Abstract:
An OFDM receiver may include OFDM-signal receiving means for receiving an orthogonal frequency division multiplexing (OFDM) signal; channel-characteristic estimating means for estimating a channel characteristic using pilot signals in the OFDM signal received by the OFDM-signal receiving means; and transmission-distortion compensating means for applying, on the basis of the channel characteristic estimated by the channel-characteristic estimating means, processing for compensating for transmission distortion to the OFDM signal received by the OFDM-signal receiving means. The channel-characteristic estimating means may include plural kinds of time-direction-channel estimating means used for the estimation of a channel characteristic, and switching control means for switching these estimating means according to a state of a channel.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority from Japanese Patent Application No. JP 2006-247097 filed in the Japanese Patent Office on Sep. 12, 2006, the entire content of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an OFDM receiver and an OFDM signal receiving method for receiving an orthogonal frequency division multiplexing (OFDM) signal and demodulating the OFDM signal.  
         [0004]     2. Description of the Related Art  
         [0005]     A modulation system called an orthogonal frequency division multiplexing (OFDM) system is used as a modulation and demodulation system of a terrestrial digital broadcasting system. This OFDM system is a system for providing a large number of orthogonal sub-carriers in a transmission band, allocating data to amplitudes and phases of the respective sub-carriers, and digitally modulating a signal according to PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation).  
         [0006]     The OFDM system has a characteristic that, since the transmission band is divided by the large number of sub-carriers, although a band per one sub-carrier is narrowed and modulation speed is reduced, transmission speed as a whole is the same as that in the modulation system in the past. The OFDM system also has a characteristic that, since the large number of sub-carriers are transmitted in parallel, symbol speed is reduced. Therefore, in the OFDM system, a time length of a multi-path relative to a time length of a symbol can be reduced and transmission is less susceptible to a multi-path interference. Further, the OFDM system has a characteristic that, since data is allocated to the plural sub-carriers, a transmission and reception circuit can be formed by using, during modulation, an IFFT (Inverse Fast Fourier Transform) arithmetic circuit that performs inverse Fourier transform and using, during demodulation, an FFT (Fast Fourier Transform) arithmetic circuit that performs Fourier transform.  
         [0007]     Since the OFDM system has the characteristics described above, the OFDM system is often applied to the terrestrial digital broadcast that is intensely affected by the multi-path interference. As the terrestrial digital broadcast employing such an OFDM system, there are standards such as DVB-T (Digital Video Broadcasting-Terrestrial), ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) and ISDB-TSB (Integrated Services Digital Broadcasting-Terrestrial Sound Broadcasting) (see, for example, “Receiver for Terrestrial Digital Sound Broadcast-Standard (Desirable Specifications) ARIB STD-B30 version 1.1”, Association of Radio Industries and Businesses, decided on May 31, 2001 and revised on Mar. 28, 2002 and “Transmission System for Terrestrial Digital Sound Broadcast ARIB STD-B29 version 1.1”, Association of Radio Industries and Businesses, decided on May 31, 2001 and revised on Mar. 28, 2002).  
         [0008]     A transmission signal in the OFDM system is transmitted by a unit of a symbol called an OFDM symbol. This OFDM symbol includes an effective symbol that is a signal period in which IFFT is performed during transmission and a guard interval in which a waveform of a part of the latter half of this effective symbol is directly copied. This guard interval is provided in the former half of the OFDM symbol. In the OFDM system, such a guard interval is provided to improve multi-path resistance. Plural OFDM symbols are collected to form one OFDM transmission frame. For example, in the ISDB-T standard, ten FDM transmission frames are formed by two hundred four OFDM symbols. Insertion positions of pilot signals are set with this unit of OFDM transmission frames as a reference.  
