Abstract:
A method and apparatus for adaptively determining weights for transmission antennas according to a channel condition in a closed-loop transmit diversity system using a plurality of transmission antennas. The antenna weights are determined using variation of channel characteristics based on time and space, to thereby improve bit error rate performance. Parameters for fixing average transmission power per symbol according to the statistical channel characteristics for symbol energy and additive white Gaussian nose energy are previously determined to estimate channel characteristics. The antenna weights are determined using the parameters and channel gains estimated upon reception of data symbols.

Description:
[0001]    This application claims priority under 35 U.S.C. § 119 to an application entitled “Method and Apparatus for Determining Antenna Weight in a Closed-Loop Transmit Diversity System” filed in the Korean Intellectual Property Office on Jun. 13, 2003 and assigned Serial No. 2003-38390, the contents of which are incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to a closed-loop transmit diversity system using a plurality of transmission antennas, and in particular, to a method and apparatus for adaptively determining weights of respective antennas according to a condition of channels.  
           [0004]    2. Description of the Related Art  
           [0005]    Transmit diversity is an effective technology for relieving degradation of a signal due to fading in a wireless communication system. In the transmit diversity technology, a transmitter transmits desired transmission data via a plurality of transmission antennas so that a receiver can receive data in a stable manner.  
           [0006]    [0006]FIG. 1 is a block diagram illustrating a simplified baseband structure of a closed-loop transmit diversity system having M transmission antennas. Here, “closed-loop” means that a transmitter uses information fed back from a receiver in transmitting data via a plurality of transmission antennas.  
           [0007]    As illustrated, in a transmitter  100 , a modulator  110  receives a bit stream including desired transmission information and generates a data symbol d having symbol energy E s  according to a predetermined modulation scheme. M multipliers  120 ,  122  and  124  multiply the data symbol d by weights w 1 , w 2 , . . . , W M  assigned to corresponding antennas, and provide their outputs to M transmission antennas  130 ,  132  and  134 . The M transmission antennas  130 ,  132  and  134  radiate the weighted data symbols dw m  to the air.  
           [0008]    In a receiver  140 , a reception antenna  150  receives a radio signal from the transmitter  100 . An adder  160  indicates that an additive white Gaussian noise (AWGN) is added while the receiver  140  receives the radio signal. The radio signal includes a previously known pilot together with the data transmitted by the transmitter  100 . A channel estimator  170  calculates channel gains h 1 , h 2 , . . . , h M  indicating channel impulse response characteristics of the receiver  140 , from the M transmission antennas  130 ,  132  and  134 , using the pilot. The data excluding the pilot is provided to a demodulator  180  so as to restore its original information.  
           [0009]    An antenna weight determiner  190  calculates antenna weights w 1 , w 2 , . . . , W M  using the calculated channel gains h 1 , h 2 , . . . , h M , and the calculated antenna weighs are transmitted to the transmitter  100  over a feedback channel  10 , using undepicted transmission elements.  
           [0010]    In the transmit diversity system constructed in this way, a procedure for calculating antenna weights for the transmission antennas  130 ,  132  and  134  by the antenna weight determiner  190  is performed as follows. First, a received signal in a receiver is expressed as shown in Equation (1):  
             r   =         ∑     m   =   1     M                         E   s            dw   m          h   m         +   n             (   1   )                               
 
           [0011]    Here, r is a received signal, m is a transmission antenna index, M is the number of transmission antennas, E s  is symbol energy used in a transmitter, and d is a data symbol. When d is a binary data symbol, E s  is equal to bit energy E b . In addition, w m  is a weight assigned to an m th  antenna, h m  is a channel characteristic from an m th  transmission antenna to the receiver, and n is a channel noise.  
           [0012]    If a data symbol d is a binary data symbol having the same probability of d=+1 and d=−1, an instantaneous signal-to-noise ratio (SNR) of a received signal is given as shown in Equation (2):  
             γ   =         E   s                   ∑     m   =   1     M            w   m          h   m              2         N   o               (   2   )                               
 
           [0013]    Here, N o  is AWGN energy. In order to maximize SNR, antenna weights are determined in accordance with Equation (3) below (see R. T. Derryberry, S. D. Gray, D. M. Ionescu, G. Mandyam, and B. Raghothaman, “Transmit Diversity in 3G CDMA System,” IEEE Commun. Mag., pp. 68-75, April 2002).  
               w   m     =       h   m   *           ∑     m   =   1     M                 h   m          2                   (   3   )                               
 
