Abstract:
Provided is an adaptive equalization processing circuit with which an adaptive equalization process converges in a stable manner without reducing the transmission efficiency. This adaptive equalization processing circuit is characterized by being equipped with: a demodulation means that demodulates a received signal, and generates and outputs a training signal; an adaptive equalization processing means that uses a tap coefficient (generated using the received signal or the training signal) to perform an adaptive equalization process for removing waveform distortion in the received signal, and then outputs an equalization output signal; and a selection means that selects the training signal when the adaptive equalization processing means is in a non-convergent state, and inputs the training signal to the adaptive equalization processing means.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an adaptive equalization processing circuit and an adaptive equalization processing method, in which adaptive equalization processing converges stably. 
       BACKGROUND ART 
       [0002]    In recent years, demand for performing high-speed data communication by wireless has been increasing. In particular, in a mobile environment and a wireless LAN (Local Area Network) environment, a diversity scheme and adaptive equalization processing have become indispensable, because multi-path (multiplexed radio wave propagation) fading occurs there. 
         [0003]    Patent Literature 1 discloses an automatic equalizer which performs adaptive equalization processing by the use of an adaptive matched filter (AMF) and a decision feedback equalizer (DFE). The adaptive matched filter symmetrizes the impulse response of a transmission line, by estimating it from a received signal and realizing the inverse characteristic of the estimated impulse response. The decision feedback equalizer estimates inter-symbol interference in the transmission line on the basis of a received signal, and removes waveform distortion due to the inter-symbol interference. 
         [0004]    In the automatic equalizer disclosed in Patent Literature 1, the adaptive matched filter and the decision feedback equalizer are connected in cascade. Then, the adaptive matched filter estimates the impulse response of the transmission line on the basis of a received signal. Further, the adaptive matched filter is connected in cascade with the transmission line, as a filter which realizes the inverse characteristic of the estimated impulse response. Thereby, matched signal, from which waveform distortion due to the impulse response of the transmission line is removed, is outputted. Subsequently, from the matched signal, the decision feedback equalizer removes waveform distortion due to inter-symbol interference. The decision feedback equalizer estimates the inter-symbol interference in the transmission line from the matched signal. Then, the decision feedback equalizer removes the inter-symbol interference by removing the temporally advanced interference component by the use of a forward equalizer inside the decision feedback equalizer, and by removing the temporally delayed interference component by the use of a backward equalizer also installed in the inside. 
         [0005]    Patent Literature 2 discloses a technology of a diversity type receiver which performs adaptive equalization processing by the use of an adaptive matched filter and a decision feedback equalizer. 
         [0006]    The technology disclosed in Patent Literature 2 is one which performs tap coefficient setting at initial start-up, by generating a known preamble signal included in a received signal also at the reception side, and by then using the generated preamble as a preamble for tap coefficient setting for the adaptive matched filter and that for the decision feedback equalizer. In the technology disclosed in Patent Literature 2, a preamble signal having a known waveform included in a received signal and a preamble signal generated at the reception side are used, and thereby, improvement in the stability of convergence of the adaptive equalization processing and reduction in a time to the convergence are conducted. That is, the technology of Patent Literature 2 makes tap coefficients of the adaptive matched filter and those of the decision feedback equalizer converge immediately, on the basis of correlation between the known preamble signal and the preamble signal generated at the reception side. Accordingly, the technology disclosed in Patent Literature 2 secures stability of convergence of the adaptive equalization processing and reduces a time required for the convergence. 
       CITATION LIST 
     Patent Literature 
       [0007]    [Patent Literature 1] Japanese Patent Application Laid-Open No. H04-271508 
         [0008]    [Patent Literature 2] Japanese Patent Application Laid-Open No. 2001-308763 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0009]    In order to perform adaptive equalization processing normally, it is necessary to make converge both of an adaptive matched filter and a decision feedback equalizer which execute the adaptive equalization processing. “To make an adaptive matched filter converge” means to make converge tap coefficients by which respective tap outputs of a transversal filter constituting the adaptive matched filter are multiplied. “To make a decision feedback equalizer converge” means to make converge tap coefficients by which respective tap outputs of a transversal filter constituting the decision feedback equalizer are multiplied. In order to make the tap coefficients converge, it is desirable that a signal used for generating the tap coefficients has correlation with a received signal. 
         [0010]    However, for generating tap coefficients of the adaptive matched filter, the technology disclosed in Patent Literature 1 uses a received signal and a matched signal which is an output from the adaptive matched filter itself. Therefore, when it is applied to a modulation method using both phase modulation and amplitude modulation, such as QAM (Quadrature Amplitude Modulation), for example, the technology disclosed in Patent Literature 1 is more susceptible to the influence of the transmission line than when applied to QPSK (Quadrature Phase Shift Keying) or the like. As a result, in the technology disclosed in Patent Literature 1, the matched signal used for generating tap coefficients of the adaptive matched filter does not become stable, and also the tap coefficients of the adaptive matched filter is less likely to converge. 
         [0011]    Further, for generating tap coefficients of the decision feedback equalizer, the technology disclosed in Patent Literature 1 uses an equalization output signal obtained by performing adaptive equalization processing on the matched signal from the adaptive matched filter and an error output signal equivalent to the difference between the equalization output signal and a decision output signal from a decision device inside the decision feedback equalizer. Accordingly, the technology disclosed in Patent Literature 1 has a problem in that, in some multipath fading state, the tap coefficients of the decision feedback equalizer do not converge within a permissible time period or never converge. 
         [0012]    In the technology disclosed in Patent Literature 2, a known preamble signal needs to be transmitted from the transmission side. Accordingly, the technology disclosed in Patent Literature 2 has a problem in that the above-described necessity leads to decrease in the transmission efficiency in high-speed data communication where improvement in the transmission efficiency is particularly demanded. 
         [0013]    The present invention has been made in view of the above-described problems, and accordingly, the objective of the present invention is to provide an adaptive equalization processing circuit and an adaptive equalization processing method in which adaptive equalization processing converges stably, without causing decrease in the transmission efficiency. 
