Abstract:
A method and a device for preventing interference in an overlapping service area are disclosed. The method for preventing interference in an overlapping service area of a wireless LAN can comprise the steps of: receiving, by an interference AP, a beacon frame broadcasted by an AP, wherein the beacon frame includes an interference prevention information element; receiving, by the interference AP, a sounding PPDU indicated on the basis of the interference prevention information element from the AP; determining, by the interference AP, a transmission control matrix on the basis of the sounding PPDU; and transmitting, by the interference AP, data through a beam generated on the basis of the transmission control matrix.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to wireless communications, and more particularly, to a method and an apparatus for preventing interference in a wireless communication system. 
         [0003]    2. Related Art 
         [0004]    In a wireless local area network (WLAN), there are two kinds of basic service sets (BSSs). A first type of BSS is an independent BSS (IBSS) that is a BSS in an ad-hoc mode, in which stations (STAs) can directly communicate with each other, not via an access point (AP). The IBSS does not allow access to a distribution system (DS). Thus, the IBSS may achieve a self-contained network. A second type of BSS may be an infrastructure BSS. The infrastructure BSS may include an AP and a plurality of STAs, in which the AP may be connected to a DS. Meanwhile, in an overlapping basic service set (OBSS) environment, a plurality of BSSs is present within a certain region and may overlap with each other in an area and overlapping service areas of the BSSs may cause interference between the BSSs. 
         [0005]    When there are a large number of BSSs or a small number of frequency channels due to use of broadband of 80 MHz/160 MHz as in TGac, an OBSS may occur. In the OBSS environment, interference between STAs may cause drastic deterioration in performance. To mitigate interference in the OBSS environment, a selection algorithm for BSSs to actively select an available channel or a mechanism for communication between overlapping BSSs may be adopted. Alternatively, a mechanism in which communications between other overlapping BSSs are suspended during communication of a target BSS may be used. In addition, studies are being conducted on a variety of methods for preventing interference occurring between an AP and an STA in the OBSS environment. 
       SUMMARY OF THE INVENTION 
       [0006]    An aspect of the present invention is to provide a method of preventing interference in an overlapping service area. 
         [0007]    Another aspect of the present invention is to provide an apparatus for performing a method of preventing interference in an overlapping service area. 
         [0008]    To achieve the aforementioned purposes of the present invention, a method of preventing interference in an overlapping service area of a wireless local area network (WLAN) according to one aspect of the present invention includes receiving, by an interfering access point (AP), a beacon frame broadcasted by an AP, the beacon frame including an interference avoidance information element; receiving, by the interfering AP, a sounding physical layer convergence procedure (PLCP) protocol data unit (PPDU) indicated based on the interference avoidance information element from the AP; determining, by the interfering AP, a transmit steering matrix based on the sounding PPDU; and transmitting, by the interfering AP, data through a beam generated based on the transmit steering matrix. 
         [0009]    To achieve the aforementioned purposes of the present invention, an AP for preventing interference in an overlapping service area of a WLAN according to another aspect of the present invention includes a radio frequency (RF) unit configured to transmit or receive a radio signal and a processor selectively connected to the RF unit, wherein the processor receives a beacon frame broadcasted by another AP, the beacon frame including an interference avoidance information element, receives a sounding PPDU indicated based on the interference avoidance information element from the other AP, determines a transmit steering matrix based on the sounding PPDU, and transmits data through a beam generated based on the transmit steering matrix. 
         [0010]    Interference which may occur in communication STAs may be prevented based on a sounding frame in an OBSS environment. Thus, interference between STAs may be reduced in an OBSS environment, such as a case where a large number of BSSs are present or a small number of frequency channels are available. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a conceptual view illustrating the structure of a wireless local area network (WLAN). 
           [0012]      FIG. 2  is a view illustrating a layer architecture of a WLAN system supported by IEEE 802.11. 
           [0013]      FIG. 3  is a conceptual view illustrating an overlapping basic service set (OBSS) environment. 
           [0014]      FIG. 4  is a conceptual view illustrating a method for reducing interference in an OBSS environment according to an embodiment of the present invention. 
           [0015]      FIG. 5  is a conceptual view illustrating a sounding PPDU transmitted to reduce interference in an OBSS environment according to an embodiment of the present invention. 
           [0016]      FIG. 6  is a conceptual view illustrating a method for reducing interference in an OBSS environment according to an embodiment of the present invention. 
           [0017]      FIG. 7  is a conceptual view illustrating a method for reducing interference in an OBSS environment according to an embodiment of the present invention. 
           [0018]      FIG. 8  is a conceptual view illustrating a method for reducing interference in an OBSS environment according to an embodiment of the present invention. 
           [0019]      FIG. 9  is a conceptual view illustrating an interference mitigation method according to an embodiment of the present invention. 
           [0020]      FIG. 10  is a conceptual view illustrating an interference mitigation method according to an embodiment of the present invention. 
           [0021]      FIG. 11  is a conceptual view illustrating an interference mitigation method in an OBSS environment according to an embodiment of the present invention. 
           [0022]      FIG. 12  is a conceptual view illustrating an interference mitigation method in an OBSS environment according to an embodiment of the present invention. 
           [0023]      FIG. 13  is a conceptual view illustrating a method of transmitting a steered sounding PPDU according to an embodiment of the present invention. 
           [0024]      FIG. 14  is a flowchart illustrating a method of transmitting a steered sounding PPDU according to an embodiment of the present invention. 
           [0025]      FIG. 15  is a block diagram illustrating a wireless device according to an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0026]      FIG. 1  is a conceptual view illustrating the structure of a wireless local area network (WLAN). 
         [0027]    The upper part of  FIG. 1  shows the structure of the IEEE (institute of electrical and electronic engineers) 802.11 infrastructure network. 
         [0028]    Referring to the upper part of  FIG. 1 , 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 STA 1  (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 . 
         [0029]    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. 
         [0030]    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  230  connected via the distribution system  110 . The APs included in one ESS  140  may have the same SSID (service set identification). 
         [0031]    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). 
         [0032]    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). 
         [0033]    The lower part of  FIG. 1  is a conceptual view illustrating an independent BSS. 
         [0034]    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. 
         [0035]    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). 
         [0036]    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. 
         [0037]      FIG. 2  is a view illustrating a layer architecture of a WLAN system supported by IEEE 802.11. 
         [0038]      FIG. 2  conceptually illustrates a layer architecture (PHY architecture) of a WLAN system. 
         [0039]    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. 
         [0040]    The MAC sub-layer  220 , the PLCP sub-layer  210 , and the PMD sub-layer  200  may conceptually include management entities. 
         [0041]    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 . 
         [0042]    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. 
         [0043]    The operation of each sub-layer is briefly described below. The PLCP sub-layer  110  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. 
         [0044]    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. 
         [0045]    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. 
