TECHNICAL FIELD

The present disclosure relates to techniques for Cooperative Radio Transmission, in particular via sidelink to support Ultra-Reliable Low-Latency Communications (URLLC) traffic in Vehicle-to-Network (V2N) communications, in particular in communication systems such as 5G NR (New Radio). The disclosure relates to a coordination device for coordinating cooperative radio transmission between User Equipments (UEs), an assistance device for assisting cooperative radio transmission between UEs and corresponding methods.

BACKGROUND

In a V2X (vehicle-to-anything) scenario 100 for cooperative retransmission, e.g. as shown in Fig. 1 , a“Car A“ 1 10 is driving on a highway, which is served by a gNB 120 with an ultra-reliable low-latency communications (URLLC) service (vehicle to network, V2N, communication). A truck 130 driving next to the car 1 10 may create a deep shadow for the car 1 10, such that its communication link 1 16 to the gNB 120 is completely obstructed. Then, other cars 1 1 1 , 1 12 in the vicinity of“Car A” 1 10 can support the communication by cooperative retransmissions via the sidelink.

For the retransmission via sidelink, the packet is cooperatively transmitted by several cars simultaneously in the same resource. Therefore, the cooperating cars use either the single-frequency-network (SFN) mode, where the cars transmit the same signal without further preprocessing, or they use the Alamouti space time code, where the transmit signal is precoded by one out of two orthogonal transmit sequences. The transmit sequence is chosen randomly by each car selected for packet retransmission.

However, by using SFN mode, no actual spatial diversity gains provided by the

independent communication links from different cars can be realized. Alamouti code can realize only two-fold spatial diversity, such that the diversity gain is strictly limited and does not scale with the number of cooperating users. SUMMARY

It is the object of the invention to provide techniques for improving cooperative

retransmission, in particular for the above-described V2X scenario.

It is further the object of the invention to provide techniques for on the proper exploitation of the link diversity made available by cars driving in the direct neighborhood of “car A” in a V2X scenario, such that the use of resources for the retransmissions on the sidelink can be kept to a minimum while attaining high reliability and fulfilling latency constraints.

These objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

A basic idea of the invention is the application of distributed spatial diversity transmission schemes for cooperative retransmission of URLLC packets using the sidelink (SL), which is coordinated by the dedicated car A in the downlink and the gNB in the uplink. This way, high reliability can be attained for the communication in a resource efficient manner. The required signaling between the coordinator, the neighboring cars and the gNB, which has been tailored to fulfill latency constraints, is a further core component. The signaling provides the exchange on information on the transmission scheme to be used, the spatial order and transmit power per each car involved in the retransmission process.

Concepts described hereinafter exploit the full link diversity provided by the neighboring cars that have been selected for the cooperative transmission by applying spatial diversity transmission schemes, such as space time block codes (STBC), space time frequency codes (SFBC) or frequency shift transmit diversity (FSTD) schemes. These concepts specify the additional exchange of control data between the coordinating entity and the neighboring cars and detail the required signaling. In the downlink (DL), the cooperative retransmission is coordinated by the dedicated user equipment (UE) (i.e. the final receiver of the URLLC packet), whereas in the uplink (UL), cooperative retransmission is coordinated by the gNB. In order to describe the invention in detail, the following terms, abbreviations and notations will be used:

URLLC: Ultra-reliable low-latency communications

V2N: Vehicle-to-Network

gNB: base station in 5G notation

V2X: Vehicle-to-anything

UE: User Equipment

V2V: Vehicle-to-Vehicle

SFN: Single-frequency-network

STBC: space time block codes

SFBC: space frequency block codes

FSTD frequency shift transmit diversity

UL: uplink

DL: downlink

SL: sidelink

TTI: transmission time interval

MCS: modulation and coding scheme

HARQ: Hybrid Automatic Repeat Request

SO: spatial order

ACK: Acknowledgement

NR: New Radio (standard)

According to a first aspect, the invention relates to a coordination device for coordinating cooperative radio transmission between user equipments, UEs, of a plurality of UEs in a wireless network, wherein the coordination device is configured to: select a subset of UEs from the plurality of UEs and a spatial diversity transmission scheme for a first retransmission of a packet; assign to each UE of the subset of UEs a spatial order and a transmit power for the first retransmission of the packet; and send a request for the first retransmission of the packet to each UE of the subset of UEs, wherein the request comprises the spatial diversity transmission scheme, the spatial order and the transmit power assigned to the respective UE. Such a coordination device allows to exploit the link diversity made available by the UEs of the subset of UEs (e.g. cars driving in the direct neighborhood of the car using the coordination device in a V2X scenario). Hence, the use of resources for the

retransmissions on the sidelink can be kept to a minimum while attaining high reliability and fulfilling latency constraints.

The coordination device may be a user equipment (UE) or a base station (BS), e.g. a gNB. The plurality of UEs may be cars or may be arranged or installed in cars. In an implementation, the plurality of UEs may be arranged in a group of UEs. Such group comprises at least two UEs.

The spatial diversity transmission scheme may be selected from different types of diversity transmission schemes providing diversity with respect to space, time and/or frequency, e.g. from space time block code, space frequency block code, or frequency shift transmit diversity schemes. Alternatively or additionally, the spatial diversity transmission scheme may be selected depending on the number of transmit and/or receive antennas of the coordination device and/or the other UEs of the plurality of UEs.

