JP2002026865A - Demodulating apparatus and demodulation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an OFDM receiver that efficiently estimates a signal after waveform equalization so as to enhance the bit error rate characteristic in the case of waveform equalization to the signal through the estimation of a frequency characteristic of a transmission channel. SOLUTION: The OFDM receiver 1 has an equalizer 8 that applies waveform equalization to quadature amplitude modulation signal adopting the 16 QAM system after being subjected to an FFT arithmetic operation. The equalizer 8 estimates the frequency characteristic H(ω) of the transmission channel from an SP signal and divides the estimated frequency characteristic H(ω) by the quadrature amplitude modulation signal to apply amplitude equalization and phase equalization to the quadrature amplitude modulation signal. In this case, the equalizer 8 selects a power P of a maximum level of the quadrature amplitude modulation signal to be a maximum value of data outputted after the waveform equalization and boosts the power of each subcarrier and applies the amplitude equalization to the quadrature amplitude modulation signal by using the power P of the maximum level of a valid signal for a reference signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to orthogonal frequency division multiplexing transmission (OFDM).
The present invention relates to a demodulation device and a demodulation method 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 re-amplitude 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 OF
As terrestrial digital broadcasting employing the DM system, for example, DVB-T (Digital Video Broadcasting-Terres
trial) and ISDB-T (Integrated Services Digital)
Broadcasting-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 effective symbol, and the subcarrier interval is 4.14 Hz.
Becomes Also, data is modulated on 1705 subcarriers out of the 2048 subcarriers in the effective symbol. The guard interval is a signal having a time length of 1/4 or 1/8 of the effective symbol.

[0006] In addition, ODM such as the DVB-T standard using QAM-based modulation as a modulation method for each subcarrier.
In the FDM system, if different distortion occurs for each subcarrier due to multipath or the like during transmission, the signal space diagram of each subcarrier is distorted as shown in FIG. 5, for example, and the amplitude and The phase characteristics will be different. Therefore, OFD
In the M receiving apparatus, an equalizer is provided at a stage subsequent to the FFT operation circuit, and equalizes the amplitude and phase of each subcarrier in the frequency domain after the FFT operation. In particular,
In the OFDM system, since 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, an equalizer monitors the amplitude and phase of the pilot signal to thereby control the frequency of a transmission path. The characteristic is estimated, and the signal modulated on each subcarrier is equalized based on the estimated frequency characteristic of the transmission path. The pilot signal used for estimating the frequency characteristic of the transmission path is called a scattered pilot signal (SP) signal.

[0007]

By the way, generally, O
In the FDM method, the power of the SP signal is set higher than the power of another signal (a signal containing information: a valid signal) in order to increase resistance to the influence of noise or the like. For example, in the DVB-T system, when the effective signal is modulated by 16QAM, as shown in FIG. 6, the SP signal has a phase of 0 ° or 180 ° and the power is the average power of the effective signal (α = It is 4/3 times that of 1).

In the conventional OFDM receiver, when performing equalization by an equalizer, the power level does not saturate all the signals including the SP signal, and represents such a signal with higher bit precision. Thus, the power level was boosted and output. Specifically, the conventional OF
In the DM receiver, the power level of the SP signal is used as a reference,
The power of each subcarrier is boosted and the amplitude is equalized so that the power level of the SP signal becomes the maximum value of the output data. For example, as shown in FIG.
When expressing each data modulated on the I channel and the Q channel with 8-bit precision and outputting the data, the power level of the SP signal is set to the maximum value (255) of 8 bits,
Other data determine levels based on the power level of the SP signal.

However, when the waveform equalization is performed such that the power level of the SP signal becomes the maximum value of the output data, the SP signal does not contain any information, but contains information. Since this is a valid signal to the last, as shown in FIG. 8, information is not included in a portion (hatched portion) of a power level higher than the maximum power P that can be taken by the valid signal.

That is, the representation of data after waveform equalization involves many redundant portions that do not contain any information, and the bit precision of the effective signal that is actually represented is expressed in the output data. The bit precision is lower than the bit precision, and it is not expressed efficiently.

For example, even if the output data is represented by 8 bits, the data assigned to the actual effective signal cannot be represented by 8 bits, and for example, DVB-T
In the case of 16QAM of the system, the precision becomes about 7.5 bits.

The present invention has been made in view of such circumstances, and when estimating frequency characteristics of a transmission path and performing waveform equalization, efficiently represents a signal after waveform equalization. An object of the present invention is to provide an OFDM signal demodulation device and a demodulation method capable of improving a bit error rate characteristic.

