Patent Publication Number: US-7225215-B2

Title: Adaptive filter employing adaptively controlled forgetting factor and adaptively controlling method of forgetting factor

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an adaptive filter employing an adaptively controlled forgetting factor and an adaptively controlling method of the forgetting factor, which are utilized in an echo canceller using at data transmission and an audio system, in an automatic equalizer for digital data transmission, and also are generally utilized in the identification of unknown systems. 
     DESCRIPTION OF THE RELATED ART 
     First, the principle of an adaptive filter is explained.  FIG. 1  is a block diagram showing the principle of the adaptive filter. The adaptive filter generates an estimator of an unknown signal based on a known filter reference input signal. And the adaptive filter updates parameters in the adaptive filter based on an error signal between the unknown signal and this estimator, and identifies an unknown system accurately. 
     Noise at the time of general observation has been added to the unknown signal. The adaptive filter converges to a final steady state by starting from an initial state being a not learning state. There are many cases that the unknown signal is given as a response of the unknown system to the filter reference input signal. An echo canceller and an automatic equalizer correspond to this case. 
     In many cases, the adaptive filter is realized by a FIR (finite impulse response) type filter, at this case, the adaptive filter controls N pieces of tap weights c 0 ,c 1 , . . . ,cN−1 by using an error signal e (n). At the present invention, a recursive least squares algorithm is used as a controlling algorithm for the tap weights. 
     The update equations for the tap weights and so forth are given by the following. Hereinafter, _ is referred to as a vector or a matrix. The update equation of the tap weights is given as  c (n+1), the update equation of a Kalman gain is given as  g (n), and the estimator for an inverse covariance matrix of the filter reference input signal vector is given as  P (n+1).
 
   c   ( n+ 1)=   c   ( n )+ e ( n )   g   ( n )  (1)
 
   g   ( n )=   P   ( n )   a   ( n )/{λ+   a     T ( n )   P   ( n )   a   ( n )}  (2)
 
   P   ( n+ 1)={   P   ( n )−   g   ( n )   a     T ( n )    P   ( n )}/λ  (3)
 
where “n” is a time instant,  c (n) is a tap weight vector and the  c (n)=[c 0 (n), c 1 (n), . . . , cN−1(n)] T ,  a (n) is a filter reference input signal vector and the  a (n)=[a(n), a(n−1), . . . , a(n−N+1)] T , and a(n) is a filter reference input signal sequence, e (n) is an error signal, λ is a parameter called a forgetting factor, and (•) T  denotes the transpose of a vector or a matrix. That is,  c ,  g ,  P , and  a  denote a vector or a matrix.
 
