Patent Publication Number: US-2023156796-A1

Title: Methods, devices and systems for wireless communication using multi-link

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 U.S.C. § 119 to Korean Patent Application Nos. 10-2021-0157094, filed on Nov. 15, 2021, and 10-2022-0054439, filed on May 2, 2022 in the Korean Intellectual Property Office, the disclosures of which are incorporated by reference herein in their entirety. 
     BACKGROUND 
     The inventive concepts relate to wireless communication, and more particularly, to a device and method for wireless communication using a multi-link. 
     As an example of wireless communication, a wireless local area network (WLAN) is technology that connects two or more devices by using a wireless signal transmission method. The WLAN technology may be based on the institute of electrical and electronics engineers (IEEE) 802.11 standard. The 802.11 standard evolved into 802.11b, 802.11a, 802.11g, 802.11n, 802.11ac, 802.11ax, etc., and may support a transmission rate of up to 1 Gbyte/s based on orthogonal frequency-division multiplexing (OFDM) technology. 
     In 802.11ac, data may be simultaneously or contemporaneously transmitted to a plurality of users through a multi-user multi-input multi-output (MU-MIMO) technique. In 802.11ax, which is referred to as high efficiency (HE), multiple access is implemented by dividing and providing available subcarriers to users by applying not only MU-MIMO but also orthogonal frequency-division multiple access ( 01 -DMA) technology. Accordingly, a WLAN system, to which 802.11ax is applied, may effectively support communication in dense areas and outdoors. 
     In 802.11be, which is referred to as extremely high throughput (EHT), 6 GHz unlicensed frequency band support, bandwidth utilization of up to 320 MHz per channel, introduction of hybrid automatic repeat and request (HARQ), support for up to 16×16 MIMO, and/or the like, are to be implemented. Accordingly, a next-generation WLAN system is expected to effectively support low latency and high-speed transmission, like new radio (NR), which is 5th generation (5G) technology. 
     SUMMARY 
     The inventive concepts provide a device and method for improving data reliability in wireless communication by using a multi-link. 
     According to an aspect of the inventive concepts, there is provided a wireless communication method of a first device with a second device using multiple links, the multiple links including a first link and a second link, the wireless communication method including obtaining transmission-related information in response to entering a transmission preparation period of a first request-to-send (RTS) frame through the first link, the transmission-related information corresponding to a second RTS frame associated with the second link, determining at least one of an additional length or cross-link-related information of the first RTS frame based on the transmission-related information, and determining whether to transmit the second RTS frame based on the at least one of the additional length of the cross-link-related information of the first RTS frame. 
     According to an aspect of the inventive concepts, there is provided a first device configured to communicate with a second device using multiple links, the multiple links including a first link and a second link, the first device including a radio frequency (RF) integrated circuit configured to provide access points respectively corresponding to the first link and the second link, and processing circuitry configured to obtain transmission-related information in a transmission preparation period of a first request-to-send (RTS) frame through the first link, the transmission-related information corresponding to a second RTS frame associated with the second link, determine at least one of an additional length or cross-link-related information of the first RTS frame based on the transmission-related information, and determine whether to transmit the second RTS frame based on the at least one of the additional length of the cross-link-related information of the first RTS frame. 
     According to an aspect of the inventive concepts, there is provided a wireless communication system including a first device and a second device, the first device and the second device being configured to communicate with each other through multiple links, wherein the first device is configured to determine at least one of an additional length or cross-link-related information of a first request-to-send (RTS) frame among a plurality of RTS frames based on transmission-related information of the plurality of RTS frames through the multiple links, and generate the first RTS frame based on the at least one of the additional length or cross-link-related information to transmit the first RTS frame to the second device through a first link among the multiple links. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the inventive concepts will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG.  1    is a diagram of a wireless communication system according to embodiments; 
         FIG.  2    is a block diagram of a wireless communication system according to embodiments; 
         FIG.  3    is a block diagram of a wireless communication system according to embodiments; 
         FIGS.  4 A and  4 B  are timing diagrams each illustrating an example of a multi-link operation; 
         FIG.  5    is a timing diagram including signaling based on a protection mechanism in a multi-link operation; 
         FIGS.  6 A and  6 B  are flowcharts illustrating an operating method of an access point multiple links device (AP MLD), according to embodiments; 
         FIG.  7    is a timing diagram illustrating an example of a multi-link operation, according to embodiments; 
         FIGS.  8 A and  8 B  are flowcharts illustrating an operating method of an AP MLD, according to embodiments; 
         FIG.  9    is a table diagram for explaining cross-link-related information, according to embodiments; 
         FIGS.  10 A and  10 B  are flowcharts illustrating an operating method of an AP MLD and a non-AP MLD, according to embodiments; 
         FIG.  11    is a table diagram for explaining cross-link-related information, according to embodiments; 
         FIG.  12    is a flowchart illustrating an example of operation S 330  of  FIG.  10 A  using cross-link-related information of  FIG.  11   ; 
         FIGS.  13 A to  13 C  are timing diagrams illustrating examples of a multi-link operation, according to embodiments; 
         FIGS.  14 A and  14 B  are diagrams illustrating formats of a request-to-send (RTS) frame, according to embodiments; 
         FIG.  15    is a diagram for explaining an example in which cross-link-related information is arranged in a format of a trigger frame, according to embodiments; 
         FIG.  16    is a diagram for explaining formats of a trigger frame that may be adopted for a format of an RTS frame, according to embodiments; 
         FIGS.  17  and  18    are diagrams for explaining an RTS frame employing an ‘MU-RTS’ format, according to embodiments; 
         FIG.  19    is a block diagram of a system according to embodiments; and 
         FIG.  20    is a diagram illustrating examples of a device for wireless communication according to embodiments. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    is a diagram of a wireless communication system  10  according to embodiments of the inventive concepts. In particular,  FIG.  1    shows a wireless local area network (WLAN) system, as an example of the wireless communication system  10 . 
     Hereinafter, embodiments are described based on an orthogonal frequency-division multiplexing (OFDM)-based or orthogonal frequency-division multiple access (OFDMA)-based communication system, in particular, an institute of electrical and electronics engineers (IEEE) standard, but in the inventive concepts, other communication systems having a similar technical background and channel type (for example, a cellular communication system, such as long term evolution (LTE), LTE-advanced (LTE-A), new radio (NR), wireless broadband (WiBro), global system for mobile communication (GSM), etc., or a short-distance communication system, such as Bluetooth and/or near-field communication (NFC), may be applicable with slight modifications within a range that does not significantly depart from the scope of the inventive concepts. 
     In addition, various functions to be described below may be implemented or supported by artificial intelligence technology or one or more computer programs, and the one or more computer programs may each be configured by computer-readable program code and executed in a computer-readable medium. The terms “application” and “program” refer to one or more computer programs, software components, sets of instructions, procedures, functions, objects, classes, instances, related data, or portions thereof suitable for implementation of suitable computer-readable program code. The term “computer-readable program code” includes computer code of any type, including source code, object code, and executable code. The term “computer-readable medium” includes any tangible medium that may be accessed by a computer, such as read only memory (ROM), random access memory (RAM), a hard disk drive, a compact disk (CD), a digital video disk (DVD), or any other type of memory. A “non-transitory” computer-readable medium excludes wired, wireless, optical, or other communication links that transmit transitory electrical or other signals. A non-transitory computer-readable medium includes a medium in which data may be permanently stored, and a medium in which data may be stored and later overwritten, such as a rewritable optical disk or a removable memory device. 
     In embodiments to be described below, a hardware approach method is described as an example. However, because embodiments include technology using both hardware and software, embodiments do not exclude a software-based approach method. 
     Referring to  FIG.  1   , the wireless communication system  10  may include first to sixth devices D 1 _ 1 , D 1 _ 2 , D 2 _ 1 , D 2 _ 2 , D 2 _ 3 , and D 2 _ 4 . The first and second devices D 1 _ 1 , D 1 _ 2  may access a network  13  including the Internet, an Internet protocol (IP) network, or any other network. The first device D 1 _ 1  may access the network  13  within a first coverage area  11 . The first device D 1 _ 1  may provide a connection to the network  13  to the third to sixth devices D 2 _ 1 , D 2 _ 2 , D 2 _ 3 , and D 2 _ 4  within the first coverage area  11 , and the second device D 1 _ 2  may provide a connection to the network  13  to the third and fourth devices D 2 _ 1  and D 2 _ 2  within a second coverage area  12 . 
     In embodiments, the first and second devices D 1 _ 1  and D 1 _ 2  may communicate with at least one of the third to sixth devices D 2 _ 1 , D 2 _ 2 , D 2 _ 3 , and D 2 _ 4  based on wireless fidelity (Wi-Fi) or any other WLAN access technology by using a multi-link. The first and second devices D 1 _ 1  and D 1 _ 2  may each correspond to an access point multiple links device (AP MLD), and the third to sixth devices D 2 _ 1 , D 2 _ 2 , D 2 _ 3 , and D 2 _ 4  may each correspond to a non-access point multiple links device (non-AP MLD). Herein, an AP MLD is a device capable of supporting a plurality of access point (APs), and the non-AP MLD may be a device capable of supporting a plurality of stations (STAs). 
