Abstract:
Disclosed is a link margin adaptation method using feedback information of a wireless communication system. For transmission and reception of data between two nodes of the wireless communication system, a transmission node requests a receiving node to transmit a link margin. The transmission node receives the link margin, and selects one of a white noise table or a delay spread table according to a delay spread value between the two nodes. Then, the transmission node adapts a link between the two nodes to a transmission mode and decides a transmission rate to transmit data at the decided transmission rate, thereby improving transmission capacity of the wireless communication system, obtaining the optimum link state, extending power utilization time of stations, and reducing interference between stations.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application is based on Korea Patent Application No. 2003-88540 filed on Dec. 8, 2003 in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   (a) Field of the Invention 
   The present invention relates to a link adaptation method using feedback information for a wireless communication system. More particularly, the invention relates to a link adaptation method using feedback information for a wireless communication system, which transmits and receives data depending on the state of a data transmission link to increase transmission capacity. 
   (b) Description of the Related Art 
   Link adaptation in a data communication system is a technique that selects a transmission power, a modulation method, a packet length, etc. depending on the state of a link through which data is transmitted and received so as to increase the transmission capacity of the data communication system. The link adaptation technique is divided into a technique of supposing symmetry of a link, and a technique of supposing asymmetry of a link. The former considers the state of a data transmission link from an access point (AP) to a station (STA) to be identical to that of a transmission link from the STA to the AP, while the latter supposes that the state of the data transmission link from the AP to the STA is different from the state of the data transmission link from the STA to the AP. 
     FIG. 1  shows a forward link and a backward link of a wireless communication system including an AP  110  and a STA  120 . Referring to  FIG. 1 , a data transmission link from the AP  110  to the STA  120  is called a forward link and a data transmission link from the STA  120  to the AP  110  is called a backward link, in general. 
   When the two links are asymmetrical, a level of interference at a receiving side is varied with the position of the receiving side and it is different from a level of interference at a transmitting side. Thus, a transmission power and a modulation method for data transmission of the transmitting side should be decided at least partially from the standpoint of the receiving side. 
   For this, the STA monitors a channel state of the forward link to propose a transmission modulation method suitable for the AP when the STA communicates with the AP. Furthermore, link adaptation through a feedback method is required when backward data needed to decide the modulation method is transmitted. 
   A conventional data transmission and reception technique depending on a wireless link state is disclosed in U.S. Pat. No. 6,567,375, entitled “Method and Apparatus for Packet Size Dependent Link Adaptation for Wireless Packet”, which proposed a technique that changes a packet length and a coding method depending on a wireless link state to reduce data transmission delay time. However, this technique did not consider various parameters in a wireless communication system other than the packet length and coding method, and proposed only a method of shortening the data transmission delay time. Furthermore, it cannot solve problems with respect to system transmission capacity and power consumption of stations. 
   SUMMARY OF THE INVENTION 
   It is an advantage of the present invention to provide a link adaptation method using feedback information for a wireless communication system, which performs link adaptation in consideration of various parameters applied to the wireless communication system to reduce data transmission delay time, increase the transmission capacity of the communication system, and decrease power consumption of the system. 
   In one aspect of the present invention, a link adaptation method using feedback information of a wireless data communication system for transmission and reception of data between at least two nodes of the wireless data communication system, comprises a step (a) in which a first node transmits a link margin information request message to a second node; a step (b) in which the second node estimates a SNR value between the first and second nodes using the link margin information request message, and compares the estimated SNR value with a predetermined SNR value designated in a white noise table to presume a link margin value; a step (c) in which the second node selects one of a predetermined white noise table and a delay spread table to presume a link margin according to the comparison result, and transmits the presumed link margin to the first node; and a step (d) in which the first node adjusts a transmission rate and transmission power according to a current data transmission mode using the presumed link margin received from the second node, and one of the white noise table and delay spread table, selected by the first node. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention, and, together with the description, serve to explain the principles of the invention: 
