Abstract:
Disclosed are method and device for indicating a PLCP header transmission mode. A method for generating a physical layer convergence protocol (PLCP) of a data frame can comprise the following steps: demodulating PLCP header mode information based on a scrambling sequence of the received frame or information elements included in the received frame; and generating a data frame including the PLCP header generated based on the PCLP header mode information. The PLCP header mode information comprises at least one indication from a space time block coding mode indication, a guard interval mode indication, and a repetition mode indication. The invention is capable of improving data transmission efficiency by adaptively determining a transmission mode of a PCLP header to a channel situation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to wireless communication and, more particularly, to a method and apparatus for indicating PLCP header transmission mode. 
         [0003]    2. Related Art 
         [0004]    A Machine-to-Machine (M2M) system has recently been in the spotlight as the next-generation communication technology. In the IEEE 802.11 WLAN, a new standard is being established in order to support M2M. An M2M system means a network over which a machine not a human being becomes the main body of communication and exchanges pieces of information. A temperature sensor, a humidity sensor, a camera, home appliances, such as TV, a process machine in a factory, or a large-sized machine, such as a vehicle, may become one of the elements of an M2M system. With the recent advent of various communication services, such as smart grid, e-Health, and ubiquitous, M2M technology is used a lot in order to support the various communication services. An M2M system has the following characteristics. 
         [0005]    1) A very large number of STAs: M2M assumes a very large number of STAs unlike an existing network. This is because not only machines owned by persons, but also sensors installed in homes and companies must be taken into consideration. Accordingly, a very large number of STAs may be connected to one AP. 
         [0006]    2) Low traffic load per STA: An M2M terminal has a traffic pattern in which surrounding information is collected and reported. Accordingly, information does not need to be frequently transmitted, and the amount of the information is small. 
         [0007]    3) Uplink-oriented: M2M is chiefly configured to receive a command in downlink, take action, and then report resulting data in uplink. Thus, uplink becomes the center in M2M because major data is commonly transmitted in uplink. 
         [0008]    4) Longer lifespan of an STA: An M2M terminal is chiefly operated by the battery, and a user may not frequently charge the M2M terminal. Accordingly, it is necessary to guarantee a longer life span by minimizing the consumption of the battery. 
         [0009]    5) Automatic recovery function: An M2M terminal needs an automatic recovery function because it is difficult for a person to directly manipulate the M2M terminal in a special situation. 
       SUMMARY OF THE INVENTION 
       [0010]    An object of the present invention is to provide a method of indicating preamble transmission mode. 
         [0011]    Another object of the present invention is to provide a device for performing a method of indicating preamble transmission mode. 
         [0012]    A method of generating the Physical Layer Convergence Protocol (PLCP) header of a data frame according to an aspect of the present invention for achieving an object of the present invention includes steps of demodulating PLCP header mode information based on the scrambling sequence of a received frame or an information element included in the received frame and generating the data frame including a PLCP header generated based on the PLCP header mode information, wherein the PLCP header mode information may include at least one indication of Space Time Block Coding (STBC) mode indication, Guard Interval (GI) mode indication, and repetition mode indication. The STBC mode indication may indicate Space Time Block Coding (STBC) used to generate the PLCP header of the data frame, the GI mode indication may indicate whether a guard interval used to generate the PLCP header of the data frame is a Long Guard Interval (LGI) or a Short Guard Interval (SGI), and the repetition mode indication may indicate whether or not the PLCP header of the data frame has been repeated and generated. The step of demodulating PLCP header mode information based on the scrambling sequence of a received frame or an information element included in the received frame may include steps of demodulating STBC mode information of the PLCP header based on B 0  and B 1  of the first 7 bits B 0 , B 1 , B 2 , B 3 , B 4 , B 5 , and B 6  of the scrambling sequence of the received frame, demodulating SGI mode information of the PLCP header based on B 2  of the first 7 bits of the scrambling sequence of the received frame, and demodulating repetition mode information of the PLCP header based on B 3  of the first 7 bits B 0 ˜B 6  of the scrambling sequence of the received frame, wherein the received frame may b a Clear To Send (CTS) frame. The information element may be information included in the data payload of the received frame and may include an STBC field including the STBC mode indication, an SGI field including the SGI mode indication, and a repetition field including the repetition mode indication. The received frame may be a probe response frame. The PLCP header mode information may be information determined adaptively depending on channel conditions. 