[0013]

A demodulating apparatus according to the present invention uses a transmission symbol generated by subjecting a divided effective signal to quadrature amplitude modulation on a plurality of subcarriers as a transmission unit, A demodulation device for demodulating an orthogonal frequency division (OFDM) signal in which a pilot signal having a specific power greater than the maximum power of the transmission signal and having a specific phase is discretely inserted into each transmission symbol, Fourier transform means for Fourier transforming the signal in units of the transmission symbol to generate a quadrature amplitude modulation signal,
The pilot signal is extracted from the quadrature amplitude modulation signal to estimate the frequency characteristic of the transmission path on which the OFDM signal is transmitted, and the quadrature amplitude modulation signal is waveform-equalized in the transmission symbol unit based on the estimated frequency characteristic. Waveform equalizing means, and decoding means for decoding the waveform-equalized quadrature amplitude modulation signal, wherein the waveform equalization means converts the effective signal of the signals modulated into the quadrature amplitude modulation signal. The quadrature amplitude modulation signal is waveform-equalized so that the maximum power has a maximum value or a predetermined value near the maximum value.

In this demodulator, the maximum power of the effective signal among the signals modulated on the quadrature amplitude modulation signal is expressed by
The quadrature amplitude modulation signal is equalized in waveform so as to have a maximum value or a predetermined value near the maximum value.

Further, in the demodulation device according to the present invention, the waveform equalizing means clips the electric power whose value output by performing the waveform equalization is equal to or more than the maximum value to the maximum value. It is characterized by doing.

This demodulator clips the electric power whose value output by performing the above-mentioned waveform equalization is equal to or more than the above-mentioned maximum value to the above-mentioned maximum value.

[0017] In the demodulation method according to the present invention, a transmission symbol generated by subjecting a divided effective signal to quadrature amplitude modulation on a plurality of subcarriers is used as a transmission unit.
Orthogonal frequency division (OFDM) in which a pilot signal having a specific power greater than the maximum power of the effective signal and having a specific phase is discretely inserted into each transmission symbol.
A demodulation method for demodulating a signal, wherein the OFDM signal is Fourier-transformed in units of the transmission symbol to generate a quadrature amplitude modulation signal, and the OFDM signal is transmitted by extracting the pilot signal from the quadrature amplitude modulation signal. And estimating the maximum power of the effective signal among the signals modulated by the quadrature amplitude modulation signal to a maximum value or a predetermined value near the maximum value based on the estimated frequency characteristic. The waveform of the quadrature amplitude modulation signal is equalized in the unit of the transmission symbol, and the waveform-equalized quadrature amplitude modulation signal is decoded.

In this demodulation method, the maximum power of the effective signal among the signals modulated on the quadrature amplitude modulation signal is calculated as follows:
The quadrature amplitude modulation signal is equalized in waveform so as to have a maximum value or a predetermined value near the maximum value.

Further, the demodulation method according to the present invention is characterized in that the electric power whose value output by performing the waveform equalization is equal to or greater than the maximum value is clipped to the maximum value.

In this demodulation method, the electric power whose value output by performing the waveform equalization is equal to or greater than the maximum value is clipped to the maximum value.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, as an embodiment of the present invention, a receiving apparatus (OFDM receiving apparatus) for digital television broadcasting based on the OFDM system to which the present invention is applied will be described. FIG. 1 is a block diagram of an OFDM receiving apparatus to which the present invention is applied. In this OFDM receiver, the present invention is applied to the DVB-T standard (2K mode). The received OFDM signal has information of 1
It is assumed that the signal is modulated by the 6QAM method. Further, 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. ing.

As shown in FIG. 1, the OFDM receiver 1 includes an antenna 2, a tuner 3, 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. 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 a signal subjected to quadrature amplitude modulation by the 16QAM method. The OFDM frequency domain signal is output from the equalizer 8
Supplied to

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 W _sync indicating the boundary position is generated. F
The FT 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, in accordance with the 16QAM system. For example, as shown in FIG. 2, a decision threshold is set for each level of the I-channel signal and the Q-channel signal, and based on the decision threshold, data represented by 4 bits per one signal point is output. I do. The data decoded by the demapping circuit 9 is transmitted to the error correction circuit 1
0 is supplied.

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.

