Patent Publication Number: US-2023156797-A1

Title: Device and method for wireless communication using multiple links

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2021-0157095, filed on Nov. 15, 2021, and Korean Patent Application No. 10-2022-0054440, filed on May 2, 2022, in the Korean Intellectual Property Office, the disclosures of which are incorporated by reference herein in their entireties. 
     BACKGROUND 
     The present disclosure relates to wireless communication, and more particularly, to a device and method for wireless communication using multiple links. 
     As an example of wireless communication, a Wireless Local Area Network (WLAN) is technology for connecting two or more apparatuses to each other by using a wireless signal transmission method. The WLAN technology may be based on Institute of Electrical and Electronics Engineers (IEEE) 802.11 standards. For example, the 802.11 standards have developed into 802.11b, 802.11a, 802.11g, 802.11n, 802.11ac, and 802.11ax and may support a transmission rate up to  1  gigabyte per second (GBps), based on Orthogonal Frequency-Division Multiplexing (OFDM) technology. 
     According to 802.11ac, data may be simultaneously transmitted to multiple users through a multi-user multi-input multi-output (MU-MIMO) technique. According to 802.11ax, which may be referred to as High Efficiency Wireless (HEW), the MU-MIMO technique, as well as, an Orthogonal Frequency-Division Multiple Access (OFDMA) technique may be used, and thus, multiple access may be realized by dividing available subcarriers and providing the divided subcarriers to users. Accordingly, a WLAN system using 802.11ax may effectively support communication in congested areas and outdoor space. 
     According to 802.11be, which may be referred to as Extremely High Throughput (EHT), a 6-GHz unlicensed spectrum support, bandwidth utilization up to 320 MHz per channel, Hybrid Automatic Repeat and Request (HARM) introduction, support for up to 16×16 MIMO, etc., are planned. To this end, a next-generation WLAN system is expected to effectively support low latency and ultra-high speed transmission like New Radio (NR) used by 5G technology. 
     SUMMARY 
     Embodiments of the disclosure provide devices and methods for improving data reliability in wireless communication using multiple links. 
     According to an aspect of an example embodiment, a wireless communication method performed by a first device, includes: entering a transmission preparation period of a first Request To Send (RTS) frame via a first link; obtaining transmission-related information of a second RTS frame via a second link; determining at least one of a first transmission timing and content of the first RTS frame, based on the transmission-related information of the second RTS frame; generating the first RTS frame based on a result of the determining; and transmitting, to a second device via the first link, the generated first RTS frame. 
     According to an aspect of an example embodiment, a first device configured to communicate with a second device via a first link and a second link, includes: a radio frequency integrated circuit (RFIC) configured to provide a plurality of access points respectively corresponding to the first link and the second link; and a processor configured to control the plurality of access points, wherein the processor is further configured to: during a transmission preparation period of a first Request To Send (RTS) frame through the first link, obtain transmission-related information of a second RTS frame via the second link; determine at least one of a first transmission timing and content of the first RTS frame, based on the transmission-related information of the second RTS frame; generate the first RTS frame based on a result of the determining; and transmit, to the second device via the first link, the generated first RTS frame. 
     According to an aspect of an example embodiment, a wireless communication system includes: a plurality of links comprising a first link and a second link; a first device; and a second device configured to communicate with the first device via the plurality of links, wherein the first device is configured to communicate with the second device via the plurality of links, and wherein the first device is further configured to: determine at least one of a transmission timing and content of a first Request To Send (RTS) frame from among a plurality of RTS frames, based on transmission-related information of the plurality of RTS frames via the plurality of links; generate the first RTS frame based on a result of the determining; and transmit, to the second device the generated first RTS frame via the first link. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of certain embodiments of the disclosure will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    is a diagram of a wireless communication system, according to an embodiment; 
         FIG.  2    is a block diagram of a wireless communication system, according to an embodiment; 
         FIG.  3    is a block diagram of a wireless communication system, according to an embodiment; 
         FIGS.  4 A and  4 B  are timing diagrams of an example of a multiple-links operation, according to an embodiment; 
         FIG.  5    is a timing diagram including signaling according to a protection mechanism in a multiple-links operation, according to an embodiment; 
         FIGS.  6 A and  6 B  are flowcharts of an operation method of an access point multiple-links device (AP MLD), according to an embodiment; 
         FIG.  7    is a timing diagram of an example of a multiple-links operation, according to an embodiment; 
         FIGS.  8 A and  8 B  are flowcharts of an operation method of an AP MLD, according to an embodiment; 
         FIG.  9    is a timing diagram of an example of a multiple-links operation, according to an embodiment; 
         FIGS.  10 A and  10 B  are flowcharts of an operation method of an AP MLD, according to an embodiment; 
         FIG.  11    is a table diagram describing cross-link-related information, according to an embodiment; 
         FIG.  12    is a flowchart of an operation method of an AP MLD and a non-AP MLD, according to an embodiment; 
         FIGS.  13 A and  13 B  are timing diagrams of an example of a multiple-links operation, according to an embodiment; 
         FIG.  14    is a block diagram of a system, according to an embodiment; and 
         FIG.  15    is a diagram of examples of a device for wireless communication, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    is a diagram of a wireless communication system  10  according to an embodiment. In detail,  FIG.  1    illustrates a Wireless Local Area Network (WLAN) system as an example of the wireless communication system  10 . 
     Hereinafter, one or more embodiments of the disclosure are described based on an Orthogonal Frequency-Division Multiplexing (OFDM) or Orthogonal Frequency-Division Multiple Access (OFDMA)-based wireless communication system, in particular, IEEE 802.11 standards. The disclosure may also be applicable, with minor modifications, to other communication systems (e.g., a cellular communication system, such as Long Term Evolution (LTE), LTE-advanced (LTE-A), New Radio (NR), Wireless Broadband (WiBro), or Global System for Mobile Communication (GSM), or a short-range communication system, such as Bluetooth or Near Field Communication (NFC)), which have similar technical backgrounds and channel forms to those of the disclosure without significantly departing from the scope of the disclosure. 
