Abstract:
A wireless communication method used in a wireless infrastructure network including multi input multi output (MIMO) stations. The wireless communication method includes generating a management frame that comprises information on a MIMO contention period during which MIMO stations contend with each other for a channel, and transmitting the management frame. The wireless communication method prevents collisions of frames transmitted from the MIMO stations and from the SISO stations.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority from Korean Patent Application No. 10-2004-0009002 filed on Feb. 11, 2004 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to wireless communications, and more particularly, a wireless communication method used in a wireless infrastructure network where single input single output (SISO) stations and multi input multi output (MIMO) stations coexist, the wireless communication method being capable of preventing collision of frames transmitted from the SISO stations and from the MIMO stations by designating part of a conventional contention free period (CFP) as a contention period where only the MIMO stations can contend with each other for a channel.  
         [0004]     2. Description of the Related Art  
         [0005]     A wireless LAN allows stations within a predetermined distance of one another to wirelessly send and receive data to and from one another without the need for floor wiring similar to that of wired LAN. Thus, within the wireless LAN, stations wirelessly communicate with one another so they are free to move from place to place.  
         [0006]     In general, the IEEE 802.11 network is built around a Basic Service Set (BSS), which is a group of stations communicating with one another. There are two specific kinds of BSS&#39;s: an independent BSS, and an infrastructure BSS.  
         [0007]     An access point (AP) in the infrastructure BSS transmits beacon frames. A basic service area is defined as an area, in which the beacon frames are transmitted between members of the infrastructure BSS.  
         [0008]     On the other hand, the AP is not used in the independent BSS, which is an IEEE 802.11 ad-hoc network, in which stations directly communicate with one another.  
         [0009]     Meanwhile, according to the IEEE 802.11 standard, in order for a station to access a data transmission medium, a Distributed Coordination Function (DCF) and a Point Coordination Function (PCF) are used.  
         [0010]      FIG. 1  is a diagram illustrating the transmission of data between stations using a point coordination function (PCF) method. The PCF method is generally used together with a distributed coordination function (DCF) method. In other words, when a PCF section ends, a DCF section begins. A contention-free period (CFP) is comprised of repetitions of the PCF section and the DCF section. Referring to  FIG. 1 , D 1  and D 2  are frames transmitted from a point coordinator, and U 1  and U 2  are frames transmitted from stations polled by the point coordinator.  
         [0011]     When the point coordinator transmits a beacon frame, a CFP based on PCF rules begins. The point coordinator, which is located in an access point (AP), polls stations in a round-robin manner to determine whether the stations have data to transmit. Each of the stations polled by the point coordinator transmits data and an acknowledgement (ACK) message to the point coordinator. Then, the point coordinator transmits the data and ACK, which are received from the station polled by the point coordinator, to a destination station and polls the destination station. The destination station polled by the point coordinator transmits an ACK message to the point coordinator. If the destination station has data to transmit, it transmits the data to the point coordinator together with the ACK message. In this manner, data is transmitted between the stations during a CFP.  
         [0012]      FIG. 2  is a diagram illustrating a DCF-based back-off procedure. A PCF method provides contention-free services, but a DCF method provides contention-based services. A DCF method adopts a rotating back-off window mechanism in order to prevent frames transmitted from stations from colliding with each other. In a DCF method, it is determined whether a predetermined medium is currently being used based on the length of a distributed inter-frame space (DIFS).  
         [0013]     Referring to  FIG. 2 , during a DCF-based CP, a contention window, CWindow, having a predetermined size is allotted to each station after a DIFS period. Random slots (back-off times) having almost the same probability of being selected through a back-off algorithm are respectively allotted to stations that contend with one another for a channel in an independent basic service set (IBSS).  
         [0014]     Specifically, when the transmission of a frame from a station A that currently uses the channel is complete, stations B, C, and D contend with one another for the channel in a first contention window period after a DIFS period. In the first contention window period, the station C that has selected a minimum amount of back-off time secures the channel using the back-off algorithm and transmits a frame when its back-off timer reaches 0.  
         [0015]     In a second contention window period after another DIFS period, the stations B and D and a station E contend with one another for the channel, and the station D successfully secures the channel using the back-off algorithm and transmits a frame. In a third contention window period, the stations B and E contend with each other for the channel, and the station E successfully secures the channel using the back-off algorithm and transmits a frame. Accordingly, only the station B is left to secure the channel. In a fourth contention window period, the station B secures the channel using the back-off algorithm and transmits a frame.  
         [0016]     In accordance with the proliferation and development of digital devices, digital technology has demanded a high-speed wireless local area network (LAN) system that will operate at data rates of 100 Mbits/sec or higher. To meet such demand, multiple input multiple output (MIMO) technology has been introduced as a candidate for one of the most promising technologies for speeding up the next generation wireless LAN systems.  
         [0017]     The MIMO technology is classified into a spatial multiplexing technique, which enables higher-speed data transmission by simultaneously transmitting different types of data using multiple transmitting and receiving antennas without the necessity of increasing the bandwidth of an entire system, and a spatial diversity technique, which enables transmission diversity by transmitting one kind of data using multiple transmitting antennas.  
