Patent Publication Number: US-2022231797-A1

Title: Nack frame for harq operation

Description:
TECHNICAL FIELD 
     The present specification relates to a method for transmitting a negative acknowledgment (NACK) frame for a hybrid automatic repeat request (HARQ) operation in a wireless local area network (WLAN) system. 
     RELATED ART 
     A wireless local area network (WLAN) has been enhanced in various ways. For example, the IEEE 802.11ax standard has proposed an enhanced communication environment by using orthogonal frequency division multiple access (OFDMA) and downlink multi-user multiple input multiple output (DL MU MIMO) schemes. 
     The present specification proposes a technical feature that can be utilized in a new communication standard. For example, the new communication standard may be an extreme high throughput (EHT) standard which is currently being discussed. The EHT standard may use an increased bandwidth, an enhanced PHY layer protocol data unit (PPDU) structure, an enhanced sequence, a hybrid automatic repeat request (HARQ) scheme, or the like, which is newly proposed. The EHT standard may be called the IEEE 802.11be standard. 
     SUMMARY 
     A method performed by a station (STA) in a wireless local area network (WLAN) system, according to various embodiments, may include a technical feature related to transmission of a negative acknowledgment (NACK) frame for a hybrid automatic repeat request (HARQ) operation. The transmitting station (STA) may transmit a PHY protocol data unit (PPDU) including a first redundancy version of the transmission data to a receiving STA. The transmitting STA may determine the second redundancy version of the transmission data to be retransmitted based on whether an acknowledgement (ACK) or negative ACK (NACK) frame is received within a threshold time. 
     According to an example of the present specification, when the receiving terminal fails to decode one MPDU or fails to decode all MPDUs included in the A-MPDU, the NACK frame may be transmitted. The receiving terminal may transmit the NACK only when decoding the PHY header of the received PPDU is successful. If the transmitting terminal transmits data and does not receive an ACK/NACK, the transmitting terminal may retransmit the same redundancy version of the transmitted data. If the transmitting terminal receives a NACK, the transmitting terminal may retransmit another redundancy version of the transmitted data. Therefore, the operation performed when decoding the PHY header fails or when the decoding the PHY header is successful may be different. Accordingly, a more efficient HARQ operation may be performed in the WLAN system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example of a transmitting apparatus and/or receiving apparatus of the present specification. 
         FIG. 2  is a conceptual view illustrating the structure of a wireless local area network (WLAN). 
         FIG. 3  illustrates a general link setup process. 
         FIG. 4  illustrates an example of a PPDU used in an IEEE standard. 
         FIG. 5  illustrates a layout of resource units (RUs) used in a band of 20 MHz. 
         FIG. 6  illustrates a layout of RUs used in a band of 40 MHz. 
         FIG. 7  illustrates a layout of RUs used in a band of 80 MHz. 
         FIG. 8  illustrates a structure of an HE-SIG-B field. 
         FIG. 9  illustrates an example in which a plurality of user STAs are allocated to the same RU through a MU-MIMO scheme. 
         FIG. 10  illustrates an operation based on UL-MU. 
         FIG. 11  illustrates an example of a trigger frame. 
         FIG. 12  illustrates an example of a common information field of a trigger frame. 
         FIG. 13  illustrates an example of a subfield included in a per user information field. 
         FIG. 14  describes a technical feature of the UORA scheme. 
         FIG. 15  illustrates an example of a channel used/supported/defined within a 2.4 GHz band. 
         FIG. 16  illustrates an example of a channel used/supported/defined within a 5 GHz band. 
         FIG. 17  illustrates an example of a channel used/supported/defined within a 6 GHz band. 
         FIG. 18  illustrates an example of a PPDU used in the present specification. 
         FIG. 19  illustrates an example of a modified transmission device and/or receiving device of the present specification. 
         FIG. 20  is a diagram illustrating an example of chase combining. Chase combining is a method in which the same coded bit as in the initial transmission is retransmitted. 
         FIG. 21  is a diagram illustrating an example of an incremental redundancy (IR) method. 
         FIGS. 22 to 28  are diagrams illustrating embodiments of a NACK frame. 
         FIGS. 29 and 30  are diagrams illustrating HARQ transmission/retransmission procedures when a NACK frame is not used. 
         FIGS. 31 to 34  are diagrams illustrating HARQ transmission/retransmission procedures using the NACK frame of  FIGS. 22 to 28 . 
         FIG. 35  is a diagram illustrating an embodiment of an operation of a transmitting STA. 
         FIG. 36  is a diagram illustrating an embodiment of an operation of a receiving terminal. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In the present specification, “A or B” may mean “only A”, “only B” or “both A and B”. In other words, in the present specification, “A or B” may be interpreted as “A and/or B”. For example, in the present specification, “A, B, or C” may mean “only A”, “only B”, “only C”, or “any combination of A, B, C”. 
     A slash (/) or comma used in the present specification may mean “and/or”. For example, “A/B” may mean “A and/or B”. Accordingly, “A/B” may mean “only A”, “only B”, or “both A and B”. For example, “A, B, C” may mean “A, B, or C”. 
     In the present specification, “at least one of A and B” may mean “only A”, “only B”, or “both A and B”. In addition, in the present specification, the expression “at least one of A or B” or “at least one of A and/or B” may be interpreted as “at least one of A and B”. 
     In addition, in the present specification, “at least one of A, B, and C” may mean “only A”, “only B”, “only C”, or “any combination of A, B, and C”. In addition, “at least one of A, B, or C” or “at least one of A, B, and/or C” may mean “at least one of A, B, and C”. 
     In addition, a parenthesis used in the present specification may mean “for example”. Specifically, when indicated as “control information (EHT-signal)”, it may mean that “EHT-signal” is proposed as an example of the “control information”. In other words, the “control information” of the present specification is not limited to “EHT-signal”, and “EHT-signal” may be proposed as an example of the “control information”. In addition, when indicated as “control information (i.e., EHT-signal)”, it may also mean that “EHT-signal” is proposed as an example of the “control information”. 
     Technical features described individually in one figure in the present specification may be individually implemented, or may be simultaneously implemented. 
     The following example of the present specification may be applied to various wireless communication systems. For example, the following example of the present specification may be applied to a wireless local area network (WLAN) system. For example, the present specification may be applied to the IEEE 802.11a/g/n/ac standard or the IEEE 802.11ax standard. In addition, the present specification may also be applied to the newly proposed EHT standard or IEEE 802.11be standard. In addition, the example of the present specification may also be applied to a new WLAN standard enhanced from the EHT standard or the IEEE 802.11be standard. In addition, the example of the present specification may be applied to a mobile communication system. For example, it may be applied to a mobile communication system based on long term evolution (LTE) depending on a 3rd generation partnership project (3GPP) standard and based on evolution of the LTE. In addition, the example of the present specification may be applied to a communication system of a 5G NR standard based on the 3GPP standard. 
     Hereinafter, in order to describe a technical feature of the present specification, a technical feature applicable to the present specification will be described. 
       FIG. 1  shows an example of a transmitting apparatus and/or receiving apparatus of the present specification. 
     In the example of  FIG. 1 , various technical features described below may be performed.  FIG. 1  relates to at least one station (STA). For example, STAs  110  and  120  of the present specification may also be called in various terms such as a mobile terminal, a wireless device, a wireless transmit/receive unit (WTRU), a user equipment (UE), a mobile station (MS), a mobile subscriber unit, or simply a user. The STAs  110  and  120  of the present specification may also be called in various terms such as a network, a base station, a node-B, an access point (AP), a repeater, a router, a relay, or the like. The STAs  110  and  120  of the present specification may also be referred to as various names such as a receiving apparatus, a transmitting apparatus, a receiving STA, a transmitting STA, a receiving device, a transmitting device, or the like. 
     For example, the STAs  110  and  120  may serve as an AP or a non-AP. That is, the STAs  110  and  120  of the present specification may serve as the AP and/or the non-AP. 
     STAs  110  and  120  of the present specification may support various communication standards together in addition to the IEEE 802.11 standard. For example, a communication standard (e.g., LTE, LTE-A, 5G NR standard) or the like based on the 3GPP standard may be supported. In addition, the STA of the present specification may be implemented as various devices such as a mobile phone, a vehicle, a personal computer, or the like. In addition, the STA of the present specification may support communication for various communication services such as voice calls, video calls, data communication, and self-driving (autonomous-driving), or the like. 
     The STAs  110  and  120  of the present specification may include a medium access control (MAC) conforming to the IEEE 802.11 standard and a physical layer interface for a radio medium. 
     The STAs  110  and  120  will be described below with reference to a sub-figure (a) of  FIG. 1 . 
     The first STA  110  may include a processor  111 , a memory  112 , and a transceiver  113 . The illustrated process, memory, and transceiver may be implemented individually as separate chips, or at least two blocks/functions may be implemented through a single chip. 
     The transceiver  113  of the first STA performs a signal transmission/reception operation. Specifically, an IEEE 802.11 packet (e.g., IEEE 802.11a/b/g/n/ac/ax/be, etc.) may be transmitted/received. 
     For example, the first STA  110  may perform an operation intended by an AP. For example, the processor  111  of the AP may receive a signal through the transceiver  113 , process a reception (RX) signal, generate a transmission (TX) signal, and provide control for signal transmission. The memory  112  of the AP may store a signal (e.g., RX signal) received through the transceiver  113 , and may store a signal (e.g., TX signal) to be transmitted through the transceiver. 
     For example, the second STA  120  may perform an operation intended by a non-AP STA. For example, a transceiver  123  of a non-AP performs a signal transmission/reception operation. Specifically, an IEEE 802.11 packet (e.g., IEEE 802.11a/b/g/n/ac/ax/be packet, etc.) may be transmitted/received. 
     For example, a processor  121  of the non-AP STA may receive a signal through the transceiver  123 , process an RX signal, generate a TX signal, and provide control for signal transmission. A memory  122  of the non-AP STA may store a signal (e.g., RX signal) received through the transceiver  123 , and may store a signal (e.g., TX signal) to be transmitted through the transceiver. 
     For example, an operation of a device indicated as an AP in the specification described below may be performed in the first STA  110  or the second STA  120 . For example, if the first STA  110  is the AP, the operation of the device indicated as the AP may be controlled by the processor  111  of the first STA  110 , and a related signal may be transmitted or received through the transceiver  113  controlled by the processor  111  of the first STA  110 . In addition, control information related to the operation of the AP or a TX/RX signal of the AP may be stored in the memory  112  of the first STA  110 . In addition, if the second STA  120  is the AP, the operation of the device indicated as the AP may be controlled by the processor  121  of the second STA  120 , and a related signal may be transmitted or received through the transceiver  123  controlled by the processor  121  of the second STA  120 . In addition, control information related to the operation of the AP or a TX/RX signal of the AP may be stored in the memory  122  of the second STA  120 . 
     For example, in the specification described below, an operation of a device indicated as a non-AP (or user-STA) may be performed in the first STA  110  or the second STA  120 . For example, if the second STA  120  is the non-AP, the operation of the device indicated as the non-AP may be controlled by the processor  121  of the second STA  120 , and a related signal may be transmitted or received through the transceiver  123  controlled by the processor  121  of the second STA  120 . In addition, control information related to the operation of the non-AP or a TX/RX signal of the non-AP may be stored in the memory  122  of the second STA  120 . For example, if the first STA  110  is the non-AP, the operation of the device indicated as the non-AP may be controlled by the processor  111  of the first STA  110 , and a related signal may be transmitted or received through the transceiver  113  controlled by the processor  111  of the first STA  110 . In addition, control information related to the operation of the non-AP or a TX/RX signal of the non-AP may be stored in the memory  112  of the first STA  110 . 
     In the specification described below, a device called a (transmitting/receiving) STA, a first STA, a second STA, a STA1, a STA2, an AP, a first AP, a second AP, an AP1, an AP2, a (transmitting/receiving) terminal, a (transmitting/receiving) device, a (transmitting/receiving) apparatus, a network, or the like may imply the STAs  110  and  120  of  FIG. 1 . For example, a device indicated as, without a specific reference numeral, the (transmitting/receiving) STA, the first STA, the second STA, the STA1, the STA2, the AP, the first AP, the second AP, the AP1, the AP2, the (transmitting/receiving) terminal, the (transmitting/receiving) device, the (transmitting/receiving) apparatus, the network, or the like may imply the STAs  110  and  120  of  FIG. 1 . For example, in the following example, an operation in which various STAs transmit/receive a signal (e.g., a PPDU) may be performed in the transceivers  113  and  123  of  FIG. 1 . In addition, in the following example, an operation in which various STAs generate a TX/RX signal or perform data processing and computation in advance for the TX/RX signal may be performed in the processors  111  and  121  of  FIG. 1 . For example, an example of an operation for generating the TX/RX signal or performing the data processing and computation in advance may include: 1) an operation of determining/obtaining/configuring/computing/decoding/encoding bit information of a sub-field (SIG, STF, LTF, Data) included in a PPDU; 2) an operation of determining/configuring/obtaining a time resource or frequency resource (e.g., a subcarrier resource) or the like used for the sub-field (SIG, STF, LTF, Data) included the PPDU; 3) an operation of determining/configuring/obtaining a specific sequence (e.g., a pilot sequence, an STF/LTF sequence, an extra sequence applied to SIG) or the like used for the sub-field (SIG, STF, LTF, Data) field included in the PPDU; 4) a power control operation and/or power saving operation applied for the STA; and 5) an operation related to determining/obtaining/configuring/decoding/encoding or the like of an ACK signal. In addition, in the following example, a variety of information used by various STAs for determining/obtaining/configuring/computing/decoding/decoding a TX/RX signal (e.g., information related to a field/subfield/control field/parameter/power or the like) may be stored in the memories  112  and  122  of  FIG. 1 . 
     The aforementioned device/STA of the sub-figure (a) of  FIG. 1  may be modified as shown in the sub-figure (b) of  FIG. 1 . Hereinafter, the STAs  110  and  120  of the present specification will be described based on the sub-figure (b) of  FIG. 1 . 
     For example, the transceivers  113  and  123  illustrated in the sub-figure (b) of  FIG. 1  may perform the same function as the aforementioned transceiver illustrated in the sub-figure (a) of  FIG. 1 . For example, processing chips  114  and  124  illustrated in the sub-figure (b) of  FIG. 1  may include the processors  111  and  121  and the memories  112  and  122 . The processors  111  and  121  and memories  112  and  122  illustrated in the sub-figure (b) of  FIG. 1  may perform the same function as the aforementioned processors  111  and  121  and memories  112  and  122  illustrated in the sub-figure (a) of  FIG. 1 . 
     A mobile terminal, a wireless device, a wireless transmit/receive unit (WTRU), a user equipment (UE), a mobile station (MS), a mobile subscriber unit, a user, a user STA, a network, a base station, a Node-B, an access point (AP), a repeater, a router, a relay, a receiving unit, a transmitting unit, a receiving STA, a transmitting STA, a receiving device, a transmitting device, a receiving apparatus, and/or a transmitting apparatus, which are described below, may imply the STAs  110  and  120  illustrated in the sub-figure (a)/(b) of  FIG. 1 , or may imply the processing chips  114  and  124  illustrated in the sub-figure (b) of  FIG. 1 . That is, a technical feature of the present specification may be performed in the STAs  110  and  120  illustrated in the sub-figure (a)/(b) of  FIG. 1 , or may be performed only in the processing chips  114  and  124  illustrated in the sub-figure (b) of  FIG. 1 . For example, a technical feature in which the transmitting STA transmits a control signal may be understood as a technical feature in which a control signal generated in the processors  111  and  121  illustrated in the sub-figure (a)/(b) of  FIG. 1  is transmitted through the transceivers  113  and  123  illustrated in the sub-figure (a)/(b) of  FIG. 1 . Alternatively, the technical feature in which the transmitting STA transmits the control signal may be understood as a technical feature in which the control signal to be transferred to the transceivers  113  and  123  is generated in the processing chips  114  and  124  illustrated in the sub-figure (b) of  FIG. 1 . 