       FIG. 1  shows a forward link and a backward link of a wireless communication system including an AP and a STA; 
       FIG. 2  shows a procedure of transmitting a link margin request and reporting a link margin for link adaptation according to an embodiment of the present invention; 
       FIG. 3  shows a procedure of exchanging information required for link adaptation between stations and operations carried out between a physical layer and a media access control layer of each station; 
       FIG. 4  is a flow chart of link adaptation in a normal mode according to an embodiment of the present invention; 
       FIG. 5  is a flow chart of link adaptation in a high-rate data transmission mode according to an embodiment of the present invention; and 
       FIG. 6  is a flow chart of link adaptation in a transmission power saving mode according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the following detailed description, only the preferred embodiment of the invention has been shown and described, simply by way of illustration of the best mode contemplated by the inventor(s) of carrying out the invention. As will be realized, the invention is capable of modification in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not restrictive. 
   The present invention proposes a link adaptation method in consideration of a feedback mechanism for a communication link or channel in order to decide the optimum modulation method and transmission power for a wireless data communication link. 
   Feedback information used for the link adaptation method of the invention is a link margin that is defined as a difference between a signal-to-noise ratio measured at a receiver and a signal-to-noise ratio suitable for a method of modulating currently received data. 
   The link margin means a power level higher than a signal power required for the current communication modulation method from the standpoint of the receiver. When the link margin is a positive number, it represents that reception power is higher than a required level. When the link margin is a negative number, it means a lack of reception power. 
   A procedure of calculating a link margin value and feeding back it is described below. 
     FIG. 2  shows a procedure of transmitting a link margin request and reporting a link margin for link adaptation according to an embodiment of the present invention. Referring to  FIG. 2 , when a STA  121  transmits/receives data to/from a STA  122 , the STA  121  delivers a link margin request packet to the STA  122  to grasp the state of a wireless data line connected to the STA  122  in step S 201 . 
   The STA  122  estimates the state of the link, which is represented by a signal-to-noise ratio (referred to as SNT hereinafter), using the link margin request packet to presume a link margin that represents a difference between a desired transmission power and currently used transmission power of the STA  121  in units of dB in step S 202 . Then, the STA  122  transmits a link margin report packet including the presumed link margin information to the STA  121  in step S 203 . 
   The STA  121  adjusts a transmission power and a modulation method depending on the received link margin report packet in step S 204 , and applies the adjusted transmission power and modulation method to transmission of next data in step S 205 . Here, the operation of requesting the link margin and adjusting the transmission power and modulation method depending on the link margin is called link adaptation. The link adaptation is carried out in each of a physical layer and a media access control layer. 
     FIG. 3  shows a procedure of exchanging information required for link adaptation between the stations and operations carried out between a physical layer and a media access control layer of each station. Referring to  FIG. 3 , first of all, the STA  121  transmits the link margin request packet to the STA  122  in step S 301 . Here, the link margin request packet includes information about a modulation method used in a corresponding link and information about a data transmission rate. 
   The STA  122  that has received the link margin request packet demodulates the packet to extract information about the modulation method and data transmission rate and estimates a SNR in step S 302 , which is carried out by the physical layer of the STA  122 . 
   The modulation method, data transmission rate, and SNR obtained by the physical layer are delivered to the media access control layer of the STA  122 . Then, the media access control layer obtains the link margin using the modulation method, data transmission rate, SNR, and a predetermined table in step S 303 . The table represents data transmission rates and SNRs required therefor and uses a white noise table in a white noise environment. 
   Table 1 shows a general form of a table in the white noise channel environment. 
   
