TECHNICAL FIELD

The present disclosure relates to wireless communications in general. More specifically, the present disclosure relates to methods and devices for adaptive cyclic delay diversity (CDD) for sidelink transmission, for instance, in the 5th generation cellular network or 5 ^th generation mobile network (5G) wireless networks.

BACKGROUND

The use of diversity transmissions from multiple transmitting antennas or multiple transmission nodes (so called "multi-point" transmissions) to a destination node in a communication network is a known simple and effective way to improve robustness and reliability. These transmissions may or may not be part of a re-transmission.

Multi-point transmissions from multiple fixed base stations (such as transmission reception points (TRPs), evolved nodeBs (eNBs), next generation nodeBs (gNBs), remote radio heads (RRHs) and the like) to user equipments, UEs, in the downlink direction, have been discussed since long term evolution (LTE) Release 10 in the context of CoMP (Coordinated Multi Point). During this time and subsequently after Release 10 was standardized, various coherent and non-coherent transmission schemes have been discussed and studied.

A fully coherent multipoint transmission scheme requires that the multiple transmitting antennas or multiple TRPs are tightly time and frequency synchronized. Although this is not a problem when the transmitting antennas are fed from the same baseband transmitter (which is normally the case for co-located antennas from the same TRP) or possibly non-collocated TRPs which are connected with optical fibers, the use of coherent schemes for transmitters which have larger frequency and/or timing offsets is very challenging.

The most commonly used coherent schemes for multipoint diversity transmissions are based on space block codes (SBC), and are typically orthogonal in the time domain (= Space Time Block Code (STBC)) or in the frequency domain (=Space Frequency Block Code (SFBC). Both STBC and SFBC, however, have one or more of the following issues:

1) A full rate transmission cannot be achieved when the number of Tx (transmitting) antennas is greater than 2.

2) For more than 2 Tx antennas, the code rate needs to be reduced, or non-orthogonal codes need to be used. 3) Block codes with reduced rate or quasi-orthogonal (QO-STBC) properties have been designed for a larger number of antennas. In this case, however, the receiver needs to know the codes used and the number of active transmitters.

4) Furthermore, a receiver architecture is required which can handle the number of transmitters, and as the number of transmitters is increased, the requirements for the channel not to change in the frequency or the time domain are increased.

For device to device (D2D) communication, sidelink is used between devices. A transmitting device can help a receiving device to forward data or signaling to the receiving device. Currently, how to enable multiple transmitting devices transmit data to the receiving device in diversity mode is not disclosed yet.

SUMMARY

It is an objective to provide improved devices, methods and systems for adaptive cyclic delay diversity (CDD) for UE sidelink transmission, for instance, in 5G networks.

The foregoing and other objective are achieved by the subject matter of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

Generally, embodiments provide devices, methods and systems for improving the reliability of sidelink connections (UE to UE transmissions) in particular for 5G New Radio (NR), by utilizing multiple diversity transmissions from different UEs (herein referred to as "Tx UEs") to a target receiving UE (herein referred to as "target Rx UE" or short "Rx UE"). Moreover, embodiments of the invention provide methods and devices for non-coherent sidelink diversity transmission and for adaptive cyclic delay diversity (CDD) for each transmitting UE for three different cases which relate to which entity proposes, determines and assigns the cyclic delay to the different UE transmitters. These cases are as follows: (i) the cyclic delays for each Tx UE are determined and assigned by a network device, e.g. base station (assuming the UEs are in network coverage); (ii) the cyclic delays for each Tx UE are determined and assigned by the target Rx UE; and (iii) the cyclic delays for each Tx UE are proposed by the Tx UEs.

More specifically, according to a first aspect, a sidelink diversity transmission method is provided. The sidelink diversity transmission method comprises: obtaining, by a user equipment, UE, cyclic delay diversity (CDD) capability information, wherein the CDD capability information indicates the CDD capability of the UE for sidelink transmission; and transmitting, by the UE, a CDD capability report to a network device or to a second UE.

Thus, an improved method for adaptive cyclic delay diversity (CDD) for UE sidelink transmission diversity, for instance, in 5G wireless networks is provided.

As used herein, obtaining is to be understood that the UE can obtain the CDD capability information from a further device (i.e. further UE) in a direct way or indirect way (via a network device, in particular base station), or that the UE can determine the CDD capability information itself. As will be described in more detail below, the UE may be a target Rx UE or one of the Tx UEs.

In a further possible implementation form the method comprises receiving, by the UE, a CDD capability request from the network device. Thus, the CDD capability information may be obtained and transmitted in response to a request from the network device, e.g. base station or the target Rx UE.

In a further possible implementation form the method comprises: receiving, by the UE, a CDD assignment for data transmission on the sidelink; and transmitting, by the UE, data on the sidelink based on the CDD assignment. Thus, depending on their respective sidelink channel states, the Tx UEs may be assigned Tx UE specific CDD values, which may be different or the same for each Tx UE (also referred to as "CDD shifts" or "CDD delays" herein) for diversification of the overall sidelink channel. The CDD values may be assigned by a network device, e.g. base station or the target Rx UE.

In a further possible implementation form the method comprises transmitting, by the UE, a proposed CDD to the network device or the second UE. Thus, a Tx UE based on its knowledge about its CDD capabilities and the state of its sidelink channel may propose a CDD value to the network device, e.g. base station or the target Rx UE.

In a further possible implementation form the method comprises receiving, by the UE, a sidelink reference signal configuration. Thus, the network device, e.g. base station may define the configuration of the sidelink reference signals and/or the measurement thereof for the Tx UEs and/or the target Rx UE.

In a further possible implementation form the method comprises transmitting, by the UE, a sidelink reference signal for estimating the sidelink transmission channel by the Rx UE. Thus, the Tx UEs and/or the target Rx UE may transmit a sidelink reference signal (in accordance with the configuration defined by the network device) for estimating the respective sidelink channel between the Tx UEs and the target Rx UE in one or both directions.

In a further possible implementation form the method comprises transmitting, by the UE, a CDD assignment for data transmission on at least one sidelink. Thus, the target Rx UE and/or the Tx UEs may operate as a relay and forward a CDD assignment, i.e. a CDD value to a further Tx UE out of network coverage.

In a further possible implementation form the method comprises receiving, by the UE, a timing measurement or a timing advance, TA, assignment of another UE. When acting as a relay for a further out of network coverage Tx UE, an in-network coverage Tx UE or target Rx UE may receive a timing measurement or a TA assignment from the further out of network coverage Tx UE.

In a further possible implementation form the method comprises receiving, by the UE, commands to receive the CDD capability and/or timing advance, TA, assignment of another UE.

In a further possible implementation form the CDD capability information comprises one or more bits to indicate whether the UE supports CDD. Thus, the CDD values may be encoded and signalled with little signaling overhead.

