Abstract:
A method for use in data communications equipment for improving convergence of a hybrid decision feedback apparatus including an adaptive feed-forward equalizer and an adaptive decision feedback equalizer. An independent error predictor component is used for better convergence and then is eliminated by converting, using z-transformations, the adaptive feed forward equalizer and adaptive decision feedback equalizer to an equivalent feed forward equalizer and an equivalent decision feedback equalizer, respectively, in which the error predictor is embedded or incorporated therein. The smaller system with a reduced number of FFE-DFE coefficients has a faster convergence rate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of communications. In digital communications each symbol transmitted over a time dispersive channel extends beyond the time interval used to represent that symbol. Distortion caused by the overlap of received symbols results in Inter-Symbol Interference (ISI), which may be reduced, by placing an equalizer in the path of the received signal. In particular, the invention relates to a method for changing the coefficients of an adaptive decision feedback equalizer, including its decision feedback and feed forward parts, to improve convergence. 
     2. Description of Related Art 
     The distortion caused by ISI may be reduced by passing the incoming signal through an adaptive decision feedback equalizer. Generally, the adaptation is performed according to some recursive method based on the minimization of means square error (MSE), for example, using a conventional least means square (LMS) or a recursive least squares (RLS) method. These conventional MSE based recursive adaptation equalizer methods exhibit convergence problems if the eigenvalues spread, e.g., the autocorrelation matrix condition number, representing a ratio between the largest to smallest eigenvalues, of the input signal is relatively large. Specifically, convergence speed depends on the spread of the eigenvalues of the input signal. Generally, the eigenvalue spread is relatively large due to the relatively high correlation of the input signal and depends on the length of the filter. A relatively large eigenvalue spread results in a relatively slow convergence requiring a very large number, for example, thousands or millions, of iterations that slowdown the adaptation process. 
     It is therefore desirable to develop a method that reduces the residual error and speeds up the convergence of any recursive mean square error based decision feedback equalizer using error prediction. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method for use in data communications equipment for improving convergence of a hybrid decision feedback apparatus including an adaptive feed-forward equalizer and an adaptive decision feedback equalizer. An independent error predictor component is used for better convergence and then is eliminated by converting, using z-transformations, the adaptive feed forward equalizer and adaptive decision feedback equalizer to an equivalent feed forward equalizer and an equivalent decision feedback equalizer, respectively, in which the error predictor is embedded or incorporated therein. The smaller system with a reduced number of FFE-DFE coefficients has a faster convergence rate. 
     In one embodiment of the method for use in data communications equipment for improving convergence of a hybrid decision feedback apparatus in accordance with the present invention the adaptive feed-forward equalizer is transformed into an equivalent feed forward equalizer represented by a z-transformation 
     
       
         FFE′( z )=FFE( z )[1−EP( Z   N )] 
       
     
     where, FFE′(z) is the equivalent feed-forward equalizer; 
     FFE(z) is an adaptive feed-forward equalizer; 
     EP(Z N ) is an adaptive error predictor, wherein N is a sampling factor. 
     Similarly, the adaptive decision feedback equalizer is transformed into an equivalent decision feedback equalizer represented by a z-transformation 
      DFE′( z )=EP( z )+DFE( z )[1−EP( z )] 
     where, DFE′(z) is the equivalent decision feedback equalizer; 
     DFE(z) is an adaptive decision feedback equalizer; 
     EP(Z) is the adaptive error predictor. 
     Another embodiment of the present invention is directed to a method for use in data communications equipment for improving convergence of a hybrid decision feedback apparatus, in which an initial tap length is set for each of the adaptive feed-forward equalizer and the adaptive decision feedback equalizer. During a first predetermined time period, the adaptive feed-forward equalizer and the adaptive decision feedback equalizer are adapted using a received distorted signal, until a desired error is obtained. An initial tap length is set for the error predictor. During a second predetermined time period, the adaptive feed-forward equalizer, the adaptive decision feedback equalizer, and the error predictor are simultaneously adapted using the received distorted signal. After that period, an equivalent feed-forward equalizer is determined using the z-transformation 
     
       
         FFE′( z )=FFE( z )[1−EP( Z   N )] 
       
     
     where, FFE′(z) is the equivalent feed-forward equalizer; 
     FFE(z) is an adaptive feed-forward equalizer; 
     EP(Z N ) is an adaptive error predictor, wherein N is a sampling factor. 
     Similarly an equivalent decision feedback equalizer is determined using the z-transformation 
     
       
         DFE′( z )=EP( z )+DFE( z )[1−EP( z )] 
       
     
     where, DFE′(z) is the equivalent decision feedback equalizer; 
     DFE(z) is an adaptive decision feedback equalizer; 
     EP(Z) is the adaptive error predictor. 
     Still another embodiment in accordance with the invention relates to a method for use in data communications equipment for improving convergence of a hybrid decision feedback apparatus, in which an initial tap length is set for each of the adaptive feed-forward equalizer and the adaptive decision feedback equalizer. During a first predetermined time period, the adaptive feed-forward equalizer and the adaptive decision feedback equalizer are adapted using a received distorted signal, until a desired error is obtained. An initial tap length is set for an error predictor. During a second predetermined time period, the error predictor is adapted using the received distorted signal. After that period, an equivalent feed-forward equalizer is determined using the z-transformation 
     
