Patent Publication Number: US-11653261-B2

Title: Wireless communication method using frame aggregation and wireless communication terminal using same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/512,027 filed on Mar. 16, 2017, which is the U.S. National Stage of International Patent Application No. PCT/KR2015/009766 filed on Sep. 17, 2015, which claims the priority to Korean Patent Application No. 10-2014-0123906 filed in the Korean Intellectual Property Office on Sep. 17, 2014, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a method for constructing an aggregated frame for efficient wireless communication in a high-density environment and a wireless communication method using the same. 
     BACKGROUND ART 
     In recent years, with supply expansion of mobile apparatuses, a wireless LAN technology that can provide a rapid wireless Internet service to the mobile apparatuses has been significantly spotlighted. The wireless LAN technology allows mobile apparatuses including a smart phone, a smart pad, a laptop computer, a portable multimedia player, an embedded apparatus, and the like to wirelessly access the Internet in home or a company or a specific service providing area based on a wireless communication technology in a short range. 
     Institute of Electrical and Electronics Engineers (IEEE) 802.11 has commercialized or developed various technological standards since an initial wireless LAN technology is supported using frequencies of 2.4 GHz. First, the IEEE 802.11b supports a communication speed of a maximum of 11 Mbps while using frequencies of a 2.4 GHz band. IEEE 802.11a which is commercialized after the IEEE 802.11b uses frequencies of not the 2.4 GHz band but a 5 GHz band to reduce an influence by interference as compared with the frequencies of the 2.4 GHz band which are significantly congested and improves the communication speed up to a maximum of 54 Mbps by using an OFDM technology. However, the IEEE 802.11a has a disadvantage in that a communication distance is shorter than the IEEE 802.11b. In addition, IEEE 802.11g uses the frequencies of the 2.4 GHz band similarly to the IEEE 802.11b to implement the communication speed of a maximum of 54 Mbps and satisfies backward compatibility to significantly come into the spotlight and further, is superior to the IEEE 802.11a in terms of the communication distance. 
     Moreover, as a technology standard established to overcome a limitation of the communication speed which is pointed out as a weak point in a wireless LAN, IEEE 802.11n has been provided. The IEEE 802.11n aims at increasing the speed and reliability of a network and extending an operating distance of a wireless network. In more detail, the IEEE 802.11n supports a high throughput (HT) in which a data processing speed is a maximum of 540 Mbps or more and further, is based on a multiple inputs and multiple outputs (MIMO) technology in which multiple antennas are used at both sides of a transmitting unit and a receiving unit in order to minimize a transmission error and optimize a data speed. Further, the standard can use a coding scheme that transmits multiple copies which overlap with each other in order to increase data reliability. 
     As the supply of the wireless LAN is activated and further, applications using the wireless LAN are diversified, the need for new wireless LAN systems for supporting a higher throughput (very high throughput (VHT)) than the data processing speed supported by the IEEE 802.11n has come into the spotlight. Among them, IEEE 802.11ac supports a wide bandwidth (80 to 160 MHz) in the 5 GHz frequencies. The IEEE 802.11ac standard is defined only in the 5 GHz band, but initial 11ac chipsets will support even operations in the 2.4 GHz band for the backward compatibility with the existing 2.4 GHz band products. Theoretically, according to the standard, wireless LAN speeds of multiple stations are enabled up to a minimum of 1 Gbps and a maximum single link speed is enabled up to a minimum of 500 Mbps. This is achieved by extending concepts of a radio interface accepted by 802.11n, such as a wider radio frequency bandwidth (a maximum of 160 MHz), more MIMO spatial streams (a maximum of 8), multi-user MIMO, and high-density modulation (a maximum of 256 QAM). Further, as a scheme that transmits data by using a 60 GHz band instead of the existing 2.4 GHz/5 GHz, IEEE 802.11ad has been provided. The IEEE 802.11ad is a transmission standard that provides a speed of a maximum of 7 Gbps by using a beamforming technology and is suitable for high bit rate moving picture streaming such as massive data or non-compression HD video. However, since it is difficult for the 60 GHz frequency band to pass through an obstacle, it is disadvantageous in that the 60 GHz frequency band can be used only among devices in a short-distance space. 
     Meanwhile, in recent years, as next-generation wireless LAN standards after the 802.11ac and 802.11ad, discussion for providing a high-efficiency and high-performance wireless LAN communication technology in a high-density environment is continuously performed. That is, in a next-generation wireless LAN environment, communication having high frequency efficiency needs to be provided indoors/outdoors under the presence of high-density stations and access points (APs) and various technologies for implementing the communication are required. 
     DISCLOSURE 
     Technical Problem 
     As described above, an object of the present invention is to provide high-efficiency/high-performance wireless LAN communication in a high-density environment. 
     Technical Solution 
     In order to achieve the objects, the present invention provides a wireless communication method and a wireless communication terminal as below. 
     First, an embodiment of the present invention provides a wireless communication terminal including: a transceiver transmits and receives a wireless signal; and a processor controls an operation of the wireless communication terminal, wherein the processor generates an aggregated MPDU (A-MPDU) including a plurality of MAC protocol data unit (MPDU) groups to which independent modulation and coding schemes (MCSs) are applied, respectively, and transmits the generated A-MPDU through the transceiver. 
     According to an embodiment of the present invention, the A-MPDU may include at least one data MPDU group and at least one control MPDU group. 
     In this case, an MCS more robust than an MCS applied to an MPDU of the data MPDU group may be applied to an MPDU of the control MPDU. 
     In an embodiment, a binary phase shift keying (BPSK) modulation may be applied to the MPDU of the control MPDU group. 
     Further, the control MPDU group may include a trigger frame soliciting simultaneous transmission of uplink data of multiple users. 
     Further, the control MPDU group may include a multi-STA block ACK frame corresponding to uplink data simultaneously transmitted by the multiple users 
     Further, the control MPDU group may include a last MPDU of the A-MPDU. 
     According to an embodiment of the present invention, the data MPDU group may be located before the control MPDU group, and an MPDU delimiter of a first MPDU of the control MPDU group may include information representing a start of the control MPDU group. 
     In this case, the information representing the start of the control MPDU group may be indicated through a predetermined bit of the MPDU delimiter. 
     According to an embodiment of the present invention, the A-MPDU may include a first MPDU group and a second MPDU group to which different MCSs are applied. 
     In an embodiment, preamble information corresponding to the A-MPDU may indicate a basic MCS applied to the A-MPDU including the first MPDU group, and MPDU delimiter information of a first MPDU of the second MPDU group may indicate that an MCS different from the basic MCS is applied to the second MPDU group. 
