JP2009141818A - Ofdm demodulation apparatus, ofdm demodulation method, ofdm demodulation program, and recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an OFDM (Orthogonal Frequency Division Multiplex) demodulation apparatus which prevents a reception rate from being reduced due to frame re-synchronization that is not required. <P>SOLUTION: In the OFDM demodulation apparatus 100, a signal quality discrimination section 152 discriminates a signal quality of a waveform-equalized data signal on the basis of a modulation error ratio calculated by a modulation error ratio calculation section 151. A synchronization section 153e then determines whether or not frame synchronization is established based on a determination result of the signal quality on the basis of result of the signal quality discrimination unit 152, so that useless frame re-synchronization is prevented while a sufficient signal quality of an output signal is obtained. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an OFDM demodulator, an OFDM demodulation method, and an OFDM demodulation program for demodulating an OFDM signal modulated by orthogonal frequency division multiplex (hereinafter, abbreviated as OFDM). The present invention also relates to a computer-readable recording medium on which such an OFDM demodulation program is recorded.

  As a modulation scheme suitable for overcoming ghost interference (fading, multipath) by buildings, OFDM (Orthogonal Frequency Division Multiplex, hereinafter abbreviated as OFDM) modulation is known. OFMD modulation is a modulation method capable of efficiently transmitting video signals and audio signals by N (about 256 to 1024) subcarriers provided in one channel band. Currently, terrestrial digital broadcasting and wireless Widely used for LAN and the like.

  In OFDM modulation, several transmission bits are collectively mapped onto a complex signal (QPSK, 16QAM, or 64QAM), and N complex signals are subjected to inverse fast Fourier transform (IFFT), A baseband (BB) OFDM signal is obtained.

  The OFDM signal obtained by OFMD modulation will be described in more detail with reference to FIG.

  The OFDM signal is transmitted in units of transmission frames composed of a plurality (about 100) of transmission symbols that are sequentially transmitted. Non-Patent Document 1 defines the number of transmission symbols per transmission frame as 204. In addition to transmission symbols for information transmission, transmission symbols for frame synchronization and service identification can be included in the transmission frame.

  FIG. 7 is a diagram showing a structure of transmission symbols constituting each transmission frame. As shown in FIG. 7, the transmission symbol 500 normally includes a guard interval (GI) 520 obtained by copying the signal waveform of the terminal end 511 of the effective symbol 510 to the effective symbol 510 corresponding to the inverse fast Fourier transform processing window. It is configured by adding. Therefore, the transmission symbol period length Ts is the sum of the effective symbol period length Tu corresponding to N cycles of the Fs clock and the guard interval period length Tg.

  In Non-Patent Document 1, the effective symbol period length Tu is defined as shown in Table 1 according to a parameter called “mode”.

  Further, in Non-Patent Document 1, the guard interval is set so that the ratio (GI ratio g) of the guard interval period Tg to the effective symbol period length Tu is one of 1/4, 1/8, 1/16, and 1/32. The period Tg is defined as shown in Table 2 according to the mode.

  Each transmission symbol included in the OFDM signal is divided into a plurality of segments, and each segment includes a subcarrier group shown in Table 3.

  In Table 3, a data carrier is a carrier for transmitting a data signal. The SP carrier is a carrier for transmitting an SP (Scattered Pilot) signal used as a reference for waveform equalization. The SP carrier is periodically inserted into each transmission frame. This period is determined in advance. Specifically, one SP carrier is arranged for every 12 carriers in the carrier direction and one for every 4 symbols in the symbol direction. The TMCC carrier is a carrier for transmitting a TMCC (Transmission and Multiplexing Configuration Control) signal including a synchronization word and transmission parameters. AC1 is a carrier for transmitting an AC1 (Auxiliary Channel) signal for transmitting additional information. Unlike the SP carrier, the TMCC carrier and the AC1 carrier are aperiodically inserted in each transmission frame.

  Next, an OFDM demodulator that demodulates the OFDM signal as described above will be described with reference to FIGS.

  FIG. 8 is a block diagram showing a typical configuration of a conventional OFDM demodulator 600 (see Non-Patent Document 1 and Patent Document 1).

  Generally speaking, the OFDM demodulator 600 includes an antenna 101, a tuner 102, and an OFDM demodulation LSI (Large Scale Integrated circuit) 103. The OFDM demodulation LSI 103 includes a baseband signal processing unit (BB) 110 and an error correction processing unit (FEC) 120. The baseband signal processing unit 110 includes an analog-digital conversion unit (ADC) 111, an orthogonal detection unit 112, a narrowband carrier frequency error correction unit 113, a fast Fourier transform calculation unit (FFT) 114, and a waveform equalization unit 115. A symbol synchronization unit 116, an automatic gain control unit (AGC) 117, a broadband carrier frequency error correction unit 118, and a TMCC decoding unit 119.

  The antenna 101 receives a broadcast wave of a digital broadcast broadcast from a broadcast station, and supplies an RF (high frequency) signal to the tuner 102. The tuner 102 converts the frequency of the RF signal and supplies the obtained IF (intermediate frequency) signal to the ADC 111 provided in the baseband signal processing unit 110.

  The analog-digital conversion unit 111 digitizes the IF signal and supplies the digitized IF signal to the quadrature detection unit 112. The quadrature detection unit 112 performs quadrature detection on the digitized IF signal using a carrier signal having a set carrier frequency, thereby obtaining a real axis component (I channel signal) and an imaginary axis component (Q channel signal). A baseband OFDM signal is obtained. The obtained baseband OFDM signal is supplied to the narrow global carrier frequency error correction unit 113.

  The narrowband carrier frequency error correction unit 113 corrects the narrowband carrier frequency error in the baseband OFDM signal, and converts the corrected baseband OFDM signal into a fast Fourier transform unit 114, a symbol synchronization unit 116, and an automatic gain control unit. 117.

  The symbol synchronization unit 116 extracts transmission parameters such as a transmission mode and a guard interval ratio from the corrected baseband OFDM signal, and identifies the head timing of each effective symbol. The automatic gain control unit 117 optimizes the intensity of the IF signal input to the OFDM demodulation LSI 103 by adjusting the gain of the tuner 102.

  The fast Fourier transform unit 114 performs fast Fourier transform (FFT operation) on the baseband OFDM signal. More specifically, by referring to the start timing of each effective symbol specified by the symbol synchronization unit 116, a signal corresponding to the effective symbol period is extracted from the baseband OFDM signal, and the extracted signal is processed at high speed. Perform Fourier transform (FFT). Here, the start point of the signal extracted as the target of the fast Fourier transform may be an arbitrary point within the guard interval period.

  The fast Fourier transform unit 114 obtains N complex signals modulated on each carrier, including data signals, SP signals, TMCC signals, and the like. The obtained complex signal is supplied to the waveform equalization unit 115, the broadband carrier frequency error correction unit 118, and the TMCC decoding unit 119.

  The broadband carrier frequency error correction unit 118 detects and corrects the broadband carrier frequency error. Of the demodulated signal obtained by the fast Fourier transform unit 114, only the complex signal modulated by the carrier in the specific band determined based on the detected wide band carrier frequency error is supplied to the waveform equalization unit 115. It has become so.

  The TMCC decoding unit 119 specifies the start timing of each transmission frame by performing DBPSK demodulation on the TMCC signal obtained by the fast Fourier transform unit 114 and detecting a synchronization word included in the demodulation result. Further, error correction decoding, specifically, differential set cyclic decoding is performed on the TMCC signal to obtain TMCC information defining transmission parameters and the like. The TMCC decoding unit 119 sends a synchronization establishment signal indicating the start timing of each transmission frame to the waveform equalization unit 115 and the error correction processing unit 120. Also, the TMCC information obtained by error correction decoding is supplied to the error correction processing unit 120.

  The waveform equalization unit 115 performs waveform equalization on the data signal obtained by the fast Fourier transform unit 114. Specifically, based on the expected value of the SP signal group obtained by the fast Fourier transform unit 114, the transfer function of the communication channel (corresponding to amplitude fluctuation / phase rotation generated in the communication channel) is estimated. Then, a waveform-equalized data signal is obtained by complex division of each of the data signal groups by the estimated transfer function. The data signal group subjected to waveform equalization by the waveform equalization unit 115 is supplied to the error correction processing unit 120.

