JP2009188602A - Ofdm receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technique for efficiently removing unwanted signals without increasing the circuit size of a receiver. <P>SOLUTION: A spurious detection circuit 94 detects a frequency f2 of a spurious signal from the reception signal of a frequency region outputted from an FFT computing unit 10, and calculates a phase rotation amount (p) in a time domain from the difference between a stop frequency f1 of a notch filter 92 and a spurious frequency f2 to set it to a rotator 91 and a derotator 93. The rotator 91 rotates the phase of the reception signal based on the phase rotation amount (p). The notch filter 92 attenuates the spurious signal having a frequency adjusted to f1. The derotator 93 reversely rotates the phase of the reception signal based on the phase rotation amount (p). The FFT computing unit 10 demodulates the reception signal from which the spurious signal has been removed. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a technique for removing a spurious signal from a received signal in an OFDM transmission system.

  In Japanese terrestrial digital television broadcasting, an OFDM (Orthogonal Frequency Division Multiplexing) system is adopted as a transmission system. The OFDM scheme is one of multicarrier transmission schemes in which transmission data is divided into a plurality of carrier waves and transmitted. The spectrum of each subchannel, which is strong against frequency selective fading of a multipath transmission path, can be densely arranged, There are advantages such as high frequency utilization efficiency.

  The OFDM demodulating IC is not only mounted on a stationary television but also applied to various applications. For example, there is an application example in which an OFDM demodulation IC is mounted on a mobile phone, a game machine, or a car navigation system.

Bernard Widrow & Samuel D. Stearns, “Adaptive signal processing”, Prentice Hall, 1985, p. 349-354

  As the application of OFDM demodulation ICs to various applications spreads, the problem of spurious interference is increasing. The spurious interference is interference that a received signal receives due to an image signal, a harmonic component of local oscillation, or a frequency of another oscillator used in the receiver and its harmonic component.

  For example, there is a system in which a one-seg (1-segment reception) terrestrial digital broadcast can be received by a game machine by inserting a cartridge containing an OFDM demodulation IC into the game machine. In this case, there is a problem that the frequency of the oscillator used in the game machine and its harmonic components are mixed in the OFDM demodulating IC and become an interference source, and the BER (Bit Error Rate) of the OFDM signal is deteriorated. It has occurred.

  Spurious interference is a challenge for OFDM demodulation ICs for the following reasons.

  (1) Spurious signals have different properties depending on the application in which the OFDM demodulation IC is mounted. The spurious frequency that falls within the band of the OFDM signal is indefinite, and a cancel filter cannot be provided in advance as in CCI (Co-Channel-Interference).

  (2) The amplitude of the spurious signal is often much larger than that of the OFDM signal. For this reason, the amplitude of the spurious signal in the signal demodulated by the FFT operation is much larger than the amplitude of the OFDM signal. However, the bit accuracy of the demodulator is limited, and if a bit is assigned to a spurious signal with a large amplitude, the quantization accuracy of an OFDM signal with a relatively small amplitude will deteriorate rapidly.

  (3) An error occurs in the frequency synchronization processing due to the spurious signal. Since the frequency error is not normally corrected, the BER deteriorates.

  (4) When spurious interference is strong, even symbol synchronization may not be achieved.

  Non-Patent Document 1 discloses a technique for removing spurious signals by an adaptive filter. This technique automatically detects the frequency of a spurious signal mixed in a received signal, forms a notch filter corresponding to the detected frequency, and cancels the spurious signal.

  However, since the adaptive filter requires a large number of taps and the OFDM signal is a complex signal, the adaptive filter must also have complex coefficients. Therefore, the amount of calculation of a complex adaptive filter having a large number of taps is enormously increased, which is not practical.

  In view of the above problems, an object of the present invention is to provide a technique for efficiently removing unnecessary signals without increasing the circuit scale of a receiving apparatus.

  In order to solve the above-described problem, an OFDM receiver according to claim 1 includes an analysis unit that detects a frequency of an unnecessary signal from a received signal converted into a frequency domain, and an unnecessary signal detected by the analysis unit in a time domain. An unnecessary signal removing unit that attenuates the frequency, and a demodulating unit that performs FFT conversion on the received signal from which the unnecessary signal has been removed by the unnecessary signal removing unit.

