JPH1075226A - Orthogonal frequency-division multiplexing signal demodulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct distortion caused by the influence of multipath interference on a transmission line, to follow up the temporal variation of transmission line characteristics as well and to improve noise resistance. SOLUTION: Fourier transformation is performed to an orthogonal frequency-division multiplexing(OFDM) signal at a Fourier transforming circuit 3, a pilot signal is generated by a pilot signal generating circuit 4 synchronously with this transformation and the pilot signal is divided with the Fourier transformed signal by a divider 5 so that the transmission line characteristics corresponding to a sub-carrier used for transmitting the pilot signal can be calculated. On the other hand, the maximum delay time of a delay wave component in a received signal is calculated at a delay time calculation circuit 10, the level of temporal variation in the transmission line characteristics is calculated at a Doppler frequency calculation circuit 12 respectively and, based on these calculated results, the output of the divider 5 is filtered by a band variable two-dimensional filter 9 so that the transmission characteristics corresponding to all the subcarriers can be calculated. Further, the output of the Fourier transforming circuit 3 is divided with the output of the band variable two-dimensional filter 9 by a divider 8 so that a demodulated signal can be provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an orthogonal frequency division multiplex signal demodulator, and more particularly to correction of transmission path characteristics on a receiving side.

[0002]

2. Description of the Related Art In recent years, orthogonal frequency division multiplexing (hereinafter, referred to as OFDM) has been used in digital audio broadcasting for mobile objects and digital television broadcasting on terrestrial systems.
ncy Division Multiplex)) A transmission system is drawing attention.

[0003] OFDM is digital data to be transmitted.
This is a method of modulating a large number of subcarriers orthogonal to each other and multiplexing and transmitting the modulated waves. When the number of subcarriers to be used is increased to several hundreds to several thousands, the symbol period of each modulated wave becomes extremely long, so that it has a feature that it is hardly affected by multipath interference.

Hereinafter, the principle of the OFDM transmission system will be described.
This will be described with reference to FIGS. FIG. 7 is a block diagram showing a principle configuration of the OFDM transmission system. In FIG. 7, thick arrows indicate complex signals, and thin arrows indicate real signals.

[0005] First, in the OFDM modulator 61, the inverse Fourier transform circuit 71 regards transmission data for one symbol as a signal in the frequency domain and performs an inverse Fourier transform to generate an effective symbol period signal.

The guard period adding circuit 72 copies the rear part of the effective symbol period signal for each symbol and adds it as a guard period signal before the effective symbol period signal, thereby providing a baseband (hereinafter, baseband) OFDM signal. Generate a signal.

FIG. 8 shows an example of a baseband OFDM signal waveform and how a guard period is added. Due to the presence of the guard period, inter-symbol interference does not occur for a delayed wave having a delay time shorter than the guard period length, so that resistance to multipath interference is further enhanced.

The quadrature modulation circuit 73 has a baseband OFD
After two carriers orthogonal to each other are modulated by the real part and the imaginary part of the M signal, the signals are added and frequency-converted into a signal of an intermediate frequency (hereinafter, IF (Intermediate Frequency)) band.

The up-converter 62 converts the IF band signal into a radio frequency (RF (Radio Frequency)) band signal and outputs the signal. The tuner 64 is connected to the transmission path 6
3 is converted to an IF band signal by an RF band signal and output to an OFDM demodulator 65.

In the OFDM demodulator 65, a quadrature demodulation circuit 81 demodulates an IF band signal into a baseband OFDM signal using two orthogonal carriers. The guard period removing circuit 82 removes the guard period added on the transmission side from the baseband OFDM signal, and extracts an effective symbol period signal. The Fourier transform circuit 83 regards the effective symbol period signal as a signal in the time domain, and outputs the result of the Fourier transform as received data.

However, in such a basic configuration, when multipath interference occurs in the transmission path, if the delay time of the delayed wave is equal to or less than the guard period length, no intersymbol interference occurs. Since the amplitude and the phase are distorted, data cannot be demodulated accurately.

