JP2008118567A - Ofdm receiving device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain excellent reception quality by controlling an FFT window to an optimal position in relatively small circuit scale. <P>SOLUTION: An OFDM receiving device includes: a Fourier transform means 16 for performing Fourier transform processing on an input OFDM time waveform; a band limiting means 30 for selecting a band to be limited from among all bands of the OFDM time waveform and performing down conversion while limiting the selected band; a storage means 37 for storing the OFDM time waveform band-limited by the band limiting means; a reception quality determining means 20 for reading the OFDM time waveform from the storage means in accordance with an FFT window specifying a read position of the OFDM time waveform stored in the storage means, and determining reception quality of the read OFDM time waveform; and FFT window generating means 25, 26 for controlling the FFT window and setting the FFT window for obtaining a prescribed determination result from the reception quality determining means to the Fourier transform means. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an OFDM receiver applied to a mobile communication system, a wireless LAN system, and the like.

  In recent years, the digitization of terrestrial television broadcasting systems has been studied. In Europe and Japan, an OFDM (Orthogonal Frequency Division Multiplexing) modulation method is adopted as a transmission method for a terrestrial television broadcasting system. This OFDM system has an advantage that delay interference characteristics in a multipath propagation path, which is an essential transmission condition in terrestrial television broadcasting, can be improved by transmitting a wide-area signal with a large number of carriers orthogonal to each other.

  In the transmission apparatus, an OFDM signal is generated by performing inverse Fourier transform on a large number of modulated carriers at once. The OFDM signal is transmitted in transmission symbol units. A transmission symbol (OFDM symbol) is transmitted in a format in which a guard interval is added before an effective symbol in order to avoid intersymbol interference due to multipath. The guard interval is obtained by copying the waveform at the end of the effective symbol as it is to the front of the effective symbol period.

  The receiving apparatus separates each carrier wave from the effective symbol by cutting out the effective symbol from the received transmission symbol and Fourier-transforming the extracted effective symbol. In addition, FFT (Fast Fourier Transform) which is a high-speed transformation algorithm is used for Fourier transformation.

  An effective symbol extraction unit in which an FFT (time) window indicating the extraction position (period) is set is employed for extracting effective symbols. The FFT window is set to the effective symbol period length. Since the guard interval is obtained by copying the waveform of the end portion of the effective symbol as it is to the front of the effective symbol period, the original effective period can be set regardless of the time position including the card interval period. Data of symbol periods can be extracted, and FFT orthogonality can be maintained. Therefore, if the multipath delay time is within the guard interval period, it is possible to extract effective symbols that are not affected by the multipath by appropriately setting the position of the FFT window.

  However, when the FFT window is set so as to be shifted to a position including the adjacent symbol, intersymbol interference occurs, and the orthogonality of the FFT cannot be maintained. For this reason, in order to obtain good reception quality, it is necessary to accurately control the setting position of the FFT window.

  Therefore, Patent Document 1 employs a method in which a circuit for controlling the FFT window is provided, and the setting position of the FFT window is continuously controlled by repeatedly changing the position of the FFT window and verifying the reception quality. The reception quality can be verified by giving an OFDM symbol to the FFT circuit and using the output from the FFT circuit.

  In particular, in mobile communication where reception is performed in a running car or the like, radio waves arrive from a plurality of different directions due to reflection of buildings, etc., and the received waves interfere with each other, and the received signal is temporally A fading phenomenon that fluctuates greatly occurs. In such a case, it is important to control the FFT window continuously and accurately, and the method of Patent Document 1 is effective in this respect.

However, in Patent Document 1, in order to repeat reception quality verification, it is necessary to repeatedly supply OFDM symbols to the FFT circuit, and a buffer memory that holds transmission symbols for at least one symbol is required. Since it is necessary to provide such a memory, the device of Patent Document 1 has a problem that the circuit scale is large.
JP 2004-336279 A

  An object of the present invention is to provide an OFDM receiver capable of continuously controlling the FFT window to an optimum position with a relatively small circuit scale and obtaining good reception quality.

  An OFDM receiver according to an aspect of the present invention selects a Fourier transform unit that performs a Fourier transform process on an input OFDM time waveform, a band that is restricted from the entire band of the OFDM time waveform, and restricts the band to the selected band. Band-limiting means for down-converting, band-limiting means for band-limiting and down-converting, storage means for storing the OFDM time waveform band-limited by the band-limiting means, and OFDM time waveform stored in the storage means The OFDM time waveform is read from the storage means in accordance with an FFT window that defines the read position of the received signal, the reception quality judgment means for judging the reception quality of the read OFDM time waveform, the FFT window is controlled, and the reception quality An FFT window for obtaining a predetermined determination result from the determination means is set in the Fourier transform means. It is obtained by and a FFT window generating means.

