JP2001292122A - Demodulation device and demodulation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To highly precisely estimate the transmission characteristic of a transmission line and to equalize amplitude and a phase from the transmission characteristic, at the time of demodulating an OFDM signal. SOLUTION: An OFDM receiver 1 is provided with an equalizer 10 for estimating the transmission characteristic of the transmission line through which the OFDM signal is transmitted and equalizing the amplitude and the phase of the OFDM signal. The equalizer 10 equalizes the amplitude and the wave of the OFDM signal by using an SP signal and a CP 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] A transmission signal according to the OFDM scheme is transmitted in a symbol unit called an OFDM symbol, as shown in FIG. This OFDM symbol is used as an IFF
An effective symbol, which is a signal period in which T is performed, and a guard interval in which a waveform of a part of the latter half of the effective symbol is copied as it is. This guard interval is provided in the first half of the OFDM symbol. For example, in the DVB-T standard (2K mode), 2048 subcarriers are included in an 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 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 the influence of multipath or the like during transmission, the characteristics of amplitude and phase for each subcarrier will be different. Therefore, on the receiving side, it is necessary to equalize (correct) the waveform of the received signal so that the amplitude and phase of each subcarrier become equal. In the OFDM system, a pilot signal having a predetermined amplitude and a predetermined phase is scattered in a transmission symbol in a transmission signal on a transmission side, and the amplitude and phase of the pilot signal are monitored on a reception side.
The characteristics of the transmission path are estimated, and the received signal is equalized based on the estimated characteristics of the transmission path. The pilot signal used for estimating the characteristics of the transmission path is called a scattered pilot signal (SP) signal.

In the DVB-T standard, it has been proposed to insert an SP signal into a subcarrier having an index number k as shown in the following equation.

[0008]

(Equation 1)

Here, k indicates an index number of a 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, as shown in FIG. 4, one SP signal is inserted into 12 subcarriers at a rate of 3 subcarriers for each OFDM symbol. Means to shift by one.

[0011] By shifting the insertion position of the SP signal for each OFDM symbol, the redundancy of the SP signal with respect to the original data is reduced.

The SP signal inserted into the subcarrier having such an index number is, for example, BPSK-modulated data in the DVB-T standard.

In the OFDM system, a baseband signal is generated by multiplying an IF signal by a SIN signal and a COS signal having a predetermined carrier frequency fc and performing quadrature demodulation. At this time, if an error occurs in the carrier frequency fc, the frequency position of each subcarrier is shifted as a whole from the original frequency position, and accurate demodulation cannot be performed. for that reason,
On the receiving side, the shift amount of the carrier frequency is accurately detected, and the carrier frequency is constantly adjusted while synchronizing so that the shift amount becomes zero.

In order to synchronize such a carrier frequency, a transmission signal of the OFDM system contains a pilot signal called a continuous pilot signal (CP signal). This CP signal is a signal that always represents a specific phase and amplitude, and is inserted into a plurality of subcarriers in an effective symbol, and the number inserted in the effective symbol and the arrangement pattern thereof are predetermined. . This CP
Unlike the SP signal described above, the signal is always inserted into the subcarrier having the same index number in each OFDM symbol. For example, in the case of the DVB-T standard (2K mode), 2048 subcarriers (0 to 2047) exist in one effective symbol, of which 45 subcarriers include a CP signal. Also, this DV
In the BT standard (2K mode), the allocation pattern of the CP signal is 0,
48, 54, 87, 141, 156, 192, 201,
255, 279, 282, 333, 432, 450, 4
83, 525, 531, 618, 636, 714, 75
9, 765, 780, 804, 873, 888, 91
8,939,942,969,984,1050,11
01, 1107, 1110, 1137, 1140, 11
46, 1206, 1269, 1323, 1377, 14
91, 1683, and 1704. Note that this C
The arrangement pattern of the P signal indicates the range of the index of the subcarrier in the effective symbol, that is, the range of Kmin to Kmax.

The receiving side detects the CP signal, detects how much the CP signal has shifted from the original subcarrier position, and feeds back the shift amount to adjust the carrier frequency error. I have.

The CP signal inserted into the subcarrier having such an index number is, for example, BPSK-modulated data in the DVB-T standard.

FIG. 5 shows a configuration example of a conventional signal receiving apparatus of the DVB-T system for estimating transmission path characteristics from such an SP signal, equalizing (correcting) a received signal and synchronizing a carrier frequency. ing.

As shown in FIG. 5, a conventional OFDM receiving apparatus 101 includes an antenna 102, a tuner 103,
/ D conversion circuit 104 and digital quadrature demodulation circuit 105
, An FFT operation circuit 106, a narrow-band fc error calculation circuit (FAFC) 107, and a wide-band fc error calculation circuit (WA
FC) 108 and numerical control oscillator (NCO)
109, an equalizer 110, and a demapping circuit 11
1 is provided.

