Patent Publication Number: US-2005129155-A1

Title: Receiver, receiving method, reception controlling program, and recording medium

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to a receiver, a receiving method, a reception controlling program, and a recording medium.  
      The present application claims priority from Japanese Patent Application No. 2003-414834, the disclosure of which is incorporated herein by reference.  
      In recent years, terrestrial digital broadcasting has adopted orthogonal frequency division multiplexing (OFDM) which is resistant to multipath and fading. To receive this terrestrial digital broadcasting on movable bodies such as a vehicle with stability, some receivers adopt diversity reception techniques for switching a plurality of antennas as appropriate (for example, see Japanese Patent Application Laid-Open No. 2003-143100).  
      In the conventional technology described above, however, the diversity operation for switching antennas is performed based on the reception power of the entire desired wave included in the received radio waves. One of the possible problems is that when distortion ascribable to multipath, fading, and the like occurs in the transmission channel and the radio waves are received with degradation at certain frequencies, the results of demodulation can involve errors even if the desired wave has sufficient reception power as a whole.  
     SUMMARY OF THE INVENTION  
      A receiver according to a first aspect of the present invention comprises: a plurality of antennas for receiving a transmission signal; a reception signal outputting unit for outputting a reception signal by using one or more antennas out of the plurality of antennas; a transmission channel distortion estimating unit for estimating distortion of a transmission channel based on the reception signal output from the reception signal outputting unit; a decision unit for deciding whether or not to change a receiving condition of the reception signal outputting unit based on the distortion of the transmission channel estimated by the transmission channel distortion estimating unit; and a control unit for controlling the receiving condition of the reception signal outputting unit if the decision unit decides to change the receiving condition of the reception signal outputting unit based on the distortion of the transmission channel.  
      A receiving method according to a second aspect of the present invention comprises: a reception signal outputting step of outputting a reception signal by using one or more antennas out of a plurality of antennas for receiving a transmission signal; a transmission channel distortion estimating step of estimating distortion of a transmission channel based on the reception signal output in the reception signal outputting step; a decision step of deciding whether or not to change a receiving condition of the reception signal outputting step based on the distortion of the transmission channel estimated in the transmission channel distortion estimating step; and a control step of controlling the receiving condition of the reception signal outputting step if it is decided in the decision step to change the receiving condition of the reception signal outputting step based on the distortion of the transmission channel.  
      A reception controlling program according to a third aspect of the present invention is one for controlling a receiver by using a computer, the receiver outputting a reception signal by using one or more antennas out of a plurality of antennas for receiving a transmission signal. The reception controlling program makes the computer estimate distortion of a transmission channel based on the reception signal and control the reception signal based on the distortion of the transmission channel estimated.  
      A recording medium according to a fourth aspect of the invention contains the reception controlling program according to the third aspect. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      These and other objects and advantages of the present invention will become clear from the following description with reference to the accompanying drawings, wherein:  
       FIG. 1  is a diagram showing an example of arrangement of pilot signals in an OFDM transmission scheme;  
       FIG. 2  is a diagram showing the frequency characteristic of the pilot signals transmitted from a transmitting side;  
       FIG. 3  is a block diagram showing the general configuration of a receiver according to a first embodiment;  
       FIG. 4  is a diagram showing the frequency characteristic of the pilot signals received at a receiving side;  
       FIG. 5  is a flowchart for explaining the operation of changing the receiving condition according to an example 1;  
       FIG. 6  is a diagram showing the characteristic of moving averages, taken across frequencies, of the pilot signals received at the receiving side;  
       FIG. 7  is a flowchart for explaining the operation of changing the receiving condition according to an example 2;  
       FIG. 8  is a block diagram showing the general configuration of a receiver according to a second embodiment;  
       FIG. 9  is a diagram showing an example of the SP impulse response characteristic of transmission signals received; and  
       FIG. 10  is a flowchart for explaining the operation of changing the receiving condition according to an example 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      Hereinafter, preferred embodiments of the receiver, the receiving method, the reception controlling program, and the recording medium according to the present invention will be described in detail with reference to the accompanying drawings.  