     The requiring amount of operation is large by the recursive least squares algorithm, however, when it is compared with a well-known LMS (least mean square) algorithm, the recursive least squares algorithm gives faster convergence for a colored reference input. 
     In case that an adaptive filter utilized the recursive least squares algorithm is used to identify a response from an unknown system, when the response is nonstationary time-variant, not being stationary time-invariant, it is well known that there exists an optimum value of the forgetting factor. In this case, the MSE (mean square error) becomes minimum for the optimum forgetting factor. Since the unknown system being nonstationary time-variant is not known beforehand, the optimum forgetting factor is also unknown. 
     Therefore, at identifying the unknown system being nonstationary time-variant, it is required that the MSE is made to be minimum, by controlling the forgetting factor optimally. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an adaptive filter employing an adaptively controlled forgetting factor and an adaptively controlling method of the forgetting factor, in which the MSE can be made to be minimum by controlling the forgetting factor optimally. 
     According to a first aspect of the present invention, for achieving the object mentioned above, there is provided an adaptive filter employing an adaptively controlled forgetting factor, adopted an adaptive filter system using a recursive least squares algorithm. And adaptive control is carried out by that the forgetting factor is updated by using a gradient of an error signal with respect to the forgetting factor. 
     According to a second aspect of the present invention, in the first aspect, the adaptive filter employing the adaptively controlled forgetting factor provides a first multiplying means that multiplies an error signal at time “n” by an error signal at time “n+1”, which was delayed by one unit time from the time “n”, an inner product of vectors calculating means that calculates an inner product of a filter reference input signal vector at time “n+1” and a Kalman gain vector at time “n”, a second multiplying means that multiplies the output from the first multiplying means by the output from the inner product of vectors calculating means, a dividing means that divides the output from the second multiplying means by a complementary forgetting factor (complement of the forgetting factor with respect to 1) at time “n”, a third multiplying means that multiplies the output from the dividing means by a coefficient for adaptation, an adding means that adds the output from the third multiplying means and the complementary forgetting factor at time “n”, a delaying means that delays the output from the adding means by one unit time, and a subtracting means that subtracts the output from the delaying means from “1”. And the complementary forgetting factor at time “n”, inputted to the dividing means and the adding means, is a feedback signal from the delaying means. 
     According to a third aspect of the present invention, in the second aspect, the adaptive filter employing the adaptively controlled forgetting factor further provides a signum detecting means that detects signums of the error signals at time “n” and at time “n+1”. 
     According to a fourth aspect of the present invention, in the second aspect, the inner product of vectors calculating means calculates the inner product of the filter reference input signal vector at time “n+1” and the filter reference input signal vector at time “n”, instead of, and the Kalman gain vector at time “n”. 
     According to a fifth aspect of the present invention, for achieving the object mentioned above, there is provided an adaptively controlling method of a forgetting factor in an adaptive filter, adopted an adaptive filter system using a recursive least squares algorithm. And adaptive control is carried out by that the forgetting factor is updated by using a gradient of an error signal with respect to the forgetting factor. 
     According to a sixth aspect of the present invention, in the fifth aspect, the adaptively controlling method of the forgetting factor in the adaptive filter provides the steps of; multiplying an error signal at time “n” by an error signal at time “n+1”, which was delayed by one unit time from the time “n”, calculating an inner product of a filter reference input signal vector at time “n+1” and a Kalman gain vector at time “n”, multiplying the multiplied result of the error signals at time “n” and at time “n+1” by the calculated inner product, dividing the multiplied result of the error signals at time “n” and at time “n+1” and the calculated inner product result by a complementary forgetting factor at time “n”, multiplying the divided result by a coefficient for adaptation, adding the multiplied result by the coefficient for adaptation and the complementary forgetting factor at time “n”, delaying the added result by one unit time, and subtracting the delayed result from “1”. And the complementary forgetting factor at time “n” is a feedback signal of the delayed result. 
     According to a seventh aspect of the present invention, in the sixth aspect, the adaptively controlling method of the forgetting factor in the adaptive filter further provides the step of detecting signums of the error signals at time “n” and at time “n+1”. 
     According to an eighth aspect of the present invention, in the sixth aspect, the calculating of the inner product calculates the inner product of the filter reference input signal vector at time “n+1” and the filter reference input signal vector at time “n”, instead of the Kalman gain vector at time “n”. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objects and features of the present invention will become more apparent from the consideration of the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a block diagram showing the principle of an adaptive filter; 
         FIG. 2  is a block diagram showing a structure of an adaptive filter employing an adaptively controlled forgetting factor at an embodiment of the present invention; and 
         FIG. 3  is a graph showing a result of a computer simulation at the embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Referring now to the drawings, an embodiment of the present invention is explained in detail. 
     As mentioned above, at the embodiment of the present invention, an adaptive filter employing an adaptively controlled forgetting factor and an adaptively controlling method of the forgetting factor are provided. For achieving this, a gradient of an error signal with respect to a forgetting factor is used. At the embodiment of the present invention, the update equations for the forgetting factor are given by the following.
 
λ( n )=1−λ c ( n )  (4)
 
λ c ( n+ 1)=λ c ( n )+ρ G   e ( n+ 1) e ( n )   a     T ( n+ 1)   g   ( n )/λ c ( n )  (5)
 
where the λ(n) is a forgetting factor at time “n”, the λc (n) is a complementary forgetting factor at time “n”, and the ρ G  is a coefficient for adaptation. As mentioned above, the  g  (n) is a Kalman gain,  a  (n) is a filter reference input signal vector, e (n) is an error signal, and (•) T  denotes the transpose of a vector or a matrix.
 
