Abstract:
A method for selecting modulation and coding scheme (MCS) for multi-antenna systems comprises the steps of: a multi-antenna system transmits signals according to MCSs of single spatial stream and determines an MCS accordingly. Subsequently, the multi-antenna system increases the number of the spatial streams applied, transmits signals according to the corresponding MCSs and determines an MCS accordingly until an optimum MCS is found.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for selecting modulation and coding schemes for a communication system, and more particularly, to a method for selecting modulation and coding schemes for a multi-antenna system. 
         [0003]    2. Description of the Related Art 
         [0004]    In Wi-Fi wireless local area networks, such as those following the IEEE 802.11in standard, a receiver is required to suggest a transmitter the modulation and coding scheme (MCS) based on transmission environment, and the MCS adopted by the transmitter is adjusted with the variation of the transmission environment so as to maintain the highest transmission throughput. 
         [0005]    Automatic rate fallback (ARF) algorithm is a popular MCS selection technique. It 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 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 higher data rate is adopted according to the priority order. Since the ARF algorithm needs to calculate the PER within a fixed amount of time for every MCS adjustment, a lot amount of time is spent on lesser MCSs, which affects the throughput of the communication system. In addition, for a multi-antenna system, the real data rates provided by every MCS depend on the signal to noise ratio (SNR) of each antenna, and therefore the priority order cannot be established based on data rates for single-antenna systems. An ill-established priority order can cause the communication system to be unable to select the optimum MCS. 
         [0006]    Another MCS selection method is based on the transmission environment, that is, adjusting the MCS for the transmitter based on the SNR.  FIG. 1  shows experiment results of the optimum MCSs for different SNRs in a wireless communication system complying with IEEE 802.11in standard. 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, wherein 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 adjusts the MCS adopted by the transmitter according to the stored experiment results. One drawback of this method is that the accuracy of the estimated SNR affects the performance of the communication system. In addition, this table requires an excessively large storage space of the receiver such that the hardware cost increases significantly. Furthermore, if a triple antenna system or a system structure with more antennas is used, the required storage space would increase exponentially such that the hardware limitations could be prohibitive. 
         [0007]    Therefore, there is a need to design a method for selecting MCS for multi-antenna systems that is fast and easy to implement. 
       SUMMARY OF THE INVENTION 
       [0008]    The method for selecting modulation and coding schemes of the present invention transmits signal based on MCSs of single spatial stream signals and increments the dimension of the single spatial stream signals until an optimum MCS is found. 
         [0009]    The method for selecting modulation and coding schemes according to one embodiment of the present invention comprises the steps of: setting the dimension of transmission spatial stream signals of a multi-antenna system to 1 and transmitting signals based on different MCSs to determine an initial MCS; repeating incrementing the dimension of the transmission spatial stream signals by 1 and transmitting signals based on different MCSs to update the MCS of the multi-antenna system until the updated MCS is equal to the MCS before update or the dimension of the transmission spatial stream signals reaches a threshold; selecting the MCS before update as the MCS of the multi-antenna system if the updated MCS is equal to the MCS before update; and selecting the updated MCS as the MCS of the multi-antenna system if the dimension of the transmission spatial stream signals reaches a threshold. 
         [0010]    The method for selecting modulation and coding schemes according to another embodiment of the present invention comprises the steps of: setting the dimension of transmission spatial stream signals of a multi-antenna system to 1 and transmitting signals based on different MCSs to determine an initial MCS; repeating incrementing the dimension of the transmission spatial stream signals by 1 and transmitting signals based on different MCSs to update the MCS of the multi-antenna system until the data rate of the multi-antenna system is smaller than that of the multi-antenna system before update or the dimension of the transmission spatial stream signals reaches a threshold; selecting the MCS before update as the MCS of the multi-antenna system if the data rate of the multi-antenna system is smaller than that of the multi-antenna system before update; and selecting the updated MCS as the MCS of the multi-antenna system if the data rate of the multi-antenna system is greater than that of the multi-antenna system before update and the dimension of the transmission spatial stream signals reaches a threshold. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    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: 
           [0012]      FIG. 1  shows experiment results of the optimum MCSs for different SNRs; 
           [0013]      FIG. 2  shows the flow chart of a method for selecting MCSs for multi-antenna systems according to an embodiment of the present invention; 
           [0014]      FIG. 3  shows a double antenna system; 
           [0015]      FIG. 4  shows the corresponding data rates of a plurality of MCSs according to an embodiment of the present invention; and 
           [0016]      FIG. 5  shows the available MCSs under selection according to an embodiment of the present invention 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]      FIG. 2  shows the flow chart of a method for selecting MCSs for multi-antenna systems according to an embodiment of the present invention. In step  201 , the dimension of the transmission spatial stream signals of a multi-antenna system is set to 1, and step  202  is executed. In step  202 , signals of different MCSs are transmitted by the multi-antenna system, and step  203  is executed. In step  203 , an optimum MCS is determined from the applied MCSs in step  201  according to the quality of the transmitted signals at the receiver, and step  204  is executed. In the present embodiment, the optimum MCS is the MCS with the highest data rate. In step  204 , the dimension of the transmission spatial stream signals is incremented by 1, and step  205  is executed. In step  205 , signals of different MCSs are transmitted by the multi-antenna system according to the updated spatial stream signals, and step  206  is executed. In step  206 , an optimum MCS is determined from the applied MCSs in step  205  and the previous determined MCS according to the quality of the transmitted signals at the receiver, and step  207  is executed. In step  207 , whether the updated optimum MCS is the previous determined MCS is checked. If the result is positive, step  208  is executed; otherwise, step  209  is executed. In step  208 , the previous determined MCS is set as the MCS of the multi-antenna system, and the selecting method is finished. In step  209 , whether the dimension of the transmission spatial stream signals reaches a threshold, e.g. the maximum dimension the multi-antenna system can provide, is checked. If the result is positive, step  204  is executed; otherwise, step  210  is executed. In step  210 , the updated MCS is set as the MCS of the multi-antenna system, and the selecting method is finished. 
