Patent Publication Number: US-2009225876-A1

Title: Method for transmitting and receiving data in multiple-input multiple-output wireless local area network environment, and a system and apparatus for performing the method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a multiple-input multiple-output (MIMO) wireless local area network (WLAN), and more particularly, to a method for transmitting and receiving data in a MIMO WLAN that can reduce data loss due to collision between stations. 
     2. Discussion of the Background 
     A WLAN is a wireless short-distance communication network. The WLAN can maintain the readiness and extensibility of a wired LAN, but may not require a cabled connection and therefore can prevent cost increases, and can also provide a user with a more convenient network access environment. 
     Currently, various types of portable devices are widely used, and may include a personal digital assistant (PDA), a portable media player (PMP), and a laptop-type personal computer (PC). Also widely used is a desktop PC. Users&#39; interests in the WLAN are rapidly increasing as the demand for network connectivity with such devices continues to increase. 
     The Institute of Electrical and Electronics Engineers (IEEE) 802.11 Wireless Fidelity (Wi-Fi) standard has been associated with WLAN. Since the IEEE 802.11 standard was initially released in 1997, the IEEE 802.11 standard has been the base of various extensions such as 802.11a, 802.11b, and 802.11g. The standardization of IEEE 802.11n is currently in progress in order to support higher WLAN performance. 
     The IEEE 802.11n standard may adopt a system configuration of an MIMO scheme to support a high data transmission rate in a physical layer. In the MIMO scheme, a transmitting end, such as a station, transmits data via a plurality of transmitting antennas, passing through various types of paths, and a receiving end, such as an access point, detects the data in a signal that is received from each path via a plurality of receiving antennas. Through this, the MIMO scheme can improve a data transmission rate and reduce interference in a multi-path environment. 
     Therefore, in an IEEE 802.11n WLAN environment, the station and/or the access point may include a plurality of antennas. Through the above configuration, it is possible to support an improved data transmission rate in the physical layer. However, despite the performance improvement in the physical layer, there are some constraints on improving the entire data transmission throughput due to the limitation of a Media Access Control (MAC) protocol. 
     An IEEE 802.11 MAC protocol adopts a contention-based Distributed Coordinate Function (DCF) scheme for readiness design and configuration, and for sharing of wireless resources. According to the DCF scheme, stations that wish to transmit data to an access point first detect or consider a data transmission state of the WLAN prior to transmitting a data frame. If data transmission by another station is detected, the station wishing to transmit identifies the channel state as busy and postpones the transmission. After a time elapses, where the time may be based on the success or failure of the existing data frame transmission, a new channel contention is performed. If the channel is in an idle state, the station will wait for frame transmission during a backoff time that is randomly selected within a predetermined backoff window range. Through this procedure, many stations can share radio resources of a WLAN. The above-described collision avoidance scheme applied to the DCF scheme refers to a Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) scheme. 
     In the above-described DCF scheme, since the backoff time is randomly selected, more than one station should not transmit a data frame in the same time slot as another station and thus collision may be reduced between the stations. However, if the backoff time of more than one station is terminated in the same time slot and two or more stations start data frame transmission in the same time slot, the collision may occur between the stations. Thus, data transmitted from these stations may be overlappingly received at the access point, and the access point may not decode each station&#39;s individual data frame. Accordingly, in this case, the stations may not receive an acknowledge signal from the access point and each station needs to be reallocated with another backoff time and wait for a corresponding period of time to elapse before retransmitting the data frames. 
     If a small number of stations are covered by a single access point, the collision probability may be relatively low. Therefore, the performance deterioration according to the frame retransmission may be insignificant. However, if many stations are covered by the single access point, the collision probability increases and the data transmission throughput may deteriorate significantly. 
     Accordingly, there is a need for a new technology that can solve the collision problem by applying an MIMO technology in a MAC layer. 
     SUMMARY OF THE INVENTION 
     This invention provides a MAC protocol that may be used with MIMO technology. 
     This invention also provides a method for transmitting data using an MIMO decoding scheme if a collision occurs between a plurality of stations. 
     This invention also provides a method for receiving data using an MIMO decoding scheme if a collision occurs between a plurality of stations. 
     This invention also provides a system for transmitting and receiving data using an MIMO decoding scheme if a collision occurs between a plurality of stations. 
     This invention also provides an apparatus for transmitting data using an MIMO decoding scheme if a collision occurs between a plurality of stations. 
     This invention also provides an apparatus for receiving data using an MIMO decoding scheme if a collision occurs between a plurality of stations. 