In one implementation, the request may be sent to each UE of the subset separately. Alternatively, a single request may be sent to the subset which may be distributed by the subset, e.g. a lead UE of the subset, to the UEs of the subset. In another implementation the request may be sent as a broadcast message to all UEs, which contains some individual settings for selected UEs (which the other UEs may discard).

In an exemplary implementation form of the coordination device, the spatial diversity transmission scheme comprises one of a space time block code, STBC, space frequency block code, SFBC or frequency shift transmit diversity, FSTD, scheme.

This provides the advantage that the best transmission scheme can be selected according to the specific environment.

In an exemplary implementation form of the coordination device, the spatial order of a respective UE indicates a U E-specific modulation sequence of the spatial diversity transmission scheme and a unique pilot pattern to be used by the respective UE. This provides the advantage that the link diversity can be easily detected and exploited.

The UE-specific modulation sequence may be a UE-specific code sequence of the spatial diversity transmission scheme.

This provides the advantage that each UE can be identified by its specific code sequence.

In an exemplary implementation form of the coordination device, the coordination device is configured to assign the transmit power to a respective UE based on information about reception power level requirements of a corresponding receiving device. The

corresponding receiving device may be the receiver of the coordination device that receives signals from the respective UE.

This provides the advantage that the maximum power level at the receiving device is not exceeded.

In one example, the coordination device may assign the same transmit power to all UEs of the subset. For this configuration, the best results with respect to overall signal and interference level could be observed.

In an exemplary implementation form of the coordination device, the coordination device is configured to: select a second subset of UEs from the plurality of UEs for a second retransmission of the packet; and send a request for the second retransmission of the packet to each UE of the second subset of UEs.

This provides the advantage that a non-optimal selected first subset can be replaced by a better selected second subset or alternatively that HARQ retransmission by different user sets can be allowed.

In an exemplary implementation form of the coordination device, the request for the second retransmission comprises a spatial diversity transmission scheme selected for the second subset of UEs, a spatial order and transmit power assigned to a respective UE of the second subset of UEs. Such a coordination device allows to even better exploit the link diversity made available by the UEs by exploiting the link diversity of different selected subsets of UEs.

In an exemplary implementation form of the coordination device, the coordination device is configured to reuse the spatial diversity transmission scheme, the spatial order and transmit power from the first subset of UEs for the second subset of UEs.

Reusing the spatial diversity transmission scheme from the first subset of UEs for the second subset of UEs means that the same spatial diversity transmission scheme selected for the first subset shall also be selected for the second subset and shall be included in the request for the second retransmission, in particular if both subsets are of the same size, i.e., have the same number of UEs. If both subsets are not of the same size, at least the same type of diversity transmission scheme with respect to space, time and/or frequency, e.g. from space time block code, space frequency block code, or frequency shift transmit diversity scheme shall be applied for the second subset of UEs.

In an exemplary implementation form of the coordination device, the coordination device is configured to select further subsets of UEs from the plurality of UEs for further retransmissions of the packet. For example, each of the subsets of UEs may be disjoint to the other subsets of UEs. Alternatively each subset may differ in one or more elements from each other subset, i.e. some elements may be equal in some or all subsets.

Such a coordination device allows to even better exploit the link diversity made available by the UEs by exploiting the link diversity of a multiple number of subsets of UEs.

In an exemplary implementation form of the coordination device, for a cooperative radio transmission in downlink direction, the coordination device is a dedicated UE from the plurality of UEs; and for a cooperative radio transmission in uplink direction, the coordination device is a base station.

This provides the advantage that the coordination device can be simply chosen by one of the available devices in the wireless network, e.g. gNB or one of the UEs.

In one example, the coordination device may be a gNB. Hence such coordination device is suitable for application in 5G networks. In an exemplary implementation form of the coordination device, the dedicated UE is configured to send the requests for retransmission of the packet via sidelink to the UEs of the subset; and the base station is configured to send the requests for retransmission of the packet via downlink to the UEs of the subset.

This provides the advantage, that use of resources for the retransmissions on the sidelink can be kept to a minimum while attaining high reliability and fulfilling latency constraints.

According to a second aspect, the invention relates to an assistance device for assisting cooperative radio transmission between user equipments, UEs, of a plurality of UEs in a wireless network, wherein the assistance device is configured to: receive a packet from a UE of the plurality of UEs or a base station; receive scheduling information for scheduling radio resources for at least one retransmission of the packet; receive a request for a first retransmission of the packet from a coordination device, wherein the request indicates a spatial diversity transmission scheme, a spatial order and a transmit power assigned to the assistance device; and retransmit the packet to the coordination device on the radio resources scheduled for a first retransmission by using the spatial diversity transmission scheme, the spatial order and the transmit power assigned to the assistance device.

Such an assistance device allows to exploit the link diversity made available by the UEs of the subset of UEs (e.g. cars driving in the direct neighborhood of the car using the coordination device in a V2X scenario). Hence, the use of resources for the

retransmissions on the sidelink can be kept to a minimum while attaining high reliability and fulfilling latency constraints.

The assistance device may be a user equipment (UE). The assistance device may be a car or may be arranged or installed in a car. The plurality of UEs may be cars as well or may be arranged or installed in cars, e.g. a platoon of cars or a plurality of cars driving within geographical neighborhood. In an implementation, the plurality of UEs may be arranged in a group of UEs. Such group comprises at least two UEs.