A broadcast wave of a digital television broadcast broadcast from a broadcast station is received by an antenna 102 of an OFDM receiving apparatus 101, and is transmitted as a RF signal to a tuner 103.
Supplied to

The RF signal received by the antenna 102 is frequency-converted by a tuner 103 into an IF signal.
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), for example, the effective symbol of this OFDM time domain signal is 20
Forty-eight samples, the guard interval is quantized by a clock sampled at, for example, 512 samples.

The digital quadrature demodulation circuit 105 quadrature demodulates the digitized IF signal using a carrier signal of a predetermined frequency (carrier frequency), and outputs 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 supplied to an FFT operation circuit 106 and a narrow band fc error calculation circuit 107.

The FFT operation circuit 106 performs an FFT operation on the OFDM time domain signal, and extracts and outputs data that is 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 (position A in FIG. 3) and the end position of the guard interval (position B in FIG. 3). This calculation range is called an FFT window.

As described above, 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. The OFDM frequency domain signal is supplied to the wideband fc error calculation circuit 108 and the equalizer 110.

The narrow-band fc error calculation circuit 107 calculates the OFD
A carrier frequency error included in the M time domain signal is calculated. Specifically, the narrow-band fc error calculation circuit 107 calculates a narrow-band carrier frequency error with an accuracy of ± 1/2 or less of the subcarrier frequency interval (4.14 Hz). The carrier frequency error is an error in the center frequency position of the OFDM time domain signal caused by a shift in the reference frequency output from the local oscillator of the tuner 103, and the error rate of the output data increases as the error increases. The narrow band carrier frequency error obtained by the narrow band fc error calculation circuit 107 is supplied to the NCO 109.

The wideband fc error calculation circuit 108
A carrier frequency error included in the M time domain signal is calculated. Specifically, the wideband fc error calculation circuit 108 calculates a wideband carrier frequency error with subcarrier frequency (for example, 4.14 Hz) interval accuracy. The wideband fc error calculation circuit 108 calculates how much the CP signal is shifted from the original CP signal insertion position with reference to the continuous pilot signal (CP signal), and calculates the shift amount. I'm asking. The wideband carrier frequency error obtained by the wideband fc error calculation circuit 108 is supplied to the NCO 109.

The NCO 109 has a subcarrier frequency interval of ± 1 calculated by the narrow band fc error calculation circuit 107.
/ 2 precision narrowband carrier frequency error and the wideband carrier frequency error of subcarrier frequency interval precision calculated by the wideband fc error calculation circuit 108, and the frequency is determined according to the carrier frequency error obtained by the addition. An increasing / decreasing carrier frequency error correction signal is output. The carrier frequency error correction signal is a complex signal and is supplied to the digital quadrature demodulation circuit 105. This carrier frequency error correction signal performs digital orthogonal demodulation while correcting the carrier frequency fc based on the carrier frequency error correction signal.

The equalizer 110 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 111.

The demapping circuit 111 includes an equalizer 1
The OFDM frequency domain signal, the amplitude and phase of which have been equalized by 10, is subjected to demapping according to the modulation method to decode data.

Then, the decoded data decoded by the demapping circuit 111 is supplied to an error correction circuit and the like at the subsequent stage.

Next, the wideband fc error calculation circuit 108 will be described in more detail.

The wideband fc error calculation circuit 108 performs the FFT
A CP signal is extracted by performing differential demodulation twice between symbols that are temporally adjacent to each other on the OFDM frequency domain signal after the calculation, and the subcarrier position of the extracted CP signal is the original subcarrier position. The carrier frequency error of the OFDM signal is calculated by calculating the degree of shift from the carrier frequency error.

Referring to FIG. 6, the principle of extracting a CP signal by performing differential demodulation between two symbols for an OFDM frequency domain signal will be described.

FIG. 6 is a diagram for explaining the phase change between normal information data and the CP signal for explaining the differential demodulation between symbols in the first stage and the differential demodulation between symbols in the second stage. It is. In FIG. 6, it is assumed that the information data is QPSK (Quadrature Phase Shift Keying) modulated, and that the CP signal is modulated with information of a signal point having a specific amplitude and phase. .

FIG. 6A shows an I channel signal and a Q signal decomposed for each frequency component of each subcarrier by FFT.
The channel signal is shown on a phase plane for each symbol (the (n−1) th symbol, the nth symbol, the (n + 1) th symbol). an and bn are the n-th OF
The index data of the subcarriers after the FFT of the DM symbol are information data of a and b, respectively.
Also, can and cbn are F of the n-th OFDM symbol.
Index numbers of subcarriers after FT are ca and cb
Are respectively shown. Note that the CP signal originally has constant amplitude and phase information,
Some phase rotation may occur for each symbol due to the influence of a reproduced carrier frequency error or the like.

FIG. 6B shows one-time differential demodulated data on the phase plane when the information of the same index number is subjected to one-time differential demodulation between symbols. Dan and dbn are one-time differential demodulated data of the (n-1) th symbol and the nth symbol whose subcarrier index numbers are a and b, respectively. Also, dcan and dcbn are one-time differential demodulated data of the (n-1) th symbol and the nth symbol whose subcarrier index numbers are ca and cb, respectively.

