Patent Publication Number: US-2020280351-A1

Title: Operation method of station in wireless local area network

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of and claims priority to U.S. patent application Ser. No. 14/843,956, filed Sep. 2, 2015, which claims priority to U.S. Patent Application No. 62/045,446, filed on Sep. 3, 2014, and Korean Patent Application No. 10-2015-0124136 filed on Sep. 2, 2015, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a wireless local area network (WLAN) technology, and more particularly to a technology for multi-user transmission and reception in the WLAN. 
     2. Related Art 
     With the development of information communication technologies, a variety of wireless communication technologies have been developed. Among these technologies, wireless local area network (WLAN) is a technology that Internet access is possible in a wireless way in homes, business or specific service providing areas, using portable terminal such as personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), or the like, based on wireless frequency technologies. 
     WLAN technologies are created and standardized by the IEEE 802.11 Working Group under IEEE 802 Standard Committee. As such WLAN technology becomes more prevalent and its applications become more diverse, there is increasing demand for new WLAN technology that can support a higher throughput than existing WLAN technologies. Very high throughput (VHT) WLAN technology is proposed to support a data rate of 1 Gbps and higher. A WLAN technology according to IEEE 802.11ac standard is a technology providing VHT in sub 6 GHz band, and A WLAN technology according to IEEE 802. 11 ad standard is a technology providing VHT in 60 GHz band. 
     In addition to the above-described standards, various standards on WLAN technologies have been developed, and are being developed. As representative recent technologies, a WLAN technology according to IEEE 802.11af standard is a technology which has been developed for WLAN operation in TV white space bands, and a WLAN technology according to IEEE 802.11ah standard is a technology which has been developed for supporting a great number of stations operating with low power in sub 1 GHz band, and a WLAN technology according to IEEE 802.11ai standard is a technology which has been developed for supporting fast initial link setup (FILS) in WLAN systems. Also, IEEE 802.11ax standard is being developed for enhancing frequency efficiency of dense environments in which numerous access points and stations exist. 
     In the system based on such the WLAN technologies, in a case that data units for multiple users are transmitted through a physical layer convergence procedure (PLCP) protocol data unit (PPDU), methods for informing which users receive the data units and which resources are used for transmitting the data units are needed. 
     SUMMARY 
     Accordingly, exemplary embodiments of the present disclosure are provided to substantially obviate one or more problems due to limitations and disadvantages of the related art. Exemplary embodiments of the present disclosure provide scheduling methods and apparatuses for multi-user transmission. 
     In order to achieve the objectives of the present disclosure, an operation method performed in a first station according to an exemplary embodiment, the method may comprise: generating a high efficiency (HE) preamble including scheduling information of a plurality of reception stations; and generating a physical layer convergence procedure (PLCP) protocol data unit (PPDU) including a legacy preamble, the HE preamble, and a payload having data units to be transmitted to the plurality of reception stations. 
     Here, the HE preamble further includes information indicating a transmission manner of the PPDU. 
     Here, the PPDU is transmitted based on at least one of a multi user-multiple input multiple output (MU-MIMO) and an orthogonal frequency division multiple access (OFDMA). 
     Here, the scheduling information is included in a HE signal (HE-SIG) field of the HE preamble. 
     Here, the scheduling information includes resource allocation information indicating a resource allocated to the plurality of reception stations and identification information for each of the plurality of reception stations. 
     Here, the scheduling information further includes identification information of a sub-group to which the plurality of reception stations belong. 
     Here, the resource allocation information is an allocation pattern of spatial streams or an allocation pattern of frequency bands. 
     Here, the HE preamble further includes scheduling information of a plurality of groups to which the plurality of reception stations respectively belong. 
     Here, the scheduling information includes resource allocation information indicating a resource allocated to the plurality of groups and identification information for each of the plurality of groups. 
     Here, when the payload included in the PPDU is divided into a plurality of time domains and at least one time domain among the plurality of time domains has a different allocation pattern of frequency bands, the HE preamble includes scheduling information for each of the plurality of time domains. 
     Here, when the payload included in the PPDU is divided into a plurality of time domains, the HE preamble further includes information indicating that the payload is divided into the plurality of time domains. 
     Here, when the payload included in the PPDU is divided into a plurality of time domains, the HE preamble further includes information indicating lengths of data units included in each of the plurality of time domains. 
     Here, when the PPDU includes data units belonging to different access categories, the HE preamble further includes information indicating that the PPDU includes data units belonging to different access categories. 
     Here, when the PPDU includes data units belonging to different access categories, the HE preamble further includes traffic identifiers (TIDs) for access categories to which the data units respectively belong. 
     Here, the PPDU includes data units belonging to different access categories and information indicating an end of data units belonging to a first access category, and the information indicating the end of data units is located between data units belonging to the different access categories. 
     In order to achieve the objectives of the present disclosure, an operation method performed in a first station according to another exemplary embodiment, the method may comprise: obtaining a legacy preamble of a physical layer convergence procedure (PLCP) protocol data unit (PPDU); obtaining a high efficiency (HE) preamble of the PPDU; and obtaining at least one data unit included in a payload of the PPDU through a resource indicated by scheduling information for a plurality of reception stations included in the HE preamble. 
     Here, the HE preamble further includes information indicating a transmission manner of the PPDU. 
     Here, the scheduling information includes resource allocation information indicating a resource allocated to the plurality of reception stations and identification information for each of the plurality of reception stations. 
     Here, the scheduling information further includes identification information of a sub-group to which the plurality of reception stations belong. 
     Here, the resource allocation information is an allocation pattern of spatial streams or an allocation pattern of frequency bands. 
     Here, when the payload included in the PPDU is divided into a plurality of time domains and at least one time domain among the plurality of time domains has a different allocation pattern of frequency bands, the HE preamble includes scheduling information for each of the plurality of time domains. 
     Here, when the payload included in the PPDU is divided into a plurality of time domains, the HE preamble further includes information indicating lengths of data units included in each of the plurality of time domains. 
     According to exemplary embodiments of the present disclosure, scheduling methods for multi-user transmission (e.g., orthogonal frequency division multiple access (OFDMA), multi user-multiple input multiple output (MU-MIMO), etc.) may be provided. According to the scheduling methods for multi-user transmission, each of spatial streams may be used for transmitting data units of different users, and each of frequency bands included in a single spatial stream may be used for transmitting data units of different users. Also, respective data units of multiple users may be multiplexed and transmitted in a time domain of a single frequency band. 
     Also, according to the scheduling methods for multi-user transmission, space, frequency, and time resources can be efficiently utilized. Also, the scheduling methods can be performed with minimum scheduling information exchanges. Thus, the performance of WLAN can be enhanced, and Quality of Service (QoS) and Quality of Experience (QoE) for users may also be enhanced. 
     Also, an aggregate-MPDU (A-MPDU) including a plurality of medium access control (MAC) protocol data units (MPDUs) belonging to different access categories (e.g., AC_VO, AC_VI, AC_BE, and AC_BK) may be generated, and an A-MPDU including MPDUs having different frame formats (e.g., data frame, management frame, and control frame) may also be generated. Therefore, since an A-MPDU can be formed in various ways, the performance of WLAN systems may be enhanced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Exemplary embodiments of the present disclosure will become more apparent by describing in detail exemplary embodiments of the present disclosure with reference to the accompanying drawings, in which: 
         FIG. 1  is a block diagram illustrating a structure of a WLAN device according to an embodiment; 
         FIG. 2  is a schematic block diagram illustrating a transmitting signal processing unit  100  according to an embodiment suitable for use in a WLAN; 
         FIG. 3  is a schematic block diagram of a receiving signal processing unit according to an embodiment suitable for use in the WLAN; 
         FIG. 4  illustrates interframe space (IFS) relationships; 
         FIG. 5  is a timing diagram illustrating a frame transmission procedure based on a CSMA (carrier sense multiple access)/CA (collision avoidance) manner for avoiding collision between frames in a channel; 
         FIG. 6  is a flow chart illustrating a method of multi-user transmission according to an exemplary embodiment; 
         FIG. 7  is a block diagram illustrating an exemplary embodiment of PPDU; 
         FIG. 8  is a block diagram illustrating scheduling information included in HE preamble; 
         FIG. 9  is a conceptual diagram illustrating an exemplary embodiment of an allocation pattern of spatial streams; 
         FIG. 10  is a conceptual diagram illustrating an exemplary embodiment of an allocation pattern of frequency bands; 
         FIG. 11  is a block diagram illustrating an exemplary embodiment of a payload included in a PPDU; and 
         FIG. 12  is a block diagram illustrating another exemplary embodiment of a payload included in a PPDU. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, only certain embodiments of the present disclosure have been shown and described, simply by way of illustration. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present disclosure. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification. 
     In a wireless local area network (WLAN), a basic service set (BSS) includes a plurality of WLAN devices. The WLAN device may include a medium access control (MAC) layer and a physical (PHY) layer according to IEEE (Institute of Electrical and Electronics Engineers) 802.11 standard. In the plurality of WLAN devices, at least one WLAN device may be an access point and the other WLAN devices may be non-AP stations (non-AP STAs). Alternatively, all of the plurality of WLAN devices may be non-AP STAs in Ad-hoc networking. In general, the AP STA and the non-AP STAs may be collectively called the STA. However, for ease of description herein, only the non-AP STAs are referred to as the STAs. 
       FIG. 1  is a block diagram illustrating a structure of a WLAN device according to an embodiment. 
     Referring to  FIG. 1 , the WLAN device  1  may include a baseband processor  10 , a radio frequency (RF) transceiver  20 , an antenna unit  30 , a memory  40 , an input interface unit  50 , an output interface unit  60 , and a bus  70 . The baseband processor  10  may perform baseband signal processing, and may include a MAC processor  11  and a PHY processor  15 . 
     In one embodiment, the MAC processor  11  may include a MAC software processing unit  12  and a MAC hardware processing unit  13 . The memory  40  may store software (hereinafter referred to as “MAC software”) including at least some functions of the MAC layer. The MAC software processing unit  12  executes the MAC software to implement the some functions of the MAC layer, and the MAC hardware processing unit  13  may implement remaining functions of the MAC layer as hardware (hereinafter referred to “MAC hardware”). However, embodiments of the MAC processor  11  are not limited to this. The PHY processor  15  may include a transmitting (Tx) signal processing unit  100  and a receiving (Rx) signal processing unit  200 . 
     The baseband processor  10 , the memory  40 , the input interface unit  50 , and the output interface unit  60  may communicate with each other via the bus  70 . The RF transceiver  20  may include an RF transmitter  21  and an RF receiver  22 . The memory may further store an operating system and applications. The input interface unit  50  receives information from a user, and the output interface unit  60  outputs information to the user. 