     In this case, when the MPDU delimiter information of the first MPDU indicates a change of the MCS, a predetermined MCS may be applied to the second MPDU group. 
     In an embodiment, the predetermined MCS may be binary phase shift keying (BPSK) modulation. 
     According to another embodiment, when the MPDU delimiter information of the first MPDU indicates a change of the MCS, an MCS having an MCS index difference of a predetermined offset may be applied to the second MPDU group. 
     In this case, an MCS applied to the second MPDU group may have an MCS index value lower than an MCS applied to the first MPDU group. 
     In this case, the MPDU delimiter information may indicate whether the MCS is changed through flag information. 
     In addition, an embodiment of the present invention provides a wireless communication method including: generating an aggregated MPDU (A-MPDU) including a plurality of MAC protocol data unit (MPDU) groups to which independent modulation and coding schemes (MCSs) are applied, respectively; and transmitting the generated A-MPDU. 
     Next, another embodiment of the present invention provides a wireless communication terminal including: a transceiver transmits and receives a wireless signal; and a processor controls an operation of the wireless communication terminal, wherein the processor transmits an A-MPDU composed of a plurality of MPDUs through the transceiver, receives a block ACK corresponding to the A-MPDU, and determines a channel access method based on block ACK bitmap information of the received block ACK. 
     According to an embodiment of the present invention, the block ACK bitmap may be divided into a plurality of sections including a first section and a second section, and the channel access method may be determined based on the number of bits having a value of 0 for each section. 
     In an embodiment, the processor may increase a contention window for a backoff procedure when the number of bits having the value of 0 in the first section is greater than or equal to a predetermined first threshold. 
     According to another embodiment, the processor may perform a link adaptation process when the number of bits having the value of 0 in the second section is equal to or greater than a predetermined second threshold. 
     In this case, the link adaptation process may include modifying a modulation and coding scheme (MCS), and the processor may transmit new data using an MCS more robust than an MCS applied to the A-MPDU. 
     According to yet another embodiment, the processor may reduce an MPDU aggregation size of the A-MPDU when the number of bits having the value of 0 in the second section is equal to or greater than a predetermined third threshold. 
     In addition, another embodiment of the present invention provides a wireless communication method including: transmitting an A-MPDU composed of a plurality of MPDUs; receiving a block ACK corresponding to the A-MPDU; and determining a channel access method based on block ACK bitmap information of the received block ACK. 
     Advantageous Effects 
     According to an embodiment of the present invention, a transmission is performed by efficiently combining a plurality of data when performing wireless communication, thereby improving a data transmission rate. 
     In addition, according to the embodiment of the present invention, it is possible to adaptively determine a collision situation of the terminal and a channel estimation error situation according to the transmission of the aggregated frame, and to perform the channel access method optimized for each situation. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating a wireless LAN system according to an embodiment of the present invention. 
         FIG.  2    is a diagram illustrating a wireless LAN system according to another embodiment of the present invention. 
         FIG.  3    is a block diagram illustrating a configuration of a station according to an embodiment of the present invention. 
         FIG.  4    is a block diagram illustrating a configuration of an access point according to an embodiment of the present invention. 
         FIG.  5    is a diagram schematically illustrating a process in which a STA establishes a link with an AP. 
         FIG.  6    is a diagram illustrating a carrier sense multiple access (CSMA)/collision avoidance (CA) method used in wireless LAN communication. 
         FIG.  7    is a diagram comparing a data communication method using an individual MAC protocol data unit (MPDU) and a data communication method using an Aggregated MPDU (A-MPDU). 
         FIG.  8    is a diagram illustrating a detailed configuration of an A-MPDU according to an embodiment of the present invention. 
         FIG.  9    is a diagram illustrating a data communication method using an A-MPDU and a corresponding block ACK according to an embodiment of the present invention. 
         FIG.  10    is a diagram illustrating an embodiment of a block ACK frame structure corresponding to an A-MPDU. 
         FIG.  11    is a diagram illustrating another embodiment of a block ACK frame structure corresponding to an A-MPDU. 
         FIG.  12    is a diagram illustrating yet another embodiment of a block ACK frame structure corresponding to an A-MPDU. 
         FIG.  13    is a diagram illustrating an embodiment for expressing a length of an extended A-MPDU using L-SIG of a preamble. 
     
    
    
     BEST MODE 
     Terms used in the specification adopt general terms which are currently widely used by considering functions in the present invention, but the terms may be changed depending on an intention of those skilled in the art, customs, and emergence of new technology. Further, in a specific case, there is a term arbitrarily selected by an applicant and in this case, a meaning thereof will be described in a corresponding description part of the invention. Accordingly, it should be revealed that a term used in the specification should be analyzed based on not just a name of the term but a substantial meaning of the term and contents throughout the specification. 
     Throughout this specification and the claims that follow, when it is described that an element is “coupled” to another element, the element may be “directly coupled” to the other element or “electrically coupled” to the other element through a third element. Further, unless explicitly described to the contrary, the word “comprise” and variations such as “comprises” or “comprising”, will be understood to imply the inclusion of stated elements but not the exclusion of any other elements. Moreover, limitations such as “or more” or “or less” based on a specific threshold may be appropriately substituted with “more than” or “less than”, respectively. 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2014-0123906 filed in the Korean Intellectual Property Office and the embodiments and mentioned items described in the respective application, which forms the basis of the priority, shall be included in the Detailed Description of the present application. 
       FIG.  1    is a diagram illustrating a wireless LAN system according to an embodiment of the present invention. The wireless LAN system includes one or more basic service sets (BSS) and the BSS represents a set of apparatuses which are successfully synchronized with each other to communicate with each other. In general, the BSS may be classified into an infrastructure BSS and an independent BSS (IBSS) and  FIG.  1    illustrates the infrastructure BSS between them. 
     As illustrated in  FIG.  1   , the infrastructure BSS (BSS 1  and BSS 2 ) includes one or more stations STA 1 , STA 2 , STA 3 , STA 4 , and STA 5 , access points PCP/AP- 1  and PCP/AP- 2  which are stations providing a distribution service, and a distribution system (DS) connecting the multiple access points PCP/AP- 1  and PCP/AP- 2 . 