  FIG. 9 is an explanatory diagram for explaining waveform equalization performed by the waveform equalization unit 115. 9A shows the constellation on the transmission side of the SP signal 601 and the data signal 602, and FIG. 9B shows the constellation on the reception side of the same SP signal 601 and data signal 602. c) shows a constellation after waveform equalization of the same SP signal 601 and data signal 602.

  When the SP signal 601 and the data signal 602 shown in FIG. 9A pass through the communication path, they are distorted as shown in FIG. 9B due to fading. Here, a distortion in which only the phase rotates is illustrated, but a distortion in which the amplitude changes or a distortion in which both the amplitude and the phase change can naturally occur. The constellation of the SP signal 601 shown in FIG. 9A is predetermined, and the waveform equalization unit 115 converts the constellation on the reception side of the SP signal 601 shown in FIG. The transfer function of the communication path can be estimated. The waveform equalization unit 115 corrects the constellation of the data signal 602 by complex-dividing the data signal 602 by the estimated transfer function. Thereby, as shown in FIG. 9, the constellation on the transmission side of the data signal 602 is restored. That is, the influence of fading can be removed from the data signal 602 by waveform equalization.

  The error correction processing unit 120 illustrated in FIG. 8 refers to the frame head timing specified by the TMCC decoding unit 119 and the TMCC information extracted by the TMCC decoding unit 119, and performs an error on the waveform-equalized data signal. Perform corrective decoding.

  A configuration example of the error correction processing unit 120 is shown in FIG. As shown in the figure, the error correction processing unit 120 includes a frequency deinterleaving unit 121, a time deinterleaving unit 122, a demapping unit 123, a bit deinterleaving unit 124, a depuncturing unit 125, and a Viterbi decoding unit. 126, a byte deinterleave unit 127, an energy despreading unit 128, a TSP generation unit 129, and a Reed-Solomon (RS) correction unit 130.

  The frequency deinterleaver 121 cancels frequency interleaving in the waveform-equalized data signal. The time deinterleaving unit 122 cancels the time interleaving. The demapping unit 123 restores the transmission bit string mapped to the complex signal by any one of QPSK, 16QAM, or 64QAM. The bit deinterleave unit 124 cancels bit interleaving in the restored transmission bit string.

  The error correction processing unit 120 performs error correction decoding using an error correction code included in the restored transmission bit string. When the inner code of the error correction code included in the transmission bit string is a punctured convolutional code, the depuncture unit 125 performs the depuncture process. Also, the Viterbi decoding unit 126 performs decoding and error correction of the encoded data convolutionally encoded as an inner code on the transmission side.

  The byte deinterleaver 127 releases the byte interleave. The energy despreading unit 128 performs an energy despreading process. The TSP generation unit 129 generates a transport stream packet (TSP: Transport Stream Packet) having a predetermined size (for example, 204 bytes) from the output signal of the energy despreading unit 128. The Reed-Solomon correction unit 130 performs decoding and error correction of Reed-Solomon encoded data as an outer code on the transmission side.

  As described above, the demodulation process in the OFDM modulation apparatus 100 is performed with reference to the SP signal and the TMCC signal inserted in each transmission frame. Therefore, when the head timing of the transmission frame cannot be specified, It is necessary to reestablish frame synchronization by resetting the entire demodulation LSI 103 or the like. Therefore, when the TMCC signal decoding unit 119 can detect the synchronization word, the TMCC signal decoding unit 119 sends a synchronization establishment signal indicating that synchronization is correctly established to a control unit (not shown) that controls the entire OFDM demodulation LSI 103. It is configured to send out. The control unit determines whether or not frame synchronization is correctly established based on the synchronization establishment signal, and resets the entire OFDM demodulation LSI 103 if it is determined that frame synchronization is lost.

  A specific configuration of the TMCC decoding unit 119 that generates such a synchronization establishment signal is disclosed in Patent Document 2, for example. FIG. 11 is a block diagram illustrating a configuration of the TMCC decoding unit 119 disclosed in Patent Document 2.

  According to Patent Document 2, as shown in FIG. 11, the TMCC decoding unit 119 includes a DBPSK demodulation unit 119a, a majority processing unit 119b, a synchronization word detection unit 119c, an error correction decoding unit 119d, a synchronization establishment / protection unit 119e, A TMCC signal acquisition unit 119f and a timing adjustment unit 119g are included.

  The DBPSK demodulator 119a performs DBPSK demodulation on the TMCC signal. The majority processing unit 119b performs majority processing on the demodulation result. The synchronization word detection unit 119c detects a synchronization word from the demodulated result subjected to the majority process. The error correction decoding unit 119d counts the number of errors in the majority-processed TMCC signal, and performs error correction decoding, specifically, differential set cyclic decoding if the number of errors is equal to or less than a predetermined number of bits.

The synchronization establishment / protection unit 119e determines that the frame synchronization has been established when the majority demodulated demodulation result sequence matches the synchronization word and error correction decoding has been performed on the TMCC signal. Is output. For this reason, it is possible to make a more accurate determination than when determining whether or not frame synchronization is established based only on the synchronization word.
JP 2002-261729 A (published on September 13, 2002) Japanese Patent Laying-Open No. 2005-278111 (released on October 6, 2005) "Transmission method for digital terrestrial television broadcasting ARIB STD-B31 1.5 edition", Radio Industry, first edition of May 31, 2001, revised version 1.5 on July 29, 2003

  However, in the conventional OFDM demodulator, frame resynchronization (reestablishment of frame synchronization) often occurs in a fading environment where DBPSK demodulation errors frequently occur, and the signal quality of the transport stream packet finally obtained is However, there was a problem of deterioration. Hereinafter, this problem will be described in a little more detail.

  In wireless communication using the atmosphere as a transmission medium, transmission path characteristics generally vary depending on weather conditions and geographical conditions. In particular, when the transmitting station and the receiving station move during communication, the change in transmission path characteristics becomes more severe due to the Doppler effect. Such a phenomenon is called “fading”, and causes deterioration in transmission quality due to fluctuations in the amplitude and phase of the received wave.

  The magnitude of amplitude fluctuation and phase fluctuation due to fading is determined by the maximum Doppler frequency, delay profile, and the like. Here, the maximum Doppler frequency is a Doppler frequency when the moving direction of the receiving station coincides with the traveling direction of the received wave. The delay profile is an impulse response of a propagation path or a communication path due to various fading. For details on such fading, see, for example, “Introduction to Digital Wireless Communication” (Fumio Takahata, Bafukan, 2002).

  In particular, DBPSK demodulation in OFMD demodulation is a demodulation method that generates a demodulated signal having a value of “1” or “0” depending on whether or not the phase difference between the previous symbol and the current symbol is π. Accordingly, DBPSK demodulation errors frequently occur in a fading environment where phase fluctuations frequently approach π. For this reason, the probability that the synchronization word is successfully detected from the TMCC signal and the probability that the error correction decoding of the TMCC signal is successful are reduced. Therefore, if frame resynchronization is performed based on the synchronization establishment signal generated according to Patent Document 2, the frequency of frame resynchronization increases in a fading environment.

  By the way, the data signal obtained by OFDM demodulation is waveform-equalized based on the SP signal. Therefore, even in a fading environment in which DBPSK demodulation errors occur frequently and error correction for the TMCC signal does not work, the waveform-equalized data signal has a signal quality sufficient to correctly restore the transmission bit string by error correction. There may be.

  In particular, when a transmission bit string is encoded using a concatenated code with a convolutional code as an inner code and a Reed-Solomon code as an outer code, the transmission bit string is restored on the receiving side by error correction. It is highly possible. This is because the error correction capability of the concatenated code is stronger than the error correction capability of the difference set cyclic code used in the TMCC signal. For the error correction capability of the concatenated code, refer to “Error correction code and its application-Introduction series of advanced technology” (edited by Eto and Kaneko, Ohmsha, 1996).