The spatial diversity transmission scheme may be from different type of diversity transmission scheme, e.g. providing diversity with respect to space, time and/or frequency, e.g. from space time block code, space frequency block code, or frequency shift transmit diversity schemes. Alternatively or additionally, the spatial diversity transmission scheme may depend on the number of transmit and/or receive antennas of the coordination device and/or the other UEs of the plurality of UEs.

In an exemplary implementation form of the assistance device, for a cooperative radio transmission in downlink direction, the packet and the scheduling information is received from the base station and the retransmission request is received from a dedicated UE from the plurality of UEs; and for a cooperative radio transmission in uplink direction, the packet is received from the dedicated UE and the retransmission request is received with the scheduling information from the base station.

This provides the advantage that the coordination device and the assistance device can be simply chosen by one of the available devices in the wireless network, e.g. gNB or one of the UEs.

In an exemplary implementation form of the assistance device, the assistance device is configured to: receive a request for a second retransmission of the packet from the coordination device, wherein the request indicates a spatial diversity transmission scheme and a spatial order and transmit power assigned to the assistance device for the second retransmission; and retransmit the packet to the coordination device on the radio resources scheduled for the second retransmission by using the spatial diversity transmission scheme and the spatial order and transmit power assigned to the assistance device for the second retransmission.

Such an assistance device allows to better exploit the link diversity made available by the UEs by exploiting the link diversity of different selected subsets of UEs.

In an exemplary implementation form of the assistance device, the assistance device is configured to: receive an acknowledgement, ACK, from the coordination device; and forward the ACK to the UE or the base station from which the packet was initially received by using the spatial diversity transmission scheme and the spatial order and transmit power assigned to the assistance device for the previous retransmission.

“Initially received” means that it is the base station/UE where the packet originates from. This provides the advantage that all network elements, i.e. UEs and base station, are informed about a successful retransmission.

According to a third aspect, the invention relates to a method for coordinating cooperative radio transmission between user equipments, UEs, of a plurality of UEs in a wireless network, wherein the method comprises: selecting a subset of UEs from the plurality of UEs and a spatial diversity transmission scheme for a first retransmission of a packet; assigning to each UE of the subset of UEs a spatial order and a transmit power for the first retransmission of the packet; and sending a request for the first retransmission of the packet to the UEs of the subset of UEs, wherein the request indicates the spatial diversity transmission scheme, the spatial order and the transmit power assigned to the respective UE.

Such a method allows to exploit the link diversity made available by the UEs of the subset of UEs (e.g. cars driving in the direct neighborhood of the car using the coordination device in a V2X scenario). Hence, the use of resources for the retransmissions on the sidelink can be kept to a minimum while attaining high reliability and fulfilling latency constraints.

According to a fourth aspect, the invention relates to a method for assisting cooperative radio transmission between user equipments, UEs, of a plurality of UEs in a wireless network, wherein the method comprises: receiving a packet from a UE of the plurality of UEs or a base station; receiving scheduling information for scheduling radio resources for at least one retransmission of the packet; receiving a request for a first retransmission of the packet from a coordination device, wherein the request indicates a spatial diversity transmission scheme, a spatial order and a transmit power assigned to the assistance device; and retransmitting the packet to the coordination device on the radio resources scheduled for a first retransmission by using the spatial diversity transmission scheme, the spatial order and the transmit power assigned to the assistance device.

Such a method allows to exploit the link diversity made available by the UEs of the subset of UEs (e.g. cars driving in the direct neighborhood of the car using the coordination device in a V2X scenario). Hence, the use of resources for the retransmissions on the sidelink can be kept to a minimum while attaining high reliability and fulfilling latency constraints. According to a fifth aspect, the invention relates to a computer program product including computer executable code or computer executable instructions that, when executed, causes at least one computer to execute the method according to the third or fourth aspect. Such a computer program product may include a non-transient readable storage medium storing program code thereon for use by a processor, the program code comprising instructions for performing the methods or the computing blocks as described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Further embodiments of the invention will be described with respect to the following figures, in which:

Fig. 1 shows a schematic diagram 100 illustrating an exemplary V2X scenario for cooperative retransmission of URLLC packets via sidelink;

Fig. 2 shows a schematic diagram 200 illustrating an exemplary cooperative transmission scheme according to the disclosure applying STBC;

Fig. 3 shows an exemplary message sequence chart for a downlink cooperative transmission scheme 300 according to the disclosure;

Fig. 4 shows an exemplary message sequence chart for an uplink cooperative transmission scheme 400 according to the disclosure;

Fig. 5 shows a block diagram illustrating a coordination device 500 for coordinating cooperative radio transmission according to the disclosure;

Fig. 6 shows a block diagram illustrating an assistance device 600 for assisting cooperative radio transmission according to the disclosure;

Fig. 7 shows a schematic diagram of a method 700 for coordinating cooperative radio transmission according to the disclosure; Fig. 8 shows a schematic diagram of a method 800 for assisting cooperative radio transmission according to the disclosure; and

Fig. 9 shows a performance diagram 900 illustrating examples of attainable reliability for cooperative retransmission via sidelink versus number of neighbor UEs.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following detailed description, reference is made to the accompanying drawings, which form a part thereof, and in which is shown by way of illustration specific aspects in which the disclosure may be practiced. It is understood that other aspects may be utilized and structural or logical changes may be made without departing from the scope of the present disclosure. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims.