         [0013]    A wireless terminal according to an aspect of the present invention for achieving an object of the present invention includes a processor for demodulating PLCP header mode information based on the scrambling sequence of a received frame or an information element included in the received frame and generating the data frame including a PLCP header generated based on the PLCP header mode information, wherein the PLCP header mode information may include at least one indication of Space Time Block Coding (STBC) mode indication, Guard Interval (GI) mode indication, and repetition mode indication, the STBC mode indication may indicate Space Time Block Coding (STBC) used to generate the PLCP header of the data frame, the GI mode indication may indicate whether a guard interval used to generate the PLCP header of the data frame is a Long Guard Interval (LGI) or a Short Guard Interval (SGI), and the repetition mode indication may indicate whether or not the PLCP header of the data frame has been repeated and generated. The processor may demodulate STBC mode information of the PLCP header based on B 0  and B 1  of the first 7 bits B 0 , B 1 , B 2 , B 3 , B 4 , B 5 , and B 6  of the scrambling sequence of the received frame, demodulate SGI mode information of the PLCP header based on B 2  of the first 7 bits of the scrambling sequence of the received frame, and demodulate repetition mode information of the PLCP header based on B 3  of the first 7 bits B 0 ˜B 6  of the scrambling sequence of the received frame, wherein the received frame may be a Clear To Send (CTS) frame. The information element may be information included in the data payload of the received frame and may include an STBC field including the STBC mode indication, an SGI field including the SGI mode indication, and a repetition field including the repetition mode indication, and the received frame may be a probe response frame. The PLCP header mode information may be information determined adaptively depending on channel conditions. 
         [0014]    In accordance with the method and device for indicating preamble transmission mode according to an embodiment of the present invention as described above, transmission mode of a PLCP header is adaptively determined depending on channel conditions because information about transmission mode of the PLCP header of a data frame is transmitted before transmitting the data frame. Accordingly, data transmission efficiency can be efficiently improved using wireless resources depending on channel conditions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a conceptual diagram showing the case where an STA erroneously senses a medium. 
           [0016]      FIG. 2  is a conceptual diagram showing a method using an RTS frame and a CTS frame in order to solve a hidden node issue and an exposed node issue. 
           [0017]      FIG. 3  is a conceptual diagram showing a CTS-to-self mechanism. 
           [0018]      FIG. 4  is a conceptual diagram showing a method of transmitting PLCP header mode information based on an RTS frame in accordance with an embodiment of the present invention. 
           [0019]      FIG. 5  is a conceptual diagram showing a method of transmitting PLCP header mode information based on a CTS frame in accordance with an embodiment of the present invention. 
           [0020]      FIG. 6  is a conceptual diagram showing a method of transmitting PLCP header mode information using a CTS-to-self frame in accordance with an embodiment of the present invention. 
           [0021]      FIG. 7  is a conceptual diagram showing a method of transmitting PLCP header mode information through a management frame in accordance with an embodiment of the present invention. 
           [0022]      FIG. 8  is a conceptual diagram showing a PLCP header mode information element in accordance with an embodiment of the present invention. 
           [0023]      FIG. 9  is a block diagram showing a wireless device to which an embodiment of the present invention may be applied. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0024]    IEEE 802.11 communication has characteristics basically different from those of a wired channel environment because it is performed in a shared wireless medium. 
         [0025]    In a wired channel environment, communication was made possible based on Carrier Sense Multiple Access/Collision Detection (CSMA/CD). For example, when a transmission STA transmits a signal once, the signal may be transmitted up to a reception STA without great signal attenuation because there is no significant change in a channel environment in a wired channel. If two or more signals collide against each other in a wired channel environment, whether or not the signals have collided against each other may be detected because power detected by a reception STA Rx becomes instantly greater than power transmitted by a transmission STA Tx. 
         [0026]    Unlike in a wired channel environment, however, in a wireless channel environment, various factors (e.g., the attenuation of a signal according to the distance and instant deep fading) affect a channel. Accordingly, a transmission STA cannot precisely perform carrier sensing on whether or not a reception STA has properly received a signal or whether or not there is a collision between signals in the reception STA. 
         [0027]    In order to solve the problem, in 802.11, a signal is transmitted based on a Distributed Coordination Function (DCF) that is a Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) mechanism. 
         [0028]    The Distributed Coordination Function (DCF) performs Clear Channel Assessment (CCA) for sensing a medium for specific duration (e.g., a DCF Inter-Frame Space (DIFS)) before stations (STAs) having data to be transmitted transmit the data. 
