JP2002344411A - Ofdm modulation apparatus and method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve information error rate by accurately predicting the transmission characteristics of transmission lines. SOLUTION: The OFDM receiver 1 has an equalizer 8 for equalizing the waveform of amplitude modulation signals, after FFT operation. The equalizer 8 comprises an SP signal extraction circuit 11 for extracting SP signals, a time direction interpolating filter 12 for interpolating the extracted SP signal in the time direction, and a frequency direction interpolating filter 13 for interpolating the SP signal that has been interpolated in the time direction further in the frequency direction. A frequency direction interpolating filter 16 reduces the number of filter taps, when the positions of carriers to be interpolated are at the end section of symbols.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to orthogonal frequency division multiplexing transmission (OFDM).
The present invention relates to an OFDM demodulation apparatus and method applied to digital broadcasting or the like using a multiplexing method.

[0002]

2. Description of the Related Art In recent years, as a system for transmitting digital signals, an orthogonal frequency division multiplexing system (OFDM) has been proposed.
A modulation method called "requency division multiplexing" has been proposed. In this OFDM system, a number of orthogonal subcarriers (subcarriers) are provided in a transmission band, data is allocated to the amplitude and phase of each subcarrier, and PSK (Phase Shift Keying) and QAM (Quadraturtu
This is a method of performing digital modulation by reamplitude modulation.

In this OFDM system, since the transmission band is divided by a large number of subcarriers, the band per subcarrier wave becomes narrow and the modulation speed becomes slow, but the total transmission speed is different from that of the conventional modulation system. There is no feature. Further, the OFDM scheme has a feature that the symbol rate is reduced because a large number of subcarriers are transmitted in parallel. Therefore, in the OFDM system, the time length of the multipath relative to the time length of the symbol can be shortened, and multipath interference is reduced. Further, in the OFDM method, data is allocated to a plurality of subcarriers, so that IFFT (Inverse Fast Fourier Transform) that performs inverse Fourier transform during modulation is performed.
An er Transform operation circuit and an FFT (Fast Fourier Transform) operation circuit for performing a Fourier transform at the time of demodulation can be used to configure a transmission / reception circuit.

[0004] From the above characteristics, the OFDM system is widely studied for application to terrestrial digital broadcasting which is strongly affected by multipath interference. Such OFD
As terrestrial digital broadcasting employing the M system, for example, in Europe, DVB-T (Digital Video Broadc
A standard called asting-Terrestrial was proposed, and in Japan ISDB-T (Integrated Services Digital Broadcas)
ting-Terrestrial) has been proposed.

[0005] A transmission signal according to the OFDM scheme is transmitted in a symbol unit called an OFDM symbol, as shown in FIG. This OFDM symbol is used as an IFF
An effective symbol, which is a signal period in which T is performed, and a guard interval in which a waveform of a part of the latter half of the effective symbol is copied as it is. This guard interval is provided in the first half of the OFDM symbol. For example, in the DVB-T standard (2K mode), 2048 subcarriers are included in an OFDM symbol. Also, 2048 in the effective symbol
Data is modulated on 1705 subcarriers among the subcarriers. The guard interval is a signal having a time length of, for example, １ ／ of the effective symbol.

In an OFDM system using QAM-based modulation as a modulation system for each subcarrier,
If distortion occurs in a signal modulated on each subcarrier due to the influence of multipath or the like during transmission, the characteristics of amplitude and phase differ for each subcarrier. Therefore, on the receiving side, it is necessary to equalize the waveform of the received signal so that the amplitude and phase of each subcarrier become equal. In the OFDM system, a pilot signal having a predetermined amplitude and a predetermined phase is scattered in a transmission symbol on a transmission side on a transmission side, and the amplitude and phase of the pilot signal are monitored on a reception side, and the frequency of a transmission path is monitored. The characteristics are determined, and the received signal is equalized based on the determined characteristics of the transmission path. A pilot signal used to calculate the characteristics of the transmission path is referred to as a scattered pilot signal (S
P) signal.