         [0046]      FIG. 3  is a conceptual view illustrating an overlapping basic service set (OBSS) environment. 
         [0047]    Referring to  FIG. 3 , in the OBSS environment, a plurality of BSSs is present within a certain region and service areas (or service ranges) of the BSSs may overlap with each other. The overlapping service areas of the different BSSs may cause interference between the BSSs. 
         [0048]    A service area may also be referred to as coverage. Generally, coverage of an STA may refer to an area (transmission (Tx)/reception (Rx) range) in which the STA transmits data to another STA or receives data from another STA. Alternatively, the coverage of the STA may refer to a carrier sensing (CS) range for performing CS on data transmitted from another STA. In the following embodiments of the present invention, it is assumed that a Tx/Rx range is the same as a CS range for convenience. 
         [0049]    An upper part of  FIG. 3  illustrates a case where a plurality of STAs  310  and  320  is present in an overlapping area of coverages of a plurality of BSSs. In an OBSS environment where the coverages of the plurality of BSSs overlap with each other, the STAs  310  and  320  communicating with a particular AP may encounter interference by data transmitted from another AP. Thus, performance deterioration in communications between the STAs  310  and  320  and the particular AP may occur by the interference. For similar reasons, interference may occur not only between the STAs and the other AP but also between the AP and the other AP in the OBSS environment. In the following embodiments of the present invention, an AP causing interference may be referred to as an interfering AP for convenience. 
         [0050]    In the embodiments of the present invention, a direction of a beam transmitted by an interfering AP may be controlled based on a sounding PPDU transmitted between an AP and the interfering AP or between the interfering AP and an STA in order to solve an interference problem. The direction of the beam from the interfering AP determined based on the sounding PPDU may be determined towards a reduction in interference between the AP and the interfering AP or between the interfering AP and the STA. 
         [0051]    A lower part of  FIG. 3  illustrates an OBSS environment where APs respectively forming a plurality of BSSs are positioned in an overlapping area of the BSSs. 
         [0052]    In the lower part of  FIG. 3 , STA 2   370  is positioned in coverage of AP 1   350  and coverage of AP 2   360 . When AP 2   360  and STA 2   370  are associated to perform communication, AP 2   360  may form a beam for transmitting data to STA 2   370 . Since AP 2   360  and STA 2   370  are positioned within the coverage of AP 1   350 , communication between AP 2   360  and STA 2   370  may encounter interference by a beam formed by AP 1   350 . 
         [0053]    The following embodiments of the present invention illustrate a method for reducing interference in an STA in an OBSS environment. Specifically, an interference mitigation procedure for mitigating interference in communication between AP 2  and STA 2  by AP 1 , which is an interfering AP, will be described. 
         [0054]      FIG. 4  is a conceptual view illustrating a method for reducing interference in an OBSS environment according to an embodiment of the present invention. 
         [0055]    Referring to  FIG. 4 , coverage of a first BSS including AP 1   410  may overlap with coverage of a second BSS including AP 2   420 . 
         [0056]    In communication, AP 2   420  may broadcast an interference avoidance request frame (I-avoidance request frame)  425  to avoid interference by a neighboring STA and/or a neighboring AP not communicating with AP 2   420 . 
         [0057]    The interference avoidance request frame  425  may be broadcast to the neighboring STA and/or the neighboring AP to prevent in advance interference by the neighboring STA and the neighboring AP which may occur in communication of AP 2   420 . Alternatively, the interference avoidance request frame  425  may be broadcast to the neighboring STA and/or the neighboring AP to prevent continuous interference while AP  2   420  encounters interference. 
         [0058]    The neighboring STA and/or the neighboring AP receiving the interference avoidance request frame  425  may transmit an interference avoidance response frame (I-avoidance response frame)  415 . The neighboring STA and/or the neighboring AP transmitting the interference avoidance response frame  415  may be an AP and/or an STA not communicating (or not associated) with an AP and/or an STA transmitting the interference avoidance request frame (AP 2   420  in  FIG. 4 ). Further, the neighboring STA or the neighboring AP assumes that a direction of a transmitted beam is changeable. 
         [0059]    In  FIG. 4 , AP 1   410  may transmit an interference avoidance response frame  415  to AP 2   420 . 
         [0060]    When AP 2   420  receives the interference avoidance response frame from AP 1   410 , AP 2   420  may transmit a sounding PPDU  435  to AP 1   410 . AP 1   410  receiving the sounding PPDU  435  transmitted by AP 2   420  may acquire channel information on a channel between AP 1   410  and AP 2   420 . 
         [0061]    AP 1   410  may determine an appropriate transmit steering matrix for transmitting data to STA 2   400  based on the sounding PPDU  435  received from AP 2   420 . When AP 1   410  forms a beam for transmitting data based on the determined transmit steering matrix, interference in AP 2   420  by AP 1   410  may be minimized. As illustrated in  FIG. 4 , the beam formed by AP 1   410  may be determined using the transmit steering matrix determined based on the sounding PPDU  435  transmitted by AP 2   420 . For example, the beam used for communication between AP 1   410  and STA 2   400  may be formed towards a reduction in interference in communication between AP 2   420  and an STA (for example, STA 2 , STA 3 , and STA 3 ) based on the transmit steering matrix. That is, interference in AP 2   420  by the beam formed by AP 1   410  may be reduced, which may improve performance of communication between STA 2 , STA 3  or STA 4  positioned within the coverage of AP 2   420  and AP 2   420 . 
         [0062]    Alternatively, downlink data transmission from AP 1   410  to STA 2   400  and uplink data transmission from STA 3  and/or STA 4  to AP 2   420  may occur simultaneously. In this case, the beam formed by AP 1   410  based on the transmit steering matrix may be formed towards a reduction in interference between AP 1   410  and AP 2   420 . Thus, overall throughput in a WLAN system may be increased. 
         [0063]    The sounding PPDU transmitted from AP 2   420  to AP 1   410  may be, for example, a null data packet (NDP) frame. The NDP frame may be a null data packet and may include only a PLCP header and a PLCP preamble excluding a data field. 
         [0064]      FIG. 5  is a conceptual view illustrating a sounding PPDU transmitted to reduce interference in an OBSS environment according to an embodiment of the present invention. 
         [0065]    Referring to  FIG. 5 , a sounding PPDU may be an NDP frame including no data field. 
         [0066]    The NDP frame may further include a legacy-short training field (L-STF)  500 , a legacy-long training field (L-LTF)  510 , a legacy signal (L-SIG) field  520 , a very high throughput (VHT)-SIG-A  530 , a VHT-STF  540 , a VHT-LTF  550 , and a VHT-SIG-B  560 . 
         [0067]    The L-STF  500 , the L-LTF  510 , and the L-SIG field  520  included in the NDP frame may be used for frequency offset adjustment, phase offset adjustment, and channel estimation for a legacy STA, respectively. 
         [0068]    Specifically, the L-STF  500  may include a short training orthogonal frequency division multiplexing (OFDM) symbol. The L-STF  500  may be used for frame detection, automatic gain control (AGC), diversity detection, and coarse frequency/time synchronization. 
         [0069]    The L-LTF  510  may include a long training OFDM symbol. The L-LTF  510  may be used for fine frequency/time synchronization and channel estimation. 
         [0070]    The L-SIG  520  may be used to transmit control information. The L-SIG  520  may include information on data rate and data length. 
         [0071]    A VHT PPDU may further include a VHT-SIG-A  530 , a VHT-STF  540 , a VHT-LTF  550 , and a VHT-SIG-B  560 . The VHT-SIG-A  530 , the VHT-STF  540 , the VHT-LTF  550 , and the VHT-SIG-B  560  may be used for an STA supporting a VHT system. 
         [0072]    The VHT-SIG-A  530  may include information for interpreting the VHT PPDU. The VHT-SIG-A  530  may include a VHT-SIG-A 1  and a VHT-SIG-A 2 . 
         [0073]    The VHT-SIG-A 1  may include bandwidth information on a used channel, information on whether space-time block coding is applied, a group identifier (ID) for multi-user (MU)-multiple-input and multiple-output (MIMO) transmission, and information on the number of streams used for MU-MIMO transmission. 
         [0074]    The VHT-SIG-A 2  may include information on whether a short guard interval (GI) is used, forward error correction (FEC) information, information on a modulation and coding scheme (MCS) for a single user, information on channel coding types for multiple users, beamforming related information, redundancy bits for cyclic redundancy checking (CRC), and tail bits of a convolutional decoder. 