         [0018]     Conventional IEEE 802.11a single input single output (SISO) stations do not recognize frames transmitted from multi input multi output (MIMO) stations. Accordingly, in an infrastructure basic service set (BSS) where conventional IEEE 802.11a SISO stations and MIMO stations coexist, the conventional IEEE 802.11a SISO stations are likely to contend for a channel currently being occupied by the MIMO stations and attempt to transmit frames, which are highly likely to collide with frames transmitted from the MIMO stations.  
       SUMMARY OF THE INVENTION  
       [0019]     The present invention provides a wireless communication method, which enables two different types of stations, i.e., single input single output (SISO) stations and multi input multi output (MIMO) stations, to coexist in a wireless infrastructure network and which can prevent collisions of frames transmitted from the SISO stations and from the MIMO stations by allotting to the MIMO stations a predetermined amount of time in which only the MIMO stations can contend with each other for a channel.  
         [0020]     According to an aspect of the present invention, there is provided a wireless communication method comprising generating a management frame that comprises information on a multi input multi output (MIMO) contention period during which MIMO stations contend with each other for a channel, and transmitting the management frame.  
         [0021]     According to another aspect of the present invention, there is provided a wireless communication method comprising allowing a point coordinator to set a contention period for MIMO stations, allowing the point coordinator to generate and transmit a management frame, that comprises information on the contention period, to stations in its basic service set, and allowing the stations that have received the management frame from the point coordinator to communicate by referring to the information on the contention period contained in the management frame. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0023]      FIG. 1  is a diagram illustrating the transmission of data between stations using a PCF method;  
         [0024]      FIG. 2  is a diagram illustrating a DCF-based back-off procedure;  
         [0025]      FIG. 3  is a diagram illustrating the structure of an IEEE 802.11 management frame;  
         [0026]      FIG. 4  is a diagram illustrating the structure of a frame body of  FIG. 3 ;  
         [0027]      FIG. 5  is a table showing various types of element identifications (IDs) recorded in an element ID field of  FIG. 4 ;  
         [0028]      FIG. 6  is a diagram illustrating the structure of a MIMO cyclic prefix (CP) parameter set of  FIG. 5 ;  
         [0029]      FIG. 7  is a diagram illustrating a CFP repetition interval; and  
         [0030]      FIG. 8  is a flowchart of a wireless communication method according to an exemplary embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]     A wireless communication method according to an exemplary embodiment of the present invention will now be described more fully with reference to the accompanying drawings.  
         [0032]      FIG. 3  is a diagram illustrating a management frame  100  based on the IEEE 802.11 standard. The management frame  100  may be a beacon frame, an association request frame, a dissociation frame, an association response frame, a probe request frame, and a probe response frame.  
         [0033]     The management frame  100  includes a media access control (MAC) header  110 , a frame body  120 , and a frame check sequence (FCS) field  130 . The MAC header  110  includes a frame control field, a duration field, a destination address field, a source address field, a basic service set identification (ID) field, and a sequence control field.  
         [0034]     The frame body  120  of the management frame  100  is filled with one or more information elements (IEs)  200 , as shown in  FIG. 4 . Each of the IEs  200  includes an element ID field  210  in which the types of the IE  200  are recorded, an information field  230  in which data to be actually transmitted through the IE  200  is recorded, and a length field  220  in which the size of the information field  230  is recorded.  
         [0035]      FIG. 5  is a table showing types of element IDs that are recorded in an element ID field  210  of  FIG. 4 . Referring to  FIG. 5 , it is possible to determine what information is included in the information element  200  by referring to element IDs recorded in the element ID field  210  of the information element  200 . The element ID field  210 , unlike a conventional element ID field, includes a MIMO CP parameter set  310  having an element ID of 41. The MIMO CP parameter set  310  may be identified by a number other than 41. The MIMO CP parameter set  310  will be described later in detail with reference to  FIG. 6 .  
         [0036]      FIG. 6  is a diagram illustrating the structure of the MIMO CP parameter set  310  of  FIG. 5 . Referring to  FIG. 6 , the MIMO CP parameter set  310  includes an element ID field  410 , a length field  420 , and a MIMO CP duration field  430 .  
         [0037]     An element ID of the MIMO CP parameter set  310  is recorded in the Element ID field  410 . For example, if the element ID of the MIMO CP parameter set  310  is 41, as shown in  FIG. 5 , 41 may be recorded in the element ID field  410 . The size of the MIMO CP duration field  430  is recorded in the length field  420 . The MIMO CP duration field  430  contains information on a MIMO contention period in which MIMO stations contend with each other for a channel. For example, the duration of the MIMO contention period may be recorded in the MIMO CP duration field  430 . Therefore, when receiving the MIMO CP parameter set  310  from a point coordinator, MIMO stations interpret information recorded in the MIMO CP duration field  430  and determine when they are able to occupy a channel based on the interpretation. A reserved field (not shown) may be added to the MIMO CP parameter set  310 , in which case, additional information on the MIMO CP parameter set  310  may be recorded in the reserved field.  
