Patent Publication Number: US-2023146147-A1

Title: Control of Reliability Target based Transmissions on LBT Carriers

Description:
TECHNICAL FIELD 
     The present invention relates to methods for controlling wireless transmissions and to corresponding devices, systems, and computer programs. 
     BACKGROUND 
     In wireless communication technologies, there is an ongoing need for improved technologies. For example, a 5 th  generation (5G) cellular technology, referred to as New Radio (NR), is being developed by 3GPP (3 rd  Generation Partnership Project). The NR technology is designed with the aim of high flexibility to support multiple and substantially different use cases. 
     One use case of the NR technology is Ultra-Reliable and Low Latency Communication (URLLC). URLLC has strict requirements on transmission reliability and latency, i.e., 99.9999% reliability within 1 ms one-way latency. In NR Rel-15, several new features and enhancements were introduced to support these requirements. Further developments of the NR technology aim at enhancing URLLC system performance as well as ensuring reliable and efficient coexistent of URLLC and other NR use cases. One example scenario is when both enhanced mobile broadband (eMBB) and URLLC services are run within the same UE (user equipment). 
     A possible application field of URLLC is integration with Time Sensitive Networking (TSN) technologies. TSN is a set of features that allow deterministic networking in Ethernet based wired communication networks. Within a TSN network the communication endpoints are called Talker and Listener. All the switches or bridges in between Talker and Listener need to support certain TSN features, like for example IEEE 802.1AS time synchronization. All nodes that are synchronized in the network are considered as belonging to a so called TSN domain. TSN communication is only possible within such a TSN domain. To allow for deterministic communication, TSN communication happens in streams, that are setup across the TSN domain before the data communication takes place. Here, a 3GPP NR system supporting URLLC could act as a virtual bridge within the TSN network. 
     In licensed operation, UL (uplink) traffic can be scheduled with dynamic UL grants or configured UL grants. In case of dynamic grants, an access node, in the NR technology referred to as gNB, provides an UL grant to the UE for each UL transmission. The UL grant may for example be a dynamic grant allocating resources for a UL transmission immediately before the UL transmission on an as-needed basis, e.g., in response to a scheduling request (SR) from the UE. Further, the UL grant could be a configured grant. Configured grants are pre-allocated, i.e., provided once to the UE, thereafter the configured UL grant is valid for usage for UL transmissions according to a configured periodicity. A configured UL grant may thus also be considered as a semi-static allocation of resources. The UE may skip an UL transmission on resource allocated by a configured UL grant if no UL data is available for transmission. In other words, the UE is not required to use the reserved UL resources and thus does not need to transmit padding on the reserved UL resources. When a UL grant is received, a Logical channel Prioritization (LCP) procedure at the MAC (Medium Access Control) layer in the UE determines the order of priority and amount of data from each logical channel to be multiplexed and transmitted in an UL transmission. Logical channel mapping restrictions can be defined, e.g., restricting certain logical channels to be transmitted on resources of an UL grant referring to certain serving cells or being of certain transmission duration. In this way, URLLC traffic requiring a certain low latency can be restricted to prevent transmission in excessively long time slots or in unreliable serving cells. 
     In addition to operation in licensed bands, the NR technology also supports operation in unlicensed or shared bands. The utilization of the NR technology in unlicensed bands, also referred to as NR-U, is for example discussed in 3GPP TR 38.889 V16.0.0 (2018 December). Allowing operation in a shared spectrum or unlicensed spectrum may help to more effectively use the available spectrum and is an attractive approach to increase system capacity. Although an unlicensed spectrum typically does not match the qualities of a licensed spectrum, solutions that allow an efficient utilization of an unlicensed spectrum, e.g., as a complement to licensed deployments, may provide significant value network operators and users. 
     When operating on a channel from an unlicensed spectrum, many regions in the world require a device to sense the channel as free before transmitting. Corresponding procedures are referred to as Clear Channel Assessment (CCA) or Listen-Before-Talk (LBT). There are many different flavors of CCA or LBT, depending on which radio technology the device uses and which type of data the device intends to transmit. A common characteristic of CCA or LBT procedures is that the sensing is done in a particular channel, e.g., corresponding to a defined carrier frequency and over a predefined bandwidth. For example, in the 5 GHz band, the sensing may be done over 20 MHz bandwidth. 
     Both of the above-mentioned scheduling options, namely using dynamic UL grants or configured UL grants, are in principle also available on unlicensed carriers. However, while the required CCA or LBT procedures work well for providing a best effort service, they may fail to provide satisfactory results for services which are subject to reliability requirements. 
     Accordingly, there is a need for techniques which allow for efficiently ensuring reliability for wireless transmissions requiring LBT or other CCA mechanisms. 
     SUMMARY 
     According to an embodiment, a method of controlling wireless transmissions in a wireless communication network is provided. According to the method, a wireless device determines a set of carriers. The carriers each require an LBT procedure before transmitting on the carrier. Further, the wireless device determines a reliability target for a wireless transmission from the wireless device to the wireless communication network. Depending on the reliability target, the wireless device controlling aggregation of carriers from the set for redundantly performing the wireless transmission on the aggregated carriers. 
     According to a further embodiment, a method of controlling wireless transmissions in a wireless communication network is provided. According to the method, an access node an access node of the wireless communication network determines a reliability target for a wireless transmission from a wireless device to the wireless communication network. Further, the access node determines a set of carriers to enable the wireless device to meet the reliability target by redundantly performing the wireless transmission on aggregated carriers from the set of carriers. The carriers each require an LBT procedure before transmitting on the carrier. 
     According to a further embodiment, a wireless device for a wireless communication network is provided. The wireless device is configured to determine a set of carriers. The carriers each require an LBT procedure before transmitting on the carrier. Further, the wireless device is configured to determine a reliability target for a wireless transmission from the wireless device to the wireless communication network. Further, the wireless device is configured to, depending on the reliability target, control aggregation of carriers from the set for redundantly performing the wireless transmission on the aggregated carriers. 
     According to a further embodiment, a wireless device for a wireless communication network is provided. The wireless device comprises at least one processor and a memory. The memory contains instructions executable by said at least one processor, whereby the wireless device is operative to determine a set of carriers. The carriers each require an LBT procedure before transmitting on the carrier. Further, the memory contains instructions executable by said at least one processor, whereby the wireless device is operative to determine a reliability target for a wireless transmission from the wireless device to the wireless communication network. Further, the wireless device is configured to, depending on the reliability target, control aggregation of carriers from the set for redundantly performing the wireless transmission on the aggregated carriers. 