     Additional features of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. 
     The present invention discloses a method for receiving data transmitted from a first station to an access point in a MIMO WLAN system. The method includes receiving a first preamble from the first station via a plurality of receiving antennas, measuring a channel coefficient between the first station and the access point based on the first preamble, detecting a collision between the first station corresponding to the first preamble and a second station corresponding to a second preamble, and constructing a channel vector using the channel coefficient if the collision is detected, and decoding data transmitted from the first station and the second station using the channel vector. 
     The present invention also discloses a method for transmitting data from a first station to an access point in a MIMO WLAN system. The method includes monitoring a system carrier to detect a data transmission state of the system, waiting for data transmission during a backoff time if data transmission from a second station to the access point is detected, and transmitting a data frame to the access point via a plurality of transmitting antennas, the data frame including a preamble associated with the first station, and the preamble including an orthogonal code or a pseudo-noise code. Further, the monitoring, receiving, and transmitting are repeated if an acknowledge signal is not received from the access point within a predetermined period of time. 
     The present invention also discloses a MIMO WLAN system. The system includes an access point including a plurality of receiving antennas, a first station including a plurality of transmitting antennas, the first station to transmit data to the access point, and a second station including a plurality of transmitting antennas, the second station to transmit data to the access point. If data transmission from the second station is detected using a system carrier, the first station is in a standby state during a first backoff time after a short interframe Space (SIFS), an acknowledgement (ACK), a Distributed Coordinate Function (DCF) interframe space (DIFS), or an extended interframe space (EISF) time has elapsed, and thereafter the first station transmits a first data frame that includes a first preamble associated with the first station. If a collision is detected via the first preamble and a second preamble respectively received from the first station and the second station, the access point decodes the first data frame and a second data frame using a channel coefficient. Further, the first data frame and the second data frame are transmitted from the first station and the second station, and the channel coefficient is measured based on the first preamble and the second preamble. 
     The present invention also discloses an access point apparatus for a MIMO WLAN system. The access point apparatus includes a radio signal processing unit to receive a first data signal from a first station, a preamble extractor to extract a first preamble from the first data signal, the first preamble corresponding to the first station, a channel estimator to estimate a channel associated with the first station based on the first preamble, and to obtain a channel estimate, a collision detector to detect a collision between the first station and a second station based on the first preamble and a second preamble in a second data signal from the second station, and a frame detector to collectively or sequentially decode the first data signal based on the obtained channel estimate if the collision is detected, and to detect a first data frame of the first station. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and together with the description serve to explain the principles of the invention. 
         FIG. 1  illustrates a configuration of an MIMO WLAN system according to an exemplary embodiment of the present invention. 
         FIG. 2  is a timing diagram illustrating data transmission according to an exemplary embodiment of the present invention. 
         FIG. 3  is a flowchart illustrating a method for receiving data according to an exemplary embodiment of the present invention. 
         FIG. 4  is a flowchart illustrating a method for transmitting data according to an exemplary embodiment of the present invention. 
         FIG. 5  is a block diagram illustrating an access point according to an exemplary embodiment of the present invention. 
         FIG. 6  is a block diagram illustrating a frame detector of the access point shown in  FIG. 5 . 
         FIG. 7  is a block diagram illustrating a data signal estimator of the frame detector shown in  FIG. 6 . 
         FIG. 8A  and  FIG. 8B  are graphs illustrating the simulation test result in an error-free environment. 
         FIG. 9A  and  FIG. 9B  are graphs illustrating the simulation test result showing the throughput and mean access delay time where constant error exists. 
         FIG. 10A  and  FIG. 10B  are graphs illustrating the simulation test result if an error rate changes. 
     
    
    
     DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS 
     The invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure is thorough, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity. Like reference numerals in the drawings denote like elements. 
     Hereinafter, a method for transmitting and/or receiving data according to exemplary embodiments of the present invention and an MIMO WLAN system and apparatus to perform the method will be described with reference to the accompanying drawings. The method will be described based on a data transmitting/receiving protocol of a Distributed Coordinate Function (DCF) scheme as defined in an IEEE 802.11 standard. Accordingly, aspects not described herein may be interpreted as being the same or substantially similar as those defined in the IEEE 802.11 DCF scheme. Descriptions of the present specification that differ from the IEEE 802.11 WLAN standard will follow the descriptions herein. 