In view of the above, a technique has been disclosed in which a pilot signal is inserted into a transmission signal on the transmission side, and the distortion received on the transmission path is corrected on the reception side using the pilot signal to accurately demodulate data. (See, for example, IEE Conference Publication 413, pp. 122-128 (IEE Conf. Publ. No. 413 P.122-1).
28 1995)). Hereinafter, the configuration and operation of such an OFDM transmission system and a conventional OFDM demodulator will be described with reference to FIG.
This will be described with reference to FIG.

FIG. 9 is a schematic diagram showing an example of a pilot signal arrangement in a transmission signal. In FIG. 9, the horizontal axis represents frequency, the vertical axis represents time, black circles represent subcarriers for transmitting pilot signals, and white circles represent subcarriers for transmitting data. The index k indicates the subcarrier number, and the index n indicates the symbol number.
In this example, an index k =
Pilot signals are allocated to kp subcarriers.

[0014]

(Equation 1) In the formula (1), p is any non-negative integer. Here, the pilot signal only needs to have a known amplitude and phase on the receiving side, and it is not necessary that all pilot signals be the same.

In the following, a signal transmitted on the k-th subcarrier of the n-th symbol is x (n, k), the transmission path characteristic for the signal is H (n, k), and the noise is n (n, n). ,
k), and the received signal is represented as y (n, k). At this time, the following relationship (2) is established between them.

[0016]

(Equation 2)

FIG. 10 is a block diagram showing a configuration of a conventional OFDM demodulator. In FIG. 10, a thick arrow indicates a complex number signal, and a thin arrow indicates a real number signal. In FIG. 10, a quadrature demodulation circuit 1 demodulates an IF band signal frequency-converted by a tuner (not shown) into a baseband OFDM signal using two carriers that are orthogonal to each other. The guard period elimination circuit 2
The guard period added on the transmission side is removed from the baseband OFDM signal, and an effective symbol period signal is extracted. The Fourier transform circuit 3 regards the effective symbol period signal as a signal in the time domain, and outputs a signal subjected to Fourier transform as a received signal.

The pilot generation circuit 4 adjusts the timing so that the Fourier transform circuit 3 outputs the signal yp (n, kp) corresponding to the subcarrier of index k = kp satisfying the expression (1), and sets the pilot signal xp (N, k
p). The divider 5 calculates yp (n, kp) as xp
By dividing by (n, kp), the transmission path characteristic Hp ′ (n, kp) estimated on the receiving side is calculated. At this time H
p ′ (n, kp) is represented by the following equation (3).

[0019]

(Equation 3)

The symbol filter 6 and the subcarrier filter 7 perform filtering processing in the symbol direction and the subcarrier direction, respectively, to calculate only the subcarriers (the black circles in FIG. 9) in which the pilot signal is transmitted. Hp '(n, kp) is interpolated to determine the transmission path characteristics H' (n, k) for all carriers. The divider 8 converts the received signal y (n, k) into the transmission path characteristic H ′ (n, k).
By dividing by k), a demodulated signal x '(n, k) in which the influence on the transmission path is corrected is obtained and output.

[0021]

In the conventional method as described above, the transmission obtained by interpolating the transmission path characteristics Hp '(n, kp) estimated based on the received pilot signal yp (n, kp). The influence of the transmission path characteristic is corrected by the path characteristic H ′ (n, k). However, Hp ′ (n,
k) is the noise np (n, n,
kp), the actual transmission path characteristics Hp (n, kp)
And different. Therefore, the demodulated signal x ′ (n, k) calculated from the original noise component n (n, k) included in the received signal y (n, k) is replaced by the noise component due to the estimation error of the transmission path characteristics. Are superimposed, so that the noise resistance deteriorates.

Therefore, the present invention corrects distortion due to the influence of multipath interference on a transmission line, follows the time variation of the characteristics of the transmission line, and has excellent noise resistance.
It is an object to provide an M demodulator.

