JP2007274630A - Ofdm demodulator, ofdm demodulation method, program, and computer-readable recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an OFDM demodulator having high transfer function estimation capability and improved noise resistance. <P>SOLUTION: The OFDM demodulator has a digital orthogonal demodulation circuit 4, an FFT operation circuit 5, and a waveform equalization circuit 11. The waveform equalization circuit 11 includes: a complex division circuit 7; a symbol filter 9; a carrier filter 6 for estimating a data carrier transfer function based on a symbol direction interpolation transfer function; a complex division circuit 12 for generating a waveform equalization data carrier, by performing complex division to a data carrier extracted from an FFT demodulation signal by the data carrier transfer function; and a filter coefficient control circuit 13 for controlling the filter coefficient of the carrier filter 6, according to a DU ratio for indicating multipath conditions and the variations of delay time. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an orthogonal frequency division multiplexing (OFDM) demodulation apparatus, an OFDM demodulation method, a program, and a computer readable medium that can efficiently transmit a video signal and an audio signal by a digital transmission method. The present invention relates to various recording media.

(OFDM broadcast)
In terrestrial digital broadcasting, a multicarrier OFDM modulation / demodulation method is known as a modulation method suitable for overcoming ghost interference (fading, multipath) by buildings. The OFDM modulation / demodulation method is a digital modulation / demodulation method in which a large number (about 256 to 1024) of sub-carriers are provided in one channel band and video signals and audio signals can be efficiently transmitted. A baseband (BB) signal in which all the carriers are OFDM-modulated by an inverse fast Fourier transform (IFFT) is generated.

FIG. 14 is a diagram illustrating an example of transmission symbols of an OFDM modulated wave. The period of the IFFT conversion processing window is an effective symbol period ts. The effective symbol period corresponds to N cycles of F _s clocks. The sum of all digitally modulated carriers with the effective symbol period ts as a basic unit is called an OFDM transmission symbol.

  As shown in FIG. 14, an actual transmission symbol is usually configured by adding a period tg called a guard interval (GI) 202a to an effective symbol period 201. The waveform of the GI period tg (202a) is obtained by repeating the signal waveform of the rear part 202b of the effective symbol period ts. The symbol period 203 of the transmission symbol is the sum of the effective symbol period 201 and the GI period 202a. For example, according to the broadcasting standard of Non-Patent Document 1, the effective symbol period length is defined as shown in the following table 1 by a parameter called MODE.

  Further, the GI period (unit: μs) is defined as shown in the following Table 2 by a parameter called GI period length (GI ratio) which is a ratio to each effective symbol period length.

  A collection of several transmission symbols is called a transmission frame. This is a collection of about 100 information transmission symbols plus frame synchronization symbols and service identification symbols. For example, in Non-Patent Document 1, one frame is defined as 204 symbols.

  Further, according to Non-Patent Document 1, a carrier shown in the following table 3 is arranged for one segment in one transmission symbol subjected to QPSK, 16QAM, or 64QAM modulation.

  In this table, SP means an SP (Scattered Pilot) signal. This SP signal is a pilot signal periodically inserted. For example, the SP signal is inserted once in 12 carriers in the carrier direction and once in 4 symbols in the symbol direction. TMCC means TMCC (Transmission and Multiplexing Configuration Control) signal. This TMCC signal is a signal for transmitting a frame synchronization signal and a transmission parameter. AC1 means an AC1 (Auxiliary Channel) signal. This AC1 signal is a signal for transmitting additional information. Unlike SP, TMCC and AC1 are arranged aperiodically in each carrier.

(Basic configuration of conventional OFDM demodulator)
An example of the configuration of a conventional OFDM demodulator is disclosed in Patent Document 1, for example. FIG. 15 is a block diagram showing a configuration of a conventional OFDM demodulator 91. The OFDM demodulator 91 includes an antenna 92, a tuner 93, a baseband signal processing unit 86, and an error correction processing unit 87. The baseband signal processing unit 86 includes an analog-to-digital converter (ADC) 88, an orthogonal demodulation circuit 94, a narrowband carrier frequency error correction circuit 73, a symbol synchronization circuit 89, an AGC circuit 70, and an FFT operation circuit 95. A broadband carrier frequency error correction circuit 72, a TMCC decoding circuit 71, and a waveform equalization circuit 81.

  The broadcast wave of the digital broadcast broadcast from the broadcast station is received by the antenna 92 of the OFDM demodulator 91 and supplied to the tuner 93 as an RF signal. The tuner 93 converts the frequency of each RF (high frequency) signal received through the antenna 92 into an IF (intermediate frequency) signal. The tuner 93 supplies the frequency-converted IF signal to the ADC 88 provided in the baseband signal processing unit 86.

  The IF signal output from the tuner 93 is digitized by the ADC 88. The digitized IF signal is supplied to the quadrature demodulation circuit 94.

  The orthogonal demodulation circuit 94 performs orthogonal demodulation on the digitized IF signal using a carrier signal having a predetermined frequency (carrier frequency), and outputs a baseband OFDM signal. As a result of orthogonal demodulation, the baseband OFDM signal becomes a complex signal composed of a real axis component (I channel signal) and an imaginary axis component (Q channel signal). The baseband OFDM signal output from the orthogonal demodulation circuit 94 is supplied to the narrowband carrier frequency error correction circuit 73.

  The narrowband carrier frequency error correction circuit 73 corrects the narrowband carrier frequency error of the baseband OFDM signal supplied from the orthogonal demodulation circuit 94 and supplies it to the symbol synchronization circuit 89, the FFT operation circuit 95 and the AGC circuit 70. .

  The symbol synchronization circuit 89 extracts a transmission parameter such as a transmission mode and a guard interval ratio from the baseband OFDM signal, and executes symbol synchronization processing for extracting the leading timing of the effective symbol.

