Patent Publication Number: US-2022217780-A1

Title: Wireless communication method for simultaneous data transmission, and wireless communication terminal using same

Description:
TECHNICAL FIELD 
     The present invention relates to a wireless communication method for simultaneous data transmission and a wireless communication terminal using the same, and more particularly, to a wireless communication method in which a plurality of terminals simultaneously transmit data for improving a data throughput in a high density environment and a wireless communication terminal 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 802.1 lad has been provided. The IEEE 802.1 lad 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 
     The present invention provides wireless LAN communication with high-efficiency/high-performance in a high-density environment. 
     The present invention also provides a method in which a plurality of terminals efficiently perform simultaneous data transmission. 
     The present invention also enables a plurality of terminals to perform distributed data transmission using multiple channels. 
     Technical Solution 
     In order to solve the above-described technical problems, the present invention provides the following wireless communication method and wireless communication terminal. 
     The present invention provides a wireless communication method for a terminal including: transmitting a trigger frame indicating simultaneous uplink data transmission of multi-users; receiving uplink data transmitted by a plurality of terminals in response to the trigger frame; and transmitting a block acknowledgement for the plurality of terminals having transmitted the uplink data. 
     In addition, the present invention provides a wireless communication terminal including: a transceiver for transmitting/receiving a wireless signal; and a processor for controlling an operation of the terminal, wherein the processor transmits a trigger frame for indicating simultaneous uplink data transmission of multi-users, receives uplink data transmitted by a plurality of terminals in response to the trigger frame, and transmits a block acknowledgement for the plurality of terminals having transmitted the uplink data. 
     At this point, the uplink data may be received through channels respectively allocated to the plurality of terminals by orthogonal frequency-division multiple access. 
     In addition, the trigger frame may indicate an uplink data transmission time of the plurality of terminals. 
     According to an embodiment, the trigger frame may indicate at least one of a designated access period and a random access period of the plurality of terminals. 
     At this point, in the designated access period, the uplink data of at least one terminal designated by the trigger frame may be received through a designated channel, and in the random access period, uplink data of at least one terminal having the uplink data to be transmitted may be received through a channel allocated to the random access period. 
     In addition, the designated access period and the random access period may be classified and allocated to different channel groups. 
     In addition, the designated access period and the random access period may be classified and allocated to different transmission times. 
     According to an embodiment of the present invention, the block acknowledgement may have a frame format of a Multi-TID block acknowledgement and include an identifier representing that the corresponding frame is a block acknowledgement for the plurality of terminals. 
     In addition, the block acknowledgement may include identifier information or partial identifier information for the plurality of terminals that are reception targets of the corresponding frame. 
     Next, the present invention provides a wireless communication method for a terminal including: receiving, from an AP, a trigger frame indicating simultaneous uplink data transmission of multi-users; transmitting uplink data to the AP in response to the trigger frame; receiving, from the AP, a block acknowledgement for the plurality of terminals in response to the uplink data transmission, wherein the uplink data transmission is simultaneously performed by the plurality of terminals through a channel allocated by orthogonal frequency-division multiple access. 
     In addition, the present invention provides a wireless communication terminal including: a transceiver for transmitting/receiving a wireless signal; and a processor for controlling an operation of the terminal, wherein the processor receives, from an AP, a trigger frame for indicating simultaneous uplink data transmission of multi-users, transmits uplink data to the AP in response to the trigger frame, and receives, from the AP, a block acknowledgement for a plurality of terminals in response to the uplink data transmission. 
     At this point, the uplink data transmission of the plurality of terminals may be performed at a predetermined time in response to the reception of the trigger frame. 
     In addition, each of the plurality of terminals may be allocated with at least one of a plurality of slots divided into at least one of a transmission time and a transmission channel, and transmit the uplink data using the at least one allocated slot. 
     In addition, the trigger frame may indicate at least one of a designated access period and a random access period of the plurality of terminals. 
     At this point, in the designated access period, at least one terminal designated by the trigger frame may transmit uplink data through a designated channel, and in the random access period, at least one terminal having uplink data to be transmitted may transmit uplink data through a channel allocated to the random access period. 
     Advantageous Effects 
     According to an embodiment of the present invention, an environment in which a plurality of terminals can simultaneously transmit data may be provided to increase the overall resource utilization ratio and improve the performance of a wireless LAN system. 
     In addition, according to an embodiment of the present invention, in the uplink transmission of multi-users in a contention-based channel access system, a block acknowledgement for a plurality of terminals may be used to perform efficient response transmission. 
     In addition, according to an embodiment of the present invention, by implementing a trigger frame and a block acknowledgement frame for simultaneous transmission of uplink data using frames used by existing legacy terminals, compatibility with the legacy terminals may be maintained and system changes may be minimized. 
