Abstract:
A communication method for an AP comprises the step of: the AP receiving multiple RTS frames from each of multiple STAs through each of multiple channels; the AP transmitting multiple CTS frames to the multiple STAs through each of multiple available channels from among the multiple channels; and the AP receiving multiple uplink data frames transmitted from each of the multiple STAs through each of the multiple available channels, wherein the AP communicates with the multiple STAs based on BSS1 and BSS2, wherein the BSS1 includes a primary channel band1 and secondary channel band1, the BSS2 includes a primary channel band2 and secondary channel band2, wherein the primary channel band1 overlaps with the secondary channel band2, the secondary channel band1 overlaps with the primary channel band2, and wherein the multiple channels can be included in the primary channel band1 and secondary channel band1.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage filing under 35 U.S.C. 371 of International Application No. PCT/KR2014/000972, filed on Feb. 5, 2014, which claims the benefit of U.S. Provisional Application No. 61/761,725, filed on Feb. 7, 2013, the contents of which are all hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a wireless local area network (WLAN) and, more particularly, to a method of transmitting and receiving data in a BSS. 
     Related Art 
     A Wireless Next Generation Standing Committee (WNG SC) of institute of electrical and electronic engineers (IEEE) 802.11 is an AD-HOC committee that a next-generation wireless local area network (WLAN) in the medium and long term. 
     In an IEEE conference in March 2013, Broadcom presented the need of discussion of the next-generation WLAN after IEEE 802.11ac in the first half of 2013 when an IEEE 802.11ac standard is finished based on a WLAN standardization history. A motion for foundation of a study group which Orange and Broadcom proposed in the IEEE conference in March 2013 and most members agreed has been passed. 
     A scope of a high efficiency WLAN (HEW) which the next-generation WLAN study group primarily discusses the next-generation study group called the HEW includes 1) improving a 802.11 physical (PHY) layer and a medium access control (MAC) layer in bands of 2.4 GHz and 5 GHz, 2) increasing spectrum efficiency and area throughput, 3) improving performance in actual indoor and outdoor environments such as an environment in which an interference source exists, a dense heterogeneous network environment, and an environment in which a high user load exists, and the like. That is, the HEW operates at 2.4 GHz and 5 GHz similarly to the existing WLAN system. A primarily considered scenario is a dense environment in which access points (APs) and stations (STAs) are a lot and under such a situation, improvement of the spectrum efficiency and the area throughput is discussed. In particular, in addition to the indoor environment, in the outdoor environment which is not considerably considered in the existing WLAN, substantial performance improvement is concerned. 
     In the HEW, scenarios such as wireless office, smart home, stadium, Hotspot, and building/apartment are largely concerned and discussion about improvement of system performance in the dense environment in which the APs and the STAs are a lot is performed based on the corresponding scenarios. 
     In the future, in the HEW, improvement of system performance in an overlapping basic service set (OBSS) environment and improvement of outdoor environment performance, and cellular offloading are anticipated to be actively discussed rather than improvement of single link performance in one basic service set (BSS). Directionality of the HEV means that the next-generation WLAN gradually has a technical scope similar to mobile communication. When a situation is considered, in which the mobile communication and the WLAN technology haven been discussed in a small cell and a direct-to-direct (D2D) communication area in recent years, technical and business convergence of the next-generation WLAN and the mobile communication based on the HEW is predicted to be further active. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for transmitting and receiving data in a multi-BSS. 
     The present invention provides an apparatus for performing the method for transmitting and receiving data in a multi-BSS. 
     In an aspect, a communication method for an access point (AP) in a wireless local area network (WLAN) comprises: receiving, by the AP, a plurality of RTS frames from each of a plurality of STAs through each of a plurality of channels; transmitting, by the AP, a plurality of CTS frames to the plurality of STAs through each of a plurality of available channels among the plurality of channels; and receiving, by the AP, a plurality of uplink data frames transmitted from each of the plurality of STAs through each of the plurality of available channels, wherein the AP communicates with the plurality of STAs based on a first basic service set (BSS) and a second BSS, wherein the first BSS includes a first main channel band and a first sub channel band, wherein the second BSS includes a second main channel band and a second sub channel band, wherein the first main channel band overlaps with the second sub channel band and the first sub channel band overlaps with the second main channel band, and wherein the plurality of channels are included in the first main channel band and the first sub channel band. 
     In another aspect, An access point (AP) performing downlink transmission in a wireless local area network (WLAN) comprises a radio frequency (RF) unit configured to transmit and receive a radio signal and a processor configured to: receive a plurality of RTS frames from each of a plurality of STAs through each of a plurality of channels, transmit a plurality of CTS frames to the plurality of STAs through each of a plurality of available channels among the plurality of channels, and receive a plurality of uplink data frames transmitted from each of the plurality of STAs through each of the plurality of available channels, wherein the AP communicates with the plurality of STAs based on a first basic service set (BSS) and a second BSS, wherein the first BSS includes a first main channel band and a first sub channel band, wherein the second BSS includes a second main channel band and a second sub channel band, wherein the first main channel band overlaps with the second sub channel band and the first sub channel band overlaps with the second main channel band, and wherein the plurality of channels are included in the first main channel band and the first sub channel band. 
     A method of independently transmitting and receiving data between an extension AP supporting an existing legacy channel band and a newly defined extension channel band and a legacy STA supporting an existing legacy channel band and an extension STA supporting an existing legacy channel band and a newly defined extension channel band may be performed. Accordingly, data throughput and frequency efficiency may be improved using a newly extended channel band. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a conceptual diagram illustrating a configuration of a wireless local area network (WLAN). 