  The FFT operation circuit 95 performs an FFT operation on the baseband OFDM signal, and extracts and outputs a signal that is orthogonally modulated on each subcarrier. The FFT operation circuit 95 extracts an effective symbol length signal from one OFDM symbol, and performs an FFT operation on the extracted signal. That is, the FFT operation circuit 95 removes a signal corresponding to the guard interval length from one OFDM symbol, and performs an FFT operation on the remaining signal. The range of the signal extracted for performing the FFT operation may be an arbitrary position of one OFDM transmission symbol as long as the extracted signal points are continuous. That is, the start position of the extracted signal range is any position during the GI period. The signal modulated by each subcarrier extracted by the FFT operation circuit 95 is a complex signal composed of a real axis component (I channel signal) and an imaginary axis component (Q channel signal). The signal extracted by the FFT operation circuit 95 is supplied to the TMCC decoding circuit 71, the broadband carrier frequency error correction circuit 72, and the waveform equivalent circuit 81.

  The wideband carrier frequency error correction circuit 72 corrects the wideband carrier frequency error of the baseband OFDM signal supplied by the FFT operation circuit 95 and supplies it to the FFT operation circuit 95.

  The waveform equivalent circuit 81 is supplied with a signal demodulated from each subcarrier output from the FFT operation circuit 95. The waveform equivalent circuit 81 performs carrier demodulation on the signal. In the case of demodulating an ISDB-T standard OFDM signal, the waveform equivalent circuit 81 performs, for example, DQPSK differential demodulation or synchronous demodulation such as QPSK, 16QAM, and 64QAM.

  The TMCC decoding circuit 71 decodes transmission control information such as TMCC that is modulated at a predetermined position in the OFDM transmission frame. The error correction processing unit 87 corrects an error in the OFDM signal waveform-equivalent by the waveform equivalent circuit 81.

FIG. 16 is a block diagram for explaining the configuration of the waveform equalization circuit 81. The waveform equalization circuit 81 has an SP extraction circuit 905. The SP extraction circuit 905 extracts the SP carrier X _FFT (n _SP , k _SP ) from the FFT demodulated signal X _FFT (n, k) output from the FFT operation circuit 95 and supplies it to the complex division circuit 97.

The complex division circuit 97 divides the SP carrier X _FFT (n _SP , k _SP ) by the reference SP carrier X _ARIB (n _SP , k _SP ) generated by the SP generation circuit 906, and the SP carrier transfer function H (n _SP , k _{SP SP} ) is supplied to the symbol filter 99.

The symbol filter 99 estimates the symbol direction interpolation transfer function H (n, k _SP ) based on the SP carrier transfer function H (n _SP , k _SP ) and supplies it to the carrier filter 96. The carrier filter 96 generates a data carrier transfer function H (n, k) based on the symbol direction interpolation transfer function H (n, k _SP ) supplied from the symbol filter 99 and supplies it to the complex division circuit 82.

The waveform equalization circuit 81 has a data extraction circuit 903. The data extraction circuit 903 extracts a data carrier from the FFT demodulated signal X _FFT (n, k) output from the FFT operation circuit 95 and supplies the data carrier to the complex division circuit 82. The complex division circuit 82 performs complex division on the data carrier extracted by the data extraction circuit 903 by the data carrier transfer function H (n, k) to generate a waveform equalized data carrier X _EQ (n, k).
JP 2002-261729 A (published September 13, 2002 (2002.9.13)) "Transmission method for digital terrestrial television broadcasting ARIB STD-B31 1.5 edition", the radio industry, the first edition of May 31, 2001, revised version 1.5 on July 29, 2003 Introduction to digital wireless communication, Fumio Takahata, Bafukan, 2002 "Digital Signal Processing." V. Openheim, R.A. W. Schaffer. Co-authored, Prentice-Hall, Englewood Cliffs, NJ, 1975.

However, in the above-described conventional configuration, when the DU ratio (Desired Undesired power Ratio, the intensity of the preceding wave ÷ the intensity of the delayed wave) decreases or the delay time increases and the multipath condition becomes severe, the carrier filter 96 becomes symbol direction. There arises a problem that the data carrier transfer function H (n, k) cannot be correctly estimated based on the interpolation transfer function H (n, k _SP ).

  In order to solve this problem, if the carrier filter 96 is configured by a filter having a high transfer function estimation capability, there arises a problem that the noise removal capability decreases.

  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 having high transfer function estimation capability and improved noise resistance.

  In order to solve the above problem, an OFDM demodulator according to the present invention includes a digital quadrature demodulator that generates a baseband signal by orthogonally demodulating an intermediate frequency signal received from an antenna and frequency-converted from a high frequency signal by a tuner. An FFT operation circuit that performs an FFT operation on the baseband signal to generate an FFT demodulated signal, and a waveform equalization circuit that equalizes the waveform of the FFT demodulated signal. A first complex division circuit for complexly dividing the extracted SP carrier by a reference SP carrier to generate an SP carrier transfer function; a symbol filter for estimating a symbol direction interpolation transfer function based on the SP carrier transfer function; and the symbol A carrier filter that estimates a data carrier transfer function based on a directional interpolation transfer function; A second complex division circuit that complex-divides a data carrier extracted from an FFT demodulated signal by the data carrier transfer function to generate a waveform equalized data carrier, and a DU ratio indicating a multipath condition and a delay time variation And a filter coefficient control circuit for controlling a filter coefficient of the carrier filter.

  According to the above feature, the filter coefficient of the carrier filter is controlled in accordance with the variation of the DU ratio indicating the multipath condition and the delay time. For this reason, the filter coefficient of the carrier filter is controlled so that the transfer function estimation capability (interpolation capability) is given priority in the case of multipath with a strict delay profile, and the noise removal capability is given priority when the delay profile is not strict. be able to. As a result, it is possible to provide an OFDM demodulator capable of improving the transfer function estimation capability when the multipath condition is severe, and improving the noise tolerance when the multipath condition is not severe.