     In addition, according to an embodiment of the present invention, the spectrum efficiency of an entire network may be increased in an overcrowded OBSS environment. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates a wireless LAN system according to an embodiment of the present invention. 
         FIG. 2  illustrates 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  schematically illustrates a process in which a STA establishes a link with an AP. 
         FIG. 6  illustrates a Carrier Sense Multiple Access (CSMA)/Collision Avoidance (CA) method used in a wireless LAN communication. 
         FIG. 7  illustrates a method for performing a Distributed Coordination Function (DCF) using a Request to Send (RTS) frame and a Clear to Send (CTS) frame. 
         FIGS. 8 and 9  illustrate embodiments in which a plurality of terminals perform simultaneous data transmission. 
         FIG. 10  illustrates a slot structure according to an embodiment of the present invention. 
         FIGS. 11 to 14  illustrate various embodiments of a block acknowledgement for a plurality of terminals according to the present invention. 
         FIGS. 15 and 16  illustrate other embodiments in which a plurality of terminals simultaneously transmit data. 
         FIG. 17  illustrates an embodiment of a trigger frame with which simultaneous data transmissions of multi-users are triggered. 
         FIG. 18  illustrates a simultaneous uplink data transmission process by a plurality of terminals and a block acknowledgement transmission process by an AP according to another embodiment of the present invention. 
     
    
    
     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 Nos. 10-2014-0080251, 10-2014-0085344 and 10-2015-0035000 filed in the Korean Intellectual Property Office and the embodiments and mentioned items described in the respective applications are 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 FTG.  2 , duplicative description of parts, which are the same as or correspond to the embodiment of FTG.  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 STA 7  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 STA 7  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 radio 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 radio 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 radio 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 radio 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 radio 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 radio signal. Meanwhile, when a radio signal is not sensed in the corresponding channel or a radio 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 allocated 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 allocated with new random numbers, respectively to perform the backoff procedure again. According to an embodiment, a random number newly allocated 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 allocated with. 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  is a diagram illustrating a method for performing a distributed coordination function using a request to send (RTS) frame and a clear to send (CTS) frame. 
     The AP and STAs in the BSS contend in order to obtain an authority for transmitting data. When data transmission at the previous step is completed, each terminal having data to be transmitted performs a backoff procedure while decreasing a backoff counter (alternatively, a backoff timer) of a random number allocated to each terminal after an AFIS time. A transmitting terminal in which the backoff counter is expired transmits the request to send (RTS) frame to notify that corresponding terminal has data to transmit. According to an exemplary embodiment of  FIG. 7 , STA 1  which holds a lead in contention with minimum backoff may transmit the RTS frame after the backoff counter is expired. The RTS frame includes information on a receiver address, a transmitter address, and duration. A receiving terminal (i.e., the AP in  FIG. 7 ) that receives the RTS frame transmits the clear to send (CTS) frame after waiting for a short IFS (SIFS) time to notify that the data transmission is available to the transmitting terminal STA 1 . The CTS frame includes the information on a receiver address and duration. In this case, the receiver address of the CTS frame may be set identically to a transmitter address of the RTS frame corresponding thereto, that is, an address of the transmitting terminal STA 1 . 
     The transmitting terminal STA 1  that receives the CTS frame transmits the data after a SIFS time. When the data transmission is completed, the receiving terminal AP transmits an acknowledgment (ACK) frame after a STFS time to notify that the data transmission is completed. When the transmitting terminal receives the ACK frame within a predetermined time, the transmitting terminal regards that the data transmission is successful. However, when the transmitting terminal does not receive the ACK frame within the predetermined time, the transmitting terminal regards that the data transmission is failed. Meanwhile, adjacent terminals that receive at least one of the RTS frame and the CTS frame in the course of the transmission procedure set a network allocation vector (NAV) and do not perform data transmission until the set NAV is terminated. In this case, the NAV of each terminal may be set based on a duration field of the received RTS frame or CTS frame. 
     In the course of the aforementioned data transmission procedure, when the RTS frame or CTS frame of the terminals is not normally transferred to a target terminal (i.e., a terminal of the receiver address) due to a situation such as interference or a collision, a subsequent process is suspended. The transmitting terminal STA 1  that transmitted the RTS frame regards that the data transmission is unavailable and participates in a next contention by being allocated with a new random number. In this case, the newly allocated random number may be determined within a range (2*CW) twice larger than a previous predetermined random number range (a contention window, CW). 
