JP2004343546A - Ofdm signal demodulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an OFDM signal demodulator for equalizing a multipath exceeding a guard interval, and capable of immediately following up variation in multipaths to demodulate properly an OFDM signal, even if there is a large variation in the multipaths. <P>SOLUTION: The OFDM signal demodulator comprises a window processing means 14, a frequency characteristic equalizing means 16, a symbol extracting means 17, and a component filtering out means 18. The window processing means 14 conducts window processing so that not only effective symbols to be demodulated but also effective symbols existing in a time axis before and after the effective symbols to be demodulated are included in a time window. The frequency characteristic equalizing means 16 equalizes a multipath which arrives early in time and a multipath which arrives late in time. The symbol extracting means 17 extracts frequency characteristics of the effective symbols to be demodulated. The component filtering-out means 18 filters out frequency components other than quadrature frequency component for the effective symbols to be demodulated. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) signal receiving apparatus, and more particularly, to an OFDM signal demodulating apparatus that correctly demodulates bit data from an OFDM signal including a multipath exceeding a guard interval.
[0002]
[Prior art]
As shown in FIG. 10A, a conventional OFDM signal demodulator 80 includes a down converter 81, an AD converter 82, a quadrature demodulator 83, a window processor 84, a Fourier transformer 85, The apparatus includes a characteristic equalizing unit 86, a region determining unit 87, and a data demodulating unit 88. The OFDM signal transmitted from the transmission source is composed of a plurality of effective symbols having a predetermined time width. The OFDM signal is received by the antenna 8 and is input to the OFDM signal demodulator 80. Unnecessary frequency components of the received signal are removed by the downconverter 81, sampled at a predetermined sample frequency by the AD converter 82, and orthogonally demodulated by the orthogonal demodulator 83. The orthogonally demodulated OFDM signal is subjected to FFT window processing for each effective symbol by window processing means 84, and FFT for each effective symbol subjected to FFT window processing is applied to Fourier transform means 85, and the time domain signal is converted. It is converted to a frequency domain signal. The propagation characteristic of the path on which the OFDM signal has propagated is calculated by the frequency characteristic equalizing means 86, and the propagation characteristic superimposed on the frequency domain signal is canceled, that is, the frequency domain signal is equalized. Then, based on the frequency domain signal after the equalization, vector data of the real axis and the imaginary axis are extracted by the area determining means 87 for each subcarrier, and the bit data based on the vector data is demodulated by the data demodulating means 88. Is done.
[0003]
Also, an OFDM signal demodulator capable of correctly reproducing bit data even if a multipath due to a delayed wave exceeding a guard interval inserted between effective symbols is included in the OFDM signal has already been proposed. . As shown in FIG. 10B, the OFDM signal demodulation device 90 includes a filter coefficient calculation unit 91 and a time domain equalization unit 92 in addition to the configuration of the OFDM signal demodulation device 80. 91 calculates a predetermined filter coefficient based on the signal fed back from the Fourier transform means 85 and the area determination means 87, and the time domain equalization means 92 uses the filter coefficient calculated by the filter coefficient calculation means 91. The signal output from the quadrature demodulation unit 83 is subjected to a filtering process (for the filter coefficient calculation unit 91 and the time domain equalization unit 92, see, for example, Patent Documents 1, 2, and 3). ). In this configuration, since the multipath signal components included in the received signal are removed by the filtering process by the time domain equalizing means 92, the bit data modulated into the OFDM signal is correctly demodulated.
[0004]
[Patent Document 1]
JP 2000-295195 A
[Patent Document 2]
JP-A-2000-341242
[Patent Document 3]
JP 2001-237749 A
[0005]
[Problems to be solved by the invention]
However, in such a conventional OFDM signal demodulation apparatus, a filter coefficient for removing a multipath signal component exceeding a guard interval is calculated by feedback, and it takes time until a useful filter coefficient is calculated. However, there is a problem that the OFDM signal cannot be correctly demodulated immediately following a large variation in the multipath.
[0006]
The present invention has been made in order to solve such a problem, and equalizes a multipath exceeding a guard interval, and immediately follows a change in the multipath even if there is a large change in the multipath. An object of the present invention is to provide an OFDM signal demodulation device capable of correctly demodulating an OFDM signal.
[0007]
[Means for Solving the Problems]
An OFDM signal demodulator according to claim 1 of the present invention is an OFDM signal demodulator that receives an OFDM signal transmitted from a predetermined transmission source and demodulates bit data for each effective symbol. Window processing means for performing window processing on the time domain of the OFDM signal including the effective symbol to be demodulated using a time window having a time width of a power of 2 times the symbol time width; Fourier transform means for transforming the applied OFDM signal from a time-domain signal to a frequency-domain signal by performing a Fourier transform, and a propagation from the frequency-domain signal of the OFDM signal to the window processing means from the transmission source. Frequency characteristic equalizing means for calculating a characteristic and equalizing a frequency characteristic of the OFDM signal in accordance with the calculated propagation characteristic; Symbol extracting means for extracting a frequency component of the demodulated effective symbol from the OFDM signal whose frequency characteristic has been equalized by the demodulating means; And a component removing unit that removes the other frequency components while leaving the orthogonal frequency components of the subcarriers of the OFDM signal transmitted from.
[0008]
With this configuration, window processing is performed so that not only the effective symbol to be demodulated but also the effective symbol existing before and after the effective symbol to be demodulated are included in the time window, and the received signal arrives with a time delay. And the multipath that arrives earlier in time are equalized, the frequency domain signal of the effective symbol to be demodulated is extracted by the symbol extracting means, and the frequency other than the orthogonal frequency component of each subcarrier of the effective symbol to be demodulated is extracted. The area signal is removed by the component removing means. Therefore, every time one effective symbol is demodulated, the signal component of the multipath can be removed in real time, and even if there is a large change in the multipath, the OFDM can be immediately followed up to the change of the multipath. The signal can be demodulated correctly. Further, conventionally, the multipath signal component that can be removed is only the multipath that arrives with a delay in time, but the OFDM signal demodulator of the present invention can also remove the multipath signal component that arrives in a short time. It can also be removed.
