JP2000022661A - Ofdm demodulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely demodulate reception signals without enlarging the circuit scale of an FFT circuit even in the case of generating large distortion by fading. SOLUTION: Inside the respective frames of the reception signals S1, known pattern signals for propagation line estimation are inserted. A timing signal generation circuit 8 generates timing signals S3 and S4 for indicating the period of the pattern signals and the selector 4 is operated corresponding to the timing signals S3, selects the output signal D1 of an A/D converter 2 and outputs it to a fast Fourier transformer(FFT) 5 in a pattern signal period. A transfer function estimation circuit 9 fetches the pattern signal outputted from the fast Fourier transformer 5 corresponding to the timing signal S4 and estimates the transfer function of a propagation line. An automatic equalizer 3 inversely Fourier transforms the reciprocal of the estimated transfer function and obtains an impulse response, a convolution operation with the signal D1 is performed by a transversal filter and the distortion of the propagation line is removed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to OFDM (Orth).
ogonal Frequency Division
The present invention relates to a demodulator for digitally modulated signals, and more particularly to an OFDM demodulator having a function of removing distortion of a propagation path.

[0002]

2. Description of the Related Art An OFDM transmission system is a system in which data to be transmitted is divided into a number of subcarriers whose frequency components are orthogonal to each other and transmitted, and it is said that the transmission data can be hardly affected by fading or multipath. Has advantages.

On the transmitting side of the OFDM transmission system, a coded signal is allocated to each subcarrier and modulated, and a digital signal in the frequency domain composed of the modulated subcarriers is subjected to inverse Fourier transform to obtain a digital signal in the time domain. And transmits it as a predetermined transmission signal. On the other hand, the receiving side obtains an encoded signal assigned to each subcarrier by Fourier-transforming the received signal into a digital signal in the frequency domain.

FIG. 4 shows an example of a conventional OFDM transmission system transmitting apparatus and receiving apparatus. Here, the data D10 of the transmission device is a symbol of the I-axis and Q-axis components converted into the code point (symbol) of the PSK system or the QAM system. Data D10 is a serial / parallel converter (S / P) 11
Are converted into parallel symbols, and the I-axis and Q-axis components of these parallel symbols are regarded as a real part and an imaginary part of a complex number, and an inverse fast Fourier transform (referred to as IFFT) circuit 1
2 is subjected to an inverse Fourier transform process, and is converted into a time domain signal of a real part and an imaginary part. The real part and the imaginary part of the time domain signal are converted into analog signals by the D / A converter 13, and are quadrature-modulated by the quadrature modulator 14 as I and Q channel baseband signals and transmitted.

On the other hand, in the receiving apparatus, the received signal is subjected to quadrature detection by a quadrature detector 21 to become baseband signals of I and Q channels, quantized by an A / D converter 22 and real and imaginary parts of a time domain signal. Output as part. And a fast Fourier transform (referred to as FFT) circuit 2
3 is subjected to Fourier transform processing, converted into a signal in the frequency domain, and output as a parallel symbol. afterwards,
After the distortion of the amplitude and phase due to the fading of the propagation path is corrected by the equalizer 24, the parallel / serial converter (P /
S) The demodulated data D converted to a serial symbol by 25
Output as 20. Incidentally, the equalizer 24 is controlled by the correction circuit 26 to perform an equalization process on the output signal of the FFT circuit 22. The correction circuit 26 extracts the reference symbol for timing synchronization periodically inserted for each frame on the transmission side, compares the reference symbol included in the received signal with the internally generated reference symbol, and , And controls the equalizer 24 so as to reduce the error.

[0006]

In the OFDM transmission system, the influence of fading and multipath can be reduced by increasing the number of divided multicarriers and narrowing the signal band per carrier. However, if the number of divided multicarriers is increased, the circuit scale of the inverse Fourier transform circuit of the transmitting device and the Fourier transform circuit on the receiving side becomes large, and if the signal band per carrier is too narrow, the signal is converted to radio frequency. Requirements for the frequency deviation and phase noise characteristics of the local oscillator are strict.