         FIG. 2  is a diagram illustrating a tier-architecture in a WLAN system supported by an IEEE 802.11. 
         FIG. 3  is a conceptual diagram illustrating a problem which may occur when the STA detects a medium. 
         FIG. 4  is a conceptual diagram illustrating a method for transmitting and receiving an RTS frame and a CTS frame in order to solve a hidden node problem and an exposed node problem 
         FIG. 5  is a conceptual diagram illustrating information on a bandwidth of the WLAN. 
         FIG. 6  is a conceptual diagram illustrating an operation of an AP which may support a multi-BSS operation according to an embodiment of the present invention. 
         FIG. 7  is a conceptual diagram illustrating a procedure of transmitting and receiving data between a STA and an AP according to an embodiment of the present invention. 
         FIG. 8  is a conceptual diagram illustrating a procedure of transmitting and receiving data between a STA and an AP according to an embodiment of the present invention. 
         FIG. 9  is a block diagram illustrating a wireless apparatus according to an embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  is a concept view illustrating the structure of a wireless local area network (WLAN). 
     An upper part of  FIG. 1(A)  shows the structure of the IEEE (institute of electrical and electronic engineers) 802.11 infrastructure network. 
     Referring to the upper part of  FIG. 1(A) , the WLAN system may include one or more basic service sets (BSSs,  100  and  105 ). The BSS  100  or  105  is a set of an AP such as AP (access point)  125  and an STA such as STA1 (station)  100 - 1  that may successfully sync with each other to communicate with each other and is not the concept to indicate a particular area. The BSS  105  may include one AP  130  and one or more STAs  105 - 1  and  105 - 2  connectable to the AP  130 . 
     The infrastructure BSS may include at least one STA, APs  125  and  130  providing a distribution service, and a distribution system (DS)  110  connecting multiple APs. 
     The distribution system  110  may implement an extended service set (ESS)  140  by connecting a number of BSSs  100  and  105 . The ESS  140  may be used as a term to denote one network configured of one or more APs  125  and  130  connected via the distribution system  110 . The APs included in one ESS  140  may have the same SSID (service set identification). 
     The portal  120  may function as a bridge that performs connection of the WLAN network (IEEE 802.11) with other network (for example, 802.X). 
     In the infrastructure network as shown in the upper part of  FIG. 1 , a network between the APs  125  and  130  and a network between the APs  125  and  130  and the STAs  100 - 1 ,  105 - 1 , and  105 - 2  may be implemented. However, without the APs  125  and  130 , a network may be established between the STAs to perform communication. The network that is established between the STAs without the APs  125  and  130  to perform communication is defined as an ad-hoc network or an independent BSS (basic service set). 
     A lower part of  FIG. 1  is a concept view illustrating an independent BSS. 
     Referring to the lower part of  FIG. 1 , the independent BSS (IBSS) is a BSS operating in ad-hoc mode. The IBSS does not include an AP, so that it lacks a centralized management entity. In other words, in the IBSS, the STAs  150 - 1 ,  150 - 2 ,  150 - 3 ,  155 - 4  and  155 - 5  are managed in a distributed manner. In the IBSS, all of the STAs  150 - 1 ,  150 - 2 ,  150 - 3 ,  155 - 4  and  155 - 5  may be mobile STAs, and access to the distribution system is not allowed so that the IBSS forms a self-contained network. 
     The STA is some functional medium that includes a medium access control (MAC) following the IEEE (Institute of Electrical and Electronics Engineers) 802.11 standards and that includes a physical layer interface for radio media, and the term “STA” may, in its definition, include both an AP and a non-AP STA (station). 
     The STA may be referred to by various terms such as mobile terminal, wireless device, wireless transmit/receive unit (WTRU), user equipment (UE), mobile station (MS), mobile subscriber unit, or simply referred to as a user. 
       FIG. 2  is a view illustrating a layer architecture of a WLAN system supported by IEEE 802.11. 
       FIG. 2  conceptually illustrates a layer architecture (PHY architecture) of a WLAN system. 
     The WLAN system layer architecture may include an MAC (medium access control) sub-layer  220 , a PLCP (Physical Layer Convergence Procedure) sub-layer  210 , and a PMD (Physical Medium Dependent) sub-layer  200 . The PLCP sub-layer  210  is implemented so that the MAC sub-layer  220  is operated with the minimum dependency upon the PMD sub-layer  200 . The PMD sub-layer  200  may serve as a transmission interface to communicate data between a plurality of STAs. 
     The MAC sub-layer  220 , the PLCP sub-layer  210 , and the PMD sub-layer  200  may conceptually include management entities. 
     The management entity of the MAC sub-layer  220  is denoted an MLME (MAC layer management entity,  225 ), and the management entity of the physical layer is denoted a PLME (PHY layer management entity,  215 ). Such management entities may offer an interface where a layer management operation is conducted. The PLME  215  is connected with the MLME  225  to be able to perform a management operation on the PLCP sub-layer  210  and the PMD sub-layer  200 , and the MLME  225  is also connected with the PLME  215  to be able to perform a management operation on the MAC sub-layer  220 . 
     There may be an SME (STA management entity,  250 ) to perform a proper MAC layer operation. The SME  250  may be operated as a layer independent component. The MLME, PLME, and SME may communicate information between the mutual components based on primitive. 
