Transmission method, receiving method, transfer method, and receiving device

The present invention relates to a method of transmitting an orthogonal frequency-multiplexed signal and a receiving device thereof. More specifically, the present invention relates to an orthogonal frequency-division-multiplexed signal for each symbol of a predetermined length from the transmitting side to the receiving side via a predetermined transmission line. And its receiver.

In recent years, communication using Orthogonal Frequency Division Multiplexing (hereinafter referred to as OFDM) signals has attracted attention in digital audio broadcasting for mobile bodies, terrestrial digital television broadcasting, and the like. This is because the OFDM signal has good frequency utilization efficiency, can transmit a large amount of data at high speed, and has little characteristic deterioration due to reflected waves even without a waveform equalizer. Further, since the signal waveform has a shape close to random noise, it is difficult to interfere with other services. A transmission method using an OFDM signal having such characteristics is described in JP-A-5-167633 (hereinafter referred to as the first prior art), Nikkei Electronics (no.574) published on February 15, 1993. "Next Generation Services at Home Beyond Television" (hereinafter referred to as "Second Advanced Technology") and EIAJ Technology Seminar Materials dated September 14, 1994, Nos. 1 to 15 described on pages 101 to 124 of the above. On the page, it is disclosed in "OFDM Method and Its Development Trend" (hereinafter referred to as "Third Advanced Technology") written by Masanori Saito of NHK Science & Technical Research Laboratories.

FIG. 11 is a diagram showing the configuration of a conventional OFDM signal. In particular, FIG. 11 (a) shows each symbol of the OFDM signal along the time axis, and FIG. 11 (b) shows the portion of FIG. 11 (a). α is enlarged and shown. As shown in FIG. 11 (a), the OFDM signal S is configured by arranging the symbols Sm (m = 1,2, ...) along the time axis. Each symbol Sm digitally modulates (for example, QPSK modulation, 16QAM, etc.) the data to be transmitted by each carrier (several tens to thousands, for example 512) having different frequencies (orthogonal to each other in the symbol time ts). ), And each modulated carrier is multiplexed on the frequency axis by an inverse FFT (Fast Fourier Transform) operation. Therefore, each symbol Sm shows a random amplitude distribution as shown in FIG. 11 (b). It should be noted that such an OFDM signal S takes the form of a complex signal in which a real part and an imaginary part are superimposed for each symbol Sm on the transmission path.

By the way, such an OFDM signal is transmitted from the transmitting side to the receiving side via a wired or wireless transmission line. In a wired transmission line, the occupied frequency band is regulated by the transmission characteristics of the transmission line. Further, in a wireless transmission line, the occupied frequency band is regulated by legal regulations. Therefore, the transmitting side converts the OFDM signal from the intermediate frequency band to the occupied frequency band of the transmission line. On the other hand, on the receiving side, when demodulating data, the received OFDM signal is converted from the occupied frequency band of the transmission line to the intermediate frequency band for demodulation work.

In the first prior art described above, a bandpass filter, a frequency converter and a lowpass filter for converting an OFDM signal transmitted from the transmitting side into a baseband OFDM signal, and a baseband OFDM signal are sampled. An A / D converter that converts to a digital signal, an FFT demodulator that Fourier-converts time-axis data to obtain data on the frequency axis for each carrier, and determines the amplitude and phase of each carrier on the complex plane. A signal point coordinate determination circuit that obtains complex data, a received data coupling circuit that converts complex data into digital data, and combines data according to the number of bits transmitted on each carrier to generate a bit stream, and bits. A receiving device including a deinterleaved matrix for obtaining received data by performing deinterleave and error correction on a stream and an error correction code circuit is disclosed.

In the third prior art described above, a bandpass filter, an orthogonal detector and a lowpass filter for converting an OFDM signal transmitted from the transmitting side into a baseband OFDM signal, and a baseband OFDM signal are sampled. A / D converter that converts to a digital signal, FFT demodulator that Fourier-converts time-axis data to obtain data on the frequency axis for each carrier, and received data by converting parallel data on the frequency axis in series. A receiver with a parallel-series conversion circuit to obtain the above is disclosed.

FIG. 12 is a block diagram showing a configuration of an OFDM signal receiving device that can be easily inferred from the first and third prior art techniques. In FIG. 12, the receiving device includes an input terminal I into which the received OFDM signal is input, a frequency converter 100, an orthogonal detector 300, a Fourier converter 400, and a demodulated data detector 500. The orthogonal detector 300 includes a demultiplexer 301, detectors 302, 303, and a carrier regenerator 304.

The OFDM signal of the occupied frequency band (center frequency fr) of the transmission line shown in FIG. 11 received by the receiving device is input to the frequency converter 100 via the input terminal I. The frequency converter 100 converts an OFDM signal in the occupied frequency band of the transmission line into an OFDM signal in the intermediate frequency band (center frequency fc) by shifting by a predetermined fixed frequency.

The demultiplexer 301 of the quadrature detector 300 demultiplexes the OFDM signal output from the frequency converter 100 into two signals, and outputs the demultiplexed OFDM signals to the detectors 302 and 303, respectively. The carrier regenerator 304 outputs an in-phase carrier having a center frequency of fc to the detector 302, and outputs an orthogonal carrier having a center frequency of fc to the detector 303. The detector 302 outputs a real part of the OFDM signal by multiplying the OFDM signal output from the demultiplexer 301 by a common mode carrier. The detector 303 outputs an imaginary part of the OFDM signal by multiplying the OFDM signal output from the demultiplexer 301 by an orthogonal carrier. That is, the quadrature detector 300 converts the intermediate frequency band OFDM signal into the baseband OFDM signal.