     The station (STA) is a predetermined device including medium access control (MAC) following a regulation of an IEEE 802.11 standard and a physical layer interface for a radio medium, and includes both a non-access point (non-AP) station and an access point (AP) in a broad sense. Further, in the present specification, a term ‘terminal’ may be used to refer to a non-AP STA, or an AP, or to both terms. A station for wireless communication includes a processor and a transceiver and according to the embodiment, may further include a user interface unit and a display unit. The processor may generate a frame to be transmitted through a wireless network or process a frame received through the wireless network and besides, perform various processing for controlling the station. In addition, the transceiver is functionally connected with the processor and transmits and receives frames through the wireless network for the station. 
     The access point (AP) is an entity that provides access to the distribution system (DS) via wireless medium for the station associated therewith. In the infrastructure BSS, communication among non-AP stations is, in principle, performed via the AP, but when a direct link is configured, direct communication is enabled even among the non-AP stations. Meanwhile, in the present invention, the AP is used as a concept including a personal BSS coordination point (PCP) and may include concepts including a centralized controller, a base station (BS), a node-B, a base transceiver system (BTS), and a site controller in a broad sense. 
     A plurality of infrastructure BSSs may be connected with each other through the distribution system (DS). In this case, a plurality of BSSs connected through the distribution system is referred to as an extended service set (ESS). 
       FIG.  2    illustrates an independent BSS which is a wireless LAN system according to another embodiment of the present invention. In the embodiment of  FIG.  2   , duplicative description of parts, which are the same as or correspond to the embodiment of  FIG.  1   , will be omitted. 
     Since a BSS 3  illustrated in  FIG.  2    is the independent BSS and does not include the AP, all stations STA 6  and STAT are not connected with the AP. The independent BSS is not permitted to access the distribution system and forms a self-contained network. In the independent BSS, the respective stations STA 6  and STAT may be directly connected with each other. 
       FIG.  3    is a block diagram illustrating a configuration of a station  100  according to an embodiment of the present invention. 
     As illustrated in  FIG.  3   , the station  100  according to the embodiment of the present invention may include a processor  110 , a transceiver  120 , a user interface unit  140 , a display unit  150 , and a memory  160 . 
     First, the transceiver  120  transmits and receives a wireless signal such as a wireless LAN packet, or the like and may be embedded in the station  100  or provided as an exterior. According to the embodiment, the transceiver  120  may include at least one transmit/receive module using different frequency bands. For example, the transceiver  120  may include transmit/receive modules having different frequency bands such as 2.4 GHz, 5 GHz, and 60 GHz. According to an embodiment, the station  100  may include a transmit/receive module using a frequency band of 6 GHz or more and a transmit/receive module using a frequency band of 6 GHz or less. The respective transmit/receive modules may perform wireless communication with the AP or an external station according to a wireless LAN standard of a frequency band supported by the corresponding transmit/receive module. The transceiver  120  may operate only one transmit/receive module at a time or simultaneously operate multiple transmit/receive modules together according to the performance and requirements of the station  100 . When the station  100  includes a plurality of transmit/receive modules, each transmit/receive module may be implemented by independent elements or a plurality of modules may be integrated into one chip. 
     Next, the user interface unit  140  includes various types of input/output means provided in the station  100 . That is, the user interface unit  140  may receive a user input by using various input means and the processor  110  may control the station  100  based on the received user input. Further, the user interface unit  140  may perform output based on a command of the processor  110  by using various output means. 
     Next, the display unit  150  outputs an image on a display screen. The display unit  150  may output various display objects such as contents executed by the processor  110  or a user interface based on a control command of the processor  110 , and the like. Further, the memory  160  stores a control program used in the station  100  and various resulting data. The control program may include an access program required for the station  100  to access the AP or the external station. 
     The processor  110  of the present invention may execute various commands or programs and process data in the station  100 . Further, the processor  110  may control the respective units of the station  100  and control data transmission/reception among the units. According to the embodiment of the present invention, the processor  110  may execute the program for accessing the AP stored in the memory  160  and receive a communication configuration message transmitted by the AP. Further, the processor  110  may read information on a priority condition of the station  100  included in the communication configuration message and request the access to the AP based on the information on the priority condition of the station  100 . The processor  110  of the present invention may represent a main control unit of the station  100  and according to the embodiment, the processor  110  may represent a control unit for individually controlling some component of the station  100 , for example, the transceiver  120 , and the like. The processor  110  controls various operations of wireless signal transmission/reception of the station  100  according to the embodiment of the present invention. A detailed embodiment thereof will be described below. 
     The station  100  illustrated in  FIG.  3    is a block diagram according to an embodiment of the present invention, where separate blocks are illustrated as logically distinguished elements of the device. Accordingly, the elements of the device may be mounted in a single chip or multiple chips depending on design of the device. For example, the processor  110  and the transceiver  120  may be implemented while being integrated into a single chip or implemented as a separate chip. Further, in the embodiment of the present invention, some components of the station  100 , for example, the user interface unit  140  and the display unit  150  may be optionally provided in the station  100 . 
       FIG.  4    is a block diagram illustrating a configuration of an AP  200  according to an embodiment of the present invention. 
     As illustrated in  FIG.  4   , the AP  200  according to the embodiment of the present invention may include a processor  210 , a transceiver  220 , and a memory  260 . In  FIG.  4   , among the components of the AP  200 , duplicative description of parts which are the same as or correspond to the components of the station  100  of  FIG.  2    will be omitted. 
     Referring to  FIG.  4   , the AP  200  according to the present invention includes the transceiver  220  for operating the BSS in at least one frequency band. As described in the embodiment of  FIG.  3   , the transceiver  220  of the AP  200  may also include a plurality of transmit/receive modules using different frequency bands. That is, the AP  200  according to the embodiment of the present invention may include two or more transmit/receive modules among different frequency bands, for example, 2.4 GHz, 5 GHz, and 60 GHz together. Preferably, the AP  200  may include a transmit/receive module using a frequency band of 6 GHz or more and a transmit/receive module using a frequency band of 6 GHz or less. The respective transmit/receive modules may perform wireless communication with the station according to a wireless LAN standard of a frequency band supported by the corresponding transmit/receive module. The transceiver  220  may operate only one transmit/receive module at a time or simultaneously operate multiple transmit/receive modules together according to the performance and requirements of the AP  200 . 
     Next, the memory  260  stores a control program used in the AP  200  and various resulting data. The control program may include an access program for managing the access of the station. Further, the processor  210  may control the respective units of the AP  200  and control data transmission/reception among the units. According to the embodiment of the present invention, the processor  210  may execute the program for accessing the station stored in the memory  260  and transmit communication configuration messages for one or more stations. In this case, the communication configuration messages may include information about access priority conditions of the respective stations. Further, the processor  210  performs an access configuration according to an access request of the station. The processor  210  controls various operations such as wireless signal transmission/reception of the AP  200  according to the embodiment of the present invention. A detailed embodiment thereof will be described below. 
