Abstract:
Disclosed are a method and an apparatus for reporting information about a transmission failure frame. The method for reporting information about a transmission failure frame in a wireless LAN may comprise the steps of: creating, by an STA, the retransmission frame includes information about the transmission failure frame, the information about the transmission failure frame includes timestamp information and duration information, the timestamp information includes information about the transmission start time of the transmission failure frame, and the duration information includes information about the transmission period of the transmission failure frame.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the invention 
         [0002]    The present invention relates to wireless communications, and more particularly, to an operation of a station (STA) when frame transmission fails. 
         [0003]    2. Related Art 
         [0004]    The Institute of Electrical and Electronics Engineers (IEEE) 802.11 Wireless Next Generation Standing Committee (WNG SC) is an ad hoc committee which conducts medium- and long-term examinations on a next-generation wireless local area network (WLAN). 
         [0005]    At the IEEE conference in March, 2013, Broadcom suggested, based on the WLAN standardization history, the need for discussions on the next-generation WLAN subsequent to IEEE 802.11ac in the first half of 2013 when the IEEE 802.11ac standards were finalized. On the basis of technical necessity and need for standardization, a motion for creating a study group for the next-generation WLAN was carried at the IEEE conference in March, 2013. 
         [0006]    The scope of the HEW mainly discussed by the study group for the next-generation WLAN so called a high efficiency WLAN (HEW) includes 1) improvement in a 802.11 physical (PHY) layer and medium access control (MAC) layer in 2.4 GHz and 5 GHz bands, 2) increase in spectrum efficiency and area throughput, and 3) performance improvement in actual indoor and outdoor environments, such as environments including interference sources, crowded heterogeneous networks and environments having high user load. The HEW mostly considers a scenario of an environment crowed with access points (APs) and stations (STAs), and the HEW conduct discussions on improvement in spectrum efficiency and area throughput in this situation. In particular, the HEW pays attention to improvement in practical performance not only in indoor environments but also in outdoor environments, which are not substantially considered in existing WLANs. 
         [0007]    The HEW pays substantial attention to scenarios for a wireless office, a smart home, a stadium, a hotspot and a building/apartment, and discussions on system performance improvement in an environment crowed with APs and STAs based on a corresponding scenario are conducted. 
         [0008]    Discussions are expected to be vigorous on system performance improvement in an overlapping basic service set (OBSS) environment and outdoor environment, instead of single link performance improvement in a single basic service set (BSS), and on cellular offloading. This HEW orientation means that the next-generation WLAN gradually has a similar technological scope to that of mobile communication. Considering that mobile communication technology is discussed along with WLAN technology in small cell and direct-to-direct (D2D) communications areas, technological and business convergence of the next-generation WLAN based on the HEW and mobile communication is expected to be further promoted. 
       SUMMARY OF THE INVENTION 
       [0009]    The present invention provides a method of reporting information about a transmission failure frame. 
         [0010]    The present invention also provides a station (STA) for reporting information about a transmission failure frame. 
         [0011]    According to one aspect of the present invention, in order to achieve the aforementioned purpose of the present invention, a method of reporting information about a transmission failure frame in a wireless local area network (WLAN) may include: creating, by a station (STA), a retransmission frame; and transmitting, by the STA, the retransmission frame to an access point (AP). The retransmission frame may include information about the transmission failure frame. The information about the transmission failure frame may include timestamp information and duration information. The timestamp information may include information about a transmission start time of the transmission failure frame. The duration information may include information about a transmission duration of the transmission failure frame. 
         [0012]    According to another aspect of the present invention, in order to achieve the aforementioned purpose of the present invention, an STA for reporting information about a transmission failure frame in a WLAN may include: a radio frequency (RF) unit for transmitting and receiving a radio signal; and a processor operatively coupled to the RF unit. The processor may be implemented to create a retransmission frame and to transmit the retransmission frame to an AP. The retransmission frame may include information about the transmission failure frame. The information about the transmission failure frame may include timestamp information and duration information. The timestamp information may include information about a transmission start time of the transmission failure frame. The duration information may include information about a transmission duration of the transmission failure frame. 
         [0013]    An access point (AP) can receive information about a transmission failure frame from a station (STA), and can determine a transmission failure cause on the basis of information about the received transmission failure frame. The AP can decrease a transmission failure rate by performing a procedure for decreasing the transmission failure rate according to the transmission failure cause. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a concept view illustrating the structure of a wireless local area network (WLAN). 
           [0015]      FIG. 2  is a view illustrating a layer architecture of a WLAN system supported by IEEE 802.11. 
           [0016]      FIG. 3  shows the concept of a DCF-based channel access process. 
           [0017]      FIG. 4  shows the concept of a backoff procedure of a plurality of STAs. 
           [0018]      FIG. 5  shows the concept of an interval between frames. 
           [0019]      FIG. 6  shows the concept of a case where a state of a medium determined by an STA is different from a state of a real medium. 
           [0020]      FIG. 7  shows the concept of a method of using an RTS frame and a CTS frame to solve a hidden node problem and an exposed node problem. 