     According to a further embodiment, an access node for a wireless communication network is provided. The access node is configured to determine a reliability target for a wireless transmission from a wireless device to the wireless communication network. Further, the access node is configured to determine a set of carriers, to enable the wireless device to meet the reliability target by redundantly performing the wireless transmission on aggregated carriers from the set of carriers. The carriers each require an LBT procedure before transmitting on the carrier. 
     According to a further embodiment, an access node for a wireless communication network is provided. The access node comprises at least one processor and a memory. The memory contains instructions executable by said at least one processor, whereby the access node is operative to determine a reliability target for a wireless transmission from a wireless device to the wireless communication network. Further, memory contains instructions executable by said at least one processor, whereby the access node is operative to determine a set of carriers, to enable the wireless device to meet the reliability target by redundantly performing the wireless transmission on aggregated carriers from the set of carriers. The carriers each require an LBT procedure before transmitting on the carrier. 
     According to a further embodiment of the invention, a computer program or computer program product is provided, e.g., in the form of a non-transitory storage medium, which comprises program code to be executed by at least one processor of a wireless device for a wireless communication network. Execution of the program code causes the wireless device to determine a set of carriers. The carriers each require an LBT procedure before transmitting on the carrier. Further, execution of the program code causes the wireless device to determine a reliability target for a wireless transmission from the wireless device to the wireless communication network. Further, execution of the program code causes the wireless device to, depending on the reliability target, control aggregation of carriers from the set for redundantly performing the wireless transmission on the aggregated carriers. 
     According to a further embodiment of the invention, a computer program or computer program product is provided, e.g., in the form of a non-transitory storage medium, which comprises program code to be executed by at least one processor of an access node for a wireless communication network. Execution of the program code causes the access node to determine a reliability target for a wireless transmission from a wireless device to the wireless communication network. Further, the access node is configured to determine a set of carriers, to enable the wireless device to meet the reliability target by redundantly performing the wireless transmission on aggregated carriers from the set of carriers. The carriers each require an LBT procedure before transmitting on the carrier. 
     Details of such embodiments and further embodiments will be apparent from the following detailed description of embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    schematically illustrates a wireless communication network according to an embodiment. 
         FIG.  2    schematically illustrates carriers utilized in an embodiment. 
         FIG.  3    schematically illustrates an example of processes for controlling wireless transmissions in accordance with a reliability target according to an embodiment. 
         FIG.  4 A  schematically illustrates an example of information which can be used as a basis for controlling aggregation of LBT carriers according to an embodiment. 
         FIG.  4 B  schematically illustrates a further example of information which can be used as a basis for controlling aggregation of LBT carriers according to an embodiment. 
         FIG.  5    shows a flowchart for schematically illustrating a method according to an embodiment. 
         FIG.  6    shows a block diagram for schematically illustrating functionalities of a wireless device according to an embodiment. 
         FIG.  7    shows a flowchart for schematically illustrating a further method according to an embodiment. 
         FIG.  8    shows a block diagram for schematically illustrating functionalities of an access node according to an embodiment. 
         FIG.  9    schematically illustrates structures of a wireless device according to an embodiment. 
         FIG.  10    schematically illustrates structures of an access node according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the following, concepts in accordance with exemplary embodiments of the invention will be explained in more detail and with reference to the accompanying drawings. The illustrated embodiments relate to ensuring reliability of transmissions on carriers which require an LBT procedure, in the following referred to as LBT carriers. The wireless communication system may be based on the NR technology. In particular, the illustrated embodiments are explained by referring to various examples in the context of NR unlicensed operation, i.e., NR-U. However, it is noted that the illustrated concepts could also be applied in other unlicensed scenarios, such as LTE LAA (Licensed Assisted Access) or MulteFire. As used herein, UL (uplink) refers to a transmission direction from the wireless device to an access node of the wireless communication system, and DL (downlink) refers to a transmission direction from an access node of the wireless communication system to the wireless device. Further, the term carrier refers to a frequency that may be used for transmission of signals. A carrier may also cover a set of subcarriers within a certain bandwidth range. The terms carrier and carrier frequency may be used in an interchangeable manner. Further, it is noted that the LBT procedure on the LBT based carriers may assess the entire carrier bandwidth or only a part of the entire carrier bandwidth. 
     In the illustrated concepts, a wireless device, in the following assumed to be a UE, may utilize one or more LBT based carriers for communication with the wireless communication network, in particular with an access node of the wireless communication network In the illustrated examples, the wireless device is assumed to be a UE, and the access node is assumed to be a gNB of the NR technology. However, it is noted that depending on the underlying technology, the concepts could also be applied in connection with other types of wireless devices or access nodes. 
     The illustrated concepts, compliance with a reliability target may be achieved by transmitting redundantly over multiple aggregated LBT carriers, e.g., by duplicating the same data packet for transmission on each of the aggregated LBT carriers. The LBT carriers to be aggregated may be selected based on estimated failure risks, e.g., a risk of failure of the LBT procedure, for the individual LBT carriers. By increasing the number of aggregated LBT carriers, reliability can be increased because there is a higher likelihood that the LBT procedure will be successful on at least one of the aggregated LBT carriers. Further, the illustrated concepts may at the same time allow for avoiding excessively high numbers of the aggregated LBT carriers, which is beneficial in view of efficient utilization of the available LBT carriers. 
       FIG.  1    illustrates an exemplary wireless communication network according to an embodiment. In the illustrated example, the wireless communication system includes access nodes  101 - 1 ,  101 - 2 ,  101 - 3 , and  101 - 4 , serving cells  100 - 1 ,  100 - 2 ,  100 - 3 ,  100 - 4  respectively. The access nodes  101 - 1 ,  101 - 2 ,  101 - 3 ,  101 - 4  may for example correspond to gNBs of the NR technology. As illustrated UEs  11 ,  12  may be connected through a radio link to the access node  101 - 1 . The radio link may be based on one or more carriers from a licensed spectrum and/or on one or more carriers from an unlicensed spectrum. An example of such carriers in illustrated in  FIG.  2   . Specifically, the example of  FIG.  2    shows licensed carriers  21 ,  22 , e.g., from a band allocated to the NR technology and licensed to an operator of the wireless communication network, and unlicensed or shared carriers  31 ,  32 ,  33 ,  34 ,  41 ,  42 ,  43 ,  44  from one or more unlicensed or shared frequency bands. For example, the carriers  31 ,  32 ,  33 ,  34  could be from a 2.4 GHz band which is also utilized by WLAN (Wireless Local Area Network) systems, and the carriers  41 ,  42 ,  43 ,  44  could be from a 5 GHz band which is also utilized by WLAN systems. 