According to a second aspect a sidelink diversity transmission method is provided, wherein the method comprises receiving, by a network device, a cyclic delay diversity, CDD, capability report from a user equipment, UE. The CDD capability report indicates the CDD capability of the UE for sidelink transmission. Thus, an improved method for adaptive cyclic delay diversity (CDD) for UE sidelink transmission diversity, for instance, in 5G wireless networks is provided.

In a further possible implementation form the method comprises sending, by the network device, a CDD capability request to the UE. Thus, the CDD capability information may be transmitted by the target Rx UE and/or a Tx UE in response to a request from the network device, e.g. base station or the target Rx UE.

In a further possible implementation form the method comprises sending, by the network device, a CDD assignment to the UE for data transmission on the sidelink. Thus, depending on their respective sidelink channel states, the network device, e.g. base station may assign the Tx UEs different (or sometimes similar or the same) CDD values (also referred to as "CDD shifts" or "CDD delays" herein) for sidelink diversification.

In a further possible implementation form the method comprises receiving, by the network device, a proposed CDD from the UE. Thus, a Tx UE based on its knowledge about its CDD capabilities and the state of its sidelink channel may propose a CDD value to the network device, e.g. base station and the network device, e.g. base station may perform the CDD assignment further on the basis of the CDD values proposed by the Tx UEs.

In a further possible implementation form the method comprises sending, by the network device, a sidelink reference signal configuration to the UE. Thus, the network device, e.g. base station may define the configuration of the sidelink reference signals and/or the measurement thereof for the Tx UEs and/or the target Rx UE.

In a further possible implementation form the method comprises sending, by the network device, a timing advance, TA, assignment to the UE. Thus, the base station may assign a TA to the Tx UEs and/or the target Rx UE. In case a Tx UE is not in coverage of the network device, e.g. base station, one of the in-coverage Tx UEs or the target Rx UE may operate as a relay and forward the TA assignment to the out-of-coverage Tx UE.

In a further possible implementation form the method comprises sending, by the network device, a request for sidelink reference signal scheduling for a second UE to a second network device. Thus, the network device, e.g. base station may coordinate reference signals from Tx UEs being served by itself and a further Tx UE being served by a further network device, e.g. base station. This request may be send via a Xn interface.

In a further possible implementation form the method comprises sending, by the network device, a CDD capability request to a second network device for acquiring CDD capability information of a second UE controlled by the second network device. Thus, the network device, e.g. base station may obtain the CDD capability information of a Tx UE being served by another network device, e.g. base station.

In a further possible implementation form the method comprises receiving, by the network device, a CDD capability report from the second network device, wherein the CDD capability report indicates the CDD capability of the second UE. Thus, the network device, e.g. base station may obtain the CDD capability information of a Tx UE being served by another network device, e.g. base station. In a further possible implementation form the method comprises sending, by the network device, a CDD assignment of the second UE to the second network device. Thus, the network device, e.g. base station may assign a CDD value to a Tx UE being served by another network device, e.g. base station.

In a further possible implementation form the CDD capability information comprises one or more bits to indicate whether the UE supports CDD. Thus, the CDD values may be encoded and signalled with little signaling overhead.

In a further possible implementation form the method comprises sending, by the network device, a CDD capability report response to the UE.

According to a third aspect a user equipment, UE, for sidelink diversity transmission is provided. The UE comprises a processor configured to: obtain cyclic delay diversity, CDD, capability information, wherein the CDD capability information indicates the CDD capability of the UE for sidelink transmission; and transmit a CDD capability report to a network device or a second UE. Thus, an improved UE for adaptive cyclic delay diversity (CDD) for UE sidelink transmission diversity, for instance, in 5G wireless networks is provided.

In addition to the processor the UE may comprise a non-transitory memory for storing and retrieving data and a communication interface for exchanging data, for instance, with the network device and/or other UEs.

The processor may be implemented in hardware and/or software. The hardware may comprise digital circuitry, or both analog and digital circuitry. Digital circuitry may comprise components such as application-specific integrated circuits (ASICs), field-programmable arrays (FPGAs), digital signal processors (DSPs), or general-purpose processors.

The non-transitory memory may store executable program code which, when executed by the processor, causes the UE to perform the functions and methods described herein. The communication interface may comprise one or more antennas and/or transmission ports for exchanging data, for instance, with the network device and/or other UEs.

The UE according to the third aspect can implement the method according to the first aspect. Further features of the UE according to the third aspect result directly from the steps and functionality of the method according to the first aspect and its different implementation forms described above and below.

According to a fourth aspect a network device is provided. The network device comprises a compressor configured to receive a cyclic delay diversity, CDD, capability report from a user equipment, UE, wherein the CDD capability report indicates the CDD capability of the UE for sidelink transmission. Thus, an improved network device, e.g. base station for adaptive cyclic delay diversity (CDD) for UE sidelink transmission diversity, for instance, in 5G wireless networks is provided.

In addition to the processor, the network device may comprises a non-transitory memory for storing and retrieving data and a communication interface for exchanging data, for instance, with the Rx UE, the Tx UEs, another network device and/or core network components.

The processor may be implemented in hardware and/or software. The hardware may comprise digital circuitry, or both analog and digital circuitry. Digital circuitry may comprise components such as application-specific integrated circuits (ASICs), field-programmable arrays (FPGAs), digital signal processors (DSPs), or general-purpose processors.

The non-transitory memory may store executable program code which, when executed by the processor, causes the network device to perform the functions and methods described herein. The communication interface may comprise signal processing circuitry, one or more antennas and/or transmission ports for exchanging data, for instance, with the Rx UE, the Tx UEs, another base station and/or core network components.

The network device according to the fourth aspect can implement the method according to the second aspect. Further features of the network device according to the fourth aspect result directly from the steps and functionality of the method according to the second aspect and its different implementation forms described above and below.

According to a fifth aspect a computer program product is provided comprising a non- transitory computer-readable storage medium carrying program code which causes a computer or a processor to perform the method according to the first aspect and/or the method according to the second aspect when the program code is executed by the computer or the processor. Details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description, drawings, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following embodiments are described in more detail with reference to the attached figures and drawings, in which:

Fig. 1a shows a schematic diagram illustrating an example of an in coverage sidelink diversity transmission scenario according to an embodiment;

Fig. 1b shows a schematic diagram illustrating an example of an out of coverage sidelink diversity transmission scenario according to an embodiment;

Fig. 2a shows a schematic diagram illustrating components of the target receiving UE and/or the transmitting UEs according to an embodiment;

Fig. 2b shows a schematic diagram illustrating components of the network device, e.g. base station according to an embodiment;

Figs. 3a and 3b show schematic diagrams illustrating examples of an in coverage sidelink diversity transmission scenario for different arrangements of the target receiving UE and the transmitting UEs according to an embodiment;

Fig. 4 shows a signaling diagram for the exemplary in coverage sidelink diversity transmission scenario of figure 1a according to an embodiment;

