JP5146929B2 - OFDM receiver - Google Patents

Description

  The present invention relates to an OFDM receiving apparatus, and more particularly to an OFDM receiving apparatus that can more accurately extract transmission path information from a received signal.

  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 a transmission signal is divided into a plurality of carrier waves and transmitted. The OFDM scheme has advantages such as being strong against frequency selective fading of a multipath transmission path, capable of densely arranging the spectrum of each subchannel, and having high frequency utilization efficiency.

  In addition, terrestrial digital television broadcasting (1 segment broadcasting) for portable devices has been started. When designing a mobile phone (or a card-type digital television receiver, etc.) capable of receiving digital television broadcasts, it is always necessary to consider miniaturization and low power consumption. Here, the mobile device generally receives a broadcast signal while moving. Accordingly, the transmission path information is important information that is indispensable when performing demodulation processing control in the OFDM receiver. The transmission path information can be extracted from the received signal received from the outside by the OFDM receiver.

  In an OFDM modulation system to which a synchronous demodulation (Coherent Demodulation) method is applied, transmission path information is not only used for synchronous demodulation control but also FFT (Fast Fourier Transform) window position control, clock synchronization, transmission path estimation. Also used in the case of.

  The transmission path information can be extracted from the transmission path transfer function in the frequency domain and the transmission path impulse response in the time domain. The time domain transmission line impulse response is mainly used in each control such as demodulation processing control. From the transmission path impulse response in the time domain, information such as relative relationship information (delay, amplitude ratio) between the direct wave and the reflected wave in the transmission path, FFT window position offset information, and FFT window position shift information is obtained. It is done. Then, the OFDM receiver controls demodulation processing of the OFDM signal included in the received signal based on the information obtained from the transmission impulse response in the time domain.

  A technique related to a frequency domain transmission path transfer function and a time domain transmission path impulse response is disclosed in Patent Document 1, for example.

JP 2006-253915 A

  However, the received signal including the transmission path information may be disturbed by disturbances such as external noise and fading. In particular, in the case of a Rayleigh fading transmission line with a high Doppler shift, the time-domain transmission line impulse response is considerably disturbed. That is, a large amount of noise that becomes a false reflection signal is included in the transmission impulse response waveform in the time domain, and the reflected wave is not detected from the transmission impulse response waveform in the time domain.

  Accurate transmission path information cannot be obtained from the disturbed time domain transmission path impulse response, and accurate OFDM signal demodulation cannot be performed. In the worst case, synchronization may be lost and reception of the OFDM signal may be disabled.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an OFDM receiver capable of more accurately extracting transmission path information from a received signal.

In order to achieve the above object, an OFDM receiving apparatus according to claim 1 according to the present invention includes an FFT unit for Fourier transforming a received signal received from outside, and a pilot signal included in an output signal from the FFT unit. Is divided by a prescribed pilot signal to generate a frequency domain transmission line transfer function, and a time interpolation unit that interpolates in the time direction the frequency domain transmission line transfer function generated by the division unit; A frequency interpolation unit for interpolating the frequency domain transmission path transfer function interpolated by the time interpolation unit in a frequency direction, and the FFT based on the frequency domain transmission path transfer function interpolated by the frequency interpolation unit. an equalization section for performing equalization processing on the output signals from the parts, without being interpolated by the frequency interpolation portion, the inverse Fourier transform of the transmission path transfer function in the frequency domain which is interpolated by the time interpolation section An IFFT unit that generates a transmission impulse response in the time domain; and a low-pass filter unit that performs a filtering process on the transmission impulse response in the time domain generated by the IFFT unit. Based on the output signal from the unit, demodulation processing is performed on the equalized signal.

  An OFDM receiving apparatus according to claim 2 of the present invention is the OFDM receiving apparatus according to claim 1, wherein the low-pass filter unit performs a symbol direction with respect to the transmission impulse response in the time domain. Has been filtered.

  An OFDM receiving apparatus according to claim 3 of the present invention is the OFDM receiving apparatus according to claim 1, wherein the low-pass filter unit performs a smoothing process on the transmission path impulse response in the time domain. IIR filter to be performed.

The OFDM receiver according to claim 1 of the present invention includes a low-pass filter unit that performs a filtering process on a time-domain transmission path impulse response generated by the IFFT unit. Then, based on the output signal from the low-pass filter unit , demodulation processing is performed on the equalized signal .