[0009]
An OFDM signal demodulator according to claim 2 of the present invention is an OFDM signal demodulator that receives an OFDM signal transmitted from a predetermined transmission source and demodulates bit data for each effective symbol, First window processing means for performing window processing on the OFDM signal including an effective symbol to be demodulated using a first time window having a time width of a power of 2 times the time width of the effective symbol; First Fourier transform means for transforming a signal in the time domain into a signal in the frequency domain by performing a Fourier transform on the OFDM signal subjected to window processing by the window processing means; Frequency characteristic equalization for calculating a propagation characteristic from a transmission source to the first window processing means and for equalizing a frequency characteristic of the OFDM signal according to the calculated propagation characteristic Transforming the OFDM signal, whose frequency characteristic has been equalized, from a frequency-domain signal to a time-domain signal by inverse Fourier transforming the OFDM signal whose frequency characteristic has been equalized by the frequency characteristic equalizing means. Inverse Fourier transforming means, and the time domain of the effective symbol to be demodulated included in the time domain of the OFDM signal converted into a signal in the time domain by the inverse Fourier transforming means. A second window processing means for performing window processing using a second time window having a time width; and a Fourier transform of the OFDM signal window-processed by the second window processing means, thereby providing a time domain And a second Fourier transforming means for converting the signal of the frequency domain into a signal of the frequency domain.
[0010]
With this configuration, window processing is performed so that not only the effective symbol to be demodulated but also the effective symbol existing before and after the effective symbol to be demodulated are included in the time window, and the received signal arrives with a time delay. After the multipath to be performed and the multipath that arrives earlier in time are equalized, and the time domain of the effective symbol to be demodulated is extracted by the second time window, the frequency characteristic of the effective symbol to be demodulated is calculated. Therefore, every time one effective symbol is demodulated, the signal component of the multipath can be removed in real time, and even if there is a large change in the multipath, the OFDM can be immediately followed up to the change of the multipath. The signal can be demodulated correctly.
[0011]
Further, in the OFDM signal demodulating apparatus according to claim 3 of the present invention, the frequency characteristic equalizing means calculates the propagation characteristic from a frequency domain signal of the OFDM signal before the frequency characteristic is equalized. It has a configuration having a calculation circuit unit and an equalization circuit unit for equalizing the frequency characteristics of the OFDM signal according to the propagation characteristics.
[0012]
With this configuration, since the frequency domain signal is equalized after the propagation characteristic is calculated using the feedforward type circuit, the propagation characteristic can be calculated in real time, and a change in the propagation characteristic can be immediately dealt with.
[0013]
According to a fourth aspect of the present invention, in the OFDM signal demodulating apparatus according to the first or second aspect, the frequency characteristic equalizing means may adjust a frequency characteristic of the OFDM signal according to the calculated propagation characteristic. And a propagation characteristic from the transmission source to the window processing means from the signal in the frequency domain of the OFDM signal whose frequency characteristics have been equalized by the equalization circuit unit and the calculated propagation. A propagation characteristic calculation circuit for calculating a difference propagation characteristic representing a difference from the characteristic; and a propagation characteristic update circuit for adding the difference propagation characteristic to the calculated propagation characteristic and updating the calculated propagation characteristic. And a part having the same.
[0014]
With this configuration, the propagation characteristics are calculated using the feedback type circuit, and the frequency characteristics of the OFDM signal are equalized. Therefore, the accuracy of equalization can be improved each time the operation is repeated, and the demodulation accuracy of the OFDM signal can be improved. Can be increased.
[0015]
An OFDM signal demodulation device according to claim 5, wherein the frequency characteristic equalizing means equalizes the frequency characteristic of the OFDM signal in accordance with the calculated propagation characteristic. And from the signal in the frequency domain of the OFDM signal, the frequency characteristic of which has been equalized by the first equalizing circuit, from the transmission characteristic from the transmission source to the window processing means and the calculated propagation characteristic. A propagation characteristic calculating circuit for calculating a difference propagation characteristic representing a difference, a propagation characteristic updating circuit for adding a difference propagation characteristic to the calculated propagation characteristic, and updating the calculated propagation characteristic; In accordance with the characteristic, a second equalizer circuit for further equalizing the frequency characteristic of the OFDM signal equalized by the first equalizer circuit is provided.
[0016]
With this configuration, the propagation characteristic is calculated using the feedback type circuit, and the frequency characteristic of the OFDM signal is equalized. Therefore, the accuracy of equalization can be improved each time the operation is repeated, and further, the feedforward type Even when a large variation occurs in the propagation characteristic of the OFDM signal using a circuit, it is possible to calculate the variation difference of the propagation characteristic in real time without delaying the variation and to equalize the frequency characteristic of the OFDM signal again. Therefore, the accuracy of OFDM signal demodulation can be improved.
[0017]
According to a sixth aspect of the present invention, in the OFDM signal demodulation apparatus according to any one of the third to fifth aspects, the frequency characteristic equalizing means may perform an inverse Fourier transform on the reciprocal of the calculated propagation characteristic. Calculate the impulse response obtained by detecting the maximum value of the absolute value of the impulse response, the time interval from the beginning of the impulse response to the impulse response at which the detected absolute value is the maximum before the equalization The impulse response is shifted in the time axis direction so as to match the time interval from the head of the time window used for the window processing applied to the OFDM signal to the effective symbol to be demodulated, and the impulse shifted in the time axis direction The configuration is such that the response is Fourier-transformed to calculate the propagation characteristic again, and the frequency characteristic of the OFDM signal is equalized according to the calculated propagation characteristic again.
[0018]
With this configuration, the frequency domain signal of the effective symbol to be demodulated can be reliably extracted, and the phase rotation of each subcarrier can be prevented to output accurate bit data.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
As shown in FIG. 1, an OFDM signal demodulation apparatus 10 according to an embodiment of the present invention includes a down converter 11, an AD converter 12, a quadrature demodulator 13, a window processor 14, a Fourier transformer 15, , Frequency characteristic equalizing means 16, symbol extracting means 17, component removing means 18, area determining means 19, and data demodulating means 20.