Further, when a large distortion occurs in the propagation path due to multipath, the level of the signal input to the FFT circuit differs greatly for each subcarrier, so that the FFT circuit must execute the FFT operation without error. The dynamic range of the circuit must be widened and the circuit scale must be increased, and there is a problem that it is very difficult to improve the anti-fading characteristics in the above-described conventional receiver.

[0008] An object of the present invention is to be able to reliably demodulate a received signal without enlarging the dynamic range of an FFT circuit and enlarging the circuit scale even when fading causes a large distortion. An object of the present invention is to provide an OFDM demodulator capable of easily realizing power consumption and no adjustment.

[0009]

SUMMARY OF THE INVENTION An OFDM demodulator according to the present invention receives a signal in which a known pattern signal for propagation path estimation is inserted in each frame of an OFDM signal on a transmission side, and performs quantization on the received signal. An automatic equalizer and a selector are provided on the input side of the fast Fourier transformer that performs Fourier transform processing on the signals in the area, and a transfer function estimating circuit is provided on the output side of the fast Fourier transformer. The selector is operated to supply a time-domain signal to the fast Fourier transformer, and the transfer function estimating circuit is caused to take in the output of the fast Fourier transformer to estimate the transfer function of the propagation path. The transfer function is supplied to the automatic equalizer to remove the distortion of the propagation path. The automatic equalizer obtains an impulse response by performing an inverse Fourier transform on the reciprocal of the estimated transfer function, and performs a convolution operation with the signal in the time domain using a transversal filter to remove distortion of a propagation path.

[0010] More specifically, an OFDM demodulator that receives an OFDM signal transmitted from an OFDM (Orthogonal Frequency Division Multiplexing) transmission type transmitting apparatus, performs a Fourier transform process on a quantized time-domain signal, and demodulates the signal. , The OFDM signal is a signal in which a frame symbol for frame synchronization and a known pattern signal for propagation path estimation are respectively inserted at predetermined positions of each frame, and the OFDM signal has a propagation path with respect to the quantized time domain signal. An automatic equalizer that performs an equalization process that removes distortion of the above, a fast Fourier transformer that performs the Fourier transform process, and a transfer function of a propagation path is estimated based on the pattern signal output from the fast Fourier transformer. A propagation path estimating unit, wherein the automatic equalizer performs equalization processing based on the transfer function estimated by the propagation path estimating unit.

In the above configuration, the automatic equalizer includes a complex division circuit for calculating a reciprocal of the transfer function estimated by the propagation path estimating means, and an inverse Fourier transform for calculating the reciprocal of the transfer function calculated by the complex division circuit. An inverse fast Fourier transform circuit for converting and outputting the impulse response; and a transversal filter for performing a convolution operation between the quantized time-domain signal and the impulse response.

Further, the propagation path estimating means detects a frame symbol included in the quantized time domain signal and generates a frame signal indicating a frame timing, based on the frame signal. A timing signal generation circuit that generates a timing signal indicating a signal period of the pattern signal; and capturing the pattern signal from an output of the fast Fourier transformer in accordance with the timing signal, and comparing a spectrum of a known pattern signal. And a transfer function estimating circuit for estimating the transfer function of the propagation path.

Further, the propagation path estimating means selects a signal to be input to the automatic equalizer when the timing signal indicates a signal period of the pattern signal, and supplies the selected signal to the fast Fourier transformer. , A selector for selecting a signal output from the automatic equalizer and supplying the selected signal to the fast Fourier transformer.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing one embodiment of the present invention. Here, the received signal S1 is an OFDM signal transmitted by a transmission apparatus using an OFDM (orthogonal frequency division multiplexing) transmission method as shown in FIG.
As shown in FIG. 3, the frame structure of the signal is such that a frame symbol for frame synchronization is arranged at the beginning, and thereafter,
It is assumed that a known pattern signal for estimating a propagation path is arranged, and a plurality of data symbols are arranged after that. That is, it is assumed that the transmitting apparatus has means for inserting a pattern signal for channel estimation at a predetermined position in each frame. The pattern signal for channel estimation is not particularly limited as long as it is a known pattern signal.

