Patent Publication Number: US-2022225339-A1

Title: Method and apparatus for transmitting sidelink control information

Description:
TECHNICAL FIELD 
     The present disclosure relates to a method and apparatus for transmitting sidelink control information. 
     BACKGROUND 
     Wireless communication systems generally aim to reduce costs for users and providers, improve service quality, and expand and improve coverage and system capacity. To achieve these goals, in some scenarios, wireless communication systems are designed to reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement. 
     SUMMARY 
     An aspect of the present disclosure is to provide a method and apparatus for transmitting sidelink control information within a time period. 
     In an aspect, a method for a wireless device in a wireless communication system is provided. The method includes determining that a sidelink (SL) resource has not been reserved within a time period related to transmission of sidelink control information (SCI), and triggering SL resource reservation for transmission of the SCI. 
     In another aspect, an apparatus for implementing the above method is provided. 
     The present disclosure can have various advantageous effects. 
     For example, a UE can transmit control information (e.g., SCI) for sidelink management within appropriate time period. 
     For example, a UE can reserve a resource and transmit control information (e.g., SCI) for a direct link with other UE, in particular when the UE has no data to be transmitted to the other UE. 
     For example, the system can reliably manage a direct link between two UEs performing sidelink communication. 
     Advantageous effects which can be obtained through specific embodiments of the present disclosure are not limited to the advantageous effects listed above. For example, there may be a variety of technical effects that a person having ordinary skill in the related art can understand and/or derive from the present disclosure. Accordingly, the specific effects of the present disclosure are not limited to those explicitly described herein, but may include various effects that may be understood or derived from the technical features of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example of a wireless communication system to which implementations of the present disclosure can be applied. 
         FIG. 2  shows a block diagram of an example of a user plane protocol stack t to which implementations of the present disclosure can be applied. 
         FIG. 3  shows a block diagram of an example of a control plane protocol stack to which implementations of the present disclosure can be applied. 
         FIG. 4  shows a frame structure in a 3GPP based wireless communication system to which implementations of the present disclosure can be applied. 
         FIG. 5  shows a data flow example in the 3GPP NR system to which implementations of the present disclosure can be applied. 
         FIG. 6  shows an example of a communication system to which implementations of the present disclosure can be applied. 
         FIG. 7  shows an example of wireless devices to which implementations of the present disclosure can be applied. 
         FIG. 8  shows an example of a wireless device to which implementations of the present disclosure can be applied. 
         FIG. 9  shows another example of wireless devices to which implementations of the present disclosure can be applied. 
         FIG. 10  shows an example of UE to which implementations of the present disclosure can be applied. 
         FIGS. 11 and 12  show an example of PC5 protocol stacks to which implementations of the present disclosure can be applied. 
         FIG. 13  shows an example of a method for a wireless device to which implementations of the present disclosure can be applied. 
         FIG. 14  shows an example of a method for performing sidelink communication for a UE to which implementations of the present disclosure can be applied. 
     
    
    
