Abstract:
A transmitting and receiving apparatus and method for optimizing performance of an adaptive modulation and coding (AMC) in a multiple input and multiple output antenna (MIMO) communication system. When the AMC is applied to the MIMO system, the optimal scheme is different depending on the MIMO channel situation, the maximum transmission power, and the maximum modulation order. The transmitting apparatus includes an ordering selector that selects a successive interference cancellation (SIC) scheme in order to obtain a maximum MIMO-AMC performance, and the receiving apparatus includes a channel quality information (CQI) generator corresponding to the SIC scheme selected at the transmitting apparatus.

Description:
PRIORITY 
   This application claims priority under 35 U.S.C. § 119 to applications entitled “Transmitting And Receiving Apparatus And Method For Optimizing Performance Of Adaptive Modulation And Coding In Multiple Input And Multiple Output Antenna Communication System” filed in the Korean Intellectual Property Office on Oct. 19, 2004 and assigned Serial. No. 2004-83705, the contents of which are herein incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to a transmitting and receiving apparatus and method for optimizing performance of an adaptive modulation and coding (AMC) in a multiple input and multiple output antenna (MIMO) communication system. More specifically, the present invention relates a transmitting apparatus of a MIMO system, which includes an ordering selector that selects a successive interference cancellation (SIC) scheme to obtain a maximum MIMO-AMC performance, and a receiving apparatus, which includes a channel quality information (CQI) generator (a MIMO equivalent channel generator) corresponding to the SIC scheme selected at the transmitting apparatus. 
   2. Description of the Related Art 
   Multiple input and multiple output antenna (MIMO) technologies have been introduced for increasing a transmission data rate. An example of MIMO technology is spatial multiplexing (SM), which enables high-speed data transmission by transmitting different data via multiple transmit (Tx) antennas. Recently, in MIMO systems, space-time coding (STC) has been proposed, which can obtain a diversity gain by transmitting the same data via multiple Tx antennas. However, high SM gain and maximum diversity gain cannot be obtained at the same time. Accordingly, the STC for maximizing the diversity gain has difficulty in maximizing the transmission data rate. Although many attempts have been made to simultaneously obtain the SM gain and the diversity gain, these technologies have not been yet implemented in real applications. 
   There is proposed a technology for increasing the transmission data rate in the MIMO system, that is, for obtaining the maximum SM gain. More specifically, there is proposed a technology for increasing transmission rate when an adaptive modulation and coding (AMC) is applied to the MIMO system. Hereinafter, the performance of the AMC in the MIMO system means a transmission data rate when AMC is applied to a MIMO system. In addition, the terms: transmitting apparatus, transmitter, or transmitting terminal will be used interchangeably. 
   As one of methods for increasing the transmission data rate in the MIMO system, MIMO channel information measured at a receiver is fed back to a transmitter and an AMC is applied, thereby maximizing system capacity. Practically, it has been known that a channel capacity in the MIMO channel can be maximized using a singular value decomposition (SVD). However, when the SVD is performed an accurate channel value must be known. In a real system, a receiver estimates a channel value and transmits it to a transmitter through a feedback channel. Serious errors may occur during this process. 
   For the MIMO channel, a plurality of receive (Rx) antenna channel estimation values, which are transmitted from a plurality of antennas, are fed back. Therefore, a large number of the feedback values cause the serious degradation of performance due to error. Accordingly, the application of the SVD to the real system is not practical. 
   Another technology is a Vertical Bell Laboratories Space Time (V-BLAST). According to V-BLAST technology, a transmitter transmits independent signals through a plurality of Tx antennas, and a receiver differentiates the transmitted data through an appropriate signal processing. The receiver can obtain satisfactory performance using a successive interference cancellation (SIC) method. The SIC method includes a forward ordering policy and a reverse ordering policy. Of the two, the forward ordering policy is typical. 
   According to the forward ordering policy, MIMO equivalent channels distorting signals transmitted through Tx antennas are removed from MIMO equivalent channel with the highest gain. Accordingly, the forward ordering policy can prevent serious performance degradation occurring in error transmission. However, this prevention of the performance degradation is achieved when the AMC is not applied to the transmitter. 
   When the AMC is applied to the MIMO system, a feedback channel value based on the forwarding ordering policy is varied depending on channel states because the AMC is determined not by a general MIMO channel but by a MIMO equivalent channel, and its value is varied depending on the ordering policies. Accordingly, when the AMC is applied, it has to be determined which of the forward ordering policy and the reverse ordering policy is good. In addition, parameters necessary for determining the ordering policy need to be selected and the reference needs to be determined. 
   SUMMARY OF THE INVENTION 
   Therefore, the present invention has been designed to substantially solve at least the above problems and/or disadvantages and to provide at least the advantages below. 
   Accordingly, an object of the present invention is to provide a transmitting apparatus and method for optimizing a MIMO-AMC performance, i.e., AMC performance when an AMC is applied to a MIMO communication system. 
   Another object of the present invention is to provide a receiving apparatus and method for optimizing MIMO-AMC performance. 
   A further object of the present invention is to provide an ordering selector for selecting an SIC ordering policy used in the transmitting apparatus so as to optimize MIMO-AMC performance, and a method of selecting the same. 
   A further object of the present invention is to provide a CQI generator for generating a CQI used in the receiving apparatus in order to optimize MIMO-AMC performance, and a method of generating the same. 
