JP3907574B2 - Demodulator for digital broadcast receiver - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a demodulator in a digital broadcast receiver that receives a broadcast signal modulated (OFDM modulated) by an orthogonal frequency division multiplexing system.
[0002]
[Prior art]
One OFDM transmission symbol period of an OFDM-modulated broadcast signal includes an effective symbol period and a period called a guard interval. The effective symbol period is a signal period necessary for data transmission. The guard interval section is for preventing inter-symbol interference such as multipath, and is obtained by cyclically copying the last predetermined period length portion of the effective symbol section to the beginning of the effective symbol section (see FIG. 11 described later). (See arrow in (a)).
[0003]
2. Description of the Related Art A demodulation device in a digital broadcast receiver that receives an OFDM-modulated broadcast signal such as a digital terrestrial television broadcast signal is known.
[0004]
In such a digital broadcast receiver, conventionally, in order to reduce the influence of intersymbol interference even when a delay wave having a delay time exceeding the guard interval period exists, a transmission path characteristic unit and a scattered pilot extraction demodulation unit are used. The multipath transfer function in the transmission path is estimated, and the FIR filter circuit that is an equalizer is operated to cancel the multipath, and the received signal is demodulated by the complex division circuit and signal demodulation circuit of the reception demodulation unit. (For example, refer to patent documents).
[0005]
[Patent Literature]
JP 2001-292120 A (page 3-7, FIG. 1-2).
[0006]
Although not found in the patent literature, the OFDM demodulator 30 in the conventional digital broadcast receiver is configured as shown in FIG.
[0007]
In the digital broadcast receiver shown in FIG. 8, the received signal is input to a tuner (not shown), and is converted to an intermediate frequency by performing amplification, frequency conversion, and band limitation. The intermediate frequency signal output from the tuner is supplied to the OFDM demodulator 30. In the OFDM demodulator 30, the intermediate frequency signal is supplied to the A / D converter 1 and converted into a digital signal, and the converted digital signal is quadrature detected by the quadrature detector 2 to generate a baseband (I, Q) signal. Is converted to
[0008]
The baseband (I, Q) signal output from the quadrature detector 2 is supplied to the effective symbol extraction circuit 3 to be sampled in the effective symbol period, before the first sampling data in the effective symbol period, and last in the effective symbol period. A predetermined range of sampling data including a predetermined number of sampling data after the sampling data is temporarily stored in the memory, and the effective symbol extraction circuit 3 takes in the effective symbols. On the other hand, based on the timing detection signal output from the timing detection circuit 11A, the effective symbol extraction circuit 3 takes in the sampling data that includes the effective symbol and is temporally one earlier than the effective symbol, or one time later. Up to sampling data is captured.
[0009]
Sampling data taken in by the effective symbol extraction circuit 3 is supplied to a fast Fourier transform (FFT) circuit 5, where FFT processing is performed in the FFT circuit 5 to demodulate the OFDM modulation signal, and information for each carrier is obtained. Transmission path characteristics are obtained from a scattered pilot (also referred to as SP), which is a carrier that is separated and supplied to the equalizer 6 and is inserted to correct the amplitude and phase of each carrier and has a known amplitude and phase. It is estimated, distortion is corrected based on the estimated value, and it is sent out as demodulated data by demapping by a demapper circuit.
[0010]
On the other hand, the output of the FFT circuit 5 is also supplied to the SP extraction circuit 7 and the TMCC decoder 9, the SP signal is extracted by the SP extraction circuit 7, and the extracted SP signal is supplied to the IFFT circuit 8 for inverse FFT operation processing. And a delay profile signal is obtained, and the TMCC decoder 9 determines whether or not the SP signal is included.
[0011]
The baseband (I, Q) signal output from the quadrature detector 2 is supplied to the guard correlator 10, and the input baseband (I, Q) signal of the guard correlator 10 and the baseband (I, Q) The product of the signal and the delayed baseband (I, Q) signal obtained by delaying the time width of the effective symbol period is integrated over the time width of the guard interval period, and the integration is performed for A / D conversion in the A / D converter 1. The guard correlation output is obtained from the integrated value by sequentially shifting the sampling periods.