FIG. 6C shows, on a phase plane, two-time differential demodulated data when information of the same index number is twice differentially demodulated between symbols. dda and ddb are obtained by differentially demodulating the (n-1) th symbol and the nth symbol whose subcarrier index numbers are a and b, respectively, and the nth symbol and the (n + 1) th symbol. And differentially demodulated data of the symbol No. 2 and differential demodulated data twice. Also, ddca and ddbc are obtained by differentially demodulating the (n-1) th symbol and the nth symbol with subcarrier index numbers ca and cb, respectively, and the nth symbol and the (n + 1) th symbol. This is twice differential demodulated data obtained as a result of further differential demodulation of the data obtained by differential demodulation of the symbol.

Since the CP signals ca and cb have a constant phase, the FFT window phase error and the carrier phase error are excluded in the first differential demodulation, and the carrier frequency error, the CPE, and the reproduced clock frequency error are eliminated. Will remain. Since any phase error remaining after the first differential demodulation does not depend on time, it remains constant between the data after differential demodulation. Therefore, by performing the second differential demodulation between the data subjected to the first differential demodulation, the phase error depending on the CPE and the reproduction clock frequency remaining in the first differential demodulation can be removed. Can be. As a result, the CP signal converges to a positive value on the I axis (see FIG. 6C).

On the other hand, since the information data a and b take a random phase between symbols, the phase becomes random for each data even after performing two differential demodulations. Are randomly distributed on the I axis.

Therefore, for example, C within one symbol
If only the I-axis data of the P signal is accumulated and added, this CP
Since the signal has converged to a certain value on the I-axis, the value of the information data is much larger than the result of the cumulative addition of the extracted I-axis data. Therefore, the subcarrier position of the CP signal can be estimated from the maximum value of the cumulative addition. Then, by calculating how much the estimated subcarrier position of the CP signal is shifted from the original subcarrier arrangement position, the carrier frequency error can be calculated by the subcarrier interval system.

Next, the equalizer 110 will be described in more detail.

As shown in FIG. 5, the equalizer 110
SP signal extraction circuit 121, hold circuit 122, reference SP signal generation circuit 123, SP signal complex division circuit 1
24, an interpolation filter 125, and a complex division circuit 126.

The OFDM frequency domain signal output from the FFT operation circuit 106 is supplied to the SP signal extraction circuit 121. The SP signal extraction circuit 121 extracts only the SP signal from the OFDM frequency domain signal. The insertion position of the SP signal is determined in advance by the standard as described above.
Since the SP signal is inserted at a different subcarrier position for each symbol, the SP signal extraction circuit 121 refers to the symbol number of the supplied OFDM frequency domain signal, and determines the index number of the subcarrier from the symbol number. Whether the SP signal is inserted is calculated based on the standard, and the SP signal is extracted. SP signal extraction circuit 121
Supplies the extracted SP signal to the hold circuit 122.

The hold circuit 122 includes a predetermined number of OFDMs.
It collects SP signals scattered in symbols, holds them, and outputs them as a group of SP signals at fixed subcarrier intervals.

In the OFDM system, although the arrangement position of the SP signal is determined to be different for each OFDM symbol, an OFDM symbol in which the arrangement position of the SP signal is the same appears repeatedly for a predetermined number of OFDM symbols. It is stipulated as follows. The hold circuit 122 collects the SP signals inserted into the predetermined number of OFDM symbols, and holds all the collected SP signals. Then, the hold circuit 122 outputs the held SP signals to the SP signal complex division circuit 24 collectively. Subsequently, the next new one OFDM symbol is input and the new one
When the SP signal is extracted from one OFDM symbol, the hold circuit 122 deletes only the SP signal at the same subcarrier position as the extracted SP signal from the plurality of held SP signals, and A new SP signal is held at the deleted position. That is, the hold circuit 122 updates and holds the newly extracted SP signal. Then, the hold circuit 122 outputs the held SP signals to the SP signal complex division circuit 24 collectively. That is, the hold circuit 122 has a new OFD
Every time an SP signal is extracted from M symbols, a predetermined number of SP signals are collected for a predetermined number of OFDM symbols while the positions of a plurality of OFDM symbols to be collected are slid one by one in time series, and are output collectively. Is working to be.

In the case of the DVB-T standard, FIG.
As shown in (A), one SP signal is inserted into twelve subcarriers, and the insertion position of the SP signal is shifted by three subcarriers for each OFDM symbol. That is, with four symbols as one cycle, OFDM symbols in which the arrangement positions of the SP signals are the same appear. Therefore, by collecting four OFDM symbols, one SP signal is detected for every three subcarriers.