     The antenna unit  30  includes one or more antennas. When multiple-input multiple-output (MIMO) or multi-user MIMO (MU-MIMO) is used, the antenna unit  30  may include a plurality of antennas. 
       FIG. 2  is a schematic block diagram illustrating a transmitting signal processing unit  100  according to an embodiment suitable for use in a WLAN. 
     Referring to  FIG. 2 , a transmitting signal processing unit  100  may include an encoder  110 , an interleaver  120 , a mapper  130 , an inverse Fourier transformer (IFT)  140 , and a guard interval (GI) inserter  150 . 
     The encoder  110  encodes input data. For example, the encoder  100  may be a forward error correction (FEC) encoder. The FEC encoder may include a binary convolutional code (BCC) encoder followed by a puncturing device, or may include a low-density parity-check (LDPC) encoder. 
     The transmitting signal processing unit  100  may further include a scrambler for scrambling the input data before the encoding to reduce the probability of long sequences of 0s or 1s. If BCC encoding is used in the encoder, the transmitting signal processing unit  100  may further include an encoder parser for demultiplexing the scrambled bits among a plurality of BCC encoders. If LDPC encoding is used in the encoder, the transmitting signal processing unit  100  may not use the encoder parser. 
     The interleaver  120  interleaves the bits of each stream output from the encoder to change an order of bits. Interleaving may be applied only when BCC encoding is used. The mapper  130  maps the sequence of bits output from the interleaver to constellation points. If the LDPC encoding is used in the encoder, the mapper  130  may further perform LDPC tone mapping besides the constellation mapping. 
     When the MIMO or the MU-MIMO is used, the transmitting signal processing unit  100  may use a plurality of interleavers  120  and a plurality of mappers  130  corresponding to a number of spatial streams Nss. In this case, the transmitting signal processing unit  100  may further include a stream parser for dividing outputs of the BCC encoders or the LDPC encoder into blocks that are sent to different interleavers  120  or mappers  130 . The transmitting signal processing unit  100  may further include a space-time block code (STBC) encoder for spreading the constellation points from the NSS spatial streams into NSTS space-time streams and a spatial mapper for mapping the space-time streams to transmit chains. The spatial mapper may use direct mapping, spatial expansion, or beamforming. 
     The IFT  140  converts a block of the constellation points output from the mapper  130  or the spatial mapper to a time domain block (i.e., a symbol) by using an inverse discrete Fourier transform (IDFT) or an inverse fast Fourier transform (IFFT). If the STBC encoder and the spatial mapper are used, the inverse Fourier transformer  140  may be provided for each transmit chain. 
     When the MIMO or the MU-MIMO is used, the transmitting signal processing unit  100  may insert cyclic shift diversities (CSDs) to prevent unintentional beamforming. The CSD insertion may occur before or after the inverse Fourier transform. The CSD may be specified per transmit chain or may be specified per space-time stream. Alternatively, the CSD may be applied as a part of the spatial mapper. When the MU-MIMO is used, some blocks before the spatial mapper may be provided for each user. 
     The GI inserter  150  prepends a guard interval (GI) to the symbol. The transmitting signal processing unit  100  may optionally perform windowing to smooth edges of each symbol after inserting the GI. The RF transmitter  21  converts the symbols into an RF signal and transmits the RF signal via the antenna unit  30 . When the MIMO or the MU-MIMO is used, the GI inserter  150  and the RF transmitter  21  may be provided for each transmit chain. 
       FIG. 3  is a schematic block diagram of a receiving signal processing unit according to an embodiment suitable for use in the WLAN. 
     Referring to  FIG. 3 , a receiving signal processing unit  200  may include a GI remover  220 , a Fourier transformer (FT)  230 , a demapper  240 , a deinterleaver  250 , and a decoder  260 . An RF receiver  22  receives an RF signal via the antenna unit  30  and converts the RF signal into a symbol. The GI remover  220  removes the GI from the symbol. When the MIMO or the MU-MIMO is used, the RF receiver  22  and the GI remover  220  may be provided for each receive chain. 
     The FT  230  converts the symbol (i.e., the time domain block) into a block of the constellation points by using a discrete Fourier transform (DFT) or a fast Fourier transform (FFT). The Fourier transformer  230  may be provided for each receive chain. When the MIMO or the MU-MIMO is used, the receiving signal processing unit  200  may include a spatial demapper for converting the Fourier transformed receiver chains to constellation points of the space-time streams, and an STBC decoder for despreading the constellation points from the space-time streams into the spatial streams. 
     The demapper  240  demaps the constellation points output from the Fourier transformer  230  or the STBC decoder to the bit streams. If the LDPC encoding is used, the demapper  240  may further perform LDPC tone demapping before the constellation demapping. The deinterleaver  250  deinterleaves the bits of each stream output from the demapper  240 . Deinterleaving may be applied only when BCC encoding is used. 
     When the MIMO or the MU-MIMO is used, the receiving signal processing unit  200  may use a plurality of demappers  240  and a plurality of deinterleavers  250  corresponding to the number of spatial streams. In this case, the receiving signal processing unit  200  may further include a stream deparser for combining the streams output from the deinterleavers  250 . 
     The decoder  260  decodes the streams output from the deinterleaver  250  or the stream deparser. For example, the decoder  100  may be an FEC decoder. The FEC decoder may include a BCC decoder or an LDPC decoder. The receiving signal processing unit  200  may further include a descrambler for descrambling the decoded data. If BCC decoding is used in the decoder, the receiving signal processing unit  200  may further include an encoder deparser for multiplexing the data decoded by a plurality of BCC decoders. If LDPC decoding is used in the decoder  260 , the receiving signal processing unit  100  may not use the encoder deparser. 
       FIG. 4  illustrates interframe space (IFS) relationships. 
     Referring to  FIG. 4 , a data frame, a control frame, or a management frame may be exchanged between WLAN devices. The data frame is used for transmission of data forwarded to a higher layer. The WLAN device transmits the data frame after performing backoff if a distributed coordination function IFS (DIFS) has elapsed from a time when the medium has been idle. 
     The management frame is used for exchanging management information which is not forwarded to the higher layer. Subtype frames of the management frame include a beacon frame, an association request/response frame, a probe request/response frame, and an authentication request/response frame. The control frame is used for controlling access to the medium. Subtype frames of the control frame include a request to send (RTS) frame, a clear to send (CTS) frame, and an acknowledgement (ACK) frame. When the control frame is not a response frame of a previous frame, the WLAN device transmits the control frame after performing backoff when the DIFS has elapsed. When the control frame is the response frame of the previous frame, the WLAN device transmits the control frame without performing backoff when a short IFS (SIFS) has elapsed. The type and subtype of frame may be identified by a type field and a subtype field in a frame control field. 
     On the other hand, a Quality of Service (QoS) STA may transmit the frame after performing backoff when an arbitration IFS (AIFS) for access category (AC), i.e., AIFS[AC] has elapsed. In this case, the data frame, the management frame, or the control frame which is not the response frame may use the AIFS [AC]. 
       FIG. 5  is a timing drawing illustrating a frame transmission procedure based on a CSMA (carrier sense multiple access)/CA (collision avoidance) manner for avoiding collision between frames in a channel. 
     Referring to  FIG. 5 , STA 1  is a transmit WLAN device for transmitting data, STA 2  is a receive WLAN device for receiving the data, and STA 3  is a third WLAN device which may be located at an area where a frame transmitted from the STA 1  and/or a frame transmitted from the STA 2  can be received by the third WLAN device STA 3 . 
     The STA 1  may determine whether the channel is busy by carrier sensing. The STA 1  may determine the channel occupation based on an energy level on the channel or correlation of signals in the channel, or may determine the channel occupation by using a network allocation vector (NAV) timer. 
     When it is determined that the channel is not in use by other devices during DIFS (that is, the channel is idle), the STA 1  may transmit an RTS frame to the STA 2  after performing backoff. Upon receiving the RTS frame, the STA 2  may transmit a CTS frame as a response of the CTS frame after a SIFS. 
     When the STA 3  receives the RTS frame, it may set the NAV timer for a transmission duration of subsequently transmitted frames (for example, a duration of SIFS+CTS frame duration+SIFS+data frame duration+SIFS+ACK frame duration) by using duration information included in the RTS frame. When the STA 3  receives the CTS frame, it may set the NAV timer for a transmission duration of subsequently transmitted frames (for example, a duration of SIFS+data frame duration+SIFS+ACK frame duration) by using duration information included in the CTS frame. Upon receiving a new frame before the NAV timer expires, the STA 3  may update the NAV timer by using duration information included in the new frame. The STA 3  does not attempt to access the channel until the NAV timer expires. 
     When the STA 1  receives the CTS frame from the STA 2 , it may transmit a data frame to the STA 2  after a SIFS elapses from a time when the CTS frame has been completely received. Upon successfully receiving the data frame, the STA 2  may transmit an ACK frame as a response of the data frame after a SIFS elapses. 
     When the NAV timer expires, the STA 3  may determine whether the channel is busy by the carrier sensing. Upon determining that the channel is not in use by the other devices during DIFS after the NAV timer has expired, the STA 3  may attempt the channel access after a contention window according to random backoff operation. 
     Meanwhile, multi-user transmission, such as orthogonal frequency division multiple access (OFDMA), MU-MIMO, and so on, may be performed in the WLAN. In the below description, methods for multi-user transmission in the WLAN will be explained. 
       FIG. 6  is a flow chart illustrating a method of multi-user transmission according to an exemplary embodiment. 
     Referring to  FIG. 6 , stations STA 1  to STA 9  may establish a BSS. Here, the STA 1  may be an AP, and each of the STA 2  to STA 9  may be a non-AP STA. The STA 1  may determine stations which will participate in multi-user transmission (S 600 ). For example, the STA 1  may transmit a first frame including an indicator requesting identification information (e.g., MAC address, association identifier (AID), partial AID (PAID), etc.) of the stations participating in multi-user transmission in a broadcast manner or a unicast manner. The STA 1  may receive second frames including identification information of the STA 2  to STA 9  as responses corresponding to the first frame. In this case, the STA 1  may determine the STA 2  to STA 9  which transmitted the second frames as stations participating in multi-user transmission. Each of the first frame and the second frames may be a management frame, a control frame, or a data frame. 
     The STA 1  may set the stations participating in multi-user transmission to at least one sub-group. The STA 1  may set a sub-group ID corresponding to each sub-group for identifying respective sub-groups, and set respective IDs for stations included in the respective sub-groups. For example, the STA 1  may set a sub-group  1  including the STA 2  to STA 5 , and set its sub-group ID to ‘00.’ Also, for the sub-group  1 , the STA 1  may set an ID of the STA 2  to ‘00,’ an ID of the STA 3  to ‘01,’ an ID of the STA 4  to ‘10,’ and an ID of the STA 5  to ‘11’ Alternatively, each of the IDs of the STA 2  to STA 5  may be set to AID or PAID. 
     Also, the STA 1  may set a sub-group  2  including the STA 6  to STA 9 , and set its sub-group ID to ‘01’. Also, for the sub-group  2 , the STA 1  may set an ID of the STA 6  to ‘00,’ an ID of the STAT to ‘01,’ an ID of the STA 8  to ‘ 10 ,’ and an ID of the STA 9  to ‘11.’ Alternatively, each of the IDs of the STA 6  to STA 9  may be set to AID or PAID. The STA 1  may set at least one group including a plurality of sub-groups, and set a group ID for identifying respective groups. For example, the STA 1  may set a group  1  including the sub-group  1  and the sub-group  2 , and set an ID of the group  1  to ‘00.’ Here, each of ‘00,’ ‘01,’ ‘10,’ and ‘11’ may be a binary number. According to the above-described manner, the relation among the group ID, the sub-group ID, and the station ID may be set as represented in the below table 1. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Group ID 
                   