The Fourier transformer 400 multiplexes on the frequency axis by collectively performing a Fourier transform operation on the real part of the OFDM signal output from the detector 302 and the imaginary part of the OFDM signal output from the detector 303. The real and imaginary parts of each digitally modulated wave are separated. The demodulated data detector 500 maps the real and imaginary parts of each digitally modulated wave to the complex plane, demodulates the data in which each carrier is modulated from the mapping position according to the threshold set inside the complex plane, and outputs the data. Output the demodulated data from terminal O.<patcit num="1"><text>Japanese Patent Application Laid-Open No. 5-167633</text></patcit><nplcit num="1"><text>"Next-generation home services go beyond television" Nikkei Electronics (no.574), February 15, 1993, pp. 101-124</text></nplcit><nplcit num="2"><text>Masanori Saito "OFDM Method and Its Development Trend" NHK Science &amp; Technical Research Laboratories, September 14, 1994, EIAJ Technical Seminar Materials, pp. 1-15</text></nplcit>

<p> The above-mentioned OFDM signal is transmitted from the transmitting device to the receiving device via a wireless or wired transmission line, and the OFDM signal is attenuated in any of the transmission lines. The amount of attenuation of the OFDM signal changes according to the change in the distance in the wireless transmission line, and changes according to the number of branches in the transmission line in the wired transmission line. When the attenuation of the OFDM signal changes, the receiving device changes the reception level of the OFDM signal. However, the receiving device of FIG. 12 performs data demodulation processing without any correction even if the receiving level of the OFDM signal fluctuates. Therefore, there is a problem that the demodulated data detector 500 frequently misdetermines the demodulated data.</p><p> By the way, FM receivers and the like are provided with an automatic gain control amplifier that corrects fluctuations in the reception level based on fluctuations in the envelope of the received signal. It is conceivable to apply such a correction method to the receiving device shown in FIG. 12, but unlike the FM signal of a single carrier, the OFDM signal has a large number of modulation carriers multiplexed on the frequency axis, so that each of them is used. The amplitude and phase patterns in the symbol section change randomly. Therefore, the envelope waveform of the OFDM signal also changes frequently on the time axis, and if the automatic gain control amplifier is controlled based on such an envelope waveform, the gain of the automatic gain control amplifier becomes unstable and stable control is performed. I can't. Further, in the OFDM signal, since the modulation data of each carrier is different from each other, the fluctuation of the envelope waveform and the fluctuation of the reception level are not always correlated. Therefore, even if the level correction method in the FM receiver is applied to the OFDM signal receiver, the fluctuation of the reception level cannot be corrected with high accuracy.</p><p> Further, in the receiving device of FIG. 12, since the frequency shift amount in the frequency converter 100 is fixedly set, even if the frequency band shift, that is, the frequency band fluctuation occurs, the fluctuation of this frequency band cannot be corrected. .. Therefore, there is a problem that the demodulated data is frequently erroneously determined.</p><p> By the way, AM receivers and the like are provided with a frequency converter that corrects fluctuations in the frequency band based on fluctuations in frequency discrimination of received signals. It is conceivable to apply such a correction method to the receiver shown in FIG. 12, but unlike the AM signal of a single carrier, the OFDM signal has a large number of modulation carriers multiplexed on the frequency axis, so that each of them is used. The amplitude and phase patterns in the symbol section change randomly. Therefore, the frequency discrimination waveform of the OFDM signal also changes frequently on the frequency axis, and if the frequency converter is controlled based on such a frequency discrimination waveform, the frequency shift amount of the frequency converter becomes unstable and stable. I can't control it. Further, in the OFDM signal, since the modulation data of each carrier is different from each other, the fluctuation of the frequency discrimination waveform and the fluctuation of the frequency shift amount do not always correlate with each other. Therefore, even if the method of frequency shift amount correction in the AM receiver is applied to the OFDM signal receiver, the fluctuation of the frequency band cannot be corrected with high accuracy.</p><p> Therefore, an object of the present invention is to provide a method for transmitting an orthogonal frequency multiplexed signal and a receiving device thereof, which can accurately correct fluctuations in reception level and do not cause erroneous determination of demodulated data as a result.</p><p> Another object of the present invention is to provide a method for transmitting an orthogonal frequency multiplex signal and a receiving device thereof, which can accurately correct fluctuations in the frequency band and do not cause erroneous determination of demodulated data as a result.</p>

<p> In order to achieve the above object, the first aspect of the present invention is directed to a method of transmitting as an orthogonal frequency division multiplex signal for each symbol including a plurality of carriers having a frequency relationship orthogonal to each other. The transmission method transmits the first symbol, and further transmits the second symbol. Here, the first symbol contains an integer number of pseudo-random information whose time axis component and / or frequency axis component is known, and the second symbol contains a plurality of carriers contained in one symbol. It is PSK-modulated or QAM-modulated by the transmitted data.</p><p> The second aspect of the present invention is directed to a method of receiving orthogonal frequency division multiplexing signals transmitted for each symbol including a plurality of carriers having a frequency relationship orthogonal to each other. The receiving method receives the first symbol and further receives the second symbol. Here, the first symbol contains an integer number of pseudo-random information whose time axis component and / or frequency axis component is known, and the second symbol contains a plurality of carriers contained in one symbol. It is PSK-modulated or QAM-modulated by the transmitted data. The receiving method further demodulates the second symbol to obtain transmission data based on the synchronization information obtained by the received first symbol.</p><p> In addition, the receiving method further corrects the fluctuation of the frequency band based on the received first symbol.</p><p> In addition, the receiving method further corrects the fluctuation of the receiving level based on the received first symbol.</p><p> Further, the third aspect of the present invention is as an orthogonal frequency division multiplex signal for each symbol including a plurality of carriers having a frequency relationship orthogonal to each other from the transmitting side to the receiving side via a wired or wireless transmission line. It is directed to the method of transmission. The sender transmits the first symbol and further transmits the second symbol. Here, the first symbol contains an integer number of pseudo-random information whose time axis component and / or frequency axis component is known, and the second symbol contains a plurality of carriers contained in one symbol. It is PSK-modulated or QAM-modulated by the transmitted data. Further, the receiving side receives the second symbol and further receives the first symbol. Here, the receiving side further demodulates the second symbol based on the synchronization information obtained by the received first symbol to obtain transmission data.</p><p> In addition, the receiving side further corrects the fluctuation of the frequency band based on the received first symbol.</p><p> In addition, the receiving side further corrects the fluctuation of the receiving level based on the received first symbol.</p><p> Further, the fourth aspect of the present invention is directed to a receiving device that receives orthogonal frequency division multiplexing signals transmitted for each symbol including a plurality of carriers having a frequency relationship orthogonal to each other. The receiving device includes receiving means for receiving the orthogonal frequency division multiplexing signal. Here, the orthogonal frequency division multiplex signal includes a first symbol and a second symbol, and the first symbol contains an integer number of pseudo-random information whose time axis component and / or frequency axis component is known. Further, in the second symbol, each of the plurality of carriers included in one symbol is PSK-modulated or QAM-modulated by the transmission data. The receiving device further includes a demodulation means for demodulating the second symbol and obtaining transmission data based on the synchronization information obtained by the received first symbol.</p><p> In addition, the receiving device corrects the fluctuation of the frequency band based on the received first symbol.</p><p> In addition, the receiving device corrects the fluctuation of the receiving level based on the received first symbol.</p>

<p> As in the first to fourth aspects above, the first symbol containing an integer number of pseudo-random information whose time axis component and / or frequency axis component is known, and a plurality of carriers included in one symbol are transmitted. By transmitting a second symbol that is PSK-modulated or QAM-modulated by the data, it becomes easier to correlate on the receiving side.</p>

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing an example of a configuration of an OFDM signal transmitted from a transmitting side to a receiving side in the present invention. In particular, FIG. 1 (a) shows each symbol of the OFDM signal along the time axis, and FIG. 1 (b) shows an enlarged part α of FIG. 1 (a).