   According to one aspect of the present invention, a transmitter of a MIMO system using an AMC includes: an AMC determiner in which a total transmission power and a maximum modulation order are preset. The AMC determiner receives MIMO equivalent channel information fed back from a receiver, determines an AMC level to be used in a transmit (Tx) antenna by using the total transmission power, the modulation order, and the MIMO equivalent channel information, and generates AMC level information. An ordering selector in which the total transmission power and the maximum modulation order are preset, receives the MIMO equivalent channel information fed back from the receiver, determines ordering policy information to be used in the receiver by using the total transmission power, the modulation order, and the MIMO equivalent channel information, and generates the ordering policy formation. A signal selector receives the AMC level information from the AMC determiner and the ordering policy information from the ordering selector, transmits data signals when a transmission interval is an interval for data signal transmission, and transmits the ordering policy information and the AMC level information when the transmission interval is an interval for control information transmission. An adaptive modulator modulates the ordering policy information and the AMC level information into predefined specific AMC level, and adaptively modulates the data signals according to the AMC level information. 
   In addition, the present invention provides a transmitting method of the transmitter. 
   According to another aspect of the present invention, a receiver of a MIMO system using an AMC includes a MIMO channel estimator for estimating a MIMO channel value by using a pilot channel or traffic channel; a SIC (successive interference cancellation)-type detector for receiving data signal and control information, the control information including an ordering policy information and an AMC level information, from a transmitter through a receive (Rx) antenna, and transmitting the data signal and the control signal to a demultiplexer, determining an SIC ordering policy according to the ordering policy information, and for determining a modulation scheme of the received data signal according to the AMC level information; the demultiplexer for receiving the data signal, the ordering policy information, and the AMC level information from the SIC-type detector, transmitting the data signal in an interval for data signal transmission; transmitting the ordering policy information and the AMC level information in an interval for control information transmission, and feeding back the AMC level information to the SIC-type detector; and a channel quality information (CQI) generator for generating a MIMO equivalent channel based on the ordering policy by using the estimated MIMO channel value and the ordering policy information, and transmitting the MIMO equivalent channel to the transmitter. 
   In addition, the present invention provides a receiving method of the receiver. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features, and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
       FIG. 1  is a block diagram of a transmitter with an ordering selector in a MIMO system according to an embodiment of the present invention; 
       FIG. 2  is a block diagram of a receiver with a CQI generator and a SIC-type detector in a MIMO system according to an embodiment of the present invention; 
       FIG. 3A  is a schematic diagram of the ordering selector that selects an SIC ordering policy for an optimal AMC performance in a transmitter of a MIMO system according to an embodiment of the present invention; 
       FIG. 3B  is a flowchart illustrating sequential procedures of selecting an SIC ordering policy according to an embodiment of the present invention; 
       FIG. 4  is a block diagram of a CQI generator for generating a MIMO equivalent channel in the receiver of the MIMO system according to an embodiment of the present invention; 
       FIGS. 5A and 5B  are flowcharts illustrating sequential procedures of generating a MIMO equivalent channel based on a forward ordering policy at a CQI generator in a receiver of a MIMO system according to an embodiment of the present invention; 
       FIGS. 6A and 6B  are flowcharts illustrating sequential procedures of generating a MIMO equivalent channel based on a reverse ordering policy at a CQI generator in a receiver of a MIMO system according to an embodiment of the present invention; 
       FIG. 7  is a flowchart illustrating a transmitting process including an ordering selection operation in a MIMO system according to an embodiment of the present invention; 
       FIG. 8  is a flowchart illustrating a receiving process including a CQI generation operation and a SIC-type detection operation in a MIMO system according to an embodiment of the present invention; 
       FIG. 9  is a flowchart illustrating the process of generating a MIMO equivalent channel in a receiver of a MIMO system according to an embodiment of the present invention; and 
       FIG. 10  is a graph illustrating performance of an ordering policy in a 4×4 MIMO system according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will be described in detail herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail. 
   According to the present invention, a transmitter includes an ordering selector that selects an SIC scheme in order to optimize AMC gain, and a receiver includes a CQI generator corresponding to the SIC scheme selected at the transmitter. 
     FIG. 1  is a block diagram of a transmitter for a MIMO system having a plurality of Tx antennas and a plurality of Rx antennas according to an embodiment of the present invention. The transmitter includes an ordering selector  107 , which receives a MIMO equivalent channel information I from a receiver stage, and determines an ordering policy to maximize an AMC performance (gain) using a preset total transmission power P T  and maximum modulation order M. The MIMO equivalent channel information I is a value fed back from a terminal used as a receiver and is used to determine an AMC level. This value has as many elements as the number of Tx antennas, i.e., N T . Because this value is equivalent channel information, it represents N T  number of channel information, not N T ×N R  number of channel information, where N T  and N R  denote the number of Tx antennas and the number of Rx antennas, respectively. 
   The total transmission power P T  represents a maximum transmission power that can be used at the transmitter stage. When M (maximum modulation order) is 1, it represents a BPSK scheme, and when M is 2, it represents a QPSK scheme. When M is 4, it represents a 16QAM scheme. The total transmission power P T  and the maximum modulation order M are values preset to the ordering selector  107 , and are important factors for determining a level of the AMC. 
   The ordering selector  107  generates an Order_In signal representing an SIC ordering information. An algorithm for calculating the Order_In signal will be described later with reference to  FIGS. 3A and 3B . 
   The Order_In signal is transmitted to a signal selector  101 . The AMC determiner  103  determines AMC levels suitable for the Tx antennas using the MIMO equivalent channel information I fed back from the receiver, the total transmission power P T , and the maximum modulation order M. The AMC level is determined by a Bit Loading Algorithm using a Greedy Algorithm developed by Levin and Campello. 