       FIG. 1  illustrates a configuration of a MIMO WLAN system according to an exemplary embodiment of the present invention. Referring to  FIG. 1 , the MIMO WLAN system includes a first station  110  to transmit data, a second station  120  to transmit data, and an access point (AP)  130  that receives data from the first station  110  and data from the second station  120 . 
     Contrary to the general WLAN system, the MIMO WLAN system includes the first station  110  including a plurality of transmitting antennas  111  and  112 , the second station  120  including a plurality of transmitting antennas  121  and  122 , and the AP  130  including a plurality of receiving antennas  131 ,  132 ,  133 , and  134 . The first station  110  may transmit data as signals via the transmitting antennas  111  and  112  through different paths, and the second station  120  may transmit data as signals via the transmitting antennas  121  and  122  through different paths. The AP  130  may receive signals transmitted through different paths via the receiving antennas  131 ,  132 ,  133 , and  134 . 
     In the WLAN system according to an exemplary embodiment of the present invention, the first station  110  and the second station  120  transmit data according to a DCF scheme. The following description will be provided for the first station  110 , but the procedures will be equivalent or substantially similar for the second station  120 . 
     Prior to transmitting a data frame, the first station  110  detects a data transmission state of the system using a carrier used for transmitting/receiving data in the WLAN system. If data transmission from another station to the AP  130  is detected, the first station  110  terminates or delays data transmission and returns to an idle state. After a DCF interframe space (DIFS) or extended interframe space (EIFS) time has elapsed, the first station  110  waits a predetermined backoff time and then transmits the data frame. In this instance, the data frame to be transmitted includes a preamble associated with the first station  110  for collision detection at the AP  130 . 
     The AP  130  extracts the preamble from a radio signal that is received from the first station  110 , and estimates a channel associated with the first station  110  based on the extracted preamble. 
     Also, the extracted preamble is used for collision detection. Thus, the first station  110  and the second station  120  may both wait a predetermined backoff time and then transmit the data frames. The data frame of the first station  110  includes a preamble associated with the first station  110 , and the data frame of the second station  120  includes a preamble associated with the second station  120  for collision detection at the AP  130 . When a preamble associated with the first station  110  and a preamble associated with the second station  120  are received in the same time slot, the AP  130  detects that the first station  110  and the second station  120  simultaneously transmitted the data frames, and detects the event as collision. Although not described, the collision may also include additional preambles respectively associated with additional stations that are received in the same time slot. If the collision is detected, the AP  130  may decode the data frames transmitted from each of the first station  110  and the second station  120  by referring to an estimated channel value. 
     In order to decode the data frames from the radio signals that are simultaneously received from the first station  110  and the second station  120 , the AP  130  may use a MIMO decoding scheme. Examples of the MIMO decoding scheme may include a zero-forcing (ZF) scheme, a minimum mean-square error (MMSE) scheme, a maximum likelihood (ML) scheme, and a Successive Interference Cancellation (CIS). 
     Hereinafter, a method for simultaneously receiving data frames from the first station  110  and the second station  120  will be described in detail. As shown in  FIG. 1 , signals are transmitted to the AP  130  via two transmitting antennas  111  and  112  of the first station  110  and two transmitting antennas  121  and  122  of the second station  120 , which may be respectively labeled as X 1 , X 2 , X 3 , and X 4 . A wireless channel between each of the transmitting antennas  111 ,  112 ,  121 , and  122 , and each ofthe receiving antennas  131 ,  132 ,  133 , and  134  may be expressed as channel vector H shown below in Equation  1 , which, according to this exemplary embodiment, includes sixteen channel coefficients h 11  through h 44 . Received signals Y 1 , Y 2 , Y 3 , and Y 4  received via the receiving antennas  131 ,  132 ,  133 , and  134  are represented as Equation 1 below. 
     
       
         
           
             
               
                 
                   
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     The channel coefficients h 11  through h 44  constituting the channel vector H are independent variables and thus may be individually measured. Each channel coefficient h 11  through h 44  may be measured based on pilot signals that are transmitted from the transmitting antennas  111 ,  112 ,  121 , and  122  to the receiving antennas  131 ,  132 ,  133 , and  134 , respectively. 
     If the channel response associated with the first station  110  and the second station  120  is estimated as the channel vector H, the AP  130  may obtain the signals X 1 , X 2 , X 3 , and X 4  by multiplying received signal matrix Y by an inverse matrix of the channel vector H. The received signal matrix Y includes the received signals Y 1 , Y 2 , Y 3 , and Y 4 . When m total transmitting antennas are included in a plurality of stations and n total receiving antennas are included in an AP, the channel vector H is an n×m matrix. Accordingly, if the channel vector H is not represented as a square matrix where m=n, the inverse matrix of the channel vector H may not be obtained. However, in this case, if using a pseudo-inverse matrix, it is possible to obtain a transmit signal from a received signal and a channel vector. 