           [0021]      FIG. 8  shows the concept of a method of detecting a hidden node according to an embodiment of the present invention. 
           [0022]      FIG. 9  shows the concept of a case where an STA fails to receive ACK for a frame transmitted from an AP according to an embodiment of the present invention. 
           [0023]      FIG. 10  shows the concept of a method of determining a cause of a frame transmission failure of an STA on the basis of information received by an AP from an STA according to an embodiment of the present invention. 
           [0024]      FIG. 11  is a flowchart showing a resource allocation method when a data collision occurs due to a hidden node according to an embodiment of the present invention. 
           [0025]      FIG. 12  shows the concept of a time resource allocated to an STA and a hidden node according to an embodiment of the present invention. 
           [0026]      FIG. 13  is a block diagram illustrating a wireless device to which an embodiment of the present invention may apply. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0027]      FIG. 1  is a concept view illustrating the structure of a wireless local area network (WLAN). 
         [0028]    An upper part of  FIG. 1  shows the structure of the IEEE (institute of electrical and electronic engineers) 802.11 infrastructure network. 
         [0029]    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 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 . 
         [0030]    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. 
         [0031]    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). 
         [0032]    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). 
         [0033]    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). 
         [0034]    A lower part of  FIG. 1  is a concept view illustrating an independent BSS. 
         [0035]    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. 
         [0036]    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). 
         [0037]    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. 
         [0038]      FIG. 2  is a view illustrating a layer architecture of a WLAN system supported by IEEE 802.11. 
         [0039]      FIG. 2  conceptually illustrates a layer architecture (PHY architecture) of a WLAN system. 
         [0040]    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. 
         [0041]    The MAC sub-layer  220 , the PLCP sub-layer  210 , and the PMD sub-layer  200  may conceptually include management entities. 
         [0042]    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 . 
         [0043]    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. 
         [0044]    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. 
         [0045]    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. 
         [0046]    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. 
         [0047]    Hereafter, a radio access method used in the IEEE 802.11 is disclosed. 
         [0048]    Basically, MAC layer uses DCF(distributed coordination function) to share radio medium In IEEE 802.11, DCF as a CSMA/CA(carrier sense multiple access/collision avoidance) mechanism is used for the channel access. 
         [0049]    Selectively, the MAC layer defines a method to share medium based on RTS (request to send)/CTS (clear to send). Channel access based on DCF is described as below. 
         [0050]      FIG. 3  shows the concept of a DCF-based channel access process. 
         [0051]    First, in the DCF-based channel access, an STA may determine whether to use a medium through a carrier sensing mechanism. If the medium is not in use longer than a DCF inter frame symbol (DIFS) duration (i.e., if a channel is idle), the STA may transmit a MAC protocol data unit (MPDU) of which transmission is imminent. 
         [0052]    On the contrary, if the medium is in use during the DIFS duration (i.e., if the channel is busy), the STA may set a backoff time according to a random backoff algorithm. 
         [0053]    The backoff time is a waiting time before transmitting a frame after the channel waits for a specific time (e.g., DIFS). The backoff time may be defined by the following equation. 
         [0000]      BackoffTime=Random( )× a SlotTime  &lt;Equation 1&gt;
 
         [0054]    Random( ) is a function of calculating a pseudo-random integer selected with uniform distribution in the interval of [0, CW]. CW may be selected from an integer greater than or equal to aCWMin less than or equal to aCWMax. aCWMin and aCWmax may be determined according to physical (PHY) characteristics. aSlotTime may be a time unit defined according to the PHY characteristics. 
         [0055]    The STA may determine whether the channel is idle, and if the channel is idle, may decrease the backoff time in unit of SlotTime. Before the backoff time is decreased in unit of SlotTime, the STA may determine again whether the channel is idle for a duration corresponding to the DIFS. If the backoff time is 0, the STA may perform the channel access. 
         [0056]      FIG. 4  shows the concept of a backoff procedure of a plurality of STAs. 
         [0057]    Referring to  FIG. 4 , a backoff time (or a size of a contention window (CW)) may be decreased after it is determined that a medium is idle for a DIFS duration. If an activity of the medium is not detected, the STA may decrease the backoff time in unit of SlotTime. If it is determined that the medium is in use during a backoff slot, the STA may defer the decrement of the backoff time. Frame transmission of the STA may start whenever a pre-set backoff timer reaches 0. 
         [0058]    After frame transmission of an STA A, a backoff time which is set by each of an STA B, an STA C, and an STA D may be decreased. Among the STA B, the STA C, and the STA D, the STA C of which the backoff time is most rapidly decreased to 0 may transmit a frame through the medium. If the STA C transmits the frame, the decrement of the backoff time of the STA B and the STA D may be deferred. 
         [0059]    Further, a DCF transmission scheme includes an RTS/CTS access mode in which control frames (e.g., RTS and CTS) are exchanged before a data frame is transmitted. This scheme can reduce a channel waste by replacing a collision which may occur when the STA transmits a data frame with a collision caused by a relatively short control frame. The RTS/CTS access mode will be described later. 