                 Sub-group ID 
                   
                 Station ID 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Group 1 
                 00 
                 Sub-group 1 
                 00 
                 STA2 
                 00 
               
               
                   
                   
                   
                   
                   
                 STA3 
                 01 
               
               
                   
                   
                   
                   
                   
                 STA4 
                 10 
               
               
                   
                   
                   
                   
                   
                 STA5 
                 11 
               
               
                   
                   
                   
                 Sub-group 2 
                 01 
                 STA6 
                 00 
               
               
                   
                   
                   
                   
                   
                 STA7 
                 01 
               
               
                   
                   
                   
                   
                   
                 STA8 
                 10 
               
               
                   
                   
                   
                   
                   
                 STA9 
                 11 
               
               
                   
                   
               
            
           
         
       
     
     According to the table 1, in a case that ‘group ID—sub-group ID—station ID’ is used for multi-user transmission, the STA 2  may be identified as ‘00 00 00,’ and the STA 7  may be identified as ‘00 01 01.’ Alternatively, in a case that ‘sub-group ID—station ID’ is used for multi-user transmission, the STA 2  may be identified as ‘00 00,’ and the STA 7  may be identified as ‘01 01.’ The set manner of ‘group ID—sub-group ID—station ID’ is not restricted to the above-described manner. For example, the set manner of ‘group ID—sub-group ID—station ID’ may vary according to various exemplary embodiments. 
     Alternatively, instead of the above-described manner (e.g., group—sub-group), the STA 1  may set stations participating in multi-user transmission to at least one group. As represented in the below table 2, the STA 2  to STA 9  may be identified by using ‘group ID—station ID.’ 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Group ID 
                   
