JP2004297216A - Demodulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a demodulator for carrying out Viterbi decoding using the transmission path characteristics without increasing the circuit scale. <P>SOLUTION: A carrier demodulation circuit 20 of an OFDM receiver 10 calculates the power of a received signal on the basis of a transmission path characteristic signal. The carrier demodulation circuit 20 discriminates whether or not the power of the received signal is within a prescribed range, and replaces the value of a complex signal after demodulating the carrier with the origin on complex signals when the power of the received signal is considerably greater or smaller than a value in the prescribed range. The demodulated carrier signal outputted from the carrier demodulation circuit 20 is subjected to e.g., de-interleaving and thereafter transferred to a demapping circuit. The demapping circuit calculates bit metric from the demodulated carrier signal. In this case, the demapping circuit discriminates whether or not the demodulated carrier signal includes the origin, and replaces the bit metric at that part with a median when the demodulated carrier signal includes the origin, and allows a Viterbi decoding circuit to process the resulting demodulated carrier signal as a vanishing point. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a demodulation device for demodulating a convolutionally encoded transmission data sequence from an orthogonal modulation signal generated by digital orthogonal modulation.
[0002]
[Prior art]
As a method for modulating digital data, a modulation method called an orthogonal frequency division multiplexing (OFDM) is known.
[0003]
In the OFDM modulation method, a number of orthogonal sub-carriers (sub-carriers) are provided in a transmission band, and data is allocated to the amplitude and phase of each sub-carrier by PSK (Phase Shift Keying) or QAM (Quadrature Amplitude Modulation). This is a digital modulation method. The OFDM system divides the transmission band by a number of subcarriers, so the band per subcarrier wave becomes narrower and the modulation speed becomes slower, but the total transmission speed is the same as that of the conventional modulation system. are doing. Further, in the OFDM system, since a number of subcarriers are transmitted in parallel, the symbol rate is reduced, the time length of the multipath relative to the time length of the symbol can be shortened, and multipath interference is reduced. It has the feature of. In the OFDM system, data is allocated to a plurality of subcarriers. Therefore, an IFFT (Inverse Fast Fourier Transform) arithmetic circuit that performs inverse Fourier transform during modulation, and an FFT (Fast Fourier Transform) that performs Fourier transform during demodulation. 3.) It has a feature that a transmission / reception circuit can be configured by using an arithmetic circuit.
[0004]
The OFDM scheme is often applied to digital terrestrial broadcasting that is strongly affected by multipath interference. As terrestrial digital broadcasting employing the OFDM system, for example, ISDB-T _SB (Integrated Services Digital Broadcasting-Terrestrial Sound Broadcasting) is a standard (Non-Patent Document 1).
[0005]
ISDB-T _SB The standard specifies that convolutional coding with a constraint length of 7 is performed on transmission data as a transmission path coding method. Therefore, ISDB-T _SB An OFDM demodulator that conforms to the standard is provided with a maximum likelihood decoding circuit that decodes a convolutionally encoded data sequence. Note that a Viterbi decoding circuit is usually used for the maximum likelihood encoding circuit.
[0006]
Also, ISDB-T _SB According to the standard, there are two types of digital quadrature modulation schemes: a differential modulation scheme (DQPSK) that can perform carrier demodulation by delay detection and a synchronous modulation scheme (QPSK, 16QAM, 64QAM) that performs carrier demodulation by synchronous detection. It is prescribed that it can be adopted. Therefore, ISDB-T _SB In an OFDM demodulation device conforming to the standard, a carrier demodulation circuit provided with both a differential demodulation circuit for performing differential demodulation and an equalization circuit for performing equalization processing is provided.
[0007]
Hereinafter, ISDB-T _SB A conventional carrier carrier demodulation circuit provided in an OFDM demodulation device conforming to the standard will be described with reference to FIG.
[0008]
A demodulated signal (complex signal) after the FFT operation is input to the conventional carrier demodulation circuit 100. Hereinafter, this signal is referred to as an input signal.
[0009]
As shown in FIG. 5, a conventional carrier demodulation circuit 100 includes a differential demodulation circuit 101 for demodulating a carrier-modulated signal using a differential modulation scheme, and a demodulation circuit for demodulating a carrier-modulated signal using a synchronous modulation scheme. , A first selector 103, and a second selector 104.
[0010]
The differential demodulation circuit 101 has a one-symbol delay circuit 111, a complex division circuit 112, and a transmission path estimation circuit 113.