      One of the objects of these embodiments is to make it possible to select an optimum receiving condition all the time.  
      The receiving condition depends on antennas in the case of an antenna switching method, and depends on phase differences or levels in the case of a phase difference feeding method. As employed in the following description of the embodiments, “changing the receiving condition” shall refer to changing the state (such as a level and a delay) of the reception signal by such means as switching the antennas and adjusting the phases or levels.  
      Initially, description will be given in detail of the digital broadcasting which is used for the receiver and the receiving method of this invention.  FIG. 1  is a diagram showing an example of arrangement of pilot signals in an OFDM transmission scheme.  
      In  FIG. 1 , the y-axis indicates the symbol (equivalent to time t), and the x-axis the sub carrier (equivalent to frequency f). In the OFDM transmission scheme for use in terrestrial digital broadcasting and the like, the transmitting side inserts pilot signals (SP: Scattered Pilot) having a known amplitude and phase into a series of data signals of the transmission signal regularly in advance, at predetermined positions on the frequency axis and the time axis. In  FIG. 1 , the black circles “•” represent the pilot signals SP, and the white circles “∘” the data signals.  
       FIG. 2  is a diagram showing the frequency characteristic of the pilot signals for the transmitting side to transmit. In  FIG. 2 , the y-axis indicates the power, and the x-axis the frequency. When the transmission channel has an ideal characteristic, i.e., the transmission channel is distortion-free, the frequency-specific powers of the pilot signals SP at the receiving side are also uniform, as in  FIG. 2 .  
     First Embodiment  
      Now, a first embodiment of this invention will be described.  FIG. 3  is a block diagram showing the general configuration of a receiver according to the first embodiment.  
      The transmission signal under the OFDM transmission scheme is received by a plurality of antennas  301   a  and  301   b . For diversity reception, the number of antennas is at least two. Reception signal outputting means  302  selects the transmission signal received by the antenna  301   a  or  301   b  which is selected based on select signals S 1  and S 2  of decision mean  309 , and outputs the same to an RF tuner  303 . This reception signal outputting means  302  comprises AGC amplifiers  311   a  and  311   b , and an adding unit  312 . The gain of the AGC amplifier  311   a  or  311   b  which is provided for the antenna  301   a  or  301   b  unselected by the select signals S 1  and S 2  is lowered to interrupt the input, or to decrease the proportion in the combining ratio of the transmission signals in the adding unit  312 . The reception signal output means  302  is not limited to the configuration of combining the transmission signals thus by using the phase difference feeding method, but may be of antenna switching method in which the AGC amplifiers  311   a  and  311   b  are switched to select either one of the antennas  301   a  and  301   b . For convenience&#39;s sake, the following description will deal with the configuration of the antenna switching method in which the antennas  301   a  and  301   b  are switched selectively based on the select signals S 1  and S 2 .  
      The RF tuner  303  includes filters  321  and  322 , variable gain amplifiers  323  and  324 , an oscillator  325 , and a mixer  326 . This RF tuner  303  tunes to the OFDM signal of the desired wave out of the transmission signals selected by the reception signal output means  302 , converts it into an OFDM signal of intermediate frequency by using the mixer  326 , and outputs the resultant to an ADC  304 .  
      The ADC  304  applies analog-to-digital conversion to the transmission signal of intermediate frequency output from the RF tuner  303 , and outputs the resultant to OFDM demodulation means  305 . The OFDM demodulation means  305  comprises an orthogonal demodulation unit  331  and an FFT circuit  332 . The orthogonal demodulation unit  331  converts the digitalized transmission signal into a baseband signal (complex baseband OFDM signal). The FFT circuit  322  receives the baseband signal, extracts signals included in a predetermined FFT window period, and performs FFT (Fast Fourier Transform) to convert the signals into ones on the frequency axis. Consequently, demodulated-wave symbol signals can be obtained from a plurality of respective orthogonal frequency signals constituting the OFDM signal. Demodulation/decoding means  306  demodulates these modulated-wave symbol signals into symbol data, and then decodes the data for reproduction and outputs the resultant through an output terminal  307 .  