A signal broadcast from a broadcasting station is distorted by a transmission path. Assuming that a transmission signal broadcast from a broadcasting station is X (ω) and a frequency characteristic of a transmission path is H (ω), a reception signal received by the OFDM receiver is X (ω).
H (ω). In the OFDM receiver, an SP signal having a specific power level and a specific phase is extracted from each of the subcarriers after the FFT operation. as well as,
It detects how much the SP signal fluctuates from the original power level, and estimates the frequency characteristic H (ω) of the transmission path from the information. Then, the reciprocal 1 / H (ω) of the estimated frequency characteristic of the transmission path is calculated as the received signal X (ω) · H
(Ω). As a result, distortion due to the influence of the transmission path can be removed, and the originally transmitted signal can be restored.

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

The SP signal extraction circuit 11 is supplied with the OFDM frequency domain signal output from the FFT operation circuit 6. The SP signal extraction circuit 11 extracts only the SP signal from the OFDM frequency domain signal. The SP signal is
The symbols are discretely inserted into the symbols, and the insertion positions are determined in advance by standards. 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 filter 12.

The time direction filter 12 is an IIR (Infini
te Impulse Response) filter, and filters the SP signal in the time axis direction to remove noise included in the SP signal. The SP signal filtered by the time direction filter 12 is supplied to the frequency interpolation filter 13 in units of one OFDM symbol.

The frequency interpolation filter 13 has a function of FIR (Fini
te Impulse Response) filter, interpolates the SP signal in the subcarrier direction, and estimates the amplitude and phase frequency characteristics for all subcarriers of the OFDM symbol. That is, the frequency characteristic H (ω) of the transmission path is estimated. For example, in the DVB-T standard, one SP signal is supplied from the time direction filter 12 to three subcarriers. Accordingly, the frequency interpolation filter 13 obtains the characteristics of the frequency in which the SP signal is not inserted by using a triple interpolation filter or the like, and obtains, for example, 1705 subcarriers in which information of 2048 is modulated. To determine the transfer characteristics. The frequency characteristics H (ω) of the transmission path for all subcarriers obtained by the frequency 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.

Here, the complex multiplying circuit 15 outputs a complex signal represented by a predetermined bit amount to the demapping circuit 9. For example, the complex multiplication circuit 15 outputs a complex signal represented by 8 bits.

At this time, as shown in FIG. 3, the complex division circuit 15 sets the maximum level power P of the valid signal to be the maximum value of the output data, and sets the maximum level power of the valid signal. Based on P, the power of each subcarrier is boosted to perform amplitude equalization. Here, the valid signal is
A signal containing information, which is 16 data expressed by 16QAM. In addition, the maximum level power P refers to the maximum power on the I channel or the maximum power on the Q channel expressed in the 16QAM system. For example, if a complex signal represented by 8 bits is output, the maximum level power P of the effective signal is 255
Then, the power of each subcarrier is boosted so as to perform amplitude equalization. Further, for a signal whose output value overflows when the waveform is equalized by boosting in this manner, the value is clipped to the maximum value. That is, all signals having power equal to or higher than the maximum power of the valid signal are clipped to the same value (for example, 255) as the maximum value of the valid signal. Therefore, for example, since the SP signal is a signal having power larger than the maximum power of the effective signal, its value is clipped to the maximum value 255 of the output data.

As described above, the OF of the embodiment of the present invention
In the DM receiver 1, the equalizer 8 extracts the SP signal discretely inserted in the OFDM signal composed of a number of subcarriers. Subsequently, the equalizer 8 converts these discretely inserted SP signals into
The noise component contained in the SP signal is removed by filtering in the time axis direction. Further, the equalizer 8 filters the SP signal from which the noise component has been removed in the frequency direction, that is, a portion of the SP signal that does not contain the SP signal. The frequency characteristics of the transmission path of the subcarrier are complemented from the SP signal. Then, the OFDM receiver 1 performs waveform equalization by complexly multiplying the OFDM signal after the FFT operation by the reciprocal of the frequency characteristic of the transmission path obtained in this manner. When performing the waveform equalization, the OFDM receiver 1 boosts the power of each subcarrier and performs amplitude equalization so that the maximum level power of the valid signal becomes the maximum value of the output data.

As a result, in the OFDM receiver 1, the bit precision for expressing each data to be supplied to the demapping circuit 9 increases, so that the interval between the decision thresholds at the time of demapping increases, and the bit error rate characteristics Can be improved.

In the above description of the embodiment to which the present invention is applied, an OFDM receiver using a 16QAM system as a modulation system has been described as an example. However, the modulation system is not limited to such 16QAM, but may be 64QAM. , 1
Any scheme may be used as long as it is a quadrature amplitude modulation scheme such as 28QAM.

