Patent Publication Number: US-2023140608-A1

Title: Beam management for device-to-device communication

Description:
TECHNICAL FIELD 
     Various examples generally relate to beam management at a wireless communication device for communication on a device-to-device link. Various examples specifically relate to beam adjustment at the wireless communication device, following beam establishment. 
     BACKGROUND 
     In order to proliferate the use of wireless communication, there is an attempt to increase reliability and/or data throughput of the wireless communication. 
     One strategy to increase reliability and/or data throughput is to operate respective wireless links at higher frequencies. For example, in the Third Generation Partnership Project (3GPP), Release 17 there are two frequency ranges FR1 (410 - 7125 MHz) and FR2 (24.25 - 52.6 GHz). 
     Multiple-input multiple-output (MIMO) techniques are sometimes used to enhance reliability and/or throughput of communication on a wireless link. Here, a transmitter node and a receiver node of a communication system both include multiple antennas that can be operated in a phase-coherent manner. Thereby, a signal can be transmitted redundantly (diversity multi-antenna mode) along multiple spatial data streams, or multiple signals can be transmitted on multiple spatial data streams (spatial multiplexing multi-antenna operational mode). Spatial data streams can be defined by focusing the transmission energy for transmitting (transmit beam, TX beam) and/or the receive sensitivity for receiving (receive beam, RX beam) to a particular spatial direction; this corresponds to beamforming. In the context of beamforming, the process of identifying the appropriate beams is often referred to as beam management. 
     For higher frequencies - e.g., FR2 - it is typically required to employ beamforming. 
     For instance, beam management can include beam establishment. As part of the beam establishment, a beam pair between two nodes of a wireless communication system. This can be done without any prior knowledge on appropriate beams - as may be the case in high-mobility situations or upon activation of a wireless communication device. 
     To establish the beam pair, it can be required to perform one or more beam sweeps: a beam sweep is time-aligned with a transmission including a burst of reference signals (RSs). The burst defines multiple transmit repetitions of the RSs. For instance, the RSs can be transmitted along different transmit beams, for a transmit (TX) beam sweep. Alternatively or additionally, for a receive (RX) beam sweep, monitoring for the RSs can be repeatedly implemented along different RX beams. 
     It has been found that beam management can require significant resources on the wireless link. Further, energy consumption at nodes participating in the beam management can increase. Further, interference with other nodes in the vicinity can be caused by the beam management. 
     This applies, in particular, to scenarios in which multiple peer nodes communicate with each other on a device-to-device (D2D) link. 
     SUMMARY 
     Accordingly, there is a need for advanced techniques of beam management, in particular for D2D links. 
     This need is met by the features of the independent claims. The features of the dependent claims define embodiments. 
     A method of operating a first wireless communication device is provided. The first wireless communication device communicates with a second wireless communication device on a D2D link and in accordance with a preestablished beam pair. The method includes communicating at least one message. The at least one message is indicative of an activation of a least one of a first transmission or a second transmission. The first transmission includes first reference signals. The first transmission is from the first wireless communication device to the second wireless communication device. The second transmission includes second reference signals. The second transmission is from the second wireless communication device to the first wireless communication device. The activation of the at least one of the first transmission of the second transmission is for use in beam adjustment of the preestablished beam pair at the first wireless communication device. The method also includes participating in the at least one of the first transmission of the second transmission, based on the communicated at least one message. 
     A computer program or a computer-program product or a computer readable storage medium or a digital signal includes program code. The program code can be loaded and executed by least one processor. Executing the program code causes the at least one processor to perform a method of operating a first wireless communication device. The first wireless communication device communicates with a second wireless communication device on a D2D link and in accordance with a preestablished beam pair. The method includes communicating at least one message. The at least one message is indicative of an activation of a least one of a first transmission or a second transmission. The first transmission includes first reference signals. The first transmission is from the first wireless communication device to the second wireless communication device. The second transmission includes second reference signals. The second transmission is from the second wireless communication device to the first wireless communication device. The activation of the at least one of the first transmission of the second transmission is for use in beam adjustment of the preestablished beam pair at the first wireless communication device. The method also includes participating in the at least one of the first transmission of the second transmission, based on the communicated at least one message. 
     A first wireless communication device is provided. The first wireless communication device communicates with a second wireless communication device on a D2D link and in accordance with a preestablished beam pair. The first wireless communication device includes control circuitry configured to communicate at least one message. The at least one message is indicative of an activation of a least one of a first transmission or a second transmission. The first transmission includes first reference signals. The first transmission is from the first wireless communication device to the second wireless communication device. The second transmission includes second reference signals. The second transmission is from the second wireless communication device to the first wireless communication device. The activation of the at least one of the first transmission of the second transmission is for use in beam adjustment of the preestablished beam pair at the first wireless communication device. The control circuitry is also configured to participate in the at least one of the first transmission of the second transmission, based on the communicated at least one message. 
     A method of operating a second wireless communication device is provided. The second wireless communication devices configured to communicate with a first wireless communication device on a D2D link and in accordance with a preestablished beam pair. The method includes communicating at least one message. The method includes communicating at least one message. The at least one message is indicative of an activation of a least one of a first transmission or a second transmission. The first transmission includes first reference signals. The first transmission is from the first wireless communication device to the second wireless communication device. The second transmission includes second reference signals. The second transmission is from the second wireless communication device to the first wireless communication device. The activation of the at least one of the first transmission of the second transmission is for used in beam adjustment of the preestablished beam pair at the first wireless communication device. The method also includes participating in the at least one of the first transmission of the second transmission, based on the communicated at least one message. 
     A computer program or a computer-program product or a computer readable storage medium or a digital signal includes program code. The program code can be loaded and executed by least one processor. Executing the program code causes the at least one processor to perform a method of operating a second wireless communication device is provided. The second wireless communication devices configured to communicate with a first wireless communication device on a D2D link and in accordance with a preestablished beam pair. The method includes communicating at least one message. The method includes communicating at least one message. The at least one message is indicative of an activation of a least one of a first transmission or a second transmission. The first transmission includes first reference signals. The first transmission is from the first wireless communication device to the second wireless communication device. The second transmission includes second reference signals. The second transmission is from the second wireless communication device to the first wireless communication device. The activation of the at least one of the first transmission of the second transmission is for used in beam adjustment of the preestablished beam pair at the first wireless communication device. The method also includes participating in the at least one of the first transmission of the second transmission, based on the communicated at least one message. 
     A second wireless communication device is provided. The second wireless communication device is configured to communicate with a first wireless communication device on a D2D link in accordance with a pre-established beam pair. the second wireless communication device includes control circuitry configured to: communicate at least one message indicative of an activation of at least one of a first transmission of first reference signals from the first wireless communication device to the second wireless communication device, or a second transmission of second reference signals from the second wireless communication device to the first wireless communication device, the activation of the at least one of the first transmission or the second transmission being for a beam adjustment of the pre-established beam pair at the first wireless communication device; and to participate in the at least one of the first transmission or the second transmission based on the at least one message. 
     A method of operating a node of a communications network is provided. The radio-access network of the communications network supports a sidelink between a first wireless communication device and a second wireless communication device. The method includes designating a master device being selected from the first wireless communication device and the second wireless communication device and a base station of the radio-access network. The master device is responsible for an activation of at least one of a first transmission of first reference signals from the first wireless communication device to the second wireless communication device, or a second transmission of second reference signals from the second wireless communication device to the first wireless communication device. The activation of the at least one of the first transmission or the second transmission is for a beam adjustment of the pre-established beam pair at the first wireless communication device. 