     DETAILED DESCRIPTION 
     In the present disclosure, “A or B” may mean “only A”, “only B”, or “both A and B”. In other words, “A or B” in the present disclosure may be interpreted as “A and/or B”. For example, “A, B or C” in the present disclosure may mean “only A”, “only B”, “only C”, or “any combination of A, B and C”. 
     In the present disclosure, slash (/) or comma (,) may mean “and/or”. For example, “A/B” may mean “A and/or B”. Accordingly, “A/B” may mean “only A”, “only B”, or “both A and B”. For example, “A, B, C” may mean “A, B or C”. 
     In the present disclosure, “at least one of A and B” may mean “only A”, “only B” or “both A and B”. In addition, the expression “at least one of A or B” or “at least one of A and/or B” in the present disclosure may be interpreted as same as “at least one of A and B”. 
     In addition, in the present disclosure, “at least one of A, B and C” may mean “only A”, “only B”, “only C”, or “any combination of A, B and C”. In addition, “at least one of A, B or C” or “at least one of A, B and/or C” may mean “at least one of A, B and C”. 
     Also, parentheses used in the present disclosure may mean “for example”. In detail, when it is shown as “control information (PDCCH)”, “PDCCH” may be proposed as an example of “control information”. In other words, “control information” in the present disclosure is not limited to “PDCCH”, and “PDDCH” may be proposed as an example of “control information”. In addition, even when shown as “control information (i.e., PDCCH)”, “PDCCH” may be proposed as an example of “control information”. 
     Technical features that are separately described in one drawing in the present disclosure may be implemented separately or simultaneously. 
     Although not limited thereto, various descriptions, functions, procedures, suggestions, methods and/or operational flowcharts of the present disclosure disclosed herein can be applied to various fields requiring wireless communication and/or connection (e.g., 5G) between devices. 
     Vehicle-to-everything (V2X) communication is the communication of information from a vehicle to an entity that may affect the vehicle, and vice versa. Examples of V2X include vehicle-to-infrastructure (V2I), vehicle-to-network (V2N), vehicle-to-vehicle (V2V), vehicle-to-pedestrian (V2P), vehicle-to-device (V2D), and vehicle-to-grid (V2G). 
     V2X systems may be designed to achieve various objectives, such as road safety, traffic efficiency, and energy savings. V2X communication technology may be classified into two types, depending on the underlying technology: wireless local area network (WLAN)-based V2X, and cellular-based V2X. 
     The 3rd generation partnership project (3GPP) long-term evolution (LTE) is a technology designed to enable high-speed packet communications. In addition, the international telecommunication union (ITU) and 3GPP have developed technical standards for new radio (NR) systems. In doing so, technology is being identified and developed to successfully standardize the new radio access technology (RAT), in order to timely satisfy both urgent market needs, as well as longer-term goals and requirements set forth by the ITU radio communication sector (ITU-R) international mobile telecommunications (IMT)-2020 process. In some scenarios, NR is being designed to use any spectrum band ranging at least up to 100 GHz, which may be made available for wireless communications even in a more distant future. 
     The NR targets a technical framework addressing various usage scenarios, requirements, and deployment scenarios, such as, for example, enhanced mobile broadband (eMBB), massive machine-type-communications (mMTC), ultra-reliable and low latency communications (URLLC), etc. 
     In some systems, one or more technical features described below may be compatible with one or more technical standards, such as those used by a communication standard by the 3GPP standardization organization, a communication standard by the institute of electrical and electronics engineers (IEEE), etc. For example, the communication standards by the 3GPP standardization organization include LTE and/or evolution of LTE systems. The evolution of LTE systems includes LTE-advanced (LTE-A), LTE-A Pro, and/or 5G NR. The communication standard by the IEEE standardization organization includes a wireless local area network (WLAN) system such as IEEE 802.11a/b/g/n/ac/ax. The above system uses various multiple access technologies such as orthogonal frequency division multiple access (OFDMA) and/or single carrier frequency division multiple access (SC-FDMA) for downlink (DL) and/or uplink (DL). For example, only OFDMA may be used for DL and only SC-FDMA may be used for UL. Alternatively, OFDMA and SC-FDMA may be used for DL and/or UL. 
     For convenience of description, implementations of the present disclosure are mainly described in regards to a 3GPP based wireless communication system. However, the technical features of the present disclosure are not limited thereto. For example, although the following detailed description is given based on a mobile communication system corresponding to a 3GPP based wireless communication system, aspects of the present disclosure that are not limited to 3GPP based wireless communication system are applicable to other mobile communication systems. 
     For terms and technologies which are not specifically described among the terms of and technologies employed in the present disclosure, the wireless communication standard documents published before the present disclosure may be referenced. 
     Hereinafter, the present disclosure will be described in more detail with reference to drawings. The same reference numerals in the following drawings and/or descriptions may refer to the same and/or corresponding hardware blocks, software blocks, and/or functional blocks unless otherwise indicated. 
       FIG. 1  shows an example of a wireless communication system to which implementations of the present disclosure can be applied. 
     Referring to  FIG. 1 , the wireless communication system includes one or more user equipment (UE), a next-generation RAN (NG-RAN) and a 5th generation core network (5GC). The NG-RAN consists of at least one NG-RAN node. The NG-RAN node consists of at least one gNB and/or at least one ng-eNB. The gNB provides NR user plane and control plane protocol terminations towards the UE. The ng-eNB provides E-UTRA user plane and control plane protocol terminations towards the UE. 
     The 5GC includes an access and mobility management function (AMF), a user plane function (UPF) and a session management function (SMF). The AMF hosts various functions, such as, for example, non-access stratum (NAS) security, idle state mobility handling, evolved packet system (EPS) bearer control, etc. The UPF hosts various functions, such as, for example, mobility anchoring, protocol data unit (PDU) handling, etc. The SMF hosts various functions, such as, for example, UE IP address allocation, PDU session control, etc. 
     The gNBs and ng-eNBs are interconnected with each other by an interface, such as the Xn interface. The gNBs and ng-eNBs are also connected by NG interfaces to the 5GC, for example, to the AMF by the NG-C interface and to the UPF by the NG-U interface. 
     An example of a protocol structure between network entities described above is described. In the example of  FIG. 1 , layers of a radio interface protocol between the UE and the network (e.g. NG-RAN) may be classified into a first layer (L1), a second layer (L2), and a third layer (L3), for example based on the lower three layers of the open system interconnection (OSI) model. 
       FIG. 2  shows a block diagram of an example of a user plane protocol stack t to which implementations of the present disclosure can be applied.  FIG. 3  shows a block diagram of an example of a control plane protocol stack to which implementations of the present disclosure can be applied. 
     Referring to the examples of  FIG. 2  and  FIG. 3 , a physical (PHY) layer belongs to L1. The PHY layer offers information transfer services to the media access control (MAC) sublayer and higher layers. For example, the PHY layer offers transport channels to the MAC sublayer, and data between the MAC sublayer and the PHY layer is transferred via the transport channels. Between different PHY layers, e.g., between a PHY layer of a transmission side and a PHY layer of a reception side, data is transferred via physical channels. 
     The MAC sublayer belongs to L2. The services and functions of the MAC sublayer include, for example, mapping between logical channels and transport channels, multiplexing/de-multiplexing of MAC service data units (SDUs) belonging to one or different logical channels into/from transport blocks (TB) delivered to/from the physical layer on transport channels, scheduling information reporting, error correction through hybrid automatic repeat request (HARD), priority handling between UEs by dynamic scheduling, priority handling between logical channels of one UE by logical channel prioritization (LCP), etc. The MAC sublayer offers to the radio link control (RLC) sublayer logical channels. 
     The RLC sublayer belong to L2. In some implementations, the RLC sublayer supports different transmission modes, e.g., transparent mode(TM), unacknowledged mode (UM), and acknowledged mode (AM). The different transmission modes may help guarantee various quality of services (QoS) required by radio bearers. The services and functions of the RLC sublayer may depend on the transmission mode. For example, in some implementations, the RLC sublayer provides transfer of upper layer PDUs for all three modes, but provides error correction through ARQ for AM only. In some implementations, such as implementations compatible with LTE/LTE-A, the RLC sublayer provides concatenation, segmentation and reassembly of RLC SDUs (only for UM and AM data transfer) and re-segmentation of RLC data PDUs (only for AM data transfer). In NR, the RLC sublayer provides segmentation (only for AM and UM) and re-segmentation (only for AM) of RLC SDUs and reassembly of SDU (only for AM and UM). In some implementations, the NR does not support concatenation of RLC SDUs. The RLC sublayer offers RLC channels to the packet data convergence protocol (PDCP) sublayer. 
     The PDCP sublayer belongs to L2. The services and functions of the PDCP sublayer for the user plane include, for example, header compression and decompression, transfer of user data, duplicate detection, PDCP PDU routing, retransmission of PDCP SDUs, ciphering and deciphering, etc. The services and functions of the PDCP sublayer for the control plane include, for example, ciphering and integrity protection, transfer of control plane data, etc. 
     The service data adaptation protocol (SDAP) sublayer belongs to L2. In some implementations, the SDAP sublayer is only defined in the user plane. The services and functions of SDAP include, for example, mapping between a QoS flow and a data radio bearer (DRB), and marking QoS flow ID (QFI) in both DL and UL packets. The SDAP sublayer offers QoS flows to 5GC. 
     A radio resource control (RRC) layer belongs to L3. In some implementations, the RRC layer is only defined in the control plane. The RRC layer controls radio resources between the UE and the network. For example, the RRC layer exchanges RRC messages between the UE and the BS. The services and functions of the RRC layer include, for example, broadcast of system information related to access stratum AS and NAS, paging, establishment, maintenance and release of an RRC connection between the UE and the network, security functions including key management, establishment, configuration, maintenance and release of radio bearers, mobility functions, QoS management functions, UE measurement reporting and control of the reporting, NAS message transfer to/from NAS from/to UE. 
     As such, in some implementations, the RRC layer controls logical channels, transport channels, and physical channels in relation to the configuration, reconfiguration, and release of radio bearers. A radio bearer refers to a logical path provided by L1 (PHY layer) and L2 (MAC/RLC/PDCP/SDAP sublayer) for data transmission between a UE and a network. In some scenarios, setting the radio bearer may include defining the characteristics of the radio protocol layer and the channel for providing a specific service, and setting each specific parameter and operation method. Radio bearers may include signaling RB (SRB) and data RB (DRB). The SRB is used as a path for transmitting RRC messages in the control plane, and the DRB is used as a path for transmitting user data in the user plane. 
     An RRC state indicates whether an RRC layer of the UE is logically connected to an RRC layer of the network. In some implementations, when the RRC connection is established between the RRC layer of the UE and the RRC layer of the network, the UE is in the RRC_connected state (RRC_CONNECTED); and otherwise, the UE is in the RRC idle state (RRC_IDLE). In implementations compatible with NR, the RRC inactive state (RRC_INACTIVE) is additionally introduced. The RRC_INACTIVE state may be used for various purposes. For example, in some scenarios, massive machine-type communications (mMTC) UEs can be efficiently managed in RRC_INACTIVE. When specific conditions are satisfied, transitions can be made from one of the above three states to others. 
     Various operations may be performed according to the RRC state. For example, in RRC_IDLE, operations such as public land mobile network (PLMN) selection, broadcast of system information (SI), cell re-selection mobility, core network (CN) paging and discontinuous reception (DRX) configured by NAS may be performed. The UE may be allocated an identifier (ID) which uniquely identifies the UE in a tracking area. In some implementations, no RRC context is stored in the base station. 
     As another example, in RRC_CONNECTED, the UE has an RRC connection with the network. Network-CN connection (both C/U-planes) is also established for UE. In some implementations, the UE AS context is stored in the network and the UE. The RAN knows the cell which the UE belongs to, and the network can transmit and/or receive data to/from UE. In some implementations, network controlled mobility including measurement is also performed. 
     One or more operations that are performed in RRC_IDLE may also be performed in RRC_INACTIVE. However, in some implementations, instead of performing CN paging as in RRC_IDLE, RAN paging may be performed in RRC_INACTIVE. For example, in RRC_IDLE, paging for mobile terminated (MT) data is initiated by a core network and paging area is managed by the core network. In RRC_INACTIVE, paging may be initiated by NG-RAN, and RAN-based notification area (RNA) is managed by NG-RAN. Further, in some implementations, instead of DRX for CN paging configured by NAS in RRC_IDLE, DRX for RAN paging is configured by NG-RAN in RRC_INACTIVE. In some implementations, in RRC_INACTIVE, 5GC-NG-RAN connection (both C/U-planes) is established for UE, and the UE AS context is stored in NG-RAN and the UE. The NG-RAN may know the RNA which the UE belongs to. 
     The NAS layer is implemented above the RRC layer, as shown in the example of  FIG. 3 . The NAS control protocol performs various functions, such as, for example, authentication, mobility management, security control, etc. 
     Physical channels, for example as utilized by the PHY layer, may be modulated according to various modulation techniques utilizing time and frequency as radio resources. For example, the physical channels may consist of a plurality of orthogonal frequency division multiplexing (OFDM) symbols in time domain and a plurality of subcarriers in frequency domain. A subframe may be implemented, which consists of a plurality of OFDM symbols in the time domain. A resource block may be implemented as a resource allocation unit, and each resource block may consist of a plurality of OFDM symbols and a plurality of subcarriers. In addition, each subframe may use specific subcarriers of specific OFDM symbols (e.g., the first OFDM symbol) of the corresponding subframe for a specific purpose, such as for a physical downlink control channel (PDCCH), e.g., an L1/L2 control channel. A transmission time interval (TTI) may be implemented as a basic unit of time, for example as used by a scheduler for resource allocation. The TTI may be defined in units of one or a plurality of slots, or may be defined in units of mini-slots. 
     Transport channels may be classified according to how and with what characteristics data are transferred over the radio interface. For example, DL transport channels include a broadcast channel (BCH) used for transmitting system information, a downlink shared channel (DL-SCH) used for transmitting user traffic or control signals, and a paging channel (PCH) used for paging a UE. As another example, UL transport channels include an uplink shared channel (UL-SCH) for transmitting user traffic or control signals and a random access channel (RACH) normally used for initial access to a cell. 
     Different kinds of data transfer services may be offered by the MAC sublayer. Different logical channel types may be defined by what type of information is transferred. In some implementations, logical channels may be classified into two groups: control channels and traffic channels. 
     Control channels are used for the transfer of control plane information only, according to some implementations. The control channels may include, for example, a broadcast control channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH) and a dedicated control channel (DCCH). The BCCH is a DL channel for broadcasting system control information. The PCCH is DL channel that transfers paging information, system information change notifications. The CCCH is a channel for transmitting control information between UEs and network. In some implementations, the CCCH is used for UEs having no RRC connection with the network. The DCCH is a point-to-point bi-directional channel that transmits dedicated control information between a UE and the network. In some implementations, the DCCH is used by UEs having an RRC connection. 
     Traffic channels are used for the transfer of user plane information only, according to some implementations. The traffic channels include, for example, a dedicated traffic channel (DTCH). The DTCH is a point-to-point channel, dedicated to one UE, for the transfer of user information. In some implementations, the DTCH can exist in both UL and DL. 
     In some scenarios, mappings may be implemented between the logical channels and transport channels. For example, in DL, BCCH can be mapped to BCH, BCCH can be mapped to DL-SCH, PCCH can be mapped to PCH, CCCH can be mapped to DL-SCH, DCCH can be mapped to DL-SCH, and DTCH can be mapped to DL-SCH. As another example, in UL, CCCH can be mapped to UL-SCH, DCCH can be mapped to UL-SCH, and DTCH can be mapped to UL-SCH. 
       FIG. 4  shows a frame structure in a 3GPP based wireless communication system to which implementations of the present disclosure can be applied. 
     The frame structure shown in  FIG. 4  is purely exemplary and the number of subframes, the number of slots, and/or the number of symbols in a frame may be variously changed. In the 3GPP based wireless communication system, OFDM numerologies (e.g., subcarrier spacing (SCS), TTI duration) may be differently configured between a plurality of cells aggregated for one UE. For example, if a UE is configured with different SCSs for cells aggregated for the cell, an (absolute time) duration of a time resource (e.g., a subframe, a slot, or a TTI) including the same number of symbols may be different among the aggregated cells. Herein, symbols may include OFDM symbols (or CP-OFDM symbols), SC-FDMA symbols (or discrete Fourier transform-spread-OFDM (DFT-s-OFDM) symbols). 
     Referring to  FIG. 4 , downlink and uplink transmissions are organized into frames. Each frame has T f =10MS duration. Each frame is divided into two half-frames, where each of the half-frames has 5 ms duration. Each half-frame consists of 5 subframes, where the duration T sf  per subframe is 1 ms. Each subframe is divided into slots and the number of slots in a subframe depends on a subcarrier spacing. Each slot includes 14 or 12 OFDM symbols based on a cyclic prefix (CP). In a normal CP, each slot includes 14 OFDM symbols and, in an extended CP, each slot includes 12 OFDM symbols. The numerology is based on exponentially scalable subcarrier spacing Δ f =2 u *15 kHz. 
     Table 1 shows the number of OFDM symbols per slot N slot   symbol , the number of slots per frame N frame,u   slot , and the number of slots per subframe N subframe,u   slot  for the normal CP, according to the subcarrier spacing Δf=2 u *15 kHz. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 u 
                 N slot   symb   
                 N frame, u   slot   
                 N subframe, u   slot   
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 0 
                 14 
                 10 
                 1 
               
               
                   
                 1 
                 14 
                 20 
                 2 
               
               
                   
                 2 
                 14 
                 40 
                 4 
               
               
                   
                 3 
                 14 
                 80 
                 8 
               
               
                   