     The operation of each sub-layer is briefly described below. The PLCP sub-layer  210  delivers an MPDU (MAC protocol data unit) received from the MAC sub-layer  220  according to an instruction from the MAC layer between the MAC sub-layer  220  and the PMD sub-layer  200  to the PMD sub-layer  200  or delivers a frame from the PMD sub-layer  200  to the MAC sub-layer  220 . The PMD sub-layer  200  is a PLCP sub-layer and the PMD sub-layer  200  may communicate data between a plurality of STAs by way of a radio medium. The MPDU (MAC protocol data unit) delivered from the MAC sub-layer  220  is denoted a PSDU (Physical Service Data Unit) on the side of the PLCP sub-layer  210 . The MPDU is similar to the PSDU, but in case an A-MPDU (aggregated MPDU), which is obtained by aggregating a plurality of MPDUs, has been delivered, each MPDUs may differ from the PSDU. 
     The PLCP sub-layer  210  adds an additional field including information required by the physical layer transceiver while receiving the PSDU from the MAC sub-layer  220  and delivering the same to the PMD sub-layer  200 . In this case, the added field may include a PLCP preamble to the PSDU, a PLCP header, and tail bits necessary to return the convolution encoder to zero state. The PLCP preamble may play a role to allow the receiver to prepare for syncing and antenna diversity before the PSDU is transmitted. The data field may include padding bits to the PSDU, a service field including a bit sequence to initialize the scrambler, and a coded sequence in which a bit sequence added with tail bits has been encoded. In this case, as the encoding scheme, one of BCC (Binary Convolutional Coding) encoding or LDPC (Low Density Parity Check) encoding may be selected depending on the encoding scheme supported by the STA receiving the PPDU. The PLCP header may include a field containing information on the PPDU (PLCP Protocol Data Unit) to be transmitted. 
     The PLCP sub-layer  210  adds the above-described fields to the PSDU to generate the PPDU (PLCP Protocol Data Unit) and transmits the same to a receiving station via the PMD sub-layer  200 , and the receiving station receives the PPDU and obtains information necessary for data restoration from the PLCP preamble and PLCP header to thus restore the same. 
       FIG. 3  is a conceptual diagram illustrating an issue which may occur when the STA senses a medium. 
     An upper end of  FIG. 3  illustrates a hidden node issue and a  FIG. 3(B)  illustrates an exposed node issue. 
     At the upper end of  FIG. 3 , it is assumed that an STA A  300  and an STA B  320  transmit and receive current data and an STA C  330  and an STA B  320  has data to be transmitted. When the data is transmitted and received between the STA A  300  and the STA B  320 , a specific channel may be busy. However, when the STA C  330  carrier-senses a medium before transmitting the data to the STA B  320  due to transmission coverage, the STA C  330  may determine that the medium for transmitting the data to the STA B  320  is in an idle state. When the STA C  330  determines that the medium is in the idle state, the data may be transmitted from the STA C  330  to the STA B  320 . Consequently, since the STA B  320  simultaneously receives information of the STA A  300  and the STA C  330 , a collision of data occurs. In this case, the STA A  300  may be a hidden node as the STA C  330 . 
     At a lower end of  FIG. 3 , it is assumed that an STA B  350  transmits data to an STA A  340 . When an STA C  360  intends to transmit data to an STA D  370 , the STA C  360  may perform carrier sensing in order to find whether the channel is busy. The STA C  360  may sense that the medium is busy due to transmission coverage of the STA B  350  because the STA B  350  transmits information to the STA A  340 . In this case, although the STA C  360  intends to transmit data to the STA D  370 , since it is sensed that the medium is busy, the STA C  360  may not transmit the data to the STA D  370 . Until it is sensed that the medium is idle after the STA B  350  completes transmitting the data to the STA A  340 , a situation in which the STA C  360  needs to unnecessarily wait occurs. That is, although the STA A  340  is out of a carrier sensing range of the STA C  360 , the STA A  340  may prevent data transmission by the STA C  360 . In this case, the STA C  360  becomes an exposed node of the STA B  350 . 
     In order to solve the hidden nose issue disclosed at the upper end of  FIG. 3  and the exposed node issue disclosed at the lower end of  FIG. 3 , it may be sensed whether the medium is busy by using an RTS frame and a CTS frame in a WLAN. 
       FIG. 4  is a conceptual diagram illustrating a method for transmitting and receiving the RTS frame and the CTS frame in order to solve the hidden node issue and the exposed node issue. 
     Referring to  FIG. 4 , short signaling frames such as the request to send (RTS) frame and the clear to send (CTS) frame may be used in order to solve the hidden node issue and the exposed node issue. It may be overheard whether data is transmitted and received among neighboring STAs based on the RTS frame and the CTS frame. 
     An upper end of  FIG. 4  illustrates a method for transmitting an RTS frame  403  and a CTS frame  405  in order to solve the hidden node issue. 
     Assumed that both an STA A  400  and an STA C  420  intend to transmit data to an STA B  410 , when the STA A  400  sends the RTS frame  403  to the STA B  410 , the STA B  410  may transmit the CTS frame  405  to both the STA A  400  and the STA C  420  therearound. The STA C  420  that receives the CTS frame  405  from the STA B  410  may obtain information indicating that the STA A  400  and the STA B  410  are transmitting data. Further, the RTS frame  403  and the CTS frame  405  include a duration field including information on a busy duration of a radio channel to configure a network allocation vector (NAV) during a predetermined duration so as to prevent the STA C  420  from using the channel. 
     The STA C  420  waits until the transmission and reception of the data between the STA A  400  and the STA B  410  is completed, and as a result, the STA C  420  may avoid the collision at the time of transmitting the data to the STA B  410 . 