  According to the present invention, it is possible to continuously control the FFT window to the optimum position with a relatively small circuit scale and obtain a good reception quality.

  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, the signal receiver 10 includes an antenna 11, a tuner 12, an AGC circuit 13, an A / D converter 14, and a quadrature detection circuit 15. An OFDM signal is induced in the antenna 11. The OFDM signal received by the antenna 11 is supplied to the tuner 12. The tuner 12 converts the frequency of the input radio frequency band OFDM signal into an IF (intermediate frequency) band OFDM signal and outputs it to the AGC circuit 13.

  The AGC circuit 13 controls the output signal level of the tuner 12 to be constant. The output of the AGC circuit 13 is given to the A / D converter 14, and the A / D converter 14 converts the analog OFDM signal into a digital signal and outputs it to the quadrature detection circuit 15. The quadrature detection circuit 15 converts the input OFDM signal into a baseband OFDM signal and outputs it.

  The OFDM signal from the quadrature detection circuit 15 is supplied to the FFT unit 16 and also to the band limiting circuit 30. The output of the quadrature detection circuit 15 is an OFDM signal in the time domain. This OFDM signal has a guard interval in which the end portion of the effective symbol is copied to the head side. That is, one symbol of the OFDM signal in the time domain has a configuration in which a guard interval is arranged in the first guard period and an effective symbol is arranged in the subsequent effective symbol period.

  The FFT unit 16 sets an FFT window having an effective symbol period width by a time window setting unit 26 to be described later, and extracts a signal in a period specified by the FFT window (hereinafter referred to as FFT window period). Thereby, an effective symbol from which the guard period is removed is obtained. An effective symbol can be extracted even if the FFT window period is shifted within the guard period length. That is, by controlling the FFT window period according to the multipath delay time, it is possible to obtain an effective symbol that avoids the influence of multipath. Therefore, the setting of the FFT window for extracting effective symbols greatly affects the reception quality.

  The FFT unit 16 converts the time domain signal into a frequency domain signal by performing FFT processing on the effective symbol. The output of the FFT unit 16 is supplied to the data demodulation unit 17. The data demodulator 17 performs reverse processing of the modulation processing performed on the transmission side on the input signal to restore the original data. For example, each subcarrier of the OFDM symbol is modulated by a modulation scheme such as PSK modulation or QAM modulation, and the data demodulator 17 restores the original data by a demodulation process corresponding to these modulation schemes.

  In the present embodiment, in order to set the FFT window to the optimum setting, the effectiveness of the setting of the FFT window is determined using the output of the quadrature detection circuit 15. That is, the effective symbol is actually cut out while changing the setting of the FFT window. The buffer memory 37 stores one symbol data of an OFDM signal in the time domain, and cuts out and reads out an effective symbol while changing the setting of the FFT window. By determining the reception quality of the extracted effective symbol, the validity of the FFT window setting is determined.

  In the present embodiment, the band limiting circuit 30 supplies the buffer memory 37 after band limiting the symbol data of the OFDM signal. The band limiting circuit 30 band-limits the input OFDM signal to an arbitrary band. For example, the band limiting circuit 30 limits the bandwidth to 1/8 of the total bandwidth of the OFDM symbol. As a result, a buffer memory having a capacity of 1/8 can be adopted. Even in this case, the accuracy of reception quality confirmation is slightly reduced, but it is considered possible to determine whether the reception quality is superior or inferior due to the difference in the FFT window setting.

  The buffer memory 37 stores the band limited OFDM signal input from the band limiting circuit 30 in accordance with a write instruction from the time window control unit 25 constituting the FFT window generating unit. Further, in response to a read instruction from the time window control unit 25, the stored band-limited OFDM signal is output to the validity determination unit 20. The readout instruction from the time window control unit 25 instructs readout of a period (hereinafter referred to as a determination window period) corresponding to the FFT window period set in the FFT unit 16. The buffer memory 37 outputs the symbol data of the determination window period to the validity determination unit 20.