                 4 
                 14 
                 160 
                 16 
               
               
                   
                   
               
            
           
         
       
     
     Table 2 shows the number of OFDM symbols per slot N slot   symb , the number of slots per frame N frame,u   slot , and the number of slots per subframe N subframe,u   slot  for the extended CP, according to the subcarrier spacing Δf2 u* 15 kHz. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 u 
                 N slot   symb   
                 N frame, u   slot   
                 N subframe, u   slot   
               
               
                   
                   
               
             
            
               
                   
                 2 
                 12 
                 40 
                 4 
               
               
                   
                   
               
            
           
         
       
     
     A slot includes plural symbols (e.g., 14 or 12 symbols) in the time domain. For each numerology (e.g., subcarrier spacing) and carrier, a resource grid of N size,u   grid,x *N RB   sc  subcarriers and N subframe,u   symb  OFDMsymbols is defined, starting at common resource block (CRB)N start,u   grid  indicated by higher-layer signaling (e.g., RRC signaling), where N size,u   grid,x  is the number of resource blocks (RBs) in the resource grid and the subscript x is DL for downlink and UL for uplink. N RB   sc  is the number of subcarriers per RB. In the 3GPP based wireless communication system, N RB   sc  is 12 generally. There is one resource grid for a given antenna port p, subcarrier spacing configuration u, and transmission direction (DL or UL). The carrier bandwidth N size,u   grid  for subcarrier spacing configuration u is given by the higher-layer parameter (e.g., RRC parameter). Each element in the resource grid for the antenna port p and the subcarrier spacing configuration u is referred to as a resource element (RE) and one complex symbol may be mapped to each RE. Each RE in the resource grid is uniquely identified by an index k in the frequency domain and an index l representing a symbol location relative to a reference point in the time domain. In the 3GPP based wireless communication system, an RB is defined by 12 consecutive subcarriers in the frequency domain. 
     In the 3GPP NR system, RBs are classified into CRBs and physical resource blocks (PRBs). CRBs are numbered from 0 and upwards in the frequency domain for subcarrier spacing configuration u. The center of subcarrier 0 of CRB 0 for subcarrier spacing configuration u coincides with ‘point A’ which serves as a common reference point for resource block grids. In the 3GPP NR system, PRBs are defined within a bandwidth part (BWP) and numbered from 0 to N size   BWP,i −1, where i is the number of the bandwidth part. The relation between the physical resource block nPRB in the bandwidth part i and the common resource block n CRB  is as follows: n PRB =n CRB +N size   BWP,i , where N size   BWP,i  is the common resource block where bandwidth part starts relative to CRB 0. The BWP includes a plurality of consecutive RBs. A carrier may include a maximum of N (e.g., 5) BWPs. A UE may be configured with one or more BWPs on a given component carrier. Only one BWP among BWPs configured to the UE can active at a time. The active BWP defines the UE&#39;s operating bandwidth within the cell&#39;s operating bandwidth. 
     The NR frequency band may be defined as two types of frequency range, i.e., FR1 and FR2. The numerical value of the frequency range may be changed. For example, the frequency ranges of the two types (FR1 and FR2) may be as shown in Table 3 below. For ease of explanation, in the frequency ranges used in the NR system, FR1 may mean “sub 6 GHz range”, FR2 may mean “above 6 GHz range,” and may be referred to as millimeter wave (mmW). 
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Frequency Range 
                 Corresponding 
                   
               
               
                 designation 
                 frequency range 
                 Subcarrier Spacing 
               
               
                   
               
             
            
               
                 FR1 
                  450 MHz-6000 MHz 
                  15, 30, 60 kHz 
               
               
                 FR2 
                 24250 MHz-52600 MHz 
                 60, 120, 240 kHz 
               
               
                   
               
            
           
         
       
     
     As mentioned above, the numerical value of the frequency range of the NR system may be changed. For example, FR1 may include a frequency band of 410 MHz to 7125 MHz as shown in Table 4 below. That is, FR1 may include a frequency band of 6 GHz (or 5850, 5900, 5925 MHz, etc.) or more. For example, a frequency band of 6 GHz (or 5850, 5900, 5925 MHz, etc.) or more included in FR1 may include an unlicensed band. Unlicensed bands may be used for a variety of purposes, for example for communication for vehicles (e.g., autonomous driving). 
     
       
         
           
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Frequency Range 
                 Corresponding 
                   
               
               
                 designation 
                 frequency range 
                 Subcarrier Spacing 
               
               
                   
               
             
            
               
                 FR1 
                  410 MHz-7125 MHz 
                  15, 30, 60 kHz 
               
               
                 FR2 
                 24250 MHz-52600 MHz 
                 60, 120, 240 kHz 
               
               
                   
               
            
           
         
       
     