     In embodiments, the first and second devices D 1 _ 1  and D 1 _ 2  may be referred to as routers, gateways, or the like, and the third to sixth devices D 2 _ 1 , D 2 _ 2 , D 2 _ 3 , and D 2 _ 4  may be referred to as terminals, mobile terminals, wireless terminals, user equipment, or the like. In addition, the third to sixth devices D 2 _ 1 , D 2 _ 2 , D 2 _ 3 , and D 2 _ 4  may be mobile devices, such as a mobile phone, a laptop computer, a wearable device, or the like, or may be stationary devices, such as a desktop computer, a smart television (TV), or the like. 
     The AP MLD may allocate at least one resource unit (RU) to at least one non-AP MLD. The AP MLD may transmit data to the non-AP MLD through the at least one allocated RU, and the non-AP MLD may receive the data through the at least one allocated RU. In 802.11be (hereinafter, referred to as EHT) or next-generation IEEE 802.11 standards (hereinafter, referred to as EHT+), the AP MLD may allocate a multi-resource unit (MRU) including two or more RUs to at least one non-AP MLD. For example, the first device D 1 _ 1  may allocate at least one MRU to at least one of the third to sixth devices D 2 _ 1 , D 2 _ 2 , D 2 _ 3 , and D 2 _ 4 , and may transmit data to the at least one of the third to sixth devices D 2 _ 1 , D 2 _ 2 , D 2 _ 3 , and D 2 _ 4  through the at least one allocated MRU. 
     In embodiments, when an AP MLD and a non-AP MLD perform communication through multiple links, a protection mechanism for ensuring or improving reliability of data communication may be selectively activated for each link. For example, in communication between the AP MLD and the non-AP MLD, the protection mechanism may be activated in communication through a first link, and the protection mechanism may be deactivated in communication through a second link. The AP MLD may adaptively control at least one of a transmission timing of a particular frame (e.g., an RTS frame) and/or content of the particular frame, so that signaling defined in the protection mechanism may be effectively performed with the non-AP MLD. In embodiments, the non-AP MLD may also adaptively control at least one of the transmission timing of a particular frame (e.g., an RTS frame) and/or the content of the particular frame, so that signaling defined in the protection mechanism may be performed with the AP MLD. Hereinafter, for convenience of description, embodiments are mainly described based on an operation of an AP MLD. However, this is only an example, and the inventive concepts are not limited thereto. Embodiments of the inventive concepts may also be applied to a non-AP MLD. 
     Hereinafter, for convenience of description, embodiments are mainly described based on an operation of an AP MLD. However, this is only an example, and the inventive concepts are not limited thereto. Embodiments may also sufficiently be applied to a non-AP MLD. 
     In addition, hereinafter, embodiments will be described with reference mainly to EHT. However, it may be understood that embodiments may also be applied to other protocol standards, such as EHT+. 
       FIG.  2    is a block diagram of a wireless communication system  20  according to embodiments of the inventive concepts. In particular, the block diagram of  FIG.  2    shows an AP MLD  100  and a non-AP MLD  200 , which communicate with each other in the wireless communication system  20 . Each of the AP MLD  100  and the non-AP MLD  200  of  FIG.  2    may be any device communicating in the wireless communication system  20  and may be referred to as a device for wireless communication or simply a device. 
     Referring to  FIG.  2   , the AP MLD  100  may include a radio frequency integrated circuit (RFIC)  110  and/or a processor  120 . The RFIC  110  may include first to n-th (where n is an integer greater than or equal to 2) transceivers  111 _ 1  to  111 _ n  and first to n-th antennas  112 _ 1  to  112 _ n.  The RFIC  110  and the processor  120  may be included in one package, or may be included in different packages, respectively. The non-AP MLD  200  may include an RFIC  210  and/or a processor  220 . The RFIC  210  may include first to m-th (where m is an integer greater than or equal to 2) transceivers  211 _ 1  to  211 _ m  and first to m-th antennas  212 _ 1  to  212 _ m.  Hereinafter, descriptions of the non-AP MLD  200  already given with reference to descriptions of corresponding features of the AP MLD  100  are omitted. 
     The first to n-th antennas  112 _ 1  to  112 _ n  may be respectively coupled to the first to n-th transceivers  111 _ 1  to  111 _ n,  and may receive signals from the non-AP MLD  200  to provide the same to the first to n-th transceivers  111 _ 1  to  111 _ n,  or may transmit signals provided from the first to n-th transceivers  111 _ 1  to  111 _ n  to the non-AP MLD  200 . In embodiments, the first to n-th antennas  112 _ 1  to  112 _ n  may include a phased array for beamforming. 
     The first to n-th transceivers  111 _ 1  to  111 _ n  may process signals received from the non-AP MLD  200  through the first to n-th antennas  112 _ 1  to  112 _ n,  and the processed signals may be provided to the processor  120 . In addition, the first to n-th transceivers  111 _ 1  to  111 _ n  may process signals provided by the processor  120 , and output the processed signals through the first to n-th antennas  112 _ 1  to  112 _ n.  In embodiments, the first to n-th transceivers  111 _ 1  to  111 _ n  may respectively include analog circuits, such as a low noise amplifier, a mixer, a filter, a power amplifier, an oscillator, or the like. The first to n-th transceivers  111 _ 1  to  111 _ n  may process signals received from the first to n-th antennas  112 _ 1  to  112 _ n  and/or signals received from the processor  120  under the control of the processor  120 . 
     In embodiments, the AP MLD  100  may perform communication based on multiple links ML with the non-AP MLD  200  by using the first to n-th transceivers  111 _ 1  to  111 _ n  and the first to n-th antennas  112 _ 1  to  112 _ n.  Frequency bands, to which the multiple links ML are respectively allocated, may be different, and the AP MLD  100  may support up to n APs through a control operation of the processor  120  by using the first to n-th transceivers  111 _ 1  to  111 _ n  and the first to n-th antennas  112 _ 1  to  112 _ n.  For example, the AP MLD  100  may support a first AP configured by the first transceiver  111 _ 1 , which is controlled by the processor  120 , and the first antenna  112 _ 1 , and may support a second AP configured by a second transceiver  111 _ 2  (among the first to n-th transceivers  111 _ 1  to  111 _ n ), which is controlled by the processor  120 , and a second antenna  112 _ 2  (among the first to n-th antennas  112 _ 1  to  112 _ n ). 
     In embodiments, the AP MLD  100  may further include first individual processors (not shown) respectively corresponding to n APs, and the processor  120  may control the first individual processors (not shown), so that each of the n APs may perform an operation according to embodiments. For example, the AP MLD  100  may support a first AP configured by a first-1 individual processor (not shown), the first transceiver  111 _ 1 , and the first antenna  112 _ 1 , and may support a second AP configured by a first-2 individual processor (not shown), the second transceiver  111 _ 2 , and the second antenna  112 _ 2 . 
     The processor  120  may extract information by demodulating and/or decoding signals received from the non-AP MLD  200  including processing signals received from the first to n-th transceivers  111 _ 1  to  111 _ n.  In addition, signals including information to be transmitted to the non-AP MLD  200  may be generated and provided to the first to n-th transceivers  111 _ 1  to  111 _ n.  For example, the processor  120  may provide signals generated by coding and/or modulating data, to be transmitted to the non-AP MLD  200 , to the first to n-th transceivers  111 _ 1  to  111 _ n.  In embodiments, the processor  120  may also include a programmable component, such as a central processing unit (CPU), a digital signal processor (DSP), or the like, a reconfigurable component, such as a field programmable gate array (FPGA), or the like, or a component that provides a fixed function, such as an intellectual property (IP) core. In embodiments, the processor  120  may include or access a memory that stores data and/or a series of instructions. 
     In embodiments, the non-AP MLD  200  may perform communication based on the multiple links ML with the AP MLD  100  by using the first to m-th transceivers  211 _ 1  to  211 _ m  and the first to m-th antennas  212 _ 1  to  212 _ m.  The non-AP MLD  200  may support up to m STAs through a control operation of the processor  220  by using the first to m-th transceivers  211 _ 1  to  211 _ m  and the first to m-th antennas  212 _ 1  to  212 _ m.  As an example, the non-AP MLD  200  may support a first STA configured by the first transceiver  211 _ 1 , which is controlled by the processor  220 , and the first antenna  212 _ 1 , and may support a second STA configured by a second transceiver  211 _ 2  (among the first to m-th transceivers  211 _ 1  to  211 _ m ), which is controlled by the processor  220 , and the second antenna  212 _ 2  (among the first to m-th antennas  212 _ 1  to  212 _ m ). In embodiments, the number of STAs supported by the non-AP MLD  200  may be less than or equal to the number of APs supported by the AP MLD  100 , and the number of multiple links ML may correspond to the number of STAs supported by the non-AP MLD  200 . 
     In embodiments, the non-AP MLD  200  may further include second individual processors (not shown) respectively corresponding to m STAs, and the processor  220  may also control the second individual processors (not shown), so that each of the m STAs may perform an operation according to embodiments. For example, the non-AP MLD  200  may support a first STA configured by a second-1 individual processor (not shown), the first transceiver  211 _ 1 , and the first antenna  212 _ 1 , and may support a second STA (not shown) configured by a second-2 individual processor (not shown), the second transceiver  211 _ 2 , and the second antenna  212 _ 2 . 