Fig. 5 shows a schematic diagram illustrating an example of a sidelink diversity transmission scenario according to an embodiment, where one of the transmitting UEs is out of network coverage;

Fig. 6 shows a signaling diagram for the exemplary sidelink diversity transmission scenario of figure 5 according to an embodiment;

Fig. 7 shows a schematic diagram illustrating an example of a sidelink diversity transmission scenario according to an embodiment, where the transmitting UEs are served by different base stations of the network; Fig. 8 shows a signaling diagram for the exemplary sidelink diversity transmission scenario of figure 7 according to an embodiment;

Fig. 9 shows a signaling diagram for an exemplary sidelink diversity transmission scenario according to an embodiment, where the CDD values for the transmitting UEs are determined and selected by the target receiving UE;

Fig. 10 shows a signaling diagram for an exemplary sidelink diversity transmission scenario according to an embodiment, where the CDD values for the transmitting UEs are proposed by the transmitting UEs to the serving base station;

Fig. 11 shows a signaling diagram for an exemplary sidelink diversity transmission scenario according to an embodiment, where the CDD values for the transmitting UEs are proposed by the transmitting UEs to the target receiving UE; and

Fig. 12 shows an exemplary table illustrating CDD configuration bits for defining different CDD configurations according to an embodiment.

In the following identical reference signs refer to identical or at least functionally equivalent features.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following description, reference is made to the accompanying figures, which form part of the disclosure, and which show, by way of illustration, specific aspects of embodiments or specific aspects in which embodiments may be used. It is understood that embodiments may be used in other aspects and comprise structural or logical changes not depicted in the figures. The following detailed description, therefore, is not to be taken in a limiting sense.

For instance, it is to be understood that a disclosure 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 one or a plurality of specific method steps are described, a corresponding device may include one or a plurality of units, e.g. functional units, to perform the described one or plurality of method steps (e.g. one unit performing the one or plurality of steps, or a plurality of units each performing one or more of the plurality of steps), even if such one or more units are not explicitly described or illustrated in the figures. On the other hand, for example, if a specific apparatus is described based on one or a plurality of units, e.g. functional units, a corresponding method may include one step to perform the functionality of the one or plurality of units (e.g. one step performing the functionality of the one or plurality of units, or a plurality of steps each performing the functionality of one or more of the plurality of units), even if such one or plurality of steps are not explicitly described or illustrated in the figures.

Further, it is understood that the features of the various exemplary embodiments and/or aspects described herein may be combined with each other, unless specifically noted otherwise.

As will be described in more detail below, embodiments are directed to the use and assignment of cyclic delay diversity (non-coherent transmitter, Tx, diversity) with different shifts for different participating UE transmitters (which may have one of more transmitting ports) for sending the same data to increase the reliability of sidelink transmission via spatial diversity. In this way, even if the different sidelink transmissions are predominantly line-of- sight, LoS, with similar delay profiles (and slightly different timing) the sidelink channel selectivity can be increased by cyclically shifting the different sidelinks in various ways.

By improving the channel selectivity of the overall link from multiple Tx UEs to the receiving UE, a better performance can be obtained since typically used transmission schemes (i.e. OFDM with channel coding) can exploit this diversity to yield a better performance. This will be described in more detail in the context of figures 1a and 1b, which show two exemplary scenarios, where embodiments can be advantageously employed.

In the scenario shown in figure 1a, a plurality of transmitting UEs 101 (referred herein to as "Tx UEs") and a target receiving UE 101 (referred herein to as "target Rx UE") are in coverage of a network device 103, such as a base station, gNB, TRP or the like.

The transmitting UE(s) 101 is/are the UE(s) used for forwarding data or signaling to the target receiving UE. Target receiving UE(s) is/are the sink UE(s) of data or signaling forwarded by transmitting UE(s). A UE may be transmitting UE and target receiving UE simultaneously, and the UE may forward data or signaling to other target receiving UE(s) and/or receiving data or signaling from other transmitting UE(s).

As illustrated in figure 1a, according to embodiments the plurality of Tx UEs 101 are configured to send the same data using different (or sometimes the same) CDD shifts via respective sidelinks to the target Rx UE 101. By way of example, the three Tx UEs 101 illustrated in figure 1a use no CDD shift ("UE 2"), a first CDD shift ("UE 1") and a second different CDD shift ("UE 3"), respectively. The UEs 101 illustrated in figure 1a may be, for instance, mobile phones, loT devices, smart vehicles and the like. As will be appreciated, in different scenarios the UEs 101 illustrated may operate sometimes as a Tx UE 101 and sometimes as a Rx UE 101 and sometimes as both.

In the scenario shown in figure 1b, the plurality of Tx UEs 101 and the target Rx UE 101 are out-of-coverage of a network device, e.g. base station 103. As illustrated in figure 1b, according to embodiments the plurality of Tx UEs 101 are configured to send the same data using different (or sometimes the same) CDD shifts via respective sidelinks to the target Rx UE 101. By way of example, the three Tx UEs 101 illustrated in figure 1b use no CDD shift ("UE 2"), a first CDD shift ("UE 1") and a second different CDD shift ("UE 3"), respectively.

Figure 2a shows a schematic diagram illustrating components of the target receiving UE 101 and/or the transmitting UEs 101 according to an embodiment. The UE 101 illustrated in figure 2a comprises a processor 111 for processing data, a non-transitory memory 113 for storing and retrieving data and a communication interface 115 for exchanging data, for instance, with the network device 103 and/or the other UEs.

The processor 111 may be implemented in hardware and/or software. The hardware may comprise digital circuitry, or both analog and digital circuitry. Digital circuitry may comprise components such as application-specific integrated circuits (ASICs), field-programmable arrays (FPGAs), digital signal processors (DSPs), or general-purpose processors.

The non-transitory memory 113 may store executable program code which, when executed by the processor 111, causes the UE 101 to perform the functions and methods described herein. The communication interface 115 may comprise one or more antennas and/or transmission ports for exchanging data, for instance, with the network device 103 and/or the other UEs.

Figure 2b shows a schematic diagram illustrating components of the network device, e.g. base station 103 according to an embodiment. The network device, in particular base station 103 illustrated in figure 2b comprises a processor 121 for processing data, a non-transitory memory 123 for storing and retrieving data and a communication interface 125 for exchanging data, for instance, with the Rx UE 101, the Tx UEs 101, another base station and/or core network components. The processor 121 may be implemented in hardware and/or software. The hardware may comprise digital circuitry, or both analog and digital circuitry. Digital circuitry may comprise components such as application-specific integrated circuits (ASICs), field-programmable arrays (FPGAs), digital signal processors (DSPs), or general-purpose processors.

The non-transitory memory 123 may store executable program code which, when executed by the processor 121, causes the network device, e.g. base station 103 to perform the functions and methods described herein. The communication interface 125 may comprise signal processing circuitry, one or more antennas and/or transmission ports for exchanging data, for instance, with the Rx UE 101, the Tx UEs 101, another base station and/or core network components.