     As described above, if AP  130  successfully obtains the signals X 1 , X 2 , X 3 , and X 4  so that data decoding succeeds despite a collision between data frames of the first station  110  and the second station  120 , the AP  130  transmits an acknowledge signal (hereinafter, referred to as “ACK”) to the first station  110  and the second station  120 . Generally, the ACK may consist of a frame with a short length. A configuration of the ACK will be later described in detail. 
     The ACK is a feedback signal to inform the first station  110  and the second station  120  that the data frames transmitted from the first station  110  and the second station  120  have been normally received at the AP  130 . If the ACK is not received by the first station  110  and the second station  120  from the AP  130  within a predetermined period of time after transmitting the data frames, the first station  110  and the second station  120  may be reassigned with backoff time for retransmission of the data frames and may then wait for the backoff time to elapse. When the backoff time has elapsed, the first station  110  and the second station  120  retransmit the data frames. The operation associated with reception of the ACK is based on the general DCF scheme. 
     Referring to  FIG. 1 , each station includes a plurality of antennas according to a MIMO scheme. However, it will be apparent that the present invention may be applicable even if one or more of the colliding stations includes a single antenna. 
       FIG. 2  is a timing diagram illustrating data transmission according to an exemplary embodiment of the present invention. Referring to  FIG. 2 , a third station  210  is included in the WLAN system in addition to the first station  110  and the second station  120 , and transmits data according to a DCF scheme. 
     Also, in  FIG. 2 , backoff time  221  of the first station  110  and the second station  120  is terminated simultaneously. Therefore, the first station  110  and the second station  120  both begin to transmit a data frame in the same time duration  222 . Also, the third station  210  detects a carrier after the first station  110  and the second station  120  begin data transmission, and detects that the channel is busy. 
     Referring to  FIG. 2 , the first station  110  terminates the data frame transmission earlier than the second station  120 . Accordingly, in another time duration  223 , the second station  120  continues to transmit the data frame, whereas the first station  110  detects the data transmission from the second station  120  and waits for an ACK with respect to its own data frame. 
     When the data frame transmission during time durations  222  and  223  is completed, the first station  110  and the second station  120  wait to receive an ACK from an AP  130  during a period of time after a short interframe space (SIFS) time has elapsed. Referring to  FIG. 2 , assuming that the simultaneously transmitted data frames are both received at the AP  130  and are successfully decoded by the AP  130 , both the first station  110  and the second station  120  receive an ACK. The first station  110  and the second station  120  each receive an ACK and wait for a DIFS time prior to a backoff time in a subsequent time duration  225 . Also, until the SIFS time, ACK waiting time, and the DIFS time have elapsed with respect to the first station  110  and the second station  120 , the third station  210  waits for a corresponding extended interframe space (EIFS) time to elapse during the time duration  224 . 
     According to a conventional contention-based DCF scheme adopting a technology different from a MIMO technology in a MAC layer, data frames that are simultaneously transmitted from the first station  110  and the second station  120  in the time durations  222  and  223  are not normally received and decoded at the AP  130 . Accordingly, the first station  110  and the second station  120  would not receive an ACK from the AP  130 , and the first station  110  and the second station  120  would wait until a predetermined backoff time has elapsed and before transmitting the data frames again. 
     To the contrary, according to the present invention, the MIMO decoding technology may be used. Accordingly, even if a collision occurs between the first station  110  and the second station  120 , the MIMO decoding technology may decode the data frames transmitted from the first station  110  and the second station  120  based on a received signal. Thus it is possible to prevent or reduce the throughput deterioration due to the data frame retransmission. 
       FIG. 3  is a flowchart illustrating a method for receiving data according to an exemplary embodiment of the present invention, and  FIG. 4  is a flowchart illustrating a method for transmitting data according to an exemplary embodiment of the present invention. 
     First,  FIG. 3  illustrating a method for receiving data transmitted from at least one station will be described. 
     In operation S 310 , the AP receives a preamble from at least one station. Operation S 310  may include detailed operations for extracting the preamble from a radio signal that is received via a plurality of antennas. 
     More specifically, operation S 310  may include receiving a radio signal that is transmitted from each station, correlating the received radio signal with predetermined preamble codes to calculate a correlation value, and extracting, as the preamble, a preamble code used for correlation if the calculated correlation value is greater than a predetermined threshold. 