     In the present disclosure, the term “cell” may refer to a geographic area to which one or more nodes provide a communication system, or refer to radio resources. A “cell” as a geographic area may be understood as coverage within which a node can provide service using a carrier and a “cell” as radio resources (e.g., time-frequency resources) is associated with bandwidth which is a frequency range configured by the carrier. The “cell” associated with the radio resources is defined by a combination of downlink resources and uplink resources, for example, a combination of a DL component carrier (CC) and a UL CC. The cell may be configured by downlink resources only, or may be configured by downlink resources and uplink resources. Since DL coverage, which is a range within which the node is capable of transmitting a valid signal, and UL coverage, which is a range within which the node is capable of receiving the valid signal from the UE, depends upon a carrier carrying the signal, the coverage of the node may be associated with coverage of the “cell” of radio resources used by the node. Accordingly, the term “cell” may be used to represent service coverage of the node sometimes, radio resources at other times, or a range that signals using the radio resources can reach with valid strength at other times. 
     In CA, two or more CCs are aggregated. A UE may simultaneously receive or transmit on one or multiple CCs depending on its capabilities. CA is supported for both contiguous and non-contiguous CCs. When CA is configured, the UE only has one RRC connection with the network. At RRC connection establishment/re-establishment/handover, one serving cell provides the NAS mobility information, and at RRC connection re-establishment/handover, one serving cell provides the security input. This cell is referred to as the primary cell (PCell). The PCell is a cell, operating on the primary frequency, in which the UE either performs the initial connection establishment procedure or initiates the connection re-establishment procedure. Depending on UE capabilities, secondary cells (SCells) can be configured to form together with the PCell a set of serving cells. An SCell is a cell providing additional radio resources on top of special cell (SpCell). The configured set of serving cells for a UE therefore always consists of one PCell and one or more SCells. For dual connectivity (DC) operation, the term SpCell refers to the PCell of the master cell group (MCG) or the primary SCell (PSCell) of the secondary cell group (SCG). An SpCell supports PUCCH transmission and contention-based random access, and is always activated. The MCG is a group of serving cells associated with a master node, comprised of the SpCell (PCell) and optionally one or more SCells. The SCG is the subset of serving cells associated with a secondary node, comprised of the PSCell and zero or more SCells, for a UE configured with DC. For a UE in RRC_CONNECTED not configured with CA/DC, there is only one serving cell comprised of the PCell. For a UE in RRC_CONNECTED configured with CA/DC, the term “serving cells” is used to denote the set of cells comprised of the SpCell(s) and all SCells. In DC, two MAC entities are configured in a UE: one for the MCG and one for the SCG. 
       FIG. 5  shows a data flow example in the 3GPP NR system to which implementations of the present disclosure can be applied. 
     Referring to  FIG. 5 , “RB” denotes a radio bearer, and “H” denotes a header. Radio bearers are categorized into two groups: DRBs for user plane data and SRBs for control plane data. The MAC PDU is transmitted/received using radio resources through the PHY layer to/from an external device. The MAC PDU arrives to the PHY layer in the form of a transport block. 
     In the PHY layer, the uplink transport channels UL-SCH and RACH are mapped to their physical channels physical uplink shared channel (PUSCH) and physical random access channel (PRACH), respectively, and the downlink transport channels DL-SCH, BCH and PCH are mapped to physical downlink shared channel (PDSCH), physical broadcast channel (PBCH) and PDSCH, respectively. In the PHY layer, uplink control information (UCI) is mapped to physical uplink control channel (PUCCH), and downlink control information (DCI) is mapped to physical downlink control channel (PDCCH). A MAC PDU related to UL-SCH is transmitted by a UE via a PUSCH based on an UL grant, and a MAC PDU related to DL-SCH is transmitted by a BS via a PDSCH based on a DL assignment. 
       FIG. 6  shows an example of a communication system to which implementations of the present disclosure can be applied. 
     The 5G usage scenarios shown in  FIG. 6  are only exemplary, and the technical features of the present disclosure can be applied to other 5G usage scenarios which are not shown in  FIG. 6 . 
     Three main requirement categories for 5G include (1) a category of enhanced mobile broadband (eMBB), (2) a category of massive machine type communication (mMTC), and (3) a category of ultra-reliable and low latency communications (URLLC). 
     Partial use cases may require a plurality of categories for optimization and other use cases may focus only upon one key performance indicator (KPI). 5G supports such various use cases using a flexible and reliable method. 
     eMBB far surpasses basic mobile Internet access and covers abundant bidirectional work and media and entertainment applications in cloud and augmented reality. Data is one of 5G core motive forces and, in a 5G era, a dedicated voice service may not be provided for the first time. In 5G, it is expected that voice will be simply processed as an application program using data connection provided by a communication system. Main causes for increased traffic volume are due to an increase in the size of content and an increase in the number of applications requiring high data transmission rate. A streaming service (of audio and video), conversational video, and mobile Internet access will be more widely used as more devices are connected to the Internet. These many application programs require connectivity of an always turned-on state in order to push real-time information and alarm for users. Cloud storage and applications are rapidly increasing in a mobile communication platform and may be applied to both work and entertainment. The cloud storage is a special use case which accelerates growth of uplink data transmission rate. 5G is also used for remote work of cloud. When a tactile interface is used, 5G demands much lower end-to-end latency to maintain user good experience. 
     Entertainment, for example, cloud gaming and video streaming, is another core element which increases demand for mobile broadband capability. Entertainment is essential for a smartphone and a tablet in any place including high mobility environments such as a train, a vehicle, and an airplane. Other use cases are augmented reality for entertainment and information search. In this case, the augmented reality requires very low latency and instantaneous data volume. 
     In addition, one of the most expected 5G use cases relates a function capable of smoothly connecting embedded sensors in all fields, i.e., mMTC. It is expected that the number of potential Internet-of-things (IoT) devices will reach 204 hundred million up to the year of 2020. An industrial IoT is one of categories of performing a main role enabling a smart city, asset tracking, smart utility, agriculture, and security infrastructure through 5G. 
     URLLC includes a new service that will change industry through remote control of main infrastructure and an ultra-reliable/available low-latency link such as a self-driving vehicle. A level of reliability and latency is essential to control a smart grid, automatize industry, achieve robotics, and control and adjust a drone. 
     5G is a means of providing streaming evaluated as a few hundred megabits per second to gigabits per second and may complement fiber-to-the-home (FTTH) and cable-based broadband (or DOCSIS). Such fast speed is needed to deliver TV in resolution of 4K or more ( 6 K,  8 K, and more), as well as virtual reality and augmented reality. Virtual reality (VR) and augmented reality (AR) applications include almost immersive sports games. 
     A specific application program may require a special network configuration. For example, for VR games, gaming companies need to incorporate a core server into an edge network server of a network operator in order to minimize latency. 
     Automotive is expected to be a new important motivated force in 5G together with many use cases for mobile communication for vehicles. For example, entertainment for passengers requires high simultaneous capacity and mobile broadband with high mobility. This is because future users continue to expect connection of high quality regardless of their locations and speeds. Another use case of an automotive field is an AR dashboard. The AR dashboard causes a driver to identify an object in the dark in addition to an object seen from a front window and displays a distance from the object and a movement of the object by overlapping information talking to the driver. In the future, a wireless module enables communication between vehicles, information exchange between a vehicle and supporting infrastructure, and information exchange between a vehicle and other connected devices (e.g., devices accompanied by a pedestrian). A safety system guides alternative courses of a behavior so that a driver may drive more safely drive, thereby lowering the danger of an accident. The next stage will be a remotely controlled or self-driven vehicle. This requires very high reliability and very fast communication between different self-driven vehicles and between a vehicle and infrastructure. In the future, a self-driven vehicle will perform all driving activities and a driver will focus only upon abnormal traffic that the vehicle cannot identify. Technical requirements of a self-driven vehicle demand ultra-low latency and ultra-high reliability so that traffic safety is increased to a level that cannot be achieved by human being. 
     A smart city and a smart home/building mentioned as a smart society will be embedded in a high-density wireless sensor network. A distributed network of an intelligent sensor will identify conditions for costs and energy-efficient maintenance of a city or a home. 
     Similar configurations may be performed for respective households. All of temperature sensors, window and heating controllers, burglar alarms, and home appliances are wirelessly connected. Many of these sensors are typically low in data transmission rate, power, and cost. However, real-time HD video may be demanded by a specific type of device to perform monitoring. 
     Consumption and distribution of energy including heat or gas is distributed at a higher level so that automated control of the distribution sensor network is demanded. The smart grid collects information and connects the sensors to each other using digital information and communication technology so as to act according to the collected information. Since this information may include behaviors of a supply company and a consumer, the smart grid may improve distribution of fuels such as electricity by a method having efficiency, reliability, economic feasibility, production sustainability, and automation. The smart grid may also be regarded as another sensor network having low latency. 
     Mission critical application (e.g., e-health) is one of 5G use scenarios. A health part contains many application programs capable of enjoying benefit of mobile communication. A communication system may support remote treatment that provides clinical treatment in a faraway place. Remote treatment may aid in reducing a barrier against distance and improve access to medical services that cannot be continuously available in a faraway rural area. Remote treatment is also used to perform important treatment and save lives in an emergency situation. The wireless sensor network based on mobile communication may provide remote monitoring and sensors for parameters such as heart rate and blood pressure. 
     Wireless and mobile communication gradually becomes important in the field of an industrial application. Wiring is high in installation and maintenance cost. Therefore, a possibility of replacing a cable with reconstructible wireless links is an attractive opportunity in many industrial fields. However, in order to achieve this replacement, it is necessary for wireless connection to be established with latency, reliability, and capacity similar to those of the cable and management of wireless connection needs to be simplified. Low latency and a very low error probability are new requirements when connection to 5G is needed. 
     Logistics and freight tracking are important use cases for mobile communication that enables inventory and package tracking anywhere using a location-based information system. The use cases of logistics and freight typically demand low data rate but require location information with a wide range and reliability. 
     Referring to  FIG. 6 , the communication system  1  includes wireless devices  100   a  to  100   f , base stations (BSs)  200 , and a network  300 . Although  FIG. 6  illustrates a 5G network as an example of the network of the communication system  1 , the implementations of the present disclosure are not limited to the 5G system, and can be applied to the future communication system beyond the 5G system. 
     The BSs  200  and the network  300  may be implemented as wireless devices and a specific wireless device may operate as a BS/network node with respect to other wireless devices. 
     The wireless devices  100   a  to  100   f  represent devices performing communication using radio access technology (RAT) (e.g., 5G new RAT (NR)) or LTE) and may be referred to as communication/radio/5G devices. The wireless devices  100   a  to  100   f  may include, without being limited to, a robot  100   a , vehicles  100   b - 1  and  100   b - 2 , an extended reality (XR) device  100   c , a hand-held device  100   d , a home appliance  100   e , an IoT device  100   f , and an artificial intelligence (AI) device/server  400 . For example, the vehicles may include a vehicle having a wireless communication function, an autonomous driving vehicle, and a vehicle capable of performing communication between vehicles. The vehicles may include an unmanned aerial vehicle (UAV) (e.g., a drone). The XR device may include an AR/VR/Mixed Reality (MR) device and may be implemented in the form of a head-mounted device (HMD), a head-up display (HUD) mounted in a vehicle, a television, a smartphone, a computer, a wearable device, a home appliance device, a digital signage, a vehicle, a robot, etc. The hand-held device may include a smartphone, a smartpad, a wearable device (e.g., a smartwatch or a smartglasses), and a computer (e.g., a notebook). The home appliance may include a TV, a refrigerator, and a washing machine. The IoT device may include a sensor and a smartmeter. 