     A lower end of  FIG. 4  illustrates a method for transmitting an RTS frame  433  and a CTS frame  435  in order to solve the exposed node issue. 
     An STA C  450  overhears transmission of the RTS frame  433  and the CTS frame  435  of an STA A  430  and an STA B  440 , and as a result, the STA C  450  may find that no collision occurs in spite of transmitting the data to another STA D  460 . That is, the STA B  440  transmits the RTS frame  433  to all neighboring terminals and transmits the CTS frame  435  to only the STA A  430  to which the STA B  440  needs to actually transmit data. Since the STA C  450  receives only the RTS frame  433  and may not receive the CTS frame  435  of the STA A  430 , it may be found that the STA A  430  is out of a carrier sensing range of the STA C  450 . Accordingly, the STA C  450  may not transmit data to the STA D  460 . 
     An RTS frame format and a CTS frame format are disclosed in 8.3.1.2 RTS frame format and 8.3.1.3 CTS frame format of “IEEE Standard for Information Technology Telecommunications and information exchange between systems Local and metropolitan area networks Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications which is IEEE Draft P802.11-REVmb™/D12 opened in November 2011. 
     The IEEE 802.11 WLAN (wireless local area network, WLAN) standard may have different transmission rate in various bands. A very high throughput (VHT) system which is the IEEE 802.11ac standard is to provide a throughput of 1 Gbps or more at a MAC service access point (SAP). 
     To this end, the VHT system may support a channel bandwidth of 80/160 MHz and 8 spatial streams. When the channel bandwidth of 160 MHz, 8 spatial streams, 256 QAM, and a short guard interval (GI) are all implemented, the VHT system may provide a maximum of 6.9 Gbps transmission rate. 
     However, VHT STAs that support multiple VHT systems need to simultaneously use the channel in order for an aggregated throughput of a VHT BSS to satisfy 1 Gbps in an actual environment. 
     An AP that supports a VHT may use space division multiple access (SDMA) or multi user-multiple input multiple output (MU-MIMO) as a method for transmitting data in order for the STAs supporting the multiple VHT systems to simultaneously use the channel. That is, the VHT systems may simultaneously transmit and receive different data among the multiple VHT STAs and the VHT AP based on multiple spatial streams. 
     In the VHT system, since legacy stations (STAs) supporting IEEE 802.11a/n are widely used in transmitting data by using the channel bandwidth of 160 MHz. 
     Accordingly, non-contiguous channels need to be aggregated and used. 
       FIG. 5  is a conceptual diagram illustrating information on the bandwidth of the WLAN. 
     With the increase in demand for high-definition multimedia transmission, a channel bandwidth for the WLAN has been extended. 
     Referring to  FIG. 5 , channel bands which the IEEE 802.11ac may currently use and bands to be newly added in a frequency band of 5 GHz are illustrated. 
     When channel bands to be newly allocated for the WLAN in channel bands of 5350 MHz to 5470 MHz and 5825 MHz to 5925 MHz are considered, the number of channel bands which may be used by the STA or AP may increase. For example, referring to the 80 MHz channel band, 80 MHz channel bands which are usable may increase from 6 channels at present to 9 channels by allocating the new channel bands. As another example, referring to the 160 MHz channel band, 160 MHz channel bands which are usable may increase from 2 channels at present to 4 channels. 
     The legacy STA (e.g., the STA supporting the IEEE 802.11a/n/ac) in the related art, which operates at 5 GHz does not support the newly allocated channels. Accordingly, the AP that supports the newly allocated channels configures a primary channel as a channel in which the legacy STA may operate to support the legacy STA. 
     Hereinafter, an embodiment of the present invention discloses channel bands allocated to 5 GHz by defining the channel bands allocated to 5 GHz as two types of channel bands. An existing channel supported by a legacy STA as in IEEE 802.11a/n/ac is defined as a legacy channel band. A channel band newly allocated to 5 GHz is defined as an extension channel band. Further, an operable STA at an extension channel band is defined as an extension STA. 
     Hereinafter, an embodiment of the present invention discloses a method of transmitting and receiving data between an AP supporting a plurality of BSS operation (multi BSS operation) and a legacy STA and an extension STA based on a legacy channel band and an extension channel band of a 5 GHz band. The AP may transmit and receive data to and from the STA by implementing a plurality of BSSs. 
     An embodiment of the present invention discloses a multi-BSS operation method for efficiently using an extension channel band which is newly extended at a frequency band of 5 GHz. 
     A multi-BSS operation method according to an embodiment of the present invention may include a method of configuring a plurality of BSSs to simultaneously provide a service to at least one STA using a plurality of BSS by the AP. Hereinafter, it is assumed that an AP is an AP capable of supporting a multi-BSS operation method. For example, the AP may operate by configuring a first BSS and a second BSS according to a channel band. Each BSS may separately define a primary channel. For example, a primary channel of a first BSS may be included in a first channel band, and a primary channel of the second BSS may be included in a second channel. 
     In this case, a first STA included in a first BSS accesses the AP through a first channel band and a second STA included in a second BSS accesses the AP through a second channel band. Inclusion of the STA in a specific BSS may mean a case where the STA configures a primary channel of an STA (STA primary channel) in a primary channel of a specific BSS. For example, the first STA may perform a competition based access using a back-off mechanism based on carrier sense multiple access/collision avoidance (CSMA/CA) at an STA primary channel of a first channel band. In the same manner, the second STA may perform a competition based access using a back-off mechanism based on the CSMA/CA at an STA primary channel of a second channel. The operation of the STA will be described in detail. 
