JP4819651B2 - OFDM signal transmission line characteristic estimation means and correction means and apparatus using the same - Google Patents

Description

  The present invention relates to a transmission path characteristic estimation unit and a correction unit of an apparatus for receiving an OFDM signal into which a pilot carrier having a known amplitude and phase is inserted, and an apparatus using the same.

  In recent years, Orthogonal Frequency Division Multiplexing (OFDM) is a modulation method suitable for digital transmission for mobiles and terrestrial digital television broadcasting. Has attracted attention. The OFDM scheme is a type of multi-carrier modulation scheme, and is a transmission scheme in which digital modulation is performed on dozens to hundreds of carriers (hereinafter referred to as carriers) orthogonal to each other. A discrete code is assigned to each of the I-axis component and Q-axis component of each carrier as a modulated signal, and the code is updated every symbol period (several μsec to msec). These carriers are added so as to maintain an orthogonal relationship with each other, and an OFDM time axis waveform is generated. This addition process can be realized by performing an inverse Fourier transform (hereinafter referred to as IFFT) process on each carrier.

  As shown in FIG. 7, the OFDM symbol is composed of an effective symbol, which is a time axis waveform after the IFFT processing, and an OFDM symbol based on a guard interval that is a part of the effective symbol copied and added before the effective symbol. Composed. By adding a guard interval, it is possible to avoid deterioration due to inter-symbol interference with respect to a delayed wave having a delay time within the guard interval period, and thus can have strong resistance against multipath fading. .

  The OFDM signal generated by the above processing is frequency-converted (up-converted) to high frequency (RF), transmitted from the antenna, received by the receiving antenna, and frequency-converted (down-converted) to an intermediate (IF) signal. , Return to OFDM signal.

  The modulation error ratio (hereinafter referred to as MER) is a value obtained by quantifying the variation of data symbol coordinates of a digital broadcast wave, and becomes a larger value as the reception state is less affected by noise and distortion. Specifically, in the demodulated constellation, it is a value defined as the power ratio between the power converted value of the vector error from the ideal constellation point and the ideal constellation, and represents the equivalent C / N. In the case of terrestrial digital broadcasting, MER is the value of a QAM signal after OFDM demodulation, and in the case of 64 QAM used in normal broadcasting, the minimum reception condition is about 20 dB or more. However, in practice, it is necessary to allow for a sufficient margin against the deterioration of the reception state due to fading, jamming waves, etc., so it is said that it is desirable to secure 25 dB or more.

  In the digital terrestrial television broadcasting system, pilot symbols with known amplitudes and phases are inserted, and the channel characteristics are estimated by interpolating the received pilot carriers in the time and frequency directions. The pilot carriers are inserted every 12 carriers in the frequency direction, and the positions of the inserted carriers are shifted by 3 carriers with time and are called discrete pilots (hereinafter abbreviated as SP).

  Since the frequency at which the pilot carrier is inserted differs from time to time, transmission path characteristics for every three carriers can be obtained by performing interpolation in the time direction first. Thereafter, the channel characteristics of the carrier in which no pilot carrier is inserted are estimated by interpolation in the frequency direction. This method is called a 4-symbol estimation method. Also, in order to set a fast Fourier transform (FFT) time window fixedly and equalize the delayed wave and the preceding wave within the guard interval period, the pass band of the interpolation filter in the frequency direction is doubled. An interval length method is often used.

In the case of transmitting a video signal, a continuous pilot (hereinafter abbreviated as CP) in which pilot carriers are continuously arranged on the same carrier in time is also used.
JP 2005-260331 A

However, the four symbol estimation method for performing interpolation in the time direction, when the tracking of the variations is disadvantageously difficult.

  In order to solve the above problem, channel characteristic estimation using all carriers is used, but the length of the low-pass filter is twice as long as the guard interval length, and the filter is applied to an unnecessary range. There is a problem that the noise component increases.

  The present invention is intended to solve such a conventional problem, and can provide an OFDM signal receiving apparatus and a relay apparatus having a high-accuracy and high-speed transmission path characteristic estimation technique.

