Abstract:
The present invention relates to a method for estimating an optimized transmission bit rate in a wireless local area network (LAN) system, which is to provide an accurate data transmission bit rate according to channel status when block data are transmitted in the wireless LAN system. According to the present invention, there is provided a method for estimating an optimized transmission bit rate according to the status of a data transmission bit rate in a wireless LAN system, comprising the steps of determining whether to change a current data transmission bit rate in a wireless LAN network; and estimating an optimized transmission bit rate on the basis of results of the determination in the determining step, and reflecting the estimated transmission bit rate upon transmission of data. The method is effective in that when data blocks are transmitted in the wireless LAN system, the performance degradation of data transmission invited due to an inappropriate selection of a transmission bit rate can be reduced.

Description:
CLAIM OF PRIORITY 
     This application claims priority under 35 USC 119 to Korean Patent Application No. 10-2007-0013583, filed on Feb. 9, 2007, which is incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for estimating an optimized transmission bit rate in a wireless local area network (LAN) system, and more particularly, to a method for estimating an optimized transmission bit rate in a wireless LAN system, which is to provide an accurate data transmission bit rate according to channel status when block data are transmitted in the wireless LAN system. 
     2. Description of the Related Art 
     The physical layer of a wireless LAN system to which IEEE 802.11 is applied supports a variety of transmission bit rates using different modulation and coding schemes. For example, IEEE 802.11a supports eight transmission bit rates between 6 Mbps to 54 Mbps, IEEE 802.11b supports four transmission bit rates between 1 Mbps to 11 Mbps, and IEEE 802.11g supports twelve transmission bit rates between 1 Mbps to 54 Mbps. Here, since each of the transmission bit rates has individual bit error characteristics, there usually exists a single optimized transmission bit rate in a given channel condition, and a lower transmission bit rate generally has further low bit error characteristics. 
     Accordingly, mobile communication network operators are encouraged to contrive a method for increasing a throughput by applying an optimized transmission bit rate, in which channel status changed in real-time according to time and space is reflected, to a mobile communication terminal and a mobile communication system. 
     In order to solve the problem, a method for estimating a transmission bit rate considering channel status has been applied to a mobile communication terminal. However, since the channel status is inaccurately measured when estimating a transmission bit rate or the channel status is rapidly changed, the mobile communication terminal frequently select a non-optimized transmission bit rate. 
     If it is attempted to transmit data at a transmission bit rate higher than an optimized transmission bit rate, the probability for the data transmission to fail is high. Contrarily, if it is attempted to transmit data at a transmission bit rate lower than the optimized transmission bit rate, the success rate of the data transmission is increased. However, there is a problem in that transmission efficiency is decreased since the channel is unnecessarily occupied for an extended period of time. 
     The side-effect of selecting an inappropriate transmission bit rate gets severer when the block acknowledgement (BA) scheme defined in the IEEE 802.11e is applied. 
     In more detail, if a plurality of data frames (MAC protocol data unit: MPDU), which are grouped as a block, are consecutively transmitted, only one block acknowledgement is transmitted in response thereto. Therefore, waste of bandwidth can be reduced as compared with transmitting an acknowledgement for each data frame. However, since a plurality of data frames are transmitted at a time before transmitting an acknowledgement, the same transmission bit rate is applied to each of corresponding data frames that are grouped as a block. 
     Accordingly, when transmitting data using the block acknowledgement scheme, inappropriate increase of a transmission bit rate invites transmission failure of a plurality of frames, and inappropriate decrease of a transmission bit rate invites waste of channel bandwidth for an extended period of time compared with transmitting a single frame. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is conceived to solve the aforementioned problems in the prior art. An object of the present invention is to provide a method for estimating an optimized transmission bit rate in a wireless LAN system, wherein channel status of a network is more accurately measured when data blocks are transmitted in the wireless LAN system, thereby increasing data transmission success rate. 
     Another object of the present invention is to provide a method for estimating an optimized transmission bit rate in a wireless LAN system, wherein when a transmission bit rate is changed in the wireless LAN system in consideration of channel status of a network, a changed transmission bit rate is first applied only to some of a plurality of frames contained in block data to gradually change the data transmission bit rate according to the channel status of the network. 