         [0060]    As another method for sharing a radio medium by a plurality of STAs in a MAC layer, a point coordination function (PCF) may be defined. The aforementioned DCF is a CSMA/CA-based channel access. Therefore, real-time transmission of data transmitted between an STA and an AP cannot be guaranteed. On the contrary, the PCF may be used as a method for providing quality of service (QoS) in real-time data transmission. Unlike the DCF, the PCF is a non-contention based transmission service. The PCF may be used alternately with a DCF-type contention based service, rather than exclusively using the entire medium transmission duration. In the PCF, a point coordinator implemented in the AP of a BSS may control a right to occupy the medium by each STA by using a polling scheme. The PCF which is an inter-frame space (IFS) in the PCF may be set to be smaller than the DIFS which is an IFS of the DCF. By using this method, an STA which has access to a medium on the basis of the PCF may have a priority over an STA having access to a channel on the basis of the DCF. The IFS denotes an interval between frames, and may be used to set a priority according to which the STA has access to the medium. The IFS may be specifically defined as follows. 
         [0061]      FIG. 5  shows the concept of an interval between frames. 
         [0062]    Referring to  FIG. 5 , an interval between two frames may be referred to as an inter-frame symbol (IFS). An STA may use a carrier sensing scheme to determine whether a channel is used for a time duration of the IFS as defined in the standard. A MAC layer using a DCF defines a plurality of IFSs. A priority of an STA occupying a radio medium may be determined by the IFS. The interval between frames is dependent on an IFS type, and is defined as follow. 
         [0063]    (1) SIFS(short inter frame symbol): It is used in RTS/CTS, ACK frame transmission. Top priority. 
         [0064]    (2) PIFS(PCF IFS): It is used in frame transmission of an STA operating based on PCF. 
         [0065]    (3) DIFS(DCF IFS): It is used in frame transmission of an STA operating based on DCF. 
         [0066]    (4) EIFS(extended IFS): It is used only when an error occurs in frame transmission, and is not a fixed duration. 
         [0067]    When DCF is used as a method of sharing a wireless medium by a plurality of STAs in a MAC layer, several problems may occur. For example, if the plurality of STAs simultaneously perform an initial access when using the DCF, frames transmitted by the plurality of STAs may collide. In addition, there is no concept on a transmission priority in the DCF. Therefore, a quality of service (QoS) cannot be guaranteed as to traffic data transmitted by the STA. In order to solve such a problem, a hybrid coordination function (HCF) is defined in IEEE 802.11e as a new coordination function. As a channel access mechanism, the HCF defines an HCF controlled channel access (HCCA) and an enhanced distributed channel access (EDCA). 
         [0068]    The EDCA and the HCCA may define traffic categories indicating transmission priorities. A priority for performing a channel access may be determined on the basis of the traffic access categories. That is, different CWs and IFSs may be defined differently according to categories of traffic data transmitted by the STA. The different CWs and IFSs may determine a channel access priority depending on the categories of traffic data. 
         [0069]    For example, if traffic data is an e-mail, the traffic data may be assigned to a low priority class for transmission. For another example, if the traffic data is voice communication through a wireless local area network (WLAN), the traffic data may be assigned to a high priority class for transmission. 
         [0070]    In case of using EDCA, traffic data having a high priority may have more transmission opportunities than traffic data having a low priority. In addition, on average, an STA which transmits high-priority traffic may have a shorter waiting time than an STA which transmits low-priority traffic before transmitting a packet. In the EDCA, a transmission priority may be implemented by allocating a shorter CW to higher-priority traffic than that of lower-priority traffic while assigning an arbitration inter-frame space (AIFS) that is shorter than the IFS which is a frame interval defined in the DCF. Further, the EDCA enables an STA to access a channel without contention during a period that is referred to as a transmit opportunity (TXOP). During the TXOP period determined within a range not exceeding a maximum duration of the TXOP, the STA may transmit as many packets as possible. If one frame is too long to be entirely transmitted during one TXOP period, it may be transmitted by being split into small frames. The use of the TXOP may reduce a situation in which an STA having a low transfer rate excessively occupies a channel, which is a problem of the conventional 802.11 DCF MAC. 
         [0071]    In the aforementioned channel access method, a collision may occur in data transmission if an STA incorrectly senses whether a medium is idle when performing medium sensing based on a carrier sensing mechanism. A case where a state of a medium determined by the STA is different from a state of a real medium is shown below in  FIG. 6 . 
         [0072]      FIG. 6  shows the concept of a case where a state of a medium determined by an STA is different from a state of a real medium. 
         [0073]    A hidden node issue is shown in an upper portion of  FIG. 6 , and an exposed node issue is shown in a lower portion of  FIG. 6 . 
         [0074]    A case where an STA A  600  is communicating with an STA B  620  and an STA C  630  has information to be transmitted to the STA B  620  is assumed in the upper portion of  FIG. 6 . If the STA A  600  transmits data to the STA B  620 , a channel medium for transmitting data to the STSA B  620  is occupied by the STA A  600 . 
