allocated for each group and its dependent members.

The parameter R may denote the number of subframes over which gNACK is sought. In eMBMS, broadcast scheduling may not be performed on every sub-frame. The number of broadcast packets scheduled in R sub-frames may be all of the information needed for the gNACK protocol.

The variable d may be the maximum number of HARQ processes defined for a group specific broadcast/multicast transmission. Hence for gNACK generation, d may need to be adaptable depending on the R (number of broadcast scheduled subframes) and K (delay parameter for sending gNACK). Although the maximum number of HARQ processes d may be an increasing function of both R and K, the need for increased buffering at the eNB may be group-specific and not user-specific. For user-specific NACK for broadcast transmissions, the buffer requirements may be much more than the buffer requirements for the gNACK scheme. In the case of user-specific NACK for broadcast transmission, some users may have more packet losses than others, and the eNB may have to buffer packets based on the user with the highest packet loss.

The HARQ feedback in unicast scenarios may only require one or two bits depending on whether it is single or double layer. In the uplink, the eNB may be aware of the unique HARQ process for which it is expecting feedback from the WTRU. However, in the case of gNACK, the feedback may indicate the explicit HARQ processes that may have been received in error. If gNACK is attempted every x HARQ processes, the feedback for a single HARQ process may be <MAT> bits.

As shown in <FIG>, the gNACK feedback for downlink sub-frames [n, (n + R - <NUM>)] may be sent out in m<NUM> uplink frames [n + <NUM>, (n + <NUM> + m<NUM> - <NUM>)]. Assuming that two bits are allocated per sub-frame for sending gNACK feedback on the PUCCH, gNACK feedback may be sent up to a total of <MAT> HARQ processes. The starting sub-frame in the uplink and the number of sub-frames (m<NUM>) for sending gNACK may be signaled by the eNB. The PUCCH resources that need to be used by WTRUs for sending gNACK may be based on the starting control channel element (CCE) location in the PDCCH used for signaling broadcast/multicast related downlink control information (DCI) , for example, in the case of SC-PTM.

Alternately, the eNB may signal the starting resource (x<NUM>) for the G-RNTI, and the WTRU may compute the resource locations as follows: <MAT>.

The WTRU may provide up to <NUM> bits of feedback on two consecutive PUCCH resources (j and j+<NUM>) on current subframe and this pattern may continue for one fourth of <MAT> consecutive subframes. <MAT> for gNACK resource location may be configured by higher layers, or be obtained through system information broadcast messages.

The PUCCH resources to be used for sending gNACK may be group-specific and not WTRU-specific. The group member that needs to transmit gNACK, which may be identified by C-RNTI, may be pre-specified, or may be signaled by the eNB. The eNB may use the G-RNTI to re-transmit, or broadcast, the lost packet to the entire group, as none of the group members may have the gNACK packet. Alternatively, the eNB may perform a unicast to any member of the group with the additional signaling indicating that the re-transmitted packet is associated with the G-RNTI and that recipient may need to perform sidelink broadcast of this packet to other group members.

The size of gNACK uplink feedback may be dependent on the number of group members, N, for example, the number of uplink sub-frames m<NUM> and the number of feedback bits per sub-frame. The probability that a packet is not available with any of the group member may decrease with an increasing N. Hence the eNB may adapt the number of uplink sub-frames m<NUM> and the number of feedback bits per sub-frame dynamically based on the number of current group members, N.

A gNACK may be sent for a group, and not on per-WTRU basis. A packet may have been received by some member of the group with a high probability, and so the probability that a packet is lost by all members may be small. The variable Bi may denote the set of packets lost by member-i. A guideline for using the gNACK scheme may be: <MAT>.

)| may denote the cardinality. That is, the number of common packets lost by all members of the group may be much less than the union of the packet losses incurred by the group. Further, a gNACK may be transmitted on blocks of R subframes. This scheme may avoid the overhead of having to send feedback on a sub-frame basis by leveraging the fact that the probability that all the members lose a packet on a sub-frame may be small.