[0012]
The guard correlation output obtained by the guard correlator 10, the delay profile signal obtained by the IFFT circuit 8, and the presence / absence identification signal of the SP signal obtained by the TMCC decoder 7 are supplied to the timing detection circuit 11A.
[0013]
The timing is detected from the peak position of the guard correlation output when the TMCC decoder 9 identifies that the SP signal does not exist, and from the delay profile signal and / or the guard correlation output when it is identified that the SP signal exists. The detection circuit 11A detects a timing signal indicating the start position of the effective symbol section of the OFDM symbol, and the effective symbol extraction circuit 3 detects the effective symbol to be captured from the sampling data one time ahead in the effective symbol. Or sampling data up to one later sampling data as sampling data included in the effective symbol period.
[0014]
This will be further described. Since the sampling clock of the OFDM demodulator 30 and the sampling clock on the transmission side are not completely synchronized, it is necessary to synchronize in the OFDM demodulator 30. The fact that the operation clock of the OFDM demodulator 30 and the sampling clock on the transmission side do not completely match means that the clock frequency is shifted, and the sampling clock of the A / D converter in the OFDM modulator and the OFDM demodulator This means that the sampling clock of the A / D converter 1 at 30 shifts. As a result of this shift, the sampling timing gradually shifts, and the received signal cannot be received correctly.
[0015]
This is specifically illustrated by FIG. The circles in FIG. 9A schematically show the output of the sampled digital part of the OFDM modulator. FIG. 9B schematically shows the waveform of the modulated wave baseband signal (analog signal) corresponding to the digital output of the modulator in FIG. In FIG. 9C, a circle indicates an input to the digital unit of the OFDM demodulator 30, and a deviation occurs between the sampling timing shown in FIG. 9C and the sampling timing shown in FIG. ing. In FIG. 9, the broken line indicates the sampling timing. The reception waveform indicated by a circle in FIG. 9C is shifted from the transmission waveform indicated by a circle in FIG. 9A.
[0016]
Here, when the sampling timings coincide, sampling data in the effective symbol period is normally extracted from the OFDM transmission symbol period as shown in FIG. However, for example, when the sampling timing gradually shifts with respect to the peak position of the guard correlation and exceeds the adjacent sample position, the number of samples of the output of the OFDM modulation apparatus obtained within the same period and the OFDM demodulation apparatus The number of input samples at 30 will not match.
[0017]
Therefore, for example, FIG. 10B in which the number of samples is increased by 1 and FIG. 10C in which the number of samples is insufficient are shown in FIG. As described above, if the sampling data taken out when the sampling data is taken out from the effective symbol section is corrected to the right and left, the timing cannot be correctly received. 10 (a) to 10 (c), GI indicates a guard interval period, and hatched lines indicate one sampling portion shifted.
[0018]
However, since the clock of the OFDM demodulator itself cannot be synchronized with the clock frequency of the modulator exactly, synchronization of the clock is conventionally detected for each OFDM symbol for each sampling clock, and the effective symbol for each OFDM symbol. The position for extracting the signal is shifted by one sampling clock unit to correct the timing. More specifically, the sampling clock shift (OFDM symbol start position shift) is detected from the peak position of the guard correlation output or the delay profile, and is extracted by the effective symbol extraction circuit 3 based on the output from the timing detection circuit 11A. The first sampling data is determined.
[0019]
The peak position of the guard correlation output is the position of the sample having the largest correlation between the received signal and the delayed signal obtained by sending the received signal by the effective symbol time. The guard interval period is a valid symbol of the last 1/4, 1/8, 1/16 or 1/32 period of a valid symbol added before the valid symbol, and the correlation between the received signal and its delayed signal. It can be said that the peak position as a result of taking is the head of the OFDM symbol, so that the start position of the OFDM symbol can be estimated. This situation is illustrated in FIG. 11 when the guard interval period is 1/4 of the effective symbol period.