       
         FFE′( z )=FFE( z )[1−EP( Z   N )] 
       
     
     where, FFE′(z) is the equivalent feed-forward equalizer; 
     FFE(z) is an adaptive feed-forward equalizer; 
     EP(Z N ) is an adaptive error predictor, wherein N is a sampling factor. 
     Similarly an equivalent decision feedback equalizer is determined using the z-transformation 
     
       
         DFE′( z )=EP( z )+DFE( z )[1−EP( z )] 
       
     
     where, DFE′(z) is the equivalent decision feedback equalizer; 
     DFE(z) is an adaptive decision feedback equalizer; 
     EP(Z) is the adaptive error predictor. 
     The present invention is also directed to a hybrid decision feedback device for use with the methods described above. The device includes an equivalent feed-forward equalizer represented by a first z-transformation 
     
       
         FFE′( z )=FFE( z )[1−EP( Z   N )] 
       
     
     where, FFE′(z) is the equivalent feed-forward equalizer; 
     FFE(z) is an adaptive feed-forward equalizer; 
     EP(Z N ) is an adaptive error predictor, wherein N is a sampling factor. 
     The device also includes an equivalent decision feedback equalizer represented by a second z-transformation equation 
     
       
         DFE′( z )=EP( z )+DFE( z )[1−EP( z )] 
       
     
     where, DFE′(z) is the equivalent decision feedback equalizer; 
     DFE(z) is an adaptive decision feedback equalizer; 
     EP(Z) is the adaptive error predictor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The foregoing and other features of the present invention will be more readily apparent from the following detailed description and drawings of illustrative embodiments of the invention wherein like reference numbers refer to similar elements throughout the several views and in which: 
     FIG. 1 a  is an example prior art block diagram of a hybrid decision feedback equalizer including a feed forward part, followed by a decision feedback part and an error predictor part in parallel. 
     FIGS. 1 b  and  1   c  show the intermediate transformation stages of the system from that shown in FIG. 1 a  to the equivalent system shown in FIG. 1 d;    
     FIG. 1 d  shows an equivalent system of FIG. 1 a  without an error predictor part and exhibiting an improved speed of convergence; 
     FIG. 2 is a flow chart of a method for improving the convergence rate of the hybrid decision feedback equalizer of FIG. 1, wherein the feed forward equalizer, the decision feedback equalizer and the error predictor are adapted during a second time period T 1 ; and 
     FIG. 3 is a flow chart of an alternative method for improving the convergence of the hybrid decision feedback equalizer of FIG. 1, wherein only the error predictor is adapted over the second time period T 1 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Conventional hybrid type decision feedback equalizer structures including an adaptive feed forward equalizer followed by an intersymbol interference decision equalizer (ISI-DFE) and a noise predictive decision feedback equalizer (NP-DFE) in parallel are described, for example, in U.S. Pat. Nos. 5,513,216 and 5,604,769, the disclosure of which is each hereby expressly incorporated by reference as if fully set forth herein. FIG. 1 a  shows an example prior art hybrid DFE structure similar to the arrangement disclosed in these U.S. Patents. The hybrid DFE  100  includes an adaptive feed forward equalizer FFE(z)  105  the output of which is connected to a decimator  110  for sampling the signal by a factor N, where N≧1. Decimator  110  is only necessary when the adaptive feed forward equalizer  105  is a fractionally spaced equalizer. It is however, within the intended scope of the present invention, to use other equalizers as the adaptive feed forward equalizer  105 , such as a symbol-spaced equalizer. A first adder  115  produces an output signal e(n)  116  representing the difference between the sampled output  11  of the decimator  110  and an output  141  of an adaptive decision feedback equalizer DFE(z)  140 . A second adder  120  generates a signal ε(n)  121  representing the difference between the received signal e(n)  116  and an output x(n)  131  of an adaptive error predictor EP(z)  130 . The output signal ε(n)  121  is transformed by a decision device  125  into the final output signal d(n)  126 , which, in turn, is fed back to a third adder  135  and to the decision feedback equalizer  140 . Adder  135  receives the output signal e(n)  116  of the first adder  115  and the output signal d(n)  126  of the decision device  125 , and generates an output signal  136 , representing the difference between the two input signals, that is fed to the adaptive error predictor EP(z)  130 . 
     Conventional hybrid DFE structures that incorporate an EP scheme, such as that shown in FIG. 1 a , have heretofore been used to track channel changes and noise, not to speed up convergence as in the present invention. Other distinguishing aspects and features will become apparent from a detailed explanation provided below of the method in accordance with the present invention for improving convergence of an adaptive decision feedback equalizer. 
     Although the hybrid equalizer shown in FIG. 1 a  is useful to achieve a desired error, the arrangement is disadvantageous for a number of applications where relatively long DFE without EP is required. On the other hand, the convergence rate of such an arrangement, with a shorter DFE, is significantly improved. In accordance with the invention, once the optimum error is obtained, the hybrid equalizer arrangement may be improved by incorporating or embedding the adaptive error predictor into the feed forward and decision feedback equalizers. To achieve this result, the hybrid equalizer shown in FIG. 1 a  may be reduced or transformed to achieve an equivalent arrangement with the error predictor embedded or incorporated into the feed forward and decision feedback equalizers, as shown in FIG. 1 d . FIGS. 1 b  and  1   c  show intermediate transformation steps between the arrangement shown in FIG. 1 a  and the equivalent structure in FIG. 1 d . In FIG. 1 d  the adaptive feed forward equalizer FFE(z)  105  in FIG. 1 a  is transformed to an equivalent feed forward equalizer FFE′(z)  105 ′ represented by the z-transformation equation 
     