     A computer program or a computer-program product or a computer readable storage medium or a digital signal includes program code. The program code can be loaded and executed by least one processor. Executing the program code causes the at least one processor to perform a method of operating a node of a communications network is provided. The radio-access network of the communications network supports a sidelink between a first wireless communication device and a second wireless communication device. The method includes designating a master device being selected from the first wireless communication device and the second wireless communication device and a base station of the radio-access network. The master device is responsible for an activation of at least one of a first transmission of first reference signals from the first wireless communication device to the second wireless communication device, or a second transmission of second reference signals from the second wireless communication device to the first wireless communication device. The activation of the at least one of the first transmission or the second transmission is for a beam adjustment of the pre-established beam pair at the first wireless communication device. 
     A node of a communications network is provided. A radio-access network of the communications network is configured to support a sidelink between a first wireless communication device and a second wireless communication device. The node includes control circuitry configured to designate a master device being selected from the first wireless communication device and the second wireless communication device and a base station of the radio-access network. The master device is responsible for an activation of at least one of a first transmission of first reference signals from the first wireless communication device to the second wireless communication device, or a second transmission of second reference signals from the second wireless communication device to the first wireless communication device. The activation of the at least one of the first transmission or the second transmission is for a beam adjustment of the pre-established beam pair at the first wireless communication device. 
     It is to be understood that the features mentioned above and those yet to be explained below may be used not only in the respective combinations indicated, but also in other combinations or in isolation without departing from the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    schematically illustrates beam management including beam establishment and beam adjustment according to various examples. 
         FIG.  2    schematically illustrates a communication system including two wireless communication devices communicating on a D2D link according to various examples. 
         FIG.  3    schematically illustrates details of the two wireless communication devices of  FIG.  2   . 
         FIG.  4    schematically illustrates a TX beam sweep at a wireless communication device according to various examples. 
         FIG.  5    schematically illustrates an RX beam sweep at a wireless communication device according to various examples. 
         FIG.  6    is a flowchart of a method according to various examples. 
         FIG.  7    is a signaling diagram of communication between two wireless communication devices on a D2D link according to various examples. 
         FIG.  8    schematically illustrates multiple beam sweeps implemented at a wireless communication device in a time-interleaved manner according to various examples. 
         FIG.  9    schematically illustrates multiple beam sweeps implemented at a wireless communication device in a time-interleaved manner according to various examples. 
         FIG.  10    is a signaling diagram of communication between two wireless communication devices on a D2D link according to various examples. 
         FIG.  11    is a signaling diagram of communication between two wireless communication devices on a D2D link according to various examples. 
         FIG.  12    is a flowchart of a method according to various examples. 
         FIG.  13    is a flowchart of a method according to various examples. 
         FIG.  14    is a flowchart of a method according to various examples. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Some examples of the present disclosure generally provide for a plurality of circuits or other electrical devices. All references to the circuits and other electrical devices and the functionality provided by each are not intended to be limited to encompassing only what is illustrated and described herein. While particular labels may be assigned to the various circuits or other electrical devices disclosed, such labels are not intended to limit the scope of operation for the circuits and the other electrical devices. Such circuits and other electrical devices may be combined with each other and/or separated in any manner based on the particular type of electrical implementation that is desired. It is recognized that any circuit or other electrical device disclosed herein may include any number of microcontrollers, a graphics processor unit (GPU), integrated circuits, memory devices (e.g., FLASH, random access memory (RAM), read only memory (ROM), electrically programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), or other suitable variants thereof), and software which co-act with one another to perform operation(s) disclosed herein. In addition, any one or more of the electrical devices may be configured to execute a program code that is embodied in a non-transitory computer readable medium programmed to perform any number of the functions as disclosed. 
     In the following, embodiments of the disclosure will be described in detail with reference to the accompanying drawings. It is to be understood that the following description of embodiments is not to be taken in a limiting sense. The scope of the disclosure is not intended to be limited by the embodiments described hereinafter or by the drawings, which are taken to be illustrative only. 
     The drawings are to be regarded as being schematic representations and elements illustrated in the drawings are not necessarily shown to scale. Rather, the various elements are represented such that their function and general purpose become apparent to a person skilled in the art. Any connection or coupling between functional blocks, devices, components, or other physical or functional units shown in the drawings or described herein may also be implemented by an indirect connection or coupling. A coupling between components may also be established over a wireless connection. Functional blocks may be implemented in hardware, firmware, software, or a combination thereof. 
     Techniques are described that facilitate wireless communication between nodes of a wireless communication system. In particular, the nodes can be peers, i.e., can communicate on a D2D link. For instance, the D2D link could be implemented by a sidelink of a radio-access network (RAN) of a cellular network, e.g., specified by the 3GPP. Hereinafter, for sake of simplicity, various examples will be described with respect to an example implementation of the D2D link by a sidelink between two wireless communication devices (UEs), but other scenarios are conceivable. 
     According to various examples, techniques are described to facilitate beam management on a sidelink. According to various examples, in particular, beam adjustment following beam establishment is described. 
       FIG.  1    schematically illustrates aspects with respect to beam management. 
     The beam management includes beam establishment  8001 . For beam establishment, one or more establishment beam sweeps can be performed by the UEs communicating on the sidelink. Typically, such one or more establishment beam sweeps each include a correspondingly large count of beams, e.g., all beams available as defined by a respective antenna interface. The beam establishing  8001  yields a beam pair. A beam pair can comprise combinations of TX and RX beams at the two UEs. Thus, a beam pair is a Tx beam on one side and an Rx on the other. The beam pair may be reciprocal, then the associated beams in the opposite communication direction can be determined from a beam pair in the first direction. This would require beam correspondence at both UEs. 
     One or more RSs are transmitted on each one of the beams of the one or more establishment beam sweeps. Channel sounding can be implemented based on the RSs to determine the quality of the respective spatial channel defined by the respective beam. Then, based on a receive property of the one or more RSs, the beam pair can be determined. 
     Various examples are based on the finding that, for a sidelink, there are only two UEs involved and continuously performing the beam establishment  8001 , including the establishment beam sweeps, would require significant resources on the sidelink. This can be a problem, especially as the resources on the sidelink are often limited by the RAN, e.g., to accommodate for resources for communication between the RAN and the UEs. Moreover, full establishment beam sweeps may increase the interference experienced by other UEs in the cell. This is why it is helpful to use beam adjustment. 
     Beam establishment is followed by beam adjustment  8002 . Various of the techniques described herein are directed to the beam adjustment  8002 . The beam adjustment is generally based on the beam pair established as part of the beam establishment  8001 . For instance, the beam adjustment can determine whether at or around the beams of the beam pairs better beams are available. The beam adjustment thus adjusts the TX and/or RX beams of the beam pair at the respective UE. Again, RSs can be transmitted and the adjustment can be based on a receive property - e.g., amplitude or phase, signal level, etc. - of the RSs. Beam adjustment is sometimes also referred to as beam tracking or beam refinement. 
     Beam adjustment, as used herein, can pertain to evaluating an already established beam pair and, if required, adapting directions of one or more beams of the beam pair. As a general rule, beam adjustment can be performed for a TX beam and/or for an RX beam. Spatial filters for transmitting and/or receiving are adjusted. The beam establishment can serve as ground truth for the beam adjustment, i.e., the beams determined during beam adjustment may be derived from the beams established during beam establishment. Beam adjustment can pertain to changing beams of a pre-established beam pair used for payload data transmission. 
     As a general rule, RSs can have a pre-defined signal shape and/or symbol sequence. RSs can have a pre-defined transmit power. In particular, a receiver node may have prior knowledge on such transmit properties of the RSs. Then, a receiver node can perform channel sounding based on the receive property - e.g., receive amplitude or phase or angle of arrival or signal level- of the RSs. RSs are sometimes also referred to as pilot signals. A specific implementation of RSs are synchronization signals (SSs). 
     As a general rule, if there is a capability for beam correspondence (BC), then it is possible to transmit RSs on a TX beam and determine, both, the appropriate TX beam, as well as the appropriate RX beam based on a receive property of the RSs. On the other hand, without a capability for BC, it may be required to transmit RSs on a TX beam, as well as to receive RSs on an RX beam and determine the appropriate TX beam and RX beam based on the receive properties of the RSs transmitted and received, respectively. 
     Occasionally, a UE may return from beam establishment to beam adjustment (box  8003 ); this could happen when the UEs experience a beam failure, or even scheduled at a sparse rate to ensure adaptation of large scale channel changes. 
     Now, considering the case of beam adjustment  8002  at a first UE of the two UEs communicating via the sidelink: According to various examples, it is possible to communicate at least one message that is indicative of an activation of at least one of a first transmission of first RSs or a second transmission of second RSs. The first UE can then participate in the at least one of the first transmission or the second transmission based on the communicated message, i.e., in accordance with the activation. This is summarized in TAB. 1 below. 
     