It is assumed that the OFDM signal demodulation device 10 naturally includes the antenna 1 that receives the OFDM signal transmitted from the transmission source. The OFDM signal transmitted from the transmission source has an effective symbol length (effective symbol time width) of Tu [seconds], N number of FFT (fast Fourier transform) points per symbol at the time of modulation, and FFT sampling rate. Is R [Hz]. Here, N is an integer of a power of 2, and R is N / Tu [Hz]. Further, the OFDM signal is configured by L subcarriers, and each subcarrier has a different frequency by 1 / Tu [Hz]. Note that the number L of subcarriers included in the OFDM signal is smaller than the number N of FFT points.
[0020]
The down-converter 11 removes unnecessary frequency components related to the OFDM signal received via the antenna 1 by filtering, and converts the frequency of the OFDM signal into a signal having a center frequency R [Hz].
The AD converter 12 samples the OFDM signal frequency-converted by the down-converter 11 at equal intervals, and converts an analog signal into a digital signal, that is, performs AD conversion. The sampling frequency in the AD converter 12 is 4R [Hz].
[0021]
The quadrature demodulation unit 13 performs quadrature demodulation on the OFDM signal that has been analog-to-digital converted by the AD conversion unit 12, and changes the sampling frequency of the quadrature demodulated signal from 4R [Hz] to R [Hz], that is, downsamples. It has become. In the following description, an OFDM signal that is orthogonally demodulated and downsampled to a sampling frequency R [Hz] is referred to as a time domain signal. In addition, the time domain signal is represented by a complex function in which a variable is time.
In the embodiment of the present invention, the OFDM signal is subjected to quadrature demodulation after AD conversion, but the OFDM signal may be subjected to AD conversion after quadrature demodulation. In this case, the sampling frequency at the time of AD conversion is R [Hz].
[0022]
The window processing means 14 applies a rectangular time window having a predetermined time width to the OFDM signal, that is, the time domain signal, which has been quadrature demodulated and downsampled by the quadrature demodulation means 13, that is, performs window processing. The time-domain signal to which the time window is applied includes an effective symbol #i to be demodulated, as shown in FIG. The time window is N 2 ^m Including (integral of a power of 2) times the number of sampling data, that is, 2 of Tu ^m It has a double time width. Here, m is a positive integer. In FIG. 2, m is 2. In FIG. 2, the time window includes an effective symbol # i-1, an effective symbol #i, an effective symbol # i + 1, a guard interval # i-1, a guard interval #i, a guard interval # i + 1, and a guard interval # i + 2. It can be applied to the time domain of the OFDM signal.
[0023]
Here, the time window as shown in FIG. 2 is based on the main wave having the largest signal power, the multipath arriving later than the main wave, and the time window more than the main wave. In order to equalize and demodulate a multipath that arrives early, an effective symbol #i to be demodulated is set to be located at the center of the time window. When the OFDM signal demodulator 10 equalizes and demodulates the main wave and the multipath that arrives later in time than the main wave, the time window is such that the effective symbol #i to be demodulated is located ahead of the time window. (Eg, the position of the effective symbol # i-1 shown in FIG. 2). When the OFDM signal demodulator 10 equalizes and demodulates the main wave and the multipath that arrives earlier than the main wave, the time window is such that the effective symbol #i to be demodulated is behind the time window. (For example, it may be set so as to exist at the position of the effective symbol # i + 1 shown in FIG. 2). In the following description, a multipath that arrives later in time than the main wave is called a delayed wave, and a multipath that arrives earlier in time than the main wave is called an early arrival wave.
[0024]
Referring back to FIG. 1, the Fourier transform unit 15 calculates the number of N of the N that has been subjected to the window processing by the window processing unit 14. ^m A time-domain signal having twice the number of points is subjected to a fast Fourier transform, and N of 2 ^m The frequency domain signal is converted into a frequency domain signal having twice the number of points, and the frequency domain signal is output to the frequency characteristic equalizing means 16. The frequency domain signal has a center frequency of 0 [Hz], a bandwidth of R [Hz], and an interval R / (2 ^m × N) A signal sampled at [Hz]. Further, the frequency domain signal is represented by a complex function using a variable as a frequency.
[0025]
The frequency characteristic equalizing means 16 calculates a propagation characteristic from the frequency domain signal, and equalizes the frequency characteristic of the frequency domain signal converted by the Fourier transform means 15 according to the calculated propagation characteristic. . Here, the frequency characteristic equalizing means 16 includes, for example, a propagation characteristic calculating circuit section 21, a delay circuit section 26, and a complex division circuit section 27 (equalizing circuit section) as shown in FIG. .
In FIG. 3, the propagation characteristic calculation circuit 21 calculates a signal representing the propagation characteristics of each subcarrier in the path from the transmission source to the frequency characteristic equalization means 16 of the OFDM signal. Note that the signal representing the propagation characteristic is represented by a complex function using a variable as a frequency. Further, as shown in FIG. 3, for example, the propagation characteristic calculating circuit 21 includes an inverse Fourier transform circuit 21a, a window processing circuit 21b, a Fourier transform circuit 21c, a training extraction circuit 21d, a training generation circuit 21e, and a 1 complex division circuit 21f, a first interpolation filter 21g, a first delay circuit 21h, a second complex division circuit 21i, a region determination circuit 21j, a second delay circuit 21k, and a third , And a second interpolation filter 21m.
[0026]
The inverse Fourier transform circuit 21a performs an inverse fast Fourier transform on the frequency domain signal output by the Fourier transform means 15 to obtain the number of points N of 2 ^m Double the frequency domain signal with the number of points N of 2 ^m The signal is converted into a double time domain signal, and the time domain signal is output to the window processing circuit 21b.
The window processing circuit 21b multiplies the time domain of the effective symbol to be demodulated in the time domain signal output by the inverse Fourier transform circuit 21a by a rectangular time window having a time width Tu [sec] to obtain a signal in the time domain having N points. Is to be extracted.