     The access nodes  101 - 1 ,  101 - 2 ,  101 - 3 ,  101 - 4  are part of a RAN (Radio Access Network) of the wireless communication network, which typically also includes further access nodes to provide a desired overall coverage of the wireless communication network. Further,  FIG.  1    shows a core network (CN)  110  of the wireless communication network. The CN  110  may provide connectivity of the UEs  11 ,  12  among each other and connectivity to other data networks, e.g., through a gateway (GVV)  120  provided in the CN  110 . Further, the CN  110  may also include various nodes for controlling operation of the UEs  11 ,  12 . 
     The radio link to the wireless communication network may be used for providing various kinds of services to the UE  11 ,  12 , e.g., a voice service, a multimedia service, or other data service. Such services may be based on applications which are executed on the UE  11 ,  12  and/or on a device linked to the UE  11 ,  12 . Further,  FIG.  1    illustrates an application service platform  150  in the CN  110  of the wireless communication network and one or more application servers  200  provided outside the wireless communication network. The application(s) executed on the UE  11 ,  12  and/or on one or more other devices linked to the UE  11 ,  12  may use the radio links for communication with one or more other UEs  11 ,  12 , the application service platform  150 , and/or the application server(s)  200 , thereby enabling the corresponding service(s) on the UE  11 ,  12 . In some scenarios, the services utilized by the UE  11 ,  12  may thus be hosted on the network side, e.g., on the application service platform  150  or on the application server(s)  200 . In the example of  FIG.  1   , the UEs  11 ,  12  are assumed to be a mobile phone or smartphone. However, it is noted that other types of UE could be used as well, e.g., an MTC device or vehicle based device. 
       FIG.  3    further illustrates an example of processes for implementing the illustrated concepts, which involve operations by a UE, e.g., one of the above-mentioned UEs  11 ,  12 , and operations of a gNB, e.g., corresponding to one of the above-mentioned access nodes  101 - 1 ,  101 - 2 ,  101 - 3 ,  101 - 4 . The processes of  FIG.  3    may be used to decide the number of aggregated-LBT carriers in real-time, e.g., for each UL transmission from the UE to the gNB, with the aim of achieve a desired reliability, defined by a reliability target, while utilizing LBT carriers as far as possible. If the desired reliability cannot be achieved while utilizing LBT carriers, transmission on a licensed carrier may be used as a fallback. In the following, this licensed carrier is also denoted as fallback carrier. 
     At block  310 , the processes involve configuration by the gNB for an intended UL transmission by the UE. In particular, the gNB selects LBT carriers which can be utilized by the UE and grants resources on these LBT carriers to the UE. In addition, the gNB grants resources on the fallback carrier to the UE. By sending an UL grant to the UE, the gNB indicates the LBT carriers and the allocated resources to the UE. Further, the UL grant may also indicate the reliability target. Optionally, the gNB may also provide various supplemental information to the UE, such as measurements and/or statistics on the LBT carriers. The gNB may also use such measurements and/or statistics as a basis for selecting the LBT carriers indicated to the UE. For example, if the measurements and/or statistics indicate that a certain LBT carrier is very busy, e.g., has an average occupancy above a threshold, the gNB may exclude this LBT carrier from the LBT carriers indicated to the UE. 
     Different options may be utilized for allocating the resources on the fallback carrier. According to one option, the resources may be allocated with a configured UL grant, i.e., in a semi-static manner. This may enable the UE to use the allocated resources of the fallback carrier without further interaction with the gNB, so that low latency can be achieved. This option may be suitable for delay critical services. According to a further option, the resources of the fallback carrier may be allocated with a configured UL grant, with the additional condition that the UE notifies the gNB if it will use the resources allocated by the configured UL grant resources. In the absence of such notification, the gNB can the re-allocate the resources and schedule other UEs on at least some of the resources. According to a still further option, the resources allocated on the fallback carrier may be limited to enable the UE to request allocation of resources on the fallback carrier, e.g., by sending a MAC CE (MAC Control Element) or by sending UCI (Uplink Control Information) on a PUCCH (Physical Uplink Control Channel). In response to such request, the gNB may then allocate additional resources on the fallback carrier to the UE, which can then be used for the intended UL transmission. 
     At block  320 , the UE initiates a variable which defines a maximum number of iterations, denoted as “max_iterations”. This may be accomplished on the basis of control information received from the gNB, e.g., in the UL grant provided at block  310  and/or on the basis of higher layer control information, e.g., provided by RRC (Radio Resource Control) signaling. As will be apparent from the following explanations, the max_iterations variable effectively corresponds to a maximum number of LBT carriers that can be aggregated by the UE for the UL transmission. The initialization of the max_iterations variable may also depend on the UE&#39;s capabilities of aggregating the LBT carriers. For example, there are UEs that cannot support parallel transmissions on more than three LBT carriers, which may depend on the UE&#39;s supported band combinations. Also the max_iteration variable can be used by the operator to impose a limit on the LBT carriers used when attempting a transmission. For example, a very noisy environment could result in an excessively high number of aggregated LBT carriers, and transmission on a licensed carrier may be preferable in such situations. 
     The UE then proceeds to aggregating at least some of the indicated LBT carriers according to an iterative process illustrated by blocks  330 ,  341 ,  342 ,  343 ,  350 , and  360 . 