       
         
          TAB 1
           
               
               
               
             
               
                 Transmissions of RSs for beam adjustment at first UE; the first and second transmission can generally be deactivated altogether or cumulatively activated. 
               
               
                 Transmission 
                 Direction 
                 Details 
               
             
            
               
                 First transmission of first RSs 
                 From first UE to second UE 
                 The first transmission may include at least one burst of the RSs. The at least one burst can be aligned with at least one TX beam sweep of the first RSs at the first UE. Different bursts can have a different count of RSs. Different bursts can be aligned with different TX beam sweeps, e.g., having 
               
               
                   
                   
                 different beams, different count of beams, and/or different beam widths. The first transmission can include a single RX beam per burst at the second UE, e.g., in accordance with pre-established beam pair. Different bursts may use different RX beams. The second UE may transmit a response message that is received by first UE. There may be one response message per burst of the first RSs; or a single response message may aggregate information across multiple bursts. The response message is indicative of an RX property of first RSs, e.g., by indicating the strongest TX beam(s), as seen by second UE, e.g., using a beam identity. The first UE may then use this strongest TX beam for transmitting, and — in case of capability for BC - use the corresponding RX beam for receiving. A selection of the RX beam of the second UE may be transparent for the first UE. The second UE may use an RX beam that is determined based on its mobility. The one or more beams used for the first transmission of the first RSs can be aligned with or generally determined based on the initial beam pair of the beam establishment  8001 . For instance, one or more TX beams used at the first UE may be determined based on the initial TX beam at the first UE, e.g., centered around the initial TX beam, etc.. This based on the assumption that the 
               
               
                   
                   
                 initial beam is a good approximation of the currently best beams, i.e., serves as a valid baseline. 
               
               
                 Second transmission of second RSs 
                 From second UE to first UE 
                 The first transmission can include an RX beam sweep at the first UE. For this, the first transmission can include at least one burst of second RSs. The at least one burst is aligned in time-domain with the at least one RX beam sweep. The second UE may use a single TX beam, e.g., in accordance with pre-established beam pair. Different bursts may use different TX beams. There is no need for reporting based on response message. The second UE should be stationary during the RX beam sweep (i.e., exhibit a low mobility level / assumed to be stationary during the RX beam sweep). This is because the measurement at the first UE can be based on the assumption that the second UE uses a single TX beam, i.e., transmits into a fixed and static direction; this, however, is only true if the second UE has low mobility, i.e., no or no significant mobility on the time scale of the RX beam sweep. The one or more beams used for the second transmission of the second RSs can be aligned with or generally determined based on the initial beam pair of the beam establishment  8001 . For instance, one or more RX beams used at the first UE may be 
               
               
                   
                   
                 determined based on the initial RX beam at the first UE, e.g., centered around the initial RX beam, etc.. This based on the assumption that the initial beam is a good approximation of the currently best beams, i.e., serves as a valid baseline. 
               
            
           
         
       