  The validity determination unit 20 determines the validity of the determination window period by verifying the reception quality from the input symbol data. Then, the validity determination unit 20 outputs a determination result of whether the reception quality is good or bad to the time window control unit 25. For example, the validity determination unit 20 instructs the time window control unit 25 to determine a determination window period during which symbol data whose reception quality is better than a predetermined threshold value is obtained. The validity determination unit 20 may instruct the time window control unit 25 of a determination window period during which symbol data with the best reception quality is obtained. During the validity determination, the validity determination unit 20 can also provide the time window control unit 25 with a control signal for switching the determination window period.

  The time window control unit 25 is given an instruction for switching the determination window period and the determination result of the effectiveness from the validity determination unit 20. When an instruction for switching the determination window period is given, the time window control unit 25 issues a read instruction from the buffer memory 37 based on this instruction.

  In addition, when an effectiveness determination result is given, the time window control unit 25 controls the time window setting unit 26 based on the determination result. The time window setting unit 26 is controlled by the time window control unit 25 to set the FFT window.

  As described above, an optimum FFT window is obtained by repeating a series of processes of reading the symbol data from the buffer memory 37 and determining the validity by the validity judgment unit 20 while changing the judgment window period by the time window control unit 25. can get.

  FIG. 2 is a block diagram showing a specific configuration of the band limiting circuit 30. As shown in FIG. 2, the band limiting circuit 30 includes a band limiting unit 21 and a downsampling unit 22. The band limiting unit 21 band-limits the input baseband digital OFDM signal. The downsampling unit 22 further downsamples the standby-limited OFDM signal. Thereby, only the symbol data of a predetermined band in one symbol is supplied to the buffer memory 37.

  FIG. 3 is a block diagram showing an example of a specific configuration of the validity determination unit 20 in FIG. The example of FIG. 3 is an example in which the symbol data input from the buffer 37 is subjected to FFT processing and the reception quality is verified. In this case, the time window setting unit 26 is configured to set a determination window for designating the determination window period in the FFT unit 31 under the control of the time window control unit 25.

  In FIG. 3, the FFT unit 31 has the same configuration as the FFT unit 16. That is, the FFT unit 31 sets a determination window period for the symbol data from the buffer memory 37 and extracts a valid symbol. The FFT unit 31 converts the time-domain OFDM signal into a frequency-domain signal by performing FFT processing on the extracted effective symbol in the determination window period. The signal converted into the frequency domain is supplied to the reception quality measurement circuit 32.

  In the present embodiment, since the symbols input to the FFT unit 31 are band-limited, the number of samples processed by the FFT unit 31 is smaller than the number of samples processed by the FFT unit 16. Therefore, the size of the working memory required by the window control FFT unit 31 at the time of calculation can be reduced.

  The reception quality measurement circuit 32 measures the reception quality of the input signal. For example, the reception quality measurement circuit 32 equalizes the frequency domain OFDM signal from the FFT unit 31 and then calculates S / N (signal-to-noise ratio) data. This S / N data becomes reception quality data indicating the reception quality of the signal obtained by the FFT operation for demodulation with respect to the effective symbol extracted from the OFDM signal received at this time using the determination window period.

  Reception quality data from the reception quality measurement circuit 32 is supplied to the reception quality judgment circuit 33. The reception quality judgment circuit 33 compares the value of the reception quality data from the reception quality measurement circuit 32 with a predetermined reference value. The reception quality determination circuit 33 determines whether the reception data satisfies the standard and the reception quality is good or the reception quality is poor without satisfying the standard by comparing the value of the reception quality data with the reference value. The reception quality judgment circuit 33 supplies the judgment result to the time window control unit 25.

  In this case, the time window control unit 25 controls the determination window period and the FFT window period based on the determination result of the reception quality determination circuit 33. That is, during the reception quality determination, the time window control unit 25 sequentially switches the determination window period, gives a read instruction to the buffer memory 37, and sets the determination window period in the FFT unit 31. When the determination of the reception quality is completed, the time window control unit 25 sets the determination window period indicated by the determination result that the reception quality is good as the FFT window period in the FFT unit 16.

  Next, the processing for determining the window position of the FFT window will be described with reference to the flowchart of FIG. 4 shows an example in which the circuit of FIG.

  The time window control unit 25 gives a read instruction to the buffer memory 37 and controls the time window setting unit 26 to determine the window position of the determination window. In step S1 of FIG. 4, the determination window is initially set. The time window setting unit 26 is controlled by the time window control unit 25 to set the determination window to the initial window position. In step S 2, the OFDM symbol from the band limiting circuit 30 is written into the buffer memory 37 under the control of the time window control unit 25.