       FIG.  5    is a diagram schematically illustrating a process in which a STA sets a link with an AP. 
     Referring to  FIG.  5   , the link between the STA  100  and the AP  200  is set through three steps of scanning, authentication, and association in a broad way. First, the scanning step is a step in which the STA  100  obtains access information of BSS operated by the AP  200 . A method for performing the scanning includes a passive scanning method in which the AP  200  obtains information by using a beacon message (S 101 ) which is periodically transmitted and an active scanning method in which the STA  100  transmits a probe request to the AP (S 103 ) and obtains access information by receiving a probe response from the AP (S 105 ). 
     The STA  100  that successfully receives wireless access information in the scanning step performs the authentication step by transmitting an authentication request (S 107   a ) and receiving an authentication response from the AP  200  (S 107   b ). After the authentication step is performed, the STA  100  performs the association step by transmitting an association request (S 109   a ) and receiving an association response from the AP  200  (S 109   b ). 
     Meanwhile, an 802.1X based authentication step (S 111 ) and an IP address obtaining step (S 113 ) through DHCP may be additionally performed. In  FIG.  5   , the authentication server  300  is a server that processes 802.1X based authentication with the STA  100  and may be present in physical association with the AP  200  or present as a separate server. 
       FIG.  6    is a diagram illustrating a carrier sense multiple access (CSMA)/collision avoidance (CA) method used in wireless LAN communication. 
     A terminal that performs a wireless LAN communication checks whether a channel is busy by performing carrier sensing before transmitting data. When a wireless signal having a predetermined strength or more is sensed, it is determined that the corresponding channel is busy and the terminal delays the access to the corresponding channel. Such a process is referred to as clear channel assessment (CCA) and a level to decide whether the corresponding signal is sensed is referred to as a CCA threshold. When a wireless signal having the CCA threshold or more, which is received by the terminal, indicates the corresponding terminal as a receiver, the terminal processes the received wireless signal. Meanwhile, when a wireless signal is not sensed in the corresponding channel or a wireless signal having a strength smaller than the CCA threshold is sensed, it is determined that the channel is idle. 
     When it is determined that the channel is idle, each terminal having data to be transmitted performs a backoff procedure after an interframe space (IFS) time depending on a situation of each terminal, for instance, an arbitration IFS (AIFS), a PCF IFS (PIFS), or the like elapses. According to the embodiment, the AIFS may be used as a component which substitutes for the existing DCF IFS (DIFS). Each terminal stands by while decreasing slot time(s) as long as a random number assigned to the corresponding terminal during an interval of an idle state of the channel and a terminal that completely exhausts the slot time(s) attempts to access the corresponding channel. As such, an interval in which each terminal performs the backoff procedure is referred to as a contention window interval. 
     When a specific terminal successfully accesses the channel, the corresponding terminal may transmit data through the channel. However, when the terminal which attempts the access collides with another terminal, the terminals which collide with each other are assigned with new random numbers, respectively to perform the backoff procedure again. According to an embodiment, a random number newly assigned to each terminal may be decided within a range (2*CW) which is twice larger than a range (a contention window, CW) of a random number which the corresponding terminal is previously assigned. Meanwhile, each terminal attempts the access by performing the backoff procedure again in a next contention window interval and in this case, each terminal performs the backoff procedure from slot time(s) which remained in the previous contention window interval. By such a method, the respective terminals that perform the wireless LAN communication may avoid a mutual collision for a specific channel. 
       FIG.  7    compares a data communication method using an individual MAC protocol data unit (MPDU) and a data communication method using an Aggregated MPDU (A-MPDU). 
     First,  FIG.  7 ( a )  illustrates a communication method using an individual MPDU. Referring to  FIG.  7 ( a ) , in a general data transmission process, an individual MPDU and corresponding ACK are sequentially transmitted. That is, when a transmitting terminal desires to transmit a plurality of MPDUs (e.g., MPDU  1 , MPDU  2 ), the transmitting terminal transmits the first MPDU (i.e., MPDU  1 ) and receives an ACK corresponding thereto, and after a channel contention interval, the transmitting terminal transmits the next MPDU (i.e., MPDU  2 ) and receives an ACK corresponding thereto. 
     However, when performing the data communication method using A-MPDU as shown in  FIG.  7 ( b ) , the transmitting terminal aggregates a plurality of MPDUs (e.g., MPDU  1 , MPDU  2 ) after a single preamble and transmits them together as a single PHY protocol data unit (PPDU). In an embodiment of the present invention, the preamble may be used as a concept including a PHY header. Likewise, a frame in which a plurality of MPDUs are aggregated in a single PPDU is referred to as an A-MPDU. According to the present invention, the A-MPDU may be composed of a plurality of data MPDUs, or may include at least one data MPDU and at least one control MPDU. A receiving terminal receiving the A-MPDU from the transmitting terminal transmits a block ACK (BA) corresponding thereto. The block ACK (BA) includes information indicating the success or failure of transmission of each of a plurality of MPDUs constituting the corresponding A-MPDU. More specifically, the block ACK (BA) may include a block ACK bitmap field representing the reception state of the receiving terminal for each MPDU. According to an exemplary embodiment, the block ACK bitmap may individually indicate the success or failure of transmissions up to 64 MPDUs by using an 8-bytes bitmap. 
     In an embodiment of the present invention, the transmitting terminal refers to a terminal that transmits data, that is, an MPDU or an A-MPDU. In addition, the receiving terminal refers to a terminal that receives the data and transmits a corresponding ACK or block ACK. In case of uplink transmission, the transmitting terminal may represent a non-AP STA, and the receiving terminal may represent an AP, respectively. Furthermore, in case of downlink transmission, the transmitting terminal may represent an AP, and the receiving terminal may represent a non-AP STA, respectively. 
       FIG.  8    illustrates a detailed configuration of an A-MPDU according to an embodiment of the present invention. The A-MPDU generating process described in  FIG.  8    can be performed by the transmitting terminal according to the present invention. 
     First, an MPDU  10  combines a MAC header (MAC HDR) and a frame check sequence (FCS) for determining whether there is an error with a MAC service data unit (MSDU) or an aggregated MSDU (A-MSDU) in an upper layer. The maximum length of the MPDU  10  according to the IEEE 802.11ac standard is set to 11,454 bytes. 