     At block  330 , the UE checks if there are available LBT carriers indicated at block  310 . If this is the case, as indicated by branch “Y”, the UE proceeds to block  341  and selects the most appropriate LBT carrier from the available indicated LBT carriers. Examples of procedures and criteria for selecting the most appropriate LBT carrier are further explained below. At block  342 , the UE appends the LBT carrier selected at block  341  in to a list of candidates. At the beginning of the iterative process, the list of candidates is empty. At block  343  the UE calculates the reliability of the UL transmission which can be expected when redundantly performing the UL transmission on the LBT carriers on the current list of candidates. At block  350 , the UE then checks if the achievable reliability calculated at block  343  meets the set reliability target. If this is not the case, as indicated by branch “N”, the UE proceeds to block  360  and checks if the maximum number of iterations defined by the max_iterations variable is reached, if this is not the case, as indicated by branch “N”, the UE proceeds to a further iteration starting with block  330 . 
     If at block  350  the UE finds that the reliability target is met, as indicated by branch “Y”, the UE proceeds to block  370  to redundantly perform the UL transmission on the aggregated LBT carriers as currently indicated by the list of candidates. In some scenarios, the UE may first inform the gNB on which LBT carriers the UL transmission will be performed, e.g., by sending a MAC CE or UCI on the fallback carrier, so that the gNB does not need to listen to all the LBT carriers indicated at block  310 . 
     If at block  360  the UE finds that the maximum number of iterations is reached, as indicated by branch “Y”, the UE proceeds to block  380  to switch to the fallback carrier. As a result, the UE then performs the UL transmission on the fallback carrier. Before doing so, the UE may first inform the gNB about the intended switch to the fallback carrier, e.g., by sending a MAC CE or UCI. 
     As can be seen, in the processes of  FIG.  3   , the UE will keep iterating and stop only if the estimated achievable reliability meets the reliability target or if there are no more available LBT carriers to be appended to the list of candidates. 
     In the following, some aspects and variants of ensuring reliability by aggregating multiple LBT carriers will be explained in more detail. For these explanations, the term targeted transmission slot (TTS) is used for denoting a time interval over which the UE/gNB are expected to be transmitting. 
     In some scenarios, a failure rate of the unlicensed LBT carriers may be used as a basis for controlling the aggregation of LBT carriers and deciding whether to alternatively transmit on the fallback carrier. The failure rates may in particular consider a rate of failures of the LBT procedure on the LBT carrier. Here, a failure of the LBT procedure may correspond to the case that the LBT carrier is found to be occupied, so that transmission on the LBT carrier needs to be deferred. Further, a failure of the LBT procedure may correspond to the case that it is not possible to perform a scheduled transmission due to the LBT procedure not being finished at the time of the scheduled transmission. Various methods may be used to measure failures of the LBT procedure, for example based on probing signals and/or corresponding reporting mechanisms. 
     Further, the failure rates may consider a rate of failures of transmissions on the LBT carrier. Here, a failure of the transmission on the LBT carrier may correspond to a failure to correctly receive the due to an LBT failure or to a failure to correctly decode the transmission due to interference, low signal quality, or the like. 
     The failure rates of the LBT carriers can be monitored at the gNB side, at the UE side, or based on a cooperative scheme involving both the UE and the gNB, e.g., involving reporting of failures or failure statistics from the UE to the gNB and/or reporting of failures or failure statistics from the gNB to the UE. The measured failure rates may be used directly or may be extrapolated to estimate a risk of failure in the TTS. The extrapolation to the TTS may be achieved by running a regression tool on the history of the measured failure rate of the LBT carrier, by running a sliding window estimator on the measured failure rate of the LBT carrier, or by considering the last LBT failure on the LBT carrier and assuming that the closer the failure to the TTS, the more probable will be a failure in the TTS.  FIG.  4 A  shows a table with exemplary failure rates, or extrapolated failure risks, determined for a set of n LBT carriers. 
     Based on the failure rates or failure risks determined for the individual LBT carriers, denoted as FR i , with i denoting an index of the respective LBT carrier, a combined failure risk FR C , corresponding to the case of a coincident failure on multiple LBT carriers, can be calculated according to 
       FR C =Π i FR i   .(1)
 
     Assuming that the same data is transmitted in a duplicated manner on each of the aggregated LBT carriers, the achievable reliability R C  may then be calculated according to: 
         X   C =1− FR   C   .(2)
 
     Further, the failure rates or failure risks determined for the individual LBT carriers may also be used for calculating an individual reliability X i  for each of the LBT carriers according to: 
         X   i =1− FR   i   ,(3)
 
     and the achievable reliability R C  may then be calculated according to: 
         X   C =1−Π i (1− X   i ).  (4)
 
     The number of LBT carriers to be aggregated to meet the reliability target, denoted by X T , can then be determined based on the criterion: 
       X C ≥X T   .(5)
 
     If this criterion is met, the number of aggregated LBT carriers is sufficient to meet the reliability target X T . 
     When now considering the exemplary values of failure rates given in the table of  FIG.  4 A  and assuming a reliability target X T  of 99.95%, the UE may operate as follows:
         The UE starts with an empty list of candidates.   The UE selects the LBT carrier with the smallest failure rate/risk, in the considered example LBT carrier 3 with FR 3 =5%.   The UE appends the selected LBT carrier to the list of candidates, so that the updated list of candidates consists of LBT Carrier 3.   The UE calculates the achievable reliability X C  according to (2) or (4), which yields X C =1-5%=95%. Since the achievable reliability Xc is smaller than the reliability target X T , the proceeds to a further iteration to select the next LBT carrier.   The UE then selects the LBT carrier with the next smallest failure rate/risk, in the illustrated example LBT carrier 1 with FR 1 =20%.   The UE appends the selected LBT carrier to the list of candidates, so that the updated list of candidates consists of LBT Carrier 3 and LBT carrier 1.   The UE calculates the achievable reliability X C  according to (2) or (4), which yields X C =1-5%*20%=1−0.01=99,9%. Since the achievable reliability X C  is still smaller than the reliability target X T , the proceeds to a further iteration to select the next LBT carrier.   The UE then selects the LBT carrier with the next smallest failure rate/risk, in the illustrated example LBT carrier 2 with FR 2 =30%.   The UE appends the selected LBT carrier to the list of candidates, so that the updated list of candidates consists of LBT Carrier 3, LBT carrier 1, and LBT carrier 2.   The UE calculates the achievable reliability X C  according to (2) or (4), which yields X C =:1-5%*20%*30%=1−0.003=99.97%.   Since now the achievable reliability X c  is higher than the reliability target X T  the UE then proceeds to use the LBT carriers on the list of candidates for redundantly performing the UL transmission.       