  In the OFDM demodulator according to the present invention, when the multipath condition becomes severe, the filter coefficient control circuit changes the filter coefficient in a direction to widen an ideal fractional delay frequency band, and when the multipath condition becomes loose, an ideal fraction. It is preferable to change the filter coefficient in the direction of narrowing the delay frequency band.

  According to the above configuration, it is possible to provide an OFDM demodulator excellent in the balance between transfer function estimation capability and noise tolerance by changing the filter coefficient of the carrier filter according to the multipath condition.

  In the OFDM demodulator according to the present invention, it is preferable that the filter coefficient control circuit changes the filter coefficient in a direction to widen an ideal fractional delay frequency band when the DU ratio decreases.

  According to the above configuration, when the DU ratio becomes small and the multipath condition becomes severe, the filter coefficient can be changed to improve the estimation capability of the carrier filter in the frequency axis direction.

  In the OFDM demodulator according to the present invention, it is preferable that the filter coefficient control circuit changes the filter coefficient in a direction of narrowing an ideal fractional delay frequency band when the DU ratio increases.

  According to the above configuration, when the DU ratio increases and the multipath condition becomes loose, it is not necessary to improve the estimation capability of the carrier filter in the frequency axis direction, and the noise removal capability can be improved by changing the filter coefficient. .

  In the OFDM demodulator according to the present invention, it is preferable that the filter coefficient control circuit changes the filter coefficient in a direction to widen an ideal fractional delay frequency band when the delay time increases.

  According to the above configuration, when the delay time becomes large and the multipath condition becomes severe, the filter coefficient can be changed to improve the estimation capability of the carrier filter in the frequency axis direction.

  In the OFDM demodulator according to the present invention, it is preferable that the filter coefficient control circuit changes the filter coefficient in a direction of narrowing an ideal fractional delay frequency band when the delay time becomes small.

  According to the above configuration, when the delay time is reduced and the multipath condition is relaxed, it is not necessary to improve the estimation capability of the carrier filter in the frequency axis direction, and the noise removal capability can be improved by changing the filter coefficient. .

  In the OFDM demodulator according to the present invention, the carrier filter is preferably constituted by a Lagrangian interpolation filter.

  According to the above configuration, a carrier filter excellent in transfer function estimation capability and noise removal capability can be obtained with a simple configuration.

  In the OFDM demodulator according to the present invention, it is preferable that the filter coefficient control circuit increases the filter order of the Lagrangian interpolation filter when the DU ratio decreases, and decreases the filter order when the DU ratio increases.

  According to the above configuration, when the DU ratio becomes small and the multipath condition becomes severe, the filter coefficient can be changed to improve the estimation capability of the Lagrangian interpolation filter in the frequency axis direction, and the DU ratio becomes large. When the multipath condition becomes loose, it is not necessary to improve the estimation capability of the Lagrangian interpolation filter in the frequency axis direction, and the noise removal capability can be improved by changing the filter coefficient.

  In the OFDM demodulator according to the present invention, it is preferable that the filter coefficient control circuit increases the filter order of the Lagrangian interpolation filter when the delay time increases, and decreases the filter order when the delay time decreases.

  According to the above configuration, when the delay time becomes large and the multipath condition becomes severe, the filter coefficient can be changed to improve the estimation capability of the Lagrangian interpolation filter in the frequency axis direction, and the delay time becomes small. When the multipath condition becomes loose, it is not necessary to improve the estimation capability of the Lagrangian interpolation filter in the frequency axis direction, and the noise removal capability can be improved by changing the filter coefficient.

  The OFDM demodulator according to the present invention further includes a delay profile estimation circuit that extracts the DU ratio and the delay time based on the baseband signal generated by the digital orthogonal demodulation circuit, and the filter coefficient control circuit includes: Preferably, the filter coefficient of the carrier filter is controlled based on the DU ratio and the delay time extracted by the delay profile estimation circuit.

  According to the above configuration, a DU ratio and a delay time indicating a multipath condition can be obtained with a simple configuration.

  In the OFDM demodulator according to the present invention, the waveform equalization circuit further includes a MER measurement circuit that measures MER based on a waveform equalization data carrier generated by the second complex division circuit, and the filter coefficient control circuit Preferably, the filter coefficient of the carrier filter is controlled based on the MER measured by the MER measurement circuit.

  According to the above configuration, when the measured MER is small and the noise power is large, the noise resistance can be improved by changing the filter coefficient.

  In order to solve the above problem, an OFDM demodulation method according to the present invention generates a baseband signal by orthogonally demodulating an intermediate frequency signal received by an antenna and frequency-converted from a high frequency signal by a tuner, and generating the baseband signal. To generate an FFT demodulated signal, complexly divide the SP carrier extracted from the FFT demodulated signal by a reference SP carrier to generate an SP carrier transfer function, and perform symbol direction interpolation based on the SP carrier transfer function Estimating a transfer function, estimating a data carrier transfer function based on the symbol direction interpolation transfer function while controlling a filter coefficient of a carrier filter according to a change in a DU ratio and a delay time indicating a multipath condition, and the FFT The data carrier extracted from the demodulated signal is expressed by the data carrier transfer function. By dividing element and generating a waveform equalizing data carrier.

  According to the above feature, the filter coefficient of the carrier filter is controlled in accordance with the variation of the DU ratio indicating the multipath condition and the delay time. For this reason, the filter coefficient of the carrier filter is controlled so that the transfer function estimation capability (interpolation capability) is given priority in the case of multipath with a strict delay profile, and the noise removal capability is given priority when the delay profile is not strict. be able to. As a result, it is possible to provide an OFDM demodulation method capable of enhancing the transfer function estimation capability when the multipath condition is severe and improving the noise tolerance when the delay profile is not severe.