       Solution to Problem 
       [0014]    An adaptive equalization processing circuit according to the present invention is characterized by that it comprises: a demodulation means which generates a training signal by demodulating a received signal and outputs the training signal; an adaptive equalization processing means which performs adaptive equalization processing to remove waveform distortion of the received signal, by the use of tap coefficients generated using the received signal or the training signal, and thereby outputs an equalization output signal; and a selection means which selects the training signal and inputs it to the adaptive equalization processing means when the adaptive equalization processing means is in an unconverged state. 
         [0015]    An adaptive equalization processing method according to the present invention is characterized by that it comprises: generating a training signal by demodulating a received signal and outputting the training signal; performing adaptive equalization processing to remove waveform distortion from the received signal by the use of tap coefficients generated using the received signal or the training signal; and selecting the training signal when the adaptive equalization processing is in an unconverged state, and inputting the selected training signal to the adaptive equalization processing. 
       Advantageous Effects of Invention 
       [0016]    According to the present invention, it is possible to make adaptive equalization processing converge stably without decreasing the transmission efficiency. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]      FIG. 1  is a block diagram showing an example of a configuration of an adaptive equalization processing circuit in a first exemplary embodiment of the present invention. 
           [0018]      FIG. 2  is a block diagram showing an example of a configuration of an adaptive equalization processing circuit in a second exemplary embodiment of the present invention. 
           [0019]      FIG. 3  is a block diagram showing an example of a configuration of an output circuit inside a decision feedback equalizer, of the adaptive equalization processing circuit in the second exemplary embodiment of the present invention. 
           [0020]      FIG. 4  is a block diagram showing an example of a configuration of a training signal generation circuit of the adaptive equalization processing circuit in the second exemplary embodiment of the present invention. 
           [0021]      FIG. 5  is a block diagram showing an example of a configuration of an adaptive equalization processing circuit in a third exemplary embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0022]    Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to drawings. 
       First Exemplary Embodiment 
       [0023]    A first exemplary embodiment of the present invention will be described.  FIG. 1  is a block diagram showing an example of a configuration of an adaptive equalization processing circuit in the first exemplary embodiment of the present invention. 
         [0024]    With reference to  FIG. 1 , the configuration of the adaptive equalization processing circuit  100  of the present exemplary embodiment will be described. The adaptive equalization processing circuit  100  comprises an adaptive equalization processing means  1 , a demodulation means  2  and a selection means  3 . 
         [0025]    The adaptive equalization processing means  1  performs adaptive equalization processing on a received signal, thus removing its waveform distortion, and thereby outputs an equalization output signal. The received signal is a signal generated by modulating a digital signal by a predetermined method. The received signal is demodulated by “symbol decision” to decide a symbol included in the received signal, and thereby the original digital signal is extracted. Here, waveform distortion occurs in the received signal owing to various causes such as multipath fading. In the present invention, there is no particular limitation on a cause of waveform distortion generation in a received signal to be a processing target. 
         [0026]    The demodulation means  2  extracts timing information from the received signal, and outputs a signal which it generates by demodulating the received signal on the basis of the extracted timing information, as a training signal. In the present invention, any optional signal demodulated from a received signal may be used as a training signal. 
         [0027]    The selection means  3  selects either of a decision output signal outputted from the adaptive equalization processing means  1  and a training signal outputted from the demodulation means  2 , and inputs the selected signal, as a feedback signal, to the adaptive equalization processing means  1 . The selection means  3  performs the selection, of either of a decision output signal or a training signal as a feedback signal, on the basis of the situation of convergence of adaptive equalization processing performed by the adaptive equalization processing means  1  at that time. Note that, the adaptive equalization processing and the feedback signal will be described later. 
         [0028]    Next, operation of the adaptive equalization processing circuit  100  will be described below. 
         [0029]    The adaptive equalization processing means  1  is provided with an adaptive matched filter (AMF) function unit and a decision feedback equalizer (DFE) function unit, for the purpose of removing waveform distortion from a received signal. 
         [0030]    Connecting in cascade with the transmission line as a filter which estimates the impulse response of the transmission line from a received signal and realizes the inverse characteristic of the estimated impulse response, the adaptive matched filter function outputs a matched signal equivalent to the received signal from which waveform distortion related to the impulse response of the transmission line has been removed. 
         [0031]    The decision feedback equalizer function estimates inter-symbol interference in the transmission line from the matched signal outputted from the adaptive matched filter function, and outputs an equalization output signal equivalent to the matched signal from which waveform distortion due to the estimated inter-symbol interference has been removed. 
         [0032]    In order for the adaptive equalization processing means  1  to normally perform adaptive equalization processing, it is necessary to make converge the processing performed by the adaptive matched filter function and decision feedback equalizer function provided in the adaptive equalization processing means  1 . In order to make converge the processing performed by the adaptive matched filter function and decision feedback equalizer function, it is required that, for a certain time period, a feedback signal is inputted to each of their tap coefficient generation functions in a manner not to enable a decision error to occur in symbol decision with respect to an input signal to each of the functions. 
         [0033]    The demodulation means  2  extracts timing information from a received signal and outputs, as a training signal, a signal generated by demodulating the received signal by the use of the timing information. Because the training signal is generated by demodulating the received signal, it becomes a signal which is synchronized with the received signal in symbol timing and carrier phase. 
         [0034]    The selection means  3  decides the situation of convergence of the adaptive equalization processing means  1  on the basis of an error output signal generated by the decision feedback equalizer function of the adaptive equalization processing means  1 . For example, when the error output signal is beyond a predetermined threshold value range, the selection means  3  decides that the situation of convergence of the adaptive equalization processing means  1  is in an unconverged state. Detecting that the error output signal is within the predetermined threshold value range, the selection means  3  decides that the situation of convergence of the adaptive equalization processing means  1  is in a converged state. 
         [0035]    When the adaptive equalization processing means  1  is in an unconverged state, the selection means  3  selects a training signal from the demodulation means  2  as a feedback signal and inputs it to the adaptive equalization processing means  1 . 