         [0029]    If, as a result of the CCA, the medium is idle (i.e., the state in which the medium has not been occupied by another STA), an STA may send a signal using the idle medium. If, as a result of the sensing, the medium is in the state in which it has been used by another STA (i.e., busy), an STA may send data after waiting for a random backoff period in addition to a DIFS, assuming that several STAs are already waiting to use the medium. 
         [0030]    In the case where Clear Channel Assessment (CCA) for sensing a medium is performed, if an STA erroneously senses whether the medium is an idle or not, a problem may occur.  FIG. 1  below shows the case where an STA erroneously senses whether or not a medium is idle or not. 
         [0031]      FIG. 1  is a conceptual diagram showing the case where an STA erroneously senses a medium. 
         [0032]      FIG. 1(A)  is a conceptual diagram showing a hidden node issue, and  FIG. 1(B)  is a conceptual diagram showing an exposed node issue. 
         [0033]    In  FIG. 1(A) , it is assumed that an STA A  100  and an STA B  120  are in communication with each other and an STA C  130  has information to be transmitted to the STA B  120 . This corresponds to a situation in which the STA A  100  sends information to the STA B  120 , and a channel medium through which data is to be transmitted to the STA B  120  has been occupied by the STA A  100 . From a viewpoint of the STA C  130 , however, there is a possibility that the STA C  130  may determine that the medium through which the data will be transmitted to the STA B  120  is in an idle state when performing carrier sensing on the medium before sending the data to the STA B  120 . In such a case, the data is transmitted from the STA C  130  to the STA B  120 . As a result, a collision is generated between the data because the STA B  120  receives the pieces of information from both the STA A  100  and the STA C  130 . In this case, the STA A  100  may be called a hidden node of the STA C  130 . 
         [0034]    In  FIG. 1(B) , it is assumed that an STA B  150  sends data to an STA A  140 . An STA C  160  may perform carrier sensing in order to know whether a medium has been occupied or not. In such a case, since the STA B  150  is sending the information to the STA A  140 , the state in which the medium has been occupied (i.e., busy) is sensed. As a result, although the STA C  160  wants to send data to an STA D  170 , the state in which the medium has been occupied (i.e., busy) is sensed. Accordingly, the STA C  160  must unnecessarily wait until the medium becomes an idle state after the STA B  150  finishes sending the data to the STA A  140 . 
         [0035]    That is, although the STA A  140  is placed outside the carrier sensing range of the STA C  160 , it may prevent the data transmission of the STA C  160 . In this case, the STA C  160  becomes an exposed node of the STA B  150 . 
         [0036]    In order to solve the hidden node issue described in  FIG. 1  (A) and the exposed node issue described in  FIG. 1(B) , whether a medium has been occupied or not can be sensed using an RTS frame and a CTS frame in a WLAN. 
         [0037]      FIG. 2  is a conceptual diagram showing a method using a Request To Send (RTS) frame and a Clear To Send (CTS) frame in order to solve a hidden node issue and an exposed node issue. 
         [0038]    Referring to  FIG. 2 , in order to solve a hidden node issue and an exposed node issue, surrounding STAs may overhear whether two STAs send data or not by introducing short signaling frames, such as an RTS frame and a CTS frame. 
         [0039]      FIG. 2(A)  shows a method of transmitting an RTS frame  203  and a CTS frame  205  in order to solve a hidden node issue. 
         [0040]    Assuming that both an STA A  200  and an STA C  220  attempt to send data to an STA B  210 , when the STA A  200  sends the RTS frame  203  to the STA B  210 , the STA B  210  may send the CTS frame  205  to all the STA A  200  and the STA C  220  placed near the STA B  210 . As a result, the STA C  220  may sense that the STA A  200  and the STA B  210  send data to each other. The STA C  220  waits until data transmission between the STA A  200  and the STA B  210  is terminated, thereby being capable of avoiding a collision. 
         [0041]      FIG. 2(B)  shows a method of transmitting an RTS frame  233  and a CTS frame  235  in order to solve an exposed node issue. 
         [0042]    It can be seen that a collision is not generated although an STA C  250  sends data to another STA D  260  by overhearing the transmission of the RTS frame  233  and the CTS frame  235  of an STA A  230  and an STA B  240 . That is, the STA B  240  sends the RTS frame  233  to all surrounding STAs and sends the CTS frame  235  to only the STA A  230  that has data to be actually transmitted. The STA C  250  may be aware that the STA A  230  is placed outside the carrier sensing range of the STA C  250  because it receives only the RTS frame  233 , but does not receive the CTS frame  235  of the STA A  230 . Accordingly, the STA C  250  can send data to the STA D  260 . 
         [0043]    The RTS frame format and the CTS frame format are disclosed in the 8.3.1.2 RTS frame format and 8.3.1.3 CTS frame format of 802.11 spec. 
         [0044]      FIG. 3  is a conceptual diagram showing a CTS-to-self mechanism. 
         [0045]      FIG. 3  shows a comparison between a case A where a medium is sensed using an RTS frame and CTS frame exchange method and a case B where a medium using a CTS-to-self frame is sensed. 
