JP6022089B2 - Receiving apparatus and receiving method - Google Patents

Description

  The present invention relates to an orthogonal frequency division multiplexing (hereinafter abbreviated as OFDM) signal receiving apparatus and receiving method.

In general, when convolutionally encoded data is transmitted by OFDM, the receiving apparatus performs a Viterbi decoding process on the demodulated signal of each subcarrier obtained from the received signal.
Here, Viterbi decoding is a decoding method that efficiently performs maximum likelihood decoding using a repetitive structure of a convolutional code.

  For example, in the case of an OFDM signal in which QPSK (quadrature phase shift keying) or multilevel QAM (quadrature phase amplitude modulation) is used as the primary modulation method, first, the Viterbi decoder first corrects the phase and amplitude of the subcarrier component. The branch metric indicating the likelihood between the received point arrangement and the signal point arrangement that is uniquely determined depending on the modulation scheme is obtained. Then, all surviving paths of the possible trellis are obtained, the branch metrics of each path are cumulatively added, and the path with the smallest cumulative addition result is selected. The Viterbi decoder outputs the state of the selected path as a decoding result and reproduces the transmission data.

It is known that the error correction capability of the Viterbi decoder is improved by taking into account the reliability of the demodulated signal for each subcarrier, that is, the probability, when calculating the branch metric.
For this reason, in an OFDM signal receiving apparatus using a subcarrier modulation scheme such as QPSK or QAM, a specific method using reliability information for each subcarrier for branch metric calculation has already been proposed.

However, the reliability information described above is calculated based on the power information for each subcarrier obtained from the estimation result of the transmission path characteristics. For this reason, as in the case of a signal (hereinafter, referred to as a DQPSK-OFDM signal) that has been subjected to primary modulation with DQPSK (differential quadrature phase shift keying) and transmitted by OFDM, the convolutionally encoded data is demodulated. It cannot be applied to signals that do not estimate transmission path characteristics.
Further, when DQPSK is differentially demodulated, straight lines Q = I, Q = −I (on the IQ plane depending on abrupt time fluctuation of transmission path characteristics, AFC (automatic frequency control) error, phase noise, etc. A constellation of demodulated signal points may be inclined with respect to a straight line having a 45-degree relationship with the I and Q axes, and reliability information cannot be generated only with power information for each subcarrier.

For example, Patent Document 1 discloses a reception technique for solving the above-described problem. The receiving apparatus described in Patent Document 1 is a receiving apparatus that receives a DQPSK-OFDM signal, and performs Viterbi decoding processing. Therefore, a fast discrete Fourier transform (FFT) output of the received signal is converted into phase information, and adjacent symbols are detected. A phase soft decision circuit that calculates a soft decision value from the phase difference information of the two and a weighting coefficient that calculates a weighting coefficient based on the amplitude (or power, or a scalar value proportional to one of these) of the FFT output of two adjacent symbols And a coefficient generation circuit.
The output of the phase soft decision circuit is a demodulated signal of the DQPSK signal, and the Viterbi decoding is performed after multiplying the output of the phase soft decision circuit by the weighting coefficient for each subcarrier generated by the weighting coefficient generation circuit. At this time, the weighting coefficient represents the high reliability of the demodulated signal, and the higher the value, the higher the reliability.

In a frequency-selective fading transmission path, not only the C / N (carrier power to noise power ratio) differs for each subcarrier, but also, as described above, the demodulated signal is affected by time fluctuation of the transmission path characteristics, AFC error, phase noise, etc. Since the quality deteriorates, the performance of Viterbi decoding cannot be sufficiently obtained only by the demodulated signal.
On the other hand, according to the technique of Patent Document 1, since the Viterbi decoding is performed using the phase rotation and amplitude information of the demodulated signal, the reception performance is improved as compared with the soft decision decoding using only the phase information. Can do.

Japanese Patent Laid-Open No. 11-196141

On the other hand, in an actual radio wave environment, in addition to the above-described quality degradation of the demodulated signal, reception performance may be significantly degraded by narrow band noise superimposed on the signal band.
However, in the conventional technique represented by Patent Document 1, it is considered that the reliability of the demodulated signal of the subcarrier is higher as the amplitude of the demodulated signal (or power or a scalar value proportional to either of them) is larger. For this reason, the demodulated signal of the subcarrier whose power is increased by superimposing the narrow band noise is considered to be highly reliable, and there is a problem that the error correction capability of the Viterbi decoding process is lowered.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a receiving apparatus and a receiving method capable of improving the error correction capability in Viterbi decoding and improving the reception performance.