In fact, even in a fading environment where DBPSK demodulation errors occur frequently and error correction for TMCC signals does not work, the bit error rate of the finally obtained signal is 10 ⁻¹¹ or less (referred to as a pseudo error free state). Can happen. In such a state, interrupting OFDM modulation and performing frame resynchronization does not contribute to an increase in the reception rate, but instead causes a decrease in the reception rate.

  The present invention has been made in view of the above problems, and an object of the present invention is to realize an OFDM demodulator that does not reduce the reception rate due to frame resynchronization that is not essential.

  In order to solve the above problems, an OFDM demodulator according to the present invention is an OFDM demodulator that obtains a control signal, a pilot signal, and a data signal by demodulating an OFDM signal for each transmission frame. And a means for equalizing the waveform of the data signal, a decoding means for performing error correction decoding of the data signal after waveform equalization, and a frame according to the signal quality of the data signal after waveform equalization. Synchronization determining means for determining whether or not synchronization is established.

  The OFDM demodulator can correctly restore a transmission bit string from a data signal even in a fading environment by waveform equalization by the equalization means and error correction decoding by the decoding means. Since the OFDM demodulator performs frame synchronization determination based on the signal quality of the data signal after waveform equalization, it performs waveform equalization like a conventional OFDM demodulator that performs frame synchronization determination based on the TMCC signal. Even though the later data signal has a signal quality sufficient to correctly restore the transmission bit string, it is not determined that frame synchronization is not established. Therefore, if frame resynchronization is performed based on the determination result by the synchronization determination means, the OFMD signal can be demodulated without interruption, and the reception rate is reduced due to unnecessary frame resynchronization. Can be prevented.

  It should be noted that the synchronization determination means only needs to determine whether or not frame synchronization is established according to the signal quality of the data signal after waveform equalization, and is not necessarily limited to the data signal after waveform equalization. It is not necessary to make a determination based only on the signal quality. For example, in addition to the signal quality of the data signal after waveform equalization, it may be determined whether frame synchronization is established by referring to a synchronization word included in the control signal. More specifically, it is determined that frame synchronization is established for the frame in which the synchronization word is detected, and the signal quality of the data signal after waveform equalization is good for the frame in which the synchronization word is not detected. For example, it may be determined that frame synchronization has been established, and otherwise, it may be determined that frame synchronization has not been established.

  The OFDM demodulator according to the present invention further includes a modulation error ratio calculation unit that calculates a modulation error ratio based on the data signal after waveform equalization, and the synchronization determination unit includes the calculated modulation error ratio. It is preferable to determine that frame synchronization is established when is equal to or greater than a predetermined threshold.

  According to the above configuration, it is possible to appropriately perform the frame synchronization determination based on the signal quality of the data signal after waveform equalization evaluated using the modulation error ratio as an index.

  The OFDM demodulator according to the present invention further comprises bit error rate calculation means for calculating a bit error rate of the data signal after waveform equalization, and the synchronization determination means is configured so that the calculated bit error rate is calculated in advance. It is preferable to determine that frame synchronization has been established when the threshold value is equal to or less than a predetermined threshold value.

  According to the above configuration, the frame synchronization determination can be appropriately performed based on the signal quality of the data signal after waveform equalization evaluated using the bit error rate as an index.

  In the OFDM demodulator according to the present invention, the decoding means includes Viterbi decoding means for Viterbi decoding the data signal after waveform equalization, and the bit error rate calculating means convolves the output of the Viterbi decoding means It is preferable to calculate the bit error rate by encoding and comparing the data signal after waveform equalization and the encoded data re-encoded by the convolutional encoding.

  According to the above configuration, the frame synchronization determination is appropriately performed based on the signal quality of the data signal after waveform equalization, which is a data signal after waveform equalization and evaluated using the bit error rate before Viterbi decoding as an index. It can be carried out.

  In the OFDM demodulator according to the present invention, the decoding means includes Reed-Solomon decoding means for performing Reed-Solomon decoding on the data signal after waveform equalization, and the bit error rate calculating means includes the Reed-Solomon decoding means. The bit error rate is preferably calculated by integrating the calculated number of errors.

  According to the above configuration, the frame synchronization determination is appropriately performed based on the signal quality of the data signal after waveform equalization, which is the data signal after waveform equalization and is evaluated using the bit error rate after Reed-Solomon decoding as an index. Can be done.

  In particular, when the OFDM demodulator is configured in accordance with the broadcast standard defined in Non-Patent Document 1, the Reed-Solomon decoding is the last error correction process for the data signal after waveform equalization. That is, the bit error rate in the data signal after Reed-Solomon decoding is the bit error rate of the output signal of the OFDM demodulator. Therefore, according to the above configuration, frame synchronization determination can be appropriately performed based on the signal quality of the data signal after waveform equalization, which is evaluated using the bit error rate of the output signal as an index.

  In the OFDM demodulator according to the present invention, the decoding means includes Reed-Solomon decoding means for performing Reed-Solomon decoding on the data signal after waveform equalization, and the number of errors calculated by the Reed-Solomon decoding means as the Reed-Solomon decoding. Means for determining the effectiveness of Reed-Solomon decoding by comparing with the maximum number of errors that can be corrected by the means, and the synchronization determining means determines the effectiveness of the determined Reed-Solomon decoding. Based on this, it is preferable to determine whether or not frame synchronization is established.

  According to the above configuration, it is possible to appropriately perform the frame synchronization determination based on the signal quality of the data signal after waveform equalization evaluated using the effectiveness of Reed-Solomon decoding as an index. Reed-Solomon decoding is linear decoding that detects and corrects errors by calculating a syndrome. When the number of errors is within the correction capability range, the effectiveness of the output signal is guaranteed 100%.

  In particular, when the OFDM demodulator is configured in accordance with the broadcast standard defined in Non-Patent Document 1, the Reed-Solomon decoding is the last error correction process for the data signal after waveform equalization. That is, the effectiveness of Reed-Solomon decoding is the effectiveness of the output signal of the OFDM demodulator. Therefore, according to the above configuration, frame synchronization determination can be appropriately performed based on the signal quality of the data signal after waveform equalization evaluated using the effectiveness of the output signal as an index.

  In the OFDM demodulator according to the present invention, it is preferable that the synchronization determination unit determines that frame synchronization is established when the data signal after the waveform or the like is a signal that can be corrected by the decoding unit.

  According to the above configuration, when frame resynchronization is performed based on the determination result by the synchronization determination unit, frame resynchronization is performed even though the data signal after the waveform or the like is a signal that can be corrected by the decoding unit. It is not performed, and therefore the reception rate is not reduced by frame resynchronization.

  In the OFDM demodulator according to the present invention, it is preferable that the synchronization determination unit determines that frame synchronization is not established when the data signal after the waveform or the like is a signal that cannot be corrected by the decoding unit.

  According to the above configuration, when frame resynchronization is performed based on the determination result by the synchronization determination means, when the data signal after the waveform or the like is a signal that cannot be corrected by the decoding means, the reception rate is improved by frame resynchronization. May improve.

  The OFDM demodulator according to the present invention controls to control itself so that frame synchronization is reestablished after a predetermined time has elapsed when it is determined by the synchronization determination means that frame synchronization has not been established. Preferably means are provided.

  In the OFDM demodulator according to the present invention, when the synchronization determination means continuously determines that frame synchronization has not been established for a predetermined number of times or more, control for controlling the own apparatus to stop the demodulation processing Preferably means are provided.

  In order to solve the above problems, an OFDM demodulation method according to the present invention is an OFDM demodulation method for obtaining a pilot signal and a data signal by demodulating an OFDM signal for each transmission frame, and refers to the pilot signal. Frame synchronization is established according to the signal equalization process for waveform equalization of the data signal, the decoding process for error correction decoding of the data signal after waveform equalization, and the signal quality of the data signal after waveform equalization And a synchronization determination step for determining whether or not it has been performed.