         [0036]    Because the feedback signal selected by the selection means  3  is a training signal generated from a received signal by demodulating it, there occurs no decision error due to the symbol timing and carrier phase in the symbol decision performed in the adaptive equalization processing means  1 . While the training signal synchronized with the received signal is thus inputted as a feedback signal from the selection means  3 , there occurs convergence of tap coefficients generated by each of the tap coefficient generation function of the adaptive matched filter function and that of the decision feedback equalizer function, in the adaptive equalization processing means  1 , and the adaptive equalization processing accordingly is started. 
         [0037]    Specifically, when the adaptive equalization processing means  1  is in an unconverged state, a received signal and a training signal generated by demodulating the received signal are inputted to the tap coefficient generation function of the adaptive matched filter function. The adaptive matched filter function starts estimating the impulse response of the transmission line from the received signal. In this case, because the tap coefficient generation function generates tap coefficients on the basis of the received signal and the training signal synchronized with the received signal, there occurs no timing-related error in the tap coefficient generation process. Accordingly, the tap coefficients converge stably. As a result, on the basis of thus generated tap coefficients, the adaptive matched filter stably starts its function as a filter to estimate the impulse response of the transmission line and be provided with the inverse characteristic of the estimated impulse response. The adaptive matched filter function accordingly starts outputting a matched signal in which waveform distortion relating to the impulse response of the transmission line has been removed. 
         [0038]    Also in that case where the adaptive equalization processing means  1  is in the unconverged state, an error output signal and the training signal are inputted to the tap coefficient generation function of the decision feedback equalizer function. The decision feedback equalizer function starts estimating inter-symbol interference in the transmission line on the basis of the matched signal. The received signal, matched signal, error output signal and training signal are signals synchronized with each other. In the tap coefficient generation function, there occurs no timing-related error in the tap coefficient generation process because the error output signal and the training signal are in synchronization with each other. Accordingly, the tap coefficients converge stably. As a result, on the basis of thus generated tap coefficients, the decision feedback equalizer function stably starts its function to estimate inter-symbol interference in the transmission line and remove waveform distortion due to the estimated inter-symbol interference. The decision feedback equalizer function accordingly starts outputting an equalization output signal from which the waveform distortion due to the inter-symbol interference has been removed. 
         [0039]    As has been described above, when the adaptive equalization processing means  1  is in an unconverged state, a training signal is fed back to the adaptive equalization processing means  1 . Accordingly, on the basis of the training signal, tap coefficients of the adaptive matched filter function and those of the decision feedback equalizer function are generated, where they converge stably. 
         [0040]    The selection means  3  detects that the processing in the adaptive equalization processing means  1  has converged, on the basis of the error output signal from the decision feedback equalizer function of the adaptive equalization processing means  1 . At this time, the selection means  3  selects a decision output signal from the decision feedback equalizer function of the adaptive equalization processing means  1  as a feedback signal and inputs it to the adaptive equalization processing means  1 . 
         [0041]    As a result, feedback for performing adaptive equalization processing in the adaptive equalization processing means  1  becomes effective. Accordingly, the adaptive equalization processing means  1  can stably continue the adaptive equalization processing automatically even when distortion of a received signal is large. 
         [0042]    Specifically, when the processing in the adaptive equalization processing means  1  has converged, the decision output signal is inputted to the tap coefficient generation function of the adaptive matched filter function. The adaptive matched filter function continues the processing of estimating the impulse response of the transmission line from a received signal. The decision output signal is a signal generated, on the basis of a received signal and a training signal, by removing waveform distortion related to the impulse response of the transmission line, further removing inter-symbol interference in the transmission line, and then performing symbol decision. After the feedback signal has been switched to the decision output signal, the tap coefficient generation function captures and traces change in received signal with reference to the decision output signal, and adaptively generates the tap coefficients. Accordingly, a change in received signal is fed back through thus generated tap coefficients, and as a result, the adaptive matched filter function can continue its function as a filter to estimate the impulse response of the transmission line and be provided with the inverse characteristic of the estimated impulse response. The adaptive matched filter function accordingly continues output of a matched signal from which waveform distortion relevant to the impulse response of the transmission line has been removed. 
         [0043]    Also in that case where the processing in the adaptive equalization processing means  1  has converged, the decision output signal is inputted to the tap coefficient generation function of the decision feedback equalizer function. The decision feedback equalizer function continues the processing of estimating inter-symbol interference in the transmission line from a matched signal. When the feedback signal has been switched from the training signal to the decision output signal, the tap coefficient generation function captures and traces change in matched signal resulting from change in received signal as change in error output signal, and adaptively generates the tap coefficients. Because a change in received signal is fed back through thus generated tap coefficients, the decision feedback equalizer function can continue its function to estimate inter-symbol interference in the transmission line from a matched signal and remove waveform distortion due to the inter-symbol interference. 
         [0044]    Here, in the tap coefficient generation, used is a decision output signal generated by symbol decision on the signal level of an equalization output signal performed by a decision device. Further, feedback of an error output signal representing a change including an error associated with the symbol decision is performed. 
         [0045]    Thus, when the processing by the adaptive equalization processing means  1  has converged, by feeding back a decision output signal to the adaptive equalization processing means  1 , generation of the tap coefficients of the adaptive matched filter function and those of the decision feedback equalizer function is started. Accordingly, the adaptive equalization processing means  1  transits from the state where the tap coefficients stably converge to the state where the tap coefficients are generated by feeding back a change in received signal in a manner to trace it. That is, feedback for performing adaptive equalization processing becomes effective. 
         [0046]    As has been described above, in the adaptive equalization reception circuit  100  of the present exemplary embodiment, when the processing by the adaptive equalization processing means  1  is in an unconverged state, a training signal generated by demodulating a received signal is fed back to the adaptive equalization processing means  1 . As a result, it is possible to make converge well the tap coefficients generated by the tap coefficient generation function of each of the adaptive matched filter function and the decision feedback equalizer function, in the adaptive equalization processing means  1 . Thus, convergence of the processing by the adaptive equalization processing means  1  is stably achieved, therefore, start of the adaptive equalization processing is accelerated. 