     Also, various functions described below may be embodied or supported according to Artificial Intelligence (AI) technology and/or one or more computer programs, and each of the one or more computer programs is implemented as computer-readable program code and executed in a computer-readable medium. The terms “application” and “program” indicate one or more computer programs, software components, a set of instructions, procedures, functions, objects, classes, instances, relevant data, or some of the applications and programs appropriate to realize computer-readable program code. The expression “computer-readable program code” includes all types of computer code including source code, object code, and execution code. The expression “computer-readable medium” includes all types of media, for example, Read Only Memory (ROM), Random Access Memory (RAM), a hard disk drive, compact disks (CDs), digital video disks (DVDs), or other types of memory, which are accessible by a computer. A “non-transitory” computer-readable recording medium excludes wired, wireless, optical, or other communication links used to transmit transitory electrical or other signals. Examples of the non-transitory computer-readable recording medium include a medium in which data may be permanently stored, and a medium, for example, a rewriteable optical disk or a removable memory device, in which data may be stored and overwritten later. 
     In one or more embodiments described below, a hardware approach method is described. However, various embodiments include technology using both hardware and software, and thus, the embodiments of the disclosure do not exclude a software-based approach method. 
     Referring to  FIG.  1   , the wireless communication system  10  may include first device D 1 _ 1 , second device D 1 _ 2 , third device D 2 _ 1 , fourth device D 2 _ 2 , fifth device D 2 _  3 , and sixth device D 2 _ 4 . The first device D 1 _ 1  and the second device D 1 _ 2  may access a network  13  that may include the Internet, an Internet protocol (IP) network, or another arbitrary network. The first device D 1 _ 1  may access the network  13  in a first coverage area  11 . The first device D 1 _ 1  may provide access to the network  13  to devices that are within the first coverage area  11  (e.g., the third device D 2 _ 1 , the fourth device D 2 _ 2 , the fifth device D 2 _ 3 , and the sixth device D 2 _ 4 ), and the second device D 1 _ 2  may provide access to the network  13  to devices that are within a second coverage area  12  (e.g., the third device D 2 _ 1  and the fourth device D 2 _ 2 ). 
     In some embodiments, based on Wireless Fidelity (Wi-Fi, e.g., 802.11) or other arbitrary WLAN access technology, the first device D 1 _ 1  and the second device D 1 _ 2  may communicate with at least one of the third to sixth devices (e.g., D 2 _ 1 , D 2 _ 2 , D 2 _ 3 , D 2 _ 4 ) by using multiple links. The first device D 1 _ 1  and the second device D 1 _ 2  may each correspond to an access point multiplelinks device (which may also be referred to as an access point multiple-links device) (AP MLD), and the third to sixth devices (e.g., D 2 _ 1 , D 2 _ 2 , D 2 _ 3 , D 2 _ 4 ) may each correspond to a non-AP MLD. In the disclosure, an AP MLD may refer to a device capable of supporting access points (APs), and a non-AP MLD may refer to a device capable of supporting stations (STAs). 
     In some embodiments, the first device D 1 _ 1  and the second device D 1 _ 2  may each be referred to as a router, a gateway, or the like, and the third to sixth devices (e.g., D 2 _ 1 , D 2 _ 2 , D 2 _ 3 , D 2 _ 4 ) may each be referred to as a terminal, a mobile terminal, a wireless terminal, user equipment, or the like. Also, the third to sixth devices (e.g., D 2 _ 1 , D 2 _ 2 , D 2 _ 3 , D 2 _ 4 ) may each be a mobile device, such as, but not limited to, a mobile phone, a laptop computer, or a wearable device, or a stationary device, such as, but not limited to, a desktop computer or a smart television (TV). 
     The AP MLD may assign at least one resource unit (RU) to at least one non-AP MLD. The AP MLD may transmit data to the non-AP MLD by using the assigned at least one RU, and the non-AP MLD may receive the data from the AP MLD by using the assigned at least one RU. According to 802.11be (hereinafter, referred to as EHT) or next-generation IEEE 802.11 standards (hereinafter, referred to as EHT+), an AP MLD may assign 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 assign an MRU to at least one of the third to sixth devices (e.g., D 2 _ 1 , D 2 _ 2 , D 2 _ 3 , D 2 _ 4 ) and may transmit data thereto by using the assigned MRU. 
     In some embodiments, in performing communication using multiple links by the AP MLD and the non-AP MLD, a protection mechanism for securing the reliability of data communication may be selectively activated in their respective links. For example, in the communication between the AP MLD and the non-AP MLD, the protection mechanism may be activated in communication using a first link and may be deactivated in communication using a second link. The AP MLD may adaptively control at least one of a transmission timing and content of arbitrary frame so as to effectively perform signaling with the non-AP MLD, the signaling being defined in the protection mechanism. In some embodiments, the non-AP MLD may also adaptively control at least one of a transmission timing and content of arbitrary frame so as to effectively perform signaling with the AP MLD, the signaling being defined in the protection mechanism. Hereinafter, for convenience of explanation, an operation of the AP MLD is mainly described, but this is merely an example. That is, one or more embodiments are not limited thereto, and embodiments may be applied to a non-AP MLD. 
     Hereinafter, embodiments are described referring to an EHT, but it is to be understood that embodiments may be applied to other protocol standards (e.g., EHT+) without deviating from the scope of the disclosure. 
       FIG.  2    is a block diagram of a wireless communication system  20  according to an embodiment. In detail, the block diagram of  FIG.  2    illustrates an AP MLD  100  and a non-AP MLD  200  that 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 an arbitrary device working in the wireless communication system  20  and may be referred to a device for wireless communication or simply as a device. 
     Referring to  FIG.  2   , the AP MLD  100  may include a Radio Frequency Integrated Circuit (RFIC)  110  and a processor  120 . The RFIC  110  may include first to n th  transceivers  111 _ 1  to  111 _ n  (hereinafter, “111” generally) and first to n th  antennas  112 _ 1  to  112 _ n  (hereinafter, “112” generally), where n is an integer greater than  1 . The RFIC  110  and the processor  120  may be included in one package or different packages, respectively. The non-AP MLD  200  may include an RFIC  210  and a processor  220 . The RFIC  210  may include first to m th  transceivers  211 _ 1  to  211 _m (hereinafter, “211” generally) and first to m th  antennas  212 _ 1  to  212 _m (hereinafter, “212” generally), where m is an integer greater than 1. Hereinafter, descriptions regarding the non-AP MLD  200  that are the same as the descriptions regarding the AP MLD  100  are omitted. 
     The antennas  112  may be respectively coupled to the transceivers  111  and may provide signals from the non-AP MLD  200  to the transceivers  111  or transmit signals from the transceivers  111  to the non-AP MLD  200 . In some embodiments, the antennas  112  may include a phased array for beam forming. 