      Transmission channel distortion estimating means  308  has an extracting unit  308   a  for extracting pilot signals. The extracting unit  308   a  extracts pilot signals SP from the signal (carrier) demodulated by the OFDM demodulation means  305 . The transmission channel distortion estimating means  308  estimates the distortion of the transmission channel from the extracted pilot signals. The pilot signals exhibit the frequency characteristic as shown in  FIG. 4 .  
       FIG. 4  is a diagram showing the frequency characteristic of the pilot signals received at the receiving side. The y- and x-axes of  FIG. 4  are the same as those of  FIG. 2 , and description thereof will be omitted. When multipath, fading, or other distortion occurs in the transmission channel, the frequency-specific powers of the pilot signals SP become uneven as shown in  FIG. 4 , causing such phenomena that pilot signals SP at some frequencies drop in power.  
      The decision means  309  compares the pilot signals SP extracted by the transmission channel distortion estimating means  308  and a reference value for comparison stored in a memory  310 , and outputs the result of comparison to control means  313 . The control means  313  exercises control for changing the receiving condition based on the result of comparison. Specifically, the control means  313  operates the reception signal outputting means  302  to switch to either one of the antennas  301   a  and  301   b  selectively. This makes it possible to conduct reception by using the antenna  301   a  or  301   b  which is in an optimum state of reception.  
      The memory  310  contains various information concerning the receiver, such as the reference value for comparison and the information on the extracted pilot signals (amplitudes and phases) Next, the changing of the receiving condition in the foregoing configuration will be described in conjunction with examples.  
     EXAMPLE 1  
      The memory  310  according to an example 1 contains a reference value for an SP carrier power difference of the pilot signals SP to be compared with. This SP carrier power difference is the differential power H−L between the pilot signals H of higher power and the pilot signals L of lower power among the pilot signals SP of the respective frequencies shown in  FIG. 4 . In the shown example, the group of pilot signals H of higher power include SP 1 -SP 3  and SP 7 -SPn. The group of pilot signals L of lower power include SP 4 -SP 6 .  
      The decision means  309  compares the differences between the frequency-specific powers of the pilot signals extracted by the transmission channel distortion estimating means  308  and the reference value stored in the memory  310 . If the differences between the frequency-specific powers of the pilot signals exceed the reference value, or threshold, the resulting decision to switch the antennas  301   a  and  301   b  selectively is output to the control means  313 . The control means  313  then operates to change the receiving condition. Specifically, given that the power difference, or reference value, set in the memory  310  is 20 dB, the decision means  309  outputs the resulting decision to start the operation of changing the receiving condition to the control means  313  when the differential power H−L is greater than or equal to the threshold value, or the power difference of 20 dB.  
       FIG. 5  is a flowchart for explaining the operation of changing the receiving condition according to the example 1. When a transmission signal is received, the individual components shown in  FIG. 3  perform signal processing on this transmission signal. A signal is thus demodulated by the FFT circuit  332  of the OFDM demodulation means  305 . The extracting unit  308   a  extracts pilot signals SP from the signal demodulated by the OFDM demodulation means  305  (step S 501 ).  
      Here, the transmission channel distortion estimating means  308  calculates the SP carrier power difference H−L of the pilot signals described with reference to  FIG. 4 , based on the pilot signals extracted by the extracting unit  308   a  (step S 502 ). The SP carrier power difference H−L is the differential power between the pilot signals H of higher power and the pilot signals L of lower power among the pilot signals SP of respective frequencies. For a specific method of calculation, the power of a pilot signal H having the highest power and that of a pilot signal L having the lowest power may be compared across the frequencies. Alternatively, the power average of a plurality of pilot signals H in a predetermined range of highest powers and that of a plurality of pilot signals L in a predetermined range of lowest powers may be compared across the frequencies.  