         [0009]     In the OFDM system in which the modulation of a QAM system is used as a modulation system for each of the sub-carriers, characteristics of the amplitude and the phase are different for each of the sub-carriers because of the influence of the multi-path and the like during transmission. Therefore, on a reception side, it is necessary to equalize a reception signal to make the amplitude and the phase for each of the sub-carriers equal. In the OFDM system, on a transmission side, pilot signals of a predetermined amplitude and a predetermined phase are discretely inserted in a transmission symbol in a transmission signal. On the reception side, a frequency characteristic of a channel is calculated using the amplitude and the phase of the pilot signals and a reception signal is equalized according to the calculated characteristic of the channel.  
         [0010]     The pilot signals used for calculating a channel characteristic are referred to as scattered pilot (SP) signals.  
       SUMMARY OF THE INVENTION  
       [0011]     As a method of estimating a time direction channel in the OFDM receiver, there are known a method of estimating a time direction channel using an average-type estimator, a method of estimating a time direction channel using an interpolation-type estimator, and a method of estimating a time direction channel using a prediction-type estimator. All of the methods have advantages and disadvantages in characteristics thereof. The prediction-type estimator can accurately estimate a channel for a static channel without temporal fluctuation and a channel in which temporal fluctuation is periodic. However, the prediction-type estimator fails in prediction and may be unable to correctly estimate a channel for a channel that fluctuates at random as known in Typical Urban. On the other hand, the interpolation-type estimator is more excellent than the prediction-type estimator in that the interpolation-type estimator can estimate a channel without a very significant error even in a channel that fluctuates at random. However, when it is attempted to attain performance equivalent to that of the prediction-type estimator in a static channel or a channel that fluctuates periodically, an enormous number of taps are necessary and, therefore, a memory for holding data is also necessary. The average-type estimator attains excellent performance when the fluctuation in a channel is extremely gentle but, when fluctuation is large, the average-type estimator may be unable to follow the fluctuation.  
         [0012]     Therefore, there is a need for providing an OFDM receiver and an OFDM signal receiving method that can receive an OFDM signal without a substantial increase in size of a circuit regardless of whether a channel is static, temporal fluctuation in the channel is periodic, or temporal fluctuation in the channel is random.  
         [0013]     Other needs and specific advantages derived therefrom will be made more obvious from the following explanations of embodiments.  
         [0014]     According to an embodiment of the present invention, in order to attain high performance without a substantial increase in size of a circuit regardless of whether a channel is static, temporal fluctuation in the channel is periodic, or temporal fluctuation in the channel is random, the average-type estimator, the interpolation-type estimator, and the prediction-type estimator may be switched and used.  
         [0015]     According to an embodiment of the present invention, there is provided an OFDM receiver which may include OFDM-signal receiving means for receiving an orthogonal frequency division multiplexing (OFDM) signal, channel-characteristic estimating means for estimating a channel characteristic using pilot signals in the OFDM signal received by the OFDM-signal receiving means, and transmission-distortion compensating means for applying, on the basis of the channel characteristic estimated by the channel-characteristic estimating means, processing for compensating for transmission distortion to the OFDM signal received by the OFDM-signal receiving means. The channel-characteristic estimating means may include plural kinds of time-direction-channel estimating means used for the estimation of a channel characteristic and switching control means for switching these estimating means according to a state of a channel.  
         [0016]     According to another embodiment of the present invention, there is provided an OFDM signal receiving method of receiving an orthogonal frequency division multiplexing (OFDM) signal, estimating a channel characteristic using pilot signals in the received OFDM signal, and applying, on the basis of the estimated channel characteristic, processing for compensating for transmission distortion to the received OFDM signal, the OFDM signal receiving method may include estimating a Doppler spectrum for the received OFDM signal and switching, according to the estimated Doppler spectrum, plural kinds of time-direction-channel estimating means used for the estimation of a channel characteristic.  