As shown in FIG. 1 (a), the OFDM signal S has a specific symbol S0 for automatic gain control shown with hatching and a symbol Sm (m = 1,2) for demodulation shown without hatching. , ...) and are arranged along the time axis. Symbol S0 is inserted at predetermined symbol intervals (for example, 15 symbol intervals). It should be noted that such an OFDM signal S takes the form of an analog complex signal in which a real part and an imaginary part are superimposed for each of the symbols S0 and Sm on the transmission path.

Each symbol Sm is composed of multiple carriers (several tens to thousands, for example 512) having different frequencies (orthogonal to each other in the symbol time ts) are multiplexed (high-speed inverse Fourier operation) on the frequency axis. ing. Each carrier is digitally modulated (eg, QPSK modulation, 16QAM, etc.) with data to be demodulated on the receiving side. Therefore, each symbol Sm shows a random amplitude distribution as shown in Fig. 1 (b).

Each symbol S0 is composed, for example, by leaving one of the above-mentioned plurality of carriers (for example, frequency fc) as an unmodulated single-tone signal and suppressing the other carriers by performing a high-speed inverse Fourier transform operation. There is. Therefore, each symbol S0 shows the amplitude distribution of a specific pattern, as shown in FIG. 1 (b). Such a symbol S0 has a known time axis component as well as a frequency axis component.

By the way, the OFDM signal S is transmitted from the transmitting side to the receiving side via a wired or wireless transmission line (not shown). Therefore, the OFDM signal S is attenuated on the transmission path. Therefore, on the receiving side, when demodulating the data, it is necessary to correct the level of the received OFDM signal S in order to compensate for the attenuation generated on the transmission path. Such an operation for correcting the reception level of the OFDM signal S is performed using the symbol S0. This is because the symbol S0 always contains the same pattern of signals, so that the change in reception level can be accurately measured from the waveform of the symbol S0.

FIG. 2 is a block diagram showing a configuration of a receiving device according to the first embodiment of the present invention. In FIG. 2, the receiving device includes an input terminal I to which the received OFDM signal is input, a band-passing filter 1, an automatic gain control amplifier 2, an orthogonal detector 3, and A / D converters 7 and 8. , The Fourier converter 4, the demodulated data detector 5, the control signal output device 6, and the output terminal O are provided. The orthogonal detector 3 includes a demultiplexer 31, detectors 32 and 33, and a carrier regenerator 34. The control signal output device 6 includes an envelope detector 61, a reference timing generator 62, a symbol timing synchronization circuit 63, a symbol energy detector 64, a control signal switch 65, a sample holder 66, and a low-pass filter 67. And include.

FIG. 3 is a waveform diagram showing signals of each part of the receiving device shown in FIG. Hereinafter, the operation of the receiving device of FIG. 2 will be described with reference to FIG.

The OFDM signal (see FIG. 1) received by the receiving device is converted from the occupied frequency band of the transmission line to the intermediate frequency band (center frequency fc) by a frequency converter (not shown), and then the band passing filter is passed through the input terminal I. Entered in 1. The band-passing filter 1 removes signal components in an unnecessary band from the OFDM signal in the intermediate frequency band, and extracts only the OFDM signal in the required band. The OFDM signal output from the bandpass filter 1 is given to the orthogonal detector 3 via the automatic gain control amplifier 2.

The demultiplexer 31 of the quadrature detector 3 demultiplexes the OFDM signal output from the automatic gain control amplifier 2 into two, and outputs the demultiplexed OFDM signal to the detectors 32 and 33, respectively. The carrier regenerator 34 outputs a common mode carrier having a center frequency fc to the detector 32, and outputs an orthogonal carrier having a center frequency fc to the detector 33. The detector 32 outputs the real part of the OFDM signal by multiplying the OFDM signal output from the demultiplexer 31 by the common mode carrier. The detector 33 outputs an imaginary part of the OFDM signal by multiplying the OFDM signal output from the demultiplexer 31 by an orthogonal carrier. That is, the orthogonal detector 3 converts the OFDM signal in the intermediate frequency band into the OFDM signal in the baseband. The A / D converter 7 converts the real part of the OFDM signal output from the detector 32 from an analog signal to a digital signal. The A / D converter 8 converts the imaginary part of the OFDM signal output from the detector 33 from an analog signal to a digital signal.

The Fourier transformer 4 collectively performs a Fourier transform on the real part of the digital OFDM signal output from the A / D converter 7 and the imaginary part of the digital OFDM signal output from the A / D converter 8. By performing the calculation, the real part and the imaginary part of each digitally modulated wave are separated on the frequency axis. The Fourier transformer 4 has a clock terminal 4c, and starts adjusting the time axis of the time window used for the Fourier transform based on the symbol synchronization signal output from the symbol timing synchronization circuit 63, and each symbol. Initiates the Fourier transform of. The demodulated data detector 5 maps the real part and the imaginary part of each digitally modulated wave on the complex plane, and demodulates the data obtained by modulating each carrier from the mapping position according to the threshold value set inside the complex plane.

The operation modes of the control signal output device 6 are the first mode in which the control signal of the automatic gain control amplifier 2 is generated based on the envelope waveform of the output signal of the orthogonal detector 3 and the symbol energy of the output signal of the Fourier converter 4. Includes a second mode in which the control signal of the automatic gain control amplifier 2 is generated based on. The control signal output device 6 operates in the first mode at the start of receiving the OFDM signal, and operates in the second mode after the operation of the Fourier transformer 4 stabilizes (that is, after synchronizing with the received signal). Hereinafter, the operation of the control signal output device 6 will be described in more detail.

