Abstract:
A method and apparatus for transmitting a frame to at least one receiver in a wireless communication system is provided. The apparatus determines at least one data length in accordance with each receiver based on a number of symbol for the frame to be transmitted, determines a maximum data length among the at least one data length, and determines a length of the frame in time domain based on the maximum data length. The apparatus generates the frame in accordance with the length of the frame, the frame including a first signal field and at least one second signal field. The first signal field indicates the length of the frame and each second signal field indicates each data length.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 13/525,220, filed Jun. 15, 2012, issued on Mar. 15, 2016, as U.S. Pat. No. 9,288,017, which is a continuation of International Application No. PCT/KR2010/009039, filed Dec. 16, 2010, which claimed priority to Korean Application No. 10-2010-0066465, filed Jul. 9, 2010, Korean Application No. 10-2010-0010590, filed Feb. 4, 2010, and Korean Application No. 10-2009-0127310, filed Dec. 18, 2009, in the Korean Intellectual Property Office, all of the disclosures of which are hereby incorporated by reference. A related application, U.S. patent application Ser. No. 13/596,328, filed on Aug. 28, 2012, issued as U.S. Pat. No. 8,995,343, on Mar. 31, 2015. 
     
    
     BACKGROUND 
       [0002]    (1) Field 
         [0003]    Exemplary embodiments of the present invention relate to a method for transmitting/receiving data when one station is to communicate with several stations at the same time, in a wireless packet communication system, in particular, a near field wireless communication system. 
         [0004]    (2) Discussion of the Background 
         [0005]    Basically, the wireless LAN supports a basic service set (BBS) including an access point (AP) and a plurality of wireless stations excluding the AP. The AP serves as an access point of a distribution system (DS). Hereafter, the AP and the stations are commonly called ‘station’. 
         [0006]    According to the IEEE 802.11n standard, when a station receives a data frame, the station transmits an acknowledgement (ACK) signal after a short inter frame space (SIFS) in order to increase the transmission efficiency in a media access control (MAC) layer, even though there is a difference depending on a policy for an ACK signal required by the corresponding data frame. 
         [0007]      FIG. 1  is a timing diagram explaining data transmission in the MAC layer according to the IEEE 802.11 standard. 
         [0008]    When a station  1  transmits a data frame  101  to a station  2 , the station  2  receives the corresponding data frame and then transmits an ACK signal  111  after an SIFS which is a predetermined time. Such a method shown in  FIG. 1  is frequently used in the MAC layer of the wireless LAN. 
         [0009]    Meanwhile, as the number of users using the wireless LAN has rapidly increased, the request to improve data throughput provided by one BSS is increasing. In the existing wireless LAN, one station may communicate with only one station at a certain moment. However, research has been conducted on technology which enables one station to communicate with several stations at the same time, in order to provide a gigabit or more throughput. As the representative technology, a multi-user MIMO (hereafter, MU-MIMO) scheme and a multi-frequency channel scheme are provided. 
         [0010]    When those schemes are used, one station may operate as if the station transmits and receives frames to and from several terminals through several independent communication paths, respectively. Accordingly, the station may transmit data to several stations at the same time. As a result, it is possible to significantly increase the throughput of the BSS. 
         [0011]    However, when using the several communication paths at the same time, the station has a limitation in which the transmission and the reception cannot be performed at the same time through all the used communication paths. For example, when a certain station uses a communication path  1  and a communication path  2  at the same time, the communication path  1  cannot be used for transmission in case where the communication path  2  is used for reception. 
         [0012]    All data frames used in the wireless LAN have a variable length. As described above, an ACK or a block ACK is transmitted immediately after a predetermined time passes from a time point when the reception of data frames is completed. Therefore, when data frames are simultaneously to several stations through several communication paths, the respective stations will transmit an ACK immediately after a predetermined time passes from a time point when the reception of the data frames having different lengths was completed. That is, a station having received a data frame having the shortest length may transmit an ACK, before the transmission of the data frames to the other stations is completed. In this case, the corresponding ACK may not be received. 
         [0013]    This will be described with reference to  FIG. 2 . 