         [0017]     According to the embodiments of the present invention, the prediction-type estimator may be used when a channel is static or when temporal fluctuation in the channel is periodic. When temporal fluctuation in the channel is random, it may be possible to switch the prediction-type estimator to the interpolation-type estimator to estimate a time direction channel. In other words, it may be possible to select an appropriate estimation method according to a state of the channel and attain excellent reception performance in all channels. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1  is a block diagram showing a structure of an OFDM receiver according to an embodiment of the present invention;  
         [0019]      FIG. 2  is a diagram for explaining transmission symbols of an OFDM signal;  
         [0020]      FIG. 3  is a diagram for explaining an arrangement pattern of SP signals in the OFDM signal;  
         [0021]      FIG. 4  is a block diagram showing a structure of a pilot-use channel estimator in the OFDM receiver;  
         [0022]      FIGS. 5A and 5B  are diagrams for explaining an average-type method of estimating a time direction channel in the pilot-use channel estimator;  
         [0023]      FIGS. 6A and 6B  are diagrams for explaining an interpolation-type method of estimating a time direction channel in the pilot-use channel estimator;  
         [0024]      FIGS. 7A  to  7 C are diagrams schematically showing an example of a Doppler spectrum;  
         [0025]      FIG. 8  is a diagram for explaining sub-carries estimated by the estimation of a time direction channel in the pilot-use channel estimator;  
         [0026]      FIG. 9  is a diagram for explaining sub-carriers estimated by a frequency-direction channel estimator in the OFDM receiver;  
         [0027]      FIG. 10  is a block diagram showing an example of another structure of the pilot-use channel estimator in the OFDM receiver;  
         [0028]      FIGS. 11A and 11B  are diagrams for explaining a prediction-type method of estimating a time direction channel in the pilot-use channel estimator;  
         [0029]      FIG. 12  is a block diagram showing an example of still another structure of the pilot-use channel estimator in the OFDM receiver;  
         [0030]      FIG. 13  is a block diagram showing an example of a structure of a fluctuation-type judging device in the pilot-use channel estimator;  
         [0031]      FIG. 14  is a flowchart showing operations of a judging device in the fluctuation-type judging device;  
         [0032]      FIGS. 15A  to  15 C are diagrams schematically showing a state of judgment of a shape of a Doppler spectrum at the time when there is no fluctuation;  
         [0033]      FIGS. 16A  to  16 C are diagrams schematically showing a state of judgment of a shape of a Doppler spectrum at the time when fluctuation is periodic; and  
         [0034]      FIGS. 17A  to  17 C are diagrams schematically showing a state of judgment of a shape of a Doppler spectrum at the time when fluctuation is random. 
     
    
     DETAILED DESCRIPTION  
       [0035]     Embodiments of the present invention will be hereinafter explained in detail with reference to the accompanying drawings. It goes without saying that the present invention is not limited to the embodiments described below and can be modified arbitrarily without departing from the spirit of the present invention.  
         [0036]     The present invention is applied to, for example, an OFDM receiver  10  having a structure shown in  FIG. 1 .  
         [0037]     The OFDM receiver  10  includes an antenna  11 , a tuner  12 , a band-pass filter (BPF)  13 , an A/D converter  14 , a digital orthogonal demodulator  15 , an FFT arithmetic circuit  16 , a pilot-use channel estimator  17 , a channel distortion compensator  18 , an error correction circuit  19 , a transmission parameter decoder  20 , a delay profile estimator  21 , and a window regenerator  22 .  
         [0038]     A broadcast wave of a digital broadcast transmitted from a broadcasting station is received by the antenna  11  of the OFDM receiver  10  and supplied to the tuner  12  as an RF signal.  
         [0039]     The tuner  12  includes a multiplication circuit  121  and a local oscillator  122 . The tuner  12  frequency-converts the RF signal received by the antenna  11  into an IF signal. The IF signal obtained by the tuner  12  is filtered by the band-pass filter (BPF)  13  and, then, digitized by the A/D converter  14  and supplied to the digital orthogonal demodulator  15 .  