     
     Referring to TAB. 1, as a general rule, the first RSs can be the same or different as the second RSs. 
     As indicated in TAB. 1, as a general rule, to facilitate the beam sweeps at the first UE, it is possible that the first transmission and/or the second transmission include at least one burst of the respective RSs. The burst can define a repetition of the respective RSs, e.g., with a predefined time-domain offset between adjacent repetitions. For instance, there may be a predefined time-domain offset of one or two symbols between adjacent RSs of the burst. Each RS of a burst may be indicative of the burst, e.g., be indicative of belonging to the burst. It would be possible that the RSs of a burst are indicative of their position with the burst. By using such a burst, the respective beam sweep is facilitated: e.g., multiple beams can be each assigned a respective repetition of the RSs defined by the burst. 
     By such techniques of selective activation of the first transmission and/or the second transmission by means of the at least one message, overhead associated with beam management can be reduced. For example, by selective activation of the first transmission and/or the second transmission, a trade-off between, firstly, interference imposed onto the spectrum, and secondly, robustness of the beam management can be addressed. It is, furthermore, possible to take into account the mobility level of the first UE and/or the second UE, as well as the capability for BC at the first UE. 
     In detail, the first transmission of first RSs can employ the TX beam sweep at the first UE, thereby being associated with an increased level of interference (the interference is spatially distributed into multiple directions in accordance with the beam sweep) - in particular, if compared to a scenario of a single TX beam at the second UE combined with a RX beam sweep as defined by the second transmission. Similarly, the first transmission may be associated with increased communication overhead if compared to the second transmission, by requiring the response message. 
     For the second transmission: By using an RX beam sweep, the interference is minimized to the spatial direction implied by the single TX beam at the second UE aligned with the spatial direction in which the first UE is located. As the first UE is responsible for its beam management (aided by the RSs), no additional overhead is needed between the first and second UEs to support the beam management at the first UE. 
     Where a beam correspondence capability is not provided at the first UE, it may be required to combine the first transmission and the second transmission, thereby further increasing allocation on the spectrum. 
       FIG.  2    schematically illustrates a communication system  100 . The communication system  100  includes two UEs  101 ,  102  that are configured to communicate with each other via a D2D link  114 , here specifically a sidelink  114  of a RAN  109  of a cellular NW. For instance, the cellular network may be 3GPP-specified, e.g., 5G or upcoming 6G. 
     For example, the UEs  101 ,  102  could be selected from the group including: cell phone; a smart phone; a smart TV; Internet of things device; a machine type communication device; etc. 
     Communication on the sidelink  114  can employ time-division duplex (TDD) and/or frequency-division duplex (FDD). Using TDD, communication in both directions takes place at different points in time using the same frequency. Using FDD, communication in the two directions takes place at the same point in time, using different frequencies. 
       FIG.  2    schematically illustrates the first transmission  181  of first RSs  191  from the UE  101  to the UE  102  and, furthermore, illustrates the second transmission  182  of second RSs  192  from the UE  102  to the UE  101  (cf. TAB 1.). 
       FIG.  2    illustrates an in-coverage scenario in which control links  110  are established between the RAN  109  and each one of the UEs  101 ,  102 . It is also conceivable that there is an out-of-coverage scenario in which there are no control links available between the RAN  109  and both the UE’s  101 ,  102 . A mixed-coverage scenario pertains to a situation in which the control link  110  is available between the RAN  109  and a single one of the two UEs  101 ,  102 . 
     The RAN  109  can include a RAN node, e.g., a base station. The base station can include control circuitry that could be implemented by a processor and a non-volatile memory. The processor can load program code that is stored in the non-volatile memory and execute the program code. Executing the program code causes the at least one processor to perform techniques as described herein, e.g.: communicating with the UE  101  on the control link  110 ; communicating with the UE  102  on the control link  110 ; designating a master UE selected from the UE  101  and the UE  102 , the master UE being responsible for the activation of the first transmission  181  and/or of the second transmission  182  or, more generally, for configuration of the beam management. 
       FIG.  3    illustrates details with respect to the UE  101 . The UE  101  includes control circuitry that is implemented by a processor  1011  and a non-volatile memory  1015 . The processor  1011  can load program code that is stored in the memory  1015 . The processor  1011  can then execute the program code. Executing the program code causes the processor to perform techniques as described herein, e.g.: communicating on the sidelink  114 ; participating in the first transmission  181 , i.e., transmitting the first RSs  191 ; participating in the second transmission  182 , i.e., receiving the second RSs  192 ; transmitting and/or receiving (communicating) on the control link  110 ; communicating at least one message indicative of an activation of the first transmission  181  or the second transmission  181 ; performing beam management of transmit beams and/or received beams, in particular, including beam establishment  8001  and/or beam adjustment  8002  (cf.  FIG.  1   ). 
       FIG.  3    also illustrates details with respect to the UE  102 . The UE  102  includes control circuitry that is implemented by a processor  1021  and a non-volatile memory  1025 . The processor  1021  can load program code that is stored in the memory  1025 . The processor can execute the program code. Executing the program code causes the processor to perform techniques as described herein, e.g.: assisting the UE  101  for beam management at the UE  101 , e.g., by participating in the first transmission  181  and/or the second transmission; communicating at least one message indicative of an activation of the first transmission  181  or the second transmission  181 ; etc.. 
       FIG.  3    also illustrates details with respect to communication between the UE  101  and the UE  102  on the sidelink  114 . The UE  101  includes an interface  1012  that can access and control multiple antennas  1014 . Likewise, the UE  102  includes an interface  1022  that can access and control multiple antennas  1024 . 
     The interfaces  1012 ,  1022  can each include one or more TX chains and or more receiver chains. For instance, such RX chains can include low noise amplifiers, analogue to digital converters, mixers, etc. Analog and/or digital beamforming would be possible. 
     Thereby, phase-coherent communicating can be implemented across the multiple antennas  1014 ,  1024 . Thereby, the UE  101  and the UE  102  implement a MIMO communication system. 
     As a general rule, the receiver of the MIMO communication system receives a signal y that is obtained from an input signal x multiplied by the transmission matrix H.  FIG.  3    includes two example labels for the components h 11  and h 13  of the transmission matrix H. 
     The transmission matrix H defines the channel impulse response of the sidelink  114 . The rank of the transmission matrix corresponds to the number of linearly independent rows or columns and, as such, indicates how many independent data streams can be used simultaneously; this is sometimes referred to as the number of layers. The rank can be set in different MIMO transmission modes. For MIMO transmission modes, the amplitude and/or phase (antenna weights) of each one of the antennas  1014 ,  1024  is appropriately controlled by the interfaces  1012 ,  1022 . According to examples, a rank equaling one can be used, for beamforming using a single TX or RX beam, respectively. By using a beam, the direction of the wave front of signals transmitted by a transmitter of the communication system is controlled. Energy is focused into a respective direction, by phase-coherent superposition of the individual signals originating from each antenna  1014 ,  1024 . Thereby, the spatial data stream can be directed. As a general rule, alternatively or additionally to such TX beams, it is possible to employ RX beams. 
     The concept of beams can be used in so-called beam sweeps. Details with respect to beam sweeping are explained next in connection with  FIG.  4    and  FIG.  5   . 
       FIG.  4    schematically illustrates a TX beam sweep  300  at the UE  101 . For example, the first transmission  181  (cf.  FIG.  1   ) can include the TX beam sweep  300 . The TX beam sweep  300  includes multiple TX beams 301-306, i.e., a well-defined count of TX beams 301-306. For example, it would be possible that the first RSs  191  of the first transmission  181  are transmitted on each one of the transmit beams 301-306 of the TX beam sweep  300 . 
     The inset of  FIG.  4    illustrates aspects with respect to a burst  390  of the first RSs  191 . The burst  390  is defined by respective allocation of time-frequency resource elements in the time-frequency resource grid next to each other, wherein different ones of the multiple transmit beams 301-306 are activated for different time-frequency resource elements (for instance, in  FIG.  4   , each square could pertain to a symbol-subcarrier of an Orthogonal Frequency Division Multiple, OFDM, multiplex modulation). A predefined time-offset (here, a single resource element) is provisioned, to enable the interface  1012  to switch between the TX beams 301-306. 
     In some examples, it would even be possible to use nested bursts. Here, multiple first RSs  191  are transmitted on each one of the multiple TX beams 301-306. This corresponds to a top-level burst to address each TX beam 301-306, and nested bursts for each element of the top-level burst. Such use of nested bursts can be helpful in a scenario in which the TX beam sweep  300  at the UE  101  is combined with multiple RX beam sweeps at the UE  102 , each RX beam sweep being time-aligned with a respective nested burst. 
     As a general rule, depending on the implementation - e.g., whether the interface  1012  of the UE  101  has analog beamforming capability or digital beamforming capability -the different transmit beams  301 - 306  may be activated consecutively (cf. inset of  FIG.  4   ) or, at least partially, in parallel. 
       FIG.  5    schematically illustrates a RX beam sweep  310  at the UE  101 . For example, the second transmission  182  can include the RX beam sweep  310 . The RX beam sweep  310  includes multiple RX beams  311 - 316 . For example, it would be possible that the RSs  192  of the second transmission  182  are transmitted and the UE  101  attempts to receive (monitors) for the second RSs  192  on each one of the RX beams  311 - 316  of the RX beam sweep  310 . For example, a burst  390  of the second RSs  192  may be defined (cf. inset of  FIG.  4   ), or even possible nested bursts. Again, the RX beams  311 - 360  may be activated consecutively or, at least partially, in parallel. 
       FIG.  6    is a flowchart of a method according to various examples. The method of  FIG.  6    may be executed by a UE. For example, the method of  FIG.  6    could be executed by the UE  101 , more specifically by the circuitry  1011  upon loading program code from the memory  1015 . The details of the method of  FIG.  6    will be explained for such as scenario of execution by the UE  101  below, for sake of simplicity; but similar techniques may be implemented for other devices and entities executing the method. 
     The method of  FIG.  6    is for beam management at the UE  101 . Accordingly, the method of  FIG.  6    can help to determine a TX beam  301 - 306  and/or a RX beam  311 - 316  at the UE  101 , for communicating with the UE  102 . 
     Optional boxes are denoted with dashed lines in  FIG.  6   . 
     At box  9010 , at least one message is communicated, i.e., transmitted and/or received by the UE  101 . This could be, e.g., a Layer 3 control message. 
     The at least one message facilitates configuring the beam adjustment  8002  of the beam management. For example, the at least one message may be communicated on a control link  110  between the UE  101  and the RAN  109 . Alternatively or additionally, the at least one message may be communicated on the sidelink  114  between the UE  101  and the UE  102 . As a general rule, there are various options available for implementing the at least one message. For example, a single message may be used or multiple messages may be used. Where multiple messages may be used, the multiple messages may be communicated in different directions between the UE  101  and the UE  102  and/or may be communicated between the RAN  109  and the UE  101 . Mixed scenarios are possible, where a first one of the at least one message is communicated on the sidelink  114 , and a second one of the at least one message is communicated on the control link  110 . 
     The at least one message is indicative of an activation of the first transmission  181  and/or of the second transmission  182 . For instance, the at least one message may indicate whether the first transmission  181  and/or the second transmission  182  is active or inactive, respectively. 
     Activation can pertain to performing the first transmission  181  and/or the second transmission  182  upon communicating the message. The indication of the at least one message may be valid until reception of a further instance of the at least one message or until expiry of a corresponding timer, wherein the timer may be preset or may be initialized by the at least one message. The first transmission  181  and/or the second transmission  182  can be performed once; or it would be possible that the first transmission  181  and/or the second transmission  182  are re-occurring (details with respect to such an implementation are described below). It would also be possible that the first transmission  181  and/or the second transmission  182  are continuously performed upon communicating the message indicative of the activation. 
     As a general rule, there are various options available for implementing the at least one message to be indicative of the activation of the at least one of the first transmission  181  or the second transmission  182 . The at least one message may include an explicit indicator indicative of the activation of the first transmission  181  and/or the second transmission  182 . For example, a one-hot encoding may be used, e.g., according to the following scheme {[first transmission  181  activated or deactivated]; [second transmission  182  activated or deactivated]}. The at least one message may indicate time-frequency resources allocated to the first RS  191  in case the first transmission  181  is activated; and may not indicate time-frequency resources allocated to the first RS  191  in case the first transmission  181  is not activated (similarly, for the second transmission  182 ). The at least one message could also be indirectly indicative of the activation of the first transmission  181  and/or the second transmission  182 . For instance, it would be possible that there are multiple predefined operational modes available for the beam adjustment  8002  at the first UE  101 . Then, the activation of the first transmission  181  and/or the second transmission  182  can be defined by the selected or configured operational mode. In particular, the at least one message may be indicative of an indicator indicating the selected operational mode. Some of these operational modes are described in TAB. 2 below. 
     