FIG. 7 shows an arrangement pattern in an OFDM symbol of an SP signal adopted in the DVB-T standard or the ISDB-T standard.

In the DVB-T standard and the ISDB-T standard,
In the subcarrier direction (frequency direction), BPSK-modulated SP signals are inserted at a rate of one per 12 subcarriers. Further, in the DVB-T standard and the ISDB-T standard, the insertion position of the SP signal is shifted in the frequency direction by three subcarriers for each OFDM symbol. As a result, for the same subcarrier in the OFDM symbol direction (time direction), S is applied once every four OFDM symbols.
The P signal will be inserted.

As described above, the DVB-T standard and the ISDB-T
According to the standard, the OFD signal is
It is inserted into M symbols to reduce the redundancy of the SP signal with respect to the original information.

When the characteristics of the transmission path are calculated using the SP signal, the characteristics can be specified for the subcarrier in which the SP signal is inserted, but the other subcarriers, ie, the original subcarriers, can be specified. The characteristics of other subcarriers containing information cannot be directly calculated. Therefore, the receiving side estimates the characteristics of the transmission path of another subcarrier including the original information by filtering the SP signal using a two-dimensional interpolation filter.

Normally, the processing of estimating transmission path characteristics using a two-dimensional interpolation filter is performed as follows.

When performing the process of estimating transmission line characteristics, first,
The information component is removed from the received OFDM signal, and FIG.
Only the SP signal inserted at the position shown in FIG.

Subsequently, as shown in FIG. 8, the extracted SP
The signal is input to a time-direction interpolation filter to perform time-direction interpolation processing, and for each OFDM symbol, the transmission path characteristics of the subcarrier in which the SP signal is arranged are estimated. As a result, as shown in FIG. 9, the transmission path characteristics can be estimated for every three subcarriers in the frequency direction for all OFDM symbols.

Subsequently, as shown in FIG. 10, the SP signal interpolated in the time direction is input to the interpolation filter in the frequency direction to perform frequency direction interpolation processing, and the transmission path characteristics of all subcarriers in the OFDM symbol are estimated. I do. As a result, transmission path characteristics can be estimated for all subcarriers of the received OFDM signal.

Here, when performing the interpolation processing, it is generally desirable to increase the number of filter taps in order to improve the attenuation characteristics and transition characteristics of the filter. However,
When estimating the transmission path characteristics of an OFDM signal, if a filter having a large number of taps is used in the interpolation process in the time direction, the delay amount of the delay line becomes extremely large, and mounting becomes difficult. For such reasons of mounting, when estimating the transmission path characteristics, it is common to use a zero-order hold filter with a small hardware scale as the time direction filter.

FIG. 11 shows an example of a configuration in the case where this zero-order hold filter is realized by an FIR filter. FIG.
The zero-order hold filter 100 shown in FIG.
, And three first to third adders 104, 105, and 106. That is, the zero-order hold filter 100 is configured as an FIR filter in which all tap coefficients are 1.

An SP signal inserted into a predetermined subcarrier number of each OFDM signal is sequentially input to the zero-order hold filter 100 in the time direction (OFDM symbol direction) as an input signal. Note that 0 is input to a portion where the SP signal is not inserted (a portion where the original information is included). The first delay element 101 delays an input signal by one timing. Second delay element 102
Delays the output signal of the first delay element 101 further by one timing. The third delay element 103 further delays the output signal of the second delay element 102 by one timing. That is, each of the delay elements 101 to 103 outputs a delayed signal delayed by one timing, a delayed signal delayed by two timings, and a delayed signal delayed by three timings. Further, the first adder 104 adds the input signal and the output signal of the first delay element 101 and outputs the result. The second adder 105 outputs the output signal of the first adder 104 and the second delay signal. The third adder 106 adds the output signal of the second adder 105 and the output signal of the third delay element 103 and outputs the result.