It is understood that comments made in connection with a described method may also hold true for a corresponding device or system configured to perform the method and vice versa. For example, if a specific method step is described, a corresponding device may include a unit to perform the described method step, even if such unit is not explicitly described or illustrated in the figures. Further, it is understood that the features of the various exemplary aspects described herein may be combined with each other, unless specifically noted otherwise.

The methods, devices and systems described herein may particularly be implemented in wireless communication networks based on 5G NR (New Radio) mobile communication standards and beyond.

The methods, devices and systems described herein may also be implemented in wireless communication networks based on mobile communication standards such as LTE, in particular 3G, 4G and 4.5G. The methods, devices and systems described herein may also be implemented in wireless communication networks, in particular

communication networks similar to WiFi communication standards according to IEEE 802.1 1 . The described devices may include integrated circuits and/or passives and may be manufactured according to various technologies. For example, the circuits may be designed as logic integrated circuits, analog integrated circuits, mixed signal integrated circuits, optical circuits, memory circuits and/or integrated passives.

The devices described herein may be configured to transmit and/or receive radio signals. Radio signals may be or may include radio frequency signals radiated by a radio transmitting device (or radio transmitter or sender). However, devices described herein are not limited to transmit and/or receive radio signals, also other signals designed for transmission in deterministic communication networks may be transmitted and/or received.

The devices and systems described herein may include processors or processing devices, memories and transceivers, i.e. transmitters and/or receivers. In the following description, the term“processor” or“processing device” describes any device that can be utilized for processing specific tasks (or blocks or steps). A processor or processing device can be a single processor or a multi-core processor or can include a set of processors or can include means for processing. A processor or processing device can process software or firmware or applications etc.

Fig. 1 shows a schematic diagram 100 illustrating an exemplary V2X scenario for cooperative retransmission of URLLC packets via sidelink.

A“Car A“ 1 10 is driving on a highway, which is served by a gNB 120 with an ultra-reliable low-latency communications (URLLC) service (vehicle to network, V2N, communication). A truck 130 driving next to the car 1 10 may create a deep shadow for the car 1 10, such that its communication link 1 16 to the gNB 120 is completely obstructed. Then, other cars 1 1 1 , 1 12 in the vicinity of“Car A” 1 10 can support the communication by cooperative retransmissions via the sidelink.

The cooperative retransmission scenario as shown in Fig.1 can be described by the following procedure, assuming URLLC downlink transmission from gNB 120 to car A 1 10:

1. Car A 1 10 provides list of neighbor cars to gNB 120. From the provided list, gNB 120 selects those cars with reasonable link quality (V2N link) and forms a user group for multicast transmission, which includes car A 1 10. 2. The URLLC packet is multicast to the pre-determined group, including scheduling information for potential packet retransmission via the sidelink.

3. If car A 1 10 did not receive the packet, it requests retransmission 102 from gNB 120 via V2N link 1 16 and from the cars 1 1 1 , 1 12 in its neighbor list via vehicle to vehicle (V2V) link 1 17.

4. The cars 1 1 1 , 1 12 which received the request 102 and successfully received the URLLC packet from the gNB 120 retransmit 103 the packet to car A 1 10 via the sidelink using the pre-scheduled frequency resource(s).

5. If the packet still cannot be decoded by car A 1 10, car A 1 10 may send another retransmission request (e.g. to a subset of the neighbor cars 1 1 1 , 1 12).

6. Car A 1 10 can combine the packets received from the different transmissions 103 and decode the packet while fulfilling the latency constraints.

For the retransmission via sidelink in step 4, existing retransmission schemes also consider the cooperative transmission of the packet by several cars simultaneously in the same resource. Therefore, the cooperating cars 1 10, 1 1 1 , 1 12 use either the single- frequency-network (SFN) mode, where the cars transmit the same signal without further preprocessing, or they use the Alamouti space time code, where the transmit signal is precoded by one out of two orthogonal transmit sequences. The transmit sequence is chosen randomly by each car selected for packet retransmission.

However, these existing solutions suffer from the following disadvantages: By using SFN mode, no actual spatial diversity gains provided by the independent communication links from different cars can be realized (see also the performance evaluations given in Fig. 9). Alamouti code can realize only two-fold spatial diversity, such that the diversity gain is strictly limited and does not scale with the number of cooperating users. By applying the novel schemes presented in this disclosure and described below, exploiting the full diversity gains available in the system provides much higher overall reliability, which comes at little additional costs for the signaling only.

Fig. 2 shows a schematic diagram 200 illustrating an exemplary cooperative transmission scheme according to the disclosure applying space-time block coding (STBC).

Space-time block coding is a technique used in wireless communications to transmit multiple copies of a data stream across a number of antennas and to exploit the various received versions of the data to improve the signal quality and hence the reliability of data transfer. The fact that the transmitted signal must traverse a potentially difficult

environment with scattering, reflection, refraction and so on and may then be further corrupted by thermal noise in the receiver means that some of the received copies of the data will be 'better' than others. This redundancy results in a higher chance of being able to use one or more of the received copies to correctly decode the received signal. In fact, space-time coding combines all the copies of the received signal in an optimal way to extract as much information from each of them as possible.

Fig. 2 is an example for using STBC 210 in a distributed fashion for cooperative

transmissions via sidelink. Transmit antennas here illustrate the spatial order, where a first spatial order 21 1 is assigned to neighbor car UE1 , 1 1 1 , a second spatial order 212 is assigned to neighbor car UE2, 1 12 and a third spatial order 213 is assigned to neighbor car UE3, 1 13.