[0011]
One-symbol delay circuit 111 is a circuit that delays an input signal by one symbol. The complex division circuit 112 is a circuit that performs complex division on the input signal by the signal output from the one-symbol delay circuit 111 (that is, the input signal delayed by one symbol). That is, the complex division circuit 112 performs the complex division of the signal of the Nth symbol by the signal of the (N-1) th symbol. Since the differential demodulation circuit 101 has the one-symbol delay circuit 111 and the complex division circuit 112 as described above, the differential demodulation processing of the input signal can be performed. The differentially demodulated signal (carrier demodulated signal) output from the complex division circuit 112 is input to the first selector 103.
[0012]
The transmission path estimation circuit 113 is a circuit that calculates the propagation characteristics of the transmission path on which the OFDM signal has been transmitted. The transmission path estimation circuit 113 calculates, for example, the signal power of each signal point as the propagation characteristic of the transmission path. In the case of the differential modulation method, since the signal power of the transmission signal is always constant (that is, the amplitude is constant), the propagation characteristic of the transmission path is estimated by calculating the signal power of the reception signal. Can be. The calculated propagation characteristic of the transmission path (transmission path characteristic information) is input to the second selector 104.
[0013]
In the case of an OFDM signal, demodulated signals demodulated from a plurality of carriers are input. In the differential demodulation circuit 101, differential demodulation processing and transmission path characteristic information calculation processing are individually performed for each subcarrier. .
[0014]
The equalization circuit 102 includes a transmission channel estimation circuit 121, an N symbol delay circuit 122, and a complex division circuit 123.
[0015]
The transmission path estimation circuit 121 is a circuit that calculates the propagation characteristics of the transmission path through which the OFDM signal has been transmitted. ISDB-T _SB According to the standard, when transmitting a signal modulated by the synchronous modulation method (QPSK, 16QAM, 64QAM), a pilot signal called a scattered pilot signal (SP signal) which is a signal having a predetermined amplitude and a predetermined phase is transmitted in the frequency direction. And scatteredly inserted in the time direction. The transmission channel estimation circuit 121 extracts an SP signal from the signal after the FFT operation, detects the amplitude and phase of the extracted SP signal, and calculates a propagation characteristic for each subcarrier frequency based on the detected amplitude and phase. The propagation characteristic of the transmission path calculated by the transmission path estimation circuit 121 is represented by a complex signal. The propagation characteristic signal of the transmission path represented by the complex signal is input to the complex division circuit 123. In addition, the transmission path estimation circuit 121 also calculates a component represented by only the signal power component among the propagation characteristics of the transmission path represented by the complex signal. The propagation characteristic of the transmission path (transmission path characteristic information) expressed only by the signal power component is input to the second selector 104.
[0016]
The N-symbol delay circuit 122 is a circuit that delays an input signal by a delay time necessary for the transmission path estimation circuit 121 to calculate the propagation characteristics of the transmission path.
[0017]
The complex division circuit 123 is a circuit that performs complex division on the signal output from the N-symbol delay circuit 122 by the propagation characteristic signal (complex signal) of the transmission path calculated by the transmission path estimation circuit 121. As a result of performing the complex division, the amplitude and phase are corrected according to the propagation characteristics of the transmission path, and the input signal is subjected to equalization processing. The signal (carrier demodulated signal) output from the complex division circuit 123 is input to the first selector 103.
[0018]
In the case of an OFDM signal, demodulated signals demodulated from a plurality of carriers are input. In the equalization circuit 102, transmission path characteristic information calculation processing and equalization processing are individually performed for each subcarrier.
[0019]
The first selector 103 receives the carrier demodulation signal output from the differential demodulation circuit 101 and the carrier demodulation signal output from the equalization circuit 102, and converts one of the signals according to the modulation scheme selection signal. Select and output. The first selector 103 selects and outputs the carrier demodulated signal output from the differential demodulation circuit 101 in the case of the differential modulation method, and outputs the carrier demodulated signal in the case of the synchronous modulation method. The selected carrier demodulated signal is output.
[0020]
The second selector 104 receives the transmission path characteristic information output from the differential demodulation circuit 101 and the transmission path characteristic information output from the equalization circuit 102, and selects one of them according to the modulation scheme selection signal. Select and output a signal. The second selector 104 selects and outputs the transmission path characteristic information output from the differential demodulation circuit 101 in the case of the differential modulation method, and outputs the information from the equalization circuit 102 in the case of the synchronous modulation method. The selected transmission path characteristic information is selected and output.
[0021]
As described above, the carrier demodulation circuit 100 can perform differential demodulation processing when a differential modulation signal is input, and can perform equalization processing when a synchronous modulation signal is input.