     
       
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
                 
               SNR: Table. Transmission rate n required 
             
             
                 
               in white noise channel environment 
             
             
               Transmission rate 
               for satisfying transmission rate n 
             
             
                 
             
           
           
             
               Transmission rate 1 
               SNRw 1   
             
             
               (modulation method 1) 
             
             
               Transmission rate 2 
               SNRw 2   
             
             
               (modulation method 2) 
             
             
               . 
               . 
             
             
               . 
               . 
             
             
               . 
               . 
             
             
               Transmission rate n 
               SNRw n   
             
             
               (modulation method n) 
             
             
               . 
               . 
             
             
               . 
               . 
             
             
               . 
               . 
             
             
               Transmission rate N 
               SNRw N   
             
             
               (modulation method N) 
             
             
                 
             
           
        
       
     
   
   In Table 1, SNRw n  denotes a signal-to-noise ratio required for achieving transmission rate n that is a function of modulation method n in the white noise environment. Table.Transmission rate n represents a signal-to-noise ratio that a given channel environment (white noise or delay spread channel) and transmission rate require. 
   When the physical layer and media access control layer of the STA  122  complete the step of presuming the link margin using the white noise table of Table 1, the STA  122  reports the link margin to the STA  121  using the link margin report packet in step S 304 . 
   The physical layer of the STA  121  demodulates the received link margin report packet to extract a link margin presumption value and estimates a delay spread value of the link between the STA  121  and STA  122  in step S 305 . Here, the delay spread value is used for deciding an actually required transmission power and transmission modulation method. Furthermore, the delay spread value is utilized to select a table in a white noise channel environment or a table in a delay spread channel environment. Table 2 shows the table in the delay spread channel environment. 
   
     
       
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
                 
               SNR: Table. Transmission rate n required 
             
             
                 
               in delay spread channel environment 
             
             
               Transmission rate 
               for satisfying transmission rate n 
             
             
                 
             
           
           
             
               Transmission rate 1 
               SNRd 1  (delay spread value) 
             
             
               (modulation method 1) 
             
             
               Transmission rate 2 
               SNRd 2  (delay spread value) 
             
             
               (modulation method 2) 
             
             
               . 
               . 
             
             
               . 
               . 
             
             
               . 
               . 
             
             
               Transmission rate n 
               SNRd n  (delay spread value) 
             
             
               (modulation method n) 
             
             
               . 
               . 
             
             
               . 
               . 
             
             
               . 
               . 
             
             
               Transmission rate N 
               SNRd N  (delay spread value) 
             
             
               (modulation method N) 
             
             
                 
             
           
        
       
     
   