[0023]

In order to solve this problem, an OFDM demodulator according to the present invention is configured as follows. (1) A demodulation device for demodulating an orthogonal frequency division multiplexed signal for transmitting a pilot signal using a predetermined subcarrier among subcarriers used for transmission, and performing a Fourier transform on the orthogonal frequency division multiplexed signal, A Fourier transform circuit for restoring the multiplexed signal, a pilot generator circuit for generating a pilot signal in synchronization with the output of the Fourier transform circuit, and dividing the output of the Fourier transform circuit by the output of the pilot generator circuit A first divider for calculating a channel characteristic for a subcarrier used for transmitting a pilot signal; a delay time calculating circuit for calculating a maximum delay time of a delay wave component included in a received signal; The passband in the subcarrier direction is controlled based on the output of the calculation circuit, and the necessary A bandwidth-variable subcarrier filter that calculates transmission path characteristics for all subcarriers and a bandwidth-variable subcarrier filter, and outputs a demodulated signal by dividing an output of the Fourier transform circuit by an output of the bandwidth-variable subcarrier filter. And a second divider.

(2) A demodulator for demodulating an orthogonal frequency division multiplexed signal for transmitting a pilot signal using a predetermined subcarrier among subcarriers used for transmission,
A Fourier transform circuit for restoring the multiplexed signal by Fourier transforming the orthogonal frequency division multiplexed signal; a pilot generating circuit for generating a pilot signal in synchronization with an output of the Fourier transform circuit; A first divider that divides an output of a circuit by an output of the pilot generation circuit to calculate a transmission path characteristic for a subcarrier used for transmitting a pilot signal;
A Doppler frequency calculating circuit for calculating the magnitude of the time variation of the transmission path characteristic, and a passband in a symbol direction is controlled based on an output of the Doppler frequency calculating circuit, and a necessary component is obtained from an output of the first divider. Extracting a demodulated signal by dividing the output of the Fourier transform circuit by the output of the band variable symbol filter and extracting the demodulated signal. And a divider.

(3) A demodulator for demodulating an orthogonal frequency division multiplex signal for transmitting a pilot signal using a predetermined subcarrier among subcarriers used for transmission,
A Fourier transform circuit for restoring the multiplexed signal by Fourier transforming the orthogonal frequency division multiplexed signal; a pilot generating circuit for generating a pilot signal in synchronization with an output of the Fourier transform circuit; A first divider that divides an output of a circuit by an output of the pilot generation circuit to calculate a transmission path characteristic for a subcarrier used for transmitting a pilot signal;
A delay time calculation circuit that calculates a maximum delay time of a delay wave component included in a received signal, a Doppler frequency calculation circuit that calculates the magnitude of a time variation of a transmission path characteristic, the delay time calculation circuit, and the Doppler frequency calculation circuit , The passband in the subcarrier direction and the symbol direction is controlled based on the output of the first divider, and only the necessary components are extracted from the output of the first divider, and the band variable 2 for calculating the transmission path characteristics for all the subcarriers. A two-dimensional filter, and a second divider that outputs a demodulated signal by dividing an output of the Fourier transform circuit by an output of the band variable two-dimensional filter.

(4) In the configuration of (3), the band variable two-dimensional filter includes a band variable symbol filter for controlling a pass band in a symbol direction based on an output of the Doppler frequency calculation circuit; A configuration is provided that includes a band variable subcarrier filter that controls the passband in the subcarrier direction based on the output of the delay time calculation circuit.

(5) In any one of the constitutions (1) to (4), further, a judgment circuit for judging an output of the second divider with a predetermined threshold value, and a Fourier transform circuit And a switch that switches between the output of the pilot generation circuit and the output of the determination circuit and supplies the output to the first divider in synchronization with the output of the first divider.

(6) In any one of the constitutions (1) to (5), the delay time calculating circuit converts a guard period signal included in the orthogonal frequency division multiplexed signal and an effective symbol period signal. The correlation is used to calculate the maximum delay time of the delay wave component included in the received signal.

(7) In any one of the constitutions (1) to (5), the delay time calculating circuit detects a change in the subcarrier direction of the pilot signal to detect a delay included in the received signal. The maximum delay time of the wave component is calculated.

(8) In any one of the constitutions (1) to (7), the Doppler frequency calculation circuit calculates the time variation of the transmission path characteristic based on the moving speed of the object on which the apparatus is mounted. The size is calculated.

(9) In any one of the constitutions (1) to (7), the Doppler frequency calculating circuit detects a change in the symbol direction of the pilot signal,
It is configured to calculate the magnitude of the time variation of the transmission path characteristics.