The right side of Fig. 2 illustrates an exemplary STBC 210 represented by a matrix which columns represent the modulation sequences for the spatial orders assigned to the respective UEs. For example, first column 221 corresponds to a modulation sequence specified for the first spatial order 21 1 assigned to UE1 , 1 1 1. Second column corresponds to a modulation sequence specified for the second spatial order 212 assigned to UE2, 1 12, etc.

Fig. 3 shows an exemplary message sequence chart for a downlink cooperative

transmission scheme 300 according to the disclosure. Data is transmitted by the gNB 120 in downlink direction 315 to Car A, 1 10, Car B, 1 1 1 and Car C, 1 12.

The message sequence 300 includes the following steps:

1. Car A 1 10 provides list of neighbor cars 1 1 1 , 1 12 to gNB 120, i.e. it shares list of neighbor cars 1 1 1 , 1 12 with good vehicle-to-vehicle (V2V) link. From the provided list, gNB 120 selects those cars with reasonable link quality (V2N link) and forms a user group 302 for multicast transmission, which includes car A 1 10.

2. The URLLC packet is multicast 303a, 303b, 303c to the pre-determined group 1 10, 1 1 1 , 1 12, including scheduling information for potential packet retransmission via the sidelink. 3. If car A 1 10 did not receive the packet, it requests retransmission 304a, 304b from the cars 1 1 1 , 1 12 in its neighbor list via vehicle to vehicle (V2V) link, i.e. sidelink.

4. The cars 1 1 1 , 1 12 which received the request 304a, 304b and successfully

received the URLLC packet from the gNB 120 retransmit 305a, 305b the packet to car A 1 10 via the sidelink using the pre-scheduled frequency resource(s).

5. If the packet still cannot be decoded 306 by car A 1 10, car A 1 10 may send

another retransmission request 307 (e.g. to a subset of the neighbor cars 1 1 1 ,

1 12).

6. Car A 1 10 can combine the packets received from the different transmissions 309 and decode the packet while fulfilling the latency constraints.

7. Car A 1 10 sends ACK 310 after successful packet reception to the subset that executed the last retransmission (e.g. Car B, 1 1 1 ).

8. Car B 1 1 1 forwards 31 1 ACK to gNB 120. It may also use latest TX scheme,

reusing spatial order and TX power from previous attempt.

With respect to the message sequence described above with respect to Fig. 1 , steps 2 to

5 can be enhanced as described in the following:

2. gNB 120 schedules radio resources for a single packet only. It may schedule

resources for additional retransmissions in successive transmission time intervals (TTIs).

3. Car A 1 10 selects a subset of the cars 1 1 1 , 1 12 and a spatial diversity

transmission scheme (e.g. STBC 210 described above with respect to Fig. 2), supporting as many transmit antennas as there are cars in the subset. It further assigns each selected car a spatial order (which specifies the particular modulation sequence used by the car for data transmission and the particular pilot pattern used for channel estimation) and its transmit power. Selected subset of users, selected spatial diversity scheme, spatial order and transmit power per user are shared via sidelink in the retransmission request.

4. The cars selected for cooperative retransmission (e.g. Car B 1 1 1 and Car C 1 12 shown in Fig. 3) use the identical pre-scheduled frequency resource in the sidelink for retransmitting the URLLC packet by applying the spatial diversity transmission scheme in a distributed fashion with the assigned transmit power.

5. If the packet still cannot be decoded by car A 1 10, car A 1 10 selects another

subset of cars for retransmission (redoing steps 3 & 4). In the solution according to the disclosure, all cooperative cars may retransmit the identical URLLC packet (encoded with a fixed modulation coding scheme, MCS), which is then only pre-coded by a sequence corresponding to the chosen spatial diversity transmission scheme. For the example of an STBC, this particular precoding is reflected by a single column of that STBC 210 (as shown in Fig. 2). For exploiting the full diversity gains at the receiver (i.e. car A 1 10), the receiver needs to properly estimate the channel responses for the link between each neighboring car 1 1 1 , 1 12 and car A 1 10, which can be done by assigning a unique pilot sequence to each neighboring car 1 1 1 , 1 12 selected for the cooperative transmission. As the sequences for spatial diversity transmission scheme as well as the pilot sequences can be predefined, these can be specified by a single value, which is termed the spatial order (see Fig. 2). This spatial order needs to be assigned by the coordinator before the retransmission can be carried out.

The other value that needs to be specified by the coordinator is the transmit power to be used by each neighboring car 1 1 1 , 1 12. This is essential since, on the one hand, an excessive power level at the receiver should be avoided, and on the other hand, it is reasonable to adjust the transmit power per each neighboring car 1 1 1 , 1 12 according to its individual distance to car A 1 10. As neighboring cars that are significantly farther away than others will not provide a substantial contribution for improving the signal level at the receiver, it is reasonable to distribute the transmit power over the cooperating cars 1 10,

1 1 1 , 1 12 in a way that the average expected performance gain can be maximized. One solution can be the application of a waterfilling algorithm based on the average signal-to- interference-plus-noise ratios (SINRs) of each link between car A 1 10 and its neighbors 1 1 1 , 1 12, which have been measured in the recent past. The power allocation is carried out by the coordinator, and the results are then shared with the neighboring cars 1 1 1 , 1 12 selected for retransmission in the retransmission request 304a, 304b. Another solution is the so called“equal power allocation”, where all neighboring cars selected to perform the retransmission are allocated the same transmit power. This solution has shown to provide the best results, given that the neighbor cars are properly selected.