     Accordingly, the failure rates of the individual LBT carriers, or failure risks extrapolated to the TTS, may be used as a basis for determining a reliability metric and selecting the LBT carriers to be appended to the list of candidates. In this case, the list of candidates may be successively expanded by that available LBT carrier which has the lowest failure rate or failure risk in the TTS. 
     In addition or as an alternative, energy measurements on the LBT carriers may be used as a basis for controlling the aggregation of LBT carriers and deciding whether to alternatively transmit on the fallback carrier. In a similar manner as the failure rates, the energy levels on the LBT carriers can be monitored at the gNB side, at the UE side, or based on a cooperative scheme involving both the UE and the gNB, e.g., involving reporting of energy measurements or statistics on measured energy levels from the UE to the gNB and/or reporting of energy measurements or statistics on measured energy levels from the gNB to the UE. The UE and/or gNB may implement the energy measurements based on detection over all or part of receiving FFT (Fast Fourier Transform) bins utilized for reception. If the gNB and the UE, and optionally also further UEs collaborate to collect the measurements on each LBT carrier, possible hidden node problems can be addressed, because the energy measurements typically originate from different locations.  FIG.  4 B  shows a table with exemplary energy levels determined for a set of n LBT carriers. 
     When using energy measurements, as for example shown  FIG.  4 B , as a basis for determining a reliability metric and selecting the LBT carriers to be appended to the list of candidates, the list of candidates may be successively expanded by that available LBT carrier which has the lowest energy floor. 
     In some scenarios, the UE may have multiple antennas for performing beamformed or spatially multiplexed transmissions. In such cases also channel ranks of the LBT carriers can be used as a basis for determining a reliability metric and selecting the LBT carriers to be appended to the list of candidates. In this case, it can be assumed that a higher channel rank may enable a higher degree of freedom, resulting in higher reliability. 
     In some scenarios, operating frequency of the LBT carriers can be used as a basis for determining a reliability metric and selecting the LBT carriers to be appended to the list of candidates. In this case, it can be taken into account that lower frequencies typically result in lower path loss. Further, it can be taken into account that lower frequencies could be more popular and thus subject to higher interference. Accordingly, based on the respective frequency of the LBT carrier, channel occupancy and/or path loss may be estimated and be taken into account as input for calculation of the reliability metric. 
     In some scenarios, potential interference and/or other effects among different UEs can be used as a basis for selecting the LBT carriers to be appended to the list of candidates. In this case, it can be taken into account that, when LBT carriers are scheduled for transmission of an eMBB UE, these LBT carriers should be selected with lower probability. 
     The above criteria may be applied by the UE when selecting the LBT carrier to be appended to the list of candidates, i.e., in the selection of block  341 . Further, these or similar criteria may also be applied by the gNB when selecting the LBT carriers at block  310 . Accordingly, the gNB and the UE may cooperatively apply one or more of the above-mentioned criteria. 
     Further, it is noted that the criteria may be used individually or that two or more of the criteria may be combined. For example, an LBT channel preselected on the basis of its failure rate or failure risk could still be rejected if the current energy measurements indicate an energy floor over a threshold or if earlier energy measurements or statistics indicate an excessively high energy level. 
     In some scenarios, the UE may also add an additional LBT carrier (if available) once the LBT carriers on the list of candidates are sufficient to meet the reliability target. This may provide some additional margin, e.g., for considering possible errors in the utilized input data (e.g., measured failure rates or energy levels). 
       FIG.  5    shows a flowchart for illustrating a method, which may be utilized for implementing the illustrated concepts. The method of  FIG.  5    may be used for implementing the illustrated concepts in a wireless device, such as one of the above-mentioned UEs  11 ,  12 . 
     If a processor-based implementation of wireless device is used, at least some of the steps of the method of  FIG.  5    may be performed and/or controlled by one or more processors of the wireless device. Such wireless device may also include a memory storing program code for implementing at least some of the below described functionalities or steps of the method of  FIG.  5   . 
     At step  510 , a wireless device determines a set of carriers. The carriers each require an LBT procedure before transmitting on the carrier. The LBT procedure may be based on sensing channel occupancy over an entire bandwidth of the carrier or on only a part of the bandwidth of the carrier, e.g., on a bandwidth part or channel within the bandwidth of the carrier. If the LBT procedure yields that the channel is occupied, the wireless device is not allowed to transmit on the carrier or at least not on the part of the bandwidth which was sensed in the LBT procedure. The carriers may be from an unlicensed frequency spectrum. The carriers may for example correspond to the above-mentioned LBT carriers. 
     The wireless device may determine the set of carriers based on a resource grant received from the wireless communication network. The resource grant may indicate the set of carriers and resources allocated on each carrier of the set of carriers. The resource grant may correspond to a configured resource grant, which allocates the resources in a semi-static manner for multiple wireless transmissions by the wireless device. Alternatively, the resource grant may correspond to a dynamic resource grant, which allocates the resources on an as-needed basis for a particular wireless transmissions by the wireless device, e.g., in response to a request from the wireless device. 
     At step  520 , the wireless device determines a reliability target for a wireless transmission from the wireless device to the wireless communication network. The reliability target may be indicated by the wireless communication network. The reliability target may for example be determined in terms of a probability of the wireless transmission being successfully received. The reliability target may be related to a data type or service type carried associated with the wireless transmission. 
     The wireless device may determine the reliability target based on control information received from the wireless communication network. For example, such control information could be indicated in the above-mentioned resource grant received at step  510 . Further, such control information could be indicated by higher layer signaling, e.g., by MAC signaling or RRC signaling. 
     At step  530 , the wireless device may determine an individual reliability metric for each carrier of the set. 
     In some scenarios, the wireless device may determine the individual reliability metrics of the carriers based on measurements performed by the wireless device, e.g., energy measurements on the carrier. Alternatively or in addition, the wireless device may determine the individual reliability metrics of the carriers based on information received from the wireless communication network. For example, such information may include information on a rank or frequency of the carrier. Further, such information may include information on measurements performed by other devices, e.g., energy measurements on the carrier. 