  The OFDM demodulation method can correctly restore a transmission bit string from a data signal even in a fading environment by performing waveform equalization in the equalization step and error correction decoding in the decoding step. In the OFDM demodulation method, since the frame synchronization determination is performed based on the signal quality of the data signal after waveform equalization, the waveform etc., as in the conventional OFDM demodulation method in which the frame synchronization determination is performed based on the TMCC signal. Although the converted data signal has signal quality sufficient to correctly restore the transmission bit string, it is not determined that frame synchronization is not established. Therefore, if frame resynchronization is performed based on the determination result obtained in the synchronization determination step, the OFMD signal can be demodulated without interruption, and the reception rate due to frame resynchronization that is not necessary originally can be reduced. A decrease can be prevented.

  The OFDM demodulator may be realized by a computer. In this case, an OFDM demodulation program for realizing the OFDM demodulator in the computer by operating the computer as each of the above means and a computer-readable recording medium recording the program also fall within the scope of the present invention.

  Since the OFDM demodulator according to the present invention is configured to perform frame synchronization determination based on the signal quality of the data signal after waveform equalization, the data signal after waveform equalization correctly restores the transmission bit string. Despite having sufficient signal quality, it is not determined that frame synchronization has not been established. Therefore, if frame resynchronization is performed based on the determination result by the synchronization determination means, the OFMD signal can be demodulated without interruption, and the reception rate is reduced due to unnecessary frame resynchronization. Can be prevented.

(Embodiment 1)
The OFDM demodulator 100 according to the first embodiment of the present invention will be described below with reference to FIG. Similar to the OFDM demodulator 600 shown in FIG. 8, the OFDM demodulator 100 roughly includes an antenna 101, a tuner 102, and an OFDM demodulator LSI (Large Scale Integrated Circuit) 103. FIG. 1 is a block diagram showing the configuration of the OFDM demodulation LSI 103.

  The OFDM demodulation LSI of the OFDM demodulator 100 includes a baseband signal processing unit (BB) 110 and an error correction processing unit (FEC) 120, as shown in FIG. The baseband signal processing unit 110 includes an analog / digital conversion unit (ADC) 111, an orthogonal detection unit 112, a narrowband carrier frequency error correction unit 113, a fast Fourier transform calculation unit (FFT) 114, and a waveform equalization. Part 115.

  Each of the blocks included in the demodulation LSI of the OFDM demodulator 100 shown in FIG. 1 has the same function as the corresponding block of the OFDM demodulator 600 shown in FIG. The description is omitted by attaching.

  The OFDM demodulator 100 according to the present embodiment includes a modulation error ratio calculation unit 151, a signal quality determination unit 152, and a TMCC decoding unit 153 as its characteristic configuration.

  The modulation error ratio calculation unit 151 is a means for calculating a modulation error ratio (MER) based on the data signal waveform-equalized by the waveform equalization unit 115.

The modulation error ratio MER is the power conversion value of the waveform-equalized data signal X _EQ (n, k) and the ideal constellation point X _EQ ′ (n, k) of the complex signal X _EQ (n, k). Is the ratio of the deviation (vector error) δX _EQ (n, k) from the power conversion value. That is, the modulation error ratio MER for the data signal X _EQ (n, k) is defined by the following equations (1) to (4).

In these equations, n is a subscript representing a symbol number, and k is a subscript representing a carrier number. Further, the sum in equation (1) is the sum of the subscripts n and k. In order to obtain a modulation error ratio MER with higher accuracy, the sum of more n and k may be calculated. If real-time performance is required at the expense of accuracy, the sum may be calculated only for all carriers in one symbol or for a specific carrier. The ideal constellation point X _EQ ′ (n, k) for the complex signal X _EQ (n, k) is all possible constellation points Xm (determined according to the narrowband modulation method) (0 ≦ m < of M), the distance dm (n, k and _{X EQ (n, k))} = | X EQ (n, k) -Xm | is that of constellation points a minimum.

  The modulation error ratio MER calculated by the modulation error ratio calculation unit 151 corresponds to a carrier noise power ratio (CNR) and can be used as an index indicating the signal quality of the waveform-equalized data signal. . The modulation error ratio MER calculated by the modulation error ratio calculation unit 151 is supplied to the signal quality determination unit 152.

  The signal quality determination unit 152 determines the signal quality of the data signal after waveform equalization by comparing the modulation error ratio MER calculated by the modulation error ratio calculation unit 151 with a preset threshold value Th1. The determination result is notified to a synchronization processing unit 153e described later. The determination of the signal quality is realized, for example, by determining that the signal quality is good when the modulation error ratio MER is equal to or greater than the threshold value Th1, and otherwise determining that the signal quality is not good. The threshold Th1 can be stored in a register (not shown) provided in the signal quality determination unit 152, for example.

  The TMCC decoding unit 153 includes a DBPSK demodulation unit 153a, a majority processing unit 153b, a synchronization word detection unit 153c, an error correction decoding unit 153d, a synchronization processing unit 153e, a TMCC signal acquisition unit 153f, a timing adjustment unit 153g, It has.

  The DBPSK demodulator 153a demodulates the TMCC signal obtained by the fast Fourier transform unit 114. The majority processing unit 153b performs majority processing on the demodulation result obtained by the DBPSK demodulation unit 153a. The synchronization word detection unit 153c detects the synchronization word from the demodulated result subjected to the majority process, and notifies the synchronization processing unit 153e of the detection result. The detection of the synchronization word is realized, for example, by calculating the correlation between the demodulation result obtained from the majority processing unit 153b and the synchronization word (known symbol sequence). In this case, the timing at which the calculated correlation value becomes maximum is the timing at which the synchronization word is detected.

  The error correction decoding unit 153d performs error correction decoding, specifically, differential set cyclic decoding, on the demodulation result obtained by the DBPSK demodulation unit 153a. Then, the error-corrected demodulation result is supplied to the TMCC signal acquisition unit 37. The TMCC signal acquisition unit 153f extracts TMCC information such as transmission parameters from the error-corrected demodulation result, and supplies the extracted TMCC information to the error correction processing unit 120.

  The synchronization processing unit 153e determines whether or not frame synchronization is correctly established based on the detection result notified from the synchronization word detection unit 153c and the determination result notified from the signal quality determination unit 152, and Based on the determination result, a synchronization establishment signal and a synchronization signal are output. Here, the synchronization establishment signal is a signal indicating that frame synchronization has been established, and is supplied to a control unit (not shown) via the timing processing unit 153g. On the other hand, the synchronization signal is a signal indicating the frame head timing, and is supplied to the correction processing unit 120 via the timing processing unit 153g.

  When the synchronization establishment signal supplied from the synchronization processing unit 153e is interrupted, the control unit (not shown) resets the entire OFDM demodulation LSI 103 so as to reestablish frame synchronization. For example, after resetting the demodulation process to the OFDM modulation LSI 103, the reset is performed by the OFDM modulation LSI 103 after initializing various parameters used by the OFDM modulation LSI 103 to execute the demodulation process. Is realized.

  The algorithm by which the synchronization processing unit 153e determines whether or not frame synchronization is correctly established is as follows. That is, (1) for a frame in which a synchronization word is detected, it is determined that frame synchronization is correctly established regardless of whether the signal quality of the data signal after the waveform or the like is good. On the other hand, for (2) a frame in which no synchronization word is detected, (2-1) if the signal quality of the data signal after the waveform or the like is good, it is determined that frame synchronization is correctly established, and (2 -2) If the signal quality of the data signal after waveform equalization is not good, it is determined that frame synchronization has not been established correctly.

  By referring to the synchronization establishment signal generated based on such determination, the control unit (not shown) can generate a waveform or the like even in a fading environment in which DBPSK demodulation errors frequently occur and a synchronization word cannot be detected. If the signal quality of the subsequent data signal is good, the frame synchronization can be protected without resetting the OFDM demodulation LSI 103.

  For the frame in which the synchronization word is detected, if the signal quality of the data signal after (1-1) waveform or the like is good, it is determined that the frame synchronization is correctly established, and (1-2) waveform If the signal quality of the subsequent data signal is not good, it may be determined that the frame synchronization is not correctly established. That is, frame synchronization determination may be performed based only on the signal quality of the data signal after waveform equalization regardless of whether or not a synchronization word is detected. Also in this case, if the signal quality of the waveform-equalized data signal is good, the control unit (not shown) can protect the frame synchronization without resetting the OFDM demodulation LSI 103.