According to an embodiment, the CDD assignment, i.e. the assignment of CDD shifts may be done by choosing and assigning to each Tx UE 101 a cyclic shift from a fixed set of CCD values, i.e. shifts (also referred to as delays herein). A possible set of shifts will be discussed further below in the context of figure 12.

To optimize performance, according to an embodiment they may be assigned different CDD shifts for the sidelink channels with similar channel delay characteristic. While sidelink channels which have different characteristics may be assigned with the same CDD shifts.

It should be noted that different Tx UEs may have the same CDD shift configuration or may have different CDD shift configurations.

As will be described in more detail below, embodiments address the challenge that usually only the target Rx UE 101 or the Tx UEs 101 (via TDD channel reciprocity) have full details of the different sidelink channel responses and the overhead of communicating the complete complex channel response (H) elsewhere is obviously too high.

Further below embodiments will be described that address this challenge and relate to one of the following three scenarios:

(i) the cyclic delays for each Tx UE 101 are determined and assigned by the network device, e.g. base station 103 (assuming the UEs 101 are in network coverage, as illustrated in the scenario of figure 1a); (ii) the cyclic delays for each Tx UE 101 are determined and assigned by the target Rx UE 101 (such as for the out-of-coverage scenario shown in figure 1b or for the in-coverage- scenario of figure 1a); and

(iii) the cyclic delays for each Tx UE 101 are proposed by the Tx UEs 101.

As will be described in more detail below, in embodiments either the target Rx UE 101 and/or a Tx UE 101 obtain cyclic delay diversity, CDD, capability information, wherein the CDD capability information indicates the CDD capability of the respective UE 101 for sidelink transmission; and transmit a CDD capability report to the network device, e.g. base station 103, the target Rx UE 101 or a Tx UE 101.

As used herein, obtaining CDD capability is to be understood that the UE 101 can obtain the CDD capability information from a further device, such as the network device, e.g. base station 103 or a further UE(s) 101, or that the UE 101 can determine the CDD capability information itself.

In the following embodiments will be described related to the first of the above scenarios, i.e. scenario (i), where the cyclic delays for each Tx UE 101 are determined and assigned by the network device, e.g. base station 103 (such as for the in-coverage scenario illustrated in figure 1a).

For this case, all the UEs 101 are in network coverage and it may be assumed that each UE 101 transmitting sidelink signals has the same timing as the uplink transmission which is controlled by the timing advance from the serving base station 103. This in line, with the current standardized systems for sidelinks in network coverage, i.e. 3GPP LTE V2x release 13 (R13), 3GPP LTE V2x R14 and 3GPP 5G NR V2x R16.

For this scenario, as can be seen in figures 3a and 3b, depending on the location of the Tx UEs 101 and the target Rx UE 101, there may or may not be an inherent time/phase shifting of the different sidelinks. Therefore, according to embodiments knowledge of the timing advance (TA) information for each Tx UE 101 and the respective sidelink channel measurement reports can be used by the serving network device 103, such as base station, eNB, gNB, TRP, RRH and the like, to estimate a timing offset and the states of the different sidelink channels, as will be described in more detail below. The TAs provide information on where the UEs 101 are located with respect to the serving base station 103. According to embodiments, sidelink channel measurements for each sidelink and in particular the CQI (channel quality information) or RSRP (Reference signal received power) at the target Rx UE 101 from the different Tx UEs 101 yield an estimate of how far the Tx UEs 101 are from the target Rx UE 101.

According to embodiments these sidelink measurements may be utilized together with the TA for each UE 101 to enable the serving base station 103 to understand the relative orientation of the Tx UEs 101 and the target Rx UE 101, so that the base station 103 can determine whether the channels are similar or not and whether they are likely to be line-of- sight, LoS, channels or not. In this way, the base station 103 can make decisions on how to assign the CDD values to each Tx UE 101.

In the following some exemplary embodiments will be described on how these inputs (sidelink measurements and/or TA for each UE 101) may be for different situations, and how this may relate to a possible decision on the CDD(s) to assign to each Tx UE 101.

As a first example, for a group of UEs (i.e. two Tx UEs 101 and the target Rx UE 101) which are very close to each other (say in a platoon) the TAs may be very similar to each other and the reported sidelink (SL) CQIs may be quite high. In this case, the sidelinks may have a very high LoS probability. Therefore, in this case, the base station 103 may assign these SLs different CDDs, i.e. different CDD delays or shifts to generate more spatial diversity in the multiple Tx to Rx channels.

As a further example, for a group of Tx UEs 101 which are further apart and have similar distances to the Rx UE 101 and also to the base station 103, the SL CQI values may be similar but lower than in the previous case. The TA values may also be similar. Therefore, in this case, the base station 103 may assign these sidelinks different CDDs for generating a more spatially diverse set of SL channels.

As a further example, for a group of Tx UEs 101 which are approximately equidistant to each other but have different distances to the serving base station 103, these may have similar SL CQIs as measured at the target Rx UE 101, but they may have very different TAs. Since the TAs may be different, the signals may arrive at the target Rx UE 101 with different timings. Therefore, in this case, the base station 103 may not need to assign different CDDs for these different sidelinks, since they may already have different time offsets. As a further example, for a group of Tx UEs 101 which have similar TAs (i.e. a similar distance from the serving base station 103), but very different CQIs because they may have different distances to the target Rx UE 101, the base station 103 may not need to assign each Tx UE 101 a different CDD value, since the different transmitted sidelinks may already be sufficiently different.

Therefore, it may be inferred that if the TAs are different, or if the sidelink CQI are different, it may not be needed to assign each Tx UE 101 a different CDD value, since there may already be sufficient diversity in the channel. However, if both the sidelink CQIs and the TAs are similar, the different sidelink channels may be very similar and assigning each Tx UE 101 a different CDD value may yield extra diversity.

An example of a possible signaling for the scenarios illustrated in figures 1a, 3a and 3b, i.e. when all UEs 101 are in network coverage is shown in Figure 4.

Here it is assumed that the candidate set of UEs that could be involved in sidelink diversity transmission, e.g. the set of Tx UEs 101 has already been established, for instance, by the base station 103 (referred to as "Serving gNB" in figure 4).

Once the UEs 101 are connected to the serving base station 103, e.g. by means of the radio access channel, RACH, (step 401 of figure 4) and are assigned their timing advances, TAs, from the serving base station 103 (steps 403 and 405 of figure 4), the UEs 101 transmit to the serving base station 103 their UE capabilities (step 407 of figure 4), including CDD capability information, e.g. information about their capability to transmit sidelink signals with cyclic delay diversity (CDD), in addition to the normal regular capability parameters (e.g. number of Tx ports). The base station 103 determines a configuration for sidelink measurements and informs the UEs 101 accordingly (steps 409 and 411 of figure 4).