  In order to solve the above-described problem, the program according to the present invention is configured to generate a baseband signal by orthogonally demodulating an intermediate frequency signal received by an antenna and frequency-converted from a high-frequency signal by a tuner. A procedure for generating an FFT demodulated signal by performing an FFT operation on a baseband signal, a procedure for generating an SP carrier transfer function by complex-dividing an SP carrier extracted from the FFT demodulated signal by a reference SP carrier, and the SP carrier transfer Data based on the symbol direction interpolation transfer function while controlling the filter coefficient of the carrier filter according to the procedure of estimating the symbol direction interpolation transfer function based on the function and the DU ratio indicating the multipath condition and the variation of the delay time A procedure for estimating the carrier transfer function and extracted from the FFT demodulated signal. The data carrier by the complex division by the data carrier transfer function, characterized in that to execute the steps of generating a waveform equalizing data carrier.

  A computer-readable recording medium according to the present invention is a computer-readable recording medium, wherein a baseband signal is generated by orthogonally demodulating an intermediate frequency signal received by an antenna and frequency-converted from a high-frequency signal by a tuner; Based on the SP carrier transfer function, a procedure for generating an FFT demodulated signal by performing an FFT operation, a procedure for generating an SP carrier transfer function by complex-dividing an SP carrier extracted from the FFT demodulated signal by a reference SP carrier, The procedure for estimating the symbol direction interpolation transfer function, and the data carrier transfer function based on the symbol direction interpolation transfer function while controlling the filter coefficient of the carrier filter according to the variation of the DU ratio indicating the multipath condition and the delay time The estimation procedure and the data extracted from the FFT demodulated signal. Characterized in that the carrier has been recorded a program for executing the steps of generating a waveform equalizing data carrier to the complex division by the data carrier transmission function.

  As described above, the OFDM demodulator according to the present invention includes the filter coefficient control circuit that controls the filter coefficient of the carrier filter in accordance with the variation of the DU ratio indicating the multipath condition and the delay time. There is an effect that it is possible to provide an OFDM demodulator with high estimation capability and improved noise tolerance.

  As described above, the OFDM demodulation method according to the present invention controls the data coefficient based on the symbol direction interpolation transfer function while controlling the filter coefficient of the carrier filter according to the DU ratio indicating the multipath condition and the variation of the delay time. Since the transfer function is estimated, it is possible to provide an OFDM demodulation method with high transfer function estimation capability and improved noise resistance.

  An embodiment of the present invention will be described below with reference to FIGS. 1 to 13A and 13B. FIG. 1 shows an embodiment of the present invention and is a block diagram showing a main configuration of an OFDM demodulator 1.

  The OFDM demodulator 1 includes an antenna 2, a tuner 3, a baseband signal processing unit 16, and an error correction processing unit 17. The baseband signal processing unit 16 includes an analog / digital converter (ADC) 18, an orthogonal demodulation circuit 4, a narrowband carrier frequency error correction circuit 23, a symbol synchronization circuit 19, an AGC circuit 20, and an FFT operation circuit 5. A broadband carrier frequency error correction circuit 22, a TMCC decoding circuit 21, and a waveform equalization circuit 11.

  A broadcast wave of a digital broadcast broadcast from a broadcast station is received by the antenna 2 of the OFDM demodulator 1 and supplied to the tuner 3 as an RF signal. The tuner 3 frequency-converts each RF (high frequency) signal received through the antenna 2 into an IF (intermediate frequency) signal. The tuner 3 supplies the frequency-converted IF signal to the ADC 18 provided in the baseband signal processing unit 16.

  The IF signal output from the tuner 3 is digitized by the ADC 18. The digitized IF signal is supplied to the quadrature demodulation circuit 4.

  The orthogonal demodulation circuit 4 orthogonally demodulates the digitized IF signal using a carrier signal having a predetermined frequency (carrier frequency), and outputs a baseband OFDM signal. As a result of orthogonal demodulation, the baseband OFDM signal becomes a complex signal composed of a real axis component (I channel signal) and an imaginary axis component (Q channel signal). The baseband OFDM signal output from the orthogonal demodulation circuit 4 is supplied to the narrowband carrier frequency error correction circuit 23.

  The narrowband carrier frequency error correction circuit 23 corrects the narrowband carrier frequency error of the baseband OFDM signal supplied from the orthogonal demodulation circuit 4, and performs symbol synchronization circuit 19, FFT operation circuit 5, and AGC (automatic gain control). Supply to circuit 20.

  The symbol synchronization circuit 19 extracts a transmission parameter such as a transmission mode and a guard interval ratio from the baseband OFDM signal, and executes symbol synchronization processing for extracting the leading timing of the effective symbol.

  The FFT operation circuit 5 performs an FFT operation on the baseband OFDM signal, and extracts and outputs a signal that is orthogonally modulated on each subcarrier. The FFT operation circuit 5 extracts a signal for an effective symbol length from one OFDM symbol, and performs an FFT operation on the extracted signal. That is, the FFT operation circuit 5 removes a signal corresponding to the guard interval length from one OFDM symbol, and performs an FFT operation on the remaining signal. The range of the signal extracted for performing the FFT operation may be an arbitrary position of one OFDM transmission symbol as long as the extracted signal points are continuous. That is, the start position of the extracted signal range is any position during the GI period. The signal modulated by each subcarrier extracted by the FFT operation circuit 5 is a complex signal composed of a real axis component (I channel signal) and an imaginary axis component (Q channel signal). The signal extracted by the FFT operation circuit 5 is supplied to the TMCC decoding circuit 21, the broadband carrier frequency error correction circuit 22, and the waveform equivalent circuit 11.

  The broadband carrier frequency error correction circuit 22 corrects the broadband carrier frequency error of the baseband OFDM signal supplied by the FFT calculation circuit 5 and supplies the corrected error to the FFT calculation circuit 5.