  Next, in step S <b> 3, the FFT unit 31 reads the OFDM signal in the initial determination window period from the buffer memory 37. The FFT unit 31 performs an FFT operation on the read OFDM signal. In the FFT unit 31, the band-limited OFDM signal is converted from a time domain signal to a frequency domain signal.

  The OFDM signal converted into the frequency domain signal is supplied to the reception quality measurement circuit 32. The reception quality measurement circuit 32 equalizes the frequency domain OFDM signal from the FFT unit 31 and then calculates S / N (step S5). This S / N data becomes reception quality data indicating reception quality.

  If this reception quality data indicates reception quality higher than a predetermined threshold, for example, the window position of the set determination window is good, that is, the window position of this determination window is set as the FFT window. This shows that sufficient reception quality can be obtained. Therefore, in this case, the reception quality determination circuit 33 outputs a signal indicating that the reception quality is good to the time window control unit 25. Then, the time window control unit 25 controls the time window setting unit 26 so as to set the window position of the determination window to the FFT window. Thus, the time window setting unit 26 sets an FFT window from which good reception quality can be obtained.

  On the other hand, reception quality may be poor at the initial window position due to the influence of fading or the like. In this case, the reception quality data indicates reception quality lower than a predetermined threshold, for example. The reception quality judgment circuit 33 outputs a signal indicating that the reception quality is poor to the time window control unit 25. Then, the time window control unit 25 moves the process to step S7, determines whether the number of window position changes exceeds the specified number, and if not, in step S8, determines whether the determination window Change the window position.

  The time window setting unit 26 sets a window position different from the previous determination window position in the FFT unit 31. Thus, the FFT unit 31 cuts out the OFDM signal held in the buffer memory 37 in the newly set determination window period in step S3. Thereafter, the processes in steps S2 to S9 are repeated.

  That is, the time window control unit 25 sequentially switches the determination window positions until a determination result that the reception quality is good is obtained. Then, the time window setting unit 26 sets the determination window period in which good reception quality is obtained in the FFT unit 16 as the FFT window period.

  If the determination window is set as an FFT window in step S9, or if the number of window position changes exceeds a predetermined number in step S7, the process returns to step S2, and the next one symbol is stored in the buffer memory. 37, and the processing of steps S3 to S9 is repeated.

  The FFT unit 16 cuts out an effective symbol from the OFDM signal from the quadrature detection circuit 15 according to the FFT window set by the time window setting unit 26, and performs an FFT operation on the effective symbol. Thus, the time domain OFDM signal is converted into a frequency domain signal. The output of the FFT unit 16 is supplied to the data demodulation unit 17. The data demodulator 17 performs reverse processing of the modulation processing performed on the transmission side on the input signal to restore the original data. In this way, demodulated data is output from the data demodulator 17.

  Thus, in the present embodiment, the OFDM signal after being band-limited by the band-limiting circuit is supplied to the buffer memory holding the OFDM signal for determining the validity of the FFT window. As a result, the capacity of the buffer memory can be reduced, and the FFT circuit used for determining the validity of the FFT window can be reduced.

  In the example of FIG. 4, the reception quality is determined in step S6, and the determination window is set to the FFT window when it is determined that the reception quality is good. However, the determination window may be shifted within a specified range. . That is, the determination window may be sequentially shifted within a predetermined band, the reception quality result in each determination window may be retained, and the determination window that obtains the best determination result may be set as the FFT window. For example, eight different window positions may be sequentially shifted to determine any one window position with the best reception quality.

  If it is determined that good reception quality cannot be obtained at any window position, the setting at the time of the previous symbol may be used as the window position of the FFT window set in the FFT unit 16, or the initial setting Window positions may be used. Moreover, although the example which sets a window position for every symbol was shown in FIG. 4, you may make it set a window position for every predetermined period.

(Modification of the first embodiment)
FIG. 5 is a flowchart showing an operation flow in the case of verifying reception quality at a plurality of predetermined window positions of a determination window according to a modification of the first embodiment. In FIG. 5, the same steps as those in FIG.

  In step S5, the reception quality measuring circuit 32 measures the reception quality. This reception quality data is stored in a memory (not shown) together with information related to the determination window used for measurement. In the next step S10, it is determined whether or not the verification has been completed for all the determination windows to be verified. That is, a plurality of preset window positions are defined as the window positions of the determination window used for verification, and it is determined whether or not verification has been performed for all of these window positions.