         [0075]    The STA C  630  may perform carrier sensing on the medium before transmitting data to the STA B  620 . Due to a transmission coverage of the STA A  600 , even if the STA A  600  is communicating with the STA B  620 , the STA C  630  may determine that the medium for transmitting the data to the STA B  620  is in an idle state. In this case, the STA C  630  may transmit the data to the STA B  620 . Eventually, the data of the STA A  600 , which is to be transmitted to the STA B  620 , may collide with the data of the STA C  630 . In this case, the STA A  600  may be a hidden node of the STA C  630 . 
         [0076]    A case where the STA B  650  transmits data to the STA A  640  is assumed in the lower portion of the  FIG. 6 . The STA C  660  may perform carrier sensing to know whether a medium is occupied by another STA. Since it is a state in which the STA B  650  transmits information to the STA A  640 , the STA C  660  may sense that it is a state in which a medium is occupied. Therefore, the STA C  660  may not be able to transmit data to an STA D  670 . The STA C  650  must defer data transmission to the STA D  670  until unnecessary data transmission caused by the STA B  640  is finished. That is, the STA A  640  may prevent data transmission of the STA C  660  even though it exists outside a carrier sensing range. In this case, the STA C  660  is an exposed node of the STA B  650 . 
         [0077]    In order to solve the aforementioned hidden node problem and exposed node problem of  FIG. 6 , whether a medium is occupied can be sensed in a WLAN by using an RTS frame and a CTS frame. 
         [0078]      FIG. 7  shows the concept of a method of using an RTS frame and a CTS frame to solve a hidden node problem and an exposed node problem. 
         [0079]    In  FIG. 7 , in order to solve the hidden node problem and the exposed node problem, a method of transmitting a short signaling frame (e.g., a request to send (RTS) frame, a clear to send (CTS) frame, etc.) by an STA is disclosed. Neighboring STAs may determine whether to transmit data on the basis of the RTS frame and CTS frame transmitted/received between two STAs performing communication. 
         [0080]    Transmission of an RTS frame  703  and a CTS frame  705  for solving a hidden node problem is shown in an upper portion of  FIG. 7 . 
         [0081]    It may be assumed a case where both of an STA A  700  and an STA C  720  intend to transmit data. When the STA A  700  transmits the RTS frame  703  to an STA B  710 , the STA B  710  may transmit the CTS frame  705 . A CTS frame  704  may be transmitted to both of the neighboring STA A  700  and STA C  720 . The STA C  720  may sense a presence of the STA A  700  for transmitting data to the STA B  710  by receiving the CTS frame  705 . After communication between the STA A  700  and the STA B  710  ends, the STA B  720  may transmit data to the STA B  710 . By using this method, a data collision caused by a node may not occur. 
         [0082]    Transmission of an RTS frame  733  and a CTS frame  735  for solving an exposed node problem is shown in a lower portion of  FIG. 7 . 
         [0083]    When performing communication, an STA A  730  and an STA B  740  may transmit the RTS frame  733  and the CTS frame  735 . An STA C  750  may receive only the RTS frame  733 , and may know that the STA A  730  is located outside a carrier sensing range of the STA C  750 . Therefore, the STA C  750  may transmit data to an STA D  760 . 
         [0084]    An RTS frame format and a CTS frame format are disclosed in the 8.3.1.2 RTS frame format and 8.3.1.3 CTS frame format of the IEEE 802.11 spec. 
         [0085]    Although a hidden node (or a hidden STA) is not detected by carrier sensing of a specific STA, the specific STA may produce interference when transmitting data to another STA. There may be a high possibility of a collision between data transmitted by the hidden node of the specific STA (hereinafter, referred to as the hidden node) and data transmitted by the specific STA. The data collision may result in a decrease in system performance. Therefore, as described above, the RTS frame and the CTS frame may be used to avoid the data collision caused by the hidden node. However, an overhead of a management frame may be increased when the RTS frame and the CTS frame are transmitted in data transmission. 
         [0086]    In an embodiment of the present invention, an AP can acquire information for detecting a hidden node of a specific STA. On the basis of information for detecting the hidden node (hereinafter, referred to as information for detecting hidden node), the AP may decrease a possibility of a collision between data transmitted by the specific STA and data transmitted by the hidden node. 
         [0087]    The information for detecting hidden node transmitted by the STA may be a variety of information. For example, the information for detecting hidden node may include identification information of another STA for transmitting a frame which is overheard by the STA, and when frame transmission caused by the STA fails, a transmission start time of the transmission failure frame, transmission duration information, or the like. 
         [0088]    Hereinafter, an embodiment of the present invention discloses a method of detecting a hidden node of an STA. 
         [0089]      FIG. 8  shows the concept of a method of detecting a hidden node according to an embodiment of the present invention. 
         [0090]    A method in which an STA transmits information included in an overheard frame to an AP as information for detecting hidden node is disclosed in  FIG. 8 . 