           [0014]    Here, h m * means a complex conjugate of the channel characteristic.  
           [0015]    [0015]FIG. 2 is a block diagram illustrating a detailed structure of the antenna weight determiner  190  according to the prior art. As illustrated, complex conjugate generators  191 - 1  to  191 -M calculate conjugates of channel gains h 1 , . . . , h M , and power gain acquisition sections  192 - 1  to  192 -M calculate channel power gains by squaring absolute values of the channel gains. A summer  193  sums up the channel power gains, a square root generator  194  calculates a square root of the summed channel power gain, and a reciprocal generator  195  calculates a reciprocal of the square root. Multipliers  196 - 1  to  196 -M calculate weights w 1 , . . . , w M  by multiplying the conjugates of the channel gains by the reciprocal of the square root.  
           [0016]    In the conventional weight determining method described above, power consumed during transmission of data symbols is normalized so that the power is always constant. That is, during transmission of each data symbol, the sum of power gains of antenna weights always becomes 1, so the SNR of a received signal is maximized by distributing transmission power according to spaces (antennas). This conventional technology considers only a space variation characteristic among time and space (antenna) variation characteristics of channels. Thus, the conventional technology fails to minimize an average bit error rate (BER) serving as an important criterion for performance evaluation.  
         SUMMARY OF THE INVENTION  
         [0017]    It is, therefore, an object of the present invention to provide a method and apparatus for determining an antenna weight considering a radio channel characteristic which varies according to time and space in a closed-loop transmit diversity system.  
           [0018]    It is another object of the present invention to provide a method and apparatus for determining an antenna weight for minimizing an average bit error rate while fixing average transmission power in a closed-loop transmit diversity system.  
           [0019]    To achieve the above and other objects, the invention provides a method for determining antenna weights for M transmission antennas in a closed-loop transmit diversity system. The method comprises the steps of estimating channel gains from the M transmission antennas to a receiver; and determining antenna weights for the M transmission antennas by using the estimated channel gains and parameters previously determined so as to fix average transmission power per transmission symbol from the M transmission antennas.  
           [0020]    To achieve the above and other objects, the invention also provides an apparatus for determining antenna weights for M transmission antennas in a closed-loop transmit diversity system. The apparatus comprises a channel estimator for estimating channel gains from the M transmission antennas to a receiver; and an antenna weight determiner for determining antenna weights for the M transmission antennas by using the estimated channel gains and parameters previously determined so as to fix average transmission power per transmission symbol from the M transmission antennas. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    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:  
         [0022]    [0022]FIG. 1 is a block diagram illustrating a simplified baseband structure of a general closed-loop transmit diversity system;  
         [0023]    [0023]FIG. 2 is a block diagram illustrating a detailed structure of the antenna weight determiner according to the prior art;  
         [0024]    [0024]FIG. 3 is a block diagram illustrating a structure of an antenna weight determiner according to an embodiment of the present invention;  
         [0025]    [0025]FIG. 4 is a block diagram illustrating a detailed structure of the power gain acquisition sections of the antenna weight determiner shown in FIG. 3; and  
         [0026]    [0026]FIG. 5 illustrates a comparison between an average bit error rate of the conventional technology and an average bit error rate of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]    A preferred embodiment of the present invention will now be described in detail with reference to the annexed drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for conciseness. The terms used herein will be defined considering their functions in the invention and can be changed according to users, intentions of the users, or usual practices. Therefore, a definition of a term is based on the entire contents of the specification.  
         [0028]    The invention determines a weight of an antenna according to a channel condition and average transmission power per symbol at an operation time. An antenna weight is determined so as to satisfy a restriction condition of average transmission power and based on channel information of channels formed from transmission antennas to a receiver.  
         [0029]    A structure of a transmit diversity system to which the present invention is applied has already been described in FIG. 1. In particular, the invention relates to an operation of the antenna weight determiner  190  in the closed-loop transmit diversity system of FIG. 1. That is, the antenna weight determiner  190  according to the present invention determines an antenna weight so as to minimize an average bit error rate of a plurality of data symbols, instead of maximizing the SNR of every data symbol.  
         [0030]    An average bit error rate in a closed-loop transmit diversity system can be expressed as shown in Equation (4):  
               P   b     =       E       h   1     ,     h   2     ,     ⋯                   h   M                [     Q        (     2                 γ     )       ]               (   4   )                               
 
