JP2006174218A - Ofdm reception apparatus and ofdm reception method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an interference removal method and reception apparatus therefor in which reception is enabled even if a great CW interference comes so that no peak appears in a guard correlation waveform. <P>SOLUTION: An OFDM reception apparatus of the present invention included an interference detection circuit for detecting an interference in a frequency domain after FFT, and a notch filter is applied at a position where an interference frequency of a signal of a time base domain is present. The notch filter varies a frequency of a notch, and the position of the notch is made variable in accordance with the interference frequency after the FFT. If a signal is greater than average power by a predetermined value or more, the interference detection circuit determines an interference. The application position of the notch filter is applied to branching for branching data before the FFT and performing synchronizing processing on a time base and is not applied to branching for FFT inputting. An application bandwidth of the notch filter is made variable in accordance with a magnitude of interference and two or more notch filter are disposed in series for a case where a number of interferences exist. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an orthogonal frequency division multiplexing signal decoding method and circuit used for digital broadcasting by an orthogonal frequency division multiplexing (OFDM) transmission method, and more particularly to a technique for improving reception performance in OFDM demodulation.

  Following terrestrial digital broadcasting in Europe and the United States, terrestrial digital broadcasting was started in Japan in three major metropolitan areas. The digital terrestrial broadcasting system in Japan and Europe adopts the OFDM (Orthogonal Frequency Division Multiplexing Multiplex) system.

  The OFDM transmission method is a method in which transmission data is distributed and transmitted over a plurality of carriers. Since the symbol period length is longer than that of single carrier transmission, it is known as a multipath interference strong system. In addition, in the OFDM transmission method, a redundancy period called a guard period is generally provided before an effective symbol period during which data is actually transmitted, and the influence of multipath delay waves can be further reduced. .

  As a method of detecting a frequency error in an OFDM receiver, a method using the guard period is known. The frequency error detection method using this guard period will be described below.

  FIG. 13 is a block diagram showing a configuration of a conventional OFDM receiver. This OFDM receiver is described in Patent Document 1. In FIG. 13, an OFDM signal received by an antenna 301 is input to a tuner 302, which selects an OFDM signal of a predetermined channel and converts it to an IF (intermediate frequency) band. The output of the tuner 302 is supplied to an A / D converter 303 and converted into a digital signal by the A / D converter 303.

  The output of the A / D converter 303 is supplied to the IQ demodulation circuit 304. The IQ demodulation circuit 304 performs quadrature detection, and the output of the A / D converter 303 is quasi-synchronized quadrature detection and converted into a complex baseband signal.

  The complex baseband signal output from the IQ demodulation circuit 304 is supplied to the FFT circuit 305. The FFT circuit 305 converts the complex baseband signal from data on the time axis to data on the frequency axis by an FFT operation. The output of the FFT circuit 305 is supplied to the equalization circuit 306, and the equalization circuit 306 demodulates the data of each carrier. The output of the equalization circuit 306 is supplied to the error correction circuit 307, and the error correction circuit 307 performs error correction decoding processing.

  The signal output from the IQ demodulation circuit 304 is branched and supplied to the correlation detection circuit 309 and the effective period delay circuit 308. The signal delayed by the effective period delay circuit 308 is supplied to the correlation detection circuit 309, and the correlation detection circuit 309 detects the correlation between the signal in the rear part of the effective symbol period and the signal in the guard period that is a duplicate.

  Next, the correlation detection operation in the correlation detection circuit 309 will be described in detail with reference to FIGS. 14 (a) to 14 (d).

  FIG. 14A shows a complex baseband signal output from the IQ demodulation circuit 304, and FIG. 14B shows an effective period delay signal output from the effective period delay circuit 308. Hereinafter, the complex baseband signal output from the IQ demodulation circuit 304 is referred to as an IQ demodulated signal. One symbol period of the IQ demodulated signal shown in FIG. 14A includes a guard period and an effective symbol period for transmitting data. In the guard period, the rear signal of the effective symbol period is copied. Therefore, when the complex conjugate correlation between the IQ demodulated signal and the effective period delayed signal is obtained, a large correlation appears at the portion where both coincide, as shown in FIG. The figure shows only the I component of the correlation signal.

  Furthermore, when the correlation signal shown in FIG. 14C is moving averaged with a guard period width, a correlation signal having a peak at the beginning of the effective symbol period is obtained as shown in FIG. 14D. When there is no frequency error in the received OFDM signal, the peak of the correlation signal in the guard period appears only in the I component, and the Q component becomes almost zero. The signal shown in FIG. 14D is the output of the correlation detection circuit 309.

