JP2002344410A - Ofdm modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve information error rate by accurately estimating the transmission characteristics of transmission lines even in the case of reception using mobile units. 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 interpolation filter 13 for interpolating the SP signal that has been interpolated in the time direction further in the frequency direction. A time direction interpolating filter 12 comprise an n-stage (n indicates an integer being 2 or larger) 0-order hold filter.

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 demodulator applied to digital broadcasting by 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] As shown in FIG. 4, a transmission signal according to the OFDM scheme is transmitted in a symbol unit called an OFDM symbol. 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 different distortion occurs for each subcarrier due to the influence of multipath or the like during transmission, the characteristics of the amplitude and phase for each subcarrier will be different. 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. OF
In the DM system, a pilot signal having a predetermined amplitude and a predetermined phase is scattered in a transmission symbol in a transmission signal on a transmission side, and the amplitude and phase of the pilot signal are monitored on a reception side, so that a transmission path is controlled. The characteristics are determined, and the received signal is equalized based on the determined characteristics of the transmission path. The pilot signal used to calculate the characteristics of the transmission path is called a scattered pilot signal (SP) signal.

FIG. 5 shows an arrangement pattern of an SP signal in an OFDM symbol 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. 6, 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. 7, the transmission path characteristics can be estimated for every three subcarriers in the frequency direction for all OFDM symbols.

Subsequently, as shown in FIG. 8, the SP signal interpolated in the time direction is input to an interpolation filter in the frequency direction to perform interpolation processing in the frequency direction, thereby estimating the channel characteristics of all subcarriers in the OFDM symbol. 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 mounting reasons, when estimating the transmission path characteristics of an OFDM signal, it is common to use a one-stage zero-order hold filter with a small hardware scale as the time direction filter.

As shown in FIG. 9, the zero-order hold filter holds the value of the actually extracted SP signal for three OFDM symbols and outputs the result. That is, the SP signal actually received is output as an estimated value of the following 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 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. 10 shows a configuration example in which a frequency direction filter is realized by a 21-tap FIR filter. The FIR filter 200 shown in FIG. 10 includes first to twentieth twenty delay elements 201 to 220, zeroth to twentieth twenty-one 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 the portion where the characteristics of the transmission path are not estimated (the portion where the characteristics are not estimated by the time direction interpolation filter).

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 of the delay element are interpolated three times.

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

[0024]

By the way, in conventional digital broadcasting, especially digital television broadcasting, fixed reception with a fixed receiver is premised, so that the time variation of transmission path characteristics is particularly considered. I didn't have to. Therefore, 0 is used as the time direction interpolation filter.
Even if the secondary hold filter was used, there was no particular problem in estimating the transmission path.

However, in the future, not only fixed reception but also mobile reception will be widely used for digital broadcasting. In the case of mobile reception, a time variation of transmission path characteristics called so-called fading occurs. Therefore, if a zero-order hold filter is used for estimating the transmission path characteristics as in the related art, it is impossible to follow the time variation due to fading, and the transmission error increases.

The present invention has been made in view of such a situation. Even in the case of performing mobile reception, an OFDM that estimates the transmission characteristics of a transmission path with high accuracy and improves the error rate of information is provided. It is an object to provide a demodulation device.

[0027]

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 subcarriers, and waveform equalization means for equalizing the waveform of the signal Fourier-transformed by the Fourier transformation means based on the transmission path characteristics of each subcarrier calculated by the interpolation means. And the time direction interpolation filter of the interpolation means is n
It is characterized by comprising a zero-order hold filter of stages (n is an integer of 2 or more).

In this OFDM demodulator, a time-direction interpolation filter for estimating a transmission path characteristic from a pilot signal is constituted by n stages (n is an integer of 2 or more) of a zero-order hold filter.

[0029]

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, an A / D converter 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 within the effective symbol length range (for example, 2048 samples) from one 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.