The specific processing procedure of the hold circuit 122 in the case of the DVB-T standard will be described. First, as shown in FIG.
The SP signal of the FDM symbol is held, and an SP signal group existing at a ratio of one to three subcarriers is output. For example, SP from OFDMsymbol # 0 to OFDMsymbol # 3
The signals are held, and the SP signals inserted at a ratio of one to three subcarriers are output at once. That is, FIG.
As shown by X in (B), the index numbers are numbers # 0, #
The SP signals inserted into the subcarrier positions of # 3, # 6, # 9, # 12, # 15, # 18, # 21,.

Subsequently, when the SP signal extracted from the next OFDM symbol (SP signal inserted into 12 subcarriers at a rate of one) is supplied from the SP signal extraction circuit 121, the hold circuit 122 Updates only the SP signal inserted into the subcarrier of the index number where the supplied SP signal exists, as shown in FIG. 7 (B), and updates one of the three subcarriers together with the other SP signals. Is output. Specifically, as shown in Y of FIG. 7B, OFDM symbol # 4 OF
When a DM symbol is input, index numbers # 0 and #
Update the SP signals inserted into the subcarriers 12, 12,..., And output the SP signals inserted into the subcarriers with other index numbers while holding the value of the previous symbol as it is.

Subsequently, the SP signal extracted from the next OFDM symbol (SP signal inserted into 12 subcarriers at a rate of one) is output to the SP signal extraction circuit 12.
When supplied from 1, only the SP signal inserted into the subcarrier of the index number in which the supplied SP signal exists is updated, and one SP signal is present in three subcarriers together with other SP signals. Outputs SP signal. Specifically, as shown in Z of FIG. 7B, when OFDM symbol # 5 is input, the SP signal inserted into the subcarriers with index numbers # 3, # 15, # 27,. The SP signal that has been updated and inserted into subcarriers of other index numbers holds the value of the previous symbol as it is, and outputs these at once.

The hold circuit 122 always repeats such processing for each OFDM symbol period,
An SP signal (SP signal for every three subcarrier intervals) collected from the FDM symbol is output.

The hold circuit 122 stores the collected SP
The signal is supplied to the SP signal complex division circuit 124.

The reference SP signal generating circuit 123 generates an SP signal (reference SP signal) having the original amplitude and phase when inserted into the transmission signal on the transmission side. For example, DVB
In the -T standard, since the SP signal is BPSK-modulated data, it generates data having a phase of 0 or π and an amplitude of a predetermined amplitude (for example, 4/3). If it is specified by the standard that SP signals having different amplitudes or phases are inserted on the transmitting side for each index number of the inserted subcarrier, the reference SP signal generation circuit 123
Generates an SP signal having an amplitude and a phase according to the standard for each index number of the subcarrier. The reference SP signal generation circuit 122 supplies the generated reference SP signal to the SP complex division circuit 124.

The SP signal complex division circuit 124 performs complex division on the SP signal supplied from the hold circuit 124 by the reference SP signal generated from the reference SP signal generation circuit 123, and calculates the amount of distortion of the received SP signal. . here,
The obtained distortion amount is a distortion amount for the subcarrier at the position where the SP signal is inserted. For example, DVB-
In the T standard, it is the amount of distortion for every three subcarrier intervals. The calculated distortion amount of the SP signal is supplied to the interpolation filter 125.

The interpolation filter 125 interpolates the amount of distortion of the SP signal in the subcarrier direction, and obtains the amount of distortion for all subcarriers of the OFDM symbol. That is, the transmission characteristics of the transmission path for all subcarriers are obtained. For example, in the DVB-T standard, a distortion amount is supplied from the SP signal complex division circuit 124 to three subcarriers at a rate of one. Therefore, interpolation filter 1
Reference numeral 25 denotes a subcarrier at a position where no SP signal is inserted (for example, index numbers # 1, # 2, # 4, # 5, # 7, # 8,...) Using a triple interpolation filter or the like. The transmission characteristics of all 2048 subcarriers are obtained, for example, by interpolating the transmission characteristics of subcarriers such as. The transfer characteristic of the transmission path for all subcarriers obtained by the interpolation filter 125 is calculated by the complex division circuit 126
Supplied to

The complex division circuit 126 is an FFT operation circuit 1
06 for the OFDM frequency domain signal supplied from
The transfer characteristic obtained by the interpolation filter 125 is complex-divided, and the OFDM frequency domain signal is equalized in amplitude and in phase. This complex division circuit 126 converts the amplitude- and phase-equalized OFDM frequency domain signal into a mapping circuit 1
11.

As described above, in the equalizer 110, the SP
By estimating the transfer characteristics of the transmission path using the signal and performing waveform equalization, the amplitude and phase of each subcarrier become equal, and the effects of multipath and the like during transmission can be removed.

[0057]

By the way, the conventional OF
In the DM receiver, the transmission characteristics of the transmission path are estimated using the SP signal as described above, and amplitude equalization and phase equalization are performed based on the estimated transmission characteristics. There is a need to estimate the transfer characteristics of the road.