     The transceivers  111  may process signals transmitted from the non-AP MLD  200  through the antennas  112  and may provide the processed signals to the processor  120 . Also, the transceivers  111  may process signals provided from the processor  120  and output the processed signals through the antennas  112 . In some embodiments, the transceivers  111  may include analog circuits, such as, but not limited to, a low noise amplifier, a mixer, a filter, a power amplifier, an oscillator, or the like. The transceivers  111  may process signals from the antennas  112  and/or the processor  120  under control by the processor  120 . 
     In an embodiment, the AP MLD  100  may perform ML-based communication with the non-AP MLD  200  by using the transceivers  111  and the antennas  112 . Frequency bands, to which multiple links ML are respectively assigned, may be different from one another, and the AP MLD  100  may support n APs through a control operation of the processor  120  that utilizes the transceivers  111  and the antennas  112 . For example, the AP MLD  100  may support a first AP including the first transceiver  111 _ 1  and the first antenna  112 _ 1  and controlled by the processor  120  and a second AP including the second transceiver  111 _ 2  and the second antenna  112 _ 2  and controlled by the processor  120 . 
     In some embodiments, the AP MLD  100  may further include first individual processors respectively corresponding to n APs, and the processor  120  may control the first individual processors so that the n APs may respectively perform operations according to embodiments. For example, the AP MLD  100  may support a first AP including a first individual processor, the first transceiver  111 _ 1 , and the first antenna  112 _ 1  and a second AP including a second individual processor, the second transceiver  111 _ 2 , and the second antenna  112 _ 2 . 
     The processor  120  may extract information by demodulating and/or decoding signals that are transmitted from the non-AP MLD  200  by processing the signals from the transceivers  111 . Also, the processor  120  may generate signals including information to be transmitted to the non-AP MLD  200  and may provide the generated signals to the transceivers  111 . For example, the processor  120  may provide the transceivers  111  with signals generated by coding and/or modulating data to be transmitted to the non-AP MLD  200 . In some embodiments, the processor  120  may include a programmable component, such as a central processing unit (CPU) or a digital signal processor (DSP), a reconfigurable component, such as a field programmable gate array (FPGA), or a component, e.g., an intellectual property (IP) core, which provides a fixed function. In some embodiments, the processor  120  may include a memory, in which data and/or a series of instructions are stored, or may access the memory. 
     In an embodiment, the non-AP MLD  200  may perform the ML-based communication with the AP MLD  100  by using the transceivers  211  and the antennas  212 . The non-AP MLD  200  may support m STAs through a control operation of the processor  220  that utilizes the transceivers  211  and the antennas  212 . For example, the non-AP MLD  200  may support a first STA including the first transceiver  211 _ 1  and the first antenna  212 _ 1  and controlled by the processor  220  and a second STA including the second transceiver  211 _ 2  and the second antenna  212 _ 2  and controlled by the processor  220 . In some embodiments, the number of STAs supported by the non-AP MLD  200  may be less than or equal to the number of AP MLDs  100 , and the number of links ML may correspond to the number of STAs supported by the non-AP MLD  200 . 
     In some embodiments, the non-AP MLD  200  may further include second individual processors respectively corresponding to m STAs, and the processor  220  may control the second individual processors so that the m STAs may respectively perform operations according to embodiments. For example, the non-AP MLD  200  may support a first STA including a first individual processor, the first transceiver  211 _ 1 , and the first antenna  212 _ 1 , and a second STA including a second individual processor, the second transceiver  211   2 , and the second antenna  212 _ 2 . 
     In an embodiment, the processor  120  may include frame alignment circuitry  121 . The frame alignment circuitry  121  may adaptively control at least one of a timing and content of an arbitrary frame so as to effectively perform signaling of a protection mechanism for securing the reliability of communication using multiple links ML, the communication being performed between the AP MLD  100  and the non-AP MLD  200 . In detail, the AP MLD  100  and the non-AP MLD  200  may mutually transmit/receive a Request To Send (RTS) frame and a Clear To Send (CTS) frame according to the protection mechanism and may mutually transmit/receive data according to a transmission/reception result. In this case, the frame alignment circuitry  121  may determine at least one of a transmission timing and content of an RTS frame to enable the AL MLD  100  to receive mutually aligned CTS frames from the non-AP MLD  200  through at least two of the multiple links ML, and may generate an RTS frame based on a result of the determination, thereby transmitting the generated RTS frame to the non-AP MLD  200  through the RFIC  110 . 
     In some embodiments, the processor  220  of the non-AP MLD  200  may also include frame alignment circuitry configured to perform a similar or identical operation to the frame alignment circuitry  121  of the AP MLD  100 . 
       FIG.  3    is a block diagram of a wireless communication system  30  according to an embodiment. In detail, the block diagram of  FIG.  3    illustrates an AP MLD  300  and a non-AP MLD  400  that communicate with each other by using first to third links L 1  to L 3  in the wireless communication system  30 . The AP MLD  300  may include or may be similar in many respects to at least one of the AP MLD  100  described in reference to  FIG.  2    and the first device D 1 _ 1  and the second device D 1 _ 2  described in reference to  FIG.  1   , and may include additional features not mentioned above. The non-AP MLD  400  may include or may be similar in many respects to at least one of the non-AP MLD  200  described in reference to  FIG.  2    and the third to sixth devices (e.g., D 2 _ 1 , D 2 _ 2 , D 2 _ 3 , D 2 _ 4 ) described in reference to  FIG.  1   , and may include additional features not mentioned above. 
     In an embodiment, the AP MLD  300  may include a first access point AP 1 , a second access point AP 2 , and a third access point AP 3 , and the non-AP MLD  400  may include a first station STA 1 , a second station STA 2 , and a third station STA 3 . In some embodiments, unlike the illustration of  FIG.  3   , the AP MLD  300  and the non-AP MLD  400  may communicate with each other by using three or more links and may respectively include APs and STAs of which the number corresponds to the number of links. 
     In some embodiments, the first access point AP 1  may setup a first link L 1  with first station STA 1 , the second access point AP 2  may setup a second link L 2  with second station STA 2 , and the third access point AP 3  may setup a third link L 3  with third station STA 3 . However, the disclosure is not limited in this regard. That is, the access points AP 1 -AP 3  may setup any number of links with any of the stations STA 1 -STA 3 . 
     For example, the AP MLD  300  and the non-AP MLD  400  may perform an access process and/or a negotiation process for a multiple-links operation. The non-AP MLD  400  may identify a frequency band usable in the communication with the AP MLD  300  and may negotiate with the AP MLD  300  to use two or more of the links, which are supported by the AP MLD  300 , for the multiple links. As described above, a method whereby two or more links are used for mutual communication may be referred to as a multiple-links operation (MLO). According to the MLO, the wireless communication system  30  may provide a high throughput. 