     In the exemplary embodiments given below, a case where a first terminal transmits an RTS frame to a second terminal may be construed as a meaning that the first terminal transmits the RTS frame in which a transmitter address is an address of the first terminal and a receiver address is an address of the second terminal unless otherwise stated. Further, a case where the first terminal transmits a CTS frame to the second terminal may be construed as a meaning that the first terminal transmits the CTS frame in which a receiver address is an address of the second terminal unless otherwise stated. 
     On the other hand, the aforementioned contention-based data transmission methods can operate well in an environment with less users, but the communication performance is drastically deteriorated in an environment where there are many users to transmit packets. Therefore, a method in which a plurality of terminals efficiently transmit and receive data in a dense user environment is required. Hereinafter, methods for transmitting and receiving data according to embodiments of the present invention will be described with reference to each drawing. In the embodiments of the drawings, duplicative description of parts which are the same as or correspond to the embodiments of the previous drawings will be omitted. 
       FIGS. 8 and 9  illustrate embodiments in which a plurality of terminals perform simultaneous data transmission. According to the embodiments of the  FIGS. 8 and 9 , a plurality of terminals may perform simultaneous data transmission for efficient data transmission in a dense user environment. At this point, the plurality of terminals in a BSS are grouped into at least one group and data transmissions of terminals are performed in units of an allocated group. 
     First, referring to  FIG. 8 , an AP transmits a trigger frame indicating simultaneous transmission of uplink data (UL_Data) by multi-users (S 802 ). According to the embodiment of the present invention, a resource for uplink data transmission to the AP is divided into a plurality of slots and simultaneous uplink data transmission may be performed for each divided slot. Here, the resource includes at least one of a transmission time and a transmission channel. The number of slots for data transmission may be determined according to various embodiments. For example, a predetermined number of slots may be used or the AP may determine the number of slots and deliver the corresponding slot number information to each terminal. In addition, according to an embodiment of the present invention to be described later, when the AP performs simultaneous data transmission and reception, the number of slots may be determined based on a duration of down link data (DL_Data) of the AP or based on a ratio between the duration of the downlink data (DL_Data) and a duration of uplink data (UL_Data) of the terminals. In the embodiment of the present invention, the duration of data represents the time necessary for transmitting the corresponding data. The duration of data may also be referred to as a length of data, a length of data frame, or a length of data packet, etc., according to an embodiment. 
     According to the embodiment of the present invention of  FIG. 8 , a modified RTS (hereinafter, RTS′) frame may be used as the trigger frame. According to an embodiment of the present invention, the RTS&#39; frame may have a format of a basic RTS frame and indicate uplink data transmission of multi-users through predetermined bits of a preamble. At this point, information for indicating the uplink data transmission of the multi-users may be included in a non-legacy preamble decodable by a non-legacy terminal (e.g. 802.1 lax terminal) and may be expressed through reserved bits in a VHT-SIG-A field. In addition, the RTS&#39; may also indicate the uplink data transmission of the multi-users through a reserved bit of a MAC header. Moreover, the RTS&#39; may further include additional information such as the number of slots used in the uplink data transmission of the multi-users. 
     The AP may perform a separate backoff procedure for transmitting the RTS′. When a backoff counter expires (S 801 ), the AP transmits the RTS&#39; as a trigger frame. A plurality of terminals receiving the RTS&#39; frame may transmit uplink data corresponding thereto and a time point of the uplink data transmission of each terminal may be determined based on the RTS&#39; frame. On the other hand, according to an additional embodiment of the present invention, the RTS&#39; may indicate that downlink data transmission of the AP may be performed together with the uplink data transmission of the plurality of terminals. In other words, the AP may transmit downlink data to a target STA while simultaneously receiving uplink data from the plurality of terminals. Further, the AP may trigger such simultaneous transmission and reception of data by transmitting the RTS′. At this point, the target STA having received the RTS′ transmits a CTS after a SIFS time (S 803 ). 
     When the simultaneous data transmission is triggered by the RTS′, the plurality of terminals transmit uplink data to the AP in response to the RTS′ (S 805 ). The uplink data transmissions by the plurality of terminals are performed in a plurality of allocated slots Slot 1 to Slot 5 and are started at a transmission time point designated by the RTS′. At this point, terminals in the BSS having uplink data to be transmitted, namely, terminals of a transmission candidate group (TX Candidate Group) attempt to transmit the uplink data through slots allocated to corresponding terminals. In an embodiment of the present invention, the plurality of slots Slot 1 to Slot 5 may be divided by a time unit or a channel unit. In addition, the plurality of slots Slot 1 to Slot 5 may be divided by a subchannel unit. In other words, in the embodiment of the present invention, a slot may indicate at least one of a specific time slot and a specific channel (or a subchannel). When the plurality of slots are divided by the time unit, the plurality of terminals may sequentially transmit uplink data using a slot allocated to each terminal. In addition, when the plurality of slots are divided by the channel or subchannel unit, the plurality of terminals may transmit the uplink data through a channel or subchannel allocated to each terminal by using Orthogonal Frequency Domain Multiple Access (OFDMA). The AP receives the uplink data transmitted by the plurality of terminals. 