  In the present invention, in order to solve the above-mentioned problem, in an apparatus having means for receiving an OFDM modulated signal in which a pilot carrier having a known amplitude and phase is arranged, and performing FFT on the received signal, the signal is obtained from the FFT means. Transmission from the first transmission path characteristic estimation means by interpolating the pilot carrier signal and all the carrier signals of the signal obtained from the FFT means using the signal obtained from the first transmission path characteristic estimation means Second transmission path characteristic estimation means for estimating path characteristics, a low-pass filter whose guard interval length is a pass band for the signal obtained from the second transmission path characteristic estimation means, and the first transmission The position of the main wave is detected from the signal obtained from the path characteristic estimating means, and the position of the main wave is corrected so as to be a predetermined position in the pass band of the low-pass filter. Providing channel estimation means and correction means of the OFDM signal, characterized in that it comprises a means.

  Alternatively, in an apparatus having means for receiving an OFDM modulated signal in which a pilot carrier having a known amplitude and phase is arranged and performing FFT on the received signal, the pilot carrier signal included in the signal obtained from the FFT means is interpolated. The first transmission path characteristic estimation means and the signal obtained from the first transmission path characteristic estimation means to estimate the transmission path characteristics from all the carrier signals of the signal obtained from the FFT means, A first modulation for calculating a modulation error ratio by equalizing a signal obtained from the FFT means using a signal obtained from the second transmission path characteristic estimation means passing through the filter and the first transmission path characteristic estimation means; The signal obtained from the FFT means is equalized using the error ratio calculating means and the signal obtained from the low-pass filter to calculate the modulation error ratio. A second modulation error ratio calculation means, a means for comparing a signal obtained from the first modulation error ratio calculation means and a signal obtained from the second modulation error ratio calculation means, and the comparison means result When the signal obtained from the first modulation error ratio computing unit is better, the signal obtained from the first transmission path characteristic estimating means is selected and obtained from the second modulation error ratio computing means. An OFDM signal transmission line characteristic estimating means and a correcting means are provided, comprising means for selecting a signal obtained from the low-pass filter when the signal is better.

  In the OFDM signal transmission path characteristic estimation unit and the main wave position correction means, a low-pass filter whose guard interval length is a pass band with respect to the signal obtained from the second transmission path characteristic estimation means, and And a means for detecting the position of the main wave from the signal obtained from the first transmission path characteristic estimating means and correcting the position of the main wave so as to be a predetermined position in the pass band of the low-pass filter. An OFDM signal transmission line characteristic estimation unit and a correction unit are provided.

Furthermore, the present invention provides an OFDM signal receiving apparatus comprising the OFDM signal transmission path characteristic estimation means and correction means.
Alternatively, there is provided an OFDM signal repeater comprising the OFDM signal transmission line characteristic estimation means and correction means.

  As described above, according to the present invention, unnecessary noise components can be reduced by using the all-carrier method for the transmission path characteristic estimation method and by reducing the range of the low-pass filter to ½ that of the conventional one. In addition, highly accurate transmission path characteristic estimation can be performed. Further, by comparing the modulation error ratio (MER) between the 4-symbol estimation method and the all-carrier method, it is possible to select a transmission channel characteristic estimation method according to the transmission path condition.

  Hereinafter, an embodiment of the present invention will be described with reference to FIG. 8 and FIG. 9, and each part will be described with reference to FIGS. 1 to 7.

  FIG. 8 is a block diagram showing the overall configuration of the OFDM signal receiving apparatus according to the embodiment of the present invention, and FIG. 9 is a block diagram showing the overall configuration of the OFDM signal relay apparatus according to the embodiment of the present invention. 8 and 9, the high-frequency signal received by the antenna 1 is converted into an IF signal by the down-converter 2 and converted into a digital signal by the analog-digital converter (A / D) 3, and the frequency is obtained by the fast Fourier transform (FFT) unit 4. It is converted into a component, and the transmission line deterioration is corrected by the transmission line characteristic estimation unit 6 and the equalization unit 5. In the OFDM signal receiving apparatus of FIG. 8, the correction output of the equalization unit 5 is returned to the transport stream (TS) signal by the demodulation unit 7. In the OFDM signal relay apparatus of FIG. 9, it is converted into a time axis component by an inverse fast Fourier transform (IFFT) unit 8, converted into an IF signal by a digital-analog converter (D / A) 9, and converted into a high-frequency signal by an up-converter 10, 11 is retransmitted.