         [0046]    In the IEEE 802.11g standard, a CTS-to-self protection mechanism has been defined. The CTS-to-self protection mechanism may be used instead of a medium sensing mechanism using an RTS frame and a CTS frame. If the CTS-to-self protection mechanism is used, the overhead of a medium can be reduced as compared with the case where a medium sensing mechanism using RTS/CTS frames is used. However, unlike in the case where RTS/CTS frames are used, CTS-to-self is not an effective solution to a hidden node issue. 
         [0047]    Referring to  FIG. 3(A) , a method of exchanging an RTS frame and a CTS frame before a transmission STA sends a data frame may be performed as follows. 
         [0048]    In  FIG. 3(A) , it is assumed that an STA A  300  attempts to send a data frame to an STA C  310 . 
         [0049]    1) First, the STA A  300  sends an RTS frame  320 . 
         [0050]    2) The transmitted to RTS frame  320  is transmitted to an STA B  305  and the STA C  310  that are present in its carrier sensing range. 
         [0051]    3) The STA B  305  and the STA C  310  send CTS frames  325  and  330 . 
         [0052]    4) The transmitted CTS frames  325  and  330  are transmitted to the STA A  300 , the STAB  305 , the STA C  310 , and an STA D  315 . 
         [0053]    The STA D  315  has not received the RTS frame  320  from the STA A  300  because it is placed outside the carrier sensing range of the STA A  300  (i.e., the STA D  315  is a hidden node of the STA A  300 ). However, the STA D  315  may be aware that the STA A  300  has occupied a medium in order to send data by receiving the CTS frame  330  from the STA C  310 . 
         [0054]    5) The STA A  300  sends the data frame to the STA C  310 . 
         [0055]    Referring to  FIG. 3(B) , a medium sensing method using a CTS-to-self frame, performed before a transmission STA sends a data frame, may be performed as follows. In  FIG. 3(B) , it is assumed that an STA A  350  attempts to send a data frame to an STA C  360 . 
         [0056]    1) The STA A  350  sends a CTS-to-self frame to an STA B  355  and the STA C  360  that are present in its carrier sensing range. 
         [0057]    2) The STA B  355  and the STA C  360  that have received the CTS-to-self frame defer the transmission of other data frames in order to receive a data frame from the STA A  350 . 
         [0058]    If such a method is used, an STA D  365  placed outside the coverage area of the STA A  350  does not receive a CTS frame  370  from the STA A  350 . Accordingly, the STA D  365  does not obtain information about whether or not data is transmitted by the STA A  350 . 
         [0059]    In such a case, a collision may occur between data frames when the STA D  365  sends a data frame to the STA A  350  or the STA C  360 . That is, the method using the CTS-to-self frame  370  cannot solve a hidden node issue. Accordingly, the method using the CTS-to-self frame  370  needs to be applied to only a case where the transmission of data frames between STAs can be sensed. In other cases, a medium needs to be sensed using the RTS/CTS frame exchange method. 
         [0060]    Hereinafter, embodiments of the present invention disclose methods of exchanging pieces of information about modes that are used to generate the PLCP header of a data frame upon exchange of a CTS frame, an RTS frame, and a CTS-to-self frame which is performed before a data frame is transmitted in a frequency band of sub 1 GHz. Detailed embodiments are described below. 
         [0061]    In IEEE 802.11ah, an operation in a frequency band of sub 1 GHz is defined. The IEEE 802.11ah standard has a significant characteristic in that it has much wider coverage (up to 1 km) than an existing indoor-based WLAN in the non-licensed band of sub 1 GHz except TV white space bands. That is, unlike existing 2.4 GHz or 5 GHz, if a WLAN is used in the sub 1 GHz band represented by 700˜900 MHz, the coverage of an AP is extended about 2 to 3 times due to the propagation characteristic of the corresponding band assuming that the power is the same. 
         [0062]    In order to increase coverage to which a transmitted frame reaches up to 1 km in an outdoor environment, robustness needs to be increased when sending a PLCP header. 
         [0063]    In order to increase the robustness of a PLCP header, a repetition structure generated by performing repetition on the PLCP header may be used. If the repetition structure based on repetition is used in the PLCP header, there is an advantage in that a range in which a data frame may reach is increased. However, there is a disadvantage in that the overhead of a preamble is increased because the length of the preamble of a data frame is too long. 
         [0064]    A data frame having a PLCP header to which repetition has been applied is required to send a frame to an STA placed at a long distance without an error. However, to send data to STAs placed at a relatively short distance from an AP using a frame format including a preamble having a repetition structure may be the waste of resources. Accordingly, if a method of generating a preamble may be adaptively changed when a frame is transmitted to an STA placed at a short distance and when a frame is transmitted to an STA placed at a long distance, the throughput of data transmission can be increased and the power consumption of an STA can be reduced. 
         [0065]    Hereinafter, in an embodiment of the present invention, when generating the PLCP header of a data frame, a terminal may flexibly generate the PLCP header depending on environments differently using a method of generating the PLCP header adaptively depending on a channel environment. 