Compared to existing solutions, the new scheme can provide the following advantages: Full exploitation of the available diversity gains (quantitative results are shown in Fig. 9); Spare use of resources: If spatial diversity schemes with rate 1 are used (such as STBC or quasi-STBC), the resources used for the cooperative transmission are basically the same as for the case of a single retransmission. Only minor additional overhead needs to be considered due to the additional pilot sequences needed to estimate the channels to all neighboring cars; By decoding the pilot sequence, car A 1 10 can determine which car responded to its retransmission request 304a, 304b. This information may also be used for selecting the UEs 1 1 1 , 1 12 for a potential second retransmission.

Fig. 4 shows an exemplary message sequence chart for an uplink cooperative

transmission scheme 400 according to the disclosure.

As described above, the solution can also be applied in the uplink, i.e. transmission from car A 1 10 to the gNB 120. Then the entire routine can be described as follows:

1. Car A 1 10 transmits 401 a its URLLC packet to the gNB 120 and broadcasts it

401 b, 401c simultaneously to the cars 1 1 1 , 1 12 in its neighbor list via the sidelink.

2. If the gNB 120 did not receive the packet, it requests retransmission 402b, 402c from the cars in the neighbor list to which the gNB 120 maintains V2N links of good quality. It further provides scheduled resources for a single packet for the retransmission, a selected spatial diversity transmission scheme (e.g. 210 as described above with respect to Fig. 2) and assigns each selected car 1 1 1 , 1 12 a spatial order (e.g. as described above with respect to Fig. 2) and its transmit power.

3. If the cars 1 1 1 , 1 12 addressed by the gNB 120 received the URLLC packet from car A 1 10, they retransmit it 403b, 403c to the gNB 120 in the identical scheduled resources by applying the transmission scheme with the assigned transmit power.

4. If gNB 120 still fails to decode the packet 404, it selects another subset of cars (e.g. including car C 1 12) with good V2N link quality and requests retransmission 405 from these 1 12. Updated scheduling information, new transmission scheme and assignment of spatial order and transmit power is provided to this subset.

5. The cars 1 12 in the subset retransmit 406 the packet in the identical scheduled resources applying the transmission scheme with the assigned transmit power.

6. gNB 120 can combine the packets 407 received from all transmissions and decode the packet while fulfilling latency constraints.

7. gNB 120 sends ACK 408 after successful packet reception to the subset that

executed the last retransmission (e.g. Car C, 1 12 in this example of Fig. 4).

8. Car C 1 12 forwards 409 ACK to Car A 1 10. Based on the above description and the signaling diagrams, the key features of the new scheme can be summarized as follows:

The STBC based cooperative retransmission is coordinated by the dedicated UE (the final receiver of the URLLC packet) in the DL and by the gNB in the UL. The coordinator may correspond to a coordination device 500 as described below with respect to Fig. 5. The other UEs of the selected subsets may be assistance devices, e.g. corresponding to the assistance device 600 as described below with respect to Fig. 6.

The coordinator selects a subset of neighboring UEs for each retransmission and a corresponding spatial diversity transmission scheme supporting as many TX antennas as there are cars in that subset to enable the full diversity gains.

The coordinator assigns the“spatial order” to each UE, which determines the particular modulation sequence from the spatial diversity transmission scheme and the unique pilot pattern to be used by each UE.

The coordinator selects the transmit power per each UE to avoid too high signal power at the receiver, which would drive the power amplifier into saturation.

The coordinator signals the selected spatial diversity transmission scheme, the spatial order and the transmit power per each UE to the subset of neighbor UEs selected for retransmission.

If more than n>1 retransmission can be performed, coordinator forms n disjoint subsets of neighbor UEs for the retransmissions. If the sets are of identical size, STBC and power control values determined for the first retransmission may be reused.

ACK forwarding is carried out by the UEs having performed the last retransmission, reusing STBC and power control values applied in that last retransmission.

Fig. 5 shows a block diagram illustrating a coordination device 500 for coordinating cooperative radio transmission according to the disclosure. The coordination device 500 may correspond to a UE, e.g. UE 1 10 described above with respect to Figures 1 to 4, or may correspond to a base station, e.g. gNB 120. The coordination device 500 can be used for coordinating cooperative radio transmission between user equipments (UEs) of a plurality of UEs 51 1 , 512, 513, 514, 515 in a wireless network.

The coordination device 500 is configured to select 501 a subset 510 of UEs from the plurality of UEs 51 1 , 512, 513, 514, 515 and a spatial diversity transmission scheme, e.g. a scheme 210 as described above with respect to Fig. 2, for a first retransmission of a packet.

The coordination device 500 is configured to assign 502 to each UE 51 1 , 512, 513, 514 of the subset 510 of UEs a spatial order, e.g. a spatial order 21 1 , 212, 213 as described above with respect to Fig. 2, and a transmit power for the first retransmission of the packet.

The coordination device 500 is configured to send 503 a request for the first

retransmission of the packet to each UE 51 1 , 512, 513, 514 of the subset 510 of UEs. The request comprises the spatial diversity transmission scheme 210, the spatial order 21 1 , 212, 213 and the transmit power assigned to the respective UE, e.g. as described above with respect to Figs. 3 and 4.

