JP2002026860A - Demodulator and demodulation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a demodulator that surely calculates an accurate S/N and uses the S/N for reliability information used for soft decision Viterbi decoding. SOLUTION: An error correction circuit 10 used by an OFDM receiver corrects an error by soft decision Viterbi decoding. An S/N calculation circuit 11 calculates the S/N with respect to each subcarrier on the basis of a SP (scattered pilot) signal and provides the calculated S/N to the error correction circuit 10 as the reliability information.

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 and communication devices 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 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.

Next, a receiving apparatus (OFDM receiving apparatus) for digital television broadcasting based on the OFDM system will be described. FIG. 3 is a block diagram of a conventional OFDM receiver. Here, an OFDM receiver applied to the DVB-T standard (2K mode) will be described.
The information of the received OFDM signal is modulated by the 16QAM method. Further, in FIG. 3, 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. 3, a conventional OFDM receiving apparatus includes an antenna 102, a tuner 103, an A / D conversion circuit 104, a digital quadrature demodulation circuit 105, and an FF.
A T operation circuit 106, a window synchronization circuit 107, an equalizer 108, a demapping circuit 109, and an error correction circuit 10 are provided.

[0007] A broadcast wave of a digital television broadcast broadcast from a broadcast station is transmitted to an antenna 10 of the OFDM receiver 1.
2 and supplied to the tuner 103 as an RF signal.

The RF signal received by the antenna 102 is frequency-converted into an IF signal by a tuner 103,
The signal is supplied to the A / D conversion circuit 104. The IF signal is A /
It is digitized by the D conversion circuit 104 and supplied to the digital quadrature demodulation circuit 105. A / D conversion circuit 1
In the DVB-T standard (2K mode), reference numeral 04 quantizes an effective symbol of the OFDM time-domain signal with a clock that samples 2048 samples and a guard interval of 512 samples, for example.

The digital quadrature demodulation circuit 105 quadrature demodulates the digitized IF signal using a carrier signal of a predetermined frequency (carrier frequency), and generates a baseband signal.
Outputs FDM signal. This digital quadrature demodulation circuit 10
5, the baseband OFDM signal is
This is a so-called time-domain signal before the T 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 105 is output to the FFT operation circuit 106 and the window synchronization circuit 10.
7 is supplied.

[0010] The FFT operation circuit 106 performs an FFT operation on the OFDM time domain signal, and extracts and outputs data orthogonally modulated on each subcarrier. This FF
The signal output from the T operation circuit 106 is a so-called frequency domain signal after the FFT. From this,
Hereinafter, the signal after the FFT operation is referred to as an OFDM frequency domain signal.

The FFT operation circuit 106 has one OFDM
A signal in the effective symbol length range (for example, 2048 samples) is extracted from a symbol, that is, one OFDM
The FFT operation is performed on the extracted OFDM time domain signal of 2048 samples excluding the guard interval range from the symbol. Specifically, the calculation start position is O
Any position between the boundary of the FDM symbol and the end position of the guard interval. This calculation range is called an FFT window.

[0012] The OFDM frequency domain signal output from the FFT operation circuit 106 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 provided to equalizer 108.

The window synchronization circuit 107 delays the input OFDM time-domain signal by an effective symbol period, obtains a correlation between a guard interval portion and a signal from which the guard interval is copied, and the correlation is high. Calculating a boundary position of the OFDM symbol based on the portion;
A window synchronization signal W _sync indicating the boundary position is generated. The FFT window synchronization circuit 107 supplies the generated window synchronization signal _Wsync to the FFT operation circuit 106.

The equalizer 108 performs phase equalization and amplitude equalization of an OFDM frequency domain signal using a scattered pilot signal (SP signal). The OFDM frequency domain signal subjected to the phase equalization and the amplitude equalization is supplied to a demapping circuit 109.

The demapping circuit 109 includes an equalizer 1
08, the OFDM frequency domain signal subjected to the amplitude equalization and the phase equalization is subjected to demapping according to the 16QAM method to decode data. For example, as shown in FIG.
A decision threshold is set for each level of the I-channel signal and the Q-channel signal, and data expressed by 4 bits per one signal point is output based on the decision threshold. The data decoded by the demapping circuit 109 is supplied to an error correction circuit 110.

The error correction circuit 110 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.

Here, in the OFDM system, generally, at the time of transmission, data is encoded by a convolutional code, and at the time of reception, soft-decision Viterbi decoding using reliability information of a received signal is performed. By performing such soft-decision decoding, the error rate characteristics are improved as compared with normal hard-decision decoding that does not use reliability information.