         [0014]    In  FIG. 2 , it is assumed that stations  1  and  2  exist and data are transmitted between the stations  1  and  2  through different communication paths. That is, when the station  1  transmits a data frame  201  and a data frame  202  having different lengths, the transmission of the data frame  201  having a shorter length may be first completed. In this case, when the length of the data frame  201  is smaller than that of the data frame  202  by an SIFS  221  or more, the station  2  transmits an ACK  211  immediately after the SIFS  221  passes from a time point when the reception of the data frame  201  is completed. 
         [0015]    However, since the station  1  is still transmitting the data frame  202  at the time point when the station  2  transmits the ACK  211 , a reception impossible section  230  occurs, in which the station  1  cannot receive the ACK  212  transmitted by the station  2 . 
       SUMMARY 
       [0016]    An embodiment of the present invention is directed to a method capable of informing stations of the length of a data frame having the largest length among data frames to be transmitted to the respective stations, when several communication paths are used. 
         [0017]    Another embodiment of the present invention is directed to a method in which a reception station transmits an ACK in a predetermined time after a time corresponding to the length of the largest data frame passes, thereby preventing a reception impossible zone from occurring. 
         [0018]    Other objects and advantages of the present invention can be understood by the following description, and become apparent with reference to the embodiments of the present invention. Also, it is obvious to those skilled in the art to which the present invention pertains that the objects and advantages of the present invention can be realized by the means as claimed and combinations thereof. 
         [0019]    In accordance with an embodiment of the present invention, there is provided a data transmission method in which a transmission station transmits different data at the same time. The data transmission method includes: acquiring length information of data having the largest length among the different data; generating a first signal field containing the length information of the data having the largest length; and transmitting the first signal field such that all stations receive the first signal field. 
         [0020]    In accordance with another embodiment of the present invention, there is provided a data reception method in which a reception station receives data from a transmission station transmitting different data to a plurality of reception stations at the same time. The data reception method includes: receiving a first signal field containing length information of data having the largest length among the different data transmitted to the respective reception stations at the same time; receiving data; and waiting for a predetermined time after a time indicated by the length information based on the length information contained in the first signal field, and transmitting an acknowledgement (ACK) of the received data. 
         [0021]    In accordance with the embodiment of the present invention, when one station transmits data to several stations at the same time by using several communication paths and receives ACK signals, a reception impossible zone does not occur. Therefore, communication may be smoothly performed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0022]      FIG. 1  is a timing diagram explaining data transmission in a MAC layer according to the IEEE 802.11 standard. 
           [0023]      FIG. 2  is a diagram showing a case in which a problem occurs due to the superposition of ACK signals depending on data lengths for several communication paths used in a wireless communication system. 
           [0024]      FIG. 3  is a diagram showing the structure a general PPDU used in a wireless communication system. 
           [0025]      FIG. 4  is a diagram showing a signal field used in a wireless LAN system. 
           [0026]      FIG. 5  is a timing diagram in a case in which the entire PHY overhead is transmitted by using a communication path through which all stations may receive data in accordance with a first embodiment of the present invention. 
           [0027]      FIG. 6  is a timing diagram in a case in which a part of the PHY overhead is transmitted by using a communication path through which all stations may receive data, and the other parts of the PHY overhead are transmitted by using communication paths independent of each other in accordance with a second embodiment of the present invention. 
           [0028]      FIG. 7  is a timing diagram in a case in which the entire PHY overhead is transmitted by using communication paths independent of each other in accordance with a third embodiment of the present invention. 
           [0029]      FIG. 8  is a timing diagram in a case in which a signal field is divided into two parts in accordance with the fourth embodiment of the present invention. 
           [0030]      FIG. 9  is a diagram showing a case in which length information contained in an L-SIG field is used in accordance with the embodiment of the present invention. 
           [0031]      FIG. 10  is a diagram showing a case in which length information contained in an L-SIG field is used to represent the time length of the longest data frame and an SIG field contains length information on the data amounts of data frames to be transmitted to the respective stations. 
           [0032]      FIG. 11  is a diagram showing a case in which length information contained in an L-SIG field is used to represent the time length of the longest data frame and SIG B fields included in the respective communication fields contain length information on the data amounts of data frames to be transmitted to the respective stations. 