As a result, as shown in FIG. 12, the 0th-order hold filter 100 holds the value of the actually extracted SP signal for three OFDM symbols and outputs it. That is, the value of the actually received SP signal is output as an estimated value for the next three OFDM symbols.

On the other hand, when performing interpolation processing in the frequency direction of the OFDM signal, the delay amount of the delay line is smaller than that in the time direction. Therefore, it is possible to improve the attenuation characteristic and the transition characteristic by using a filter having a larger number of taps than the time direction filter.

FIG. 13 shows a configuration example in which a frequency direction filter is realized by a 21-tap FIR filter. The FIR filter 200 illustrated in FIG. 13 includes 20 first to 20th delay elements 201 to 220, 0th to 20th 21 multipliers 230 to 250, and an adder 2
51.

To the FIR filter 200, signals whose transmission path characteristics are estimated at three subcarrier intervals are sequentially input from the time direction filter in the frequency direction (subcarrier direction) as input signals. Note that 0 is input to a portion where the characteristics of the transmission path are not estimated (a portion where the original information is included).

The first delay element 201 delays an input signal by one timing. The second delay element 202 has a first
Is further delayed by one timing. The third delay element 203 is the second delay element 2
02 is further delayed by one timing. Thereafter, each delay element 204 delays the output signal of the immediately preceding delay element by one timing. That is, each delay element 20
From 1 to 120, a delay signal delayed by 1 to 20 timings is output. The zeroth multiplier 230 multiplies the undelayed input signal by a coefficient k0, the first multiplier 231 multiplies the output signal of the first delay element 201 by a coefficient k1, and the second multiplier 231 232 multiplies the output signal of the second delay element 202 by a coefficient k2, and thereafter, each of the multipliers 232 to 250 multiplies the output signal of the corresponding delay element 203 to 220 by a coefficient k3 to k20. Then, the adder 251 adds and outputs the multiplied outputs of all the multipliers 230 to 250.

The coefficients k0 to k20 are set in advance so that the transmission path characteristics of the subcarrier at the center position of the delay element are tripled.

As a result, the FIR filter 200 can estimate the transmission path characteristics for each subcarrier in the OFDM symbol.

As described above, by performing two-dimensional interpolation processing using the time direction interpolation filter and the frequency direction interpolation filter, the transmission path characteristics of all subcarriers can be estimated on the receiving side.

[0026]

When performing interpolation in the frequency direction, the filtering process must be completed in OFDM symbol units. That is, it is necessary to perform the filtering process for each certain data unit instead of continuous sequential data.

For this reason, OF interpolation is performed in the frequency direction.
When a subcarrier at an end portion (low-frequency portion or high-frequency portion) in the DM symbol is used as an interpolation point, as shown in FIG. 14, a signal component outside the band of the OFDM symbol is included in the filter as a sample signal for interpolation. As a result, a true value cannot be supplied to a delay element at a position where an actually received signal must be input.

In general, when an interpolation process is performed, if a true value cannot be supplied to a delay element at a predetermined position, an interpolation point is obtained by assuming an appropriate value.

However, assuming an appropriate value, the error in interpolation increases. Therefore, an error occurs in the estimated value of the transmission path characteristic. That is,
As shown in FIG. 15, the estimated value of the channel characteristics at the band edge (low frequency portion and high frequency portion) of the OFDM symbol has a larger error than the estimated value of the channel characteristics at the center of the band of the OFDM symbol. turn into.

Therefore, the OF is estimated using the estimated transmission path characteristics.
Even if the reception frequency characteristic of the DM signal is corrected, OFD
The information transmitted on the subcarrier at the band edge of the M signal is C
Even if / N is high, transmission errors have increased.

The present invention has been made in view of such circumstances, and has as its object to provide an OFDM demodulator and a method for estimating the transmission characteristics of a transmission path with high accuracy and improving the error rate of information. I do.