In FIG. 1, a quadrature detector 1 performs quadrature detection on a received signal S1 and outputs baseband signals of I and Q channels. The A / D converter 2 quantizes the quadrature detection output and outputs it as a real part signal D1 (I) and an imaginary part signal D1 (Q) in the time domain. These signals D1 (I), D
1 (Q) is supplied to the automatic equalizer 3, the selector 4, and the frame synchronization circuit 7, respectively.

The automatic equalizer 3 outputs signals D1 (I), D1
(Q) is subjected to an equalization process for removing distortion of the propagation path,
The signals are output as signals D2 (I) and D2 (Q).

The frame synchronization circuit 7 detects a frame symbol inserted at the head of the frame, generates a frame signal S2 indicating the timing of the frame and the symbol, and outputs the frame signal S2 to the timing signal generation circuit 8. The timing signal generation circuit 8 generates a timing signal S3 for controlling the operation of the selector 4 and a timing signal S4 for controlling the transfer function estimation circuit 9 based on the frame signal S2.
Here, the timing signals S3 and S4 are signals indicating the period of the pattern signal included in each frame.

The selector 4 operates according to the timing signal S3, selects the output signals D1 (I) and D1 (Q) of the A / D converter 2 during the pattern signal period, and automatically equalizes during the other periods. The output signals D 2 (I) and D 2 (Q) of the device 3 are selected and output to the fast Fourier transformer 5.

A fast Fourier transformer (FFT) 5 performs a Fourier transform process on an output signal of the selector 4 to convert the output signal into a frequency domain signal D3 and output it. That is, the pattern signal of each frame is directly received from the A / D converter 2 and subjected to Fourier transform processing, and the other signals are received via the automatic equalizer 3 and subjected to Fourier transform processing.

The parallel-to-serial conversion circuit (P / S) 6 performs parallel-to-serial conversion on the signal D3 output from the fast Fourier transformer 5 and separated into subcarriers, and outputs the result as demodulated data D4.

The transfer function estimating circuit 9 fetches the pattern signal output from the fast Fourier transformer 5 according to the timing signal S4 from the timing signal generating circuit 3, and obtains the spectrum of the received pattern signal and the known pattern signal. The transfer function of the propagation path is estimated by comparing the spectrum with the spectrum.
Send to Note that the timing signal S4 is delayed from the timing signal S3 by the time required for the Fourier transform processing in the fast Fourier transformer 5.

If the known pattern signal spectrum is Pt (ω), the transfer function of the propagation path is H (ω), and the received pattern signal spectrum is Pr (ω), the transfer function H ( ω) = Pr (ω) / Pt
(Ω) and Pt (ω) is known, so Pr
By performing the calculation of (ω) / Pt (ω), the transfer function H (ω) of the propagation path can be estimated.

Next, the automatic equalizer 3 will be described.

FIG. 2 is a block diagram showing an example of the automatic equalizer 4, which includes a complex division circuit 31, an inverse fast Fourier transform circuit (IFFT) 32, and a transversal filter 33.
And Then, based on the transfer path transfer function H (ω) estimated by the transfer function estimating circuit 9, the time domain signals D1 (I) and D1 output from the A / D converter 2 are output.
For (Q), an equalization process for removing distortion of the propagation path is performed.

The complex dividing circuit 31 includes a transfer function estimating circuit 9
Calculate the reciprocal of the transfer function H (ω) of the propagation path estimated by The inverse fast Fourier transform circuit 32 calculates the inverse 1 / of the transfer function of the propagation path calculated by the complex division circuit 31.
The impulse response is output by performing an inverse Fourier transform on H (ω). Transversal filter 33
Performs a convolution operation on the signals D1 (I) and D1 (Q) and the impulse response output from the inverse fast Fourier transform circuit 32, thereby removing distortion in the propagation path.

Now, assume that the spectrum of the original signal transmitted from the transmitting device is S (ω), and the transfer function of the propagation path is H
(Ω) and the received signal spectrum is R (ω), then R (ω) = S (ω) · H (ω). Therefore,
The equalization process in the frequency domain may be performed by S (ω) / H (ω).