         [0075]    The VHT-STF  540  may be used to improve automatic gain control estimation in an MIMO environment. 
         [0076]    The VHT-LTF  550  may be used for channel estimation in an MIMO environment. An STA may perform channel estimation for 20/40/80/160/80+80 MHz channel bandwidths based on the VHT-LTF  550 . 
         [0077]    The VHT-SIG-B  560  may include information on each STA, that is, information on PSDU length and a MCS, tail bits, or the like. 
         [0078]    In the embodiment of the present invention, an STA or AP receiving an interference avoidance request frame may perform channel estimation based on the training fields included in the NDP frame and determine a transmit steering matrix for forming a beam based on channel estimation. 
         [0079]      FIG. 6  is a conceptual view illustrating a method for reducing interference in an OBSS environment according to an embodiment of the present invention. 
         [0080]      FIG. 6  illustrates a method in which an STA or AP receiving an interference avoidance request frame transmits a sounding PPDU after transmitting an interference avoidance response frame. Unlike in  FIG. 4 , the STA or AP transmitting the interference avoidance response frame may transmit the sounding PPDU, instead of the STA or AP transmitting the interference avoidance request frame. 
         [0081]    Referring to  FIG. 6 , AP 2   620  may broadcast an interference avoidance request frame, and AP 1   610  may receive the interference avoidance request frame  625 . AP 1   610  receiving the interference avoidance request frame  625  may transmit a sounding PPDU (for example, an NDP frame)  620  after a certain amount of time (for example, short inter-frame space (SIFS)). 
         [0082]    When AP 1   610  transmits a sounding PPDU  635  to AP 2   620 , AP 2   620  may perform channel estimation for a channel between AP 1   610  and AP 2   620  based on the sounding PPDU  635 . AP 2   620  may transmit a result of channel estimation (channel estimation information or channel state information)  645  performed based on the sounding PPDU  635  to AP 1   610 . 
         [0083]    AP 1   610  may determine an appropriate transmit steering matrix for transmitting data to STA 2   600  based on the channel state information  645  received from AP 2   620 . AP 1   610  may generate a beam for transmitting data to STA 2   600  using the transmit steering matrix determined based on the channel state information. In this case, interference in AP 2   620  by the beam formed by AP 1   610  may be reduced. 
         [0084]      FIG. 7  is a conceptual view illustrating a method for reducing interference in an OBSS environment according to an embodiment of the present invention. 
         [0085]      FIG. 7  illustrates an interference mitigation procedure (or interference mitigation protocol) using a beacon frame including an interference avoidance request information element without a request with an interference avoidance request frame. A beacon frame may be used to reduce signaling overhead by the interference avoidance request frame and the interference avoidance response frame used in the procedure for reducing interference in the OBSS illustrated in  FIGS. 4 to 6 . 
         [0086]    Specifically, a beacon frame may be used for the interference mitigation procedure in the OBSS environment, instead of an interference avoidance response frame. The beacon frame may include an interference avoidance request information element (or interference avoidance information element). The beacon frame including the interference avoidance request information element may serve the same function as the interference avoidance request frame. 
         [0087]    Referring to  FIG. 7 , AP 2   720  may periodically transmit a beacon frame  725  including an interference avoidance request information element for interference mitigation. The interference avoidance request information element may indicate that transmission of a sounding PPDU  750  is performed SIFS after transmitting the beacon frame  725 . AP 2   720  may transmit the sounding PPDU (for example, an NDP frame) to AP 1   710  for sounding SIFS after transmitting the beacon frame  725  including the interference avoidance request information element. 
         [0088]    The transmitted sounding PPDU  725  may be a steered sounding PPDU for estimation of an effective channel. The steered sounding PPDU may be generated using a precoding matrix acquired based on another sounding PPDU transmitted by STA 2   700  to AP 2   720 , which will be described below in a specific embodiment. 
         [0089]    AP 1   710  receiving the beacon frame  725  and the sounding PPDU  750  from AP 2   720  may perform channel estimation for a channel between AP 1   710  and AP 2   720  based on training fields (for example, VHT-LTF) included in the sounding PPDU  750 . AP 1   710  may determine a transmit steering matrix based on a result of channel estimation (or channel state information). AP 1   710  may form a beam based on the determined transmit steering matrix. The beam of AP 1   710  formed based on the transmit steering matrix may reduce interference by AP 1   710  in communication between AP 2   720  and STA 2   700 . 
         [0090]      FIG. 8  is a conceptual view illustrating a method for reducing interference in an OBSS environment according to an embodiment of the present invention. 
         [0091]      FIG. 8  illustrates an interference mitigation procedure (or interference mitigation protocol) using a beacon frame including an interference avoidance response information element without a request with an interference avoidance request frame. A beacon frame may be used to reduce signaling overhead by the interference avoidance request frame and the interference avoidance response frame used in the procedure for reducing interference in the OBSS illustrated in  FIGS. 4 to 6 . 
         [0092]    Specifically, a beacon frame may be used for the interference mitigation procedure in the OBSS environment, instead of an interference avoidance response frame. The beacon frame may include an interference avoidance response information element (or interference avoidance information element). The beacon frame including the interference avoidance response information element may serve the same function as the interference avoidance response frame. 
         [0093]    Referring to  FIG. 8 , AP 1   810  may receive no interference avoidance request frame from AP 2   820 . AP 1   810  may periodically transmit a beacon frame  825  including an interference avoidance response information element for interference mitigation. The interference avoidance response information element (or interference avoidance information element) may indicate that transmission of a sounding PPDU  840  is performed SIFS after transmitting the beacon frame  825 . AP 1   810  may transmit the sounding PPDU (for example, an NDP frame)  840  to AP 2   820  for sounding SIFS after transmitting the beacon frame  825  including the interference avoidance response information element. 
         [0094]    The transmitted sounding PPDU  840  may be a steered sounding PPDU for estimation of an effective channel. The steered sounding PPDU may be generated using a precoding matrix acquired based on another sounding PPDU transmitted by STA 2   800  to AP 1   810 , which will be described below in a specific embodiment 
         [0095]    AP 2   820  receiving the beacon frame  825  and the sounding PPDU  840  from AP 1   810  may perform channel estimation for a channel between AP 1   810  and AP 2   820  based on training fields (for example, VHT-LTF) included in the sounding PPDU  840 . AP 2   820  may transmit a result of channel estimation (or channel state information  850  to AP 1   810 . AP 2   820  may transmit the channel state information, which is included in a compressed beamforming report frame, to AP 1   810 . AP 2   820  may include the channel state information in compressed beamforming report information included in the compressed beamforming report frame for transmission to AP 1   810 . 
         [0096]    AP 1   810  may determine a transmit steering matrix based on the channel state information  850  included in the frame transmitted from AP 2   820 . AP 1   810  may form a beam based on the determined transmit steering matrix. The beam of AP 1   810  formed based on the transmit steering matrix may reduce interference by AP 1   810  in communication between AP 2   820  and STA 2   800 . 
         [0097]    That is, in the OBSS environment illustrated in  FIGS. 7 and 8 , the interference mitigation procedures are performed not by exchanges of an interference avoidance request frame/interference avoidance response frame between AP 1  and AP 2 . The interference mitigation procedures may be performed by AP 1  in an unsolicited manner. 
         [0098]    Table 1 illustrates the channel state information included in the compressed beamforming report information. 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 (Channel 
                   