     In the present disclosure, the wireless devices  100   a  to  100   f  may be called user equipments (UEs). A UE may include, for example, a cellular phone, a smartphone, a laptop computer, a digital broadcast terminal, a personal digital assistant (PDA), a portable multimedia player (PMP), a navigation system, a slate personal computer (PC), a tablet PC, an ultrabook, a vehicle, a vehicle having an autonomous traveling function, a connected car, an UAV, an AI module, a robot, an AR device, a VR device, an MR device, a hologram device, a public safety device, an MTC device, an IoT device, a medical device, a FinTech device (or a financial device), a security device, a weather/environment device, a device related to a 5G service, or a device related to a fourth industrial revolution field. 
     The UAV may be, for example, an aircraft aviated by a wireless control signal without a human being onboard. 
     The VR device may include, for example, a device for implementing an object or a background of the virtual world. The AR device may include, for example, a device implemented by connecting an object or a background of the virtual world to an object or a background of the real world. The MR device may include, for example, a device implemented by merging an object or a background of the virtual world into an object or a background of the real world. The hologram device may include, for example, a device for implementing a stereoscopic image of 360 degrees by recording and reproducing stereoscopic information, using an interference phenomenon of light generated when two laser lights called holography meet. 
     The public safety device may include, for example, an image relay device or an image device that is wearable on the body of a user. 
     The MTC device and the IoT device may be, for example, devices that do not require direct human intervention or manipulation. For example, the MTC device and the IoT device may include smartmeters, vending machines, thermometers, smartbulbs, door locks, or various sensors. 
     The medical device may be, for example, a device used for the purpose of diagnosing, treating, relieving, curing, or preventing disease. For example, the medical device may be a device used for the purpose of diagnosing, treating, relieving, or correcting injury or impairment. For example, the medical device may be a device used for the purpose of inspecting, replacing, or modifying a structure or a function. For example, the medical device may be a device used for the purpose of adjusting pregnancy. For example, the medical device may include a device for treatment, a device for operation, a device for (in vitro) diagnosis, a hearing aid, or a device for procedure. 
     The security device may be, for example, a device installed to prevent a danger that may arise and to maintain safety. For example, the security device may be a camera, a closed-circuit TV (CCTV), a recorder, or a black box. 
     The FinTech device may be, for example, a device capable of providing a financial service such as mobile payment. For example, the FinTech device may include a payment device or a point of sales (POS) system. 
     The weather/environment device may include, for example, a device for monitoring or predicting a weather/environment. 
     The wireless devices  100   a  to  100   f  may be connected to the network  300  via the BSs  200 . An AI technology may be applied to the wireless devices  100   a  to  100   f  and the wireless devices  100   a  to  100   f  may be connected to the AI server  400  via the network  300 . The network  300  may be configured using a 3G network, a 4G (e.g., LTE) network, a 5G (e.g., NR) network, and a beyond-5G network. Although the wireless devices  100   a  to  100   f  may communicate with each other through the BSs  200 /network  300 , the wireless devices  100   a  to  100   f  may perform direct communication (e.g., sidelink communication) with each other without passing through the BSs  200 /network  300 . For example, the vehicles  100   b - 1  and  100   b - 2  may perform direct communication (e.g., vehicle-to-vehicle (V2V)/vehicle-to-everything (V2X) communication). The IoT device (e.g., a sensor) may perform direct communication with other IoT devices (e.g., sensors) or other wireless devices  100   a  to  100   f.    
     Wireless communication/connections  150   a ,  150   b  and  150   c  may be established between the wireless devices  100   a  to  100   f  and/or between wireless device  100   a  to  100   f  and BS  200  and/or between BSs  200 . Herein, the wireless communication/connections may be established through various RATs (e.g., 5G NR) such as uplink/downlink communication  150   a , sidelink communication (or device-to-device (D2D) communication)  150   b , inter-base station communication  150   c  (e.g., relay, integrated access and backhaul (IAB)), etc. The wireless devices  100   a  to  100   f  and the BSs  200 /the wireless devices  100   a  to  100   f  may transmit/receive radio signals to/from each other through the wireless communication/connections  150   a ,  150   b  and  150   c . For example, the wireless communication/connections  150   a ,  150   b  and  150   c  may transmit/receive signals through various physical channels. To this end, at least a part of various configuration information configuring processes, various signal processing processes (e.g., channel encoding/decoding, modulation/demodulation, and resource mapping/de-mapping), and resource allocating processes, for transmitting/receiving radio signals, may be performed based on the various proposals of the present disclosure. 
       FIG. 7  shows an example of wireless devices to which implementations of the present disclosure can be applied. 
     Referring to  FIG. 7 , a first wireless device  100  and a second wireless device  200  may transmit/receive radio signals to/from an external device through a variety of RATs (e.g., LTE and NR). In  FIG. 7 , {the first wireless device  100  and the second wireless device  200 } may correspond to at least one of {the wireless device  100   a  to  100   f  and the BS  200 }, {the wireless device  100   a  to  100   f  and the wireless device  100   a  to  100   f } and/or {the BS  200  and the BS  200 } of  FIG. 6 . 
     The first wireless device  100  may include one or more processors  102  and one or more memories  104  and additionally further include one or more transceivers  106  and/or one or more antennas  108 . The processor(s)  102  may control the memory(s)  104  and/or the transceiver(s)  106  and may be configured to implement the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts described in the present disclosure. For example, the processor(s)  102  may process information within the memory(s)  104  to generate first information/signals and then transmit radio signals including the first information/signals through the transceiver(s)  106 . The processor(s)  102  may receive radio signals including second information/signals through the transceiver(s)  106  and then store information obtained by processing the second information/signals in the memory(s)  104 . The memory(s)  104  may be connected to the processor(s)  102  and may store a variety of information related to operations of the processor(s)  102 . For example, the memory(s)  104  may store software code including commands for performing a part or the entirety of processes controlled by the processor(s)  102  or for performing the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts described in the present disclosure. Herein, the processor(s)  102  and the memory(s)  104  may be a part of a communication modem/circuit/chip designed to implement RAT (e.g., LTE or NR). The transceiver(s)  106  may be connected to the processor(s)  102  and transmit and/or receive radio signals through one or more antennas  108 . Each of the transceiver(s)  106  may include a transmitter and/or a receiver. The transceiver(s)  106  may be interchangeably used with radio frequency (RF) unit(s). In the present disclosure, the first wireless device  100  may represent a communication modem/circuit/chip. 
     The second wireless device  200  may include one or more processors  202  and one or more memories  204  and additionally further include one or more transceivers  206  and/or one or more antennas  208 . The processor(s)  202  may control the memory(s)  204  and/or the transceiver(s)  206  and may be configured to implement the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts described in the present disclosure. For example, the processor(s)  202  may process information within the memory(s)  204  to generate third information/signals and then transmit radio signals including the third information/signals through the transceiver(s)  206 . The processor(s)  202  may receive radio signals including fourth information/signals through the transceiver(s)  106  and then store information obtained by processing the fourth information/signals in the memory(s)  204 . The memory(s)  204  may be connected to the processor(s)  202  and may store a variety of information related to operations of the processor(s)  202 . For example, the memory(s)  204  may store software code including commands for performing a part or the entirety of processes controlled by the processor(s)  202  or for performing the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts described in the present disclosure. Herein, the processor(s)  202  and the memory(s)  204  may be a part of a communication modem/circuit/chip designed to implement RAT (e.g., LTE or NR). The transceiver(s)  206  may be connected to the processor(s)  202  and transmit and/or receive radio signals through one or more antennas  208 . Each of the transceiver(s)  206  may include a transmitter and/or a receiver. The transceiver(s)  206  may be interchangeably used with RF unit(s). In the present disclosure, the second wireless device  200  may represent a communication modem/circuit/chip. 
     Hereinafter, hardware elements of the wireless devices  100  and  200  will be described more specifically. One or more protocol layers may be implemented by, without being limited to, one or more processors  102  and  202 . For example, the one or more processors  102  and  202  may implement one or more layers (e.g., functional layers such as physical (PHY) layer, media access control (MAC) layer, radio link control (RLC) layer, packet data convergence protocol (PDCP) layer, radio resource control (RRC) layer, and service data adaptation protocol (SDAP) layer). The one or more processors  102  and  202  may generate one or more protocol data units (PDUs) and/or one or more service data unit (SDUs) according to the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure. The one or more processors  102  and  202  may generate messages, control information, data, or information according to the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure. The one or more processors  102  and  202  may generate signals (e.g., baseband signals) including PDUs, SDUs, messages, control information, data, or information according to the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure and provide the generated signals to the one or more transceivers  106  and  206 . The one or more processors  102  and  202  may receive the signals (e.g., baseband signals) from the one or more transceivers  106  and  206  and acquire the PDUs, SDUs, messages, control information, data, or information according to the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure. 
     The one or more processors  102  and  202  may be referred to as controllers, microcontrollers, microprocessors, or microcomputers. The one or more processors  102  and  202  may be implemented by hardware, firmware, software, or a combination thereof. As an example, one or more application specific integrated circuits (ASICs), one or more digital signal processors (DSPs), one or more digital signal processing devices (DSPDs), one or more programmable logic devices (PLDs), or one or more field programmable gate arrays (FPGAs) may be included in the one or more processors  102  and  202 . descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure may be implemented using firmware or software and the firmware or software may be configured to include the modules, procedures, or functions. Firmware or software configured to perform the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure may be included in the one or more processors  102  and  202  or stored in the one or more memories  104  and  204  so as to be driven by the one or more processors  102  and  202 . The descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure may be implemented using firmware or software in the form of code, commands, and/or a set of commands. 
     The one or more memories  104  and  204  may be connected to the one or more processors  102  and  202  and store various types of data, signals, messages, information, programs, code, instructions, and/or commands. The one or more memories  104  and  204  may be configured by read-only memories (ROMs), random access memories (RAMs), electrically erasable programmable read-only memories (EPROMs), flash memories, hard drives, registers, cash memories, computer-readable storage media, and/or combinations thereof. The one or more memories  104  and  204  may be located at the interior and/or exterior of the one or more processors  102  and  202 . The one or more memories  104  and  204  may be connected to the one or more processors  102  and  202  through various technologies such as wired or wireless connection. 
     The one or more transceivers  106  and  206  may transmit user data, control information, and/or radio signals/channels, mentioned in the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure, to one or more other devices. The one or more transceivers  106  and  206  may receive user data, control information, and/or radio signals/channels, mentioned in the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure, from one or more other devices. For example, the one or more transceivers  106  and  206  may be connected to the one or more processors  102  and  202  and transmit and receive radio signals. For example, the one or more processors  102  and  202  may perform control so that the one or more transceivers  106  and  206  may transmit user data, control information, or radio signals to one or more other devices. The one or more processors  102  and  202  may perform control so that the one or more transceivers  106  and  206  may receive user data, control information, or radio signals from one or more other devices. 