       FIG. 2  is a block diagram showing a structure of the adaptive filter employing an adaptively controlled forgetting factor at the embodiment of the present invention. As shown in  FIG. 2 , the adaptive filter employing the adaptively controlled forgetting factor at the embodiment of the present invention consists of a multiplier  11 , an inner product of vectors calculator  17 , a multiplier  12 , a divider  16 , a multiplier  13 , an adder  14 , a delay circuit  18 , and a subtracter  15 . 
     An input signal  101  is an error signal e (n+1) at time “n+1”, and an input signal  102  is an error signal e (n) at time “n”. The multiplier  11  calculates the product of the input signals  101  and  102 . An input vector  103  is a filter reference input signal vector  a (n+1) at time “n+1”, and an input vector  104  is a Kalman gain  g (n) at time “n”. The inner product of vectors calculator  17  calculates the inner product  a   T  (n+1)  g (n). 
     The multiplier  12  calculates the product of the output from the multiplier  11  and the output from the inner product of vectors calculator  17 . A signal  106 , which is transmitted from the delay circuit  18  via a feedback route, is a complementary forgetting factor λc (n), and the divider  16  divides the output from the multiplier  12  by the signal  106 . The quotient being the output from the divider  16  is inputted to the multiplier  13 . The multiplier  13  calculates the product of the quotient and a coefficient for adaptation  107 , which is inputted to the multiplier  13 . 
     The adder  14  adds the signal  106  (the complementary forgetting factor λc (n)), which was transmitted from the delay circuit  18  via the feedback route, to the output from multiplier  13 . The sum at the adder  14  is given to the delay circuit  18 , whose delay time is one unit time, and the sum is delayed for one unit time. The output from the delay circuit  18  is the signal  106 , and the signal  106  is also inputted to subtracter  15 , and the subtracter  15  subtracts the signal  106  from an input signal  108  being a value “1”. The subtracted result becomes an output  105 . The output  105  is none other than the forgetting factor λ(n). 
     The embodiment mentioned above is a circuit that the equations (4) and (5) were realized as a concrete circuit. At the block diagram shown in  FIG. 2 , the order of the multipliers and divider can be changed. 
     The modified equation of the equation (5) is given by the following equation (6) or (7).
 
λ c ( n+ 1)=λ c ( n )+ρ G   sgn{e ( n+ 1)} sgn{e ( n )}   a     T ( n+ 1)   g   ( n )/λ c ( n )   (6)
 
λ c ( n+ 1)=λ c ( n )+ρ G   e ( n+ 1) e ( n ) a   T ( n+ 1)   a   ( n )/λ c ( n )   (7)
 
In the equation (6), sgn (•) denotes a signum function, and in the equation (7), the filter reference input signal vector  a (n) is used, instead of using the Kalman gain  g (n). By using the equation (6) or (7), the effect being the same as that using the equation (5) can be obtained.
 
       FIG. 3  is a graph showing a result of a computer simulation at the embodiment of the present invention. In this, an unknown system being nonstationary time-variant is assumed to be a random walk model. Since at the initial state, an unknown system being stationary time-invariant is assumed, and the complementary forgetting factor λc (n) is selected as a small value so that the MSE becomes small, in case that the unknown system is the nonstationary time-variant, the MSE becomes −8 dB after the adaptive filter was converged. 
     When an adaptively controlling method of the forgetting factor is started at time n=10000, the MSE reached to −20 dB, and the MSE was improved by the value being more than 10 dB. Actually, in this example, the minimum MSE value for the optimum forgetting factor is −20 dB. That is, the optimum controlling of the forgetting factor is achieved. In  FIG. 3 , the change of the complementary forgetting factor λc (n) is also shown, and it can be seen that the complementary forgetting factor λc (n) is controlled to the optimum value from the initial small value. 
     At the embodiment of the present invention mentioned above, an adaptive filter employing an adaptively controller forgetting factor is realized by using a circuit, however, the adaptive filter employing the adaptively controlled forgetting factor can be realized by digital processing using a DSP (digital signal processor). 
     As mentioned above, according to the present invention, the MSE is made to be minimum by controlling the forgetting factor optimally, consequently, an unknown system can be identified accurately. 
     While the present invention has been described with reference to the particular illustrative embodiment, it is not to be restricted by that embodiment but only by the appended claims. It is to be appreciated that those skilled in the art can change or modify the embodiment without departing from the scope and spirit of the present invention.