     For example, a technical feature in which the receiving STA receives the control signal may be understood as a technical feature in which the control signal is received by means of the transceivers  113  and  123  illustrated in the sub-figure (a) of  FIG. 1 . Alternatively, the technical feature in which the receiving STA receives the control signal may be understood as the technical feature in which the control signal received in the transceivers  113  and  123  illustrated in the sub-figure (a) of  FIG. 1  is obtained by the processors  111  and  121  illustrated in the sub-figure (a) of  FIG. 1 . Alternatively, the technical feature in which the receiving STA receives the control signal may be understood as the technical feature in which the control signal received in the transceivers  113  and  123  illustrated in the sub-figure (b) of  FIG. 1  is obtained by the processing chips  114  and  124  illustrated in the sub-figure (b) of  FIG. 1 . 
     Referring to the sub-figure (b) of  FIG. 1 , software codes  115  and  125  may be included in the memories  112  and  122 . The software codes  115  and  126  may include instructions for controlling an operation of the processors  111  and  121 . The software codes  115  and  125  may be included as various programming languages. 
     The processors  111  and  121  or processing chips  114  and  124  of  FIG. 1  may include an application-specific integrated circuit (ASIC), other chipsets, a logic circuit and/or a data processing device. The processor may be an application processor (AP). For example, the processors  111  and  121  or processing chips  114  and  124  of  FIG. 1  may include at least one of a digital signal processor (DSP), a central processing unit (CPU), a graphics processing unit (GPU), and a modulator and demodulator (modem). For example, the processors  111  and  121  or processing chips  114  and  124  of  FIG. 1  may be SNAPDRAGON™ series of processors made by Qualcomm®, EXYNOS™ series of processors made by Samsung®, A series of processors made by Apple®, HELIO™ series of processors made by MediaTek®, ATOM™ series of processors made by Intel® or processors enhanced from these processors. 
     In the present specification, an uplink may imply a link for communication from a non-AP STA to an SP STA, and an uplink PPDU/packet/signal or the like may be transmitted through the uplink. In addition, in the present specification, a downlink may imply a link for communication from the AP STA to the non-AP STA, and a downlink PPDU/packet/signal or the like may be transmitted through the downlink. 
       FIG. 2  is a conceptual view illustrating the structure of a wireless local area network (WLAN). 
     An upper part of  FIG. 2  illustrates the structure of an infrastructure basic service set (BSS) of institute of electrical and electronic engineers (i.e.EE) 802.11. 
     Referring the upper part of  FIG. 2 , the wireless LAN system may include one or more infrastructure BSSs  200  and  205  (hereinafter, referred to as BSS). The BSSs  200  and  205  as a set of an AP and a STA such as an access point (AP)  225  and a station (STA1)  200 - 1  which are successfully synchronized to communicate with each other are not concepts indicating a specific region. The BSS  205  may include one or more STAs  205 - 1  and  205 - 2  which may be joined to one AP  230 . 
     The BSS may include at least one STA, APs providing a distribution service, and a distribution system (DS)  210  connecting multiple APs. 
     The distribution system  210  may implement an extended service set (ESS)  240  extended by connecting the multiple BSSs  200  and  205 . The ESS  240  may be used as a term indicating one network configured by connecting one or more APs  225  or  230  through the distribution system  210 . The AP included in one ESS  240  may have the same service set identification (SSID). 
     A portal  220  may serve as a bridge which connects the wireless LAN network (i.e. EE 802.11) and another network (e.g., 802.X). 
     In the BSS illustrated in the upper part of  FIG. 2 , a network between the APs  225  and  230  and a network between the APs  225  and  230  and the STAs  200 - 1 ,  205 - 1 , and  205 - 2  may be implemented. However, the network is configured even between the STAs without the APs  225  and  230  to perform communication. A network in which the communication is performed by configuring the network even between the STAs without the APs  225  and  230  is defined as an Ad-Hoc network or an independent basic service set (IBSS). 
     A lower part of  FIG. 2  illustrates a conceptual view illustrating the IBSS. 
     Referring to the lower part of  FIG. 2 , the IBSS is a BSS that operates in an Ad-Hoc mode. Since the IBSS does not include the access point (AP), a centralized management entity that performs a management function at the center does not exist. That is, in the IBSS, STAs  250 - 1 ,  250 - 2 ,  250 - 3 ,  255 - 4 , and  255 - 5  are managed by a distributed manner. In the IBSS, all STAs  250 - 1 ,  250 - 2 ,  250 - 3 ,  255 - 4 , and  255 - 5  may be constituted by movable STAs and are not permitted to access the DS to constitute a self-contained network. 
       FIG. 3  illustrates a general link setup process. 
     In S 310 , a STA may perform a network discovery operation. The network discovery operation may include a scanning operation of the STA. That is, to access a network, the STA needs to discover a participating network. The STA needs to identify a compatible network before participating in a wireless network, and a process of identifying a network present in a particular area is referred to as scanning Scanning methods include active scanning and passive scanning. 
       FIG. 3  illustrates a network discovery operation including an active scanning process. In active scanning, a STA performing scanning transmits a probe request frame and waits for a response to the probe request frame in order to identify which AP is present around while moving to channels. A responder transmits a probe response frame as a response to the probe request frame to the STA having transmitted the probe request frame. Here, the responder may be a STA that transmits the last beacon frame in a BSS of a channel being scanned. In the BSS, since an AP transmits a beacon frame, the AP is the responder. In an IBSS, since STAs in the IBSS transmit a beacon frame in turns, the responder is not fixed. For example, when the STA transmits a probe request frame via channel 1 and receives a probe response frame via channel 1, the STA may store BSS-related information included in the received probe response frame, may move to the next channel (e.g., channel 2), and may perform scanning (e.g., transmits a probe request and receives a probe response via channel 2) by the same method. 
     Although not shown in  FIG. 3 , scanning may be performed by a passive scanning method. In passive scanning, a STA performing scanning may wait for a beacon frame while moving to channels. A beacon frame is one of management frames in IEEE 802.11 and is periodically transmitted to indicate the presence of a wireless network and to enable the STA performing scanning to find the wireless network and to participate in the wireless network. In a BSS, an AP serves to periodically transmit a beacon frame. In an IBSS, STAs in the IBSS transmit a beacon frame in turns. Upon receiving the beacon frame, the STA performing scanning stores information about a BSS included in the beacon frame and records beacon frame information in each channel while moving to another channel. The STA having received the beacon frame may store BSS-related information included in the received beacon frame, may move to the next channel, and may perform scanning in the next channel by the same method. 
     After discovering the network, the STA may perform an authentication process in S 320 . The authentication process may be referred to as a first authentication process to be clearly distinguished from the following security setup operation in S 340 . The authentication process in S 320  may include a process in which the STA transmits an authentication request frame to the AP and the AP transmits an authentication response frame to the STA in response. The authentication frames used for an authentication request/response are management frames. 
     The authentication frames may include information about an authentication algorithm number, an authentication transaction sequence number, a status code, a challenge text, a robust security network (RSN), and a finite cyclic group. 
     The STA may transmit the authentication request frame to the AP. The AP may determine whether to allow the authentication of the STA based on the information included in the received authentication request frame. The AP may provide the authentication processing result to the STA via the authentication response frame. 
     When the STA is successfully authenticated, the STA may perform an association process in S 330 . The association process includes a process in which the STA transmits an association request frame to the AP and the AP transmits an association response frame to the STA in response. The association request frame may include, for example, information about various capabilities, a beacon listen interval, a service set identifier (SSID), a supported rate, a supported channel, RSN, a mobility domain, a supported operating class, a traffic indication map (TIM) broadcast request, and an interworking service capability. The association response frame may include, for example, information about various capabilities, a status code, an association ID (AID), a supported rate, an enhanced distributed channel access (EDCA) parameter set, a received channel power indicator (RCPI), a received signal-to-noise indicator (RSNI), a mobility domain, a timeout interval (association comeback time), an overlapping BSS scanning parameter, a TIM broadcast response, and a QoS map. 
     In S 340 , the STA may perform a security setup process. The security setup process in S 340  may include a process of setting up a private key through four-way handshaking, for example, through an extensible authentication protocol over LAN (EAPOL) frame. 
       FIG. 4  illustrates an example of a PPDU used in an IEEE standard. 
     As illustrated, various types of PHY protocol data units (PPDUs) are used in IEEE a/g/n/ac standards. Specifically, an LTF and a STF include a training signal, a SIG-A and a SIG-B include control information for a receiving STA, and a data field includes user data corresponding to a PSDU (MAC PDU/aggregated MAC PDU). 
       FIG. 4  also includes an example of an HE PPDU according to IEEE 802.11ax. The HE PPDU according to  FIG. 4  is an illustrative PPDU for multiple users. An HE-SIG-B may be included only in a PPDU for multiple users, and an HE-SIG-B may be omitted in a PPDU for a single user. 
     As illustrated in  FIG. 4 , the HE-PPDU for multiple users (MUs) may include a legacy-short training field (L-STF), a legacy-long training field (L-LTF), a legacy-signal (L-SIG), a high efficiency-signal A (HE-SIG A), a high efficiency-signal-B (HE-SIG B), a high efficiency-short training field (HE-STF), a high efficiency-long training field (HE-LTF), a data field (alternatively, an MAC payload), and a packet extension (PE) field. The respective fields may be transmitted for illustrated time periods (i.e., 4 or 8 μs). 
     Hereinafter, a resource unit (RU) used for a PPDU is described. An RU may include a plurality of subcarriers (or tones). An RU may be used to transmit a signal to a plurality of STAs according to OFDMA. Further, an RU may also be defined to transmit a signal to one STA. An RU may be used for an STF, an LTF, a data field, or the like. 
       FIG. 5  illustrates a layout of resource units (RUs) used in a band of 20 MHz. 
     As illustrated in  FIG. 5 , resource units (RUs) corresponding to different numbers of tones (i.e., subcarriers) may be used to form some fields of an HE-PPDU. For example, resources may be allocated in illustrated RUs for an HE-STF, an HE-LTF, and a data field. 
     As illustrated in the uppermost part of  FIG. 5 , a 26-unit (i.e., a unit corresponding to 26 tones) may be disposed. Six tones may be used for a guard band in the leftmost band of the 20 MHz band, and five tones may be used for a guard band in the rightmost band of the 20 MHz band. Further, seven DC tones may be inserted in a center band, that is, a DC band, and a 26-unit corresponding to 13 tones on each of the left and right sides of the DC band may be disposed. A 26-unit, a 52-unit, and a 106-unit may be allocated to other bands. Each unit may be allocated for a receiving STA, that is, a user. 
     The layout of the RUs in  FIG. 5  may be used not only for a multiple users (MUs) but also for a single user (SU), in which case one 242-unit may be used and three DC tones may be inserted as illustrated in the lowermost part of  FIG. 5 . 
     Although  FIG. 5  proposes RUs having various sizes, that is, a 26-RU, a 52-RU, a 106-RU, and a 242-RU, specific sizes of RUs may be extended or increased. Therefore, the present embodiment is not limited to the specific size of each RU (i.e., the number of corresponding tones). 
       FIG. 6  illustrates a layout of RUs used in a band of 40 MHz. 
     Similarly to  FIG. 5  in which RUs having various sizes are used, a 26-RU, a 52-RU, a 106-RU, a 242-RU, a 484-RU, and the like may be used in an example of  FIG. 6 . Further, five DC tones may be inserted in a center frequency, 12 tones may be used for a guard band in the leftmost band of the 40 MHz band, and 11 tones may be used for a guard band in the rightmost band of the 40 MHz band. 
     As illustrated in  FIG. 6 , when the layout of the RUs is used for a single user, a 484-RU may be used. The specific number of RUs may be changed similarly to  FIG. 5 . 
       FIG. 7  illustrates a layout of RUs used in a band of 80 MHz. 
     Similarly to  FIG. 5  and  FIG. 6  in which RUs having various sizes are used, a 26-RU, a 52-RU, a 106-RU, a 242-RU, a 484-RU, a 996-RU, and the like may be used in an example of  FIG. 7 . Further, seven DC tones may be inserted in the center frequency, 12 tones may be used for a guard band in the leftmost band of the 80 MHz band, and 11 tones may be used for a guard band in the rightmost band of the 80 MHz band. In addition, a 26-RU corresponding to 13 tones on each of the left and right sides of the DC band may be used. 
     As illustrated in  FIG. 7 , when the layout of the RUs is used for a single user, a 996-RU may be used, in which case five DC tones may be inserted. 
     In the meantime, the fact that the specific number of RUs can be changed is the same as those of  FIGS. 5 and 6 . 
     The RU arrangement (i.e., RU location) shown in  FIGS. 5 to 7  can be applied to a new wireless LAN system (e.g. EHT system) as it is. Meanwhile, for the 160 MHz band supported by the new WLAN system, the RU arrangement for 80 MHz (i.e., an example of  FIG. 7 ) may be repeated twice, or the RU arrangement for the 40 MHz (i.e., an example of  FIG. 6 ) may be repeated 4 times. In addition, when the EHT PPDU is configured for the 320 MHz band, the arrangement of the RU for 80 MHz (i.e., an example of  FIG. 7 ) may be repeated 4 times or the arrangement of the RU for 40 MHz (i.e., an example of  FIG. 6 ) may be repeated 8 times. 
     One RU of the present specification may be allocated for a single STA (e.g., a single non-AP STA). Alternatively, a plurality of RUs may be allocated for one STA (e.g., a non-AP STA). 
     The RU described in the present specification may be used in uplink (UL) communication and downlink (DL) communication. For example, when UL-MU communication which is solicited by a trigger frame is performed, a transmitting STA (e.g., an AP) may allocate a first RU (e.g., 26/52/106/242-RU, etc.) to a first STA through the trigger frame, and may allocate a second RU (e.g., 26/52/106/242-RU, etc.) to a second STA. Thereafter, the first STA may transmit a first trigger-based PPDU based on the first RU, and the second STA may transmit a second trigger-based PPDU based on the second RU. The first/second trigger-based PPDU is transmitted to the AP at the same (or overlapped) time period. 
     For example, when a DL MU PPDU is configured, the transmitting STA (e.g., AP) may allocate the first RU (e.g., 26/52/106/242-RU. etc.) to the first STA, and may allocate the second RU (e.g., 26/52/106/242-RU, etc.) to the second STA. That is, the transmitting STA (e.g., AP) may transmit HE-STF, HE-LTF, and Data fields for the first STA through the first RU in one MU PPDU, and may transmit HE-STF, HE-LTF, and Data fields for the second STA through the second RU. 
     Information related to a layout of the RU may be signaled through HE-SIG-B. 
       FIG. 8  illustrates a structure of an HE-SIG-B field. 
     As illustrated, an HE-SIG-B field  810  includes a common field  820  and a user-specific field  830 . The common field  820  may include information commonly applied to all users (i.e., user STAs) which receive SIG-B. The user-specific field  830  may be called a user-specific control field. When the SIG-B is transferred to a plurality of users, the user-specific field  830  may be applied only any one of the plurality of users. 
     As illustrated in  FIG. 8 , the common field  820  and the user-specific field  830  may be separately encoded. 
     The common field  820  may include RU allocation information of N*8 bits. For example, the RU allocation information may include information related to a location of an RU. For example, when a 20 MHz channel is used as shown in  FIG. 5 , the RU allocation information may include information related to a specific frequency band to which a specific RU (26-RU/52-RU/106-RU) is arranged. 
     An example of a case in which the RU allocation information consists of 8 bits is as follows. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 8 bits indices 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 (B7 B6 B5 B4 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Number of 
               