The present invention has been made in view of such a situation, and an OFDM signal demodulating apparatus for estimating transmission characteristics of a transmission path with high accuracy and performing amplitude equalization and phase equalization based on the transmission characteristics is provided. It is an object to provide a demodulation method.

[0059]

According to the present invention, there is provided a demodulation apparatus comprising: an effective symbol generated by dividing and modulating information into frequency components within a predetermined band; and a part of the effective symbol. A scattered pilot in which a transmission symbol including a guard interval generated by copying a signal waveform is used as a transmission unit, and a predetermined amplitude and phase are inserted into frequency components at different positions for each transmission symbol. A signal and a continuous pilot signal inserted into a frequency component at the same position for each transmission symbol and having a predetermined amplitude and phase are demodulated in an orthogonal frequency division multiplexing (OFDM) signal included in the effective symbol. A demodulation device, a Fourier transform unit for performing a Fourier transform on the OFDM signal in units of the effective symbol to demodulate the signal, The scattered pilot signal and the continuous pilot signal are extracted from the Fourier-transformed and demodulated signal, and the extracted scattered pilot signal and the continuous pilot signal are collected for a predetermined transmission symbol and collected. An equalizing means for equalizing the amplitude and phase of all frequency components of the Fourier-transformed signal based on the scattered pilot signal and the continuous pilot signal.

In this demodulator, the amplitude and the waveform of all the frequency components of the OFDM signal after the Fourier transform using the continuous pilot signal together with the scattered pilot signal are equalized.

In the demodulation method according to the present invention, an effective symbol generated by dividing and modulating information into frequency components within a predetermined band and a signal waveform of a part of the effective symbol are copied. A scattered pilot signal having a predetermined amplitude and phase inserted into a frequency component at a different position for each transmission symbol with a transmission symbol including a guard interval generated by the transmission unit as a transmission unit, and each transmission symbol A demodulation method for demodulating an orthogonal frequency division multiplexing (OFDM) signal in which a continuous pilot signal having a predetermined amplitude and phase inserted into a frequency component at the same position every time is included in the effective symbol. Fourier-transform the signal in units of the effective symbol to demodulate the signal. Extracting the scattered pilot signal and the continuous Al pilot signal,
The extracted scattered pilot signal and the continuous pilot signal are collected for a predetermined transmission symbol, and the amplitudes of all frequency components of the Fourier-transformed signal based on the collected scattered pilot signal and the continuous pilot signal are calculated. It is characterized in that the phases are equalized.

In this demodulation method, the amplitude and the waveform of all the frequency components of the OFDM signal after the Fourier transform using the continuous pilot signal together with the scattered pilot signal are equalized.

[0063]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, as an embodiment of the present invention, a digital broadcast receiving apparatus (OFDM receiving apparatus) based on the OFDM system to which the present invention is applied will be described.

As shown in FIG. 1, the OFDM receiver 1 includes an antenna 2, a tuner 3, and an A / D conversion circuit 4.
Digital quadrature demodulation circuit 5, FFT operation circuit 6,
Narrow band fc error calculation circuit (FAFC) 7 and wide band fc
An error calculation circuit (WAFC) 8, a numerical control oscillation circuit (NCO) 9, an equalizer 10, and a demapping circuit 11 are provided.

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. Become

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 narrow band fc error calculation 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 a range of an effective symbol length (for example, 2048 samples) from one OFDM symbol, that is, removes a range of 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 having a real axis component (I channel signal) and an imaginary axis component (Q channel signal), like the OFDM time domain signal. Signal.
The OFDM frequency domain signal is converted to a wideband fc error calculation circuit 8
And supplied to the equalizer 10.

The narrow-band fc error calculation circuit 7 calculates a carrier frequency error included in the OFDM time domain signal.
Specifically, the narrow-band fc error calculation circuit 7 calculates a narrow-band carrier frequency error with an accuracy of ± 1/2 or less of the subcarrier frequency interval (4.14 Hz). The carrier frequency error is an error in the center frequency position of the OFDM time domain signal caused by a shift of the reference frequency output from the local oscillator of the tuner 3, and the error rate of the output data increases as the error increases. The narrow band carrier frequency error obtained by the narrow band fc error calculation circuit 7 is:
It is supplied to NCO9.

The wideband fc error calculation circuit 8 calculates a carrier frequency error included in the OFDM time domain signal.
Specifically, the wideband fc error calculation circuit 8 calculates a wideband carrier frequency error with subcarrier frequency (for example, 4.14 Hz) interval accuracy. This wide-band fc error calculation circuit 8 outputs a continuous pilot signal (CP signal).
, The amount of shift of the CP signal from the original insertion position of the CP signal is calculated to determine the shift amount. The wideband carrier frequency error obtained by the wideband fc error calculation circuit 8 is supplied to the NCO 9.