                   
                 (Subcarriers for which Compressed Feedback Beamforming Matrix subfield 
               
               
                 Width) 
                 Ng 
                 Ns 
                 is sent: scidx(0), scidx(1), . . . , scidx(Ns − 1)) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 20 MHz 
                 1 
                 52 
                 −28, −27, −26, −25, −24, −23, −22, −20, −19, −18, −17, −16, −15, −14, −13, −12, −11, −10, 
               
               
                   
                   
                   
                 −9, −8, −6, −5, −4, −3, −2, −1, 1, 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 
               
               
                   
                   
                   
                 19, 20, 22, 23, 24, 25, 26, 27, 28 
               
               
                   
                   
                   
                 (NOTE - Pilot subcarriers (±21, ±7) and DC subcarrier (0) are skipped) 
               
               
                   
                 2 
                 30 
                 −28, −26, −24, −22, −20, −18, −16, −14, −12, −10, −8, −6, −4, −2, −1, 1, 2, 4, 6, 8, 10, 12 
               
               
                   
                   
                   
                 14, 16, 18, 20, 22, 24, 26, 28 
               
               
                   
                 4 
                 16 
                 −28, −24, −20, −16, −12, −8, −4, −1, 1, 4, 8, 12, 16, 20, 24, 28, 
               
               
                 40 MHz 
                 1 
                 108 
                 −58, −57, −56, −55, −54, −52, −51, −50, −49, −48, −47, −46, −45, −44, −43, −42, −41, −40, 
               
               
                   
                   
                   
                 −39, −38, −37, −36, −35, −34, −33, −32, −31, −30, −29, −28, −27, −26, −24, −23, −22, −21, 
               
               
                   
                   
                   
                 −20, −19, −18, −17, −16, −15, −14, −13, −12, −10, −9, −8, −7, −6, −5, −4, −3, −2, 2, 3, 4, 5, 
               
               
                   
                   
                   
                 6, 7, 8, 9, 10, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 26, 27, 28, 29, 30, 
               
               
                   
                   
                   
                 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 
               
               
                   
                   
                   
                 54, 55, 56, 57, 58 
               
               
                   
                   
                   
                 (NOTE - Pilot subcarriers (±53, ±25, ±11) and DC subcarrier (0, ±1) are 
               
               
                   
                   
                   
                 skipped) 
               
               
                   
                 2 
                 58 
                 −58, −56, −54, −52, −50, −48, −46, −44, −42, −40, −38, −36, −34, −32, −30, −28, −26, −24, 
               
               
                   
                   
                   