         [0066]    For example, a PLCP header having a different format may be generated by differently applying 1) Space Time Block Coding (STBC) used to generate the PLCP header, 2) a guard interval included in the PLCP header, and 3) whether or not to use a repetition format in the PLCP header depending on channel environments. 
         [0067]    STBC, an SGI, and a repetition format that may be differently applied when generating a PLCLP header are described below. 
         [0068]    (1) Space Time Block Coding (STBC) 
         [0069]    In a WLAN, several types of STBC are defined and used, and an appropriate constellation mapper of STBC has been designated depending on the number of antennas used in transmission. A system has different performance and the data transmission rate is also different depending on whether STBC is used or not and what constellation mapper is used. Terminals to which MIMO may be applied may use STBC in order to send a preamble as well as in order to send a data payload. In this case, the length of a preamble and a reachable length may be different depending on the type of STBC applied to the preamble. 
         [0070]    Various STBC methods have been disclosed in IEEE 802.11 20.3.11.9.2 Space-Time Block Coding (STBC). 
         [0071]    (2) a Short Guard Interval (SGI) 
         [0072]    In IEEE 802.11n and 802.11ac, a Short Guard Interval (SGI) is supported in order to prevent interference between symbols. Even in 802.11ah, an SGI is supported. Currently, information about whether or not an SGI and an LGI are used is included in the SIG field of a PLCP header and indicated. Information about whether a Short Guard Interval (SGI) will be used or a Long Guard Interval (LGI) will be used in a payload that follows a PLCP header may be aware based on a SIG field. In an existing PLCP header, only an LGI is used, and an SGI may not be selectively used. However, if an LGI and an SGI are selectively used even in a PLCP header, the length of a preamble can be reduced. Accordingly, if an LGI and an SGI may be adaptively used depending on a channel environment, it may help reduce the length of a preamble. 
         [0073]    (3) Repetition Mode 
         [0074]    In IEEE 802.11ah, in order to increase coverage to which a frame reaches, repetition mode is sought to be introduced into a preamble. A preamble using repetition is advantageous in that a range to which the preamble reaches without an error can be increased. However, if repetition is used, there is a disadvantage in that the length of a PLCP header is much lengthened. If repetition mode is used in the PLCP header of a frame that will be transmitted to a terminal placed at a short distance in a unicast environment, resources may be unnecessarily wasted. If whether or not to apply repetition to a preamble can be controlled depending on environments in order to prevent such an unnecessary waste of resources, it may help prevent an unnecessary increase in the length of the preamble. 
         [0075]    In accordance with an embodiment of the present invention, information about the type of STBC used to generate a PLCP header, whether or not a Short Guard Interval (SGI) has been used in a PLCP header, and whether or not a repetition structure has been used in a PLCP header may be indicated using a CTS frame/RTS frame, a CTS-to-self frame, and a management frame, that is, frames exchanged prior to the transmission of a data frame. 
         [0076]    That is, STBC, a GI, and repetition to be used in the PLCP header of a data frame may be selected and indicated by including information about STBC mode indication, SGI/LGI mode indication, and repetition mode indication in the RTS frame, the CTS frame, and the CTS-to-self frame or the management frame so that a data frame can be transmitted and received depending on a selected option. 
         [0077]    Hereinafter, in an embodiment of the present invention, information, such as STBC mode indication, SGI/LGI mode indication, and repetition mode indication, is defined and used as PLCP header mode information. 
         [0078]    The PLCP header mode information may include other transmission mode in addition to STBC mode indication, SGI/LGI mode indication, and repetition mode indication. Furthermore, some transmission modes of the STBC mode indication, the SGI/LGI mode indication, and the repetition mode indication may be used as PLCP header mode information. 
         [0079]    Hereinafter, in an embodiment of the present invention, an example in which all the STBC mode indication, the SGI/LGI mode indication, and the repetition mode indication are used as indication information is described for convenience of description. 
         [0080]      FIG. 4  is a conceptual diagram showing a method of transmitting PLCP header mode information based on an RTS frame in accordance with an embodiment of the present invention. 
         [0081]    Referring to  FIG. 4 , it is assumed that the PLCP header mode information includes all of STBC mode indication, SGI/LGI mode indication, and repetition mode indication. 
         [0082]    An STA 2  450  sends an RTS frame  410  including PLCP header mode information. 
         [0083]    In accordance with an embodiment of the present invention, PLCP header mode information may be transited through the RTS frame  420 . The PLCP header mode information may be transmitted through the first 7 bits of a scrambling sequence for scrambling the RTS frame  410 . 
         [0084]    Table 1 below shows the first 7 bits of the scrambling sequence. 
         [0000]    
       