       
         
          TAB 2
           
               
               
               
             
               
                 Operational modes for defining the activation of the first transmission  181  and/or the second transmission  182 . The operational modes can be predefined at both UEs 101-102. The operational modes can be specified in the communication specification and, e.g., hardcoded to respective memories of the UEs 101-102. 
               
               
                 Operational mode 
                 Transmission(s) activated as defined by operational mode 
                 Details 
               
             
            
               
                 I 
                 Second transmission  182 , but not first transmission  181 
 
                 UE  101  monitors for second RSs  192  from UE  102 , e.g., using multiple RX beams to implement the RX beam sweep  310 . If a received second RS  192  is stronger or shows better capacity in a given RX beam different to the RX beam defined by the pre-established beam pair, the UE  101  can adjust to use the given RX beam for an upcoming data 
               
               
                   
                   
                 transmission. For BC capability, the UE  101  can also use the corresponding given TX beam. 
               
               
                 II 
                 First transmission  181 , but not second transmission  182 
 
                 UE  101  transmits the first RSs  191  using various TX beams, i.e., uses a TX beam sweep  300 . The UE  102  monitors for the first RSs  191  using a fixed RX beam and reports back the desired TX beam: the desired TX beam can be selected based on, e.g. the strongest one or the one with the best capacity, as seen from the UE  102 . If UE  102  reports back a TX beam which is different from the active one, the UE  101  is expected to change the TX beam. For BC capability, the UE  101  can also use the corresponding given RX beam. 
               
               
                 III 
                 First transmission  181  and second transmission  182 
 
                 Without BC capability, both the first transmission  181 , as well as the second transmission  182  are activated. The UE  102  receives the first RSs  191  in a TX beam sweep from UE  101  and reports back the desired TX beam for UE  101 . The UE  102  transmits the second RSs  192  to the UE  101  - e.g., using a single TX beam - which can use a RX beam sweep to identify the desired RX beam for UE  101 . 
               
               
                 IV 
                 First transmission  181  and second transmission  182 
 
                 A further mode to improve robustness is to transmit a nested burst of first RSs  191  on each beam 301-306 of a TX beam sweep  300  at the UE  101 . This will enable the receiving UE  102  to perform RX beam sweeps, one for each nested burst. 
               
               
                   
                   
                 For example, mode IV may be a pre-requisite for transitioning back to the beam establishment. I.e., failure of mode IV operation may be checked at box  8003  in  FIG.  1 
 , to decide whether to fallback to beam establishment  8001 . 
               
            
           
         
       
     
     As described above, the at least one message is indicative of the activation of the at least one of the first transmission  181  or the second transmission  182 . Alternatively or additionally, it is possible that the at least one message is indicative of one or more properties of the first transmission  181  and/or the second transmission  182 . This is, however, optional - it would be possible that such properties of the first transmission  181  and/or the second transmission  182  are predefined, e.g., according to a standard. For example, a first one of the at least one message can be indicative of the activation; and a second one of the at least one message can be indicative of one or more properties of the of the first transmission  181  and/or the second transmission  182 . 
     For example, the at least one message can be indicative of the first transmission  181  including at least one burst of the first reference signals  191 . The at least one message could be indicative of the first transmission  181  including a single receive beam at the second UE  102 . The at least one message could be indicative of the first transmission  181  including at least one TX beam sweep at the UE  101 . The at least one message could be indicative of the first UE  101  performing the at least one TX beam sweep. The at least one message could be indicative of the second transmission  182  including at least one burst of the second reference signals  192 . The at least one message could be indicative of the second transmission  182  including at least one receive beam sweep at the first UE  101 . The at least one message could be indicative of the first UE  101  performing the at least one RX beam sweep. The at least one message could be indicative of the second transmission  182  including a single transmit beam at the second UE  102 . 
     Such indication of one or more properties can be implemented indirectly, e.g., by specifying a respective operation mode as described in TAB. 2, the operational mode being associated with respective one or more properties; or directly by expressly specifying respective properties, e.g., by indicating associated values. Some examples of properties that may be indicated by the at least one message are summarized in TAB. 3. 
     
       
         
          TAB 3
           
               
               
               
             
               
                 Examples of properties that can be indicated by the at least one message. It is possible that combinations of such scenarios are used. Different messages of the at least one message may indicate different properties. It is also possible that a single message indicates multiple properties. 
               
               
                 Scenario 
                 Property indicated by at least one message 
                 Details 
               
             
            
               
                 1 
                 Count of RSs per burst 
                 For example, the first transmission  181  may include at least one burst  390  of the first RSs  191  and the at least one message can be indicative of the count of the first RSs  191  for each one of the at least one burst. Alternatively or additionally, it would be possible that the second transmission  182  includes at least one burst  390  of the second RSs  192  and the at least one message can be indicative of the count of the second RSs  192  for each one of the at least one burst. 
               
               
                 2 
                 Time-frequency resources 
                 It would be possible that the at least one message is indicative of time-frequency resources of a time-frequency resource grid allocated to the at least one of the first transmission  181  or the second transmission  182 . As such, the at least one message can provide for scheduling. The time-frequency resources can be indicated on the level of elements (symbol/subcarrier), or on the level of blocks (multiple elements), or otherwise. 
               
               
                   
                   
                 It would be possible that the at least one message specifies a periodicity of re-occurrence of the first transmission  181  and/or the second transmission  182 . For example, periodicities may be specified for multiple bursts of the first transmission  181  and/or the second transmission  182 . 
               
               
                 3 
                 Scheduling type 
                 The first transmission  181  and/or the second transmission  182  can be re-occurring in time. For instance, the first transmission  181  and/or the second transmission  182  can be periodically re-occurring or in accordance with a timing schedule. For instance, one or more bursts  390  of the first transmission  181  and/or the second transmission  182  can be re-occurring in time. The at least one message can specify whether the time-frequency resources (cf. scenario 2) for the re-occurring first transmission  181  and/or the second transmission  182  are persistently scheduled or are scheduled on-demand. For instance, re-occurring time-frequency resources for multiple repetitions of the RSs could be persistently scheduled or be scheduled on demand, i.e., with a respective dedicated scheduling for each repetition. Each repetition could, e.g., pertain to a respective burst. 
               
               
                 4 
                 Count of beam sweeps 
                 It would be possible that the first transmission  181  and/or the second transmission  182  include one or more TX and/or RX beam 
               
               
                   
                   
                 sweeps at the UE  101 . It would be possible that the at least one message indicates the count of beam sweeps. Different beam sweeps can include a different count of beams. Multiple beam sweeps can be arranged interleaved in time domain. Different beam sweeps can be time-aligned with different bursts  390  of the first and/or second RSs  191 ,  192 , respectively. 
               