  If the verification of all window positions is not completed, the process proceeds to step S8, a new determination window position is set, and the process returns to step S3. If the verification of the full window position is completed, the process proceeds to step S6, and the reception quality determination circuit 33 includes a predetermined reference in the verification result (reception quality data) for the stored full window position. It is determined whether or not good quality reception quality data satisfying the value is included.

  If it is determined in step S6 that the stored verification results for all window positions do not include good quality, the process returns to step S2. In step S 2, the band-limited OFDM signal stored in the buffer memory 37 is discarded, and the band-limited OFDM signal received at that time is written in the buffer memory 37. Then, the subsequent processing is repeated.

  On the other hand, if it is determined in step S6 that the stored verification results for all window positions include good quality, reception quality data indicating the best reception quality is stored from the stored data. The determination window corresponding to the received quality data is determined as the optimum determination window. Next, the window position of the optimum determination window is set in the FFT unit 16 as an FFT window by the time window setting unit 26 (step S9).

  In step S6, when the measured reception quality data and the set reference value for quality judgment are largely different and do not satisfy the judgment condition for good judgment, it is not judged as good forever and becomes an infinite loop and the FFT window is updated. It may fall into a state where it is not done. Therefore, it is necessary to avoid this by providing a suitable reference value setting, timeout processing, and the like.

  As described above, in this modification, the window period during which the best reception quality is obtained is set to the FFT window based on the verification results at all the window positions, and data demodulation with good reception quality can be performed. it can.

(Second Embodiment)
FIG. 6 is a block diagram showing a second embodiment of the present invention. In FIG. 6, the same components as those in FIG.

  In the first embodiment, the band limiting circuit 30 limits the OFDM signal to an arbitrary band and stores it in the buffer memory 37. On the other hand, in the present embodiment, it is possible to improve reception quality verification accuracy by designating a band to be limited.

  This embodiment is different from the first embodiment in that a band limiting circuit 50 is employed instead of the band limiting circuit 30. The band limiting circuit 50 limits the band of the OFDM signal from the quadrature detection circuit 15. In this case, the band limiting circuit 50 limits the band to a band having a good reception state using, for example, information on S / N or bit error rate. The band limiting circuit 50 determines the reception state by using the output of the FFT unit 16.

  FIG. 7 is a block diagram showing a specific configuration of the band limiting circuit 50 in FIG.

  The frequency domain OFDM signal is input from the FFT circuit 16 to the band-specific reception state measurement circuit 51. The band-specific reception state measurement circuit 51 measures the reception state using power, S / N, bit error rate, or the like for each band of the input frequency domain OFDM signal. The band-specific reception state measurement circuit 51 outputs a signal indicating the reception state for each band to the band selection circuit 52.

  The band selection circuit 52 uses the signal indicating the reception state input from the band-specific reception state measurement circuit 51 to select a frequency band having a good reception state, and outputs a signal indicating the band (hereinafter referred to as a selection band). Output to the frequency shift circuit 53.

  The time-domain OFDM signal from the quadrature detection circuit 15 is given to the low-pass filter 54 via the frequency shift circuit 53. The low-pass filter 54 limits the band of the input OFDM signal, and the down-sampling unit 55 down-samples the output of the low-pass filter 54. That is, the low-pass filter 54 and the downsampling unit 55 have the same configuration as the band limiting circuit 30. The frequency shift circuit 53 receives the signal indicating the selection band from the band selection circuit 52 and shifts the frequency so that the selection band of the OFDM signal input from the quadrature detection circuit 15 becomes the pass band of the low-pass filter 54. The output of the downsampling unit 55 is supplied to the buffer memory 37.

  Thus, the band limiting circuit 50 selects a frequency band with a good reception state from the baseband OFDM signal and limits the other frequency bands.

  In the embodiment configured as described above, the method for limiting the bandwidth of the OFDM signal held in the buffer memory 37 is different from that of the first embodiment.

  The OFDM signal held in the buffer memory 37 is symbol data in a band having a good reception state, and as a result, the accuracy of determining the validity of the determination window can be improved.

  Other operations and effects are the same as those of the first embodiment.

(Third embodiment)
FIG. 8 is a block diagram showing a third embodiment of the present invention. In FIG. 8, the same components as those of FIG. In the first and second embodiments, an example in which an FFT unit or the like, which is a circuit for determining the validity of the window position, is provided in the window control unit. In the present embodiment, an example is shown in which the FFT unit for determining the validity of the window position is also used by the main line FFT unit 16.