[0022]
The carrier demodulated signal output from the carrier demodulation circuit 100 is subjected to demapping by a demapping circuit and re-allocated to a bit string. For example, ISDB-T _SB If so, reassignment processing is performed on bit strings corresponding to QPSK, 16QAM, and 64QAM. Then, the bit string after the re-allocation processing is subjected to the maximum likelihood decoding using the likelihood information, and the convolutional coded data string is decoded.
[0023]
The carrier demodulation circuit 100 performs carrier demodulation (differential demodulation or equalization processing) and also generates transmission line characteristic information. The transmission path characteristic information output from the carrier demodulation circuit 100 is used when calculating likelihood information at the time of maximum likelihood decoding (for example, at the time of Viterbi decoding). In a normal transmission-system demodulator such as an OFDM demodulator, a reliability is weighted with respect to a data sequence after the re-allocation processing with reference to transmission path characteristic information, and the weight is used as likelihood information. It is generally used (Patent Document 1).
[0024]
Note that the likelihood information for a bit is information indicating whether a certain bit is likely to be 0 or 1 by a numerical value. For example, as the likelihood information, an integer x of 0 to 14 is used, and when a certain demodulation bit has a high probability of being 0, x is set close to 0 (decreasing the value) and the probability that the bit is 1 is high. Sometimes, the probability information indicates that x is close to 14 (value is large). In the case of this example, x = 7 represents likelihood information that the probability that the bit is 0 is equal to the probability that the bit is 1, that is, it cannot be determined.
[0025]
However, the range of the value of the likelihood information and its meaning may be given in any manner depending on the convenience of implementation. For example, when the probability that the bit is 0 is high, x may be close to 14, or the range of x may be 0 to 256 instead of 0 to 14. However, when the amount of information increases, the bit width of the signal line is required accordingly, so that the circuit scale increases. Further, likelihood information can be added to not only one bit but also a bit string having a predetermined bit width.
[0026]
The likelihood information for such a bit (or a bit string having an arbitrary bit width) is hereinafter referred to as a bit metric, and when the probability that a bit is 1 is equal to the probability that a bit is 0 (or the generation of all bit strings). The value of the bit metric when the probabilities are equal) is hereinafter referred to as the median value of the bit metric.
[0027]
[Patent Document 1]
U.S. Pat. No. 5,134,635
[Non-patent document 1]
"Digital Terrestrial Audio Broadcasting Receiver Standard (ARIB STD-B30 Version 1.1)", Established on May 31, 2001, Revised on March 28, 2002, 1.1
[0028]
[Problems to be solved by the invention]
As described above, in a normal transmission-system demodulation device such as an OFDM demodulation device, demodulation is performed in the order of carrier demodulation, reallocation, and maximum likelihood decoding (for example, Viterbi decoding).
[0029]
However, in order to use the transmission path characteristic information generated during carrier demodulation for maximum likelihood decoding, the transmission path characteristic information and the demodulated signal must be transmitted to the subsequent stage in synchronization.
[0030]
However, a special processing circuit for the demodulated signal may be required between the carrier demodulation process and the re-allocation process according to the transmission standard. In such a case, it is necessary to transmit the transmission path characteristic information to the subsequent stage while delaying the transmission path characteristic information in synchronization with the delay of the demodulated signal, which increases the circuit scale of the demodulation circuit. For example, ISDB-T _SB The standard stipulates that a frequency deinterleave circuit, a time deinterleave circuit, and the like are inserted between the carrier demodulation process and the demapping circuit, and the transmission line characteristic information is also synchronized with these deinterleave processes. The demodulation circuit has to be large.
[0031]
Therefore, an object of the present invention is to provide a demodulation device capable of performing maximum likelihood decoding of a convolutional code using transmission path characteristics without increasing the circuit scale.
[0032]
[Means for Solving the Problems]
A demodulation device according to the present invention is a demodulation device that demodulates a convolutionally encoded transmission data sequence from an orthogonal modulation signal generated by digital orthogonal modulation, and is a complex signal obtained by orthogonally demodulating the orthogonal modulation signal. Orthogonal demodulation means for extracting a demodulated signal; carrier demodulation means for performing differential demodulation or equalization processing on the demodulated signal and calculating transmission path characteristics for each signal point of the demodulated signal; and The output demodulation signal is assigned to a bit string, and the demapping means for generating likelihood information indicating the existence probability of the assigned bit string, and convolution-encoded by performing maximum likelihood decoding with the likelihood information as an input. Convolutional code decoding means for decoding a transmission data sequence, wherein the carrier demodulation means performs demodulation of a demodulated signal based on the transmission path characteristics. Determining the reliability of each signal point, for a low signal point above reliability, and outputs by replacing the values of the signal point to a specific value on the complex coordinates.