     The preamble that is individually assigned to each station may be located in the head of, for example, a data frame to be transmitted. The preamble may include orthogonal code having a predetermined length, or a pseudo-noise (PN) code that is similar to a PN code used in a mobile communication system of a code division multiple access (CDMA) scheme. By including the orthogonal code or the PN code in the preamble, it is possible to extract overlapping preambles that are received at the AP. 
     Also, a different preamble may be allocated to each station at all times, so that the AP may definitely detect the collision between stations. Specifically, the preamble may include a code pattern unique to each station. However, a relatively large preamble code pool may be maintained if a number of stations to be located within the communication coverage of the AP is not known in advance. Accordingly, the size of the preamble code pool that includes preambles to be assigned to the stations may be reasonably determined based on various parameters associated with the WLAN system. 
     According to an exemplary embodiment of the present invention, each station may randomly select a code from the preamble code pool. The selected code may be included in the preamble and transmitted together when a data frame is transmitted. 
     According to another exemplary embodiment of the present invention, the preamble code pool may be maintained and managed by the AP. The AP may select a code from the preamble codes included in the preamble code pool, and individually allocate the selected code to a station located within the communication coverage of the AP. 
     According to still another exemplary embodiment of the present invention, the preamble may be allocated from the AP at the time when a station performs association on the AP. 
     In operation S 320 , the AP estimates a wireless channel between at least one station and the AP based on the preamble received from that station. Specifically, in operation S 320 , the AP may estimate the wireless channel based on a pilot signal transmitted from a station. To do so, the AP compares a received pilot signal with a known pattern, and estimates channel response based on the difference thereof. Thus, to eliminate an effect of noise, the AP may receive the pilot signal multiple times, or may continuously receive the pilot signal during a predetermined period of time. Further disclosure of various methods for estimating a wireless channel in an orthogonal frequency division multiple access (OFDMA)-based wireless terminal, which may be similarly applicable in a WLAN according to an exemplary embodiment of the present invention, are disclosed in co-pending U.S. patent application Ser. No. 11/694,243, the disclosure of which is hereby incorporated by reference for all purposes. 
     In operation S 330 , the AP detects the collision associated with that station based on the preamble received from that station. Specifically, in operation S 330 , when preambles are received in the same time slot, it is possible to detect that the collision occurred between stations corresponding to the received preambles. 
     As described above, the preamble allocated to each station may be an orthogonal code or a PN code, and thus the AP may receive the overlapping preambles. Accordingly, when N preambles are received in operation S 310 , this indicates that at least N stations simultaneously transmitted the data frames in a time slot. In operation S 335 , if N is greater than one, the collision may be detected. 
     If the collision is detected in operation S 335 , the AP applies a MIMO decoding scheme to the signal received from each station by referring to a wireless channel estimate in operation S 340 . For example, the AP decodes the data frame transmitted from each station by applying a ZF scheme, an MMSE scheme, or an ML scheme to the received signal. 
     Also, an SIC scheme may be applicable. In this case, the SIC scheme sequentially and repeatedly decodes a data frame received via a channel with the best channel response among a plurality of channels, estimates a received signal corresponding to the decoded data frame, and then eliminates the estimated signal in the entire radio signal. Through this, all the data frames may be sequentially decoded one by one from the data frame associated with the best channel. 
     Conversely, if the collision is undetected in operation S 335 , the AP extracts a data frame using a single decoding scheme in operation S 350 , without applying the MIMO decoding scheme in operation S 340 . Specifically, if there is no collision, the AP assumes that the data frame was transmitted from a single station, just as in the general DCF environment. 
     If the decoding succeeds through the above described operations in operation S 345 , the AP transmits the ACK to the station that transmitted the data frame in operation S 360 . According to the exemplary embodiments of the present invention, it is possible to successfully decode data regardless of whether the collision occurs. Accordingly, the ACK may include information associated with the collision detection. For example, the ACK may include a field including one or more bits to indicate the presence or absence, i.e. the detection, of a collision. 
     Also, if the collision is detected and decoding succeeds, the ACK may be transmitted to stations with colliding signals. Therefore, the ACK may include an identifier associated with each station. The preamble used for detecting the collision in operation S 330  is assigned to each station and thus it is possible to identify the station using the preamble. Similarly, each station may verify that the transmitted data frame is normally received at the AP by receiving the ACK that includes an index of the preamble associated with the station. Specifically, the ACK may include the index of the received preamble as an identifier. 