                 −22, −20, −18, −16, −14, −12, −10, −8, −6, −4, −2, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 
               
               
                   
                   
                   
                 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58 
               
               
                   
                 4 
                 30 
                 −58, −54, −50, −46, −42, −38, −34, −30, −26, −22, −18, −14, −10, −6, −2, 2, 6, 10, 14, 18, 
               
               
                   
                   
                   
                 22, 26, 30, 34, 38, 42, 46, 50, 54, 58 
               
               
                 80 MHz 
                 1 
                 234 
                 −122, −121, −120, −119, −118, −117, −116, −115, −114, −113, −112, −111, −110, −109, 
               
               
                   
                   
                   
                 −108, −107, −106, −105, −104, −102, −101, −100, −99, −98, −97, −96, −95, −94, −93, 
               
               
                   
                   
                   
                 −92, −91, −90, −89, −88, −87, −86, −85, −84, −83, −82, −81, −80, −79, −78, −77, −76, −74, 
               
               
                   
                   
                   
                 −73, −72, −71, −70, −69, −68, −67, −66, −65, −64, −63, −62, −61, −60, −59, −58, −57, −56, 
               
               
                   
                   
                   
                 −55, −54, −53, −52, −51, −50, −49, −48, −47, −46, −45, −44, −43, −42, −41, −40, −38, −37, 
               
               
                   
                   
                   
                 −36, −35, −34, −33, −32, −31, −30, −29, −28, −27, −26, −25, −24, −23, −22, −21, −20, −19, 
               
               
                   
                   
                   
                 −18, −17, −16, −15, −14, −13, −12, −10, −9, −8, −7, −6, −5, −4, −3, −2, 2, 3, 4, 5, 6, 7, 8, 9, 
               
               
                   
                   
                   
                 10, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 
               
               
                   
                   
                   
                 33, 34, 35, 36, 37, 38, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 
               
               
                   
                   
                   
                 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 76, 77, 78, 
               
               
                   
                   
                   
                 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 
               
               
                   
                   
                   
                 100, 101, 102, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 
               
               
                   
                   
                   
                 117, 118, 119, 120, 121, 122 
               
               
                   
                   
                   
                 NOTE - Pilot subcarriers (±103, ±75, ±39, ±11) and DC subcarrier (0, ±1) are 
               
               
                   
                   
                   
                 skipped. 
               
               
                   
                 2 
                 122 
                 −122, −120, −118, −116, −114, −112, −110, −108, −106, −104, −102, −100, −98, −96, 
               
               
                   
                   
                   
                 −94, −92, −90, −88, −86, −84, −82, −80, −78, −76, −74, −72, −70, −68, −66, −64, −62, −60, 
               
               
                   
                   
                   
                 −58, −56, −54, −52, −50, −48, −46, −44, −42, −40, −38, −36, −34, −32, −30, −28, −26, −24, 
               
               
                   
                   
                   
                 −22, −20, −18, −16, −14, −12, −10, −8, −6, −4, −2, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 
               
               
                   
                   
                   
                 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 
               
               
                   
                   
                   
                 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96, 98, 100, 102, 104, 106, 
               
               
                   
                   
                   
                 108, 110, 112, 114, 116, 118, 120, 122 
               
               
                   
                 4 
                 62 
                 −122, −118, −114, −110, −106, −102, −98, −94, −90, −86, −82, −78, −74, −70, −66, −62, 
               
               
                   
                   
                   
                 −58, −54, −50, −46, −42, −38, −34, −30, −26, −22, −18, −14, −10, −6, −2, 2, 6, 10, 14, 18, 
               
               
                   
                   
                   
                 22, 26, 30, 34, 38, 42, 46, 50, 54, 58, 62, 66, 70, 74, 78, 82, 86, 90, 94, 98, 102, 
               
               
                   
                   
                   
                 106, 110, 114, 118, 122 
               
               
                   
               
             
          
         
       
     