                 Station ID 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Group 1 
                 00 
                 STA2 
                 00 
               
               
                   
                   
                   
                 STA3 
                 01 
               
               
                   
                   
                   
                 STA4 
                 10 
               
               
                   
                   
                   
                 STA5 
                 11 
               
               
                   
                 Group 2 
                 01 
                 STA6 
                 00 
               
               
                   
                   
                   
                 STA7 
                 01 
               
               
                   
                   
                   
                 STA8 
                 10 
               
               
                   
                   
                   
                 STA9 
                 11 
               
               
                   
                   
               
            
           
         
       
     
     The STA 1  may generate a Physical Layer Convergence Procedure (PLCP) protocol data unit (PPDU) to be transmitted according to multi-user transmission manner (S 610 ). Here, the PPDU may be constructed as described below. 
       FIG. 7  is a block diagram illustrating an exemplary embodiment of PPDU. 
     Referring to  FIG. 7 , when multi-user transmission is performed based on MU-MIMO, the STA 1  may generate the PPDU transmitted through spatial streams SS- 1 , SS- 2 , . . . , SS-n. Here, n is a natural number larger than 3. The PPDU may be transmitted through frequency bandwidth 20 MHz, 40 MHz, 80 MHz, 160 MHz, 320 MHz, etc. In the PPDU, fields subsequent to a HE-signal A (HE-SIG-A) field may be transmitted through frequency bandwidth smaller than 20 MHz (e.g., 2.5 MHz, 5 MHz, 10 MHz, etc.) Each of the frequency bands FB- 1 , FB- 2 , FB- 3 , and FB- 4  may have a frequency bandwidth 20 MHz. Also, in the frequency bands, subcarrier spacing may be 312.5 kHz, 156.25 kHz, or 78.125 kHz. In the present disclosure, the PPDU transmitted through frequency bandwidth 80 HMz will be described. 
     The PPDU may include a legacy preamble, a High Efficiency (HE) preamble, and a payload. The legacy preamble may include a Legacy-Short Training Field (L-STF), a Legacy-Long Training Field (L-LTF), and a Legacy-Signal (L-SIG) field. The HE preamble may include the HE-SIG-A field, a HE-STF field, at least one HE-LTF, and HE-SIG-B field. The HE-SIG-A field may include a HE-SIG-A1 field, a HE-SIG-A2 field, etc. The HE-SIG-A field may be duplicated in unit of 20 MHz. The number of HE-LTFs included in the HE preamble may be determined according to the number of spatial streams through which the PPDU is transmitted. For example, when the PPDU is transmitted through a single spatial stream in frequency bandwidth 20 MHz, a single HE-LTF may be included in the HE preamble. Meanwhile, when the number of spatial streams through which the PPDU is transmitted is 4, 4 HE-LTFs may be included in the HE preamble. The HE-SIG-B field may include a HE-SIG-B1 field, a HE-SIG-B2 field, etc. The structure of the HE preamble is not restricted to the above-described example. That is, the HE preamble may have various structures. 
     The HE-SIG fields (e.g., the HE-SIG-A field, the HE-SIG-B field, etc.) may include a first identifier. The first identifier may indicate which transmission manner is used for transmitting the PPDU including the HE-SIG fields (or, the payload included in the PPDU). For example, the first identifier may be set as represented in the below table 3. 
     
       
         
           
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 First Identifier 
                 Transmission Manner 
               
               
                   
               
             
            
               
                 00 
                 Non-MU-MIMO (SU-MIMO), non-OFDMA (OFDM) 
               
               
                 01 
                 MU-MIMO 
               
               
                 10 
                 OFDMA 
               
               
                 11 
                 MU-MIMO, OFDMA 
               
               
                   
               
            
           
         
       
     