         [0047]    Further, in the adaptive equalization reception circuit  100  of the present exemplary embodiment, there is no necessity of transmitting an additional signal such as a preamble signal on the transmission line. Therefore, according to the present exemplary embodiment, it is possible to make the adaptive equalization processing converge without decreasing the transmission efficiency. 
         [0048]    Also according to the present exemplary embodiment, when the processing by the adaptive equalization processing means  1  has converged, the selection means  3  selects a decision output signal from the decision feedback equalizer function and feed it back to the adaptive equalization processing means  1 , and thereby the adaptive equalization processing can be continued. 
         [0049]    Thus, in the present exemplary embodiment, it is possible to achieve an excellent convergent property for the adaptive equalization processing in an unconverged state, without decreasing the transmission efficiency, and to stably continue the adaptive equalization processing automatically in a converged state even if a received signal suffers a large waveform distortion. 
         [0050]    While it has been described, in the present exemplary embodiment, that the situation of convergence of the adaptive equalization processing means is decided on the basis of an error output signal from the decision feedback equalizer function, the decision of whether the situation is in an unconverged state or in a converged state is not limited to the one using the error output signal. 
         [0051]    For example, whether the processing by the adaptive equalization processing means is in an unconverged state or in a converged state may be decided after demodulation by a subsequent stage having received an equalization output signal, on the basis of whether or not a known bit string exists at a known timing point. Alternatively, whether the processing by the adaptive equalization processing means is in an unconverged state or in a converged state may be decided on the basis of the number of error bits at a time of performing error correction after demodulation by a subsequent stage having received an equalization output signal. That is, it may be decided that the adaptive equalization processing means is in an unconverged state if error bits are included in a proportion equal to or more than a predetermined one in a signal demodulated from an equalization output signal, and that the adaptive equalization processing means is in a converged state if error bits are included in less than the predetermined proportion. 
         [0052]    Even when the selection means decides whether the adaptive equalization processing means is in a converged state or in an unconverged state in the way just described above, the selection means may select a training signal generated by demodulating a received signal by the demodulation means when in an unconverged state, and may select a decision output signal from the adaptive equalization processing means when in a converged state, on the basis of a result of the decision of whether in a converged state or in an unconverged state, similarly to in the present exemplary embodiment. 
       Second Exemplary Embodiment 
       [0053]    A second exemplary embodiment of the present invention will be described below.  FIG. 2  is a block diagram showing an example of a configuration of an adaptive equalization processing circuit in the second exemplary embodiment of the present invention.  FIG. 3  is a block diagram showing an example of a configuration of an output circuit inside a decision feedback equalizer of the adaptive equalization processing circuit in the second exemplary embodiment of the present invention.  FIG. 4  is a block diagram showing an example of a configuration of a training signal generation circuit in the second exemplary embodiment of the present invention. 
         [0054]    With reference to  FIG. 2 , the configuration of the adaptive equalization processing circuit  101  of the present exemplary embodiment will be described. The adaptive equalization processing circuit  101  comprises a delay circuit  10 , an adaptive matched filter (AMF)  11 , a decision feedback equalizer (DFE)  12 , a tap coefficient generation circuit  13  for the adaptive matched filter (AMF), a tap coefficient generation circuit  14  for the decision feedback equalizer (DFE), a delay circuit  15 , a training signal generation circuit  20 , a delay circuit  30 , a selection circuit (SEL)  31  and a selection circuit (SEL)  32 . 
         [0055]    The adaptive matched filter  11  estimates the impulse response of the transmission line from a received signal on the basis of tap coefficients received from the AMF tap coefficient generation circuit  13 . Further, the adaptive matched filter  11  is connected in cascade with the transmission line as a filter to realize the inverse characteristic of the estimated impulse response. Accordingly, the adaptive matched filter  11  outputs a matched signal equivalent to the received signal from which waveform distortion related to the impulse response of the transmission line has been removed. 
         [0056]    From the matched signal, which is the output from the adaptive matched filter at the preceding stage, the decision feedback equalizer  12  estimates inter-symbol interference in the transmission line on the basis of tap coefficients received from the DFE tap coefficient generation circuit  14 , and outputs an equalization output signal generated by removing waveform distortion due to the inter-symbol interference from the matched signal. 
         [0057]      FIG. 3  shows a configuration of an output circuit  120  which outputs the equalization output signal, a decision output signal and an error output signal from the decision feedback equalizer  12 . With reference to  FIG. 3 , the output circuit  120  will be described below. The output circuit  120  comprises a decision device  121  and a subtractor  122 . 
         [0058]    The decision feedback equalizer  12  inputs an equalization output signal from a transversal filter constituting the decision feedback equalizer  12  to the decision device  121  of the output circuit  120 , where symbol decision is performed. Specifically, the decision device  121  samples the equalization output signal, decides a symbol for the sampled signal level, and then outputs a decision output signal representing a symbol value of the decision result. The subtractor  122  subtracts the decision output signal from the equalization output signal, and outputs the resultant signal as an error output signal. 
         [0059]    Thus, from the matched signal, which is the output from the adaptive matched filter  11  at the preceding stage, the decision feedback equalizer  12  generates and outputs the equalization output signal, the decision output signal and the error output signal. 
         [0060]    The training signal generation circuit  20  outputs a training signal generated by demodulating a received signal. A configuration and operation of the training signal generation circuit  20  will be described later. 
         [0061]    The delay circuit  10  delays the received signal to be inputted to the AMF tap coefficient generation circuit  13 . The delay circuit  15  delays the decision output signal to be inputted to the AMF tap coefficient generation circuit  13  and to the DFE tap coefficient generation circuit  14  via the respective selection circuits  31  and  32 . The delay circuit  30  delays the training signal to be inputted to the AMF tap coefficient generation circuit  13  and to the DFE tap coefficient generation circuit  14  via the respective selection circuits  31  and  32 . 