[0033]
In the demodulation device, the reliability of each signal point of the demodulated signal is determined based on the transmission path characteristics, and for the signal point having the low reliability, the value of the signal point is set to a specific value on complex coordinates. Has been replaced. Therefore, erasure correction processing can be performed when the specific value is input during demapping.
[0034]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an ISDB-T according to an embodiment of the present invention will be described. _SB A standard OFDM receiver will be described.
[0035]
FIG. 1 shows a block diagram of an OFDM receiver 10 according to an embodiment of the present invention.
[0036]
As shown in FIG. 1, the OFDM receiver 10 includes an antenna 11, a tuner 12, a band-pass filter (BPF) 13, an A / D conversion circuit 14, a DC cancellation circuit 15, a digital quadrature demodulation circuit 16, , FFT operation circuit 17, frame detection circuit 18, synchronization circuit 19, carrier demodulation circuit 20, frequency deinterleave circuit 21, time deinterleave circuit 22, demapping circuit 23, and bit deinterleave circuit 24. , Depuncturing circuit 25, Viterbi decoding circuit 26, byte deinterleaving circuit 27, spreading signal removing circuit 28, transport stream generating circuit 29, RS decoding circuit 30, channel selecting circuit 32, transmission control information decoding And a circuit 31.
[0037]
The transmission wave transmitted from the OFDM transmission device is received by the antenna 11 of the OFDM reception device 10 and supplied to the tuner 12 as an RF signal.
[0038]
The RF signal received by the antenna 11 is frequency-converted into an IF signal by a tuner 12 including a multiplier 12a and a local oscillator 12b, and supplied to a BPF 13. The oscillation frequency of the received carrier signal oscillated from the local oscillator 12b is switched according to the channel selection signal supplied from the channel selection circuit 32.
[0039]
The IF signal output from the tuner 12 is filtered by the BPF 13 and digitized by the A / D conversion circuit 14. The DC component of the digitized IF signal is removed by a DC cancel circuit 15 and supplied to a digital quadrature demodulation circuit 16.
[0040]
The digital orthogonal demodulation circuit 16 orthogonally demodulates the digitized IF signal using a carrier signal of a predetermined frequency (carrier frequency), and outputs a baseband OFDM signal. As a result of orthogonal demodulation, the baseband OFDM signal becomes a complex signal composed of a real axis component (I channel signal) and an imaginary axis component (Q channel signal). The baseband OFDM signal output from the digital quadrature demodulation circuit 16 is supplied to an FFT operation circuit 17 and a synchronization circuit 19.
[0041]
The FFT operation circuit 17 performs an FFT operation on the baseband OFDM signal, and extracts and outputs a signal orthogonally modulated on each subcarrier. The FFT operation circuit 17 extracts a signal of an effective symbol length from one OFDM symbol, and performs an FFT operation on the extracted signal. That is, the FFT operation circuit 17 removes the signal of the guard interval length from one OFDM symbol and performs the FFT operation on the remaining signal.
[0042]
The signal modulated on each subcarrier extracted by the FFT operation circuit 17 is a complex signal composed of a real axis component (I channel signal) and an imaginary axis component (Q channel signal). The signal extracted by the FFT operation circuit 17 is supplied to a frame detection circuit 18, a transmission control information decoding circuit 31, a synchronization circuit 19, and a carrier demodulation circuit 20.
[0043]
The frame detection circuit 18 extracts a TMCC signal from a predetermined subcarrier of the signal demodulated by the FFT operation circuit 17, detects a synchronization signal from the TMCC signal, detects a boundary of the OFDM transmission frame, and detects a boundary of the detected frame. The position is supplied to the synchronization circuit 19 and the like.
[0044]
The synchronization circuit 19 uses a baseband OFDM signal, a signal modulated on each subcarrier after being demodulated by the FFT operation circuit 17, a boundary between OFDM symbols, a channel selection signal supplied from the channel selection circuit 32, and the like. Thus, the FFT operation circuit 17 performs various kinds of synchronous processing such as synchronous processing of the operation range of the FFT operation and its timing.
[0045]
The carrier demodulation circuit 20 is supplied with a demodulated signal from each subcarrier output from the FFT operation circuit 17 and performs carrier demodulation on the signal. Specifically, the carrier demodulation circuit 20 performs a differential demodulation process on the differential modulation signal (DQPSK) and an equalization process on the synchronous modulation signal (QPSK, 16QAM, 64QAM).