[0027]
The Fourier transform circuit 21c performs a fast Fourier transform on the time-domain signal extracted by the window processing circuit 21b to convert the time-domain signal into a frequency domain signal having the number of points N, and converts the frequency domain signal into a training extraction circuit 21d. To the delay circuit 21h and the second delay circuit 21k.
The training extracting circuit 21d extracts a training component from the frequency domain signal output by the Fourier transform circuit 21c. Here, the training is transmitted from the transmission source together with the OFDM signal, and differs depending on the OFDM signal system. The training may be a special symbol obtained by modulating a known pattern or a pilot symbol. In the case of the terrestrial digital broadcasting system, the training uses a pilot signal called SP (Scattered Pilot) or CP (Coinual Pilot). Note that the training is a signal that exists for each subcarrier frequency.
[0028]
The training generation circuit 21e generates the same training as the training transmitted from the transmission source.
The first complex division circuit 21f uses the training component extracted by the training extraction circuit 21d as a numerator and the training generated by the training generation circuit 21e as a denominator to perform complex division so that both frequencies match. Is output to the first interpolation filter 21g. Here, the signal obtained by the complex division by the first complex division circuit 21f is called a pseudo propagation characteristic signal. The pseudo propagation characteristic signal is a signal obtained for each subcarrier frequency.
[0029]
The first interpolation filter 21g inputs the signal output by the first complex division circuit 21f to the second complex division circuit 21i, and furthermore, no training is extracted by the training extraction circuit 21d. Also in this case, the signal output by the first complex division circuit 21f is output to the second complex division circuit 21i.
The signal in the frequency domain output by the Fourier transform circuit 21c is input to the first delay circuit 21h, and the first delay circuit 21h has a time corresponding to the time required to calculate the pseudo propagation characteristic signal. The delayed frequency domain signal is output to the second complex divider 21i.
The second complex division circuit 21i uses the frequency domain signal output from the first delay circuit 21h as a numerator and the pseudo propagation characteristic signal output from the first interpolation filter 21g as a denominator so that both frequencies match. Is performed, and a signal in the frequency domain obtained by the complex division is output to the domain determination circuit 21j. The signal output by the second complex division circuit 21i is a signal obtained for each subcarrier frequency.
[0030]
The domain determination circuit 21j determines the values of the real axis and the imaginary axis of each subcarrier based on the frequency domain signal output by the second complex division circuit 21i, and divides the obtained value for each subcarrier frequency. Is output to the third complex division circuit 211.
The frequency domain signal output from the Fourier transform circuit 21c is input to the second delay circuit 21k. The second delay circuit 21k includes a training extraction circuit 21d, a training generation circuit 21e, The first complex division circuit 21f, the first interpolation filter 21g, the second complex division circuit 21i, and the input signal in the frequency domain are delayed by the time required for the processing of the area determination circuit 21j to perform the third complex division. It outputs to the circuit 211.
[0031]
The third complex division circuit 211 uses the frequency domain signal output from the second delay circuit 21k as a numerator and the subcarrier frequency output value output from the area determination circuit 21j as a denominator so that both frequencies match. A complex division is performed to calculate a signal representing a propagation characteristic of each subcarrier frequency.
The second interpolation filter 21m interpolates the propagation characteristic of the frequency between the subcarriers based on the signal representing the propagation characteristic for each frequency of the subcarrier calculated by the third complex division circuit 211, A signal representing the propagation characteristic obtained by interpolation and interpolation is output to the complex division circuit unit 27.
[0032]
The frequency domain signal output by the Fourier transform means 15 is input to the delay circuit unit 26, and the delay circuit unit 26 converts the input frequency domain signal into the propagation characteristic calculated by the propagation characteristic calculation circuit unit 21. Is output to the complex division circuit 27 after being delayed by the time required for the calculation.
The complex division circuit 27 receives the signal representing the propagation characteristic output from the propagation characteristic calculation circuit 21 and the frequency domain signal output from the delay circuit 26, and receives the complex division circuit 27. Is configured to equalize the frequency characteristics of an OFDM signal represented by a frequency domain signal by performing complex division using a frequency domain signal as a numerator and a signal representing a propagation characteristic as a denominator. In the complex division performed by the complex division circuit 27, the frequency characteristic equalizing means 16 is configured so that the frequencies indicated by both the frequency domain signal and the signal representing the propagation characteristic match.
[0033]
In FIG. 1 again, the symbol extracting means 17 performs a predetermined filtering process on the frequency domain signal equalized by the frequency characteristic equalizing means 16 and emphasizes only the frequency domain signal relating to the effective symbol to be demodulated to remove the component. 18 is output. Here, the filter used in the symbol extracting means 17 is a FIR (Finite Impulse Response) type filter using a filter coefficient obtained by performing a Fourier transform on a signal in a time domain of a rectangular window having a time width Tu [sec]. The filtering by the symbol extracting unit 17 is performed by convolving the filter coefficient obtained as described above with the frequency domain signal equalized by the frequency characteristic equalizing unit 16. Note that the number of points of the frequency domain signal output by the symbol extracting means 17 is N = 2 ^m It is twice.
Note that the rectangular window serving as a source of the filter used by the symbol extracting means 17 is located near the time domain of the effective symbol to be demodulated in the time domain of the time window used by the window processing means 14. As shown in FIG. 2, a part of the guard interval #i located immediately before the effective symbol #i to be demodulated may be included.
[0034]
Referring again to FIG. 1, the component removing unit 18 outputs N (2) from the frequency domain signal output by the symbol extracting unit 17. ^m -1) By removing data having twice the number of points, 2 times the number of points N ^m The double frequency domain signal is converted into a frequency domain signal having the number of points N, and the converted frequency domain signal having the number of points N is output to the domain determination means 19.
In the component removing means 18, a function obtained when Fourier transform is performed on the time domain signal of the rectangular window having the time width Tu [sec] used as a filter in the symbol extracting means 17, sin (ω) / ω is 0. A frequency domain signal related to a frequency that is not required is removed. That is, the component removing means 18 removes frequency components not necessary for demodulation, while leaving only the frequency components of each orthogonal subcarrier relating to the effective symbol to be demodulated.