     Conversely, if decoding fails in operation S 345 , the AP may not transmit any feedback to the station during the ACK waiting time, or may transmit a negative ACK (NACK) within the ACK waiting time in operation S 361 . In the latter situation, the NACK also may include an identifier associated with each station, just like described above with respect to the ACK. 
     In another exemplary embodiment, decoding in operation S 345  may succeed for one or more stations, but may fail for one or more stations. In this instance, the AP may transmit an ACK including an identifier associated with each station for which decoding was successful, and may transmit no feedback or a NACK including an identifier associated with each station for which decoding was unsuccessful. 
     The MIMO WLAN environment adopting the data receiving method according to an exemplary embodiment of the present invention uses a MAC protocol of a DCF scheme. Accordingly, if stations do not receive the ACK from the AP within the ACK waiting time, the stations are reassigned with a predetermined backoff time in order to retransmit the data frames. 
       FIG. 4  is a flowchart illustrating a method for transmitting a data frame from a station to an AP in a MIMO WLAN system according to an exemplary embodiment of the present invention. 
     In operation S 4   10 , a station detects a data transmission state of the WLAN system by monitoring a carrier used for transmitting and receiving data in the WLAN system. Prior to transmitting the data frame, in operation S 415 , a station may verify whether an encoded signal is being transmitted from another station to the AP through activity in the carrier. 
     If data transmission from another station is detected in operation S 415 , the station waits for data transmission until a predetermined backoff time has elapsed according to the DCF scheme in operation S 420 . 
     In operation S 420 , if the data transmission of the other station is detected, a station remains in a standby state until the new backoff time has elapsed after the data transmission of another station is complete. If the new backoff time has elapsed, data transmission may be performed. In operation S 430 , the station transmits the data frame to the AP via one or more transmitting antennas. The data frame includes a preamble associated with the station. 
     As described above, the preamble transmitted in the data frame may include a PN code or an orthogonal code associated with the station that transmits the data frame. The preamble functions to identify the corresponding station and is also used to estimate and detect the collision at the AP. The preamble may be randomly selected by each station from preambles included in a preamble code pool. As described above, the preamble code pool may be generally maintained and managed by the AP. Alternatively, the preamble may be allocated from the AP to a station at a time when the station performs association on the AP. Selecting or allocating of the preamble may be performed prior to operation S 410 . 
     In operation S 440 , the station that transmits the first data frame is in a standby state to receive an ACK from the AP during the ACK waiting time. If the ACK is received within the ACK waiting time in operation S 445 , thus acknowledging that the first data frame was received and successfully decoded, operation S 410  may be performed for transmitting a subsequent or second data frame. Conversely, if the ACK is not received, operations S 410  through S 440  may be performed again for retransmitting the first data frame. 
     The backoff time may be randomly selected within a predetermined range for the size of backoff window. The size of the backoff window may be adaptively adjusted according to the collision occurrence frequency. For example, if an ACK is not received until the ACK waiting time has elapsed, it is possible to determine a statistically better backoff time by increasing the size of the backoff window. Specifically, if the data frame is not normally received at the AP, it is possible to reduce the collision occurrence probability by determining the subsequent backoff time as a larger value. 
     Specifically, according to an exemplary embodiment of the present invention, it is possible to reduce the collision probability using a CSMA/CA scheme. Even if the collision occurs, it is possible to successfully decode data frames that are simultaneously transmitted from more than one station using a MIMO decoding scheme. If using a scheme of adaptively adjusting the size of backoff window, it is possible to further reduce the collision probability for more frequent collision occurrences. 
     The data transmitting/receiving method according to the above-described exemplary embodiments may be recorded in computer-readable media including program instructions to implement various operations embodied by a computer. The media may also include, alone or in combination with the program instructions, data files, data structures, and the like. Examples of computer-readable media include magnetic media such as hard disks, floppy disks, and magnetic tape; optical media such as CD-ROM disks and DVD; magneto-optical media such as optical disks; and hardware devices that are specially configured to store and perform program instructions, such as read-only memory (ROM), random access memory (RAM), flash memory, and the like. 
     Examples of program instructions include both machine code, such as that produced by a compiler, and files containing higher level code that may be executed by the computer using an interpreter. The described hardware devices may be configured to act as one or more software modules in order to perform the operations of the above-described embodiments of the present invention. 
       FIG. 5  is a block diagram illustrating an AP apparatus according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 5 , the AP apparatus includes a radio signal processing unit  510  to receive data signals  511  from one or more stations. The data signals  511  may be transmitted from the one or more stations according to a DCF scheme. 