     According to the table 3, when the first identifier is set to ‘00,’ it may indicate that the PPDU is transmitted in based on OFDM (e.g., a transmission manner except multi-user transmission manners such as MU-MIMO and OFDMA). When the first identifier is set to ‘01,’ it may indicate that the PPDU is transmitted based on MU-MIMO. When the first identifier is set to ‘ 10 ,’ it may indicate that the PPDU is transmitted based on OFDMA. Also, when the first identifier is set to ‘11,’ it may indicate that the PPDU is transmitted based on MU-MIMO/OFDMA. Here, each of ‘00,’ ‘01,’ ‘10,’ and ‘11’ may be a binary number. The set manner of the first identifier is not restricted to the above-described example. That is, the first identifier may be set in various manners. For example, when the first identifier is set to binary number ‘0,’ it may indicate that the PPDU is transmitted based on non-OFDMA. When the first identifier is set to binary number ‘1,’ it may indicate that the PPDU is transmitted based on OFDMA. Alternatively, in a case that there is scheduling information for multi-user transmission which will be described below in the HE preamble, the scheduling information may indicate that the PPDU is transmitted according to a transmission manner corresponding to the scheduling information. On the contrary, when the scheduling information do not exist in the HE preamble, it may indicate that the PPDU is transmitted based on OFDM. Alternatively, the first identifier may indicate that the PPDU is transmitted based on single user-single input single output (SU-SISO) (or, SU-MIMO). According to the transmission manner indicated by the first identifier, the length of the HE-SIG-B field included in the HE preamble may vary. For example, in a case that the transmission manner of the PPDU indicated by the first identifier is SU-SISO or SU-MIMO, since scheduling information for reception stations is not necessary, the length of HE-SIG-B field of the HE preamble may be 0. 
     Meanwhile, the HE preamble may include scheduling information for multi-user transmission, and the scheduling information included in the HE preamble may vary according to the transmission manner of the PPDU. Alternatively, instead of including the scheduling information in the HE preamble, an additional frame including the scheduling information may be generated. In this case, the STA 1  may transmit the frame including the scheduling information, and then transmit the PPDU based on the scheduling information. In the below description, the HE preamble used for PPDU transmissions based on MU-MIMO, OFDMA, and MU-MIMO/OFDMA will be described. 
       FIG. 8  is a block diagram illustrating scheduling information included in HE preamble. 
     Referring to  FIG. 8 , the HE-SIG field  810  (e.g., the HE-SIG-A field, the HE-SIG-B field, etc.) of the HE preamble used for transmitting the PPDU based on MU-MIMO may include a sub-group ID field  811 , a resource allocation information field  812 , and a station ID list field  813 . The sub-group ID field  811  may indicate a sub-group ID according to the above-described table 1. The resource allocation information field  812  may indicate an allocation pattern of at least one spatial stream. The size of the resource allocation information field  812  may vary according to the resource allocation pattern or the number of reception stations. In a case that the size of the resource allocation information field  812  varies, the HE preamble may include information indicating that the size of the resource allocation information field  812  varies and further include information indicating the size of the scheduling information (or, the size of the resource allocation information field  812 ). In this case, each of the information indicating the size of the resource allocation information field  812  varies and the information indicating the size of the scheduling information or the size of the resource allocation information field  812  may be positioned prior to the field including the scheduling information (or, the resource allocation information field  812 ) in the HE preamble. 
     Meanwhile, the scheduling information may be included in the HE-SIG-A field or the HE-SIG-B field among the HE-SIG fields of the HE preamble. Here, the HE-SIG-B field may include a common filed or a user-specific field. Among the scheduling information, common information for reception stations may be included in the common field of the HE-SIG-B field, and specific information for a specific reception station may be included in the user-specific field of the HE-SIG-B field. Alternatively, the scheduling information may be included in the HE-SIG-A field or the HE-SIG-B field of the HE preamble. For example, among the scheduling information, a part of common information for reception stations may be included in the HE-SIG-A field, and the rest of them may be included in the common field of the HE-SIG-B field. Also, the information used for a specific reception station may be included in the user-specific field of the HE-SIG-B filed. 
       FIG. 9  is a conceptual diagram illustrating an exemplary embodiment of an allocation pattern of spatial streams. 
     Referring to  FIG. 9 , in a case that the PPDU is transmitted through four spatial streams SS- 1 , SS- 2 , SS- 3 , and SS- 4 , when the resource allocation information is set to ‘00,’ it may indicate that each of the spatial streams SS- 1 , SS- 2 , SS- 3 , and SS- 4  is allocated for a different station. Also, when the resource allocation information is set to ‘01,’ it may indicate that two spatial streams SS- 1  and SS- 2  are allocated for a station, the spatial stream SS- 3  is allocated for another station, and the spatial stream SS- 4  is allocated for the other station. Also, when the resource allocation information is set to ‘10,’ it may indicate that the spatial streams SS- 1  and SS- 3  are allocated for a station, the spatial stream SS- 2  is allocated for another station, and the spatial stream SS- 4  is allocated for the other station. Also, when the resource allocation information is set to ‘11,’ it may indicated that the spatial stream SS- 1  is allocated for a station, the spatial streams SS- 2  and SS- 3  are allocated for another station, and the spatial stream SS- 4  is allocated for the other station. Here, each of ‘00,’ ‘01,’ ‘10,’ and ‘11’ may be a binary number. The set manner of the resource allocation information and the allocation pattern of spatial streams are not restricted to the above-described example. That is, the resource allocation information and the allocation pattern of spatial streams may vary according to exemplary embodiments. 
     Re-referring to  FIG. 8 , the station ID list field  813  may include respective IDs of the stations participating in the PPDU transmission based on the MU-MIMO among stations included in the sub-group indicated by the sub-group ID. Here, the station ID may be a station ID described by referring to the table 1. For example, when the sub-group ID is set to ‘00’, among the STA 2  to STA 5 , the station ID list field  813  may include respective IDs (e.g., 00, 01, 10, and 11) of stations participating in the PPDU transmission based on the MU-MIMO. 
     According to the above description, in a case that the scheduling information ‘sub-group ID field  811 +resource allocation information field  812 +station ID list field  813 ’ is set to ‘00 00 10110100,’ it may indicate that at least one data unit (e.g., MAC protocol data unit (MPDU) or aggregated-MPDU (A-MPDU)) for the STA 4  is transmitted through the spatial stream SS- 1 , at least one data unit for the STA 5  is transmitted through the spatial stream SS- 2 , at least one data unit for the STA 3  is transmitted through the spatial stream SS- 3 , and at least one data unit for the STA 2  is transmitted through the spatial stream SS- 4 . In other words, data units for respective stations indicated by the first, second, third, and fourth station IDs included in the station ID list field  813  may be transmitted through each of the spatial streams SS- 1 , SS- 2 , SS- 3 , and SS- 4 . 
     In a case that the scheduling information ‘sub-group ID field  811 +resource allocation information field  812 +station ID list field  813 ’ is set to ‘00 01 100100,’ it may indicate that data units for the STA 4  are transmitted through the spatial streams SS- 1  and SS- 2 , at least one data unit for the STA 3  is transmitted through the spatial stream SS- 3 , and at least one data unit for the STA 2  is transmitted through the spatial stream SS- 4 . Here, since the data units for the STA 4  are transmitted through two contiguous spatial streams SS- 1  and SS- 2 , the ID of the STA 4  may be included in the station ID list field  813  only once. That is, since the station allocated to the spatial stream SS- 1  and the station allocated to the spatial stream SS- 2  are identical, the first ID included in the station ID list field  813  may indicate not only the station allocated to the spatial stream SS- 1  but also the station allocated to the spatial stream SS- 2 . Alternatively, in this case, the scheduling information ‘sub-group ID field  811 +resource allocation information field  812 +station ID list field  813 ’ may be set to ‘00 01 10100100.’ That is, the ID of the STA 4  may not be omitted in the station ID list field  813 . 
     In a case that the scheduling information ‘sub-group ID field  811 +resource allocation information field  812 +station ID list field  813 ’ is set to ‘00 10 100100,’ it may indicate that data units for the STA 4  are transmitted through the spatial streams SS- 1  and SS- 3 , at least one data unit for the STA 3  is transmitted through the spatial stream SS- 2 , and at least one data unit for the STA 2  is transmitted through the spatial stream SS- 4 . Here, since the data units for the STA 4  are transmitted through two non-contiguous spatial streams SS- 1  and SS- 3 , the ID of the STA 4  may be included in the station ID list field  813  only once. That is, since the station allocated to the spatial stream SS- 1  and the station allocated to the spatial stream SS- 3  are identical, the first ID included in the station ID list field  813  may indicate not only the station allocated to the spatial stream SS- 1  but also the station allocated to the spatial stream SS- 3 . Alternatively, in this case, the scheduling information ‘sub-group ID field  811 +resource allocation information field  812 +station ID list field  813 ’ may be set to ‘00 10 10011000.’ That is, the ID of the STA 4  may not be omitted in the station ID list field  813 . Alternatively, the HE-SIG field  810  (e.g., HE-SIG-A field, HE-SIG-B field, etc.) of the HE preamble used for transmitting PPDU based on MU-MIMO may include a sub-group ID field  811 , a N STS  field (not illustrated), and a station allocation information field (not illustrated). Also, the HE-SIG field  810  of the HE preamble may further include a Space-Time Block Coding (STBC) field. The sub-group ID field may indicate a sub-group ID described by referring to the table 1. The number of spatial streams allocated for stations indicated by the station allocation information field may be determined according to the values which are respectively indicated by the N STS  field and the STBC field. For example, the number of spatial streams may be determined according to the below table 4. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                 Value indicated 
               
               
                   
                 N STS  × N SS   
                 by STBC field 
               
               
                   
                   
               
             
            
               
                   
                 2 × 1 
                 1 
               
               
                   
                 3 × 2 
                 1 
               
               
                   
                 4 × 2 
                 2 
               
               
                   
                 4 × 3 
                 1 
               
               
                   
                   
               
            
           
         
       