  The invention according to claim 2 is the OFDM receiver according to claim 1, wherein the analysis unit includes a calculation unit that calculates a phase rotation amount for making the detected frequency of the unnecessary signal coincide with the target frequency, The unnecessary signal removing unit includes a first phase rotating unit that rotates the phase of the received signal based on the phase rotation amount, and a signal of the target frequency included in the received signal whose phase is rotated by the first phase rotating unit. And a second phase rotation unit that rotates the phase of the reception signal output from the filter based on the phase rotation amount in a direction opposite to the rotation direction of the first phase rotation unit. It is characterized by.

  According to a third aspect of the present invention, in the OFDM receiver according to the first or second aspect, the analysis unit is unnecessary from a reception signal converted into a frequency domain using an FFT operation circuit included in the demodulation unit. It is characterized by detecting the frequency of the signal.

  According to a fourth aspect of the present invention, in the OFDM receiver according to the third aspect, the FFT operation circuit generates a reception signal in a frequency domain to be provided to the analysis unit until an unnecessary signal is removed. Then, after the unnecessary signal is removed, a reception signal in the frequency domain as a demodulated signal is generated.

  The OFDM receiver of the present invention detects the frequency of an unnecessary signal in the frequency domain, attenuates the signal of the frequency detected in the time domain, and performs FFT conversion on the received signal from which the unnecessary signal has been removed. As a result, since the unnecessary signal is not included in the received signal after the FFT conversion, accurate demodulation processing can be performed.

  Further, by rotating the phase of the received signal in the time domain, the frequency of the detected unnecessary signal is matched with the stop frequency of the filter. Thereby, an unnecessary signal can be attenuated using a filter whose stop frequency is fixed. Compared to the case of using an adaptive filter, the circuit scale of the filter can be significantly reduced.

  Moreover, the frequency of an unnecessary signal is detected from the received signal converted into the frequency domain using the FFT arithmetic circuit with which a demodulation part is provided. By sharing the FFT operation circuit, the circuit scale of the receiving device can be reduced.

{1. Overall structure and processing flow of OFDM receiver}
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an OFDM receiving apparatus according to the present embodiment. An RF (Radio Frequency) signal 1 transmitted from an OFDM transmitter (not shown) is received by a receiving antenna 2 through a transmission path. The received RF signal is frequency-converted by the tuner 3 into an IF (Intermediate Frequency) signal. The IF signal is input to the mixer 5 via the BPF (band pass filter) 4, multiplied by the signal supplied from the carrier wave oscillator 6, and then output to the LPF (low pass filter) 7.

  The reception signal from which the high-frequency component has been removed by the LPF 7 is output to the A / D converter 8, and is converted to a digital signal (symbol signal) at a predetermined sampling frequency by the A / D converter 8. The received signal converted into the digital signal is output to an FFT (Fast Fourier Transform) calculator 10 after the spurious signal removing unit 9 removes the spurious component. The processing content of the spurious signal removal unit 9 will be described later.

  The FFT calculator 10 Fourier-transforms the input time domain symbol signal into a frequency domain signal. The reception signal output from the spurious signal removal unit 9 is also input to the symbol synchronization circuit 11.

  In a state where the spurious signal is not removed, the symbol synchronization circuit 11 cannot perform symbol synchronization with high accuracy. Even after performing symbol synchronization in such a state and performing FFT calculation, an effective demodulated signal cannot be obtained. Therefore, in the present embodiment, the symbol synchronization circuit 11 is not operated until the spurious signal is removed. The start of symbol synchronization processing in the symbol synchronization circuit 11 is delayed, and the FFT operation unit 10 is used for detection of spurious signals during the delayed period.

  The symbol synchronization circuit 11 detects the symbol period using the guard interval and outputs a detection signal to the FFT calculator 10. Based on this detection signal, the FFT calculator 10 determines a symbol period and performs an FFT calculation. By performing the FFT operation, the received signal is converted into a frequency domain signal.

  The frequency domain received signal is input to an equalizer 12 that performs waveform equalization. The equalizer 12 equalizes the received signal based on the calculation result in the transmission path estimation circuit (not shown). The transmission path estimation circuit calculates a transmission path function based on a pilot signal included in the received signal, and estimates the calculated transmission path function for other signals.

  On the other hand, the received signal that has been equalized in the equalizer 12 is subjected to Viterbi decoding and Reed-Solomon decoding in the channel decoder 13, and then in the source decoder 14, MPEG (Moving Picture Experts Group) − After being decoded by the two methods, it is converted to analog by the D / A converter 15 and output.

{2. Configuration of Spurious Signal Removal Unit}
Next, the configuration of the spurious signal removal unit 9 will be described. As shown in FIG. 1, the spurious signal removal unit 9 includes a rotator 91, a notch filter 92, a de-rotator 93, and a spurious detection circuit 94.