     The data signals  511  that are received by the radio signal processing unit  510  and are respectively associated with the stations are input into a preamble extractor  520 . The preamble extractor  520  extracts from each data signal a preamble  521  corresponding to a station. 
     The extracted preambles  521  may be input into a channel estimator  530  and may be used to estimate a channel associated with the station associated with each extracted preamble  521 . Also, the extracted preambles  521  may be input into a collision detector  540  and may be used to detect whether the collision occurs between two or more stations. 
     A channel estimate  531  is obtained by the channel estimator  530  based on the preamble  521 , and is input into a frame detector  550 . If the collision detector  540  detects a collision between stations, it sends a signal to the frame detector  550 , which collectively or sequentially decodes the data signals  511  transmitted from the stations based on the input channel estimate  531 , and detects a data frame  551  associated with each station. 
     The frame detector  550  may successfully decode data frames  551  that are respectively transmitted from the stations by applying a MIMO decoding scheme such as a ZF scheme, an MMSE scheme, or an ML scheme. 
     Also, the frame detector  550  may decode the data frames  551  using an SIC scheme. 
       FIG. 6  is a block diagram illustrating a frame detector of the access point apparatus shown in  FIG. 5 . More specifically,  FIG. 6  is a block diagram illustrating an internal configuration of the frame detector  550  adopting the SIC scheme. 
     A decoder  610  decodes data signals  511  and extracts data frames  551  respectively associated with the stations. 
     A frame selector  620  selects a channel with the best channel response from among the channels associated with the stations based on the channel estimate  531 . The frame selector  620  selects a decoded data frame from a data signal  511  received via the selected channel, and outputs the selected data frame. 
     The selected data frame is input into a cyclic redundancy check (CRC) unit  630 . If the CRC unit  630  does not detect any error, the data frame is output as the detected data frame  551  and is also input into a data signal estimator  640 . The data signal estimator  640  applies to the selected data frame  551  a modulation and encoding scheme that is applied to the WLAN system and the channel estimate  531 , and thereby estimates a data signal  641  corresponding to the selected data frame  551  to generate the estimated data signal  641 . 
     An interference eliminator including a subtractor/summator  650  eliminates, one by one, the estimated data signal  641  from the data signals  511 . Thus, sequential detection of a data frame  511  received via a channel with poor channel response is enabled from a data frame  551  received via the channel with better channel response. 
       FIG. 7  is a block diagram illustrating an internal configuration of a data signal estimator of the frame detector shown in  FIG. 6 . More specifically,  FIG. 7  is a block diagram illustrating an internal configuration of the data signal estimator  640 . 
     The data signal estimator  640  includes an encoder  710  that encodes the selected data frame  551  using the same scheme and level as that used in the WLAN system according to exemplary embodiments of the present invention. 
     A data frame  711  encoded again by the encoder  710  is input into a modulator  720 . The modulator  720  generates a modulation signal  721  by applying to the encoded data frame  711  the same modulation scheme as used in the WLAN system. 
     A channel response emulation unit  730  emulates the channel estimate  531  in the modulation signal  721 , and estimates and generates the data signal  641  that is received when the selected data frame  551  is transmitted and is received from any one of the plurality of stations via a wireless channel. 
     Through the above construction, even if the data frames are simultaneously transmitted from more than one station and a collision occurs between the data frames, the AP apparatus may successfully decode the data frames. 
     The configuration of the WLAN AP apparatus according to an exemplary embodiment of the present invention has been described above with reference to  FIG. 5 ,  FIG. 6 , and  FIG. 7 . Detailed descriptions made according to exemplary embodiments with reference to  FIG. 1 ,  FIG. 2 ,  FIG. 3 ,  FIG. 4 , and  FIG. 5  may be applicable to the AP apparatus, and thus further detailed descriptions will be omitted here. 
       FIG. 8A ,  FIG. 8B ,  FIG. 9A ,  FIG. 9B ,  FIG. 10A , and  FIG. 10B  show performance measurement results obtained from a simulation test by a data transmitting/receiving method according to an exemplary embodiment of the present invention. The simulation test was performed based on that data is transmitted at a speed of 24 Mbps in a basic mode of an IEEE 802.11a DCF scheme.  FIG. 8A ,  FIG. 8B ,  FIG. 9A ,  FIG. 9B ,  FIG. 10A , and  FIG. 10B  are graphs illustrating a measurement value of data transmission throughput when changing a number of users in different error environments. 