  In this embodiment, the FFT unit for determining the validity of the window position is omitted. The output of the quadrature detection circuit 15 is supplied to the FFT unit 16 via the selector 61. The selector 61 is also given the output of the buffer memory 37. The selector 61 is controlled by the time window control unit 64 to selectively output the OFDM signal from the quadrature detection circuit 15 or the band limited OFDM signal stored in the buffer memory 37 to the FFT unit 16.

  That is, the selector 61 selects the output of the buffer memory 37 during the window position control period, and selects the output of the quadrature detection circuit 15 when demodulating the OFDM signal. The window position control period is set to an idle (idle) time during which FFT processing for data demodulation is not performed in the FFT unit 16. The selector 61 makes it possible to use the circuit resources of the FFT unit 16 and the data demodulation unit 17 in time division for data demodulation and window control.

  The FFT unit 16 extracts the data of the effective symbol period length through the FFT window set in the time window setting unit 26, performs FFT processing, and outputs it to the data demodulation unit 17. The data demodulator 17 equalizes the frequency domain OFDM signal and then demodulates it. The data demodulator 17 outputs a demodulation result, calculates S / N data from the equalized data during the window control period, and receives reception quality determination unit 63 of the window controller 62 as data indicating reception quality. To supply.

  Reception quality determination unit 63 is provided with reception quality data during the window control period, and compares the value of reception quality data with a predetermined reference value. The reception quality determination circuit 63 determines whether the reception data satisfies the standard and the reception quality is good by comparing the value of the reception quality data with the reference value, or the reception quality is poor without satisfying the standard. The reception quality determination circuit 63 supplies the determination result to the time window control unit 64. Since reception quality data is output from the data demodulator 17, it is not necessary to provide a reception quality measurement circuit in the window controller 62.

  Similar to the time window control unit 25, the time window control unit 64 controls the buffer memory 37 and the time window setting unit 26. When a reception quality determination result is given from the reception quality determination unit 63, the time window control unit 64 controls the time window setting unit 26 based on the determination result.

  Next, the operation of the embodiment configured as described above will be described with reference to FIGS. FIG. 9 is an explanatory diagram illustrating an example of a time schedule for FFT processing in the FFT unit 16. FIG. 10 is a flowchart showing a method for controlling the window position of the FFT window by the window control unit 62.

  The time-domain OFDM signal is output from the quadrature detection circuit 15 and the band-limited OFDM signal is held in the buffer memory 37 as in the first embodiment. In the present embodiment, the OFDM signal from the quadrature detection circuit 15 and the OFDM signal from the buffer memory 37 are supplied to the FFT unit 16 via the selector 61.

  That is, in the present embodiment, not only the OFDM signal from the quadrature detection circuit 15 but also the OFDM signal stored in the buffer memory 37 for controlling the window position of the FFT window is stored in the FFT unit 16 of the main line system. It comes to supply. The FFT unit 16 performs time division on FFT processing for obtaining output data (hereinafter referred to as demodulation FFT processing) and FFT processing for FFT window validity determination (hereinafter referred to as window control FFT processing). . FIG. 9 illustrates time division processing in the FFT unit 16.

  As shown in FIG. 9, the FFT unit 16 performs window control FFT processing (window control 1, 2,...) In a free time of demodulation FFT processing (demodulation processing). In the example of FIG. 9, the window position of the FFT window is switched during each idle time, and the validity of the FFT window is determined at each of the two window positions.

  FIG. 10 shows the window control method. In FIG. 10, the same steps as those in FIG.

  The time window control unit 64 causes the selector 61 to select the output of the quadrature detection circuit 15 (select 0) during the demodulation FFT processing period in the FFT unit 16, and the output of the buffer memory 37 (select 1) during the free time of the FFT unit 16. ) Is selected. In step S <b> 11, select 0 is selected by the time window control unit 64. In the next step S12, the time window control unit 64 determines whether or not the FFT unit 16 is idle (idle) time.

  Now, it is assumed that it is the demodulation processing time of the FFT unit 16. In this case, the time window control unit 64 causes the selector 61 to select select 0. As a result, the OFDM signal from the quadrature detection circuit 15 is supplied to the FFT unit 16. The FFT unit 16 uses the main-line (demodulation) FFT window set by the time window setting unit 26 to extract data of the effective symbol period length from the input OFDM signal, and performs an FFT operation on the extracted data. Do.