FIG. 1 shows the configuration of the transmission path characteristic estimation unit 6 of the OFDM signal receiving apparatus according to the first embodiment of the present invention. Figure 2 is a schematic diagram showing a noise included in the filter passband the filter passband 4 symbol estimation method, Fig. 3 is low in the case of the conventional is not applied twice and optimization process of the guard interval length D _GI is a schematic diagram illustrating a band pass filter and a noise, FIG. 4 is a schematic diagram showing a low-pass filter and the noise of the guard interval length D _GI. Figure 5 is a schematic diagram showing a Dα no attenuation of the shortest and the filter from the temporal position Dm and band edge D _{GI / 2} of the low-pass filter of the guard interval length D _GI and noise and the main wave. FIG. 6 is a block diagram of the transmission path characteristic estimation unit in the second embodiment of the present invention, and FIG. 6 is a modification of the configuration of the transmission path characteristic estimation unit in the first embodiment.

  In FIG. 1, the channel characteristic estimation unit 6 includes a pilot carrier (P) extraction / provisional estimator 13, an equalization processor 14, a determination processor 15, a divider 16, an LPF 17, and an LPF optimization controller 18. The LPF optimization control unit 18 includes an IFFT unit 51, a main wave detector 52, a position detector 53, and a position controller 54.

The correction unit (LPF optimization rotator) 12 performs optimization position correction based on the output signal D _CONT from the LPF optimization control unit 18. The FFT unit 4 performs an FFT operation on the input signal.

The output signal R is connected to a P extraction / provisional estimation unit 13, an equalization processor 14, and a divider 16. The P extraction / provisional estimator 13 extracts a discrete pilot (SP) from the input signal R, and first performs interpolation in the time direction to obtain transmission path characteristics for every three carriers. Thereafter, provisional estimation of the channel characteristics of the carrier in which no pilot carrier is inserted is performed by interpolation in the frequency direction, and a signal H ₁ ′ is output to the equalization processor 14 and the LPF optimization controller 18.

The equalization processor 14 performs complex division between the input signal R and the signal H ₁ ′, and outputs the equalized signal R / H ₁ ′ to the determination processor 15. The equalized signal R / H ₁ ′ indicates a reception constellation when a signal modulated by 64QAM, 16QAM, or the like is received. Further, in the terrestrial digital television broadcasting system, a system with a different modulation system for each carrier is adopted.

The determination processor 15 performs determination based on the modulation scheme for each carrier on the input equalized signal R / H ₁ ′, and outputs the determination signal X ′ to the divider 16. The output signal R of the FFT unit 4 is distorted by the influence of the transmission path given by H. In the output signal R, the desired carrier component X is subjected to amplitude / phase distortion due to the influence of the transmission path characteristic H. This can be expressed as R = H · X. Rewriting the transmission line characteristic H, it is expressed as H = R / X. In an environment where there are few determination errors, the determination signal X ′ becomes X≈X ′, and the determination signal X ′ is calculated as an ideal value of the desired carrier component.

By performing the complex division of the determination signal X 'and the divider 16 the input signal R, it is possible to output calculated channel characteristics H which follow the time variation.

LPF17 is a low-pass filter, such as filter passband is _{-D GI} / _2~D GI / 2 when a guard interval length _{D GI,} remove noise such as unnecessary components from the signal _{H 2} 'which is input Output the signal H _LPF . FIG. 2 shows the filter passband of the 4-symbol estimation method and the noise contained in the filter passband. FIG. 3 shows an application example of a low-pass filter in the conventional case where the filter pass band is −D _{GI to} D _GI and the optimization process is not applied. It can be seen from FIG. 3 that the actually required filter passband length is D. Further, FIG. 4 is an example of a low-pass filter applications the filter passband is _{-D GI} / _2~D GI / 2. The influence of noise components can be halved by setting the filter passband to ½ that of the prior art.

The LPF optimization control unit 18 performs position correction control processing that optimizes the LPF processing, and outputs the correction information D _CONT to the LPF optimization rotator 12.