         [0040]     The digital orthogonal demodulator  15  orthogonally demodulates the digitized IF signal using a carrier signal of a predetermined frequency (a carrier frequency) and outputs an OFDM signal of a baseband. The OFDM signal of the baseband outputted from the digital orthogonal demodulator  15  is a signal in a so-called time domain before being subjected to an FFT operation. Therefore, a baseband signal after digital orthogonal demodulation and before the FFT operation is hereinafter referred to as an OFDM time domain signal. As a result of orthogonal demodulation, this OFDM time domain signal changes to a complex signal including a real axis component (an I channel signal) and an imaginary axis component (a Q channel signal). The OFDM time domain signal outputted by the digital orthogonal demodulator  15  is supplied to the FFT arithmetic circuit  16 , the window regenerator  22 , and the delay profile estimator  21 .  
         [0041]     The FFT arithmetic circuit  16  applies the FFT operation to the OFDM time domain signal, extracts data orthogonally modulated in each of sub-carriers, and outputs the data. The signal outputted from the FFT arithmetic circuit  16  is a signal in a so-called frequency domain after being subjected to the FFT operation. Therefore, the signal after the FFT operation is referred to as an OFDM frequency domain signal.  
         [0042]     The FFT arithmetic circuit  16  extracts a signal in a range of an effective symbol length from one OFDM symbol, i.e., excludes a range of a guard interval from one OFDM symbol, and applies the FFT operation to the extracted OFDM time domain signal. Specifically, as shown in  FIG. 2 , a position where the arithmetic operation is started is any position from a boundary of the OFDM symbol (a position of A in  FIG. 2 ) to an end position of the guard interval (a position of B in  FIG. 2 ). This arithmetic operation range is referred to as an FFT window.  
         [0043]     A transmission signal in the OFDM system is transmitted by a unit of a symbol called an OFDM symbol. This OFDM symbol includes an effective symbol that is a signal period in which IFFT is performed during transmission and a guard interval in which a waveform of a part of the latter half of this effective symbol is directly copied. This guard interval is provided in the former half of the OFDM symbol. In the OFDM system, such a guard interval is provided to improve multi-path resistance. Plural OFDM symbols are collected to form one OFDM transmission frame. For example, in the ISDB-T standard, ten FDM transmission frames are formed by two hundred four OFDM symbols. Insertion positions of pilot signals are set with this unit of OFDM transmission frames as a reference.  
         [0044]     In the OFDM system in which the modulation of a QAM system is used as a modulation system for each of the sub-carriers, characteristics of the amplitude and the phase are different for each of the sub-carriers because of the influence of the multi-path and the like during transmission. Therefore, on a reception side, it is necessary to equalize a reception signal to make the amplitude and the phase for each of the sub-carriers equal. In the OFDM system, on a transmission side, pilot signals of a predetermined amplitude and a predetermined phase are discretely inserted in a transmission symbol in a transmission signal. On the reception side, a frequency characteristic of a channel is calculated using the amplitude and the phase of the pilot signals and a reception signal is equalized according to the calculated characteristic of the channel.  
         [0045]     The pilot signals used for calculating a channel characteristic are referred to as scattered pilot (SP) signals. An arrangement pattern in the OFDM symbol of the SP signals adopted in the DVB-T standard and the ISDB-T standard is shown in  FIG. 3 .  
         [0046]     In the OFDM receiver  10 , the designation of this FFT window position is performed by the window regenerator  22 . As the window regenerator  22 , for example, means for performing window regeneration according to detection of a correlation value of a guard interval period using the OFDM time domain signal and means for estimating a delay profile of a channel using the delay profile estimator  21  and performing window regeneration are used.  
         [0047]     The pilot-use channel estimator  17  extracts the SP signals inserted in the OFDM frequency domain signal calculated by the FFT arithmetic circuit  16  and estimates a channel characteristic of the sub-carriers in which the SP signals are arranged.  