     That is, an AP capable of supporting the multi-BSS operation may operate so that data may be transmitted and received between the AP and the second STA simultaneously with at a time when transmission and reception of the data are achieved between the AP and the first STA. Hereinafter, the embodiment of the present invention discloses an operation of an AP capable of supporting the multi-BSS operation in detail. 
       FIG. 6  is a conceptual diagram illustrating an operation of an AP which may support a multi-BSS operation according to an embodiment of the present invention. 
     Referring to  FIG. 6 , an AP  600  may transmit and receive data to and from STAs  650  and  660  based on a 5 GHz frequency band including two 80 MHz channel bandwidths. One 80 MHz channel bandwidth may refer to a first channel band, and a remaining one 80 MHz channel bandwidth may refer to a second channel band. It is assumed that the first channel band is a legacy channel band and a second channel band is an extension band. 
     The AP  600  supporting the multi-BSS operation may transmit and receive data based on at least two TX/RX blocks  610  and  620 . A plurality of TX/RX blocks  610  and  620  may be a constituent element configured to separately transmit and receive the data at different channel bands. Hereinafter, the embodiment of the present invention is described on the assumption that an AP  600  supporting a multi-BSS operation is configured based on two TX/RX blocks  610  and  620 . 
     A TX/RX block configured to be operated at a first channel band may be defined as a first TX/RX block  610 . A TX/RX block configured to be operated at a second channel band may be defined as a second TX/RX block  620 . According to an embodiment of the present invention, the TX/RX block  610  and the second TX/RX block  620  may be independently operated. For example, the TX/RX block  610  may transmits data to the STA and the second TX/RX block  620  may receive the data from another STA. 
     The AP  600  may configure two BSS based on the first TX/RX block  610  and the second TX/RX block  620 . Each BSS may perform an operation by configuring one channel bandwidth as a primary channel and configuring a remaining channel bandwidth as a secondary channel. The primary channel of the specific BSS may be a second channel of another BSS. In the same manner, the secondary channel of the specific BSS may be a primary channel of another BSS. The primary channel may be expressed as a main channel, and a second channel may be expressed as a sub-channel. 
     For example, the first BSS may configure a first channel band corresponding to a legacy channel as a primary channel and may configure a second channel band being a remaining channel band corresponding to an extension channel as a secondary channel. Conversely, the second BSS may configure a second channel band corresponding to the extension channel as a primary channel and may configure a first channel band being a remaining channel band corresponding to the legacy channel as a second channel. 
     A first STA  650  included in the first BSS and a second STA  660  included in the second BSS may include TX/RX blocks  670  and  680  which operate at a first channel band and a second channel band, respectively. The TX/RX blocks  670  and  680  of the first STA  650  and the second STA  660  may operate at one 80 MHz band, two non-continuous 80 MHz bands (80 MHz+80 MHz) or a 160 MHz band. 
     Hereinafter, it is assumed in the embodiment of the present invention that a primary channel (hereinafter referred to as ‘first STA primary channel’) of the first STA  650  is included in the first channel band and a primary channel (hereinafter referred to as ‘second STA primary channel’) of the second STA  660  is included in the second channel band. Each STA  650  or  660  may perform initial access through each STA primary channel. Remaining available channels except for the primary channel of the STA may be defined as a secondary channel of the STA. When the primary channel is expressed as the main channel and the secondary channel is expressed as a sub-channel, the STA primary channel may expressed as a primary channel. 
     The first STA  650  may perform channel access at a first STA primary channel of the first channel band. For example, the first STA  650  may perform channel access through a CSMA/CA back-off procedure at the first STA primary channel. The first STA  650  may perform the channel access at a first STA primary channel and may determine channel status information on a remaining first STA secondary channel band except for the first STA primary channel in order to determine whether to transmit and receive the data at another channel. 
     As the determination result of the first STA  650 , only statuses of the first STA primary channel and the first STA secondary channel included in the first channel band may be idle and a remaining secondary channel included in the second channel band may not be idle. In this case, the first STA  650  may transmit and receive data to and from the AP  600  using only a first channel band. In this case, the PHY protocol data unit (PPDU) transmitted from the first STA  650  may include a PPDU transmitted through an 80 MHz band corresponding to a first channel band. 
     As the determination result of the first STA  650 , a status of the secondary channel included in the first channel band and the second channel band may be idle. In this case, the first STA  650  may transmit data to and from the AP  600  using both of the first channel band and the second channel band. The PPDU transmitted from the first STA  650  may include a PPDU transmitted at two non-continuous 80 MHz bands or a PPDU transmitted at two continuous 80 MHz bands (160 MHz) according to whether the first channel band and the second channel band are a continuous channel band. It is assumed that the above operation is a case where the first STA is an extension STA to support both of a legacy channel band and an extension channel band. If the first STA is the legacy STA, the first STA may determine channel status information on only the first STA primary channel corresponding to the first channel band. 
     In the same manner, the second STA  660  may perform channel access at a second STA primary channel of the second channel band. For example, the second STA  660  may perform channel access through a CSMA/CA based back-off procedure at the second STA primary channel. The second STA  660  may perform channel access at the second STA primary channel to determine channel status information on a remaining second STA secondary channel band except for the second STA primary channel. 
     As the determination result of the second STA  660 , only statuses of the second STA primary channel and the second STA secondary channel included in the second channel band may be idle but a remaining secondary channel included in the second channel band may not be idle. In this case, the second STA  660  may transmit and receive the data to and from the AP  600  using only the first channel band. In this case, the PHY protocol data unit (PPDU) transmitted from the first STA  650  may include a PPDU transmitted through an 80 MHz band corresponding to the first channel band. 
