JP5109878B2 - Demodulator - Google Patents

Description

  The present invention relates to a demodulation device.

  The ISDB-T (Integrated Services Digital Broadcasting-Terrestrial), which is a Japanese terrestrial digital broadcasting standard, employs OFDM (Orthogonal Frequency Division Multiplexing). OFDM is a multi-carrier transmission scheme that performs transmission using a number of orthogonal carriers. By making the subcarrier band narrow, resistance to frequency selective fading is increased, and by increasing the length of one symbol period, resistance to delayed waves is increased.

  Japanese Patent Application Publication No. 2002-539669 describes a system for generating a Doppler correction coefficient for compensating for a Doppler shift of a signal transmitted between a mobile station and a base station in a mobile communication system.

  Japanese Patent Laid-Open No. 2005-286636 describes a digital broadcast receiving apparatus that can receive a reception signal of an orthogonal frequency division division transmission system that is specified by a combination of an effective symbol period length and a guard interval length. Yes.

Special Table 2002-539669 JP 2005-286636 A

  In a demodulator in a terrestrial digital broadcast receiving terminal, when moving reception is assumed, it is necessary to estimate the moving speed of the receiving terminal.

  An object of the present invention is to provide a demodulator capable of performing processing according to the moving speed.

  The demodulator according to the present invention includes an orthogonal demodulator that orthogonally demodulates a first signal in symbol units, a Fourier transform unit that Fourier transforms a second signal orthogonally demodulated by the orthogonal demodulator, and a Fourier transform performed by the Fourier transform unit. A Doppler frequency estimator that calculates a Doppler frequency from the pilot signal of the third signal, and the third signal Fourier-transformed by the Fourier transform unit according to the Doppler frequency calculated by the Doppler frequency estimator, etc. An inverse Fourier transform that generates an impulse response by performing an inverse Fourier transform on the pilot signal of the third signal Fourier-transformed by the Fourier transform unit. The maximum impulse response generated by the transform unit and the inverse Fourier transform unit. Based on the impulse response of the maximum position detected by the maximum position detector and the maximum position detected by the maximum position detector, the current position resulting from the difference in the frequency allocation of the pilot signal between the current symbol and the previous symbol A phase difference correction unit that corrects a phase difference between the impulse response of the symbol and the preceding symbol, and a phase rotation amount between the impulse response of the current symbol and the preceding symbol corrected by the phase difference correction unit are calculated. It has a phase deviation calculation part and a Doppler frequency calculation part which calculates a Doppler frequency based on the amount of phase rotation computed by the phase deviation calculation part.

  Since the moving speed can be estimated from the Doppler frequency, it is possible to equalize the signal according to the moving speed. Further, it is possible to prevent demodulation errors by correcting the phase difference caused by the difference in the frequency arrangement of the pilot signals.

(Reference technology)
FIG. 1 is a diagram illustrating a configuration example of a Doppler frequency estimation unit in a demodulation device. Due to the influence of mobile reception, the pilot signal undergoes phase rotation. The Doppler frequency can be estimated from the phase rotation amount. The Doppler frequency in OFDM is calculated from the phase rotation amount of the pilot signal.

  FIG. 2 is a diagram illustrating an OFDM frame structure. In an OFDM frame, all subcarriers (frequency) of one symbol are pilot signals, and data symbols are arranged in such a manner as to be sandwiched between such symbols.

  The pilot symbol memory unit 101 stores pilot symbols in a signal obtained by performing fast Fourier transform on a received signal. The inverse fast Fourier transform unit 102 obtains an impulse response by performing an inverse fast Fourier transform on the pilot signal supplied from the pilot symbol memory unit 101. The impulse response delay memory unit 103 holds an impulse response. The maximum position detection unit 104 detects the maximum position of the impulse response and outputs the position to the impulse response delay memory unit 103. The maximum position detection processing of the maximum position detection unit 104 makes it possible to use a signal having the highest S / N and is less susceptible to noise. The maximum value delay memory unit 105 holds the impulse response of the maximum position detected by the maximum position detection processing unit 104. The phase deviation calculation unit 106 calculates the maximum position in the impulse response of the current symbol held in the impulse response delay memory unit 103 and the maximum value of the impulse response in the previous symbol held in the maximum value delay memory unit 105. The phase rotation amount is calculated by the inner product calculation and is output to the Doppler frequency calculation unit 107. The Doppler frequency calculation unit 107 calculates the Doppler frequency from the phase rotation amount calculated by the phase deviation calculation unit 106 and outputs it.