     According to an aspect of the present invention for achieving the object, there is provided a method for estimating an optimized transmission bit rate according to the status of a data transmission bit rate in a wireless LAN system, comprising the steps of determining whether to change a current data transmission bit rate in a wireless LAN network; and estimating an optimized transmission bit rate on the basis of results of the determination in the determining step, and reflecting the estimated transmission bit rate upon transmission of data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view briefly illustrating a method for estimating an optimized transmission bit rate according to the present invention; 
         FIG. 2  is a flowchart illustrating an embodiment of the method for estimating an optimized transmission bit rate according to the present invention; 
         FIG. 3  is a flowchart illustrating another embodiment of the method for estimating an optimized transmission bit rate according to the present invention; and 
         FIG. 4  is a flowchart illustrating a further embodiment of the method for estimating an optimized transmission bit rate according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the accompanying drawings. 
     Hereinafter, a method for estimating an optimized transmission bit rate in a wireless LAN system, which is recited in the present invention, can be applied to an access point (AP) or a mobile communication terminal, such as a handheld phone, cellular phone, personal data assistant (PDA), personal computer (PC) capable of short distance communication, which can wireless communicate with the wireless LAN system. 
     First,  FIG. 1  is a view briefly illustrating a method for estimating an optimized transmission bit rate according to the present invention. 
     As shown in  FIG. 1 , a terminal (not shown) that transmits data in a wireless LAN system changes a current data transmission bit rate considering channel status of a network according to the status of a transmission bit rate applied to the terminal itself. 
     In more detail, when the terminal transmits a data block, i.e., a plurality of data frames configured into a group, data in each of a plurality of the data frames are transmitted at the same transmission bit rate or at a transmission bit rate i or i+1. As the channel environment of the wireless LAN network is changed, the terminal sequentially changes the status of a data transmission bit rate (transmission bit rate i→transmission bit rate i, i+1→transmission bit rate i+1, or transmission bit rate i+1→transmission bit rate i, i+1→transmission bit rate i) in consideration of the status of the data transmission bit rate applied to the terminal itself as shown in  FIG. 1 . 
     For example, the status of a data transmission bit rate is divided into a single transmission bit rate where the same transmission bit rate i or i+1 is applied to all of a plurality of data frames contained in the same data block, and a composite transmission bit rate where transmission bit rates i and i+1 are compositely applied to a plurality of data frames contained in the same data block. 
     The purpose of the composite transmission bit rate is to reduce transmission failure that can be generated since only a transmission bit rate i or a transmission bit rate i+1 is applied to all data frames in a data block when the data transmission bit rate of the terminal is changed, and to gradually change the data transmission bit rate according to the channel environment of the wireless LAN network by applying a transmission bit rate i to some of the frames in a data block and applying a transmission bit rate i+1 to the other data frames in the data block. 
     The terminal shown in  FIG. 1  is either an AP or a mobile communication terminal capable of communicating with a wireless LAN system. 
       FIG. 2  is a flowchart illustrating an embodiment of the method for estimating an optimized transmission bit rate according to the present invention, which will be described with an example where the status of the data transmission bit rate of a terminal is a composite transmission bit rate (both of transmission bit rates i and i+1 are compositely applied to the data frames). 
     First, the terminal calculates a first block throughput U* for the current data transmission bit rate and a second block throughput U+ for a data transmission bit rate that is one level higher than the current data transmission bit rate, using pre-set mathematical expression 1, as data blocks are transmitted in the wireless LAN network (step S 101 ).