[0032]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to FIGS. However, in these figures, thick arrows indicate complex signals, and thin arrows indicate real signals. In addition, general control signals such as a clock necessary for the operation of each component are omitted to prevent the description from being complicated. Further, hereinafter, the effective symbol period is represented by Tes, the guard period length is represented by Tg, and the symbol period length is represented by Ts.

(First Embodiment) FIG. 1 is a block diagram showing a configuration of an OFDM demodulator according to a first embodiment of the present invention.

In FIG. 1, an IF band signal frequency-converted by a tuner (not shown) is supplied to a quadrature demodulation circuit 1. The output of the quadrature demodulation circuit 1 is distributed to two systems and supplied to the guard period elimination circuit 2 and the delay time calculation circuit 10, respectively.

The output of the guard period removing circuit 2 is supplied to a Fourier transform circuit 3, and the output of the Fourier transform circuit 3 is distributed to two systems and supplied to dividers 5 and 8, respectively. The output of pilot signal generation circuit 4 is supplied to divider 5. The output of the divider 5 is supplied to a band variable two-dimensional filter 9.

On the other hand, the output of the speedometer 11 is supplied to a Doppler frequency calculation circuit 12. The output of the Doppler frequency calculation circuit 12 is supplied to the band variable two-dimensional filter 9.

The output of the delay time calculation circuit 10 is supplied to the band variable two-dimensional filter 9. The output of the band variable two-dimensional filter 9 is supplied to the divider 8. The output of the divider 8 is output to the outside as a demodulated signal.

FIG. 2 is a block diagram showing the internal configuration of the band variable two-dimensional filter 9 in FIG. In FIG. 2, a first input from the outside of the variable band two-dimensional filter 9 (output of the Doppler frequency calculation circuit 12) is supplied to a variable band symbol filter 21. Second input from outside of the variable band two-dimensional filter 9 (output of the divider 5)
Is supplied to the variable band symbol filter 21. The output of the band variable symbol filter 21 is supplied to the band variable subcarrier filter 22.

A third input from the outside of the variable band two-dimensional filter 9 (the output of the delay time calculating circuit 10) is supplied to the variable band subcarrier filter 22. The output of the band variable subcarrier filter 22 is output to the outside as the output of the band variable two-dimensional filter 9.

In the variable band symbol filter 21,
A first external input (output of the divider 5) is a subtractor 3
1 is supplied. The output of the subtractor 31 is supplied to a coefficient unit 32. A second external input (the output of the Doppler frequency calculation circuit 12) is supplied to a coefficient unit 32. The output of the coefficient unit 32 is supplied to the adder 33. Adder 33
Is supplied to the one-symbol delay circuit 34, and at the same time, becomes the output of the band variable symbol filter 21. The output of the one-symbol delay circuit 34 is supplied to the subtractor 31 and the adder 33.

In the band variable subcarrier filter 22, a first external input (output of the band variable symbol filter 21) is supplied to subcarrier filters 41, 42, 4
3 is supplied. Subcarrier filters 41, 42, 4
All the outputs of 3 are supplied to the selector 44. A second external input (output of the delay time calculation circuit 10) is supplied to the selector 44. The output of the selector 44 becomes the output of the band variable subcarrier filter 22.

FIG. 3 shows the delay time calculating circuit 1 in FIG.
FIG. 3 is a block diagram showing an internal configuration of the ０0 ’. In FIG.
An input from the outside of the delay time calculation circuit 10 (the output of the quadrature demodulation circuit 1) is supplied to the one effective symbol period delay circuit 51 and the multiplier 53. One effective symbol period delay circuit 5
The output of 1 is supplied to the complex conjugate circuit 52. The output of the complex conjugate circuit 52 is supplied to a multiplier 53. Multiplier 5
The output of 3 is supplied to a coefficient unit 54. The output of the coefficient unit 54 is supplied to an adder 55.

The output of the adder 55 is supplied to a one-symbol delay circuit 56 and an absolute value circuit 58. The output of the one-symbol delay circuit 56 is supplied to a coefficient unit 57. Coefficient unit 57
Is supplied to the adder 55. The output of the absolute value circuit 58 is supplied to a determination circuit 59. The output of the determination circuit 59 becomes the output of the delay time calculation circuit 10.