     The one or more transceivers  106  and  206  may be connected to the one or more antennas  108  and  208  and the one or more transceivers  106  and  206  may be configured to transmit and receive user data, control information, and/or radio signals/channels, mentioned in the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure, through the one or more antennas  108  and  208 . In the present disclosure, the one or more antennas may be a plurality of physical antennas or a plurality of logical antennas (e.g., antenna ports). 
     The one or more transceivers  106  and  206  may convert received radio signals/channels, etc., from RF band signals into baseband signals in order to process received user data, control information, radio signals/channels, etc., using the one or more processors  102  and  202 . The one or more transceivers  106  and  206  may convert the user data, control information, radio signals/channels, etc., processed using the one or more processors  102  and  202  from the base band signals into the RF band signals. To this end, the one or more transceivers  106  and  206  may include (analog) oscillators and/or filters. For example, the transceivers  106  and  206  can up-convert OFDM baseband signals to a carrier frequency by their (analog) oscillators and/or filters under the control of the processors  102  and  202  and transmit the up-converted OFDM signals at the carrier frequency. The transceivers  106  and  206  may receive OFDM signals at a carrier frequency and down-convert the OFDM signals into OFDM baseband signals by their (analog) oscillators and/or filters under the control of the transceivers  102  and  202 . 
     In the implementations of the present disclosure, a UE may operate as a transmitting device in uplink (UL) and as a receiving device in downlink (DL). In the implementations of the present disclosure, a BS may operate as a receiving device in UL and as a transmitting device in DL. Hereinafter, for convenience of description, it is mainly assumed that the first wireless device  100  acts as the UE, and the second wireless device  200  acts as the BS. For example, the processor(s)  102  connected to, mounted on or launched in the first wireless device  100  may be configured to perform the UE behavior according to an implementation of the present disclosure or control the transceiver(s)  106  to perform the UE behavior according to an implementation of the present disclosure. The processor(s)  202  connected to, mounted on or launched in the second wireless device  200  may be configured to perform the BS behavior according to an implementation of the present disclosure or control the transceiver(s)  206  to perform the BS behavior according to an implementation of the present disclosure. 
     In the present disclosure, a BS is also referred to as a node B (NB), an eNode B (eNB), or a gNB. 
       FIG. 8  shows an example of a wireless device to which implementations of the present disclosure can be applied. 
     The wireless device may be implemented in various forms according to a use-case/service (refer to  FIG. 6 ). 
     Referring to  FIG. 8 , wireless devices  100  and  200  may correspond to the wireless devices  100  and  200  of  FIG. 7  and may be configured by various elements, components, units/portions, and/or modules. For example, each of the wireless devices  100  and  200  may include a communication unit  110 , a control unit  120 , a memory unit  130 , and additional components  140 . The communication unit  110  may include a communication circuit  112  and transceiver(s)  114 . For example, the communication circuit  112  may include the one or more processors  102  and  202  of  FIG. 7  and/or the one or more memories  104  and  204  of  FIG. 7 . For example, the transceiver(s)  114  may include the one or more transceivers  106  and  206  of  FIG. 7  and/or the one or more antennas  108  and  208  of  FIG. 7 . The control unit  120  is electrically connected to the communication unit  110 , the memory  130 , and the additional components  140  and controls overall operation of each of the wireless devices  100  and  200 . For example, the control unit  120  may control an electric/mechanical operation of each of the wireless devices  100  and  200  based on programs/code/commands/information stored in the memory unit  130 . The control unit  120  may transmit the information stored in the memory unit  130  to the exterior (e.g., other communication devices) via the communication unit  110  through a wireless/wired interface or store, in the memory unit  130 , information received through the wireless/wired interface from the exterior (e.g., other communication devices) via the communication unit  110 . 
     The additional components  140  may be variously configured according to types of the wireless devices  100  and  200 . For example, the additional components  140  may include at least one of a power unit/battery, input/output (I/O) unit (e.g., audio I/O port, video I/O port), a driving unit, and a computing unit. The wireless devices  100  and  200  may be implemented in the form of, without being limited to, the robot ( 100   a  of  FIG. 6 ), the vehicles ( 100   b - 1  and  100   b - 2  of  FIG. 6 ), the XR device ( 100   c  of  FIG. 6 ), the hand-held device ( 100   d  of  FIG. 6 ), the home appliance ( 100   e  of  FIG. 6 ), the IoT device ( 100   f  of  FIG. 6 ), a digital broadcast terminal, a hologram device, a public safety device, an MTC device, a medicine device, a FinTech device (or a finance device), a security device, a climate/environment device, the AI server/device ( 400  of  FIG. 6 ), the BSs ( 200  of  FIG. 6 ), a network node, etc. The wireless devices  100  and  200  may be used in a mobile or fixed place according to a use-example/service. 
     In  FIG. 8 , the entirety of the various elements, components, units/portions, and/or modules in the wireless devices  100  and  200  may be connected to each other through a wired interface or at least a part thereof may be wirelessly connected through the communication unit  110 . For example, in each of the wireless devices  100  and  200 , the control unit  120  and the communication unit  110  may be connected by wire and the control unit  120  and first units (e.g.,  130  and  140 ) may be wirelessly connected through the communication unit  110 . Each element, component, unit/portion, and/or module within the wireless devices  100  and  200  may further include one or more elements. For example, the control unit  120  may be configured by a set of one or more processors. As an example, the control unit  120  may be configured by a set of a communication control processor, an application processor (AP), an electronic control unit (ECU), a graphical processing unit, and a memory control processor. As another example, the memory  130  may be configured by a RAM, a DRAM, a ROM, a flash memory, a volatile memory, a non-volatile memory, and/or a combination thereof. 
       FIG. 9  shows another example of wireless devices to which implementations of the present disclosure can be applied. 
     Referring to  FIG. 9 , wireless devices  100  and  200  may correspond to the wireless devices  100  and  200  of  FIG. 7  and may be configured by various elements, components, units/portions, and/or modules. 
     The first wireless device  100  may include at least one transceiver, such as a transceiver  106 , and at least one processing chip, such as a processing chip  101 . The processing chip  101  may include at least one processor, such a processor  102 , and at least one memory, such as a memory  104 . The memory  104  may be operably connectable to the processor  102 . The memory  104  may store various types of information and/or instructions. The memory  104  may store a software code  105  which implements instructions that, when executed by the processor  102 , perform the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure. For example, the software code  105  may implement instructions that, when executed by the processor  102 , perform the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure. For example, the software code  105  may control the processor  102  to perform one or more protocols. For example, the software code  105  may control the processor  102  may perform one or more layers of the radio interface protocol. 
     The second wireless device  200  may include at least one transceiver, such as a transceiver  206 , and at least one processing chip, such as a processing chip  201 . The processing chip  201  may include at least one processor, such a processor  202 , and at least one memory, such as a memory  204 . The memory  204  may be operably connectable to the processor  202 . The memory  204  may store various types of information and/or instructions. The memory  204  may store a software code  205  which implements instructions that, when executed by the processor  202 , perform the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure. For example, the software code  205  may implement instructions that, when executed by the processor  202 , perform the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure. For example, the software code  205  may control the processor  202  to perform one or more protocols. For example, the software code  205  may control the processor  202  may perform one or more layers of the radio interface protocol. 
       FIG. 10  shows an example of UE to which implementations of the present disclosure can be applied. 
     Referring to  FIG. 10 , a UE  100  may correspond to the first wireless device  100  of  FIG. 7  and/or the first wireless device  100  of  FIG. 9 . 
     A UE  100  includes a processor  102 , a memory  104 , a transceiver  106 , one or more antennas  108 , a power management module  110 , a battery  1112 , a display  114 , a keypad  116 , a subscriber identification module (SIM) card  118 , a speaker  120 , and a microphone  122 . 
     The processor  102  may be configured to implement the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure. The processor  102  may be configured to control one or more other components of the UE  100  to implement the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure. Layers of the radio interface protocol may be implemented in the processor  102 . The processor  102  may include ASIC, other chipset, logic circuit and/or data processing device. The processor  102  may be an application processor. The processor  102  may include at least one of a digital signal processor (DSP), a central processing unit (CPU), a graphics processing unit (GPU), a modem (modulator and demodulator). An example of the processor  102  may be found in SNAPDRAGON™ series of processors made by Qualcomm, EXYNOS series of processors made by Samsung®, A series of processors made by Apple®, HELLO™ series of processors made by MediaTek®, ATOM™ series of processors made by Intel® or a corresponding next generation processor. 
     The memory  104  is operatively coupled with the processor  102  and stores a variety of information to operate the processor  102 . The memory  104  may include ROM, RAM, flash memory, memory card, storage medium and/or other storage device. When the embodiments are implemented in software, the techniques described herein can be implemented with modules (e.g., procedures, functions, etc.) that perform the descriptions, functions, procedures, suggestions, methods and/or operational flowcharts disclosed in the present disclosure. The modules can be stored in the memory  104  and executed by the processor  102 . The memory  104  can be implemented within the processor  102  or external to the processor  102  in which case those can be communicatively coupled to the processor  102  via various means as is known in the art. 
     The transceiver  106  is operatively coupled with the processor  102 , and transmits and/or receives a radio signal. The transceiver  106  includes a transmitter and a receiver. The transceiver  106  may include baseband circuitry to process radio frequency signals. The transceiver  106  controls the one or more antennas  108  to transmit and/or receive a radio signal. 
     The power management module  110  manages power for the processor  102  and/or the transceiver  106 . The battery  112  supplies power to the power management module  110 . 
     The display  114  outputs results processed by the processor  102 . The keypad  116  receives inputs to be used by the processor  102 . The keypad  16  may be shown on the display  114 . 
     The SIM card  118  is an integrated circuit that is intended to securely store the international mobile subscriber identity (IMSI) number and its related key, which are used to identify and authenticate subscribers on mobile telephony devices (such as mobile phones and computers). It is also possible to store contact information on many SIM cards. 
     The speaker  120  outputs sound-related results processed by the processor  102 . The microphone  122  receives sound-related inputs to be used by the processor  102 . 
     V2X communication in 5G NR is described. Sections 5.2 and 5.6 of 3GPP TS 23.287 V0.3.0 can be referred. 
     For V2X communication, two types of PC5 reference points exist: the LTE based PC5 reference point, and the NR based PC5 reference point. A UE may use either type of PC5 or both for V2X communication depending on the services the UE supports. The V2X communication over PC5 reference point supports roaming and inter-public land mobile network (PLMN) operations. V2X communication over PC5 reference point is supported when UE is “served by NR or E-UTRA” or when the UE is “not served by NR or E-UTRA”. 
     A UE is authorized to transmit and receive V2X messages when it has valid authorization and configuration. 
     The V2X communication over PC5 reference point has the following characteristics:
         V2X communication over LTE based PC5 reference point is connectionless, i.e., broadcast mode at access stratum (AS) layer, and there is no signaling over PC5 for connection establishment.   V2X communication over NR based PC5 reference point supports broadcast mode, groupcast mode, and unicast mode at AS layer. The UE will indicate the mode of communication for a V2X message to the AS layer. Signaling over control plane over PC5 reference point for unicast mode communication management is supported.   V2X services communication support between UEs over PC5 user plane.   V2X messages are exchanged between UEs over PC5 user plane. Both internet protocol (IP) based and non-IP based V2X messages are supported over PC5 reference point. For IP based V2X messages, only IP version 6 (IPv6) is used. IP version 4 (IPv4) is not supported.       