[0032]

SUMMARY OF THE INVENTION OFDM according to the present invention
The demodulation device has a transmission symbol generated by dividing and orthogonally modulating information for a plurality of subcarriers within a predetermined band as a transmission unit, and has a specific power and a specific phase. What is claimed is: 1. An OFDM demodulator for demodulating an orthogonal frequency division (OFDM) signal in which a pilot signal is discretely inserted into a predetermined subcarrier in the transmission symbol. Means, a pilot signal extracting means for extracting the pilot signal for each transmission symbol from the signal Fourier-transformed by the Fourier transform means, a time-direction interpolation filter and a frequency filter for extracting the pilot signal extracted by the pilot signal extracting means. By interpolating using a directional interpolation filter, all Interpolation means for calculating the transmission path characteristics of the subcarrier, and a waveform equalization means for waveform equalizing the signal Fourier-transformed by the Fourier transformation means based on the transmission path characteristics of each subcarrier calculated by the interpolation means, Tap number control means for controlling the number of taps for the frequency direction interpolation filter of the interpolation means, wherein the tap number control means performs the frequency direction interpolation in accordance with the position of the subcarrier to be interpolated in the transmission symbol. The number of taps of the filter is changed.

In this OFDM demodulator, the number of taps of the frequency direction interpolation filter for estimating the transmission path characteristics from the pilot signal is changed according to the position of the subcarrier to be estimated in the transmission symbol. For example, the number of taps of the frequency direction interpolation filter is controlled according to the symbol position so that only the pilot signal obtained by reception is included in the delay element as a sample signal for interpolation.

In the OFDM receiving method according to the present invention, a transmission symbol generated by dividing information and orthogonally modulating a plurality of subcarriers in a predetermined band is used as a transmission unit, and a specific power is used. Orthogonal frequency division (OFD) in which pilot signals having a specific phase are discretely inserted into predetermined subcarriers in the transmission symbol.
M) An OFDM demodulation method for demodulating a signal, wherein
Fourier-transforming the FDM signal for each transmission symbol,
The pilot signal is extracted for each transmission symbol from the Fourier-transformed signal, and the frequency direction interpolation is performed using a time direction interpolation filter and a frequency direction interpolation filter according to the position of the subcarrier to be interpolated in the transmission symbol. By interpolating the extracted pilot signal while changing the number of taps of the filter, the channel characteristics of all the subcarriers in the transmission symbol are calculated. The waveform of the converted signal is equalized.

In this OFDM demodulation method, the number of taps of the frequency direction interpolation filter for estimating the transmission path characteristics from the pilot signal is changed according to the position of the subcarrier to be estimated in the transmission symbol. For example, the number of taps of the frequency direction interpolation filter is controlled according to the symbol position so that only the pilot signal obtained by reception is included in the delay element as a sample signal for interpolation.

[0036]

Embodiments of the present invention will be described below with reference to the drawings.

A digital television broadcast receiving apparatus (OFDM receiving apparatus) based on the OFDM method will be described. FIG. 1 is a block diagram of the OFDM receiver. In FIG. 1, when the signal transmitted between the blocks is a complex signal, the signal component is represented by a thick line, and when the signal transmitted between the blocks is a real number signal, the signal component is represented by a thin line. .

As shown in FIG. 1, the OFDM receiver 1 includes an antenna 2, a tuner 3, and an A / D conversion circuit 4.
Digital quadrature demodulation circuit 5, FFT operation circuit 6,
It includes a window synchronization circuit 7, an equalizer 8, a demapping circuit 9, and an error correction circuit 10.

A broadcast wave of a digital television broadcast broadcast from a broadcast station is received by the antenna 2 of the OFDM receiver 1 and supplied to the tuner 3 as an RF signal.

The RF signal received by the antenna 2 is
The frequency is converted to an IF signal by the tuner 3 and supplied to the A / D conversion circuit 4. The IF signal is supplied to the A / D conversion circuit 4
And supplied to the digital quadrature demodulation circuit 5. In the DVB-T standard (2K mode), the A / D conversion circuit 4 quantizes the effective symbol of the OFDM time domain signal with a clock that samples 2048 samples and a guard interval with, for example, 512 samples. .