         [0038]     The MIMO CP parameter set  310  is preferably, but not necessarily, included in a beacon frame, which is one type of management frame.  
         [0039]      FIG. 7  is a diagram illustrating a CFP repetition interval according to an exemplary embodiment of the present invention.  
         [0040]     Referring to  FIG. 7 , the CFP repetition interval is comprised of a CFP  510 , a MIMO contention period  520 , and a contention period  530 .  
         [0041]     Part of a conventional CFP is designated as the MIMO contention period  520 . During the CFP  510 , stations transmit frames in a predetermined order in a conventional manner. In other words, the station that is polled first by a point coordinator transmits a frame ahead of the rest of the stations, and the rest of the stations wait until they are polled by the point coordinator.  
         [0042]     During the MIMO contention period  520 , MIMO stations contend with each other for a channel using the back-off algorithm described above with reference to  FIG. 2 . The MIMO stations receive a management frame containing a MIMO CP parameter set, which has been described above with reference to  FIG. 6 , and obtain information on the MIMO contention period  520  by interpreting the MIMO CP parameter set.  
         [0043]     However, conventional stations based on the IEEE 802.11a standard, i.e., SISO stations, are not able to interpret the MIMO CP parameter set by themselves. Thus, the SISO stations keep communicating in a PCF manner throughout a maximum CFP  540  including the CFP  510  and the MIMO contention period  520 .  
         [0044]     If the maximum CFP  540  ends in response to a CFP end frame transmitted by the point coordinator, the contention period  540  begins. During the contention period  540 , the SISO stations contend with each other for the channel to transmit SISO data. The MIMO stations may participate in the contention for the channel along with the SISO stations.  
         [0045]      FIG. 8  is a flowchart of a wireless communication method according to an exemplary embodiment of the present invention.  
         [0046]     Referring to  FIGS. 7 and 8 , in operation S 110 , a point coordinator generates a management frame including information on the MIMO contention period  520  and transmits the management frame to stations in its wireless network. The management frame generated in operation S 110  may include information elements, such as a CFP parameter set containing information on the maximum CFP  540  and a MIMO CP parameter set described above with reference to  FIG. 6 . The management frame generated in operation S 110  is preferably, but not necessarily, a beacon frame ( 512  of  FIG. 7 ) that follows the IEEE 802.11 standard.  
         [0047]     In operation S 120 , each of the stations receives the management frame generated in operation S 10  from the point coordinator and waits to be polled by the point coordinator. Of the stations, MIMO stations interpret the MIMO CP parameter set contained in the management frame received from the point coordinator and set their network allocation vector (NAV) values using only the CFP  510 . However, since SISO stations cannot interpret the MIMO CP parameter set by themselves, they spend more time than the MIMO stations in setting their NAV values. In other words, the SISO stations set their NAV values using the entire maximum CFP  540 . The stations secure a channel in a predetermined order using the PCF method described above with reference to  FIG. 1  while setting their NAV values.  
         [0048]     In operation S 130 , the CFP  510  ends, and the MIMO contention period  520  begins. Specifically, when the transmission of data from the station that has been most recently polled by the point coordinator during CFP  510  is complete, the MIMO stations contend with each other for the channel. For example, when a distributed inter-frame space (DIFS) period following the CFP  510  ends, a contention window having a predetermined size is set for each of the MIMO stations. Random slots (i.e., back-off time) having the same probability of being selected through a back-off algorithm are respectively allotted to the MIMO stations that participate in the contention for the channel.  
         [0049]     The MIMO station having minimum back-off time secures the channel and transmits a frame using the channel ahead of the rest of the MIMO stations. In the same way, the rest of the MIMO stations secure the channel in the order determined through the back-off algorithm. The order in which the MIMO stations secure the channel may be determined using a DCF method, as described above with reference to  FIG. 2 .  
         [0050]     However, since the SISO stations are not able to interpret by themselves the MIMO CP parameter set contained in the management frame transmitted from the point coordinator, they can secure the channel only when they are polled by the point coordinator even during the MIMO contention period.  
         [0051]     The MIMO contention period  520  ends along with the maximum CFP  540 . For example, when the point coordinator transmits a CFP end frame following the IEEE 802.11 standard, the maximum CFP  540  including the MIMO contention period  520  ends, and the contention period  530  begins in operation S 140 . During the contention period  530 , the SISO stations contend for the channel. This type of contention-based channel securing method may be performed using the back-off algorithm described above with reference to  FIG. 2 . During the contention period  530 , the MIMO stations participate in the contention for the channel along with the SISO stations.  
         [0052]     Although the present invention has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom. Therefore, the described embodiments are to be considered in all respects only as illustrative and not restrictive and the scope of the invention.  
         [0053]     As described above, according to the present invention, it is possible to prevent collision of frames transmitted from MIMO stations and from conventional stations based on the IEEE 802.11a standard in a wireless infrastructure network.