  Therefore, even if the time domain transmission line impulse response is disturbed by disturbance such as external noise or fading, the low pass filter unit smooths the noise part included in the time domain transmission line impulse response output from the IFFT unit. be able to. That is, a transmission impulse response in the time domain in which the peak of the direct wave and the peak of the reflected wave are clear can be generated. Therefore, the OFDM receiver can extract accurate transmission path information from the time domain transmission path impulse response after filtering. Accordingly, the OFDM apparatus can demodulate the OFDM signal included in the received signal with higher accuracy.

  In the OFDM receiver according to claim 2 of the present invention, the low-pass filter section performs filtering in the symbol direction on the transmission impulse response in the time domain.

  Therefore, filtering processing is performed using the randomness of the appearance phase of disturbance noise in the time direction and the determinism of the appearance phase of the direct wave (or reflected wave) in the time direction. Therefore, only the disturbance noise whose appearance phase changes at random is effectively smoothed by the low-pass filter unit. That is, it is possible to leave only a direct wave and a reflected wave whose appearance phase in the time direction is definite in the output signal from the low-pass filter unit.

  In the OFDM receiver according to claim 3 of the present invention, the low-pass filter unit is an IIR filter that performs a smoothing process on a time-domain transmission path impulse response.

  Therefore, even if a low-pass filter unit is added, an increase in circuit scale in the OFDM receiver can be suppressed, and simple design changes are sufficient.

  Hereinafter, the present invention will be specifically described with reference to the drawings showing embodiments thereof.

<Embodiment>
FIG. 1 is a block diagram showing a main configuration of OFDM receiving apparatus 100 according to the present embodiment. The OFDM receiver 100 is mounted on a mobile phone, a portable digital television, or the like.

  The OFDM receiver 100 includes an FFT (Fast Fourier Transform) unit 1, a division unit 2, a pilot pattern unit 3, a time interpolation unit 4, a frequency interpolation unit 5, an equalization unit 6, an IFFT (Inverse Fast Fourier Transform). : Inverse FFT) 7 parts and a low-pass filter (LPF) part 8.

  The OFDM receiver 100 receives a received signal (= OFDM signal + transmission path function) via an antenna (not shown). The received signal is subjected to predetermined processing such as A / D conversion (that is, after generation of the OFDM symbol received signal in the time domain), and then the OFDM symbol received signal in the time domain is input to the FFT unit 1. .

  The FFT unit 1 performs Fourier transform on the received signal received from the outside (more specifically, the OFDM symbol received signal in the time domain). That is, the FFT unit 1 can convert a time-domain OFDM symbol signal into a frequency-domain OFDM symbol signal.

  An output signal (frequency-domain OFDM symbol signal) from the FFT unit 1 is transmitted to the equalization unit 6. Further, the pilot signal embedded in the frequency-domain OFDM symbol signal is transmitted to division section 2. Here, the pilot signal is a signal extracted by a pilot signal extraction unit (not shown), and is a PRBS (Pseudo Random Binary Series) signal having a known subcarrier position, amplitude, and phase.

  The equalization unit 6 executes equalization processing of the OFDM symbol signal in the frequency domain. That is, the OFDM signal is extracted by dividing the transmission path function included in the received signal using the estimated transmission path response. Here, as will be described later, the estimated transmission line response is obtained by calculating a frequency domain transmission path transfer function from a frequency domain OFDM symbol signal using a pilot signal having a known amplitude and phase, and transmitting the frequency domain transmission path. The function is interpolated in the symbol direction and the carrier direction.

  Then, the equalization unit 6 performs equalization processing of the frequency-domain OFDM symbol signal using the interpolated estimated transmission line response. The equalized OFDM signal is output to a demapping unit (not shown), FEC (forward error coding) or the like for demodulation processing.

  On the other hand, the pilot pattern section 3 generates a defined pilot signal whose amplitude and phase are known in synchronization with the pilot extraction section (not shown) described above, and its output is supplied to the division section 2. As can be seen from the above, the dividing unit 2 includes a pilot signal (received pilot signal) extracted from the OFDM symbol signal in the frequency domain, a specified pilot signal (specified pilot signal) generated by the pilot pattern unit 3, and Is entered.

  The division unit 2 calculates a transmission path transfer function in the frequency domain by dividing the received pilot signal by the specified pilot signal. The transmission path transfer function in the frequency domain is supplied to the time interpolation unit 4.

  The time interpolation unit 4 interpolates the transmission path transfer function in the frequency domain in the time axis direction. The frequency-domain transmission path transfer function after the interpolation is supplied to the frequency interpolation unit 5 and the IFFT unit 7.