Once the candidate Tx UEs 101 have transmitted sidelink reference signals to the target Rx UE 101 (step 413 of figure 4) in accordance with the configuration provided by the base station 103, the target Rx UE 101 sends measurement reports to the serving base station 103 (step 415 of figure 4). These measurement reports may, for instance, take the form of CQI, RSRP, or RSRQ measurements.

As illustrated in figure 4, optionally the Tx UEs 101 may send measurement reports to the base station 103 as well. The base station 103 then uses these measurements and the knowledge of the timing advance to pick the final set of Tx UEs 101 and to assign them the most suitable cyclic delay diversity (CDD) value for each Tx UE 101 (step 417 of figure 4).

According to an embodiment, the CDD values may be selected from a set of fixed values (i.e. 2 or 4 values) which can be represented by 1 or 2 bits of information, as will be described in more detail below. Once each of the selected Tx UEs 101 has been assigned their CDD (step 419 of figure 4), they then participate in the diversity transmission to the target Rx UE 101 (step 421 of figure 4).

A further embodiment is directed to the scenario, when one or more of the Tx UEs 101 used for the diversity connection to the target Rx UE 101 are out of network coverage. Such a scenario is shown in figure 5, where the target Rx UE 101 and a first Tx UE 101 (referred to as "UE A" in figure 5) are in coverage of the serving base station 103, while a second Tx UE 101 (referred to as "UE B" in figure 5) is out of network coverage.

An exemplary embodiment of the signaling, when one of the Tx UEs 101 is out-of-network coverage (e.g. for the scenario of figure 5) is shown in figure 6. The capability of the out-of- network second Tx UE 101 ("UE B") is relayed by a UE in network coverage (in this example, the Rx UE 101) and the out-of-network second Tx UE 101 ("UE B") is using the in network coverage UE (e.g. the Rx UE 101) as a synchronization source.

As shown in Figure 6, the in network coverage Rx UE 101 may optionally change the timing advance, TA, of the out-of-network coverage second Tx UE 101 ("UE B"), and report this to the serving base station 103. The configuration of the required transmitted sidelink reference signals from the second Tx UE 101 ("UE B") is transmitted to the second Tx UE 101 ("UE B") from the in network coverage Rx UE 101.

The final CDD configuration for the out of network Tx UE, e.g. "UE B" may also be conveyed via the in-network Rx UE. As will be appreciated, the out of network UE, e.g. "UE B" 101 may be synchronized and have its messages relayed over any in-network UE, such as the first Tx UE 101 ("UE A"). Thus, the use of the target in-network Rx UE 101 for this purpose here is just an example.

More specifically, the signaling diagram of figure 6 shows the following steps:

Step 601: The target Rx UE 101 and the first Tx UE 101 ("UE A") connect to the serving network device, e.g. gNB 103. This initial connection may be made using the RACH. Step 603: Following the initial connection of the target Rx UE 101 and the first Tx UE 101 ("UE A") the serving base station 103 assigns timing advance, TA, values thereto.

Step 605: The serving base station 103 provides the TA values to the target Rx UE 101 and the first Tx UE 101 ("UE A").

Step 607: The target Rx UE 101 and the first Tx UE 101 ("UE A") provide the serving base station 103 with CDD capability information (and possibly further information, such as the number of Tx ports), wherein the CDD capability information indicates the CDD capability of the respective UE 101 for sidelink transmission, such as whether or not the respective UE 101 can transmit via its sidelink using a CDD value.

Step 608: The out-of-coverage second Tx UE 101 ("UE B") provides its CDD capability information via its sidelink to the target Rx UE 101. As illustrated in figure 6, this may be triggered by a corresponding request from the target Rx UE 101.

Step 609: The target Rx UE 101 relays the CDD capability information from the out-of- coverage second Tx UE 101 ("UE B") to the serving base station 103.

Step 611 : The target Rx UE 101 may perform a timing measurement relative to the out-of- coverage second Tx UE 101 ("UE B") or a TA assignment thereto.

Step 613: The target Rx UE 101 provides the time offset of the out-of-coverage second Tx UE 101 ("UE B") relative to the Rx UE 101 or the assigned TA to the serving base station 103.

Step 615: Based on the CDD capability information provided in steps 607 and 609 the serving base station 103 determines a configuration for the sidelink reference signals and measurements.

Step 617: The serving base station 103 transmits information about the configuration for the sidelink reference signals and measurements to the in-coverage first Tx UE 101 ("UE A").

Step 619: The serving base station 103 transmits information about the configuration for the sidelink reference signals and measurements for the out-of-coverage second Tx UE 101 ("UE B") to the target Rx UE 101. Step 620: The target Rx UE 101 relays the information about the configuration for the sidelink reference signals and measurements for the out-of-coverage second Tx UE 101 ("UE B") to the out-of-coverage second Tx UE 101 ("UE B").

Step 622: The in-coverage first Tx UE 101 ("UE A") and the out-of-coverage second Tx UE 101 ("UE B") transmit sidelink reference signals in accordance with the configuration defined by the serving base station 103 to the target Rx UE 101 for sensing the respective sidelink in the downlink direction.

Step 623: Based on the sidelink reference signals received from the in-coverage first Tx UE 101 ("UE A") and the out-of-coverage second Tx UE 101 ("UE B") the target Rx UE 101 provides a respective measurement report to the serving base station 103. As described above, these measurement reports may comprise, for instance, CQI, RSRP, and/or RSRQ measurements.

Step 625: Based on the respective TA assignments (steps 603 and 613) and the sidelink measurement reports received in step 623 the base station 103 determines and assigns a CDD value for each Tx UE 101. In an embodiment, the determination of the CDD value for each Tx UE 101 may comprise a selection of the CDD value for each Tx UE 101 from a finite set of predefined CDD values.

Step 627: The serving base station 103 provides the determined/selected CDD values to the in-coverage first Tx UE 101 ("UE A") and the target Rx UE 101. This feedback may encode the respective determined/selected actual CDD value for each Tx UE 101 using 1 or more bits.

Step 628: The target Rx UE 101 relays the CDD value assigned by the base station 103 to the out-of-coverage second Tx UE 101 ("UE B") to the out-of-coverage second Tx UE 101 ("UE B").

Step 629: Using the respective CDD value provided by the serving base station 103 and the target Rx UE 101 acting as a relay, the in-coverage first Tx UE 101 ("UE A") and the out-of- coverage second Tx UE 101 ("UE B") transmit the data via their respective sidelink to the target Rx UE 101. Thus, providing a more robust sidelink data transmission to the target Rx UE 101. A further embodiment is directed to the scenario, when the Tx UEs 101 in the diversity connection are being served by different base stations. Such a scenario is shown in figure 7, where the target Rx UE 101 and a first Tx UE 101 (referred to as "UE A" in figure 7) are served by the base station 103 (referred to as "gNB 1" in figure 7), while a second Tx UE 101 (referred to as "UE B" in figure 7) is served by the further base station 103' (referred to as "gNB 2" in figure 7).