[0046]
The carrier-demodulated signal is subjected to a frequency deinterleaving process by a frequency deinterleaving circuit 21 and then to a time deinterleaving process by a time deinterleaving circuit 22, and then supplied to a demapping circuit 23. .
[0047]
The demapping circuit 23 performs a data reassignment process (demapping process) on the carrier-demodulated signal (complex signal) to restore the transmission data sequence. For example, ISDB-T _SB When demodulating a standard OFDM signal, the demapping circuit 23 performs a demapping process corresponding to QPSK, 16QAM or 64QAM.
[0048]
The transmission data sequence output from the demapping circuit 23 passes through a bit deinterleave circuit 24, a depuncture circuit 25, a Viterbi decoding circuit 26, a byte deinterleave circuit 27, and a spread signal elimination circuit 28, thereby dispersing the error of the multi-level symbol. Deinterleaving processing corresponding to bit interleaving, depuncturing processing corresponding to puncturing processing for reducing transmission bits, Viterbi decoding processing for decoding convolutionally coded bit strings, data An energy despreading process corresponding to the interleaving process and the energy spreading process is performed, and is input to the transport stream generation circuit 29.
[0049]
The transport stream generation circuit 29 inserts data specified by each broadcasting system, such as a null packet, at a predetermined position in the stream. Further, the transport stream generation circuit 29 performs a so-called smoothing process in which the bit intervals of the intermittently supplied streams are smoothed to obtain temporally continuous streams. The transmission data sequence subjected to the smoothing process is supplied to the RS decoding circuit 30.
[0050]
The RS decoding circuit 30 performs a Reed-Solomon decoding process on the input transmission data sequence and outputs it as a transport stream defined by MPEG-2 Systems.
[0051]
The transmission control information decoding circuit 31 receives the TMCC signal synchronized by the frame detection circuit 18, decodes the TMCC information (transmission control information) from the TMCC signal, and converts the decoded TMCC information into a carrier demodulation circuit. 20, a time deinterleave circuit 22, a demapping circuit 23, a bit deinterleave circuit 24, and a transport stream generation circuit 29 to control the demodulation and reproduction of each circuit.
[0052]
Next, the carrier demodulation circuit 20 will be further described.
[0053]
FIG. 2 shows a block diagram of the carrier demodulation circuit 20.
[0054]
The demodulated signal (complex signal) after the FFT operation is input from the FFT operation circuit 17 to the carrier demodulation circuit 20. Hereinafter, in the description of the carrier demodulation circuit 20, a signal output from the FFT operation circuit 17 is referred to as an input signal.
[0055]
As shown in FIG. 2, the carrier demodulation circuit 20 includes a differential demodulation circuit 41 for demodulating a carrier-modulated signal in a differential modulation scheme and a demodulation circuit for demodulating a carrier-modulated signal in a synchronous modulation scheme. The circuit includes a conversion circuit 42, a first selector 43, a second selector 44, a power calculation circuit 45, a comparison circuit 46, and a third selector 47.
[0056]
The differential demodulation circuit 41 has a one-symbol delay circuit 51 and a complex division circuit 52.
[0057]
One-symbol delay circuit 51 is a circuit that delays an input signal by one symbol. The complex division circuit 52 is a circuit that performs complex division on the input signal by the signal output from the one-symbol delay circuit 51 (that is, the input signal delayed by one symbol). That is, the complex division circuit 52 is a circuit that performs complex division on the signal of the N-th symbol by the signal of the (N-1) -th symbol (where N is an integer). Therefore, the differential demodulation circuit 41 outputs the phase difference between the (N-1) th symbol phase and the Nth symbol phase, and the differential demodulation circuit 41 performs differential demodulation. The signal (carrier demodulated signal) output from the complex division circuit 52 is input to the first selector 43.
[0058]
In the case of an OFDM signal, demodulated signals demodulated from a plurality of carriers are input, but the differential demodulation circuit 41 performs differential demodulation processing individually for each subcarrier.
[0059]
Further, the differential demodulation circuit 41 is provided with a transmission line that supplies the input signal as it is to the second selector 44 as transmission path estimation information.
[0060]
The equalization circuit 42 has a transmission channel estimation circuit 53, an N symbol delay circuit 54, and a complex division circuit 55.