   A “Greedy Algorithm” in the bit and power allocation is an algorithm that allocates bit to a channel of the lowest power consumption among a plurality of equivalent channels when 1 bit increases. By repeating this algorithm until the total power is dissipated, the largest number of total bits can be transmitted with a restricted transmission power. The term “Greedy” is derived from the meaning that a greedy and competent person shares more profits. More specifically, an algorithm in which such a basic algorithm is applied to the bit and power allocation is called “Levin-Campello Algorithm”. The algorithm is a known technology disclosed in “Optimal Discrete Bit Loading for Multicarrier Modulation Systems” (Information Theory, 1998. Proceedings. 1998 IEEE International Symposium on, 16-21 Aug. 1998, Page. 193) published by J. Campello in 1998. Accordingly, a detailed description about this algorithm will be omitted herein. 
   AMC_In is a value containing AMC level information corresponding to each of the Tx antennas. The value AMC_In is transmitted to an adaptive modulator  105  through the signal selector  101 . The value AMC_In transmitted to the signal selector  101  is modulated into control information by the adaptive modulator  105  and is then transmitted to the receiver. The receiver determines the AMC level using the control information. In addition, the value AMC_In transmitted to the adaptive modulation  105  is used to determine the AMC level of the transmission data. 
   At a data transmission time interval, the signal selector  101  transmits data to the adaptive modulator  105 . However, at a control information transmission interval, the signal selector  101  transmits the ordering policy information Order_In and/or the AMC level information AMC_In, which are generated from the ordering selector  107  and the AMC determiner  103 , respectively. 
   In general, the data is modulated by the selected AMC level, and the Order_In signal and the AMC_In signal are the control information. Therefore, the data and the control information can be modulated by predetermined specific AMC levels. The data or control information selected by the signal selector  101  pass through the adaptive modulator  105  and are transmitted through the Tx antennas  109 ,  111 , and  113  to the receiver. 
     FIG. 2  is a block diagram of a receiver having a CQI generator  211  and a SIC-type detector  207  in the MIMO system according to an embodiment of the present invention. Referring to  FIG. 2 , data signals are received through a plurality of Rx antennas  201 ,  203 , and  205  and are demodulated into data form through a SIC-type detector  207 . The SIC-type detector determines an ordering policy, i.e., a forward ordering policy or a reverse ordering policy, according to the value of Order_In, and determines a demodulation scheme according to the value of AMC_In. Preferably, Order_In and AMC_In are values transmitted to the receiver before the data from the transmitter are demodulated. 
   The Order_In signal has two kinds of values, which will be described later in detail. 
   When Order_In=“FORWARD”, the SIC-type detector performs the SIC based on the forward ordering policy. When Order_In=“REVERSE”, the SIC-type detector performs the SIC based on the reverse ordering policy. In addition, the SIC-type detector can use both a zero-forcing scheme and a minimum mean square error (MMSE) scheme. That is, the SIC-type detector  207  can selectively use the zero forcing scheme and the MMSE scheme and use the forward ordering policy or the reverse ordering policy in the selected scheme. 
   A MIMO channel estimator  209  estimates a MIMO channel value using a pilot channel or a traffic channel. The estimated MIMO channel value is transmitted to the SIC-type detector  207  and is used for the detection of the SIC type. 
   In addition, the estimated MIMO channel value is transmitted to the CQI generator (MIMO equivalent channel generator)  211  and is used for the generation of the MIMO equivalent channel. Examples of the channel estimation methods are a maximum likelihood (ML), a minimum mean square error (MMSE), a least squares (LS), etc. Accordingly, the present invention is not limited to the specific method. 
   The CQI generator (MIMO equivalent channel generator)  211  generates the MIMO equivalent channel based on the ordering policy using the value of the MIMO channel and the value of Order_In. This process is needed because not the MIMO channel value but the equivalent channel is required for the application of the AMC. The real MIMO equivalent channel value is smaller than the MIMO channel value by the times of the number of Rx antennas. For example, if the number of the Rx antennas is four, the MIMO equivalent channel value is four times smaller than the MIMO channel value. The generation of the MIMO equivalent channel will be described later in more detail with reference to  FIG. 4 . 
   A demultiplexer (demux)  213  transmits data in the interval for data transmission and transmits the ordering policy information Order_In and/or the AMC level information AMC_In in the interval for control information transmission. Signals input to the SIC-type detector  207  through the Rx antennas include “data” signal and “control information (AMC_In or Order_In)” signal. These signals are used for signal detection at the SIC-type detector  207  and are transmitted to the demultiplexer  213 . The demultiplexer  213  receives the data signal and the control information, i.e., the ordering policy information and the AMC level information, from the SIC-type detector  207 . The demultiplexer  213  transmits the data signal in the interval for the data transmission, and transits the ordering policy information and the AMC level information in the interval for control information transmission. That is, the demultiplexer  213  separately transmits the “data” and the “control information” depending on the signal intervals. 
   Among the control information input to the demultiplexer  213 , AMC_In is transmitted to the SIC-type detector  207  because it is a value required by the SIC-type detector  207 . In a prior stage, a previous AMC_In is used. In a next stage, AMC_In is updated. That is, the AMC_In value transmitted from the demultiplexer  213  is an updated value of a previous AMC_In value used in the SIC-type detector  207 . The SIC-type detector  207  performs a predetermined function by using a previous value of the AMC_In value transmitted to the demultiplexer  213 . The AMC_In value transmitted after the AMC_In value is transmitted to the demultiplexer  213  is used as an updated value of the previous AMC_In value. 
     FIG. 3A  is a schematic diagram of the ordering selector  107  that selects a SIC ordering policy for the optimal AMC performance in the transmitter of the MIMO system according to an embodiment of the present invention. Referring to  FIG. 3A , a bit and power allocating part  301  performs a bit and power allocation based on Greedy algorithm by using an initial value of the MIMO equivalent channel information transmitted from the terminal. This algorithm is equal to the algorithm that has been used in determining the AMC level. Herein, one-bit allocation is only performed. P* T  represents a sum of powers allocated so far. 