The NCO 9 calculates the narrow-band carrier frequency error of ± 1/2 accuracy of the sub-carrier frequency interval calculated by the narrow-band fc error calculation circuit 7 and the sub-carrier frequency interval accuracy calculated by the wide-band fc error calculation circuit 8. , And outputs a carrier frequency error correction signal whose frequency increases or decreases in accordance with the carrier frequency error obtained by the addition. This carrier frequency error correction signal is a complex signal and is supplied to the digital quadrature demodulation circuit 5. This carrier frequency error correction signal is
Carrier frequency f based on carrier frequency error correction signal
Digital orthogonal demodulation is performed while correcting c.

The equalizer 10 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 11.

The demapping circuit 11 includes the equalizer 10
The OFDM frequency domain signal subjected to amplitude equalization and phase equalization is subjected to demapping according to the modulation scheme to decode data.

The decoded data decoded by the demapping circuit 11 is supplied to an error correction circuit and the like at the subsequent stage.

Next, the equalizer 10 will be described in more detail.

The equalizer 10 includes a CP / SP signal extraction circuit 21, a hold circuit 22, a reference CP / SP signal generation circuit 23, a CP / SP signal complex division circuit 24, an interpolation filter 25, and a complex division circuit 26. And

The OFDM frequency domain signal output from the FFT operation circuit 6 is supplied to the CP / SP signal extraction circuit 21. The CP / SP signal extraction circuit 21 extracts an SP signal and a CP signal from an OFDM frequency domain signal. SP
The insertion position of the signal is determined in advance by the standard as described above. Similarly, the insertion position of the CP signal is predetermined by the standard as described above. Since the SP signal is inserted at a different subcarrier position for each symbol, the CP / SP signal extraction circuit 21
Referring to the symbol number of the M frequency domain signal, SP
Whether the signal is inserted is calculated based on the standard, and the SP signal is extracted. Further, since the CP signal is always inserted into the same subcarrier position even if the symbol changes, the CP / SP signal extraction circuit 21
The CP signal is extracted from subcarriers having the same index number regardless of the symbol number of the M frequency domain signal. The CP / SP signal extraction circuit 21 supplies the extracted CP signal and SP signal to the hold circuit 22.

The hold circuit 22 collects SP signals scattered in a predetermined number of OFDM symbols and CP signals arranged in predetermined positions of each OFDM symbol.
Hold them and set the CP for each fixed subcarrier interval.
・ Output as SP signal group.

Specifically, the hold circuit 22 collects SP signals interspersed and inserted into a predetermined number of OFDM symbols, and holds all the collected SP signals. At the same time, the hold circuit 22 holds the CP signal inserted in each OFDM symbol. And
The hold circuit 22 outputs the held CP signal and SP signal to the CP / SP signal complex division circuit 24 collectively. Here, in the case of the DVB-T standard, the CP signal is always inserted into the subcarrier of the index number in which the SP signal is arranged. Specifically, it is defined that the data is inserted into a subcarrier having an index number divisible by 3. Therefore, even if the CP signal and the SP signal are simultaneously collected for four symbols, the S sub-carrier is divided into three subcarriers at a rate of one as in the case where only the SP signal is collected.
The state is such that the P signal and the CP signal are detected.

Subsequently, when the next new OFDM symbol is input and the CP signal and the SP signal are extracted from the new one OFDM symbol, the hold circuit 2
2 deletes only the CP signal and the SP signal at the same subcarrier position as the extracted CP signal and the SP signal from the plurality of held CP signals and SP signals, and adds a new signal to the deleted position. Hold the appropriate CP signal and SP signal. That is, the hold circuit 22 updates and holds the newly extracted CP signal and SP signal. Here, the SP signal is 4OFD in the DVB-T system.
It will be updated to a new signal every M symbols,
Since the CP signal is always arranged at the same subcarrier position, it is updated every symbol.

Then, the hold circuit 22 outputs the held CP signal and SP signal to the CP / SP signal complex division circuit 24 collectively. That is, the hold circuit 22
Each time a CP / SP signal is extracted from a new OFDM symbol, a predetermined number of OFD symbols are slid one by one in a time series manner.
It operates to collect CP signals and SP signals for M symbols and output them collectively.

In the case of the DVB-T standard, FIG.
As shown in (A), one SP signal is inserted into twelve subcarriers, and the insertion position of the SP signal is shifted by three subcarriers for each OFDM symbol. Further, the CP signal is always a subcarrier having the same index number (# 0, # 48, # 87, # 141, # 156)
Has been inserted. Note that the CP signal is inserted into the subcarrier of the index number where the SP signal is arranged.

The specific processing procedure of the hold circuit 22 in the case of the DVB-T standard will be described. First, as shown in FIG.
Hold CP signal and SP signal of M symbol,
An SP signal group existing at a rate of one subcarrier is output. For example, the CP signal and the SP signal from OFDMsymbol # 0 to OFDMsymbol # 3 are held, and the SP signal inserted into three subcarriers at a rate of one is output collectively. At this time, since the CP signal is inserted at the same position in all four OFDM symbols, the hold circuit 22
Holds the CP signal of the last symbol (OFDM symbol # 3). Then, as shown by X in FIG. 2B, the index numbers are numbers # 0, # 3, # 6, # 9, # 12, # 15, # 18, # 21,.
And collectively output the CP signal and SP signal inserted at the subcarrier position.