  The output of the correlation detection circuit 309 is supplied to the timing detection circuit 310, and the symbol detection position is determined by the timing detection circuit 310. The output of the timing detection circuit 310 is supplied to the FFT circuit 305, and the FFT circuit 305 performs FFT on the signal for the effective period length from this symbol cut-out position.

The output of the correlation detection circuit 309 is supplied to the tan ^-1 circuit 311. The tan ⁻¹ circuit 311 obtains the phase angle tan ⁻¹ (Q / I) from the I component and Q component of the correlation signal in this guard period. When there is no frequency error in the received OFDM signal, the signal in the rear part of the effective symbol period of the IQ demodulated signal matches the signal in the guard period, so that as shown in FIGS. 14 (a) to 14 (d), the correlation signal The phase angle of becomes zero. That is, the output of the tan ⁻¹ circuit 311 is zero.

When there is a frequency error in the received OFDM signal, a phase shift corresponding to the frequency error occurs between the signal in the latter part of the effective symbol period of the IQ demodulated signal and the signal in the guard period. The size of the corner is a value proportional to the frequency error. Therefore, the output of the tan ^-1 circuit 311 is an error signal proportional to the frequency error.

The error signal output from the tan ^-1 circuit 311 is supplied to the frequency control circuit 312. The frequency control circuit 312 integrates the supplied error signal by applying a gain coefficient, and generates a control signal for controlling the detection frequency of the IQ demodulation circuit 304. The control signal output from the frequency control circuit 312 is supplied to the IQ demodulation circuit 304, and the IQ demodulation circuit 304 controls the detection frequency so as to achieve frequency synchronization according to the control signal. As a result, frequency synchronization processing is performed on the received OFDM signal. The above is the frequency synchronization method using the correlation signal in the guard period.

  However, when there is interference such as CW interference, the output of the correlation detection circuit 309 is a combination of the correlation signal when there is no interference wave and the correlation signal due to the interference wave. For this reason, even if the frequency error of the received OFDM signal is zero, the phase angle between the I component and the Q component of the correlation signal does not become zero, and a correct frequency error cannot be detected.

  For such interference, for example, Patent Document 2 performs an operation of interference wave detection and correction. FIG. 15 is a block diagram showing a configuration of the OFDM receiver. FIG. 16A to FIG. 16E are diagrams illustrating operations of interference wave detection and correction in the OFDM receiver.

  The output of the correlation detection circuit 309 branches and is supplied to the offset detection circuit 313. The operation of the offset detection circuit 313 will be described in detail with reference to FIGS. 16 (c) to 16 (f). When there is an interference wave such as CW interference, the correlation signal between the IQ demodulated signal and the effective period delay signal is obtained by adding the correlation component of the interference wave to the correlation component when there is no interference wave, for example, FIG. ), The entire signal is a signal deviated by a certain amount from “0”. That is, the correlation signal in the effective period when there is no interfering wave is “0”, but when there is an interfering wave, a signal corresponding to the magnitude of the interfering wave is added to the entire correlation signal, as shown in FIG. As shown in c) and FIG. 16D, the signal deviates from “0” by a certain amount. The shift at this time is called an offset. The offset amount indicates the amount of deviation caused by the interference wave, and refers to the difference between the correlation signals when the interference wave does not exist and when it exists.

  The offset detection circuit 313 applies a timing signal other than the guard period (within the valid period) to each of the I component and Q component of the correlation signal according to the timing signal as shown in FIG. ) Of the correlation signal in a predetermined period T. When there is no interference, the average value of the correlation signal in the predetermined period T is almost 0. Therefore, the average value of the correlation signal is the I component and Q component (offset amount) of the correlation signal caused by the interference wave, respectively. Indicates. The output of the offset detection circuit 21 (the signal shown in FIG. 16F) is supplied to the correction circuit 314. The correction circuit 314 corrects the offset amount for each of the I component and Q component of the correlation signal in the guard period using the signal shown in FIG. In this offset amount correction, the I component of the correlation signal in the guard period is subtracted from the I component of the signal shown in FIG. 16F at the timing of the signal shown in FIG. From this, the amount of deviation caused by the interference wave is removed. Similar processing is performed for the Q component.