       FIGS.  4 A and  4 B  are timing diagrams of an example of an MLO. In detail,  FIGS.  4 A and  4 B  illustrate examples of an MLO performed by the wireless communication system  20  of  FIG.  2    and/or the wireless communication system  30  of  FIG.  3   . Hereinafter, descriptions regarding  FIGS.  4 A and  4 B  are provided with reference to  FIG.  3   . 
     The AP MLD  300  and the non-AP MLD  400  of  FIG.  3    may support Simultaneous Transmit And Receive (STR). For example, the AP MLD  300  may receive data from the non-AP MLD  400  through the second link L 2  while transmitting the data to the non-AP MLD  400  through the first link L 1  at the same time. The first to third APs AP 1  to AP 3  may have different Medium Access Control (MAC) addresses and manage the first to third links L 1  to L 3 , respectively. Accordingly, each of the first to third APs AP 1  to AP 3  may function as an independent AP. Also, the first to third STAs STA 1  to STA 3  may have different MAC addresses and function as independent STAs, respectively. The AP MLD  300  and the non-AP MLD  400  may mutually communicate 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., 160 MHz) in a 6 GHz band. 
     Referring to  FIG.  4 A , at time t 1 , the first AP AP 1  may start transmitting first data D 1  on the first link L 1 , and the first STA STA 1  may receive the first data D 1 . While the first AP AP 1  transmits the first data D 1  to the first STA STA 1 , the second STA STA 2  may start transmitting second data D 2  on the second link L 2  to the second AP AP 2 , and the second AP AP 2  may receive the second data D 2  from the second STA STA 2  at time t 21 . Although the transmission of the first data D 1  overlaps the transmission of the second data D 2  in a time domain, 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  from the first AP AP 1  and the second STA STA 2 , according to the STR. Also, while the first AP AP 1  transmits the first data D 1  to the first STA STA 1  and the second STA STA 2  transmits the second data D 2  to the second AP AP 2 , the third AP AP 3  may start transmitting third data D 3  on the third link L 3  to the third STA STA 3 , and the third STA STA 3  may receive the third data D 3  from the third AP AP 3 , at time t 31 . 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 each other in the time domain, the first STA STA 1 , the second AP AP 2 , and the third STA STA 3  may respectively receive the first data D 1 , the second data D 2 , and the third data D 3  according to the STR. 
     In addition, there may be links in which the STR is difficult to be performed in the MLO. For example, when frequency bands of links are not sufficiently spaced, interference may occur between the links, and thus, the links may be links on which the STR may be difficult to be performed. Also, when interference occurs between links because of various reasons including a structural limitation resulting from the weight and size reduction of hardware in the non-AP MLD, it may be difficult to perform the STR on the above links. As described, the links in the MLO may include a non-STR link group including the links on which the STR may be difficult to be performed, and in particular, two links, on which the STR is difficult to be performed because of mutual interference in the 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  to L 3 . 
     Referring to  FIG.  4 B , it is assumed that the first link L 1  and the second link L 2  form a non-STR link pair. For example, the first link L 1  may use a 6 GHz band, and the second link L 2  may use a 5 GHz band. At time t 12 , the first AP AP 1  may obtain a transmit opportunity (TXOP) and start transmitting the first data D 1  on the first link L 1  to the first STA STA 1 , and the first STA STA 1  may receive the first data D 1  from the first AP AP 1 . At time t 22 , the second STA STA 2  may start transmitting the second data D 2  on the second link L 2 , but when the first link L 1  and the second link L 2  are the non-STR link pair, the interference may occur between the transmission of the second data D 2  and the reception of the first data D 1  in the non-AP MLD  400 . For example, the first data D 1  is leaked in the second STA STA 2  through a path in which the second data D 2  is transmitted to the second AP AP 2 , and the second data D 2  may not be appropriately transmitted to the second AP AP 2  through the second link L 2 . The above interference may be caused during the signaling of the 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  described with reference to the drawings below are a non-STR link pair. 
       FIG.  5    is a timing diagram including signaling based on the protection mechanism in an MLO. In detail,  FIG.  5    illustrates an example of the MLO performed by the wireless communication system  20  of  FIG.  2    and/or the wireless communication system  30  of  FIG.  3   . Hereinafter, the description regarding  FIG.  5    is provided with reference to  FIG.  3   , and it is assumed that the protection mechanism is activated in the first and second links L 1  and L 2 . 
     Referring to  FIG.  5   , at time t 13 , the first AP AP 1  may start transmitting the first RTS frame RTS 1  through 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 a time t 23 , the first STA STA 1  may start transmitting the first CTS frame CTS 1  through 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 SIFS from a time t 53 , the first AP AP 1  may start transmitting a first aggregated-MAC protocol data unit (A-MPDU) A-MPDU 1  through the first link L 1  in response to the first CTS CTS 1 , and the first STA STA 1  may receive the first A-MPDU A-MPDU 1 . 
     In the time t 23 , the second AP AP 2  may start transmitting a second RTS frame RTS 2  through the second link L 2 , and the second STA STA 2  may receive the second RTS frame RTS 2 . In the time t 53 , after SIFS from the time t 43 , the second STA STA 2  may start transmitting the second CTS frame CTS 2  through the second link L 2  in response to the second RTS frame RTS 2 , and the second AP AP 2  may receive the second CTS frame CTS 2 . At time t 83 , after the SIFS from a time t 73 , the second AP AP 2  may start transmitting a second A-MPDU A-MPDU 2  through 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 . 
     Because the first link L 1  and the second link L 2  form the non-STR link pair in the time t 33  to the time t 43 , the interference may occur between the transmission of the first CTS frame CTS 1  and the reception of the second RTS frame RTS 2  in the non-AP MLD  400 . Because of the above interference, the signaling of the protection mechanism may not be smoothly performed, which may negatively affect the reliability of the data communication. 
     The AP MLD  300  may determine at least one of the transmission timing and the content of the first RTS frame RTS 1  and avoid the 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  to align the first CTS frame CTS 1  with the second CTS frame CTS 2 , thus transmitting the generated first RTS frame RTS 1  to the non-AP MLD  400  through the first link L 1 . A method whereby the AP MLD  300  generates the first RTS frame RTS 1  may vary, and examples of the method are described with reference to  FIGS.  6 A to  13 B . Hereinafter, various embodiments are independently described, but they are merely examples. It would be understood that various embodiments may be applied in combination according to the idea of the disclosure. 