               
                 B3 B2 B1 B0) 
                 #1 
                 #2 
                 #3 
                 #4 
                 #5 
                 #6 
                 #7 
                 #8 
                 #9 
                 entries 
               
               
                   
               
             
            
               
                 00000000 
                 26 
                 26 
                 26 
                 26 
                 26 
                 26 
                 26 
                 26 
                 26 
                 1 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 00000001 
                 26 
                 26 
                 26 
                 26 
                 26 
                 26 
                 26 
                 52 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 00000010 
                 26 
                 26 
                 26 
                 26 
                 26 
                 52 
                 26 
                 26 
                 1 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 00000011 
                 26 
                 26 
                 26 
                 26 
                 26 
                 52 
                 52 
                 1 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 00000100 
                 26 
                 26 
                 52 
                 26 
                 26 
                 26 
                 26 
                 26 
                 1 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 00000101 
                 26 
                 26 
                 52 
                 26 
                 26 
                 26 
                 52 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 00000110 
                 26 
                 26 
                 52 
                 26 
                 52 
                 26 
                 26 
                 1 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 00000111 
                 26 
                 26 
                 52 
                 26 
                 52 
                 52 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 00001000 
                 52 
                 26 
                 26 
                 26 
                 26 
                 26 
                 26 
                 26 
                 1 
               
               
                   
               
            
           
         
       
     
     As shown the example of  FIG. 5 , up to nine 26-RUs may be allocated to the 20 MHz channel. When the RU allocation information of the common field  820  is set to “00000000” as shown in Table 1, the nine 26-RUs may be allocated to a corresponding channel (i.e., 20 MHz). In addition, when the RU allocation information of the common field  820  is set to “00000001” as shown in Table 1, seven 26-RUs and one 52-RU are arranged in a corresponding channel. That is, in the example of  FIG. 5 , the 52-RU may be allocated to the rightmost side, and the seven 26-RUs may be allocated to the left thereof. 
     The example of Table 1 shows only some of RU locations capable of displaying the RU allocation information. 
     For example, the RU allocation information may include an example of Table 2 below. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 8 bits indices 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 (B7 B6 B5 B4 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 Number of 
               
               
                 B3 B2 B1 B0) 
                 #1 
                 #2 
                 #3 
                 #4 
                 #5 
                 #6 
                 #7 
                 #8 
                 #9 
                 entries 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 01000y 2 y 1 y 0   
                 106 
                 26 
                 26 
                 26 
                 26 
                 26 
                 8 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 01001y 2 y 1 y 0   
                 106 
                 26 
                 26 
                 26 
                 52 
                 8 
               
               
                   
               
            
           
         
       
     
     “01000y2y1y0” relates to an example in which a 106-RU is allocated to the leftmost side of the 20 MHz channel, and five 26-RUs are allocated to the right side thereof. In this case, a plurality of STAs (e.g., user-STAs) may be allocated to the 106-RU, based on a MU-MIMO scheme. Specifically, up to 8 STAs (e.g., user-STAs) may be allocated to the 106-RU, and the number of STAs (e.g., user-STAs) allocated to the 106-RU is determined based on 3-bit information (y2y1y0). For example, when the 3-bit information (y2y1y0) is set to N, the number of STAs (e.g., user-STAs) allocated to the 106-RU based on the MU-MIMO scheme may be N+1. 
     In general, a plurality of STAs (e.g., user STAs) different from each other may be allocated to a plurality of RUs. However, the plurality of STAs (e.g., user STAs) may be allocated to one or more RUs having at least a specific size (e.g., 106 subcarriers), based on the MU-MIMO scheme. 
     As shown in  FIG. 8 , the user-specific field  830  may include a plurality of user fields. As described above, the number of STAs (e.g., user STAs) allocated to a specific channel may be determined based on the RU allocation information of the common field  820 . For example, when the RU allocation information of the common field  820  is “00000000”, one user STA may be allocated to each of nine 26-RUs (e.g., nine user STAs may be allocated). That is, up to 9 user STAs may be allocated to a specific channel through an OFDMA scheme. In other words, up to 9 user STAs may be allocated to a specific channel through a non-MU-MIMO scheme. 
     For example, when RU allocation is set to “01000y2y1y0”, a plurality of STAs may be allocated to the 106-RU arranged at the leftmost side through the MU-MIMO scheme, and five user STAs may be allocated to five 26-RUs arranged to the right side thereof through the non-MU MIMO scheme. This case is specified through an example of  FIG. 9 . 
       FIG. 9  illustrates an example in which a plurality of user STAs are allocated to the same RU through a MU-MIMO scheme. 
     For example, when RU allocation is set to “01000010” as shown in  FIG. 9 , a 106-RU may be allocated to the leftmost side of a specific channel, and five 26-RUs may be allocated to the right side thereof. In addition, three user STAs may be allocated to the 106-RU through the MU-MIMO scheme. As a result, since eight user STAs are allocated, the user-specific field  830  of HE-SIG-B may include eight user fields. 
     The eight user fields may be expressed in the order shown in  FIG. 9 . In addition, as shown in  FIG. 8 , two user fields may be implemented with one user block field. 
     The user fields shown in  FIG. 8  and  FIG. 9  may be configured based on two formats. That is, a user field related to a MU-MIMO scheme may be configured in a first format, and a user field related to a non-MIMO scheme may be configured in a second format. Referring to the example of  FIG. 9 , a user field 1 to a user field 3 may be based on the first format, and a user field 4 to a user field 8 may be based on the second format. The first format or the second format may include bit information of the same length (e.g., 21 bits). 
     Each user field may have the same size (e.g., 21 bits). For example, the user field of the first format (the first of the MU-MIMO scheme) may be configured as follows. 
     For example, a first bit (i.e., B0-B10) in the user field (i.e., 21 bits) may include identification information (e.g., STA-ID, partial AID, etc.) of a user STA to which a corresponding user field is allocated. In addition, a second bit (i.e., B11-B14) in the user field (i.e., 21 bits) may include information related to a spatial configuration. Specifically, an example of the second bit (i.e., B11-B14) may be as shown in Table 3 and Table 4 below. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 N STS   
                 N STS   
                 N STS   
                 N STS   
                 N STS   
                 N STS   
                 N STS   
                 N STS   
                 Total 
                 Number of 
               
               
                 N user   
                 B3 . . . B0 
                 [1] 
                 [2] 
                 [3] 
                 [4] 
                 [5] 
                 [6] 
                 [7] 
                 [8] 
                 N STS   
                 entries 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 2 
                 0000-0011 
                 1-4 
                 1 
                   
                   
                 2-5 
                 10 
               
               
                   
                 0100-0110 
                 2-4 
                 2 
                   
                   
                 4-6 
               
               
                   
                 0111-1000 
                 3-4 
                 3 
                   
                   
                 6-7 
               
               
                   
                 1001 
                 4 
                 4 
                   
                   
                 8 
               
               
                 3 
                 0000-0011 
                 1-4 
                 1 
                 1 
                   
                 3-6 
                 13 
               
               
                   
                 0100-0110 
                 2-4 
                 2 
                 1 
                   
                 5-7 
               
               
                   
                 0111-1000 
                 3-4 
                 3 
                 1 
                   
                 7-8 
               
               
                   
                 1001-1011 
                 2-4 
                 2 
                 2 
                   
                 5-8 
               
               
                   
                 1100 
                 3 
                 3 
                 2 
                   
                 8 
               
               
                 4 
                 0000-0011 
                 1-4 
                 1 
                 1 
                 1 
                 4-7 
                 11 
               
               
                   
                 0100-0110 
                 2-4 
                 2 
                 1 
                 1 
                 5-8 
               
               
                   
                 0111 
                 3 
                 3 
                 1 
                 1 
                 8 
               
               
                   
                 1000-1001 
                 2-3 
                 2 
                 2 
                 1 
                 7-8 
               
               
                   
                 1010 
                 2 
                 2 
                 2 
                 2 
                 8 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                 N STS   
                 N STS   
                 N STS   
                 N STS   
                 N STS   
                 N STS   
                 N STS   
                 N STS   
                 Total 
                 Number of 
               
               
                 N user   
                 B3 . . . B0 
                 [1] 
                 [2] 
                 [3] 
                 [4] 
                 [5] 
                 [6] 
                 [7] 
                 [8] 
                 N STS   
                 entries 
               
               
                   
               
             
            
               
                 5 
                 0000-0011 
                 1-4 
                 1 
                 1 
                 1 
                 1 
                   
                   
                   
                 5-8 
                 7 
               
               
                   
                 0100-0101 
                 2-3 
                 2 
                 1 
                 1 
                 1 
                   
                   
                   
                 7-8 
               
               
                   
                 0110 
                 2 
                 2 
                 2 
                 1 
                 1 
                   
                   
                   
                 8 
               
               
                 6 
                 0000-0010 
                 1-3 
                 1 
                 1 
                 1 
                 1 
                 1 
                   
                   
                 6-8 
                 4 
               
               
                   
                 0011 
                 2 
                 2 
                 1 
                 1 
                 1 
                 1 
                   
                   
                 8 
               
               
                 7 
                 0000-0001 
                 1-2 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                   
                 7-8 
                 2 
               
               
                 8 
                 0000 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 1 
                 8 
                 1 
               
               
                   
               
            
           
         
       
     
