JP4929323B2 - OFDM receiver - Google Patents

Description

  The present invention relates to an OFDM receiver suitable for mobile communication systems, wireless LAN systems, and the like.

  In recent years, digital modulation systems have been actively developed for transmission of audio signals and video signals. In particular, in digital terrestrial broadcasting, attention is paid to an orthogonal frequency division multiplexing (hereinafter referred to as OFDM) modulation method having characteristics such as resistance to multipath interference and high frequency utilization efficiency.

  In Japanese digital broadcasting, the ISDB-T system is adopted. In the ISDB-T system, signal processing such as error correction coding, interleave coding, digital modulation, and the like is performed on a TS (transport stream) defined by the MPEG2 standard, and further, OFDM modulation is performed to generate a broadcast signal. Has been obtained.

  In the ISDB-T system, in the frequency domain, 108 symbols of OFDM symbols are used as one block, and 1, 2, and 4 blocks constitute one segment according to the mode. That is, the number of carriers in one segment is 108, 216, or 432. In the ISDB-T system, transmission is performed in a band for 13 segments.

  Further, in the ISDB-T system, one frame is composed of 204 OFDM symbols in the time domain. Then, TS transmission and energy diffusion processing are performed in units of frames.

  Thus, an SP (scattered pilot) signal for estimating the frequency response of the transmission path is inserted into the OFDM frame in which carriers are arranged in the frequency domain and the time domain. The SP signals are arranged at predetermined intervals in both the time direction and the frequency direction. In the OFDM receiving apparatus, in order to perform equalization processing using the SP signal, first, the SP signal is interpolated in the time direction and the frequency direction by an interpolation filter to obtain a transmission line response for each subcarrier. The receiving apparatus corrects the transmission path distortion by equalization processing using the interpolated SP signal and performs data demodulation.

  In this case, by selecting a narrow-band interpolation filter as an interpolation filter, noise can be suppressed and S / N can be improved.

  By the way, in reception by a mobile unit, each subcarrier of the received OFDM signal is affected by Doppler shift. Under a multipath environment, the effect of Doppler shift differs for each frequency, and the orthogonality between subcarriers is broken, causing intersubcarrier interference (ICI: Inter Carrier Interference). Intersubcarrier interference becomes a cause of reception performance degradation in mobile communication.

  Therefore, Patent Document 1 discloses a proposal for an ICI canceller that cancels intersubcarrier interference. The ICI canceller is also described in detail in Non-Patent Document 1.

  In the ICI canceller, it is necessary to interpolate the SP signal in the time direction using a sufficiently wide band interpolation filter in order to accurately obtain the transmission path response due to the Doppler shift. However, when a wide-band interpolation filter is used, there is a drawback that noise components are easily mixed in the interpolated SP signal, and the S / N of the demodulated output is deteriorated.

JP 2003-134010 A

“A Study on MMSE-type ICI Canceller for OFDM Mobile Reception”, Journal of the Institute of Image Information and Television Engineers, January 2004, vol. 58, no. 1, P.I. 83-90

  An object of the present invention is to provide an OFDM receiver capable of improving reception performance and reducing power consumption by controlling the operation of an ICI canceller according to the influence of Doppler shift.

  An OFDM receiver according to an aspect of the present invention is an OFDM receiver that receives an orthogonal frequency division multiplex signal including pilot signals periodically arranged in a frequency direction and a time direction, wherein the orthogonal frequency division is a time domain signal. A Fourier transform unit that converts the multiplexed signal into a frequency domain signal, and the pilot signal included in the output of the Fourier transform unit is interpolated in the time direction and the frequency direction to estimate the transmission path characteristics, and based on the estimation result The first equalization unit for equalizing the output of the Fourier transform unit, the intercarrier interference removal unit for removing the intercarrier interference of the output of the Fourier transform unit, and the intercarrier interference removal by the intercarrier interference removal unit The transmission path characteristic is estimated from the removed output of the Fourier transform unit, and the output of the Fourier transform unit is equalized based on the estimation result. And 2 of the equalizer, characterized by comprising a selection unit for selectively outputting one of the outputs of said first and second equalizing section.