The envelope detector 61 outputs an envelope signal representing the envelope of each symbol by performing envelope detection on each symbol of the OFDM signal output from the detectors 32 and 33. The envelope signal output from the envelope detector 61 is given to the reference timing generator 62, and is also given to the control signal input terminal 65a of the control signal switcher 65 via a low-pass filter 67 that smoothes the fluctuation.

The reference timing generator 62 stores in advance the single tone data corresponding to the specific pattern of the symbol S0. Then, the reference timing generator 62 obtains the correlation between the envelope signal output from the envelope detector 61 and the stored single tone data for each symbol along the time axis, so that the symbol S0 Outputs a reference timing signal indicating whether or not has been detected. That is, as shown in FIGS. 3A and 3B, the reference timing generator 62 outputs a high level (voltage Vhigh) reference timing signal when the symbol S0 is detected, and the symbol does not include a specific pattern. When Sm is detected, a low level (voltage Vlow) reference timing signal is output. Note that the reference timing generator 62 has a low level (that is, during the asynchronous period) until the detection operation stabilizes (synchronizes) with respect to the received signal, even when the symbol S0 is received. Voltage Vlow ) Reference timing signal is output. The reference timing signal output from the reference timing generator 62 is input to the clock terminals 66c of the symbol timing synchronization circuit 63 and the sample holder 66, respectively.

The symbol timing synchronization circuit 63 outputs a symbol synchronization signal (see FIG. 3C) that synchronizes with each symbol based on the reference timing signal given from the reference timing generator 62. That is, the symbol timing synchronization circuit 63 has a clock circuit inside thereof, and each time the rising edge of the reference timing signal is detected, the clock pulse synchronized from the clock circuit to the beginning of each symbol (symbol time ts is one cycle). The clock pulse), that is, the symbol synchronization signal is output. This symbol synchronization signal is input to the clock terminal 4c of the Fourier transformer 4 and the clock terminal 64c of the symbol energy detector 64, respectively.

Further, the symbol timing synchronization circuit 63 outputs a lock / unlock signal (see FIG. 3D) based on the reference timing signal given from the reference timing generator 62. This lock / unlock signal indicates an unlocked state at a low level and a locked state at a high level. At the beginning of reception, the lock / unlock signal is in the unlocked state. The symbol timing synchronization circuit 63 includes a counter that counts the clock pulse inside, and resets the counter every time the rising edge of the reference timing signal is detected. The symbol timing synchronization circuit 63 repeats the state of being reset when the internal counter reaches a predetermined count value (the symbol interval at which the symbol S0 is inserted, which is 15 in this case) a predetermined number of times (that is, the symbol S0 is inserted). When the input is stable a predetermined number of times), it is determined that the adjustment of the time window in the Fourier transform circuit 4 is completed, and the lock / unlock signal is switched from the unlocked state to the locked state. This lock / unlock signal is input to the clock terminal 65c of the control signal switch 65.

The symbol energy detector 64 includes a D / A converter (not shown) inside. The symbol energy detector 64 synchronizes with the symbol synchronization signal given from the symbol timing synchronization circuit 63, and digitally calculates the symbol period of the signal component of each carrier on the frequency axis of each symbol output from the Fourier transformer 4. By integrating the squares in ts (integrating the squared ones), the energy of each symbol is once obtained as a digital value. Then, by converting the obtained digital energy value into an analog value by the above D / A converter, an analog symbol energy signal representing the energy of each symbol is output. This energy is directly proportional to the average level of each symbol. Moreover, the square is to take the absolute value because the amplitude of each carrier fluctuates positively and negatively along the time axis. Moreover, the reason for integrating is to obtain the average. The symbol energy signal output from the symbol energy detector 64 is input to the control signal input terminal 65b of the control signal switch 65.

The control signal switch 65 selects the envelope signal output from the envelope detector 61 when the lock / unlock signal input to the clock terminal 65c is in the locked state, and the symbol energy detector 64 when the lock / unlock signal is in the unlocked state. The symbol energy signals output from are selected and output as control signals of the automatic gain control amplifier 2 respectively.

The sample hold device 66 controls when a reference timing signal having a voltage Vhigh is input to the clock terminal 66c from the reference timing generator 62, that is, when a specific symbol S0 is output from the automatic gain control amplifier 2. The control signal selected by the signal switch 65 is sampled and held. The control signal held by the sample hold device 66 is given to the control terminal 2c of the automatic gain control amplifier 2. The gain A of the automatic gain control amplifier 2 changes according to the voltage level of the control signal given by the sample holder 66.

As the reception level of the OFDM signal increases, the level of the envelope signal or symbol energy signal of symbol S0 also increases in direct proportion to this, so that the voltage level of the control signal given to the automatic gain control amplifier 2 increases. At this time, the automatic gain control amplifier 2 reduces the gain A so as to reduce the level of the received OFDM signal. On the other hand, when the reception level of the OFDM signal becomes smaller, the level of the envelope signal or the symbol energy signal of the symbol S0 also becomes smaller in direct proportion to this, so that the voltage level of the control signal given to the automatic gain control amplifier 2 becomes smaller. At this time, the automatic gain control amplifier 2 increases the gain A so as to increase the level of the received OFDM signal. As a result, the automatic gain control amplifier 2 can correct the fluctuation of the reception level of the OFDM signal to an appropriate level.

By the way, since the symbol energy signal is the energy of each symbol S0 and is obtained by digital calculation, it contains almost no error. On the other hand, since the envelope signal is an envelope connecting the vertices of the waveform of each symbol S0, the difference between the waveform of each symbol S0 and the envelope is included as an error. Moreover, the envelope signal requires a filtering process (performed by the low-pass filter 67) in order to be used as a control signal of the automatic gain control amplifier 2, and an error occurs in this filtering process as well. Therefore, it is possible to improve the gain control accuracy of the automatic gain control amplifier 2 by using the symbol energy signal rather than the envelope signal.

However, when the symbol synchronization signal is output from the symbol timing synchronization circuit 63, the Fourier transformer 4 starts adjusting the time axis of the time window used for the Fourier transform. Since it takes time, when the reception of the OFDM signal is started, the time window and the received symbol may not be synchronized (that is, the time window is set across a plurality of adjacent symbols). .. In such a state, the normal operation of the Fourier transformer 4 and the symbol energy detector 64 is not guaranteed.