      In another method of calculation, moving averages are obtained for a plurality of pilot signals SP of adjoining frequencies, and the SP carrier power difference H−L is calculated from the moving averages.  
       FIG. 6  is a diagram showing the characteristic of moving averages, taken across frequencies, of the pilot signals received at the receiving side. The y- and x-axes of  FIG. 6  are the same as those of  FIG. 2 , and description thereof will be omitted. When multipath, fading, or other distortion occurs in the transmission channel, the frequency-specific powers of the pilot signals SP become uneven, causing such phenomena that pilot signals SP at some frequencies drop in power as shown in  FIG. 4  seen above. Taking the moving averages of the powers taken across frequencies in the state of  FIG. 4  results in the state of  FIG. 6 .  
      Description will now be given in the concrete of the processing of calculating the moving averages of the powers across frequencies. Suppose, for example, that the number of adjoining signals is four (i=4). The sum of the powers of the four pilot signals SP 1  to SP 4  shown in  FIG. 4  is divided by the number of signals, or  4 , and the result is regarded as the power of the pilot signal SP 1  shown in  FIG. 6 . Similarly, the sum of the powers of the four pilot signals SP 2  to SP 5  shown in  FIG. 4  is divided by the number of signals, or  4 , and the result is regarded as the power of the pilot signal SP  2  shown in  FIG. 6 . In this way, the values for the respective pilot signals SP 1  to SPn are calculated.  FIG. 6  shows the powers of the respective pilot signals SP after the calculation. In this state, the SP carrier power difference H−L is determined.  
      Returning to  FIG. 5 , the decision means  309  compares the SP carrier power difference H−L determined as described above and the reference value (step S 501 ). If the result of comparison shows that the SP carrier power difference H−L is greater than the reference value (step S 503 : Yes), it is determined that the operation of changing the receiving condition is required since the SP carrier power difference H−L shows a large difference in power. The changing of the receiving condition is thus started (step S 504 ). Consequently, the control means  313  outputs the select signals S 1  and S 2  for switching the antenna selected so far to the other antenna (see  FIG. 3 ). The reception signal outputting means  302 , if it has selected the antenna  301  so far, selects the other antenna  301   b.    
      Now, if the SP carrier power difference H−L is smaller than the reference value (step S 503 : No), it is determined that the operation of changing the receiving condition is unnecessary since the SP carrier power difference H−L shows a small difference in power. The receiving condition is thus kept as is (unchanged), and the operation is ended (step S 505 ).  
      The operation of changing the receiving condition shown in the foregoing steps S 501  to  505  is performed at timing in minimum units of a single symbol, or a plurality of symbols. This timing of the operation of changing the receiving condition may be set depending on the receiving situation. For example, based on the modulation methods of the receiver and other information, the number of symbols for use in calculating the SP carrier power difference at step S 502  may be changed dynamically (with a lapse of time) so that the operation of changing the receiving condition is performed in units of the number of symbols after this dynamic change. Incidentally, the description that the receiving condition is kept as is at step S 505  means that the antenna switching processing is not performed and the antenna selected so far is kept selected.  
      According to the example 1 described above, the operation of changing the receiving condition can be effected by brief processing of simple comparison alone, using the SP carrier power difference of the transmission signals for comparison. This allows the stabilization of the state of reception.  
      In a modification of this example 1, the transmission channel distortion estimating means  308  estimates the distortion of the transmission channel based on phase differences between a plurality of pilot signals. More specifically, the transmission channel distortion estimating means  308  calculates phase differences from a plurality of pilot signals extracted by the extracting unit  308   a , and the decision unit  309  compares the phase differences and a reference value previously stored in the memory  310  to determine whether or not to perform the operation of changing the receiving condition. For example, if adjoining pilot signals have a phase difference greater than or equal to the reference value, the operation of changing the receiving condition is performed. As with the power differences, the phase differences can thus be used as the condition for performing the operation of changing the receiving condition.  