  FIG. 3 is a diagram illustrating an arrangement of pilot signals in ISDB-T. The horizontal axis indicates the subcarrier (frequency axis), and the vertical axis indicates the symbol (time axis). Black circles indicate pilot signals (pilot symbols), and white circles indicate data signals (data symbols). The pilot signal is inserted once into 12 subcarriers in the subcarrier direction, and inserted in a form shifted by 3 subcarriers in the symbol direction. In the OFDM signal in ISDB-T, pilot signals called SP signals are inserted so as to be scattered in the respective directions of frequency and time. The arrangement of pilot signals in ISDB-T is different from the arrangement of pilot signals in the OFDM frame of FIG.

  In terrestrial digital broadcasting, a transmission path is estimated using a pilot signal, and equalization processing is performed on a received signal. There are a plurality of equalization processing methods, and the reception characteristics can be improved by adaptively switching the method according to the state of the receiving terminal such as mobile reception.

  Although there is a method using an external speedometer as a method for estimating the state of the receiving terminal, the moving speed of the receiving terminal is estimated by obtaining the Doppler frequency in the Doppler shift in which the carrier frequency shifts due to mobile reception. It is possible.

  Due to the influence of mobile reception, the pilot signal undergoes phase rotation. The Doppler frequency can be estimated from the phase rotation amount. The Doppler frequency can be calculated from the phase rotation amount of the pilot signal.

However, the Doppler frequency estimator in FIG. 1 is applied only to intervals of four symbols in which the pilot signal arrangement is the same in the case of the scattered pilot scheme adopted in ISDB-T, which is the terrestrial digital broadcasting standard in FIG. I can't do it. This is because there are phase differences Δφ1 to Δφ3 due to the arrangement of pilot signals as shown in FIG. 4 between the impulse responses i _n to i _n-4 of the symbols n to n-4 shown in FIG. is there. The phase differences Δφ1 to Δφ3 change according to the deviation of the fast Fourier transform point from the ideal sample point. In the case of estimation at intervals of four symbols, since the elapsed time between symbols becomes long, there is a problem that the calculation of the Doppler shift representing the speed of time variation is limited.

  Therefore, in order to solve the above-described problem, a technique capable of estimating the Doppler frequency at intervals shorter than 4 symbol intervals in the scattered pilot OFDM adopted in ISDB-T is as follows. The embodiment will be described.

(Embodiment)
FIG. 5 is a block diagram illustrating a configuration example of a demodulation device in the reception terminal according to the embodiment of the present invention. The antenna 500 receives the terrestrial digital broadcast signal of FIGS. 3 and 4 from the broadcast station. The tuner unit 501 selects a signal received via the antenna 500. The orthogonal demodulator 502 orthogonally demodulates the symbol unit signal to generate an I signal and a Q signal. A fast Fourier transform (FFT) unit 503 performs fast Fourier transform (time-frequency conversion) on the signal demodulated by the orthogonal demodulation unit 502. The Doppler frequency estimation unit 504 calculates a Doppler frequency from the pilot signal of the signal subjected to the fast Fourier transform by the fast Fourier transform unit 503. The transmission path equalization unit 505 equalizes the signal subjected to the fast Fourier transform by the fast Fourier transform unit 503 according to the Doppler frequency calculated by the Doppler frequency estimation unit 504. There are a plurality of methods (for example, four methods) in the equalization processing method of the transmission path equalization unit 505, and the reception characteristics can be improved by adaptively switching the method according to the Doppler frequency. The disturbance can be removed by the equalization process. The demapping unit 506 generates code points of the signal equalized by the transmission path equalization unit 505 by demapping. The error correction unit 507 corrects the error of the code point generated by the demapping unit 506 and outputs it to the display system.

  FIG. 6 is a block diagram illustrating a configuration example of the Doppler frequency estimation unit 504 in FIG. The Doppler frequency estimation unit 504 includes a pilot signal memory unit 601, an inverse fast Fourier transform (IFFT) unit 602, a first impulse response delay memory unit 603, a maximum position detection unit 604, a second impulse response delay memory unit 605, A phase difference correction unit 606, a phase deviation calculation unit 607, and a Doppler frequency calculation unit 608 are included.