Here, the signal component of the minimum frequency among the frequency domain signals of the number N of points output by the component removing means 18 is two times the number of points N output by the symbol extracting means 17. ^m It is designed to match the signal component of the minimum frequency of the double frequency domain signal. The signal component located at the k-th position from the minimum frequency in the frequency domain signal having the number of points N output by the component removing means 18 is 2 of the number of points N output by the symbol extracting means 17. ^m K × 2 from the minimum frequency of the double frequency domain signal ^m It matches with the signal component located at the second position. That is, assuming that the output signal of the component removing means 18 is Fo and the output signal of the symbol extracting means 17 is Fi, Fo (k) = Fi (k × 2 ^m ) Is established. Note that k is a positive integer.
[0035]
The region determining unit 19 determines the values of the real axis and the imaginary axis of each subcarrier based on the frequency domain signal of each subcarrier output by the component removing unit 18, and outputs the obtained value to the data demodulating unit 20. Output.
The data demodulation unit 20 determines the bits allocated to each subcarrier based on the values of the real axis and imaginary axis of each subcarrier output by the region determination unit 19, and performs deinterleaving and error correction according to the OFDM signal system. After performing such operations, bit data is output to an external device.
[0036]
Here, the operation of the OFDM signal demodulation device 10 according to the embodiment of the present invention will be described.
When the OFDM signal (transmission signal) transmitted from the transmission source, the characteristics of the propagation path on which the transmission signal propagates (propagation path characteristics), and the time window are represented as signals in the frequency domain, as shown in FIG. (Ω), H (ω) and W (ω). Here, ω is an angular velocity representing a frequency. The frequency characteristic of the signal received by the OFDM signal demodulator 10 is expressed as X (ω) · H (ω) because it is the product of the characteristic at the time of transmission and the characteristic of the propagation path. In addition, since the signal received by the OFDM signal demodulator 10 is subjected to window processing by the window processing means 14, the frequency domain signal after the window processing and FFT is X (ω) · H (ω) · W ( ω). Further, the propagation characteristic for each subcarrier of the OFDM signal calculated in the propagation characteristic calculation circuit unit 21 of the frequency characteristic equalization unit 16 is represented by H ′ (ω), and is equalized by the frequency characteristic equalization unit 16. The subsequent frequency domain signal is expressed as X (ω) · H (ω) · W (ω) / H ′ (ω). Note that the ideal equalization condition is H ′ (ω) = H (ω) · W (ω).
[0037]
Here, it is assumed that the OFDM signal demodulator 10 receives not only the main wave but also a delayed wave or an early arrival wave exceeding the guard interval. The effective symbol to be demodulated is an effective symbol existing before the effective symbol to be demodulated included in the delayed wave (hereinafter, referred to as “post-ghost”) or an effective symbol to be demodulated included in the early arrival wave. It is affected by interference due to an effective symbol (hereinafter, referred to as “pre-ghost”) that exists later in time.
If the time window applied to the time domain signal by the window processing means 14 includes only the effective symbol to be demodulated, even if H ′ (ω) can be estimated correctly, the signal extracted in the time window can be used. Since no effective symbol serving as a source of a rear ghost or an effective symbol serving as a source of a front ghost is included therein, it is impossible to perform equalization correctly in the frequency characteristic equalizer 16.
In the window processing means 14 according to the OFDM signal demodulation device 10 according to the embodiment of the present invention, not only the effective symbol to be demodulated but also the effective symbol existing before the effective symbol to be demodulated or the effective symbol to be demodulated is displayed. The window processing is performed so that an effective symbol that is temporally later than the symbol is also included in the time window. As a result, the rear ghost or the front ghost can be correctly equalized by the frequency characteristic equalizing means 16, so that the frequency characteristic of the effective symbol to be demodulated can be correctly equalized.
[0038]
Then, the frequency domain signal of the effective symbol to be demodulated is extracted by the symbol extracting means 17, the frequency domain signal other than the orthogonal frequency component of the effective symbol to be demodulated is removed by the component removing means 18, and the bit data included in the effective symbol to be demodulated is removed. Is reproduced by the area determining means 19 and the data demodulating means 20.
[0039]
As described above, the OFDM signal demodulation apparatus according to the embodiment of the present invention includes not only the effective symbols to be demodulated but also the effective symbols existing before and after the effective symbols to be demodulated in the time window. The window processing is performed so as to be included, the multipath arriving later in time and the multipath arriving earlier in time are equalized, and the frequency domain signal of the effective symbol to be demodulated is extracted by the symbol extracting means, Frequency domain signals other than the orthogonal frequency component of each subcarrier of the effective symbol to be demodulated are removed by the component removing means. Therefore, every time one effective symbol is demodulated, the signal component of the multipath can be removed in real time, and even if there is a large change in the multipath, the OFDM can be immediately followed up to the change of the multipath. The signal can be demodulated correctly. Furthermore, conventionally, the multipath signal component that can be removed is only the multipath that arrives with a time delay, but the OFDM signal demodulator according to the embodiment of the present invention has the multipath signal component that arrives with a time delay. It can also remove signal components.
[0040]
Note that the OFDM signal demodulation device of the present invention may be configured as shown in FIG. 5, and in this configuration, the same effect as that of the above-described OFDM signal demodulation device 10 can be obtained. As shown in FIG. 5, the OFDM signal demodulator 40 includes a down converter 11, an AD converter 12, a quadrature demodulator 13, a first window processor 44, a first Fourier transformer 45, It comprises a frequency characteristic equalizing means 16, an inverse Fourier transforming means 41, a second window processing means 42, a second Fourier transforming means 43, a region judging means 19, and a data demodulating means 20. In the OFDM signal demodulation device 40, the same components as those of the above-described OFDM signal demodulation device 10 are denoted by the same reference numerals as those of the OFDM signal demodulation device 10, and a description thereof will be omitted. Further, the first window processing means 44 and the first Fourier transform means 45 correspond to the above-described window processing means 14 and Fourier transform means 15, respectively, and therefore the description thereof is omitted.