     As the determination result of the second STA  660 , both of statuses of the secondary channel included in the first channel band and the second channel band may be idle. In this case, the second STA  660  may transmit and receive data to and from the AP  600  using both of the first channel band and the second channel band. In this case, the PPDU transmitted from the second STA  660  may include a PPDU transmitted from two non-continuous 80 MHz band or two continuous 80 MHz bands (160 MHz). 
     Further, it may be assumed that the first STA is a legacy STA not to support a channel of the second channel bandwidth (extension channel). In this case, the AP  600  supporting the multi-BSS operation may serve an extension STA by configuring a separate BSS supporting an extension STA at a second channel bandwidth while serving the legacy STA based on the first channel bandwidth (legacy channel). That is, the frequency efficiency may be increased by serving the first STA through the legacy channel and supporting the second STA through an extension channel. 
     An initial access frame (e.g., beacon frame), an authentication response frame, a probe response frame transmitted by the AP  600  when the STA firstly performs channel access, may include information on a plurality of BSSs supported from the AP. 
     For example, the initial access frame transmitted to the first channel band and the second channel band from the AP  600  may include information necessary when the STA operates at the first BSS and the second BSS (e.g., operation parameter). 
     Assuming that the first BSS is the legacy channel, a HT operation element and a VHT operation element supported from the existing IEEE 802.11a/n/ac may be used as an operation parameter for an operation of a legacy STA operating in the first BSS. The HT operation element and the VHT operation element may include information for operation in the first BSS by the STA (for example, primary channel information, secondary channel offset information, and information on the operation channel bandwidth). 
     Assuming that the second BSS is for the purpose of the extension STA, a multi-BSS operation element for an extension STA operated in the second BSS may be defined. The multi-BSS operation element may be transmitted while being included in the channel access frame. The multi-BSS operation element may include information on the channel band of the second BSS (e.g., channel numbers of 20 MHz primary channel, 20 MHz secondary channel, a 40 MHz secondary channel, and an 80 MHz/160 MHz secondary channel). 
     Hereinafter, the embodiment of the present invention discloses a detailed operation between the STA and the AP. 
       FIG. 7  is a conceptual diagram illustrating a procedure of transmitting and receiving data between a STA and an AP according to an embodiment of the present invention. 
     Referring to  FIG. 7 , one 80 MHz channel bandwidth may refer to a first channel band and a remaining one 80 MHz channel bandwidth may refer to the second channel band. It is assume that the first channel band is the legacy channel and the second channel band is an extension band. Further, for the purpose of convenience, it is assumed that the first STA is the legacy STA and the second STA is an extension STA. 
     It is assumed that the first BSS is a BSS where a first channel band corresponding to the legacy channel is configured as the primary channel and a second channel band being a remaining channel band corresponding to an extension channel is configured as the secondary channel. Conversely, it is assumed that the second BSS is a BSS where the second channel band corresponding to the extension channel is the primary channel and a first channel band being a remaining channel band corresponding to the legacy channel is the secondary band. 
     Further, the first BSS and the second BSS includes each primary channel with 20 MHz             STA primary channel of 20 MHz (a first STA primary channel  720  and a second STA primary channel  700 ). The first STA primary channel may be defined in a first BSS and the second STA primary channel may be defined in the second BSS.
     The STAs included in each BSS may perform channel access based on the STA primary channels  700  and  720 . 
     The transmission and reception of the channel access and the frame may be independently performed in the first BSS and the second BSS. For example, the AP may transmit a beacon frame  740  to a first STA primary channel  720  of the first channel band and a second STA primary channel  700  of the second channel band. The beacon frame  740  may be simultaneously or independently transmitted from the first channel bandwidth and the second channel bandwidth. The beacon frame  740  transmitted from the first channel band and a beacon frame  740  transmitted from the second channel band may include an operation parameter necessary when the STA operates in the first BSS and the second BSS. 
       FIG. 7  illustrates a case where the first STA access the STA primary channel  720  and the second STA accesses the second STA primary channel  700 , based on the beacon frame transmitted from the AP. 
     The first STA may perform channel access through a CSMA/CA based back-off procedure. The first STA may perform channel access at the first STA primary channel  720  to determine channel status information on remaining first STA secondary channel bands  725  and  730  except for the first STA primary channel. The first channel band may include a first STA primary channel  720 , a first STA secondary channel  725  of 20 MHz, and a first STA secondary channel  725  of 40 MHz. The first STA may determine channel status information on whether remaining secondary channels  725  and  730  except for the first STA primary channel  720  is available. As the determination result of the channel status information, the first STA may transmit RTS frames  750  and  760  to an available channel among an available first channel band. 
     The first STA may determine a status of the channel for a point coordination function (PCF) inter-frame space (PIFS) time before transmitting the RTS frames  750  and  760  in order to determine whether the secondary channels  725  and  730  is idle or busy. If the secondary channels  725  and  730  are idle for a PIFS time before transmitting the RTS frames  750  and  760 , the first STA may determine whether a corresponding channel is available. 