         [0048]     The pilot-use channel estimator  17  in the OFDM receiver  10  includes, for example, as in a pilot-use channel estimator  17 A shown in  FIG. 4 , an SP-signal extraction circuit  171 , an average-type time-direction-channel estimator  172 , an interpolation-type time-direction-channel estimator  173 , a selector  174 , a Doppler spectrum estimator  175 , and a maximum-Doppler-frequency judging circuit  176 .  
         [0049]     In the pilot-use channel estimator  17 A, the OFDM frequency domain signal is supplied to the SP-signal extraction circuit  171  and the Doppler spectrum estimator  175 .  
         [0050]     The SP-signal extraction circuit  171  extracts only SP signals inserted in positions shown in  FIG. 3  and removes modulation components of the pilot signals to calculate channel characteristics in the SP positions. Channel characteristics in the SP positions calculated by the SP-signal extraction circuit  171  are supplied to the average-type time-direction-channel estimator  172  and the interpolation-type time-direction-channel estimator  173 .  
         [0051]     The average-type time-direction-channel estimator  172  includes a primary IIR filter having a structure, for example, shown in  FIG. 5A . The average-type time-direction-channel estimator  172  averages channel estimated values in the SP positions estimated by the SP-signal extraction circuit  171  as shown in  FIG. 5B . An IIR output is repeatedly used during the SP signals adjacent to one another in the time direction.  
         [0052]     The interpolation-type time-direction-channel estimator  173  includes a linear interpolation circuit having a structure, for example, shown in  FIG. 6A . The interpolation-type time-direction-channel estimator  173  interpolates the channel estimated values in the SP signal positions, which are estimated by the SP-signal extraction circuit  171 , in the time direction to estimate a channel during three symbols as shown in  FIG. 6B .  
         [0053]     The Doppler spectrum estimator  175  estimates a Doppler spectrum from the OFDM frequency domain signal. The maximum-Doppler-frequency judging circuit  176  calculates a maximum Doppler frequency from the Doppler spectrum estimated by the Doppler spectrum estimator  175 .  
         [0054]     A Doppler spectrum corresponding to fluctuation in a channel is shown in  FIGS. 7A  to  7 C. When there is no fluctuation or fluctuation is extremely gentle, as shown in  FIG. 7A , a spectrum is a linear spectrum centered in 0 [Hz]. When fluctuation is periodic, since the fluctuation can be approximated by adding up several sine waves, the Doppler spectrum can be represented by several linear spectra. A state of the Doppler spectrum represented by two linear spectra is shown in  FIG. 7B . When fluctuation is random, a spectrum has a spread and, as shown in  FIG. 7C , shows a well-known well-type spectrum.  
         [0055]     The pilot-use channel estimator  17 A in the OFDM receiver  10  calculates the Doppler spectrum shown in  FIGS. 7A  to  7 C from the OFDM frequency domain signal and selects an optimum method of estimating a time direction channel from a shape of the spectrum and a maximum Doppler frequency to perform the estimation of a time direction channel corresponding to the fluctuation in the channel.  
         [0056]     The selector  174  switches outputs of the average-type time-direction-channel estimator  172  and the interpolation-type time-direction-channel estimator  173  according to the maximum Doppler frequency outputted from the maximum-Doppler-frequency judging circuit  176 . When the maximum Doppler frequency is extremely small, the selector  174  selects the average-type time-direction-channel estimator  172  that executes average-type estimation of a time direction channel. When there is fluctuation, the selector  174  selects the interpolation-type time-direction-channel estimator  173  that executes interpolation-type estimation of a time direction channel. Consequently, in both a case in which temporal fluctuation in the channel is slow and a case in which temporal fluctuation in the channel is fast, it is possible to perform high-performance channel estimation and, as shown in  FIG. 8 , estimate channel characteristics for every three sub-carriers in the frequency direction for all OFDM symbols.  
         [0057]     The channel distortion compensator  18  includes a compensator  181  and a frequency-direction-channel estimator  182 .  