       
         FFE′( z )=FFE( z )[1−EP( Z   N )] 
       
     
     While the adaptive decision feedback equalizer DFE(z)  140  in FIG. 1 a  is transformed to an equivalent decision feedback equalizer DFE′(z)  140 ′ represented by the z-transformation equation 
     
       
         DFE′( z )=EP( z )+DFE( z )[1−EP( z )] 
       
     
     In the equivalent hybrid arrangement shown in FIG. 1 d , the equivalent equalizers FFE′(z)  105 ′ and DFE′(z)  140 ′ are the same or longer in length than FFE(z)  105  and DFE(z)  140 , respectively, in FIG. 1 a , while the EP(z)  130  has been eliminated. FIGS. 1 a  through  1   d  confirm that the convergence rate can be improved by solving for an optimal error predictor and then lengthening the equalizers by embedding the obtained error predictor in the FFE and DFE. Obviously, the convergence rate of the smaller system with less DFE-FFE parameters and without the EP(z), has a faster convergence rate, but larger residual error. 
     FIG. 2 is a flow chart of the operation of the method for increasing the speed of convergence in a hybrid DFE structure, such as that shown in FIG. 1, in accordance with the present invention. In operation, a transmitted signal x is received as a distorted signal y(n)  145 . In step  200 , the tap lengths of the FFE(z)  105  and the DFE(z)  140  are set to M 0  and L 0  initial values, respectively. In step  205 , during a first time period T 0  (T 0 ≧0), FFE(z)  105  and DFE(z)  140  are adapted using the distorted signal y(n)  145 . Thereafter, in step  210 , the length of the tap of the EP(z)  130  is set to an initial predetermined value K 0 . For a second time period T 1 (T 1 ≧0), FFE(z)  105 , DFE(z)  140 , EP(z)  130  are adjusted simultaneously in step  215 . In step  220  an equivalent feed forward equalizer FFE′(z) and equivalent decision feedback equalizer DFE′(z) of lengths M 1  and L 1 , respectively, are determined using the z-transformation equations 
     
       
         FFE′( z )=FFE( z )[1−EP( Z   N )] 
       
     
     
       
         DFE′( z )=EP( z )+DFE( z )[1−EP( z )] 
       
     
     In this equivalent scheme all values of EP′(z) are set to zero. The z-transformation equations incorporate or embed the influence of the EP(z)  130  to produce equivalent equalizers FFE′(z)  105 ′ and DFE′(z)  140 ′ of the same or longer in length than FFE(z)  105  and DFE(z)  140 , respectively, and exhibit a faster convergence. In step  225  a determination is made whether a desired performance level has been achieved. If not, then a new tap length of EP(z) is set in step  230  and steps  215  through  220  are repeated until a desired performance level is achieved. 
     Prior to incorporation of the influence of the error predictor, the equalizers FFE(z) and DFE(z) have lengths, M 0  and L 0  respectively, while EP(z) has a length K 0 . Accordingly, the matrix size for inversion is (M 0 +L 0 )×(M 0 +L 0 ), and the matrix size of EP(z) is K 0 ×K 0 . After the EP has been incorporated into the FFE and DFE, the equivalent length of FFE′(z) is M 1 =M 0 +N*K 0 , where the decimation factor N≧1, while the length of DFE′(z) is L 1 =L 0 +K 0 . Therefore, the equivalent matrix size for inversion, without EP, is (M 0 +L 0 +(N+1)K 0 )×(M 0 +L 0 +(N 0 +1) K 0 ). 
     A flow chart of an alternative method in accordance with the present invention for improving the convergence of an adaptive decision feedback equalizer is shown in FIG.  3 . The steps are numbered the same as those in the flow chart in FIG. 2, except step  215 ′. Step  215 ′ differs from that of step  215  in FIG. 2 in that instead of the FFE, DFE and EP being adapted simultaneously over a time period T 1 , only the EP is adapted using the distorted signal y(n) during the same time period T 1 . 
     The method in accordance with the present invention for speeding up the convergence rate has a wide range of implementations in communication systems. 
     Thus, while there have been shown, described, and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions, substitutions, and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the invention. For example, it is expressly intended that all combinations of those elements and/or steps which perform substantially the same function, in substantially the same way, to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated. It is also to be understood that the drawings are not necessarily drawn to scale, but that they are merely conceptual in nature. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.