  The pilot signal memory unit 601 receives the pilot signal in the signal subjected to the fast Fourier transform by the fast Fourier transform unit 503, holds the pilot signal, and outputs it to the inverse fast Fourier transform unit 602.

  The inverse fast Fourier transform unit 602 generates an impulse response by performing inverse fast Fourier transform (frequency-time conversion) on the pilot signal held by the pilot signal memory unit 601, and outputs the impulse response to the first impulse response delay memory unit 603. To do. By this processing, a delay profile as shown in FIG. 7 is obtained, and a reception wave that is composed of any combination of a main wave, a preceding wave, and a delay wave and forms a multipath is separated into respective paths.

  FIG. 8 is a diagram illustrating the relationship between the main wave, the delayed wave, and the fast Fourier transform (FFT) point. Ts is an effective symbol length, and Tg is a guard interval (GI). In order to avoid intersymbol interference, a redundant portion called a guard interval Tg obtained by copying the same signal as the latter half of the OFDM symbol is added before the effective symbol having the effective symbol length Ts. Thereby, an effective symbol can be appropriately cut out.

  The first impulse response delay memory unit 603 holds the impulse response generated by the inverse fast Fourier transform unit 602 and outputs the impulse response to the maximum position detection unit 604 and the second impulse response delay memory unit 605.

The maximum position detection unit 604 detects the maximum position among the impulse responses held by the first impulse response delay memory unit 603, and determines the position as the first impulse response delay memory unit 603 and the second impulse response. The response is output to the response delay memory unit 605. That is, the maximum position detection unit 604 separates the delayed wave component and the preceding wave component from the delay profile, and detects the main wave component as the maximum impulse response. By detecting the maximum position, the main wave component is found, and the maximum position has the highest S / N condition, so that the effect of noise is reduced. At this time, the maximum position detection unit 604 detects the maximum position based on the power (I ² + Q ² ) of the I signal and the Q signal.

  The first impulse response delay memory unit 603 receives the position detected by the maximum position detection unit 604, and outputs the impulse response value at the corresponding position to the phase deviation calculation unit 607.

  The second impulse response delay memory unit 605 stores the impulse response at the maximum position detected by the maximum position detection unit 604 in order to delay, and outputs the impulse response of the symbol before the current symbol. The number of impulse response symbols held by the second impulse response delay memory unit 605 depends on the symbol interval for obtaining the Doppler frequency. As an example, assuming that processing is performed at intervals of two symbols, the impulse response of the current symbol is held in the first impulse response delay memory unit 603, and the impulse responses before and after two symbols are the second impulse response delay. It is held in the memory unit 605. Therefore, the memory amount of the second impulse response delay memory unit 605 increases or decreases in accordance with the number of impulse response symbols held.

  The second impulse response delay memory unit 605 receives the maximum position detected by the maximum position detector 604, and in the held impulse response, the maximum position detector 604 according to the symbol interval for obtaining the Doppler frequency. The value of the impulse response corresponding to the input from is output to the phase deviation calculator 607. As an example, when obtaining the Doppler frequency at intervals of two symbols, the output of the first impulse response delay memory unit 603 is the value of the maximum position of the impulse response of the current symbol, and the second impulse response delay memory unit 605 As an output, the value of the impulse response two symbols before the current symbol input from the maximum position detector 604 is output.

The phase difference correction unit 606 receives the impulse response stored in the second impulse response delay memory unit 605 and corrects the phase difference caused by the difference in the frequency arrangement of the pilot signals in FIG. As an example, as shown in FIG. 9, the phase difference existing between the impulse response i _n-1 of the impulse response i _n and the symbol n-1 of the symbol n is the difference in frequency allocation of the pilot signals of FIG. 3 This is the sum of the resulting Δφ and the phase rotation amount Δθ due to fading (moving reception). The phase difference Δφ changes according to the deviation of the FFT point from the ideal sample point. The value of the phase difference Δφ corresponding to each shift is held in the phase difference table 901 or is calculated sequentially. The phase difference table 901 is provided in the phase difference correction unit 606 in FIG. 6 and outputs the phase difference Δφ according to the FFT point shift. The shift of the FFT point can be detected based on the guard interval Tg in FIG. The phase difference correction unit 606 removes the phase difference Δφ by multiplying the impulse response by the opposite phase (e ⁻ Δφ) of the phase difference Δφ caused by the pilot signal arrangement. As a result, the phase difference between the impulse responses i _n and i _n−1 is only the phase rotation amount Δθ due to fading.