               
                 5 
                 Count of beams per beam sweep 
                 It would be possible that the count of beams per beam sweep as indicated, for a scenario in which the first transmission  181  and/or the second transmission  182  include multiple beam sweeps (cf. scenario 4). This number can correspond to the count of RSs per burst, as indicated in the scenario 1. For instance, there may be pre-defined a default count of beams and an extended count of beams. Then, an information element of the at least one message can indicate whether the extended count of beams is - e.g., temporarily - activated. 
               
               
                 6 
                 Beam width for beams of multiple beam sweeps 
                 For illustration, it would be possible to specify a beam width of the beams 301-306, 311-316 used in multiple beam sweeps  300 ,  310 . For instance, different beam sweeps may rely on beams of different beam width. For example, there may be a tendency that beam sweeps employing a smaller count of beams use beams having a wider beam width, to thereby cover a corresponding solid angle of the environment, 
               
               
                   
                   
                 as a beam sweep employing a large count of beams having a narrower beam width. 
               
               
                 7 
                 Selection of specific beams 
                 It would be possible to select specific beams available at the respective UE for a beam sweep  300 ,  310 , from a candidate set of beams (codebook). 
               
            
           
         
       
     
     At box  9020 , an initial beam pair is established at the UE  101  and the UE  102 . Thus, box  9020  implements the beam establishment  8001  (cf.  FIG.  1   ). At box  9020 , a TX beam and a RX beam can be determined for the UE  101 ; similarly, a TX beam and a RX beam can be determined for the UE  102 . 
     The particular implementation of box  9020  is out-of-scope of this disclosure and not germane for the functioning of the various techniques described herein. 
     Box  9020  can be implemented using reference techniques, e.g., using one or more establishment beam sweeps. Typically, a count of beams of the establishment beam sweep is comparably large, e.g., larger than 10 or even larger than 50. Thus, the establishment beam sweeps require significant resources on the sidelink  114 . 
     In response to establishing the initial beam pair at box  9020 , the method commences with the activated first transmission  181  and/or second transmission  182 , as explained in connection with boxes  9030  to  9090 . 
     At box  9030 , it is checked whether the beam adjustment  8002  is required at the UE  101 . For instance, the beam adjustment  8002  may be triggered by expiry of a timer and/or upon UE mobility. Other trigger criteria are possible. 
     If the beam adjustment  8002  is required, then the method commences at box  9040 . 
     At box  9040  it is checked whether the first transmission  181  is activated. This is based on the at least one message of box  9010 . If the first transmission  181  is activated, then the UE  101  participates in the first transmission at box  9050 , by transmitting the RSs  191 . For instance, the first transmission  181  can include at least one burst  390  of the first RSs  191  and it would be possible that the at least one message of box  9010  specifies the count of the first RSs  191  for each one of the at least one burst of the first transmission  181  (cf. TAB. 3: scenario 1; scenario 5). 
     More specifically, it would be possible that the first transmission  181  includes at least one TX beam sweep  300  at the UE  101 . The at least one TX beam sweep  300  is time-aligned with the at least one burst of the first transmission  181 , as described in  FIG.  4   . 
     At box  9060 , the UE  101  receives a response message from the UE  102 . The response message is indicative of a receive property of the first RSs  191  transmitted at box  9050 . There are various options available for implementing the response message to be indicative of the receive property of the first RSs  191 . Some examples are summarized in TAB. 4 below. 
     
       
         
          TAB 4
           
               
               
               
             
               
                 Implementation options for the response message 
               
               
                 Scenario 
                 Response message indicates 
                 Details 
               
             
            
               
                 1 
                 Received signal strength (RSRP) or signal-to-interference-and noise (SINR) or other quality metric for the sidelink. 
                 For example, it would be possible that the response message indicates, for the first RSs  191  transmitted on at least one or even multiple ones of the TX beams 301-306, the respective RSRP or SINR. Thereby, the UE  101  can conclude on the relative performance of multiple TX beams 301-306 with respect to each other. This helps to more accurately implement the beam adjustment  8002 . 
               
               
                 2 
                 Beam identity 
                 The response message could also be indicative of a beam identity of one or more strongest beams of the multiple TX beams 301-306. For instance, a descending or ascending sequence of multiple TX beams 301-306 - sorted with respect to, e.g., RSRP or SINR - could be signaled. 
               
            
           
         
       