         [0062]    Delay time settings for the delay circuits  10 ,  15  and  30  will be described below. Among the delay time settings, the one for the delay circuit  15  is performed first, the one for the delay circuit  30  next, and lastly the one for the delay circuit  10 . 
         [0063]    In the delay circuit  15 , a delay time of the decision output signal is set such that a timing point T 1 , at which the decision output signal from the decision feedback equalizer  12  is inputted to the DFE tap coefficient generation circuit  14  via the selection circuit  32 , becomes the same as a timing point T 2  at which the error output signal from the decision feedback equalizer  12  is inputted to the DFE tap coefficient generation circuit  14 . 
         [0064]    Next, in the delay circuit  30 , a delay time of the training signal is set such that a timing point T 3 , at which the training signal from the training signal generation circuit  20  is inputted to the DFE tap coefficient generation circuit  14  via the selection circuit  32 , becomes the same as the timing point T 2 . 
         [0065]    By the delay time setting in the delay circuit  15 , the timing points T 2  and T 1  become the same, and then by the delay time setting in the delay circuit  30 , the timing points T 2  and T 3  become the same. Accordingly, by the delay time settings in the delay circuits  15  and  30 , all of the timing points T 1 , T 2  and T 3  become the same. 
         [0066]    Here, between a route through which the decision output signal is inputted to the DFE tap coefficient generation circuit  14  and a route through which the decision output signal is inputted to the AMF tap coefficient generation circuit  13 , there is only a difference in which one of the selection circuits  31  and  32  is included in each of the routes. Therefore, a timing point T 4  at which the decision output signal is inputted to the AMF tap coefficient generation circuit  13  via the selection circuit  31  can be regarded as equivalent to the timing point T 1 . 
         [0067]    Here, between a route through which the training signal is inputted to the DFE tap coefficient generation circuit  14  and a route through which the training signal is inputted to the AMF tap coefficient generation circuit  13 , there is only a difference in which one of the selection circuits  31  and  32  is included in the route. Therefore, a timing point T 5  at which the training signal is inputted to the AMF tap coefficient generation circuit  13  via the selection circuit  31  can be regarded as equivalent to the timing point T 3 . 
         [0068]    Consequently, all of the timing points T 1  to T 5  become the same by the delay time settings in the delay circuits  15  and  30 . 
         [0069]    Lastly, in the delay circuit  10 , a delay time of the received signal is set such that a timing point T 6  at which the received signal is inputted to the AMF tap coefficient generation circuit  13  becomes the same as the timing point T 4  or T 5 . 
         [0070]    By the above settings, adjustment of all of the timing points T 1  to T 6  is completed. 
         [0071]    On the basis of the situation of convergence of processing by the adaptive matched filter  11  and the decision feedback equalizer  12 , which are in charge of the adaptive equalization processing, the selection circuits  31  and  32  select and output the training signal from the training signal generation circuit  20  when the situation of convergence is in an unconverged state. When the situation of convergence is in a converged state, the selection circuits  31  and  32  select and output the decision output signal from the decision feedback equalizer  12 . 
         [0072]    The AMF tap coefficient generation circuit  13  generates tap coefficients of the adaptive matched filter  11  on the basis of the received signal whose input timing is adjusted and the signal from the selection circuit  31 . On the basis of the received signal and the signal from the selection circuit  31 , the AMF tap coefficient generation circuit  13  estimates the impulse response of the transmission line and generates the tap coefficients such that the adaptive matched filter  11  realizes the inverse characteristic of the estimated impulse response. 
         [0073]    The DFE tap coefficient generation circuit  14  generates tap coefficients of the decision feedback equalizer  12  on the basis of the error output signal from the decision feedback equalizer  12  and the signal from the selection circuit  32 . On the basis of the error output signal and the signal from the selection circuit  32 , the DFE tap coefficient generation circuit  14  estimates inter-symbol interference in the transmission line and generates the tap coefficients of the decision feedback equalizer  12  such that waveform distortion due to the inter-symbol interference is removed from an output signal of the adaptive matched filter  11 . 
         [0074]    For the above-described tap coefficient generations, for example, the LMS (Least Mean Square) algorithm, the RLS (Recursive Least Squares) algorithm and the like are used. In the present invention, there is no restriction on an algorithm used for the tap coefficient generations. 
         [0075]    With reference to  FIG. 4 , a configuration of the training signal generation circuit  20  of the present exemplary embodiment will be described below. 
         [0076]    The training signal generation circuit  20  comprises a symbol timing reproduction circuit  21 , a carrier reproduction circuit  22  and a decision device  23 . 
         [0077]    The symbol timing reproduction circuit  21  reproduces symbol timing from a received signal. For example, the symbol timing reproduction circuit  21  extracts symbol timing information by performing envelope detection, zero-crossing detection or the like on the received signal. The carrier reproduction circuit  22  reproduces the carrier signal by performing phase synchronization of the carrier signal by the use of the received signal and the symbol timing information obtained by the symbol timing reproduction circuit  21 . The decision device  23  performs decision on the received signal using the symbol timing information and the carrier signal obtained by, respectively, the symbol timing reproduction circuit  21  and the carrier reproduction circuit  22 . The decision device  23  samples the received signal on the basis of the timing information and performs symbol decision on thus sampled signal level, and thereby demodulates the received signal. In this way, the decision device  23  outputs a signal generated by demodulating the received signal as a training signal. 
         [0078]    With reference to  FIGS. 2, 3 and 4 , operation of the adaptive equalization processing circuit  101  of the present exemplary embodiment will be described below. 
         [0079]    On the basis of the tap coefficients received from the AMF tap coefficient generation circuit  13 , the adaptive matched filter  11  estimates the impulse response of the transmission line, realizes the inverse characteristic of the estimated impulse response, and thereby outputs a matched signal equivalent to the received signal from which waveform distortion related to the impulse response of the transmission line has been removed. On the basis of the tap coefficients received from the DFE tap coefficient generation circuit  14 , the decision feedback equalizer  12  at the subsequent stage estimates inter-symbol interference in the transmission line from the matched signal, and outputs an equalization output signal equivalent to the matched signal from which waveform distortion due to the inter-symbol interference has been removed. 