[0020]
FIG. 11 (a) shows a received signal, FIG. 11 (b) shows a delayed signal obtained by delaying the received signal by an effective symbol period, and the last 1/4 effective symbol in the effective symbol period in the received signal The symbol of the delayed signal in the guard interval period is exactly the same, and schematically shows that the start position of the OFDM symbol can be estimated from the peak of the correlation result shown in FIG.
[0021]
Also, the delay profile signal is calculated from the SP signal in the OFDM demodulated signal, and the “main signal” from the delay profile signal and the size of the “delay signal” of the main signal generated by reflection on the transmission path and the “delay”. Time "is estimated. As schematically shown in FIG. 12 showing the case where the extraction positions are correct and FIG. 13 showing the case where the extraction positions are shifted, the head of the effective symbol can be estimated from the position of the main signal. . FIGS. 12A and 13A show OFDM modulated signals, and arrows indicate that the last 1/4 OFDM symbol of the effective symbol period is used as the guard interval period.
[0022]
FIG. 12B shows the extracted 1 OFDM symbol, FIG. 12C shows the extracted effective symbol, and FIG. 12D shows the delay profile signal. When the extraction positions are shifted, FIG. 13B shows one OFDM symbol extracted from the OFDM modulated signal shown in FIG. 13A, and FIG. 13C shows the extracted effective symbol. FIG. 13 (d) shows a delay profile signal.
[0023]
In the arrow extending over FIG. 13 (b) and FIG. 13 (d), the head portion indicated by the horizontal arrow located before the guard interval period in FIG. 13 (b) corresponds to the head portion of FIG. 13 (d). It shows that. However, when the SP signal is not included, the delay profile cannot be calculated. For this reason, it is used together with the guard correlation output.
[0024]
[Problems to be solved by the invention]
However, if the timing is corrected in units of one sampling clock interval as described above, the continuity between the previous OFDM symbol and the next OFDM symbol is not maintained, and an equalization error occurs. The problem arises that correction is required.
[0025]
An object of the present invention is to provide a demodulator in a digital broadcast receiver in which a substantial discontinuity between two consecutive OFDM symbols is reduced to a negligible level.
[0026]
[Means for Solving the Problems]
The demodulator in the digital broadcast receiver of the present invention is a demodulator in a digital broadcast receiver that receives an OFDM-modulated signal in which one OFDM symbol period includes an effective symbol period and a guard interval period.
Obtaining a spline function based on a plurality of sampling data before and after the sampling data including the sampling data within the effective symbol interval;
The period from the start time of the effective symbol period to the sampling time of the first sampling data in the effective symbol period and the first sampling data in the effective symbol period immediately before the effective symbol period immediately before the effective symbol period Find the difference from the period until the sampling time as an interpolation value,
The sampling time of the sampling data of the effective symbol interval is corrected based on the interpolation value to obtain an interpolation position, and the spline interpolation data at the interpolation position is obtained based on the spline function,
The obtained spline interpolation data is sent to a fast Fourier transform circuit in place of the sampling data in the effective symbol section.
[0027]
According to the demodulating device in the digital broadcast receiver of the present invention, a spline function is obtained based on a plurality of sampling data including the sampling data in the effective symbol period and before and after the sampling data, and the spline function is obtained from the start time of the effective symbol period. The difference between the period until the sampling point of the first sampling data in the effective symbol period and the period from the start point of the immediately preceding effective symbol period to the sampling point of the first sampling data in the immediately preceding effective symbol period for the effective symbol period is Obtained as an interpolation value, the sampling time of the sampling data of the effective symbol period is corrected based on the interpolation value to be an interpolation position, and the spline interpolation data at the interpolation position is obtained based on the spline function. The Spline interpolation data is sent to the Fourier transform circuit in place of the sampling data of the effective symbol in the interval. Therefore, discontinuity occurring between corresponding sampling data in two consecutive effective symbol sections can be reduced to a negligible level.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a demodulator in a digital broadcast receiver according to the present invention will be described with reference to an embodiment.