[0041]
The inverse Fourier transform unit 41 performs an inverse fast Fourier transform on the equalized frequency domain signal output by the frequency characteristic equalizing unit 16 to obtain the number of points N of 2 ^m Double the frequency domain signal with the number of points N of 2 ^m The signal is converted into a double time domain signal, and the time domain signal is output to the second window processing means 42. The second window processing means 42 applies a rectangular time window (second time window) having a time width Tu [sec] to the time domain of the effective symbol to be demodulated in the time domain signal output by the inverse Fourier transform means 41. , And a time domain signal having the number of points N is extracted. Since the frequency domain signal has already been equalized by the frequency characteristic equalization unit 16, the time domain signal to which the time window is applied by the second window processing unit 42 is the same as the time domain signal representing the effective symbol to be demodulated. The time domain signal to which the time window is applied does not include the components of other effective symbols. The second Fourier transform unit 43 performs a fast Fourier transform on the time domain signal extracted by the second window processing unit 42 to convert the time domain signal into a frequency domain signal having N points. That is, the OFDM signal demodulation device 40 performs the processing performed by the symbol extraction means 17 and the component removal means 18 of the OFDM signal demodulation device 10 by the inverse Fourier transform means 41, the second window processing means 42, and the second Fourier transform means. 43 to reproduce bit data from an OFDM signal including a multipath.
[0042]
With this configuration, window processing is performed so that not only the effective symbol to be demodulated but also the effective symbol existing before and after the effective symbol to be demodulated are included in the time window, and the received signal arrives with a time delay. After the multipath to be performed and the multipath that arrives earlier in time are equalized, and the time domain of the effective symbol to be demodulated is extracted by the second time window, the frequency characteristic of the effective symbol to be demodulated is calculated. Therefore, every time one effective symbol is demodulated, the signal component of the multipath can be removed in real time, and even if there is a large change in the multipath, the OFDM can be immediately followed up to the change of the multipath. The signal can be demodulated correctly.
[0043]
The time width of the time window in the window processing means 14 or the first window processing means 44 according to the OFDM signal demodulation device 10 or the OFDM signal demodulation device 40 according to the embodiment of the present invention is, as shown in FIG. / H ′ (ω) is subjected to inverse Fourier transform to check the impulse response characteristics in the time domain, and is set so as to be larger than the time width from the first point A to the last point B where the DU ratio is equal to or larger than the threshold value. It has become so. Here, the DU ratio is obtained by calculating the maximum value D of the absolute value of the impulse response characteristic obtained by performing an inverse Fourier transform of 1 / H ′ (ω), and calculating the ratio of the absolute value U of each impulse response to the maximum value D. It means the value obtained by calculation. Note that the threshold value of the DU ratio is set in advance.
[0044]
Further, the frequency characteristic equalizer 16 according to the OFDM signal demodulator 10 or the OFDM signal demodulator 40 according to the embodiment of the present invention is configured as shown in FIG. 3 and uses a feedforward type circuit. Since the propagation characteristic is calculated and the frequency domain signal is equalized, the propagation characteristic can be calculated in real time and the fluctuation of the propagation characteristic can be immediately dealt with. It may be configured as shown in FIG.
[0045]
In FIG. 7, the frequency characteristic equalizing means 16 includes a propagation characteristic calculating circuit unit 37, a complex division circuit unit 28 (equalizing circuit unit), a propagation characteristic updating circuit unit 29, a first delay circuit unit 30, And a second delay circuit section 31.
The complex division circuit unit 28 performs complex division using the frequency domain signal output by the Fourier transform unit 15 as a numerator and a signal representing the propagation characteristic output by the first delay circuit unit 30 as a denominator. The output frequency domain signal is equalized.
The propagation characteristic calculation circuit unit 37 calculates a difference representing a difference between a propagation characteristic used for equalization in the complex division circuit unit 28 and a propagation characteristic superimposed on the frequency domain signal before being equalized by the complex division circuit unit 28. The propagation characteristics are calculated. Note that the propagation characteristic calculation circuit unit 37 may have the same configuration as the propagation characteristic calculation circuit unit 21 illustrated in FIG. The difference between the propagation characteristic calculation circuit unit 37 and the propagation characteristic calculation circuit unit 21 is that the propagation characteristic calculation circuit unit 21 calculates the propagation characteristic based on the output signal of the Fourier transform unit 15 or the first Fourier transform unit 45. On the other hand, the propagation characteristic calculation circuit unit 37 calculates the propagation characteristics based on the frequency domain signal once equalized by the complex division circuit unit 28.
[0046]
The propagation characteristic updating circuit unit 29 updates the propagation characteristic by adding the signal representing the difference propagation characteristic calculated by the propagation characteristic calculation circuit unit 37 and the signal output by the second delay circuit unit 31. , And outputs a signal representing the updated propagation characteristic to the first delay circuit unit 30 and the second delay circuit unit 31.
A signal representing the propagation characteristic updated by the propagation characteristic update circuit unit 29 is input to the second delay circuit unit 31, and the second delay circuit unit 31 After the signal to be represented is delayed by the time corresponding to one effective symbol length, the signal is input to the propagation characteristic updating circuit unit 29 again.
A signal representing the propagation characteristic updated by the propagation characteristic updating circuit unit 29 is input to the first delay circuit unit 30, and the first delay circuit unit 30 outputs the signal to the complex division circuit unit 28. The input signal representing the propagation characteristic is delayed and output so that the frequency indicated by the input frequency domain signal matches the frequency indicated by the signal representing the propagation characteristic.
With the configuration of the frequency characteristic equalizing means 16 as shown in FIG. 7, the propagation characteristic is calculated using a feedback type circuit, and the frequency characteristic of the OFDM signal is equalized. This can improve the accuracy of demodulation of the OFDM signal.