  According to the present invention, by controlling the operation of the ICI canceller according to the influence of the Doppler shift, it is possible to improve the reception performance and reduce the power consumption.

1 is a block diagram showing an OFDM receiving apparatus according to a first embodiment of the present invention. FIG. 3 is an explanatory diagram showing a part of the configuration of an OFDM frame with a frequency domain in the horizontal direction in subcarrier units and a time domain in the vertical direction in symbol units. The graph which shows the characteristic of the interpolation filter of SP interpolation part 21 and SP interpolation part 23 for ICI, taking time on a horizontal axis and taking a spectrum on a vertical axis | shaft. The graph which shows the receiving characteristic by the interpolation process of SP interpolation part 21 and SP interpolation part 23 for ICI, taking Doppler frequency on a horizontal axis and taking reception S / N on a vertical axis | shaft. The block diagram which shows the 2nd Embodiment of this invention. The block diagram which shows the modification of 2nd Embodiment.

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

(First embodiment)
FIG. 1 is a block diagram showing an OFDM receiving apparatus according to the first embodiment of the present invention.

  In FIG. 1, an OFDM signal is input to the input terminal 11. This OFDM signal is obtained, for example, by selecting an OFDM signal received by an antenna (not shown) using a tuner. The OFDM signal input via the input terminal 11 is given to the orthogonal demodulation unit 12. The quadrature demodulation unit 12 performs quadrature detection on the input OFDM signal to obtain a baseband in-phase detection axis signal (I signal) and a quadrature detection axis signal (Q signal). A baseband OFDM signal composed of these I and Q signals is supplied to the FFT unit 13.

  The FFT unit 13 removes the guard interval from the baseband OFDM signal, and converts the time-domain OFDM signal into a frequency-domain OFDM symbol by FFT (Fast Fourier Transform) processing. An OFDM symbol is a data string representing the phase and amplitude of each carrier. This OFDM symbol is supplied to the demodulator 15 via the equalizer 14.

  The equalization unit 14 includes an SP interpolation unit 21 and an ICI SP interpolation unit 23. The SP interpolation unit 21 extracts the SP signal included in the OFDM symbol and interpolates in the time direction and the frequency direction. In this case, the SP interpolation unit 21 performs interpolation by filtering processing in a relatively narrow band.

  FIG. 2 is an explanatory diagram showing a part of the structure of an OFDM frame, with the frequency domain in the horizontal direction in subcarrier units and the time domain in the vertical direction in symbol units. The circles in FIG. 2 indicate each subcarrier, the hatched circles are SP carriers, and the plain circles are data carriers.

  Each column in FIG. 2 represents an OFDM symbol. FIG. 2 shows only the first 16 carriers. When one segment is composed of four blocks, the number of carriers in the OFDM1 symbol is 5616.

  As shown in FIG. 2, SP carriers are inserted every 12 carriers of each symbol, and SP carriers are shifted by 3 subcarriers between adjacent symbols. Therefore, in the time domain, an SP carrier is arranged every 4 OFDM symbols.

  First, the SP interpolation unit 21 interpolates the SP signal inserted every four symbols for each subcarrier using an interpolation filter in the time direction. Thereby, SP signals of all symbols are obtained in the time direction. Next, the SP interpolation unit 21 interpolates the SP signal with an interpolation filter in the frequency direction. Thereby, SP signals corresponding to all carriers are obtained. The SP interpolation unit 21 estimates the channel characteristics at each carrier position using the SP signal obtained by the interpolation, and outputs the estimated channel characteristics together with the data to the equalization processing unit 22.

  The equalization processing unit 22 equalizes the waveform of each carrier data according to the estimated transmission path characteristics. As a result, data from which transmission path distortion has been removed is obtained from the equalization processing unit 22. This data is given to the selection circuit 25 and the determination unit 26.

  On the other hand, the output of the FFT unit 13 is also given to the ICI SP interpolation unit 23. The ICI SP interpolating unit 23 interpolates the SP signals inserted for every four symbols for each subcarrier with an interpolation filter in the time direction. In this case, the ICI SP interpolation unit 23 performs interpolation in the time direction using a sufficiently wide band interpolation filter. This makes it possible to detect a Doppler shift component that affects inter-subcarrier interference. The ICI SP interpolation unit 23 estimates the Doppler shift component using the SP signal interpolated in the time direction.