The IFFT unit 51 performs an IFFT (Inverse Fast Fourier Transform) operation on the temporary estimation signal H ₁ ′ of the transmission path characteristic, calculates a delay profile of the transmission path as shown in FIG. 3, and outputs it to the main wave detector 52. To do. In the delay profile shown in FIG. 3, the right end path indicates the path that has arrived most first in time, and the left end indicates the most delay path.

  The main wave detector 52 detects the main wave component from the delay profile signal from the IFFT unit 51, and outputs it to the position detector 53. The operation of the main wave detector 52 will be described. In the delay profile shown in FIG. 3, a path that exceeds a threshold provided so as not to confuse the incoming path signal and noise and that has arrived the earliest in time is detected as the main wave.

  The position detector 53 detects the temporal position Dm of the main wave based on the output of the main wave detector 52 and outputs the position information to the position controller 54.

The position controller 54 outputs position correction information D _CONT such that Dm = D _GI / 2−Dα (Dα> 0) based on the Dm information output from the target wave selector 54 to the LPF optimization rotator 12. . The optimization rotator 12 performs rotation calculation on the complex frequency axis for the output signal from the FFT unit 4 based on the control information from the LPF optimization control unit 18 and equivalently corrects the time position of the delay profile signal. .

The position Dm of the main wave of the delay profile signal is a position that is shortest from the band edge D _GI / 2 and away from Dα where there is no filter attenuation. By maintaining such a position, it is possible to arrange delay waves from as many main waves as possible in the passband as shown in FIG.

  According to the first embodiment described above, it is possible to follow time fluctuation by estimating the channel characteristics using all carriers, and by reducing the passband of the LPF to 1/2 of the conventional one, It is possible to reduce the influence, and it is possible to estimate transmission path characteristics with high speed and high accuracy.

  Next, a second embodiment will be described. In the second embodiment, the configuration of the channel characteristic estimation unit is shown in FIG. 6, and the channel characteristic estimation method using all carriers and the four symbol estimation method are compared by modulating error ratio (MER) values. Selectable. Since the other configuration is the same as that of the first embodiment, detailed description thereof is omitted.

  FIG. 6 shows a modification of the configuration of the transmission path characteristic estimation unit in the first embodiment, and an equalization processor 71, MER calculators 72 and 73, and a transmission path estimation selection processor 74 are added to the configuration of FIG. It is a thing.

The equalization processor 71 performs complex division between the input signal R and the signal H ′ _LPF , and outputs the equalized signal R / H ′ _LPF to the MER calculator 72. The MER calculator 72 calculates the MER value from the output signal R / H ′ _LPF of the equalization processor 71 and outputs MER [R / H ′ _LPF ] to the selection processor 63. MER [α] is a function that outputs C / N of α. The MER calculator 73 calculates the MER value from the output signal R / H ₁ ′ of the equalization processor 14 and outputs MER [R / H ₁ ′] to the selection processor 73. The transmission path estimation / selection processor 74 compares the input MER [R / H ′ _LPF ] and MER [R / H ₁ ′]. As a result of the comparison, if MER [R / H ′ _LPF ] is large, the input H ′ _LPF is selected. If MER [R / H ₁ ′] is large, the input H ₁ ′ is selected, and the selection signal H ′ is selected. _SEL is output.

According to the second embodiment described above, if the deterioration of MER [R / H ′ _LPF ] is severe due to extremely severe fluctuations in the transmission path and the erroneous determination of R / H ₁ ′, the MER value is By comparison, the channel characteristic estimation result of the 4-symbol estimation method is selected. That is, a better transmission path characteristic estimation result is selected according to the transmission path status.

  In FIG. 6 of the configuration of the transmission line characteristic estimation unit of the second embodiment described above, there is also a simple method in which the optimization rotator 12 and the pre-LPF control unit 18 are omitted and the LPF 17 is twice the guard interval length. Is possible.

Although the above explanation has been mainly focused on the digital terrestrial television broadcasting system in which pilot carriers are distributed and arranged in the time and frequency directions, in the continuous pilot (CP) system in which pilot carriers are continuously arranged on the same carrier, With the present invention, it is possible to estimate transmission line characteristics. The P extraction / provisional estimation unit 13 in FIGS. 1 and 6 is the same operation except that the interpolation is performed only in the frequency direction and the transmission path characteristics of the carrier in which the pilot carrier is not inserted is provisionally estimated. Detailed description will be omitted.