       FIG. 8A  and  FIG. 8B  are graphs illustrating the simulation test result when no error occurs. In the graph shown in  FIG. 8A , the data transmission throughput in a MAC protocol of a conventional DCF scheme is compared with the data transmission throughput in a MAC protocol using a MIMO decoding scheme according to an exemplary embodiment of the present invention. As shown in  FIG. 8A , the data transmitting/receiving method according to a MIMO decoding scheme according to an exemplary embodiment of the present invention may significantly improve the data transmission throughput. It can be seen that the level of the improvement increases as a number of users, that is, a number of stations accessing the AP increases. Referring to the graph, when the number of users was 50, the throughput was improved by about 8 Mpbs. 
       FIG. 8B  is a mean access delay time graph that is computed based on a second unit. It can be seen that the difference of the mean access delay time increases in a nearly linear fashion as the number of users increases. Referring to the graph, when the number of users was 50, the mean access delay time was reduced by about 0.01 seconds. 
       FIG. 9A  and  FIG. 9B  are graphs illustrating the simulation test result about the throughput and mean access delay time when an error continuously exists. 
     The graph shown in  FIG. 9A  is similar to the graph of  FIG. 8A . Referring to  FIG. 9A , the performance when using a MIMO decoding scheme of a Vertical Bell Laboratories Layered Space-Time (V-BLAST) scheme is slightly better than the performance when using a ZF scheme. However, in comparison to the conventional scheme without adopting the MIMO decoding scheme, it can be seen that the throughput was significantly improved when using the V-BLAST scheme and when using the ZF scheme. Referring to the graph, when the number of users was 50, the throughput was improved by about  8  Mbps in both tests. 
     A decreasing pattern of the mean access delay time shown in the graph of  FIG. 9B  is nearly the same as the pattern of the graph shown in  FIG. 8B . Even in this case, when using the V-BLAST, the performance was improved slightly more than when using the ZF scheme. However, in comparison to the conventional scheme, it can be seen that the mean access delay time was significantly decreased when using the V-BLAST scheme and when using the ZF scheme. Referring to the graph, when the number of users was 50, about 0.01 seconds of mean access delay time was reduced in both tests. 
       FIG. 10A  and  FIG. 10B  are graphs illustrating the simulation test result when an error rate changes. In the graph shown in  FIG. 10A , the throughput when using the ZF scheme or the V-BLAST scheme is compared with the throughput when using the conventional scheme. Referring to  FIG. 10A , although the throughput is decreasing as the number of users increases, the throughput improvement with respect to the conventional scheme is maintained to be nearly constant. Specifically, when applying the ZF scheme or the V-BLAST scheme based on 100 users, the throughput was improved by about 7.5 Mbps in comparison to the conventional data transmitting/receiving method. 
     Also, like the graphs shown in  FIG. 8B  and  FIG. 9B , the graph shown in  FIG. 10B  shows that the deterioration of the mean access delay time increases in a nearly linear fashion as the number of users increases. When adopting the ZF scheme or the V-BLAST scheme based on 100 users, the mean access delay time is decreased by about 0.02 seconds in comparison to the conventional scheme. 
     A method for transmitting/receiving data according to exemplary embodiments of the present invention can apply a MIMO technology to a physical layer and a MAC layer and thereby support a MAC protocol that effectively supports an improved data transmission rate in the physical layer. 
     Also, according to exemplary embodiment of the present invention, if a collision occurs between stations, data from each station is decoded using a MIMO decoding scheme. Accordingly, it is possible to prevent the deterioration of the data transmission throughput in the entire WLAN system, which may occur due to the frame retransmission. Also, it is possible to reduce a network access delay caused by the throughput deterioration. 
     Also, according to exemplary embodiments of the present invention, it is possible to detect a collision between a plurality of stations using a preamble consisting of an orthogonal code or a PN code. Also, it is possible to provide various types of schemes for allocating a preamble corresponding to each station. 
     Also, according to exemplary embodiments of the present invention, it is possible to provide a station side with more accurate information regarding a failure cause of data reception by including information associated with a collision in an ACK. 
     Also, according to exemplary embodiments of the present invention, if an ACK is not received from an AP within a predetermined period of time, it is possible to increase the size of a backoff window and thereby reduce the collision probability for a subsequent frame transmission. 
     Also, according to exemplary embodiments of the present invention, it is possible to provide a construction of an AP apparatus that includes a function of decoding data simultaneously transmitted from a plurality of stations, using various types of MIMO decoding technologies. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.