  The rotator 91 rotates the phase of the reception signal output from the A / D converter 8 based on the phase rotation amount p input from the spurious detection circuit 94.

  The notch filter 92 attenuates the signal having the stop frequency f1 from the received signal. The stop frequency f1 of the notch filter 92 is fixed.

  The de-rotator 93 rotates the phase of the reception signal output from the notch filter 92 based on the phase rotation amount p input from the spurious detection circuit 94. However, the derotator 93 rotates the phase of the received signal in the direction opposite to the rotation direction in the rotator 91. In other words, the phase of the received signal is rotated by p phase in the rotator 91, and the phase of the received signal is rotated by −p phase in the de-rotator 93.

  The spurious detection circuit 94 detects the frequency f2 of the spurious signal from the reception signal converted into the frequency domain by the FFT calculator 10. Since the OFDM signal is a signal like white noise, if the FFT operation is performed in a state where symbol synchronization is not achieved, the output after the operation also has a waveform like white noise. On the other hand, since a spurious signal is single-frequency interference, if a signal of one period is acquired, a spectrum peak can always be obtained by FFT calculation. Therefore, the spurious frequency f2 can be detected by obtaining a signal starting from an arbitrary point without taking symbol synchronization with respect to the received signal mixed with the spurious signal and performing the FFT operation. Further, while the power of the OFDM signal is dispersed in all frequency components, the spurious power is concentrated in one frequency component. For this reason, even if the spurious power is smaller than the power of the OFDM signal by a dozen dB in the time domain, the spectrum height of the spurious is higher than the spectrum height of the OFDM signal in the frequency domain. Therefore, it is possible to easily detect the frequency f2 of the spurious signal mixed in the received signal.

  When detecting the spurious frequency f2, the spurious detection circuit 94 obtains the phase rotation amount p from the difference between the fixed stop frequency f1 set in the notch filter 92 and the detected spurious frequency f2. That is, the phase rotation amount p in the time domain corresponding to the frequency difference (f1-f2) in the frequency domain is obtained. When the phase of the received signal in the time domain is rotated, the spectrum shifts in the frequency domain. As described above, the spurious detection circuit 94 outputs the calculated phase rotation amount p to the rotator 91 and the de-rotator 93. The rotator 91 and the de-rotator 93 set the input phase rotation amount p in the circuit.

  Thus, the phase of the received signal input to the notch filter 92 is rotated by the rotator 91, and the frequency of the spurious signal is adjusted to f1. The notch filter 92 attenuates the spurious signal based on a preset stop frequency f1. After the spurious signal is attenuated, the received signal is again subjected to reverse phase rotation in the derotator 93, and the phase is returned to the original state.

  The received signal from which the spurious signal has been removed is output to the symbol synchronization circuit 11. Since the spurious signal is removed from the reception signal input to the symbol synchronization circuit 11, the symbol synchronization circuit 11 can perform symbol synchronization with high accuracy using the guard interval signal.

{3. Flow of spurious signal removal processing}
Next, the flow of spurious signal removal processing will be described with reference to FIG. First, when the power of the receiving apparatus is turned on and the OFDM receiving apparatus is activated, the rotator 91 and the de-rotator 93 are reset. Specifically, 0 is set as the phase rotation amount of the rotator 91 and the de-rotator 93 (step S1).

  Subsequently, when the OFDM receiver starts receiving an OFDM signal, the FFT calculator 10 starts operating and outputs an output signal after the FFT calculation to the spurious detection circuit 94 (step S2).

  The spurious detection circuit 94 detects the frequency f2 of the spurious signal, and calculates the phase rotation amount p in the time domain from the difference between the stop frequency f1 preset in the notch filter 92 and the detected frequency f2 of the spurious signal. Calculate (step S3).

  In the processing of steps S1 to S3, the rotator 91 and the de-rotator 93 do not apply phase rotation to the received signal. Therefore, the spurious signal is not removed in the notch filter 92. In this state, the symbol synchronization circuit 11 is not operating. Therefore, the FFT calculator 10 performs an FFT operation on the received signal mixed with the spurious signal in a state where symbol synchronization is not established. However, as described above, since the spurious signal is a single frequency signal, the spurious detection circuit 94 can detect the spectrum peak of the spurious signal from the frequency domain signal.