     EXAMPLE 2  
      An example 2 provides a configuration in which the processing of the example 1 described above is performed in units of a plurality of symbols.  FIG. 7  is a flowchart for explaining the operation of changing the receiving condition according to the example 2. As employed in the processing shown in  FIG. 7 , the parameter n represents the number of symbols of the pilot signals SP to be used for deciding on the operation of changing the receiving condition. The parameter m represents the number of symbols of the pilot signals SP stored in the memory  310 . This m has an initial value of zero.  
      When a transmission signal is received, the individual components shown in  FIG. 3  perform signal processing on this transmission signal. A signal is thus demodulated by the FFT circuit  332  of the OFDM demodulation means  305 . The extracting unit  308   a  extracts a pilot signal SP from the signal demodulated by the OFDM demodulation means  305  (step S 701 ). Next, the extracted pilot signal SP is stored into the memory  310  (step S 702 ). By this storage, the value of m is incremented by one (step S 703 ).  
      Then, it is determined if the number n of symbols of the pilot signals SP to be used for deciding on the operation of changing the receiving condition coincides with the number m of symbols stored in the memory  310  (n=m) (step S 704 ). If the number m of symbols stored in the memory  310  is yet to reach the number n of symbols of the pilot signals to be used for deciding on the operation of changing the receiving condition (step S 704 : No), the processing returns to step S 701  for repetition.  
      If the number m of symbols stored in the memory  310  coincides with the number n of symbols of the pilot signals SP to be used for deciding on the operation of changing the receiving condition (n=m) (step S 704 : Yes), the n symbols of pilot signals SP stored in the memory  310  are read successively in descending order of time of storage (step S 705 ). Here, the transmission channel distortion estimating means  308  derives the SP carrier power difference H−L of the pilot signals SP, described with reference to  FIG. 4  (step S 706 ). As mentioned previously, the SP carrier power difference H−L can be calculated by using various specific techniques.  
      Next, the decision means  309  compares the SP carrier power difference H−L determined as described above and the reference value (step S 707 ). If the result of comparison shows that the SP carrier power difference H−L is greater than the reference value (step S 707 : Yes), it is determined that the operation of changing the receiving condition is required since the SP carrier power difference H−L itself shows a large difference in power. The operation of changing the receiving condition is thus started (step S 708 ), and the single round of processing is ended. Consequently, the decision means  309  outputs the select signals S 1  and S 2  for switching the antenna selected so far to the other antenna (see  FIG. 3 ). Specifically, if the antenna  301  has been selected so far, the other antenna  301   b  is selected.  
      Now, if the SP carrier power difference H−L is smaller than the reference value (step S 707 : No), it is determined that the operation of changing the receiving condition is unnecessary since the SP carrier power difference H−L shows a small difference in power. The receiving condition is thus kept as is (unchanged) (step S 709 ), and the single round of processing is ended. The operation of changing the receiving condition can be started and ended at timing in units of a single symbol or a plurality of symbols after the transmission signals as many as the number n of symbols in use are received.  
      Note that the processing described above is not restrictive. At step S 706 , SP carrier power differences H−L for a plurality of symbols may be calculated and stored into the memory. In this configuration, the SP carrier power differences H−L for the plurality of symbols stored in the memory are averaged and used for comparison.  
      According to the configuration of the example 2 described above, whether or not to change the receiving condition is controlled by using the plurality of symbols of pilot signals SP. It is therefore possible to perform the antenna switching with stability even when the SP carrier power difference varies symbol by symbol.  