The spatial diversity transmission scheme 210 may comprise one of a space time block code, STBC 210, space frequency block code, SFBC or frequency shift transmit diversity, FSTD, scheme, e.g. as described above with respect to Fig. 2.

The spatial order 21 1 , 212, 213 of a respective UE may indicate a UE-specific modulation sequence 221 of the spatial diversity transmission scheme 210 and a unique pilot pattern to be used by the respective UE, e.g. as described above with respect to Fig. 2.

The coordination device 500 may assign the transmit power to a respective UE (e.g. UE 1 12) based on information about reception power level requirements of a corresponding receiving device. The corresponding receiving device may be the receiver of the coordination device that receives signals from the respective UE. The coordination device 500 may select a second subset of UEs from the plurality of UEs 51 1 , 512, 513, 514, 515 for a second retransmission of the packet; and may send a request for the second retransmission of the packet to each UE of the second subset of UEs. The request for the second retransmission may comprise a spatial diversity transmission scheme (e.g. as scheme 210 according to Fig. 2) selected for the second subset of UEs, a spatial order (e.g. a spatial order 21 1 , 212, 213 according to Fig. 2) and transmit power assigned to a respective UE of the second subset of UEs.

The coordination device 500 may reuse the spatial diversity transmission scheme (e.g. scheme 210 according to Fig. 2), the spatial order (e.g. 21 1 , 212, 213 according to Fig. 2) and transmit power from the first subset (510) of UEs for the second subset of UEs, in particular if both subsets are of the same size.

The coordination device 500 may select further subsets of UEs from the plurality of UEs 51 1 , 512, 513, 514, 515 for further retransmissions of the packet. One option is that each of the subsets of UEs is disjoint to the other subsets of UEs. However, subsets do not necessarily need to be disjoint. Another option is to let the same user retransmit the packet in another retransmission, in particular if it can be allowed to use more power (which is the case if the subsets do not have the same size).

For a cooperative radio transmission 300 in downlink direction 315, e.g. as described above with respect to Fig. 3, the coordination device 500 may be a dedicated UE 1 10 from the plurality of UEs. For a cooperative radio transmission 400 in uplink direction 415, e.g. as described above with respect to Fig. 4, the coordination device 500 may be a base station 120.

The dedicated UE 1 10 may send the requests for retransmission of the packet via sidelink to the UEs 51 1 , 512, 513, 514 of the subset 510. The base station 120 may send the requests for retransmission of the packet via downlink to the UEs 51 1 , 512, 513, 514 of the subset 510.

Fig. 6 shows a block diagram illustrating an assistance device 600 for assisting cooperative radio transmission according to the disclosure. The assistance device 600 may correspond to a UE, e.g. one of the UEs 1 1 1 , 1 12 described above with respect to Figures 1 to 4. The assistance device 600 can be used for assisting cooperative radio transmission between user equipments (UEs) of a plurality of UEs 51 1 , 512, 513, 514, 515 in a wireless network.

The assistance device 600 is configured to receive 601 a packet from a UE of the plurality of UEs 51 1 , 512, 513, 514 or a base station 120, e.g. as described above with respect to Figures 5, 3 and 4. The assistance device 600 is configured to receive 602 scheduling information for scheduling radio resources for at least one retransmission of the packet. The assistance device 600 is configured to receive 603 a request for a first retransmission of the packet from a coordination device (e.g. coordination device 500 according to Fig.

5), wherein the request indicates a spatial diversity transmission scheme (e.g. scheme 210 according to Fig. 2), a spatial order (e.g. 21 1 , 212, 213 according to Fig. 2) and a transmit power assigned to the assistance device 600. The assistance device 600 is configured to retransmit 604 the packet to the coordination device 500 on the radio resources scheduled for a first retransmission by using the spatial diversity transmission scheme 210, the spatial order 21 1 , 212, 213 and the transmit power assigned to the assistance device 600.

For a cooperative radio transmission 300 in downlink direction 315, e.g. as shown above with respect to Fig. 3, the packet and the scheduling information is received from the base station 120 and the retransmission request is received from a dedicated UE (e.g. UE 1 10, i.e. car A) from the plurality of UEs. For a cooperative radio transmission 400 in uplink direction 415, e.g. as shown above with respect to Fig. 4, the packet is received from the dedicated UE 1 10 and the retransmission request is received with the scheduling information from the base station 120.

The assistance device 600 may receive a request for a second retransmission of the packet from the coordination device 500, wherein the request indicates a spatial diversity transmission scheme 210 and a spatial order 21 1 , 212, 213 and transmit power assigned to the assistance device 600 for the second retransmission. The assistance device 600 may retransmit the packet to the coordination device 500 on the radio resources scheduled for the second retransmission by using the spatial diversity transmission scheme 210 and the spatial order and transmit power assigned to the assistance device 600 for the second retransmission. The assistance device 600 may receive an acknowledgement, ACK 310, 408, e.g. as shown above with respect to Figs. 3 and 4, from the coordination device 500; and forward 31 1 , 409 the ACK to the UE or the base station from which the packet was initially received by using the spatial diversity transmission scheme and the spatial order and transmit power assigned to the assistance device for the previous retransmission.

Fig. 7 shows a schematic diagram of a method 700 for coordinating cooperative radio transmission according to the disclosure. The method 700 can be applied for coordinating cooperative radio transmission between user equipments, UEs, of a plurality of UEs in a wireless network, e.g. as described above with respect to Figures 1 to 6.