         [0058]     In the channel distortion compensator  18 , the frequency-direction-channel estimator  182  subjects the channel characteristics calculated for every three sub-carriers by the pilot-use channel estimator  17 A to processing in the frequency direction to calculate channel characteristics of all the sub-carriers in the OFDM symbol as shown in  FIG. 9 . As a result, it is possible to estimate channel characteristics for all the sub-carriers of the OFDM signal. The compensator  181  removes distortion due to the channel from the OFDM frequency domain signal calculated by the FFT arithmetic circuit  16  using the channel characteristics of all the sub-carriers supplied from the frequency-direction-channel estimator  182 .  
         [0059]     The transmission parameter decoder  20  extracts transmission parameter information from the OFDM frequency domain signal by decoding a sub-carrier in which the transmission parameter information is inserted and supplies the transmission parameter information to the error correction circuit  19 .  
         [0060]     The error correction circuit  19  applies, in accordance with the transmission parameter information supplied from the transmission parameter decoder  20 , de-interleave processing to the OFDM frequency domain signal, from which the channel distortion is removed by the channel-distortion compensator  18 . The error correction circuit  19  outputs the OFDM frequency domain signal as decoded data through depuncture, Viterbi, diffused signal removal, and RS decoding.  
         [0061]     The delay profile estimator  21  calculates an impulse response of the channel and supplies the impulse response to the window regenerator  22 . As a method of delay profile estimation, for example, a method of using a matched filter that sets a guard interval period as a tap coefficient using the OFDM time domain signal and a method of calculating a delay profile by subjecting a channel characteristic supplied from the pilot-use channel estimator  17  to IFFT are adopted.  
         [0062]     As the pilot-use channel estimator  17 , instead of the pilot-use channel estimator  17 A in which the average-type time-direction-channel estimator  172  and the interpolation-type time-direction-channel estimator  173  are switched by the selector  174 , a pilot-use channel estimator  17 B having a structure shown in  FIG. 10  or a pilot-use channel estimator  17 C having a structure shown in  FIG. 12  may be adopted.  
         [0063]     The pilot-use channel estimator  17 B shown in  FIG. 10  includes the SP-signal extraction circuit  171 , the interpolation-type time-direction-channel estimator  173 , a prediction-type time-direction-channel estimator  177 , the selector  174 , the Doppler spectrum estimator  175 , and a fluctuation-type judging device  178 .  
         [0064]     In the pilot-use channel estimator  17 B, an OFDM frequency domain signal is supplied to the SP-signal extraction circuit  171  and the Doppler spectrum estimator  175 . The SP-signal extraction circuit  171  extracts only the SP signals inserted in the positions shown in  FIG. 3  and removes modulation components of the pilot signals to calculate channel characteristics in the SP positions. The channel characteristics in the SP positions calculated by the SP-signal extraction circuit  171  are supplied to the interpolation-type time-direction-channel estimator  173  and the prediction-type time-direction-channel estimator  177 .  
         [0065]     The interpolation-type time-direction-channel estimator  173  includes a variable-coefficient FIR filter having the structure shown in  FIG. 6A . The interpolation-type time-direction-channel estimator  173  interpolates a channel estimated value in an SP position, which is estimated by the SP-signal extraction circuit  171 , in the time direction to estimate a channel during three symbols as shown in  FIG. 6B .  
         [0066]     The prediction-type time-direction-channel estimator  177  includes a primary IIR filter having a structure, for example, shown in  FIG. 11A . As shown in  FIG. 11B , the prediction-type time-direction-channel estimator  177  predicts a channel in the next SP position with the channel estimated value in the SP position estimated by the SP-signal extraction circuit  171  as an input. Until the next SP signal is inputted, the prediction-type time-direction-channel estimator  177  interpolates a predicted value to generate an estimated value. As a method of updating a coefficient of the filter, there is a method of using a least mean square (LMS) algorithm or the like.  