The demapping circuit 109 decodes data by demapping an actual received signal into a decision area divided by a decision threshold. For example, as shown in FIG. 4, if the reception point of the actual reception signal modulated in 16QAM is at the position indicated by Z in the figure, the 4-bit “1011” indicated by the determination area that separates the position Output the value.

At this time, the demapping circuit 109
Outputs the decoded value, calculates the S / N ratio of the actual received signal, and outputs the calculated S / N ratio as reliability information.

The S / N ratio is, as shown in FIG. 5, the distance from the reception point Z of the actual reception signal (indicated by ● in the figure) to the signal point of the ideal reception signal (indicated by ○ in the figure). Is the noise component, the distance from the origin of the ideal received signal is the signal component, and the ratio of the noise component to the signal component is obtained. That is, the S / N ratio is determined by calculating the following equation (1), where S is the vector indicating the signal point of the ideal received signal and S 'is the vector of the signal point of the actual received signal. Can be S / N = | S | / | (S−S ′) | (1)

[0021]

However, the noise distribution of the actual received signal is not always distributed in the area divided by the decision threshold. For this reason, the reception point of the actual reception signal may exceed the determination region in which it should originally be located and enter, for example, an adjacent determination region. In this case, not only does the data obtained by demapping become erroneous, but also an error occurs in the calculated S / N ratio.

For example, as shown in FIG.
It is assumed that the actual received signal Z that must be located in the determination area of the signal point D is located in the determination area of the signal point D adjacent to the determination area of the signal point C. In this case, this actual signal point Z
Is not calculated based on the original noise component N (ie, the distance between the signal point C and the signal point Z), but the signal point D
Is calculated based on the erroneous noise component N 'between

As described above, in the conventional OFDM receiver,
Since the S / N ratio of each data is calculated at the time of demapping, an accurate S / N ratio cannot be calculated accurately, so that erroneous reliability information is output to the subsequent soft decision decoding means. Had been lost.

The present invention has been made in view of such a situation, and an object of the present invention is to provide an OFDM signal demodulation apparatus and a demodulation method capable of reliably obtaining an accurate S / N ratio. is there.

[0025]

In order to solve the above-mentioned problems, a demodulation apparatus according to the present invention provides a transmission symbol generated by dividing an effective signal into a plurality of subcarriers and orthogonally modulating the divided signals. Is a transmission unit, and a pilot signal having a specific power and a specific phase is discretely inserted into each transmission symbol.
DM) for demodulating a signal, wherein the OFDM signal is demodulated.
Fourier transform means for performing a Fourier transform on the signal in units of the transmission symbol to generate a frequency domain signal, pilot signal extracting means for extracting the pilot signal from the frequency domain signal, and extracting a noise component of the extracted pilot signal A noise detecting unit; and an S / N ratio calculating unit that calculates an S / N ratio at a subcarrier position where the pilot signal is inserted based on the noise component.

In this demodulator, the S / N ratio at each subcarrier position is calculated based on the extracted pilot signal.

[0027] In the demodulation method according to the present invention, a transmission symbol generated by dividing an effective signal into a plurality of subcarriers and orthogonally modulating the divided signal is used as a transmission unit. And a demodulation method for demodulating an orthogonal frequency division (OFDM) signal in which a pilot signal having a phase of (i) is discretely inserted into each transmission symbol. Is generated, the pilot signal is extracted from the frequency domain signal, a noise component of the extracted pilot signal is extracted, and an S / N ratio at a subcarrier position where the pilot signal is inserted is calculated based on the noise component. It is characterized by doing.

In this demodulation method, the S / N ratio at each subcarrier position is calculated based on the extracted pilot signal.

[0029]

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.
Antenna 2, tuner 3, A / D conversion circuit 4, digital quadrature demodulation circuit 5, FFT operation circuit 6, window synchronization circuit 7, equalizer 8, demapping circuit 9, error correction circuit 10, , S / N ratio calculation circuit 11.

A broadcast wave of a digital television broadcast broadcast from a broadcasting 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 but 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, 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 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 performs phase equalization and amplitude equalization of the OFDM frequency domain signal using the scattered pilot signal (SP 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 performing demapping on the OFDM frequency domain signal, which has been subjected to amplitude equalization and phase equalization by the equalizer 8, in accordance with the 16QAM system. A decision threshold is set for each level of the I-channel signal and the Q-channel signal, and data expressed by 4 bits per one signal point is output based on the decision threshold. 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.
The error correction circuit 10 includes an S / N calculation circuit 11
Performs soft-decision Viterbi decoding using the reliability information calculated by.

The S / N ratio calculation circuit 11 has an SP signal extraction circuit 12, an S / N ratio calculation circuit 13, a time direction filter 14, and a frequency interpolation filter 15.