  A receiving apparatus according to the present invention is a receiving apparatus that receives an OFDM signal in which a subcarrier is DQPSK modulated, a Fourier transform section that performs discrete Fourier transform on the received signal for each OFDM symbol, and an output of the Fourier transform section A reference symbol extraction unit that extracts and outputs an output signal corresponding to a known reference symbol from the signal, and detects a narrowband noise component included in the reception signal based on the output signal of the reference symbol extraction unit, A narrowband noise detector that outputs the magnitude of the narrowband noise component, a differential demodulator that differentially demodulates the output signal of the Fourier transform unit and outputs a demodulated signal for each subcarrier, a differential demodulator, and a narrower Based on each output signal of the band noise detector, the signal of the demodulated signal is transmitted according to the information including the presence and the magnitude of the narrow band noise component. Sex comprises a reliability information generation unit for generating and outputting a reliability information for high and low, and a Viterbi decoding unit by using the reliability information Viterbi decoding is performed for the demodulated signal.

  According to the present invention, there is an effect that error correction capability in Viterbi decoding can be improved and reception performance can be improved.

It is a block diagram which shows the structure of the receiver which concerns on Embodiment 1 of this invention. 3 is a block diagram illustrating a configuration of a narrowband noise detection unit according to Embodiment 1. FIG. 3 is a block diagram illustrating a configuration of a reliability information generation unit according to Embodiment 1. FIG. 3 is a flowchart showing an operation of the receiving apparatus according to the first embodiment. It is a figure which shows the frequency spectrum of CIR (channel impact response) of a transmission line, and OFDM transmission / reception signal in case narrow-band noise exists. It is a block diagram which shows the structure of the reliability information generation part in Embodiment 2 of this invention. It is a block diagram which shows the structure of the reliability information generation part in Embodiment 3 of this invention. FIG. 10 is a block diagram showing another configuration of a reliability information generation unit in the third embodiment.

Hereinafter, in order to describe the present invention in more detail, modes for carrying out the present invention will be described with reference to the accompanying drawings.
Embodiment 1 FIG.
FIG. 1 is a block diagram showing a configuration of a receiving apparatus according to Embodiment 1 of the present invention. The receiving apparatus illustrated in FIG. 1 is a receiving apparatus that receives an OFDM signal in which subcarriers, which are OFDM carriers, are DQPSK modulated. The transmission side uses a convolutional code as an error correction code of transmission data, and transmits by an OFDM transmission scheme having a plurality of subcarriers using differential modulation such as DQPSK and π / 4 shift DQPSK as primary modulation.

The Fourier transform unit 1 performs discrete Fourier transform on the received signal S1 converted into the baseband band for each OFDM symbol, and outputs the result. Each subcarrier component transmitted by the OFDM method is obtained as a frequency domain signal output from the Fourier transform unit 1.
The differential demodulator 2 differentially demodulates the output signal of the Fourier transform unit 1 and outputs a demodulated signal for each subcarrier. The demodulation process is performed by complex multiplication of the complex conjugate signal of the subcarrier component of the previous OFDM symbol and the subcarrier component (complex signal) of the current OFDM symbol.
Note that the output signal of the differential demodulator 2 is a demodulated signal of the transmission signal transmitted on each subcarrier.

  The reference symbol extraction unit 3 extracts a Fourier transform output signal corresponding to the reference symbol from the output signal of the Fourier transform unit 1 and outputs it. The reference symbol is a known signal that serves as a phase reference for the subcarrier. For example, in the case of DAB (Digital Audio Broadcasting), the reference symbol is inserted into the transmission signal at a constant period. The narrowband noise detection unit 4 detects a narrowband noise component included in the received signal based on the output signal of the reference symbol extraction unit 3, and outputs the magnitude of the narrowband noise component for each subcarrier.

  Based on the output signals of the differential demodulator 2 and the narrowband noise detector 4, the reliability information generator 5 determines the reliability of the demodulated signal according to the information including the presence or absence of the narrowband noise component and the magnitude thereof. Generate and output high and low reliability information. For example, a value obtained by multiplying the distance between the straight line Q = I or Q = -I on the IQ plane and the signal point of the demodulated signal by a predetermined constant and the detected value of the narrowband noise component from the instantaneous power value of the demodulated signal The subtracted value is used as reliability information. In this case, the reliability information is considered to have a higher reliability of the demodulated signal as its value is larger, and is used for branch metric calculation by the Viterbi decoding unit 6.

  The Viterbi decoder 6 obtains a branch metric indicating the likelihood of the demodulated signal based on the output signals of the differential demodulator 2 and the reliability information generator 5, performs Viterbi decoding on the demodulated signal, and Viterbi-decoded data S2 is output. For example, the Viterbi decoder 6 obtains a branch metric indicating the likelihood of the demodulated signal based on the demodulated signal (I-Q signal) from the differential demodulator 2 and the reliability information from the reliability information generator 5. . Further, the Viterbi decoding unit 6 obtains all the surviving paths of the possible trellises, and reproduces the transmission data by selecting the path with the smallest result of accumulating the branch metrics of the respective paths.