Here, G (ω) = 1 / H (ω), and F
Assuming that (ω) = R (ω) · G (ω), F (ω) = R (ω) · G (ω) ← →
Since f (t) = r (t) * g (t) (* indicates a convolution operation), the inverse 1 / H of the transfer function of the propagation path
By performing an inverse Fourier transform on (ω) and performing a convolution operation with the received signal, it is possible to perform an equalization process for removing distortion in the propagation path.

By doing so, the signal from which the distortion of the propagation path has been removed can be input to the FFT circuit 5, so that even if a large distortion occurs due to fading, the dynamic range of the FFT circuit 5 can be reduced. The received signal can be reliably demodulated without increasing the circuit scale.

[0030]

As described above, according to the present invention, on the transmitting side, a known pattern signal for propagation path estimation is inserted into each frame of an OFDM signal.
An automatic equalizer and a selector are provided on the input side of the fast Fourier transformer, and a transfer function estimating circuit is provided on the output side of the fast Fourier transformer, and the selector is operated during the pattern signal period of each frame to operate the signal in the time domain. Is supplied to the fast Fourier transformer, and the transfer function estimating circuit is caused to take in the output of the fast Fourier transformer to estimate the transfer function of the propagation path.The estimated transfer function is supplied to the automatic equalizer to provide the propagation path. By removing the distortion of the automatic equalizer,
When inverse reciprocal of the transfer function is inverse Fourier transformed to obtain an impulse response, and a convolution operation with a signal in the time domain is performed by a transversal filter to remove the distortion of the propagation path, thereby causing a large distortion due to fading. In addition, demodulation can be reliably performed without increasing the dynamic range of the fast Fourier transformer without increasing the circuit scale, so that downsizing, low power consumption, and no adjustment by LSI can be easily realized.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a block diagram showing an example of an automatic equalizer 3 shown in FIG.

FIG. 3 is a diagram showing a frame configuration of a reception signal S1 shown in FIG.

FIG. 4 is a block diagram showing an example of a conventional OFDM transmission type transmitting apparatus and receiving apparatus.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Quadrature detector 2 A / D converter 3 Automatic equalizer 4 Selector 5 Fast Fourier transform (FFT) 7 Frame synchronization circuit 8 Timing signal generation circuit 9 Transfer function estimation circuit 31 Complex division circuit 32 Inverse fast Fourier transform circuit ( IFFT) 33 Transversal filter S1 Received signal S3, S4 Timing signal

[Procedure amendment]

[Submission date] September 9, 1999 (1999.9.9)

[Procedure amendment 1]

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[Claims]

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[Correction method] Change

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[0010] More specifically, an OFDM demodulator that receives an OFDM signal transmitted from an OFDM (Orthogonal Frequency Division Multiplexing) transmission type transmitting apparatus, performs a Fourier transform process on a quantized time-domain signal, and demodulates the signal. In the above, the OFDM signal is a signal in which a frame symbol for frame synchronization and a known pattern signal for propagation path estimation are respectively inserted at predetermined positions of each frame, and is estimated with respect to the quantized time domain signal. an automatic equalizer that performs equalization processing for removing the distortion of <br/> channel based on the transfer function, this
A fast Fourier transformer that performs the Fourier transform process on the signal that has passed through the automatic equalizer, and estimates the transfer function of the propagation path based on the pattern signal output from the fast Fourier transformer to the automatic equalizer. Channel estimating means for transmitting, the channel estimating means comprising:
Detecting the frame symbol included in the signal of the area
A frame that generates a frame signal indicating frame timing
A frame synchronization circuit and the pattern based on the frame signal.
To generate a timing signal indicating the signal period of the
An imaging signal generation circuit and the timing signal
When the signal period of the turn signal is indicated, the automatic equalizer
Select the pattern signal to be input and select the fast Fourier
When the power is supplied to the converter and other periods are indicated,
The signal output from the dynamic equalizer is selected to perform the fast Fourier transform.
And a selector for supplying the signal to the converter.
The pattern output from the fast Fourier transformer
Transfer function estimation for acquiring a signal and estimating the transfer function
And a circuit .

