Abstract:
A method for adjusting a modulation and coding scheme based on signal quality comprises the steps of: determining a reference modulation and coding scheme based on signal reception qualities of transmitted signals according to a plurality of modulation and coding schemes; determining a temporary modulation and coding scheme according to a modulation and coding scheme selection method; determining a final modulation and coding scheme according to a first table and the differences between the reference modulation and coding scheme and the temporary modulation and coding scheme.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for adjusting a modulation and coding scheme of a communication system, and more particularly, to a method for adjusting a modulation and coding scheme based on signal reception quality. 
         [0003]    2. Description of the Related Art 
         [0004]    In Wi-Fi wireless local area networks, such as those following the IEEE 802.11n standard, a receiver is required to suggest transmitter modulation and coding schemes (MCS) based on transmission environment, and the MCS adopted by the transmitter is selected according to variations in the transmission environment so as to maintain optimal transmission throughput. 
         [0005]    Among the many methods for determining an MCS, automatic rate fallback (ARF) algorithm is a widely used technique. The ARF method establishes a priority order for every MCS for the applied communication system, and calculates the packet error rate (PER) for a fixed amount of time in the receiver. If, within a fixed amount of time, the PER in the receiver exceeds an upper threshold, an MCS with a lower data rate is adopted according to the priority order. If, in the fixed amount of time, the PER in the receiver drops below a lower threshold, another MCS with a higher data rate is adopted according to the priority order. 
         [0006]    Another popular MCS selection method is based on the transmission environment; that is, selecting the MCS for the transmitter based on the signal-to-noise ratio (SNR). For example,  FIG. 1  shows experiment results of the optimum MCSs for different SNRs in an IEEE 802.11n wireless communication system. As shown in  FIG. 1 , the system structure is a double antenna system, wherein a double transmission antenna and a double receiving antenna are included. There are 16 MCSs available, of which number  0  to number  7  are single spatial stream MCSs, and number  8  to number  15  are double spatial stream MCSs. The receiver stores the experiment results shown in  FIG. 1  in a table and selects the MCS adopted by the transmitter according to the stored experiment results. 
         [0007]    However, most MCS selection methods (such as the two aforementioned methods) are established based on simulated communication environment or experiment results. If a significant difference exists between the simulated communication environment or the experiment results and the actual communication environment, these MCS selection methods will fail to find the optimum MCS for the environment. In addition, some communication systems can utilize suggested MCSs provided by the receiver. Such receiver may also generate the suggested MCSs based on simulated communication environment or experiment results. Likewise, if a significant difference exists between the simulated communication environment or the experiment results and the actual communication environment, the suggested MCSs may not be the optimum MCS for the environment. Therefore, there is a need to design a method for adjusting an MCS based on signal reception qualities. 
       SUMMARY OF THE INVENTION 
       [0008]    The method for adjusting an MCS according to one embodiment of the present invention comprises the steps of: determining a reference modulation and coding scheme based on signal reception qualities of transmitted signals according to a plurality of modulation and coding schemes; determining a temporary modulation and coding scheme based on a modulation and coding scheme selection method; and determining a final modulation and coding scheme based on a first table and the difference between the temporary modulation and coding scheme and the reference modulation and coding scheme. 
         [0009]    The method for adjusting an MCS according to another embodiment of the present invention comprises the steps of: determining a reference modulation and coding scheme based on signal reception qualities of transmitted signals according to a plurality of modulation and coding schemes; determining a temporary modulation and coding scheme based on a first modulation and coding scheme selection method; determining adjusted signal-to-noise ratios based on the signal-to-noise ratios of the transmitted signals and the difference between the temporary modulation and coding scheme and the reference modulation and coding scheme; and determining a final modulation and coding scheme based on the determined adjusted signal-to-noise ratios and a second modulation and coding scheme selection method. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The objectives and advantages of the present invention will become apparent upon reading the following description and upon referring to the accompanying drawings of which: 
           [0011]      FIG. 1  shows experiment results of the optimum MCSs for different SNRs; 
           [0012]      FIG. 2  shows a method for adjusting an MCS based on signal reception qualities according to an embodiment of the present invention; 
           [0013]      FIG. 3  shows a method for adjusting an MCS based on signal reception qualities according to another embodiment of the present invention; and 
           [0014]      FIG. 4  shows a method for adjusting an MCS based on signal reception qualities according to yet another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]      FIG. 2  shows a method for adjusting an MCS based on signal reception qualities according to an embodiment of the present invention. As shown in  FIG. 2 , a transmitter of a communication system determines a temporary modulation and coding scheme MCS′ based on an MCS selection method  210 . The transmitter then determines a reference modulation and coding scheme MCS″ based on a counter vector  220  and signal reception qualities of a plurality of transmitted signals according to various MCSs. The transmitter then determines a modulation and coding scheme MCSs′″ based on a table  230  and the difference between the temporary modulation and coding scheme MCS′ and the reference modulation and coding scheme MCS″. 
         [0016]    The following example illustrates a communication system adjusting an MCS according to the method shown in  FIG. 2 , wherein the communication system exhibits at least two antennas, and is in accordance with the IEEE 802.11n standard. The communication system has 16 MCSs, of which eight are single spatial stream MCSs (MCS 0  to MCS 7 ), and eight are double spatial stream MCSs (MCS 8  to MCS 15 ). The MCS selection method  210  could be the ARF algorithm, an MCS selection method based on the transmission environment or any other method for adjusting an MCS. The counter vector  220  is for the measurement of the signal reception qualities of the communication system, wherein the length of the counter vector  220  is 16 and the initial value of the counter vector  220  is 0. 
         [0017]    When utilizing the MCS selection method  210 , the communication system transmits signals with different MCSs, and the counting algorithm of the counter vector  220  is based on the following pseudo code: 
         [0018]    If (Ack==1) 
         [0000]        mcs Score[ m]=mcs Score[ m]+ 1; 
         [0019]    Else 
         [0000]        mcs Score[ m]=mcs Score[ m]− 1; 
         [0020]    wherein ACK is the acknowledge signal of the transmitted signal, mcsScore is the counter vector  220  and m is an integer ranging from 1 to 16. 
         [0021]    As shown in the pseudo code, when the communication system transmits a signal with an MCS, such as MCS 3 , the counter vector  220  records the signal reception quality of the transmitted signal. If the ACK signal of the transmitted signal is  1 , the corresponding value of the counter vector  220  is incremented by 1; i.e. the value of mcsScore[3] is incremented by 1. If the ACK signal of the transmitted signal is  0 , the corresponding value of the counter vector  220  is decremented by 1, i.e. the value of mcsScore[3] is decremented by 1. 
         [0022]    After the execution of the MCS selection method  210  is finished, a temporary modulation and coding scheme MCS′ is obtained. The communication system then selects an MCS corresponding to the greatest value recorded in the counter vector  220  as the reference modulation and coding scheme MCS″. In this example, the temporary modulation and coding scheme MCS′ is MCS 10 , and the reference modulation and coding scheme MCS″ is MCS 3 . Next, a table is referenced to obtain the values of Mx and My, which correspond to MCS′ and MCS″ respectively, wherein both Mx and My represent the value of each antenna of the corresponding MCS. The table is established based on the experiment results shown in  FIG. 1  and is shown as follows: 
         [0000]    
       