     As shown in Table 3 and/or Table 4, the second bit (e.g., B11-B14) may include information related to the number of spatial streams allocated to the plurality of user STAs which are allocated based on the MU-MIMO scheme. For example, when three user STAs are allocated to the 106-RU based on the MU-MIMO scheme as shown in  FIG. 9 , N_user is set to “3”. Therefore, values of N_STS[1], N_STS[2], and N_STS[3] may be determined as shown in Table 3. For example, when a value of the second bit (B11-B14) is “0011”, it may be set to N_STS[1]=4, N_STS[2]=1, N_STS[3]=1. That is, in the example of  FIG. 9 , four spatial streams may be allocated to the user field 1, one spatial stream may be allocated to the user field 1, and one spatial stream may be allocated to the user field 3. 
     As shown in the example of Table 3 and/or Table 4, information (i.e., the second bit, B11-B14) related to the number of spatial streams for the user STA may consist of 4 bits. In addition, the information (i.e., the second bit, B11-B14) on the number of spatial streams for the user STA may support up to eight spatial streams. In addition, the information (i.e., the second bit, B11-B14) on the number of spatial streams for the user STA may support up to four spatial streams for one user STA. 
     In addition, a third bit (i.e., B15-18) in the user field (i.e., 21 bits) may include modulation and coding scheme (MCS) information. The MCS information may be applied to a data field in a PPDU including corresponding SIG-B. 
     An MCS, MCS information, an MCS index, an MCS field, or the like used in the present specification may be indicated by an index value. For example, the MCS information may be indicated by an index 0 to an index 11. The MCS information may include information related to a constellation modulation type (e.g., BPSK, QPSK, 16-QAM, 64-QAM, 256-QAM, 1024-QAM, etc.) and information related to a coding rate (e.g., 1/2, 2/3, 3/4, 5/6e, etc.). Information related to a channel coding type (e.g., LCC or LDPC) may be excluded in the MCS information. 
     In addition, a fourth bit (i.e., B19) in the user field (i.e., 21 bits) may be a reserved field. 
     In addition, a fifth bit (i.e., B20) in the user field (i.e., 21 bits) may include information related to a coding type (e.g., BCC or LDPC). That is, the fifth bit (i.e., B20) may include information related to a type (e.g., BCC or LDPC) of channel coding applied to the data field in the PPDU including the corresponding SIG-B. 
     The aforementioned example relates to the user field of the first format (the format of the MU-MIMO scheme). An example of the user field of the second format (the format of the non-MU-MIMO scheme) is as follows. 
     A first bit (e.g., B0-B10) in the user field of the second format may include identification information of a user STA. In addition, a second bit (e.g., B11-B13) in the user field of the second format may include information related to the number of spatial streams applied to a corresponding RU. In addition, a third bit (e.g., B14) in the user field of the second format may include information related to whether a beamforming steering matrix is applied. A fourth bit (e.g., B15-B18) in the user field of the second format may include modulation and coding scheme (MCS) information. In addition, a fifth bit (e.g., B19) in the user field of the second format may include information related to whether dual carrier modulation (DCM) is applied. In addition, a sixth bit (i.e., B20) in the user field of the second format may include information related to a coding type (e.g., BCC or LDPC). 
       FIG. 10  illustrates an operation based on UL-MU. As illustrated, a transmitting STA (e.g., an AP) may perform channel access through contending (e.g., a backoff operation), and may transmit a trigger frame  1030 . That is, the transmitting STA may transmit a PPDU including the trigger frame  1030 . Upon receiving the PPDU including the trigger frame, a trigger-based (TB) PPDU is transmitted after a delay corresponding to SIFS. 
     TB PPDUs  1041  and  1042  may be transmitted at the same time period, and may be transmitted from a plurality of STAs (e.g., user STAs) having AIDs indicated in the trigger frame  1030 . An ACK frame  1050  for the TB PPDU may be implemented in various forms. 
     A specific feature of the trigger frame is described with reference to  FIG. 11  to  FIG. 13 . Even if UL-MU communication is used, an orthogonal frequency division multiple access (OFDMA) scheme or a MU MIMO scheme may be used, and the OFDMA and MU-MIMO schemes may be simultaneously used. 
       FIG. 11  illustrates an example of a trigger frame. The trigger frame of  FIG. 11  allocates a resource for uplink multiple-user (MU) transmission, and may be transmitted, for example, from an AP. The trigger frame may be configured of a MAC frame, and may be included in a PPDU. 
     Each field shown in  FIG. 11  may be partially omitted, and another field may be added. In addition, a length of each field may be changed to be different from that shown in the figure. 
     A frame control field  1110  of  FIG. 11  may include information related to a MAC protocol version and extra additional control information. A duration field  1120  may include time information for NAV configuration or information related to an identifier (e.g., AID) of a STA. 
     In addition, an RA field  1130  may include address information of a receiving STA of a corresponding trigger frame, and may be optionally omitted. A TA field  1140  may include address information of a STA (e.g., an AP) which transmits the corresponding trigger frame. A common information field  1150  includes common control information applied to the receiving STA which receives the corresponding trigger frame. For example, a field indicating a length of an L-SIG field of an uplink PPDU transmitted in response to the corresponding trigger frame or information for controlling content of a SIG-A field (i.e., HE-SIG-A field) of the uplink PPDU transmitted in response to the corresponding trigger frame may be included. In addition, as common control information, information related to a length of a CP of the uplink PPDU transmitted in response to the corresponding trigger frame or information related to a length of an LTF field may be included. 
     In addition, per user information fields  1160 # 1  to  1160 #N corresponding to the number of receiving STAs which receive the trigger frame of  FIG. 11  are preferably included. The per user information field may also be called an “allocation field”. 
     In addition, the trigger frame of  FIG. 11  may include a padding field  1170  and a frame check sequence field  1180 . 
     Each of the per user information fields  1160 # 1  to  1160 #N shown in  FIG. 11  may include a plurality of subfields. 
       FIG. 12  illustrates an example of a common information field of a trigger frame. A subfield of  FIG. 12  may be partially omitted, and an extra subfield may be added. In addition, a length of each subfield illustrated may be changed. 
     A length field  1210  illustrated has the same value as a length field of an L-SIG field of an uplink PPDU transmitted in response to a corresponding trigger frame, and a length field of the L-SIG field of the uplink PPDU indicates a length of the uplink PPDU. As a result, the length field  1210  of the trigger frame may be used to indicate the length of the corresponding uplink PPDU. 
     In addition, a cascade identifier field  1220  indicates whether a cascade operation is performed. The cascade operation implies that downlink MU transmission and uplink MU transmission are performed together in the same TXOP. That is, it implies that downlink MU transmission is performed and thereafter uplink MU transmission is performed after a pre-set time (e.g., SIFS). During the cascade operation, only one transmitting device (e.g., AP) may perform downlink communication, and a plurality of transmitting devices (e.g., non-APs) may perform uplink communication. 
     A CS request field  1230  indicates whether a wireless medium state or a NAV or the like is necessarily considered in a situation where a receiving device which has received a corresponding trigger frame transmits a corresponding uplink PPDU. 
     An HE-SIG-A information field  1240  may include information for controlling content of a SIG-A field (i.e., HE-SIG-A field) of the uplink PPDU in response to the corresponding trigger frame. 
     A CP and LTF type field  1250  may include information related to a CP length and LTF length of the uplink PPDU transmitted in response to the corresponding trigger frame. A trigger type field  1260  may indicate a purpose of using the corresponding trigger frame, for example, typical triggering, triggering for beamforming, a request for block ACK/NACK, or the like. 
     It may be assumed that the trigger type field  1260  of the trigger frame in the present specification indicates a trigger frame of a basic type for typical triggering. For example, the trigger frame of the basic type may be referred to as a basic trigger frame. 
       FIG. 13  illustrates an example of a subfield included in a per user information field. A user information field  1300  of  FIG. 13  may be understood as any one of the per user information fields  1160 # 1  to  1160 #N mentioned above with reference to  FIG. 11 . A subfield included in the user information field  1300  of  FIG. 13  may be partially omitted, and an extra subfield may be added. In addition, a length of each subfield illustrated may be changed. 
     A user identifier field  1310  of  FIG. 13  indicates an identifier of a STA (i.e., receiving STA) corresponding to per user information. An example of the identifier may be the entirety or part of an association identifier (AID) value of the receiving STA. 
     In addition, an RU allocation field  1320  may be included. That is, when the receiving STA identified through the user identifier field  1310  transmits a TB PPDU in response to the trigger frame, the TB PPDU is transmitted through an RU indicated by the RU allocation field  1320 . In this case, the RU indicated by the RU allocation field  1320  may be an RU shown in  FIG. 5 ,  FIG. 6 , and  FIG. 7 . 
     The subfield of  FIG. 13  may include a coding type field  1330 . The coding type field  1330  may indicate a coding type of the TB PPDU. For example, when BCC coding is applied to the TB PPDU, the coding type field  1330  may be set to ‘1’, and when LDPC coding is applied, the coding type field  1330  may be set to ‘0’. 
     In addition, the subfield of  FIG. 13  may include an MCS field  1340 . The MCS field  1340  may indicate an MCS scheme applied to the TB PPDU. For example, when BCC coding is applied to the TB PPDU, the coding type field  1330  may be set to ‘1’, and when LDPC coding is applied, the coding type field  1330  may be set to ‘0’. 
     Hereinafter, a UL 01-DMA-based random access (UORA) scheme will be described. 
       FIG. 14  describes a technical feature of the UORA scheme. 
     A transmitting STA (e.g., an AP) may allocate six RU resources through a trigger frame as shown in  FIG. 14 . Specifically, the AP may allocate a 1st RU resource (AID 0, RU 1), a 2nd RU resource (AID 0, RU 2), a 3rd RU resource (AID 0, RU 3), a 4th RU resource (AID 2045, RU 4), a 5th RU resource (AID 2045, RU 5), and a 6th RU resource (AID 3, RU 6). Information related to the AID 0, AID 3, or AID 2045 may be included, for example, in the user identifier field  1310  of  FIG. 13 . Information related to the RU 1 to RU 6 may be included, for example, in the RU allocation field  1320  of  FIG. 13 . AID=0 may imply a UORA resource for an associated STA, and AID=2045 may imply a UORA resource for an un-associated STA. Accordingly, the 1st to 3rd RU resources of  FIG. 14  may be used as a UORA resource for the associated STA, the 4th and 5th RU resources of  FIG. 14  may be used as a UORA resource for the un-associated STA, and the 6th RU resource of  FIG. 14  may be used as a typical resource for UL MU. 
     In the example of  FIG. 14 , an OFDMA random access backoff (OBO) of a STA1 is decreased to 0, and the STA1 randomly selects the 2nd RU resource (AID 0, RU 2). In addition, since an OBO counter of a STA2/3 is greater than 0, an uplink resource is not allocated to the STA2/3. In addition, regarding a STA4 in  FIG. 14 , since an AID (e.g., AID=3) of the STA4 is included in a trigger frame, a resource of the RU 6 is allocated without backoff. 
     Specifically, since the STA1 of  FIG. 14  is an associated STA, the total number of eligible RA RUs for the STA1 is 3 (RU 1, RU 2, and RU 3), and thus the STA1 decreases an OBO counter by 3 so that the OBO counter becomes 0. In addition, since the STA2 of FIG.  14  is an associated STA, the total number of eligible RA RUs for the STA2 is 3 (RU 1, RU 2, and RU 3), and thus the STA2 decreases the OBO counter by 3 but the OBO counter is greater than 0. In addition, since the STA3 of  FIG. 14  is an un-associated STA, the total number of eligible RA RUs for the STA3 is 2 (RU 4, RU 5), and thus the STA3 decreases the OBO counter by 2 but the OBO counter is greater than 0. 
       FIG. 15  illustrates an example of a channel used/supported/defined within a 2.4 GHz band. 
     The 2.4 GHz band may be called in other terms such as a first band. In addition, the 2.4 GHz band may imply a frequency domain in which channels of which a center frequency is close to 2.4 GHz (e.g., channels of which a center frequency is located within 2.4 to 2.5 GHz) are used/supported/defined. 
     A plurality of 20 MHz channels may be included in the 2.4 GHz band. 20 MHz within the 2.4 GHz may have a plurality of channel indices (e.g., an index 1 to an index 14). For example, a center frequency of a 20 MHz channel to which a channel index 1 is allocated may be 2.412 GHz, a center frequency of a 20 MHz channel to which a channel index 2 is allocated may be 2.417 GHz, and a center frequency of a 20 MHz channel to which a channel index N is allocated may be (2.407+0.005*N) GHz. The channel index may be called in various terms such as a channel number or the like. Specific numerical values of the channel index and center frequency may be changed. 
       FIG. 15  exemplifies 4 channels within a 2.4 GHz band. Each of 1st to 4th frequency domains  1510  to  1540  shown herein may include one channel. For example, the 1st frequency domain  1510  may include a channel 1 (a 20 MHz channel having an index 1). In this case, a center frequency of the channel 1 may be set to 2412 MHz. The 2nd frequency domain  1520  may include a channel 6. In this case, a center frequency of the channel 6 may be set to 2437 MHz. The 3rd frequency domain  1530  may include a channel 11. In this case, a center frequency of the channel 11 may be set to 2462 MHz. The 4th frequency domain  1540  may include a channel 14. In this case, a center frequency of the channel 14 may be set to 2484 MHz. 
       FIG. 16  illustrates an example of a channel used/supported/defined within a 5 GHz band. 
     The 5 GHz band may be called in other terms such as a second band or the like. The 5 GHz band may imply a frequency domain in which channels of which a center frequency is greater than or equal to 5 GHz and less than 6 GHz (or less than 5.9 GHz) are used/supported/defined. Alternatively, the 5 GHz band may include a plurality of channels between 4.5 GHz and 5.5 GHz. A specific numerical value shown in  FIG. 16  may be changed. 
     A plurality of channels within the 5 GHz band include an unlicensed national information infrastructure (UNII)-1, a UNII-2, a UNII-3, and an ISM. The INII-1 may be called UNII Low. The UNII-2 may include a frequency domain called UNII Mid and UNII-2Extended. The UNII-3 may be called UNII-Upper. 
     A plurality of channels may be configured within the 5 GHz band, and a bandwidth of each channel may be variously set to, for example, 20 MHz, 40 MHz, 80 MHz, 160 MHz, or the like. For example, 5170 MHz to 5330 MHz frequency domains/ranges within the UNII-1 and UNII-2 may be divided into eight 20 MHz channels. The 5170 MHz to 5330 MHz frequency domains/ranges may be divided into four channels through a 40 MHz frequency domain. The 5170 MHz to 5330 MHz frequency domains/ranges may be divided into two channels through an 80 MHz frequency domain. Alternatively, the 5170 MHz to 5330 MHz frequency domains/ranges may be divided into one channel through a 160 MHz frequency domain. 