The digital quadrature demodulation circuit 5 quadrature demodulates the digitized IF signal using a carrier signal of a predetermined frequency (carrier frequency), and outputs a baseband OFD signal.
Output M signal. The baseband OFDM signal output from the digital quadrature demodulation circuit 5 is a so-called time domain signal before the FFT operation. For this reason, the baseband signal after the digital quadrature demodulation and before the FFT operation is hereinafter referred to as an OFDM time domain signal. This OFDM time domain signal is a complex signal including a real axis component (I channel signal) and an imaginary axis component (Q channel signal) as a result of quadrature demodulation. The OFDM time domain signal output from the digital quadrature demodulation circuit 5 is supplied to an FFT operation circuit 6 and a window synchronization circuit 7.

The FFT operation circuit 6 performs an FFT operation on the OFDM time domain signal, and extracts and outputs data orthogonally modulated on each subcarrier. The signal output from the FFT operation circuit 6 is a so-called frequency domain signal after the FFT. From this, hereinafter, F
The signal after the FT operation is called an OFDM frequency domain signal.

The FFT operation circuit 6 extracts a signal in the effective symbol length range (for example, 2048 samples) from one OFDM symbol, that is, removes the range of the guard interval from one OFDM symbol and extracts 2048 samples of the extracted OFDM symbol. F for time domain signals
Perform FT operation. Specifically, the calculation start position is OFD
Any position between the boundary of the M symbols and the end position of the guard interval. This calculation range is called an FFT window.

As described above, the OFDM frequency domain signal output from the FFT operation circuit 6 is a complex composed of a real axis component (I channel signal) and an imaginary axis component (Q channel signal), like the OFDM time domain signal. Signal.
This complex signal is, for example, 16QAM system or 64QAM
This is a signal that has been subjected to quadrature amplitude modulation by a method or the like. The OFDM frequency domain signal is supplied to the equalizer 8.

The window synchronization circuit 7 receives the input OF
The DM time domain signal is delayed by an effective symbol period to obtain a correlation between a guard interval portion and a signal from which the guard interval is copied, and a boundary position of an OFDM symbol is calculated based on the high correlation portion, A window synchronization signal _Wsync indicating the boundary position is generated. The FFT window synchronization circuit 7 supplies the generated window synchronization signal _Wsync to the FFT operation circuit 6.

The equalizer 8 uses the scattered pilot signal (SP signal) to perform phase equalization and amplitude equalization of the OFDM frequency domain signal. The OFDM frequency domain signal subjected to the phase equalization and the amplitude equalization is supplied to the demapping circuit 9.

A demapping circuit 9 decodes data by performing demapping on the OFDM frequency domain signal that has been equalized in amplitude and phase by the equalizer 8 in accordance with the 16QAM system. The data decoded by the demapping circuit 9 is supplied to an error correction circuit 10.

The error correction circuit 10 performs error correction on the supplied data using, for example, Viterbi decoding or Reed-Solomon code. The error-corrected data is supplied to, for example, a subsequent MPEG decoding circuit.

Next, the equalizer 8 will be described in more detail.

The equalizer 8 includes an SP signal extraction circuit 11
, A time direction interpolation filter 12, a frequency direction interpolation filter 13, a 1 / X circuit 14, and a complex multiplication circuit 15.

The OFDM frequency domain signal output from the FFT operation circuit 6 is supplied to the SP signal extraction circuit 11. The SP signal extraction circuit 11 extracts only the SP signal from the OFDM frequency domain signal. The SP signal is
They are discretely inserted into the symbol, and the insertion position is determined in advance by a standard. SP signal extraction circuit 11
Refers to the symbol number of the supplied OFDM frequency domain signal because the SP signal is inserted at a different subcarrier position for each symbol, and the SP signal is inserted into the subcarrier of any index number from the symbol number. Is calculated based on the standard, and the SP signal is extracted. The SP signal extraction circuit 11 supplies the extracted SP signal to the time direction interpolation filter 12.