     
     Alternatively, the station allocation information field may be represented in a bitmap form. For example, each of the first, second, third, and fourth fields included in the station allocation information field may indicate whether a spatial stream is allocated for the STA 2 , STA 3 , STA 4 , and STA 5  among stations belonging to the sub-group indicated by the sub-group ID field  811 . When the station allocation information field is set to binary number ‘1001,’ it may indicate that the number of spatial streams determined according to values indicated by the N STS  field and the STBC field is allocated for the STA 2  and STA 5 , and spatial streams are not allocated to the STA 3  and STA 4 . 
     Meanwhile, the HE-SIG field  820  (e.g., HE-SIG-A field, HE-SIG-B field, etc.) of the HE preamble used for transmitting PPDU based on OFDMA may include at least one scheduling information  821 - 1 ,  821 - 2 , . . . ,  821 - n . That is, in a case that the payload of the PPDU is constituted by a plurality of time domains, scheduling information for respective time domains may be included in the HE-SIG field  820 . For example, in a case that the payload of the PPDU has a first time domain and a second time domain, the scheduling information  821 - 1  for the first time domain and the scheduling information  821 - 2  for the second time domain may be included in the HE-SIG field  820 . The HE-SIG field  820  may include a sub-group ID field  821 - 1 - 1 , a resource allocation information field  821 - 1 - 2 , and a station ID list field  821 - 1 - 3  as the scheduling information. The sub-group ID field  821 - 1 - 1  may indicate a sub-group ID described by referring to the table 1. The resource allocation information field  821 - 1 - 2  may indicate an allocation pattern of frequency bands. 
     The size of the resource allocation information field  821 - 1 - 2  may vary according to resource allocation patterns or the number of reception stations. In a case that the size of the resource allocation information field  821 - 1 - 2  is variable, the HE preamble may include information indicating that the size of the resource allocation information field  821 - 1 - 2  is variable, and may further include information indicating the size of scheduling information (or, the size of the resource allocation information field  821 - 1 - 2 ). In this case, each of the information indicating that the size of the resource allocation information field  821 - 1 - 2  is variable and the information indicating that the size of the scheduling information (or, the size of the resource allocation information field  821 - 1 - 2 ) may be positioned prior to the field including the scheduling information (or, the resource allocation information field  821 - 1 - 2 ) in the HE preamble. 
     Meanwhile, the scheduling information may be included in the HE-SIG-A field or the HE-SIG-B field of the HE-SIG field in the HE preamble. Among the scheduling information, common information for reception stations may be included in the common field of the HE-SIG-B field, and specific information for a specific reception station may be included in the user-specific field of the HE-SIG-B field. Alternatively, the scheduling information may be included in the HE-SIG-A field and the HE-SIG-B field of the HE preamble. For example, among the scheduling information, a part of common information for reception stations may be included in the HE-SIG-A field, and the rest of them may be included in the common field of the HE-SIG-B field. Also, the information used for a specific reception station may be included in the user-specific field of the HE-SIG-B filed. 
       FIG. 10  is a conceptual diagram illustrating an exemplary embodiment of an allocation pattern of frequency bands. 
     Referring to  FIG. 10 , in a case that a PPDU is transmitted through four frequency bands FB- 1 , FB- 2 , FB- 3 , and FB- 4 , when the resource allocation information is set to ‘ 00 ,’ it may indicate that the respective frequency bands FB- 1 , FB- 2 , FB- 3 , and FB- 4  are allocated for different stations. Also, when the resource allocation information is set to ‘ 01 ,’ it may indicate that two frequency bands FB- 1  and FB- 2  are allocated for a station, the frequency band FB- 3  is allocated for another station, and the frequency band FB- 4  is allocated for the other station. When the resource allocation information is set to ‘10’ or ‘11,’ it may indicate the allocation pattern of frequency bands in the same manner. Here, each of ‘00’, ‘01’, ‘10,’ and ‘11’ may be a binary number. 
     In a case that a PPDU is transmitted through a frequency bandwidth equal to or larger than 40 MHz, the allocation pattern of frequency bands may be differently set in unit of predetermined bandwidth (e.g., 20 MHz), and frequency resources within the predetermined bandwidth may be allocated to at least one reception station. For example, when a PPDU is transmitted through a frequency band including contiguous first, second, . . . , nth sub-frequency bands in unit of 20 MHz, each of odd-numbered sub-frequency bands (e.g., first sub-frequency band, third sub-frequency band, etc.) may be allocated according to a first allocation pattern, and each of even-numbered sub-frequency bands (e.g., second sub-frequency band, fourth sub-frequency band, etc.) may be allocated according to a second allocation pattern. Here, n is a natural number larger than 3. 
     The set manners of the resource allocation information and the allocation patterns of frequency bands indicated by the resource allocation manner are not restricted to the above-described example, and they may vary according to exemplary embodiments. 
     Re-referring to  FIG. 8 , the station ID list field  821 - 1 - 3  may include IDs of respective stations participating in the PPDU transmission based on OFDMA among stations included in the sub-group indicated by the sub-group ID. Here, the station IDs may be station IDs described by referring to the table 1. For example, when the sub-group ID is set to ‘00,’ the station ID list field  821 - 1 - 3  may include IDs of the stations participating in the PPDU transmission based on OFDMA (e.g., ‘00’, ‘01,’ ‘10,’ and ‘11’) among the STA 2  to STA 5 . 
     According to the above description, in a case that the scheduling information ‘sub-group ID field  821 - 1 - 1 +resource allocation information field  821 - 1 - 2 +station ID list field  821 - 1 - 3 ’ is set to ‘00 00 10110100,’ it may indicate that data units for each of the stations STA 4 , STA 5 , STA 3 , and STA 2  are respectively transmitted through each of the frequency bands FB- 1 , FB- 2 , FB- 3 , and FB- 4 , similarly to the above-described case of the HE-SIG field  810 . Also, in a case that the scheduling information ‘sub-group ID field  821 - 1 - 1 +resource allocation information field  821 - 1 - 2 +station ID list field  821 - 1 - 3 ’ is set to ‘00 01 100100’ (i.e., when data units for a single station are transmitted through two frequency bands FB- 1  and FB- 2 ), it may indicate that data units for the STA 3  are transmitted through the frequency bands FB- 1  and FB- 2 , and data units of each of the STA 3  and STA 2  are respectively transmitted through each of the frequency bands FB- 3  and FB- 4 . Also, in a case that the scheduling information ‘sub-group ID field  821 - 1 - 1 +resource allocation information field  821 - 1 - 2 +station ID list field  821 - 1 - 3 ’ is set to ‘00 10 100100’ (i.e., when data units for a single station are transmitted through non-contiguous frequency bands FB- 1  and FB- 3 ), it may indicate that data units for the STA 4  are transmitted through frequency bands FB- 1  and FB- 3 , and data units for each of the STA 3  and STA 2  are transmitted through each of the frequency bands FB- 2  and FB- 4 , similarly to the case of the HE-SIG field  810 . 
     Meanwhile, in the transmission based on OFDMA, data for different stations may be multiplexed and transmitted in time domain. 
       FIG. 11  is a block diagram illustrating an exemplary embodiment of a payload included in a PPDU. 
     Referring to  FIG. 11 , the PPDU payload may include a first time domain and a second time domain. The length of the first time domain may be identical to or different from the length of the second time domain. The lengths of data units for respective frequency bands FB- 1 , FB- 2 , FB- 3 , and FB- 4  may be different in each of the time domains. In the first time domain, a frequency band allocation pattern may be indicated by the resource allocation information ‘00’ described by referring to  FIG. 10 . Also, in the second time domain, a frequency band allocation pattern may be indicated by the resource allocation information ‘01’ described by referring to  FIG. 10 . The HE-SIG field  820  may include a first scheduling information field  821 - 1  including scheduling information for the first time domain, and a second scheduling information field  821 - 2  including scheduling information for the second time domain. The first scheduling information field  821 - 1  may be set to ‘00 00 10110100’ and the second scheduling information field  821 - 2  may be set to ‘00 01 100100.’ 
     Re-referring to  FIG. 8 , the HE-SIG field  830  (e.g., HE-SIG-A field, HE-SIG-B field, etc.) of the HE preamble used for the PPDU transmission based on MU-MIMO/OFDMA may include a group ID field  831 , a first resource allocation information field  832 , a sub-group ID list field  833 , and at least one SS scheduling information field  834 - 1 ,  834 - 2 , . . . ,  834 - n , as scheduling information. Alternatively, the scheduling information may be included in the HE-SIG-A field or the HE-SIG-B field in the HE-SIG field of the HE preamble. Among the scheduling information, common information for reception stations may be included in a common field of the HE-SIG-B field, and information for a specific reception station may be included in a user-specific field of the HE-SIG-B filed. Alternatively, the scheduling information may be included in the HE-SIG-A field and the HE-SIG-B field in the HE-SIG field of the HE preamble. For example, among the scheduling information, a part of common information for reception stations may be included in the HE-SIG-A field, and the rest of them may be included in the common field of the HE-SIG-B field. Also, the information used for a specific reception station may be included in the user-specific field of the HE-SIG-B filed. 