  The synchronization processing unit 153e periodically outputs a synchronization signal while the frame synchronization is maintained. That is, if the synchronization processing unit 153e determines that frame synchronization is correctly established for a certain frame, the synchronization processing unit 153e outputs the first synchronization signal at the start timing of the next frame of the frame, and thereafter frame synchronization is correctly established. The synchronization signal is continuously output until it is determined that there is no such signal.

  There is a discrepancy between the timing at which the synchronization processing unit 153e sends the synchronization signal and the timing at which the waveform equalization unit 14 sends the first symbol of each transmission frame. This is because the time required for the TMCC decoding unit 153 to output the synchronization signal is different from the time required for the waveform equalizing unit 115 to equalize the waveform. The timing adjustment unit 153g delays the synchronization signal so as to eliminate this timing shift. The synchronization signal whose timing is adjusted by the timing adjusting unit 153g is referred to as a frame clock in the error correction processing unit 120.

  In the present embodiment, the configuration is shown in which the signal quality of the data signal after waveform equalization is determined using the modulation error ratio MER as an index. However, the present invention is not limited to this. It is also possible to change to a configuration for determining the signal quality of the data signal after conversion. Some such embodiments are shown below.

(Embodiment 2)
An OFDM demodulator 200 according to the second embodiment of the present invention will be described below with reference to FIGS.

  FIG. 2 is a block diagram showing the configuration of the OFDM demodulation LSI of the OFDM demodulator 200. As shown in FIG. Each block included in the OFDM demodulation LSI of the OFDM demodulator 200 shown in FIG. 2 has the same function as the corresponding block of the OFDM demodulator 100 shown in FIG. The description is omitted by attaching. Although not shown, the internal configuration of the error correction processing unit 120 is the same as that shown in FIG.

  The difference between the OFDM demodulator 200 shown in FIG. 2 and the OFDM demodulator 100 shown in FIG. 2 is that the waveform equalized data signal input to the Viterbi decoder 126 instead of the modulation error ratio calculator 151. Is provided with a bit error rate calculation unit 154 for calculating a bit error rate (BER) (hereinafter abbreviated as BER before Viterbi).

  In this embodiment, the signal quality determination unit 152 determines the signal quality of the waveform-equalized data signal by comparing the pre-Viterbi BER calculated by the bit error rate calculation unit 154 with a preset threshold Th2. To do. Specifically, when the pre-Viterbi BER is less than or equal to the threshold Th2, it is determined that the signal quality is good, and otherwise, it is determined that the signal quality is not good. This threshold value Th2 can be stored in a register (not shown) provided in the signal quality determination unit 152, for example.

  The synchronization processing unit 153e determines whether or not frame synchronization has been established based on the determination result notified from the synchronization word detection unit 153c and the determination result notified from the signal quality determination unit 152. When it is determined that it has been established, a point of sending a synchronization establishment signal to the control unit is the same as in the first embodiment.

  Convolutional code decoding differs from linear code in which an error is detected by calculating a syndrome, and plausible decoded data is generated by viterbi decoding, which is one method of estimation processing. Therefore, an error can be detected by comparing the encoded data before Viterbi decoding and the re-encoded data obtained by re-encoding the decoded data after Viterbi decoding.

  For example, as shown in FIG. 3, the bit error rate calculation unit 154 in this embodiment gives a delay corresponding to the latency of the Viterbi decoding process to the waveform-equalized data signal input to the Viterbi decoding unit 126. A convolutional encoder 154b that re-encodes the output of the Viterbi decoding unit 126, and a comparator that compares the encoded data delayed by the FIFO 154a with the re-encoded data re-encoded by the convolutional encoder 154b. 154c and a counter 154d that counts the comparison result of the comparator 154c. If the encoded data and the re-encoded data that are compared by the comparator 154c match, this means that error correction has not been performed by the Viterbi decoding unit 126. If they are different, an error is detected by the Viterbi decoding unit 126. Means that corrections have been made. The counter 154d counts the number of errors in the data signal after waveform equalization input to the Viterbi decoding unit 126 by counting the latter number of times. The bit error rate calculation unit 154 sends the number of errors stored in the counter 154d or the pre-Viterbi BER calculated from the number of errors to the signal quality determination unit 152.

(Embodiment 3)
An OFDM demodulator 300 according to the third embodiment of the present invention will be described below with reference to FIGS.

  FIG. 4 is a block diagram showing the configuration of the OFDM demodulation LSI of the OFDM demodulator 300. Each block included in the OFDM demodulation LSI of the OFDM demodulator 300 shown in FIG. 4 has the same function as the corresponding block of the OFDM demodulator 100 shown in FIG. The description is omitted by attaching. Although not shown, the internal configuration of the error correction processing unit 120 is the same as that shown in FIG.

  The OFDM demodulator 300 shown in FIG. 4 is different from the OFDM demodulator 100 shown in FIG. 1 in that the bit error rate (BER) in the output data of the Reed-Solomon corrector 130 is replaced with the modulation error ratio calculator 151. : Bit Error Rate) (hereinafter abbreviated as BER after RS).

  FIG. 5 is a block diagram showing an internal configuration of the Reed-Solomon correction unit 130 included in the error correction processing unit 120. As shown in FIG. 5, the Reed-Solomon correction unit 130 includes a syndrome calculation unit 131, an error count calculation unit 132, an error position calculation unit 133, and an error correction processing unit 134. The bit error rate in the output data of the Reed-Solomon correction unit 130 can be calculated from the number of errors calculated by the error number calculation unit 132.

  The Reed-Solomon correction unit 130 performs processing in units of bytes described in a Galois field, not in units of bits described in binary numbers. The syndrome calculation unit 131 calculates a syndrome. The error number calculation unit 132 calculates the number of errors from the syndrome. The error position calculation unit 133 calculates an error position based on the syndrome and the number of errors. The error correction processing unit 134 performs error correction on a byte basis based on the error position.

  For example, the RS correction unit 130 compliant with the standard of Non-Patent Document 1 performs error correction processing in units of transport stream packets (TSP: Transport Stream Packet, 204 bytes). In this case, the error number calculation unit 132 calculates the number of error bytes or the number of bits in 204 bytes (= 1632 bits).

  The bit error rate calculation unit 155 calculates the bit error rate by integrating the number of errors (number of bits or number of bytes) calculated by the error number calculation unit 132 over a plurality of transport streams. Thus, by integrating the number of error bytes or the number of bits over a predetermined number of transport streams, the accuracy of the bit error rate can be improved.

  The signal quality determination unit 152 determines the signal quality of the data signal after waveform equalization by comparing the post-RS BER calculated by the bit error rate calculation unit 155 with a preset threshold Th3. Specifically, when the post-RS BER is equal to or less than the threshold Th3, it is determined that the signal quality is good, and otherwise, it is determined that the signal quality is not good. This threshold value Th3 can be stored in a register (not shown) provided in the signal quality determination unit 152, for example.

  The synchronization processing unit 153e determines whether frame synchronization is established based on the determination result notified from the synchronization word detection unit 153c and the determination result notified from the signal quality determination unit 152, and frame synchronization is established. If it is determined that the synchronization is established, a synchronization establishment signal is transmitted to the control unit as in the first embodiment.

  Note that the process of calculating the post-RS BER, which is an indicator of signal quality in the present embodiment, includes a step of calculating a syndrome and a step of calculating the number of errors based on the calculated syndrome. Since all of them calculate a known amount by a known algorithm, a detailed explanation is omitted. Details of these are, for example, “Error Correction Codes and Their Applications-Advanced Technology Introduction Series” (edited by Kato Eto, edited by Ohmsha) and “Digital Transmission of Information” (Morikita Publishing REBlahut by Arimoto et al.) Please refer to.

(Embodiment 4)
An OFDM demodulator 400 according to the fourth embodiment of the present invention will be described below with reference to FIG.

  FIG. 6 is a block diagram showing a configuration of an OFDM demodulation LSI of the OFDM demodulator 400. Each block included in the OFDM demodulation LSI of the OFDM demodulator 400 shown in FIG. 6 has the same function as the corresponding block of the OFDM demodulator 100 shown in FIG. The description is omitted by attaching. Although not shown, the internal configuration of the error correction processing unit 120 is the same as that shown in FIG.