Therefore, the control signal output device 6 is in the first operation mode for a while after the start of receiving the OFDM signal (until the adjustment of the time axis of the time window of the Fourier transformer 4 is completely completed), that is, The gain of the automatic gain control amplifier 2 is controlled based on the envelope signal of the symbol S0. The control signal outputr 6 then controls the gain of the automatic gain control amplifier 2 in the second mode of operation, i.e., based on the symbol energy signal of symbol S0.

As described above, according to the embodiment of FIG. 2, the reference timing generator 62 periodically detects the specific symbol S0, and the sample holder 66 samples and holds the envelope signal or symbol energy signal for this symbol S0. Since the feedback is fed back to the control terminal 2c of the automatic gain control amplifier 2, the accuracy of the gain control of the automatic gain control amplifier 2 can be improved. Further, since the gain control complements the attenuation in the transmission line, that is, the reception level is corrected, it is possible to prevent erroneous determination of the demodulated data.

In the above embodiment, the symbol S0 is inserted at intervals of 15 symbols, but it may be inserted at intervals of other symbols. Further, in the above embodiment, the symbol S0 is constructed by using only one carrier as an unmodulated single tone signal and suppressing the other carriers. However, the symbol S0 has a time axis component and a frequency axis component. Any known signal whose amplitude and phase changes along the time axis indicate a predetermined specific pattern may be configured by other methods. For example, the amplitude of one carrier may be amplitude-modulated by a plurality of known data (for example, "1" data and "2" data). In this case, although the envelope of the envelope signal output from the envelope detector 61 has some irregularities, it is smoothed by the low-pass filter 67 and can be used as a control signal.

Further, in the above embodiment, the baseband OFDM signal output from the orthogonal detector 3 is input to the envelope detector 61, but if the automatic gain control amplifier 2 or later is used, the automatic gain control amplifiers 2 and A are used. The output of any of the / D converters 7 and 8 and the Fourier converter 4 may be input to the envelope detector 61.

Further, in the above embodiment, the output of the Fourier transformer 4 is input to the symbol energy detector 64, but if the automatic gain control amplifier 2 or later is used, the automatic gain control amplifier 2, the orthogonal detector 3, A / The output of any of the D converters 7 and 8 may be input to the symbol energy detector 64.

Further, in the above embodiment, the A / D converters 7 and 8 are provided, but the A / D converters 7 and 8 may be deleted to perform Fourier transform and symbol energy detection as they are in analog.

Further, in the above embodiment, the control signal output device 6 is configured to operate in two operation modes, but may be configured to operate only in the first operation mode. In this case, the control signal output device will include only the envelope detector 61, the reference timing generator 62 and the sample hold device 66.

Further, the control signal output device 6 may be configured to operate only in the second operation mode. In this case, the control signal output device will include only the envelope detector 61, the reference timing generator 62, the symbol timing synchronization circuit 63, the symbol energy detector 64, and the sample holder 66.

FIG. 4 is a diagram showing another example of the configuration of the OFDM signal transmitted from the transmitting side to the receiving side in the present invention. In particular, FIG. 4 (a) shows each symbol of the OFDM signal along the time axis, and FIG. 4 (b) shows an enlarged part α of FIG. 4 (a).

As shown in FIG. 4 (a), the OFDM signal S has a specific symbol S0 for frequency conversion control shown with hatching and a symbol Sm (m = 1,2) for demodulation shown without hatching. , ...) and are arranged along the time axis. Symbol S0 is inserted at predetermined symbol intervals (for example, 15 symbol intervals). It should be noted that such an OFDM signal S takes the form of a complex signal in which a real part and an imaginary part are superimposed for each of the symbols S0 and Sm on the transmission path.

Each symbol Sm is composed of multiple carriers (several tens to thousands, for example 512) having different frequencies (orthogonal to each other in the symbol time ts) are multiplexed (high-speed inverse Fourier operation) on the frequency axis. ing. Each carrier is digitally modulated (eg, QPSK modulation, 16QAM, etc.) with data to be demodulated on the receiving side. Therefore, each symbol Sm shows a random amplitude distribution as shown in Fig. 4 (b).

Each symbol S0 is generated as a pseudo-random signal by, for example, amplitude-modulating one of the plurality of carriers (for example, frequency fc) with a binary (for example, "1" and "2") pseudo-random code. It is composed by performing a high-speed inverse Fourier operation on what is left and suppresses other carriers. Therefore, each symbol S0 shows the amplitude distribution of a specific pattern, as shown in FIG. 4 (b). Such a symbol S0 has a known time axis component as well as a frequency axis component.

The data speed of the pseudo-random code is preferably selected to be an integral multiple of the OFDM symbol rate. By doing so, an integer number of pseudo-random code information can be stored in one symbol S0, and it becomes easy to synchronize on the receiving side. Further, it is preferable that the repetition period (repetition period) of the pattern of the pseudo-random code used is the same as the symbol period. In this case, the number of times one code (for example, "1") appears and the number of times the other code (for example, "2") appears are equal, and it becomes easy to obtain a correlation on the receiving side.

By the way, the OFDM signal S shown in FIG. 4 is transmitted from the transmitting side to the receiving side via a wired or wireless transmission line (not shown). Therefore, the transmitting side (not shown) converts the OFDM signal S from the intermediate frequency band (center frequency fc) to the occupied frequency band (center frequency fr) of the transmission line. On the other hand, on the receiving side, when demodulating the data, the received OFDM signal S is converted from the occupied frequency band of the transmission line to the intermediate frequency band (center frequency fc) for the demodulation work. In the embodiment described below, the operation of frequency-converting the OFDM signal S from the occupied frequency band to the intermediate frequency band is performed using the symbol S0. This is because the symbol S0 always contains the same pattern of signals, so that the change in the frequency band can be accurately measured from the waveform of the symbol S0.

FIG. 5 is a block diagram showing a configuration of a receiving device according to a second embodiment of the present invention. In FIG. 5, the receiving device includes an input terminal I to which the received OFDM signal is input, a frequency converter 10, an orthogonal detector 3, a Fourier transformer 4, a demodulated data detector 5, and a control signal output device. It has 60 and an output terminal O. The orthogonal detector 3 includes a turnout 31, detectors 32 and 33, and a carrier regenerator 34. The control signal output device 60 includes an envelope detector 61, a reference timing generator 62, a symbol timing synchronization circuit 63, a sample hold device 66, and a frequency discriminator 68. For the purpose of clarifying the correspondence, in the embodiment of FIG. 5, the same reference numbers are assigned to the same components as those of the embodiment of FIG.