     In embodiments, the processor  120  may include frame alignment circuitry  121 . The frame alignment circuitry  121  may adaptively control at least one of an additional length and/or cross-link-related information of a frame (e.g., a request-to-send frame, a clear-to-send frame, etc.), so that signaling of a protection mechanism performed to ensure or improve reliability of communication using the multiple links ML between the AP MLD  100  and the non-AP MLD  200  may be effectively performed. In particular, the AP MLD  100  and the non-AP MLD  200  may transmit and receive a request-to-send (RTS) frame and a clear-to-send (CTS) frame to/from each other based on a protection mechanism, and may transmit and receive mutual data to/from each other based on a result of the transmission/reception. At this time, the frame alignment circuitry  121  may determine at least one of an additional length and/or cross-link-related information of an RTS frame, generate the RTS frame based on a result of the determination, and then transmit the generated RTS frame to the non-AP MLD  200  through the RFIC  110 , so that the AP MLD  100  may receive CTS frames, which are in a mutually aligned state, from at least two of the multiple links ML from the non-AP MLD  200 . In addition, in embodiments, the frame alignment circuitry  121  may determine whether to transmit another RTS frame based on a result of the determination. For example, in determining cross-link-related information for another link, when the frame alignment circuitry  121  determines that the other link is in a cross-link relationship with a link that transmits the RTS frame, the frame alignment circuitry  121  may omit transmission of the RTS frame through the other link. As another example, in determining cross-link-related information for another link, when the frame alignment circuitry  121  determines that the other link is not in a cross-link relationship with a corresponding link, the frame alignment circuitry  121  may transmit the RTS frame through the other link. In addition, in embodiments in which only the additional length of the RTS frame is determined, the frame alignment circuitry  121  may transmit the RTS frame through another link. A detailed description of a cross-link relationship is given below. 
     In embodiments, the processor  220  of the non-AP MLD  200  may also include frame alignment circuitry that performs the same operation as, or a similar operation to, that of the frame alignment circuitry  121  of the AP MLD  100 . 
       FIG.  3    is a block diagram of a wireless communication system  30  according to embodiments of the inventive concepts. In particular, the block diagram of  FIG.  3    shows an AP MLD  300  and a non-AP MLD  400 , which communicate with each other by using first to third links L 1 , L 2 , and L 3  in the wireless communication system  30 . 
     In embodiments, the AP MLD  300  may include first to third APs AP 1 , AP 2 , and AP 3 , and the non-AP MLD  400  may include first to third STAs STA 1 , STA 2 , and STA 3 . In embodiments, different from that shown in  FIG.  3   , the AP MLD  300  and the non-AP MLD  400  may communicate with each other by using two or more links, and may respectively include APs and STAs in a number corresponding to the number of links. 
     The first to third APs AP 1 , AP 2 , and AP 3  and the first to third STAs STA 1 , STA 2 , STA 3  may set up the first to third links L 1 , L 2 , and L 3 , respectively. For example, the AP MLD  300  and the non-AP MLD  400  may perform an access procedure and/or a negotiation procedure for a multi-link operation. The non-AP MLD  400  may identify a band usable in communication with the AP MLD  300  and may negotiate with the AP MLD  300  to use two or more links among the links supported by the AP MLD  300  for a multi-link. A method of performing mutual communication using two or more links may be referred to as a multi-link operation (MLO). Due to MLO, the wireless communication system  30  may provide high throughput. 
       FIGS.  4 A and  4 B  are timing diagrams each illustrating an example of an MLO. In particular,  FIGS.  4 A and  4 B  show examples of a multi-link operation performed in the wireless communication system  30  of  FIG.  3   . Hereinafter,  FIGS.  4 A and  4 B  are described with reference to  FIG.  3   . 
     The AP MLD  300  and the non-AP MLD  400  of  FIG.  3    may support simultaneous (or contemporaneous) transmit and receive (STR). For example, the AP MLD  300  may transmit data to the non-AP MLD  400  over the first link L 1 , and at the same time (or contemporaneously), receive data from the non-AP MLD  400  over the second link L 2 . The first to third APs AP 1 , AP 2 , and AP 3  may respectively have different medium access control (MAC) addresses, and may be in charge (e.g., in control) of the first to third links L 1 , L 2 , and L 3 , respectively. Accordingly, each of the first to third APs AP 1 , AP 2 , and AP 3  may function as an independent AP. In addition, the first to third STAs STA 1 , STA 2 , and STA 3  may each have different MAC addresses, and each of the first to third STAs STA 1 , STA 2 , and STA 3  may function as an independent STA. The AP MLD  300  and the non-AP MLD  400  may communicate with each other in a multi-band. For example, the first link L 1  may use a bandwidth (e.g., 40 MHz) in a 2.4 GHz band, the second link L 2  may use a bandwidth (e.g., 160 MHz) in a 5 GHz band, and the third link L 3  may use a bandwidth (e.g., MHz) in a 6 GHz band. 
     Referring to  FIG.  4 A , at time t 11 , the first AP AP 1  may initiate transmission of first data D 1  over the first link L 1 , and the first STA STA 1  may receive the first data DE While the first AP AP 1  transmits the first data D 1 , at time t 21 , the second STA STA 2  may initiate transmission of second data D 2  over the second link L 2 , and the second AP AP 2  may receive the second data D 2 . Although the transmission of the first data D 1  and the transmission of the second data D 2  overlap in the time domain, due to the STR, the first STA STA 1  and the second AP AP 2  may respectively receive the first data D 1  and the second data D 2 . In addition, while the first AP AP 1  transmits the first data D 1  and the second STA STA 2  transmits the second data D 2 , at time t 31 , the third AP AP 3  may initiate transmission of third data D 3  over the third link L 3 , and the third STA STA 3  may receive the third data D 3 . Although the transmission of the first data D 1 , the transmission of the second data D 2 , and the transmission of the third data D 3  overlap in the time domain, due to the STR, the first STA STA 1 , the second AP AP 2 , and the third STA STA 3  may receive the first data D 1 , the second data D 2 , and the third data D 3 , respectively. 
     It may be difficult to implement STR on links in a multiple links operation. For example, when frequency bands of links are insufficiently spaced apart, interference may occur between the links, and accordingly, the links may have difficulty in performing STR. In addition, when there is interference between links due to various causes including structural limitations due to lightness, compactness, and/or miniaturization of hardware in a non-AP MLD, the links may have difficulty in performing STR. As described above, links of a multi-link operation may include a non-STR link group including links over which performance of STR may be difficult. In particular, two links having difficulty in performing STR due to mutual interference in a non-STR link group may be referred to as a non-STR link pair. The non-STR link group may be identified while the AP MLD  300  and the non-AP MLD  400  set up the first to third links L 1 , L 2 , and L 3 . 
     Referring to  FIG.  4 B , it is assumed that the first link L 1  and the second link L 2  are a non-STR link pair. For example, the first link L 1  may use a band of 6 GHz, and the second link L 2  may use a band of 5 GHz. At time  12 , the first AP AP 1  may obtain a transmit opportunity TXOP and initiate transmission of the first data D 1  over the first link L 1 , and the first STA STA 1  may receive the first data DE At time t 22 , the second STA STA 2  may initiate transmission of the second data D 2  over the second link L 2 , but when the first link L 1  and the second link L 2  are a non-STR link pair, interference between the transmission of the second data D 2  and the reception of the first data D 1  on a side of the non-AP MLD  400  may occur. For example, the first data D 1  may leak through a path over which the second data D 2  is transmitted from the second STA STA 2 , so that the second data D 2  may not be properly transmitted through the second link L 2 . Such interference may also occur in signaling of a protection mechanism between the AP MLD  300  and the non-AP MLD  400 , as described below with reference to  FIG.  5   . It is assumed that the first link L 1  and the second link L 2  to be described in the drawings below are a non STR link pair. 
       FIG.  5    is a timing diagram including signaling based on a protection mechanism in a multi-link operation. In particular,  FIG.  5    shows an example of a multi-link operation performed in the wireless communication system  30  of  FIG.  3   . Hereinafter,  FIG.  5    is described with reference to  FIG.  3   . It is assumed that the protection mechanism is activated in the first link L 1  and the second link L 2 . 
     Referring to  FIG.  5   , at time  13 , the first AP AP 1  may initiate transmission of a first RTS frame RTS 1  over the first link L 1 , and the first STA STA 1  may receive the first RTS frame RTS 1 . At time t 33  after a short interframe space SIFS from time t 23 , the first STA STA 1  may initiate transmission of a first CTS frame CTS 1  over the first link L 1  in response to the first RTS frame RTS 1 , and the first AP AP 1  may receive the first CTS frame CTS 1 . At time t 63  after the short interframe space SIFS from time t 53 , the first AP AP 1  may initiate transmission of a first aggregated-MAC protocol data unit (A-MPDU) A-MPDU 1  over the first link L 1  in response to the first CTS frame CTS 1 , and the first STA STA 1  may receive the first A-MPDU A-MPDU 1 . 