  The above operation is repeated for each symbol to remove the influence of the interference wave. In Patent Document 3, the output of the FFT unit 405 in FIG. 17 indicates the phase and amplitude of each carrier of the OFDM signal and is supplied to the equalization circuit 406. The equalization circuit 406 performs demodulation processing by synchronous detection on the input OFDM signal in accordance with the modulation method. Here, synchronous detection uses a scattered pilot signal inserted at a rate of 1/3 in the frequency direction and 1/4 in the time direction to detect an error signal of each carrier, and perform amplitude equalization and phase detection. Equalization is performed.

  In synchronous detection, first, SPs are arranged in a 4-symbol period in the received OFDM signal, so that a 4-carrier SP reference signal is obtained with a 4-symbol SP signal. Therefore, the reference signals of all carriers are obtained by interpolating these in the frequency direction. The demodulated result is output from the output terminal after an error occurring during transmission is corrected by the error correction unit 407.

  On the other hand, the output of the FFT unit 405 is also input to the interference detection unit 408. The interference detection unit 408 determines the carrier affected by frequency selective interference (spurious, multipath, and co-channel interference) by determining the state of the received pilot signal. Is output to the equalization circuit 406, the error correction unit 407, and the synchronous reproduction unit 409 to improve the demodulation performance.

That is, the equalization circuit 406 detects the error signal of each carrier using the scattered pilot signal at the time of synchronous detection, and performs amplitude equalization and phase equalization. This is not used when the frequency of the scattered pilot signal matches the frequency of the scattered pilot signal, and the demodulation performance is improved by detecting the error signal from the signal interpolated by the scattered pilot signal that is not affected by interference. Make improvements. Further, the error correction unit 407 performs weighting processing such as erasure correction on the carrier information affected by the interference.
Japanese Patent No. 3074103 JP 2002-290371 A Japanese Patent No. 3363806

  However, the method described in Patent Document 2 discloses a method of correcting the offset when the guard correlation waveform is shifted by an offset, and the peak of the guard correlation waveform does not appear. There is no disclosure about the case where there is a large CW disturbance.

  Further, Patent Document 3 has the same disturbance detection position as the method disclosed in the present invention, but is different from the object of the present invention.

  An object of the present invention is to provide a disturbance removal method and a receiving apparatus thereof that can receive even when a large CW disturbance that does not cause a peak in a guard correlation waveform is included.

  In order to solve the above problems, an OFDM receiving apparatus according to the present invention includes an effective period in which a data signal is transmitted, and an OFDM composed of a guard period obtained by copying a signal at the rear of the effective period to the front of the effective period. An FFT circuit that receives a signal, performs symbol synchronization processing in the time domain, and performs FFT processing on the OFDM signal, an interference detection circuit that detects interference in the frequency domain after FFT, and an interference frequency of the signal in the time domain A notch filter is applied to a position where the The notch filter can change the frequency of the notch, and the interference detection circuit determines that the interference is present when the signal is larger than the average power by a predetermined power or more.

  Further, the notch filter is applied to a branch that branches data before FFT and performs synchronization processing on a time axis.

  The applicable bandwidth of the notch filter may be varied according to the magnitude of interference.

  Further, in preparation for a case where there are a large number of disturbances, the notch filter comprises a multi-stage notch filter arranged in series of two or more.

  As described above, according to the present invention, the CW interference is removed by applying the notch filter to the position where the CW interference exists, and the guard correlation process is performed using the signal after removing the CW interference.

  Therefore, an object of the present invention is to provide a disturbance removal method and a receiving apparatus thereof that can receive even when a large CW disturbance that does not cause a peak in the guard correlation waveform is entered in normal processing. .

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Embodiment 1)
FIG. 1 is a block diagram according to Embodiment 1 of the present invention. A signal received by the antenna 100 selects a predetermined channel by the tuner 101 and is A / D converted by the A / D converter 102. Thereafter, quadrature detection is performed by the quadrature detection circuit 103, frequency shift is performed by the frequency shifter 104, and a frequency error is removed. The FFT circuit 105 converts the signal in the time axis domain into a signal in the frequency domain. The signal after the FFT is equalized by the equalization circuit 106, and then error correction processing is performed by the error correction circuit 107.

  The signal after the FFT is detected by a broadband AFC circuit 108 for a carrier unit frequency error and added to the time axis processing circuit 109.