         [0091]    Referring to  FIG. 8 , a first STA  810  may overhear a second frame  825  transmitted by a second STA  820 . Since the first STA  810  can sense the frame transmitted by the second STA  820 , the second STA  820  may not be a hidden node of the first STA  810 . That is, another STA for transmitting the frame to be overheard by the STA may not be the hidden node of the STA. 
         [0092]    The first STA  810  may transmit, to an AP  850 , identification information of the second STA  820  for transmitting the second frame  825 . The identification information of the second STA  820  may be transmitter address (TA) field information included in an MAC header of the overheard second frame  825 . The AP  850  may generate a hidden node map of the first STA  810  on the basis of the received identification of the second STA  820 . 
         [0093]    The AP  850  may acquire information indicating that the second STA  820  is not a hidden node for the first STA  810  on the basis of the identification of the second STA  820 , which is transmitted by the first STA  810 . By using this information, the AP  850  may generate the hidden node map of the first STA  810 . The hidden node map of the first STA  810  may include information regarding the hidden node of the first STA  810 . 
         [0094]    Specifically, the AP  850  may configure the hidden node map of the first STA  810  including the hidden node of the first STA  810  other than the second STA  820 . Alternatively, the AP  850  may configure the hidden node map including information about a non-hidden node, rather than the hidden node. For example, the AP  850  may generate the hidden node map for the first STA  810  by configuring the second STA  820  as the non-hidden node of the first STA  810 . 
         [0095]    According to another embodiment of the present invention, the AP  850  may configure not only the hidden node map of the first STA  810  but also the hidden node map of the second STA  820  on the basis of identification information of the second STA  820  for transmitting the second frame  825  to be overheard by the first STA  810 . The AP  850  may determine the first STA  810  and the second STA  820  as an STA which is present in a location where mutual carrier sensing is possible. Therefore, it may be determined that the first STA  810  is not the hidden node of the second STA  820  on the basis of the identification information of the second STA  820  for transmitting the second frame  825  to be overheard by the first STA  810 . 
         [0096]    The first STA may transmit information included in the overheard frame to the AP  850  through various methods. For example, the first STA  810  may collect TA information included in the overheard frame. The first STA  810  may overhear a frame transmitted by not only the second STA  820  but also another STA, and may collect the TA information included in the overheard frame. If there is data to be transmitted to the AP  850 , the first STA  810  may transmit the collected TA information by piggybacking it on a data frame. Alternatively, the first STA  810  may transmit the TA information included in the overheard frame to the AP  850  by using an additional PPDU format defined to transmit the TA information included in the overheard frame. 
         [0097]      FIG. 9  shows the concept of a case where an STA fails to receive ACK for a frame transmitted from an AP according to an embodiment of the present invention. 
         [0098]    A collision between frames transmitted respectively by a plurality of STAs when the plurality of STAs transmit a frame in the same time slot is shown in an upper portion of  FIG. 9 . In a case where each of the plurality of STAs transmits a frame at the same transmission start time (e.g., in case of the same time slot), a delay time for a channel access of each of the plurality of STAs may be identical with respect to a specific time. 
         [0099]    Referring to the upper portion of  FIG. 9 , a first STA  910  may wait by a sum of a DIFS and a first backoff time to transmit a first frame  915 . A second STA  920  may wait by a sum of an AIFS and a second backoff time to transmit a second frame  925 . If the sum of the DIFS and the first backoff time is equal to the sum of the AIFS and the second backoff time with respect to the specific time, the first STA  910  may not be able to perform sensing on transmission of the second frame  925  of the second STA  920 , and the second STA  920  may not be able to perform sensing on transmission of the first frame  915  of the first STA  910 . Therefore, the first STA  910  and the second STA  920  may respectively transmit the first frame  915  and the second frame  925  at the same time, and the first frame  915  and the second frame  925  transmitted respectively from the first STA  910  and the second STA  920  may collide. The first STA  910  and the second STA  920  may not be able to receive ACK for the first frame  915  and the second frame  925  respectively from an AP  930  due to a collision of the first frame  915  and the second frame  925 . 
         [0100]    That is, a case where a plurality of STAs perform a channel access in the same time resource (e.g., a time slot) by chance during a DCF-based channel access and thus medium occupation of another STA is not sensed is shown in the upper portion of  FIG. 9 . That is, an inter-frame collision disclosed in the upper portion of  FIG. 9  may not be an inter-frame collision generated by a hidden node. 
         [0101]    The concept of an inter-frame collision caused by a hidden node is shown in a middle portion of  FIG. 9 . 
         [0102]    In the middle portion of  FIG. 9 , a first STA  950  may be a hidden node of a second STA  960 . Also in a case where the first STA  950  transmits a first frame  955  to an AP  970 , the second STA  960  cannot sense transmission of the first frame  955  of the first STA  950  through carrier sensing. Therefore, during the first STA  950  transmits the first frame  955  to the AP  970 , the second STA  960  may transmit a second frame  965  to the AP  970 . In this case, the first frame  955  transmitted by the first STA  950  may collide with the second frame  965  transmitted by the second STA  960 . Therefore, the first STA  950  may not be able to receive ACK for the first frame  955  from the AP  970 , and the second STA  960  may not be able to receive ACK for the second frame  965  from the AP  970 . 