       FIG. 17  illustrates an example of a channel used/supported/defined within a 6 GHz band. 
     The 6 GHz band may be called in other terms such as a third band or the like. The 6 GHz band may imply a frequency domain in which channels of which a center frequency is greater than or equal to 5.9 GHz are used/supported/defined. A specific numerical value shown in  FIG. 17  may be changed. 
     For example, the 20 MHz channel of  FIG. 17  may be defined starting from 5.940 GHz. Specifically, among 20 MHz channels of  FIG. 17 , the leftmost channel may have an index 1 (or a channel index, a channel number, etc.), and 5.945 GHz may be assigned as a center frequency. That is, a center frequency of a channel of an index N may be determined as (5.940+0.005*N) GHz. 
     Accordingly, an index (or channel number) of the 2 MHz channel of  FIG. 17  may be 1, 5, 9, 13, 17, 21, 25, 29, 33, 37, 41, 45, 49, 53, 57, 61, 65, 69, 73, 77, 81, 85, 89, 93, 97, 101, 105, 109, 113, 117, 121, 125, 129, 133, 137, 141, 145, 149, 153, 157, 161, 165, 169, 173, 177, 181, 185, 189, 193, 197, 201, 205, 209, 213, 217, 221, 225, 229, 233. In addition, according to the aforementioned (5.940+0.005*N) GHz rule, an index of the 40 MHz channel of  FIG. 17  may be 3, 11, 19, 27, 35, 43, 51, 59, 67, 75, 83, 91, 99, 107, 115, 123, 131, 139, 147, 155, 163, 171, 179, 187, 195, 203, 211, 219, 227. 
     Although 20, 40, 80, and 160 MHz channels are illustrated in the example of  FIG. 17 , a 240 MHz channel or a 320 MHz channel may be additionally added. 
     Hereinafter, a PPDU transmitted/received in a STA of the present specification will be described. 
       FIG. 18  illustrates an example of a PPDU used in the present specification. 
     The PPDU  1800  depicted in  FIG. 18  may be referred to as various terms such as an EHT PPDU, a TX PPDU, an RX PPDU, a first type or N-th type PPDU, or the like. In addition, the EHT PPDU may be used in an EHT system and/or a new WLAN system enhanced from the EHT system. 
     The subfields  1801  to  1810  depicted in  FIG. 18  may be referred to as various terms. For example, a SIG A field  1805  may be referred to an EHT-SIG-A field, a SIG B field  1806  may be referred to an EHT-SIG-B, an STF field  1807  may be referred to an EHT-STF field, and an LTF field  1808  may be referred to an EHT-LTF. 
     The subcarrier spacing of the L-LTF, L-STF, L-SIG, and RL-SIG fields  1801 ,  1802 ,  1803 , and  1804  of  FIG. 18  can be set to 312.5 kHz, and the subcarrier spacing of the STF, LTF, and Data fields  1807 ,  1808 , and  1809  of  FIG. 18  can be set to 78.125 kHz. That is, the subcarrier index of the L-LTF, L-STF, L-SIG, and RL-SIG fields  1801 ,  1802 ,  1803 , and  1804  can be expressed in unit of 312.5 kHz, and the subcarrier index of the STF, LTF, and Data fields  1807 ,  1808 , and  1809  can be expressed in unit of 78.125 kHz. 
     The SIG A and/or SIG B fields of  FIG. 18  may include additional fields (e.g., a SIG C field or one control symbol, etc.). The subcarrier spacing of all or part of the SIG A and SIG B fields may be set to 312.5 kHz, and the subcarrier spacing of all or part of newly-defined SIG field(s) may be set to 312.5 kHz. Meanwhile, the subcarrier spacing for a part of the newly-defined SIG field(s) may be set to a pre-defined value (e.g., 312.5 kHz or 78.125 kHz). 
     In the PPDU of  FIG. 18 , the L-LTF and the L-STF may be the same as conventional L-LTF and L-STF fields. 
     The L-SIG field of  FIG. 18  may include, for example, bit information of 24 bits. For example, the 24-bit information may include a rate field of 4 bits, a reserved bit of 1 bit, a length field of 12 bits, a parity bit of 1 bit, and a tail bit of 6 bits. For example, the length field of 12 bits may include information related to the number of octets of a corresponding Physical Service Data Unit (PSDU). For example, the length field of 12 bits may be determined based on a type of the PPDU. For example, when the PPDU is a non-HT, HT, VHT PPDU or an EHT PPDU, a value of the length field may be determined as a multiple of 3. For example, when the PPDU is an HE PPDU, the value of the length field may be determined as “a multiple of 3”+1 or “a multiple of 3”+2. In other words, for the non-HT, HT, VHT PPDI or the EHT PPDU, the value of the length field may be determined as a multiple of 3, and for the HE PPDU, the value of the length field may be determined as “a multiple of 3”+1 or “a multiple of 3”+2. 
     For example, the transmitting STA may apply BCC encoding based on a ½ coding rate to the 24-bit information of the L-SIG field. Thereafter, the transmitting STA may obtain a BCC coding bit of 48 bits. BPSK modulation may be applied to the 48-bit coding bit, thereby generating 48 BPSK symbols. The transmitting STA may map the 48 BPSK symbols to positions except for a pilot subcarrier {subcarrier index −21, −7, +7, +21} and a DC subcarrier {subcarrier index 0}. As a result, the 48 BPSK symbols may be mapped to subcarrier indices −26 to −22, −20 to −8, −6 to −1, +1 to +6, +8 to +20, and +22 to +26. The transmitting STA may additionally map a signal of {−1, −1, −1, 1} to a subcarrier index {−28, −27, +27, +28}. The aforementioned signal may be used for channel estimation on a frequency domain corresponding to {−28, −27, +27, +28}. 
     The transmitting STA may generate an RL-SIG which is identical to the L-SIG. BPSK modulation may be applied to the RL-SIG. The receiving STA may figure out that the RX PPDU is the HE PPDU or the EHT PPDU, based on the presence of the RL-SIG. 
     After RL-SIG of  FIG. 18 , for example, an EHT-SIG-A or one control symbol may be inserted. A symbol located after the RL-SIG (i.e., the EHT-SIG-A or one control symbol in the present specification) may be referred as various names, such as a U-SIG (Universal SIG) field. 
     A symbol consecutive to the RL-SIG (e.g., U-SIG) may include information of N bits, and may include information for identifying the type of the EHT PPDU. For example, the U-SIG may be configured based on two symbols (e.g., two consecutive OFDM symbols). Each symbol (e.g., OFDM symbol) for U-SIG may have a duration of 4 us. Each symbol of the U-SIG may be used to transmit 26-bit information. For example, each symbol of the U-SIG may be transmitted/received based on 52 data tones and 4 pilot tones. 
     Through the U-SIG (or U-SIG field), for example, A-bit information (e.g., 52 un-coded bits) may be transmitted. A first symbol of the U-SIG may transmit first X-bit information (e.g., 26 un-coded bits) of the A-bit information, and a second symbol of the U-SIG may transmit the remaining Y-bit information (e.g. 26 un-coded bits) of the A-bit information. For example, the transmitting STA may obtain 26 un-coded bits included in each U-SIG symbol. The transmitting STA may perform convolutional encoding (i.e., BCC encoding) based on a rate of R=1/2 to generate 52-coded bits, and may perform interleaving on the 52-coded bits. The transmitting STA may perform BPSK modulation on the interleaved 52-coded bits to generate 52 BPSK symbols to be allocated to each U-SIG symbol. One U-SIG symbol may be transmitted based on 65 tones (subcarriers) from a subcarrier index −28 to a subcarrier index+28, except for a DC index 0. The 52 BPSK symbols generated by the transmitting STA may be transmitted based on the remaining tones (subcarriers) except for pilot tones, i.e., tones −21, −7, +7, +21. 
     For example, the A-bit information (e.g., 52 un-coded bits) generated by the U-SIG may include a CRC field (e.g., a field having a length of 4 bits) and a tail field (e.g., a field having a length of 6 bits). The CRC field and the tail field may be transmitted through the second symbol of the U-SIG. The CRC field may be generated based on 26 bits allocated to the first symbol of the U-SIG and the remaining 16 bits except for the CRC/tail fields in the second symbol, and may be generated based on the conventional CRC calculation algorithm. In addition, the tail field may be used to terminate trellis of a convolutional decoder, and may be set to, for example, “000000”. 
     The A-bit information (e.g., 52 un-coded bits) transmitted by the U-SIG (or U-SIG field) may be divided into version-independent bits and version-dependent bits. For example, the version-independent bits may have a fixed or variable size. For example, the version-independent bits may be allocated only to the first symbol of the U-SIG, or the version-independent bits may be allocated to both of the first and second symbols of the U-SIG. For example, the version-independent bits and the version-dependent bits may be called in various terms such as a first control bit, a second control bit, or the like. 
     For example, the version-independent bits of the U-SIG may include a PHY version identifier of 3 bits. For example, the PHY version identifier of 3 bits may include information related to a PHY version of a TX/RX PPDU. For example, a first value of the PHY version identifier of 3 bits may indicate that the TX/RX PPDU is an EHT PPDU. In other words, when the transmitting STA transmits the EHT PPDU, the PHY version identifier of 3 bits may be set to a first value. In other words, the receiving STA may determine that the RX PPDU is the EHT PPDU, based on the PHY version identifier having the first value. 
     For example, the version-independent bits of the U-SIG may include a UL/DL flag field of 1 bit. A first value of the UL/DL flag field of 1 bit relates to UL communication, and a second value of the UL/DL flag field relates to DL communication. 
     For example, the version-independent bits of the U-SIG may include information related to a TXOP length and information related to a BSS color ID. 
     For example, when the EHT PPDU is classified into various types (e.g., EHT PPDU supporting SU, EHT PPDU supporting MU, EHT PPDU related to Trigger Frame, EHT PPDU related to Extended Range transmission, etc.), information related to the type of the EHT PPDU may be included in version-independent bits or version-dependent bits of the U-SIG. 
     For example, the U-SIG field includes 1) a bandwidth field including information related to a bandwidth, 2) a field including information related an MCS scheme applied to the SIG-B, 3) a dual subcarrier modulation in the SIG-B (i.e., an indication field including information related to whether the dual subcarrier modulation) is applied, 4) a field including information related to the number of symbols used for the SIG-B, 5) a field including information on whether the SIG-B is generated over the entire band, 6) a field including information related to a type of the LTF/STF, and/or  7 ) information related to a field indicating a length of the LTF and the CP. 
     The SIG-B of  FIG. 18  may include the technical features of HE-SIG-B shown in the example of  FIGS. 8 to 9  as it is. 
     An STF of  FIG. 18  may be used to improve automatic gain control estimation in a multiple input multiple output (MIMO) environment or an OFDMA environment. An LTF of  FIG. 18  may be used to estimate a channel in the MIMO environment or the OFDMA environment. 
     The EHT-STF of  FIG. 18  may be set in various types. For example, a first type of STF (e.g., lx STF) may be generated based on a first type STF sequence in which a non-zero coefficient is arranged with an interval of 16 subcarriers. An STF signal generated based on the first type STF sequence may have a period of 0.8 μs, and a periodicity signal of 0.8 μs may be repeated 5 times to become a first type STF having a length of 4 μs. For example, a second type of STF (e.g., 2×STF) may be generated based on a second type STF sequence in which a non-zero coefficient is arranged with an interval of 8 subcarriers. An STF signal generated based on the second type STF sequence may have a period of 1.6 μs, and a periodicity signal of 1.6 μs may be repeated 5 times to become a second type STF having a length of 8 μs. For example, a third type of STF (e.g., 4×STF) may be generated based on a third type STF sequence in which a non-zero coefficient is arranged with an interval of 4 subcarriers. An STF signal generated based on the third type STF sequence may have a period of 3.2 μs, and a periodicity signal of 3.2 μs may be repeated 5 times to become a second type STF having a length of 16 μs. Only some of the first to third type EHT-STF sequences may be used. In addition, the EHT-LTF field may also have first, second, and third types (i.e., 1×, 2×, 4×LTF). For example, the first/second/third type LTF field may be generated based on an LTF sequence in which a non-zero coefficient is arranged with an interval of 4/2/1 subcarriers. The first/second/third type LTF may have a time length of 3.2/6.4/12.8 μs. In addition, Guard Intervals (GIs) with various lengths (e.g., 0.8/1/6/3.2 μs) may be applied to the first/second/third type LTF. 
     Information related to the type of STF and/or LTF (including information related to GI applied to the LTF) may be included in the SIG A field and/or the SIG B field of  FIG. 18 . 
     The PPDU of  FIG. 18  may support various bandwidths. For example, the PPDU of  FIG. 18  may have a bandwidth of 20/40/80/160/240/320 MHz. For example, at least one field (e.g., STF, LTF, data) of  FIG. 18  may be configured based on RUs illustrated in  FIGS. 5 to 7 , and the like. For example, when there is one receiving STA of the PPDU of  FIG. 18 , all fields of the PPDU of  FIG. 18  may occupy the entire bandwidth. For example, when there are multiple receiving STAs of the PPDU of  FIG. 18  (i.e., when MU PPDU is used), some fields (e.g., STF, LTF, data) of  FIG. 18  may be configured based on the RUs shown in  FIGS. 5 to 7 . For example, the STF, LTF, and data fields for the first receiving STA of the PPDU may be transmitted/received through a first RU, and the STF, LTF, and data fields for the second receiving STA of the PPDU may be transmitted/received through a second RU. In this case, the locations/positions of the first and second RUs may be determined based on  FIGS. 5 to 7 , and the like. 
     The PPDU of  FIG. 18  may be determined (or identified) as an EHT PPDU based on the following method. 
     A receiving STA may determine a type of an RX PPDU as the EHT PPDU, based on the following aspect. For example, the RX PPDU may be determined as the EHT PPDU: 1) when a first symbol after an L-LTF signal of the RX PPDU is a BPSK symbol; 2) when RL-SIG in which the L-SIG of the RX PPDU is repeated is detected; and 3) when a result of applying “module 3” to a value of a length field of the L-SIG of the RX PPDU is detected as “0”. When the RX PPDU is determined as the EHT PPDU, the receiving STA may detect a type of the EHT PPDU (e.g., an SU/MU/Trigger-based/Extended Range type), based on bit information included in a symbol after the RL-SIG of  FIG. 18 . In other words, the receiving STA may determine the RX PPDU as the EHT PPDU, based on: 1) a first symbol after an L-LTF signal, which is a BPSK symbol; 2) RL-SIG contiguous to the L-SIG field and identical to L-SIG; 3) L-SIG including a length field in which a result of applying “modulo 3” is set to “0”; and 4) a 3-bit PHY version identifier of the aforementioned U-SIG (e.g., a PHY version identifier having a first value). 
     For example, the receiving STA may determine the type of the RX PPDU as the EHT PPDU, based on the following aspect. For example, the RX PPDU may be determined as the HE PPDU: 1) when a first symbol after an L-LTF signal is a BPSK symbol; 2) when RL-SIG in which the L-SIG is repeated is detected; and 3) when a result of applying “module 3” to a value of a length field of the L-SIG is detected as “1” or “2”. 
     For example, the receiving STA may determine the type of the RX PPDU as a non-HT, HT, and VHT PPDU, based on the following aspect. For example, the RX PPDU may be determined as the non-HT, HT, and VHT PPDU: 1) when a first symbol after an L-LTF signal is a BPSK symbol; and 2) when RL-SIG in which L-SIG is repeated is not detected. In addition, even if the receiving STA detects that the RL-SIG is repeated, when a result of applying “modulo 3” to the length value of the L-SIG is detected as “0”, the RX PPDU may be determined as the non-HT, HT, and VHT PPDU. 