         
               
               
             
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 DYN_BANDWIDTH —   
                   
               
               
                 IN_NON_HT 
                 First 7 bits of scrambling sequence 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Not present 
                 5 bit pseudo-random nonzero integer of 
                 0(NON_HT_CBW20) 
               
               
                   
                 CH_BANDWIDTH_IN_NON_HT 
                 1(NON_HT_CBW40) 
               
               
                   
                 equals NON_HT_CBW20 and 5 bit 
                 2(NON_HT_CBW80) 
               
               
                   
                 pseudo-random integer otherwise 
                 3(NON_HT_CBW160)/ 
               
             
          
           
               
                 present 
                 STBC 
                 SGI 
                 Repetition 
                 0(Static) 
                 (NON_HT_CBW80 + 80) 
               
               
                   
                 (1 bit or 2 bit) 
                 (1 bit) 
                 (1 bit) 
                 1(Dynamic) 
               
             
          
           
               
                   
                 B0 
                 B1 
                 B2 
                 B3 
                 B4 
                 B5 
                 B6 
               
               
                   
                   
               
             
          
         
       
     
         [0085]    Referring to Table 1, B 0 ˜B 4  that belong to the first 7 bits of a scrambling sequence for generating an existing RTS frame have been set as pseudo random non-zero integer values. In an embodiment of the present invention, information about the PLCP header mode of a data frame may be transmitted using the B 0 ˜B 4  used as the pseudo random non-zero integers. 
         [0086]    Information about the PLCP header mode of a data frame that will be subsequently transmitted may be indicated using B 0 ˜B 4  as STBC mode indication, B 2  as SGI mode indication, and B 3  as repetition mode indication, of B 0 ˜B 1 . 
         [0087]    As described above, Table 1 shows an embodiment in which all of STBC mode indication, SGI/LGI mode indication, and repetition mode indication will be used in PLCP header mode information. A mapping relationship between the scrambling sequence bits and the mode indication (i.e., STBC mode indication, SGI/LGI mode indication, and repetition mode indication) is random and may be changed. Furthermore, the mode indication may be transmitted using another sequence other than other reserved bits or a scrambling sequence. Furthermore, if parameters used as PLCP header mode information are changed, a mapping relationship between the scrambling sequence bits and the mode indication may be changed. 
         [0088]    The mode indication may be adaptively changed depending on channel environments. For example, if the STA 2  450  and an STA 1  400  are placed at a close distance from each other, transmission may be performed by setting a PLCP guard interval as an SGI and setting PLCP header mode information so that repetition is not performed. Furthermore, STBC may be selectively used depending on channel environments. 
         [0089]    In response to the RTS frame  410  transmitted by the STA 2  450 , the STA 1  400  sends a CTS frame  420 . 
         [0090]    The STA 1  400  may receive the RTS frame  410  transmitted by the STA 2  450  and obtain PLCP header mode indication information based on the scrambling sequence of the RTS frame  410 . The STA 1  400  may obtain information regarding that the PLCP header  430 - 1  of a data frame to be transmitted by the STA 1  400  will be transmitted with what PLCP header mode. 
         [0091]    The STA 2  450  receives the CTS frame  420  transmitted by the STA 1  400  and sends the data frame  430 . 
         [0092]    The PLCP header  430 - 1  of the data frame may be generated by determining a STBC method for the preamble, a guard interval used in the preamble, and whether or not to repeat the preamble based on STBC mode indication, SGI/LGI mode indication, and repetition mode indication information included in the scrambling sequence of the RTS frame  410  that has been previously transmitted. 
         [0093]    The STA 1  400  sends an ACK frame  440  in response to the transmitted data frame  430 . 
         [0094]    Furthermore, if the STA 1  400  has a data frame to be transmitted to the STA 2  450 , the STA 1  400  may generate the data frame including the PLCP header  430 - 1  generated in the same PLCP header mode as that of the STA 2  450  using the PLCP header mode information transmitted through the RTS frame  410  of the STA 1  400 . 
         [0095]    In  FIG. 4 , PLCP header mode indication information has been illustrated as being included in an RTS frame and transmitted, but PLCP header mode indication information may also be included in a CTS frame and transmitted. 
         [0096]      FIG. 5  is a conceptual diagram showing a method of transmitting PLCP header mode information based on a CTS frame in accordance with an embodiment of the present invention. 
         [0097]    Referring to  FIG. 5 , (1) an STA 2  550  may send an RTS frame  510  to an STA 1  500 . 
         [0098]    (2) The STA 2  550  may send a CTS frame  520  to the STA 1  500  in response to the RTS frame  510 . 