The block diagram of the transmission-line characteristic estimation part in 1st Example of this invention Schematic diagram showing the filter passband and noise of the 4-symbol estimation method Schematic view showing a 2-fold lowpass filter and a noise guard interval length D _GI Schematic diagram showing a low-pass filter and the noise of the guard interval length D _GI Schematic diagram showing a low-pass filter and noise and temporal position Dm and Dα no attenuation of the shortest and the filter from the band edge D _{GI / 2} of the main wave of the guard interval length D _GI The block diagram of the transmission-line characteristic estimation part in 2nd Example of this invention Schematic diagram showing symbol structure of OFDM signal The block diagram which shows the structure of the OFDM receiver of the Example of this invention The block diagram which shows the structure of the OFDM relay apparatus of the Example of this invention

Explanation of symbols

1, 11: antenna, 2: down converter,
3: Analog-digital converter (A / D),
4: Fast Fourier transform (FFT) unit 5: equalization unit, 6: transmission path characteristic estimation unit, 7: demodulation unit,
8, 51: Inverse Fast Fourier Transform (IFFT) part,
9: Digital-analog converter (D / A), 10: Upconverter 12: Correction unit (LPF optimization rotator), 13: P extraction / temporary estimation unit,
14: Equalizer, 15: Decision processor, 16: Divider 17: Low-pass filter (LPF), 18: LPF optimization controller 52: Main wave detector, 53: Main wave position detector, 54 : Main wave position controller 71: Equalizer, 72: MER calculator, 73: MER calculator 74: Transmission path characteristic estimation means selector

Claims (5)

In an OFDM signal receiving apparatus having means for receiving an OFDM modulated signal in which a pilot carrier having a known amplitude and phase is arranged and performing FFT on the received signal,
First transmission line characteristic estimation means by interpolating a pilot carrier signal included in the signal obtained from the FFT means;
A determination signal is obtained by equalizing and determining the signal obtained from the FFT means using the signal obtained from the first transmission line characteristic estimation means, and complex division of the signal obtained from the FFT means and the determination signal A second transmission path characteristic estimating means for estimating the transmission path characteristics based on all carrier signals;
A low-pass filter that performs low-pass filtering on the signal obtained from the second transmission path characteristic estimation means , the width of the pass band corresponding to the guard interval length;
The position of the main wave is detected from the signal obtained from the first transmission path characteristic estimation means, and the signal obtained from the FFT means is set so that the position of the main wave becomes a predetermined position in the pass band of the low-pass filter. An OFDM signal receiving apparatus comprising correction means for performing correction . A first modulation error ratio arithmetic means for calculating a first modulation error ratio equalizes the signal obtained from the FFT unit by using a signal obtained from the first transmission channel characteristic estimating means,
A second modulation error ratio calculation means for calculating the low-pass by using a signal obtained from the filter equalizes the signal obtained from the FFT unit second modulation error ratio,
Wherein the first and compares the second modulation error ratio, if the the first towards the modulation error ratio better selects the signal obtained from the first transmission channel characteristic estimating unit, wherein the second modulation error The OFDM signal receiving apparatus according to claim 1 , further comprising means for selecting a signal obtained from the low-pass filter when the ratio is better. The first transmission line characteristic estimation means estimates the transmission line characteristic by interpolating the pilot carrier signal in the time and frequency directions,
The pass band of the low-pass filter is −D _GI / 2 to D _GI / 2 when the guard interval length is D _GI .
The correction means performs a rotation operation on the frequency axis on the signal obtained from the FFT means, and equivalently corrects the time position of the main wave in the delay profile. It is controlled so that it is in the vicinity of the band edge of the low-pass filter and at a position where there is substantially no attenuation by the low-pass filter,
3. The OFDM according to claim 1, wherein the first and second transmission line characteristic estimation means are input with signals obtained from the FFT means and corrected by the correction means. 4. Signal receiving device . An equalizer for equalizing the signal corrected by the correction means using a transmission path characteristic output from the low-pass filter, and a demodulator for demodulating the output of the equivalent unit, The OFDM signal receiving apparatus according to claim 1 . An OFDM signal repeater comprising the OFDM signal receiving device according to claim 1.

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