  FIG. 10 is a diagram illustrating a configuration example of the phase difference correction unit 606 of FIG. The phase difference correction unit 606 includes a rotation matrix calculation unit 1006 and a phase difference correction value table unit 1007. In the phase difference correction value table unit 1007, a value necessary for phase difference correction is calculated and stored in advance from the phase difference corresponding to the shift of the FFT point. The phase difference correction value table unit 1007 outputs a value necessary for phase difference correction according to the FFT point shift. The rotation matrix calculation unit 1006 corrects the phase difference according to the value output from the phase difference correction value table unit 1007 and outputs the result to the phase deviation calculation unit 607.

  FIG. 11 is a diagram illustrating another configuration example of the phase difference correction unit 606 of FIG. The phase difference correction unit 606 includes a rotation matrix calculation unit 1006 and a phase difference correction value calculation unit 1008. The phase difference correction value calculation unit 1008 sequentially calculates values necessary for phase difference correction from the phase difference corresponding to the FFT point shift. The rotation matrix calculation unit 1006 corrects the phase difference according to the value calculated by the phase difference correction value calculation unit 1008 and outputs the result to the phase deviation calculation unit 607.

  As described above, the phase difference correction unit 606 is based on the difference in the frequency arrangement of the pilot signal between the current symbol and the previous symbol based on the impulse response at the maximum position detected by the maximum position detection unit 604. Correct the phase difference between the resulting current symbol and the impulse response of the previous symbol.

  Further, the phase difference correction unit 606 corrects the phase difference according to the deviation of the FFT point of the fast Fourier transform unit 503 from the ideal sample point. Further, the phase difference correction unit 606 corrects the phase difference according to a symbol interval (for example, two symbols) between the current symbol and the preceding symbol. That is, the content of the phase difference correction value table unit 1007 differs depending on whether the symbol interval between the current symbol and the preceding symbol is one symbol or two symbols, and the calculation method of the phase difference correction value calculation unit 1008 differs.

  The phase deviation calculation unit 607 receives the inputs from the first impulse response delay memory unit 604 and the phase difference correction unit 606, and calculates the phase rotation amount between the impulse responses of the current symbol and the preceding symbol. That is, the phase deviation calculation unit 607 calculates the phase rotation amount Δθ due to fading existing between impulse responses. The phase rotation amount Δθ may be calculated by obtaining the respective phases and calculating the difference, or by using an inner product calculation.

  The Doppler frequency calculation unit 608 calculates a Doppler frequency based on the phase rotation amount Δθ calculated by the phase deviation calculation unit 607 and outputs the calculated Doppler frequency to the transmission line equalization unit 505. Since the Doppler frequency is proportional to the phase rotation amount Δθ, if the phase rotation amount Δθ is known, the Doppler frequency can be calculated.

  The demodulator in FIG. 1 can only perform calculation at intervals of four symbols with which the pilot signals match, and therefore cannot cope with a case where the Doppler frequency is high. The calculation method of the Doppler frequency in the demodulator in the terrestrial digital broadcast receiving terminal of the present embodiment can calculate the Doppler frequency in a shorter section by correcting the phase difference caused by the frequency arrangement of the pilot signal, It is possible to calculate the Doppler frequency at the time of mobile reception at a higher speed than the demodulator of FIG. For example, when the interval between symbols calculated by the phase deviation calculation unit 607 is two symbols, it can handle mobile reception of about 400 km / h, and when it is one symbol, it can support mobile reception of about 800 km / h.

  The demodulating device (integrated circuit) of the present embodiment can be used effectively by being incorporated in a terrestrial digital broadcast receiving terminal for a mobile unit.

  As described above, in a digital terrestrial broadcast demodulator (LSI), when mobile reception is assumed, it is necessary to estimate the moving speed of the receiving terminal. The moving speed can be estimated from the Doppler shift amount that is a shift of the carrier frequency. Therefore, the Doppler frequency is calculated and equalization processing according to the Doppler frequency is performed.

  The received signal undergoes phase rotation due to the influence of Doppler shift. Therefore, if the phase rotation amount Δθ due to fading is known, the Doppler shift amount can be calculated. However, in the case of the scattered pilot system employed in terrestrial digital broadcasting, there is a limit to the Doppler frequency that can be calculated due to the difference in the frequency arrangement of the pilot signal. Therefore, this embodiment can calculate the Doppler frequency even in the case of high-speed movement by incorporating a mechanism for removing the influence due to the difference in frequency arrangement.