On the other hand, the following,, <MAT>
may imply that roughly each packet may be lost by every member with high probability, and it may not be possible to reconcile lost packets using sidelink communications. In this regime, acknowledgement/negative acknowledgement (ACK/NACK) may be performed instantaneously. There may be several procedures for users to receive or send feedback on a lost packet.

For all the users in the same G-RNTI (broadcast/multicast), the eNB may allocate a common resource element in the PUCCH which is signaled to the group members at connection setup. All the users may use the common resource to send feedback if they have a packet loss (i.e., a NACK).

Every user may send the feedback using the same signaled common resource, for example energy or sequence, if the packet was incorrectly received. All users may send the feedback in uplink frame n for a broadcast packet that was received in downlink frame (n - <NUM>).

Depending on the number of members that have not decoded the packet and are transmitting NACK, the received energy, as sensed by the eNB, may vary. For example, if the group members transmit a unique signal in case of NACK, the received energy as sensed by the eNB may be Nx, where N may represent the number of members transmitting NACK at a particular transmission time interval (TTI) and x may be the transmitted NACK sequence.

The eNB may sense the energy of the common resource to estimate how many users may have received the packet incorrectly. For example, the received energy in the common resource (E) may be checked against different thresholds to come up with an estimation as follows. If E > Threshold<NUM>, all group members may have sent a NACK in the common resource. If Threshold<NUM> < E < Threshold<NUM>, half of the members may have sent NACK in the common resource. If Threshold<NUM> < E < Threshold<NUM>, <NUM>/<NUM> of the members may have sent NACK in the common resource. The thresholds may be set taking into account the number of members in the group, a power p the eNB signaled for NACK transmission, and the path loss/expected fading profile of the group members.

One common resource element may be used by all group members to send an energy/sequence feedback. Alternately, multiple resource elements may be allocated, wherein, mutually exclusive subsets of group members may be allocated unique resource elements for energy transmission. For example, group member <NUM> and member <NUM> may both be allocated a common resource element f<NUM>, while group member <NUM> and member <NUM> may both be allocated a different common resource element, f<NUM>, for energy transmission. This may provide finer information on the packet loss scenario experienced by the group members to the eNB. Extending this procedure, whereby unique resource elements are allocated to mutually exclusive subsets of group members of size one, may reduce to the per-WTRU feedback.

WTRUs/UAVs belonging to a specific G-RNTI may be divided into a total of K sub-groups of size ck (k = <NUM>,<NUM>. Each sub-group may be assigned a unique PUCCH resource Yk (k = <NUM>,<NUM>. K) by the eNB to transmit the gNACK. The WTRU/UAV, and the eNB protocol may be as follows. Each member of sub-group k may use the common assigned resource Yk to transmit if a NACK is being sent. The eNB may decode the presence of NACK in resources Yk (k = <NUM>,<NUM>. K) and may retransmit the packet if it decodes NACK in all the K resources.

The sub-group gNACK PUCCH resource assignment Yk for a WTRU/UAV may be done using RRC reconfiguration when the UE/UAV is assigned GRNTI as shown in Table <NUM> below.

Group members may operate in a correlated packet loss regime, which may satisfy |{∩i Bi}| ≈ |{∪i Bi}|. Group members may operate in an independent packet loss regime, which may satisfy |{∩i Bi}| « |{∪i Bi}|. Depending on the energy sensing approach used over a period of time (e.g., several subframes/frames), the eNB may signal through DCI or a higher layer signaling, the mode of packet loss recovery the members of a group may need to follow.

As described above, priority may be assigned to users for accessing the channel based on the loss probability. A higher priority may be assigned to members with less packet loss. Alternatively, higher priority may be assigned to members with high packet losses to maximize the packet loss dissemination information among group members, given a fixed amount of transmission opportunities. The set of packet losses experienced by users that have less packet losses may be a subset of packet losses experienced by users that have higher losses. It may be optimal for group members to report the difference in lost packet information. An example for access class determination for this scenario is provided in Table <NUM>, which shows an example categorization of users into access class and contention window minimum and maximum for each access class for packet loss dissemination with fixed transmission opportunity.