The time direction interpolation filter 12 comprises a zero hold filter, performs an interpolation process by filtering the SP signal in the time axis direction, and estimates the transmission path characteristics. Specifically, this time-direction interpolation filter 1
No. 2 performs an interpolation process by holding the value of the actually extracted SP signal for three OFDM symbols. The SP signal subjected to the time direction interpolation processing is supplied to the frequency direction interpolation filter 13 in OFDM symbol units.

The frequency direction interpolation filter 13 has an FIR
(Finite Impulse Response) filter,
Interpolate the SP signal in the frequency direction (subcarrier direction)
Estimate the amplitude and phase frequency characteristics for all subcarriers in an OFDM symbol. That is, the frequency characteristic H (ω) of the transmission path is estimated. The frequency characteristics H (ω) of the transmission path for all subcarriers obtained by the frequency direction interpolation filter 13 are supplied to a 1 / X circuit 14.

The 1 / X circuit 14 performs a reciprocal operation on the estimated frequency characteristic H (ω) of the transmission path. The frequency characteristic 1 / H (ω) of the transmission line on which the reciprocal operation has been performed is supplied to the complex multiplication circuit 15.

The complex multiplication circuit 15 performs a complex multiplication of the OFDM frequency domain signal from the FFT operation circuit 6 and the frequency characteristic 1 / H (ω) of the transmission line on which the inverse operation has been performed, and performs waveform equalization.

Next, the configuration of the frequency direction interpolation filter 13 in the equalizer 8 will be further described.

The frequency direction interpolation filter 13 has an FIR filter configuration for performing triple interpolation in which the number of taps can be controlled. The frequency direction interpolation filter 13 is, for example, as shown in FIG.
As shown in the figure, the first to twentieth twenty delay elements 21
To 40 and 21 multipliers 50 to 50 from the 0th to the 20th
70, an adder 71, and a coefficient control unit 72.

Signals whose transmission path characteristics are estimated at three subcarrier intervals are sequentially input from the time direction filter 12 in the frequency direction (subcarrier direction) to the frequency direction interpolation filter 13 as input signals. In a portion where the characteristics of the transmission channel are not estimated (a portion where the transmission channel is not estimated by the time direction interpolation filter 12), 0 is set.
Is entered.

The first delay element 21 delays the input signal by one timing. The second delay element 22 further delays the output signal of the first delay element 21 by one timing. The third delay element 23 further delays the output signal of the second delay element 22 by one timing. Thereafter, each of the delay elements 24 to 40 delays the output signal of the immediately preceding delay element by one timing. That is, each of the delay elements 21 to 40
Outputs a delay signal delayed by 1 to 20 timings. The 0th multiplier 50 multiplies the undelayed input signal by a coefficient k0, the first multiplier 51 multiplies the output signal of the first delay element 21 by a coefficient k1, and
The second multiplier 52 multiplies the output signal of the second delay element 22 by a coefficient k2, and thereafter each of the multipliers 52 to 70 multiplies the output signal of the corresponding delay element 23 to 40 by a coefficient k3 to k20. Then, the adder 71 includes all the multipliers 50 to 7
The multiplied outputs of 0 are added and output.

The coefficient control section 72 generates coefficients k0 to k20 and supplies them to the multipliers 50 to 70.

Here, the coefficient control unit 72 determines the coefficient k0 according to the index number of the subcarrier to be interpolated.
To k20 to control the number of taps of the FIR filter. Specifically, the O in the low frequency part and the high frequency part
When the subcarrier at the end of the FDM symbol is to be interpolated, the number of taps is reduced. When the subcarrier at the center of the OFDM symbol is to be interpolated, the number of taps is increased (maximum). Value).

For example, there are 2048 subcarriers in an OFDM symbol, and as a result of interpolation by the time direction interpolation filter 12, the index of each OFDM symbol is #
SP for subcarriers such as 1, # 4, # 7, # 10 ...
When the signal is inserted and the number of delay elements of the filter is 20 (21 taps), the coefficient control unit 72 changes the values of the coefficients k0 to k20 as follows, Control the number.