FIG. 6 is a waveform diagram showing signals of each part of the receiving device shown in FIG. Hereinafter, the operation of the receiving device of FIG. 5 will be described with reference to FIG.

The OFDM signal (see FIG. 6 (a)) of the occupied frequency band (center frequency fr) of the transmission line received by the receiving device is input to the input terminal I, and is input to the input terminal I by the frequency converter 10 in the intermediate frequency band (center frequency fc). After being converted into an OFDM signal, it is input to the orthogonal detector 3.

The demultiplexer 31 of the orthogonal detector 3 splits the OFDM signal output from the frequency converter 10 into two, and outputs the demultiplexed OFDM signal to the detectors 32 and 33, respectively. The carrier generator 34 outputs an in-phase carrier having a center frequency fc to the detector 32, and outputs an orthogonal carrier having a center frequency fc to the detector 33. The detector 32 outputs a real part of the OFDM signal by multiplying the OFDM signal output from the demultiplexer 31 by a common mode carrier. The detector 33 outputs an imaginary part of the OFDM signal by multiplying the OFDM signal output from the demultiplexer 31 by an orthogonal carrier. That is, the orthogonal detector 3 converts the OFDM signal in the intermediate frequency band into the OFDM signal in the baseband.

The Fourier transformer 4 collectively performs a Fourier transform operation on the real part of the OFDM signal output from the detector 32 and the imaginary part of the OFDM signal output from the detector 33 on the frequency axis. The real and imaginary parts of each digitally modulated wave are separated. The demodulated data detector 5 maps the real part and the imaginary part of each digitally modulated wave to the complex plane, and demodulates the data obtained by modulating each carrier from the mapping position according to the threshold value set inside the complex plane.

Next, the operation of the control signal output device 60 will be described in more detail. The envelope detector 61 outputs an envelope signal representing the envelope of each symbol by performing envelope detection on each symbol of the OFDM signal output from the frequency converter 10. The envelope signal output from the envelope detector 61 is given to the reference timing generator 62.

The reference timing generator 62 stores in advance binary pseudo-random data corresponding to the specific pattern of the symbol S0. Then, the reference timing generator 62 obtains the correlation between the envelope signal output from the envelope detector 61 and the stored binary pseudo-random data for each symbol along the time axis, so that the symbol S0 Outputs a reference timing signal indicating whether or not has been detected. That is, as shown in FIGS. 6 (a) and 6 (b), the reference timing generator 62 outputs a high level (voltage V1) reference timing signal when it detects the symbol S0 including the specific pattern, and outputs the specific pattern. When a symbol Sm that does not include is detected, a low level (voltage V2) reference timing signal is output. The reference timing signal output from the reference timing generator 62 is input to the clock terminal 66c of the sample hold device 66 and the symbol timing synchronization circuit 63.

The symbol timing synchronization circuit 63 outputs a symbol synchronization signal synchronized with each symbol based on the reference timing signal given from the reference timing generator 62. That is, the symbol timing synchronization circuit 63 has a clock circuit inside thereof, and each time the rising edge of the reference timing signal is detected, the clock pulse synchronized from the clock circuit to the beginning of each symbol (symbol time ts is one cycle). Clock pulse), that is, the symbol synchronization signal is output. This symbol synchronization signal is input to the clock terminal 64c of the clock terminal 4c of the Fourier transformer 4.

The Fourier transformer 4 collectively performs a Fourier transform operation on the real part of the digital OFDM signal output from the detector 32 and the imaginary part of the digital OFDM signal output from the detector 33. The real and imaginary parts of each digitally modulated wave are separated on the frequency axis. The Fourier transformer 4 has a clock terminal 4c, and starts adjusting the time axis of the time window used for the Fourier transform based on the symbol synchronization signal output from the symbol timing synchronization circuit 63, and each symbol. Initiates the Fourier transform of. The demodulated data detector 5 maps the real part and the imaginary part of each digitally modulated wave on the complex plane, and demodulates the data obtained by modulating each carrier from the mapping position according to the threshold value set inside the complex plane.

The frequency discriminator 68 generates a voltage corresponding to the frequency of each symbol by frequency discriminating each symbol. The sample hold device 66 discriminates the frequency when the reference timing signal of the voltage V1 is input from the reference timing generator 62 to the clock terminal 66c, that is, when the specific symbol S0 is output from the frequency converter 10. The frequency discrimination signal output from the device 68 is sampled and held. The frequency discrimination signal held by the sample hold device 66 is given to the control terminal 10c of the frequency converter 10 as a control signal. The frequency shift amount of the frequency converter 10 changes according to the voltage level of the control signal given by the sample holder 66.

As the frequency band of the OFDM signal output from the frequency converter 10 increases, the level of the frequency discrimination signal of symbol S0 output from the frequency discriminator 68 also increases in direct proportion to this, so it is given to the frequency converter 10. The voltage level of the control signal is increased. At this time, the frequency converter 10 increases the frequency shift amount so as to lower the frequency band of the output OFDM signal. On the other hand, when the frequency of the OFDM signal becomes low, the level of the frequency discrimination signal of the symbol S0 also becomes small in direct proportion to this, so that the voltage level of the control signal given to the frequency converter 10 becomes small. At this time, the frequency converter 10 reduces the frequency shift amount so as to raise the frequency band of the output OFDM signal. As a result, the frequency converter 10 can correct the fluctuation of the frequency band of the OFDM signal to an appropriate intermediate frequency band (center frequency fc).

As described above, according to the second embodiment of FIG. 5, the reference timing generator 62 periodically detects a specific symbol S0, and the frequency discrimination signal at this symbol S0 is sample-held as a control signal for control. Since the signal is fed back to the control terminal 10c of the frequency converter 10, the accuracy of the frequency shift amount control of the frequency converter 10 can be improved. Further, since the fluctuation of the frequency band is corrected by the frequency shift amount control, the deviation from the intermediate frequency band is eliminated, and erroneous determination of the demodulated data can be prevented.

FIG. 7 is a block diagram showing a configuration of a receiving device according to a third embodiment of the present invention. The same reference number is assigned to the part corresponding to the receiving device in FIG. 5, and the description thereof will be omitted. What should be noted in this third embodiment is that the control signal output device 70 is configured by using the frequency domain energy detector 71 instead of the frequency classifier 68 in FIG.