The operation of the OFDM demodulator of FIG. 1 configured as described above will be described below. In FIG. 1, an orthogonal demodulation circuit 1 demodulates an IF band signal frequency-converted by a tuner (not shown) into a baseband OFDM signal using two orthogonal carriers. The guard period elimination circuit 2 includes a baseband O
The guard period added on the transmission side is removed from the FDM signal to extract an effective symbol period signal. The Fourier transform circuit 3 regards the effective symbol period signal as a signal in the time domain, and outputs a signal subjected to Fourier transform as a received signal.

The pilot generation circuit 4 adjusts the timing so that the Fourier transform circuit 3 outputs the signal yp (n, kp) corresponding to the subcarrier of index k = kp satisfying the expression (1), and sets the pilot signal xp (N, k
p). The divider 5 calculates yp (n, kp) as xp
By dividing by (n, kp), the transmission path characteristic Hp ′ (n, kp) estimated on the receiving side is calculated.

When the present demodulator is mounted on a moving body, the speedometer 11 measures the speed v of the moving body. The Doppler frequency calculation circuit 12 calculates the maximum Doppler frequency fd from the velocity v of the moving object by the equation (4), and generates the coefficient α based on the calculated Doppler frequency fd based on the idea described later.

[0047]

(Equation 4) In equation (4), c represents the speed of light, and f represents the frequency used for transmission.

The delay time calculating circuit 10 has a baseband O
The multipath delay time is calculated from the FDM signal using the correlation between the signal in the guard period and the signal in the effective symbol period, and the control signal S is generated based on the multipath delay time.

The band variable two-dimensional filter 9 has a symbol direction and a subcarrier having a pass band characteristic set based on the coefficient α output from the Doppler frequency calculation circuit 12 and the control signal S output from the delay time calculation circuit 10. Directional filters 21 and 22 are provided, and a filtering process is performed using these filters. As a result, Hp ′ (n, kp) calculated only for the subcarriers (black circles in FIG. 9) in which the pilot signal is transmitted is interpolated, and the transmission path characteristics H ′ (n, k) for all carriers are obtained. Ask.

The divider 8 divides the received signal y (n, k) by the transmission path characteristic H ′ (n, k) to obtain a demodulated signal x ′ (n, k) in which the influence on the transmission path is corrected, and outputs the demodulated signal x ′ (n, k). I do. In FIG. 2, the variable band symbol filter 21 includes:
An infinite impulse response (hereinafter, IIR) filter composed of a subtractor 31, a coefficient unit 32, an adder 33, and a one-symbol delay circuit 34. By controlling a coefficient α of the coefficient unit, Its frequency characteristics can be changed.

The band variable subcarrier filter 22
Are composed of subcarrier filters 41 to 43 having different passband characteristics and a selector for selecting one of the outputs based on the control signal S. The subcarrier filters 41 to 43 include, for example, a finite impulse response in the subcarrier direction having different tap coefficients (hereinafter, F
It can be realized by an IR (Finite Impule Response) filter.

Here, a method of setting the pass band characteristic of the band variable two-dimensional filter 9 in FIG. 1 will be described with reference to FIG. In FIG. 4, the horizontal axis represents the frequency in the relationship between the frequency on the horizontal axis in FIG. 9 and the Fourier transform pair, and the vertical axis represents the frequency in the relationship between the time on the vertical axis in FIG. 9 and the Fourier transform pair.

As described above, Hp ′ (n, kp) calculated only for the subcarriers (black circles in FIG. 9) on which the pilot signal is transmitted is interpolated, and the transmission path characteristics H ′ ( In order to obtain (n, k), filtering may be performed to remove aliasing components caused by sparsely arranged pilot signals. For example, in the example of FIG. 9, 1 / (4
Ts) is a pass band, and Tes /
A filter having a pass band of 3 corresponds to this.

However, considering the band of the transmission line characteristic, as shown in FIG. 4, in the symbol direction, the maximum Doppler frequency f obtained from the speed of the moving object by the equation (4)
If an interpolation process is performed by a filter having a passband twice as large as d and a passband with the maximum delay time δ of the multipath in the carrier direction, it is possible to correct the distortion due to the influence of the multipath interference on the transmission path, It is possible to follow the time variation of the road characteristic and suppress noise components outside the pass band.