     In the following example, a signal represented as a (TX/RX/UL/DL) signal, a (TX/RX/UL/DL) frame, a (TX/RX/UL/DL) packet, a (TX/RX/UL/DL) data unit, (TX/RX/UL/DL) data, or the like may be a signal transmitted/received based on the PPDU of  FIG. 18 . The PPDU of  FIG. 18  may be used to transmit/receive frames of various types. For example, the PPDU of  FIG. 18  may be used for a control frame. An example of the control frame may include a request to send (RTS), a clear to send (CTS), a power save-poll (PS-poll), BlockACKReq, BlockAck, a null data packet (NDP) announcement, and a trigger frame. For example, the PPDU of  FIG. 18  may be used for a management frame. An example of the management frame may include a beacon frame, a (re-)association request frame, a (re-)association response frame, a probe request frame, and a probe response frame. For example, the PPDU of  FIG. 18  may be used for a data frame. For example, the PPDU of  FIG. 18  may be used to simultaneously transmit at least two or more of the control frame, the management frame, and the data frame. 
       FIG. 19  illustrates an example of a modified transmission device and/or receiving device of the present specification. 
     Each device/STA of the sub-figure (a)/(b) of  FIG. 1  may be modified as shown in  FIG. 19 . A transceiver  630  of  FIG. 19  may be identical to the transceivers  113  and  123  of  FIG. 1 . The transceiver  630  of  FIG. 19  may include a receiver and a transmitter. 
     A processor  610  of  FIG. 19  may be identical to the processors  111  and  121  of  FIG. 1 . Alternatively, the processor  610  of  FIG. 19  may be identical to the processing chips  114  and  124  of  FIG. 1 . 
     A memory  620  of  FIG. 19  may be identical to the memories  112  and  122  of  FIG. 1 . Alternatively, the memory  620  of  FIG. 19  may be a separate external memory different from the memories  112  and  122  of  FIG. 1 . 
     Referring to  FIG. 19 , a power management module  611  manages power for the processor  610  and/or the transceiver  630 . A battery  612  supplies power to the power management module  611 . A display  613  outputs a result processed by the processor  610 . A keypad  614  receives inputs to be used by the processor  610 . The keypad  614  may be displayed on the display  613 . A SIM card  615  may be an integrated circuit which is used to securely store an international mobile subscriber identity (IMSI) and its related key, which are used to identify and authenticate subscribers on mobile telephony devices such as mobile phones and computers. 
     Referring to  FIG. 19 , a speaker  640  may output a result related to a sound processed by the processor  610 . A microphone  641  may receive an input related to a sound to be used by the processor  610 . 
     Hybrid automatic repeat request (HARQ) is a method of using a forward error correcting (FEC) scheme and an automatic error request (ARQ) scheme together. Unlike general automatic repeat request (ARQ), HARQ may additionally transmit information related to an FEC code capable of detecting an error. The receiving terminal may attempt error recovery through the FEC code, and when the error recovery fails, the receiving terminal may request retransmission to the transmitting terminal through ARQ. HARQ is used in standards such as high-speed downlink packet access (HSDPA), IEEE802.16e, and long term evolution (LTE), but HARQ has never been used in a contention-based wireless local area network (WLAN) environment. 
     In extreme high throughput (EHT), a standard being discussed after IEEE802.11ax, the introduction of HARQ is being considered. When HARQ is introduced, coverage can be widened in a low signal-to-noise ratio (SNR) environment, that is, an environment where the distance between the transmitting terminal and the receiving terminal is long, and a higher throughput may be obtained in a high SNR environment. 
     The receiving terminal, which receives the HARQ retransmitted frame, may perform decoding by combining the previously received original frame and the retransmitted frame. Here, it may be necessary to discuss the retransmission unit. HARQ retransmission may be performed in units of codewords at the PHY level, or HARQ retransmission may be performed in units of MPDUs at the MAC level. 
     Hereinafter, a hybrid automatic repeat request (HARQ) scheme applied to an embodiment of the present specification will be described. The terminal described below may be the apparatus of  FIGS. 1 and/or 19 , and the PPDU may be the PPDU of  FIG. 18 . The UE may be an AP or a non-AP STA. 
     The HARQ scheme is a scheme combining a forward error correction (FEC) scheme and an automatic repeat request (ARQ) scheme. According to the HARQ scheme, the physical layer checks whether the received data contains an error that cannot be decoded, and if an error occurs, the performance is improved by requesting retransmission. 
     The HARQ receiver basically attempts error correction on the received data and determines whether to retransmit using an error detection code. The error detection code may be various codes. For example, in the case of using a cyclic redundancy check (CRC), when an error in the received data is detected through a CRC detection process, the receiver transmits a negative-acknowledgement (NACK) signal to the transmitter. Upon receiving the NACK signal, the transmitter transmits appropriate retransmission data according to the HARQ mode. The receiver, which receives the retransmission data, improves reception performance by combining and decoding the previous data and the retransmission data. 
     The mode of HARQ can be divided into chase combining and incremental redundancy (IR). Chase combining is a method of obtaining a signal-to-noise ratio (SNR) gain by combining data with detected errors with retransmitted data without discarding the data. IR is a method of obtaining a coding gain by incrementally including additional redundant information in retransmitted data. 
       FIG. 20  is a diagram illustrating an example of chase combining. Chase combining is a method in which the same coded bit as in the initial transmission is retransmitted. 
       FIG. 21  is a diagram illustrating an example of an incremental redundancy (IR) method. In the incremental redundancy (IR) method, the coded bits that are initially transmitted and subsequently retransmitted may be different as follows. Accordingly, when the IR method is used, the STA performing retransmission generally delivers the IR version (or packet version/retransmission version) to the receiving STA. In the following drawings, the transmitting STA is an example of performing retransmission in the order of IR version 1, IR Version 2, IR Version 3, and IR Version 1. The receiving STA may combine and decode the received packet/signal. 
     HARQ may have the effect of expanding coverage in a low SNR environment (for example, an environment in which a transmitting terminal and a receiving terminal are far apart). HARQ may have the effect of increasing throughput in a high SNR environment. 
     In order to perform HARQ, the receiving terminal needs to inform the transmitting terminal of a response (i.e., an acknowledgment (ACK) or a negative acknowledgment (NACK)) to the data. When receiving a response (for example, a block ack (BA) frame, etc.) to a plurality of HARQ units (for example, an aggregated-MPDU (A-MPDU) including a plurality of MAC protocol data units (MPDU)), the transmitting terminal may consider an MPDU not included in the block ACK (BA) bitmap as a NACK, and the transmitting terminal may perform various operations (for example, HARQ incremental redundancy (IR)) based on this. However, if the receiving terminal successfully decodes the PHY header but fails to decode the S-MPDU (single-MPDU), or fails to decode all of the multiple HARQ units (for example, MPDU) due to an error, in the existing environment, the receiving terminal cannot transmit feedback on this. Therefore, the transmitting terminal cannot distinguish whether the transmitted PPDU has a collision or reception failure due to a channel error. That is, the transmitting terminal cannot know whether decoding the PHY header succeeds in the receiving terminal but only the data decoding fails, or whether decoding both the PHY header and the data fails. 
     Hereinafter, a NACK frame that can be used in the HARQ operation is defined. 
     Through this, the transmitting terminal could know whether there is a collision of transmitted data. A collision may mean a case in which decoding both the PHY header and data of the received signal by the receiving terminal fails. 
     When the receiving terminal fails to decode the PHY header of the reception signal, the receiving terminal may not store the data of the received signal in the buffer. That is, when the receiving terminal fails to decode the PHY header of the received signal (for example, the first signal), combining the retransmitted signal (for example, the second signal) and the received signal (for example, the first signal) may not be performed. When the receiving terminal fails to decode the PHY header of the reception signal, it may be determined that it will be difficult to use the data of the received signal as well. Relatively, since the PHY header is transmitted at a slower speed compared to the data, the probability of decoding failure is small compared to the data. Therefore, when decoding the PHY header fails, data included in the same signal may not be stored in a buffer for HARQ combining. 
     On the other hand, when the receiving terminal succeeds in decoding the PHY header of the received signal, but only the decoding the data of the received signal fails, the receiving terminal may store the data of the received signal in a buffer for the HARQ operation. The receiving terminal may combine the data of the received signal stored in the buffer and the retransmitted data. For HARQ combining, both a case combining (CC) scheme and an incremental redundancy (IR) scheme may be used. Alternatively, a separate HARQ operation may be performed based on the NACK signal. 
     When the PHY header is successfully decoded but the S-MPDU (single-MPDU) is not decoded, alternatively, if all MPDUs in an aggregated-MPDU (A-MPDU) are not decoded, the receiving terminal cannot directly find out the transmitter address (TA) of the transmitting terminal, because the MAC header has not been decoded. However, the receiving terminal may estimate whether it is a PPDU transmitted from an access point (AP) associated with it based on an uplink/downlink (UL/DL) field and a BSS Color field included in the PHY header. Otherwise, for example, when information for HARQ operation (for example, transmitter identifier (TXID), basic service set (BSS) color, short BSSID, receiver identifier (RXID), STA ID, association identifier (AID), New Data/ReTx indicator (initial transmission/retransmission indicator, etc.)) is included in the PHY header of the PPDU, the receiving terminal may know whether the PPDU and/or HARQ unit (for example, MPDU) is a signal transmitted to the corresponding terminal. 
       FIGS. 22 and 23  are diagrams illustrating an embodiment of a NACK frame. 
     Referring to  FIG. 22 , a NACK frame similar to an ACK frame used in an existing wireless local area network (WLAN) system may be configured. In IEEE802.11, when the bitstream value of the type field of the MAC frame is 01 and the bitstream value of the subtype field is 1101, the corresponding MAC frame may be related to information that it is an ACK frame. That is, the use of the MAC frame may be determined based on the values of the type field and the subtype field of the MAC frame. 
     The use of the MAC frame may be determined through a combination of a type field and a subtype field value. Some of the type field and subtype field values may be reserved values. For example, when the bitstream value of the type field is 01 and the bitstream value of the subtype field is 0000 to 0011, it may be a reserved value. For example, when the bitstream value of the type field is 01 and the bitstream value of the subtype field is 0000, the corresponding MAC frame may be defined to be used for the purpose of the NACK frame. 
     The bitstream value of the type field and the subtype field for the NACK frame is not limited to the values of the above embodiment. For example, the bitstream value of the type field and the subtype field for the NACK frame may be selected from the reserved values. 
     Referring to  FIG. 23 , a NACK frame may be configured by using another frame type already defined in an existing wireless local area network (WLAN) system. In IEEE802.11, when the bitstream value of the type field of a MAC frame is 01 and the bitstream value of the subtype field of the MAC frame is 0110, the corresponding MAC frame may include a control frame extension field. The use of the corresponding MAC frame may be determined based on the value of the control frame extension field. 
     For example, when the bitstream value of the type field is 01, the bitstream value of the subtype field is 0110, and the bitstream value of the control frame extension field is 0000, the corresponding MAC frame may be defined to be used for the purpose of the NACK frame. 
     The bitstream value of the control frame extension field for the NACK frame is not limited to the value of the above embodiment. For example, the bitstream value of the control frame extension field for the NACK frame may be selected from the reserved values. For example, the bitstream value of the control frame extension field for the NACK frame may be selected from other arbitrary values. 
       FIG. 24  is a diagram illustrating an embodiment of a NACK frame. 
     Referring to  FIG. 24 , a NACK frame having the same type field and subtype field values as the ACK frame used in the existing wireless local area network (WLAN) system may be configured. In IEEE802.11, when the bitstream value of the type field of the MAC frame is 01 and the bitstream value of the subtype field of the MAC frame is 1101, the corresponding MAC frame may be related to information that it is an ACK frame. The existing ACK frame includes To/From DS, Retry, Power Management, More Data, Protected Frame, Order fields, and etc. To/From DS, Retry, Power Management, More Data, Protected Frame, and Order field values included in the existing ACK frame are normally set to 0. The proposed NACK frame has the same type field and subtype field values as the ACK frame, but unlike the ACK frame, it may include information that the corresponding frame is a NACK frame by using any one of the To/From DS, Retry, Power Management, More Data, Protected Frame, and Order fields. For example, in the NACK frame, the bitstream value of the type field is 01, the bitstream value of the subtype field is 1101, the values of the To/From DS, Power Management, More Data, Protected Frame, and Order fields are 0, and the value of the Retry field may be set to 1. The field for the NACK frame is not limited to the Retry field of the above embodiment. A field having information that the corresponding frame is a NACK frame may be set to another field. For example, information that the corresponding frame is a NACK frame may be included by using at least one of the To DS, From DS, and Retry fields that are not used in the control frame. 
       FIG. 25  is a diagram illustrating an embodiment of a NACK frame. 
     Referring to  FIG. 25 , a NACK frame having the same structure as a block ACK (BA) frame used in an existing wireless local area network (WLAN) system may be configured. Like the BA frame, the NACK frame may include a MAC header, a BA control field, a BA information field, and an FCS field. The BA information field may include an AID TID information (info) field, a Block Ack Starting Sequence Control field, and a Block Ack Bitmap field. The AID TID info field may include AID 11, an ACK type subfield, and a TID subfield. Table 5 is a table showing the context defined in the existing BA frame. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                 Presence of Block 
                   