FIG. 8 is a waveform diagram for explaining the operation of the frequency domain energy detector 71 of FIG. In particular, FIG. 8 (a) shows the power spectrum of symbol S0 along the frequency axis, FIG. 8 (b) shows the integrated value of the power spectrum of FIG. 8 (a), and FIG. 8 (c) shows the frequency domain energy. Shows a signal. Hereinafter, the operation of the receiving device of FIG. 7 will be described with reference to FIG.

The frequency domain energy detector 71 performs a series of operations as described below for each symbol in synchronization with the symbol synchronization signal given from the symbol timing synchronization circuit 63 to the clock terminal 71c. First, as shown in FIG. 8A, the frequency domain energy detector 71 is amplitude-modulated by carriers (binary pseudo-random signals) distributed in the frequency range of 0 to fs in the output of the Fourier transformer 4. Is divided into two regions α1 and α2 with (1/2) fs as the boundary. Here, fs is the frequency of the sampling clock used in the Fourier transformer 4. Also, since the spectrum of each symbol is folded around (1/2) fs, the high frequency component is in the region α1 whose frequency is lower than (1/2) fs, and the low frequency component is (1 / 2). 2) It appears in the region α2, which has a higher frequency than fs.

Next, the frequency domain energy detector 71 squares and integrates the power spectrum component of the region α1 and the power spectrum component of the region α2, respectively, as shown in FIG. 8 (b), so that the energy E1 of the region α1 is integrated. And the energy E2 in the region α2. These energies E1 and E2 are proportional to the average level of each symbol. The square is to take the absolute value because the amplitude of each carrier fluctuates positively and negatively along the time axis. Also, the integration is to find the average of each symbol.

Next, the frequency domain energy detector 71 compares the energy E1 in the region α1 with the energy E2 in the region α2, and as shown in FIG. 8 (c), the voltage value corresponding to the energy difference (E1-E2). Generates a frequency domain energy signal with. This frequency domain energy signal shows a positive voltage value VHIGH when the energy E1 in the region α1 is larger, and a negative voltage value VLOW when the energy E2 in the region α2 is larger. By the way, in the symbol S0, when there is no deviation in the frequency band, the power distributions in the regions α1 and α2 become equal, and the voltage value of the frequency domain energy signal becomes 0. Therefore, the deviation direction and the deviation amount from the center frequency fc can be known based on the polarity and the voltage value of the frequency domain energy signal of the symbol S0.

The sample holder 66 has a frequency domain when a reference timing signal of voltage V1 is input from the reference timing generator 62 to the clock terminal 66c, that is, when a specific symbol S0 is output from the frequency converter 10. The frequency domain energy signal output from the energy detector 71 is sampled and held. The frequency domain energy signal held by the sample hold device 66 is given to the control terminal 10c of the frequency converter 10 as a control signal. The frequency shift amount of the frequency converter 10 changes according to the voltage level of the control signal given by the sample holder 66.

When the frequency band of the OFDM signal output from the frequency converter 10 becomes high, the voltage value VHIGH of the frequency domain energy signal of the symbol S0 output from the frequency domain energy detector 71 increases in the positive direction. The voltage of the control signal given to is also increased in the positive direction. At this time, the frequency converter 10 increases the frequency shift amount so as to lower the frequency band of the output OFDM signal. On the other hand, when the frequency band of the OFDM signal becomes low, the voltage VLOW of the frequency domain energy signal of the symbol S0 increases in the negative direction, so that the voltage of the control signal given to the frequency converter 10 also increases in the negative direction. At this time, the frequency converter 10 reduces the frequency shift amount so as to increase the frequency of the output OFDM signal. As a result, the frequency converter 10 can correct the fluctuation of the frequency band of the OFDM signal to an appropriate intermediate frequency band (center frequency fc). The control signal sample-held by the sample-holder 66 may be averaged over a plurality of cycles of the symbol S0.

As described above, according to the third embodiment of FIG. 7, the reference timing generator 62 periodically detects the specific symbol S0, and the frequency domain energy signal at this symbol S0 is sample-held as a control signal. Since the feedback is fed back to the control terminal 10c of the frequency converter 10, the accuracy of the frequency shift amount control of the frequency converter 10 can be improved. Further, since the fluctuation of the frequency band is corrected by the frequency shift amount control, the deviation from the intermediate frequency band is eliminated, and erroneous determination of the demodulated data can be prevented.

FIG. 9 is a block diagram showing a configuration of a receiving device according to a fourth embodiment of the present invention. The same reference number is assigned to the part corresponding to the receiving device in FIG. 5, and the description thereof will be omitted. It should be noted in this embodiment that the control signal output device 80 is configured by using the correlation detector 81 and the peak value frequency detector 82 instead of the frequency discriminator 68 in FIG.

FIG. 10 is a waveform diagram showing signals of each part of the control signal output device 80 of FIG. In particular, FIG. 10 (a) shows the correlation signal along the frequency axis, and FIG. 10 (b) shows the peak value frequency detection signal. Hereinafter, the operation of the receiving device of FIG. 9 will be described with reference to FIG. 10.

The correlation detector 81 stores information on the ideal frequency component for the specific symbol S0 in advance as reference information. The correlation detector 81 outputs a correlation signal as shown in FIG. 10A by obtaining the correlation between this reference information and the data on the frequency axis output from the Fourier transformer 4. The correlation detection operation in the correlation detector 81 is performed for each symbol in synchronization with the symbol synchronization signal given from the symbol timing synchronization circuit 63 to the clock terminal 81c. In particular, the specific symbol S0 from the Fourier transformer 4 is used. It will have meaning when it is output. Therefore, to explain the case where the specific symbol S0 is output from the Fourier transformer 4, the correlation detector 81 once has the symbol S0. Information is stored in the internal memory (not shown) as detection target information. Here, the reference information stored in advance by the correlation detector 81 and the detection target information stored in the internal memory thereof are both digital pseudo-random signals that exist discretely on the frequency axis. The correlation detector 81 superimposes the detection target information and the reference information on the frequency axis, multiplies the code information contained in each, and further obtains the sum of them. At this time, the correlation detector 81 obtains the sum of the multiplication results with the reference information while shifting the position of the detection target information on the frequency axis for each code unit. Then, the set of this sum becomes a correlation signal. The correlation signal peaks when the correspondence between the code information included in the detection target information and the code information included in the reference information matches on the frequency axis.