  The ICI SP interpolation unit 23 equalizes the SP signal using the estimated Doppler shift component. Thereby, the Doppler shift component included in the SP signal is removed. Next, the ICI SP interpolation unit 23 interpolates the SP signal from which the Doppler shift component has been removed by using an interpolation filter in the frequency direction. Thereby, SP signals corresponding to all carriers are obtained. The ICI SP interpolation unit 23 estimates the channel characteristics at each carrier position using the SP signal obtained by interpolation, and outputs the estimated channel characteristics to the equalization processing unit 22 together with the data.

  The equalization processing unit 24 equalizes the waveform of each carrier data according to the estimated transmission path characteristics. Thereby, the data from which the transmission line distortion is removed is obtained from the equalization processing unit 24. This data is given to the selection circuit 25 and the determination unit 26.

  FIG. 3 is a graph showing the characteristics of the interpolation filters of the SP interpolation unit 21 and the ICI SP interpolation unit 23 with time on the horizontal axis and spectrum on the vertical axis. 3, the band of the interpolation filter (SP interpolation filter) of the SP interpolation unit 21 is compared with the characteristic (thin line) of the time direction interpolation filter (ICI interpolation filter) of the ICI SP interpolation unit 23. It is sufficiently narrow. Thereby, the interpolation process of the SP interpolation unit 21 can suppress the mixing of Gaussian noise and improve the S / N compared to the interpolation process of the ICI SP interpolation unit 23.

  In the present embodiment, the data from the equalization processing units 22 and 24 is given to the determination unit 26. The determination unit 26 obtains reception quality for the data from the equalization processing units 22 and 24 and determines which data has good reception quality. For example, the determination unit 26 may determine the reception quality based on the reception S / N. The reception S / N can be easily obtained by using a MER (Modulation Error Ratio) in which a vector error on the constellation map is expressed as a power ratio. The determination unit 26 outputs a determination result in a predetermined unit such as every symbol, every several symbols, every frame, or the like.

  The determination unit 26 outputs the reception quality determination result to the selection circuit 25. The selection circuit 25 is given a reception quality determination result, and selects and outputs an output determined to have good reception quality from the outputs of the equalization processing units 22 and 24.

  Note that the determination unit 26 may use the error detection result as a criterion for determining the reception quality. That is, the determination unit 26 obtains a determination result based on the result of error correction of the data from the equalization processing units 22 and 24. In this case, the selection circuit 25 may select and output data with few errors.

  The output of the selection circuit 25 is given to the demodulator 15. The demodulator 15 restores the original data from the input equalized OFDM symbol and outputs it as a demodulated output.

  Next, the operation of the embodiment configured as described above will be described with reference to FIG. FIG. 4 is a graph showing reception characteristics by interpolation processing of the SP interpolation unit 21 and the ICI SP interpolation unit 23, with the Doppler frequency on the horizontal axis and the reception S / N on the vertical axis.

  The OFDM signal input via the input terminal 11 is given to the orthogonal demodulation unit 12. The quadrature demodulator 12 performs quadrature detection on the input OFDM signal to obtain baseband I and Q signals. The baseband OFDM signal composed of these I and Q signals is given to the FFT unit 13, and the FFT unit 13 performs an FFT operation on the OFDM signal in the effective symbol period excluding the guard period. Thereby, an OFDM symbol in the frequency domain is obtained from the OFDM signal in the time domain. The OFDM symbol from the FFT unit 13 is supplied to the equalization unit 14.

  The OFDM symbol is supplied to the SP interpolation unit 21 and the ICI SP interpolation unit 23 of the equalization unit 14. The SP interpolation unit 21 interpolates the SP signal in the time direction and the frequency direction using an interpolation filter, and obtains an SP signal corresponding to each data carrier. The SP interpolation unit 21 estimates the channel characteristics at each carrier position using the interpolated SP signal, and gives the estimation result and data of the channel characteristics to the equalization processing unit 22. The equalization processing unit 22 performs waveform equalization on each data using the transmission path characteristics at each carrier position. As a result, the data from which the transmission path distortion has been removed is output from the equalization processing unit 22.