         
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Mcs 
               
             
          
           
               
                   
                 0 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 9 
                 10 
                 11 
                 12 
                 13 
                 14 
                 15 
               
               
                   
                   
               
             
          
           
               
                 Mx 
                 0 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 1 
                 3 
                 3 
                 5 
                 6 
                 6 
                 6 
                 6 
               
               
                 My 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1 
                 1 
                 2 
                 1 
                 1 
                 2 
                 3 
                 4 
               
               
                   
               
             
          
         
       
     
         [0023]    Following the table, the value of Mx′ corresponding to MCS′ is 3, the value of My′ corresponding to MCS′ is 2, the value of Mx″ corresponding to MCS″ is 3 and the value of My″ corresponding to MCS″ is 0. Next, a modulation and coding scheme MCS′″ is determined based on the table  230  and the difference between MCS′ and MCS″, wherein the difference between MCS′ and MCS″ is calculated as follows: 
         [0000]      Δ 1   =Mx′−Mx″;    
         [0000]      Δ 2   =My′−My″;    
         [0024]    Following the equations shown above, it can be obtained that Δ 1  is 0 and Δ 2  is 2. The indexes of the table  230  are obtained according to the difference between MCS′ and MCS″, wherein the indexes are calculated as follows: 
         [0000]        X =rowIndex=min{7, max[ Mx ′−floor(Δ 1   *c   1 ),0]+1}; 
         [0025]    Y columnIndex=min{4, max[My′−floor(Δ 2 *c 2 ),0]+1}; wherein min is the operation of the selection of the minimum value, max is the operation of the selection of the maximum value, floor is the floor operation and c 1  and c 2  are constants greater than 0. 
         [0026]    Following the equations shown above, if c 1  and c 2  are both equal to 0.5, it can be obtained that X is 4 and Y is 2. Based on the indexes, the table  230  is referenced to obtain the modulation and coding scheme MCS″, wherein the table  230  is established based on the experiment results shown in  FIG. 1  and is shown as follows: 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
               