[0047]
In FIG. 8, the frequency characteristic equalizing means 16 includes a propagation characteristic calculating circuit unit 37, a propagation characteristic updating circuit unit 29, a first complex division circuit unit 32 (first equalizing circuit unit), One delay circuit unit 33, a second delay circuit unit 34, a third delay circuit unit 35, and a second complex division circuit unit 36 (second equalization circuit unit) are provided. In addition, the propagation characteristic calculation circuit unit 37, the propagation characteristic update circuit unit 29, the first complex division circuit unit 32, the first delay circuit unit 33, and the second delay circuit unit 34 perform the propagation characteristic calculation shown in FIG. The circuit section 37, the propagation characteristic updating circuit section 29, the complex division circuit section 28, the first delay circuit section 30 and the second delay circuit section 31 respectively correspond to the circuit section 37 and the third delay circuit section 35 shown in FIG. Since it corresponds to the delay circuit unit 26 described above, its description is omitted.
The frequency characteristic equalizer 16 as shown in FIG. 8 is configured by combining the components of the frequency characteristic equalizer 16 shown in FIG. 3 and the frequency characteristic equalizer 16 shown in FIG. In addition, the second complex division circuit unit 36 re-equalizes the frequency domain signal once equalized by the first complex division circuit unit 32 with the difference propagation characteristic calculated by the propagation characteristic calculation circuit unit 37. Has become. With such a configuration of the frequency characteristic equalizing means 16, the propagation characteristic is calculated using the feedback type circuit and the frequency characteristic of the OFDM signal is equalized, so that the accuracy of the equalization is improved each time the operation is repeated. Further, even when a large variation occurs in the propagation characteristic of the OFDM signal using a feedforward type circuit, the variation difference in the propagation characteristic is calculated in real time without delaying the variation, and the OFDM signal Since the frequency characteristics can be equalized again, the demodulation accuracy of the OFDM signal can be improved.
[0048]
Further, the frequency characteristic equalizing means 16 constituting the OFDM signal demodulation device 10 or the OFDM signal demodulation device 40 according to the embodiment of the present invention calculates the propagation characteristics H ′ (ω) for each subcarrier, and then describes the following. May be configured to perform such processing.
The frequency characteristic equalizer 16 calculates an impulse response by performing an inverse Fourier transform of 1 / H ′ (ω) based on the calculated H ′ (ω), and calculates the maximum value of the absolute value of the calculated impulse response. The value is detected. Further, as shown in FIG. 9A, the frequency characteristic equalizing means 16 sets each impulse response such that the impulse response having the maximum detected absolute value is located at the τ · Rth position from the head of the impulse response. Is shifted in the time axis direction. The impulse response having the maximum absolute value is the impulse response of the main wave, and the absolute value of the impulse response of the multipath such as the delayed wave or the early arrival wave is larger than the absolute value of the impulse response of the main wave. Is also small. Further, τ is, as shown in FIG. 9B, a value from the top of the time window applied to the time domain signal by the window processing unit 14 or the first window processing unit 44 to the top of the effective symbol #i to be demodulated. Time interval. Here, τ is a value that is an integer multiple of 1 / R, so τ · R is naturally an integer. The frequency characteristic equalizer 16 equalizes the frequency domain signal according to a signal obtained by Fourier-transforming the impulse response shifted in the time axis direction. In this case, when the frequency domain signal after the equalization output by the frequency characteristic equalizing means 16 is subjected to inverse Fourier transform, as shown in FIG. 9C, the head of the time domain of the demodulated symbol is inverse Fourier transformed. Coincides with the beginning of the time domain of the signal obtained by Therefore, it is possible to easily match the rectangular window in the time domain used by the symbol extracting means 17 or the second window processing means 42 with the time domain of the effective symbol to be demodulated. As a result, it is possible to reliably extract the frequency domain signal of the effective symbol to be demodulated in the symbol extracting means 17, and to output accurate bit data while preventing the phase rotation of each subcarrier.
[0049]
【The invention's effect】
As described above, the present invention equalizes a multipath that exceeds a guard interval, and immediately demodulates an OFDM signal by immediately following the multipath fluctuation even if there is a large fluctuation in the multipath. It is possible to provide an OFDM signal demodulation device capable of performing the above.
[Brief description of the drawings]
FIG. 1 is a block diagram of a configuration of an OFDM signal demodulation device according to an embodiment of the present invention.
FIG. 2 is a conceptual diagram of a time domain signal for explaining a time window set by a window processing unit according to the OFDM signal demodulating apparatus according to the embodiment of the present invention.
FIG. 3 is a block diagram illustrating a configuration of a frequency characteristic equalizing unit according to the OFDM signal demodulating apparatus according to the embodiment of the present invention.
FIG. 4 is a diagram illustrating the principle of frequency characteristic equalization according to the OFDM signal demodulation device according to the embodiment of the present invention.
FIG. 5 is a block diagram of a configuration of an OFDM signal demodulation device having a configuration different from that of the OFDM signal demodulation device according to the embodiment of the present invention shown in FIG.
FIG. 6 is a diagram illustrating a method of setting a time width of a time window set by a window processing unit according to the OFDM signal demodulating apparatus according to the embodiment of the present invention.
FIG. 7 is a block diagram of a configuration of a frequency characteristic equalizer having a different configuration from the frequency characteristic equalizer shown in FIG. 3 according to the OFDM signal demodulator according to the embodiment of the present invention;
FIG. 8 is a block diagram of a configuration of a frequency characteristic equalizer having a different configuration from the frequency characteristic equalizer shown in FIGS. 3 and 7 according to the OFDM signal demodulator according to the embodiment of the present invention;
FIG. 9A is a conceptual diagram of an impulse response after each impulse response is shifted in a time axis direction such that an impulse response having a maximum absolute value is located at the τ · Rth position from the beginning.
(B) A conceptual diagram of a time-domain signal illustrating an example of a relative position between a time window and an effective symbol to be demodulated.
(C) A conceptual diagram of a time-domain signal illustrating a time-domain signal obtained by performing an inverse Fourier transform on a frequency-domain signal equalized by the propagation characteristic in which the impulse response is shifted as shown in FIG. It is.
FIG. 10A is a block diagram of a configuration of a conventional OFDM signal demodulation device.