  The frequency interpolation unit 5 interpolates the frequency domain transmission path transfer function interpolated in the time axis direction in the frequency axis direction. The estimated transmission path response generated by performing each interpolation is input to the equalization unit 6. The equalization process using the estimated transmission path response in the equalization unit 6 is as described above.

  As described above, in the configuration shown in FIG. 1, the time-interpolated frequency domain transmission path transfer function is also supplied to the IFFT unit 7. In the IFFT unit 7, the time-interpolated frequency domain transmission line transfer function is subjected to fast inverse Fourier transform and converted to a time domain signal. That is, in the IFFT unit 7, the frequency domain transmission path transfer function is converted into a time domain transmission path impulse response.

  By the way, as described in “Problems to be solved by the invention”, a received signal received from the outside may be disturbed by disturbances such as external noise and fading. In particular, in the case of a Rayleigh fading transmission line with a high Doppler shift, the time-domain transmission line impulse response is considerably disturbed.

  FIG. 2 is a diagram showing a transmission path impulse response in the time domain without noise and pseudo reflection. However, when external noise is mixed in the received signal, as shown in FIG. 3, the distinction between the direct wave and the reflected wave and the noise (white noise and false reflection due to Doppler shift) becomes unclear. Accurate transmission path information cannot be obtained from the disturbed time domain transmission path impulse response, and accurate OFDM signal demodulation cannot be performed. Here, the vertical axis of FIGS. 2 and 3 represents the signal intensity, and the horizontal axis represents time.

  Therefore, in the OFDM receiver 100 according to the present invention, an LPF unit 8 is connected to the subsequent stage of the IFFT unit 7 as shown in FIG. That is, in the OFDM receiver 100, the LPF 8 performs a filtering process on the transmission impulse response in the time domain.

  More specifically, the LPF unit 8 applies a filtering process in the symbol direction to the transmission impulse response in the time domain. For example, considering the roll-off characteristics of the filter, it is appropriate to employ an IIR (Infinite-duration Impulse Response) filter that is a digital filter as the LPF unit 8. The IIR filter performs a smoothing process on the transmission impulse response in the time domain.

  FIG. 4 is a diagram illustrating a simulation result of a signal output from the LPF unit 8 when the signal illustrated in FIG. 3 is processed by the LPF unit 8. As shown in FIG. 4, noise is smoothed, and the peak of the direct wave Dw and the peak of the reflected wave Rw become clear.

  The transmission impulse response in the time domain subjected to the filtering process in the LPF unit 8 is supplied to a control unit (not shown). The control unit controls the demodulation process of the OFDM signal based on the output signal from the LPF unit 8. That is, the control unit extracts each transmission path information from the transmission impulse response in the time domain on which the filtering process has been performed. And the said control part controls the demodulation process with respect to the signal by which the equalization part 6 performed the equalization process using each said extracted transmission line information.

  As described above, the OFDM receiving apparatus 100 according to the present embodiment includes the LPF unit 8 that performs low-pass filter processing on a time-domain transmission path impulse response generated by inverse Fourier transform.

  Therefore, even if the transmission impulse response in the time domain is disturbed due to disturbance such as external noise or fading, the noise portion included in the transmission impulse response in the time domain output from the IFFT unit 7 can be smoothed. That is, it is possible to generate a transmission impulse response in the time domain in which the direct wave peak and the reflected wave peak are clarified. Therefore, the OFDM receiver 100 according to the present invention can extract accurate transmission path information from the time domain transmission path impulse response after filtering. Therefore, the OFDM receiver 100 can demodulate the OFDM signal included in the received signal with higher accuracy.

  Here, when there is Rayleigh fading, the waveform of the transmission impulse response in the time domain (that is, the output from the IFFT unit 7) differs for each symbol. However, the direct wave and the reflected wave in the transmission path impulse response in the time domain are deterministic signals. Therefore, the direct wave and the reflected wave have a small phase change of data at the same position in the time direction for each symbol. On the other hand, the noise portion (including pseudo reflection) included in the output signal from the IFFT unit 7 has a random and large phase change of data at the same position in the time direction for each symbol (that is, the phase where noise appears is a symbol Random).

  Therefore, in OFDM receiving apparatus 100 according to the present embodiment, LPF section 8 performs filtering in the symbol (time) direction for the transmission impulse response in the time domain for each subcarrier.

  As described above, the filtering process using the randomness of the appearance phase of the disturbance noise and the determinism of the appearance phase of the direct wave (or reflected wave) is performed. Therefore, only disturbance noise whose appearance phase changes at random is effectively smoothed. That is, only a direct wave and a reflected wave having a definite appearance phase can be left in the output signal from the LPF unit 8.