  The difference between the OFDM demodulator 400 shown in FIG. 6 and the OFDM demodulator 100 shown in FIG. 1 is calculated by the error number calculator 132 of the Reed-Solomon corrector 130 instead of the modulation error ratio calculator 151. An error correction information extraction unit 156 that determines the validity of the transport stream based on the number of errors is provided.

  For example, the Reed-Solomon correction unit 130 compliant with the standard of Non-Patent Document 1 employs a code (204,188) that can correct an error of up to 8 bytes per transport stream packet (TSP: Transport Stream Packet, 204 bytes). It has become the composition. Therefore, if the number of errors is 8 bytes or less, the transport stream packet is considered to be a valid packet, and otherwise, it is considered to be an invalid packet.

  When the number of errors detected by the error number calculation unit 132 is 8 bytes or less, the error correction information extraction unit 156 determines that the transport stream packet being processed is a valid packet. On the other hand, when the detected error number is 9 bytes or more, it is determined that the transport stream packet being processed is an invalid packet. Error correction information extraction section 156 then sends error correction information indicating the determination result to signal quality determination section 152.

  The signal quality determination unit 152 determines the signal quality of the waveform-equalized data signal based on the error correction information. For example, when the number of consecutive invalid packets exceeds the threshold Th4, it is determined that the signal quality is not good, and when the number of consecutive invalid packets is equal to or less than the threshold Th4, it is determined that the signal quality is good. The threshold Th4 can be stored in a register (not shown) provided in the signal quality determination unit 152, for example.

  The synchronization processing unit 153e determines whether or not frame synchronization is established based on the determination result notified from the synchronization word detection unit 153c and the determination result notified from the signal quality determination unit 152. When it is determined that the synchronization is established, a synchronization establishment signal is transmitted to the control unit as in the first embodiment.

(Threshold setting)
Here, the setting of the threshold value in each embodiment described above will be briefly described.

  One of the performance indexes of the OFDM demodulator is a so-called required CN. The required CN refers to the bit error rate of the output signal of the OFDM demodulator (that is, the output data of the Reed-Solomon correction unit 130) when additive white Gaussian noise (AWGN) is considered as a communication channel. This is a signal-to-noise ratio (CNR) that is 10-11 or less, that is, pseudo error free.

  It can be considered that each threshold value is determined so as to be pseudo error free with reference to the required CN. That is, the threshold Th1 can be set to a modulation error ratio when the signal-to-noise power ratio matches the required CN. For example, when the narrowband modulation scheme is QPSK and the coding rate is ½, the threshold Th1 may be set to about 4.0 dB. As another example, when the narrowband modulation scheme is QPSK and the coding rate is 2/3, the threshold Th1 may be set to about 4.0 dB. As another example, when the narrowband modulation scheme is 16QAM and the coding rate is 1/2, the threshold Th1 may be set to about 10 dB. However, since these settings depend on various system parameters such as waveform equalization noise removal capability, the values that can be used as threshold values for realizing pseudo error free are limited to the specific examples described above. is not.

  The other threshold values Th2 to Th4 may be set to be pseudo error free. That is, the threshold Th2 is set to the pre-Viterbi BER when the signal-to-noise power ratio matches the required CN, and the threshold Th3 is set to the post-RS BER when the signal-to-noise power ratio matches the required CN. Can be considered. For example, the threshold Th4 may be set to an average number of consecutive invalid packets when the signal-to-noise power ratio matches the required CN.

  Note that the above threshold value determination method exemplifies the threshold value determination method for ensuring that the reception rate is 100%. For example, the reception rate is guaranteed to be 95% or more. The threshold value may be set, the threshold value may be set so as to guarantee that the reception rate is 90% or more, and the threshold value may be set so that the lower limit of the reception rate is further reduced.

  In fact, a reception rate of 95% or more may be ensured even in a radio wave environment where the signal-to-noise power ratio is smaller than the required CN. In this case, if re-synchronization is applied, reception cannot be performed at all during that period, and the reception rate decreases instead. Therefore, setting the threshold value to allow the signal-to-noise power ratio to be smaller than the required CN may lead to an improvement in the final reception rate. That is, ultimately, it is desirable to experimentally optimize the threshold value using an actual system, a prototype system, or the like while considering the configuration of the entire receiving system and the required specifications.

(Modification 1)
For example, when receiving in a mobile body such as a train, if the mobile body enters the shadow of a radio wave environment such as a tunnel or a mountain, the synchronization word cannot be detected temporarily from the TMCC signal, or waveform equalization is performed. The signal quality of the received data signal may deteriorate. In this case, the frame synchronization cannot be established again unless it escapes from the shadow of the radio wave environment.

Therefore, when the synchronization processing unit 153e determines that the frame synchronization is lost, the above embodiments are modified so that the frame resynchronization is performed after waiting for a certain time T ₀ without performing the frame resynchronization immediately. It is possible. For example, this modification does not reset the OFDM demodulation LSI 103 when the synchronization establishment signal is interrupted, but resets the OFDM demodulation LSI 103 when the predetermined time T _{0 has} elapsed since the synchronization establishment signal was interrupted. This can be realized by configuring the control unit as described above.

Note that once frame synchronization is lost, the next synchronization timing does not come for at least one transmission frame period length. Therefore, the fixed time T ₀ is preferably set to one transmission frame period length or an integral multiple thereof.

However, it is desirable to optimize the fixed time T ₀ experimentally with an actual system, a prototype system, or the like. When the time T ₀ becomes shorter, since the frequency of the frame resynchronization increases, it can be shortened time required to establish frame synchronization again correspondingly increased power consumption. Conversely, when the time T ₀ is increased, the frequency of frame resynchronization is decreased, so that the time required to establish frame synchronization again is increased, but power consumption can be reduced by that amount. Thus, since the time required to establish synchronization again and the power consumption are in a trade-off relationship with respect to the time T _0, it is desirable to optimize experimentally in consideration of the required specifications.

(Modification 2)
For example, when receiving in a mobile body such as a train, if the mobile body enters the shadow of a radio wave environment such as a tunnel or a mountain, the synchronization word cannot be detected temporarily from the TMCC signal, or waveform equalization is performed. The signal quality of the received data signal may deteriorate. In this case, the frame synchronization cannot be established again unless it escapes from the shadow of the radio wave environment.

Therefore, when the frame synchronization is the not established synchronization unit 153e determines repeatedly threshold count N ₀ or more, power to the tuner 102 and the OFDM demodulation LSI 103, and, by stopping the supply of the operation clock, It is conceivable to modify the above embodiments so as to stop the demodulation process. This means that if the frame synchronization is the not established synchronization unit 153e determines repeatedly threshold count N ₀ or more, such that the OFDM demodulation device enters a radio wave environmental shade, such as a tunnel or a mountain is envisioned Because. In this modification, for example, when the control unit counts the number of frames for which the synchronization processing unit 153e did not output the synchronization establishment signal and the number exceeds the threshold number N ₀ , the control unit performs the OFDM demodulation with the tuner 102. This can be realized by stopping the demodulation processing by stopping the supply of power and operation clock to the LSI 103. As a result, useless resynchronization processing can be reduced, so that power consumption can be reduced. The threshold number N ₀ may be stored in a register that can be read by the control unit.

The threshold number N ₀ is preferably optimized experimentally with an actual system, a prototype system, or the like. Setting small threshold number N _0, consumption the effect of power reduction is large, it becomes easier to correspondingly stopped. Conversely, if the threshold number N ₀ is set large, the effect of reducing power consumption is suppressed, but the resynchronization process will continue to persist until the limit. This way it becomes a trade-off relationship with respect to a threshold number N ₀ is the resynchronization successive number and power consumption, to optimize empirically taking into account the required specifications is desirable.

For example, when the OFDM demodulating LSI 103 is mounted so as to always establish frame synchronization regardless of the radio wave condition, it is assumed that the radio wave to be received does not exist as a situation in which the frame synchronization is lost. The Therefore, the threshold number N ₀ is preferably 2 times.