         [0103]    That is, an inter-frame collision caused by a hidden node is shown in the middle portion of  FIG. 9 , which may occur also in a case where a plurality of STAs do not perform a channel access in the same time resource (e.g., a time slot) by chance. Although it has been described in the middle portion of  FIG. 9  that two STAs are present for convenience of explanation, if a plurality of STAs are hidden nodes to each other, a collision may also occur in frames respectively transmitted by the plurality of STAs. 
         [0104]    A case where ACK is not received for a frame  985  transmitted by one STA  980  is shown in an lower portion of  FIG. 9 . If the inter-frame collision does not occur, the STA  980  may not be able to receive ACK for the frame  985  from an AP  990  due to various causes. For example, if a channel state is not good due to interference or the like, the frame  985  transmitted from the AP  990  by the STA  980  may not be decoded by the AP  990 , and may not be able to receive ACK from the AP  990 . 
         [0105]    According to an embodiment of the present invention, if ACK is not received from an AP in response to a frame transmitted by an STA, the STA may transmit information about a time resource for starting transmission of a frame not receiving ACK (hereinafter, referred to as timestamp information of a transmission failure frame) and/or duration information to the AP. 
         [0106]    Timestamp information of the transmission failure frame may be determined on the basis of a value of a time synchronization function (TSF) timer which is set by a beacon frame. A reference TSF timer value included in the beacon frame may be used to synchronize a TSF timer of STAs included in a BSS. If a TSF timer value of the STA is different from a reference timestamp value included in the received beacon frame, the STA may adjust the reference timestamp value included in the beacon frame. The reference timestamp value may be the reference TSF timer value. 
         [0107]    That is, the timestamp information of the transmission failure frame may be determined through a TSF timer of the STA, which is set on the basis of the reference timestamp value. 
         [0108]    The duration information may include information about a time resource related to frame transmission. The duration information may be defined in various forms. For example, the duration information may be information included in a duration field included in a medium access control (MAC) header. For another example, the duration information may include information about a time resource regarding a time until a frame is transmitted and ACK for the frame is received. For another example, frame length information (e.g., length information of a frame defined in a legacy(L)-signal(S) included in a physical layer convergence procedure (PLCP)) or the like may be used as the duration information. 
         [0109]      FIG. 10  shows the concept of a method of determining a cause of a frame transmission failure of an STA on the basis of information received by an AP from an STA according to an embodiment of the present invention. 
         [0110]    According to the embodiment of the present invention, the STA may transmit to the AP the duration information and the timestamp information of the transmission failure frame for which ACK cannot be received from the AP. The AP may determine whether frame transmission performed by a hidden node has failed on the basis of the duration information and the timestamp information of the transmission failure frame received from at least one STA. The timestamp information of the transmission failure frame is one example of information about a time resource for starting transmission of the transmission failure frame. 
         [0111]    In an upper portion of  FIG. 10 , if transmission on a first frame  1015  has failed, a first STA  1010  may transmit first timestamp information  1017  and first duration information  1019  for the first frame  1015 . Likewise, if transmission on a second frame  1025  has failed, a second STA  1020  may transmit second timestamp information  1027  and second duration information  1029  for the second frame  1025 . 
         [0112]    The first timestamp information  1017  and the second timestamp information  1027  may indicate the same timestamp. Further, the first duration information  1019  and the second duration information  1029  may indicate that the first frame  1015  and the second frame  1025  have an overlapping transmission duration. 
         [0113]    Therefore, if the first timestamp information  1017  of the first frame  1015  and the second timestamp information  1027  of the second frame  1025  indicate the same timestamp and if the first duration information  1019  of the first frame  1015  and the second duration information  1029  of the second frame  1025  indicate the overlapping of the transmission duration of the first frame  1015  and the second frame  1025 , an AP  1030  may determine the first STA  1010  and the second STA  1020  as STAs in which an inter-frame collision occurs, by selecting the same time resource when performing a channel access. 
         [0114]    In this case, the AP  1030  may differently set a first channel access parameter for the first STA  1010  and a second channel access parameter for the second STA  1020 . For example, the AP may differently set a contention window size and/or an arbitration inter frame symbol (AIFS) size with respect to the first STA  1010  and the second STA  1020 , thereby differently setting delay times before the channel access. Alternatively, the AP  1030  may differently set a time resource and/or a frequency resource for performing the channel access by the first STA  1010  and the second STA  1020  through load balancing. 
         [0115]    In case of a middle portion of  FIG. 10 , first timestamp information  1057  of a first frame  1055  transmitted by a first STA  1050  and second timestamp information  1067  of a second frame  1065  transmitted by a second STA  1060  may not be equal to each other. 