In an embodiment, the two serving base stations 103, 103' may be using the same band and may be time synchronized using the same TDD frame format. In an embodiment, the different UEs 101 may be scheduled to transmit data at the same time, while the base station 103 may know the time advance (TA) ranges of the further base station 103'.

Figure 8 shows a signaling diagram for the scenario shown in figure 7 according to an embodiment, wherein the signaling diagram illustrates the following steps:

Step 801: The target Rx UE 101 and the first Tx UE 101 ("UE A") connect to the base station 103. This initial connection may be made using the RACH.

Step 802: The second Tx UE 101 ("UE B") connects to the further base station 103'. This initial connection may be made using the RACH.

Step 803: Following the initial connection of the UEs, the base station 103 assigns timing advance TA values to the target Rx UE 101 and the first Tx UE 101 ("UE A").

Step 804: Following the initial connection of the UE, the further base station 103' assigns a timing advance TA value to the second Tx UE 101 ("UE B").

Step 805: The base station 103 provides the TA values to the target Rx UE 101 and the first Tx UE 101 ("UE A").

Step 806: The further base station 103' provides the TA value to the second Tx UE 101 ("UE B").

Step 807: The target Rx UE 101 and the first Tx UE 101 ("UE A") provide the base station 103 with their CDD capability information (and possibly further information, such as the number of transmission ports), wherein the CDD capability information indicates the CDD capability of the respective UE 101 for sidelink transmission, such as whether or not the respective UE 101 can transmit via its sidelink using a CDD value.

Step 808: The second Tx UE 101 ("UE B") provides the further base station 103' with its CDD capability information (and possibly further information, such as the number of transmission ports), wherein the CDD capability information indicates the CDD capability of the second Tx UE 101 for sidelink transmission, such as whether or not the second Tx UE 101 can transmit via its sidelink using a CDD value.

Step 809: Based on the CDD capability information provided in step 807 the base station 103 determines a configuration for the sidelink reference signals and measurements.

Step 811: The base station 103 transmits a request to the further base station 103' for scheduling a sidelink reference signal from the second Tx UE 101 ("UE B"). This request may be transmitted via the Xn interface.

Step 813: Optionally, the base station 103 transmits a further request to the further base station 103' for the TA assignment and the CDD capability information of the second Tx UE 101 ("UE B"). This request may be transmitted via the Xn interface as well.

Step 815: Optionally, the further base station 103' responds to the request of step 813 by providing the TA assignment and the CDD capability information of the second Tx UE 101 ("UE B") to the base station 103.

Step 816: Based on the CDD capability information of the second Tx UE 101 ("UE B") provided in step 808 and the scheduling request of step 811 the further base station 103' determines a configuration for the sidelink reference signal and measurements for the second Tx UE 101 ("UE B").

Step 817: The base station 103 transmits the information about the configuration for the sidelink signals and measurements to the target Rx UE 101 and the first Tx UE 101 ("UE A").

Step 818: The further base station 103' transmits the information about the configuration for the sidelink signals and measurements to the second Tx UE 101 ("UE B"). Step 819: The first Tx UE 101 ("UE A") transmits one or more sidelink reference signals in accordance with the configuration defined by the base station 103 to the target Rx UE 101 for sensing the sidelink.

Step 820: The second Tx UE 101 ("UE B") transmits one or more sidelink reference signals in accordance with the configuration defined by the further base station 103' to the target Rx UE 101 for sensing the sidelink.

Step 821: Based on the sidelink reference signals received from the first Tx UE 101 ("UE A") and the second Tx UE 101 ("UE B") the target Rx UE 101 provides a respective measurement report to the base station 103. As described above, these measurement reports may, for instance, comprise CQI, RSRP, and/or RSRQ measurements.

Step 823: Based on the respective TA assignments (steps 803 and 815) and the sidelink measurement reports received in step 821 the base station 103 determines and assigns a CDD value for each Tx UE 101. In an embodiment, the determination of the CDD value for each Tx UE 101 may comprise a selection of the CDD value for each Tx UE 101 from a finite set of predefined CDD values.

Step 825: The base station 103 provides the determined CDD value for the second Tx UE 101 ("UE B") to the further base station 103'. Moreover, the base station 103 provides a scheduling request to the further base station 103' for scheduling the sidelink transmission of the second Tx UE 101 ("UE B") using its CDD value. This request may be transmitted via the Xn interface.

Step 826: Based on the data provided by the base station 103 in step 825 the further base station 103' schedules and determines the CDD configuration of the second Tx UE 101 ("UE B").

Step 827: The base station 103 provides the determined CDD value to the first Tx UE 101 ("UE A"). This feedback may encode the respective determined/selected CDD value for the first Tx UE 101 using 1 or more bits.

Step 828: The further base station 103' provides the CDD value to the second Tx UE 101 ("UE B"). This may be done by encoding the respective determined/selected CDD value for the first Tx UE 101 using 1 or more bits. Step 829: Using the respective CDD value provided by the base station 103 and the further base station 103' the first Tx UE 101 ("UE A") and the second Tx UE 101 ("UE B") transmit the data via their sidelinks to the target Rx UE 101. Thus, providing a more robust sidelink data transmission to the target Rx UE 101.

It will be described in the following embodiments, where the cyclic delay for each Tx UE 101 is determined by the target Rx UE 101, e.g. scenario (ii) described above.

This is advantageous, since the target Rx UE 101 has the complete channel response of all the received side-link channels, so it is in the best position to accurately estimate the different side-link channels (from the Tx UEs 101 to itself) and how these channels should be modified to maximize the overall channel selectivity and spatial diversity.

According to embodiments, the target Rx UE 101 may or may not be involved in scheduling decisions of which Tx UEs 101 to use in the diversity sidelink transmission. It is important to note, however, that any changes in the configuration of the cyclic delay diversity for the set of Tx UEs 101 has no effect on the resource allocation and selection of these Tx UEs 101. Therefore, changes made to the cyclic diversity for each Tx UE 101 (made by the target Rx UE 101) may do not have to be communicated to the serving base station 103 when the UEs 101 are in coverage.

The following description therefore will concentrate on the sidelink cyclic delay diversity (CDD) signaling from the target Rx UE 101 to the Tx UEs 101.

In an embodiment, this may be part of the sidelink control channel or the sidelink feedback signaling, such as the Physical Sidelink Control CHannel (PS-CCH) or the Physical Sidelink Feedback CHannel (PS-FCH). Such signaling choices should be very low overhead. Therefore, selecting a sidelink CDD from a set of fixed choices may be advantageous, such as 1 bit CDD encoding "on'V'off" or a 2 bit encoding "no CDD", "short CDD", "medium CDD", and "long CDD".