  In addition, in OFDM receiving apparatus 100 according to the present embodiment, an IIR filter can be employed as LPF unit 8. The IIR filter has a small circuit scale and is easy to design.

  Therefore, even if an IIR filter is added, an increase in circuit scale in the OFDM receiver 100 can be suppressed, and a simple design change is sufficient.

  In the configuration of FIG. 1, the signal from the division unit 2 may be directly input to the IFFT unit 7 without performing the time interpolation process on the signal from the division unit 2 (case 1). However, in the above description, the signal after time interpolation processing is input to the IFFT unit 7 (Case 2).

  Since the case 2 is adopted in the present invention, the IFFT unit 7 can perform (fast) inverse Fourier transform processing on the transmission path transfer function in the frequency domain with a larger amount of information. That is, the IFFT unit 7 can generate a more accurate time domain transmission path impulse response.

  Here, in the configuration of FIG. 1, time interpolation and frequency interpolation may be performed on the signal output from the division unit 2, and the signal after each interpolation processing may be input to the IFFT unit 7 (case 3).

  However, even if a frequency domain transmission path transfer function (estimated transmission path response) subjected to frequency interpolation in addition to time interpolation is input to the IFFT unit 7, only the amount of calculation increases. That is, compared with the accuracy of the time domain transmission line impulse response generated in case 2 above, the accuracy of the time domain transmission line impulse response generated in case 3 is commensurate with the amount of calculation. There is no improvement by minutes.

  Therefore, it can be concluded that the configuration of case 2 is the most appropriate from the viewpoint of the computational complexity of the inverse Fourier transform and the high accuracy of the transmission impulse response in the time domain.

  When the configuration shown in FIG. 1 does not include the LPF unit 8, the multipath characteristic (reflection wave delay tolerance) and the fading characteristic (Doppler shift tolerance) cannot be simultaneously made highly accurate. That is, the multipath characteristic is better as the symbol is longer, while the fading characteristic is better as the symbol is shorter, and both characteristics are in a trade-off relationship.

  However, in OFDM receiving apparatus 100 according to the present embodiment, LPF unit 8 is separately connected after IFFT unit 7. Therefore, the multipath characteristic can be improved by lengthening the symbol, and the fading characteristic can be improved by providing the LPF unit 8. That is, the OFDM receiver 100 can make the multipath characteristic and the fading characteristic at the same time highly accurate.

  The OFDM receiver 100 can be applied to a terrestrial digital one-segment OFDM receiver, a 1 / 3-segment OFDM receiver, and the like.

It is a block diagram which shows the principal part structure of the OFDM receiver which concerns on embodiment. It is a figure which shows the waveform of the transmission-path impulse response of the time domain which does not contain noise and a false reflection. It is a figure which shows the waveform of the transmission-line impulse response of the time domain containing noise and pseudo reflection. It is a figure which shows the result of having performed the filtering process with respect to the transmission path impulse response of a time domain.

Explanation of symbols

1 FFT unit 2 Division unit 3 Pilot pattern unit 4 Time interpolation unit 5 Frequency interpolation unit 6 Equalization unit 7 IFFT unit 8 LPF
100 OFDM receiver

Claims (3)

An FFT unit for Fourier transforming a received signal received from the outside;
A division unit that divides a pilot signal included in an output signal from the FFT unit by a specified pilot signal to generate a transmission path transfer function in a frequency domain;
A time interpolation unit for interpolating in a time direction the transmission path transfer function of the frequency domain generated by the division unit;
A frequency interpolation unit that interpolates in the frequency direction the transmission path transfer function of the frequency domain interpolated by the time interpolation unit;
An equalization unit that performs an equalization process on the output signal from the FFT unit based on the transmission path transfer function in the frequency domain interpolated by the frequency interpolation unit ;
An IFFT unit that performs an inverse Fourier transform on the frequency domain transmission path transfer function that is not interpolated by the frequency interpolation unit, and generates a time domain transmission line impulse response;
A low pass filter unit that performs a filtering process on the time domain transmission line impulse response generated by the IFFT unit,
Based on the output signal from the low-pass filter unit, a demodulation process is performed on the signal after the equalization process,
An OFDM receiver characterized by that. The low-pass filter unit is
Filtering is performed in the symbol direction for the transmission impulse response in the time domain,
The OFDM receiver according to claim 1. The low-pass filter unit is
An IIR filter that performs a smoothing process on the transmission impulse response in the time domain.
The OFDM receiver according to claim 1.