         [0116]    Further, first duration information  1059  of the first frame  1055  transmitted by the first STA  1050  and second duration information  1069  of the second frame  1065  transmitted by the second STA  1060  may indicate that the first frame  1055  and the second frame  1065  have an overlapping transmission duration. 
         [0117]    Therefore, the first timestamp information  1057  and the second timestamp information  1067  indicate different timestamps and if the first duration information  1059  and the second duration information  1069  indicate the overlapping of the transmission duration of the first frame  1055  and the second frame  1065 , an AP  1070  may determine this as an inter-frame collision caused by a hidden node. Specifically, the AP  1070  may determine the first STA  1050  of which a timestamp is relatively fast as a hidden node for the second STA  1060  of which a timestamp is relatively slow. Alternatively, the AP  1070  may determine that the first STA  1050  and the second STA  1060  are hidden nodes with each other. For convenience of explanation, it is assumed that the AP  1070  determines the first STA  1050  as a hidden node for the second STA  1060 . 
         [0118]    In this case, the AP  1070  may differently set a first channel access parameter of the first STA  1050  and a second channel access parameter of the second STA  1060 . For example, since a contention window size and/or an AIFS size for the first STA  1050  and the second STA  1060  are set differently, a delay time before the channel access may be set differently, thereby being able to decrease a probability of a collision occurrence caused by a hidden node. 
         [0119]    In another method, the AP  1070  may allocate different transmission resources to the second STA  1060  and the first STA  1050  which is a hidden node of the second STA  1060 . The first STA  1050  and the second STA  1060  can avoid a frame collision caused by a channel access of the hidden node by differently allocating a time resource and/or a frequency resource for performing the channel access. 
         [0120]    In another method, the AP  1070  may mandate transmission and/or reception of a CTS frame and an RTS frame to the STA. Upon receiving the CTS frame, the second STA  1060  may not perform the channel access during a specific time duration. Upon receiving the RTS frame from each of the first STA  1050  and the second STA  1060 , the AP  1070  may not transmit the CTS frame. The AP  1070  may mandate transmission and reception of the CTS frame and the RTS frame on the basis of an announcement frame. 
         [0121]    Although two STAs are assumed in the upper portion of  FIG. 10  and the middle portion of  FIG. 10  for convenience of explanation, the decision on a frame collision of an AP may also be applied to a plurality of STAs. 
         [0122]    In a lower portion of  FIG. 10 , an STA  1080  may determine that a frame transmitted by the STA  1080  on the basis of timestamp information  1087  and duration information  1089  for a transmission failure frame  1085  received from the STA  1080  does not collide with a frame transmitted from another STA. For example, upon receiving the timestamp information  1087  and duration information  1089  for the transmission failure frame  1085  only from one STA  1080 , an AP  1090  may determine this not as a collision with other frames but as a transmission failure caused by other factors such as interference. 
         [0123]    In this case, the AP may change a transmission (Tx) rate on the basis of a method such as link adaptation. 
         [0124]    According to anther embodiment of the present invention, as shown in the upper portion of  FIG. 10 , if the first timestamp information  1017  of the first frame  1015  transmitted by the first STA  1010  and the second timestamp information  1027  of the second frame  1025  transmitted by the second STA  1020  indicate the same timestamp, it may be determined that the frames  1015  and the  1025  transmitted respectively by the first STA  1010  and the second STA  1020  overlap, and thus whether the overlapping occurs may not have to be determined additionally through a duration field. 
         [0125]    The timestamp information and duration information of the transmission failure frame disclosed in  FIG. 10  may be transmitted by being included in a retransmission frame of the STA. For example, the timestamp information and the duration information may be transmitted by being piggybacked on the retransmission frame. Alternatively, the STA may transmit the timestamp information and the duration information to the AP by using an additional PPDU format for transmitting the timestamp information and the duration information. 
         [0126]      FIG. 11  is a flowchart showing a resource allocation method when a data collision occurs due to a hidden node according to an embodiment of the present invention. 
         [0127]    In  FIG. 11 , if an AP determines that an inter-frame collision is caused by a hidden node as shown in the middle portion of  FIG. 10 , a method of allocating a resource to an STA and the hidden node is disclosed. A first STA is assumed as a hidden node for a second STA. 
         [0128]    Referring to  FIG. 11 , a first frame  1115  transmitted by a first STA  1110  may collide with a second frame  1125  transmitted by a second STA  1120 . 
         [0129]    If the first frame  1115  and the second frame  1125  collide, the first STA  1110  and the second STA  1120  may receive ACK for the first frame  1115  and the second frame  1125 , respectively. If the ACK for each of the first frame  1115  and the second frame  1125  is not received, the first STA  1110  and the second STA  1120  may retransmit a frame. A first retransmission frame  1130  to be retransmitted may include first timestamp information and first duration information of the first frame  1115 . A second retransmission frame  1140  to be retransmitted may include second timestamp information and second duration information of the second frame  1125 . 