         [0099]    Referring to Table 1, Ns may indicate the total number of indexes of subcarrier for transmitting a beamforming feedback matrix. Ng may indicate a difference between indexes of neighboring subcarriers. In a channel bandwidth of 80 MHz, when Ng is 1, Ns may indicate 234 subcarriers. When Ng is 2, Ns may indicate 122 subcarriers, and when Ng is 4, Ns may indicate 62 subcarriers. Channel state information may include information on Ns subcarriers. Specifically, the channel state information may be determined based on a beamforming feedback matrix for Ns subcarriers. As described above, an interfering AP may determine a transmit steering matrix based on the channel state information. The interfering AP may form a beam based on the determined transmit steering matrix. 
         [0100]      FIGS. 4 to 8  illustrate the interference mitigation procedures of a plurality of APs positioned in an overlapping basic service area (BSA) when BSAs of a plurality of BSSs overlap with each other. That is, in the OBSS environment, an interfering AP is positioned in the coverage of the first BSS including an AP and the AP is positioned in the coverage of the second BSS including the interfering AP. The interference mitigation procedures may be performed not only between APs but also between an AP and an STA. 
         [0101]      FIG. 9  is a conceptual view illustrating an interference mitigation method according to an embodiment of the present invention. 
         [0102]      FIG. 9  illustrates a method in which an STA performs an interference mitigation procedure by transmitting a sounding PPDU to an interfering AP. 
         [0103]    Referring to  FIG. 9 , it may be assumed that AP 2   920  transmits data to STA 2   925 , simultaneously with AP 1   910  transmitting data to STAS  915 . In this case, a beam formed by AP 1   910  may cause interference in communication between AP 2   920  and STA 2   925  to deteriorate communication performance. 
         [0104]    In order to avoid deterioration in communication performance by interference, STA 2   925  may broadcast an interference avoidance request frame  930 . AP 1   910  may receive the interference avoidance request frame  930  from STA 2   925  and transmit an interference avoidance response frame  940  to STA 2   925  in response to the interference avoidance request frame  930 . STA 2   925  may receive the interference avoidance response frame  940  from AP 1   910  and transmit a sounding PPDU  950  to AP 1   910 . AP 1   910  may acquire channel state information on a channel between STA 2   925  and AP 1   910  based on the sounding PPDU  950  received from STA 2   925 . AP 1   910  may determine a transmit steering matrix based on the channel state information. When AP 1   910  forms a beam based on the determined transmit steering matrix, interference by the beam formed by AP 1   910  may be reduced in communication between STA 2   925  and AP 2   920 . 
         [0105]    The sounding PPDU  950  transmitted by STA 2   925  to AP 1   910  may be an NDP frame. As described above, AP 1   910  may perform channel estimation for 20/40/80/160/80+80 MHz channel bandwidths based on a training field (for example, VHT-LTF) included in the NDP frame. Further, AP 1   910  may determine the transmit steering matrix based on a result of channel estimation (channel state information). 
         [0106]      FIG. 10  is a conceptual view illustrating an interference mitigation method according to an embodiment of the present invention. 
         [0107]      FIG. 10  illustrates an interference mitigation procedure performed based on a sounding PPDU transmitted from an STA transmitting an interference avoidance response frame. 
         [0108]    Referring to  FIG. 10 , STA 2   1025  may broadcast an interference avoidance request frame  1030 . AP 1   1015  may receive the interference avoidance request frame  1030  from STA 2   1025  and transmit an interference avoidance response frame  1040  to STA 2   1025  in response to the interference avoidance request frame  1030 . AP 1   1010  may transmit a sounding PPDU  1050  to STA 2   1025  SIFS after transmitting the interference avoidance response frame  1040 . 
         [0109]    STA 2   1025  may acquire channel state information  1060  on a channel between AP 1   1010  and STA 2   1025  based on the sounding PPDU  1050  received from AP 1   1010 . STA 2   1025  may transmit the acquired channel state information  1060  to AP 1   1010 . AP 1   1010  may determine a transmit steering matrix based on the channel state information  1060  received from STA 2   1025 . When AP 1   1010  forms a beam based on the determined transmit steering matrix, interference in STA 2   1025  by the beam formed by AP 1   1010  may be reduced. 
         [0110]      FIG. 11  is a conceptual view illustrating an interference mitigation method in an OBSS environment according to an embodiment of the present invention. 
         [0111]    In  FIG. 11 , AP 1   1110  may transmit a beacon frame  1130 , instead of an interference avoidance response frame, in order to reduce signaling overhead caused by an interference mitigation request frame and an interference mitigation response frame in an interference mitigation procedure. That is, the interference mitigation procedure (or interference mitigation protocol) may be performed using the beacon frame  1130  including an interference avoidance response information element (or interference avoidance information element) without a request with an interference avoidance request frame. The beacon frame  1130  including the interference avoidance response information element may serve a similar function to an interference avoidance response frame. 
         [0112]    Referring to  FIG. 11 , AP 1   1110  may periodically transmit the beacon frame  1130  including the interference avoidance response information element for interference mitigation. The interference avoidance response information element (or interference avoidance information element) may indicate that transmission of a sounding PPDU  1150  is performed SIFS after transmitting the beacon frame  1130 . 
         [0113]    AP 1   1110  may transmit the sounding PPDU (for example, an NDP frame)  1150  for sounding SIFS after transmitting the beacon frame  1130 . The transmitted sounding PPDU  1150  may be a steered sounding PPDU for estimation of an effective channel. The steered sounding PPDU may be generated using a precoding matrix acquired based on another sounding PPDU transmitted by AP 2   1120  to AP 1   1110 , which will be described below in a specific embodiment. 
         [0114]    STA 2   1125  receiving the beacon frame  1130  and the sounding PPDU  1150  from AP 1   1110  may acquire channel state information on a channel between AP 1   1110  and STA 2   1125  based on a training field (for example, VHT-LTF) included in the sounding PPDU  1150 . STA 2  may transmit the acquired channel state information  1160  to AP 1   1110 . STA 2   1125  may transmit the channel state information  1160 , which is included in a compressed beamforming report frame, to AP 1   1110 . Specifically, STA 2   1125  may include the channel state information  1160  in compressed beamforming report information included in the compressed beamforming report frame for transmission to AP 1   1110 . The channel state information  1160  may include information on Nx subcarriers for a particular channel bandwidth. Specifically, the channel state information  1060  may be determined based on a beamforming feedback matrix for Ns subcarriers. 
         [0115]    AP 1   1110  may determine a transmit steering matrix based on the channel state information  1160  included in the frame transmitted from STA 2   1125  to form a beam. The transmit steering matrix determined by AP 1   1110  may reduce interference by AP 1   1110  in communication between STA 2   1125  and AP 2   1120 . 
         [0116]    That is, in the OBSS environment illustrated in  FIG. 11 , the interference mitigation procedure is performed not by exchanges of an interference avoidance request frame/interference avoidance response frame between AP 1   1110  and STA 2   1125 . The interference mitigation procedure may be performed by AP 1   1110  in an unsolicited manner. 
         [0117]      FIG. 12  is a conceptual view illustrating an interference mitigation method in an OBSS environment according to an embodiment of the present invention. 
         [0118]      FIG. 12  illustrates a method in which AP 2  starts transmitting a sounding PPDU based on a request to send (RTS) frame/clear to send (CTS) frame. 
         [0119]    AP 2   1220  may transmit an RTS frame  1225  or a frame performing similar functions to the RTS frame  1225  (similar RTS frame) to STA 2   1200  before transmitting a data frame to STA 2   1200 . 
         [0120]    STA 2   1200  may transmit a CTS frame  1205  or a frame performing similar functions to the CTS frame (similar CTS frame) to AP 1   1210  or AP 2   1220 . The CTS frame  1205  or similar CTS frame may serve the same function as a sounding PPDU. AP 1   1210  and/or AP 2   1220  may receive the CTS frame  1205  or similar CTS frame and generate channel state information based on the received CTS frame  1205  or similar CTS frame. Hereinafter, in the embodiment of the present invention, the CTS frame  1205  or similar CTS frame serving as a sounding PPDU may be referred to as a sounding CTS frame. 
         [0121]    AP 1   1210  may determine a transmit steering matrix using the channel state information on a channel between STA 2   1200  and AP 1   1210  acquired based on the sounding CTS frame  1205 . The transmit steering matrix determined by AP 1   1210  may be used to form a beam for mitigating interference by AP 1   1210  in communication between STA 2   1200  and AP 2   1220 . 
         [0122]    Alternatively, an STA receiving an RTS frame may transmit a separate sounding PPDU to the AP, instead of a CTS frame. For example, AP 2  may transmit an RTS frame or a frame performing similar functions to the RTS frame (similar RTS frame) to STA 2  before transmitting a data frame to STA 2 . STA 2  may transmit a CTS frame or a similar CTS frame to AP 1  or AP 2 . STA 2  may transmit a separate sounding PPDU after transmitting the CTS frame or similar CTS frame. Specifically, STA 2  may transmit the sounding PPDU (for example, an NDP frame) SIFS after transmitting the CTS frame or similar CTS frame. 
         [0123]    AP 1  receiving the sounding PPDU may generate channel state information on a channel between STA 2  and AP 1 . AP 1  may determine a transmit steering matrix using the channel state information on the channel between STA 2  and AP 1  acquired based on the sounding PPDU. The transmit steering matrix determined by AP 1  may reduce interference by AP 1  in communication between STA 2  and AP 2 . 
         [0124]    The CTS frame transmitted by STA 2  may include an interference mitigation indication field. The interference mitigation indication field may indicate that transmission of a sounding PPDU for sounding for interference mitigation is performed after transmission of the CTS frame. 
         [0125]    According to another embodiment of the present invention, the sounding PPDUs transmitted in  FIGS. 4 to 12  may be a steered sounding PPDU. The steered sounding PPDU may be used for estimation of an effective channel between an AP and/or an STA transmitting the steered sounding PPDU and an AP and/or an STA receiving the steered sounding PPDU. 
         [0126]      FIG. 13  is a conceptual view illustrating a method of transmitting a steered sounding PPDU according to an embodiment of the present invention. 
         [0127]    Referring to  FIG. 13 , STA 2   1325  may transmit a steered sounding PPDU to AP 1   1310 . Here, STA 2   1325  may decompose an estimated channel between AP 2   1320  and STA 2   1325  (for example, using singular value decomposition (SVD)) to acquire a receiving matrix. STA 2   1325  may use the acquired receiving matrix as a precoding matrix for the steered sounding PPDU. That is, STA 2   1325  may transmit the steered sounding PPDU, precoded using the receiving matrix determined by decomposing the channel between AP 2   1320  and STA 2   1325 , to AP 1   1310 . AP 1   1310  may estimate an effective channel between AP 1   1310  and STA 2   1325  based on the steered sounding PPDU to form a beam. According to this method, interference by AP 1   1310  in STA 2   1325  may be reduced. 
         [0128]    Defining the channel between AP 1   1310  and STA 2   1325  as H 1  and the channel between AP 2   1320  and STA 2   1325  as H 2 , a received signal r of STA 2   1325  may be represented by Equation 1. 
         [0000]        r=H   2   P   2   x   2   +H   1   P   1   x   1   +n   &lt;Equation 1&gt;
 