  The Fourier transform unit 1, the differential demodulation unit 2, the reference symbol extraction unit 3, the narrowband noise detection unit 4, the reliability information generation unit 5, and the Viterbi decoding unit 6 can be realized as a hardware circuit. In addition, the above-described components 1 to 6 can be realized as specific means in which hardware and software cooperate by, for example, a microcomputer executing a program in which processing unique to the present invention is described. it can.

FIG. 2 is a block diagram showing the configuration of the narrowband noise detector in the first embodiment. As shown in FIG. 2, the narrowband noise detection unit 4 includes a CIR detection unit 41, an LPF unit 42, a noise component extraction unit 43, a determination threshold calculation unit 44, and a narrowband noise determination unit 45.
The CIR detector 41 detects and outputs the CIR (channel impulse response) of each subcarrier by dividing the output signal of the reference symbol extractor 3 by the known reference symbol value corresponding thereto.

  The LPF unit 42 smoothes and outputs the output signal of the CIR detection unit 41 using a low-pass filter. The pass band of the low-pass filter has a bandwidth necessary for passing the reflected wave component generated in the transmission path, that is, a bandwidth through which the reflected wave component generated in the transmission path can pass. By adopting such a pass band, the CIR can be appropriately smoothed, and a narrow band noise component can be accurately detected.

  The noise component extraction unit 43 extracts and outputs a noise component superimposed on the subcarrier based on the output signals of the CIR detection unit 41 and the LPF unit 42. For example, the CIR signal of each subcarrier output from the CIR detection unit 41 is delayed by a signal delay (for example, about 200 μs) generated in the LPF unit 42, and the CIR signal smoothed by the LPF unit 42 from the delayed CIR signal Is subtracted. This subtraction result can be regarded as a noise component superimposed on each subcarrier. By calculating the instantaneous power of this subtraction result, the noise power superimposed on the subcarrier can be obtained.

The determination threshold calculation unit 44 inputs the noise power for each subcarrier obtained by the noise component extraction unit 43, calculates an average value thereof, and calculates a determination threshold based on the average value.
Examples of the determination threshold include a value obtained by adding or multiplying a predetermined value to the average value. Note that, in a transmission line in which no narrowband noise exists, the noise component is mainly thermal noise, and the component is white noise that is distributed almost uniformly over the entire signal band. In this case, it can be considered that the output signal of the noise component extraction unit 43 is substantially constant regardless of the subcarrier, and the value thereof is close to the average value calculated as described above.
On the other hand, when not only thermal noise but also narrow band noise is superimposed, the noise power of the subcarrier is increased by the narrow band noise component. Therefore, it is possible to determine the presence or absence of narrowband noise by using the determination threshold value calculated based on the average value.

The narrowband noise determination unit 45 determines the presence or absence of narrowband noise for each subcarrier based on the result of comparing the output signal of the noise component extraction unit 43 with the determination threshold calculated by the determination threshold calculation unit 44. The presence / absence of the component and its size are converted into binary or multi-value information and output.
For example, the binary information indicating the presence / absence and the magnitude of the narrowband noise component outputs a predetermined value when it is determined that narrowband noise is present, and is 0 when it is determined that narrowband noise is not present. (Zero value) is output. When outputting as multi-level information, for example, a value proportional to the magnitude of the power is output if narrowband noise is present, and 0 (zero value) is output if no narrowband noise is present. In any case, the larger the output value of the narrowband noise determination unit 45, the larger the narrowband noise power.

FIG. 3 is a block diagram showing a configuration of the reliability information generation unit in the first embodiment. As shown in FIG. 3, the reliability information generation unit 5 includes a subcarrier power detection unit 50, a signal point quadrant conversion unit 51, and a reliability information calculation unit 52.
The subcarrier power detection unit 50 demodulates each subcarrier based on the Q signal which is the in-phase component and the quadrature component of the demodulated signal (hereinafter also referred to as IQ signal) output from the differential demodulation unit 2. Generate and output signal power or a signal proportional to it.

The signal point quadrant converter 51 receives the IQ signal from the differential demodulator 2, converts all quadrants having signal points on the IQ plane to the first quadrant, and outputs the first quadrant.
The reliability information calculation unit 52 demodulates each subcarrier based on the output signal of the subcarrier power detection unit 50, the output signal of the signal point quadrant conversion unit 51, the output signal of the narrowband noise detection unit 4, and the fixed coefficient k. Reliability information that increases or decreases the reliability of the demodulated signal is calculated according to the power of the signal, the constellation of the demodulated signal, and the magnitude of the narrowband noise component. Note that the reliability information calculated by the reliability information calculation unit 52 is used to generate a branch metric by the Viterbi decoding unit 6 at the subsequent stage.