               
               
                   
                   
                 Ack Starting 
               
               
                 Ack 
                   
                 Sequence Control 
               
               
                 Type 
                 TID 
                 subfield and Block 
               
               
                 subfield 
                 subfield 
                 Ack Bitmap 
                 Context of a Per AID TID Info subfield in a Multi-STA 
               
               
                 values 
                 values 
                 subfields 
                 BlockAck frame 
               
               
                   
               
             
            
               
                 0 
                 0-7 
                 Present 
                 Block acknowledgment context: 
               
               
                   
                   
                   
                 Sent as an acknowledgment to QoS Data frames that solicit 
               
               
                   
                   
                   
                 a BlockAck frame response or to a BlockAckReq frame. 
               
               
                 1 
                 0-7 
                 Not present 
                 Acknowledgment context: 
               
               
                   
                   
                   
                 Sent as an acknowledgment to a QoS Data or QoS Null 
               
               
                   
                   
                   
                 frame that solicits an Ack frame response. 
               
               
                 0 or 1 
                  8-13 
                 N/A 
                 Reserved 
               
               
                 0 
                 14 
                 N/A 
                 Reserved 
               
               
                 1 
                 14 
                 Not present 
                 All ack context: 
               
               
                   
                   
                   
                 Sent as an acknowledgment to an A-MPDU or multi-TID 
               
               
                   
                   
                   
                 A-MPDU that contains an MPDU that solicits an immediate 
               
               
                   
                   
                   
                 response and all MPDUs contained in the A-MPDU or 
               
               
                   
                   
                   
                 multi-TID A-MPDU are received successfully. 
               
               
                 0 
                 15 
                 N/A 
                 Reserved 
               
               
                 1 
                 15 
                 Not present 
                 Management frame/PS-Poll acknowledgment context: 
               
               
                   
                   
                   
                 Sent as an acknowledgment to a Management frame carried 
               
               
                   
                   
                   
                 in an A-MPDU or S-MPDU, or PS-Poll frame in an S- 
               
               
                   
                   
                   
                 MPDU. 
               
               
                   
               
               
                 NOTE 
               
               
                 Additional rules for acknowledgment, block acknowledgment and all ack are defined in 26.4.2 (Acknowledgment context in a Multi-STA BlockAck frame) for a multi-TID A-MPDU. 
               
            
           
         
       