     According to an embodiment, each terminal may determine a slot to be used by the corresponding terminal and may transmit the uplink data through the determined slot. At this point, a slot number to be used by the terminal may be determined based on information included in the trigger frame, namely, the RTS′. For example, the terminal may obtain the information on the number of slots used for the uplink data transmission and determine the slot number based on a result value of modulo-calculating an identifier of the corresponding terminal with the number of slots. Alternatively, the terminal may obtain the slot number based on hashing operation using the identifier of the corresponding terminal and the number of slots. At this point, as the identifier of the terminal, a MAC address or an association identification (AID) of the corresponding terminal, etc., may be used. According to another embodiment, a slot through which each terminal transmits the uplink data may be allocated by the AP and the terminal may also receive the allocated slot information from the AP. The slot allocation process may be individually performed for each terminal in the BSS, and thus a plurality of terminals may be allocated to one slot. 
     On the other hand, according to an additional embodiment of the present invention, whether to transmit the uplink data by each terminal may be determined based on reception power of the CTS transmitted corresponding to the RTS′. When the reception power of the CTS is equal to or greater than a predetermined threshold value, the terminal may be considered to be in an interference relationship with the target STA, which has transmitted the CTS, and may not participate in the uplink data transmission. At this point, the corresponding terminal sets a network allocation vector (NAV) based on a duration field value of the CTS and/or RTS′ and does not perform data transmission until the set NAV is expired. However, when the reception power of the CTS is smaller than the predetermined threshold value or the CTS is not received, the terminal attempts to transmit the uplink data in a slot allocated to the corresponding terminal. 
     When the uplink data reception from the plurality of terminals is completed, the AP transmits an acknowledgement (ACK) in response to each uplink data. 
     According to an embodiment of the present invention, the AP may transmit a block acknowledgement for the plurality of terminals (Multi-STA Block ACK, BA′) in response to the uplink data reception from the plurality of terminals (S 807 ). In other words, the AP may transmit the ACK(s) for the plurality of terminals by using one BA′. According to an embodiment of the present invention, the BA′ may have a frame format of a basic block ACK (BA) or a Multi-traffic ID (TID) BA. Furthermore, the BA′ may include identifier information on the plurality of terminals to indicate the ACK for the uplink data transmitted by each terminal. A detailed description thereof will be described later. On the other hand, according to the above-described embodiment, when simultaneous data transmission and reception of the AP is triggered by the RTS′, the target STA having received downlink data from the AP also transmits an ACK (S 806 ). According to an embodiment, the ACK transmission of the target STA and BA′ transmission of the AP may be performed at the same time. 
       FIG. 9  illustrates a more detailed embodiment in which the uplink data of the plurality of terminals is transmitted. In the embodiment of  FIG. 9 , duplicative descriptions of parts which are the same as or corresponding to the embodiment of  FIG. 8  will be omitted. 
     When a simultaneous uplink data transmission is triggered by the RTS′, each terminal in the BSS transmits uplink data through an allocated slot. As described above, the slot may represent a specific channel (or a subchannel) or a specific transmission time. In the embodiment of  FIG. 9 , an interval for the uplink data transmission is composed of total 5 slots Slot 1 to Slot 5, and each terminal in the BSS is allocated with at least one of the 5 slots. Terminals of a TX Candidate Group having uplink data to be transmitted perform the uplink data transmission through the slot allocated to the corresponding terminal. According to an embodiment of the present invention, a plurality of terminals may be allocated to each slot. Accordingly, when the plurality of terminals simultaneously perform the uplink data transmission in the same slot, a collision may occur. 
     Referring to  FIG. 9 , in Slot 1, STA 1 - a  and STA 1 - e  simultaneously transmit uplink data and thereby a collision occurs. However, in Slot 2, only STA 2 - c  transmits uplink data and the AP may normally receive the corresponding data. Similarly, in Slot 3, two or more terminals simultaneously perform uplink data transmission and thereby a collision occurs. In Slots 4 and 5, one terminal transmits uplink data and the AP normally receives the corresponding data, respectively. In response to reception of the uplink data from a plurality of terminals, the AP transmits a block acknowledgement (BA′) for the plurality of terminals according to the above-described embodiment. At this point, the block acknowledgement (BA′) may include acknowledgement information for the normally received uplink data in Slots 2, 4 and 5. 