[0029]
FIG. 1 is a block diagram showing a configuration of a demodulating device in a digital broadcast receiver according to an embodiment of the present invention. The same components as those of the OFDM demodulating device 30 shown in FIG. It is shown.
[0030]
The received signal is input to the tuner and is converted into an intermediate frequency by performing amplification, frequency conversion, and band limitation. The intermediate frequency signal output from the tuner is supplied to the OFDM demodulator 20 shown in FIG. In the OFDM demodulator 20, the intermediate frequency signal is supplied to the A / D converter 1 and converted into a digital signal, and the converted digital signal is quadrature detected by the quadrature detector 2 to generate a baseband (I, Q) signal. Is converted to
[0031]
The baseband (I, Q) signal output from the quadrature detector 2 is supplied to the effective symbol extraction circuit 3, and the sampling data in the effective symbol period and the first sampling data in the effective symbol period are the last in the effective symbol period. A predetermined range of sampling data including a predetermined number of sampling data after the sampling data is temporarily stored in the memory, whereby the effective symbol extraction circuit 3 uses the sampling data in the effective symbol period and before and after the effective symbol period. A plurality of, for example, two sampling data are extracted and captured.
[0032]
On the other hand, the timing detection circuit 11 is based on the input guard correlation output, the delay profile signal, and the presence / absence signal of the SP signal, and the first in the effective symbol period immediately before the effective symbol period schematically shown in FIG. The magnitude and direction of the deviation (correction value) between the sampling clock of the sampling data (see FIG. 2B) and the sampling clock of the first sampling data in the effective symbol section shown in FIG. 2C is detected. The magnitude and direction signals are sent to the interpolation position / extraction position control circuit 12. In FIG. 2, ◯ indicates the sampling timing, and the correction value is constant in the effective symbol period.
[0033]
In the interpolation position / extraction position control circuit 12, based on the output signal from the timing detection circuit 11, in the effective symbol extraction circuit 3, the sampling data in the effective symbol period, the first sampling data in the effective symbol period, and the last in the effective symbol period A plurality of sampling data in a predetermined range after the sampling data is extracted. The sampling data extracted by the effective symbol extraction circuit 3 is supplied to the interpolation circuit 4.
[0034]
As shown in FIGS. 3 and 4 to be described later, the interpolation circuit 4 uses a predetermined range of sampling data including the sampling data of the effective symbol period that has been input, and converts the spline function based on four consecutive sampling data. In addition, the time interval between adjacent sampling data is divided into a plurality of, for example, 2048, and interpolation coefficient data for obtaining spline interpolation data at each divided time position is calculated from the obtained spline function, The calculated interpolation coefficient data is stored in a memory having each time position as an address, and the interpolation coefficient data stored in the address position based on the interpolation position control signal output from the interpolation position / extraction position control circuit 12 is read out. And spline interpolation based on the read interpolation coefficient data Calculating a spline interpolation data, computed spline interpolation data is sent to the FFT circuit 5 as an active symbol data.
[0035]
The FFT circuit 5 that has received the spline interpolation data interpolated by the interpolation filter 4 performs FFT processing to demodulate the OFDM modulated signal, separates the information for each carrier, and supplies it to the equalizer 6. The channel characteristics are estimated from the SP signal, which is a carrier having a known amplitude and phase inserted to correct the amplitude and phase of each carrier, and distortion is corrected based on the SP characteristics, which are supplied to the demapper circuit. Then, it is transmitted as demodulated data by demapping.
[0036]
On the other hand, the output of the FFT circuit 5 is also supplied to the SP extraction circuit 7 and the TMCC decoder 9, the SP signal is extracted by the SP extraction circuit 7, and the extracted SP signal is supplied to the IFFT circuit 8 for inverse FFT operation processing. To obtain a delay profile signal. On the other hand, the presence / absence information of the SP signal is detected by the TMCC decoder 9. The baseband (I, Q) signal output from the quadrature detector 2 is supplied to the guard correlator 10, and the guard correlation output is obtained by the guard correlator 10.