  The demodulator in FIG. 1 can estimate the Doppler frequency only in a limited section due to the difference in the frequency arrangement of the pilot signals in FIG. The demodulator according to the present embodiment corrects the phase difference due to the difference in the frequency arrangement of the pilot signals, thereby making it possible to estimate in any section, and to cope with high-speed mobile reception in which the Doppler frequency becomes high. Will be able to.

  Since the moving speed can be estimated from the Doppler frequency, it is possible to equalize the signal according to the moving speed. Further, it is possible to prevent demodulation errors by correcting the phase difference caused by the difference in the frequency arrangement of the pilot signals.

  The above-described embodiments are merely examples of implementation in carrying out the present invention, and the technical scope of the present invention should not be construed in a limited manner. That is, the present invention can be implemented in various forms without departing from the technical idea or the main features thereof.

It is a figure which shows the structural example of the Doppler frequency estimation part in a demodulation apparatus. It is a figure which shows an OFDM frame structure. It is a figure which shows arrangement | positioning of the pilot signal in ISDB-T. It is a figure which shows the phase difference by the difference in the frequency arrangement | positioning of a pilot signal. It is a block diagram which shows the structural example of the demodulation apparatus in the receiving terminal by embodiment of this invention. It is a block diagram which shows the structural example of the Doppler frequency estimation part of FIG. It is a figure which shows the delay profile by an impulse response. It is a figure which shows the relationship between a main wave, a delayed wave, and a fast Fourier transform (FFT) point. It is a figure which shows the correction method of the phase difference by the difference in the frequency arrangement | positioning of a pilot signal. It is a figure which shows the structural example of the phase difference correction | amendment part of FIG. It is a figure which shows the other structural example of the phase difference correction | amendment part of FIG.

Explanation of symbols

500 Antenna 501 Tuner unit 502 Orthogonal demodulation unit 503 Fast Fourier transform unit 504 Doppler frequency estimation unit 505 Transmission path equalization unit 506 Demapping unit 507 Error correction unit 601 Pilot signal memory unit 602 Inverse fast Fourier transform unit 603 First impulse response Delay memory unit 604 Maximum position detection unit 605 Second impulse response delay memory unit 606 Phase difference correction unit 607 Phase deviation calculation unit 608 Doppler frequency calculation unit

Claims (5)

An orthogonal demodulator that orthogonally demodulates the first signal in symbol units;
A Fourier transform unit for Fourier transforming the second signal orthogonally demodulated by the orthogonal demodulator;
A Doppler frequency estimation unit that calculates a Doppler frequency from a pilot signal of the third signal Fourier-transformed by the Fourier transform unit;
A transmission path equalization unit that equalizes the third signal Fourier-transformed by the Fourier transform unit according to the Doppler frequency calculated by the Doppler frequency estimation unit;
The Doppler frequency estimator is
An inverse Fourier transform unit that generates an impulse response by performing an inverse Fourier transform on the pilot signal of the third signal Fourier-transformed by the Fourier transform unit;
A maximum position detection unit for detecting a maximum position in the impulse response generated by the inverse Fourier transform unit;
Based on the impulse response at the maximum position detected by the maximum position detector, the impulse response of the current symbol and the previous symbol resulting from the difference in the frequency arrangement of the pilot signal between the current symbol and the previous symbol A phase difference correction unit that corrects the phase difference between,
A phase deviation calculation unit that calculates a phase rotation amount between the impulse response of the current symbol corrected by the phase difference correction unit and the previous symbol;
And a Doppler frequency calculation unit configured to calculate a Doppler frequency based on the phase rotation amount calculated by the phase deviation calculation unit.   2. The demodulator according to claim 1, wherein the maximum position detecting unit separates a delay wave component and a preceding wave component from a delay profile and detects a main wave component as the maximum impulse response.   The demodulator according to claim 1, wherein the phase difference correction unit corrects the phase difference according to a deviation of a Fourier transform point of the Fourier transform unit from a sample point.   The demodulation apparatus according to claim 1, wherein the phase difference correction unit corrects the phase difference according to a symbol interval between the current symbol and the preceding symbol. .   The Doppler frequency estimation unit includes a delay memory unit that stores an impulse response at a maximum position detected by the maximum position detection unit to delay and outputs an impulse response of the preceding symbol. The demodulator according to any one of claims 1 to 4.

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