  4 indicates the output characteristics of the SP interpolation unit 21. The band of the SP interpolation filter is sufficiently narrow. Therefore, the SP interpolation unit 21 can perform interpolation processing with low noise. As shown in FIG. 4, the SP interpolation unit 21 is in a region where the influence of Doppler shift is small, that is, when the Doppler frequency is relatively low. The reception S / N of the output of is sufficiently high.

  On the other hand, the SP interpolation unit 23 for ICI interpolates the SP signal inserted for every 4 symbols for each subcarrier with an interpolation filter in the time direction. In this case, the ICI SP interpolation unit 23 performs interpolation in the time direction using a sufficiently wide band interpolation filter. As a result, the ICI SP interpolation unit 23 can reliably detect the Doppler shift component that affects inter-subcarrier interference. The ICI SP interpolation unit 23 estimates the Doppler shift component using the SP signal interpolated in the time direction, and equalizes the SP signal using the estimated Doppler shift component. Thereby, the Doppler shift component included in the SP signal is removed. Next, the ICI SP interpolation unit 23 interpolates the SP signal from which the Doppler shift component has been removed by using an interpolation filter in the frequency direction, and estimates the transmission path characteristics at each carrier position using the interpolated SP signal.

  The ICI SP interpolation unit 23 gives the estimation result and data of the transmission path characteristics to the equalization processing unit 24. The equalization processing unit 24 equalizes each data waveform using the transmission path characteristics at each carrier position. Thereby, the data from which the transmission line distortion is removed is obtained from the equalization processing unit 24.

  The thin line in FIG. 4 indicates the output characteristics of the ICI SP interpolation unit 23. The band of the ICI interpolation filter is sufficiently wide. As a result, the ICI SP interpolation unit 23 can reliably detect and remove the Doppler shift component. As shown in FIG. 4, the ICI SP interpolation unit can be used even in a region where the influence of the Doppler shift is large. The output of 23 can obtain a relatively high reception S / N.

  However, since the ICI interpolation filter has a wide band, noise is easily mixed and the S / N is likely to deteriorate.

  For this reason, when the Doppler frequency is lower than ft, if only the ICI SP interpolation unit 23 is used, the reception S / N due to noise mixing is improved rather than the improvement of the reception S / N by removing the Doppler shift component. Deterioration is greater. For this reason, when the Doppler frequency is ft or less, the output S / N of the output of the SP interpolation unit 21 is higher than the output of the SP interpolation unit 23 for ICI.

  In the present embodiment, the determination unit 26 detects reception quality (for example, reception S / N) for the waveform equalized data from the equalization processing units 22 and 24. The determination result of the determination unit 26 is given to the selection circuit 25. The selection circuit 25 selects an output with good reception quality from the outputs of the equalization processing units 22 and 24 according to the determination result, and outputs the selected output to the demodulation unit 15.

  That is, in the example of FIG. 4, as indicated by the bold line, when the Doppler frequency is ft or less, the output of the SP interpolation unit 21 is selected, and when the Doppler frequency exceeds ft, the output of the ICI SP interpolation unit 23 is selected and output. To do. Thereby, data with low noise and sufficiently suppressed subcarrier interference can be obtained.

  The data equalized by the equalizer 14 is given to the demodulator 15. The demodulator 15 demodulates the output of the equalizer 14 to restore the original data and outputs it as a demodulated output.

  As described above, in this embodiment, the time direction interpolation filter of the SP interpolation unit 21 is designed to be narrower than the time direction interpolation filter of the ICI SP interpolation unit 23, and the Gaussian noise of the SP signal is reduced. The S / N improvement is achieved. The determination unit determines the reception quality between the output of the SP interpolation unit 21 and the output of the ICI SP interpolation unit 23, and based on the determination result, one of the output of the SP interpolation unit 21 and the output of the ICI SP interpolation unit 23 Is selected and output. As a result, data with good reception quality is always obtained.

  Note that the determination unit may determine the reception quality using both the reception S / N and the error correction result.

(Second Embodiment)
FIG. 5 is a block diagram showing a second embodiment of the present invention. In FIG. 5, the same components as those of FIG.