[0037]
Based on the guard correlation output obtained by the guard correlator 10, the delay profile signal obtained by the IFFT circuit 8, and the SP signal presence / absence information identified by the TMCC decoder 7, an effective symbol is obtained by the guard correlation output and / or the delay profile. As described above, the magnitude and direction of the deviation between the sampling clock of the first sampling data in the section and the sampling clock of the first sampling data in the immediately preceding effective symbol section are detected.
[0038]
Here, the interpolation operation of the interpolation circuit 4 will be described with reference to FIGS. 3 and 4 taking the case of the sampling data β as an example.
[0039]
In the interpolating circuit 4, as shown in FIG. 3, a cubic spline function is obtained based on four consecutive sampling data α, β, γ, and δ including the sampling data β. Based on the interpolation position control signal indicating the input interpolation position, the interpolation position for the sampling data β is determined, and the sampling data at the interpolation position is obtained by interpolation using a spline function.
[0040]
In this case, the time interval between adjacent sampling data is divided into 2048, and one of the divided times is determined based on the interpolation position control signal. As shown in FIG. 3B, this will be simplified and described as an example in which the time is divided into six and each time point is t1, t2, tt3, t4, and t5. Using the cubic spline function obtained based on four consecutive sampling data α, β, γ, and δ, the spline interpolation data at the interpolation position at each time point t1, t2, tt3, t4, and t5 is obtained. Coefficient data (a0, b0, c0, d0), ..., (a4, b4, c4, d4) are respectively obtained.
[0041]
The obtained coefficient data (a0, b0, c0, d0),..., (A4, b4, c4, d4) are schematically shown in FIG. 3C at memory addresses t1, t2, tt3, t4, and t5. As shown in FIG. Therefore, when the interpolation position is determined at the time point t3 based on the interpolation position control signal, the coefficient data (a3, b3, c3, d3) is read from the memory address t3, and the sampling data α, β, γ , Δ and coefficient data (a3, b3, c3, d3), the spline interpolation data B at the interpolation position is obtained by (a3 × α + b3 × β + c3 × γ + d3 × δ), and is interpolated.
[0042]
However, since the interpolation position is divided into 2048 and its resolution is set to 2047 as schematically shown in FIG. 4, the addresses in the memory are t1, t2,..., T2047, and the interpolation position in FIG. Then, the interpolated spline interpolation data is (ai × α + bi × β + ci × γ + di × δ). Here, i = 1 to 2047.
[0043]
Further, when the time position of the sampling data γ is 0, the time position of the sampling data δ is 1, and in the above interpolation, the time corresponding to the interpolation position is t as shown in FIG. , Δ does not fall within the sampling clock, that is, when t <0, the interpolated spline interpolation data B1 is B1 = (aj × ω + bj × α + cj × β + dj × γ), and when t> 1 The interpolated spline interpolation data B2 is B2 = (ak × β + bk × γ + ck × δ + dk × ε). Of course, when 0 <t <1, the spline interpolation B is B = (ai × α + bi × β + ci × γ + di × δ).
[0044]
In the above, the time point position of the sampling data β is set to 0 and the time point position of the sampling data γ is set to 1 as shown in FIG. 5. In this interpolation, when 0 <t <1, as shown in FIG. As shown in FIG. 5B, when t <0 as shown in FIG. 5B, the effective symbol extraction circuit 3 performs interpolation by shifting the extraction position by one in the direction of delaying in time as shown in FIG. When> 1, the effective symbol extraction circuit 3 may perform interpolation by shifting the extraction position by one in the direction of time advance as shown in FIG.
[0045]
As described above, the spline interpolation data B is obtained from the four consecutive sampling data α, β, γ, and δ and replaced with the sampling data β. Similarly, the four consecutive sampling data β, γ, δ, and ε are splined. The next spline interpolation data (B + 1) after the interpolation data B is obtained and replaced with the sampling data γ, and this operation is repeated until the entire effective symbol section is reached. In this case, the interpolation position is the same as that for the sampling data γ at the time ti for the sampling data β.