Therefore, when at least one of the maximum delay time δ of the multipath and the Doppler frequency fd is small, the transmission path characteristic H ′ (n, k) is smaller than that of the conventional example in which the pass band characteristic of the filter is fixed. Since the estimation error can be reduced, the noise resistance of the demodulation characteristics can be improved.

Next, the delay time calculating circuit 10 in FIG.
Will be described with reference to FIGS. 3 and 5. As shown in FIG. 3, the delay time calculation circuit 10
The conjugate complex multiplication of the DM signal and the signal delayed by one effective symbol period Tes, the IIR filter in the symbol direction, and the absolute value calculation calculate the absolute value of the correlation signal of those signals.

In FIG. 5, G0 to G2 represent signals in the guard period, and S0 to S2 represent signals in the effective symbol period. 3A shows a desired wave component in the input baseband OFDM signal of the one effective symbol delay circuit 51 in FIG. 3, FIG. 3B shows the one effective symbol delay signal of FIG.
(C) represents components corresponding to these desired wave components in the output correlation signal of the absolute value circuit 58. At this time, since the signal after the effective symbol period and the guard period signal are the same signal, a signal component appears only in a portion corresponding to them in the correlation signal (c).

(D) to (f) are waveforms similar to (a) to (c) for the delayed wave component in the baseband OFDM signal, and the correlation signal (f) is a delayed wave for (c). The signal component appears only in the portion delayed by the delay time δ. Since the output signal of the absolute value circuit 58 is the sum of (c) and (f), the waveform is as shown in (g).

The decision circuit 59 in FIG.
Is measured from the time when the output exceeds a certain threshold value to when the output again falls below the threshold value, and the delay time δ of the delayed wave is calculated by subtracting the guard period length Tg from the measured time. Control signal S for selecting a carrier filter having a wide pass band.

(Second Embodiment) FIG. 6 is a block diagram showing a configuration of an OFDM demodulator according to a second embodiment of the present invention. The OFD in this embodiment
The configuration of the M demodulator includes a decision circuit 13 for judging the demodulated signal output of the divider 8 with a certain threshold value, and a Fourier transform circuit 3
The output of the pilot generation circuit 4 is supplied to the divider 5 in synchronization with the output of the signal yp (n, kp) corresponding to the subcarrier of the index k = kp satisfying the expression (1). Except that a switch 14 for supplying the output of the decision circuit 13 to the divider 5 is added.
Since the configuration is the same as that of the first embodiment, the description of the configuration is omitted.

In the present embodiment, unlike the first embodiment, the transmission line characteristics H ′ (n,
Since k) can be obtained, the band variable two-dimensional filter 9 does not need to perform interpolation processing, and only needs to perform filtering for noise suppression.

At this time, in the example of FIG.
/ (4Ts), which eliminates the upper limit of the pass band of the symbol filter for removing aliasing components caused by the sparse arrangement of pilot signals, makes the transmission path more time-efficient than the first embodiment. The ability to follow changes can be improved.

In the OFDM demodulators shown in FIGS. 1 and 6, the Doppler frequency fd is obtained based on the speed v of the moving object output from the speedometer 11, and the delay time δ is the signal of the guard period and the effective symbol period. Are obtained by utilizing the correlation with the signal of the pilot signal, but they can also be obtained by other methods, for example, by observing changes in the symbol direction and the subcarrier direction of the pilot signal.

In FIG. 2, the coefficient of the IIR filter is controlled in the symbol direction, and the outputs of a plurality of FIR filters having different passband characteristics are switched in the subcarrier direction, thereby changing the band variable two-dimensional filter 9. This is realized by other methods, for example, by switching the outputs of a plurality of FIR filters having different passband characteristics in the symbol direction and controlling the coefficients of the IIR filter in the subcarrier direction. Is also possible.

In addition, in FIG. 2, the filtering process in the subcarrier direction is performed after the filtering process in the symbol direction, but the order may be reversed.

In this embodiment, the maximum delay time δ
And two parameters of the Doppler frequency fd are calculated and the passband characteristics of the two-dimensional filter in the subcarrier direction and the symbol direction are respectively controlled by using the two parameters. No problem.