 
 U (( r   i   ,r   j ),( p   s   i   ,p   s   j ))=( p   s   i   k   i   +p   s   j   k   j )/ T   b ( r   i   ,r   j )  [Mathematical expression 1]
 
where r i  denotes the bit rate of a transmission bit rate i, r j  denotes the bit rate of a transmission bit rate j, p s   i  denotes transmission success probability of a data frame transmitted at the transmission bit rate i, p s   j  denotes transmission success probability of a data frame transmitted at the transmission bit rate j, k i  denotes the total number of bytes of payload data transmitted at the transmission bit rate i in a block, k j  denotes the total number of bytes of payload data transmitted at the transmission bit rate j in a block, and T b (r i ,r j ) denotes time needed for block transmission and block acknowledgement therefor. The transmission success probability of a data frame is shown in mathematical expression 2.
 
 p   s   i   =l   i   /N   i   [Mathematical expression 2]
 
     where l i  denotes the number of data frames successfully transmitted at the transmission bit rate i in a block, and N i  denotes the number of data frames transmitted at the transmission bit rate i in a block. 
     Next, the terminal compares the first block throughput U* and the second block throughput U+ that are calculated in step S 101 , and confirms whether the first block throughput U* is lower than the second block throughput U+ (U((r i+1 ,r i+1 ),(p s   i+1 ,p s   i+1 ))&gt;U((r i ,r i+1 ),(p s   i ,p s   i+1 )) of  FIG. 1 ) (step S 103 ). 
     If the first block throughput U* is lower than the second block throughput U+ in step S 103 , the terminal increases a data transmission success count by one (the step of increasing a data transmission success count by one in  FIG. 2 ) (step S 105 ), and compares it with a pre-set success threshold count S th  (step S 107 ). 
     If the data transmission success count (the data transmission success count increased by one) is larger than or equal to the pre-set success threshold count in step S 107 , the terminal increases the current data transmission bit rate by one level and reflects it when the data transmission (step S 109 ). That is, an optimized transmission bit rate is a value that is one level higher than the current data transmission bit rate. 
     In the meantime, if the first block throughput U* is not lower than the second block throughput U+ in step S 103 , the terminal terminates the process of estimating a transmission bit rate. 
     On the other hand, if the data transmission success count is smaller than the pre-set success threshold count in step S 107 , the terminal performs the steps after step S 101 . 
       FIG. 3  is a flowchart illustrating another embodiment of the method for estimating an optimized transmission bit rate according to the present invention, which will be described with an example where the status of the data transmission bit rate of a terminal is a composite transmission bit rate (both of transmission bit rates i and i+1 are compositely applied to the data frames), and an example where the status of the data transmission bit rate of a terminal is a single transmission bit rate (either of a transmission bit rate i or i+1 is applied to the data frames) and it is determined whether to decrease the current data transmission bit rate. 
     First, the terminal calculates a first block throughput U* for the current data transmission bit rate and a third block throughput U− for a data transmission bit rate that is one level lower than the current data transmission bit rate, using pre-set mathematical expression 1, as data blocks are transmitted in the wireless LAN network (step S 201 ). 
     Here, if the status of the data transmission bit rate is a single transmission bit rate and it is determined whether to decrease the current data transmission bit rate, when the terminal calculates the third block throughput U−, the transmission success probability of a data transmission bit rate that is one level lower than the current data transmission bit rate is assumed to be one. 
     Next, the terminal compares the first block throughput U* and the third block throughput U− that are calculated in step S 201 , and confirms whether the first block throughput U* is lower than the third block throughput U− (step S 203 ). 
     At this time, a case where the status of the data transmission bit rate of the terminal is a composite transmission bit rate can be expressed in mathematical expression U((r i ,r i+1 ),(p s   i ,p s   i+1 ))&lt;U((r i ,r i ),(p s   i ,p s   i )) of  FIG. 1 , and a case where the status of the data transmission bit rate of the terminal is a single transmission bit rate and it is determined whether to decrease the current data transmission bit rate can be expressed in mathematical expression U((r i+1 ,r i+1 ), (p s   i+1 ,p s   i+1 ))&lt;U((r i ,r i+1 ),(1,p s   i+1 )) of  FIG. 1 . 
     If the first block throughput U* is lower than the third block throughput U− in step S 203 , the terminal increases a data transmission failure count by one (the step of increasing a data transmission failure count by one in  FIG. 3 ) (step S 205 ), and compares it with a pre-set failure threshold count F th  (step S 207 ). 