Next, the operation will be described.
FIG. 4 is a flowchart showing an operation of the receiving apparatus according to Embodiment 1. For the configuration of the receiving apparatus according to Embodiment 1, reference is made to FIGS.
First, the Fourier transform unit 1 performs discrete Fourier transform on the received signal S1 for each OFDM symbol (step ST1). Here, among the output signals of the Fourier transform unit 1, the Fourier transform output signal corresponding to the reference symbol is extracted by the reference symbol extraction unit 3 and output to the narrowband noise detection unit 4 (step ST2; YES). On the other hand, the differential demodulator 2 performs processing on the Fourier transform output signal corresponding to other than the reference symbol among the output signals of the Fourier transform unit 1 (step ST2; NO).

In the narrowband noise detection unit 4, the CIR detection unit 41 divides the output signal of the reference symbol extraction unit 3 by a known reference symbol value corresponding to the output signal to detect the CIR for each subcarrier (step ST3). Here, C (n) is a CIR corresponding to the nth subcarrier in the OFDM symbol.
Subsequently, the LPF unit 42 of the narrowband noise detection unit 4 performs smoothing by performing a low-pass filter (LPF) process on C (n) detected by the CIR detection unit 41 (step ST4). Let the CIR of the nth subcarrier smoothed in this way be L (n).

  Next, the noise component extraction unit 43 extracts the narrowband noise component superimposed on the subcarrier based on the output signals of the CIR detection unit 41 and the LPF unit 42 (step ST5). For example, the CIR signal C (n) of the nth subcarrier output from the CIR detection unit 41 is delayed by the signal delay generated in the LPF unit 42. Then, by subtracting the CIR signal L (n) of the nth subcarrier smoothed by the LPF unit 42 from the delayed CIR signal C (n), it is superimposed on the nth subcarrier. Noise component Nz (n) (power value) is obtained. Note that the noise component extraction processing is executed for all subcarriers.

  Next, the determination threshold calculation unit 44 sequentially inputs the noise component Nz (n) for each subcarrier extracted by the noise component extraction unit 43 to calculate an average value, and calculates the determination threshold T based on the average value. (Step ST6). The determination threshold T is, for example, a value obtained by adding or multiplying a predetermined value to the average value.

The narrowband noise determination unit 45 determines the presence or absence of narrowband noise for each subcarrier based on whether or not the noise component Nz (n) is greater than the determination threshold T (step ST7).
When the noise component Nz (n) is larger than the determination threshold T, that is, when a narrowband noise component is detected (step ST7; YES), the narrowband noise determination unit 45 has a binary value indicating the magnitude of the narrowband noise component. Information or multi-value information is output (step ST8).

In addition, when the noise component Nz (n) is equal to or less than the determination threshold T, that is, when the narrowband noise component is not detected (step ST7; NO), the narrowband noise determination unit 45 has binary information indicating that there is no narrowband noise component. Alternatively, multi-value information is output (step ST9).
For example, the narrowband noise determination unit 45 outputs Z (n) indicating the magnitude of the narrowband noise component superimposed on the nth subcarrier. When narrowband noise is detected, Z (n) is greater than 0 (zero value), and when no narrowband noise is detected, Z (n) is 0 (zero value).

  The differential demodulator 2 differentially demodulates the Fourier transform output signals corresponding to those other than the reference symbol among the output signals of the Fourier transform unit 1, and outputs a demodulated signal for each subcarrier (step ST10). The demodulation process is a process of performing complex multiplication of the complex conjugate signal of the subcarrier component of the previous OFDM symbol and the subcarrier component (complex signal) of the current OFDM symbol. Here, the in-phase component I (n) and the quadrature component Q (n) of the demodulated signal of the nth subcarrier are obtained.

Next, in the reliability information generation unit 5, the subcarrier power detection unit 50 determines each subcarrier based on the in-phase component I (n) and the quadrature component Q (n) of the demodulated signal output from the differential demodulation unit 2. The demodulated signal power or P (n) proportional thereto is detected (step ST11).
P (n) is, for example, a value obtained by adding the square value of the quadrature component Q (n) to the square value of the in-phase component I (n). Since the input signal of the subcarrier power detection unit 50 is a differential demodulation result by the differential demodulation unit 2, P (n) does not strictly represent the power of the demodulated signal of the subcarrier. However, since the demodulated signal by differential demodulation is a signal obtained by complex multiplication between adjacent OFDM symbols, P (n) is a value proportional to the power of the demodulated signal of the subcarrier.