When the symbol S0 is output from the Fourier transformer 4, for example, when the frequency shift Δf is 0, the correlation detector 81 moves the position of the center frequency fc as shown in β1 of FIG. 10 (a). Outputs a correlation signal with a peak value in. Further, when the symbol S0 is output, for example, when there is a frequency shift Δf on the higher side, the correlation detector 81 shifts the appearance of the peak value as shown in β2 of FIG. 10 (a), and the peak value becomes The correlation signal generated on the higher side of the frequency axis is output. Therefore, the frequency shift direction and the shift amount can be detected from such a correlation signal.

The peak value frequency detector 82 compares the location of the peak value of the correlation signal output from the correlation detector 81 with the center frequency fc, and has a peak value frequency signal having a voltage value ΔV corresponding to the difference Δf (Fig.). 10 (b)) is output.

The sample holder 66 has a peak value when a reference timing signal of voltage V1 is input from the reference timing generator 62 to the clock terminal 66c, that is, when a specific symbol S0 is output from the frequency converter 10. The peak value frequency signal output from the frequency detector 82 is sampled and held. The peak value frequency signal held by the sample hold device 66 is given to the control terminal 10c of the frequency converter 10 as a control signal. The frequency shift amount of the frequency converter 10 changes according to the voltage level of the control signal given by the sample holder 66.

When the frequency band of the OFDM signal output from the frequency converter 10 becomes higher, the level ΔV of the peak value frequency signal of the symbol S0 output from the peak value frequency detector 82 also increases in the positive direction. The voltage level of the given control signal increases. At this time, the frequency converter 10 increases the frequency shift amount so as to lower the frequency band of the output OFDM signal. On the other hand, when the frequency of the OFDM signal becomes low, the level ΔV of the peak value frequency signal of the symbol S0 also increases in the negative direction, so that the voltage level of the control signal given to the frequency converter 10 increases in the negative direction. At this time, the frequency converter 10 reduces the frequency shift amount so as to raise the frequency band of the output OFDM signal. As a result, the frequency converter 10 can correct the fluctuation of the frequency band of the OFDM signal to an appropriate intermediate frequency band (center frequency fc).

As described above, according to the fourth embodiment of FIG. 9, the reference timing generator 62 periodically detects a specific symbol S0, and the peak value frequency signal at this symbol S0 is sample-held as a control signal. Since the feedback is fed back to the control terminal 10c of the frequency converter 10, the accuracy of the frequency shift amount control of the frequency converter 10 can be improved. Further, since the fluctuation of the frequency band is corrected by the frequency shift amount control, the deviation from the intermediate frequency band is eliminated, and erroneous determination of the demodulated data can be prevented.

In the second to fourth embodiments, the symbols S0 are inserted at intervals of 15 symbols, but they may be inserted at intervals of other symbols. Further, in the second to fourth embodiments, each symbol S0 is constructed by amplitude-modulating only one carrier with a binary pseudo-random code and suppressing the other carriers. As long as the time axis component and the frequency axis component are known and the amplitude and phase changes along the time axis indicate a predetermined specific pattern, the signal may be configured by another method. For example, it may be composed of a signal (see FIG. 1) in which only one carrier is used as an unmodulated single tone signal and the other carriers are suppressed.

Further, in the second to fourth embodiments, the OFDM signal of the intermediate frequency band output from the frequency converter 10 is input to the envelope detector 61 (in the second embodiment, the frequency discriminator 68). However, if the frequency converter is 10 or later, the output of either the orthogonal detector 3 or the Fourier converter 4 may be input to the envelope detector 61 (and the frequency discriminator 68).

Further, in the third and fourth embodiments, the outputs of the Fourier transformer 4 are input to the frequency domain energy detector 71 and the correlation detector 81, respectively, but if the frequency converter 10 or later is used, the output is input to the frequency domain energy detector 71 and the correlation detector 81, respectively. , The output of any one of the frequency converter 10 and the orthogonal detector 3 may be input to the frequency domain energy detector 71 and the correlation detector 81.

Further, the first embodiment is configured to correct the fluctuation of the reception level, and the second to fourth embodiments are configured to correct the fluctuation of the frequency band, but the second to fourth embodiments are configured. By combining any of the embodiments with the first embodiment, a receiving circuit may be configured that can correct both the fluctuation of the reception level and the fluctuation of the frequency band.

The transmission method and the reception method according to the present invention are useful for digital audio broadcasting, terrestrial digital television broadcasting, and the like, which are required not to cause erroneous determination of demodulated data.

<figref num="1">In the present invention, a diagram showing an example of the configuration of an OFDM signal transmitted from the transmitting side.</figref><figref num="2">A block diagram showing a configuration of a receiving device according to the first embodiment of the present invention.</figref><figref num="3">Waveform diagram showing signals of each part of the receiving device shown in FIG.</figref><figref num="4">In the present invention, a diagram showing another example of the configuration of the OFDM signal transmitted from the transmitting side.</figref><figref num="5">A block diagram showing a configuration of a receiving device according to a second embodiment of the present invention.</figref><figref num="6">Waveform diagram showing signals of each part of the receiving device shown in FIG.</figref><figref num="7">A block diagram showing a configuration of a receiving device according to a third embodiment of the present invention.</figref><figref num="8">Waveform diagram for explaining the operation of the frequency domain energy detector 71 shown in FIG.</figref><figref num="9">A block diagram showing a configuration of a receiving device according to a fourth embodiment of the present invention.</figref><figref num="10">Waveform diagram showing signals of each part of the control signal output device 80 shown in FIG.</figref><figref num="11">The figure which shows the structure of the conventional OFDM signal transmitted from the transmitting side.</figref><figref num="12">A block diagram showing a configuration of an OFDM signal receiver inferred from the first and third prior arts.</figref>

Code description

1 Band-pass filter 2 Automatic gain control amplifier 3 Orthogonal detector 31 Demultiplexer 32,33 Detector 34 Carrier regenerator 4 Fourier converter 5 Demodulation data detector 6 Control signal output device 61 Envelope detector 62 Reference timing generator 63 Symbol Timing Sync Circuit 64 Symbol Energy Detector 65 Control Signal Switch 66 Sample Holder 67 Low Pass Filter 7,8 A / D Converter 10 Frequency Converter 60 Control Signal Output 68 Frequency Discriminator 70 Control Signal Output 71 Frequency Region Energy detector 80 Control signal output device 81 Correlation detector 82 Peak value Frequency detector S OFDM signal S0, Sm Symbol ts Symbol time