     In some scenarios, the individual reliability metrics may be based on a failure rate associated with wireless transmissions on the carrier. Here, the failure rate may in particular The failure rate correspond to or at least include a rate of failures of the LBT procedure on the carrier. Further, the failure rate may also include a rate of failures of a wireless transmission after success of the LBT procedure, e.g., correspond to an overall failure rate of wireless transmissions including either a failure of the LBT procedure or a failure of a wireless transmission initiated in response to a success of the LBT procedure. The failure rate may be used to extrapolate or otherwise estimate a failure risk of the current wireless transmission intended by the wireless device. Alternatively, the individual reliability metrics may be based on an energy level measured on the carrier, e.g., an energy floor on the carrier. 
     At step  540 , the wireless device may determine a candidate set of aggregated carriers from the set of carriers determined at step  510 . The wireless device may determine the candidate set of aggregated carriers by selecting carriers from the set determined at step  510 . 
     At step  550 , the wireless device may determine a combined reliability metric for the candidate set of aggregated carriers determined at step  540 . The combined reliability metric may be based on the individual reliability metrics determined at step  530  and represent a probability that the wireless transmission is successfully received when being performed redundantly on the candidate set of aggregated carriers. 
     At step  560 , the wireless device controls aggregation of carriers from the set, in particular aggregation with the purpose of redundantly performing the wireless transmission on the aggregated carriers. Specifically, the wireless device controls the aggregation of carriers depending on the reliability target determined at step  520 . 
     In some scenarios, step  560  may involve that the wireless device controls the aggregation of carriers further based on individual reliability metrics of the carriers of the set, e.g., as determined at step  530 . 
     In some scenarios, step  560  may be based on the candidate set of aggregated carriers determined at step  540  and the combined reliability metric determined at step  550 . For example, based on a comparison of the combined reliability metric to the reliability target the wireless device may decide whether to use the candidate set of aggregated carriers for performing the wireless transmission. For example, in response to the combined reliability metric being at least equal to the reliability target, the wireless device may decide to use the candidate set of aggregated carriers for performing the wireless transmission. 
     Further, step  560  may involve that, based on a comparison of the combined reliability metric to the reliability target, the wireless device decides whether to add a further carrier of the set to the candidate set of aggregated carriers. 
     According to a further example, in response to the combined reliability metric being at least equal to the reliability target, the wireless device may decide to add one further carrier of the set to the candidate set of aggregated carriers and then use the candidate set of aggregated carriers for performing the wireless transmission. 
     Further, step  560  may involve that, in response to deciding to add a further carrier of the set to the candidate set of aggregated carriers, the wireless device selects, from the set of carriers, the carrier which has the highest individual reliability metric and is not yet member of the candidate set of aggregated carriers and adds the selected carrier as the further carrier to the candidate set of aggregated carriers. 
     In some scenarios, step  560  may involve that the wireless device controls the aggregation of carriers further based on a maximum limit of aggregated carriers, e.g., as defined by the above-mentioned max_iterations variable. For example, in response to a number of carriers in the candidate set of aggregated carriers exceeding the maximum limit of aggregated carriers, the wireless device may decide to perform the wireless transmission on an alternative carrier not requiring an LBT procedure before transmitting on the carrier, such as the above-mentioned fallback carrier. The alternative carrier may be from a licensed frequency spectrum. 
     Step  560  may thus involve updating the candidate set of aggregated carriers. This may be based on an iterative process, e.g., as explained in connection with  FIG.  3   . 
     At step  570 , the wireless device performs the wireless transmission. In some scenarios, this may involve that, in response to determining that it is not possible to meet the reliability target by redundantly transmitting on the aggregated carriers, the wireless device performs the wireless transmission on an alternative carrier not requiring an LBT procedure before transmitting on the carrier, such as the above-mentioned fallback carrier. The alternative carrier may be from a licensed frequency spectrum. 
     In some scenarios, the wireless device may perform the wireless transmission on the alternative carrier on radio resources which correspond to radio resources allocated on the set of carriers. Accordingly, the resource grant mentioned in connection with step  510  may also allocate resources on the alternative carrier. In this case, before performing the wireless transmission on the alternative carrier, the wireless device may indicate to the wireless communication network that the wireless transmission will be performed on the alternative carrier. This may enable the wireless communication network to re-allocate the resource on the alternative carrier if there is no indication from the wireless communication device that the wireless transmission will be performed on the alternative carrier. 
     In some scenarios, the wireless device may before performing the wireless transmission on the alternative carrier, send a request for allocation of resources of the alternative carrier to the wireless communication network, in response to the request, receive a resource grant indicating resources allocated on the alternative carrier; and performing the wireless transmission on the resources allocated on the alternative carrier. The resources to be used for performing the wireless transmission on the alternative carrier may thus be allocated on an as-needed basis by a dynamic resource grant. 
     In some scenarios, the wireless device may perform the wireless transmission by redundantly transmitting on the aggregated carriers from step  560 . Here, redundantly performing the wireless transmission may involve duplicating a data packet for transmission on each of the aggregated carriers. For example, the duplicated data packet may be a PDCP (Packet Data Convergence Protocol) packet. 
       FIG.  6    shows a block diagram for illustrating functionalities of a wireless device  600  which operates according to the method of  FIG.  5   . The wireless device  600  may for example correspond to one of the above-mentioned UEs  11 ,  12 . As illustrated, the wireless device  600  may be provided with a module  610  configured to determine a set of carriers, such as explained in connection with step  510 . Further, the wireless device  600  may be provided with a module  620  configured to configure determine a reliability target, such as explained in connection with step  520 . Further, the wireless device  600  may be provided with a module  630  configured to determine individual reliability metrics, such as explained in connection with step  530 . Further, the wireless device  600  may be provided with a module  640  configured to determine a candidate set of aggregated carriers, such as explained in connection with step  540 . Further, the wireless device  600  may be provided with a module  650  configured to determine a combined reliability metric, such as explained in connection with step  550 . Further, the wireless device  600  may be provided with a module  660  configured to control aggregation of carriers, such as explained in connection with step  560 . Further, the wireless device  600  may be provided with a module  670  configured to perform a wireless transmission, such as explained in connection with step  570 . 
     It is noted that the wireless device  600  may include further modules for implementing other functionalities, such as known functionalities of various kinds of UE. Further, it is noted that the modules of the wireless device  600  do not necessarily represent a hardware structure of the wireless device  600 , but may also correspond to functional elements, e.g., implemented by hardware, software, or a combination thereof. 