     The identifiers used in the V2X communication over PC5 reference point are described below in detail. UE decides on the type of PC5 reference point and Tx Profile to use for the transmission of a particular packet based on the configuration. 
     If the UE has an active emergency PDU session, the communication over the emergency PDU session shall be prioritized over V2X communication over PC5 reference point. 
     Broadcast mode of communication is supported over both LTE based PC5 reference point and NR based PC5 reference point. Therefore, when broadcast mode is selected for transmission over PC5 reference point, PC5 RAT selection needs to be performed based on configuration. 
     For LTE based PC5 reference point, broadcast mode is the only supported communication mode. 
     For NR based PC5 reference point, the broadcast mode also supports enhanced QoS handling. 
     Groupcast mode of communication is only supported over NR based PC5 reference point. 
     Unicast mode of communication is only supported over NR based PC5 reference point. When application layer initiates a V2X service which requires PC5 unicast communication, the UE establishes a PC5 unicast link with the corresponding UE. 
     After successful PC5 unicast link establishment, UE A and UE B use a same pair of Layer-2 IDs for subsequent PC5-S signaling message exchange and V2X service data transmission. V2X layer of the transmitting UE indicates to AS layer whether the message is for PC5-S signaling message (i.e., Direct Communication Accept, Link Layer Identifier Update Request/Response, Disconnect Request/Response) or service data transmission when it sends message over the established PC5 link. V2X layer of receiving UE handles message if it is PC5-S signaling message whilst the V2X layer of receiving UE forwards the message to the upper layer if it is application data message. 
     The unicast mode supports per-flow QoS model. During the unicast link establishment, each UEs self-assign PC5 link identifier and associate the PC5 link identifier with the unicast link profile for the established unicast link. The PC5 link identifier is a unique value within the UE. The unicast link profile identified by PC5 link identifier includes application layer identifier and Layer-2 ID of UE A, application layer identifier and Layer-2 ID of UE B and a set of PC5 QoS flow identifier(s) (PFI(s)). Each PFI is associated with QoS parameters (i.e., PC5 QoS indicator (PQI) and optionally range). The PC5 link identifier and PFI(s) are unchanged values for the established unicast link regardless of the change of application layer identifier and Layer-2 ID. The UE uses PFI to indicate the PC5 QoS flow to AS layer, therefore AS layer identifies the corresponding PC5 QoS flow even if the source and/or destination Layer-2 IDs are changed due to, e.g., privacy support. The UE uses PC5 link identifier to indicate the PC5 unicast link to V2X application layer, therefore V2X application layer identifies the corresponding PC5 unicast link even if there are more than one unicast link associated with one service type (e.g., the UE establishes multiple unicast links with multiple UEs for a same service type). 
     Identifiers for V2X communication is described. 
     Each UE has one or more Layer-2 IDs for V2X communication over PC5 reference point, consisting of:
         Source Layer-2 ID(s); and   Destination Layer-2 ID(s).       