       FIG. 10  illustrates a slot structure according to an embodiment of the present invention. When a plurality of wireless terminals attempt to access through consecutive slots, data in adjacent slots may overlap due to a transmission delay, or the like. According to an embodiment of the present invention, in order to prevent such data overlap, a guard interval may be set between each slot as shadowed in  FIGS. 10( a ) to 10( c ) . Accordingly, a slot duration including the guard interval may be determined to be longer than a duration of uplink data of a terminal. 
     The length of the guard interval may be determined based on a maximum transmission delay value of the terminal. In other words, as illustrated in  FIG. 10( b ) , the minimum length of the guard interval may be set to be a maximum transmission delay value allowed by a corresponding BSS. The maximum transmission delay value represents a prediction value of delay time taken when a terminal located farthest in a service area of the AP transmits data. On the other hand, referring to  FIG. 10( c ) , the maximum length of the guard interval may be set to be equal to or shorter than an AIFS or DIFS time, which is a waiting time when terminals in the BSS attempt data transmission. Accordingly, it is possible to prevent a legacy terminal or the like from transmitting data during the guard interval in an idle state. 
     On the other hand, although the guard interval is illustrated as being configured with a predetermined time in  FIG. 10 , the present invention is not limited thereto. In other words, when slots are divided by a channel unit or a subchannel unit according to an embodiment of the present invention, the guard interval may be configured with a predetermined number of subcarriers. 
       FIGS. 11 to 14  illustrate various embodiments of a block acknowledgement for a plurality of terminals according to the present invention. In respective embodiments of  FIGS. 11 to 14 , duplicative descriptions of parts which are the same as or corresponding to the embodiments of previous drawings will be omitted. 
       FIG. 11  illustrates a frame structure of a block acknowledgement BA′ according to an embodiment of the present invention. Referring to  FIG. 11 , the BA′ has a frame format of a Multi-TID BA and includes acknowledgement information for a plurality of terminals. In other words, a BA′ frame includes a duration field, a receiver address (RA) field, a transmitter address (TA) field, a block acknowledgement control (BA Control) field and a block acknowledgement information (BA Information) field, and may represent information of block acknowledgement for the plurality of terminals (Multi-STA Block ACK) through at least one of the above fields. According to an embodiment, the BA′ may indicate, through a separate identifier, that the corresponding frame is a Multi-STA BA. In addition, according to an embodiment of the present invention, the acknowledgement information for the plurality of terminals may be represented through the BA information field. The BA information field may include identifier information or partial identifier information for reception target terminals of the BA′. At this point, a MAC address, an AID or the like of the corresponding terminal may be used as the identifier of the terminal and the partial identifier represents a part of the identifier. 
     According to a more detailed embodiment, the block acknowledgement information field may include a Starting Slot Number field and a Slotmap field. First, the Starting Slot Number field may indicate a smallest slot number in which data is normally received. Referring to the embodiment of  FIG. 9 , since data is normally received in slots Slot 2, Slot 4 and Slot 5, the Starting Slot number field indicates the number of Slot 2, namely, 010 that is the smallest number among these slots. Next, each bit of the Slotmap field may indicate a data reception state of each slot subsequent to the starting slot. At this point, a bit value 1 indicates a normal reception state, and a bit value 0 indicates a state where data is not received. Referring to the embodiment of  FIG. 9 , since data is normally received in Slot 2, Slot 4, and Slot 5 and data is not received in Slot 3, the Slotmap field represents  1011 . On the other hand, the Starting Slot number field and the Slotmap field illustrated in  FIG. 11  indicate an embodiment of a method for configuring the BA information field, but the present invention is not limited thereto. For example, the BA information field may represent, instead of the foregoing information on the slots, an identifier of each terminal in which data transmission is normally performed. At this point, the identifier information of each terminal may be represented in a bitmap format. 
     According to an additional embodiment of the present invention, a receiver address (RA) of the BA′ frame may be set as an address designated for the Multi-STA Block ACK. For example, the receiver address (RA) may be set as a multicast address targeting a plurality of terminals in the BSS. At this point, the first bit of the multicast address may be set to 1. 
       FIGS. 12 to 14  illustrate more detailed embodiments of a BA′ frame structure. First, referring to  FIG. 12 , the BA′ includes a block acknowledgement control (BA Control) field and a block acknowledgement information (BA information) field and may represent block acknowledgement information for a plurality of terminals through at least one of the fields. The BA information field is set to have a variable length and may include a Per TID information field, a Block ACK Starting Sequence Control field, and a Block ACK Bitmap field. The Per TID information field includes a Reserved bit field B 0  to B 11  and a TID Value field B 12  to B 15 . 