The SP signal extraction circuit 12 is supplied with the OFDM frequency domain signal output from the FFT operation circuit 6. The SP signal extraction circuit 12 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.

For example, the DVB-T standard proposes to insert an SP signal into a subcarrier having an index number k as shown in the following equation.

[0044]

(Equation 1)

Here, k indicates the index number of the subcarrier into which the SP signal is inserted. l indicates the symbol number of the OFDM symbol. Kmin indicates the minimum index number of the effective subcarrier in the OFDM symbol, and Kmax indicates the maximum index number of the effective subcarrier in the OFDM symbol. Also, p is
Indicates an integer of 0 or more. Note that the index number k is K
It is assumed to take a value in the range of min to Kmax.

That is, in this equation, one SP signal is inserted into 12 subcarriers, and
This means that the insertion position of the SP signal shifts by three subcarriers for each symbol.

Since the SP signal is inserted at a different subcarrier position for each symbol, the SP signal extraction circuit 12 refers to the symbol number of the supplied OFDM frequency domain signal, and determines which index number from the symbol number. Whether the SP signal is inserted into the subcarrier is calculated based on the standard, and the SP signal is extracted. The SP signal extraction circuit 12 supplies the extracted SP signal to the S / N ratio calculation circuit 13. The SP signal is subjected to M-sequence random encoding, and its phase is set to 0 ° or 180 °. S
The P signal extraction circuit 12 extracts an original signal component by adding an M-sequence random code to the SP signal.

The S / N ratio calculation circuit 13 calculates the S / N ratio of the extracted SP signal. The ideal signal point of the SP signal is predetermined in the standard. For example, in the case of 16QAM modulation of the DVB-T standard (2k mode), the phase is 0.
° or 180 °, and the amplitude is 4/3 times the average power of the effective signal (α = 1).

As shown in FIG. 2, the S / N ratio calculation circuit 13 sets a vector indicating an ideal SP signal as S, a vector indicating an actually received SP signal as S ', and Assuming that the noise component of the SP signal is N, an operation as shown in the following equation is performed to obtain the S / N ratio.

[0050]

(Equation 2)

Here, assuming that the level of the ideal SP signal is 1, the S / N ratio is calculated as follows.

[0052]

(Equation 3)

The S / N ratio calculation circuit 13 calculates the S / N ratio by performing the above calculation on the SP signal. The S / N ratio component of the actually received SP signal obtained by the S / N ratio calculation circuit 13 in this manner is supplied to the time direction filter 14.

The time direction filter 14 is, for example, an IIR
(Infinite Impulse Response) filter, which filters the S / N ratio component of the SP signal in the time axis direction and removes the noise in the time direction included in the obtained S / N ratio component. The S / N component filtered by the time direction filter 14 is supplied to the frequency interpolation filter 15 in units of 1 OFDM symbol.

The frequency interpolation filter 15 is, for example, an FIR
(Finite Impulse Response) filter,
The SN ratio component is interpolated in the subcarrier direction, and the S / N ratio at all subcarrier positions of the OFDM symbol is estimated. For example, in the DVB-T standard, one SP signal is supplied to three subcarriers. Accordingly, the frequency interpolation filter 15 obtains the characteristics of the frequency in which the SP signal is not inserted by interpolating using a triple interpolation filter or the like. Find the S / N ratio in the subcarrier. The S / N ratio for all subcarriers obtained by the frequency interpolation filter 15 is supplied to the error correction circuit 10 as reliability information.

As described above, the OFD according to the embodiment of the present invention
In the M receiving apparatus 1, the S / N ratio of the SP signal is calculated, and the S / N ratio for each subcarrier is calculated based on the S / N ratio of the SP signal.
/ N ratio is estimated and obtained. The S / N ratio of the SP signal is
It can be calculated no matter where the actual SP signal reception point is located. Therefore, for example, it is possible to reliably calculate an accurate S / N ratio even when the noise distribution range is large. it can.

In the 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 QPSK. , 64
Other modulation schemes such as QAM and 128QAM may be used.

In the OFDM receiver 1, the SP
Although the S / N ratio for each subcarrier is calculated by calculating the S / N ratio of the signal, a continuous pilot signal CP for synchronizing the carrier frequency may be used together with the SP signal.

The OFDM receiver 1 may not only supply the calculated S / N ratio as reliability information to the error correction circuit 10 but also output it externally. For example, the S / N ratio may be displayed by outputting to an external monitor indicating the reception sensitivity of the OFDM receiver 1 or a monitor indicating the reception sensitivity of the antenna.