         [0129]    In Equation 1, P 1  may be a precoding matrix of a signal transmitted from AP 1   1310  to STA 2   1325 , and P 2  may be a precoding matrix of a signal transmitted from AP 2   1320  to STA 2   1325 . 
         [0130]    x 1  may be data transmitted from AP 1   1310  to STA 2   1325 , x 2  may be data transmitted from AP 2   1320  to STA 2   1325 , and n may be noise. Here, signal H 1 P 1   x   1  transmitted by AP 1   1310  is an interfering signal for STA 2   1325 , and a signal to be received by STA 2   1325  is H 2 P 2   x   2 . 
         [0131]    Since AP 2   1320  has information on the channel between AP 2   1320  and STA 2   1325 , AP 2   1320  may properly select precoding matrix P 2 . For example, using channel decomposition, STA 2   1325  may select U 2  as a receiving matrix and V 2  as P 2  according to Equation 2. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             U 
                             2 
                             H 
                           
                            
                           r 
                         
                         = 
                           
                          
                         
                           
                             
                               U 
                               2 
                               H 
                             
                              
                             
                               H 
                               2 
                             
                              
                             
                               P 
                               2 
                             
                              
                             
                               x 
                               2 
                             
                           
                           + 
                           
                             
                               U 
                               2 
                               H 
                             
                              
                             
                               H 
                               1 
                             
                              
                             
                               P 
                               1 
                             
                              
                             
                               x 
                               1 
                             
                           
                           + 
                           
                             
                               U 
                               2 
                               H 
                             
                              
                             n 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             
                               U 
                               2 
                               H 
                             
                              
                             
                               U 
                               2 
                             
                              
                             Σ 
                              
                             
                                 
                             
                              
                             
                               V 
                               2 
                               H 
                             
                              
                             
                               P 
                               2 
                             
                              
                             
                               x 
                               2 
                             
                           
                           + 
                           
                             
                               U 
                               2 
                               H 
                             
                              
                             
                               H 
                               1 
                             
                              
                             
                               P 
                               1 
                             
                              
                             
                               x 
                               1 
                             
                           
                           + 
                           
                             
                               U 
                               2 
                               H 
                             
                              
                             n 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           
                             Σ 
                              
                             
                                 
                             
                              
                             
                               x 
                               2 
                             
                           
                           + 
                           
                             
                               U 
                               2 
                               H 
                             
                              
                             
                               H 
                               1 
                             
                              
                             
                               P 
                               1 
                             
                              
                             
                               x 
                               1 
                             
                           
                           + 
                           
                             
                               U 
                               2 
                               H 
                             
                              
                             n 
                           
                         
                       
                     
                   
                 
               
               
                 
                   〈 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   〉 
                 
               
             
           
         
       
     