Further, the value of the maximum power P of the effective signal may be a value near the maximum value instead of the maximum value of the output data. For example, when the output data is represented by 8 bits, the value of the maximum value of the valid signal is set to, for example, 244, 243, instead of the maximum value 255 of the valid signal.
242... May be used.

[0047]

In the demodulation apparatus and the demodulation method according to the present invention, the maximum power of the effective signal among the signals modulated by the quadrature amplitude modulation signal is set to a maximum value or a predetermined value near the maximum value. The quadrature amplitude modulation signal is equalized in waveform.

As a result, in the present invention, the bit precision for expressing the output quadrature amplitude modulation signal is increased,
The accuracy of the determination at the time of decoding performed by demapping or the like is increased, and the bit error rate characteristics can be improved.

[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 illustrating a determination threshold when decoding a quadrature amplitude modulation signal according to the 16QAM method.

FIG. 3 is a diagram illustrating a boost amount of amplitude equalization by an equalizer of the OFDM receiver and clip processing.

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

FIG. 5 is a diagram illustrating a signal space diagram in which a quadrature amplitude modulation signal by the 16QAM method is distorted by distortion due to a transmission path.

FIG. 6 is a diagram illustrating an SP signal inserted into a quadrature amplitude modulation signal of the 16QAM system.

FIG. 7 is a diagram illustrating an expression range of output data when waveform equalization is performed.

FIG. 8 is a diagram illustrating a redundant portion in the expression of output data when performing waveform equalization.

[Explanation of symbols]

1 OFDM receiver, 5 digital quadrature demodulator, 6
FFT operation circuit, 7 window synchronization circuit, 8 equalizer, 9 demapping circuit

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Takahiro Okada 6-35, Kita-Shinagawa, Shinagawa-ku, Tokyo Sony Corporation F-term (reference) 5C025 AA15 AA18 5K004 AA08 JA03 JG01 JH02 5K022 DD01 DD13 DD18 DD19 DD33 DD34 DD42

Claims (4)

[Claims] 1. A transmission symbol generated by subjecting a divided effective signal to quadrature amplitude modulation on a plurality of subcarriers as a transmission unit, and having a specific power larger than the maximum power of the effective signal. And a demodulator for demodulating an orthogonal frequency division (OFDM) signal in which a pilot signal having a specific phase is discretely inserted into each transmission symbol, wherein the OFDM signal is Fourier-transformed on a per transmission symbol basis to perform quadrature amplitude modulation. Fourier transform means for generating a signal; extracting the pilot signal from the quadrature amplitude modulation signal; estimating the frequency characteristics of a transmission path on which the OFDM signal is transmitted; and estimating the quadrature amplitude modulation signal based on the estimated frequency characteristics. Waveform equalizing means for equalizing the waveform in transmission symbol units, and decoding the quadrature amplitude modulated signal having the waveform equalized. Decoding means, wherein the waveform equalizing means sets the quadrature amplitude so that the maximum power of the effective signal among the signals modulated to the quadrature amplitude modulation signal is a maximum value or a predetermined value near the maximum value. A demodulator for equalizing a waveform of a modulated signal. 2. The apparatus according to claim 1, wherein the waveform equalizing means clips the electric power whose value output by performing the waveform equalization is equal to or greater than the maximum value to the maximum value. 2. The demodulation device according to 1. 3. A transmission symbol generated by subjecting a divided effective signal to quadrature amplitude modulation on a plurality of subcarriers is used as a transmission unit, and has a specific power larger than the maximum power of the effective signal. And a demodulation method for demodulating an orthogonal frequency division (OFDM) signal in which a pilot signal having a specific phase is discretely inserted into each transmission symbol, wherein the OFDM signal is Fourier-transformed on a per transmission symbol basis to perform quadrature amplitude modulation. Generating a signal, extracting the pilot signal from the quadrature amplitude modulation signal, estimating a frequency characteristic of a transmission path through which the OFDM signal is transmitted, and modulating the frequency characteristic into the quadrature amplitude modulation signal based on the estimated frequency characteristic. The quadrature amplitude modulation signal is transmitted so that the maximum power of the effective signal among the signals being output is a maximum value or a predetermined value near the maximum value. A demodulation method comprising equalizing a waveform in units of transmission symbols and decoding the quadrature amplitude modulated signal having the equalized waveform. 4. The demodulation method according to claim 3, wherein the electric power whose value output by performing the waveform equalization is equal to or larger than the maximum value is clipped to the maximum value.