     When a plurality of MPDUs  10  are to be aggregated to generate an A-MPDU  14 , the transmitting terminal may generate an MPDU subframe  12  in which an MPDU delimiter an a pad are combined before and after the MPDU  10 . In this case, the MPDU delimiter comprises an end-of-frame (EOF) field representing whether the corresponding MPDU subframe  12  is the last MPDU subframe  12  of the A-MPDU  14 , an MPDU length field representing the length of each MPDU  10 , a cyclic redundancy check (CRC) field, a signature field, and the like. 
     The A-MPDU  14  includes a plurality of MPDU subframes (e.g., MPDU subframe  1 , . . . , MPDU subframe N) and may be composed of a maximum of N MPDU subframes  12 . According to an embodiment, when the block ACK bitmap transmitted corresponding to the A-MPDU has a size of 8 bytes, the N may be set to 64. 
     Meanwhile, a preamble located at the front of the A-MPDU  14  includes a legacy signal field (L-SIG), and the L-SIG includes a legacy rate (L-Rate) field and a legacy length (L-Length) field. The L-Rate represents such as the transmission rate, the modulation scheme, and the coding rate of the transmission data. The L-Length represents the size (e.g. number of bytes) of the transmission data. 
     The receiving terminal may obtain information on the number of transmission bits per OFDM symbol of the transmission data based on a modulation and coding scheme (MCS) indicated by the L-Rate field, and may obtain the number of OFDM symbols constituting the transmission data by dividing the L-Length field information by the information on the number of transmission bits. According to an embodiment, when the L-Rate is set to 6 Mbps, which is the minimum transmission rate of an orthogonal frequency division multiplexing (OFDM) packet, the maximum number of symbols that can be represented through a combination of L-Rate/L-Length is 1365 or 1366. In this case, the length (i.e., duration) of one OFDM symbol may be set to 4 us, and the L-SIG may set the length (i.e., duration) of the corresponding data packet up to 5.460 ms or 5.464 ms through a combination of L-Rate/L-Length. In this manner, the L-SIG of the A-MPDU  14  may represent the packet duration information of the A-MPDU  14  in units of symbols through a combination of the L-Rate field and the L-Length field. 
     The receiving terminal receiving the A-MPDU  14  sets a network allocation vector (NAV) based on the number of OFDM symbols obtained through the combination of the L-Rate field and the L-Length field of the L-SIG, and defers transmission for the corresponding channel. In this case, since the L-SIG can be recognized by legacy terminals such as IEEE 802.11a, n, etc., interference by other terminals during the transmission period of the A-MPDU  14  can be prevented. 
       FIG.  9    illustrates a data communication method using an A-MPDU and a corresponding block ACK according to an embodiment of the present invention. In  FIG.  9   , STA 1  indicates a transmitting terminal which transmits an A-MPDU  14 , and STA 2  indicates a receiving terminal which receives the A-MPDU  14  and transmits a block ACK  20 . 
     Referring to  FIG.  9 ( a ) , STA 1  generates an A-MPDU  14  including a plurality of MPDUs (e.g., MPDU  1 , MPDU  2 , . . . , MPDU N), and transmits the generated A-MPDU  14 . The STA 2  receiving the A-MPDU  14  may fail to normally receive a specific MPDU according to a channel condition or a collision. In this case, the STA 2  may notify the STA 1  of the MPDU that has successfully received and the MPDU that failed to receive by indicating them through the bitmap of the block ACK  20 . 
       FIG.  9 ( b )  illustrates a detailed structure of the block ACK  20  according to an embodiment of the present invention. The block ACK  20  includes ACK information for a plurality of MPDUs constituting the A-MPDU  20 . Referring to  FIG.  9 ( b ) , the block ACK  20  may include a frame control field, a duration field, a receiver address (RA) field, a transmitter address (TA) field, a block ACK control (BA control) field, starting sequence control field, a block ACK bitmap field, and a frame check sequence (FCS) field. 
     The block ACK control (BA control) field comprises setting information relating to the configuration of the block ACK  20 . The starting sequence control field represents the sequence number of the first MPDU among MPDUs received through the A-MPDU  14 . In addition, each bit of the block ACK bitmap field represents the success or failure of reception of MPDUs starting from the first MPDU by flag information. The first bit of the block ACK bitmap field corresponds to the MPDU of the sequence number matched with the starting sequence control field. When the value of each bit of the block ACK bitmap field is 1, it is indicated that the MPDU corresponding to the relevant bit has been successfully received. Accordingly, when the n-th bit of the block ACK bitmap is 1, it is indicated that the MPDU having the sequence number of ‘block ACK sequence control+n’ has been successfully received. However, when the n-th bit of the block ACK bitmap is 0, it is indicated that the MPDU having the sequence number of ‘block ACK sequence control+n’ is not received. If the block ACK bitmap has a size of 8 bytes, the ‘n’ may have a value between 0 and 63. 
     According to the embodiment of the present invention, communication can be performed by variously modifying the A-MPDU  14  and the block ACK  20  in order to improve data transmission performance in a high-density environment. The transmitting terminal and the receiving terminal of the present invention can perform data communication according to the embodiments described below or a combination thereof. 
     If an ACK corresponding to the data is not received after transmitting the data, the WLAN terminal determines that a collision has occurred between the terminals as described above with reference to  FIG.  6   , and increases the contention window twice to access the channel. In addition, the WLAN terminal may selectively perform a link adaptation process to change the modulation and coding scheme (MCS) of transmission data. That is, the terminal may increase the probability of transmission success of data by transmitting new data using an MCS that is more robust than an MCS used in the previous transmitted data. 
     According to an embodiment of the present invention, the transmitting terminal may adjust the channel access method based on the block ACK bitmap information of the received block ACK  20 . In this case, the channel access method indicates at least one of the size of the contention window for the backoff procedure and the MCS applied to the data to be transmitted, but the present invention is not limited thereto. The transmitting terminal may adjust the channel access method based on the number and distribution of bits having a value of 0 (hereinafter, referred to as ‘MPDU reception failure information’) in the block ACK bitmap of the block ACK  20 . 
     When a plurality of MPDUs are transmitted together through the A-MPDU  14 , the probability of reception failure of the receiving terminal increases as the MPDU is located at the rear side. This phenomenon is more likely to occur due to a decrease in accuracy of channel estimation based on a preamble at a receiving terminal as data transmission time increases, rather than due to a collision with another terminal. Therefore, according to the embodiment of the present invention, the channel access method of the transmitting terminal can be determined based on the distribution of the MPDU reception failure information in the block ACK bitmap. 