     At time t 23 , the second AP AP 2  may initiate transmission of a second RTS frame RTS 2  over the second link L 2 , and the second STA STA 2  may receive the second RTS frame RTS 2 . At time t 53  after the short interframe space SIFS from time t 43 , the second STA STA 2  may initiate transmission of a second CTS frame CTS 2  over the second link L 2  in response to the second RTS frame, and the second AP AP 2  may receive the second CTS frame CTS 2 . At time t 83  after the short interframe space SIFS from time t 73 , the second AP AP 2  may initiate transmission of a second A-MPDU A-MPDU 2  over the second link L 2  in response to the second CTS frame CTS 2 , and the second STA STA 2  may receive the second A-MPDU A-MPDU 2 . The AP MLD  300  may control an end of the first A-MPDU A-MPDU 1  to be aligned with an end of the second A-MPDU A-MPDU 2  at time t 93 . 
     From time t 33  to time t 43 , because the first link L 1  and the second link L 2  are a non-STR link pair, interference between transmission of the first CTS frame CTS 1  and the reception of the second RTS frame RTS 2  may occur on a side of the non-AP MLD  400 . Due to the above interference, signaling of a smooth protection mechanism may be difficult, which may adversely affect the reliability of data communication. 
     The AP MLD  300  according to embodiments may determine at least one of an additional length and/or cross-link-related information of the first RTS frame RTS 1  to avoid interference between the reception of the second RTS frame RTS 2  and the transmission of the first CTS frame CTS 1  in the non-AP MLD  400 . That is, the AP MLD  300  may generate the first RTS frame RTS 1  for aligning the first CTS frame CTS 1  with the second CTS frame to transmit the generated first RTS frame RTS 1  to the non-AP MLD  400  through the first link L 1 . In embodiments, for the format of the first RTS frame RTS 1 , any one of a plurality of trigger formats defined in a standard specification is adopted or newly developed so that padding data or cross-link-related information corresponding to an additional length may be appropriately arranged, and thus, signaling of an efficient protection mechanism may be supported. 
     A method of generating the first RTS frame RTS 1  of the AP MLD  300  may vary, and examples thereof are described with reference to  FIGS.  6 A to  13 C . The format of the first RTS frame RT 1  may vary, and examples thereof are described with reference to  FIGS.  14 A to  18   . 
       FIGS.  6 A and  6 B  are flowcharts illustrating an operating method of an AP MLD, according to embodiments. It is assumed that a k-th AP and other APs described below are included in the AP MLD. In addition, hereinafter, a case in which an RTS frame is transmitted to a non-AP MLD from an AP MLD is mainly described. However, the inventive concepts are not limited thereto, and the inventive concepts may be applied even when the RTS frame is transmitted from the non-AP MLD to an AP MLD. 
     Referring to  FIG.  6 A , in operation S 100 , the k-th AP may enter a transmission preparation period of a k-th RTS frame. Herein, an operation of a particular AP may be interpreted as an operation of an AL MLD. In addition, herein, a transmission preparation period of an RTS frame is a period that determines at least one of an additional length and/or cross-link-related information of the RTS frame, and may be set before a transmission timing of the RTS frame arrives. However, the transmission preparation period of the RTS frame is arbitrarily named to help understanding of embodiments, and the inventive concepts are not limited thereto. Also, in embodiments, the AP MLD may perform an operation of generating an RTS frame according to another period without a separate transmission preparation period of the RTS frame. For example, the transmission preparation period of an RTS frame may be entered in response to a back-off count for transmission of an RTS frame of a corresponding AP reaching a reference value. For example, when the back-off count of the AP gradually decreases from a certain value to reach the reference value, the AP may enter the transmission preparation period of an RTS frame. 
     In operation S 110 , the k-th AP may obtain transmission-related information of RTS frames of APs other than the k-th AP. The other APs may be limited to APs corresponding to links having a non-STR link pair relationship with a k-th link corresponding to the k-th AP. In embodiments, the transmission-related information of RTS frames of the other APs may include whether (e.g., an indication of whether) a protection mechanism is activated in links corresponding to the other APs and back-off counts of the other APs for transmission of the RTS frame. In the transmission preparation period of the k-th RTS frame, the k-th AP may identify in advance other APs capable of transmitting RTS frames and an RTS frame having a transmission timing close to a transmission timing of the k-th RTS frame through the transmission-related information of RTS frames. 
     In operation S 120 , the k-AP may generate a k-th extended (E)-RTS frame based on the transmission-related information obtained in operation S 110 . In embodiments, the k-th AP may determine an additional length of the k-th RTS frame based on a transmission timing of an RTS frame of the other AP, and add padding data corresponding to the additional length to the k-th RTS frame to generate the k-th E-RTS frame. 
     In operation S 130 , the k-th AP may end the transmission preparation period and transmit the k-th E-RTS frame. Thereafter, the other AP may transmit an RTS frame, and an end of the k-th E-RTS frame transmitted from the k-th AP may be aligned with an end of the RTS frame transmitted from the other AP. 
       FIG.  6 B  is a flowchart particularly illustrating operation S 120  and operation S 130  of  FIG.  6 A . Referring to  FIG.  6 B , in operation S 121  after operation S 110  ( FIG.  6 A ), the k th  AP may identify whether there is an RTS frame that may be aligned with the k th  RTS frame. At this time, the RTS frame that may be aligned with the k-th RTS frame may refer to an RTS frame scheduled to be transmitted to have an end that may be aligned with an end of the k-th RTS frame. 
     When the result of operation S 121  is ‘YES’, operation S 122  may be subsequently performed, and the k-th AP may generate the k-th E-RTS frame from the k-th RTS frame. In particular, the k-th AP may determine an additional length based on a difference between a transmission timing of an alignment target RTS frame of an identified AP and a transmission timing of the k-RTS frame of the k-th AP, and generate padding data corresponding to the additional length to add the generated padding data to the k-th RTS frame, thereby generating the k-th E-RTS frame. In embodiments, in operation S 121 , it may be identified that there are a plurality of RTS frames that may be aligned with the k th  RTS frame, and in this case, APs including the k th  AP may generate the k-th -ETS frame and alignment target E-RTS frames by adding padding data to each of the k-RTS frame and the alignment target RTS frames. 
     In operation S 131 , the k-th AP may transmit the k-th E-RTS frame. The k-th E-RTS frame may be longer than an alignment target RTS frame. 
     When the result of operation S 121  is ‘NO’, operation S 132  may be subsequently performed, and the AP MLD may transmit the k-th RTS frame to the non-AP MLD. The k-th RTS frame may have the same length as (or a similar length to) that of an alignment target RTS frame. According to embodiments, when the result of operation S 121  is ‘NO’, operation S 132  is performed without performing operation S 122 . 
       FIG.  7    is a timing diagram illustrating an example of a multiple links operation, according to embodiments. 
     Referring to  FIG.  7   , a back-off count of the first AP AP 1  corresponding to the first link L 1  may decrease while the first link L 1  is not in a busy state, and a back-off count of the second AP AP 2  corresponding to the second link L 2  may decrease while the second link L 2  is not in a busy state. At time t 14 , the first link L 1  may enter a busy state due to other transmissions, and the back-off count of the first AP AP 1  may stop decreasing. Similarly, at time t 24 , the second link L 2  may enter a busy state due to other transmissions, and the back-off count of the second AP AP 2  may stop decreasing. 
     At time t 34 , the busy state of the first link L 1  may be released, and accordingly, the back-off count of the first AP AP 1  may gradually decrease from 4 (e.g., may resume from the back-off count prior to the busy state). In embodiments, the release of the busy state may be referred to as a ready state (or an idle state). Similarly, at time t 44 , the busy state of the second link L 2  may be released, and accordingly, the back-off count of the second AP AP 2  may gradually decrease from 3 (e.g., may resume from the back-off count prior to the busy state). In addition, at time t 44 , the first AP AP 1  may enter a transmission preparation period TPP of the first RTS frame RTS 1  in response to the back-off count of the first AP AP 1  reaching 1. However, this is an example, and the inventive concepts are not limited thereto. The first AP AP 1  may enter the transmission preparation period TPP of the first RTS frame RTS 1  in response to that the back-off count of the first AP AP 1  reaches another reference value (e.g., a threshold reference value). 
     In the transmission preparation period TPP, the first AP AP 1  may obtain transmission-related information of the second RTS frame of the second AP AP 2 . In particular, the first AP AP 1  may obtain the back-off count of the second AP AP 2  (e.g., the back-off count of the second link L 2 ). At this time, at time t 44 , the first AP AP 1  may identify that a difference between the transmission timing (e.g., back-off count, which is 1) of the first AP AP 1 , and the transmission timing (e.g., back-off count, which is 3) of the second AP AP 2 , falls within a reference difference to determine the second RTS frame as an alignment target RTS frame. In the transmission preparation period TPP, at time t 44 , the first AP AP 1  may determine an additional length for the first RTS frame based on the difference between the transmission timing (e.g., back-off count, which is 1) of the first AP AP 1 , and the transmission timing (e.g., back-off count, which is 3) of the second AP AP 2 , and add padding data corresponding to the additional length (e.g., 2) to generate a first E-RTS frame E-RTS 1 . In embodiments, a unit length of padding data may correspond to a unit length of a back-off count. 