The signal point quadrant conversion unit 51 receives the demodulated signal (IQ signal) output from the differential demodulation unit 2, and converts all quadrants having signal points on the IQ plane into the first quadrant (step S1). ST12).
For example, when the signal point of the demodulated signal of the input nth subcarrier exists in the first quadrant of the IQ plane, the in-phase component I (n) and the quadrature component Q (n) of the input demodulated signal are Output as is. That is, the converted signal points (Id (n), Qd (n)) are set to Id (n) = I (n) and Qd (n) = Q (n).
When the signal point of the demodulated signal exists in the second quadrant of the IQ plane, the converted signal point (instead of the in-phase component I (n) and quadrature component Q (n) of the demodulated signal ( Id (n), Qd (n)) is output as Id (n) = − I (n), Qd (n) = Q (n).
Further, when the signal point of the demodulated signal exists in the third quadrant of the IQ plane, the converted signal point (instead of the in-phase component I (n) and quadrature component Q (n) of the demodulated signal ( Id (n), Qd (n)) is output as Id (n) = − I (n), Qd (n) = − Q (n).
Further, when the signal point of the demodulated signal exists in the fourth quadrant of the IQ plane, the converted signal point (instead of the in-phase component I (n) and quadrature component Q (n) of the demodulated signal ( Id (n), Qd (n)) is output as Id (n) = I (n), Qd (n) = − Q (n).

  Next, the reliability information calculation unit 52 outputs the demodulated signal points (Id (n), Qd (n)) converted into the first quadrant by the signal point quadrant conversion unit 51 and the output signal Z of the narrowband noise detection unit 4. Based on (n), a distance D (n) between the straight line Q = I on the IQ plane and the demodulated signal point is calculated (step ST13). In the above description, the signal point of the demodulated signal is converted into the first quadrant. However, the signal point in the second quadrant or the fourth quadrant is converted into the distance from the straight line Q = −I to D (n ).

Next, the reliability information calculation unit 52 calculates power information P (n) of the demodulated signal of the nth subcarrier, a straight line Q = I on the IQ plane, and a demodulated signal point of the nth subcarrier. And the detected value Z (n) of the narrowband noise superimposed on the nth subcarrier and the positive fixed coefficient k, the following formula (1) is used to calculate the nth subcarrier: The reliability information R (n) of the demodulated signal is calculated (step ST14). Thus, the reliability information R (n) is a value that increases or decreases depending on the presence or absence of the narrowband noise component superimposed on the subcarrier and its magnitude.
R (n) = P (n) −k × D (n) −Z (n) (1)

Thereafter, the Viterbi decoding unit 6 receives the demodulated signal of the n-th subcarrier input from the differential demodulation unit 2 (I (based on the reliability information R (n) input from the reliability information generation unit 5). n) and branch metrics indicating the likelihood of Q (n)) are obtained, Viterbi decoding is performed on the demodulated signal, and Viterbi-decoded data S2 is output (step ST15).
When performing error correction by viterbi decoding, the conventional receiver performs viterbi decoding using the power value of the demodulated demodulated signal and phase error information which is the slope of the constellation of the demodulated signal in the IQ plane. It was.
In contrast, according to the present invention, Viterbi decoding is performed in consideration of the presence and magnitude of narrowband noise superimposed on the signal band, in addition to the power value of the demodulated signal and the slope of the constellation of the demodulated signal. . For this reason, the error correction capability for the DQPSK-OFDM signal is improved, and the reception performance can be improved.

  In OFDM transmission, as shown in FIG. 5A, the transmission side assigns a plurality of subcarriers to transmission data, and each subcarrier is digitally modulated by a method such as DQPSK. When the transmission path is affected by multipath or frequency selective fading, as shown in FIG. 5B, the signal power for each subcarrier differs depending on the frequency of the subcarrier.

If the phase of the demodulated signal is not deviated from the original phase (π / 4, 3π / 4, −3π / 4, −π / 4), there is a special channel that has the same channel interference at a specific frequency. Except for the radio wave environment, it can be said that the higher the subcarrier signal power is, the less susceptible to thermal noise, the higher the reliability of the demodulated signal.
However, when narrowband noise is superimposed on the signal band, the power of the noise component is also added to the signal power of the subcarrier as shown in FIG. Not necessarily.

In DQPSK, the ideal signal point of the demodulated signal exists on the straight line Q = I or Q = −I on the IQ plane, but information is superimposed on the phase instead of the amplitude, and so on, as in QPSK. It is not possible to define four ideal signal points.
In this case, it can be said that the C / N is relatively better when the signal power of the subcarrier is larger, but even if the power is the same, the demodulated signal point becomes closer to the straight line Q = I or Q = −I. Reliability increases.