[Procedure amendment 3]

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[Procedure amendment 6]

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The transfer function estimating circuit 9 fetches the pattern signal output from the fast Fourier transformer 5 according to the timing signal S4 from the timing signal generating circuit 3, and obtains the spectrum of the received pattern signal and the known pattern signal. The transfer function of the propagation path is estimated by comparing the spectrum with the spectrum.
Send to Note that the pattern signal passes through the automatic equalizer.
Therefore, the timing signal S4 is delayed by the time required for the Fourier transform processing in the fast Fourier transformer 5 from the timing signal S3.

[Procedure amendment 7]

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[0030]

As described above, according to the present invention, on the transmitting side, a known pattern signal for propagation path estimation is inserted into each frame of an OFDM signal.
An automatic equalizer and a selector are provided on the input side of the fast Fourier transformer, a transfer function estimating circuit is provided on the output side of the fast Fourier transformer, and the selector is operated during the pattern signal period of each frame to operate the time domain pattern. While supplying the signal to the fast Fourier transformer, the transfer function estimating circuit quickly takes in the pattern signal output from the fast Fourier transformer and estimates the transfer function of the propagation path.The estimated transfer function is sent to the automatic equalizer. Transfer function estimation by supply
In addition, since the distortion of the propagation path can be removed by early updating the coefficient of the automatic equalizer, even if a large distortion occurs due to fading, the dynamic range of the fast Fourier transformer is expanded and the circuit scale is increased. Demodulation can be performed without any need for downsizing.
Low power consumption and no adjustment can be easily realized.

Claims (6)

[Claims] 1. An OFDM demodulator for receiving an OFDM signal transmitted from an OFDM (Orthogonal Frequency Division Multiplexing) transmission system and performing a Fourier transform process on a quantized time-domain signal to demodulate the signal. An OFDM signal is a signal in which a frame symbol for frame synchronization and a known pattern signal for propagation path estimation are inserted at predetermined positions of each frame, and distortion of a propagation path with respect to the quantized time domain signal is reduced. An automatic equalizer for performing an equalization process for removing, a fast Fourier transformer for performing the Fourier transform process, and a channel estimation for estimating a transfer function of a channel based on the pattern signal output from the fast Fourier transformer Means, wherein the automatic equalizer performs equalization processing based on the transfer function estimated by the propagation path estimation means. OFDM demodulator. 2. The automatic equalizer obtains an impulse response having an inverse characteristic of a transfer function estimated by the propagation path estimating means and performs a convolution operation with the quantized time domain signal to perform propagation. 2. The OFDM demodulator according to claim 1, wherein the distortion of the road is removed. 3. An automatic equalizer, comprising: a complex divider for calculating a reciprocal of a transfer function estimated by the propagation path estimating means; and an inverse Fourier transform of the reciprocal of the transfer function calculated by the complex divider. 2. An OFDM according to claim 1, further comprising: an inverse fast Fourier transform circuit for outputting the impulse response; and a transversal filter for performing a convolution operation of the quantized time-domain signal and the impulse response. Demodulator. 4. The channel estimating means detects a frame symbol included in the quantized time domain signal and generates a frame signal indicating a frame timing, A timing signal generation circuit for generating a timing signal indicating a signal period of the pattern signal based on the pattern signal; and a transfer function for capturing the pattern signal from an output of the automatic equalizer in accordance with the timing signal and estimating a transfer function of the propagation path. 2. The OFDM according to claim 1, further comprising an estimation circuit.
Demodulator. 5. The propagation path estimating means selects a signal to be input to the automatic equalizer when the timing signal indicates a signal period of the pattern signal, and supplies the selected signal to the fast Fourier transformer. 5. The OFDM demodulator according to claim 4, further comprising a selector for selecting a signal output from the automatic equalizer when the period indicates a period other than the above and supplying the selected signal to the fast Fourier transformer. 6. The transfer function estimating circuit in the frequency domain by comparing a spectrum of a pattern signal taken from an output of the fast Fourier transformer with a spectrum of a known pattern signal set in advance. 6. The OFDM demodulator according to claim 5, wherein the transfer function is estimated.