         [0080]    In order to normally perform the adaptive equalization processing by the adaptive matched filter  11  and the decision feedback equalizer  12 , it is necessary to make converge the processing performed by the adaptive matched filter  11  and that by the decision feedback equalizer  12 . In order to make converge the processing by the adaptive matched filter  11  and that by the decision feedback equalizer  12 , it is required that, for a certain time period, a signal not causing a decision error is inputted to each of the respective tap coefficient generation circuits  13  and  14 , 
         [0081]    The training signal generation circuit  20  extracts timing information from a received signal by means of the symbol timing reproduction circuit  21  and the carrier reproduction circuit  22 . On the basis of the timing information, the decision device  23  performs symbol decision on the received signal and then demodulates it. Thus, the decision device  23  outputs the signal generated by the demodulation as a training signal. Because the training signal is generated by demodulating the received signal, it is a signal whose symbol timing and carrier phase are synchronized with the received signal. 
         [0082]    When the adaptive matched filter  11  and the decision feedback equalizer  12  in charge of the adaptive equalization processing are in an unconverged state, the selection circuits  31  and  32  each select a training signal from the training signal generation circuit  20  and input it to, respectively, the tap coefficient generation circuits  13  and  14 . 
         [0083]    The signal selected by the selection circuits  31  and  32  is a training signal generated by demodulating a received signal. Accordingly, in the adaptive equalization processing performed by the adaptive matched filter  11  and the decision feedback equalizer  12 , there occurs no decision error related to the symbol timing and carrier phase. 
         [0084]    While the training signal synchronized with the received signal is inputted from the selection circuits  31  and  32 , the tap coefficients generated by the tap coefficient generation circuit  13  of the adaptive matched filter  11  and those by the tap coefficient generation circuit  14  of the decision feedback equalizer  12  converge, and the adaptive equalization processing is started. 
         [0085]    On the basis of an error output signal from the decision feedback equalizer, the selection circuits  31  and  32  detect that the adaptive matched filter  11  and the decision feedback equalizer  12  in charge of the adaptive equalization processing have converged. At this time, the selection circuits  31  and  32  each select a decision output signal from the decision feedback equalizer  12  and input it to the respective tap coefficient generation circuits  13  and  14 . 
         [0086]    As a result, feedback for performing the adaptive equalization processing, of which the adaptive matched filter  11  and the decision feedback equalizer  12  are in charge, becomes effective. Accordingly, on the basis of the feedback of the tap coefficients generated by the tap coefficient generation circuit  13  and those by the tap coefficient generation circuit  14 , respectively, the adaptive matched filter  11  and the decision feedback equalizer  12  can stably continue the adaptive equalization processing automatically even if distortion of a received signal becomes large. 
         [0087]    As has been described above, when the adaptive equalization processing is in an unconverged state, the adaptive equalization reception circuit  101  of the present exemplary embodiment uses a training signal generated by demodulating a received signal. Accordingly, in the adaptive equalization reception circuit  101 , it is possible to make converge well the tap coefficients generated by the tap coefficient generation circuit  13  of the adaptive matched filter  11  and those by the tap coefficient generation circuit  14  of the decision feedback equalizer  12 . Start of the adaptive equalization processing, of which the adaptive matched filter  11  and the decision feedback equalizer  12  are in charge, is accordingly accelerated because convergence of their tap coefficients is stably achieved. 
         [0088]    Further, in the adaptive equalization reception circuit  101  of the present exemplary embodiment, there is no necessity of transmitting an additional signal such as a preamble signal on the transmission line. Therefore, in the present exemplary embodiment, it is possible to make the adaptive equalization processing converge without decreasing the transmission efficiency. 
         [0089]    Furthermore, in the present exemplary embodiment, when the adaptive equalization processing has converged, a decision output signal from the decision feedback equalizer  12  is inputted to each of the tap coefficient generation circuits  13  and  14  and then fed back to the adaptive matched filter  11  and the decision feedback equalizer  12  in the form of their tap coefficients. As a result, in the present exemplary embodiment, the adaptive equalization processing can be continued stably. 
         [0090]    Thus, in the present exemplary embodiment, similarly to the first exemplary embodiment, it is possible to achieve an excellent convergent characteristic for the adaptive equalization processing in an unconverged state, without decreasing the transmission efficiency, and to stably continue the adaptive equalization processing automatically even if a received signal suffers large waveform distortion in a converged state. 
         [0091]    While it has been described, in the present exemplary embodiment, that the situation of convergence of the adaptive equalization processing is decided on the basis of an error output signal from the decision feedback equalizer  12 , decision of whether the situation is in an unconverged state or in a converged state is not limited to the one using the error output signal. 
         [0092]    For example, whether the adaptive equalization processing is in an unconverged state or in a converged state may be decided after demodulation performed by a subsequent stage having received an equalization output signal, on the basis of whether or not a known bit string exists at a known timing point. Alternatively, whether the adaptive equalization processing is in an unconverged state or in a converged state may be decided after demodulation performed by a subsequent stage having received an equalization output signal, on the basis of the number of error bits at a time of performing error correction. That is, it may be decided that the adaptive equalization processing is in an unconverged state if error bits are included in a proportion equal to or more than a predetermined one in a signal demodulated from an equalization output signal, and that the adaptive equalization processing is in a converged state if error bits are included in less than the predetermined proportion. 
         [0093]    Even in a case where the selection circuits  31  and  32  decide whether the adaptive equalization processing is in a converged state or in an unconverged state in the way just described above, the selection circuits  31  and  32  may select a training signal generated by demodulating a received signal when in an unconverged state, and may select a decision output signal from the decision feedback equalizer when in a converged state, on the basis of a result of the decision of whether in a converged state or in an unconverged state, similarly to the present exemplary embodiment. 
       Third Exemplary Embodiment 
       [0094]    A third exemplary embodiment of the present invention will be described below.  FIG. 5  is a block diagram showing an example of a configuration of an adaptive equalization processing circuit in the third exemplary embodiment of the present invention. 