     
     Next, at box  9070 , it is checked whether the second transmission  182  is activated. If the second transmission  182  is activated, then at box  9080  the UE  101  participates in the second transmission  182  by monitoring for the second RSs  192  transmitted by the UE  102 . The UE  101  can then determine a receive property of the second RSs  192  based on said monitoring. 
     At box  9090 , it is possible to adjust the preestablished beam pair of box  9020 , or a previous iteration of box  9090 . This is based on the receive property as indicated by the response message  9060  and/or the receive property is determined based on said monitoring for the second RSs  192  at box  9080 . 
     The sequence of the boxes in  FIG.  6    is only an example. Other sequences are conceivable. For instance, box  9010  may be executed after execution of box  9020 . For example, boxes 9040-9060 may be executed after execution of boxes 9070-9080. 
       FIG.  7    is a signaling diagram of communication between the UE  101  and the UE  102  on the sidelink  114 . 
     The signaling of  FIG.  7    can implement the method of  FIG.  6   . 
     At  5001 , the UE  101  transmits a message  4101  to the UE  102 . In the example of  FIG.  7   , message  4101  is indicative of the activation of the operational mode II (cf. TAB. 2). As such, the message  4101  is indicative of activation of the first transmission  181 . 
     The UE  102  transmits a message  4102  to the UE  101  and confirms the activation of the operational mode II, at  5002 . 
     Such an implementation using the messages  4101 - 4102  is optional. Other scenarios are conceivable. For example, the message  4102  is optional. For example, in the illustrated scenario, the UE  101  acts as the master device responsible for the activation of the transmission  181  and/or the transmission  182 . In other scenarios, the UE  102  may act as the master device, then the message  4101  may be transmitted from the UE  102  to the UE  101  (i.e., the UE  102  can decide for the UE  101  whether the UE  101  employs the first transmission  181  and/or the second transmission  182  for the beam adjustment  8002  at the UE  101 ). 
     Respective information designating the master device may be obtained by the UE  101  and the UE  102 , e.g., based on a predefined rule or via the control link  110  from the RAN  109 . For instance, the RAN  109  may designate the master device in an in-coverage scenario or a mixed-coverage scenario; while the UEs  101 - 102  may also autonomously designate the master device and on out-of-coverage scenario, e.g., based on a random selection scheme. For example, the master device may select the respective operational mode (cf. TAB. 2). 
     At  5003 , the beam establishment  8001  is performed. An initial beam pair is determined. The beam pair is used for a payload transmission  4001  at  5004 . 
     At  5005 , the UE  101  and the UE  102  participate in the transmission  181  of the RSs  191 , for the beam adjustment  8002 . The UE  101  performs the TX beam sweep  300 , i.e., the transmission  181  includes a burst of RSs  191 . The beams  301 - 306  of the TX beam sweep  300  could be centered around the initial TX beam of the pre-established initial beam pair of the beam establishment  8001 . 
     One or more RSs are transmitted per TX beam  301 - 306  of the TX beam sweep  300 . The transmission  181  includes a single RX beam at the UE  102 ; i.e., the UE  102  does not perform a RX beam sweep. For instance, the UE  102  can select the RX beam in accordance with the initial beam pair. 
     At  5006 , the UE  102  responds with the response message  4011 , cf. TAB. 4. In the illustrated example, the strongest TX beam  301 - 306  of the TX beam sweep  300  is indicated (cf. TAB. 4, scenario 2). 
     The UE  101  can adjust the initial beam pair by selecting the strongest TX beam 301-306 and then perform a further payload transmission  4001  at  5007  using the adjusted beam pair. 
     In the scenario  FIG.  7   , there is a single TX beam sweep  300 . In some examples, it is possible that the first transmission  181  includes multiple TX beam sweeps. Such a scenario is illustrated in  FIG.  8   . 
       FIG.  8    illustrates a scenario in which the first transmission  181  includes 2 TX beam sweeps  300 - 1 ,  300 - 2 . The two TX beam sweeps  300 - 1 ,  300 - 2  include a different count of TX beams. For example, this can pertain to a coarse beam adjustment and a fine beam adjustment, depending on the count of TX beams. 
     Each one of the beam sweeps  300 - 1 ,  300 - 2  is time-aligned with a respective burst  390 - 1 ,  390 - 2  (cf.  FIG.  4   ) of the first RSs  191 . Different bursts  390 - 1 ,  390 - 2  can have different allocation schemes for the time-frequency resources, etc. 
       FIG.  8    also illustrates aspects with respect to the time-domain arrangement of the multiple TX beam sweeps  300 - 1 ,  300 - 2 . The TX beam sweeps  300 - 1 ,  300 - 2  are arranged interleaved in time domain. 
     Respective properties associated with the multiple TX beam sweeps  300 - 1 ,  300 - 2  of the first transmission  181  can be indicated by the messages  4101 ,  4102  (cf. TAB 3: scenario 4). In particular, it would be possible that the count of beams is indicated by least one of the messages  4101 ,  4102  (cf. TAB. 3, scenario 1 and 5), e.g., implicitly by referring to the respective bursts  390 - 1 ,  390 - 2 , or explicitly. 
     While  FIG.  8    illustrates a scenario of multiple TX beam sweeps  300 - 1 ,  300 - 2  of the first transmission  181 , similar scenarios can be readily applied for multiple RX beam sweeps of the second transmission  182  at the UE  101 . 
       FIG.  8    illustrates a scenario in which the first transmission  181  is re-occurring in time at a fixed periodicity - and, along with this, both TX beam sweeps  300 - 1 ,  300 - 2  are re-occurring at a fixed periodicity. Hence, corresponding time-frequency resources may be persistently or semi-persistently scheduled, but other scenarios are conceivable. One example scenario is illustrated in  FIG.  9   . 
       FIG.  9    also illustrates a scenario in which the first transmission  181  includes two TX beam sweeps  300 - 1 ,  300 - 2 . The two TX beam sweeps  300 - 1 ,  300 - 2  include a different count of TX beams. 
     In particular, the TX beam sweep  300 - 1  includes an extended count of beams, wherein the TX beam sweep  300 - 2  includes a default count of beams. Here, the time-frequency resources for the TX beam sweep  300 - 2  are persistently scheduled, while the TX beam sweep  300 - 1  is scheduled on-demand by a message  4103 . Thus, the TX beam sweep  300 - 1  can be labeled aperiodic. 
     In other words, the re-occurring first transmission  181  includes the TX beam sweep  300 - 2  by default; and includes the TX beam sweep  300 - 1  on demand. 
     As a general rule - with reference to  FIG.  8    and  FIG.  9    - it would be possible that the multiple beam sweeps  300 - 1 ,  300 - 2  employ beams having different beam width. 
       FIG.  10    is a signaling diagram of communication between the UE  101  and the UE  102  on the sidelink  114 . 
     The signaling of  FIG.  10    can be in accordance with the method of  FIG.  6   .  5101 - 5104  correspond to  5001 - 5004 , respectively (cf.  FIG.  7   ). 
     At  5105 , the UE  101  and the UE  102  both participate in the second transmission  182  of the RSs  192 . The UE  102  transmits the RSs  192 , e.g., by implementing a respective burst  390  of the RSs  192 . For instance, the message  4101  could indicate the count of the RSs  192  included in the burst  390  (cf. TAB. 3, scenario 1). The UE  101  monitors for the RSs  192 ,  5105 . The second transmission  182  includes the RX beam sweep  310  at the UE  101 . The RX beam sweep  310  is aligned in time domain with the burst  390  of the second transmission  182  implemented at the UE  102 . I.e., the UE  101  can switch between the different RX beams  311 - 316  in accordance with a timing of the burst. The UE  102  may use a single TX beam, e.g., the TX beam defined by the beam establishment at  5103 , for transmitting the RSs  192  at  5105 . 
     As a general rule, referring to  FIG.  7    and  FIG.  10   , it would be possible that the beam sweeps  300 ,  310  of the first and second transmissions  181 ,  182  implemented at the UE  101  have a comparably limited count of beams. In particular, the count of beams used for the beam sweeps  300 ,  310  during the beam adjustment  8002  can be smaller than the count of beams for one or more establishment beam sweeps used during the beam establishment  8001 . For example, typically, the count of beams used for one or more beam sweeps during the beam establishment  8001  may be larger than 10 or even larger than 50. In contrast, the count of beams used for the one or more beam sweeps during the beam adjustment  8002  may be smaller than ten or even smaller than six. In other words, the UE  101  may refrain from using all available beams; typically, the hardware capability of the interface  1012  defines the count of available beams. 
     As a general rule, it would be possible to implement both the first transmission  181 , as well as the second transmission  182  (cf. TAB. 2: mode III). This is illustrated in  FIG.  11   . Here,  5201 - 5206  correspond to  5001 - 5006 , respectively.  5207  corresponds to  5105 .  5208  corresponds to  5106  and  5007 . 
       FIG.  12    is a flowchart of a method according to various examples. The method of  FIG.  12    may be executed by a master device. The method of  FIG.  12    is for configuring beam management at the UE  101 . For instance, the master device may be the UE  101  or even the UE  102 . It would also be possible that the master device is implemented by a node of a cellular network, e.g., by a base station of the RAN  109 . Hereinafter, for sake of simplicity,  FIG.  12    will be explained in connection with an implementation in which the method is executed by the UE  101 , but other scenarios are possible. 
     At box  9000 , the UE  101  can obtain respective information designating the UE  101  as the master device, e.g., from a negotiation process with the UE  102  or from the RAN  109 . As a general rule, it would be possible that the UE  101  and the UE  102  agree upon the role of the master device. It would also be possible that the master device is designated by the RAN  109 . 
     Thus, a flexible designation of the master device is conceivable. 
     The method of  FIG.  12    may, e.g., precede the method of  FIG.  6   , or at least partly overlap with the method of  FIG.  6   . 
     At box  9110 , the UE  101  determines whether to activate the first transmission  181  and/or the second transmission  182 . As such, the at least one message communicated at box  9010  of the method of  FIG.  6    may reflect an outcome of the determination at box  9110 . 
     I.e., the UE  101  — responsible for the activation, as the master device - can determine whether to activate or deactivate the first transmission  181  of the RSs  191 ; and can determine whether to activate or deactivate the second transmission  182  of the RSs  192 . For instance, the UE  101  could select between one of the operational modes I-IV of TAB. 3. 
     As a general rule (e.g., irrespective of the particular choice of the master device), various decision criteria can be considered when determining whether to activate the first transmission  181  and/or the second transmission  182 . Some of the decision criteria are listed below in table 5. 
     
       
         
          TAB 5
           
               
               
             
               
                 Decision criteria for activation of the first transmission  181  and/or the second transmission  182 . Multiple such decision criteria can be used in an aggregated manner. There may be a prioritization between different one of such decision criteria. 
               
               
                 Decision criterion 
                 Explanation 
               
             
            
               
                 Channel quality 
                 For instance, depending on the channel quality, the first transmission  181  and/or the second transmission  182  may be activated. 
               