Subsequently, the CP signal and the SP signal extracted from the next OFDM symbol are output to the CP / SP signal extraction circuit 2.
When supplied from 1, the hold circuit 22 updates the CP signal and the SP signal inserted in the subcarrier of the index number in which the supplied CP signal and SP signal exist, and updates the CP signal and the SP signal together with other SP signals that are not updated. An SP signal existing at a rate of one per subcarrier is output. Specifically, as shown by Y in FIG.
When the OFDM symbol of bol # 4 is input, the SP number and index numbers # 0, # 48, # 54, # 87, and SP signals inserted into the subcarriers with index numbers # 0, # 12, # 24,. # 141, # 1
The CP signal inserted into the 56... Subcarriers is updated, and the SP signal inserted into the subcarriers of other index numbers is output while holding the value of the previous symbol as it is.

Subsequently, the CP signal and the SP signal (SP signal inserted into 12 subcarriers at a rate of one) extracted from the next OFDM symbol are converted into CP · S
When supplied from the P signal extraction circuit 21, the supplied CP
The SP signal inserted into the subcarrier of the index number in which the signal and the SP signal exist is updated, and the SP signal existing in three subcarriers is output together with the other SP signals that are not updated. Specifically, FIG.
As shown in Z of (B), when OFDM symbol # 5 is input, the SP signals inserted into the subcarriers having index numbers # 3, # 15, # 27, and the index numbers # 0 and # 48. , #
C inserted into subcarriers of 54, # 87, # 141, # 156 ...
The P signal is updated, and the SP signal inserted into the subcarriers of other index numbers holds the value of the previous symbol as it is, and outputs these at once.

The hold circuit 22 performs such a process for 1
While repeating every OFDM symbol period, always 4 OF
The CP signal and the SP signal (SP signal for every three subcarrier intervals) collected from the DM symbol are output.

The hold circuit 22 supplies the collected CP signal and SP signal to the CP / SP signal complex division circuit 24.

The reference CP / SP signal generation circuit 23 generates the CP signal and the SP signal having the original amplitude and phase when the transmission signal is inserted into the transmission signal (the reference CP signal and the reference SP signal).
Signal). For example, in the DVB-T standard, since the CP signal and the SP signal are BPSK-modulated data, data having a phase of 0 or π and an amplitude of a predetermined amplitude (for example, 4/3) is generated. Note that if it is specified by the standard that CP signals and SP signals having different amplitudes or phases are inserted on the transmission side for each index number of the inserted subcarrier, the reference CP / SP signal generation circuit 23 A CP signal and an SP signal having amplitude and phase according to the standard are generated for each index number of the subcarrier. The reference SP signal generation circuit 122 converts the generated reference CP signal and reference SP signal into an SP complex division circuit 124.
To supply.

The CP / SP signal complex division circuit 24 performs complex division of the CP signal and the SP signal supplied from the hold circuit 124 with the reference CP signal and the reference SP signal generated from the reference CP / SP signal generation circuit 23. , Received C
The amount of distortion of the P signal and the SP signal is calculated. Here, the obtained distortion amount is a distortion amount for the subcarrier at the position where the CP signal and the SP signal are inserted. For example, in the DVB-T standard, it is a distortion amount for every three subcarrier intervals. The calculated distortion amounts of the CP signal and the SP signal are supplied to the interpolation filter 25.

[0092] The interpolation filter 25 interpolates the distortion amounts of the CP signal and the SP signal in the subcarrier direction, and obtains the distortion amounts for all the subcarriers of the OFDM symbol. That is, the transmission characteristics of the transmission path for all subcarriers are obtained. For example, in the DVB-T standard, CP
A distortion amount is supplied from the SP signal complex division circuit 24 to three subcarriers at a rate of one. Therefore, the interpolation filter 25 uses the triple interpolation filter or the like to
The transmission characteristics of subcarriers at positions where signals are not inserted (for example, subcarriers such as index numbers # 1, # 2, # 4, # 5, # 7, # 8 ...) are obtained by interpolation. For example, transfer characteristics for all 2048 subcarriers are obtained. The transfer characteristics of the transmission path for all subcarriers obtained by the interpolation filter 25 are supplied to a complex division circuit 26.

The complex multiplication circuit 26 performs complex division of the transfer characteristic obtained by the interpolation filter 25 on the OFDM frequency domain signal supplied from the FFT operation circuit 6,
The FDM frequency domain signal is equalized in amplitude and in phase. This complex division circuit 26 is an OFDM that has been subjected to amplitude equalization and phase equalization.
The DM frequency domain signal is supplied to the mapping circuit 11 by the.