  As shown in FIG. 4, whether an equalization process using the SP interpolation unit 21 or the ICI SP interpolation unit 23 is selected can ensure a sufficiently high reception S / N. As shown, it can be determined based on the Doppler frequency ft. That is, if the Doppler frequency is detected, the equalization process can be selected without determining the reception S / N.

  The first embodiment is that a Doppler frequency estimation unit 32 is provided, a determination unit 33 is employed instead of the determination unit 26, and an equalization processing unit 34 is employed instead of the equalization processing units 22 and 24. Different from the embodiment. The OFDM symbol input to the equalization unit 31 is supplied not only to the SP interpolation unit 21 and the ICI SP interpolation unit 23 of the equalization unit 31 but also to the Doppler frequency estimation unit 32.

  The Doppler frequency estimation unit 32 estimates the Doppler frequency from the OFDM symbol. For example, the Doppler frequency estimation unit 32 may add the correlation between SP signals having the same subcarrier frequency but different times for a plurality of subcarriers to which the SP signal is transmitted, and estimate the Doppler frequency from the magnitude. it can. The Doppler frequency estimation unit 32 outputs information on the estimated Doppler frequency to the determination unit 33.

  The determination unit 33 determines whether the Doppler frequency estimated by the Doppler frequency estimation unit 32 is higher or lower than the Doppler frequency ft of FIG. 4, and based on the determination result, the output of the SP interpolation unit 21 and the ICI SP interpolation unit 23 Determine which output to select.

  That is, the determination unit 33 controls the selection circuit 25 to select the output of the SP interpolation unit 21 and the ICI SP when the Doppler frequency estimated by the Doppler frequency estimation unit 32 is equal to or lower than the threshold frequency ft. When the operation of the interpolation unit 23 is stopped and the estimated Doppler frequency exceeds the frequency ft, the selection circuit 25 is controlled to select the output of the ICI SP interpolation unit 23 and the operation of the SP interpolation unit 21 is stopped. Let

  The selection circuit 25 is controlled by the determination unit 33 to select the output of the SP interpolation unit 21 or the ICI SP interpolation unit 23 and output the selected output to the equalization processing unit 34. The equalization processing unit 34 equalizes each data waveform using the transmission path characteristics at each carrier position. As a result, data from which transmission path distortion has been removed is obtained from the equalization processing unit 34.

  In addition, the determination unit 33 can selectively operate only one of the SP interpolation unit 21 and the ICI SP interpolation unit 23 and stop the other operation, thereby suppressing power consumption.

  Next, the operation of the embodiment configured as described above will be described.

  The OFDM signal from the FFT unit 13 is supplied to the SP interpolation unit 21, the ICI SP interpolation unit 23, and the Doppler frequency estimation unit 32 of the equalization unit 31. The SP interpolation unit 21 interpolates the SP signal in the time direction and the frequency direction by an interpolation filter to obtain an SP signal corresponding to each data carrier, and estimates the transmission path characteristics at each carrier position. On the other hand, the ICI SP interpolator 23 detects the Doppler shift component by interpolating the SP signal in the time direction, and equalizes the waveform of the SP signal. Further, the ICI SP interpolation unit 23 estimates the channel characteristics at each carrier position using the equalized SP signal. The estimation results of the transmission path characteristics from the SP interpolation unit 21 and the ICI SP interpolation unit 23 are supplied to the selection circuit 25.

  On the other hand, the Doppler frequency estimation unit 32 estimates the Doppler frequency for the input OFDM signal. The Doppler frequency estimation unit 32 outputs the Doppler frequency estimation result to the determination unit 33. When the estimated Doppler frequency is equal to or lower than the frequency ft in FIG. 4, the determination unit 33 stops the operation of the ICI SP interpolation unit 23 and causes the selection circuit 25 to select the output of the SP interpolation unit 21. The selection circuit 25 supplies the output and data of the SP interpolation unit 21 to the equalization processing unit 34 to equalize the waveform. Thereby, in this case, waveform equalization with low noise is possible.