  The waveform equivalent circuit 11 is supplied with a signal demodulated from each subcarrier output from the FFT operation circuit 5. The waveform equivalent circuit 11 performs carrier demodulation on the signal. In the case of demodulating the ISDB-T standard OFDM signal, the waveform equivalent circuit 11 performs, for example, DQPSK differential demodulation or synchronous demodulation such as QPSK, 16QAM, and 64QAM.

  The TMCC decoding circuit 21 decodes transmission control information such as TMCC that is modulated at a predetermined position in the OFDM transmission frame. The error correction processing unit 17 corrects an error in the OFDM signal waveform-equivalent by the waveform equivalent circuit 11.

FIG. 2 is a block diagram for explaining the configuration of the waveform equalization circuit 11. The waveform equalization circuit 11 has an SP extraction circuit 105. The SP extraction circuit 105 extracts the SP carrier X _FFT (n _SP , k _SP ) from the FFT demodulated signal X _FFT (n, k) output from the FFT operation circuit 5 and supplies it to the complex division circuit 7.

The complex division circuit 7 divides the SP carrier X _FFT (n _SP , k _SP ) by the reference SP carrier X _ARIB (n _SP , k _SP ) generated by the SP generation circuit 106, and the SP carrier transfer function H (n _SP , k _SP) . _SP ) is supplied to the symbol filter 9.

Based on the SP carrier transfer function H (n _SP , k _SP ) generated by the complex division circuit 7, the symbol filter 9 performs symbol direction interpolation transfer function H (n, k _SP ) through symbol direction interpolation based on the SP carrier. Is supplied to the carrier filter 6.

The carrier filter 6 is configured by a Lagrangian interpolation filter, and estimates the data carrier transfer function H (n, k) by interpolation in the carrier direction based on the symbol direction interpolation transfer function H (n, k _SP ). This is supplied to the division circuit 12.

  The symbol synchronization circuit 19 includes a complex correlation circuit 15 and a delay profile estimation circuit 8. The complex correlation circuit 15 calculates the complex correlation of the baseband OFDM signal output from the orthogonal demodulation circuit 4 and supplies the calculated complex correlation to the delay profile estimation circuit 8. The delay profile estimation circuit 8 extracts a DU ratio representing a multipath condition and a delay time from the complex correlation of the baseband OFDM signal supplied by the complex correlation circuit 15, and the filter coefficient control circuit 13 of the waveform equalization circuit 11. To supply.

  The filter coefficient control circuit 13 controls the filter order of the Lagrangian interpolation filter constituting the carrier filter 6 according to the DU ratio and delay time fluctuation extracted by the delay profile estimation circuit 8. Specifically, the filter coefficient control circuit 13 increases the filter order when the DU ratio decreases, and decreases the filter order when the DU ratio increases. Further, the filter coefficient control circuit 13 increases the filter order when the delay time increases, and decreases the filter order when the delay time decreases. In this way, the filter coefficient control circuit 13 changes the filter order in a direction in which the interpolation capability increases when the multipath condition becomes severe, and changes the filter order in a direction in which the noise removal capability increases when the multipath condition becomes loose. To do.

The waveform equalization circuit 6 has a data extraction circuit 103. The data extraction circuit 103 extracts a data carrier from the FFT demodulated signal X _FFT (n, k) output from the FFT operation circuit 5 and supplies the data carrier to the complex division circuit 12.

The complex division circuit 12 complex-divides the data carrier extracted from the FFT demodulated signal X _FFT (n, k) by the data carrier transfer function H (n, k) to generate a waveform equalized data carrier.

  The waveform equalization circuit 11 is provided with a MER measurement circuit 14. The MER measurement circuit 14 calculates MER (Modulation Error Rate) based on the waveform equalization data carrier generated by the complex division circuit 12 and supplies it to the filter coefficient control circuit 13.

  MER is a value defined as the power ratio between the power conversion value of the vector error from the estimated ideal constellation point and the power value of the ideal constellation point in the demodulated constellation, and is expressed by the following equation. The

The MER is equivalent to CNR (Carrier to Noise Ratio), and the filter coefficient control circuit 13 corresponding to noise added during the communication path and BB signal processing is the MER calculated by the MER measuring circuit 14. Based on, the filter coefficient of the Lagrange interpolation filter constituting the carrier filter 6 is controlled.

  FIG. 3 illustrates a method in which a Lagrangian interpolation filter constituting the carrier filter 6 provided in the waveform equalization circuit 11 estimates the transfer function between SP carriers based on the output of the symbol filter 9 by interpolation processing. It is a graph of.

  A Lagrange interpolation filter is a filter whose filter coefficient h (k, D) is described by the following (Formula 1).

here,
N: the order of the Lagrange interpolation filter,
It is.

  When the FIR filter shown in the following (Expression 2) is configured by the filter coefficient h (k, D), the following (Expression 3) is obtained.

  The coefficient D shown in (Expression 1), (Expression 2), and (Expression 3) is not limited to an integer. When the coefficient D has a decimal part, a value between sampling points is obtained by interpolation processing.

  As shown in FIG. 3, the Lagrangian interpolation filter is based on the symbol direction interpolation transfer function 17 corresponding to the data carrier 20 and the symbol direction interpolation transfer function 18 corresponding to the data carrier 21. The data carrier transfer function 19 corresponding to the data carrier 22 is estimated by interpolation processing.

  4A is a diagram for explaining Lagrangian interpolation of degree 1, FIG. 4B is a diagram for explaining Lagrangian interpolation of degree 3, and FIG. It is a figure for demonstrating the Lagrange interpolation of.

  The Lagrangian interpolation filter of order N = 1 is arranged between the data carriers 23a and 23b on the basis of the symbol direction interpolation transfer functions respectively corresponding to the data carriers 23a and 23b arranged at a distance of 3 carriers along the frequency axis direction. A data carrier transfer function corresponding to the received data carrier 24 is estimated by interpolation processing.