   In Table 2, SNRd n  represents a signal-to-noise ratio required for achieving transmission rate n in the delay spread channel environment. Transmission 1 is lower than transmission N, and transmission 2 through transmission N−1 are sequentially increased. Table.Transmission rate n, that is, a required transmission power, is increased as transmission N is increased. 
   The link margin presumption value extracted by the physical layer of the STA  121  and the delay spread value estimated by the physical layer are delivered to the media access control layer. The media access control layer estimates a SNR value based on the link margin presumption value and decides a SNR value required for the modulation method depending on the delay spread value in step S 306 . The media access control layers of the STA  121  and STA  122  share a table such as Table 1 and Table 2. 
   SNR a2b  in step S 306 , shown in  FIG. 3 , is an estimated SNR of the channel that links the STA  121  to the STA  122 . 
   The media access control layer of the STA  121  delivers a transmission rate and transmission power of subsequent data to the physical layer. Then, the physical layer applies the received values to the current transmission rate and transmission power to adjust them in step S 307 . 
   Data subsequently transmitted from the STA  121  to the STA  122  is delivered with the adjusted transmission rate and transmission power in step S 308 . 
   The link adaptation procedure in various modes is explained below in more detail. 
     FIG. 4  is a flow chart of link adaptation in a normal mode according to an embodiment of the present invention. Referring to  FIG. 4 , the link adaptation includes a process S 410  of selecting a table and a process S 429  of selecting a transmission rate and a transmission power. 
   Specifically, the STA  121  compares the delay spread value estimated using the link margin report received from the STA  122  with a predetermined delay spread threshold value in step S 411 . When the estimated delay spread value is greater than the predetermined delay spread threshold value, the STA  121  selects the delay spread table of Table 2 as a reference table in step S 413 . When the estimated delay spread value is less than or identical to the predetermined delay spread threshold value, the STA  121  selects the white noise table of Table 1 as the reference table in step S 412 . 
   When the reference table is selected, the STA  121  compares the estimated SNR a2b  transmitted from the STA  122  with the SNR supporting a maximum transmission rate in the reference table, that is, Table.Transmission rate N (=SNRd N  or SNRw N ) in step S 421 . When SNR a2b  is higher than Table.Transmission rate N, which means that the transmission power is unnecessarily used, the STA  121  reduces the transmission power until SNR a2b  corresponds to Table.Transmission rate N in step S 422 . When the reduced transmission power corresponds to transmission rate N, the transmission power is used for transmitting data in step S 427 . 
   When SNR a2b  is lower than Table.Transmission rate N in step S 421 , the STA  121  compares SNR a2b  with Table.Transmission rate 1 supporting a minimum transmission rate in step S 423 . If SNR a2b  is lower than Table.Transmission rate 1, the STA  121  increases the transmission power of SNR a2b  until it corresponds to the transmission power of Table.Transmission rate 1 in step S 424 , and sets a wireless communication transmission rate to transmission rate 1 in step S 426 . 
   If SNR a2b  is lower than Table.Transmission rate 1 even when the transmission power is increased to the maximum power in step S 425 , the minimum transmission rate, transmission rate 1, is selected as the transmission rate because the maximum power is restricted by the wireless communication system. The minimum transmission rate is used when subsequent data is transmitted in step S 426 . Here, a frame length can be decreased if required. 
   When SNR a2b  is increased to become higher than Table.Transmission rate 1 in step S 424 , a transmission rate is selected depending on the table selected in step S 410  and used for data transmission in step S 428 . 
     FIG. 5  is a flow chart of link adaptation in a high-rate data transmission mode according to an embodiment of the present invention. In the high-speed data transmission mode, a table-selecting process S 510  corresponds to the table-selecting process  410  shown in  FIG. 4 . 
   After the table is selected, the STA  121  compares SNR a2b  with Table.Transmission rate N in step S 521 . When SNR a2b  is higher than Table.Transmission rate N, the STA  121  reduces the transmission power until SNR a2b  corresponds to Table.Transmission rate N in step S 522  and uses transmission rate N as the transmission rate in step S 523 . 
   When SNR a2b  is lower than Table.Transmission rate N in step S 521 , the STA  121  increases the transmission power to the maximum power permitted by the system, and calculates SNR a2b  depending on the maximum power in step S 524 . When SNR a2b  is higher than Table.Transmission rate 1, the STA  121  selects a transmission rate according to the selected table in step S 526 . When SNR a2b  is lower than Table.Transmission rate 1, the STA uses transmission rate 1 as the transmission rate in step S 527 . Here, the frame length can be reduced if required. 
   The above-described operation selects a transmission rate for transmitting data at a high rate while minimizing unnecessary power consumption. 
     FIG. 6  is a flow chart of link adaptation in a transmission power saving mode according to an embodiment of the present invention. In the transmission power saving mode, a table-selecting process S 610  corresponds to those of  FIGS. 4 and 5 . 
   When the table is selected in step S 610 , the STA  121  compares SNR a2b  with Table.Transmission rate 1 in step S 621  in order to select a transmission power depending on Table.Transmission rate 1 corresponding to the minimum transmission power. When SNR a2b  is higher than Table.Transmission rate 1, the STA  121  reduces the transmission power until SNR a2b  corresponds to Table.Transmission rate 1 in step S 622 , and uses Table.Transmission rate 1 as the transmission rate in step S 625 . 
   When SNR a2b  is lower than Table.Transmission rate 1 in step S 621 , the STA  121  increases the transmission power until SNR a2b  corresponds to Table.Transmission rate 1 in step S 623 . Here, the transmission power is restricted by the wireless communication system. Accordingly, If SNR a2b  is lower than Table.Transmission rate 1 even when the transmission power has been increased to the maximum power, transmission rate 1 is used as the transmission rate in step S 625 . When SNR a2b  becomes higher than or identical to Table.Transmission rate 1, data is transmitted at transmission rate N in step S 626 . 
   According to the aforementioned methods, link adaptation between the receiver and transmitter in the wireless data communication system is carried out in the normal, high-rate data transmission and transmission power saving modes to obtain the optimum result in each mode. 
   While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 
   As described above, the link adaptation method using feedback information for a wireless communication system according to the present invention can perform link adaptation between a transmitter and a receiver using a feedback mechanism between the transmitter and receiver to improve the transmission capacity of the system. Furthermore, the present invention can optimize a link state, extend power utilization time of a station, and reduce interference between stations.