     At time t 54 , as the back-off count of the first AP AP 1  reaches 0, the first AP AP 1  may end the transmission preparation period TPP to transmit the first E-RTS frame E-RTS 1  to the first STA STA 1  through the first link L 1 . 
     At time t 64 , as the back-off count of the second AP AP 2  reaches 0, the second AP AP 2  may transmit the second RTS frame RTS 2  to the second STA STA 2  through the second link L 2 . Although a transmission timing of the first E-RTS frame E-RTS 1  is different from a transmission timing of the second frame RTS 2 , an end (of transmission timing) of the first E-RTS frame E-RTS 1  may be aligned with an end (of transmission timing) of the second RTS frame RTS 2 . According to embodiments, a length of the first E-RTS frame E-RTS 1  is greater than a length of the second RTS frame RTS 2 . 
     When the first STA STA 1  identifies that the first link L 1  is in a ready state by sensing the first link L 1 , at time t 84  after the short interframe space SIFS from time t 74 , the first STA STA 1  may transmit the first CTS frame CTS 1  to the first AP AP 1  through the first link L 1 . When the second STA STA 2  identifies that the second link L 2  is in a ready state by sensing the second link L 2 , at time t 84  after the short interframe space SIFS from time t 74 , the second STA STA 2  may transmit the second CTS frame CTS 2  to the second AP AP 2  through the second link L 2 . Accordingly, the first CTS frame CTS 1  and the second CTS frame CTS 2  may be aligned with each other. In embodiments, a sensing operation for the first link L 1  of the first STA STA 1  and a sensing operation for the second link L 2  of the second STA STA 2  may be performed from time t 74  to time t 84 . Herein, a sensing operation for a link may be referred to as a link sensing operation. 
     At time t 104  after the short interframe space SIFS from time t 94 , the first AP AP 1  may transmit a first A-MPDU A-MPDU 1  to the first STA STA 1  through the first link L 1 , and the second AP AP 2  may transmit a second A-MPDU A-MPDU 2  to the second STA STA 2  through the second link L 2 . 
       FIGS.  8 A and  8 B  are flowcharts illustrating an operating method of an AP MLD, according to embodiments. Hereinafter, a case in which an RTS frame is transmitted to a non-AP MLD from an AP MLD is mainly described. However, the inventive concepts are not limited thereto, and the inventive concepts may be applied even when the RTS frame is transmitted from the non-AP MLD to an AP MLD. In addition, hereinafter, descriptions already given with reference to  FIGS.  6 A and  6 B  are omitted. 
     Referring to  FIG.  8 A , in operation S 200 , a k-AP may enter a transmission preparation period of a k-RTS frame. 
     In operation S 210 , the k-th AP may obtain transmission-related information of RTS frames of APs other than the k-th AP. The other APs may be limited to APs corresponding to links having a non-STR link pair relationship with a k-th link corresponding to the k-th AP. 
     In operation S 220 , the k-th AP may generate a k-th cross-link (CL)-RTS frame based on the transmission-related information obtained in operation S 210 . In embodiments, the k-th AP may determine cross-link-related information of the kth RTS frame based on a transmission timing of an RTS frame of another AP and fill the k-th RTS frame with data corresponding to the determined cross-link-related information, thereby generating the k-th CL-RTS frame. In embodiments, the cross-link-related information may include at least one of a link index indicating multiple links (e.g., indicating or identifying the first link L 1 , the second link L 2 , etc.), information indicating whether sensing of each of the multiple links is to be performed (e.g., whether sensing of the first link L 1 , the second link L 2 , etc., is to be performed, or is required), and/or resource information (e.g., RUs, such as frequency and/or timing resources) allocated to the multiple links (e.g., allocated to the first link L 1 , the second link L 2 , etc.) for transmission of CTS frames. In embodiments, the cross-link-related information may further include information indicating whether transmission of a CTS frame is possible through an alternative link with respect to each of the multiple links (e.g., with respect to the first link L 1 , the second link L 2 , etc.). 
     In embodiments, the format of the k-th RTS frame may include a plurality of fields corresponding to cross-link-related information, and the plurality of fields may include a first field indicating multiple links (e.g., indicating or identifying the first link L 1 , the second link L 2 , etc.), a second field indicating whether link sensing for multiple links is to be performed (e.g., whether sensing of the first link L 1 , the second link L 2 , etc., is to be performed, or is required), and/or a third field indicating resources (e.g., RUs, such as frequency and/or timing resources) allocated to the multiple links (e.g., the first link L 1 , the second link L 2 , etc.) for transmission of CTS frames. In embodiments, the format of the kth RTS frame may further include at least one of a fourth field indicating whether transmission of a CTS frame is possible through an alternative link with respect to each of the multiple links (e.g., with respect to the first link L 1 , the second link L 2 , etc.) and/or a padding field filled with padding data having a variable length according to an additional length. 
     In embodiments, the format of the k-th RTS may include a plurality of individual fields corresponding to cross-link-related information, and each of the plurality of individual fields may include a first subfield indicating a specific link (e.g., indicating or identifying one of the first link L 1 , the second link L 2 , etc.), a second subfield indicating whether link sensing is to be performed for the specific link (e.g., whether sensing of one of the first link L 1 , the second link L 2 , etc., is to be performed, or is required), and/or a third subfield indicating resources (e.g., RUs, such as frequency and/or timing resources) allocated to the specific link (e.g., one of the first link L 1 , the second link L 2 , etc.) for transmission of a CTS frame. In embodiments, each of the plurality of individual fields included in the format of the k-th RTS frame may further include a fourth subfield indicating whether transmission of a CTS frame is possible through an alternative link with respect to a specific link (e.g., with respect to one of the first link L 1 , the second link L 2 , etc.). In addition, in embodiments, the format of the k-th RTS frame may further include a padding field filled with padding data having a variable length according to an additional length described above with reference to  FIGS.  6 A to  7   . 
     In embodiments, the format of the k-th RTS frame may correspond to any one of a plurality of trigger frame types proposed in the IEEE 802.11ax standard. At this time, the format of the k-th RTS frame may include a common information field and/or a user information field, and cross-link-related information may be arranged in any one of the common information field and/or the user information field. 
     In operation S 230 , the k-th AP may end the transmission preparation period and transmit a k-th CL-RTS frame to a k-th STA. The transmission of the k-th CL-RTS frame may replace transmission of an RTS frame of another AP, and accordingly, the transmission of an RTS frame of the other AP to other STA may be omitted. 
       FIG.  8 B  is a flowchart particularly illustrating operation S 220  of  FIG.  8 A . Referring further to  FIG.  8 B , in operation S 221  after operation S 210  ( FIG.  8 A ), the k-th AP may detect a link for which sensing is to be performed (e.g., is required) among (other) links other than a k-th link corresponding to the k-th AP. The detected link may be referred to as being in a cross-link relationship with the k-th link. Herein, sensing for a link may refer to an operation of sensing whether a state of the link is a busy state. In embodiments, the k-th AP may detect a link in which transmission of an RTS frame having a transmission timing close to a transmission timing of the k-th RTS frame is scheduled. 
     In operation S 222 , the k-th AP may generate the k-th CL-RTS frame based on a detection result in operation S 221 . For example, based on information indicating that link sensing is to be performed for a link index corresponding to the detected link, resource information allocated to the k-th link for transmission of a CTS frame may be filled in the k-th RTS frame to generate the k-th CL-RTS frame. Then, operation S 230  ( FIG.  8 A ) may follow. 
       FIG.  9    is a table diagram for explaining cross-link-related information INFO 1 , according to embodiments. 
     Referring to  FIG.  9   , the cross-link-related information INFO 1  may include ‘Link Index’ information indicating each of multiple links, ‘Link Sensing Required’ information indicating whether sensing of each of multiple links is to be performed (e.g., is required), and ‘RU Allocation’ information indicating resources allocated to multiple links for transmission of CTS frames. The cross-link-related information INFO 1  may further include information used for CTS frame transmission of an AP (e.g., the AP detected in operation S 221  in  FIG.  8 B ) that does not directly receive an RTS frame through a link allocated to itself, and may include a ‘Reserved’ space in which such information may be arranged. 
       FIGS.  10 A and  10 B  are flowcharts illustrating an operating method of an AP MLD  500  and a non-AP MLD  600 , according to embodiments. As shown in  FIG.  10 A , the AP MLD  500  may include (e.g., support) a k-th AP APk and other APs APs, and the non-AP MLD  600  may include (e.g., support) a k-th STA STAk communicating with the k-th APk through a k-th link and other STAs STAs. Hereinafter, a case in which an RTS frame is transmitted to the non-AP MLD  600  from the AP MLD  500  is mainly described. However, the inventive concepts are not limited thereto, and the inventive concepts may be applied even when the RTS frame is transmitted from the non-AP MLD  600  to the AP MLD  500 . 