  The Lagrangian interpolation filter of order N = 3 is based on the symbol direction interpolation transfer function corresponding to each of the four data carriers 25a, 25b, 25c, and 25d arranged every three carriers along the frequency axis direction. A data carrier transfer function corresponding to the data carrier 26 arranged between the carriers 25b and 25c is estimated by interpolation processing.

  The Lagrangian interpolation filter of order N = 5 has symbol direction interpolation transfer functions respectively corresponding to six data carriers 27a, 27b, 27c, 27d, 27e, and 27f arranged every three carriers along the frequency axis direction. Based on this, a data carrier transfer function corresponding to the data carrier 28 arranged between the data carriers 27c and 27d is estimated by interpolation processing.

  The relationship between the order N and the coefficient D of the Lagrangian interpolation filter is as shown in (Table 4) below. That is, when the order N = 1, the coefficients D = 1/3 and 2/3. When the order N = 3, the coefficients D = 1 + (1/3), 1+ (2/3). When the order N = 5, the coefficients D = 2 + (1/3), 2+ (2/3), and when the order N = 7, the coefficients D = 3 + (1/3), 3+ (2/3). ).

  FIG. 5A is a graph showing the amplitude characteristics of a Lagrangian interpolation filter constituting the carrier filter 6, and FIG. 5B is a graph showing the phase characteristics thereof.

  It has been proved strictly in terms of signal processing that the transfer function of the ideal fractional delay can be expressed by the following (Equation 4) (Non-patent Document 3).

  Referring to FIG. 5A, the amplitude characteristic 29a represents the amplitude characteristic in the ideal fractional delay. The amplitude characteristic 29b represents the amplitude characteristic when the Lagrange interpolation filter has an order N = 1. The amplitude characteristic 29c represents the amplitude characteristic when the order N = 5, and the amplitude characteristic 29d represents the amplitude characteristic when the order N = 7. It can be seen that the amplitude characteristic approaches the ideal amplitude characteristic 29a as the order of the Lagrangian interpolation filter increases to the first, fifth, and seventh orders.

  In this specification, the “ideal fractional delay frequency band” means an area where the amplitude characteristic 29a in the ideal fractional delay and the amplitude characteristic 29d of the Lagrange interpolation filter overlap in FIG. 5A (normalized frequency 0.25). Means the following frequency band). In this ideal fractional delay frequency band, the amplitude characteristic is 0 dB. In the system of digital signal processing, if frequency characteristics (amplitude characteristics / phase characteristics) as shown in FIGS. 5A and 5B are defined, the Remez method that calculates filter coefficients that approximate the given frequency characteristics is used. A filter coefficient generation method has been established.

  Referring to FIG. 5B, the phase characteristic 30a represents the phase characteristic in the ideal fractional delay. The phase characteristic 30b represents the phase characteristic when the order N of the Lagrange interpolation filter is N = 1. The phase characteristic 30c represents the phase characteristic when the order N = 5, and the phase characteristic 30d represents the phase characteristic when the order N = 7. It can be seen that the phase characteristic approaches the ideal phase characteristic 30a as the order of the Lagrangian interpolation filter increases to the first, fifth, and seventh orders.

Since the interpolation characteristic is good near the center of the Lagrangian interpolation filter, the coefficient D is
When the order N = 1: D = 0 + (1/3),
When the order N = 5: D = 2 + (1/3),
When the order N = 7: D = 3 + (1/3),
It set as follows.

  The noise removal capability of the Lagrange interpolation filter is proportional to the gap between the amplitude characteristics 29b, 29c, and 29d of the Lagrange interpolation filter and the amplitude characteristics 29a of the ideal filter. Accordingly, as the order N of the Lagrange interpolation filter increases, the noise removal capability decreases, and the relationship between the Lagrange interpolation filter order and the noise removal capability is between the Lagrange interpolation filter order and the transfer function interpolation capability. The opposite of the relationship.

here,
τ: delay time,
ρ: DU ratio,
Then, the transfer function theoretical formula in frequency selective fading (static multipath) is expressed by the following (Formula 5) (Non-Patent Document 2).

  FIG. 6A is a graph showing the amplitude of the transfer function when τ = 5 μs and DU ratio = 0 [dB], and FIG. 6B is a graph showing the phase thereof. FIG. 7A is a graph showing the amplitude of the transfer function when τ = 5 μs and DU ratio = 6 [dB], and FIG. 7B is a graph showing the phase thereof.

  The DU ratio = 0 [dB] in FIGS. 6A and 6B is smaller than the DU ratio = 6 [dB] in FIGS. 7A and 7B, and the multipath condition is severe. In FIG. 7A, the amplitude of the transfer function is always 0.2 or more, but in FIG. 6A, carrier numbers −200, −100, 0, 100, and 200 disappear due to interference in the transmission path. The amplitude of the transfer function is zero.

  FIG. 8A is a graph showing the amplitude of the transfer function when τ = 10 μs and DU ratio = 0 [dB], and FIG. 8B is a graph showing the phase thereof. FIGS. 9A and 9B are enlarged views of FIGS. 8A and 8B, respectively.

  The delay time τ = 10 μs in FIGS. 8A and 8B is larger than the delay time τ = 5 μs in FIGS. 6A and 6B, and the multipath condition is severe. The fluctuation period along the carrier number direction of the amplitude and phase of the transfer function is shorter than in the case of FIGS. 6A and 6B, and it is necessary to increase the transfer function estimation capability of the Lagrange interpolation filter.

  FIG. 10 is a graph showing the relationship between the DU ratio and the maximum / minimum ratio of the amplitude of the transfer function. As the DU ratio decreases, the maximum / minimum ratio of the amplitude of the transfer function increases, and the multipath condition becomes severe. Therefore, when the DU ratio is small, it is necessary to increase the transfer function estimation capability of the Lagrange interpolation filter.