The equalizer 8 of the OFDM receiver 1 extracts the SP signal and estimates the transmission characteristic of the transmission path based on the amount of phase change and the amount of amplitude change of the SP signal.
Waveform equalization for each subcarrier is performed based on the estimated transfer characteristics. The equalizer 8 extracts an SP signal, an IIR filter that filters the SP signal in the time direction to remove noise, and filters the SP signal in the frequency direction to interpolate the transfer characteristics for each subcarrier. FIR filter and FFT
In general, it comprises a complex division circuit that performs complex division on the transfer characteristic of each subcarrier with respect to the OFDM frequency domain signal obtained by the operation. Therefore, the SP signal extraction circuit, the time direction filter, and the frequency direction filter of the S / N ratio calculation circuit 11 may be used in common with the circuit provided in the equalizer.

[0061]

According to the demodulation device and the demodulation method of the present invention, the S / N ratio at each subcarrier position is calculated based on the extracted pilot signal.

As a result, in the present invention, an accurate S / N ratio can be reliably calculated for each subcarrier. Further, when the soft decision decoding means is provided at the subsequent stage, it is possible to output accurate reliability information to the soft decision decoding means.

[Brief description of the drawings]

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

FIG. 2 shows the S / S of the received scattered pilot signal.
It is a figure explaining an N ratio.

FIG. 3 is a block diagram of a conventional OFDM receiver.

FIG. 4 is a diagram illustrating a process of demapping and decoding a 16QAM modulated signal.

FIG. 5 is a diagram illustrating a process of calculating an S / N ratio by a demapping circuit of the conventional OFDM receiver.

FIG. 6 is a diagram illustrating a case where the S / N ratio cannot be accurately calculated by the demapping circuit of the conventional OFDM receiver.

[Explanation of symbols]

1 OFDM receiver, 2 antennas, 3 tuners,
4 A / D conversion circuit, 5 digital quadrature demodulation circuit, 6
FFT operation circuit, 7 window control circuit, 8 equalizer, 9 demapping circuit, 10 error correction circuit, 1
1 S / N ratio calculation circuit

Claims (8)

[Claims] A transmission signal generated by dividing an effective signal into a plurality of subcarriers and performing orthogonal modulation is used as a transmission unit, and a pilot signal having a specific power and a specific phase is generated. A demodulator for demodulating an orthogonal frequency division (OFDM) signal discretely inserted into each transmission symbol, comprising: a Fourier transform unit for performing a Fourier transform on the OFDM signal in transmission symbol units to generate a frequency domain signal; Pilot signal extracting means for extracting the pilot signal from the area signal; noise detecting means for extracting a noise component of the extracted pilot signal; S / N at a subcarrier position where the pilot signal is inserted based on the noise component S / N to calculate ratio
A demodulation device comprising a ratio calculation unit. 2. The S / N ratio calculating means calculates an S / N ratio at all subcarrier positions including a valid signal from an S / N ratio at a subcarrier position where a pilot signal is inserted.
2. The demodulation device according to claim 1, wherein the / N ratio is estimated. 3. A decoding device for decoding transmission data from a frequency domain signal output from the Fourier transform device, and an S / N ratio estimated by the S / N ratio calculation device as reliability information, 3. The demodulator according to claim 2, further comprising: soft decision decoding means for performing soft decision decoding of the transmission data decoded by the decoding means. 4. The S / N ratio calculation means, wherein the calculated S / N ratio
The demodulator according to claim 1, wherein the N ratio is filtered in a time direction. 5. A transmission symbol generated by dividing an effective signal into a plurality of subcarriers and performing quadrature modulation is used as a transmission unit, and a pilot signal having a specific power and a specific phase is generated. In a demodulation method for demodulating an orthogonal frequency division (OFDM) signal discretely inserted into each transmission symbol, a Fourier transform of the OFDM signal is performed in units of the transmission symbol to generate a frequency domain signal, and the frequency domain signal is generated from the frequency domain signal. A demodulation method comprising: extracting a pilot signal; extracting a noise component of the extracted pilot signal; and calculating an S / N ratio at a subcarrier position where the pilot signal is inserted based on the noise component. 6. The demodulation method according to claim 5, wherein the S / N ratio at all the subcarrier positions including the effective signal is estimated from the S / N ratio at the subcarrier position where the pilot signal is inserted. . 7. Soft decision of the decoded transmission data by decoding transmission data from the frequency domain signal and using S / N ratios at all subcarrier positions including the estimated valid signal as reliability information. 7. The demodulation method according to claim 6, wherein decoding is performed. 8. The demodulation method according to claim 5, wherein the calculated S / N ratio is filtered in a time direction.