             
             
               
                 0 
                 8 
                 8 
                 8 
                 8 
               
               
                 1 
                 8 
                 9 
                 9 
                 9 
               
               
                 2 
                 9 
                 10 
                 11 
                 11 
               
               
                 3 
                 9 
                 10 
                 11 
                 12 
               
               
                 4 
                 11 
                 12 
                 13 
                 13 
               
               
                 5 
                 11 
                 12 
                 13 
                 14 
               
               
                 6 
                 12 
                 13 
                 14 
                 15 
               
               
                 7 
                 12 
                 13 
                 14 
                 15 
               
               
                   
               
             
          
         
       
     
         [0027]    According to table  230 , the obtained MCS′″ is MCS 9 . 
         [0028]      FIG. 3  shows a method for adjusting an MCS based on signal reception qualities according to another embodiment of the present invention. As shown in  FIG. 3 , a transmitter of a communication system determines a temporary modulation and coding scheme MCS′ based on an MCS selection method  310 . The transmitter then determines a reference modulation and coding scheme MCS″ based on a counter vector  320  and signal reception qualities of a plurality of transmitted signals according to various MCSs. Then, the transmitter calculates the difference between the temporary modulation and coding scheme MCS′ and the reference modulation and coding scheme MCS″. A signal-to-noise ratio (SNR) adjusting method  330  is then utilized to obtain an adjusted SNR based on the difference between MCS′ and MCS″ and the SNR of the transmitted signals. An MCS selection method  340  is utilized to determine a modulation and coding scheme MCS′″ based on the adjusted SNR. In some embodiments of the present invention, the MCS selection method  310  is equal to the MCS selection method  340 . 
         [0029]    The following example illustrates a communication system adjusting an MCS according to the method shown in  FIG. 3 , wherein the communication system, similar to the communication system in the previous example, exhibits at least two antennas, and is in accordance with the IEEE 802.11n standard. The counting algorithm of the counter vector  320 , the criteria to obtain the values of Mx and My corresponding to MCS′ and MCS″ and the calculation of the difference between MCS′ and MCS″ are all the same as those in the previous example. The calculation of the SNR adjusting method  330  is shown as follows: 
         [0000]      SNR1′=SNR1+Δ 1   *r   1 ; 
         [0030]    SNR 2 ′=SNR 2 +Δ 2 *r 2 ; wherein SNR 1  and SNR 2  are the SNRs obtained at each antenna of the receiver, SNR 1 ′ and SNR 2 ′ are the adjusted SNRs and r 1  and r 2  are constants. The MCS selection method  340  is then utilized based on the adjusted signal-to-noise ratios SNR 1 ′ and SNR 2 ′ to determine the modulation and coding scheme MCS′″. 
         [0031]      FIG. 4  shows a method for adjusting an MCS based on signal reception qualities according to yet another embodiment of the present invention. Compared with  FIG. 3 , there is an additional step  440  to calculate the standard deviation in  FIG. 4 , wherein the standard deviation of the suggested MCSs provided by the receiver at different times is calculated. The weightings of these suggested MCSs are then determined based on the calculated standard deviation such that the modulation and coding scheme MCS′″ is determined. If the calculated standard deviation is too great to be reliable, the weightings of these suggested MCSs can be lowered or even ignored. 
         [0032]    In conclusion, the method for adjusting an MCS of the present invention is based on signal reception qualities to adjust the original MCS. Therefore, if a significant difference exists between the simulated communication environment or the experiment results and the actual communication environment, the adjusted MCS will be more suitable to the actual communication environment than the original MCS. 
         [0033]    The above-described embodiments of the present invention are intended to be illustrative only. Those skilled in the art may devise numerous alternative embodiments without departing from the scope of the following claims.