     According to an embodiment of the present invention, the transmitting terminal receiving the block ACK  20  may divide the block ACK bitmap of the block ACK  20  into a plurality of sections and obtain the number of MPDU reception failure information for each section. According to an embodiment, the block ACK bitmap may be divided into two or more sections including a first section and a second section. In this case, the first section is a section preceding the second section. Each of the sections may indicate the first half and the second half of the block ACK bitmap, or may indicate a section of a predetermined length, respectively. According to an embodiment of the present invention, when the number of MPDU reception failure information in the first section of the block ACK bitmap is equal to or greater than a predetermined first threshold, the transmitting terminal may determine that a collision has occurred between the terminals and increase the contention window for the backoff procedure. On the other hand, when the number of MPDU reception failure information in the second section of the block ACK bitmap is equal to or greater than a predetermined second threshold, the transmitting terminal may determine that a channel estimation error has occurred and perform the link adaptation process. Furthermore, when the number of MPDU reception failure information in the second section of the block ACK bitmap is equal to or greater than a predetermined third threshold, the transmitting terminal may reduce the number (i.e., aggregation size) of MPDUs aggregated in the A-MPDU  14 . In this case, the first threshold, the second threshold, and the third threshold may be set to the same value or may be set to different values. According to an embodiment, the third threshold may be set to a value greater than the second threshold. 
     Meanwhile, although it is described that the block ACK bitmap is divided into two sections in the above embodiment, the present invention is not limited thereto, and the block ACK bitmap may be divided into further sections. For example, the block ACK bitmap may be divided into four sections including a first section, a second section, a third section, and a fourth section. The transmitting terminal may increase the contention window based on the number of MPDU reception failure information of the first section and perform the link adaptation process based on the number of MPDU reception failure information of the fourth section. According to another embodiment of the present invention, instead of the number of MPDU reception failure information in each section, an MPDU reception failure rate can be used to adjust the channel access method. That is, the transmitting terminal may perform the contention window increase and/or a link adaptation process based on whether the ratio of the number of MPDU reception failure information to the total number of bits in each segment is equal to or greater than a predetermined threshold. 
     According to another embodiment of the present invention, the transmitting terminal may adjust the channel access method based on the cumulative distribution of bits having a value of 1 (hereinafter, ‘MPDU reception success information’) in the block ACK bitmap of the block ACK  20 . The transmitting terminal may generate a graph based on the cumulative distribution. The variable x in the graph represents the bit order ‘n’ in the block ACK bitmap and the variable y in the graph represents the cumulative number of MPDU reception success information up to the bit order ‘n’. If all the MPDUs included in the A-MPDU  14  are successfully received, the graph represents y=x (the first shape). However, if the reception of some intermediate MPDUs in the A-MPDU  14  fails and subsequent MPDUs are successfully received again, the graph will represent a multi-stepped form (the second shape). In addition, if the reception of all the MPDUs after a specific time point in the A-MPDU  14  fails, the graph will represent a single step form (the third shape). According to the embodiment of the present invention, when the graph based on the cumulative distribution represents the second shape, the transmitting terminal may determine that a collision with another terminal has occurred and increase the contention window value. In addition, when the graph based on the cumulative distribution represents the third shape, the transmitting terminal may determine that a channel estimation error has occurred after a specific MPDU and perform the link adaptation process. 
     According to yet another embodiment of the present invention, the transmitting terminal may additionally consider the maximum number (hereinafter, referred to as ‘N_Max’) of MPDUs that can be aggregated and the number (hereinafter, referred to as ‘N_Used’) of actually aggregated MPDUs of the A-MPDU  14  to adjust the channel access method. In this case, N_max may be a predetermined value or a value determined by mutual negotiation between the transmitting terminal and the receiving terminal. The transmitting terminal may adjust the channel access method based on the N_Used value or the ratio of N_Used to N_Max. More specifically, the transmitting terminal may adjust at least one of thresholds (i.e., the first threshold, the second threshold, and the third threshold) of each section of the block ACK bitmap for adjusting the channel access method based on the N_Used value. The transmitting terminal may increase at least one of the thresholds for each section as the N_Used value increases, since the error probability of data reception becomes higher as the N_Used value increases. Also, the transmitting terminal may increase at least one of the thresholds for each section as the ratio of N_Max to N_Used increases. 
     According to still another embodiment of the present invention, the transmitting terminal may adjust the channel access method based on the position (hereinafter, referred to as a ‘reception failure start position’) of the bit in which the MPDU reception failure information first appears in the block ACK bitmap. The reception failure start position may represent a period during which the channel estimation of the reception terminal is effectively applied, that is, a coherence time. The transmitting terminal may obtain the coherence time information based on the reception failure start position information. The transmitting terminal may increase the MCS or reduce the number (i.e., aggregation size) of MPDUs which are aggregated so that the duration of the A-MPDU  14  transmitted thereafter to be within the coherence time. 
     According to still yet another embodiment of the present invention, the transmitting terminal may classify the MPDUs included in the A-MPDU  14  into a plurality of groups, and transmit them by applying separate MCSs to each group. That is, the transmitting terminal may generate an A-MPDU  14  including a plurality of MPDU groups to which independent MCSs are applied, respectively, and transmit the generated A-MPDU  14 . In this case, each MPDU group is composed of at least one MPDU. 
     The A-MPDU  14  according to the embodiment of the present invention may include different types of MPDU groups. For example, the A-MPDU  14  may include at least one data MPDU group and at least one control MPDU group. The data MPDU group is composed of at least one data MPDU (i.e., data frame), and may be located before the control MPDU group according to an embodiment. The control MPDU group is composed of at least one control MPDU (i.e., control frame), and the first MPDU of the control MPDU group may indicate the start of the control MPDU group through an MPDU delimiter, and the like. Information indicating the start of the control MPDU group can be indicated through a predetermined bit of the MPDU delimiter. According to an embodiment, a reserved bit of the MPDU delimiter defined in the legacy wireless LAN (e.g., IEEE 802.11ac) standard may be used as the predetermined bit. 