   After passing through the bit and power allocating part  301 , if the sum P* T  of the power allocated so far exceeds a maximum transmission power P T , a power determining part  303  sets A to 1 and transmits it to an ordering policy determining part  305 . If P* T  is less than or equal to P T , the power determining part  303  sets A to 0 and transmits it to a maximum modulation order determining part  307 . 
   If max 1≦t≦N     T    C t &gt;M, the maximum modulation order determining part  307  sets B to 1 and transmits it to the ordering policy determining part  305 . If max 1≦t≦N     T    C t ≦M, the maximum modulation order determining part  307  sets B to 0 and transmits it to the bit and power allocating part  301 . Therefore, the bit and power allocation by Greedy algorithm is again performed. 
   Above, C t  represents the number of bits allocated to a t-th Tx antenna and does not exceed the maximum modulation order M according to a specific rule. That is, if the maximum modulation order of the system is defined, the number of bits allocated to the Tx antennas exists within this limitation. Accordingly, if B is equal to 1, C t  exceeds the maximum modulation order M. Additionally, when B is equal to one, this can also indicate that the maximum transmission power P T  is sufficiently great. 
   If A is equal to 1, it means that the power allocated so far exceeds the maximum transmission power, and can also represent that the maximum transmission power P T  is not sufficiently great. 
   In order to maximize the AMC gain, the ordering policy determining part  305  has to use the forward ordering policy when the maximum transmission power P T  is sufficient, and use the reverse ordering policy when the maximum transmission power P T  is insufficient. A detailed reason for this will be described later with reference to  FIG. 10 . 
   If (A, B)=(1, 0), the ordering policy determining part  305  sets the ordering value to Order_In=“REVERSE”. If (A, B)=(0, 1), the ordering policy determining part  305  sets the ordering value to “FORWARD”. 
     FIG. 3B  is a flowchart illustrating sequential procedures of selecting the SIC ordering policy according to an embodiment of the present invention. Referring to  FIG. 3B , in step  309 , a bit and power allocation is performed based on a Greedy algorithm using an initial value of the MIMO equivalent channel information transmitted from the terminal. This algorithm is equal to the algorithm that has been used in determining the AMC level. Herein, one-bit allocation is only performed, and P* T  represents a sum of powers allocated so far. 
   In step  311 , the power determining part  303  determines whether P* T  exceeds P T  (maximum transmission power). In step  311 , if P* T  exceeds P T , the power determining part  303  sets A to 1 and transmits it to the ordering policy determining part  305  with B=0 from the status of max 1≦t≦N     T    C t ≦M. The ordering policy determining part outputs Order_IN=“REVERSE” in step  315  according to (A,B)=(1,0). 
   However, if P* T  is less than or equal to P T , in step  313 , the maximum modulation order determining part determines if max 1≦t≦N     T    C t &gt;M. Further, the power determining part  303  sets A to 0 and transmits it to the maximum modulation order determining part  307 . 
   In step  313 , if max 1≦t≦N     T    C t &gt;M, the maximum modulation order determining part  307  sets B to 1 and transmits it to the ordering policy determining part  305  with A=0 from the status that P* T  is less than or equal to P T . The ordering policy determining part  305  outputs Order_IN=“FORWARD” in step  317  according to (A,B)=(0,1). 
   However, if max 1≦t≦N     T    C t ≦M, the maximum modulation order determining part  307  sets B to 0 and transmits it to the bit and power allocating part  301  which performs nit and power allocation based on greedy algorithm in step  309 . Thus, the bit and power allocation by Greedy algorithm is again performed. 
   In steps  315  (the ordering policy determining part outputs Order_IN=“REVERSE”) and  317  (the ordering policy determining part  305  outputs Order_IN=“FORWARD”), in order to maximize the AMC gain, the ordering policy determining part  305  selects the forward ordering policy when the maximum transmission power P T  is great, and selects the reverse ordering policy when the maximum transmission power P T  is small. That is, if (A, B)=(1, 0), the ordering policy determining part  305  sets the ordering value to Order_In=“REVERSE”. If (A, B)=(0, 1), the ordering policy determining part  305  sets the ordering value to “FORWARD”. 
     FIG. 4  is a block diagram of the CQI generator  211  for generating a MIMO equivalent channel in a receiver of the MIMO system according to an embodiment of the present invention. Referring to  FIG. 4 , a forward ordering MIMO equivalent channel generator  401  generates a MIMO equivalent channel based on a forward ordering policy using MIMO channel information obtained by the MIMO channel estimator  209  of the receiver illustrated in  FIG. 2 . This process will be described later in more detail with reference to  FIG. 5 . 
   Similarly, a reverse ordering MIMO equivalent channel generator  403  generates MIMO equivalent channel based on the reverse ordering policy using the MIMO channel information obtained by the MIMO channel estimator  209 . This process will be described later in more detail with reference to  FIG. 6 . 
   The MIMO equivalent channel based on the forward ordering policy and the MIMO equivalent channel based on the revere ordering policy are input to a selector  405 . If Order_In=“FORWARD”, the selector  405  outputs the MIMO equivalent channel based on the forward ordering policy. However, if Order_In=“REVERSE”, the selector  405  outputs the MIMO equivalent channel based on the reverse ordering policy. 
     FIGS. 5A and 5B  are flowcharts illustrating sequential procedures of generating a MIMO equivalent channel based on a forward ordering policy at a CQI generator in the receiver of the MIMO system according to an embodiment of the present invention. First, the process of generating the MIMO equivalent channel based on the forward ordering policy will be described with reference to  FIG. 5A . 
   Referring to  FIG. 5A , a pseudo inverse matrix (H H H) −1 H H  of the MIMO channel H is generated in step  503 . The MIMO equivalent channel is generated using a predetermined equation relationship in step  505 . Because the forward ordering SIC is used, the maximum equivalent channel is selected in step  507 . A channel corresponding to a third Tx antenna is removed to generate a reduced MIMO channel in step  509 . In order to ascertain if the SIC is finished, in step  511 , it is determined if a rank value of the reduced MIMO channel is zero. If the rank value is not zero, the process returns to the step  503 . However, if the rank value is zero, the process is terminated. 
   In  FIG. 5B , it is assumed that the number of Tx antennas is N T =3, the number of Rx antennas is N R =3, and the flat fading channel gain is given by Equation (1), 
   