       FIGS.  6 A and  6 B  are flowcharts of an operation method of an AP MLD (e.g., AP MLD  100  of  FIG.  2   , AP MLD  300  of  FIG.  3   ), according to an embodiment. It is assumed that a k th  AP and other APs are included in the 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, where k is an integer equal to or greater than  1 . In the disclosure, an operation of a certain AP may be understood as an operation of the AP MLD. Also, in the disclosure, a transmission preparation period of an RTS frame may refer to a period in which a transmission timing and content of the RTS frame are determined and may be set before the transmission timing of the RTS frame has occurred. The transmission preparation period of the RTS frame is temporarily named for the understanding of embodiments, and thus, one or more embodiments are not limited thereto. Also, in some embodiments, the AP MLD may not separately include a transmission preparation period of an RTS frame and may perform, in an arbitrary period, an operation of generating an RTS frame according to an embodiment. For example, the entry to the transmission preparation period of the RTS frame may be made in response to the a back-off count for the transmission of the RTS frame of the AP reaching a reference value. For example, when the back-off count of the AP gradually decreases from a certain value and reaches a reference value, the AP may enter the transmission preparation period of the RTS frame. Also, in some embodiments, the transmission preparation period of the RTS frame may include a space in which the transmission of the RTS frame is waited for. 
     In operation S 110 , the k th  AP may obtain information regarding transmissions of RTS frames of other APs except for the k th  AP. It may be limited that other APs respectively correspond to links having a non-STR link pair relationship with a k th  link corresponding to the k th  AP. In an embodiment, the information regarding the transmissions of the RTS frames of other APs may include whether the protection mechanism is activated in links respectively corresponding to the other APs and back-off counts of other APs for the transmission of the RTS frame. In a transmission preparation period of a k th  RTS frame, the k th  AP may check in advance other APs capable of transmitting the RTS frames and an RTS frame having a transmission timing close to a transmitting timing of the k th  frame, based on the information regarding the transmissions of the RTS frames. 
     In operation S 120 , the k th  AP may determine the transmission timing of the k th  RTS frame, based on the information obtained in operation S 110 . In an embodiment, the k th  AP may delay the transmission timing of the k th  RTS frame to make the transmission timing of the k th  RTS frame correspond to the transmission timing of the RTS frame of another AP. To this end, the k th  AP may transmit the k th  RTS frame at the same timing as the transmission timing of the RTS frame of another AP. 
       FIG.  6 B  is a flowchart illustrating in detail operation S 120  of  FIG.  6 A . Referring further 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. In this case, the RTS frame that may be aligned with the k th  RTS frame may indicate an RTS frame scheduled to be transmitted to have a start and an end that may be respectively aligned with a start and an end of the k th  RTS frame. In an embodiment, the k th  AP may identify an RTS frame of another AP having a transmission timing close to a transmission timing of the k th  RTS frame. In an embodiment, whether the transmission timing of the RTS frame is close to that of the k th  RTS frame may be identified based on whether a difference between back-off counts of other APs for the transmission of the RTS frame and a back-off count of the k th  AP is included in a reference difference. 
     When a result from S 121  is ‘YES,’ operation S 122  may be subsequently performed, and the k th  AP may defer the transmission of the k th  RTS frame of the k th  AP. 
     In operation S 123 , the k th  AP may end the transmission preparation period of the k th  RTS frame stated in operation S 100  and transmit the k th  RTS frame at a transmission timing of an alignment target RTS frame of the other AP that is identified in operation S 121 . In some 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 defer the transmission of the k th  RTS frame and the RTS frames, which may be aligned with the k th  RTS frame, according to a transmission timing of an RTS frame that is the latest among transmission timings of the RTS frames. 
     When a result from operation S 121  is ‘NO,’ operation S 124  may be subsequently performed, and the k th  AP may end the transmission preparation period of the k th  RTS frame in operation S 100  and immediately transmit the k th  RTS frame. 
       FIG.  7    is a timing diagram of an example of an MLO according to an embodiment. 
     Referring to  FIG.  7   , a back-off count BC 1  of the first AP AP 1  corresponding to the first link L 1  may decrease while the first link L 1  is not busy, and a back-off count BC 2  of the second AP AP 2  corresponding to the second link L 2  may decrease while the second link L 2  is not busy. At time t 14 , the first link L 1  may enter a busy state according to other transmissions, and the back-off count BC 1  of the first AP AP 1  may stop decreasing. Similarly, at time t 24 , the second link L 2  may enter a busy state according to other transmissions, and the back-off count BC 2  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 thus, the back-off count BC 1  of the first AP AP 1  may gradually decrease from 4, for example. In some 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 thus, the back-off count BC 2  of the second AP AP 2  may gradually decrease from 3, for example. Also, 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 BC 1  reaching 1, for example. However, this is merely an example, and one or more embodiments 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 the back-off count BC 1  reaching another arbitrary reference value. 
     In the transmission preparation period TPP, the first AP AP 1  may obtain transmission-related information of the second RTS frame RTS 2  of the second AP AP 2 . In detail, the first AP AP 1  may obtain the back-off count BC 2  of the second AP AP 2 . In this case, the first AP AP 1  may identify that a difference between the back-off count BC 1  of the first AP AP 1  (e.g., 1) in the time t 44  and the back-off count BC 2  of the second AP AP 2  (e.g., 3) is included in a reference difference and may determine the second RTS frame RTS 2  to be an alignment target RTS frame. In the transmission preparation period TPP, the first AP AP 1  may set a standby space SS from a time t 54  to a time t 64 , based on the difference between the back-off count BC 1  of the first AP AP 1  (e.g., 1) in the time t 44  and the back-off count BC 2  of the second AP AP 2  (e.g., 3) and may delay the transmission of the first RTS frame RTS 1  by as much as the standby space SS. In an embodiment, a unit length of the standby space SS may correspond to a unit length of a back-off count. 
     In the time t 64 , because the back-off count BC 2  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 , and the first AP AP 1  may end the transmission preparation period TPP and transmit the first RTS frame RTS 1  to the first STA STA 1  through the first link L 1 . 