  Furthermore, the constellation of the demodulated signal that has been differentially demodulated may be tilted on the IQ plane due to time fluctuation or frequency error of the transmission path. In this case, it becomes more difficult to evaluate the reliability only with the power of the demodulated signal, but even if the demodulated signal has the same power value, the demodulated signal point is closer to the straight line Q = I or Q = −I. Reliability increases.

The above equation (1) takes into account the above characteristics in the OFDM signal in which the subcarrier is DQPSK modulated, and means that the greater the calculated value of the above equation (1), the higher the reliability of the demodulated signal. .
For example, the larger the subcarrier power information P (n), the larger the R (n) value and the higher the reliability. Further, if the demodulated signal of the subcarrier exists on the straight line Q = I or Q = −I which is an ideal signal point, D (n) = 0 and R (n) becomes a large value.
On the other hand, even if the demodulated signal constellation is tilted on the IQ plane, the closer the demodulated signal point is to the straight line Q = I or Q = −I, the smaller D (n) becomes, and R (n) becomes a larger value. Become.
Furthermore, since the detection value Z (n) of the narrowband noise component superimposed on the subcarrier is a value according to the presence / absence of the noise component and its magnitude, if there is no noise component and Z (n) is zero, , R (n) becomes a large value, and when narrow band noise exists and Z (n) increases, R (n) decreases accordingly.

As described above, according to the first embodiment, the Fourier transform unit 1 that outputs the received signal by performing discrete Fourier transform for each OFDM symbol, and the output corresponding to a known reference symbol from the output signal of the Fourier transform unit 1 A reference symbol extraction unit 3 that extracts and outputs a signal; a narrowband noise component included in the received signal is detected based on an output signal of the reference symbol extraction unit 3; A narrowband noise detector 4 for outputting a magnitude, a differential demodulator 2 for differentially demodulating the output signal of the Fourier transform unit 1 and outputting a demodulated signal for each subcarrier, a differential demodulator 2 and a narrowband Based on each output signal of the noise detection unit 4, reliability information is generated so that the reliability of the demodulated signal is high or low according to information including the presence or absence and the magnitude of the narrowband noise component. It comprises a reliability information generating unit 5 to and output, and a Viterbi decoding unit 6 for performing Viterbi decoding of the demodulated signal using the reliability information.
With this configuration, it is possible to generate reliability information in consideration of the presence / absence and the size of the narrowband noise component superimposed on the subcarrier. By performing Viterbi decoding using this reliability information, the influence of narrowband noise in the demodulated signal is suppressed. For this reason, the error correction capability in Viterbi decoding is improved and the reception performance can be improved.

  Further, according to the first embodiment, the narrowband noise detection unit 4 obtains the transmission path CIR signal C (n) obtained by dividing the output signal corresponding to the reference symbol by a known value and the CIR signal. It is determined that a narrowband noise component exists in a frequency band in which the difference value from the signal L (n) smoothed by the LPF is greater than a predetermined determination threshold T. In this way, it is possible to accurately detect the narrowband noise superimposed on the subcarrier.

  Furthermore, according to the first embodiment, since the pass band of the low-pass filter has a bandwidth that allows the reflected wave component generated in the transmission path to pass, the CIR can be appropriately smoothed and the narrow-band noise component is obtained. Can be accurately detected.

  Further, according to the first embodiment, the reliability information generation unit 5 performs the demodulation signal power information P (n), the Q = I or Q = −I straight line on the IQ plane, and the signal point of the demodulation signal. Reliability information in which the reliability of the demodulated signal is high or low is generated in accordance with the distance D (n), the presence / absence of a narrowband noise component, and the magnitude Z (n). This enables Viterbi decoding using reliability information corresponding to Z (n) in addition to P (n) and D (n). Even if the demodulated signal constellation is tilted, the reliability of the demodulated signal can be appropriately represented by D (n).

Embodiment 2. FIG.
FIG. 6 is a block diagram showing a configuration of the reliability information generation unit according to the second embodiment of the present invention. As shown in FIG. 6, in the reliability information generation unit 5A, an intra-symbol averaging unit 53 is added to the configuration of the first embodiment, and instead of the reliability information calculation unit 52, an intra-symbol averaging unit A reliability information calculation unit 52A that calculates reliability information using 53 information is provided. In FIG. 6, the same components as those in FIG.