     
       
         
           
             
               
                 H 
                 = 
                 
                   ( 
                   
                     
                       
                         1.2 
                       
                       
                         0.5 
                       
                       
                         1.5 
                       
                     
                     
                       
                         1.5 
                       
                       
                         1.0 
                       
                       
                         0.4 
                       
                     
                     
                       
                         1.3 
                       
                       
                         0.2 
                       
                       
                         1.2 
                       
                     
                   
                   ) 
                 
               
             
             
               
                 ( 
                 1 
                 ) 
               
             
           
         
       
     
   
   The reception types of the receiver include a Zero-Forcing (ZF) scheme and a Minimum Mean Square Error (MMSE) scheme. Hereinafter, the ZF scheme will be taken as an example. Further, when the ZF scheme is used, a following reception signal has to be multiplied by the pseudo inverse matrix of the MIMO channel H. This process refers to Equations (2) and (3) below.
 
 y=HPx+n   (2)
 
( H   H   H ) −1   H   H   y=Px +( H   H   H ) −1   H   H   n   (3)
 
   In Equations (2) and (3), P is a 3×3 diagonal matrix for power allocation and is expressed as 
   
     
       
         
           P 
           = 
           
             ( 
             
               
                 
                   
                     
                       P 
                       1 
                     
                   
                 
                 
                   0 
                 
                 
                   0 
                 
               
               
                 
                   0 
                 
                 
                   
                     
                       P 
                       2 
                     
                   
                 
                 
                   0 
                 
               
               
                 
                   0 
                 
                 
                   0 
                 
                 
                   
                     
                       P 
                       3 
                     
                   
                 
               
             
             ) 
           
         
       
     
   
   the transmission signal x=(x 1 x 2 x 3 ) T , and 
   noise n=(n 1 n 2 n 3 ) T . 
   In order to express the MIMO equivalent channel, (H H H) −1 H H  is defined by Equation (4). 
   