     When the first STA STA 1  senses the first link L 1  and identifies that the first link L 1  is in a ready state, 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  at time t 94  after SIFS from the time t 84 . When the second STA STA 2  senses the second link L 2  and identifies that the second link L 2  is in a ready state, 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  in the time t 94  after the SIFS from the time t 84 . Accordingly, the first CTS frame CTS 1  and the second CTS frame CTS 2  may be aligned with each other. In some embodiments, an operation of sensing the first link L 1  by the first STA STA 1  and an operation of sensing the second link L 2  by the second STA 2  may be performed between a time t 74  and a time t 84 . In the disclosure, an operation of sensing a link may be referred to as a link sensing operation. 
     At time t 104  after the SIFS from the time t 94 , 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 , and the second AP AP 2  may transmit the 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 of an operation method of an AP MLD (e.g., AP MLD  100  of  FIG.  2   , AP MLD  300  of  FIG.  3   ), according to an embodiment. It is assumed that the k th  AP and other APs described below are included in the AP MLD. Also, hereinafter, a case where an RTS frame is transmitted from the AP MLD to a non-AP MLD is mainly described, but one or more embodiments are not limited thereto. It is to be fully understood that the disclosure may also be applied to a case where an RTS frame is transmitted from the non-AP MLD to the AP MLD without departing from the scope of the disclosure. Hereinafter, the descriptions already provided with reference to  FIGS.  6 A and  6 B  are omitted. 
     Referring to  FIG.  8 A , in operation S 200 , the k th  AP may enter the transmission preparation period of the k th  RTS frame of the k th  AP. 
     In operation S 210 , the k th  AP may obtain transmission-related information of RTS frames of other APs except for the k th  AP. It may be limited that other APs correspond to links having a non-STR link pair relationship with the k th  link corresponding to the k th  AP. 
     In operation S 220 , the k th  AP may generate a k th  extended RTS (E-RTS) frame based on the information obtained in operation S 210 . In an embodiment, the k th  AP may determine an additional length of the k th  RTS frame based on a transmission timing of the RTS frame of another AP and may add, to the k th  RTS frame, padding data corresponding to the additional length, thereby generating the k th  E-RTS frame. 
     In operation S 230 , the k th  AP may transmit the k th  E-RTS frame after the transmission preparation ends. Then, another 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.  8 B  is a flowchart illustrating in detail operations S 220  and S 230  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 identify whether there is an RTS frame that may be aligned with the k th  RTS frame. In this case, the RTS frame, which may be aligned with the k th  RTS frame, may indicate an RTS frame scheduled to be transmitted to have an end that may be aligned with the end of the k th  RTS frame. 
     When a result from operation S 221  is ‘YES,’ operation S 222  may be subsequently performed, and the k th  AP may generate the k th  E-RTS frame from the k th  RTS frame. In detail, the k th  AP may determine an additional length based on a difference between a transmission timing of the alignment target RTS frame of the identified AP and the transmission timing of the k th  RTS frame of the k th  AP, generate padding data corresponding to the additional length, and add the padding data to the k th  RTS frame, thus generating the k th  E-RTS frame. In some embodiments, in operation S 221 , 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 add the padding data respectively to the k th  RTS frame and the alignment target RTS frames, based on the latest transmission timing of the RTS frame, and may generate the k th  E-RTS frame and alignment target E-RTS frames. 
     In operation S 231 , the k th  AP may transmit the k th  E-RTS frame. The k th  E-RTS frame may be greater in length than the alignment target RTS frame. 
     When the result from operation S 221  is ‘NO,’ operation S 232  may be subsequently performed, and the AP MLD may transmit the k th  RTS frame. The k th  RTS frame may have the same length as the alignment target RTS frame. 
       FIG.  9    is a timing diagram of an example of an MLO according to an embodiment. Hereinafter, descriptions that are the same as the descriptions regarding  FIG.  7    are omitted. 
     Referring to  FIG.  9   , at time t 15 , the first link L 1  may enter a busy state according to other transmissions, and the back-off count BC 1  of the first AP AP 1  may stop decreasing. Similarly, at time t 25 , the second link L 2  may enter a busy state according to other transmissions, and the back-off count BC 2  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 thus, the back-off count BC 1  of the first AP AP 1  may gradually decrease from 4, for example. Similarly, at time t 45 , the busy state of the second link L 2  may be released, and thus, the back-off count BC 2  of the second AP AP 2  may gradually decrease from 3, for example. Also, in the time t 45 , the first AP AP 1  may enter the transmission preparation period TPP of the first RTS frame in response to the back-off count BC 1  of the first AP AP 1  reaching  1 , for example. 
     In the transmission preparation period TPP, the first AP AP 1  may obtain the transmission-related information of the second RTS frame RTS 2  of the second AP AP 2 . In detail, the first AP AP 1  may obtain information, which indicates that the protection mechanism is activated in the second link L 2  corresponding to the second AP AP 2 , and the back-off count BC 2  of the second AP AP 2 . In this case, the first AP AP 1  may identify that the difference between the back-off count BC 1  of the first AP AP 1  (e.g., 1) and the back-off count BC 2  of the second AP AP 2  (e.g., 3) in the time t 45  is included in the reference difference and may determine the second RTS frame RTS 2  as the alignment target RTS frame. In the transmission preparation period TPP, the first AP AP 1  may determine an additional length regarding the first RTS frame, based on the difference between the back-off count BC 1  of the first AP AP 1  (e.g., 1) and the back-off count BC 2  of the second AP AP 2  (e.g., 3) in the time t 45 , and may add padding data corresponding to the additional length to the first RTS frame, thus generating the first E-RTS frame E-RTS 1 . 
     In the time T 55 , because the back-off count BC 1  of the first AP AP 1  reaches 0, the first AP AP 1  may end the transmission preparation period TPP and transmit the first E-RTS frame E-RTS 1  to the first STA STA 1  through the first link L 1 . 
     At time t 65 , because the back-off count BC 2  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 . The transmission timing of the first E-RTS frame E-RTS 1  is different from the transmission timing of the second RTS frame RTS 2 , but an end of the first E-RTS frame E-RTS 1  may be aligned with an end of the second RTS frame RTS 2 . 
     When the first STA STA 1  senses the first link L 1  and identifies that the first link L 1  is in a ready state, 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  in the time t 95  after the SIFS from the time t 85 . When the second STA STA 2  senses the second link L 2  and identifies that the second link L 2  is in a ready state, 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  in the time t 95  after the SIFS from the time t 85 . 
     At time t 105  after the SIFS from the time t 95 , 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 , and the second AP AP 2  may transmit the second A-MPDU A-MPDU 2  to the second STA STA 2  through the second link L 2 . 