     Referring to  FIG.  10 A , in operation S 300 , the k-th AP APk may transmit a k-th CL-RTS frame through the k-th link. As described above, the k-th CL-RTS frame may indicate a link for which link sensing is to be performed in response to the k-th CL-RTS frame among other links of the other APs APs, and may indicate resources for transmission of a CTS frame. The AP MLD  500  may omit transmission of an RTS frame through a link having a cross-link relationship with the k-th link. For example, when a second link is sensed based on a CL-RTS frame transmitted through a first link (e.g., in response to the transmission of the CL-RTS frame), the first link and the second link may be referred to as having a mutual cross-link relationship. As the k-th CL-RTS frame is used for sensing a link in a cross-link relationship with the k-th link, transmission of an RTS frame through the corresponding link may be omitted. 
     In operation S 310 , the non-AP MLD  600  (e.g., the k-th STA STAk) may provide cross-link-related information of the k-th CL-RTS frame to other STAs STAs. 
     In operation S 320 , an STA selected from among the other STAs STAs may perform link sensing based on the cross-link-related information. The selected STA may be an STA corresponding to a link for which sensing is to be performed (as indicated) in the cross-link-related information. Herein, an operation of a particular STA may be interpreted as an operation of a non-AP MLD. 
     The STA selected in operation S 330  may transmit a CTS frame to an AP corresponding to the STA selected from among the other APs APs through a link allocated to the STA selected based on a sensing result in operation S 320 . Although not shown in  FIG.  10 A , a k-th STA STAk may sense the k-th link in response to the k-th CL RTS frame, and transmit the k-th CTS frame to the k-th AP APk through the k-th link based on a sensing result. 
       FIG.  10 B  is a flowchart particularly illustrating operation S 330  of  FIG.  10 A . Referring further to  FIG.  10 B , in operation S 331   a  after operation S 320  ( FIG.  10 A ), the selected STA may respond to the k-th CL-RTS frame to identify whether a corresponding link, that is, a link allocated to itself, is in a busy state. 
     When the result of operation S 331   a  is ‘YES’, operation S 332   a  may be subsequently performed, and the selected STA may not transmit a CTS frame through a corresponding link. When the result of operation S 331   a  is ‘NO’, operation S 333   a  may be subsequently performed, and the selected STA may transmit a CTS frame through a corresponding link. 
       FIG.  11    is a table diagram for explaining cross-link-related information INFO 2 , according to embodiments. Hereinafter, a difference from the cross-link-related information INFO 1  of  FIG.  9    is mainly described. 
     Referring to  FIG.  11   , as compared with the cross-link-related information INFO 1  of  FIG.  9   , the cross-link-related information INFO 2  may further include ‘non-Primary TX’ information indicating whether a CTS frame may be transmitted through an alternative link with respect to each of multiple links. In particular, the ‘non-Primary TX’ information may indicate whether to transmit a CTS frame through an alternative link by detecting an alternative link in a ready state among other links for each link when STAs or APs of a device (e.g., an AP MLD or a non-AP MLD) may not transmit a CTS frame through links allocated to themselves because the links allocated to themselves are currently in a busy state. 
       FIG.  12    is a flowchart illustrating an example of operation S 330  of  FIG.  10 A  using the cross-link-related information INFO 2  of  FIG.  11   . 
     Referring to  FIG.  12   , in operation S 331   b  after operation S 320  ( FIG.  10 A ), the selected STA, in response to the k-th CL-RTS frame, may identify whether a corresponding link, that is, a link allocated to itself, is in a busy state. 
     When the result of operation S 331   b  is ‘YES’, operation S 332   b  may be subsequently performed, and the selected STA may identify whether a value corresponding to the link is ‘1’ with reference to the ‘non-Primary TX’ information of  FIG.  11   . 
     When the result of operation S 332   b  is ‘YES’, operation S 333   b  may be subsequently performed, and the selected STA may perform a detection operation on an alternative link currently in a ready state among other links, and may identify whether an alternative link is detected. 
     When the result of operation S 333   b  is ‘YES’, operation S 334   b  may be subsequently performed, and the selected STA may transmit a CTS frame through a detected alternative link. 
     When the result of operation S 331   b  is ‘NO’, operation S 335   b  may be subsequently performed, and the selected STA may transmit a CTS frame through a corresponding link. 
     When the result of at least one of operation S 332   b  and/or operation S 333   b  is ‘NO’, operation S 336   b  may be subsequently performed, and the selected STA may not transmit a CTS frame. 
       FIGS.  13 A to  13 C  are timing diagrams illustrating examples of a multi-link operation, according to embodiments. Hereinafter, descriptions already given with reference to  FIG.  7    are omitted. 
     Referring to  FIG.  13 A , at time t 15 , the first link L 1  may enter a busy state due to other transmissions, and a back-off count of the first AP AP 1  may stop decreasing. Similarly, at time t 25 , the second link L 2  may enter a busy state due to other transmissions, and a back-off count of the second AP AP 2  may stop decreasing. 
     At time t 35 , the busy state of the first link L 1  may be released, and accordingly, the back-off count of the first AP AP 1  may gradually decrease from 4. Similarly, at time t 45 , the busy state of the second link L 2  may be released, and accordingly, the back-off count of the second AP AP 2  may gradually decrease from 3. In addition, at time t 45 , the first AP AP 1  may enter a transmission preparation period TPP of a first RTS frame in response to the back-off count of the first AP AP 1  reaching 1. 
     In the transmission preparation period TPP, the first AP AP 1  may obtain transmission-related information of a second RTS frame of the second AP AP 2 . For example, the first AP AP 1  may receive the transmission-related information from the second AP AP 2 . In particular, the first AP AP 1  may obtain information indicating that a protection mechanism is activated in the second link L 2  corresponding to the second AP AP 2  and the back-off count of the second AP AP 2 . At this time, the first AP AP 1  may identify that a difference between the transmission timing (e.g., back-off count, which is 1) of the first AP AP 1 , and the transmission timing (e.g., back-off count, which is 3) of the second AP AP 2 , falls within a reference difference to detect that the second link L 2  as a link for which sensing is to be performed. In the transmission preparation period TPP, the first AP AP 1  may determine cross-link-related information based on a detection result. In particular, the first AP AP 1  may determine cross-link-related information to indicate that sensing for the second link L 2  is to be performed and to indicate resources allocated for transmission of the second CTS frame CTS 2  through the second link L 2 . In the transmission preparation period TPP, the first AP AP 1  may generate the first CL-RTS frame CL-RTS 1  by filling the first RTS frame with data corresponding to the determined cross-link-related information. 
     At time t 55 , as the back-off count of the first AP AP 1  reaches 0, so that the first AP AP 1  may end the transmission preparation period TPP to transmit the first CL-RTS frame CL-RTS 1  to the first STA STA 1  through the first link L 1 . 
     At time t 65 , the back-off count of the second AP AP 2  reaches 0, and transmission of the second RTS frame of the second AP AP 2  may be omitted. 
     At time t 75  to time t 95 , the second STA STA 2  may receive the cross-link-related information included in the first CL-RTS frame CL-RTS 1  and perform a sensing operation SO on the second link L 2  based on the cross-link-related information. For example, the second STA STA 2  may receive the cross-link-related information from the first STA STA 1 . The second STA STA 2  may identify that the second link L 2  is in a ready state through the sensing operation SO. A performance timing and a performance time of the sensing operation SO shown in  FIG.  13 A  are only examples, and the inventive concepts are not limited thereto. The sensing operation SO for the second link L 2  may be performed at an appropriate timing and performance time, so that the first CTS frame CTS 1  and the second CTS frame CTS 2  may be aligned with each other. 
     When the first STA STA 1  identifies that the first link L 1  is in a ready state by sensing the first link L 1 , at time t 95  after a short interframe space SIFS from time t 85 , the first STA STA 1  may transmit the first CTS frame CTS 1  to the first AP AP 1  through the first link L 1 . When the second STA STA 2  identifies that the second link L 2  is in a ready state by sensing the second link L 2 , at time t 95  after a short interframe space SIFS from time t 85 , the second STA STA 2  may transmit the second CTS frame CTS 2  to the second AP AP 2  through the second link L 2 . 
     At time t 115  after a short interframe space SIFS from time t 105 , the first AP AP 1  may transmit a first A-MPDU A-MPDU 1  to the first STA STA 1  through the first link L 1 , and the second AP AP 2  may transmit a second A-MPDU A-MPDU 2  to the second STA STA 2  through the second link L 2 . 
     In  FIG.  13 B , an operation from time t 16  to time t 76  may be the same as (or similar to) an operation from time t 15  to time t 75  of  FIG.  13 A , and accordingly, an operation after time t 76 , which is different from that shown in  FIG.  13 A , is mainly described. 
     Referring further to  FIG.  13 B , at time t 76  to time t 96 , the second STA STA 2  may receive the cross-link-related information included in the first CL-RTS frame CL-RTS 1  and perform a sensing operation SO on the second link L 2  based on the cross-link-related information. The second STA STA 2  may identify that the second link L 2  is in a busy state through the sensing operation SO. 
     When the first STA STA 1  identifies that the first link L 1  is in a ready state by sensing the first link L 1 , at time t 96  after a short interframe space SIFS from time t 86 , the first STA STA 1  may transmit the first CTS frame CTS 1  to the first AP AP 1  through the first link L 1 . When the second STA STA 2  identifies that the second link L 2  is in a busy state by sensing the second link L 2 , the second STA STA 2  may not transmit the second CTS frame. 