           [0033]      FIG. 12  is a diagram showing a case in which a tail is additionally attached to the back of an MPDU or A-MPDU. 
           [0034]      FIG. 13  is a diagram showing a case in which a tail is additionally attached to the back of an MPDU or A-MPDU. 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    Exemplary embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be constructed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. 
         [0036]    A wireless communication system includes a MAC layer and a physical (PHY) layer, each of which attaches overhead information required for processing data. Therefore, a transmission unit of the PHY layer attaches information for data processing of the PHY layer to data received from the MAC layer and then transmits the data through a wireless channel, and a reception unit of the PHY layer extracts data to be transferred to the MAC layer by using the attached information. 
         [0037]    At this time, a PHY protocol data unit (PPDU) which is outputted to the wireless channel by the PHY layer has a structure as shown in  FIG. 3 . 
         [0038]    The overhead information used in the PHY layer may be roughly divided into a signal field  301  and a training field  302 . The training field  302  is used for synchronization detection and wireless channel estimation, and the signal field  302  is used for extracting MAC data (MPDU or A-MPDU)  303 . 
         [0039]    A signal field defined in the IEEE 802.11n standard which provides the highest data rate, among current wireless LAN standards, has a structure as shown in  FIG. 4 . 
         [0040]    Among elements composing the signal field, Length  401  is information representing the length of MAC data included in the corresponding PPDU, and the PHY layer is set to transfer the extracted MAC data and the length information to the MAC layer. 
         [0041]    In this embodiment of the present invention, the length information is used to inform the respective stations of the largest value among the time lengths of the data frames transmitted to several stations at the same time. A communication method using this will be described briefly as follows. 
         [0042]    A station which is to simultaneously transmit data to several stations by using several communication channels at the same time attaches a PHY layer overhead in front of the data frames to be transmitted to the respective stations. At this time, length information included in a signal field is set to represent the largest value among the time lengths of the data frames to be transmitted to the respective stations at the same time. Therefore, a station receiving data will transmit an ACK after a time indicated by the length information of the signal field passes, although the reception of the data frames was completed. 
         [0043]    Then, the several stations which have simultaneously received the data frames may transmit ACK signals at the same time. Therefore, a reception impossible zone does not occur, which has occurred when the conventional technology was used. 
         [0044]    Here, a method of representing a time length needs to be calculated in the same time unit for all modulation and coding schemes (MCS). Therefore, the number of symbols constructing a data frame or a time-unit length such as microsecond may be used. 
         [0045]    A method of attaching a PHY overhead, to which the embodiment of the present invention is applied, will be described below. 
       First Embodiment 
       [0046]    In a first embodiment of the present invention, the entire PHY overhead is transmitted by using a communication path through which all stations may receive data. This will be described with reference to  FIG. 5 . 
         [0047]      FIG. 5  is a timing diagram in a case in which the entire PHY overhead is transmitted by using a communication path through which all stations may receive data in accordance with the first embodiment of the present invention. 
         [0048]    In  FIG. 5 , it is assumed that station  1  transmits data frames  503  and  504  having different lengths to stations  2  and  3 , respectively. 
         [0049]    The station  1  transmits a signal field  510  and a training field  502  through bands or paths of the data frames  503  and  504 , in order to transmit the entire PHY overhead to all the stations. At this time, length information included in the signal field  501  is set to indicate the length of the data frame  504 , because the data frame  504  has the largest length. That is, the length information represents the length of the data frame  504 , as indicated by an arrow  500  in the signal field  501 . 
         [0050]    Then, the stations  2  and  3  receive the data frames  503  and  504  by using the length information of the signal field  501 , and transmit ACK signals  511  and  512  corresponding to the respective data frames  503  and  504  after waiting for a predetermined SIFS  521 . 
       Second Embodiment 
       [0051]    Next, a second embodiment of the present invention will be described. In the second embodiment, a part of a PHY overhead is transmitted by using a communication path through which all stations may receive data, and the other parts of the PHY overhead are transmitted by using communication paths independent of each other. This will be described with reference to  FIG. 6 . 