       FIGS.  10 A and  10 B  are flowcharts of an operation method of an AP MLD, according to an embodiment. It is assumed that the k th  AP and other APs described below are included in the AP MLD. Also, hereinafter, a case where the RTS frame is transmitted from the AP MLD to the non-AP MLD is mainly described, but one or more embodiments are not limited thereto. It is to be fully understood that the disclosure may be applied to a case where the RTS frame is transmitted from the non-AP MLD to the AP MLD without departing from the scope of the disclosure. Hereinafter, descriptions that are the same as the descriptions regarding  FIGS.  6 A and  6 B  are omitted. 
     Referring to  FIG.  10 A , in operation S 300 , the k th  AP may enter a transmission preparation period of the k th  RTS frame of the k th  AP. 
     In operation S 310 , the k th  AP may obtain transmission-related information of RTS frames of other APs except for the k th  AP. It may be limited that other APs correspond to links having a non-STR link pair relationship with a k th  link corresponding to the k th  AP. 
     In operation S 320 , the k th  AP may generate a k th  cross link-RTS (CL-RTS) frame based on the information obtained in operation S 310 . In an embodiment, the k th  AP may determine cross-link-related information of the k th  RTS frame based on a transmission timing of the RTS frame of another AP and may generate the k th  CL-RTS frame by filling data matching cross-link-related information determined with regard to some subfields of the k th  RTS frame. In an embodiment, the cross-link-related information may include at least one of link indices indicating links, information used to indicate whether each link needs to be sensed, and information regarding resources assigned for the transmission of a CTS frame of each link. 
     In operation S 330 , the k th  AP may end the transmission preparation period and transmit the k th  CL-RTS frame. The transmission of the k th  CL-RTS frame may replace the transmission of the RTS frame of another AP, and thus, the transmission of the RTS frame of the other AP may be omitted. 
       FIG.  10 B  is a flowchart illustrating operation S 320  of  FIG.  10 A  in detail. Referring further to  FIG.  10 B , in operation S 321  performed after operation S 310  ( FIG.  10 A ), the k th  AP may detect a link that needs to be sensed from among links except for the k th  link corresponding to the k th  AP. In the disclosure, sensing of a link may denote an operation of sensing whether the link is in a busy state. In an embodiment, the k th  AP may detect a link through which transmission of an RTS frame is scheduled, the RTS frame having a transmission timing close to the transmission timing of the k th  RTS frame. 
     In operation S 322 , the k th  AP may generate the k th  CL-RTS frame based on a result of the detection obtained in operation S 321 . For example, the k th  CL-RTS frame may be generated by filling, in some of the subfields of the k th  RTS frame, information indicating that link sensing is required for a link index corresponding to the detected link and information regarding resources assigned for the transmission of the CTS frame. Then, operation S 330  ( FIG.  10 A ) may be subsequently performed. 
       FIG.  11    is a table diagram describing cross-link-related information INFO according to an embodiment. 
     Referring to  FIG.  11   , the cross-link-related information INFO may include ‘Link Index’ indicating each link, ‘Link Sensing Required’ indicating whether each link needs to be sensed, and ‘RU Allocation’ indicating resources assigned to transmit a CTS frame of each link. The cross-link-related information INFO may further include information required to transmit a CTS frame of an AP (e.g., the AP detected in operation S 321  of  FIG.  10 B ) that does not directly receive an RTS frame through a link assigned to the AP, and there may be a ‘Reserved’ space to enable such information to be arranged later. The format of the RTS frame may further include subfields related to cross-links in which the cross-link-related information INFO is filled. 
       FIG.  12    is a flowchart of an operation method of an AP MLD  500  and a non-AP MLD  600 , according to an embodiment. In detail, the AP MLD  500  may include a k th  AP APk and other APs, and the non-AP MLD  600  may include a k th  STA STAk and other STAs communicating with the k th  AP APk through a k th  link. The AP MLD  500  may include or may be similar in many respects to at least one of the AP MLD  300  of  FIG.  3   , the AP MLD  100  of  FIG.  2    and the first device D 1 _ 1  and the second device D 1 _ 2  of  FIG.  1   , and may include additional features not mentioned above. The non-AP MLD  600  may include or may be similar in many respects to at least one of the non-AP MLD  400  of  FIG.  3   , the non-AP MLD  200  of  FIG.  2   , and the third to sixth devices (e.g., D 2 _ 1 , D 2 _ 2 , D 2 _ 3 , D 2 _ 4 ) of  FIG.  1   , and may include additional features not mentioned above. 
     Hereinafter, a case where an RTS frame is transmitted from the AL MLD  500  to the non-AP MLD  600  is mainly described, but one or more embodiments are not limited thereto. It is to be fully understood that the disclosure may be applied to a case where the RTS frame is transmitted from the non-AP MLD  600  to the AP MLD  500  without departing from the scope of the disclosure. 
     Referring to  FIG.  12   , in operation S 400 , 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 that needs to be sensed in response to the k th  CL-RTS frame from among other links APs and indicate resources for the transmission of the CTS frame. The AP MLD  500  may omit the transmission of the RTS frame through a link that is in a cross-link relationship with the k th  link. In the disclosure, when the second link is sensed based on the CL-RTS frame transmitted through the first link, the first link and the second link may be referred to as being in a mutual cross-link relationship. That is, because the k th  CL-RTS frame is used to sense a link having a cross-link relationship with the k th  link, the transmission of the RTS frame through the link may be omitted. 
     In operation S 410 , the non-AP MLD  600  may provide other STAs with cross-link-related information of the k th  CL-RTS frame. 
     In operation S 420 , an STA selected from among the other STAs may perform link sensing based on the cross-link-related information. The selected STA may be an STA corresponding to a link requiring the sensing in the cross-link-related information. In the disclosure, an operation of a certain STA may be interpreted as an operation of the non-AP MLD. 
     The STA selected in operation S 430  may transmit the CTS frame to an AP corresponding to the selected STA from among the other APs, through a link assigned to the STA selected based on the sensing result obtained in operation S 420 . The 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 the sensing result. 
       FIGS.  13 A and  13 B  are timing diagrams of an example of an MLO according to an embodiment. Hereinafter, descriptions that are the same as the descriptions regarding  FIG.  7    are omitted. 
     Referring to  FIG.  13 A , at time t 16 , the first link L 1  may enter a busy state according to other transmissions, and the back-off count BC 1  of the first AP AP 1  may stop decreasing. Similarly, at time t 26 , the second link L 2  may enter a busy state according to other transmissions, and the back-off count BC 2  of the second AP AP 2  may stop decreasing. 