[0046]
Also, for the effective symbol period immediately before the target effective symbol period, the interpolation position is obtained in the same manner as the effective symbol period immediately after, and the spline interpolation data at the calculated interpolation position is used instead of the sampling data. . By doing so, discontinuity between corresponding data between the spline interpolation data corresponding to the sampling data in the effective symbol period and the spline interpolation data corresponding to the sampling data in the immediately subsequent effective sampling period is reduced. .
[0047]
As described above, in the OFDM demodulator 20 according to the embodiment of the present invention, as shown in FIG. 2, the correction value is the sampling point of the first sampling data in the immediately preceding effective symbol section and the next effective symbol. The difference between the sampling time of the first sampling data in the section and the case where it is assumed to be constant over the effective symbol section is exemplified. However, as indicated by the correction value in FIG. It may be constant.
[0048]
【The invention's effect】
As described above, according to the digital broadcast receiver according to the present invention, since the sampling data interval and level between adjacent effective symbols are corrected, it occurs between corresponding sampling data in two consecutive effective symbol sections. Discontinuities can be reduced to a negligible extent.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a demodulator according to an embodiment of the present invention.
FIG. 2 is a schematic explanatory diagram illustrating a temporal difference between sampling data in an effective symbol section of the demodulator according to the embodiment of the present invention.
FIG. 3 is a schematic explanatory diagram of spline correction in the correction circuit of the demodulator according to the embodiment of the present invention.
FIG. 4 is a schematic explanatory diagram of a correction position in the correction circuit of the demodulator according to the embodiment of the present invention.
FIG. 5 is a schematic explanatory diagram of spline correction in the correction circuit of the demodulator according to the embodiment of the present invention.
FIG. 6 is a schematic explanatory diagram of spline correction in the correction circuit of the demodulator according to the embodiment of the present invention.
FIG. 7 is another schematic explanatory diagram illustrating a temporal difference between sampling data in an effective symbol section of the demodulating device according to the embodiment of the present invention.
FIG. 8 is a block diagram showing a configuration of a conventional demodulator.
FIG. 9 is a schematic explanatory diagram showing a temporal relationship between sampling data in the case of a conventional demodulating device and sampling data in a modulating device.
FIG. 10 is a schematic explanatory diagram for explaining effective symbol interval extraction in the case of a conventional demodulator.
FIG. 11 is a schematic explanatory diagram of start position estimation of an effective symbol period by guard correlation.
FIG. 12 is a schematic explanatory diagram of estimation of the start position of an effective symbol period when the timing is correct.
FIG. 13 is a schematic explanatory diagram of start position estimation of an effective symbol period when the timing is not correct.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 A / D converter 2 Quadrature detector 3 Effective symbol extraction circuit 4 Interpolation circuit 5 FFT circuit 6 Equalizer 7 SP extraction circuit 8 IFFT circuit 9 TMCC decoder 10 Guard correlator 11 Timing detection circuit 12 Interpolation position and cutout position control Circuit 20 OFDM demodulator

Claims (1)

In a demodulator in a digital broadcast receiver that receives an OFDM-modulated signal in which one OFDM symbol period includes an effective symbol period and a guard interval period,
Obtaining a spline function based on a plurality of sampling data before and after the sampling data including the sampling data within the effective symbol interval;
The period from the start time of the effective symbol period to the sampling time of the first sampling data in the effective symbol period and the first sampling data in the effective symbol period immediately before the effective symbol period immediately before the effective symbol period Find the difference from the period until the sampling time as an interpolation value,
The sampling time of the sampling data of the effective symbol interval is corrected based on the interpolation value to obtain an interpolation position, and the spline interpolation data at the interpolation position is obtained based on the spline function,
A demodulating apparatus in a digital broadcast receiver, wherein the obtained spline interpolation data is sent to a fast Fourier transform circuit in place of the sampling data in the effective symbol section.

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