         [0095]    An adaptive equalization processing circuit  102  of the third exemplary embodiment shown in  FIG. 5  is a diversity type adaptive equalization processing circuit, which is different from the one in the second exemplary embodiment in that it deals with respective received signals from a plurality of branches, and that an adder circuit  16  and a maximum value output circuit (MAX)  40  are added. Accordingly, to the same constituent element as that in the second exemplary embodiment, the same sign as that used in the second exemplary embodiment will be assigned, and its description will be omitted here. 
         [0096]    With reference to  FIG. 5 , the configuration of the adaptive equalization processing circuit  102  of the present exemplary embodiment will be described below. 
         [0097]    To the adaptive equalization processing circuit  102 , branch-specific received signals B 1  to Bn (n is an integer equal to or larger than 2) from a plurality of branches are inputted. Adaptive matched filters  11 - 1  to  11 - n  respectively deal with the branch-specific received signals B 1  to Bn. The adder circuit  16  combines together matched signals which are respective outputs from the adaptive matched filters (AMFs)  11 - 1  to  11 - n , and it outputs the combined signal to a decision feedback equalizer (DFE)  12  at the subsequent stage. 
         [0098]    Delay circuits  10 - 1  to  10 - n , AMF tap coefficient generation circuits  13 - 1  to  13 - n  and selection circuits  31 - 1  to  31 - n  deal with the branch-specific received signals B 1  to Bn, respectively. The maximum value output circuit  40  outputs a branch-specific received signal Bi (i is an integer in the range from 1 to n) whose reception level value is the largest, among the branch-specific received signals B 1  to Bn, to a training signal generation circuit  20  at the subsequent stage. 
         [0099]    With reference to  FIG. 5 , operation of the adaptive equalization processing circuit  102  of the present exemplary embodiment will be described. 
         [0100]    The branch-specific received signals B 1  to Bn which are received by a diversity method are inputted to respectively corresponding ones of the adaptive matched filters  11 - 1  to  11 - n , where adaptive matched filter processing is performed on the inputted signals, and the resultant signals are combined together by the adder circuit  16 . The combined signal generated by the adder circuit  16  is inputted to the decision feedback equalizer  12 , where waveform distortion due to inter-symbol interference is removed from the combined signal and the resultant signal is outputted as an equalization output signal. 
         [0101]    The maximum value output circuit  40  inputs a branch-specific received signal Bi whose reception level value is the largest, among the branch-specific received signals B 1  to Bn, to the training signal generation circuit  20 . The training signal generation circuit  20  outputs a signal generated by demodulating the branch-specific received signal Bi inputted from the maximum value output circuit  40 , as a training signal. 
         [0102]    The selection circuits  31 - 1  to  31 - n  and  32  decide the situation of convergence of the adaptive equalization processing on the basis of an error output signal from the decision feedback equalizer  12 . 
         [0103]    When the adaptive equalization processing is in an unconverged state, the selection circuits  31 - 1  to  31 - n  and  32  each select a signal equivalent to the training signal whose timing has been adjusted by a delay circuit  30 , and input the selected signal to respective ones of the AMF tap coefficient generation circuits  13 - 1  to  13 - n  and a DFE tap coefficient generation circuit  14 . 
         [0104]    Further, the branch-specific received signals B 1  to Bn receive timing adjustment performed by the delay circuits  10 - 1  to  10 - n , respectively, and then are inputted to respectively corresponding ones of the AMF tap coefficient generation circuits  13 - 1  to  13 - n . Each of the AMF tap coefficient generation circuits  31 - 1  to  31 - n  generates its own tap coefficients on the basis of the training signal selected by the corresponding one of the selection circuits  31 - 1  to  31 - n  and the corresponding one of the branch-specific received signals B 1  to Bn, and inputs the generated tap coefficients to the corresponding one of the adaptive matched filters  11 - 1  to  11 - n.    
         [0105]    The DFE tap coefficient generation circuit  14  generates tap coefficients on the basis of the training signal selected by the selection circuit  32  and an error output signal from the decision feedback equalizer  12 , and inputs the generated tap coefficients to the decision feedback equalizer  12 . 
         [0106]    Because the signal selected by the selection circuits  31 - 1  to  31 - n  and  32  is the training signal generated by demodulating the received signal, there occurs no decision error related to the symbol timing and carrier phase in the adaptive equalization processing. 
         [0107]    While the training signal synchronized with the received signal is inputted from the selection circuits  31 - 1  to  31 - n  and  32 , tap coefficients generated by each of the tap coefficient generation circuits  13 - 1  to  13 - n  of the respective adaptive matched filters  11 - 1  to  11 - n  and those generated by the tap coefficient generation circuit  14  of the decision feedback equalizer  12  converge, and the adaptive equalization processing is started. 
         [0108]    On the basis of an error output signal from the decision feedback equalizer  12 , the selection circuits  31 - 1  to  31 - n  and  32  detect that the adaptive equalization processing has converged. At this time, the selection circuits  31 - 1  to  31 - n  and  32  each select a decision output signal from the decision feedback equalizer  12  and input it to the respective ones of the tap coefficient generation circuits  13 - 1  to  13 - n  and  14 . 
         [0109]    As a result, feedback for performing the adaptive equalization processing in the adaptive matched filters  11 - 1  to  11 - n  and the decision feedback equalizer  12  becomes effective. Accordingly, on the basis of the feedback of the tap coefficients generated by the tap coefficient generation circuits  13 - 1  to  13 - n  and those by the tap coefficient generation circuit  14 , the adaptive matched filters  11 - 1  to  11 - n  and the decision feedback equalizer  12  can stably continue the adaptive equalization processing automatically even if distortion of a received signal becomes large. 