  Further, the signal after the FFT is branched and input to the interference detection circuit 110, and the interference detection circuit 110 detects CW interference. The notch of the variable notch filter is controlled by the output of the interference detection circuit 110 so as to be set at the interference presence position. This variable notch filter is applied to the signal branched from the output of the frequency shifter 104, and the signal from which the interference is removed by the variable notch filter is used to perform symbol synchronization, narrow band AFC, and clock recovery in the time axis processing circuit. . Details of the time axis processing circuit are shown in FIGS. Since this part is conventional, it will not be described in detail herein.

  FIG. 4 shows the detection of disturbance. CW interference is narrow band and extremely high power. Accordingly, a certain threshold is set, and if it is equal to or higher than this threshold, it is determined as an obstruction and the position is detected.

  In the present embodiment, the power of each carrier is obtained, the OFDM band is divided into about 10, the average of the power in the band is obtained, and interference is present in the band where the average power is prominent among the divided bands. Judge that it exists. The detection of this interference can be determined that there is interference, for example, when there is power that protrudes 20 dB or more from the average level. Even in this case, if it is determined that the interference has not been effectively removed, the same determination is performed by increasing the number of band divisions. Further, the detection level of interference is lowered from 20 dB to 10 dB so that small interference is detected.

  Even if the synchronization cannot be obtained properly due to the presence of interference, a large interference appears in the vicinity of the position where the interference exists in the waveform after the FFT. Therefore, the position of the disturbance can be easily detected.

  In addition, since the position where the notch filter is inserted is a position for synchronization processing in the time axis region, distortion due to the notch filter does not appear in the data processing system. Further, since the position where the notch filter is applied is determined in the data processing system, the position of the disturbance can always be detected even after the notch filter is applied.

  Next, FIG. 6 shows a general DC blocking filter. As shown in FIG. 7, the input signal is rotated by the input of the filter, and after the filtering process, the rotation is reversed, so that a notch filter is equivalently obtained. By changing this rotational speed, the frequency position where the notch is inserted can be controlled. Further, when this FIG. 7 is deformed, it can be simplified as shown in FIG. In the present embodiment, the notch filter of FIG. 5 is used. The characteristics are shown in FIG. In the rotation operator exp (jωnt) in FIG. 5, ωn indicates the position of the notch, and the position of the notch can be controlled by changing ωn. The transfer function is expressed by the following equation.

  The shape of the notch filter can be changed by changing the value of k.

  FIG. 8 shows filter characteristics when two types of k values are changed. Therefore, at the initial pull-in time when the synchronization is unstable, the value of k is increased to perform initial synchronization, and then the value of k is decreased to remove only unnecessary components.

  Even when the synchronization is stable, the shape of the notch filter is changed according to the width of the interference signal.

  If the value of k is increased, the overall attenuation level is increased, so that the passband is normalized as 0 dB in the figure.

(Embodiment 2)
In the present embodiment, there are two CW interferences. This embodiment is shown in FIG. The difference from the first embodiment is that two CW interferences can be removed. Specifically, the variable notch filter 113 is added. The interference detection circuit 112 also detects two interferences. As for detection, as shown in FIG. 10, a threshold is provided for each carrier as in the first embodiment, and detection is performed with power.

  FIG. 11 shows the notch filter used. A single notch filter is arranged in series. FIG. 12 shows the combined filter characteristics of two notch filters.

  In the present embodiment, two variable notch filters are used. However, when there are a large number of interference waves, it is only necessary to connect the variable notch filters in multiple stages.

(Embodiment 3)
This embodiment is shown in FIG. This embodiment is an example in which the position of disturbance detection is after the FFT circuit 105 and after the equalization circuit 106 in the first embodiment. The present embodiment can be applied to a received signal having a relatively small disturbance and a level that can be synchronized.

  In the present embodiment, the disturbance is detected by monitoring the fluctuation of the SP signal transmitted every four symbols at the same carrier position, and applying a variable notch filter on the assumption that the disturbance is present in a portion where the fluctuation is large. .

(Embodiment 4)
This embodiment is an example when co-channel interference exists. Beyond the service area, analog co-channel interference may enter the digital broadcast band. In the present embodiment, notch filters are fixedly set at the video carrier position and the audio carrier position. In the present embodiment, the position of one notch filter is fixed at the video carrier position and the other notch filter is fixed at the audio carrier position in the second embodiment.

  In analog broadcasting, the offset is set to either 0 or ± 10 kHz depending on the transmitting station, but since the notch width of the notch filter is relatively wide, the offset of the FFT circuit 105 is assumed to be 0. It can be fixedly applied to later signals. It is also possible to turn on / off application of the notch filter by providing a disturbance detection circuit.