         [0052]      FIG. 6  is a timing diagram in a case in which a part of the PHY overhead is transmitted by using a communication path through which all stations may receive data, and the other parts of the PHY overhead are transmitted by using communication paths independent of each other in accordance with the second embodiment of the present invention. 
         [0053]    In  FIG. 6 , it is also assumed that the station  1  transmits data frames  604  and  605  having different lengths to the stations  2  and  3 , respectively. 
         [0054]    The station  1  transmits a signal field  601  by using a communication path through which all stations may receive data. Furthermore, training fields  602  and  603  are transmitted only through communication paths for the respective data frames  604  and  605 . That is, the training field  602  for detecting the data frame  604  is transmitted only through the communication path for transmitting the data frame  604 , and the training field  603  for detecting the data frame  605  is transmitted only through the communication path for transmitting the data frame  605 . 
         [0055]    Although the transmission is performed in such a manner, the stations  2  and  3  may confirm the length of the data frame  605 , because the signal field  601  represents the length of the data frame  605  which is the longest frame between the data frames  604  and  605 , as indicated by reference numeral  600 . 
         [0056]    Therefore, the stations  2  and  3  may wait for an SIFS  621  which is a predetermined time, after a time corresponding to the value indicated by the length information, and then transmit ACK signals  611  and  612  corresponding to the respective data frames  604  and  605 . 
       Third Embodiment 
       [0057]    In a third embodiment of the present invention, the entire PHY overhead is transmitted by using communication paths independent of each other. This will be described with reference to  FIG. 7 . 
         [0058]      FIG. 7  is a timing diagram in a case in which the entire PHY overhead is transmitted by using communication paths independent of each other in accordance with the third embodiment of the present invention. 
         [0059]    In  FIG. 7 , it is also assumed that the station  1  transmits data frames  704  and  705  having different lengths to the stations  2  and  3 , respectively. 
         [0060]    In  FIG. 7 , training fields  701  and  702  and signal fields  703  and  704  are transmitted in correspondence to the data frames  704  and  705 . That is, the training field  701  and the signal field  703  are transmitted through the communication path of the data frame  705 , and the training field  702  and the signal field  704  are transmitted through the communication path of the data frame  706 . 
         [0061]    At this time, the signal fields  703  and  704  represent the length of the data frame  706  which is the longest frame between the data frames  705  and  706 , as indicated by reference numeral  700 . That is, even when the signal fields  703  and  704  are transmitted by using communication paths independent of each other, the length information contained in the signal field  703  should be identical to the length information contained in the signal field  704 . 
         [0062]    Therefore, when receiving the data frame  705  and the data frame  706 , the stations  2  and  3  wait for an SIFS  721  which is a predetermined time, after the reception of the data frame  706  is completed, and then transmit ACK signals  711  and  712  corresponding to the respective data frames  705  and  706 . 
       Fourth Embodiment 
       [0063]    In a fourth embodiment of the present invention, a part of a signal field is transmitted by using a communication path through which all stations may receive data, and the other part of the signal field is transmitted by using communication fields independent of each other. This will be described with reference to  FIG. 8 . 
         [0064]      FIG. 8  is a timing diagram in a case in which a signal field is divided into two parts, one signal field is transmitted through the communication path through which all stations may receive data, and the other signal field is transmitted by using communication fields independent of each other in accordance with the fourth embodiment of the present invention. 
         [0065]    In  FIG. 8 , it is also assumed that the station  1  transmits data frames  806  and  807  having different lengths to the stations  2  and  3 . 
         [0066]    Referring to  FIG. 8 , a signal field  801  transmitted by using a communication path through which all stations may receive data is set to represent a length corresponding to the data frame  807  having the largest length between the respective data frames  806  and  807 . Through this process, response times of different frames may be confirmed as described above. 