The method 700 comprises selecting 701 a subset of UEs from the plurality of UEs and a spatial diversity transmission scheme for a first retransmission of a packet, e.g. as described above with respect to Fig. 5.

The method 700 comprises assigning 702 to each UE of the subset of UEs a spatial order and a transmit power for the first retransmission of the packet, e.g. as described above with respect to Fig. 5.

The method 700 comprises sending 703 a request for the first retransmission of the packet to the UEs of the subset of UEs, wherein the request indicates the spatial diversity transmission scheme, the spatial order and the transmit power assigned to the respective UE, e.g. as described above with respect to Fig. 5.

Fig. 8 shows a schematic diagram of a method 800 for assisting cooperative radio transmission according to the disclosure. The method 800 can be applied for assisting cooperative radio transmission between user equipments, UEs, of a plurality of UEs in a wireless network, e.g. as described above with respect to Figs. 1 to 6.

The method 800 comprises receiving 801 a packet from a UE of the plurality of UEs or a base station, e.g. as described above with respect to Fig. 6. The method 800 comprises receiving 802 scheduling information for scheduling radio resources for at least one retransmission of the packet, e.g. as described above with respect to Fig. 6.

The method 800 comprises receiving 803 a request for a first retransmission of the packet from a coordination device, wherein the request indicates a spatial diversity transmission scheme, a spatial order and a transmit power assigned to the assistance device, e.g. as described above with respect to Fig. 6.

The method 800 comprises retransmitting 804 the packet to the coordination device on the radio resources scheduled for a first retransmission by using the spatial diversity transmission scheme, the spatial order and the transmit power assigned to the assistance device, e.g. as described above with respect to Fig. 6.

Fig. 9 shows a performance diagram 900 illustrating examples of attainable reliability for cooperative retransmission via sidelink versus number of neighbor UEs. The diagram 900 shows the exemplary performance of the scheme in terms of the attainable reliability versus the number of neighboring cars involved in the cooperative retransmission process.

The following scenario is applied here: Each neighbor UE receives the packet dedicated for car A 1 10 in Uu downlink with probability p1. Neighbor UEs 1 1 1 , 1 12 retransmit packet via sidelink to car A 1 10. The performance diagram shows the differences between the disclosed novel cooperative transmission scheme (using STBC) 904, 905, 906, where UEs fully utilize the available diversity gains, and existing cooperative transmission schemes using single frequency network (SFN) mode 901 , 902, 903, where all UEs transmit the same signal on the identical resource without further precoding (baseline).

Figure 9 shows reliability vs. number of neighbor UEs for a guaranteed channel gain of -10 dB (compared to avg. SNR) and for a single retransmission (1 rtx) and HARQ and ARQ retransmissions (2 rtx) on V2V link.

The following assumptions apply: V2V channels are i.i.d. Rayleigh fading; 1 rtx (901 , 904): All simultaneously transmitting UEs share the total available transmit power equally. 2 rtx HARQ (903, 906): Half the UEs transmit in 1 st slot, other half in 2nd slot. 2nd half can listen to 1 st slot -> increased probability of successful packet reception. From Fig. 9, the following performance can be observed: The disclosed novel cooperative transmission scheme (using STBC) 904, 905, 906 significantly outperforms the simple SFN mode 901 , 902, 903, which shows saturation behavior. Dividing the set of neighboring cars into two subgroups and letting them transmit in two HARQ

retransmission cycles (graph 906) improves the reliability by two orders of magnitude compared to letting all neighboring users retransmit in a single retransmission cycle (graph 904). The disclosed novel cooperative transmission scheme (with HARQ retransmission) 906 can attain high reliabilities for small number of UEs; e.g. < 10-5 for 6 neighbor UEs.

The present disclosure also supports a computer program product including computer executable code or computer executable instructions that, when executed, causes at least one computer to execute the performing and computing steps described herein, in particular the methods and procedures described above. Such a computer program product may include a readable non-transitory storage medium storing program code thereon for use by a computer. The program code may perform the processing and computing steps described herein, in particular the methods and procedures described above.

While a particular feature or aspect of the disclosure may have been disclosed with respect to only one of several implementations, such feature or aspect may be combined with one or more other features or aspects of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms "include", "have", "with", or other variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term "comprise". Also, the terms "exemplary", "for example" and "e.g." are merely meant as an example, rather than the best or optimal. The terms“coupled” and “connected”, along with derivatives may have been used. It should be understood that these terms may have been used to indicate that two elements cooperate or interact with each other regardless whether they are in direct physical or electrical contact, or they are not in direct contact with each other.

Although specific aspects have been illustrated and described herein, it will be

appreciated by those of ordinary skill in the art that a variety of alternate and/or equivalent implementations may be substituted for the specific aspects shown and described without departing from the scope of the present disclosure. This application is intended to cover any adaptations or variations of the specific aspects discussed herein. Although the elements in the following claims are recited in a particular sequence with corresponding labeling, unless the claim recitations otherwise imply a particular sequence for implementing some or all of those elements, those elements are not necessarily intended to be limited to being implemented in that particular sequence. Many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the above teachings. Of course, those skilled in the art readily recognize that there are numerous applications of the invention beyond those described herein. While the present invention has been described with reference to one or more particular embodiments, those skilled in the art recognize that many changes may be made thereto without departing from the scope of the present invention. It is therefore to be understood that within the scope of the appended claims and their equivalents, the invention may be practiced otherwise than as specifically described herein.