         [0067]     The Doppler spectrum estimator  175  estimates a Doppler spectrum from the OFDM frequency domain signal. The fluctuation-type judging device  178  judges a shape of the Doppler spectrum estimated by the Doppler spectrum estimator  175 .  
         [0068]     The selector  174  switches outputs of the interpolation-type time-direction-channel estimator  173  and the prediction-type time-direction-channel estimator  177  according to an output of the judgment by the fluctuation-type judging device  178 . When fluctuation in a channel is a linear spectrum, the selector  174  selects the prediction-type time-direction-channel estimator  177  that executes prediction-type estimation of a time direction channel. When fluctuation is random, i.e., when a spectrum has a spread, the selector  174  selects the interpolation-type time-direction-channel estimator  173  that executes interpolation-type estimation of a time direction channel. Consequently, in both a case in which temporal fluctuation in the channel is periodic (including a case in which there is no fluctuation) and a case in which the channel fluctuates at random, it is possible to perform high-performance channel estimation and, as shown in  FIG. 8 , estimate channel characteristics for every three sub-carriers in the frequency direction for all OFDM symbols.  
         [0069]     The pilot-use channel estimator  17 C shown in  FIG. 12  includes the SP-signal extraction circuit  171 , the average-type time-direction-channel estimator  172 , the interpolation-type time-direction-channel estimator  173 , the prediction-type time-direction-channel estimator  177 , the selector  174 , the Doppler spectrum estimator  175 , the maximum-Doppler-frequency judging circuit  176 , and the fluctuation-type judging device  178 .  
         [0070]     The pilot-use channel estimator  17 C is obtained by combining the pilot-use channel estimator  17 A shown in  FIG. 4  and the pilot-use channel estimator  17 B shown in  FIG. 10 . In the pilot-use channel estimator  17 C, the Doppler spectrum estimator  175  estimates a Doppler spectrum from the OFDM frequency domain signal. The maximum-Doppler-frequency judging circuit  176  calculates a maximum Doppler frequency. When this maximum Doppler frequency is small, the average-type method of estimating a time direction channel is selected. When fluctuation is large, the fluctuation-type judging device  178  judges whether the fluctuation is periodic fluctuation or random fluctuation. When the fluctuation is periodic fluctuation, the prediction-type method of estimating a time direction channel is selected. When the fluctuation is random fluctuation, the interpolation-type method of estimating a time direction channel is selected. This makes it possible to select an appropriate estimation method according to presence or absence of fluctuation in the channel and a type of the fluctuation and perform high-performance channel estimation.  
         [0071]     The fluctuation-type judging device  178  includes, for example, as shown in  FIG. 13 , a center clip circuit  1781 , a positive-maximum-Doppler search device  1782 , a negative-maximum-Doppler search device  1783 , an fd-section-0-count circuit  1784 , and a judging device  1785 .  
         [0072]     In the fluctuation-type judging device  178 , first, in order to remove noise components, the center clip circuit  1781  applies center clip processing to a spectrum. The center clip circuit  1781  subtracts a threshold from the spectrum and forcibly replaces a negative portion with 0 to perform the center clip processing. The spectrum subjected to the center clip processing is supplied to the positive-maximum-Doppler search device  1782 , the negative-maximum-Doppler search device  1783 , and the fd-section-0-count circuit  1784 . The positive-maximum-Doppler-search device  1782  searches for a maximum positive index of a non-zero value. The negative-maximum-Doppler search device  1783  searches for a negative maximum index of a non-zero value. The fd-section-0-count circuit  1784  counts an index of 0 between the positive maximum Doppler index and the negative maximum Doppler index.  
         [0073]     The judging device  1785  judges a shape of the spectrum in accordance with a procedure shown in a flowchart in  FIG. 14 .  
         [0074]     First, the judging device  1785  subtracts the negative maximum index from the positive maximum index to calculate a Doppler spread (hereinafter referred to as “Fds”) (step S 1 ).  