It is important to note, that the contents of the PS-FCH for the sidelink is presently under discussion at 3GPP for release 16 of 5G V2x, so the format is still fluid, however it is generally accepted that the that timing and the resources for PS-FCH is mapped to the PS- CCH of the forward sidelink signal. PS-FCH is envisaged to convey feedback signals for the sidelink (including HARQ and CQI feedback). Thus, according to embodiments, the PS-FCH may convey 1 or 2 extra bits (as described above) for the purpose of CDD assignment. The signaling according to an embodiment for scenario (ii) is shown in Figure 9. The configuration of the sidelink reference signals and sidelink measurements may be configured and scheduled by the serving base station 103 (shown here as example) or by the target Rx UE 101 for out of network scenarios (not shown).

For the in coverage case, the serving base station 103 may have to decide which UEs 101 will be used for the SL CDD diversity transmissions. This may be based on reports from the target Rx UE 101. The set of possible CDDs for the Rx UE 101 to select from (e.g. 2 bits (none, short, medium, long CDD etc)) for the Rx UE 101 may be configured by the network (i.e. serving base station 103), based on its knowledge of the Tx UEs 101.

More specifically, the signaling diagram of figure 9 shows the following steps:

Step 901: The serving base station 103 determines a configuration for the sidelink measurements.

Step 903: The serving base station 103 transmits information about the configuration for the sidelink measurements to the target Rx UE 101 and the Tx UEs 101.

Step 905: The Tx UEs 101 transmit respective sidelink reference signals in accordance with the configuration defined by the serving base station 103 to the target Rx UE 101 for sensing the respective sidelink.

Step 907: Based on the received sidelink reference signals, i.e. the channel responses received from the Tx UEs 101 the target Rx UE 101 determines a respective CDD delay for the Tx UEs 101. In an embodiment, the determination of the CDD value for each Tx UE 101 based on the channel responses may comprise a selection of the CDD value for each Tx UE 101 from a finite set of predefined CDD values.

Step 909: The serving base station 103 provides the determined/selected CDD values to each Tx UE 101. This feedback may be provided via the PS-CCH or the PS-FCH and may encode the respective determined/selected CDD value for each Tx UE 101 using 1 or more bits. Step 911 : Using the respective CDD value provided by the target Rx UE 101 each Tx UE 101 transmits the data via its sidelink to the target Rx UE 101. Thus, providing a more robust sidelink data transmission to the target Rx UE 101.

With regards to Figure 9, there may be a fixed timing relationship and potential resource mapping between the sidelink reference signals and the feedback (especially when PS-FCH is used) signaling. In this way, it is simpler for the Tx UEs 101 to decode the information regarding the assignment of the CDD.

If any of the Tx UEs 101 are not CDD capable and the Rx UE 101 and/or the serving base station 103 are unaware of this, one option for the CDD incapable Tx UE 101 may be to just ignore the CDD configuration sent to it, and transmit the required data without CDD.

It will be described in the following embodiments, where the Tx UE(s) 101 propose a CDD shift/delay based on their estimate of the channel between them and the Rx UE 101 (e.g. scenario (iii) already described above). This estimate may be made at the Tx UE 101 using sidelink channel reciprocity (assuming the Tx and Rx section of the Tx UE 101 are calibrated) and the channel reference signal measurements made at the Tx UE 101, when it receives reference signals from the target Rx UE 101 (target Rx UE to Tx UE).

Such embodiments are particularly useful, when the serving base station 103 (or the target Rx UE 101) has no knowledge of the CDD capability of the Tx UEs 101. Since each Tx UE 101 knows its own CDD capability, but unfortunately does not have channel knowledge between the other Tx UEs 101 and the target Rx UE 101, it can only propose a CDD delay/shift based on its own received channel knowledge and capabilities. Thus, according to embodiments, the final CDD delay/shift to be used is decided by the serving base station 103 (or the target Rx UE 101) based on all of the knowledge that is available.

According to embodiment, a Tx UE 101 may propose a Tx CDD delay that could make sense. For instance, if the "estimated' channel from the target Rx UE 101 to the Tx UE 101 is measured to be very close to a line-of-sight, LoS, channel, it may propose a certain CDD delay for the reverse direction (Tx UE to Rx UE). On the other hand, if the estimated channel has more dispersion, the Tx UE 101 may propose that a CDD delay may not be necessary.

An example of a corresponding signaling scheme is shown Figure 10. It is a variation of the scheme which is shown in Figure 4. In this case the network device, i.e. serving base station (e.g. gNB) 103 makes the final decision about the CDD for each Tx UE 101, but uses the proposed CDD from each Tx UE 101 as an input to its decision. Of particular importance here is that the sidelink reference signals (referred to as "SL Ref. signals" in figure 10) are transmitted in both directions Tx to Rx UE and Rx to Tx UE.

More specifically, the signaling diagram of figure 10 shows the following steps:

Step 1001: The target Rx UE 101 and the Tx UEs 101 connect to the serving base station, e.g. gNB 103. This initial connection may be made using the RACH.

Step 1003: Following the initial connection of the UEs 101 the serving base station 103 assigns timing advance TA values to each of the UEs 101.

Step 1005: The serving base station 103 provides the TA values to the UEs 101.

Step 1007: The UEs 101 provide the serving base station 103 with CDD capability information (and possibly further information, such as the number of transmission ports), wherein the CDD capability information indicates the CDD capability of the respective UE 101 for sidelink transmission, such as whether or not the respective UE 101 can transmit via its sidelink using a CDD value.

Step 1009: Based on the CDD capability information provided in the previous step the serving base station 103 determines a configuration for the sidelink measurements, which may include measurements for one or more of the sidelinks from the target Rx UE 101 to the respective Tx UE 101.

Step 1011: The serving base station 103 transmits information about the configuration for the sidelink measurements to the target Rx UE 101 and the Tx UEs 101.

Step 1013: The target Rx UE 101 transmits sidelink reference signals in accordance with the configuration defined by the serving base station 103 to the respective Tx UEs 101 for sensing the respective sidelink in the uplink direction. Moreover, the Tx UEs 101 transmit sidelink reference signals in accordance with the configuration defined by the serving base station 103 to the target Rx UE 101 for sensing the respective sidelink in the downlink direction.

Step 1015: Based on the sidelink reference signals received from the target Rx UE 101 the Tx UEs 101 provide a respective measurement report to the serving base station 103. Also, the target Rx UE 101 provides one or more measurement reports to the serving base station 103. As described above, these measurement reports may comprise, for instance, CQI, RSRP, and/or RSRQ measurements.

Step 1017: Moreover, based on the sidelink reference signals received from the target Rx UE 101 the respective Tx UE 101 determines a proposed Tx CDD value, i.e. delay.

Step 1018: The Tx UEs 101 transmit the respective proposed Tx CDD value to the serving base station 103. In an embodiment, the respective proposed Tx CDD value may be transmitted via the PS-CCH or the PS-FCH and encoded, for instance, by 1 or more bits, as will be described in more detail further below in the context of figure 12.