         [0018]    In another embodiment of the present invention, in step  206 , the optimum MCS is determined only from the applied MCSs in step  205 , and therefore the updated optimum MCS is not the same as the previous determined MCS. Therefore, the check condition in step  207  can be revised to determine whether the data rate of the multi-antenna system is lower than that of the multi-antenna system before update. If the result is positive, step  208  is executed; otherwise, step  209  is executed. 
         [0019]    In one embodiment of the present invention, in step  202 , signals are transmitted by the multi-antenna system with all MCSs of single spatial stream signals. In another embodiment of the present invention, in step  205 , signals are transmitted by the multi-antenna system according to all MCSs of the updated spatial stream signals. In yet another embodiment of the present invention, in step  205 , signals are transmitted by the multi-antenna system according to a part of MCSs of the updated spatial stream signals. For example, if the data rate of the determined MCS in steps  203  or  206  is R, in step  205 , under the updated spatial stream signals, the MCSs of the transmitted signals can be selected such that the data rates of the transmitted signal are between R and a x R, wherein a is a positive integer. For another example, if the determined MCS in steps  203  or  206  is MCS k , in step  205 , under the updated spatial stream signals, the MCSs of the transmitted signals can be derived from the previous MCS k  according to experiment data. 
         [0020]      FIG. 3  shows a double antenna system  300 , comprising a transmitting end  310  and a receiving end  320 . The double antenna system  300  uses the method shown in  FIG. 2  to select the applied MCS. The double antenna system  300  is implemented based on the IEEE 802.11in wireless communication network standard, and comprises MCS 0  to MCS 15 , a total of 16 MCSs, wherein MCS 0  to MCS 7  are single spatial stream MCSs, and MCS 8  to MCS 15  are double spatial stream MCSs.  FIG. 1  shows the experiment results of the double antenna system  300  of the optimum MCSs for different SNRs.  FIG. 4  shows the data rates for every MCS of the double antenna system  300 . 
         [0021]    Following step  201 , the dimension of the transmission spatial stream signals of the double antenna system  300  is set to 1. Following step  202 , signals of different MCSs are transmitted by the double antenna system  300 . In one embodiment of the present invention, signals are transmitted by the double antenna system  300  with all MCSs of single spatial stream signals, i.e., MCS 0  to MCS 7 . Following step  203 , the double antenna system  300  compares MCS 0  to MCS 7  according to the quality of the transmitted signals at the receiver and determined MCS 5  as the optimum MCS, wherein the data rate of MCS 5  is 52 Mbps as shown in  FIG. 4 . Following step  204 , the dimension of the transmission spatial stream signals of the double antenna system  300  is incremented by 1 to be 2. Following step  205 , signals of different MCSs are transmitted by the double antenna system  300  according to the updated spatial stream signals, i.e., double spatial stream signals. In one embodiment of the present invention, signals are transmitted by the double antenna system  300  according to all MCSs of the updated spatial stream signals, i.e., MCS 8  to MCS 15 . In yet another embodiment of the present invention, the MCSs of the transmitted signals are selected from the double spatial MCSs such that the data rates of the transmitted signal are between R and a×R, wherein if a is 3, the selected MCSs are MCS 11 , MCS 12 , MCS 13 , MCS 14  and MCS 15 . In yet another embodiment of the present invention, MCS 11 , MCS 12 , MCS 13  and MCS 14  are the derived MCSs from MCS 5  according to the experiment results shown in  FIG. 1  and are thus selected as the MCSs of the transmitted signals. Following step  206 , from the applied MCSs in step  205  (MCS 8  to MCS 15 , MCS 11  to MCS 15  or MCS 8  to MCS 14 ) and the previous determined MCS 5 , MCS 5  is determined as the optimum MCS according to the quality of the transmitted signals at the receiver. Following step  207 , since the updated optimum MCS is the previous determined MCS, step  208  is executed, MCS 5  is set as the MCS of the double antenna system  300 , and the selecting method is finished. 
         [0022]      FIG. 5  shows MCS data for the double antenna system  300  including MCS values selected in step  203  from MCS 0  to MCS 7 , and the available MCSs under selection in step  205 . The first row shows all the double spatial MCSs; the second row shows the MCSs for which the data rates of the transmitted signal are between R and a×R, and a is 3; the third row shows the MCSs derived from MCS 0  to MCS 7  according to the experiment results shown in  FIG. 1 . 
         [0023]    In conclusion, the method for selecting modulation and coding schemes for a multi-antenna system disclosed by the present invention quickly an optimum MCS according to a simple determining procedure, and is not affected by poorly established priority order or inaccurate estimated SNR and can be easily implemented. 
         [0024]    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.