     At time t 116  after a short interframe space SIFS from time t 106 , the first AP AP 1  may transmit the first A-MPDU A-MPDU 1  to the first STA STA 1  through the first link L 1 . 
     In  FIG.  13 C , an operation from time t 17  to time t 77  may be the same as (or similar to) an operation from time t 15  to time t 75  of  FIG.  13 A , and accordingly, an operation after time t 77 , which is different from that shown in  FIG.  13 A , is mainly described. 
     Referring further to  FIG.  13 C , at time t 77  to time t 97 , the second STA STA 2  may receive the cross-link-related information included in the first CL-RTS frame CL-RTS 1  and perform a sensing operation SO on the second link L 2  based on the cross-link-related information. The second STA STA 2  may identify that the second link L 2  is in a busy state through the sensing operation SO. At this time, the second STA STA 2  may sense a state of a third link L 3 . 
     When the first STA STA 1  identifies that the first link L 1  is in a ready state by sensing the first link L 1 , at time t 97  after a short interframe space SIFS from time t 87 , the first STA STA 1  may transmit the first CTS frame CTS 1  to the first AP AP 1  through the first link L 1 . When the second STA STA 2  identifies that the third link L 3  is in a ready state through the sensing operation SO for the third link L 3 , the second STA STA 2  may detect the third link L 3  as an alternative link, and at time t 97  after a short interframe space SIFS from time t 87 , the second STA STA 2  may transmit the second CTS frame CTS 2  to the second AP AP 2  through the third link L 3 , which is an alternative link. 
     At time t 117  after a short interframe space SIFS from time t 107 , the first AP AP 1  may transmit a first A-MPDU A-MPDU 1  to the first STA STA 1  through the first link L 1 , and the second AP AP 2  may transmit a second A-MPDU A-MPDU 2  to the second STA STA 2  through the third link L 3 , which is an alternative link. In embodiments, the second AP AP 2  may transmit the second link L 2  to the second STA STA 2  through the second link L 2 . 
       FIGS.  14 A and  14 B  are diagrams illustrating formats of an RTS frame, according to embodiments. 
     Referring to  FIG.  14 A , a format FF 1  of an RTS frame may include a common field CF in which cross-link-related information for multiple links are arranged and a ‘Padding’ field in which padding data is arranged. The common field CF may include a ‘Link ID’ field indicating multiple links, a ‘Link Sensing Required’ field indicating whether link sensing is to be performed (e.g., is required) for multiple links, an ‘RU Allocation’ field indicating resources allocated to multiple links for transmission of CTS frames, and a ‘non-Primary TX’ field indicating whether a CTS frame may be transmitted through an alternative link with respect to multiple links. However, the format FF 1  of the RTS frame shown in  FIG.  14 A  is only an example and embodiments are not limited thereto, and the format FF 1  of the RTS frame may not include some fields or may further include a field which is not shown in  FIG.  14 A . In addition, names of fields may be defined as other names. Herein, the common field CF may be defined as a field in which cross-link-related information commonly decoded by a plurality of APs or a plurality of STAs of a device receiving an RTS frame is arranged. 
     Referring further to  FIG.  14 B , a format FF 2  of an RTS frame may include a plurality of individual fields IF_ 1  to IF_N and a ‘Padding’ field in which padding data is arranged. The plurality of individual fields IF_ 1  to IF_N may each include a ‘Specific Link ID’ subfield indicating a specific link (e.g., a link identifier (ID)), a ‘Link Sensing Required’ subfield indicating whether link sensing of a specific link is to be performed (e.g., is required), an RU Allocation’ subfield indicating resources allocated to a specific link for CTS frame transmission, and a ‘non-Primary TX’ subfield indicating whether a CTS frame may be transmitted through an alternative link with respect to a specific link. However, the format FF 2  of the RTS frame shown in  FIG.  14 B  is only an example and embodiments are not limited thereto, and the format FF 2  of the RTS frame may not include some subfields or may further include a subfield which is not shown in  FIG.  14 A . In addition, names of subfields may be defined as other names Herein, the plurality of individual fields IF_ 1  to IF_N may be defined as fields in which cross-link-related information decoded by a specific AP or a specific STA of a device receiving an RTS frame is arranged. 
       FIG.  15    is a diagram for explaining an example in which cross-link-related information is arranged in a format T_FF of a trigger frame, according to embodiments. 
     Referring to  FIG.  15   , a format of an RTS frame may correspond to the format T_FF of the trigger frame. The format T_FF of the trigger frame may include a ‘Frame Control’ field, a ‘Duration’ field, a ‘receiver address (RA)’ field, a ‘transmitting AP&#39;s address (TA)’ field, a ‘Common Info’ field (or a common information field) (CIF), ‘User Info’ fields (or user information fields) (UIFs), a ‘Padding’ field, and/or an ‘FCS’ field. A ‘MAC header’ may include the ‘Frame Control’ field, the ‘Duration’ field, the ‘RA’ field, and the ‘TA’ field. 
     In embodiments, cross-link-related information according to the inventive concepts may be arranged with any one of the CIF and the UIFs. 
       FIG.  16    is a diagram for explaining formats of a trigger frame that may be adopted for a format of an RTS frame, according to embodiments. 
     Referring to  FIG.  16   , according to IEEE 802.11.ax, formats of a trigger frame may include a ‘Basic Trigger Frame’ format, a ‘Beamforming Report (BFRP)’ format, a ‘Multi-user block ack request (MU-BAR)’ format, a ‘Multi-user request to send (MU-RTS)’ format, a ‘Buffer Status Report (BSRP)’ format, a ‘GCR (groupcast with retries) MU-BAR’ format, a ‘Bandwidth Query Report (BQRP)’ format, and/or an ‘neighbor discovery protocol (NDP) Feedback Report Poll (NFRP)’ format. 
     In embodiments, any one of a plurality of formats may be employed for the format of an RTS frame. However, this is an example, and the inventive concepts are not limited thereto. For the format of an RTS frame, any one of the formats of any frame defined in more various standards may be employed.  FIGS.  17  and  18    are described on a premise that a ‘Multi-user request to send’ format (hereinafter, referred to as a ‘MU-RTS’ format) is employed in the format of an RTS frame. 
       FIGS.  17  and  18    are diagrams for explaining an RTS frame employing an ‘MU-RTS’ format, according to embodiments. Hereinafter, descriptions are made with further reference to  FIG.  15    for better understanding. 
     Referring to  FIGS.  15  and  17   , a common information field CIF may include a ‘Trigger Type’ subfield, a ‘UL Length’ subfield, a ‘More TF subfield, a ‘CS Required’ subfield, a ‘UL BW’ subfield, a ‘GI AND HE-LTF Type’ subfield, a ‘MU-MIMO HE-LTF Mode’ subfield, a ‘Number of EHT-LTF Symbols And Midamble Periodicity’ subfield, a ‘UL STBC’ subfield, a ‘LDPC Extra Symbol Segment’ subfield, an ‘Ap Tx Power’ subfield, a ‘Pre-FEC Padding Factor’ subfield, a ‘PE Disambiguity’ subfield, a ‘UL Spatial Reuse’ subfield, a ‘Doppler’ subfield, a ‘HE/EHT P 160 ’ subfield, a ‘Special User Info Field Present’ subfield, ‘Reserved’ subfields, and/or a ‘Trigger Dependent Common Info’ subfield. 
     In a ‘MU-RTS’ format, the ‘UL Length’ subfield, the ‘GI And HE-LTF Type’ subfield, the ‘MU-MIMO HE-LTF Mode’ subfield, the ‘Number Of EHT-LTF Symbols And Midamble Periodicity’ subfield, the ‘UL STBC’ subfield, the ‘LDPC Extra Symbol Segment’ subfield, the ‘AP Tx Power’ subfield, the ‘Pre-FEC Padding Factor’ subfield, the ‘PE Disambiguity’ subfield, the ‘UL Spatial Reuse’ subfield, the ‘Doppler’ subfield, the ‘HE/EHT P160’ subfield, and/or the ‘Special User Info Field Present’ subfield may not be used, and the ‘MU-RTS’ format may be in a reserved state so that other data may be arranged. 
     In embodiments, cross-link-related information CF-CLI arranged in the common field CF in  FIG.  14 A  may be arranged in at least one of the ‘UL Length’ subfield, the ‘GI And HE-LTF Type’ subfield, the ‘MU-MIMO HE-LTF Mode’ subfield, the ‘Number Of EHT-LTF Symbols And Midamble Periodicity’ subfield, the ‘UL STBC’ subfield, the ‘LDPC Extra Symbol Segment’ subfield, the ‘AP Tx Power’ subfield, the ‘Pre-FEC Padding Factor’ subfield, the ‘PE Disambiguity’ subfield, the ‘UL Spatial Reuse’ subfield, the ‘Doppler’ subfield, the ‘HE/EHT P160’ subfield, and/or the ‘Special User Info Field Present’ subfield. In embodiments, the cross-link-related information CF-CLI may also be arranged in the ‘Reserved’ subfields. 
     Referring further to  FIG.  18   , at least one user information field UIF may include an ‘AID12’ subfield, a ‘PHY Version ID’ subfield, a ‘UL Bandwidth Extension’ subfield, a ‘Spatial Reuse  1 ’ subfield, a ‘Spatial Reuse  2 ’ subfield, a ‘U-SIG Disregard And Validate’ subfield, a ‘Reserved’ subfield, and/or a ‘Trigger Dependent User Info’ subfield. 