         [0031]    Here, P b  is a probability density function indicating an average bit error rate for channel gains h 1 , h 2 , . . . , h M  representing channel impulse response characteristics, from M transmission antennas to a receiver, and E means an average function. In addition, Q( ) is a known function defined as in Equation (5):  
                 Q        (   x   )       =       1       2      π                ∫   x   ∞                   -     t   2       /   2                          t             ,                x   ≥   0             (   5   )                               
 
         [0032]    Q(2γ) means an instantaneous bit error rate, and monotonically decreases as an instantaneous SNR γ of a received signal is increased.  
         [0033]    Average transmission power per symbol is represented by  
         E        [       ∑     m   =   1     M                            w   m          2       ]       ,                         
 
         [0034]    and minimizing an average bit error rate when the average transmission power is fixed is equivalent to calculating Equation (6) below:  
                     min       w   1     ,     w   2     ,     ⋯                   w   M                {       P   b     =     E        [     Q        (       2      γ       )       ]         }                   E        [       ∑     m   =   1     M                            w   m          2       ]       =   1                 (   6   )                               
 
         [0035]    Here, E[ ] is an average function. Equation (6) above means calculating antenna weights w 1 , w 2 , . . . , w M  that minimize a formula in { }. For the convenience of calculation, it is assumed herein that the average transmission power is fixed to 1.  
         [0036]    Then, as set forth in Equation (7), the restriction condition of average transmission power being fixed to 1 is identical to  
                       E        [       ∑     m   =   1     M                 w   m          2       ]       =       1   ⇔       ∑     m   =   1     M                 w   m          2         =   α       ,             E        [   α   ]       =   1                 (   7   )                               
 
         [0037]    Here, α represents an amount of transmission power used per symbol transmission in a time domain. Referring to Equation (8), according to the Cauchy-Schwartz inequality, SNR is restricted by  
               γ   ≤           E   s          (       ∑     m   =   1     M                 w   m          2       )            (       ∑     m   =   1     M                 h   m          2       )         N   o         =       α                   E   s            ∑     m   =   1     M                 h   m          2           N   o               (   8   )                               
 
         [0038]    In Equation (8), in order for SNR to be maximized, Equation (9) below should be satisfied.  
                       w   m     =         α          h   m   *             ∑     m   =   1     M                 h   m          2             ,             m   =   1     ,   2   ,     ⋯                 M                   (   9   )                               
 
         [0039]    Since Q( ) is a monotonically decreasing function, if antenna weights are determined by Equation (9), instantaneous SNR is maximized for a given α, and as a result, an instantaneous bit error rate is minimized.  
         [0040]    Then, minimizing an average bit error rate when average transmission power is fixed can be expressed with a simpler formula given in Equation (10):  
                 min   α          {       P   b     =       ∫   0   ∞            Q        (         2      α                 g                   E   s         N   o         )              P   G          (   g   )               g           }            
            E           α          =         ∫   0   ∞          α                     P   G          (   g   )               g         =   1               (   10   )                               
 
         [0041]    Here, g or G is  
         ∑     m   =   1     M                            h   m          2                           
 
         [0042]    and is the sum of channel gains representing channel characteristics of transmission antennas. A probability density function P G ( ) of the G can be previously known according to statistical properties of radio channels formed from transmission antennas to a receiver. More specifically, a channel characteristic model based on Rayleigh fading which is commonly considered in wireless communication technology is known. Then, the G is calculated as the sum of channel gains based on the known channel characteristic model.  
         [0043]    Equation (10) above is an optimization problem when there is a restriction condition, and it is a typical problem known by Variational Calculus. Therefore, α satisfying Equation (10) is represented as shown in Equation (11):  
             α   =       W        (       (       gE   s           2      π          λ                   N   o         )     2     )         2          gE   s     /     N   o                   (   11   )                               
 
         [0044]    Here, W( ) is a known Lambert W function (see R. M. Coreless, G. H. Gonnet, D. E. G. Hare, D. J. Jerrey, and D. E. Knuth, “On The Lambert W Function,” Advances in Computational Mathematics, vol. 5, pp. 329-359, 1996), and is defined in Equation (12) as:  
           W ( z ) e   w(z)   =Z    (12)  
         [0045]    In addition, λ is a constant determined so as to satisfy the restriction condition E[α]=1 of average transmission power. Then, respective antenna weights for minimizing an average bit error rate are calculated as shown in Equation (13):  
                       w   m     =         h   m   *                N   o       2                   E   s              W        (       (         E   s           2                 π          λ                   N   o                ∑     i   =   1     M                 h   i          2         )     2     )                 ∑     i   =   1     M                 h   i          2           ,             m   =   1     ,     2                 ⋯                 M                   (   13   )                               
 