         [0099]    In accordance with an embodiment of the present invention, PLCP header mode information may be transmitted through the CTS frame  520 . The PLCP header mode information may be transmitted through the first 7 bits of a scrambling sequence for scrambling the CTS frame  520 . 
         [0100]    If STBC mode indication, SGI/LGI mode indication, and repetition mode indication are used as the PLCP header mode information, B 0 ˜B 4  that belong to the first 7 bits of the scrambling sequence may be used to represent the PLCP header mode information as in Table 1. 
         [0101]    For example, as in Table 1, information about the PLCP header mode of a data frame that will be subsequently transmitted by the STA 2  550  or the STA 1  500  may be transmitted using B 0 ˜B 1  as STBC mode indication, B 2  as an SGI mode indication, and B 3  as repetition mode indication, of B 0 ˜B 4 . A mapping relationship between the PLCP header information and the scrambling sequence is random and may be changed. 
         [0102]    (3) The STA 2  550  receives the CTS frame  520  and generates a data frame  530  based on a scrambling sequence included in the CTS frame  520 . 
         [0103]    The STA 2  550  may receive the CTS frame  520  and demodulate PLCP header mode information (e.g., STBC mode indication, SGI/LGI mode indication, and repetition mode indication) based on a scrambling sequence included in the CTS frame  520 . The STA 2  550  may generate the PLCP header  530 - 1  of the data frame  530  based on the demodulated PLCP header information. By using a method of transmitting PLCP header mode through the CTS frame  520 , the PLCP header  530 - 1  of the data frame may be generated adaptively depending on channel conditions. Accordingly, in the case of a close distance or an excellent channel state, medium efficiency can be increased because the length of a PLCP header is reduced (e.g., if a channel state is not good, the PLCP header of a data frame is generated using a Long Guard Interval (LGI) and repetition). 
         [0104]    (4) The STA 1  500  sends ACK  540  to the data frame  530  transmitted by the STA 2  550 . 
         [0105]    The STA 1  500  may receive the data frame  530  including the PLCP header  530 - 1  that has been generated using the PLCP header mode information transmitted by the STA 2  550  through the CTS frame  520 . If there is no error in the received data frame  530 , the STA 2  550  may send the ACK frame  540 . 
         [0106]    Furthermore, if the STA 1  500  sends a data frame to the STA 2  550 , the STA 1  500  may generate the PLCP header  530 - 1  of the data frame using the same information as the PLCP header mode information that has been transmitted to the STA 2  550  through the CTS frame  520 . 
         [0107]      FIGS. 4 and 5  disclose a method of transmitting PLCP header mode information when using a method of exchanging an RTS frame and a CTS frame. In accordance with an embodiment of the present invention, PLCP header mode information may be transmitted using a CTS-to-self frame. 
         [0108]      FIG. 6  is a conceptual diagram showing a method of transmitting PLCP header mode information using a CTS-to-self frame in accordance with an embodiment of the present invention. 
         [0109]    Referring to  FIG. 6 , (1) an STA 2  650  may send a CTS-to-self frame  610  to an STA 1  600 . 
         [0110]    Like in a method of transmitting PLCP header mode through an RTS frame and a CTS frame, PLCP header mode information may be transmitted through the first 7 bits of a scrambling sequence for scrambling the CTS-to-self frame  610 . For example, information about the PLCP header mode of a data frame to be subsequently transmitted by the STA 2  650  or the STA 1  600  may be transmitted using B 0 ˜B 1  as STBC mode indication, B 2  as SGI mode indication, and B 3  as repetition mode indication, of B 0 ˜B 4 , even in the CTS-to-self frame  610 , as in Table 1. A mapping relationship between the PLCP header information and the scrambling sequence is random and may be changed. The PLCP header mode information may include STBC mode indication, SGI/LGI mode indication, and repetition mode indication. 
         [0111]    (2) The STA 2  650  sends a data frame to the STA 1  600 . 
         [0112]    The STA 2  650  may send the data frame including a PLCP header  620 - 1  that has been generated based on PLCP header mode information transmitted through the CTS-to-self frame  610 . If such a method is used, the PLCP header  620 - 1  may be generated adaptively depending on channel conditions. Medium efficiency can be increased because the length of a PLCP header is reduced in the case of a close distance or an excellent channel state. 
         [0113]    (3) The STA 1  600  sends ACK  630  to the STA 2  650 . 
         [0114]    The STA 2  650  may send the ACK frame  630  to the STA 2  650  if the data frame  620  generated by the STA 1  600  is received without an error. 
         [0115]    Furthermore, if the STA 1  600  has a data frame to be transmitted to the STA 2  650 , the STA 1  600  may generate the PLCP header  620 - 1  of the data frame based on PLCP header mode information transmitted through the CTS-to-self frame  610 . 
         [0116]    The PLCP header information may be transmitted using an RTS frame, a CTS frame, and a CTS-to-self frame as described with reference to  FIGS. 4 to 6 , but may be transmitted using management frames used in scanning/authentication/association steps. For example, the PLCP header information may be transmitted through a probe request frame and a probe response frame. The PLCP header information may be transmitted through an information element of a management frame. 