         [0075]     The judging device  1785  judges whether the Doppler spread (Fds) calculated in step S 1  is smaller than the threshold (step S 2 ).  
         [0076]     When a result of the judgment in step S 2  is TRUE, i.e., the Fds is smaller than the threshold, the judging device  1785  judges that a channel is a channel without fluctuation (step S 4 ) and finishes the processing for judging a shape of the spectrum.  
         [0077]     A state of the judgment of a shape of a Doppler spectrum at the time when there is no fluctuation is shown in  FIGS. 15A  to  15 C.  
         [0078]     The center clip circuit  1781  applies, as shown in  FIG. 15A , the center clip processing to the Doppler spectrum calculated by the Doppler spectrum estimator  175  to obtain a Doppler spectrum from which noise is removed as shown in  FIG. 15B . As shown in  FIG. 15C , when a Doppler spread (Fds) of the Doppler spectrum is smaller than the threshold, the judging device  1785  judges that the channel is a channel without fluctuation.  
         [0079]     When a result of the judgment in step S 2  is FALSE, i.e., the Fds is equal to or larger than the threshold, the judging device  1785  judges whether fluctuation is periodic fluctuation or random fluctuation (step S 3 ).  
         [0080]     The judgment processing in step S 3  can be performed on the basis of a ratio of a section of 0 in the Doppler spread. When the number of 0s (hereinafter referred to as nzero) supplied from the fd-section-0-count circuit  1784  is larger than Fds* scaling (e.g., 0.9) (step S 3 : TRUE), the judging device  1785  regards the fluctuation as periodic fluctuation (step S 5 ). When the number of 0s is not larger than Fds* scaling (step S 3 : FALSE), the judging device  1785  regards the fluctuation as random fluctuation (step S 6 ) and finishes the processing for judging a shape of the spectrum.  
         [0081]     A state of the judgment of a shape of a Doppler spectrum at the time when fluctuation is periodic is shown in  FIGS. 16A  to  16 C.  
         [0082]     The center clip circuit  1781  applies, as shown in  FIG. 16A , the center clip processing to the Doppler spectrum calculated by the Doppler spectrum estimator  175  to obtain a Doppler spectrum from which noise is removed as shown in  FIG. 16B . As shown in  FIG. 16C , when the number of indexes of 0 between the positive maximum Doppler index and the negative maximum Doppler index is larger than Fds* scaling, the judging device  1785  judges that the channel is a channel that fluctuates periodically.  
         [0083]     A state of the judgment of a shape of a Doppler spectrum at the time when fluctuation is random is shown in  FIGS. 17A  to  17 C.  
         [0084]     The center clip circuit  1781  applies, as shown in  FIG. 17A , the center clip processing to the Doppler spectrum calculated by the Doppler spectrum estimator  175  to obtain a Doppler spectrum from which noise is removed as shown in  FIG. 17B . As shown in  FIG. 17C , when the number of indexes of 0 between the positive maximum Doppler index and the negative maximum Doppler index is equal to or smaller than Fds* scaling, the judging device  1785  judges that the channel is a channel that fluctuates at random.  
         [0085]     In the OFDM receiver  10  according to this embodiment, according to an output of the fluctuation-type judging device  178 , the selector  174  selects the average-type time-direction-channel estimator  172  when a channel is static, selects the prediction-type time-direction-channel estimator  177  in the case of periodic temporal fluctuation, and selects the interpolation-type time-direction-channel estimator  173  in the case of random temporal fluctuation.  
         [0086]     As described above, the selector  174  selectively switch, according to an output of the fluctuation-type judging device  178 , any one of the average-type time-direction-channel estimator  172 , the prediction-type time-direction-channel estimator  177 , and the interpolation-type time-direction-channel estimator  173 . Thus, it is possible to select an appropriate estimation method according to a state of a channel without increasing sizes of the circuits and attain excellent reception performance in all channels.  
         [0087]     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations, and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.