         [0067]    Next, training fields  802  and  803  are transmitted through the corresponding communication paths of the respective data frames  806  and  807 . Then, signal fields  804  and  805  for indicating the lengths of the respective data frames  806  and  807  are contained. That is, the signal field  801  transmitted through the communication path through which all stations may receive data indicates Length 1   810  representing the length of the longest data frame, the signal field  804  corresponding to the data frame  806  indicates Length 2   811 , and the signal field  805  corresponding to the data frame  807  indicates Length 3   812 . Therefore, the length  810  of the actual data frame and the data frames  806  and  807  may be transmitted by the respective signal fields. At this time, the length information contained in the signal field  804  may include a time length or a length representing a data amount, for example, a length in the unit of byte. 
         [0068]    Then, the stations  2  and  3  receiving the data frames wait for an SIFS  831  which is a predetermined time from a time point when the transmission of the data frame  807  which is the longest frame between the data frames  806  and  807  is completed, and then transmit ACK signals  821  and  822  corresponding to the respective data frames  806  and  807 . 
         [0069]    In the above-described four embodiments of the present invention, it has been described that stations which have simultaneously received data frames transmit ACK signals or block ACK frames corresponding to the data frames at the same time. However, the stations may sequentially transmit ACK signals or block ACK frames, depending on a protocol. In this case, the transmission time of an ACK signal or block ACK frame which is to be first transmitted may be decided by the method in accordance with the embodiments of the present invention, and the transmission time of an ACK signal or block ACK frame which is to be transmitted later may be decided by another method. 
         [0070]    Furthermore, the structure of the frames used in the four embodiments described with reference to the above drawings is a conceptual structure. Therefore, when the frames are actually applied, the frames may have a variety of specific forms. 
         [0071]    In the wireless LAN, the backward compatibility with the existing technology is considered to be very important. Therefore, even when a more advanced method is used, a disadvantage should not be given to a station using the existing method. Accordingly, a PHY layer overhead of the existing method needs to be added in front of a frame using the new method such that the station using the existing method may perceive the length of the frame having a new structure. In this case, length information contained in an L-SIG field which is one of the PHY layer overheads in the existing method may be used to represent the largest value among time lengths of data frames which are to be transmitted to the respective terminals at the same time. 
         [0072]    A method of representing the length information of a frame may include a method based on time such as several microseconds or several symbol lengths and a method based on data amount such as several bytes or several words. Depending on which method is used between them, the position of a tail used for coding and decoding of the PHY layer may be changed. 
         [0073]    Based on this, specific examples of the transmitted frame will be described with reference to  FIGS. 9 to 13 . At this time, a long training field (LTF) may be transferred by using a communication path through which all stations may receive data, as shown in  FIG. 5 , or may be transferred by using a communication path through only each station may receive data, as shown in  FIGS. 6 to 8 . 
         [0074]      FIG. 9  is a diagram showing a case in which length information contained in the L-SIG field is used to represent only a time length of the longest data frame. 
         [0075]      FIG. 9  shows a case in which different MPDUs or A-MPDUs  902  and  912  are transmitted. In the following descriptions, it is assumed that the MPDUs  902  and  912  having different lengths from each other are transmitted. 
         [0076]    Since the end positions of frames for transmitting the MPDUs  902  and  912  having different lengths to several stations at the same time should be matched with each other, pads should be inserted into the MAC layer and the PHY layer in order to remove a difference between the positions. That is, a MAC pad  903  is added to the back of the MPDU  902 , and a PHY pad  904  and a tail  905  are sequentially added. Here, Service  1  ( 901 ) indicates a field for representing a scramble seed of the PHY layer. Furthermore, a MAC pad  913  is added to the back of the MPDU  912 , and a PHY pad  914  and a tail  915  are sequentially added. Here, Service  2  ( 902 ) indicates a field for representing a scramble seed of the PHY layer. 
         [0077]    As described above, information for representing the entire length information according to an MPDU having the largest length between the MPDUs  902  and  912  having different lengths may be transmitted by using the length value of the L-SIG field  900 , as indicated by reference numeral  910 . At this time, the length value of the L-SIG field may be used as a time length as it is, or may be converted into a time length by another method. 
         [0078]      FIG. 10  is a diagram showing a case in which length information contained in an L-SIG field is used to represent the time length of the longest data frame and an SIG field contains length information on the data amounts of data frames to be transmitted to the respective stations. 