(B) It is a block diagram of the structure of the conventional OFDM signal demodulation device which can equalize the multipath exceeding a guard interval.
[Explanation of symbols]
1, 8 antenna
10, 40, 80, 90 OFDM signal demodulator
11,81 Down converter means
12, 82 AD conversion means
13,83 quadrature demodulation means
14, 84 window processing means
15, 85 Fourier transform means
16,86 Frequency characteristic equalization means
17 Symbol extraction means
18 Component removal means
19,87 area determination means
20,88 Data demodulation means
21, 37 Propagation characteristics calculation circuit unit
21a Inverse Fourier transform circuit
21b Window processing circuit
21c Fourier transform circuit
21d Training extraction circuit
21e Training generation circuit
21f first complex division circuit
21g First interpolation filter
21h first delay circuit
21i Second complex division circuit
21j area determination circuit
21k second delay circuit
21l Third complex division circuit
21m second interpolation filter
26 Delay circuit section
27, 28 Complex division circuit
29 Propagation characteristics update circuit
30, 33 first delay circuit section
31, 34 second delay circuit section
32. First complex division circuit section
35 Third delay circuit section
36 Second complex division circuit section
41 inverse Fourier transform means
42 second window processing means
43 Second Fourier Transform Means
44 first window processing means
45 First Fourier Transform Means
91 Filter coefficient calculation means
92 Time domain equalization means

Claims (6)

An OFDM signal demodulator that receives an OFDM signal transmitted from a predetermined transmission source and demodulates bit data for each effective symbol, and has a time width that is a power of 2 times the time width of the effective symbol. Window processing means for performing window processing on the time domain of the OFDM signal including effective symbols to be demodulated using a time window, and performing a Fourier transform on the OFDM signal which has been subjected to the window processing by the window processing means; Fourier transform means for converting a signal in a domain to a signal in a frequency domain, and calculating a propagation characteristic from the transmission source to the window processing means from a signal in the frequency domain of the OFDM signal, according to the calculated propagation characteristic. Frequency characteristic equalizing means for equalizing the frequency characteristic of the OFDM signal, and the OF whose frequency characteristic is equalized by the frequency characteristic equalizing means Symbol extracting means for extracting a frequency component of the demodulated effective symbol from the M signals; and a subcarrier of the OFDM signal transmitted from the transmission source among the extracted frequency components of the demodulated effective symbol. An OFDM signal demodulator comprising: a component removing unit that removes other frequency components while leaving orthogonal frequency components. An OFDM signal demodulator for receiving an OFDM signal transmitted from a predetermined transmission source and demodulating bit data for each effective symbol, comprising: First window processing means for performing window processing on the OFDM signal including an effective symbol to be demodulated using one time window, and Fourier transform of the OFDM signal subjected to window processing by the first window processing means. A first Fourier transform unit for converting a signal in the time domain into a signal in the frequency domain, and a propagation characteristic from the transmission source to the first window processing unit from the frequency domain signal of the OFDM signal. Frequency characteristic equalizing means for equalizing the frequency characteristic of the OFDM signal according to the calculated propagation characteristic; and frequency characteristic equalization by the frequency characteristic equalizing means. Inverse Fourier transform of the obtained OFDM signal, thereby transforming the OFDM signal, whose frequency characteristics have been equalized, from a frequency domain signal to a time domain signal, and a time domain signal by the inverse Fourier transform means. Window processing using a second time window having the same time width as the effective symbol time width with respect to the time domain of the demodulated effective symbol included in the time domain of the OFDM signal converted into the signal A second Fourier transform for converting a time domain signal into a frequency domain signal by performing a Fourier transform on the OFDM signal subjected to window processing by the second window processing means; Means for demodulating an OFDM signal. A frequency characteristic equalizing unit configured to calculate the propagation characteristic from a signal in a frequency domain of the OFDM signal before the frequency characteristic is equalized; a frequency characteristic of the OFDM signal according to the propagation characteristic; 3. The OFDM signal demodulator according to claim 1, further comprising an equalizing circuit unit for equalizing characteristics. The frequency characteristic equalizing means equalizes a frequency characteristic of the OFDM signal in accordance with the calculated propagation characteristic, and an OFDM signal whose frequency characteristic is equalized by the equalization circuit. A propagation characteristic calculation circuit unit that calculates a difference propagation characteristic representing a difference between a propagation characteristic from the transmission source to the window processing unit and the calculated propagation characteristic from a signal in a frequency domain, and the calculated propagation The OFDM signal demodulation device according to claim 1 or 2, further comprising a propagation characteristic updating circuit unit that adds the difference propagation characteristic to the characteristic and updates the calculated propagation characteristic. A frequency equalization unit configured to equalize a frequency characteristic of the OFDM signal according to the calculated propagation characteristic; and a frequency characteristic equalized by the first equalization circuit unit. From the signal of the frequency domain of the OFDM signal, a propagation characteristic calculation circuit unit that calculates a difference propagation characteristic representing a difference between the propagation characteristic from the transmission source to the window processing unit and the calculated propagation characteristic, A propagation characteristic updating circuit unit that adds the difference propagation characteristic to the calculated propagation characteristic to update the calculated propagation characteristic, and equalizes by the first equalization circuit unit according to the difference propagation characteristic. 3. The OFDM signal demodulator according to claim 1, further comprising a second equalizer circuit for further equalizing a frequency characteristic of the OFDM signal. The frequency characteristic equalizing means calculates an impulse response obtained by performing an inverse Fourier transform on the calculated inverse of the propagation characteristic, detects a maximum value of an absolute value of the impulse response, and detects the impulse response from the beginning. The time interval until the impulse response at which the absolute value becomes maximum coincides with the time interval from the head of the time window used for the window processing applied to the OFDM signal before equalization to the effective symbol to be demodulated. Shifting the impulse response in the time axis direction, Fourier transforming the impulse response shifted in the time axis direction to calculate the propagation characteristic again, and the frequency characteristic of the OFDM signal according to the calculated propagation characteristic again. The OFDM signal demodulation device according to any one of claims 3 to 5, wherein Eq.

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