When the subcarrier number # 1 is the interpolation point (when the lowest frequency subcarrier is the interpolation point), FIG.
As shown in A, the coefficient k0 is set so that the number of taps becomes 5.
Ｋk20 are set. Also, subcarrier number # 2
In the case of, as shown in FIG. 3B, the values of the coefficients k0 to k20 are set so that the number of taps becomes seven. In the case of subcarrier number # 3, the values of coefficients k0 to k20 are set such that the number of taps becomes 9, as shown in C of FIG.
Thereafter, as the number of interpolation points increases by one, the number of taps is increased by two. In the case of subcarrier number # 8, coefficients k0 to k are set so that the number of taps becomes 19 as shown in FIG.
Set the value of 20. After the subcarrier number # 9, the values of the coefficients k0 to k20 are set so that the number of taps becomes 21 as shown by F and G in FIG. 3, and the coefficients are fixed.

On the other hand, the interpolation point is the subcarrier number # 203
Up to 9, as shown in FIGS.
The values of the coefficients k0 to k20 are set in advance so that Subsequently, when the subcarrier number # 2040 is an interpolation point, the values of the coefficients k0 to k20 are set so that the number of taps becomes 19, as shown in D of FIG. Subsequently, when the subcarrier number # 2041 is an interpolation point, the coefficients k0 to k2 are set so that the number of taps becomes 17 as shown in E of FIG.
Set a value of 0. Thereafter, as the number of interpolation points increases by one, the number of taps is reduced by two and the subcarrier number # 204
In the case of 7, as shown in F of FIG. 3, the values of the coefficients k0 to k20 are set so that the number of taps becomes 5, and in the case of the subcarrier number # 2048, as shown in G of FIG. To
The values of the coefficients k0 to k20 are set so that the number of taps becomes three.

When the number of taps is controlled as described above, when triple interpolation is performed, it is possible to perform interpolation using only signals actually received and obtained without inserting dummy data as reference values. It becomes possible. That is, the number of taps is changed so that only a valid value (true value) output from the time direction interpolation filter 12 can be used as an input signal.

As described above, the OFD according to the embodiment of the present invention
When estimating the channel characteristics by interpolation, the M receiving apparatus 1 does not input a dummy signal into the delay element if the subcarrier to be interpolated is a low-frequency part or a high-frequency part in an OFDM symbol. Then, the number of taps is controlled so that the value of the subcarrier to be interpolated can be detected.

For this reason, it is possible to reduce an error in the estimated value of the transmission path characteristic, and it is possible to estimate the transmission characteristic of the transmission path with high accuracy and to improve the information error rate.

The configuration of the frequency interpolation filter 13 in the OFDM receiver 1 is not limited to the configuration shown in FIG. 2, but may be a configuration capable of controlling the number of taps according to the subcarrier number to be interpolated. Any configuration may be used.

For example, as shown in FIG.
Tap filters, 7 taps, 9 taps... 21 taps are provided in parallel with interpolation filters 80 to 89, and input signals are supplied to all of these interpolation filters 80 to 89. The selector 90 may be configured to select and output any one of the interpolation filters according to the subcarrier number to be interpolated.

[0070]

According to the OFDM demodulation apparatus and method according to the present invention, the number of taps of an interpolation filter for estimating a transmission path characteristic from a pilot signal is changed according to the position of a subcarrier to be estimated in a transmission symbol. . Therefore, according to the present invention, it is possible to highly accurately estimate the transfer characteristic of the transmission path and improve the information error rate.

[Brief description of the drawings]

FIG. 1 is a block diagram of an OFDM receiver according to an embodiment of the present invention.

FIG. 2 is a diagram for explaining a configuration of a frequency direction interpolation filter.

FIG. 3 is a diagram for explaining a specific setting value of the number of taps in a low frequency portion in an OFDM symbol.

FIG. 4 is a diagram illustrating a specific setting value of the number of taps in a high frequency portion in an OFDM symbol.