         [0079]      FIG. 10  shows a case in which different MPDUs or A-MPDUs  1012  and  1022  are transmitted. In the following descriptions, it is assumed that the MPDUs  1012  and  1022  having different lengths from each other are transmitted. 
         [0080]    Since the end positions of frames for transmitting the MPDUs  1012  and  1022  having different lengths to several stations at the same time should be matched, pads should be inserted into the MAC layer and the PHY layer in order to remove a difference between the positions. That is, a MAC pad  1013  is added to the back of the MPDU  1012 , and a PHY pad  1014  and a tail  1015  are sequentially added. Here, Service  1  ( 1011 ) indicates a field for representing a scramble seed of the PHY layer. Furthermore, a MAC pad  1023  is added to the back of the MPDU  1022 , and a PHY pad  1024  and a tail  1025  are sequentially added. Here, Service  2  ( 1021 ) indicates a field for representing a scramble seed of the PHY layer. 
         [0081]    As described above, information for representing the entire length information according to an MPDU having the largest length between the MPDUs  1012  and  1022  having different lengths may be transmitted by using the length value of the L-SIG field  1000 , as indicated by reference numeral  1030 . Furthermore, the information of the respective MPDUs  1012  and  1022  may be represented by SIG Length 1  and SIG Length 2  in an SIG field  1001 . 
         [0082]    In such a structure as shown in  FIG. 10 , the PHY layer of a reception station may turn off a reception function of the station, after decoding of data corresponding to the length of a frame transmitted to the PHY layer is completed. Therefore, power consumption may be effectively reduced. 
         [0083]      FIG. 11  is a diagram showing a case in which length information contained in an L-SIG field is used to represent the time length of the longest data frame and SIG B fields included in the respective communication fields contain length information on the data amounts of data frames to be transmitted to the respective stations. 
         [0084]      FIG. 11  shows a case in which different MPDUs or A-MPDUs  1112  and  1122  are transmitted. In the following descriptions, it is assumed that the MPDUs  1112  and  1122  having different lengths from each other are transmitted. 
         [0085]    Since the end positions of frames for transmitting the MPDUs  1112  and  1122  having different lengths to several stations at the same time should be matched, pads should be inserted into the MAC layer and the PHY layer in order to remove a difference between the positions. That is, a MAC pad  1113  is added to the back of the MPDU  1112 , and a PHY pad  1114  and a tail  1115  are sequentially added. Here, Service  1  ( 1111 ) indicates a field for representing a scramble seed of the PHY layer, and SIG B ( 1110 ) indicates a field for representing the length of the MPDU  1112 . 
         [0086]    Furthermore, a MAC pad  1123  is added to the back of the MPDU  1122 , and a PHY pad  1124  and a tail  1125  are sequentially added. Here, Service 2  ( 1121 ) indicates a field for representing a scramble seed of the PHY layer, and SIG B 2  ( 1120 ) indicates a field for representing the length of the MPDU 1   1122 . 
         [0087]    As described above, information for representing the entire length information according to an MPDU having the largest length between the MPDUs  1112  and  1122  having different lengths may be transmitted by using the length value of the L-SIG field  1000 , as indicated by reference numeral  1130 . Furthermore, the information of the respective MPDUs  1112  and  1122  may be indicated by SIG Length 1  and SIG Length 2  in an SIG B 1  field  1110  and an SIG B 2  field  1120 , respectively. 
         [0088]    In such a structure as shown in  FIG. 11 , the PHY layer of a reception station may turn off a reception function of the station, after decoding of data corresponding to the length of a frame transmitted to the PHY layer is completed. Therefore, power consumption may be effectively reduced. 
         [0089]      FIGS. 12 and 13  show the same structures as shown in  FIGS. 10 and 11 , respectively, except that a tail is additionally attached to the back of a MPDU or A-MPUD. When such a structure is used, decoding of the MPDU or A-MPUD may be terminated immediately after Tail A. Therefore, the power consumption in the reception station may be more effectively reduced than in the structures of  FIGS. 10 and 11 . 
         [0090]    While the present invention has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims. 
         [0091]    Embodiments of the present invention are applied to a WLAN system using multiple user and multiple access.