  The OFDM receiving apparatus according to the present invention relates to an orthogonal frequency division multiplexing signal decoding method and circuit used for digital broadcasting by an orthogonal frequency division multiplexing (OFDM) transmission system, and is particularly useful as a receiving apparatus for improving reception performance in OFDM demodulation. .

The block block diagram which shows one embodiment of this invention Block diagram showing one form of time axis processing circuit Block diagram showing one form of time axis processing circuit Diagram explaining how to detect interference Configuration diagram of a notch filter used in an embodiment of the present invention Diagram showing a typical DC blocking filter Configuration diagram of a notch filter used in an embodiment of the present invention The figure which shows the characteristic of the notch filter used by one embodiment of this invention The figure which shows the concept of one embodiment of this invention Diagram explaining how to detect interference Configuration diagram of a notch filter used in an embodiment of the present invention The figure which shows the characteristic of the notch filter used by one embodiment of this invention Block configuration diagram showing the configuration of a conventional OFDM receiver The figure explaining operation | movement of the conventional OFDM receiver Block configuration diagram showing the configuration of a conventional OFDM receiver The figure explaining operation | movement of the conventional OFDM receiver Block configuration diagram showing the configuration of a conventional OFDM receiver The block block diagram which shows one embodiment of this invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Antenna part 101 Tuner part 102 A / D converter 103 Quadrature detection circuit 104 Multiplier 105 FFT circuit 106 Equalization circuit 107 Error correction circuit 108 Broadband AFC
109 Time axis processing circuit 110 Interference detection circuit 111 Variable notch filter 112 Interference detection circuit 113 Variable notch filter 114 Interference detection circuit 115 Multistage notch filter 301 Antenna 302 Tuner 303 A / D converter 304 IQ demodulation circuit 305 FFT circuit 306 Equalization circuit 307 Error correction circuit 308 Valid period delay circuit 309 Correlation detection circuit 310 Timing detection circuit 311 Tan ^-1 circuit 312 Frequency control circuit 313 Offset detection circuit 314 Correction circuit

Claims (12)

An OFDM receiver for receiving an OFDM (Orthogonal Frequency Division Multiplex) signal, wherein the OFDM signal is a guard in which a data signal is transmitted and a signal after the effective period is copied to the front of the effective period. Consists of a period,
A receiver that performs symbol synchronization processing in the time domain,
An FFT circuit that performs FFT processing on the OFDM signal, a disturbance detection circuit that detects disturbance in the frequency domain after FFT, and a notch filter that is applied to a position where the disturbance frequency of the signal in the time axis domain exists. OFDM receiving apparatus. 2. The OFDM receiver according to claim 1, wherein the notch filter is a frequency variable notch filter. 3. The OFDM receiver according to claim 1, wherein the interference detection circuit determines that the interference is present when the signal is larger than the average power by a predetermined power or more. 3. The OFDM receiving apparatus according to claim 1, wherein the notch filter is applied to a branch that branches data before FFT and performs synchronization processing on a time axis. 4. The OFDM receiving apparatus according to claim 1 or 2, wherein an applicable bandwidth of the notch filter is varied according to a magnitude of interference. The OFDM receiving apparatus according to claim 1 or 2, wherein two or more notch filters are multi-stage notch filters arranged in series. An OFDM receiver for receiving an OFDM (Orthogonal Frequency Division Multiplex) signal, wherein the OFDM signal is a guard in which a data signal is transmitted and a signal after the effective period is copied to the front of the effective period. Consists of a period,
A receiver that performs symbol synchronization processing in the time domain,
FFT processing means for performing FFT processing on the OFDM signal, and interference detection means for detecting interference in the frequency domain after FFT, and a notch filter to be applied at a position where the interference frequency of the signal in the time axis domain exists. An OFDM receiving method characterized by the above. The OFDM reception method according to claim 7, wherein the notch filter is a frequency variable notch filter. 9. The OFDM reception method according to claim 7, wherein the interference detection means determines that the interference is present when the signal is larger than the average power by a predetermined power or more. 9. The OFDM reception method according to claim 7, wherein the notch filter is applied to a branch that branches data before FFT and performs synchronization processing on a time axis. 9. The OFDM reception method according to claim 7, wherein an applicable bandwidth of the notch filter is varied according to a magnitude of interference. 9. The OFDM reception method according to claim 7, wherein the notch filter is a multistage notch filter in which two or more notch filters are arranged in series.

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