<FIG> is a flowchart illustrating the gNACK determination process, according to the claimed invention. In step <NUM>, a UAV may determine packet loss for the Uu interface between the UAV and the eNB in the configured time window. In step <NUM>, the UAV may determine an initial sidelink access class based on its determined Uu packet loss. In step <NUM>, the UAV may receive Uu packet loss information from other members of its group over sidelink communications. In step <NUM>, the UAV may determine a differential NACK/ACK of Uu packets within the group and may update its access class for sidelink communications. In step <NUM>, the UAV may determine if gNACK is complete. If gNACK is not complete, the process may return to step <NUM>. If gNACK is complete, the process may proceed to step <NUM>. In step <NUM>, the UAV may transmit a single gNACK to the eNB over the Uu interface. The gNACK may reflect a coordinated NACK across the group based on the differential NACK/ACK of Uu packets within the group.

In group packet repair, each member of the group may know the packet loss of all other members, and how each member can assist other members so that all members are able to reconcile their lost packets.

This procedure may require packet loss/received information of every packet for every group member. However gNACK protocol may not be able to provide information on every packet for each member, as it is designed for each member to provide the maximum unreported difference information. Each member may provide information on every packet which incurs a maximum overhead of Nn bits transmitted, where N may be the number of group members, and n may the number of packets for which the lost/received information is to be reported.

In group packet repair, it may be desirable to optimize ways by which all group members obtain their lost packets in the minimum amount of transmission time possible. In addition, it may be desirable, given fixed transmission opportunities, to maximize the amount of packets that can be repaired for the whole group.

<FIG> show examples of group packet repair. Group members may be able to help others based on the packet losses experienced by other members and the availability of the lost packet at a member that wants to help. <FIG> shows a simple broadcast, wherein the same packet is requested by many members. <FIG> shows loss patterns for a two packet network coding. <FIG> shows loss patterns for a three packet network coding. <FIG> shows loss patterns for a four packet network coding.

A group member may determine how many group members may be simultaneously served in one transmission opportunity. This may depend on the packet losses experienced by other group members and the availability of the lost packet with the group member that wants to help. In <FIG>, the same packet (packet #<NUM>) may be lost by members U<NUM>,. UN, and this packet may be available with another member, U<NUM>. Hence one transmission may be sufficient to repair all users, N users, with this packet loss.

In <FIG>, U<NUM> may receive packet-<NUM> and may lose packet-<NUM>, while the reverse scenario exists for U<NUM>. Another member, U<NUM> that received both packet-<NUM> and packet-<NUM> may broadcast exclusive or XOR of these packets. Accordingly, it may be possible to repair the packet losses of both members U<NUM>, U<NUM> simultaneously with a single transmission.

In <FIG>, U<NUM> may lose packet-<NUM>, but may receive the other two, U<NUM> may lose packet-<NUM>, but may receive the other two, U<NUM> may lose packet-<NUM>, but may receive the other two, and U<NUM> may receive all the packets. It may be possible to repair packet losses of three members U<NUM>, U<NUM>, U<NUM> simultaneously with a single transmission.

In <FIG>, four members may be served in one transmission. It may be possible to simultaneously serve multiple members in one transmission opportunity by carefully considering the lost packet patterns of group members. The variable ℓ may be the number of users that can be simultaneously served by a group member and N may be the total numbers of group members. An example for access class determination of group members for packet repair phase is shown in Table <NUM>.

In order to signal a network coded packet, the MAC header may indicate the sequence number (e.g., media access control protocol data unit (MPDU) sequence numbers) of the packets that have been used to form the network coded packet. As the group members may use a network code in the sidelink, the MAC packet header for the sidelink may have bits allocated for representing the sequence number for the MPDUs that may have been used to form the network code in addition to the existing bit allocation for representing the sequence number for the current MPDU. The MAC packet header updates required to enable network coding may only be performed for sidelink communications.