  In addition, when the OFDM demodulation LSI 103 is mounted so that frame synchronization is not necessarily established in a specific fading environment, the preferable number of repetitions is two or more. In this case, the establishment of frame synchronization is determined in combination from the symbol synchronization performance of the symbol synchronization unit, the correction performance of the broadband carrier frequency error correction unit, and the frame synchronization performance of the TMCC unit. Therefore, it is desirable to experimentally confirm to what level the number of repetitions is increased under conditions where the establishment of frame synchronization is weak.

(Additional notes)
The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and embodiments obtained by appropriately combining technical means disclosed in different embodiments. Is also included in the technical scope of the present invention.

  For example, the present invention can be expressed or modified as follows.

  1. An error correction encoded data signal, a frame synchronization signal, a control signal, and a pilot signal serving as a reference for waveform equalization processing are subjected to orthogonal frequency division multiplexing to generate an effective symbol, and the effective symbol, An OFDM demodulator for receiving and demodulating a digital transmission wave according to an orthogonal frequency division multiplex modulation (OFDM) system including a transmission symbol having a guard interval obtained by copying the same content as a part of the effective symbol. AGC means for automatically adjusting power, symbol synchronization means for extracting the effective symbol head of the received wave, AFC means for correcting a carrier frequency error of the received wave, FFT means for OFDM demodulation of the received wave, Frame synchronization detection means for performing frame synchronization processing from the frame synchronization signal, and extracting the control signal Control signal demodulation means, waveform equalization processing means for performing waveform equalization processing based on the pilot signal, communication path information detection means for detecting communication path information of the received wave, and frame synchronization based on the communication path information An OFDM demodulator comprising: frame synchronization processing means for performing protection and frame resynchronization.

  According to the above configuration, frame synchronization synchronization protection and resynchronization processing can be appropriately performed using communication channel information, not TMCC error correction information. As a result, the user can receive and demodulate communications and broadcasts without interruption even when demodulation is interrupted when judged by TMCC information. On the other hand, when the communication path information deteriorates, the output of the OFDM demodulation LSI becomes invalid, and there is a possibility that it will be restored to valid data by resynchronization processing.

  2. The OFDM demodulator according to 2.1, further comprising a MER calculating unit for waveform equalization output, wherein the channel information detecting unit detects channel information from the MER.

  According to the above configuration, by using the waveform equalization output MER as the communication path information, it becomes possible to appropriately perform synchronization protection and resynchronization processing of frame synchronization. As a result, the user can receive and demodulate communications and broadcasts without interruption even when demodulation is interrupted when judged by TMCC information. Conversely, when the MER deteriorates, the output of the OFDM demodulating LSI becomes ineffective, and there is a possibility that it will be restored to valid data by resynchronization processing.

  3.1 OFDM demodulator according to 3.1, further comprising a BER calculating means for error correction, wherein the communication path information detecting means detects communication path information from the BER.

  According to the above configuration, by using the BER of the error correction code as the communication path information, it becomes possible to appropriately perform synchronization protection and resynchronization processing for frame synchronization. As a result, the user can receive and demodulate communications and broadcasts without interruption even when demodulation is interrupted when judged by TMCC information. Conversely, when the BER deteriorates, the output of the OFDM demodulating LSI becomes ineffective, and there is a possibility that it will be restored to valid data by resynchronization processing.

  The OFDM demodulator according to 4.3, wherein the BER calculation means is a pre-Viterbi BER calculation means when the error correction code is a convolutional code.

  According to the above configuration, when a convolutional code is employed as the error correction code, it is possible to appropriately perform synchronization protection and resynchronization processing of frame synchronization by using the pre-Viterbi BER meter. As a result, the user can receive and demodulate communications and broadcasts without interruption even when demodulation is interrupted when judged by TMCC information. Conversely, when the pre-Viterbi BER deteriorates, the output of the OFDM demodulation LSI becomes ineffective, and there is a possibility that the data will be restored to valid data by resynchronization processing.

  The broadcast standard demodulation of Non-Patent Document 1 employs a concatenated code of a convolutional code and a Reed-Solomon code. There is a required CN as a performance index of the OFDM demodulation LSI. This is defined by BER at the time of Viterbi output. By using the pre-Viterbi BER as the communication path information, it is possible to set the conditions for synchronization protection / resynchronization processing in association with the required CN.

  The OFDM demodulator according to 5.3, wherein when the error correction code is a Reed-Solomon code, the BER calculating means is a Reed-Solomon BER calculating means.

  According to the above configuration, when a Reed-Solomon code is used as the error correction code, it is possible to appropriately perform synchronization protection and resynchronization processing of frame synchronization by using Reed-Solomon BER. As a result, the user can receive communications and broadcasts without interruption. Conversely, when the Reed-Solomon BER deteriorates, the output of the OFDM demodulating LSI becomes ineffective, and there is a possibility that it will be restored to valid data by resynchronization processing.

  In the broadcast standard, the Reed-Solomon decoding process is the last error correction process, so the Reed-Solomon BER corresponds to the BER of the OFDM demodulated LSI output signal. According to the above configuration, by using the Reed-Solomon BER as communication path information, it is possible to set the conditions for synchronization protection / resynchronization processing in association with the state of the output signal of the OFDM demodulation LSI.

  In the OFDM demodulator according to 6.1, when the error correction code is a Reed-Solomon code, the OFDM demodulation apparatus further includes error correction information extraction means for generating a valid signal of decoded output, and the channel information detection means An OFDM demodulator for detecting communication path information.

  According to the above configuration, when a Reed-Solomon code is employed as the error correction code, it is possible to appropriately perform synchronization protection and resynchronization processing of frame synchronization by using an effective signal. As a result, the user can receive and demodulate communications and broadcasts without interruption even when demodulation is interrupted when judged by TMCC information. Conversely, when the Reed-Solomon BER deteriorates, the output of the OFDM demodulating LSI becomes ineffective, and there is a possibility that it will be restored to valid data by resynchronization processing.

  Reed-Solomon is a linear code that detects and corrects errors by calculating syndromes. When the number of errors is within the correction capability range, the Reed-Solomon decoding unit guarantees the validity of the output signal 100%.

  The broadcast standard demodulation of Non-Patent Document 1 employs a concatenated code of a convolutional code and a Reed-Solomon code. In the broadcasting standard, since the Reed-Solomon decoding process is the last error correction process, the Reed-Solomon output valid signal corresponds to the OFDM demodulated LSI output signal valid signal. By using the valid signal as the communication path information, it is possible to set conditions for synchronization protection / resynchronization processing in association with the state of the output signal of the OFDM demodulation LSI.

  In the OFDM demodulating device described in 7.1 to 6, when the frame synchronization signal cannot be detected and the communication path information detection unit detects that the radio wave state can output the effective TSP, the frame synchronization processing unit An OFDM demodulator characterized by performing frame synchronization protection.

  According to the above configuration, when the communication path information detection unit detects that valid data can be output even in a state where the frame synchronization signal cannot be detected correctly, frame synchronization is stopped by performing frame synchronization synchronization protection. And resynchronization is suppressed. As a result, the user can receive and demodulate communications and broadcasts without interruption even when demodulation is interrupted when judged by TMCC information.

  In the OFDM demodulator according to 8.1 to 6, when the frame synchronization signal cannot be detected and the communication path information detection unit detects that the radio wave state is not an effective TSP output, the frame synchronization processing unit An OFDM demodulator that performs resynchronization processing.

  According to the above configuration, when the frame synchronization signal cannot be detected correctly and the communication path information detection unit also detects that the valid data cannot be output, the valid data is obtained by performing resynchronization of the frame synchronization. May be restored to a state where can be output.

  9. The OFDM demodulator according to 9.8, wherein the frame synchronization processing unit intermittently performs resynchronization processing.

  According to the above configuration, when the frame synchronization signal cannot be detected correctly and the communication path information detection unit detects that the valid data cannot be output, even if the frame synchronization is immediately resynchronized, There is a possibility that it cannot be restored to a state where valid data can be output. Therefore, power consumption can be suppressed by intermittently performing resynchronization processing.

  1. The OFDM demodulator according to 10.8, wherein the frame synchronization processing unit stops the OFDM demodulation processing when the resynchronization processing exceeds a set number of times.