     The group ID field  831  may indicate a group ID described by referring to the table 1. The first resource allocation information field  832  may indicate a resource allocation pattern of spatial streams identically to the above-described resource allocation information field  812  of the HE-SIG field  810 . The size of the first resource allocation information field  832  may vary according to the resource allocation pattern or the number of reception stations. In a case that the size of the first resource allocation information field  832  is variable, the HE preamble may include information indicating that the size of the first resource allocation field  832  is variable and may further information indicating the size of the scheduling information (or, the size of the first resource allocation information field  832 ). In this case, each of the information indicating that the size of the first resource allocation information field  832  is variable and the information indicating the size of the scheduling information (or, the size of the first resource allocation information field  832 ) may be positioned prior to the field including the scheduling information (or, the first resource allocation information field  832 ) in the HE preamble. 
     The sub-group ID list field  833  may include respective IDs of the sub-groups participating in the PPDU transmission based on MU-MIMO among sub-groups included in the group indicated by the group ID. Here, the sub-group ID may be a sub-group ID described by referring to the table 1. 
     Each of the SS scheduling information fields  834 - 1 ,  834 - 2 , . . . ,  834 - n  may indicate scheduling information of each PPDU transmitted through the corresponding spatial stream. For example, a SS- 1  scheduling information field  834 - 1  may indicate scheduling information of a PPDU transmitted through the spatial stream SS- 1 . The number of the SS scheduling information fields  834 - 1 ,  834 - 2 , . . . ,  834 - n  may correspond to the number of spatial streams through which PPDUs are transmitted. For example, when PPDUs are transmitted through four spatial streams, the HE-SIG field  830  may include four SS scheduling information fields  834 - 1 ,  834 - 2 ,  834 - 3 , and  834 - 4 . 
     Each of the SS scheduling fields  834 - 1 ,  834 - 2 , . . . ,  834 - n  may include at least one scheduling information fields (e.g.,  834 - 1 - 1 ,  834 - 1 - 2 , . . . ,  834 - 1 - n ). That is, when the PPDU payload includes multiple time domains, scheduling information for each of the multiple time domains may be included in the SS scheduling information fields  834 - 1 ,  834 - 2 , . . . ,  834 - n . For example, when the PPDU payload includes the first time domain and the second time domain, scheduling information  834 - 1 - 1  for the first time domain and scheduling information  834 - 1 - 2  for the second time domain may be included in the SS- 1  scheduling information field  834 - 1 . 
     The first scheduling information field  834 - 1 - 1  may include a sub-group ID field  834 - 1 - 1 - 1 , a second resource allocation information field  834 - 1 - 1 - 2 , and a station ID list field  834 - 1 - 1 - 3 . The sub-group ID field  834 - 1 - 1 - 1  may indicate a sub-group ID described by referring to the table 1. The second resource allocation information field  834 - 1 - 1 - 2  may indicate an allocation pattern of frequency bands, identically to the above-described resource allocation information field  821 - 1 - 2  of the HE-SIG field  820 . The size of the second resource allocation information field  834 - 1 - 1 - 2  may vary according to the resource allocation pattern or the number of reception stations. In a case that the size of the second resource allocation information field  834 - 1 - 1 - 2  is variable, the HE preamble may include information indicating that the size of the second resource allocation information field  834 - 1 - 1 - 2  is variable and may further information indicating the size of the scheduling information (or, the size of the second resource allocation information field  834 - 1 - 1 - 2 ). In this case, each of the information indicating that the size of the second resource allocation information field  834 - 1 - 1 - 2  is variable and the information indicating the size of the scheduling information (or, the size of the second resource allocation information field  834 - 1 - 1 - 2 ) may be positioned prior to the field including the scheduling information (or, the second resource allocation information field  834 - 1 - 1 - 2 ) in the HE preamble. 
     The station ID list field  834 - 1 - 1 - 3  may include respective IDs of the stations participating in the PPDU transmission based on the OFDMA among stations included in the sub-group indicated by the sub-group ID. Here, the station ID may be a station ID described by referring to the table 1. That is, ‘sub-group ID field  834 - 1 - 1 - 1 +second resource allocation information field  834 - 1 - 1 - 2 +station ID list field  834 - 1 - 1 - 3 ’ may act the same role as that of ‘sub-group ID list field  821 - 1 - 1 +resource allocation information field  821 - 1 - 2 +station ID list field  821 - 1 - 3 ’ included in the above-described HE-SIG field  820 . 
     Re-referring to  FIG. 7 , the HE-SIG-B field may include information needed for reception of the corresponding payload. The HE-SIG-B field may include different information for each of the frequency bands FB- 1 , FB- 2 , FB- 3 , and FB- 4 . Also, the HE-SIG-B field may include a second indicator on whether the corresponding payload includes multiple time domains (or, whether data for different stations are multiplexed and transmitted in time domain). For example, when the second indicator is set to ‘0,’ it may indicate that the corresponding payload includes a single time domain, and when the second indicator is set to ‘1,’ it may indicate that the corresponding payload includes multiple time domains (e.g., time domains illustrated in  FIG. 11 ). 
     Also, the HE-SIG-B field may include a third indicator on whether the corresponding payload includes data units belonging to different access categories (e.g. AC_VO, AC_VI, AC_BE, and AC_BK). For example, when the third indicator is set to ‘0,’ it may indicate that the corresponding payload includes at least one data unit belonging to a single access category (AC), and when the third indicator is set to ‘1,’ it may indicate that the corresponding payload includes data units belonging to different access categories. Here, the HE-SIG-B field may further include a traffic ID (TID) list field. The TID list field may include TIDs for access categories to which each of data units included in the payload belongs. 
     Also, the HE-SIG-B field may further include a fourth indicator on whether the corresponding payload includes data units having different frame formats (e.g., control frame, management frame, and data frame). For example, when the fourth indicator is set to ‘0,’ it may indicate that the payload includes data units belonging to a single frame format, and when the fourth indicator is set to ‘1,’ it may indicate that the payload includes data units belonging to different frame formats. Here, the HE-SIG-B field may further include a frame format ID list field. The frame format ID list field may include frame format IDs for respective data units included in the payload. For example, in a case that frame format IDs ‘00,’ ‘01,’ and ‘10’ respectively indicate a control frame, a management frame, and a data frame, when the frame format ID is set to ‘0010,’ it may indicate that the payload includes a data unit having a control frame format and a data unit having a data frame format. 
     The payload including data units belonging to different access categories or data units having different MAC frame formats may have the following structure. 
       FIG. 12  is a block diagram illustrating another exemplary embodiment of a payload included in a PPDU. 
     Referring to  FIG. 12 , the payload may include A-MPDUs belonging to different access categories or A-MPDUs having different MAC frame formats. For example, the payload may include A-MPDU subframes  1210 - 1  and  1210 - 2  belonging to AC_VO (or, having a data frame format), a Null subframe  1220 , A-MPDU subframes  1230 - 1  and  1230 - 2  belonging to AC_BE (or, having a control frame format), and a pad field  1240 . The Null subframe  1220  may be located between A-MPDUs belonging to different access categories (or, between A-MPDUs having different frame formats). At least one Null subframe  1220  may be included in the payload, and each of the multiple Null subframes may have the same information. 
     The Null subframe  1220  may include a reserved field  1221 , an End-of-AC (EOA) field (or, End-of-Type (EOT) field)  1222 , a length field  1223 , a cyclic redundancy check (CRC) field  1224 , and a delimiter signature field  1225 . The EOA field  1222  may indicate that an AC to which the A-MPDU subframes  1230 - 1  and  1230 - 2  subsequent to the Null subframe  1220  belong is different from an AC to which the A-MPUD subframes  1210 - 1  and  1210 - 2  prior to the Null subframe  1220  belong. That is, the EOA field  1222  may indicate an end of A-MPDU subframes  1210 - 1  and  1210 - 2  belonging to AC_VO. Alternatively, the EOT field  1222  may indicate that a frame format of the A-MPDU subframes  1230 - 1  and  1230 - 2  subsequent to the Null subframe  1220  is different from a frame format of the A-MPUD subframes  1210 - 1  and  1210 - 2  prior to the Null subframe  1220 . That is, the EOT field  1222  may indicate an end of A-MPDU subframes  1210 - 1  and  1210 - 2  having a data frame format. The length field  1223  may indicate the length of a MPDU included in the Null subframe  1220 , and may be set to ‘0.’ 