               
                 Mobility level 
                 For instance, if the mobility level of, both, the first UE  101  and the second UE  102  is below a threshold, then it would be possible to activate at least one of the first 
               
               
                   
                 transmission  181  and/or of the second transmission  182 . Conversely, the mobility level of at least one of the UE  101  and the UE  102  exceeds a threshold, then there may be a tendency to use beam establishment  8001  rather than beam adjustment  8002 . At significant mobility levels, there can be significant and fast changes in the required beam pair such that the initial assumption of the preestablished beam pair has only a limited validity duration. In other examples, it would be possible to execute the beam adjustment  8002  more often or, e.g., increase the solid angle covered by the beam sweeps at the UE  101 , e.g., by increasing the count of beams of the beam sweeps depending on the mobility level and/or a predicted movement of the UE  101 . 
               
               
                 Interference level 
                 For instance, if the interference level of the sidelink  114  - e.g., caused by other devices accessing the spectrum - is exceeding a certain threshold level, then it may be appropriate to use the second transmission  182 , rather than the first transmission  181 , because the second transmission  182  is characterized by the RX beam sweep at the UE  101  and does not require a TX beam sweep that spatially distributes additional interference on top of the interference level already present. 
               
               
                 BC capability 
                 For instance, if there is a BC capability at the UE  101  (e.g., if TDD is used), then it may be possible to activate either one of the first transmission  181  or the second transmission  182 . On the other hand, if there is no BC capability, then it may be preferable to activate, both, the first transmission  181 , as well as the second transmission  182 , or even rely on the full establishment beam sweeps of the beam 
               
               
                   
                 establishment  8001 . For example, BC capability may depend on one or more of the following parameters: TDD or FDD; distance between uplink and downlink bands for FDD; signal quality on the sidelink. 
               
            
           
         
       
     
     Box  9010  may be re-executed from time to time, e.g., to account for changes in the conditions underlying the one or more decision criteria. One example would be that the second transmission  182  is activated and the first transmission  181  is deactivated, but upon experiencing link degradation of the sidelink  114 , the second transmission  182  is deactivated and the first transmission  181  is activated. A similar mechanism is that when the first transmission  181  is activated and the second transmission  182  is deactivated, and a stable condition of the sidelink  114  is experienced, then the first transmission  181  can be deactivated and the second transmission  182  can be activated. 
     Next, at box  9120 , — upon activation of the at least one of the first transmission  181  or the second transmission  182  - it would be possible to trigger a change of a time domain density of beacon RSs that are periodically scheduled on the sidelink  114 , e.g., sidelink SS blocks (SL-SSBs). For example, upon activation of the first transmission  181  and/or of the second transmission  182 , this may correspond to scheduling the RSs  191  and/or the RSs  192 , such that for the purpose of beam management there are sufficient RSs  191 ,  192  available. Then, it may not be required to have a significant number of additional beacon RSs and the time-domain density can be decreased. 
     The triggering of the change of the time-domain density can be implemented differently in different scenarios. In one example, e.g., in an out-of-coverage scenario, it would be possible that the UE  101  and the UE  102  autonomously agree on the change in the time-domain density. In other scenarios, a corresponding request may be transmitted via the control link  110  to the RAN  109 . 
       FIG.  13    is a flowchart of a method according to various examples. For instance, the method of  FIG.  13    can be executed by a UE. The example of  FIG.  13    will be described by a scenario in which the method is executed by the UE  102 , e.g., by the processor  1021  upon loading program code from the memory  1025  (cf.  FIG.  3   ). 
     The method of  FIG.  13    is for assisting the UE  101  in the beam adjustment  8002  at the UE  101 . 
     At box  9210 , at least one messages communicated, the at least one message being indicative of an activation of the first transmission  181  and/or of the second transmission  182 . Thus, box  9210  is interrelated with box  9010 . 
     At box  9220 , the UE  102  participates in the first transmission  181  and/or the second transmission  182 , based on the at least one message communicated at box  9210 . 
       FIG.  14    is a flowchart of a method according to various examples. For example, the method of  FIG.  14    can be executed by a base station of the RAN  109 , e.g., by a respective processor, upon loading program code from a respective memory. 
     At box  9310 , a master device is designated. For instance, the UE  101  or the UE  102  could be designated as master device. It would also be possible that the base station designates itself as the master device. 
     The master device is responsible for the activation of the first transmission  181  and/or the second transmission  182 . It would be possible that corresponding information being indicative of the master device is determined. This can form the basis of box  9100  of  FIG.  12   . 
     Summarizing, above, at least the following techniques have been described. 
     Sidelink control channel instructions for activation of one or more transmissions of RSs, including a first transmission from a first UE to a second UE and the second transmission from the second UE to the first UE. This can be aligned with mode selection for beam management. The first UE can be aware of the respective mode for beam management selected at the second UE, and vice versa. 
     SL control channel instruction for a number of RX beams, e.g., by specifying a count of resources allocated to the RSs and how they are located in a time-frequency resource grid. 
     SL control channel instruction for a number of TX beams, e.g., by specifying a count of resources allocated to the RSs and how they are located in a time-frequency resource grid. 
     SL control channel instruction for an extended beam sweep (not a full establishment beam sweep, i.e., including fewer beams that a full establishment beam sweep). A corresponding request for the extended beam sweep can provide an on-demand allocation of resources. It would also be possible to persistently schedule the resources at a lower rate than a default beam sweep having a smaller count of beams than the extended beam sweep. The extended beam sweep may be performed occasionally. 
     A response message may feedback information on the best TX beam on the SL control channel. Optionally, information for multiple TX beams may be fed back. 
     A RAN control link can be used to define one UE as master device and one or more further UE as slave devices, e.g., with respect to the determination which transmissions to activate. 
     Signaling from one or more UEs to the RAN to indicate that the activity of the beacon RSs can be reduced (e.g., by reducing the amount of SL-SS blocks (SL-SSBs) and/or SL-SS bursts, and/or by extending the SL-SS block periodicity, or the SL-SSB beacon can be shut completely. A corresponding request to activate the SL-SSBs, or to increase its activity level. 
     The technical effect is that the UEs can maintain a high-quality beam pair without the need of a full beam sweep as in beam establishment. This has the advantage of requiring less overhead, in terms of resources spent on RSs. 
     A further technical effect is that interference experienced by other devices can be reduced by using RX beam sweeps (only). 
     A further technical effect is that the RSs need not to be SL-SSBs, which then reduces overhead. 
     Summarizing, at least the following EXAMPLEs have been described above. 
     EXAMPLES 
     It will be appreciated that the above description for clarity has described embodiments with reference to different functional units, circuitry and/or processors. However, it will be apparent that any suitable distribution of functionality between different functional units, circuitry and/or processors may be used without detracting from the embodiments. 
     Described embodiments may be implemented in any suitable form including hardware, software, firmware or any combination of these. Described embodiments may optionally be implemented at least partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of any embodiment may be physically, functionally and logically implemented in any suitable way. Indeed, the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the disclosed embodiments may be implemented in a single unit or may be physically and functionally distributed between different units, circuitry and/or processors. 
     Although the present disclosure has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in any manner suitable to implement the technique. 
     For illustration, above, various options have been described with respect to beam management at the UE  101 . Similar techniques may be readily applied with respect to beam management at the UE  102 . However, beam management at the UE  102  may be decoupled from the beam management at the UE  101 , and vice versa. 
     For further illustration, above, various option have been described in which resources for one or more beam sweeps are persistently or semi-persistently schedule. Other options for scheduling are available, e.g., on-demand scheduling or periodic scheduling. 
     For still further illustration, various examples have been described in connection with the implementation of the D2D link by a sidelink of a RAN. Similar techniques may be readily applied for other kinds and types of D2D links, e.g., in WiFi or Bluetooth communication protocols. In some examples, it would even be possible to employ the techniques described herein to other wireless links, beyond D2D links. 
     For further illustration, above, various examples have been described in which a burst of RSs is followed by payload data transmission. It would also be possible that the burst of RSs and the payload data transmission are at least partly executed in parallel. I.e., the burst can be distributed over time with longer gaps in time-domain between adjacent RSs of the burst.