     According to an embodiment of the present invention, acknowledgement information for the plurality of terminals may be represented by using the Reserved bit field of the Per TID information field. In more detail, the Reserved bit field includes AID information of a reception target terminal of the BA′ and flag information for indicating Multi-STA BA. For example, the Reserved bit field may be composed of 12 bits B 0  to B 11 . Among these, a specific bit, for example B 11 , may indicate whether the corresponding frame is the Multi-STA BA frame according to the embodiment of the present invention or a typical ACK frame. In addition, remaining bits of the Reserved bit field, for example, 11 bits of B 0  to B 11  may represent AID information of the reception target terminal of the corresponding frame BA′. 
     The BA information field having the above-described configuration may be repeated for each Traffic ID (TID). Since the BA information field has a variable length, AIDs for all terminals participating in multi-user uplink transmission may be inserted into the BA information field through the Reserved bit field. On the other hand, when flag information B 11  of the Reserved bit field indicates a typical ACK frame, the BA Starting Sequence Control field and the BA Bitmap field may be omitted from the BA information field. 
     Next, according to an embodiment of  FIG. 13 , information for indicating Multi-STA BA may be included in the BA Control field. In more detail, the BA Control field includes a Multi-TID field B 1 , a Compressed Bitmap field B 2  and a Reserved bit field B 3  to B 11 , and may indicate whether the corresponding frame is Multi-STA BA through at least one of the fields. For example, a specific bit among the Reserved bit field B 3  to B 11  may be used as a bit for indicating the Multi-STA BA. According to another embodiment, the BA Control field may indicate the Multi-STA BA with a combination of the Multi-TIM field B 1  and the Compressed Bitmap field B 2 . Typically, since the Compressed Bitmap field B 2  is not activated either when the Multi-TIM field B 1  is not activated, the Multi-STA BA may be indicated using an exceptional combination in which B 1  is assigned to 0 and B 2  is assigned to 1. 
       FIG. 14  illustrates an additional embodiment using the BA information field of  FIG. 12 . As described above, the Reserved bit field B 0  to B 11  of the Per TID information field may include AID information on a reception target terminal of the BA′ and information indicating the Multi-STA BA. When the channel condition is good, since most of data transmissions are normally performed, it is highly possible that data of all terminals participating in the multi-user uplink transmission is successfully transmitted. At this point, the Reserved bit field of the Per TID information field may represent respective AIDs for all participating terminals. But alternatively, the Reserved hit field may represent positive ACK for all the terminals (All ACK) at once. According to an embodiment of the present invention, a preset value (for example, B 0  to B 10 : 111111111111) other than the values between 1 and 2007 allocated to the AID may be used as information for indicating the positive ACK for all the terminals (All ACK). Similarly, other than the values allocated to the AID, another preset value (for example, B 0  to B 10 : 00000000000) may be used as information indicating negative ACK for all the terminals (All NACK). When the Reserved bit field of the Per TID information field is set as the ALL ACK or ALL NACK, a field for indicating the AID of an individual terminal may be omitted. 
       FIGS. 15 and 16  illustrate other embodiments in which a plurality of terminals simultaneously transmit data. According to the embodiments of  FIGS. 15 and 16 , an uplink data transmission period of a plurality of terminals may include at least one of a dedicated Access (DA) period and a random Access (RA) period. To this end, according to an embodiment of the present invention, a modified Traffic Indicator Map (TIM) may be used as a trigger frame for indicating simultaneous uplink data transmission of multi-users. At this point, as illustrated in  FIG. 15 , the modified TIM (hereinafter, TIM′) may be transmitted SIFS time after the AP has received the CTS, or alternatively, as illustrated in  FIG. 16 , right after the AP has transmitted RTS′. However, the method of transmitting TIM′ in the present invention is not limited to those illustrated in  FIGS. 15 and 16 . 
     According to an embodiment of the present invention, the trigger frame may indicate at least one of a DA period and an RA period. The DA period consists of at least one DA slot, and similarly, the RA period consists of at least one RA slot. In the DA slot, a terminal designated to the corresponding slot transmits uplink data. Furthermore, in the RA slot, a plurality of terminals in the BSS contend to transmit uplink data without a separately designated terminal. At this point, information about the terminal designated to the DA slot may be indicated by the trigger frame. When one terminal is designated to one DA slot, non-contention based uplink data transmission may be performed. The AP may indicate at least one of the DA period and RA period through a trigger frame to determine an uplink data transmission scheme for the plurality of terminals. According to an embodiment of the present invention, the trigger frame may represent AID information for each terminal allocated to the DA period. When the trigger frame indicates the RA period, a preset value (for example, AID=0) other than the values between 1 and 2007 allocated to the AID of a terminal may be designated as a value of an AID field included in the trigger frame. 