     At time t 36 , the busy state of the first link L 1  may be released, and thus, the back-off count BC 1  of the first AP AP 1  may gradually decrease from 4, for example. Similarly, at time t 45 , the busy state of the second link L 2  may be released, and thus, the back-off count BC 2  of the second AP AP 2  may gradually decrease from 3, for example. Also, at time t 46 , the first AP AP 1  may enter the transmission preparation period TPP of the first RTS frame in response to the back-off count BC 1  reaching 1. 
     In the transmission preparation period TPP, the first AP AP 1  may obtain transmission-related information of the second RTS frame RTS 2  of the second AP AP 2 . In detail, the first AP AP 1  may obtain the back-off count BC 2  of the second AP AP 2 . In this case, the first AP AP 1  may identify that the difference between the back-off count BC 1  of the first AP AP 1  (e.g., 1) and the back-off count BC 2  of the second AP AP 2  (e.g., 3) in the time t 46  is included in the reference difference and may detect the second link L 2  as a link requiring the sensing. In the transmission preparation period TPP, the first AP AP 1  may determine cross-link-related information based on a detection result. In detail, the AP MLD may indicate that the second link L 2  needs to be sensed and determine the cross-link-related information to indicate a resource assigned for the 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 fill data matching the determined cross-link-related information in some of the sub-fields of the first RTS frame and thus may generate the first CL-RTS frame CL-RTS 1 . 
     At time t 56 , because the back-off count BC 1  of the first AP AP 1  reaches 0, the first AP AP 1  may end the transmission preparation period TPP and transmit the first CL-RTS frame CL-RTS 1  to the first STA STA 1  through the first link L 1 . 
     At time t 66 , the back-off count BC 2  of the second AP AP 2  may reach 0, and the transmission of the second RTS frame of the second AP AP 2  may be omitted. 
     At time t 76  to time t 96 , the second STA STA 2  may receive cross-link-related information included in the first CL-RTS frame CL-RTS 1  and perform a sensing operation SO regarding 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 ready state by performing the sensing operation SO. A performance timing and a performance time of the sensing operation SO of  FIG.  13 A  are merely examples, and one or more embodiments are not limited thereto. The sensing operation SO may be performed on the second link L 2  at appropriate timing and time so that the first CTS frame CTS 1  and the second CTS frame CTS 2  may be aligned. 
     When the first STA STA 1  senses the first link L 1  and identifies that the first link L 1  is in a ready state, 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  at time t 96  after the SIFS from the time t 86 . When the second STA STA 2  senses the second link L 2  and identifies that the second link L 2  is in a ready state, 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  in the time t 96  after the SIFS from the time t 86 . 
     At time t 116  after the 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 , and the second AP AP 2  may transmit the second A-MPDU A-MPDU 2  through the second link L 2 . 
     In  FIG.  13 B , an operation from the times t 17  to t 77  may be identical to an operation from the times t 16  to t 66 , and thus, an operation after the time t 77 , which is different from the operation of  FIG.  13 A , is mainly described. 
     Referring further to  FIG.  13 B , 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 CL-RTS 1  and perform the 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 the busy state by performing the sensing operation SO. 
     When the first STA STA 1  senses the first link L 1  and the first link L 1  is in the ready state, 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  in the time t 97  after the SIFS from the time t 87 . When the second STA STA 2  senses the second link L 2  and identifies that the second link L 2  is in the busy state, the second STA STA 2  may not transmit the second CTS frame. 
     At time t 117  after the SIFS from a time t 107 , 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 . 
       FIG.  14    is a block diagram of a system  1000  according to an embodiment. In some embodiments, the system  1000  of  FIG.  14    may be realized as at least one chip, and the system  1000  realized as one chip may be referred to as a system-on-chip. 
     Referring to  FIG.  14   , the system  1000  may include an application processor  1010 , a memory  1020 , an input/output interface  1030 , a hardware accelerator  1040 , and a communication interface  1050 , and the application processor  1010 , the memory  1020 , the input/output interface  1030 , the hardware accelerator  1040 , and the communication interface  1050  may mutually 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 some embodiments, the application processor  1010  may execute an operating system (OS) and applications on the OS. The application processor  1010  may control other components of the system  1000 . For example, the application processor  1010  may instruct the hardware accelerator  1040  to perform jobs by providing data thereto, and may obtain results of the jobs performed by the hardware accelerator  1040 . Also, the application processor  1010  may provide data, which is to be externally transmitted, to the communication interface  1050  and instruct the communication interface  1050  to transmit the data, and may obtain data received from the outside through the communication interface  1050 . 
     The memory  1020  may be accessed by other components through the bus  1060 . The memory  1020  may have an arbitrary structure in which data may be stored, and may include volatile memory such as static random access memory (SRAM) or dynamic random access memory (DRAM) and/or non-volatile memory such as flash memory or resistive random access memory (RRAM). The memory  1020  may store instructions executed by the application processor  1010  or may store data that is read or written by other components. 
     The input/output interface  1030  may provide an interface regarding inputs to the system  1000  and outputs therefrom. 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 an input from a user or provide an output to the user through the input/output device. Also, 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 functions 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 used to communicate with an external subject of the system  1000 . For example, the communication interface  1050  may provide a wired communication channel and/or a wireless communication channel. In some embodiments, the communication interface  1050  may perform at least one operation included in a wireless communication method using the multiple links described with reference to the attached drawings. For example, the communication interface  1050  may include at least one processor  1055 , and the at least one processor  1055  may execute instructions and thus may perform at least one operation included in the wireless communication method using the multiple links. In some 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 some embodiments, the memory  1020  or the memory included in the communication interface  1050  may store information collected with regard to links and may be accessed by the at least one processor  1055 . 
       FIG.  15    is a diagram illustrating examples of a device for mobile communication, according to an embodiment. In detail,  FIG.  15    illustrates home gadgets  2010 , home appliances  2020 , entertainment devices  2030 , and an Internet of Things (IoT) network system including an access point  2040 . 
     In some embodiments, in the devices for wireless communication of  FIG.  15   , as described above with reference to the drawings, an operation using the multiple links may be performed. Accordingly, devices for wireless communication may smoothly perform signaling of a protection mechanism by reducing interference between links in receiving/transmitting RTS frames and CTS frames in a mutual protection mechanism by using a multiple links. Accordingly, high reliability of data communication between devices may be secured. 
     While embodiments of the disclosure has been particularly shown and described, it is to be understood that various changes in form and details may be made therein without departing from the spirit and scope of the following claims.