  When the frame resynchronization process is performed by the OFDM demodulator according to 8, the resynchronization process may be repeated a plurality of times. In this case, there is a high possibility that the radio wave state of the communication path cannot be received. In this case, since the OFDM demodulation processing only wastes power consumption, power consumption can be suppressed by stopping the OFDM demodulation processing and the tuner.

  In each part of the OFDM demodulator of each of the above embodiments, arithmetic means such as a CPU executes a program stored in a storage means such as a ROM (Read Only Memory) or RAM, and controls communication means such as an interface part. This can be realized. Therefore, various functions and various processes of the OFDM demodulator according to the present embodiment can be realized simply by a computer having these means reading the recording medium storing the program and executing the program. In addition, by recording the program on a removable recording medium, the various functions and various processes described above can be realized on an arbitrary computer.

  As the recording medium, a program medium such as a memory (not shown) such as a ROM may be used for processing by the microcomputer, and a program reading device is provided as an external storage device (not shown). It may be a program medium that can be read by inserting a recording medium there.

  In any case, the stored program is preferably configured to be accessed and executed by the microprocessor. Furthermore, it is preferable that the program is read out, and the read program is downloaded to a program storage area of the microcomputer and the program is executed. It is assumed that this download program is stored in advance in the main unit.

  The program medium is a recording medium configured to be separable from the main body, such as a tape system such as a magnetic tape or a cassette tape, a magnetic disk such as a flexible disk or a hard disk, or a disk such as a CD / MO / MD / DVD. Fixed disk, IC card (including memory card), etc., or semiconductor ROM such as mask ROM, EPROM (Erasable Programmable Read Only Memory), EEPROM (Electrically Erasable Programmable Read Only Memory), flash ROM, etc. In particular, there are recording media that carry programs.

  In addition, if the system configuration is capable of connecting to a communication network including the Internet, the recording medium is preferably a recording medium that fluidly carries the program so as to download the program from the communication network.

  Further, when the program is downloaded from the communication network as described above, it is preferable that the download program is stored in the main device in advance or installed from another recording medium.

  The present invention can be applied to an OFDM demodulation device, an OFDM demodulation method, a program, and a computer-readable recording medium that can efficiently transmit a video signal and an audio signal by a digital transmission method. The present invention is also applied to a device that receives a signal in accordance with the OFDM system, for example, a demodulator for a wireless LAN, a demodulator for receiving BS digital broadcast and CS digital broadcast, and a demodulator for cable television. be able to.

1 is a block diagram illustrating a configuration of an OFDM demodulator according to a first embodiment of the present invention. FIG. FIG. 7 is a block diagram illustrating a configuration of an OFDM demodulator according to a second embodiment of the present invention. It is a block diagram which shows the 2nd Embodiment of this invention and shows the structural example of a bit error rate calculation part. FIG. 9 is a block diagram illustrating a configuration of an OFDM demodulator according to a third embodiment of the present invention. It is a block diagram which shows the 3rd Embodiment of this invention and shows the structure of a Reed-Solomon correction part. FIG. 10 is a block diagram illustrating a main configuration of an OFDM demodulator according to a fourth embodiment of the present invention. It is a figure which shows a prior art and shows the structure of the transmission symbol in an OFDM signal. It is a block diagram which shows a prior art and shows the structure of an OFDM demodulation apparatus. It is an explanatory view for showing a prior art and explaining waveform equalization by a waveform equalization unit. It is a block diagram which shows a prior art and shows the structural example of an error correction process part. It is a block diagram which shows a prior art and shows the structural example of a TMCC decoding part.

Explanation of symbols

100 OFDM demodulator 101 antenna 102 tuner 103 OFDM demodulation LSI
110 Baseband signal processing unit 111 Analog-digital conversion unit 112 Orthogonal detection unit 113 Narrow band carrier frequency error correction unit 114 Fast Fourier transform unit 115 Waveform equalization unit (equalization means)
116 Symbol synchronization unit 117 Automatic gain control unit 118 Wideband carrier error correction unit 120 Error correction processing unit 126 Viterbi decoding unit (Viterbi decoding means)
130 Reed-Solomon Correction Unit (Reed-Solomon decoding means)
151 Modulation error ratio calculation unit (modulation error rate calculation means)
152 Signal Quality Discriminator (Determination Unit)
153 TMCC decoding unit 153a DBPSK demodulation unit 153b Majority processing unit 153c Synchronization word detection unit (detection means)
153d Error correction decoding unit 153e Synchronization processing unit (synchronization determination means)
153f TMCC signal acquisition unit 153g Timing adjustment unit 154 Bit error rate calculation unit (bit error calculation means)
156 Error correction information extraction unit (discrimination means)

Claims (13)

An OFDM demodulator that obtains a control signal, a pilot signal, and a data signal by demodulating an OFDM signal for each transmission frame,
Equalizing means for waveform equalizing the data signal with reference to the pilot signal;
Decoding means for performing error correction decoding of the data signal after waveform equalization;
Synchronization determination means for determining whether or not frame synchronization is established according to the signal quality of the data signal after waveform equalization,
An OFDM demodulator characterized by the above. A modulation error ratio calculating means for calculating a modulation error ratio based on the data signal after waveform equalization;
The synchronization determination means determines that frame synchronization is established when the calculated modulation error ratio is equal to or greater than a predetermined threshold;
The OFDM demodulator according to claim 1. A bit error rate calculating means for calculating a bit error rate of the data signal after waveform equalization;
The synchronization determination unit determines that frame synchronization is established when the calculated bit error rate is equal to or less than a predetermined threshold value.
The OFDM demodulator according to claim 1, wherein: The decoding means includes Viterbi decoding means for Viterbi decoding the data signal after waveform equalization,
The bit error rate calculating means convolutionally encodes the output of the Viterbi decoding means, and compares the data signal after the waveform equalization with the encoded data re-encoded by the convolutional encoding, Calculating the bit error rate,
The OFDM demodulator according to claim 3, wherein The decoding means includes Reed-Solomon decoding means for performing Reed-Solomon decoding on the data signal after waveform equalization,
The bit error rate calculating means calculates the bit error rate by integrating the number of errors calculated by the Reed-Solomon decoding means;
The OFDM demodulator according to claim 3, wherein The decoding means includes Reed-Solomon decoding means for performing Reed-Solomon decoding on the data signal after waveform equalization, and the number of errors calculated by the Reed-Solomon decoding means is set to a maximum number of errors that can be corrected by the Reed-Solomon decoding means. A validity judging means for judging the effectiveness of Reed-Solomon decoding by comparing, and
The synchronization determination means determines whether or not frame synchronization is established based on the determined Reed-Solomon decoding effectiveness.
The OFDM demodulator according to claim 1. The synchronization determination means determines that frame synchronization is established when the data signal after the waveform or the like is a signal that can be corrected by the decoding means.
The OFDM demodulator according to claim 1. The synchronization determination means determines that frame synchronization is not established when the data signal after the waveform or the like is a signal that cannot be corrected by the decoding means,
The OFDM demodulator according to claim 1 or 7, characterized in that When it is determined by the synchronization determination means that frame synchronization has not been established, the synchronization determination means includes a control means for controlling the own apparatus so as to reestablish frame synchronization after a predetermined time has elapsed.
9. The OFDM demodulator according to any one of 1 to 8, wherein When the synchronization determination means continuously determines that frame synchronization is not established more than a predetermined number of times, the synchronization determination means includes a control means for controlling the own apparatus to temporarily stop the demodulation process.
The OFDM demodulator according to any one of claims 1 to 8, characterized in that An OFDM demodulation method for obtaining a pilot signal and a data signal by demodulating an OFDM signal for each transmission frame,
An equalization step of waveform equalizing the data signal with reference to the pilot signal;
A decoding step for performing error correction decoding of the data signal after waveform equalization;
And a synchronization determination step of determining whether or not frame synchronization is established according to the signal quality of the data signal after waveform equalization. An OFDM demodulation program for operating the OFDM demodulator according to any one of claims 1 to 9,
An OFDM demodulation program for causing a computer to function as each of the above means.   A computer-readable recording medium on which the OFDM demodulation program according to claim 12 is recorded.

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