Also, it has been described above that the present demodulator can correct the transmission path characteristics following the movement of the object on which the demodulator is mounted as a factor of the time variation of the transmission path, and following it. , The behavior due to the phase noise of the tuner is similar to the variation due to movement (however,
Since there is no amplitude fluctuation), if the pass band of the filter in the symbol direction is controlled in consideration of the phase noise, it is possible to follow the time fluctuation due to the control. Furthermore, in the above description, the pilot signal arrangement shown in FIG. 9 has been described as an example, but it is needless to say that other arrangements may be used.

[0068]

As described above, according to the present invention, transmission is performed by performing filtering with the maximum Doppler frequency as the pass band in the symbol direction and the maximum multipath delay time as the pass band in the subcarrier direction. It is possible to correct distortion due to the influence of multipath interference on the path, follow the time variation of the transmission path characteristics, and suppress noise components outside the pass band. Therefore, a remarkable effect that the estimation error of the transmission path characteristic can be reduced and the noise resistance of the demodulation characteristic can be improved as compared with the conventional example can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration example of an OFDM demodulator according to a first embodiment of the present invention.

FIG. 2 is a block diagram showing an example of an internal configuration of a variable band two-dimensional filter 9 in FIG.

FIG. 3 is a block diagram showing an example of an internal configuration of a delay time calculation circuit 10 in FIG. 1;

FIG. 4 is a schematic diagram showing a principle of setting a pass band characteristic of the band variable two-dimensional filter 9 in FIG.

5A and 5B are diagrams for explaining the operation of the delay time calculation circuit 10 in FIG. 3, wherein FIG. 5A shows a desired wave component in a baseband OFDM signal, and FIG. 5B shows one effective symbol period delay signal of FIG. , (C) are correlation components corresponding to (a) and (b), (d) is a delayed wave component in a baseband OFDM signal, (e) is a delayed signal of one effective symbol period of (d),
(F) is a correlation component corresponding to (d) and (e), (g)
FIG. 9 is a waveform diagram showing a correlation signal obtained by adding (c) and (f).

FIG. 6 is a block diagram illustrating a configuration example of an OFDM demodulator according to a second embodiment of the present invention.

FIG. 7 is a block diagram illustrating an example of a principle configuration of an OFDM transmission system.

FIG. 8 is a waveform chart showing an example of a time waveform of an OFDM signal.

FIG. 9 is a schematic diagram showing a pilot signal arrangement example.

FIG. 10 is a block diagram illustrating a configuration example of a conventional OFDM demodulator.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Quadrature demodulation circuit 2 ... Guard period removal circuit 3 ... Fourier transform circuit 4 ... Pilot generation circuit 5, 8 ... Divider 9 ... Band variable two-dimensional filter 10 ... Delay time calculation circuit 11 ... Speedometer 12 ... Doppler frequency calculation circuit

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tomohiro Kimura 5-2-8 Akasaka, Minato-ku, Tokyo Inside the Next Generation Digital Television Broadcasting System Research Laboratories (72) Inventor Sadaji Kageyama 5 Akasaka, Minato-ku, Tokyo No. 2-8, Next Generation Digital Television Broadcasting System Laboratory Co., Ltd. (72) Inventor Yasuo Harada 5-2-2, Akasaka, Minato-ku, Tokyo Next Generation Digital Television Broadcasting System Co., Ltd. Laboratory (72) Inventor Akira Kisoda 5-2-8 Akasaka, Minato-ku, Tokyo Inside the Next Generation Digital Television Broadcasting System Research Laboratories (72) Inventor Seiji Sakashita 1006 Kazuma Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd.

Claims (9)