Group members may transmit randomly. Each group member may be assigned to transmit a specific set of packets in each iteration. The assigned set of packets may not have been received correctly by the member. In this case, other members may end up transmitting this packet in a different iteration.

In an example there may be N members numbered {<NUM>,<NUM>,. N - <NUM>} and M packets numbered {<NUM>,<NUM>,. M - <NUM>}. User-j may transmit packet k, if k satisfies the following: <MAT> <MAT>.

The variable Δ may represent the iteration and m may represent the number of packets transmitted.

In an example, M = <NUM>, N = <NUM>, and m = <NUM>,<NUM>,<NUM>. The packets transmitted by a user in each iteration as shown in Table <NUM> may be obtained by using the above equations. Table <NUM> shows packets transmitted by each user during a cycle. Each packet may be uniquely transmitted by a user per cycle. Each user may transmit all packets across all cycles.

In each iteration, a packet may be uniquely transmitted by a user. For example, in iteration Δ = <NUM>, User-<NUM> may transmit packets <NUM>, <NUM>, <NUM> corresponding to m = <NUM>, <NUM>, <NUM> respectively. Across all iterations, each member may transmit all packets. If a packet is received by at least one member after iteration Δ = <NUM>, each member may have recovered its lost packets.

If a member has lost a packet that it is supposed to transmit in an iteration, it may eventually be transmitted by another member in a different iteration. For example, if user-<NUM> has lost packet-<NUM>, it will not be able to transmit it during Δ = <NUM>. However, packet-<NUM> may be transmitted by user-<NUM> during Δ = <NUM>. Each member may be assigned fair loading for packet transmission. In the example above, each member may have a requirement to transmit three packets in every iteration. Alternately, members may transmit unequal packets in every iteration. Even if the members transmit unequal packets in every iteration, each member may still transmit an equal number of packets across every iteration. For example, in Δ = <NUM>, user-<NUM> and user-<NUM> may transmit two and six packets, respectively but they may transmit six and two packets in Δ = <NUM>. Though this scheme may be unfair to users across each iteration, it may be fair across all iterations.

Each member may signal to the group once it has repaired its lost packets so that the iterations may be stopped after receiving this message from all members of the group. The network may configure the starting time, for example a system frame number-subframe number (SFN-SF) combination, to begin the packet repair process. A time that a user should transmit its packets may be preconfigured based on the user-id as shown in Table <NUM>. For example, the network may be configured to start the packet repair process every even or odd frame number. From this absolute time, each user-id may calculate its opportunity for transmission.

A packet repair mode using priority based probabilistic schema may be efficient when there is a constraint in sidelink resources, as the scheme may attempt to repair packet losses in minimum transmission opportunities. A packet repair mode using a random packet loss scheme may become useful when there are enough sidelink resources/bandwidth and/or packet losses are high.

Although the description herein considers 3GPP specific protocols, it should be understood that embodiments are not restricted to this scenario and are applicable to other wireless systems. In addition, although examples described herein include drones and aerial vehicles, it should be understood that embodiments apply to all wireless terminals.

<FIG> is a flowchart illustrating group packet repair. In step <NUM>, a UAV may determine if it has packets received over the Uu interface that are being requested by others. If no, in step <NUM>, the UAV may wait for others to transmit. If yes, in step <NUM>, the UAV may determine a sidelink access class for repair based on the number of group members requesting the packets. In step <NUM>, the UAV may transmit the packets to other group members using sidelink communications.

<FIG> shows an example UAV <NUM> that may be used in a UTM system. The UAV <NUM> may be any type of conventional aerial vehicle that has the ability to fly without a pilot, The UAV <NUM> may be a fixed wing drone, a multi-rotor drone, a single-rotor drone, an inflatable drone or a hybrid drone. The UAV <NUM> is illustrated as a four propeller drone, but any drone configuration that can fly within an anticipated area of deployment can be used. The UAV <NUM> may have an electronic processing circuit configured to execute machine instructions to carry out the tasks described herein. The UAV <NUM> may communicate with a UTM and/or other UAVs wirelessly during flight, for example using a short or long range wireless communication protocol, examples including WiFi, WiMAX, BLUETOOTH, SIGFOX, <NUM>, <NUM>, LTE, or another protocol, for example using a publicly available frequency.