      In possible configurations of the first embodiment, the power of the transmission signal may be detected based on any of the RF signal from the RF tuner  303 , the IF signal past the mixer  326 , and the baseband signal past the filter  322 . In another possible configuration, the detection may be based on the digital signal past the ADC  304 .  
      According to the first embodiment described above, even when the receiving situation varies to make the frequency-specific powers of the pilot signals SP uneven so that the powers of the pilot signals SP drop at some frequencies, the frequency-specific SP carrier power differences can be used to determine whether or not to perform the operation of changing the receiving condition. This can provide the effect of stabilizing the state of reception.  
     Second Embodiment  
      Next, description will be given of a second embodiment of this invention. While the first embodiment is configured to use the SP carrier power differences for comparison, the second embodiment is configured to use SP impulse responses of the pilot signals SP for comparison.  
       FIG. 8  is a block diagram showing the general configuration of a receiver according to the second embodiment. In  FIG. 8 , the same components as those of  FIG. 3  described in the first embodiment will be designated by identical numerals, and description thereof will be omitted. The configuration of  FIG. 8  differs from that of  FIG. 3  in that the transmission channel distortion estimating means  308  is provided with an SP impulse response difference calculating unit  801 .  
      The SP impulse response difference calculating unit  801  is formed by subjecting the pilot signal SP obtained by the FFT circuit  332  of the OFDM demodulation means  305  to additional FFT (Fast Fourier Transform).  FIG. 9  is a diagram showing an example of the SP impulse response characteristic of a received transmission signal. In  FIG. 9 , the y-axis represents the power, and the x-axis the time. The characteristics of the pilot signals at the frequencies shown in  FIG. 4  seen above are represented by respective crosses. Given an ideal distortion-free transmission channel, the powers of the SP impulse responses would concentrate at time 0, with no delay or lead in time.  
      Nevertheless, when distortion occurs in the transmission channel and causes the state that the powers of the pilot signals SP drop at some frequencies as shown in  FIG. 4 , the pilot signals SP exhibit the characteristic that they are also distributed over the leading side and trailing side of the time axis, aside from the pilot signal SPR falling on the area of time 0. In particular, the pilot signal SPM shown in  FIG. 9  appears due to multipath distortion of T in delay time. If the multipath wave SPM has high power with respect to the desired wave of the pilot signal SP, or SPR, it interferes with the desired wave to contribute a degraded state of reception. For this reason, the operation of changing the receiving condition shall be performed when the difference H−L between the power H of the desired wave of the pilot signal SP, or SPR, and the power L of the multipath or other interference wave SPM falls to or below a certain value.  
      The memory  310  shown in  FIG. 8  contains a reference value (threshold) for an SP impulse response difference of the pilot signals SP to be compared with. The decision means  309  shown in  FIG. 8  compares the SP impulse response difference of the pilot signals calculated by the SP impulse response difference calculating unit  801  and the reference value stored in the memory  310 . Then, the operation of changing the receiving condition through selective switching of the antennas  301   a  and  301   b  is performed when the SP impulse response difference between a plurality of pilot signals is smaller than the reference value.  
      Next, description will be given of an example of the operation of changing the receiving condition according to the foregoing configuration. The following example will deal chiefly with the reference value stored in the memory  310  and examples of operation of the decision means  309  which makes comparing operations using the reference value.  
     EXAMPLE 3  
       FIG. 10  is a flowchart for explaining the operation of changing the receiving condition according to the example 3. When a transmission signal is received, the individual components shown in  FIG. 8  perform signal processing on this transmission signal. A signal is thus demodulated by the FFT circuit  332  of the OFDM demodulation means  305 . The extracting unit  308   a  extracts pilot signals SP from the signal demodulated by the OFDM demodulation means  305  (step S 1001 ).  
      The SP impulse response difference calculating unit  801  calculates the SP impulse responses of a plurality of pilot signals extracted by the extracting unit  308   a . Then, as shown in  FIG. 9 , the SP impulse response difference calculating unit  801  calculates the difference H−L between the power H of the desired wave SPR at time 0 and the power L of the multipath or other interference wave SPM having the second highest power (step S 1002 ).  