     Re-referring to  FIG. 7 , the HE-SIG-B field may include modulation and coding scheme (MCS) index information and length information for the corresponding payload. The length information may indicate a length from an end point (or, a start point of the payload) of the HE-SIG-B field to an end point of a data unit included in the payload (e.g., a start point of the pad field). When the payload include a plurality of time domains, the HE-SIG-B field may include MSC index information and length information for each of the plurality of time domains. For example, in the case of the PPDU payload illustrated in  FIG. 11 , the HE-SIG-B field may include a HE-SIG-B1 field for the first time domain and a HE-SIG-B2 field for the second time domain. The HE-SIG-B1 field may include MCS index information and length information for the first time domain. Here, the length information may indicate a length from a start point of the payload (or, an end point of the HE-SIG-B1 field) to an end point of the data unit belonging to the first time domain. The HE-SIG-B2 field may include MCS index information and length information for the second time domain. Here, the length information may indicate a length from a start point of the second time domain to an end point of the data unit belonging to the second time domain. 
     Also, in a case that the payload includes data units belonging to different access categories (or, data units having different MAC frame formats), the HE-SIG-B field may include MCS index information and length information for each of the multiple data units. Here, the length information may be included in a header of the payload. For example, in the payload of the PPDU illustrated in  FIG. 12 , the HE-SIG-B field may include a HE-SIG-B1 field for A-MPDU subframes  1210 - 1  and  1210 - 2  belonging to AC_VO (or, A-MPDU subframes having a data frame format) and a HE-SIG B2 field for A-MPDU subframes  1230 - 1  and  1230 - 2  belonging to AC_BE (or, A-MPDU subframes having a control frame format). The HE-SIG-B1 field may include MCS index information and length information of the A-MPDU subframes  1210 - 1  and  1210 - 2 . Here, the length information may indicate a length from a start point of the payload (or, an end point of the HE-SIG-B1 field) to an end point of the A-MPDU subframe  1210 - 2  (e.g., a start point of the Null subframe  1220 ). The HE-SIG-B2 field may include MCS index information and length information for the A-MPDU subframe  1230 - 1  and  1230 - 2 . Here, the length information may indicate a length from a start point of the A-MPDU subframe  1230 - 1  (e.g., an end point of the Null subframe  1220 ) to an end point of the A-MPDU subframe  1230 - 2 . 
     Re-referring to  FIG. 6 , the STA 1  may transmit the PPDU generated in the above-described manner based on multi-user transmission (S 602 ). In a case that the PPDU is transmitted based on MU-MIMO, the STA 1  may transmit the PPDU to one of the STA 2  to STA 5  through the spatial stream SS- 1 , and transmit the PPDU to one of the STA 6  to STA 9  through the spatial stream SS- 2 . In a case that the PPDU is transmitted based on OFDMA, the STA 1  may transmit the PPDU to the STA 2  to STA 9 . In a case that the PPDU is transmitted based on MU-MIMO/OFDMA, the STA 1  may transmit the PPDU to the STA 2  to STA 9  in the OFDMA through the spatial stream SS- 1 , and transmit the PPDU to the STA 2  to STA 9  in the OFDMA through the spatial stream SS- 2 . 
     Meanwhile, upon receiving the PPDU, reception stations (e.g., the STA 2  to STA 9 ) may obtain the legacy preamble and the HE preamble of the PPDU, and obtain the first indicator included in the HE-SIG-A field of the HE preamble. Since the first indicator indicates a transmission manner described in the table 2, the reception stations may identify a transmission manner of the corresponding PPDU by using the first indicator. Since scheduling information is varied according to the transmission manner of the PPDU, the reception stations predict which scheduling information is transmitted. For example, when the reception station identifies that the PPDU is transmitted based on MU-MIMO, the reception station may predict that the ‘sub-group ID field  811 +resource allocation information field  812 +station ID list field  813 ’ illustrated in  FIG. 8  is transmitted as scheduling information, and the reception station may identify a resource (e.g., spatial stream) through which its data can be received. The reception station may obtain information needed for receiving a data unit (e.g., MCS index information, length information, etc.) by receiving the HE-SIG-B field related to the identified resource, and receive and decode the data unit based on the obtained information. 
     Also, when the reception station identifies that the PPDU is transmitted based on OFDMA, the reception station may predict that the ‘at least one scheduling information field  821 - 1 ,  821 - 2 , . . . ,  821 - n ’ illustrated in  FIG. 8  is transmitted as scheduling information, and the reception station may identify a resource (e.g., frequency and time) through which its data can be received by receiving the scheduling information. The reception station may obtain information needed for receiving a data unit (e.g., MCS index information, length information, etc.) by receiving the HE-SIG-B field related to the identified resource, and receive and decode the data unit based on the obtained information. 
     Also, when the reception station identifies that the PPDU is transmitted based on MU-MIMO/OFDMA, the reception station may predict that the ‘group ID field  831 +first resource allocation information field  832 +sub-group ID list field  833 +at least one SS scheduling information field  834 - 1 ,  834 - 2 , . . . ,  834 - n ’ illustrated in  FIG. 8  is transmitted as scheduling information, and the reception station may identify a resource (e.g., spatial stream, time, frequency) through which its data can be received. The reception station may obtain information needed for receiving a data unit (e.g., MCS index information, length information, etc.) by receiving the HE-SIG-B field related to the identified resource, and receive and decode the data unit based on the obtained information. 
     For example, when the first scheduling information field  821 - 1  and the second scheduling information field  821 - 2  are respectively set to ‘00 00 10110100’ and ‘00 01 100100,’ the STA 4  may receive and decode at least one data unit transmitted through the first frequency band FB- 1  in the first time domain, and receive and decode data units transmitted through the frequency bands FB- 1  and FB- 2  in the second time domain. Meanwhile, the STA 4  may receive a pilot signal transmitted through the frequency band FB- 2  in the first time domain, and decode the data unit based on the pilot signal. That is, when the length of the data unit received in the first time domain is longer than a predetermined length, error of channel estimation based on the HE-STF, the HE-LTF, and so on may be increased. In this case, the STA 4  can perform channel estimation (e.g., phase-tracking) by receiving the pilot signal transmitted through the first frequency band FB- 1  in the first time domain before it receives the data transmitted through the second frequency band FB- 2  in the second time domain. 
     When the reception station successfully receives the data, in response to the data, the reception station may transmit a response frame (e.g. an acknowledgement (ACK) frame) to the STA 1  (S 603 , S 604 ). The response frame may be transmitted in various manners. For example, according to the above-described  FIG. 11 , when the first scheduling information field  821 - 1  and the second scheduling information field  821 - 2  are respectively set to ‘00 00 10110100’ and ‘00 01 100100,’ each of the STA 4 , STA 5 , STA 3 , and STA 2  may transmit a response frame to the STA 1  in a third time domain (not illustrated), as a response corresponding to the data unit received in the first time domain. In this case, each of the STA 4 , STA 5 , STA 3 , and STA 2  may transmit the response frame to the STA 1  in one or more manners including OFDMA, code division multiple access (CDMA), sequential transmission manner, etc. Here, when the sequential transmission manner is used, the STA 4 , STA 5 , STA 3 , and STA 2  may sequentially transmit their response frames to the STA 1  in the third time domain. The interval between the response frames transmitted by the STA 4 , STA 5 , STA 3 , and STA 2  may be a SIFS. The third time domain may be a time domain after a SIFS from an end of the PPDU transmitted from the STA 1 , or may be indicated by the STA 1 . For example, the STA 1  may indicate information on a resource allocated for the third time domain by transmitting a first request frame after the PPDU transmission, and request a transmission of the response frame in the third time domain as the response of the data unit. The first request frame may be a frame performing function of a block ACK request (BAR) frame, a beamforming poll frame, etc. 
     Also, each of the STA 4 , STA 3 , and STA 2  may transmit a response frame for the data unit received in the second time domain to the STA 1  in a fourth time domain (not illustrated). Identically to the above-described transmission manner of the response frame in the third time domain, the STA 4 , STA 3 , and STA 2  may transmit the response frames to the STA 1 . Here, the fourth time domain may be indicated by the above-described first request frame, or indicated by a second request frame transmitted by the STA 1  after the third time domain. 
     Also, the response frames for the data units transmitted in the first time domain and the second time domain may be aggregated into a single response frame, and the single response frame is transmitted. For example, the response frames for the data units transmitted to the STA 4  through the first frequency band FB- 1  in the first time domain and the second time domain may be aggregated into a single response frame, and the single response frame may be transmitted to the STA 1  in the third time domain. Also, the response frames for the data units transmitted respectively to the STA 3  and STA 4  through the second frequency band FB- 2  in the first time domain and the second time domain may be aggregated into a single response frame, and the single response frame may be transmitted to the STA 1  in the third time domain. Also, the response frames for the data units transmitted through the frequency bands FB- 3  and FB- 4  may be transmitted to the STA 1  in the third time domain in the manner identical to the above-described manner. 
     Meanwhile, when the STA 1  receives the response frames respectively from the STA 2  to STA 9 , the STA 1  may determine that the data is successfully received at the corresponding station. 
     While the example embodiments of the present disclosure and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations may be made herein without departing from the scope of the disclosure.