     In the ‘MU-RTS’ format, the ‘UL Bandwidth Extension’ subfield, the ‘Spatial Reuse  1 ’ subfield, the ‘Spatial Reuse  2 ’ subfield, and/or the ‘U-SIG Disregard And Validate’ subfield may not be used, and the ‘MU-RTS’ format may be in a reserved state so that other data may be arranged. 
     In embodiments, cross-link-related information IF-CLI arranged in any one of the plurality of individual fields IF_ 1  to IF_N in  FIG.  14 B  may be arranged in at least one of the ‘UL Bandwidth Extension’ subfield, the ‘Spatial Reuse  1 ’ subfield, the ‘Spatial Reuse  2 ’ subfield, and/or the ‘U-SIG Disregard And Validate’ subfield. In embodiments, the cross-link-related information IF-CLI may also be arranged in the ‘Reserved’ subfield. 
       FIG.  19    is a block diagram of a system  1000  according to embodiments. In embodiments, the system  1000  of  FIG.  19    may be implemented with at least one chip, and the system  1000  implemented with one chip may be referred to as a system-on-chip. 
     Referring to  FIG.  19   , the system  1000  may include an application processor  1010 , a memory  1020 , an input/output interface  1030 , a hardware accelerator  1040 , and/or a communication interface  1050 . The application processor  1010 , the memory  1020 , the input/output interface  1030 , the hardware accelerator  1040 , and/or the communication interface  1050  may communicate with each other through a bus  1060 . 
     The application processor  1010  may control the system  1000 . For example, the application processor  1010  may include at least one core, and each of the at least one core may execute a series of instructions stored in the memory  1020 . In embodiments, the application processor  1010  may execute an operating system (OS), and may execute applications on the OS. The application processor  1010  may control other components of the system  1000 . For example, the application processor  1010  may provide data to the hardware accelerator  1040  to instruct the hardware accelerator  1040  to perform an operation, and may obtain a result of the operation performed by the hardware accelerator  1040 . Also, the application processor  1010  may provide data to be transmitted to the outside to the communication interface  1050  to instruct transmission of the data, and may also obtain data received from the outside through the communication interface  1050 . 
     The memory  1020  may be accessed by other components via the bus  160 . The memory  1020  may have any structure capable of storing data, for example, a volatile memory, such as static random access memory (SRAM), and dynamic random access memory (DRAM), and/or a non-volatile memory, such as flash memory, and resistive random access memory (RRAM). The memory  1020  may store instructions executed by the application processor  1010  and may also store data read or written by other components. 
     The input/output interface  1030  may provide an interface for input to the system  1000  and output from the system  1000 . For example, the input/output interface  1030  may communicate with an input/output device included in a product together with the system  1000 , and may receive a user&#39;s input or provide an output to the user through the input/output device. In addition, the input/output interface  1030  may communicate with peripherals included in the product together with the system  1000 , and may enable the application processor  1010  to control the peripherals. 
     The hardware accelerator  1040  may be hardware designed to perform a function at high speed. For example, the hardware accelerator  1040  may be designed to perform encoding and decoding of data at high speed. Also, the hardware accelerator  1040  may be designed to perform neural processing at high speed. The hardware accelerator  1040  may process data stored in the memory  1020  and store the processed data in the memory  1020 . 
     The communication interface  1050  may provide a communication channel with an external object of the system  1000 . For example, the communication interface  1050  may provide a wired communication channel and/or a wireless communication channel In embodiments, the communication interface  1050  may perform at least one operation included in the method for wireless communication using a multi-link described with reference to the drawings. For example, the communication interface  1050  may include at least one processor  1055 , and the at least one processor  1055  may execute instructions to perform at least one operation included in a method for wireless communication using a multi-link described above. In embodiments, the at least one processor  1055  may execute instructions stored in the memory  1020  or a memory included in the communication interface  1050 . In embodiments, the memory  1020  or a memory included in the communication interface  1050  may store information collected about links, and may be accessed by the at least one processor  1055 . 
       FIG.  20    is a diagram illustrating examples of a device for wireless communication according to embodiments. In particular,  FIG.  20    shows an Internet of Things (IoT) network system including a home gadget  2010 , a home appliance  2020 , an entertainment device  2030 , and/or an AP  2040 . 
     In embodiments, in devices for wireless communication of  FIG.  20   , as described above with reference to the drawings, an operation using a multi-link may be performed. Accordingly, devices for wireless communication may smoothly perform signaling of a protection mechanism by minimizing or reducing interference between links when transmitting and receiving RTS frames and CTS frames in a mutual protection mechanism using a multi-link. Accordingly, high reliability of data communication between devices may be ensured or improved. 
     Conventional devices and methods for performing multi-link communication experience excessive amounts of mutual interference in communication links (e.g., between non-STA link pairs) between the devices. This excessive interference interferes with protection mechanism signaling (e.g., RTS signaling, CTS signaling, etc.) between the devices, thereby reducing the effectiveness of the protection mechanism signaling and reducing the reliability of data communication between the devices. 
     However, according to embodiments, improved devices and methods are provided for multi-link communication. For example, the improved devices and methods may generate an RTS frame based on a determination of at least one of an additional length and/or cross-link-related information. Through the use of the at least one of the additional length and/or cross-link-related information, the protection mechanism signaling (e.g., RTS signaling, CTS signaling, etc.) between devices may be aligned in a time-dimension, thereby reducing mutual interference in communication links between the devices. Alternatively, the protection mechanism signaling transmitted by one of the devices may be omitted based on the cross-link-related information to reduce the mutual interference in the communication links. Therefore, the improved devices and methods overcome the deficiencies of the conventional devices and methods to at least improve the effectiveness of the protection mechanism signaling by reducing the mutual interference, and thus, improve the reliability of data communication between the devices. 
     According to embodiments, operations described herein as being performed by the wireless communication system  10 , the first to sixth devices D 1 _ 1 , D 1 _ 2 , D 2 _ 1 , D 2 _ 2 , D 2 _ 3 , and D 2 _ 4 , the AP MLD  100 , the non-AP MLD  200 , the wireless communication system  20 , the RFIC  110 , the processor  120 , the first to n-th transceivers  111 _ 1  to  111 _ n,  the RFIC  210 , the processor  220 , the first to m-th transceivers  211 _ 1  to  211 _ m,  the frame alignment circuitry  121 , AP MLD  300 , the non-AP MLD  400 , the wireless communication system  30 ; the first to third APs AP 1 , AP 2 , and AP 3 , the first to third STAs STA 1 , STA 2 , and STA 3 , the AP MLD  500 , the non-AP MLD  600 , the k-th AP APk, the other APs APs, the a k-th STA, the other STAs STAs, the system  1000 , the application processor  1010 , the input/output interface  1030 , the hardware accelerator  1040 , the communication interface  1050 , the at least one processor  1055 , the home gadget  2010 , the home appliance  2020 , the entertainment device  2030 , and/or the AP  2040  may be performed by processing circuitry. The term ‘processing circuitry,’ as used in the present disclosure, may refer to, for example, hardware including logic circuits; a hardware/software combination such as a processor executing software; or a combination thereof. For example, the processing circuitry more specifically may include, but is not limited to, a central processing unit (CPU), an arithmetic logic unit (ALU), a digital signal processor, a microcomputer, a field programmable gate array (FPGA), a System-on-Chip (SoC), a programmable logic unit, a microprocessor, application-specific integrated circuit (ASIC), etc. 
     The various operations of methods described above may be performed by any suitable device capable of performing the operations, such as the processing circuitry discussed above. For example, as discussed above, the operations of methods described above may be performed by various hardware and/or software implemented in some form of hardware (e.g., processor, ASIC, etc.). 
     The software may comprise an ordered listing of executable instructions for implementing logical functions, and may be embodied in any “processor-readable medium” for use by or in connection with an instruction execution system, apparatus, or device, such as a single or multiple-core processor or processor-containing system. 
     The blocks or operations of a method or algorithm and functions described in connection with embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a tangible, non-transitory computer-readable medium. A software module may reside in Random Access Memory (RAM), flash memory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM), Electrically Erasable Programmable ROM (EEPROM), registers, hard disk, a removable disk, a CD ROM, or any other form of storage medium known in the art. 
     Embodiments may be described with reference to acts and symbolic representations of operations (e.g., in the form of flow charts, flow diagrams, data flow diagrams, structure diagrams, block diagrams, etc.) that may be implemented in conjunction with units and/or devices discussed in more detail below. Although discussed in a particular manner, a function or operation specified in a specific block may be performed differently from the flow specified in a flowchart, flow diagram, etc. For example, functions or operations illustrated as being performed serially in two consecutive blocks may actually be performed concurrently, simultaneously, contemporaneously, or in some cases be performed in reverse order. 
     It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it may be directly connected or coupled to the other element or intervening elements may be present. As used herein the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     While the inventive concepts have been particularly shown and described with reference to embodiments thereof, it will be understood that various changes in form and details may be made therein without departing from the spirit and scope of the following claims.