         [0117]      FIG. 7  is a conceptual diagram showing a method of transmitting PLCP header mode information through a management frame in accordance with an embodiment of the present invention. 
         [0118]    Referring to  FIG. 7 , (1) if an STA  750  performs active scanning, it may send a probe request frame  710  to an AP  700 . 
         [0119]    (2) The AP  700  may send a probe response frame  720 , including PLCP header mode information as an information element, to the STA  750  in response to the probe request frame  710 . 
         [0120]    The probe response frame  720  may include an information element indicative of PLCP header mode information. Hereinafter, in an embodiment of the present invention, PLCP header mode information included as an information element of a management frame is defined and used as a PLCP header mode Information Element (IE). 
         [0121]      FIG. 8  is a conceptual diagram showing a PLCP header mode information element in accordance with an embodiment of the present invention. 
         [0122]    Referring to  FIG. 8 , the PLCP header mode information element  800  may include an element ID  810 , a length  820 , and preamble mode  830 . 
         [0123]    The element ID  810  may include indication information for classifying information elements included in a frame, and the length  820  may include information about the size of data assigned to information elements. 
         [0124]    The preamble mode  830  may include STBC  840 , an SGI  850 , and repetition  860 . 
         [0125]    The STBC  840  may indicate STBC mode information used in the PLCP header of a data frame. 
         [0126]    The SGI  850  may indicate the type of a GI used in the PLCP header. 
         [0127]    The repetition  860  may indicate whether coding will be used or not using repetition in the PLCP header. 
         [0128]    As described above, the PLCP header mode information may include only some of the STBC  840 , the SGI  850 , and the repetition  860  or may additionally include another indication information in addition to the STBC  840 , the SGI  850 , and the repetition  860 . 
         [0129]    Referring to back  FIG. 7 , 
         [0130]    (3) The STA  750  may receive a probe response frame  720  including a PLCP header mode information element and send a data frame  730 , generated based on the PLCP header mode information element, to the AP  740 . 
         [0131]    If an authentication process and an association process are omitted for convenience of description, the STA  750  may generate the PLCP header of a data frame based on the PLCP header mode information element transmitted by the AP  700 . 
         [0132]    (4) If the data frame  730  is received without an error, the AP  700  sends ACK  740  to the STA  750 . 
         [0133]    When sending a data frame, the AP  700  may send the data frame  730  including a PLCP header generated based on a PLCP header mode information element, to the STA  750 . 
         [0134]    In  FIG. 7 , an example in which the probe response frame  720  including the PLCP header mode information element is transmitted has been described, but the PLCP header mode information element may be transmitted using another management frame (e.g., a probe request frame). 
         [0135]    For example, the probe request frame  710  including the PLCP header mode information element may be transmitted in order to send information about preamble mode of a data frame to be received by the AP  700 . 
         [0136]      FIG. 9  is a block diagram showing a wireless device to which an embodiment of the present invention may be applied. 
         [0137]    The wireless device  70  is a terminal that may implement the aforementioned embodiments and may be an AP or a non-AP STA. 
         [0138]    The wireless device  70  includes a processor  72 , memory  74 , and a transceiver  76 . The transceiver  76  transmits/receives radio signals, and the physical layer of IEEE 802.11 is implemented in the transceiver  76 . The processor  72  is functionally connected to the transceiver  76  and implements the MAC layer and physical layer of IEEE 802.11. The processor  72  may demodulate PLCP header mode information based on the scrambling sequence of a received frame or an information element included in a received frame and may generate a data frame including a PLCP header generated based on the PLCP header mode information. That is, the processor may perform an operation for performing the aforementioned embodiments of the present invention. 
         [0139]    The processor  72  and/or the transceiver  76  may include Application-Specific Integrated Circuits (ASICs), other chipsets, logic circuits and/or data processors. The memory  74  may include Read-Only Memory (ROM), Random Access Memory (RAM), flash memory, memory cards, storage media and/or other storage devices. The transceiver  76  may include baseband circuits for processing radio signals. When the embodiment is implemented in software, the aforementioned scheme may be implemented as a module (process or function) that performs the aforementioned function. The module may be stored in the memory  74  and executed by the processor  72 . The memory  74  may be placed inside or outside the processor  72  and may be connected to the processor  72  using a variety of well-known means.