When the demodulated signal point (Id (n), Qd (n)) converted into the first quadrant by the signal point quadrant converting unit 51 is input to the intra-symbol averaging unit 53, the demodulated signal point and the straight line Q = I Are averaged within one OFDM symbol, and an average value D ′ (n) is output.
The reliability information calculation unit 52A uses the power information P (n) of the demodulated signal, the average value D ′ (n), the presence / absence and the magnitude Z (n) of the narrowband noise component, and the positive fixed coefficient k. The reliability information R (n) for calculating the reliability of the demodulated signal is calculated from the following equation (2).
R (n) = P (n) −k × D ′ (n) −Z (n) (2)

  In the above description, the signal point of the demodulated signal is converted to the first quadrant. However, the distance from the straight line Q = −I is converted to D (n) by converting the signal point to the second quadrant or the fourth quadrant. And an average value of these values in the OFDM symbol may be used.

As described above, according to the second embodiment, the reliability information generating unit 5A determines that the demodulated signal power information P (n), the straight line Q = I or Q = −I on the IQ plane, and the demodulated signal Reliability information in which the reliability of the demodulated signal is high or low is generated according to the average value D ′ (n) for each OFDM symbol of the distance to the signal point, the presence / absence of a narrowband noise component, and the magnitude Z (n). .
In this way, it is possible to generate reliability information in consideration of a phase rotation component common to all subcarriers in the demodulated signal, and the accuracy of the reliability information is improved as compared with the first embodiment. Can be made.

Embodiment 3 FIG.
FIG. 7 is a block diagram showing a configuration of the reliability information generation unit according to Embodiment 3 of the present invention. As shown in FIG. 7, the reliability information generation unit 5B has a phase error detection unit 54 and a variation coefficient conversion unit 55 added to the configuration of the second embodiment, and instead of the reliability information calculation unit 52A, A reliability information calculation unit 52B that calculates reliability information using information from the variation coefficient conversion unit 55 is provided. In FIG. 7, the same components as those in FIGS. 3 and 6 are denoted by the same reference numerals, and redundant description is omitted.

Based on the signal points (I (n), Q (n)) of the demodulated signal input from the differential demodulator 2, the phase error detector 54 calculates the phase error θ (n) of the signal point on the IQ plane. Calculate, average within 1 OFDM symbol, and output. Note that θ (n) may be calculated from the arc tangent based on the signal points (I (n), Q (n)), or may be substituted with Q (n) / I (n) as an approximate value. .
The value obtained by averaging θ (n) within one OFDM symbol is a value proportional to the average phase rotation amount in the differential demodulated signal of this OFDM symbol.

The variation coefficient converting unit 55 calculates a positive variation coefficient m multiplied by the distance D (n) between the straight line Q = I or Q = −I on the IQ plane and the signal point of the demodulated signal as an average in the demodulated signal. The larger the value proportional to the amount of phase rotation, the larger the value.
The reliability information calculation unit 52B uses the power information P (n) of the demodulated signal, the average value D ′ (n), the presence / absence and the magnitude Z (n) of the narrowband noise component, and the variation coefficient m as follows: From (3), the reliability information R (n) for calculating the reliability of the demodulated signal is calculated.
R (n) = P (n) −m × D ′ (n) −Z (n) (3)

  In the above description, the signal point of the demodulated signal is converted to the first quadrant. However, the distance from the straight line Q = −I is converted to D (n) by converting the signal point to the second quadrant or the fourth quadrant. And an average value of these values in the OFDM symbol may be used.

Further, as shown in FIG. 8, the reliability information calculation unit 52C performs power information P (n) of the demodulated signal, distance D (n), presence / absence of a narrowband noise component, its magnitude Z (n), and a variation coefficient. Using m, the reliability information R (n) that makes the demodulated signal highly reliable may be calculated from the following equation (4).
R (n) = P (n) −m × D (n) −Z (n) (4)

  As described above, according to the third embodiment, the reliability information generating unit 5B determines that the demodulated signal power information P (n), the straight line Q = I or Q = −I on the IQ plane, and the demodulated signal A value m × D ′ (n) obtained by changing the average value of each distance from the signal point for each OFDM symbol according to the slope of the constellation of the demodulated signal, the presence / absence of a narrowband noise component, and its magnitude Z (n) Accordingly, reliability information in which the reliability of the demodulated signal is high or low is generated. With this configuration, it is possible to obtain reliability information that more accurately reflects the phase offset of the demodulated signal, that is, the constellation slope, than in the first and second embodiments.

  Further, according to the third embodiment, the reliability information generation unit 5C performs the demodulation signal power information P (n), the Q = I or Q = −I straight line on the IQ plane, and the signal point of the demodulation signal. The reliability of the demodulated signal increases or decreases depending on the value m × D (n) obtained by changing the distance to the constellation of the demodulated signal, the presence / absence of the narrowband noise component, and the magnitude Z (n) Generate sex information. Even with this configuration, the same effect as described above can be obtained.

  In the present invention, within the scope of the invention, any combination of each embodiment, any component of each embodiment can be modified, or any component can be omitted in each embodiment. .