As described above, the equalizer 10 estimates the transfer characteristic of the transmission path using the CP signal and the SP signal and equalizes the waveform, so that the amplitude and phase of each subcarrier become equal, and The effects of multipath and the like can be eliminated. In particular, in the equalizer 10, S
Since the transfer characteristics are estimated using the CP signal, which is constantly updated for each OFDM symbol, together with the P signal, the transfer characteristics of the transmission path are estimated with higher accuracy, and amplitude equalization and phase equalization are performed from the transfer characteristics. , The effects of multipath and the like during transmission can be eliminated.

In the above-described equalizer 10, the transfer characteristics are estimated using all the CP signals (45 CP signals in the DVB-T standard). , CP signals (D
In the VB-T standard, for example, index numbers # 0 and # 170
4 may be used. When performing interpolation, since the interpolation accuracy is not good because both ends have a small number of data to be interpolated, the transmission characteristics can be accurately obtained by replacing both ends having poor interpolation accuracy with CP signals updated for each symbol. Can be estimated.

[0096]

According to the demodulation apparatus and the demodulation method according to the present invention, the O after the Fourier transform is performed using the continuous pilot signal together with the scattered pilot signal.
Equalize the amplitude and waveform of all frequency components of the FDM signal.
As a result, in the present invention, it is possible to highly accurately estimate the transfer characteristics of the transmission path, perform amplitude equalization and phase equalization from the transfer characteristics, and remove the influence of multipath and the like during transmission.

[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 continuous pilot signal and a scattered pilot signal collected by an equalizer of the OFDM receiver.

FIG. 3 is a diagram for explaining a guard interval of an OFDM signal.

FIG. 4 is a diagram for explaining a scattered pilot signal inserted into a DVB-T OFDM signal;

FIG. 5 is a block diagram of a conventional OFDM receiver.

FIG. 6 is a diagram for explaining a principle that a CP signal can be extracted by performing differential demodulation between symbols twice for an OFDM frequency domain signal.

FIG. 7 is a diagram for explaining a scattered pilot signal collected by an equalizer of the OFDM receiver.

[Explanation of symbols]

0 OFDM receiver, 5 digital quadrature demodulation circuit, 6
FFT operation circuit, 7 Narrow band fc error calculation circuit, 8
Broadband fc error calculation circuit, 9 numerically controlled oscillator, 10
Equalizer, 11 demapping circuit, 21 CP · S
P signal extraction circuit, 22 hold circuit, 23 reference CP
・ SP signal generation circuit, 24 CP / SP signal complex division circuit, 25 interpolation filter, complex division circuit

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Takahiro Okada 6-35, Kita-Shinagawa, Shinagawa-ku, Tokyo Sony Corporation F-term (reference) 5K022 DD01 DD13 DD18 DD19 DD33 DD34 5K046 AA05 BB03 DD14 EE16 EE37 EE42 EE56

Claims (2)

[Claims] 1. An effective symbol generated by dividing and modulating information into frequency components within a predetermined band, and a guard generated by copying a signal waveform of a part of the effective symbol. A transmission symbol including an interval is used as a transmission unit, a scattered pilot signal having a predetermined amplitude and phase inserted into a frequency component at a different position for each transmission symbol, and a scattered pilot signal having the same position for each transmission symbol. An orthogonal frequency division multiplex (OFD) in which a continuous pilot signal having a predetermined amplitude and phase inserted into a frequency component is included in the effective symbol.
M) In a demodulation device for demodulating a signal, a Fourier transform means for Fourier transforming the OFDM signal in units of the effective symbol to demodulate the signal; and a scattered pilot signal from the Fourier transformed and demodulated signal; A continuous pilot signal is extracted, the extracted scattered pilot signal and the continuous pilot signal are collected for a predetermined transmission symbol, and the Fourier transform is performed based on the collected scattered pilot signal and the continuous pilot signal. A demodulator comprising: an equalizing means for equalizing the amplitude and phase of all frequency components of a signal. 2. An effective symbol generated by dividing and modulating information into each frequency component within a predetermined band, and a guard generated by copying a signal waveform of a part of the effective symbol. A transmission symbol including an interval is used as a transmission unit, a scattered pilot signal having a predetermined amplitude and phase inserted into a frequency component at a different position for each transmission symbol, and a scattered pilot signal having the same position for each transmission symbol. An orthogonal frequency division multiplex (OFD) in which a continuous pilot signal having a predetermined amplitude and phase inserted into a frequency component is included in the effective symbol.
M) In a demodulation method for demodulating a signal, the OFDM signal is Fourier-transformed in units of the effective symbols to demodulate the signal, and the scattered pilot signal and the continuous pilot signal are demodulated from the Fourier-transformed signal. Is extracted, the extracted scattered pilot signal and the continuous pilot signal are collected for a predetermined transmission symbol, and all frequencies of the Fourier-transformed signal based on the collected scattered pilot signal and the continuous pilot signal are collected. A demodulation method characterized by equalizing the amplitude and phase of a component.