         [0046]    Here, α is calculated according to Equation (11).  
         [0047]    In Equation (13),  
           E   s           2      π          λ                   N   o                       and                     N   o       2        E   s                               
 
         [0048]    are constants that can be previously calculated regardless of channel gains. Therefore, the antenna weight determiner previously calculates the constants, stores the calculated constants in a memory, and calculates antenna weights using channel gains estimated upon every reception of a data symbol and the previously stored constants.  
         [0049]    [0049]FIG. 3 is a block diagram illustrating a structure of an antenna weight determiner  190  according to an embodiment of the present invention. As illustrated, complex conjugate generators  205 - 1  to  205 -M calculate conjugates of channel gains h 1 , . . . , h M , and power gain acquisition sections  210 - 1  to  210 -M calculate channel power gains by squaring absolute values of the channel gains. A summer  215  sums up the channel power gains, and a reciprocal generator  220  calculates a reciprocal of the sum of the channel power gains.  
         [0050]    A multiplier  225  multiplies the sum of the channel power gains by the previously stored constant  
           E   s           2                 π          λ                   N   o         ,                         
 
         [0051]    a square generator  230  squares an output of the multiplier  225 , and a Lambert W function generator  235  calculates a Lambert W function output for an output of the square generator  230 . A multiplier  240  multiplies the Lambert W function output by the previously stored function  
           N   o       2        E   s         ,                         
 
         [0052]    and a square root generator  245  calculates a square root for an output of the multiplier  240 . A multiplier  250  multiplies an output of the square root generator  245  by an output of the reciprocal generator  220 .  
         [0053]    Multipliers  255 - 1  to  255 -M calculate weights w 1 , . . . , W M  by multiplying the conjugates of the channel gains by an output of the multiplier  250 .  
         [0054]    [0054]FIG. 4 is a block diagram illustrating a detailed structure of the power gain acquisition sections  210 - 1  to  210 -M shown in FIG. 3. As illustrated, in an m th  power gain acquisition section  210 -m, an absolute calculator  310  calculates an absolute value |h m | of an input channel gain h m , and a square calculator  320  calculates a square |h m | 2  of the absolute value and outputs the calculated square as a channel power gain.  
         [0055]    When a radio channel for each transmission antenna is a Rayleigh fading channel, an average bit error rate when determining an antenna weight by the conventional technology is represented as shown in Equation (14):  
                     P   b     =                (       1   -   μ     2     )     M            ∑     m   =   0       M   -   1                         (       M   -   1   +   m     m     )            (       1   +   μ     2     )     m                       Here   ,            μ                 is                           E   s     /     N   o             E   s     /     N   o       +   1         .                     (   14   )                               
 
         [0056]    An average bit error rate when determining an antenna weight according to the invention is represented as shown in Equation (15):  
               P   b     =       1       (     M   -   1     )     !              ∫   0   ∞            Q        (       W        (       (       gE   s           2                 π          λ                   N   o         )     2     )         )            g     M   -   1            e     -   g               g                   (   15   )                               
 
         [0057]    Here, λ is a constant that satisfies a restriction condition of average transmission power being equal to 1, shown in Equation (16) below.  
               E        [   α   ]       =           N   o       2          (     M   -   1     )     !          E   s                ∫   0   ∞            W        (       (       gE   s           2      π          λ                   N   o         )     2     )            g     M   -   2                 -   g                          g           =   1             (   16   )                               
 
         [0058]    [0058]FIG. 5 illustrates average bit error rates calculated using Equation (15) and Equation (16), wherein white points represent average bit error rates according to the conventional technology, while black points represent average bit error rates according to the present invention. As illustrated, compared to the conventional antenna weight determining method, the new antenna weight determining method shows a very low average bit error rate in the same SNR environment. For example, when the number of transmission antennas is M=3, the invention shows performance improvement of about 7.4 dB in terms of an average bit error rate.  
         [0059]    The invention has the following advantages. In the invention, a transmitter optimizes antenna weights according to characteristics of radio channels which vary according to time and space, thereby minimizing an average bit error rate of a closed-loop transmit diversity system. The reduction in an average bit error rate brings about a reduction in average power required in a transmitter, thus contributing to an increase in battery run-time and a decrease in size of a battery of the transmitter. Also, the number of transmission antennas needed to satisfy a required average bit error rate can be reduced, contributing to a decrease in cost required in installing a transmit diversity system.  
         [0060]    While the invention has been shown and described with reference to certain preferred embodiment 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 invention as defined by the appended claims.