     
     In IEEE802.11, when the ACK type subfield value is 1 and the traffic identifier (TID) subfield value is 14, the BA frame may include information related to all ack context. All ack context may be used as information that all MPDUs included in the A-MPDU have been successfully received. 
     For example, if the transmitting terminal supports All ACK (i.e., if the All Ack Support subfield value of the HE Capabilities element field is 1), the receiving terminal may transmit a BA frame having an ACK type subfield value of 1 and a TID subfield value of 14 (i.e., a BA frame indicating that all MPDUs of the A-MPDU or multi-TID A-MPDU have been successfully received). The transmitting terminal may receive the BA frame from the receiving terminal. When the transmitting terminal receives the BA frame including the all ack context in response to single-MPDU (S-MPDU) transmission, the transmitting terminal may regard the BA frame including the all ack context as a NACK. 
     The receiving terminal generally does not transmit a BA frame in response to the S-MPDU. Also, in general, the receiving terminal does not transmit a BA frame including information related to the All ack context in response to the S-MPDU. However, if the transmitting terminal transmits the S-MPDU and the receiving terminal transmits a BA frame including information related to the all ack context in response to the S-MPDU, the transmitting terminal may regard the BA frame as a NACK. That is, the transmitting terminal could know that the receiving terminal has failed to receive the S-MPDU. 
     Although  FIG. 25  shows the NACK frame using the All ack context as above, the reserved subfield of Table 5 may be used for the NACK frame. For example, the receiving terminal may transmit a BA frame having an ACK type subfield value of 1 and a TID subfield value of 8, and the transmitting terminal may regard the BA frame as a NACK frame. That is, the transmitting terminal can know that the receiving terminal has failed to receive data. 
     In the case of an environment in which a channel error is frequent, the BA frame may also have a high probability of experiencing a channel error. Accordingly, the BA frame may be transmitted in a narrow-band. For example, the BA frame may be transmitted using 26 tone RUs or 52 tone RUs. 
       FIG. 26  is a diagram illustrating an embodiment of a NACK frame. 
     Referring to  FIG. 26 , a NACK frame having a structure similar to that of a block ACK (BA) frame used in an existing wireless local area network (WLAN) system may be configured. For example, the NACK frame may include a MAC header, a BA control field, and an FCS field like a BA frame, but may not include a BA information field. The MAC header may include a frame control field, a duration field, an RA field, and a TA field. 
     The TA field may include information related to the NACK frame. For example, when the individual/group bit included in the TA field is set to a specific value (for example, 1), the BA frame, including the TA field, may include information that it is a NACK frame. For example, when the private/group bit included in the TA field is set to 1, the TA field may have a bandwidth signaling TA value. A BA frame in which the private/group bit included in the TA field is set to 1 may be regarded as a NACK frame. Whether it is a NACK frame may be indicated through the private/group bit of the TA frame, so the BA information field may be omitted. 
     For example, the transmitting terminal may transmit the S-MPDU to the receiving terminal. When decoding the S-MPDU fails, the receiving terminal may transmit a BA frame to the transmitting terminal. The BA frame may include a TA field. The private/group bit of the TA field may be set to a value indicating a bandwidth signaling TA, and may include the MAC address of the receiving terminal. The transmitting terminal may receive the BA frame from the receiving terminal, and may know that the receiving terminal has failed to receive the S-MPDU. 
       FIG. 27  is a diagram illustrating an embodiment of a NACK frame. 
     Referring to  FIG. 27 , the BA control field may be omitted from the NACK frame of  FIG. 26 . Other technical features are the same as those of the embodiment of  FIG. 26 , and since a corresponding frame may be notified as a NACK frame through the TA field, the BA information field and the BA control field may be omitted. 
       FIG. 28  is a diagram illustrating an embodiment of a NACK frame. 
     Referring to  FIG. 28 , the NACK frame of  FIG. 28  may have the same structure as the NACK frame of  FIG. 26 . The NACK frame may include a MAC header, a duration field, an RA field, a TA field, a BA control field, and an FCS field. The BA control field may include a BA type subfield, a reserved field, and a TID INFO subfield. That is, when the value of the BA type subfield is 4, it may indicate that the BA frame including the BA type subfield is a NACK frame. 
       FIGS. 29 and 30  are diagrams illustrating HARQ transmission/retransmission procedures when a NACK frame is not used. 
     The signal (i.e., PPDU) transmitted in  FIGS. 29 and 30  may include only one MPDU (i.e., S-MPDU). Alternatively, although a plurality of MPDUs are included, the receiving terminal may fail to decode all MPDUs included in the PPDU. 
     Referring to  FIG. 29 , the receiving (Rx) terminal may fail to decode the PHY header (HDR) and data of the received signal. Hereinafter, a case in which the receiving terminal fails to decode both the PHY header and the data of the received signal is referred to as a collision case. 
     Referring to  FIG. 30 , the receiving (Rx) terminal may decode the PHY header of the received signal successfully, and may fail to decode the data. Hereinafter, a case in which the receiving terminal can decode the PHY header of the received signal, but fails to decode the data is referred to as a low link margin case. 
     In the collision case (i.e.,  FIG. 29 ), the transmitting terminal may transmit a signal (for example, PPDU) to the receiving terminal. The receiving terminal may fail to decode both the PHY header and the data of the signal, and may not transmit an ACK. When the transmitting terminal transmits a signal and does not receive an ACK from the receiving terminal within a threshold time, the transmitting terminal may retransmit the signal. For example, since the transmitting terminal cannot distinguish whether it is a collision case or a low link margin case, retransmission may be performed at the same speed (for example, code rate) as the initial transmission. For example, since the transmitting terminal cannot know the state of the receiving terminal, it can transmit the same signal as the previously transmitted signal. For example, even in the case of HARQ IR, the same redundancy version may be transmitted. The receiving terminal may receive the retransmitted signal. The receiving terminal may succeed in decoding the PHY header and the data of the retransmitted signal. When the receiving terminal succeeds in decoding the PHY header and the data of the retransmitted signal, it may transmit an ACK for the signal. HARQ combining may not be performed in the receiving terminal. 
     In the low link margin case (i.e.,  FIG. 30 ), the transmitting terminal may transmit a signal (for example, PPDU) to the receiving terminal. The receiving terminal may succeed in decoding the PHY header of the signal, but may fail in decoding the data. Since the receiving terminal has failed to decode the data, it may not transmit the ACK. The receiving terminal may store data in a buffer. When the transmitting terminal transmits a signal and does not receive an ACK from the receiving terminal within a threshold time, the transmitting terminal may retransmit the signal. For example, since the transmitting terminal cannot distinguish whether it is a collision case or a low link margin case, retransmission may be performed at the same speed (for example, code rate) as the initial transmission. For example, since the transmitting terminal cannot know the state of the receiving terminal, it can transmit the same signal as the previously transmitted signal. For example, even in the case of HARQ IR, the same redundancy version may be transmitted. The receiving terminal may receive the retransmitted signal. For example, the receiving terminal may perform HARQ combining (for example, HARQ IR or HARQ CC) on the data stored in the buffer and the retransmitted data. The receiving terminal may succeed in decoding the PHY header and the data of the retransmitted signal. When the receiving terminal succeeds in decoding the PHY header and the data of the retransmitted signal, it may transmit an ACK for the signal. 
       FIGS. 31 to 34  are diagrams illustrating HARQ transmission/retransmission procedures using the NACK frame of  FIGS. 22 to 28 . 
     A signal (i.e., PPDU) transmitted in  FIGS. 31 to 34  may include only one MPDU (i.e., S-MPDU). Alternatively, although a plurality of MPDUs are included, the receiving terminal may fail to decode all MPDUs included in the PPDU. 
     Referring to  FIG. 31 , a transmitting terminal may transmit a signal (for example, a PPDU) to a receiving terminal. The receiving terminal may fail to decode both the PHY header and the data of the signal. The receiving terminal cannot transmit a NACK frame. When the transmitting terminal transmits a signal and does not receive an ACK/NACK from the receiving terminal within a threshold time, the transmitting terminal may retransmit the signal. Since the transmitting terminal has not received even a NACK from the receiving terminal, it can be determined that it is a collision case. Accordingly, the transmitting terminal may retransmit the same code (for example, the same redundancy version) as the signal transmitted to the receiving terminal. For example, the transmitting terminal may perform retransmission at the same rate (for example, code rate) as the initial transmission. The receiving terminal may receive the retransmitted signal. The receiving terminal may succeed in decoding the PHY header and the data of the retransmitted signal. When the receiving terminal succeeds in decoding the PHY header and the data of the retransmitted signal, it may transmit an ACK for the signal. HARQ combining may not be performed in the receiving terminal. 
     Referring to  FIG. 32 , a transmitting terminal may transmit a signal (for example, a PPDU) to a receiving terminal. The receiving terminal may succeed in decoding the PHY header of the signal, but may fail in decoding the data. The receiving terminal may store the data in a buffer. The receiving terminal may transmit a NACK frame. The transmitting terminal may receive a NACK frame from the receiving terminal. The transmitting terminal may retransmit the signal based on the NACK frame. Since the transmitting terminal has received the NACK frame from the receiving terminal, it can be determined that it is a low link margin case. Accordingly, the transmitting terminal may retransmit a code different from the signal transmitted to the receiving terminal (for example, a different redundancy version). That is, the transmitting terminal retransmits the same data as the initial transmission, but the retransmitted signal may include a redundancy version different from the redundancy version of the initially transmitted signal. For example, the transmitting terminal may perform retransmission at a slower rate (for example, code rate) than the initial transmission. The receiving terminal may receive the retransmitted signal. The receiving terminal may perform HARQ combining on the data stored in the buffer and the retransmitted data. The receiving terminal may succeed in decoding the PHY header and the data of the retransmitted signal. When the receiving terminal succeeds in decoding the PHY header and the data of the retransmitted signal, it may transmit an ACK for the signal. 
     Referring to  FIG. 33 , in the same situation as in  FIG. 32 , the transmitting terminal may retransmit only additional redundant information, rather than retransmitting a self-decodable redundancy version. The receiving terminal may perform HARQ combining on the data stored in the buffer and the retransmitted additional extra information. The receiving terminal may succeed in decoding the PHY header and the data of the retransmitted signal. When the receiving terminal succeeds in decoding the PHY header and the data of the retransmitted signal, it may transmit an ACK for the signal. 
     Referring to  FIG. 34 , a transmitting terminal may transmit a signal (for example, a PPDU) to a receiving terminal. The receiving terminal may succeed in decoding the PHY header of the signal, but may fail in decoding the data. The receiving terminal may store the data in a buffer. The receiving terminal may transmit a NACK frame. For example, the NACK frame may be a BA frame based on  FIGS. 25 to 28 . For example, the NACK frame may be transmitted over a narrowband (for example, 26 tone RU or 52 tone RU). The transmitting terminal may receive a NACK frame from the receiving terminal. The transmitting terminal may retransmit the signal based on the NACK frame. Since the transmitting terminal has received the NACK frame from the receiving terminal, it can be determined that it is a low link margin case. Accordingly, the transmitting terminal may retransmit a code different from the signal transmitted to the receiving terminal (for example, a different redundancy version). That is, the transmitting terminal retransmits the same data as the initial transmission, but the retransmitted data may be a redundancy version different from the redundancy version of the initially transmitted data. For example, the transmitting terminal may perform retransmission at a slower rate (for example, code rate) than the initial transmission. The receiving terminal may receive the retransmitted signal. The receiving terminal may perform HARQ combining on the data stored in the buffer and the retransmitted data. The receiving terminal may succeed in decoding the PHY header and the data of the retransmitted signal. When the receiving terminal succeeds in decoding the PHY header and the data of the retransmitted signal, it may transmit an ACK (for example, Block ACK frame) for the signal. 
     In the procedure of  FIGS. 31 to 34 , the NACK frame may be generated in the MAC layer (i.e., the unit of HARQ may be an MPDU). Therefore, when the MAC layer of the receiving terminal fails to decode the received data, it may inform the PHY layer of whether the decoding the received data has failed. The PHY layer may determine whether to store data in the buffer based on information received from the MAC layer. 
       FIG. 35  is a diagram illustrating an embodiment of an operation of a transmitting STA. 
     The structure of the NACK frame may be determined based on  FIGS. 22 to 28 . 
     Referring to  FIG. 35 , a transmitting STA may perform association with a receiving STA (S 3510 ). For example, if the transmitting STA is a non-AP STA, it may perform association with a receiving STA that is an AP, and if the transmitting STA is an AP, it may perform association with a receiving STA that is a non-AP STA. 
     The transmitting STA may generate an MPDU (S 3520 ). For example, when the MAC layer of the transmitting STA receives, from the upper layer, data to be transmitted (for example, transmission control protocol/internet protocol (TCP/IP), protocol data unit (PDU), etc.), it may generate an MPDU based on data received from the higher layer. For example, information capable of mapping an MPDU and a codeword (for example, information related to a subframe size) may be included in the MAC level signal. For HARQ operation, the same data may have multiple redundancy versions. Even if the same data is retransmitted, different redundancy versions for the same data may be transmitted. Each redundancy version may contain information related to all of the same data. The receiving STA could obtain benefits from HARQ combining for redundancy versions. 
     The transmitting STA may generate a PPDU (S 3530 ). For example, the PHY layer of the transmitting STA may generate the PPDU by combining the PHY header with the MPDU. 
     The transmitting STA may transmit the PPDU to the receiving STA (S 3540 ). 
     The transmitting STA may determine a redundancy version to retransmit to the receiving STA (S 3550 ). For example, if the transmitting STA does not receive an ACK or NACK from the receiving STA within a threshold time, the transmitting STA may determine the redundancy version of the data to be retransmitted as the same redundancy version (for example, the first redundancy version) as the previously transmitted redundancy version (for example, the first redundancy version). The threshold time may be a preset value. For example, when the transmitting STA receives a NACK from the receiving STA within a threshold time, the transmitting STA may determine the redundancy version of the data to be retransmitted as a redundancy version (for example, the second redundancy version) different from the previously transmitted redundancy version (for example, the first redundancy version). When the transmitting terminal receives the NACK frame from the receiving terminal, the transmitting terminal may determine that it is a low link margin case. Accordingly, the transmitting terminal may retransmit a code (for example, the second redundancy version) different from the signal (for example, the first redundancy version) transmitted to the receiving terminal. That is, the transmitting terminal retransmits the same data as the initial transmission, but the retransmitted data may include a redundancy version different from the redundancy version of the initially transmitted data. 
     The transmitting STA may perform retransmission (S 3560 ). For example, the transmitting STA may retransmit the redundancy version determined in S 3550 . For example, the transmitting terminal may perform retransmission at a slower rate (for example, code rate) than the initial transmission. 
     The transmitting STA may receive an ACK from the receiving STA (S 3570 ). For example, the transmitting STA may receive an ACK for the retransmitted data in S 3560 . When the transmitting STA receives the ACK for the transmitted MPDU and there is no more data to transmit, the transmission/reception process may be terminated. 
       FIG. 36  is a diagram illustrating an embodiment of an operation of a receiving terminal. 
     The structure of the NACK frame may be determined based on  FIGS. 22 to 28 . 
     Referring to  FIG. 36 , a receiving STA may perform association with a transmitting STA (S 3610 ). For example, if the receiving STA is a non-AP STA, it may perform association with a transmitting STA that is an AP, and if the receiving STA is an AP, it may perform association with a transmitting STA that is a non-AP STA. 
     The receiving STA may receive the PPDU (S 3620 ). For example, the receiving STA may store the MPDU included in the received PPDU in a buffer. 
     The receiving STA may attempt to decode the received data (S 3630 ). For example, the MAC layer of the receiving STA may perform error checking at the MAC level using a frame check sum (FCS) field. For example, the decoding may be a procedure for confirming whether data is successfully received at the receiving STA. For example, the receiving terminal may succeed in decoding the PHY header of the signal, but may fail in decoding the data. The receiving terminal may store data in a buffer. 
     The receiving STA may transmit a NACK based on the decoding result (S 3640 ). For example, the receiving STA may transmit a MAC-level NACK based on the error check performed by the MAC layer. For example, the receiving STA may transmit a NACK when there is an error in the received MPDU. For example, the receiving STA may transmit a block ACK (BA) according to the error check result of each MPDU. 
     The receiving STA may receive the retransmitted PPDU (S 3650 ). For example, the receiving STA may receive a PPDU including the retransmitted MPDU. The receiving terminal may perform HARQ combining on the data stored in the buffer (for example, the first redundancy version) and the retransmitted data (for example, the second redundancy version) (S 3660 ). The receiving terminal may succeed in decoding the PHY header and the data of the retransmitted signal. When the receiving terminal succeeds in decoding the PHY header and the data of the retransmitted signal, it may transmit an ACK for the signal. 
     Some of the detailed steps shown in the example of  FIGS. 35 and 36  may not be essential steps and may be omitted. For example, steps S 3510 , S 3520 , S 3530 , and S 3570  of  FIG. 35  may be omitted. In addition, steps other than the steps shown in  FIGS. 35 and 36  may be added, and the order of the steps may vary. Some of the above steps may have their own technical meaning. 
     The technical features of the present specification described above may be applied to various devices and methods. For example, the above-described technical features of the present specification may be performed/supported through the apparatus of  FIGS. 1 and/or 19 . For example, the technical features of the present specification described above may be applied only to a part of  FIGS. 1 and/or 19 . For example, the technical features of the present specification described above are implemented based on the processing chips  114  and  124  of  FIG. 1 , or implemented based on the processors  111  and  121  and the memories  112  and  122  of  FIG. 1 , or, may be implemented based on the processor  610  and the memory  620  of  FIG. 19 . For example, an apparatus herein may include a memory and a processor operatively coupled to the memory, wherein the processor configured to transmit a PHY protocol data unit (PPDU) including a first redundancy version of transmission data to a receiving STA, and determine a second redundancy version of the transmission data to be retransmitted based on whether an acknowledgement (ACK) or negative ACK (NACK) frame is received within a threshold time. 
     The technical features of the present specification may be implemented based on a computer readable medium (CRM). For example, the CRM proposed by the present specification may store instructions being executed by at least one processor of a station (STA) in a wireless local area network (WLAN) system and causing the STA to perform operations. The operations may include transmitting a PHY protocol data unit (PPDU) including a first redundancy version of transmission data to a receiving STA, and determining a second redundancy version of the transmission data to be retransmitted based on whether an acknowledgement (ACK) or negative ACK (NACK) frame is received within a threshold time. 
     The instructions stored in the CRM of the present specification may be executed by at least one processor. At least one processor related to CRM in the present specification may be the processors  111  and  121  or the processing chips  114  and  124  of  FIG. 1 , or the processor  610  of  FIG. 19 . Meanwhile, the CRM of the present specification may be the memories  112  and  122  of  FIG. 1 , the memory  620  of  FIG. 19 , or a separate external memory/storage medium/disk. 
     The foregoing technical features of the present specification are applicable to various applications or business models. For example, the foregoing technical features may be applied for wireless communication of a device supporting artificial intelligence (AI). 
     Artificial intelligence refers to a field of study on artificial intelligence or methodologies for creating artificial intelligence, and machine learning refers to a field of study on methodologies for defining and solving various issues in the area of artificial intelligence. Machine learning is also defined as an algorithm for improving the performance of an operation through steady experiences of the operation. 
     An artificial neural network (ANN) is a model used in machine learning and may refer to an overall problem-solving model that includes artificial neurons (nodes) forming a network by combining synapses. The artificial neural network may be defined by a pattern of connection between neurons of different layers, a learning process of updating a model parameter, and an activation function generating an output value. 
     The artificial neural network may include an input layer, an output layer, and optionally one or more hidden layers. Each layer includes one or more neurons, and the artificial neural network may include synapses that connect neurons. In the artificial neural network, each neuron may output a function value of an activation function of input signals input through a synapse, weights, and deviations. 
     A model parameter refers to a parameter determined through learning and includes a weight of synapse connection and a deviation of a neuron. A hyper-parameter refers to a parameter to be set before learning in a machine learning algorithm and includes a learning rate, the number of iterations, a mini-batch size, and an initialization function. 
     Learning an artificial neural network may be intended to determine a model parameter for minimizing a loss function. The loss function may be used as an index for determining an optimal model parameter in a process of learning the artificial neural network. 
     Machine learning may be classified into supervised learning, unsupervised learning, and reinforcement learning. 
     Supervised learning refers to a method of training an artificial neural network with a label given for training data, wherein the label may indicate a correct answer (or result value) that the artificial neural network needs to infer when the training data is input to the artificial neural network. Unsupervised learning may refer to a method of training an artificial neural network without a label given for training data. Reinforcement learning may refer to a training method for training an agent defined in an environment to choose an action or a sequence of actions to maximize a cumulative reward in each state. 
     Machine learning implemented with a deep neural network (DNN) including a plurality of hidden layers among artificial neural networks is referred to as deep learning, and deep learning is part of machine learning. Hereinafter, machine learning is construed as including deep learning. 
     The foregoing technical features may be applied to wireless communication of a robot. 
     Robots may refer to machinery that automatically process or operate a given task with own ability thereof. In particular, a robot having a function of recognizing an environment and autonomously making a judgment to perform an operation may be referred to as an intelligent robot. 
     Robots may be classified into industrial, medical, household, military robots and the like according uses or fields. A robot may include an actuator or a driver including a motor to perform various physical operations, such as moving a robot joint. In addition, a movable robot may include a wheel, a brake, a propeller, and the like in a driver to run on the ground or fly in the air through the driver. 
     The foregoing technical features may be applied to a device supporting extended reality. 
     Extended reality collectively refers to virtual reality (VR), augmented reality (AR), and mixed reality (MR). VR technology is a computer graphic technology of providing a real-world object and background only in a CG image, AR technology is a computer graphic technology of providing a virtual CG image on a real object image, and MR technology is a computer graphic technology of providing virtual objects mixed and combined with the real world. 
     MR technology is similar to AR technology in that a real object and a virtual object are displayed together. However, a virtual object is used as a supplement to a real object in AR technology, whereas a virtual object and a real object are used as equal statuses in MR technology. 
     XR technology may be applied to a head-mount display (HMD), a head-up display (HUD), a mobile phone, a tablet PC, a laptop computer, a desktop computer, a TV, digital signage, and the like. A device to which XR technology is applied may be referred to as an XR device. 
     The claims recited in the present specification may be combined in a variety of ways. For example, the technical features of the method claim of the present specification may be combined to be implemented as a device, and the technical features of the device claims of the present specification may be combined to be implemented by a method. In addition, the technical characteristics of the method claim of the present specification and the technical characteristics of the device claim may be combined to be implemented as a device, and the technical characteristics of the method claim of the present specification and the technical characteristics of the device claim may be combined to be implemented by a method.