         [0132]    Here, when a signal U 2   H H 1 P 1  transmitted from AP 1   1310 , which causes interference for STA 2   1325 , has a value close to 0, there is less interference in communication STA 2   1325  and AP 2   1320 . 
         [0133]    A precoding matrix P 1  for U 2   H H 1 P 1  to be 0 is selected by AP 1   1310 . However, in a situation where AP 1   1310  is capable of estimating only H 1  as a channel between AP 1   1310  and STA 2   1325 , interference avoidance to make U 2   H H 1 P 1  0 (or to minimize U 2   H H 1 P 1  close to 0) is restricted. 
         [0134]    For effective interference avoidance, AP 1   1310  need to estimate an effective channel U 2   H H 1 . STA 2   1325  may transmit a sounding PPDU precoded with U 2   H  to AP 1   1310  so that AP 1   1310  may estimate the effective channel U 2   H H 1 . AP 1   1310  may determine the precoding matrix P 1  based on the received precoded sounding PPDU such that U 2   H H 1 P 1  has a value of 0. 
         [0135]      FIG. 14  is a flowchart illustrating a method of transmitting a steered sounding PPDU according to an embodiment of the present invention. 
         [0136]    Referring to  FIG. 14 , STA 2   1400  may transmit an interference avoidance request frame  1450 , and AP 1   1410  may transmit an interference avoidance response frame  1460  in response. 
         [0137]    In the foregoing embodiment of the present invention, STA 2  may transmit a sounding PPDU to AP 1 , and AP 1  may acquire channel state information based on the received sounding PPDU and calculate a transmit steering matrix based on the channel state information. A beam formed based on the calculated transmit steering matrix may be used to transmit data to STAS. 
         [0138]    In  FIG. 14 , STA 2   1400  may transmit effective channel information to AP 1   1410  so that AP 1   1410  may estimate an effective channel. Specifically, STA 2   1400  may transmit the effective channel information to AP 1   1410  through a steered sounding PPDU  1480 . A precoding matrix multiplied by the steered sounding PPDU  1480  may be obtained based on a sounding PPDU  1470  transmitted by AP 2   1420  to STA 2   1400 . That is, STA 2   1400  may acquire a receiving matrix (precoding matrix) based on the sounding PPDU  1470  transmitted by AP 2   1420 , and precode a sounding PPDU transmitted to AP 1   1410  based on the acquired receiving matrix. The sounding PPDU precoded based on the acquired receiving matrix, which is the steered sounding PPDU  1480 , may be transmitted to AP 1   1410 . 
         [0139]    Subsequently, AP 1   1410  may form a beam towards minimization of interference in STA 2   1400  based on the steered sounding PPDU  1480  to transmit data. 
         [0140]    The embodiment of  FIG. 14  illustrates the method in which AP 1   1400  receives the steered sounding PPDU reflecting the effective channel information from STA 2   1400  and acquires the effective channel information. According to another embodiment, however, AP 1  may obtain the effective channel information U 2   H H 1  not by channel estimation, but STA 2  may transmit the effective channel information U 2   H H 1  directly to AP  1 . That is, STA 2  may transmit a frame including the effective channel information, instead of the steered sounding PPDU, to AP 1 . Here, the effective channel information U 2   H H 1  may be transmitted as quantized data. 
         [0141]    According to an alternative method, a covariance of a channel between STA 2  and AP 2  as compressed information may be transmitted in addition to H 1 , thereby obtaining the same result as in the steered sounding PPDU is transmitted. That is, transmitting the frame including the effective channel information may be replaced with transmitting the covariance of the channel between STA 2  and AP 2  as compressed information in addition to H 1 . 
         [0142]    Alternatively, there is a case where it is impossible to reduce interference by AP 1  or it is necessary to selectively reduce only part of the interference. In this case, a sounding PPDU is precoded based on a receiving matrix to transmit a steered sounding PPDU to AP 1 , the receiving matrix being a matrix representing an interference subspace causing greatest interference for STA 2  or a matrix for a signal to selectively attenuate. 
         [0143]    In a similar manner to the interference mitigation procedures based on the sounding PPDUs illustrated in  FIGS. 4 to 12 , the steered sounding PPDUs may be used or the frame including the effective channel information may be directly transmitted in  FIGS. 13 and 14 . 
         [0144]      FIG. 15  is a block diagram illustrating a wireless device according to an embodiment of the present invention. 
         [0145]    Referring to  FIG. 15 , the wireless device  1500  may be an STA to implement the foregoing embodiments, which may be an AP  1550  or a non-AP STA (or STA)  1500 . 
         [0146]    The STA  1500  includes a processor  1510 , a memory  1520 , and a radio frequency (RF) unit  1530 . 
         [0147]    The RF unit  1530  may be connected to the processor  1520  to transmit/receive a radio signal. 
         [0148]    The processor  1520  may implement functions of an STA, processes and/or methods suggested in the present invention. For example, the processor  1520  may be configured to perform operations of a wireless device according to the foregoing embodiments of the present invention. The processor may perform operations of an STA illustrated in the embodiments of  FIGS. 4 to 14 . 
         [0149]    For example, the processor  1520  may be configured to transmit an interference avoidance request frame to an AP and to receive an interference avoidance response frame from the AP in response to the interference avoidance request frame. Further, the processor may be configured to transmit a sounding PPDU to the AP transmitting the interference avoidance response frame. 
         [0150]    The processor may be configured to receive a broadcast beacon frame, which includes an interference avoidance information element, to receive a sounding PPDU indicated based on the interference avoidance information element from another AP, to determine transmit steering matrix based on the sounding PPDU, and to transmit data through a beam generated based on the transmit steering matrix. 
         [0151]    The AP  1550  includes a processor  1560 , a memory  1570 , and an RF unit  1580   
         [0152]    The RF unit  1580  may be connected to the processor  1560  to transmit/receive a radio signal. 
         [0153]    The processor  1560  may implement functions, processes and/or methods suggested in the present invention. For example, the processor  1520  may be configured to perform operations of a wireless device according to the foregoing embodiments of the present invention. The processor may perform operations of an AP illustrated in the embodiments of  FIGS. 4 to 14 . 
         [0154]    For example, the processor  1560  may be configured to receive a beacon frame broadcast by another AP, which includes an interference avoidance information element, to receive a sounding PPDU indicated based on the interference avoidance information element from the other AP, to determine transmit steering matrix based on the sounding PPDU, and to transmit data through a beam generated based on the transmit steering matrix. 
         [0155]    The processors  1510  and  1560  may include an application-specific integrated circuit (ASIC), other chipsets, a logic circuit, a data processor and/or a converter to convert a baseband signal and a radio signal from one to the other. The memories  1520  and  1570  may include a read-only memory (ROM), a random access memory (RAM), a flash memory, a memory card, a storage medium and/or other storage devices. The RF units  1530  and  1580  may include at least one antenna to transmit and/or receive a radio signal. 
         [0156]    When the embodiments are implemented with software, the foregoing techniques may be implemented by a module (process, function, or the like) for performing the foregoing functions. The module may be stored in the memories  1520  and  1570  and be executed by the processors  1510  and  1560 . The memories  1520  and  1570  may be disposed inside or outside the processors  1510  and  1560  or be connected to the processors  1510  and  1560  via various well-known means.