[0061]
The transmission path estimation circuit 53 is a circuit that calculates the propagation characteristics of the transmission path through which the OFDM signal has been transmitted. ISDB-T _SB According to the standard, when transmitting a signal modulated by the synchronous modulation method (QPSK, 16QAM, 64QAM), a pilot signal called a scattered pilot signal (SP signal) which is a signal having a predetermined amplitude and a predetermined phase is transmitted in the frequency direction. And scatteredly inserted in the time direction. The transmission path estimation circuit 53 extracts the SP signal from the signal after the FFT operation, detects the amplitude and phase of the extracted SP signal, and calculates the propagation characteristics for each subcarrier frequency based on the detected amplitude and phase. Note that the propagation characteristic of the transmission path estimated by the transmission path estimation circuit 53 is represented by a complex signal. The propagation characteristic signal of the transmission path represented by the complex signal is input to the complex division circuit 55. Further, the transmission path estimation circuit 53 supplies the propagation characteristic of the transmission path represented by the complex signal to the second selector 44 as transmission path characteristic information.
[0062]
The N-symbol delay circuit 54 is a circuit that delays an input signal by a delay time required for the transmission path estimation circuit 53 to calculate the propagation characteristics of the transmission path.
[0063]
The complex division circuit 55 is a circuit that performs complex division on the signal output from the N symbol delay circuit 54 by the propagation characteristic signal (complex signal) of the transmission path estimated by the transmission path estimation circuit 53. As a result of performing the complex division, the amplitude and phase are corrected according to the propagation characteristics of the transmission path, and the input signal is subjected to equalization processing. The signal (carrier demodulated signal) output from the complex division circuit 55 is input to the first selector 43.
[0064]
In the case of an OFDM signal, demodulated signals demodulated from a plurality of carriers are input. In the equalization circuit 42, transmission path characteristic information calculation processing and equalization processing are individually performed for each subcarrier.
[0065]
The first selector 43 receives the carrier demodulation signal output from the differential demodulation circuit 41 and the carrier demodulation signal output from the equalization circuit 42, and converts one of the signals according to the modulation scheme selection signal. Select and output. The first selector 43 selects and outputs the carrier demodulated signal output from the differential demodulation circuit 41 in the case of the differential modulation method, and outputs the carrier demodulated signal in the case of the synchronous modulation method. The selected carrier demodulated signal is output.
[0066]
The second selector 44 receives the transmission path characteristic information output from the differential demodulation circuit 41 and the transmission path characteristic information output from the equalization circuit 42, and receives one of them according to the modulation scheme selection signal. Select and output a signal. The second selector 44 selects and outputs the transmission line characteristic information output from the differential demodulation circuit 41 in the case of the differential modulation method, and outputs the information from the equalization circuit 42 in the case of the synchronous modulation method. The selected transmission path characteristic information is selected and output.
[0067]
The power calculation circuit 45 receives the transmission path characteristic information (complex signal) output from the second selector 44 and performs carrier demodulation on the signal power of the received signal based on the real component level and the imaginary component level of the input signal. It is calculated for each signal point of the signal. The calculated power value is supplied to the comparison circuit 46.
[0068]
The three values of the power value, the upper threshold value, and the lower threshold value output from the power calculation circuit 45 are input to the comparison circuit 46. The upper threshold and the lower threshold are fixed values. The comparison circuit 46 determines whether the power value output from the power calculation circuit 45 has a value in a range between the upper threshold and the lower threshold or a value outside the range. If the comparison result indicates that the power value output from the power calculation circuit 45 has a value in the range between the upper threshold value and the lower threshold value, the signal of the power value has high reliability. On the other hand, if the power value is out of the range, it is determined that the reliability is low. The comparison circuit 46 sets the output flag to high when the signal reliability is high (that is, when the power value is within the range), and sets the output flag to high when the signal reliability is low (that is, when the power value is out of the range). Output flag is low. The output flag of the comparison circuit 46 is supplied to the third selector 47.
[0069]
The third selector 47 receives a specific value on the complex coordinates (hereinafter, this value is referred to as an erasure signal point) and the carrier demodulated signal output from the second selector 43, and receives a signal from the comparison circuit 46. Either value is selected and output according to the output flag that has been output. Specifically, the third selector 47 outputs the carrier demodulated signal when the output flag of the comparison circuit 46 is high (when the power value output from the power calculation circuit 45 is within the range between the upper threshold and the lower threshold). When the output flag of the comparison circuit 46 is low (when the power value output from the power calculation circuit 45 is out of the range between the upper threshold value and the lower threshold value), the lost signal point is selected. Output. That is, when the signal power of the received signal is equal to or greater than the upper threshold or equal to or less than the lower threshold, the third selector 47 specifies the value of the signal point of the carrier demodulated signal. (A lost signal point).