     In an embodiment of the present invention, the DA period and the RA period may be classified into different transmission times or different channel groups. Similarly, the DA slot and the RA slot respectively constituting the DA period and the RA period may also be classified by a time unit or by a channel (or subchannel) unit. At this point, the DA/RA period and the DA/RA slot constituting the corresponding period may be set to an identical domain, or alternatively set to different domains. For example, when the DA slot and the RA slot are respectively allocated to specific channels (or subchannels), the DA period may be set to a channel group including at least one DA slot (i.e., a DA channel) and the RA period may be set to a channel group including at least one RA slot (i.e., an RA channel) (identical domain). However, according to another embodiment, the DA period and the RA period may be set to different transmission times. In this case, DA slots and RA slots within each DA period and RA period in a time domain may be allocated by a channel (or subchannel) unit (different domains). 
     The numbers of the DA slots and the RA slots may be set to fixed values, or alternatively set to variable values. When the numbers of the DA slots and the RA slots are set to variable values, the AP may transmit, through the trigger frame, at least one of information about the number of DA slots and information about the number of RA slots. When the total number of transmission slots (TX slots) including the DA slots and the RA slots is fixed, the AP may transmit, through the trigger frame, only one of the information about the number of DA slots and information about the number of RA slots. When the trigger frame only includes the information about the number of DA slots, the number of RA slots may be determined as a value obtained by subtracting the number of DA slots from the total number of transmission slots. When the DA slots and the RA slots are allocated by a channel unit, channels pre-designated among the total transmission channels (TX Slot) may be allocated to the DA channels and RA channels, respectively. According to another embodiment, the AP may determine channels to be allocated to the DA channel and RA channel, and transmit such channel allocation information through the trigger frame. 
     According to an embodiment, when the DA period and the RA period are classified into different transmission times, the DA period may be disposed after the RA period. Thus, in consideration of data round trip times and processing delays, etc., of target terminals to be allocated to each DA slot, each target terminal may secure a time to prepare uplink data transmission through a designated DA slot. However, the disposition of the DA period and the RA period is not limited thereto. 
     When the DA period is established, the AP may determine terminals to be allocated to each DA slot of the DA period and transmit each DA slot allocation information through the trigger frame. The AP may determine a terminal to be allocated to the DA slot according to various embodiments. According to an embodiment, in order to guarantee accesses of terminals expected to transmit an uplink TCP ACK, the AP may allocate the DA slots to the corresponding terminals. The transmission possibility of TCP ACK may be obtained through TCP information of messages exchanged between the AP and the terminals in the BSS. Among TCP sessions delivered through the AP, for a session in which a TCP FIN packet is not transmitted after a TCP SYN packet has been transmitted, the probability that a terminal will transmit a TCP ACK packet is high. Accordingly, the AP may set a terminal of a session satisfying the above condition as a candidate terminal to be allocated to the DA slot. According to an additional embodiment, the AP decodes each TCP packet, and when a terminal having received data and then not having transmitted an ACK is identified, the AP may set the corresponding terminal as a candidate terminal to be allocated to the DA slot. When the number of candidate terminals is equal to or smaller than the predetermined number of DA slots, the AP allocates respective candidate terminals to predetermined DA slots. However, when the number of candidate terminals exceeds the predetermined number of DA slots, the AP selects a part of the candidate terminals to allocate the selected candidate terminals to the predetermined DA slots. 
     In the DA period, terminals designated by the trigger frame transmit uplink data. At this point, the DA slots (DA slots 1, 2 and 3) through which respective terminals will transmit the uplink data may be designated by the trigger frame. Each terminal in the BSS obtains DA slot allocation information in the trigger frame and transmits the uplink data through the allocated DA slot, when the corresponding terminal is allocated to a specific DA slot. The AP receives, through each DA slot (DA slots 1, 2, and 3), the uplink data of at least one terminal designated by the trigger frame in the DA period. On the other hand, in the RA period, at least one terminal having uplink data to be transmitted transmits the uplink data through the RA slots (RA slots 1, 2, and 3) allocated to the RA period. At this point, the RA slot to be used by each terminal may be determined by the individual terminal. According to an embodiment, each terminal may determine the RA slot to be used by the corresponding terminal based on RA slot information allocated to the RA period and an identifier of the corresponding terminal, etc. When both the DA period and the RA period arm used, terminals not allocated to the DA slots may randomly transmit the uplink data in the RA period. In other words, among terminals having uplink data to be transmitted, terminals not allocated with DA slots may randomly select RA slots to transmit the uplink data in the RA period. When an uplink data transmission period including the DA period and RA period is terminated, the AP transmits a block ACK (BA′) for a plurality of terminals having transmitted uplink data through each DA slot and RA slot. 