  Here, it is assumed that the FFT unit 16 has a free time shown in the window control 1 of FIG. In this case, the time window control unit 64 selects select 1 in step S13. As a result, the OFDM symbol is read from the buffer memory 37 and supplied to the FFT unit 16.

  In the FFT unit 16, an initial value of the FFT window for window control is set from the time window setting unit 26. In step S4, the FFT unit 16 uses the set window control FFT window to extract effective symbol period length data, and performs an FFT operation on the extracted data (window control 1 in FIG. 9). . Thereby, the time-domain OFDM signal is converted into a frequency-domain OFDM signal. The FFT unit 16 outputs the frequency domain OFDM signal to the data demodulation unit 17.

  The data demodulator 17 demodulates the OFDM signal and outputs reception quality data to the window controller 62. The reception quality determination unit 63 of the window control unit 62 determines whether or not the input reception quality data is good quality reception quality data that satisfies a predetermined reference value (step S6).

  Now, it is assumed that the reception quality data does not satisfy the reference value and the reception quality is poor (NG in FIG. 9). In this case, the time window control unit 64 proceeds to step S7 and determines whether or not the number of window position changes has been exceeded. If not, the time window control unit 64 uses the FFT for window control FFT processing. Change the window position of the window. If the free time of the FFT unit 16 has not expired, the process proceeds from step S11, S12 to step S13, and the time window control unit 64 selects select 1. Thereby, the FFT unit 16 cuts out the data of the effective symbol period length using the newly set FFT window (window control 2 in FIG. 9). The FFT unit 16 performs an FFT operation on the cut out OFDM symbol, and the data demodulation unit 17 outputs reception quality data.

  The example of FIG. 9 shows an example in which the reception quality data in the window control 2 is also bad. In this case, the time window control unit 64 further changes the window position (step S8) and determines the validity. Here, it is assumed that the free time of the FFT unit 16 ends. In this case, the time window control unit 64 detects that it is not in the idle state in step S12, and causes the selector 61 to select select 0. Thereby, the demodulation process is continued in the FFT unit 16. In this case, an FFT window for demodulation is used.

  When the free time of the FFT unit 16 arrives again, the time window control unit 64 proceeds to step S13 and continues the determination process for window control. Thereafter, the same operation is repeated. In the example of FIG. 9, it is shown that data having good reception quality is obtained by the FFT processing and the demodulation processing on the data of the effective symbol period length extracted using the FFT window set in the period of the window control 6. ing. The time window control unit 64 receives a signal indicating that the reception quality is good from the reception quality determination unit 63, and performs control so that the FFT window at that time is set as an FFT window used for main line processing. (Step S9). In the next free time, a new symbol is written in the buffer memory 37 in step S2, and the same operation is repeated.

  In this way, the time window setting unit 26 outputs the instructed FFT window to the FFT unit 16 as an FFT window used for clipping the OFDM signal from the quadrature detection circuit 15. The FFT unit 16 cuts out effective symbol period length data from the OFDM signal using the designated FFT window during the demodulation processing period, and performs OFDM calculation. The data demodulator 17 demodulates the input frequency domain OFDM signal to restore the original data.

  When the measured reception quality data and the set reference value for quality determination are far apart from each other and the reception quality does not satisfy the determination condition for good or bad, in step S6, a good determination is not made forever but an infinite loop is generated. It is conceivable that the FFT window used for processing is not updated. Therefore, there is a need for a means for avoiding this by setting a suitable reference value and providing a time-out process such as step S7.

  As described above, also in this embodiment, the optimum FFT window position is detected using the narrowband OFDM signal, and the buffer memory 37 may store the bandlimited OFDM signal. The capacity of the memory 37 can be reduced. In the present embodiment, the FFT unit 16 performs the demodulation FFT process and the window control FFT process in a time-sharing manner, and can be controlled to the optimum window position by one FFT circuit. There is an advantage. Also, a reception quality measurement circuit that measures reception quality data from the frequency domain OFDM signal can be omitted. Thereby, it is possible to continuously set the FFT window to the optimum window position by continuously performing the control to the optimum window position without increasing the circuit scale.

  Note that the data demodulating unit 17 can equalize and demodulate the frequency domain OFDM signal from the FFT unit 16 and further perform error correction processing. In this case, the result of calculating the bit error rate of the output signal of the FFT unit 16 by using the data used for the error correction processing by the data demodulation unit 17 is supplied to the reception quality determination unit 63 as reception quality data. May be. That is, the window position of the FFT window can be controlled based on the bit error rate of the output signal of the FFT circuit 16.