     
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         
                           H 
                           H 
                         
                         ⁢ 
                         H 
                       
                       ) 
                     
                     
                       - 
                       1 
                     
                   
                   ⁢ 
                   
                     H 
                     H 
                   
                 
                 = 
                 
                   ( 
                   
                     
                       
                         
                           w 
                           
                             1 
                             , 
                             1 
                           
                           H 
                         
                       
                     
                     
                       
                         
                           w 
                           
                             1 
                             , 
                             2 
                           
                           H 
                         
                       
                     
                     
                       
                         
                           w 
                           
                             1 
                             , 
                             3 
                           
                           H 
                         
                       
                     
                   
                   ) 
                 
               
             
             
               
                 ( 
                 4 
                 ) 
               
             
           
         
       
     
   
   Using Equation (4), Equation (3) can be rewritten as shown in Equation (5).
 
 w   1,j   H   y =√{square root over ( p   j )} x   j   +w   1,j   H   n   (5)
 
   If a noise part (n) is normalized, the result is given by Equation (6) below and the MIMO equivalent channel with respect to the transmission signal x j  can be obtained. 
   
     
       
         
           
             
               
                 
                   
                     
                       w 
                       
                         1 
                         , 
                         j 
                       
                       H 
                     
                     
                        
                       
                         w 
                         
                           1 
                           , 
                           j 
                         
                       
                        
                     
                   
                   ⁢ 
                   y 
                 
                 = 
                 
                   
                     
                       
                         
                           P 
                           j 
                         
                       
                       
                          
                         
                           w 
                           
                             1 
                             , 
                             j 
                           
                         
                          
                       
                     
                     ⁢ 
                     
                       x 
                       j 
                     
                   
                   + 
                   
                     
                       
                         w 
                         
                           1 
                           , 
                           j 
                         
                         H 
                       
                       
                          
                         
                           w 
                           
                             1 
                             , 
                             j 
                           
                         
                          
                       
                     
                     ⁢ 
                     n 
                   
                 
               
             
             
               
                 ( 
                 6 
                 ) 
               
             
           
         
       
     
   
   In Equation (6), because the noise part is normalized, it can be seen that the MIMO equivalent channel with respect to the transmission signal x j  is equal to 
   
     
       
         
           
             1 
             
                
               
                 w 
                 
                   1 
                   , 
                   j 
                 
               
                
             
           
           . 
         
       
     
   
   When the forward ordering SIC receiver is used, the MIMO equivalent channel 
           1          w     1   ,   j                  
varies depending on the ordering policy (the forward ordering policy or the reverse ordering policy).
 
     FIG. 5B  illustrates an example when the forward ordering policy is used. Referring to  FIG. 5B , the pseudo inverse matrix (H H H) −1 H H  of the MIMO channel H is calculated. Then, the MIMO equivalent channel is calculated using the relationship of Equation (4). The MIMO equivalent channel is obtained as
 ∥w 1,j ∥ −1 ={0.4570,03549,0.6799}. 
   The maximum equivalent channel is selected because the forward ordering SIC is used. That is, the equivalent channel 0.6799 of the third Tx antenna is selected. Next, the channel corresponding to the third Tx antenna part is removed to generate the reduced MIMO channel H 2 , which is expressed as shown in Equation 7 below. 
   
     
       
         
           
             
               
                 
                   H 
                   2 
                 
                 = 
                 
                   ( 
                   
                     
                       
                         1.2 
                       
                       
                         0.5 
                       
                     
                     
                       
                         1.5 
                       
                       
                         1.0 
                       
                     
                     
                       
                         1.3 
                       
                       
                         0.2 
                       
                     
                   
                   ) 
                 
               
             
             
               
                 ( 
                 7 
                 ) 
               
             
           
         
       
     
   
   Next, in order to ascertain whether the SIC is finished, it is checked whether the rank value of H 2  is zero. If the rank value is not zero, the above processes are repeated. If the rank value is zero, the process is terminated. 
   Starting from the reduced MIMO channel H 2 , the pseudo inverse matrix is generated as shown in Equation (8). 
   
     
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         
                           H 
                           2 
                           H 
                         
                         ⁢ 
                         
                           H 
                           2 
                         
                       
                       ) 
                     
                     
                       - 
                       1 
                     
                   
                   ⁢ 
                   
                     H 
                     2 
                     H 
                   
                 
                 = 
                 
                   [ 
                   
                     
                       
                         0.269 
                       
                       
                         
                           - 
                           0.310 
                         
                       
                       
                         0.879 
                       
                     
                     
                       
                         
                           - 
                           0.104 
                         
                       
                       
                         1.343 
                       
                       
                         
                           - 
                           1.453 
                         
                       
                     
                   
                   ] 
                 
               
             
             
               
                 ( 
                 8 
                 ) 
               
             
           
         
       
     
   
   Using Equation (8), the equivalent channel can be obtained as
 
∥w 2,j ∥ −1 ={1.031,0.505, . . . }.
 
   A first Tx antenna with the maximum equivalent channel is selected and removed to generate a reduced MIMO channel H 3  expressed as shown in Equation (9). 
   