  The receiving apparatus according to the present invention is suitable for, for example, a receiver for terrestrial digital broadcasting using the OFDM method, since error correction capability in Viterbi decoding can be improved and reception performance can be improved.

  1 Fourier transform unit, 2 differential demodulation unit, 3 reference symbol extraction unit, 4 narrowband noise detection unit, 5, 5A, 5B, 5C reliability information generation unit, 6 Viterbi decoding unit, 41 CIR detection unit, 42 LPF unit , 43 Noise component extraction unit, 44 Determination threshold calculation unit, 45 Narrow band noise determination unit, 50 Subcarrier power detection unit, 51 Signal point quadrant conversion unit, 52, 52A, 52B, 52C Reliability information calculation unit, 53 Within symbol An averaging unit, 54 a phase error detection unit, 55 a variation coefficient conversion unit.

Claims (8)

A receiving device for receiving an OFDM signal whose subcarrier is DQPSK modulated,
A Fourier transform unit that outputs a received signal by discrete Fourier transform for each OFDM symbol; and
A reference symbol extraction unit that extracts and outputs an output signal corresponding to a reference symbol from the output signal of the Fourier transform unit;
A narrowband noise detection unit that detects a narrowband noise component included in the received signal based on an output signal of the reference symbol extraction unit, and outputs the presence / absence and the magnitude of the narrowband noise component for each subcarrier; ,
A differential demodulator that differentially demodulates the output signal of the Fourier transform unit and outputs a demodulated signal for each subcarrier;
Based on the respective output signals of the differential demodulator and the narrowband noise detector, reliability information that makes the demodulated signal highly reliable according to information including the presence or absence of the narrowband noise component and its magnitude A reliability information generator to generate;
And a Viterbi decoding unit that performs Viterbi decoding on the demodulated signal using the reliability information.   The narrowband noise detector includes a channel impulse response signal of a transmission path obtained by dividing an output signal corresponding to the reference symbol by a known value, and a signal obtained by smoothing the channel impulse response signal with a low-pass filter, The receiving apparatus according to claim 1, wherein it is determined that the narrowband noise component is present in a frequency band in which a difference value of is greater than a predetermined threshold.   The receiving apparatus according to claim 2, wherein a pass band of the low-pass filter has a bandwidth through which a reflected wave component generated in the transmission path can pass.   The reliability information generation unit determines power information of the demodulated signal, a distance between a Q = I or Q = -I line on the IQ plane and a signal point of the demodulated signal, and a size of the narrowband noise component. The receiving apparatus according to claim 1, wherein reliability information is generated in response to the reliability of the demodulated signal.   The reliability information generation unit includes power information of the demodulated signal, an average value for each OFDM symbol of a distance between a straight line Q = I or Q = -I on the IQ plane and a signal point of the demodulated signal, the narrowness The receiving apparatus according to claim 1, wherein reliability information is generated in which reliability of the demodulated signal is high or low according to presence / absence of a band noise component and its magnitude.   The reliability information generation unit uses the power information of the demodulated signal, the distance between the Q = I or Q = -I line on the IQ plane and the signal point of the demodulated signal as the slope of the constellation of the demodulated signal. The receiving apparatus according to claim 1, wherein reliability information that increases or decreases reliability of the demodulated signal is generated in accordance with a value varied according to the presence or absence and a magnitude of the narrowband noise component.   The reliability information generation unit demodulates the power information of the demodulated signal, the average value for each OFDM symbol of the distance between the straight line Q = I or Q = -I on the IQ plane and the signal point of the demodulated signal. The reliability information in which the reliability of the demodulated signal is high or low is generated according to a value changed according to a slope of a signal constellation, presence / absence of the narrowband noise component, and a magnitude thereof. The receiving device according to 1. An OFDM signal reception method in which subcarriers are DQPSK modulated,
A Fourier transform unit for performing discrete Fourier transform on the received signal for each OFDM symbol and outputting the received signal;
A reference symbol extracting unit extracting and outputting a Fourier transform output signal corresponding to a known reference symbol from the output of the Fourier transform unit; and
A narrowband noise detection unit detects a narrowband noise component included in the received signal based on the output signal of the reference symbol extraction unit, and outputs the presence / absence and the magnitude of the narrowband noise component for each subcarrier And steps to
A differential demodulator that differentially demodulates an output signal of the Fourier transform unit and outputs a demodulated signal for each subcarrier;
A reliability information generator configured to determine the reliability of the demodulated signal according to information including the presence or absence of the narrowband noise component and the magnitude thereof based on the output signals of the differential demodulator and the narrowband noise detector; Generating and outputting reliability information that is high and low,
A reception method comprising: a Viterbi decoding unit performing Viterbi decoding on the demodulated signal using the reliability information.

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