The UAV <NUM> may have varying amounts of processing capability, but includes at least sufficient processing capacity to fly, and further includes the components normally associated with a UAV, such as a means of propulsion, for example one or more propellers <NUM> driven by a motor <NUM>, a power source <NUM>, one or more cameras <NUM>, and a control circuit <NUM>. The control circuit <NUM> may include flight related sensors, electronics, and software as well as communications electronics, including wireless two way communication for remote or semi-autonomous control, although the UAV <NUM> may be implemented as fully autonomous. The control circuit <NUM> may include electronics necessary to carry out flight in accordance with an intended mission, whether that is remote piloted, semi-autonomous, or fully autonomous. The electronics in the control circuit <NUM> may be similar to those of the WTRU <NUM> described above with reference to <FIG>. The electronics may include one or more processors <NUM>, one or more transceivers <NUM>, one or more antennae <NUM>, a GPS chipset <NUM>, etc..

A UAV may determine packet losses in the Uu interface in a configured time window. The UAV may determine an access class for accessing the sidelink, and may perform a random back off based on the determined access class. Any member of a group with the GRNTI may generate a gNACK for the common packet loss experienced by the group, or the eNB may configure a member of the group to transmit the gNACK.

UAVs in a group may receive a GRNTI specific configuration, including a subgroup PUCCH gNACK resource. UAVs may transmit NACK in the assigned subgroup PUCCH gNACK resource if decoding fails. An eNB may re-transmit the packet if NACK is decoded in all the subgroup PUCCH gNACK resources.

An eNB may decode a presence/absence of NACK in all subgroup PUCCH gNACK resources consecutively for T sub-frames. A UAV may receive signaling for a mode change between block-based gNACK & instantaneous gNACK from the eNB. The UAV may further receive configuration information for transmitting block-based gNACK feedback (e.g., starting sub-frame number, length, number bits per sub frame).

A UAV may determine packets lost/received by other members of the group in a configured time window. The UAV may determine the access class for accessing the sidelink based on the number of users that may be simultaneously repaired in the current transmission opportunity. The UAV may perform a random back off based on the determined access class.

An eNB may determine Uu group packet loss and or sidelink bandwidth constraints. UAVs in the group may receive configuration for sidelink packet repair mode (e.g., random packet transmission repair mode and/or network coding) based on Uu group packet loss.

The group member may signal the eNB requesting the MAC packets to be of equal size amenable for network coding so as to implement the access class based schema described above. Alternately, based on the energy thresholding scheme described above, the eNB may determine wether the MAC packets need to be of equal size when transmitting its Uu broadcast packets.

Claim 1:
A method performed by a wireless transmit/receive unit, WTRU, the method comprising:
receiving a downlink communication (<NUM>) from a network over a first interface, wherein the downlink communication is transmitted to a group of WTRUs, wherein the group of WTRUs includes the WTRU;
determining an access class (<NUM>) of the WTRU based on a packet loss percentage of the downlink communication, wherein the determined access class of the WTRU is associated with a contention window for accessing a second interface;
transmitting packet loss information (<NUM>) to other WTRUs of the group of WTRUs over the second interface in the contention window associated with the determined access class of the WTRU;
receiving packet loss feedback (<NUM>) from the other WTRUs of the group of WTRUs over the second interface;
determining that the access class (<NUM>) of the WTRU is a highest access class of the group of WTRUs; and
transmitting a single groupcast negative acknowledgement, gNACK, (<NUM>) to the network on behalf of the group of WTRUs over the first interface, wherein the gNACK is based on the received packet loss feedback and the packet loss information.