     If the data transmission failure count (the data transmission failure count increased by one) is larger than or equal to the pre-set failure threshold count in step S 207 , the terminal decreases the current data transmission bit rate by one level and reflects it when the data transmission (step S 209 ). That is, an optimized transmission bit rate is a value that is one level lower than the current data transmission bit rate. 
     In the meantime, if the first block throughput U* is not lower than the third block throughput U− in step S 203 , the terminal terminates the process of estimating a transmission bit rate. 
     On the other hand, if the data transmission failure count is smaller than the pre-set failure threshold count in step S 207 , the terminal performs the steps after step S 201 . 
       FIG. 4  is a flowchart illustrating a further embodiment of the method for estimating an optimized transmission bit rate according to the present invention, which will be described with an example where the status of the data transmission bit rate of a terminal is a single transmission bit rate (either of a transmission bit rate i or i+1 is applied to the data frames) and it is determined whether to increase the current data transmission bit rate. 
     First, the terminal calculates a difference between the number m of successfully transmitted data frames and the number n of unsuccessfully transmitted data frames in a specific data block (step S 301 ). 
     Next, the terminal compares the difference calculated in step S 301  with a pre-set threshold M th , and confirms whether the difference between the number m of successfully transmitted data frames and the number n of unsuccessfully transmitted data frames is larger than or equal to the threshold M th  (m−n≧M th  in  FIG. 1 ) (step S 303 ). 
     If the difference between the number m of successfully transmitted data frames and the number n of unsuccessfully transmitted data frames is larger than or equal to the threshold M th  in step S 303 , the terminal increases a data transmission success count by one (the step of increasing a data transmission success count by one in  FIG. 4 ) (step S 305 ), and compares it with a pre-set success threshold count S th  (step S 307 ). 
     If the data transmission success count (the data transmission success count increased by one) is larger than or equal to the pre-set success threshold count in step S 307 , the terminal increases the current data transmission bit rate by one level and reflects it when the data transmission (step S 309 ). That is, an optimized transmission bit rate becomes a value that is one level higher than the current data transmission bit rate. 
     For example, in a case where it is assumed that a data block includes four data frames and the threshold M th  is two, if all of the data frames in the data block are successfully transmitted (m=4, n=0) or three of the data frames are successfully transmitted (m=3, n=1), the terminal determines that a one-level-higher data transmission bit rate is preferable to the current data transmission bit rate. 
     In the meantime, if the difference between the number m of successfully transmitted data frames and the number n of unsuccessfully transmitted data frames is smaller than the threshold M th  in step S 303 , the terminal maintains the current data transmission bit rate (step S 311 ) and terminates the process of estimating a transmission bit rate. 
     On the other hand, if the data transmission success count is smaller than the pre-set success threshold count in step S 307 , the terminal performs the steps after step S 301 . 
     Accordingly, a method for estimating an optimized transmission bit rate in a wireless LAN system of the present invention is effective in that when transmitting a data block, the wireless LAN system changes a transmission bit rate for some frames in the block before changing the data transmission bit rate according to a channel environment of a network to thereby reduce the performance degradation of data transmission invited due to an inappropriate selection of a transmission bit rate. 
     Further, the present invention is effective in that a channel can be further accurately measured since the success probability of a plurality of data transmission bit rates can be simultaneously confirmed. 
     Furthermore, since a method for estimating an optimized transmission bit rate in a wireless LAN system of the present invention can be implemented in a terminal to which IEEE 802.11e is applied without modifying the IEEE 802.11e, it is effective in that the costs needed for implementing the technique can be reduced. 
     The present invention can be implemented in other specific forms without changing the technical spirit and essential feature of the present invention, so that it should be understood by those skilled in the art that the embodiments described and illustrated above are merely for illustrative purposes and not for limitation purposes. The true scope of the present invention should be defined by the technical spirit of the appended claims rather than the above detailed descriptions. In addition, the present invention should be interpreted to encompass all modifications and changes that would occur from the spirit and scope of the claims and the equivalence thereof.