The demapping circuit 9 decodes data by demapping the OFDM frequency domain signal, which has been equalized in amplitude and phase by the equalizer 8, according to the 16QAM method. 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 performs an interpolation process by filtering the SP signal in the time axis direction, and estimates the transmission path characteristics. 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 time direction interpolation filter 12 in the equalizer 8 will be further described.

As shown in FIG. 2, the time direction interpolation filter 12 has a configuration in which n (n is an integer of 2 or more) zero-order hold filters 20-1 to 20-n are connected in series. I have.

FIG. 3 shows an example of the internal configuration of each zero-order hold filter 20. 0-order hold filter 20 shown in FIG.
Are the first to third three delay elements 21, 22, 23
And first to third adders 24, 25, and 26. That is, the zero-order hold filter 20
Is configured as an FIR filter in which all tap coefficients are 1.

First-order zero-order hold filter 20-1
, A signal having a predetermined subcarrier number of each OFDM signal is sequentially input as an input signal in the time direction (OFDM symbol direction). Note that 0 is input to a portion where the SP signal is not inserted (a portion where the original information is included). Output signals from the preceding 0th-order hold filters 20-1 to 20- (n-1) are sequentially input as input signals to the 0th-order hold filters 20-2 to 20-n of the second and subsequent stages. Is done.

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. That is, each of the delay elements 21 to 23 outputs a delayed signal delayed by one timing, a delayed signal delayed by two timings, and a delayed signal delayed by three timings. Also, the first
Adder 24 adds the input signal and the output signal of first delay element 21 and outputs the result. Second adder 25 outputs the output signal of first adder 24 and the output of second delay element 22. The third adder 26 adds the signals to the second adder 25.
And the output signal of the third delay element 23 are added and output.

As described above, each of the zero-order hold filters 20 constituting the time-direction interpolation filter 12 is:
It has a function of holding the value of the actually extracted SP signal for three OFDM symbols and outputting it.

On the other hand, for example, when the zero-order hold filter 20 is connected in two stages in series (that is, n =
2) 1 to interpolate between two SP signals with a primary linear line
It becomes a next-order linear filter. When the zero-order hold filters 20 are connected in series in three stages (that is, n = 3),
A quadratic curve filter interpolating with a quadratic curve using three SP signals. Similarly, in the case of four stages, a cubic curve, 5
In the case of stages, a quartic curve... In the case of n stages, (n-
1) It becomes a filter for interpolating with the next curve.

Therefore, as a time direction interpolation filter, n
By providing the zero-order hold filter 20 of the stage, the interpolation process can be performed by the (n-1) th-order straight line while the circuit scale is relatively small. Transmission path characteristics can be accurately estimated.

[0057]

According to the OFDM demodulator according to the present invention,
A time-direction interpolation filter for estimating transmission path characteristics from a pilot signal uses a zero-order hold filter of n stages (n is an integer of 2 or more).

Therefore, the OFDM demodulator according to the present invention can improve the information error rate by estimating the transfer characteristics of the transmission path with high accuracy even when performing mobile reception.

[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 time direction interpolation filter.

FIG. 3 is a diagram illustrating a configuration of a zero-order hold filter that forms the time-direction interpolation filter.

FIG. 4 is a diagram for explaining a guard interval of an OFDM signal.

FIG. 5 is a diagram for explaining an insertion position of a scattered pilot signal of an OFDM signal.

FIG. 6 is a diagram illustrating an interpolation filter process in a time direction when estimating a transmission path characteristic.

FIG. 7 is a diagram for describing subcarriers whose transmission path characteristics have been estimated by a time-direction interpolation filter.

FIG. 8 is a diagram for describing interpolation filter processing in the frequency direction when estimating transmission path characteristics.

FIG. 9 is a diagram for explaining transmission path characteristics obtained by interpolation using a 0 hold filter.

FIG. 10 is a diagram for describing a configuration of a 21-tap FIR filter.

[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,
20-1 to 20-n 0th-order hold filter

Claims (1)

[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 The time direction interpolation filter of the interpolation means has n stages (n is 2
An OFDM demodulator comprising a zero-order hold filter of the above integer).