       FIG.  7    shows a flowchart for illustrating a method, which may be utilized for implementing the illustrated concepts. The method of  FIG.  7    may be used for implementing the illustrated concepts in an access node, such as one of the above-mentioned access nodes  101 - 1 ,  101 - 2 ,  101 - 3 ,  101 - 4 . The access node may serve one or more cells of the wireless communication network. 
     If a processor-based implementation of access node is used, at least some of the steps of the method of  FIG.  7    may be performed and/or controlled by one or more processors of the access node. Such access node may also include a memory storing program code for implementing at least some of the below described functionalities or steps of the method of  FIG.  7   . 
     At step  710 , the access node determines a reliability target for a wireless transmission from a wireless device to the wireless communication network. The reliability target may for example be determined in terms of a probability of the wireless transmission being successfully received. The access node may determine the reliability target on the basis of a data type or service type carried associated with the wireless transmission. 
     At step  720 , the access node may determine individual reliability metrics for carriers requiring an LBT procedure before transmitting on the carrier. The LBT procedure may be based on sensing channel occupancy over an entire bandwidth of the carrier or on only a part of the bandwidth of the carrier, e.g., on a bandwidth part or channel within the bandwidth of the carrier. If the LBT procedure yields that the channel is occupied, a device is not allowed to transmit on the carrier or at least not on the part of the bandwidth which was sensed in the LBT procedure. The carriers may be from an unlicensed frequency spectrum. The carriers may for example correspond to the above-mentioned LBT carriers. 
     In some scenarios, the access node may determine the individual reliability metrics of the carriers based on measurements performed by the access node, e.g., energy measurements on the carrier. Alternatively or in addition, the access node may determine the individual reliability metrics of the carriers based on information received from the wireless device, from one or more other wireless devices, and/or from one or more other access nodes of the wireless communication network. In some scenarios, the access node may also send such information to the wireless device, to enable determination of individual reliability metrics at the wireless device. For example, such information may include information on a rank or frequency of the carrier. 
     In some scenarios, the individual reliability metrics may be based on a failure rate associated with wireless transmissions on the carrier. Here, the failure rate may in particular The failure rate correspond to or at least include a rate of failures of the LBT procedure on the carrier. Further, the failure rate may also include a rate of failures of a wireless transmission after success of the LBT procedure, e.g., correspond to an overall failure rate of wireless transmissions including either a failure of the LBT procedure or a failure of a wireless transmission initiated in response to a success of the LBT procedure. The failure rate may be used to extrapolate or otherwise estimate a failure risk of the current wireless transmission intended by the wireless device. Alternatively, the individual reliability metrics may be based on an energy level measured on the carrier, e.g., an energy floor on the carrier. 
     At step  730 , the access node determines a set of carriers. The carriers require an LBT procedure before transmitting on the carrier, e.g., like the carriers mentioned in step  720 . In particular, the access node may determine the set of carriers from carriers mentioned in step  720 , e.g., by selecting the carriers based on the respective individual reliability metric of each carrier. Further, the access node may determine the set of carriers based on the reliability target determined at step  710 . 
     In step  730 , the access node determines the set of carriers with the aim of enabling the wireless device to meet the reliability target determined at step  710  by redundantly performing the wireless transmission on aggregated carriers from the set of carriers. Here, redundantly performing the wireless transmission may involve duplicating a data packet for transmission on each of the aggregated carriers. For example, the duplicated data packet may be a PDCP packet. 
     At step  740 , the access node may allocate resources to the wireless device. For example, the access node may allocate corresponding resources for performing the wireless transmission on each carrier of the set. Further, the access node may allocate corresponding resources for performing the wireless transmission on each carrier of the set and on an alternative carrier. The alternative carrier may be from a licensed frequency spectrum. The alternative carrier may for example correspond to the above-mentioned fallback carrier. 
     At step  750 , the access node may send control information to the wireless communication network. The control information may for example indicate the reliability target determined at step  710 . The control information may be transmitted in a resource grant, and/or by higher layer signaling, e.g., by MAC signaling or RRC signaling. 
     At step  760 , the access node may send a resource grant to the wireless communication device. The resource grant may indicate the set of carriers and resources allocated on each carrier of the set of carriers. In some scenarios, the resource grant may include at least a part of the control information of step  750 . 
     In some scenarios, the access node may indicate a maximum limit of aggregated carriers to the wireless device, e.g., as defined by the above-mentioned max_iterations variable. The access node may for example indicate the maximum limit of aggregated carrier by the control information sent at step  750  and/or the resource grant sent at step  760 . 
     In some scenarios, the access node may indicate an alternative carrier for performing the wireless transmission, e.g., by the control information sent at step  750  and/or the resource grant sent at step  760 . The alternative carrier does not require an LBT procedure before transmitting on the carrier. The alternative carrier may be from a licensed frequency spectrum. The alternative carrier may for example correspond to the above-mentioned fallback carrier. 
     In some scenarios the access node may send information to the wireless device, e.g., by the control information sent at step  750  and/or the resource grant sent at step  760 , and the information may enable the wireless device to determine a reliability metric of each carrier of the set of carriers. This information may for example include information on measurements as mentioned in connection with step  720 . The information may be based on statistics collected by the access node, e.g., statistics on failure rates or energy measurements. Alternatively or in addition, the information may be based on measurements performed by the access node, one or more other access nodes, and/or one or more other wireless devices. These measurements may for example relate to failure rates of wireless transmissions on the carriers of the set or to energy levels measured on the carriers of the set. 
     At step  770 , the access node may receive the wireless transmission from the wireless device. In some scenarios, this may involve that the access node receives the wireless transmission on an alternative carrier. The alternative carrier may be from a licensed frequency spectrum. The alternative carrier may for example correspond to the above-mentioned fallback carrier. In this case, before the wireless transmission is performed on the alternative carrier, the access node may expect an indication from the wireless device that the wireless transmission will be performed on the alternative carrier. In response to not receiving the indication, the access node may re-allocating the resources on the alternative carrier. Further, before the wireless transmission is performed on the alternative carrier, the access node could receive a request for allocation of resources on the alternative carrier from the wireless device, in response to the request, send a resource grant indicating resources allocated on the alternative carrier; receiving the wireless transmission on the resources allocated on the alternative carrier. 