     Source and destination Layer-2 IDs are included in layer-2 frames sent on the layer-2 link of the PC5 reference point identifying the layer-2 source and destination of these frames. Source Layer-2 IDs are always self-assigned by the UE originating the corresponding layer-2 frames. 
     The selection of the source and destination Layer-2 ID(s) by a UE depends on the communication mode of V2X communication over PC5 reference point for this layer-2 link, as described below in detail. The source Layer-2 IDs may differ between different communication modes. 
     When IP-based V2X communication is supported, the UE configures a link local IPv6 address to be used as the source IP address. The UE may use this IP address for V2X communication over PC5 reference point without sending Neighbor Solicitation and Neighbor Advertisement message for Duplicate Address Detection. 
     If the UE has an active V2X application that requires privacy support in the current geographical area, as identified by configuration, in order to ensure that a source UE (e.g., vehicle) cannot be tracked or identified by any other UEs (e.g., vehicles) beyond a certain short time-period required by the application, the source Layer-2 ID shall be changed over time and shall be randomized. For IP-based V2X communication over PC5 reference point, the source IP address shall also be changed over time and shall be randomized. The change of the identifiers of a source UE must be synchronized across layers used for PC5, e.g., when the application layer identifier changes, the source Layer-2 ID and the source IP address need to be changed. 
     For broadcast mode of V2X communication over PC5 reference point, the UE is configured with the destination Layer-2 ID(s) to be used for V2X services. The destination Layer-2 ID for a V2X communication is selected based on the configuration. 
     The UE self-selects a source Layer-2 ID. The UE may use different source Layer-2 IDs for different types of PC5 reference points, i.e., LTE based PC5 and NR based PC5. 
     For groupcast mode of V2X communication over PC5 reference point, the V2X application layer may provide group identifier information. When the group identifier information is provided by the V2X application layer, the UE converts the provided group identifier into a destination Layer-2 ID. When the group identifier information is not provided by the V2X application layer, the UE determines the destination Layer-2 ID based on configuration of the mapping between service type (e.g., PSID/ITS-AID) and Layer-2 ID. 
     The UE self-selects a source Layer-2 ID. 
     For unicast mode of V2X communication over PC5 reference point, the destination Layer-2 ID used depends on the communication peer, which is discovered during the establishment of the unicast link. The initial signaling for the establishment of the unicast link may use a default destination Layer-2 ID associated with the service type (e.g., PSID/ITS-AID) configured for unicast link establishment. During the unicast link establishment procedure, Layer-2 IDs are exchanged, and should be used for future communication between the two UEs. 
     The UE needs to maintain a mapping between the application layer identifiers and the source Layer-2 IDs used for the unicast links, as the V2X application layer does not use the Layer-2 IDs. This allows the change of source Layer-2 ID without interrupting the V2X applications. 
     When application layer identifiers changes, the source Layer-2 ID(s) of the unicast link(s) shall be changed if the link(s) was used for V2X communication with the changed application layer identifiers. 
     A UE may establish multiple unicast links with a peer UE and use the same or different source Layer-2 IDs for these unicast links. 
       FIGS. 11 and 12  show an example of PC5 protocol stacks to which implementations of the present disclosure can be applied. 
       FIG. 11  illustrates an example of a PC5 control plane (PC5-C) protocol stack between UEs. The AS protocol stack for the control plane in the PC5 interface consists of at least RRC, PDCP, RLC and MAC sublayers, and the physical layer. 
       FIG. 12  illustrates an example of a PC5 user plane (PC5-U) protocol stack between UEs. The AS protocol stack for user plane in the PC5 interface consists of at least PDCP, RLC and MAC sublayers, and the physical layer. 
     Radio link failure related actions are described. Section 5.3.10 of 3GPP TS 38.331 V15.5.0 can be referred. 
     For detection of physical layer problems in RRC_CONNECTED, the UE shall: 
     1&gt; upon receiving N310 consecutive “out-of-sync” indications for the SpCell from lower layers while neither T300, T301, T304, T311 nor T319 are running: 
     2&gt; start timer T310 for the corresponding SpCell. 
     For recovery of physical layer problems, upon receiving N311 consecutive “in-sync” indications for the SpCell from lower layers while T310 is running, the UE shall: 
     1&gt; stop timer T310 for the corresponding SpCell. 
     The UE maintains the RRC connection without explicit signalling, i.e., the UE maintains the entire radio resource configuration. 
     Periods in time where neither “in-sync” nor “out-of-sync” is reported by L1 do not affect the evaluation of the number of consecutive “in-sync” or “out-of-sync” indications. 
     For detection of radio link failure, the UE shall: 
     1&gt; upon T310 expiry in PCell; or 
     1&gt; upon random access problem indication from MCG MAC while neither T300, T301, T304, T311 nor T319 are running; or 
     1&gt; upon indication from MCG RLC that the maximum number of retransmissions has been reached: 
     2&gt; if the indication is from MCG RLC and CA duplication is configured and activated, and for the corresponding logical channel allowedServingCells only includes SCell(s): 
     3&gt; initiate the failure information procedure to report RLC failure. 
     2&gt; else: 
     3&gt; consider radio link failure to be detected for the MCG, i.e., RLF; 
     3&gt; if AS security has not been activated: 
     4&gt; perform the actions upon going to RRC_IDLE, with release cause ‘other’; 
     3&gt; else if AS security has been activated but SRB 2  and at least one DRB have not been setup: 
     4&gt; perform the actions upon going to RRC_IDLE, with release cause ‘RRC connection failure’; 
     3&gt; else: 
     4&gt; initiate the connection re-establishment procedure. 
     The UE shall: 
     1&gt; upon T310 expiry in PSCell; or 
     1&gt; upon random access problem indication from SCG MAC; or 
     1&gt; upon indication from SCG RLC that the maximum number of retransmissions has been reached: 
     2&gt; if the indication is from SCG RLC and CA duplication is configured and activated; and for the corresponding logical channel allowedServingCells only includes SCell(s): 
     3&gt; initiate the failure information procedure to report RLC failure. 
     2&gt; else: 
     3&gt; consider radio link failure to be detected for the SCG, i.e., SCG RLF; 
     3&gt; initiate the SCG failure information procedure to report SCG radio link failure. 
     As mentioned above, for radio link monitoring (RLM) on Uu interface, a UE measures signals transmitted by the base station. Then, a lower layer (e.g., physical layer) of a UE determines in-sync or out-of-sync and periodically indicates in-sync (IS) or out-of-sync ( 00 S) to an upper layer (e.g., RRC layer) of the UE. Based on the number of out-of-sync indications, the upper layer of the UE determines whether the radio link failure occurs or not. 
     For NR V2X sidelink communication, a UE (e.g., RX UE) may be connected to another UE (e.g., TX UE) via a PC5-RRC connection and receive sidelink data from the TX UE. The RX UE may measure the sidelink control information (SCI) transmitted by the TX UE for the PC5-RRC connection and then determine in-sync and/or out-of-sync based on the received SCI. However, the TX UE may transmit the SCI only when SL data transmission occurs. Therefore, the RX UE may not receive the SCI sometimes. In this case, it is not clear how the RX UE can determine in-sync or out-of-sync based on the SCI. 
     In addition to the purpose of RLM, for the purpose of sidelink management, there may be need to transmit SCI without SL data. 
     For example, SCI without SL data needs to be transmitted within appropriate time period for sidelink management. For example, SCI transmission should be transmitted within a latency bound. 
     The following drawings are created to explain specific embodiments of the present disclosure. The names of the specific devices or the names of the specific signals/messages/fields shown in the drawings are provided by way of example, and thus the technical features of the present disclosure are not limited to the specific names used in the following drawings. 
     In some implementations, the method in perspective of the wireless device described below may be performed by first wireless device  100  shown in  FIG. 7 , the wireless device  100  shown in  FIG. 8 , the first wireless device  100  shown in  FIG. 9  and/or the UE  100  shown in  FIG. 10 . 
     In some implementations, the method in perspective of the wireless device described below may be performed by control of the processor  102  included in the first wireless device  100  shown in  FIG. 7 , by control of the communication unit  110  and/or the control unit  120  included in the wireless device  100  shown in  FIG. 8 , by control of the processor  102  included in the first wireless device  100  shown in  FIG. 9  and/or by control of the processor  102  included in the UE  100  shown in  FIG. 10 . 
       FIG. 13  shows an example of a method for a wireless device to which implementations of the present disclosure can be applied. 
     In step S 1300 , the wireless device determines that SL resource has not been reserved within a time period related to transmission of SCI. In step S 1310 , the wireless device triggers SL resource reservation for transmission of the SCI. For example, wireless device triggers SL resource reservation based on that SL resource has not been reserved within a time period related to transmission of SCI. 
     In some implementations, the time period may be a SCI period. 
     In some implementations, the SL resource may not been reserved within the period after latest transmission of the SCI. 
     In some implementations, the wireless device may transmit the SCI by using the SL resource reserved based on the SL resource reservation. The SL resource may be reserved on a resource pool with a specific priority. 
     In some implementations, the SCI may indicates the specific priority. The specific priority may be a priority configured by a network and/or a pre-configuration stored in the first wireless device. The specific priority may be a highest priority or a lowest priority. The specific priority may indicate transmission of the SCI without transmitting sidelink data. 
     In some implementations, the SCI may indicate no sidelink shared channel (SL-SCH) transmission. 
     In some implementations, the SCI may indicate a specific identifier (ID) which indicate no SL-SCH transmission. The specific ID may include at least one of a source ID, a destination ID and/or an ID associated with a link between the first wireless device and the second wireless device. 
     In some implementations, the wireless device may be in communication with at least one of a mobile device, a network, and/or autonomous vehicles other than the wireless device. 
     In some implementations, the SCI transmission described above may correspond to SL channel state information (SCI) reporting. 
       FIG. 14  shows an example of a method for performing sidelink communication for a UE to which implementations of the present disclosure can be applied. 
     In some implementations, the first UE, e.g., TX UE, may establish a connection with network (e.g., gNB). The first UE may perform initial access towards the cell. The first UE and the cell may perform random access procedure. The first UE may establish and/or resume a connection with the network and enters RRC_CONNECTED. The first UE may perform AS security activation upon receiving security mode command from the network. The first UE may configure radio bearers and radio configuration upon receiving RRC reconfiguration and/or resumes radio bearers and radio configuration upon receiving RRC resume. 
     In step S 1400 , the first UE may be configured with a SCI period for management of the direct link by the network and/or pre-configuration. 
     In step S 1402 , the first UE and the second UE, e.g., RX UE, establish PC5-RRC connection. 
     In step S 1404 , the first UE establishes a direct link with the second UE for sidelink unicast transmission and/or for sidelink groupcast transmission. 
     In some implementations, one or more resource pools may be configured for sidelink transmissions on the direct link. The resource pools may be configured on the same BWP of the same carrier, different BWPs of the same carrier, and/or different carriers. The resource pools may be associated with the direct link, e.g., with a pair of a source ID and destination ID, and/or a link ID. 
     In step S 1406 , the first UE may inform the second UE about configuration of the SCI period. 
     In step S 1408 , the second UE starts PC5 RLM. 
     In step S 1410 , the first UE transmits SCI to the second UE. The SCI may indicate link ID. 
     In some implementations, the SCI may indicate a certain SCI period, e.g., the next SCI period. During the indicated SCI period, if data is not available for STCH associated with one of the resource pools, the first UE may not transmit a SCI. 
     Alternatively, when the first UE transmits SCI, the SCI may indicate time duration which starts from the SCI transmission or transmission of a MAC PDU indicated by the SCI. During the time duration, if data is not available for STCH associated with one of the resource pools, the first UE may not transmit a SCI. 
     In step S 1412 , the first UE transmits a SL-SCH to the second UE. The SL-SCH may be scheduled by the SCI. 
     In some implementations, whenever data and a resource are available for transmission, the first UE may transmit SCI and a MAC PDU on the SL-SCH to the second UE via at least one resource pool. The first UE may transmit multiple SCIs and multiple MAC PDUs to the second UE on the SL-SCH. Each MAC PDU may be indicated and scheduled based on the SCI. 
     In some implementations, if data is available for STCH associated with one of the resource pools, and if a resource has been reserved on one of the resource pools and an interval between the resource and the latest SCI transmission is within the SCI period, the first UE may transmit the SCI and a MAC PDU on the SL-SCH based on the SCI by using the resource. 
     In some implementations, the HARQ entity of the first UE may trigger the transmission of the SCI and a MAC PDU for a HARQ process. 
     In some implementations, different SCIs transmitted to the second UE may indicate different IDs of the same ID type. For example, different SCIs may indicate different source layer-2 IDs, different destination layer-2 IDs, and/or different link IDs. But, different IDs may be associated with the direct link between the first UE and the second UE. 
     In some implementations, the ID may be a particular ID indicating PC5 RLM or no SCI transmission. 
     In some implementations, upon transmission of a PC5-RRC request message to the second UE, upon reception of a PC5-RRC response message from the second UE, upon transmission of the first SCI to the second UE and/or upon the new transmission or the last retransmission of the first MAC PDU for the direct link to the second UE, the first UE may start a timer for a SCI period. Upon expiry of the timer, the first UE may restart a timer for a next SCI period. In  FIG. 14 , the timer starts upon transmission of the first SCI to the second UE. 
     In some implementations, if the second UE receives SCI indicating at least one ID in step S 1410 , and the ID is associated with the direct link, the second UE may consider the SCI transmission for management of the direct link. 
     In some implementations, upon receiving the SCI, the second UE may not transmit HARQ feedback to the first UE. 
     In some implementations, upon reception of a PC5-RRC request message from the first UE, upon transmission of a PC5-RRC response message to the first UE, upon reception of the first SCI from the first UE and/or upon reception of the new transmission or the last retransmission of the first MAC PDU for the direct link from the first UE, the second UE may start a timer for a SCI period. Upon expiry of the timer, the first UE may restart a timer for a next SCI period. 
     In step S 1414 , the second UE determines either in-sync and/or out-of-sync based on each of the SCIs and provides each indication to an upper layer of the second UE for every SCI period, e.g., whenever the timer expires. 
     In some implementations, if the second UE does not receive any SCI from the first UE for a SCI period, e.g., whenever the timer expires, the second UE may determine out-of-sync for the SCI period. Alternatively, in this case the second UE may determine in-sync for the SCI period. 
     In some implementations, if the second UE receives a SCI which indicates a certain SCI period, e.g., the next SCI period, and if any SCI is not received for the indicated SCI period, the second UE may determine in-sync for the indicated SCI period. 
     Alternatively, if the second UE receives a SCI which indicates time duration period, the second UE may start the time duration from the SCI transmission or transmission of a MAC PDU indicated by the SCI. During the time duration, if any SCI is not received for time duration, the second UE may determine in-sync for the time duration. 
     In some implementations, the upper layer (e.g., RRC layer) of the second UE may determine whether link failure occurs or not for the direct link if a certain number of out-of-sync indications occur consecutively. 
     In some implementations, the second UE may indicate the number of out-of-sync indications to the first UE and/or the network. 
     In some implementations, upon link failure detection, the second UE may consider the direct link is released. 
     In step S 1416 , the data is available for link ID in the first UE. 
     In step S 1418 , the first UE transmits SCI to the second UE. The SCI may indicate the link ID. 
     In step S 1420 , the first UE transmits a SL-SCH to the second UE. The SL-SCH may be scheduled by the SCI. 
     In step S 1422 , the second UE determines either in-sync and/or out-of-sync based on each of the SCIs and provides each indication to an upper layer of the second UE for every SCI period, e.g., whenever the timer expires. 
     In step S 1424 , data is not available for link ID in the first UE. 
     In step S 1426 , if data is not available for any STCH associated with one of the resource pools, and if a resource has been not reserved on any of the resource pools within the SCI period after the latest SCI transmission, the first UE triggers SL resource reservation procedure for a SCI transmission in which the UE reserves at least one resource on one of the resource pools associated with the direct link by associating a particular priority to this SCI transmission. 
     In step S 1428 , the first UE transmits SCI to the second UE. The SCI may indicate the link ID and/or no data transmission. 
     In some implementations, the particular priority may be a priority configured by the network and/or pre-configuration. The particular priority may be a certain fixed priority for this type of SCI transmission (i.e., for PC5 RLM or for no SL-SCH transmission). The particular priority may be the highest priority and/or the lowest priority. The SCI may indicate the particular priority to the RX UE. 
     In some implementations, the HARQ entity of the first UE may trigger transmission of the SCI without triggering SL-SCH transmission. 
     In some implementations, the SCI may indicate PC5 RLM, no SL-SCH transmission and/or no HARQ feedback. 
     In some implementations, the SCI may indicate a particular ID which is used to indicate PC5 RLM, no SL-SCH transmission and/or no HARQ feedback. The particular ID may be one of the IDs associated with the direct link, e.g., the source layer-2 ID, the destination layer-2 ID and/or the link ID. 
     In some implementations, after the resource has been reserved for this SCI transmission, if data becomes available for STCH associated with one of the resource pools before this SCI transmission, the UE may reserve a new resource on any of the resource pools within the SCI period after the latest SCI transmission for a certain MAC PDU. If the new resource is reserved for a certain MAC PDU, the UE may cancel the previously reserved resource for this SCI transmission and/or stops this SCI transmission. 
     In some implementations, if data is not available for any STCH associated with one of the resource pools, and if a resource has been reserved on at least one of the resource pools within the SCI period after the latest SCI transmission, the first UE may transmit the SCI by using the resource and may skip SL-SCH transmission indicated by the SCI. 
     In some implementations, the HARQ entity of the first UE may trigger transmission of the SCI without triggering SL-SCH transmission. 
     In some implementations, the SCI may indicate PC5 RLM, no SL-SCH transmission and/or no HARQ feedback. 
     In some implementations, the SCI may indicate a particular ID which is used to indicate PC5 RLM, no SL-SCH transmission and/or no HARQ feedback. The particular ID may be one of the IDs associated with the direct link, e.g., the source layer-2 ID, the destination layer-2 ID, and/or the link ID. 
     Alternatively, if data is not available for any STCH associated with one of the resource pools, and if a resource is reserved on one of the resource pools and an interval between the resource and the latest SCI transmission is within the SCI period, the first UE may transmit the SCI and SL-SCH transmission by using the resource. 
     In some implementations, the HARQ entity of the first UE may create a MAC PDU having no MAC SDU and trigger transmission of the MAC PDU on the SL-SCH based on the SCI. The MAC PDU may include a MAC header indicating one of the IDs, PC5 RLM and/or no SL data without MAC SDU. Or, the MAC PDU may include a MAC header indicating a MAC control element (CE) indicating one of the IDs, PC5 RLM and/or no SL data. 
     In some implementations, the MAC header may include the particular logical channel ID (LCID) value allocated for no SL data or SCI only transmission. 
     In some implementations, the MAC header may include the particular source (SRC) value and the particular destination (DST) value allocated for no SL data or SCI only transmission. 
     In step S 1430 , the second UE determines either in-sync and/or out-of-sync based on each of the SCIs and provides each indication to an upper layer of the second UE for every SCI period, e.g., whenever the timer expires. 
     In the present disclosure, SCI may be replaced by a reference signal and/or any type of a physical channel for PC5 RLM. 
     The present disclosure can have various advantageous effects. 
     For example, a UE can transmit control information (e.g., SCI) for sidelink management within appropriate time period. 
     For example, a UE can reserve a resource and transmit control information (e.g., SCI) for a direct link with other UE, in particular when the UE has no data to be transmitted to the other UE. 
     For example, the system can reliably manage a direct link between two UEs performing sidelink communication. 
     Advantageous effects which can be obtained through specific embodiments of the present disclosure are not limited to the advantageous effects listed above. For example, there may be a variety of technical effects that a person having ordinary skill in the related art can understand and/or derive from the present disclosure. Accordingly, the specific effects of the present disclosure are not limited to those explicitly described herein, but may include various effects that may be understood or derived from the technical features of the present disclosure. 
     Claims in the present disclosure can be combined in a various way. For instance, technical features in method claims of the present disclosure can be combined to be implemented or performed in an apparatus, and technical features in apparatus claims can be combined to be implemented or performed in a method. Further, technical features in method claim(s) and apparatus claim(s) can be combined to be implemented or performed in an apparatus. Further, technical features in method claim(s) and apparatus claim(s) can be combined to be implemented or performed in a method. Other implementations are within the scope of the following claims.