      Next, the decision means  309  compares the SP impulse response difference H−L determined as described above and the reference value (step S 1003 ). If the result of comparison shows that the calculated SP impulse response difference H−L is smaller than the SP impulse response difference shown by the reference value (step S 1003 : Yes), it is determined that the SP impulse response of the interference wave SPM is high in power, requiring the operation of changing the receiving condition. The operation of changing the receiving condition is thus started (step S 1004 ). Consequently, the decision means  309  outputs the select signals S 1  and S 2  for switching the antenna selected so far to the other antenna (see  FIG. 8 ). Specifically, if the antenna  301  has been selected so far, the other antenna  301   b  is selected. On the other hand, if the calculated SP impulse response difference H−L is greater than the SP impulse response difference shown by the reference value (step S 1003 : No), it is determined that the SP impulse response of the interference wave SPM is low in power, not requiring the operation of changing the receiving condition. The processing is thus ended with the receiving condition unchanged (step S 1005 ). Each of the processes of the foregoing steps S 1001  to S 1005  is performed in minimum units of a single symbol, or a plurality of symbols.  
      According to the example 3 described above, the operation of changing the receiving condition can be effected by brief processing of simply comparing the SP impulse response difference of the transmission signals by using the reference value. This allows the stabilization of the state of reception.  
      According to the second embodiment described above, even when the receiving situation varies to make the frequency-specific powers of the pilot signals SP uneven so that the powers of the pilot signals SP drop at some frequencies, the frequency-specific SP impulse response differences can be used to determine whether or not to perform the operation of changing the receiving condition. This can provide the effect of stabilizing the state of reception.  
      Note that the foregoing embodiments have dealt with the method of estimating distortion of the transmission channel based on a power difference or phase difference of pilot signals SP or an impulse response difference of pilot signals. The method of estimating distortion of the transmission channel is not limited thereto, however. For example, in QPSK and other modulation methods with on amplitude variation, FFT may be performed in units of a single symbol. Here, the powers of the respective carriers are determined to derive the frequency characteristic, and the distortion of the transmission channel is estimated based on the frequency characteristic. In this way, the distortion of the transmission channel can be estimated by deriving the frequency characteristic of the transmission channel from information on transmission signals other than pilot signals SP.  
      The embodiments have dealt with the configuration of receiving transmission signals of OFDM transmission scheme. The transmission signals to be received are not limited thereto, however. As long as the modulation method uses a wide range of frequencies, the operation of changing the receiving condition can be performed with stability in response to degradations in the frequency-specific powers of the received transmission signal or the occurrence of multipath and other interference waves, as in the foregoing embodiments.  
      In the foregoing embodiments, single-tuner diversity receivers have been described as the concrete examples. Nevertheless, the present invention is also applicable to two-tuner diversity receivers which have two tuners and combine the outputs of the plurality of tuners, and receivers which have more tuners.  
      The receivers described in the embodiments can be controlled through the execution of a reception controlling program prepared in advance on a computer such as a personal computer. This program is recorded on a computer-readable recording medium such as a hard disk, flexible disk, CD-ROM, MO, and DVD, and is read by the computer from the recording medium for execution. This program may also be on a transmission medium capable of distribution over a network such as the Internet.  
      As has been described, the receiver, the receiving method, the reception controlling program, and the recording medium according to the embodiments are applicable to the field of application of broadcasting and communication, and can be applied to radios, television sets, and navigation systems implementing the same, as well as wide-band radio and the like. Stable reception quality can be provided with such applications as vehicle-mounted (such as car, train, and ship) or portable receivers in particular.  
      While there has been described what are at present considered to be preferred embodiments of the present invention, it will be understood that various modifications may be made thereto, and it is intended that the appended claims cover all such modifications as fall within the true spirit and scope of the invention.