FIG. 5 is a diagram for explaining another configuration example of the frequency direction interpolation filter.

FIG. 6 is a diagram for explaining a guard interval of an OFDM signal.

FIG. 7 is a diagram for explaining an insertion position of a scattered pilot signal of an OFDM signal.

FIG. 8 is a diagram illustrating an interpolation filter process in a time direction when estimating a transmission path characteristic.

FIG. 9 is a diagram for explaining subcarriers whose transmission path characteristics have been estimated by a time-direction interpolation filter.

FIG. 10 is a diagram for describing interpolation filter processing in the frequency direction when estimating transmission path characteristics.

FIG. 11 is a diagram for explaining a configuration of a zero hold filter.

FIG. 12 is a diagram for explaining transmission path characteristics obtained by interpolation using a 0 hold filter.

FIG. 13 is a diagram for explaining a configuration of a 21-tap FIR filter.

FIG. 14 is a diagram for describing interpolation processing in the frequency direction at an end portion of an OFDM symbol.

FIG. 15 is a diagram illustrating an area where an estimation error occurs because temporary data is input as an estimation source.

[Explanation of symbols]

1 OFDM receiver, 5 digital quadrature demodulator, 6
FFT operation circuit, 7 window synchronization circuit, 8 equalizer, 9 demapping circuit, 11 SP signal extraction circuit, 12 time direction interpolation filter, 13 frequency direction interpolation filter, 14 1 / X circuit, 15 complex multiplication circuit

Claims (4)

[Claims] 1. A transmission symbol generated by dividing and orthogonally modulating information on a plurality of subcarriers within a predetermined band is used as a transmission unit and has a specific power and a specific phase. In an OFDM demodulator for demodulating an orthogonal frequency division (OFDM) signal in which a pilot signal is inserted discretely into predetermined subcarriers in the transmission symbol, a Fourier transform means for performing a Fourier transform on the OFDM signal in units of the transmission symbol And a pilot signal extracting means for extracting the pilot signal for each transmission symbol from the signal Fourier-transformed by the Fourier transforming means; a time-direction interpolation filter and a frequency direction for extracting the pilot signal extracted by the pilot signal extracting means; Everything in the transmission symbol is obtained by interpolating using the interpolation filter. Interpolation means for calculating the transmission path characteristics of the subcarriers, and waveform equalization means for waveform equalizing a signal Fourier-transformed by the Fourier transformation means based on the transmission path characteristics of each subcarrier calculated by the interpolation means And a tap number control means for controlling the number of taps for the frequency direction interpolation filter of the interpolation means, wherein the tap number control means determines a frequency of the subcarrier to be interpolated in accordance with a position in a transmission symbol. Characterized in that the number of taps of the direction interpolation filter is changed.
FDM demodulator. 2. The tap number control means controls the tap number of the frequency-direction interpolation filter so that only a pilot signal obtained by reception is included in a delay element as a sample signal for interpolation. 2. The OFDM demodulator according to claim 1, wherein: 3. A transmission symbol generated by dividing and orthogonally modulating information on a plurality of subcarriers within a predetermined band is used as a transmission unit, and has a specific power and a specific phase. In an OFDM demodulation method for demodulating an orthogonal frequency division (OFDM) signal in which a pilot signal is inserted discretely into predetermined subcarriers in the transmission symbol, a Fourier transform is performed on the OFDM signal in units of the transmission symbol. The pilot signal is extracted for each transmission symbol from the obtained signal, and a time-direction interpolation filter and a frequency-direction interpolation filter are used. By interpolating the extracted pilot signal while changing the number of taps, the transmission symbol , Calculating the channel characteristics of all the subcarriers in the sub-carrier, and waveform-equalizing the Fourier-transformed signal based on the calculated channel characteristics of each subcarrier.
FDM demodulation method. 4. The number of taps of the frequency direction interpolation filter is controlled so that only a pilot signal obtained by reception is included in a delay element as a sample signal for interpolation. OFDM demodulation method.