       FIG. 17  illustrates an embodiment of a trigger frame that triggers simultaneous data transmission of a plurality of terminals. As described above in the embodiments of  FIGS. 15 and 16 , a modified TIM (TIM′) may be used as the trigger frame for indicating simultaneous uplink data transmission of multi-users. 
     Referring to  FIG. 17 , the TIM′ may have an Element ID field value modified from a TIM frame of legacy 802.11 and include a Slot Count field of 1 byte instead of a DTIM Period field and a DTIM Count field. The Element ID field value of the TIM′ may be allocated to one of values remaining in a reserved state among the Element ID list of a legacy 802.11 management frame. In addition, the Slot Count field represents the number of slots to be used for uplink data transmission. According to an embodiment, the Slot Count field may represent the number of DA slots allocated to the DA period. Next, a Bitmap Control field may represent a minimum AID value of terminals in the BSS, which are allocated to the DA slots. A Partial Virtual Bitmap field may represent whether to allocate the DA slot for each subsequent AID starting from the AID value of the Bitmap Control field. At this point, bit value 1 represents a DA slot allocation state and bit value 0 represents a DA slot non-allocation state. Accordingly, the number of 1s in the Partial Virtual Bitmap field may be set to be equal to a value of the Slot Count field. When TIM′ indicates the RA period, the Bitmap Control field may represent a preset value (for example AID=0) other than values between 1 and 2007 allocated to an AID of a terminal. 
       FIG. 18  illustrates a simultaneous uplink data transmission process by a plurality of terminals and a block ACK transmission process by an AP according to another embodiment of the present invention. 
     The process of uplink data transmission of multi-users may be managed by the AP, which operates the BSS. In order to prevent a collision of resource allocation and data transmission, the AP may obtain information such as the presence of uplink data in a buffer and Transmission Opportunities (TXOP) of each terminal in the BSS, before the simultaneous uplink data transmission is started. In addition, the AP is required to transmit uplink data transmission timing information to each terminal. To this end, the multi-user uplink data transmission includes a trigger process and a scheduling process. A trigger of the multi-user uplink data transmission may be performed by trigger frame transmission of the AP according to the above-described embodiment. In addition, as described above, the scheduling may include an allocation of the DA period and/or the RA period, an allocation of individual slots in each period, and an allocation of a terminal for each DA slot, etc. According to an embodiment, the AP may perform the trigger after collecting information for scheduling of the multi-user uplink data transmission in advance. Alternatively, the AP may perform the trigger first and then collect the scheduling information thereafter. 
     According to an embodiment of the present invention, the trigger process and the scheduling process may include a process in which the AP and the terminals in the BSS exchange channel information available for the data transmission. For example, the AP may define available channels and transmit information thereon to each terminal. The terminals may feedback information on channels available to the corresponding terminal among the available channels defined by the AP. The AP may perform the scheduling process for the multi-user uplink data transmission based on the feedback information of each terminal. 
     The trigger process may include a NAV setting for a multi-user uplink data transmission period. As described in the aforementioned embodiment, when the trigger frame is a frame (for example, RTS′) conforming to a legacy format, legacy terminals may set a NAV based on a duration field value included in a MAC header of the trigger frame. On the other hand, since the trigger frame is transmitted only by the AP, terminals distant from the AP may have difficulty in setting the NAV based on the trigger frame. Thus, according to an embodiment of the present invention, the AP may transmit the trigger frame with a power greater than that of a typical frame to prevent a hidden node problem. 
     When the trigger frame is transmitted, each terminal performs uplink data transmission at a transmission time indicated by the trigger frame. At this point, each terminal transmits the uplink data through a channel designated by the AP or through a channel randomly selected in a channel group allocated by the AP. When the uplink data transmissions of all the terminals are completed, the AP transmits a block acknowledgement for the plurality of terminals having transmitted the uplink data. On the other hand, in the multi-user uplink data transmission, an air time of each channel may be different due to various factors. Typically, such an air time of each channel may appear to be longer for a primary channel CH 1  than for subchannels CH 2  to CH 4  as illustrated in  FIG. 18 . 
     Thus, according to an additional embodiment of the present invention, when the air times for each of the channels are different from each other, the AP may multiplex block acknowledgements for all the channels having been used for the multi-user uplink data transmission and transmit the multiplexed block acknowledgment through the primary channel CH 1 . When such a multiplexed group acknowledgment (Multiplexed Group ACK) is used, the subchannels CH 2  to CH 4  may be used by other terminals (for example, other terminals of an OBSS) right after a time when the air time of the uplink data transmission is over. Accordingly, the spectrum efficiency of the entire network may be improved. 
     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.