[0070]
Here, the value of the lost signal point is set on a complex coordinate which cannot be originally present as a carrier demodulated signal. For example, in all modulation schemes, an ISDB-T in which an allocated bit string is symmetrically arranged on the origin on complex coordinates. _SB In the case of a transmission system such as a standard, it is desirable to set the lost signal point to the origin {(I, Q) = (0, 0)} where the influence of noise is small.
[0071]
As described above, the carrier demodulation circuit 20 performs differential demodulation processing when a signal of the differential modulation scheme is input, and performs equalization processing when a signal of the synchronous modulation scheme is input, Generate a carrier demodulated signal. At the same time, the carrier demodulation circuit 20 detects the transmission path characteristic information (for example, the signal power of the received signal), and if there is a signal point whose transmission path characteristic is worse than a predetermined range, the corresponding signal of the carrier demodulation signal The points are replaced with vanishing signal points and output.
[0072]
Next, the demapping circuit 23 will be described.
[0073]
FIG. 3 shows a block diagram of the demapping circuit 23.
[0074]
The carrier demodulated signal output from the carrier demodulation circuit 20 is supplied to the demapping circuit 23 after being interleaved in the frequency direction and the time direction by the frequency deinterleave 21 and the time deinterleave 22.
[0075]
As shown in FIG. 3, the demapping circuit 23 includes a bit likelihood conversion circuit 61, a coincidence comparison circuit 62, and a selector 63.
[0076]
The carrier demodulation signal and the modulation scheme selection signal are input to the bit likelihood conversion circuit 61. The modulation scheme selection signal is a signal that specifies the carrier modulation scheme (QPSK, 16QAM or 64QAM) of the carrier demodulated signal. The bit likelihood conversion circuit 61 generates and outputs a bit metric from the input carrier demodulated signal (complex signal) with reference to the modulation scheme selection signal. The bit metric output from the bit likelihood conversion circuit 61 is supplied to the selector 63.
[0077]
The match comparison circuit 62 receives the carrier demodulated signal. The coincidence comparison circuit 62 determines whether or not the value of each signal point of the input carrier demodulated signal is an erasure signal point (for example, the origin), and determines the high / low of the erasure flag according to the determination result. Control. The coincidence comparison circuit 62 sets the erasure flag to high when the value of the signal point of the carrier demodulated signal is the erasure signal point (for example, the origin), and sets the erasure flag to low when the value is different.
The selector 63 receives the bit metric generated by the bit likelihood conversion circuit 61 and the median of the bit metric. The selector 63 selects and outputs either the bit metric generated by the bit likelihood conversion circuit 61 or the median of the bit metric according to the high / low of the erasure flag output from the coincidence comparison circuit 62. . Specifically, as shown in FIG. 4, the selector 63 outputs the bit metric generated by the bit likelihood conversion circuit 61 as it is when the erasure flag is low, and outputs the bit metric when the erasure flag is high as shown in FIG. The median value (for example, 7 if the bit metric range is 0 to 14) is output. That is, the selector 63 performs a process of replacing the value of the bit metric output from the bit likelihood conversion circuit 61 with the median value for the portion where the carrier demodulated signal has been the erasure signal point (for example, the origin).
[0078]
As described above, the demapping circuit 23 demaps the input carrier demodulated signal to generate a bit metric, and replaces the bit metric with the median when the value of the carrier demodulated signal matches the erasure signal point.
[0079]
The bit metric thus generated is transferred from the demapping circuit 23 to the Viterbi decoding circuit 26.
[0080]
The Viterbi decoding circuit 26 performs maximum likelihood decoding of the convolutional code based on the bit metric. When the median value of the bit metric is input, the Viterbi decoding circuit 26 performs the maximum likelihood decoding on the assumption that the probability that the bit is 1 is equal to the probability that the bit is 0. That is, when the median value of the bit metric is input, the Viterbi decoding circuit 26 performs the vanishing point correction processing.
[0081]
In addition, the demapping circuit 23 generates a bit metric having the same probability that the bit is 1 and the probability that the bit is 0 as the likelihood information, that is, generates a bit metric having the same probability for the bit string to be allocated. Any bit metric may be generated if possible.