(Modification of the third embodiment)
FIG. 11 is a flowchart showing an operation flow in a case where reception quality is verified at a plurality of window positions of a predetermined determination window according to a modification of the third embodiment. In FIG. 11, the same steps as those in FIG.

  In step S5, the reception quality measuring circuit 32 measures the reception quality. This reception quality data is stored in a memory (not shown) together with information related to the determination window used for measurement. In the next step S20, it is determined whether or not the verification has been completed for all the determination windows to be verified. That is, a plurality of preset window positions are defined as the window positions of the determination window used for verification, and it is determined whether or not verification has been performed for all of these window positions.

  If the verification of all window positions has not been completed, the process proceeds to step S8, a new determination window position is set, and the process returns to step S11. If the verification of the full window position is completed, the process proceeds to step S6, and the reception quality determination circuit 33 includes a predetermined reference in the verification result (reception quality data) for the stored full window position. It is determined whether or not good quality reception quality data satisfying the value is included.

  If it is determined in step S6 that the stored verification results for all window positions do not include good quality, the process returns to step S2. In step S 2, the band-limited OFDM signal stored in the buffer memory 37 is discarded, and the band-limited OFDM signal received at that time is written in the buffer memory 37. Then, the subsequent processing is repeated.

  On the other hand, if it is determined in step S6 that the stored verification results for all window positions include good quality, reception quality data indicating the best reception quality is stored from the stored data. The determination window corresponding to the received quality data is determined as the optimum determination window. Next, the window position of the optimum determination window is set in the FFT unit 16 as an FFT window by the time window setting unit 26 (step S9).

  As described above, in this modification, the window period during which the best reception quality is obtained is set to the FFT window based on the verification results at all the window positions, and data demodulation with good reception quality can be performed. it can.

1 is a block diagram showing an OFDM receiving apparatus according to a first embodiment of the present invention. FIG. 2 is a block diagram showing a specific configuration of a band limiting circuit 30 in FIG. 1. The block diagram which shows an example of the specific structure of the effectiveness determination part 20 in FIG. The flowchart for demonstrating operation | movement of 1st Embodiment. The flowchart which shows the modification of 1st Embodiment. The block diagram which shows the 2nd Embodiment of this invention. FIG. 7 is a block diagram showing a specific configuration of a band limiting circuit 50 in FIG. 6. The block diagram which shows the 3rd Embodiment of this invention. Explanatory drawing for demonstrating the operation | movement of 3rd Embodiment. The flowchart for demonstrating operation | movement of 3rd Embodiment. The flowchart which shows the modification of 3rd Embodiment.

Explanation of symbols

    DESCRIPTION OF SYMBOLS 10 ... Signal receiving part, 16 ... FFT part, 17 ... Data demodulation part, 19 ... Window control part, 20 ... Effectiveness determination part, 25 ... Time window control part, 26 ... Time window setting part, 30 ... Band-limiting circuit, 37: Buffer memory.

Claims (5)

Fourier transform means for performing a Fourier transform process on the input OFDM time waveform;
Band limiting means for selecting a band to be limited from the entire band of the OFDM time waveform and band-limiting to the selected band and down-converting;
Storage means for storing the OFDM time waveform band-limited by the band-limiting means;
A reception quality determination unit that reads out the OFDM time waveform from the storage unit according to an FFT window that defines a readout position of the OFDM time waveform stored in the storage unit, and determines reception quality of the read OFDM time waveform;
An OFDM receiving apparatus comprising: an FFT window generating unit that controls the FFT window and sets an FFT window for obtaining a predetermined determination result from the reception quality determining unit in the Fourier transform unit.   2. The OFDM receiving apparatus according to claim 1, wherein the reception quality judgment means includes a window control Fourier transform means for performing a Fourier transform process on the read OFDM time waveform.   The reception quality judging means drives the Fourier transform means in a time division manner, a Fourier transform process for the input OFDM time waveform and a window control Fourier transform process for the OFDM time waveform read from the storage means. The OFDM reception apparatus according to claim 1, wherein the reception quality is determined by the following.   2. The OFDM receiver according to claim 1, wherein the band limiting unit selects a band to be limited based on a signal power, a signal-to-noise ratio, or a bit error rate of each band of the OFDM time waveform.   2. The OFDM receiving apparatus according to claim 1, wherein the reception quality determining means determines the reception quality based on a signal-to-noise ratio or a bit error rate of the OFDM time waveform read from the storage means.

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