       FIG. 7  illustrates a case where a first STA secondary channel  725  of 20 MHz is available. If the first STA secondary channel  725  of 20 MHz is available, the first STA may transmit RTS frames  750  and  760  through the first STA primary channel  720  and the first STA secondary channel  725  of 20 MHz. For example, the first RTS frame  750  transmitted through the first STA primary channel  720  and the second RTS frame  760  transmitted through the first STA secondary channel  725  may be simultaneously transmitted from the STA. The AP may transmit the first CTS frame  770  as a response to the first RTS frame  750 . The AP may transmit the second CST frame  780  as a response to the second RTS frame  760 . The AP may transmit CTS frames  770  and  780  being a response to the RTS frames  750  and  760  at the same channel bandwidth to which the RTS frame is transmitted based on a channel to which respective RTS frames  750  and  760  are transmitted. The AP may simultaneously transmit the first CTS frame  770  and the second CTS frame  780 . The first STA may equally configure a time point when the AP transmits the first CTS frame  770  and the second CTS frame  780  based on a field (e.g., duration field) included in the RTS frames  750  and  760 . 
     The first STA may transmit a data frame through the first STA primary channel  720  and the first STA secondary channel  725  of 20 MHz. If the first STA receives the CTS frames  770  and  780 , the first STA may transmit the data frame  785 . The data frame  785  transmitted from the first STA may include an aggregated MAC protocol data unit (A-MPDU) format. 
     The AP may receive the data frame  785  from the first STA and may transmit a block ACK  790  being a response to the data frame  785  through the first STA primary channel  720  and the first STA secondary channel  725  of 20 MHz. The first STA may equally configure a time point when the AP transmits the block ACK  790  based on a field (e.g., duration field) included in the data frame  785 . 
     The second STA operating at the second channel band may perform uplink transmission to the AP by the above procedure. However, when the second STA is an extension STA and supports an existing legacy channel, the second STA may determine the first channel band as the second STA secondary channel to determine the status of a channel when performing a procedure of determining availability of the channel in order to transmit the RTS frame. If an available second STA second channel is included in the first channel band, a second STA secondary channel included in the first channel band may be used for uplink transmission. It is assumed in  FIG. 7  that a second STA secondary channel includes only a 20 MHz secondary channel  705 . The second STA may transmit data based on the second STA primary channel and the 20 MHz            2 STA secondary channel.
     A procedure between the second STA and the AP may be independently performed from a procedure between the first STA and the AP. That is, the AP may independently transmit and receive the data to and from the first STA and the second STA at the first channel band and the second channel band. 
     Although  FIG. 7  illustrates that the STA transmit the RTS frame, when the AP perform downlink transmission, the AP may determine the availability of the channel to transmit the RTS frame to the first STA and the second STA. In this case, the RTS frame may be transmitted to the STA from the AP, and the CTS frame may be transmitted to the AP from the STA. In addition, the A-MPDU frame may be transmitted to the STA from the AP, and a block ACK may be transmitted to the AP from the STA. 
     That is, the AP may receive a plurality of RTS frames from a plurality of STAs through a plurality of channels, respectively, and may transmit a plurality of CTS frames to a plurality of STAs through a plurality of available channels among the plurality of channels, respectively. 
     The AP may receive a plurality of uplink data transmitted from the plurality of STAs through a plurality of available channels, respectively. The AP may serve the plurality of STAs based on the first BSS and the second BSS. The first BSS may include a first main channel band and a first sub-channel band and the second BSS may include a second main channel band and a second sub-channel band. As described above, the first main channel band may overlap with the second sub-channel band and the first sub-channel band may overlap with the second main channel band. The plurality of channels receiving a plurality of RTS frames from the plurality of STAs through the plurality of channels may be included in the first main channel band and the first sub-channel band. 
     According to another embodiment of the present invention, the AP may determine whether to perform a service to a plurality of STAs based on the RTS frame transmitted from a plurality of STAs. For example, the AP may determine to independently perform a first transmit (TX)/receive (RX) process and a second TX/RX at a specific time period with respect to the plurality of STAs on the basis of the channel status information measured based on the RTS frame. 
       FIG. 8  is a conceptual diagram illustrating a procedure of transmitting and receiving data between a STA and an AP according to an embodiment of the present invention. 
       FIG. 8  illustrates on the assumption that one STA performs uplink transmission using both of the first channel band and the second channel band. 
     One 80 MHz channel bandwidth may refer to a first channel band and a remaining one 80 MHz channel bandwidth may refer to a second channel band. It is assumed that the first channel band is a legacy channel and the second channel band is an extension band. Unlike  FIG. 7 ,  FIG. 8  illustrates on the assumption that both of the first STA and the second STA are an extension STA. 
     It is assumed that the first BSS is a BSS in which a first channel bandwidth corresponding to the legacy channel is configured as the primary channel and a second channel band being a remaining channel band corresponding to the extension channel is configured as a secondary channel. Conversely, it is assumed that the second BSS is a BSS in which a second channel band corresponding to the extension channel is configured as the primary channel and the first channel band being a remaining channel band corresponding to the legacy channel is configured as the secondary channel. In detail, the first BSS and the second BSS includes 20 MHz STA primary channel (a first STA primary channel  820  and a second STA primary channel  800 ) in each primary channel. STAs included in each BSS may perform channel access based on the STA primary channels  800  and  820 . 
     The first STA may perform channel access by performing a back-off procedure at the first STA primary channel  820 . The first STA may determine channel availability with respect to a remaining secondary channel 20 MHz first STA secondary channel  825 , a 40 MHz first STA secondary channel  830 , a 80 MHz first STA secondary channel (second channel band) after a back-off timer is terminated. Since the first STA is an extension STA, the first STA may determine channel availability with respect to both of the first channel band and the second channel band. 