[Claims] 1. A demodulator for demodulating an orthogonal frequency division multiplex signal transmitting a pilot signal using a predetermined subcarrier among subcarriers used for transmission, wherein the demodulation apparatus performs a Fourier transform on the orthogonal frequency division multiplex signal. A Fourier transform circuit for restoring a multiplexed signal, a pilot generator circuit for generating a pilot signal in synchronization with an output of the Fourier transform circuit, and an output of the Fourier transform circuit as an output of the pilot generator circuit. By dividing, a first channel characteristic for the subcarrier used for transmitting the pilot signal is calculated.
A divider for calculating a maximum delay time of a delay wave component included in a received signal; and controlling a pass band in a subcarrier direction based on an output of the delay time calculation circuit. And a band variable subcarrier filter for calculating transmission path characteristics for all subcarriers, and dividing the output of the Fourier transform circuit by the output of the band variable subcarrier filter. And a second divider for outputting a demodulated signal. 2. A demodulator for demodulating an orthogonal frequency division multiplexed signal for transmitting a pilot signal using a predetermined subcarrier among subcarriers used for transmission, wherein the demodulation apparatus performs a Fourier transform on the orthogonal frequency division multiplexed signal. A Fourier transform circuit for restoring a multiplexed signal, a pilot generator circuit for generating a pilot signal in synchronization with an output of the Fourier transform circuit, and an output of the Fourier transform circuit as an output of the pilot generator circuit. By dividing, a first channel characteristic for the subcarrier used for transmitting the pilot signal is calculated.
A Doppler frequency calculation circuit that calculates the magnitude of the time variation of the transmission path characteristic; a passband in a symbol direction is controlled based on an output of the Doppler frequency calculation circuit; Extracting only the necessary components from the output, and a band variable symbol filter that calculates the transmission path characteristics for all subcarriers, dividing the output of the Fourier transform circuit by the output of the band variable symbol filter to obtain a demodulated signal An orthogonal frequency division multiplexing signal demodulation device, comprising: a second divider for outputting. 3. A demodulator for demodulating an orthogonal frequency division multiplex signal transmitting a pilot signal using a predetermined subcarrier among subcarriers used for transmission, wherein the demodulation apparatus performs a Fourier transform on the orthogonal frequency division multiplex signal. A Fourier transform circuit for restoring a multiplexed signal, a pilot generator circuit for generating a pilot signal in synchronization with an output of the Fourier transform circuit, and an output of the Fourier transform circuit as an output of the pilot generator circuit. By dividing, a first channel characteristic for the subcarrier used for transmitting the pilot signal is calculated.
, A delay time calculation circuit that calculates the maximum delay time of the delay wave component included in the received signal, a Doppler frequency calculation circuit that calculates the magnitude of the time variation of the transmission path characteristics, the delay time calculation circuit, Based on the output of the Doppler frequency calculation circuit, the passband in the subcarrier direction and the symbol direction is controlled, and only necessary components are extracted from the output of the first divider. A quadrature filter comprising: a variable band two-dimensional filter to be calculated; and a second divider that outputs a demodulated signal by dividing an output of the Fourier transform circuit by an output of the variable band two-dimensional filter. Frequency division multiplex signal demodulator. 4. The variable band two-dimensional filter includes: a variable band symbol filter that controls a pass band in a symbol direction based on an output of the Doppler frequency calculating circuit; The orthogonal frequency division multiplex signal demodulation device according to claim 3, further comprising a band variable subcarrier filter that controls a pass band in a carrier direction. 5. A judgment circuit for judging an output of the second divider with a predetermined threshold value, and an output of the pilot generation circuit and an output of the judgment circuit in synchronization with an output of the Fourier transform circuit. Switch between output and
5. The orthogonal frequency division multiplex signal demodulator according to claim 1, further comprising: a switch for supplying the signal to the first divider. 6. The maximum delay time of a delay wave component included in a received signal using a correlation between a guard period signal included in the orthogonal frequency division multiplexed signal and an effective symbol period signal. The orthogonal frequency division multiplexed signal demodulation device according to any one of claims 1 to 5, wherein 7. The delay time calculating circuit calculates a maximum delay time of a delay wave component included in a received signal by detecting a change in a subcarrier direction of a pilot signal. 6. The orthogonal frequency division multiplexed signal demodulator according to any one of 5. 8. The apparatus according to claim 1, wherein the Doppler frequency calculation circuit calculates a magnitude of a time variation of a transmission path characteristic based on a moving speed of an object on which the apparatus is mounted.
8. The orthogonal frequency division multiplexed signal demodulator according to any one of claims 1 to 7. 9. The Doppler frequency calculation circuit according to claim 1, wherein the change in the symbol direction of the pilot signal is detected to calculate the magnitude of the time variation of the transmission path characteristic. 4. The orthogonal frequency division multiplexed signal demodulator according to item 1.