     
       
         
           
             
               
                 
                   H 
                   3 
                 
                 = 
                 
                   ( 
                   
                     
                       
                         0.5 
                       
                     
                     
                       
                         1.0 
                       
                     
                     
                       
                         0.2 
                       
                     
                   
                   ) 
                 
               
             
             
               
                 ( 
                 9 
                 ) 
               
             
           
         
       
     
   
   A rank of the reduced MIMO channel H 3  is calculated so as to check whether the SIC is finished. The above processes are repeated until the rank becomes zero. 
   Starting from the reduced MIMO channel H 3 , the pseudo inverse matrix is generated as shown in Equation (10).
 
( H   3   H   H   3 ) −1   H   3   H =[0.388 0.775 0.155]  (10)
 
   Using Equation (12), the equivalent channel can be obtained as
 
∥w 3,j ∥ −1 ={ . . . , 1.136, . . . }.
 
   A second Tx antenna with the maximum equivalent channel is selected and removed to generate a reduced MIMO channel H 4 . Because the rank of the reduced MIMO channel H 4  is zero, the SIC is finished. 
     FIGS. 6A and 6B  are flowcharts illustrating sequential procedures of generating a MIMO equivalent channel based on a reverse ordering policy at a CQI generator in the receiver of a MIMO system according to an embodiment of the present invention. The process of generating the MIMO equivalent channel based on the reverse ordering policy will be described with reference to  FIG. 6A . 
   Referring to  FIG. 6A , a pseudo inverse matrix (H H H) −1 H H  of the MIMO channel H is calculated in step  603 . The MIMO equivalent channel is calculated using a predetermined equation relationship in step  605 . Because the reverse ordering SIC is used, the minimum equivalent channel is selected in step  607 . A channel corresponding to a second Tx antenna is removed to generate a reduced MIMO channel in step  609 . In order to ascertain whether the SIC is finished, it is checked whether a rank value of the reduced MIMO channel is zero in step  611 . If the rank value is not zero, the process returns to the step  603 . However, if the rank value is zero, the process is terminated. 
   In  FIG. 6B , it is assumed that the number of Tx antennas is N T =3, the number of Rx antennas is N R =3, and the flat fading channel gain is given by Equation (12). 
   
     
       
         
           
             
               
                 H 
                 = 
                 
                   ( 
                   
                     
                       
                         1.2 
                       
                       
                         0.5 
                       
                       
                         1.5 
                       
                     
                     
                       
                         1.5 
                       
                       
                         1.0 
                       
                       
                         0.4 
                       
                     
                     
                       
                         1.3 
                       
                       
                         0.2 
                       
                       
                         1.2 
                       
                     
                   
                   ) 
                 
               
             
             
               
                 ( 
                 12 
                 ) 
               
             
           
         
       
     
   
   The reception types of the receiver include a Zero-Forcing (ZF) scheme and a Minimum Mean Square Error (MMSE) scheme. Hereinafter, the ZF scheme will be taken as an example. 
   The pseudo inverse matrix (H H H) −1 H H  of the MIMO channel H is calculated and then the MIMO equivalent channel is calculated. The MIMO equivalent channel is obtained as shown in Equation (13).
 
∥w 1,j ∥ −1 ={0.4570,03549,0.6799}  (13)
 
   The minimum equivalent channel is selected because the reverse ordering SIC is used. That is, the equivalent channel 0.3549 of the second Tx antenna is selected. 
   Next, the channel corresponding to the second Tx antenna part is removed to generate the reduced MIMO channel H 2 , which is expressed as shown in Equation (14). 
   
     
       
         
           
             
               
                 
                   H 
                   2 
                 
                 = 
                 
                   ( 
                   
                     
                       
                         1.2 
                       
                       
                         0.5 
                       
                     
                     
                       
                         1.5 
                       
                       
                         0.4 
                       
                     
                     
                       
                         1.3 
                       
                       
                         0.2 
                       
                     
                   
                   ) 
                 
               
             
             
               
                 ( 
                 14 
                 ) 
               
             
           
         
       
     
   
   In order to ascertain whether the SIC is finished, it is checked whether the rank value of H 2  is zero. If the rank value is not zero, the above processes are repeated. 
   Starting from the reduced MIMO channel H 2 , the pseudo inverse matrix is generated as shown in Equation (15). 
   
     
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         
                           H 
                           2 
                           H 
                         
                         ⁢ 
                         
                           H 
                           2 
                         
                       
                       ) 
                     
                     
                       - 
                       1 
                     
                   
                   ⁢ 
                   
                     H 
                     2 
                     H 
                   
                 
                 = 
                 
                   [ 
                   
                     
                       
                         
                           - 
                           0.2624 
                         
                       
                       
                         0.8330 
                       
                       
                         0.0503 
                       
                     
                     
                       
                         0.6595 
                       
                       
                         
                           - 
                           0.7529 
                         
                       
                       
                         0.2600 
                       
                     
                   
                   ] 
                 
               
             
             
               
                 ( 
                 15 
                 ) 
               
             
           
         
       
     
   
   Using Equation (15), the equivalent channel can be obtained as shown in Equation (16).
 
∥w 2,j ∥ −1 ={1.1432, . . . , 0.9671}  (16)
 
   A third Tx antenna with the minimum equivalent channel is selected and removed to generate a reduced MIMO channel H 3  expressed as shown in Equation (17). 
   