     The control MPDU constituting the control MPDU group may include various types of control frames. For example, the control MPDU group may include a trigger frame that solicits simultaneous transmission of uplink data of multiple users. In addition, the control MPDU group may include a multi-STA block ACK corresponding to uplink data simultaneously transmitted by multiple users. According to an embodiment, the AP may transmit an A-MPDU  14  including the trigger frame, and the plurality of non-AP STAs receiving the A-MPDU  14  may simultaneously transmit uplink data at the timing indicated by the trigger frame. The AP receiving uplink data from the plurality of non-AP STAs may transmit an A-MPDU  14  including multi-STA block ACK frame corresponding thereto. According to an embodiment of the present invention, the control MPDU group is located after the data MPDU group and includes the last MPDU of the A-MPDU  14 . That is, the trigger frame and/or the multi-STA block ACK frame may constitute the last MPDU of the A-MPDU  14 . However, specific positions of the data MPDU group and the control MPDU group in the present invention are not limited thereto. That is, according to another embodiment of the present invention, the control MPDU group may be located before the data MPDU group and include the first MPDU of the A-MPDU  14 . That is, the trigger frame and/or the multi-STA block ACK frame may constitute the first MPDU of the A-MPDU  14 . 
     The transmitting terminal may apply a specific MCS selected from an MCS set including a plurality of MCSs to the MPDU. A specific MCS can be indicated by an MCS index in the MCS set, and a lower MCS index value may indicate a robust MCS. According to the embodiment of the present invention, an MCS that is more robust than an MCS applied to the MPDU of the data MPDU group may be applied to the MPDU of the control MPDU group. For example, if an MCS of index  9  is applied to the data MPDU group, then a more robust MCS of index  8  may be applied to the control MPDU. According to another embodiment, an MCS indicated by the preamble information of the corresponding PPDU may be applied to the data MPDU group, and a predetermined MCS such as the binary phase shift keying (BPSK) modulation may be applied to the control MPDU. In this manner, the transmitting terminal may apply a more robust MCS to the control MPDU among the plurality of MPDUs constituting the A-MPDU  14 , thereby increasing the probability of the transmission success of the control MPDU. 
     MPDU groups to which different MCSs are applied in the A-MPDU  14  can be identified in various ways. According to an embodiment, the A-MPDU  14  may sequentially include a first MPDU group and a second MPDU group to which different MCSs are applied. In this case, the preamble information corresponding to the A-MPDU  14  (i.e., the preamble information of the PPDU) may indicate a basic MCS applied to the A-MPDU  14 , and the basic MCS may be applied to the first MPDU group. On the other hand, the second MPDU group to which an MCS different from the basic MCS is applied may be identified through information transmitted by the first MPDU of the corresponding group. For example, MPDU delimiter information of the first MPDU of the second MPDU group may indicate that an MCS different from the basic MCS is applied to the second MPDU group. According to an embodiment, the MPDU delimiter information may indicate whether the MCS is changed by flag information through a predetermined bit. As described above, the reserved bit of the MPDU delimiter defined in the legacy wireless LAN (e.g., IEEE 802.11ac) standard may be used as the predetermined bit. 
     In this manner, when the specific MPDU in the A-MPDU  14  includes information indicating the change of the MCS, the changed MCS is applied to the corresponding MPDU and the subsequent MPDUs. According to an embodiment of the present invention, when the MPDU delimiter information of the first MPDU of the second MPDU group indicates the change of the MCS, a predetermined MCS, for example, binary shift keying (BPSK) modulation may be applied to the second MPDU group. According to another embodiment, an MCS having an MCS index difference of a predetermined offset with an MCS applied to the first MPDU group may be applied to the second MPDU group. In this case, the MCS applied to the second MPDU group may have an MCS index lower than the MCS applied to the first MPDU group. For example, if the predetermined offset is 3 and the MCS of index  5  is applied to the first MPDU group, the MCS of index  2  may be applied to the second MPDU group. 
     Meanwhile, the A-MPDU  14  includes the first MPDU group and the second MPDU group in the above-described embodiment, but the present invention is not limited thereto, and the A-MPDU  14  may include a larger number of MPDU groups. In this case, the first MPDU of each MPDU group of the A-MPDU  14  may indicate information indicating the change of the MCS through the MPDU delimiter or the like. The MCSs applied to each MPDU group may be set to have sequentially lower MCS index values. However, the present invention is not limited thereto, and the MCS of an MCS index value higher than that of the previous MPDU group may be applied to a particular MPDU group. 
     In this manner, the transmitting terminal may change the MCS of the entire MPDUs or some MPDUs of the A-MPDU  14  and transmit the A-MPDU  14  according to various embodiments described above. If a second A-MPDU to which an MCS different from the previously transmitted first A-MPDU is transmitted, the transmitting terminal may perform the adjusting the channel access method by placing a higher weight on the block ACK information (e.g., the block ACK bitmap) corresponding to the second A-MPDU. 
       FIG.  10    to  FIG.  12    illustrate various embodiments of the block ACK corresponding to the A-MPDU. When data is transmitted in a wide frequency bandwidth, a high MCS, and a multi-spatial stream, a plurality of MPDUs can be aggregated into one A-MPDU. However, in the case of the block ACK used in the legacy WLAN standard, information that can be represented through the block ACK bitmap is only 64 bits. Therefore, when the A-MPDU is composed by aggregating more than 64 MPDUs, it is necessary to provide a new structure of the block ACK indicating whether or not each MPDU is successfully received. 
       FIG.  10    illustrates an embodiment of a block ACK frame structure corresponding to an A-MPDU. According to an embodiment of the present invention, the block ACK bitmap field of the block ACK  20   a  may be set to a size of 8 bytes by default. The transmitting terminal may transmit an A-MPDU aggregated with multiple of 64 MPDUs. When the block ACK  20   a  corresponding to the A-MPDU is transmitted, the receiving terminal indicates success or failure of reception of at least one MPDU by each bit of the block ACK bitmap. More specifically, when the number of MPDUs aggregated in the A-MPDU is n times  64 , each bit of the block ACK bitmap of the block ACK  20   a  indicates success or failure of reception of n MPDUs. If all of the n MPDUs corresponding to a particular bit of the block ACK bitmap have been successfully received, the corresponding bit may be set to one. However, if at least one of the n MPDUs corresponding to a particular bit of the block ACK bitmap is not received, the corresponding bit may be set to zero. When a value of a particular bit of the block ACK bitmap is set to 0, the transmitting terminal retransmits n MPDUs corresponding to the particular bit. 
     For example, if the transmitting terminal transmits 128 MPDUs (n=2), the receiving terminal may successfully receive the 1st to 127th MPDUs and fail to receive the 128th MPDU. In this case, the receiving terminal may transmit the block ACK  20   a  in which the 1st to 63rd bits of the block ACK bitmap field are set to 1 and the 64th bit of the block ACK bitmap field is set to 0. The 64th bit of the block ACK bitmap field corresponds to the 127th MPDU and the 128th MPDU of the A-MPDU transmitted by the transmitting terminal. The transmitting terminal retransmits the 127th MPDU and the 128th MPDU in response to the block ACK  20   a  of the receiving terminal. The receiving terminal receiving the 127th MPDU and the 128th MPDU retransmitted may selectively process only the 128th MPDU that was not received in the previous step. 