  FIG. 11 is a graph showing the relationship between the delay time τ and the oscillation period of the transfer function. A curve 31 represents the vibration period of the transfer function, a straight line 42 represents the frequency interval of SP carriers in mode 2, and a straight line 43 represents the frequency interval of SP carriers in mode 3. As indicated by the curve 31, the longer the delay time τ, the shorter the oscillation period of the transfer function and the more severe the delay profile. Therefore, it is necessary to increase the transfer function estimation capability of the Lagrangian interpolation filter.

  FIG. 12A is a graph showing the amplitude of the transfer function in the case of Lagrangian interpolation of order = 1, and FIG. 12B is a graph showing its phase.

  Referring to FIG. 12A, a circle 33b represents the amplitude of the transfer function estimated from the SP carrier of the circle 33a by the Lagrange interpolation filter of order = 1, and the waveform 34 represents the true transfer function. It represents the amplitude. Referring to FIG. 12B, the circle 35b represents the phase of the transfer function estimated from the SP carrier of the circle 35a by the Lagrange interpolation filter of order = 1, and the waveform 36 represents the true transfer function. It represents the phase.

  FIG. 13A is a graph showing the amplitude of the transfer function in the case of Lagrangian interpolation of order 11, and FIG. 13B is a graph showing its phase.

  Referring to FIG. 13A, the circle 37b represents the amplitude of the transfer function estimated from the SP carrier of the circle 37a by the Lagrange interpolation filter of order = 11, and the waveform 38 represents the true transfer function. It represents the amplitude. Referring to FIG. 13B, the circle 39b represents the phase of the transfer function estimated from the SP carrier of the circle 39a by the Lagrange interpolation filter of order = 11, and the waveform 40 represents the true transfer function. It represents the phase.

  As shown in FIGS. 12A and 12B and FIGS. 13A and 13B, when the order N of the Lagrangian interpolation filter is increased from 1 to 11, the estimated transfer function approaches the true transfer function and becomes Lagrangian. The transfer function estimation capability of the G interpolation filter is enhanced.

  In the present embodiment, an example of an OFDM demodulator for receiving terrestrial digital broadcasting has been described, but the present invention is not limited to this. Any device that receives signals in accordance with the OFDM system may be used. For example, the present invention is also applied to a demodulator for wireless LAN, a demodulator for receiving BS digital broadcast and CS digital broadcast, and a demodulator for cable television. Can be applied.

  Moreover, although the example which comprised the carrier filter 6 by the Lagrange interpolation filter was shown in this Embodiment, this invention is not limited to this. The carrier filter 6 may be configured by an interpolation filter other than the Lagrange interpolation filter. For example, the carrier filter 6 can be configured by a spline interpolation filter, a Hermite interpolation filter, and an interpolation processing filter using a combination of upsampling and an FIR filter.

  The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims. That is, embodiments obtained by combining technical means appropriately changed within the scope of the claims are also included in the technical scope of the present invention.

  Note that in each part and each processing step of the OFDM demodulator according to the above-described embodiment, a calculation unit such as a CPU executes a program stored in a storage unit such as a ROM (Read Only Memory) or a RAM, and a communication such as an interface circuit. This can be realized by controlling the means. 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 this recording medium, a program medium such as a memory (not shown) such as a ROM may be used for processing by the microcomputer, or a program reader is provided as an external storage device (not shown). It may be a program medium that can be read by inserting a recording medium therein.

  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, showing an embodiment of the present invention, is a block diagram illustrating a main configuration of an OFDM demodulator. FIG. It is a block diagram for demonstrating the structure of the waveform equalization circuit provided in the said OFDM demodulation apparatus. It is a graph for demonstrating the method in which the Lagrangian interpolation filter which comprises the carrier filter provided in the said waveform equalization circuit estimates the transfer function between SP carriers based on the output of a symbol filter. (A) is a figure for demonstrating Lagrange interpolation of order 1, (b) is a figure for demonstrating Lagrange interpolation of order 3, (c) is explaining Lagrange interpolation of order 5. It is a figure for doing. (A) is a graph which shows the amplitude characteristic of the Lagrange interpolation filter which comprises the carrier filter provided in the said waveform equalization circuit, (b) is a graph which shows the phase characteristic. (A) is a graph showing the amplitude of the transfer function when τ = 5 μs and DU = 0 [dB], and (b) is a graph showing the phase thereof. (A) is a graph which shows the amplitude of a transfer function in case of (tau) = 5 microseconds and DU = 6 [dB], (b) is a graph which shows the phase. (A) is a graph showing the amplitude of the transfer function when τ = 10 μs and DU = 0 [dB], and (b) is a graph showing its phase. FIGS. 8A and 8B are enlarged views of FIGS. 8A and 8B, respectively. It is a graph which shows the relationship between DU ratio and the maximum / minimum ratio of the amplitude of a transfer function. It is a graph which shows the relationship between delay time (tau) and the vibration period of a transfer function. (A) is a graph which shows the amplitude of the transfer function in the case of Lagrange interpolation of degree 1, and (b) is a graph which shows the phase. (A) is a graph which shows the amplitude of the transfer function in the case of order 11 Lagrangian interpolation, and (b) is a graph which shows the phase. It is a figure which shows an example of the transmission symbol of an OFDM modulation wave. It is a block diagram which shows the structure of the conventional OFDM demodulator. It is a block diagram for demonstrating the structure of the waveform equalization circuit provided in the conventional OFDM demodulation apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 OFDM demodulator 4 Digital orthogonal demodulation circuit 5 FFT operation circuit 6 Carrier filter 7 Complex division circuit (1st complex division circuit)
8 Delay profile estimation circuit 9 Symbol filter 11 Waveform equalization circuit 12 Complex division circuit (second complex division circuit)
13 Filter Coefficient Control Circuit 14 MER Measurement Circuit 15 Complex Correlation Circuit 16 Baseband Signal Processing Unit 17 Error Correction Processing Unit 18 ADC
19 Symbol Synchronization Circuit 20 AGC Circuit 21 TMCC Decoding Circuit 22 Broadband Carrier Frequency Error Correction Circuit 23 Narrow Band Carrier Frequency Error Correction Circuit 103 Data Extraction Circuit 105 SP Extraction Circuit 106 SP Generation Circuit