         [0110]    As has been described above, when the adaptive equalization processing is in an unconverged state, the adaptive equalization reception circuit  102  of the present exemplary embodiment uses a training signal generated by demodulating a received signal. Accordingly, in the adaptive equalization reception circuit  102 , it is possible to make converge well the tap coefficients generated by each of the tap coefficient generation circuits  13 - 1  to  13 - n  of the respective adaptive matched filters  11 - 1  to  11 - n  and those generated by the tap coefficient generation circuit  14  of the decision feedback equalizer  12 . In each of the adaptive matched filters  11 - 1  to  11 - n  and the decision feedback equalizer  12 , start of the adaptive equalization processing of which it is in charge is accordingly accelerated because convergence of their tap coefficients is stably achieved as above. 
         [0111]    Further, in the adaptive equalization reception circuit  102  of the present exemplary embodiment, there is no necessity of transmitting an additional signal such as a preamble signal on the transmission line. Therefore, in the present exemplary embodiment, it is possible to make the adaptive equalization processing converge without decreasing the transmission efficiency. 
         [0112]    Furthermore, in the present exemplary embodiment, when the adaptive equalization processing has converged, a decision output signal from the decision feedback equalizer  12  is inputted to each of the tap coefficient generation circuits  13 - 1  to  13 - n  and  14  and then fed back to each of the adaptive matched filters  11 - 1  to  11 - n  and the decision feedback equalizer  12  in the form of their tap coefficients, and thereby, the adaptive equalization processing can be continued stably. 
         [0113]    Thus, in the present exemplary embodiment, similarly to the first and second exemplary embodiments, it is possible to achieve an excellent convergent characteristic for the adaptive equalization processing in an unconverged state, without decreasing the transmission efficiency, and to stably continue the adaptive equalization processing automatically even if a received signal suffers large waveform distortion in a converged state. 
         [0114]    While it has been described, in the present exemplary embodiment, that the situation of convergence of the adaptive equalization processing is decided on the basis of an error output signal from the decision feedback equalizer  12 , decision of whether the situation is in an unconverged state or in a converged state is not limited to the one using the error output signal. 
         [0115]    For example, whether the adaptive equalization processing is in an unconverged state or in a converged state may be decided after demodulation performed by a subsequent stage having received an equalization output signal, on the basis of whether or not a known bit string exists at a known timing point. Alternatively, whether the adaptive equalization processing is in an unconverged state or in a converged state may be decided after demodulation performed by a subsequent stage having received an equalization output signal, on the basis of the number of error bits at a time of performing error correction. That is, it may be decided that the adaptive equalization processing is in an unconverged state if error bits are included in a proportion equal to or more than a predetermined one in a signal demodulated from an equalization output signal, and that the adaptive equalization processing is in a converged state if error bits are included in less than the predetermined proportion. 
         [0116]    Even in a case where the selection circuits  31 - 1  to  31 - n  and  32  decide whether the adaptive equalization processing is in a converged state or in an unconverged state in the way just described above, the selection circuits  31 - 1  to  31 - n  and  32  may select a training signal generated by demodulating a received signal when in an unconverged state, and may select a decision output signal from the decision feedback equalizer  12  when in a converged state, on the basis of a result of the decision of whether in a converged state or in an unconverged state, similarly to in the present exemplary embodiment. 
         [0117]    Further, it has been described, in the present exemplary embodiment, that a training signal is generated by using one of branch-specific received signals received from respective ones of a plurality of branches of a diversity method whose reception level value is the largest among the branch-specific received signals. However, it is not necessarily required to use a branch-specific received signal having the largest reception level value. 
         [0118]    For example, as a signal used for generating a training signal, one of branch-specific received signals received from respective ones of a plurality of branches whose S/N (Signal to Noise) ratio value is the largest may be detected and used. Alternatively, as a signal for generating a training signal, one of the branch-specific received signals whose bit error rate value before error correction is the largest may be detected and used. As a signal for generating a training signal, one of the branch-specific received signals whose bit error rate value after error correction is the smallest may be detected and used alternatively. Furthermore, as a signal for generating a training signal, one of the branch-specific received signals whose frequency characteristic in a specific frequency range is the flattest may be detected and used alternatively. 
         [0119]    Here, in the present exemplary embodiment, the description has been made supposing that the diversity method is a combining diversity. However, the present exemplary embodiment can be applied to even a case where the diversity method is a selection diversity, an equal gain diversity or the like. 
         [0120]    While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill 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 claims. 
         [0121]    For example, the present invention is applicable not only to QAM but also to phase modulation such as QPSK, amplitude modulation, frequency modulation, amplitude phase modulation such as APSK (Amplitude Phase Shift Keying), and any other digital modulation methods. 
         [0122]    Further, in the exemplary embodiments of the present invention, the descriptions have been made assuming that signals to be used respectively for generating tap coefficients of the adaptive matched filter and for generating those of the decision feedback equalizer are switched simultaneously, on the basis of decision of whether the adaptive equalization processing is in a converged state or in an unconverged state. However, it is not necessarily required that the switchings of the signals used for generating the tap coefficients are switched simultaneously. 
         [0123]    For example, the signal switchings may be performed such that switching of the signal for AMF tap coefficient generation is performed first, and then that of the signal for DFE tap coefficient generation is performed after a delay of a predetermined time period. The signal switchings may be performed in the reverse order. 
         [0124]    This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-112910, filed on May 29, 2013, the disclosure of which is incorporated herein in its entirety by reference. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1  adaptive equalization processing means 
               2  demodulation means 
               3  selection means 
               10 ,  10 - 1 ,  10 - n ,  15 ,  30  delay circuit 
               11 ,  11 - 1 ,  11 - n  adaptive matched filter (AMF) 
               12  decision feedback equalizer (DFE) 
               13 ,  13 - 1 ,  13 - n  tap coefficient generation circuit for adaptive matched filter (AMF) 
               14  tap coefficient generation circuit for decision feedback equalizer (DFE) 
               16  adder circuit 
               20  training signal generation circuit 
               21  symbol timing reproduction circuit 
               22  carrier reproduction circuit 
               23 ,  121  decision device 
               31 ,  31 - 1 ,  31 - n ,  32  selection circuit 
               40  maximum value output circuit 
               100 ,  101 ,  102  adaptive equalization reception circuit 
               120  output circuit 
               122  subtractor