       FIG.  11    illustrates another embodiment of a block ACK frame structure corresponding to an A-MPDU. Referring to  FIG.  11   , the block ACK  20   b  may include a bitmap set  25  composed of a plurality of block ACK bitmap fields. Each block ACK bitmap field constituting the bitmap set  25  is set to a size of M bytes, and the number K of block ACK bitmap fields can be indicated by a block ACK control (BA Control) field. Therefore, the bitmap set  25  of the block ACK  20   b  is composed of a total of M*K*8 bits, and may indicate information on the success or failure of reception of the number of MPDUs corresponding to the bits. 
       FIG.  12    illustrates yet another embodiment of a block ACK frame structure corresponding to an A-MPDU. Referring to  FIG.  12   , the block ACK  20   c  may include a plurality of bitmap sets  25 - 1 ,  25 - 2 , and  25 - 3 . Each bitmap set  25 - 1 ,  25 - 2 , and  25 - 3  is composed of a plurality of block ACK bitmap fields as described above with reference to  FIG.  11   . Each block ACK bitmap field constituting the bitmap sets  25 - 1 ,  25 - 2  and  25 - 3  is set to a size of N bytes, and the number of block ACK bitmap fields constituting one bitmap set may be set differently for each of the bitmap sets  25 - 1 ,  25 - 2 , and  25 - 3 . Information on the number (e.g., J, K, L) of bitmap fields for each bitmap set  25 - 1 ,  25 - 2 , and  25 - 3  may be indicated by a block ACK control (BA Control) field. 
     According to an embodiment, each bitmap set  25 - 1 ,  25 - 2 , and  25 - 3  may include a separate starting sequence control field, thereby transmitting ACK information for a plurality of MPDUs having sequence numbers of various and discontinuous combinations. In the embodiment of  FIG.  12   , the first bitmap set  25 - 1 , the second bitmap set  25 - 2 , and the third bitmap set  25 - 3  included in the block ACK  20   c  are composed of J, K and L block ACK bitmap fields, respectively. Therefore, the plurality of bitmap sets  25 - 1 ,  25 - 2 , and  25 - 3  of the block ACK  20   c  are composed of a total of (J*N+K*N+L*N)*8 bits, and it is possible to indicate information on the success or failure of reception of the number of MPDUs corresponding to the bits. As described above, the terminal of the present invention may transmit a response to a plurality of MPDUs using an extended type of block ACK frame according to the embodiment of  FIGS.  10  to  12   . 
       FIG.  13    illustrates an embodiment for expressing a length of an extended A-MPDU using L-SIG of a preamble. As described in the embodiment of  FIG.  8   , the duration information of the A-MPDU can be obtained through a combination of the L-Rate field and the L-Length field of the L-SIG. However, in the legacy WLAN standard, the minimum value of the OFDM-based L-Rate is 6 Mbps, and the maximum value of the duration that L-Length can represent is 1366 symbols. Assuming that one symbol is 4 us, the maximum value of A-MPDU duration that can be expressed through L-SIG is limited to 5.464 ms. 
     According to an embodiment of the present invention, the transmitting terminal may set the value of the L-Rate field of the corresponding L-SIG to 1 Mbps or 2 Mbps lower than 6 Mbps, which is the lowest rate allowed in the OFDM frame, when the A-MPDU is transmitted. Accordingly, the transmitting terminal can transmit the A-MPDU with a maximum duration of 6 times or 3 times longer by combining the existing L-Length field and the new L-Rate. 
     According to another embodiment of the present invention, the transmitting terminal may combine the reserved field of the L-SIG with the L-length to indicate the length information of the packet. In this case, the transmitting terminal can indicate a duration of a value higher than a maximum of 1366 symbols through the L-SIG. According to an embodiment, the transmitting terminal may set the value of the L-Length field to a maximum value, and scale the L-Length value by 2 or 1 times by setting the value of the reserved field to 1 or 0. Since the legacy terminals do not use the information of the reserved field, it is possible to obtain the duration information of the packet only by using the information of the L-Length field, and the increase of the packet duration through the reserved field can be interpreted only by the non-legacy terminals. 
     Although the present invention is described by using the wireless LAN communication as an example, the present invention is not limited thereto and the present invention may be similarly applied even to other communication systems such as cellular communication, and the like. Further, the method, the apparatus, and the system of the present invention are described in association with the specific embodiments, but some or all of the components and operations of the present invention may be implemented by using a computer system having universal hardware architecture. 
     The detailed described embodiments of the present invention may be implemented by various means. For example, the embodiments of the present invention may be implemented by a hardware, a firmware, a software, or a combination thereof. 
     In case of the hardware implementation, the method according to the embodiments of the present invention may be implemented by one or more of Application Specific Integrated Circuits (ASICSs), Digital Signal Processors (DSPs), Digital Signal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), Field Programmable Gate Arrays (FPGAs), processors, controllers, micro-controllers, micro-processors, and the like. 
     In case of the firmware implementation or the software implementation, the method according to the embodiments of the present invention may be implemented by a module, a procedure, a function, or the like which performs the operations described above. Software codes may be stored in a memory and operated by a processor. The processor may be equipped with the memory internally or externally and the memory may exchange data with the processor by various publicly known means. 
     The description of the present invention is used for exemplification and those skilled in the art will be able to understand that the present invention can be easily modified to other detailed forms without changing the technical idea or an essential feature thereof. Thus, it is to be appreciated that the embodiments described above are intended to be illustrative in every sense, and not restrictive. For example, each component described as a single type may be implemented to be distributed and similarly, components described to be distributed may also be implemented in an associated form. 
     The scope of the present invention is represented by the claims to be described below rather than the detailed description, and it is to be interpreted that the meaning and scope of the claims and all the changes or modified forms derived from the equivalents thereof come within the scope of the present invention. 
     MODE FOR INVENTION 
     As above, related features have been described in the best mode. 
     INDUSTRIAL APPLICABILITY 
     Various exemplary embodiments of the present invention have been described with reference to an IEEE 802.11 system, but the present invention is not limited thereto and the present invention can be applied to various types of mobile communication apparatus, mobile communication system, and the like.