[0082]
As described above, in the OFDM receiving apparatus 10 according to the embodiment of the present invention, the carrier demodulation circuit 20 detects the transmission path characteristic information and determines whether or not the transmission path characteristic is lower than a predetermined value based on the information. . When the transmission path characteristic is lower than a predetermined value, the carrier demodulation circuit 20 sets the signal point of the carrier demodulated signal obtained by performing the differential demodulation or the equalization process to a specific signal point (erased signal point). replace. Subsequently, the demapping circuit 23 detects a signal component of the carrier demodulated signal input from the carrier demodulation circuit 20 and, if the signal component is the above-mentioned lost signal point, changes the value of the bit metric of the signal component. Replace with the median. By performing such a process, the OFDM receiver 10 can cause the Viterbi decoding circuit 26 to perform the vanishing point correction process on a signal point having poor transmission path characteristics.
[0083]
For this reason, the OFDM receiver 10 does not provide a path for transferring the transmission path characteristic information between the carrier demodulation circuit 20 and the demapping circuit 23. Correction processing can be performed, and the circuit scale can be reduced.
[0084]
【The invention's effect】
In the demodulation device according to the present invention, for a signal point of a demodulated signal whose transmission path characteristic is worse than a predetermined value, at the time of demapping, by outputting likelihood information having the same probability for all bit strings to be allocated. , Causing the convolutional code decoding means to perform erasure correction.
[0085]
Further, in the demodulation device according to the present invention, the carrier demodulating means replaces the value of the signal point with a specific value of a complex domain for a signal point of a demodulated signal whose transmission path characteristic is lower than a predetermined value. , The demapping means judges whether the demodulated signal has a specific value in the complex area, and if the demodulated signal has the specific value, outputs likelihood information having the same probability for all bit strings to be allocated. This causes the convolutional code decoding means to perform erasure correction processing.
[0086]
For this reason, the demodulation device according to the present invention can perform erasure correction processing without transmitting the channel characteristics from the demapping unit to the convolutional code decoding unit. Therefore, since there is no need to provide a transfer circuit for transferring the transmission path characteristics, for example, even if various circuits are provided between the demapping unit and the convolutional code decoding unit, the circuit scale does not increase.
[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 block diagram of a carrier demodulation circuit in the OFDM receiver.
FIG. 3 is a block diagram of a demapping circuit in the OFDM receiver.
FIG. 4 is a timing chart showing processing timing of the demapping circuit.
FIG. 5 is a block diagram of a conventional carrier demodulation circuit.
[Explanation of symbols]
Reference Signs List 10 OFDM receiver, 11 antenna, 12 tuner, 13 bandpass filter, 14 A / D conversion circuit, 15 DC cancel circuit, 16 digital quadrature demodulation circuit, 17 FFT operation circuit, 18 frame detection circuit, 19 synchronization circuit, 20 carrier Demodulation circuit, 21 frequency deinterleaving circuit, 22 time deinterleaving circuit, 23 demapping circuit, 26 Viterbi decoding circuit, 27 byte deinterleaving circuit, 28 spreading signal removing circuit, 29 transport stream generating circuit, 30 RS decoding circuit, 31 Transmission control information decoding circuit

Claims (5)

In a demodulator that demodulates a convolutionally encoded transmission data sequence from a quadrature modulation signal generated by digital quadrature modulation,
Quadrature demodulation means for quadrature demodulating the quadrature modulated signal and extracting a demodulated signal that is a complex signal;
Carrier demodulation means for performing differential demodulation or equalization processing on the demodulated signal and calculating transmission path characteristics for each signal point of the demodulated signal,
Demapping means for allocating the demodulated signal output from the carrier demodulation means to a bit string, and generating likelihood information indicating the existence probability of the allocated bit string,
Convolutional code decoding means for decoding the convolutionally encoded transmission data sequence by performing maximum likelihood decoding with the likelihood information as an input,
The carrier demodulating means determines the reliability of each signal point of the demodulated signal based on the transmission path characteristics, and for a signal point having a low reliability, the value of the signal point is determined by a specific value on complex coordinates. A demodulation device characterized in that the value is output after being replaced with a value. The demapping means determines whether or not the value of the signal point of the demodulated signal is a specific value on the complex coordinates, and outputs a predetermined likelihood information when the value is the specific value. The demodulator according to claim 1, wherein 3. The demodulation device according to claim 2, wherein as the predetermined likelihood information, likelihood information having the same probability for all bit strings to be allocated is used. 2. The demodulator according to claim 1, wherein the specific value on the complex coordinates is an origin on the complex coordinates. The carrier demodulating means calculates a power value of each signal point of the demodulated signal as a transmission path characteristic, and when the power value is out of a predetermined power range, the reliability of the signal point of the corresponding demodulated signal is low. The demodulation device according to claim 1, wherein the determination is made as follows.

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