  The symbol synchronization processing by the symbol synchronization circuit 11 requires a certain amount of time. In order to start the demodulation process, the FFT operator 10 needs to wait for the symbol synchronization to be completed. However, in the state where the spurious signal is not removed, unnecessary symbol synchronization processing is cut, so that the FFT calculation processing is started at an early timing, and the time until spurious frequency detection is shortened.

  Subsequently, the spurious detection circuit 94 sets the phase rotation amount p in the rotator 91 and the de-rotator 93, and the spurious signal removal processing in the spurious signal removal unit 9 is started (step S4). That is, the phase of the received signal is rotated by the rotator 91 based on the phase rotation amount p, whereby the frequency of the spurious signal is adjusted to f1, and the spurious signal is removed by the notch filter 92.

  When the FFT calculator 10 confirms that the spurious signal has been removed from the input received signal, the FFT calculator 10 locks the phase rotation amount p of the rotator 91 and the derotator 93 and operates the symbol synchronization circuit 11. As a result, symbol synchronization processing is performed with high accuracy from the received signal from which the spurious signal has been removed. An FFT operation is performed on the symbol-synchronized reception signal. Thereafter, the FFT computing unit 10 is used for demodulation processing (step S5).

  Thus, in the present embodiment, the stop frequency f1 of the notch filter 92 can be fixed. Therefore, a filter bank is unnecessary and the circuit scale can be greatly reduced as compared with the case of using an adaptive filter whose filter coefficient is variable according to the spurious frequency. Further, since the rotator 91 and the de-rotator 93 that perform phase rotation have a small circuit scale, the spurious signal removing unit 9 as a whole can also reduce the circuit scale.

  Further, the FFT calculator used for detecting the spurious signal is shared with the FFT calculator used for the demodulation process. This also makes it possible to reduce the scale of the circuit related to spurious detection.

  The detection of the spurious frequency f2 and the calculation of the phase rotation amount p in the spurious detection circuit 94 may be performed once when the OFDM receiver is activated. Thereafter, the rotator 91 and the de-rotator 93 rotate the phase of the received signal based on the set phase rotation amount p. However, since the spurious frequency f2 may be changed due to a change in the external environment, the spurious frequency f2 may be detected and the phase rotation amount p may be calculated periodically from the output of the FFT that has entered the demodulation process. . Then, the updated phase rotation amount p may be set on the rotator 91 and the de-rotator 93 periodically. Even in this case, the stop frequency f1 of the notch filter 92 can be fixed.

  As described above, the OFDM receiving apparatus according to the present embodiment removes spurious signals in the time domain before the FFT processing unit 10 performs the demodulation process. As a result, as described in “Problems to be Solved by the Invention”, it is possible to solve various problems when a spurious signal is mixed in the received signal after the FFT operation.

It is a block diagram of the OFDM receiver in this Embodiment. It is a flowchart which shows a spurious signal removal process.

Explanation of symbols

9 Spurious signal removal unit 10 FFT calculator 91 Rotator 92 Notch filter 93 De-rotator 94 Spurious detection circuit

Claims (4)

An analysis unit for detecting the frequency of the unnecessary signal from the received signal converted into the frequency domain;
An unnecessary signal removing unit for attenuating the frequency of the unnecessary signal detected by the analysis unit in the time domain;
A demodulator that performs FFT conversion on the received signal from which unnecessary signals have been removed in the unnecessary signal remover;
An OFDM receiving apparatus comprising: The OFDM receiver according to claim 1, wherein
The analysis unit
A calculation unit for calculating a phase rotation amount for making the frequency of the detected unnecessary signal coincide with the target frequency;
Including
The unnecessary signal removing unit is
A first phase rotation unit that rotates the phase of the received signal based on the phase rotation amount;
A filter for attenuating the signal of the target frequency included in the received signal whose phase has been rotated in the first phase rotation unit;
A second phase rotation unit that rotates the phase of the reception signal output from the filter based on the phase rotation amount in a direction opposite to the rotation direction in the first phase rotation unit;
An OFDM receiving apparatus comprising: In the OFDM receiver according to claim 1 or 2,
The OFDM receiver according to claim 1, wherein the analysis unit detects a frequency of an unnecessary signal from a reception signal converted into a frequency domain using an FFT arithmetic circuit provided in the demodulation unit. The OFDM receiver according to claim 3,
The FFT operation circuit generates a reception signal in the frequency domain to be given to the analysis unit until the unnecessary signal is removed, and receives the frequency domain as a demodulated signal after the unnecessary signal is removed. An OFDM receiver that generates a signal.

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