  On the other hand, when the estimated Doppler frequency exceeds the frequency ft in FIG. 4, the determination unit 33 stops the operation of the SP interpolation unit 21 and selects the output of the ICI SP interpolation unit 23 to the selection circuit 25. Let The selection circuit 25 supplies the output and data of the ICI SP interpolation unit 23 to the equalization processing unit 34 to equalize the waveform. Thereby, in this case, waveform equalization with suppressed inter-subcarrier interference due to Doppler shift is possible. In this way, the equalizer 31 can supply data with sufficient reception quality to the demodulator 15.

  As described above, in the present embodiment, the Doppler frequency estimation unit estimates the Doppler frequency, and based on the estimation result, the outputs of the SP interpolation unit 21 and the ICI SP interpolation unit 23 are selected to perform waveform equalization. . This enables waveform equalization with low noise when the Doppler frequency is relatively low, and waveform equalization with reduced inter-subcarrier interference when the Doppler frequency is relatively high. Can be improved. Further, when one output of the SP interpolation unit 21 and the ICI SP interpolation unit 23 is selected, the operation of the other is stopped, so that power consumption can be reduced.

(Modification)
FIG. 6 is a block diagram showing a modification of the second embodiment. In FIG. 6, the same components as those in FIG.

  In mobile communication, the Doppler frequency changes mainly according to the speed of the mobile body. That is, in mobile communication, it is possible to estimate the Doppler frequency based on the speed information of the mobile body.

  The modification of FIG. 6 differs from FIG. 5 in that an equalization unit 41 that employs a Doppler frequency estimation unit 42 instead of the Doppler frequency estimation unit 32 is used. The Doppler frequency estimation unit 42 is input with speed information of the moving object. Information about the reception channel (frequency) is also input to the Doppler frequency estimation unit 42 (not shown). As the speed information, information from a speedometer of a moving body, a GPS signal, or the like can be used.

  The Doppler frequency in the moving body increases as the reception frequency increases and the movement speed increases. The Doppler frequency estimation unit 42 estimates the Doppler frequency based on the information on the reception channel and the speed information. The Doppler frequency estimation unit 42 is configured to output the estimated Doppler frequency to the determination unit 33.

  Other configurations and operational effects are the same as those of the embodiment of FIG.

  As described above, this modification has an advantage that the Doppler frequency can be estimated based on the speed of the moving body and the reception channel, and the apparatus can be simplified.

    DESCRIPTION OF SYMBOLS 12 ... Orthogonal demodulation part, 13 ... FFT part, 14 ... Equalization part, 15 ... Demodulation part, 21 ... SP interpolation part, 22, 24 ... Equalization process part, 23 ... SP interpolation part for ICI, 25 ... Selection part, 26: Determination unit.

Claims (5)

In an OFDM receiver that receives an orthogonal frequency division multiplex signal including pilot signals periodically arranged in a frequency direction and a time direction,
A Fourier transform unit for transforming the orthogonal frequency division multiplexed signal, which is a time domain signal, into a frequency domain signal;
The pilot signal included in the output of the Fourier transform unit is interpolated in the time direction and the frequency direction to estimate the transmission path characteristic, and the output of the Fourier transform unit is equalized based on the estimation result And
An inter-carrier interference removing unit for removing inter-carrier interference at the output of the Fourier transform unit;
A second equalization unit that estimates transmission path characteristics from the output of the Fourier transform unit from which inter-carrier interference is removed by the inter-carrier interference removal unit, and equalizes the output of the Fourier transform unit based on the estimation result When,
An OFDM receiving apparatus comprising: a selection unit that selectively outputs one of the outputs of the first and second equalization units.   2. The OFDM reception according to claim 1, further comprising: a determination unit that determines reception quality of outputs of the first and second equalization units and controls selection of the selection unit based on a determination result. apparatus.   The OFDM receiver according to claim 2, wherein the determination unit determines the reception quality based on at least one of a reception S / N and an error determination result.   The OFDM receiving apparatus according to claim 1, further comprising a determination unit that determines inter-carrier interference included in an output of the Fourier transform unit and controls selection of the selection unit based on a determination result.   The OFDM receiver according to claim 4, wherein the determination unit determines the inter-carrier interference based on a Doppler shift component of an output of the Fourier transform unit.

Images