     
       
         
           
             
               
                 
                   H 
                   3 
                 
                 = 
                 
                   ( 
                   
                     
                       
                         1.2 
                       
                     
                     
                       
                         1.5 
                       
                     
                     
                       
                         1.3 
                       
                     
                   
                   ) 
                 
               
             
             
               
                 ( 
                 17 
                 ) 
               
             
           
         
       
     
   
   A rank of the reduced MIMO channel H 3  is calculated so as to check whether the SIC is finished. The above processes are repeated because the rank of the reduced MIMO channel H 3  is not zero. 
   Starting from the reduced MIMO channel H 3 , the pseudo inverse matrix is generated as shown in Equation (18).
 
( H   3   H   H   3 ) −1   H   3   H =[0.2230 0.2788 0.2416]  (18)
 
   Using Equation (18), the equivalent channel can be obtained as shown in Equation (19).
 
∥w 3,j ∥ −1 ={2.3195, . . . }  (19)
 
   A first Tx antenna with the minimum equivalent channel is selected and removed to generate a reduced MIMO channel H 4 . Because the rank of the reduced MIMO channel H 4  is zero, the SIC is finished. 
     FIG. 7  is a flowchart illustrating a transmitting process including an ordering selection operation in a MIMO system according to an embodiment of the present invention. Referring to  FIG. 7 , in step  703 , AMC levels of the Tx antennas are determined using the MIMO channel equivalent information fed back from the receiver, the maximum modulation order, and the total transmission power. In step  705 , an ordering policy of the MIMO receiver stage is determined using the MIMO channel equivalent information fed back from the receiver, the maximum modulation order, and the total transmission power. In step  707 , at a data reception interval, data is selected and mapped. However, if it is a control reception interval, the control information, i.e., the ordering policy information Order_In and/or the AMC level information signal AMC_IN, is selected and mapped. 
   In step  709 , AMC modulation is performed on the data corresponding to the Tx antennas using the AMC level information signal AMC_In. In step  711 , the modulated data signals x 1 , x 2 , . . . , x N     T    are transmitted to their corresponding Tx antennas. 
     FIG. 8  is a flowchart illustrating a receiving process including a CQI generation operation and a SIC-type detection operation in the MIMO system according to an embodiment of the present invention. In step  803 , the MIMO channel is estimated using a pilot channel or traffic channel. An ML scheme or an MMSE scheme can be used to estimate the MIMO channel. 
   In step  805 , the SIC detection is performed using the AMC level information AMC_IN and the ordering policy information Order_In transmitted from the transmitter, and the MIMO channel value calculated by the receiver. In step  807 , at a data reception interval, data is received. However, if it is a control reception interval, the control information, i.e., the ordering policy information Order_In and/or the AMC level information AMC_In, is received. 
   In step  809 , the MIMO equivalent channel is generated using the ordering policy information Order_In. In step  811 , the MIMO equivalent channel is fed back to the transmitter and then process is terminated. 
     FIG. 9  is a flowchart illustrating a process of generating a MIMO equivalent channel in a receiver of a MIMO system according to an embodiment of the present invention. In step  903 , the MIMO equivalent channel based on the forward ordering policy is generated using the MIMO channel information. In step  905 , the MIMO equivalent channel based on the reverse ordering policy is generated using the MIMO channel information. In step  909 , if the ordering policy information Order_In is “FORWARD”, the forward MIMO equivalent channel is selected. However, if the ordering policy information Order_In is “REVERSE”, the reverse MIMO equivalent channel is selected. Thereafter, the process is terminated. 
     FIG. 10  is a graph illustrating performance, i.e., data rate, of the ordering policy in a 4×4 MIMO system according to an embodiment of the present invention. In  FIG. 10 , x-axis and y-axis represent the total transmission power and the total data rate, respectively. 
   As illustrated in  FIG. 10 , if the total transmission power is insufficient, the reverse ordering policy can obtain higher data rate than that of the forward ordering policy. However, if the total transmission power is sufficiently high, the forward ordering policy has higher performance. 
   A cross point occurs between the forward ordering and the reverse ordering because the maximum modulation order M is fixed to four in the system. That is, even though a lot of bits can be allocated because of good channel values, it is impossible to exceed four to the maximum. When the total transmission power is insufficient, the excess of the maximum modulation order does not almost occur. Therefore, the reverse ordering policy exhibits higher performance than that of the forward ordering policy. 
   However, when the total transmission power is very high, the excess of the maximum order occurs very frequently. In this case, power is allocated to even antennas with bad channel situation. Therefore, the equivalent channel gain is distributed relatively uniformly, such that the forward ordering policy exhibits higher gain than the reverse ordering policy. 
   According to the present invention, the reverse ordering policy is selected when the total transmission power is low, while the forward ordering policy is selected when the total transmission power is high. In this manner, the optimal AMC gain can be obtained. 
   When the AMC is applied in the MIMO system, the SIC scheme is selected according to the MIMO channel situation, the maximum transmission power, and the maximum modulation order, thereby obtaining the maximum performance of the AMC. 
   While the present invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.