         [0130]    If a time resource indicated by the timestamp information of the first frame  1115  and a time resource indicated by the timestamp information of the second frame  1125  are not equal to each other and if a transmission duration of a first transmission failure frame determined based on the first duration information overlaps with a transmission duration of a second transmission failure frame determined based on the second duration information, an AP  1150  may determine a collision between the frames  1115  and  1125  as a collision caused by a hidden node. 
         [0131]    In this case, the AP  1150  may differently allocate a time resource and/or frequency resource allocated to the first STA  1110  and the second STA  1120 . 
         [0132]    For example, the AP  1150  may allocate a first channel  1150  to the first STA  1110  through an uplink channel, and may allocate a second channel  1160  to the second STA  1120 . By using this method, the second STA  1120  and the first STA  1110  which is a hidden node for the second STA  1120  may not operate in the same channel, and a collision between frames transmitted respectively by the first STA  1110  and the second STA  1120  can be avoided. 
         [0133]    For another example, the AP  1150  may allocate a first time resource  1170  to the first STA  1110 , and may allocate a second time resource  1180  to the second STA  1120 . By using this method, the second STA  1120  and the first STA  1110  which is a hidden node for the second STA  1120  may not operate in the same time resource, and a collision between frames transmitted respectively by the first STA  1110  and the second STA  1120  can be avoided. 
         [0134]    Specifically, the first time resource  1170  and the second time resource  1180  may be distinguished with respect to a transmission duration of a beacon frame. 
         [0135]      FIG. 12  shows the concept of a time resource allocated to an STA and a hidden node according to an embodiment of the present invention. 
         [0136]    Referring to an upper portion of  FIG. 12 , a first time resource  1210  may correspond to a period from a time at which a (2N) th  beacon frame is transmitted (where N is a natural number) to the STA to a time at which a (2N+1) th  beacon frame is transmitted. A second time resource  1220  may correspond to a period from a time at which the (2N+1)th beacon frame is transmitted to the STA to a time at which a (2N+2)th beacon frame is transmitted. That is, the first time resource  1210  may correspond to a period from a time at which a beacon frame of an even number index is transmitted to a time at which a beacon frame of an odd number index is transmitted, and the second time resource  1220  may correspond to a period from a time at which the beacon frame of the odd number index is transmitted to a time at which the beacon frame of the even number index is transmitted. 
         [0137]    Referring to a lower portion of  FIG. 12 , a first time resource  1250  may be a precedent time resource duration among time resources corresponding to a transmission duration of a beacon frame which is split by half. A second time resource  1260  may be the remaining time resource durations among the time resources corresponding to the beacon frame which is split by half. 
         [0138]      FIG. 13  is a block diagram illustrating a wireless device to which an embodiment of the present invention may apply. 
         [0139]    Referring to  FIG. 13 , the wireless device may be an STA that may implement the above-described embodiments, and the wireless device may be an AP  1300  or a non-AP STA (or STA) ( 1350 ). 
         [0140]    The AP  1300  includes a processor  1310 , a memory  1320 , and an RF (Radio Frequency) unit  1330 . 
         [0141]    The RF unit  1330  may be connected with the processor  1620  to transmit/receive radio signals. 
         [0142]    The processor  1310  implements functions, processes, and/or methods as proposed herein. For example, the processor  1310  may be implemented to perform the operation of the above-described wireless device according to an embodiment disclosed in  FIG. 8  to  FIG. 12  of the present invention. 
         [0143]    For example, the processor  1310  may analyze a transmission failure cause on the basis of information about a transmission failure frame transmitted from the STA. The AP may control an operation of the STA according to a collision cause between the STAs. For example, if it is determined that the transmission failure of the STA is caused by a hidden node, the AP may determine an access parameter for a channel access of the STA, or may determine a frame transmission resource of the STA. 
         [0144]    The STA  1350  includes a processor  1360 , a memory  1370 , and an RF (Radio Frequency) unit  1380 . 
         [0145]    The RF unit  1380  may be connected with the processor  1360  to transmit/receive radio signals. 
         [0146]    The processor  1360  implements functions, processes, and/or methods as proposed herein. For example, the processor  1360  may be implemented to perform the operation of the above-described wireless device according to an embodiment disclosed in  FIG. 8  to  FIG. 12  of the present invention. 
         [0147]    For example, the processor  1360  may be implemented to generate a retransmission frame and to transmit the retransmission frame to an access point (AP). The retransmission frame may include information about a transmission failure frame, and the information about the transmission failure frame may include timestamp information and duration information. The timestamp information may include information about a transmission start time of the transmission failure frame, and the duration information may include information about a transmission duration of the transmission failure frame. 
         [0148]    The processor  1310 ,  1360  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  1320 ,  1370  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  1330 ,  1380  may include one or more antennas that transmit and/or receive radio signals. 
         [0149]    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  1320 ,  1370  and may be executed by the processor  1310 ,  1360 . The memory  1320 ,  1370  may be positioned in or outside the processor  1310 ,  1360  and may be connected with the processor  1310 ,  1360  via various well-known means.