Claims (14)

A digital quadrature demodulation circuit that generates a baseband signal by orthogonally demodulating an intermediate frequency signal received by an antenna and frequency-converted from a high-frequency signal by a tuner;
An FFT operation circuit that performs an FFT operation on the baseband signal to generate an FFT demodulated signal;
A waveform equalizing circuit for equalizing the FFT demodulated signal,
The waveform equalization circuit includes: a first complex division circuit configured to complex-divide an SP carrier extracted from the FFT demodulated signal by a reference SP carrier to generate an SP carrier transfer function;
A symbol filter for estimating a symbol direction interpolation transfer function based on the SP carrier transfer function;
A carrier filter for estimating a data carrier transfer function based on the symbol direction interpolation transfer function;
A second complex division circuit for complex-dividing the data carrier extracted from the FFT demodulated signal by the data carrier transfer function to generate a waveform equalized data carrier;
An OFDM demodulator comprising: a filter coefficient control circuit that controls a filter coefficient of the carrier filter according to a change in a DU ratio indicating a multipath condition and a delay time.   The filter coefficient control circuit changes the filter coefficient in a direction to widen an ideal fractional delay frequency band when the multipath condition becomes severe, and reduces the ideal fractional delay frequency band in a direction to narrow an ideal fractional delay frequency band when the multipath condition becomes loose. The OFDM demodulator according to claim 1, wherein the coefficient is changed.   2. The OFDM demodulator according to claim 1, wherein the filter coefficient control circuit changes the filter coefficient in a direction to widen an ideal fractional delay frequency band when the DU ratio becomes small.   The OFDM demodulator according to claim 1, wherein the filter coefficient control circuit changes the filter coefficient in a direction to narrow an ideal fractional delay frequency band when the DU ratio increases.   2. The OFDM demodulator according to claim 1, wherein the filter coefficient control circuit changes the filter coefficient in a direction to widen an ideal fractional delay frequency band when the delay time increases.   2. The OFDM demodulator according to claim 1, wherein the filter coefficient control circuit changes the filter coefficient in a direction to narrow an ideal fractional delay frequency band when the delay time is reduced.   The OFDM demodulator according to claim 1, wherein the carrier filter is configured by a Lagrangian interpolation filter.   8. The OFDM demodulator according to claim 7, wherein the filter coefficient control circuit increases the filter order of the Lagrangian interpolation filter when the DU ratio decreases, and decreases the filter order when the DU ratio increases.   8. The OFDM demodulator according to claim 7, wherein the filter coefficient control circuit increases the filter order of the Lagrangian interpolation filter when the delay time increases, and decreases the filter order when the delay time decreases. A delay profile estimation circuit that extracts the DU ratio and the delay time based on the baseband signal generated by the digital quadrature demodulation circuit;
The OFDM demodulator according to claim 1, wherein the filter coefficient control circuit controls the filter coefficient of the carrier filter based on the DU ratio and the delay time extracted by the delay profile estimation circuit. The waveform equalization circuit further includes a MER measurement circuit that measures MER based on the waveform equalization data carrier generated by the second complex division circuit,
The OFDM demodulator according to claim 1, wherein the filter coefficient control circuit controls a filter coefficient of the carrier filter based on the MER measured by the MER measurement circuit. A baseband signal is generated by orthogonally demodulating an intermediate frequency signal received by an antenna and frequency-converted from a high frequency signal by a tuner,
FFT calculation of the baseband signal to generate an FFT demodulated signal,
An SP carrier transfer function is generated by complex division of the SP carrier extracted from the FFT demodulated signal by a reference SP carrier,
Estimating a symbol direction interpolation transfer function based on the SP carrier transfer function;
Estimating the data carrier transfer function based on the symbol direction interpolation transfer function while controlling the filter coefficient of the carrier filter according to the variation of the DU ratio indicating the multipath condition and the delay time,
An OFDM demodulation method, wherein a waveform equalized data carrier is generated by complex-dividing a data carrier extracted from the FFT demodulated signal by the data carrier transfer function. A procedure for generating a baseband signal by orthogonally demodulating an intermediate frequency signal received by an antenna and frequency-converted from a high frequency signal by a tuner to a computer;
A procedure for performing an FFT operation on the baseband signal to generate an FFT demodulated signal;
A procedure of generating a SP carrier transfer function by complex-dividing an SP carrier extracted from the FFT demodulated signal by a reference SP carrier;
Estimating a symbol direction interpolation transfer function based on the SP carrier transfer function;
A procedure for estimating a data carrier transfer function based on the symbol direction interpolation transfer function while controlling a filter coefficient of a carrier filter according to a change in a DU ratio indicating a multipath condition and a delay time;
A program for executing a complex division of a data carrier extracted from the FFT demodulated signal by the data carrier transfer function to generate a waveform equalized data carrier. A procedure for generating a baseband signal by orthogonally demodulating an intermediate frequency signal received by an antenna and frequency-converted from a high frequency signal by a tuner to a computer;
A procedure for performing an FFT operation on the baseband signal to generate an FFT demodulated signal;
A procedure of generating a SP carrier transfer function by complex-dividing an SP carrier extracted from the FFT demodulated signal by a reference SP carrier;
Estimating a symbol direction interpolation transfer function based on the SP carrier transfer function;
A procedure for estimating a data carrier transfer function based on the symbol direction interpolation transfer function while controlling a filter coefficient of a carrier filter according to a change in a DU ratio indicating a multipath condition and a delay time;
A computer-readable recording medium having recorded thereon a program for executing a procedure of generating a waveform equalized data carrier by complex-dividing a data carrier extracted from the FFT demodulated signal by the data carrier transfer function.

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