     In some scenarios, the access node may also receive the wireless transmission from at least one of the set of carriers determined at step  730 . In this case, before the wireless transmission is performed, the access node may receive from the wireless device an indication on which carriers of the set the wireless transmission will be performed. 
       FIG.  8    shows a block diagram for illustrating functionalities of an access node  800  which operates according to the method of  FIG.  7   . The access node  800  may for example correspond to one the above-mentioned access nodes  101 - 1 ,  101 - 2 ,  101 - 3 , or  101 - 4 . As illustrated, the access node  800  may be provided with a module  810  configured to send SI, such as explained in connection with step  710 . Further, the access node  800  may be provided with a module  820  configured to configure one or more carriers for communication with a wireless device, such as explained in connection with step  720 . Further, the access node  800  may be provided with a module  830  configured to detect radar interference, such as explained in connection with step  730 . Further, the access node  800  may be provided with a module  840  configured to send DCI with an avoidance indication, such as explained in connection with step  740 . 
     It is noted that the access node  800  may include further modules for implementing other functionalities, such as known functionalities of a gNB of the NR technology. Further, it is noted that the modules of the access node  800  do not necessarily represent a hardware structure of the access node  800 , but may also correspond to functional elements, e.g., implemented by hardware, software, or a combination thereof. 
       FIG.  9    illustrates a processor-based implementation of a wireless device  900  which may be used for implementing the above-described concepts. For example, the structures as illustrated in  FIG.  9    may be used for implementing the concepts in one of the above-mentioned UEs  11 ,  12 . 
     As illustrated, the wireless device  900  includes one or more radio interfaces  910 . The radio interface(s)  910  may for example be based on the NR technology and may also support beamformed transmission. However, other wireless technologies could be supported as well, e.g., the LTE technology. 
     Further, the wireless device  900  may include one or more processors  950  coupled to the radio interface(s)  910  and a memory  960  coupled to the processor(s)  950 . By way of example, the radio interface(s)  910 , the processor(s)  950 , and the memory  960  could be coupled by one or more internal bus systems of the wireless device  900 . The memory  960  may include a Read-Only-Memory (ROM), e.g., a flash ROM, a Random Access Memory (RAM), e.g., a Dynamic RAM (DRAM) or Static RAM (SRAM), a mass storage, e.g., a hard disk or solid state disk, or the like. As illustrated, the memory  960  may include software  970  and/or firmware  980 . The memory  960  may include suitably configured program code to be executed by the processor(s)  950  so as to implement the above-described functionalities for controlling wireless transmissions, such as explained in connection with  FIGS.  5  and  6   . 
     It is to be understood that the structures as illustrated in  FIG.  9    are merely schematic and that the wireless device  900  may actually include further components which, for the sake of clarity, have not been illustrated, e.g., further interfaces or further processors. Also, it is to be understood that the memory  960  may include further program code for implementing known functionalities of a wireless device, e.g., functionalities of various kinds of UE. According to some embodiments, also a computer program may be provided for implementing functionalities of the wireless device  900 , e.g., in the form of a physical medium storing the program code and/or other data to be stored in the memory  960  or by making the program code available for download or by streaming. 
     It is noted that the functionalities described in connection with  FIGS.  5  to  8    could also be combined, e.g., in a system which includes a wireless device operating according to the method of  FIG.  6    and an access node operating according to the method of  FIG.  7    and receiving the wireless transmission from the wireless device. 
       FIG.  10    illustrates a processor-based implementation of an access node  1000  which may be used for implementing the above-described concepts. For example, the structures as illustrated in  FIG.  10    may be used for implementing the concepts in any of the above-mentioned access nodes  101 - 1 ,  101 - 2 ,  101 - 3 ,  101 - 4 . 
     As illustrated, the access node  1000  includes one or more radio interfaces  1010 . The radio interface(s)  1010  may for example be based on the NR technology and may also support beamformed transmission. However, other wireless technologies could be supported as well, e.g., the LTE technology. 
     Further, the access node  1000  may include one or more processors  1050  coupled to the radio interface(s)  1010  and a memory  1060  coupled to the processor(s)  1050 . By way of example, the radio interface(s)  1010 , the processor(s)  1050 , and the memory  1060  could be coupled by one or more internal bus systems of the access node  1000 . The memory  1060  may include a ROM, e.g., a flash ROM, a RAM, e.g., a DRAM or SRAM, a mass storage, e.g., a hard disk or solid state disk, or the like. As illustrated, the memory  1060  may include software  1070  and/or firmware  1080 . The memory  1060  may include suitably configured program code to be executed by the processor(s)  1050  so as to implement the above-described functionalities for controlling wireless transmissions, such as explained in connection with  FIGS.  7  and  8   . 
     It is to be understood that the structures as illustrated in  FIG.  10    are merely schematic and that the access node  1000  may actually include further components which, for the sake of clarity, have not been illustrated, e.g., further interfaces or further processors. Also, it is to be understood that the memory  1060  may include further program code for implementing known functionalities of an access node, e.g., functionalities of a gNB of the NR technology. According to some embodiments, also a computer program may be provided for implementing functionalities of the access node  1000 , e.g., in the form of a physical medium storing the program code and/or other data to be stored in the memory  1060  or by making the program code available for download or by streaming. 
     As can be seen, the concepts as described above may be used for efficiently controlling contention-based wireless transmissions, in particular in view of achieving high reliability for wireless transmissions in unlicensed or shared bands. Specifically, the illustrated concepts may help to mitigate or compensate for extra risks due to the operation in the unlicensed band or shared band, in particular an increased risk of failure due to failure of the required LBT procedure. Further, the possibility of a fallback to a carrier from a licensed spectrum may help to meet the reliability target while at the same time utilizing the unlicensed or shared spectrum as far as possible, so that the licensed spectrum can be utilized in an efficient manner. 
     It is to be understood that the examples and embodiments as explained above are merely illustrative and susceptible to various modifications. For example, the illustrated concepts may be applied in connection with various kinds of wireless technologies, without limitation to the NR-U technology. Further, the concepts may be applied with respect to various types of UEs. Moreover, it is to be understood that the above concepts may be implemented by using correspondingly designed software to be executed by one or more processors of an existing device or apparatus, or by using dedicated device hardware. Further, it should be noted that the illustrated apparatuses or devices may each be implemented as a single device or as a system of multiple interacting devices or modules.