Step 1019: Based on the proposed CDD values the serving base station 103 determines the actual CDD value for each Tx UE 101. In the embodiment shown in figure 10, this determination may be further based on the respective TA assignments of step 1003 and/or the sidelink measurement reports received in step 1015. In an embodiment, the determination of the actual CDD value for each Tx UE 101 based on the proposed CDD values may comprise a selection of the actual CDD value for each Tx UE 101 from a finite set of predefined CDD values.

Step 1021: The serving base station 103 provides the determined/selected CDD values to each Tx UE 101. This feedback may encode the respective determined/selected actual CDD value for each Tx UE 101 using 1 or more bits.

Step 1023: Using the respective CDD value provided by the serving base station 103 each Tx UE 101 transmits the data via its sidelink to the target Rx UE 101. Thus, providing a more robust sidelink data transmission to the target Rx UE 101.

A further exemplary embodiment is shown Figure 11. It is a variation of the embodiment which is shown in Figure 10. In this embodiment the Tx UEs 101 propose a suitable CDD to the target Rx UE 101, which then makes a final decision based on the knowledge it has available. The set of possible CDD choices to select from (i.e. 2 bits (none, short, medium, long etc) could be configured by the network, e.g. the serving base station 103, or pre configured (i.e. from the resource pool configuration).

More specifically, the signaling diagram of figure 11 shows the following steps: Step 1101: The serving base station 103 determines a configuration for the sidelink measurements.

Step 1103: The serving base station 103 transmits information about the configuration for the sidelink measurements to the target Rx UE 101 and the Tx UEs 101.

Step 1105: The target Rx UE 101 transmits sidelink reference signals in accordance with the configuration defined by the serving base station 103 to the respective Tx UEs 101 for sensing the respective sidelink in the uplink direction.

Step 1107: Optionally, the Tx UEs 101 may transmit sidelink reference signals in accordance with the configuration defined by the serving base station 103 to the target Rx UE 101 for sensing the respective sidelink in the downlink direction.

Step 1109: Based on the sidelink reference signals received from the target Rx UE 101 the respective Tx UE 101 determines a proposed Tx ODD value, i.e. delay.

Step 1110: The Tx UEs 101 transmit the respective proposed Tx ODD value to the target Rx UE 101. In an embodiment, the respective proposed Tx CDD value may be transmitted via the PS-CCH or the PS-FCH and encoded, for instance, by 1 or more bits, as will be described in more detail further below in the context of figure 12.

Step 1113: Based on the proposed CDD values the target Rx UE 101 determines the actual CDD value for each Tx UE 101. In case the sidelink channel responses are available to the target Rx UE 101 (i.e. if the optional step 1107 has been performed), this determination may be based on the sidelink channel responses as well. In an embodiment, the determination of the actual CDD value for each Tx UE 101 based on the proposed CDD values may comprise a selection of the actual CDD value for each Tx UE 101 from a finite set of predefined CDD values.

Step 1115: The target Rx UE 101 provides the determined/selected CDD values to each Tx UE 101. This feedback may be provided via the PS-CCH or the PS-FCH and may encode the respective determined/selected actual CDD value for each Tx UE 101 using 1 or more bits. Step 1117: Using the respective CDD value provided by the target Rx UE 101 each Tx UE 101 transmits the data via its sidelink to the target Rx UE 101. Thus, providing a more robust sidelink data transmission to the target Rx UE 101.

The embodiments described above provide the signaling to achieve adaptive sidelink cyclic delay diversity (CDD) for three different scenarios. For all of these scenarios, the actual set of CDDs may be defined as well. According to an embodiment, a finite set of phase shifts may be used to minimize the signaling overhead.

According to an embodiment, a set of CDDs similar to the large delay CDD scheme for LTE- A may be used with the flexibility to assign the cyclic delay from a set of values for each sidelink in the diversity transmission from multiple Tx UEs 101 to the target Rx UE 101.

According to an embodiment, each Tx UE 101 may have one or more Tx ports each. Therefore, the total number of Tx ports in the slide-link transmission to the target Rx UE 101 is then the sum of all Tx ports of all Tx UEs 101. However, since the ports from one Tx UE 101 may not necessarily be phase coherent with those of another Tx UE 101, each Tx UE 101 should be configured separately.

According to an embodiment, the set of candidate cyclic delays may comprise one or more of the following cyclic delays: 1, e ^~i7t/ , e ^~i7t/2 , e ^~Jn , wherein j denotes the imaginary unit.

When each Tx UE 101 has for example two Tx ports, the set of candidate cyclic delays may comprise one or more cyclic delays as defined by the matrices below.

For the case of no CDD for the sidelink, the matrix for sidelink u and subcarrier i may be as follows: ° 1l-T wherein j denotes the imaginary unit, u denotes a Tx UE index and i denotes a subcarrier index. For this case, the diagonal entries are both 1 since there is no phase rotation.

For the case of short CDD for the sidelink, the matrix for sidelink u and subcarrier i may be as follows: _e -jm/4 0

D _u ( 0 0 r-ίpί/2

For this case, Tx ports from the same sidelink (from the same UE) have a 45 degree shift (i.e. p/4) from each other.

For the case of a medium CDD for the sidelink, the matrix for sidelink u and subcarrier i may be as follows:

For this case, Tx ports from the same sidelink (from the same UE) have a 90 degree shift (i.e. p/2) from each other.

For the case of a long CDD for the sidelink, the matrix for sidelink u and subcarrier i may be as follows:

For this case, Tx ports from the same sidelink (from the same UE) have a 180 degree shift (i.e. p) from each other.

To cover all of these CDD sets, a general equation for the case of 2 Tx ports is defined as follows: where the values of A and B in the equation are listed in the table shown in figure 12, with a corresponding example of configuration bits.

The person skilled in the art will understand that the "blocks" ("units") of the various figures (method and apparatus) represent or describe functionalities of embodiments (rather than necessarily individual "units" in hardware or software) and thus describe equally functions or features of apparatus embodiments as well as method embodiments. Different step(s) in the method embodiments may be implemented in one unit. This application will not restrict the implementation of unit which may comprise one or more steps in the method embodiments.

In the several embodiments provided in the present application, it should be understood that the disclosed system, apparatus, and method may be implemented in other manners. For example, the described apparatus embodiment is merely exemplary. For example, the unit division is merely a logical function division and may be a different division in an actual implementation. For example, a plurality of units or components may be combined or integrated into another system, or some features may be ignored or not performed. In addition, the displayed or discussed mutual couplings or direct couplings or communication connections may be implemented by using some interfaces. The indirect couplings or communication connections between the apparatuses or units may be implemented in electronic, mechanical, or other forms. The units described as separate parts may or may not be physically separate, and parts displayed as units may or may not be physical units, may be located in one position, or may be distributed across a plurality of network units. Some or all of the units may be selected according to actual needs to achieve the objectives of the solutions of the embodiments. In addition, functional units in the embodiments may be integrated into one processing unit, or each of the units may exist alone physically, or two or more units are integrated into one unit.