     In order to determine the availability of the first STA secondary channel, the first STA may determine whether the first STA secondary channel is idle or busy. The first STA may determine whether a status of the channel is idle for a PIFS time to transmit the RTS frame  850 . The first STA may determine a channel in an idle status for a PIFS time among the first STA secondary channels as an available channel to transmit the RTS frame to the available channel. The first STA may perform 40 MHz/80 MHz/160 MHz/80+80 MHz transmission according to which first STA secondary channels are idle. A 160 MHz channel band represents a case where the first channel band and the second channel band continue. 
       FIG. 8  illustrates on the assumption that all first STA secondary channels are available. When all the first STA secondary channels are available, the first STA may transmit the data frame  870  using both of the first channel band and the second channel band. 
     When the AP supports transmission of a 80 MHz channel bandwidth at the first channel band and the second channel band, the AP may support 160 MHz/80+80 MHz using both of the first channel band and the second channel band. 
     In this case, the first STA may transmit the RTS frame  850  through the first channel band and the second channel band. The AP may transmit a CTS frame  860  as a response to the RTS frame  850 . The CTS frame  860  transmitted through the first STA primary channel  820  and the first STA secondary channel (20 MHz secondary channel  825 , 40 MHz secondary channel  830 , second band) by the AP may be configured to be transmitted at the same time. The first STA may configure a transmission time point of the CTS frame  860  based on a field (e.g., duration field) included in the RTS frame  850 . 
     If the first STA receives the CTS frame  860  from the AP, the first STA may transmit the data frame  870  to the AP. The first STA may equally configure a start time point and an end time point of transmission of the data frame  870  transmitted through the first STA primary channel and the first STA secondary channel. If the AP receives the data frame  870  from the first STA, the AP may transmit a block ACK  880  to the first STA as a response with respect to the data frame  870 . A time point of transmitting the block ACK  880  through the first STA primary channel and the first STA secondary channel may be equally configured. 
     In the side of the second STA, since the first STA uses the first channel band and the second channel band for a predetermined period, the first STA may determine that the channel is not available at a corresponding period. In detail, while the first STA included in the first BSS transmits and receives the data to and from the AP using both of the first channel band and the second channel band, it may be determined that the second channel band being a primary channel band of the second BSS is busy. Accordingly, the second STA included in the second BSS defers a back-off mechanism at a channel access procedure. After transmission from the first STA is terminated, the second STA included in the second BSS may perform channel access using a back-off mechanism at the second STA primary channel  800  of the second BSS during the same procedure. The second STA may determine channel availability of the first channel band and the second channel band of the second BSS to determine a channel for transmitting the data frame.  FIG. 8  illustrates a case where all of the 20 MHz second STA secondary channel  805 , a secondary channel  830  of a 40 MHz second STA, and a secondary channel (first channel band) of the 80 MHz second STA. The second STA may transmit the data frame to the AP by performing the same procedure as that of the above first STA and the AP. 
       FIG. 9  is a block diagram illustrating a wireless device to which an embodiment of the present invention may apply. 
     Referring to  FIG. 9 , the wireless device may be an STA that may implement the above-described embodiments, and the wireless device may be an AP  950  or a non-AP STA (or STA) ( 900 ). 
     The STA  900  includes a processor  910 , a memory  920 , and an RF (Radio Frequency) unit  930 . 
     The RF unit  930  may be connected with the processor  920  to transmit/receive radio signals. 
     The processor  920  implements functions, processes, and/or methods as proposed herein. For example, the processor  920  may be implemented to perform the operation of the above-described wireless device according to an embodiment disclosed in  FIG. 6  to  FIG. 8  of the present invention. 
     For example, the processor  920  may access the AP by executing back-off based on the BSS including each STA among a plurality of BSSs implemented by the AP. 
     The AP  950  includes a processor  960 , a memory  970 , and an RF (Radio Frequency) unit  980 . 
     The RF unit  980  may be connected with the processor  960  to transmit/receive radio signals. 
     The processor  960  implements functions, processes, and/or methods as proposed herein. For example, the processor  960  may be implemented to perform the operation of the above-described wireless device according to an embodiment disclosed in  FIG. 6  to  FIG. 8  of the present invention. 
     For example, the processor  960  may receive a plurality of RTS frames from a plurality of STAs through a plurality of channels, and may receive a plurality of CTS frames to the plurality of STAs through a plurality of available channels among the plurality of channels. Further, the processor  960  may be configured to receive a plurality of uplink data frames transmitted from a plurality of STAs through a plurality of available channels. 
     Further, the processor  960  may be configured to communicate with a plurality of STAs based on the first BSS and the second BSS. The first BSS may include a first main channel band and a first sub-channel band. The second BSS may include a second main channel band and a second sub-channel band. The first main channel band may overlap with the second sub-channel band. The first sub-channel band may overlap with the second main channel band. A plurality of channels may be included in the first main channel band and the first sub-channel band. 
     The processor  920  may include an ASIC (Application-Specific Integrated Circuit), other chipset, a logic circuit, a data processing device, and/or a converter that performs conversion between a baseband signal and a radio signal. The memory  940  may include a ROM (Read-Only Memory), a RAM (Random Access Memory), a flash memory, a memory card, a storage medium, and/or other storage device. The RF unit  960  may include one or more antennas that transmit and/or receive radio signals. 
     When an embodiment is implemented in software, the above-described schemes may be embodied in modules (processes, or functions, etc.) performing the above-described functions. The modules may be stored in the memory  920 ,  970  and may be executed by the processor  910 ,  960 . The memory  920 ,  970  may be positioned in or outside the processor  910 ,  960  and may be connected with the processor  910 ,  960  via various well-known means.