JP5275304B2 - OFDM receiver - Google Patents

Description

  Embodiments of the present invention relate to an OFDM receiving apparatus that receives a transmission signal based on an orthogonal frequency division multiplexing (OFDM) modulation method, and more particularly to cutout window control for setting a cutout window position for Fourier transform.

  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. Hereinafter, conventional techniques related to the present invention will be described.

  In domestic terrestrial digital broadcasting, 5617 subcarriers are OFDM-modulated and transmitted. In the receiving apparatus, a desired OFDM modulated signal is selected by a tuner, converted to a digital signal by an A / D conversion circuit, and data demodulation is performed by digital signal processing. At this time, transmission path distortion is estimated using scattered pilot signals (hereinafter referred to as SP signals) sparsely arranged in the frequency direction and the time direction, and corrected by equalization processing.

  The OFDM signal includes an effective symbol period and a guard interval period obtained by copying the rear end portion of the effective symbol period. A method of setting which range is cut out and calculated when performing FFT calculation on the signal in the time domain is a well-known technique. According to this conventional technique, the FFT window is determined by evaluating the transmission line characteristics from the impulse response of the SP signal. Further, by determining whether the interfering wave is a delayed wave or a preceding wave, the multipath interference can cope with a delay time up to 1/4 of the effective symbol length.

  As described above, the conventional OFDM receiver can cope with the delay time of the delayed wave up to 1/4 of the effective symbol length. However, with the development of error correction technology and inter-symbol interference cancellation technology, there is a possibility that it can be received even when there is a delayed wave exceeding the guard interval period (hereinafter referred to as the guard period). When there is a delayed wave exceeding 1/4 of the effective symbol length, the FFT window cannot be controlled. Further, even when the FFT window is searched and controlled by trial and error, if there is a delayed wave exceeding 1/3 of the effective symbol length, the ratio of the number of SP symbols prepared corresponding to the number of received symbols is all. Since the impulse response using the SP signal is limited to 1/3 in the received symbol, a delayed wave exceeding 1/3 of the effective symbol length cannot be expressed, and the reference position of the FFT window search cannot be obtained. Therefore, the FFT window cannot be controlled at a position where the reception quality is the best.

  Therefore, there is a demand for an OFDM receiver that can control the FFT window at a position where reception quality is optimal even when a delay wave exceeding 1/3 of the effective symbol length exists.

JP 2004-179816 A

  An object of the present invention is to provide an OFDM receiver capable of controlling an FFT window at a position where reception quality is optimal even when a delay wave exceeding 1/3 of an effective symbol length exists. To do.

An OFDM receiver according to an embodiment receives an OFDM signal including pilot signals periodically arranged in a frequency direction and a time direction.
Fourier transform means for cutting out the OFDM signal from the time domain signal by a cutout window signal and converting it to a frequency domain signal by Fourier transform;
Data demodulating means for demodulating the output of the Fourier transform means to obtain demodulated data;
Pilot extraction means for extracting pilot signals periodically arranged in the frequency direction and time direction from the output of the Fourier transform means;
Impulse response detection means for detecting an impulse response from the output of the pilot extraction means;
Peak detection means for detecting a peak position from the output of the impulse response detection means;
Quality detection means for detecting the reception quality of the OFDM signal;
A main wave search for detecting a true main wave position from which the influence of aliasing is eliminated using the peak detection signal from the peak detection means and the quality detection signal from the quality detection means, and detection by this main wave search A precise search for the cutout window for determining the Fourier transform cutout window position where the quality detection signal is the best by performing a search by stepping the Fourier transform cutout window position within a predetermined range including the main wave position. Cutting window control means for performing
Equipped with,
The clipping window control means uses the peak position detected by the peak detection means as a temporary main wave position, and determines the position of the main wave search clipping window relative to the main wave position by an impulse response using a pilot signal. Compare each reception quality obtained when switching while shifting back and forth by a predetermined expressible delay time, and compare the reception quality obtained at the temporary main wave position with the cutout window position with the best reception quality. A main wave search for detecting a peak of the impulse response as a true main wave position, and the Fourier transform cut-out window position within a predetermined range with the true main wave position by the main wave search as a window reference While changing the delay amount by a predetermined amount, a clipping window precision search is performed to detect the position of the Fourier transform clipping window with the best reception quality. And it outputs a square frame signal obtained by Chi said Fourier transform means.

The block diagram which shows the OFDM receiver of the 1st Embodiment of this invention. The figure which shows the impulse response which used the main wave, the delay wave, and SP signal. The block diagram which shows the FFT window control circuit in embodiment of this invention. Explanatory drawing of the impulse response detection concerning embodiment of this invention. The figure explaining arrangement | positioning of SP carrier in the OFDM system concerning embodiment of this invention. 3 is a flowchart for explaining an operation according to the first embodiment of the present invention. The block diagram which shows the OFDM receiver of the 2nd Embodiment of this invention. The block diagram which shows the OFDM receiver of the 3rd Embodiment of this invention. The flowchart explaining the operation | movement concerning the 3rd Embodiment of this invention.

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 a configuration of an OFDM receiving apparatus according to the first embodiment of the present invention. In FIG. 1, an OFDM receiving apparatus 100 includes an OFDM modulated signal input terminal 101, a tuner 102, an A / D conversion circuit 103, an orthogonal detection circuit 104, an FFT circuit 105 as Fourier transform means, and data demodulation means. Demodulation circuit 106, demodulated signal output terminal 107, SP extraction circuit 108 as pilot extraction means, impulse response detection circuit 109 as impulse response detection means, peak detection circuit 110 as peak detection means, An FFT window control circuit 111 and an S / N detection circuit 112 as reception quality detection means are provided.

The OFDM receiver 100 is a receiver that receives an OFDM signal including an SP signal that is a pilot signal periodically arranged in the frequency direction and the time direction.
The FFT circuit 105 cuts out an OFDM signal from a time domain signal using an FFT window signal, which is a cutout window signal, and converts the signal into a frequency domain signal by Fourier transform.
The SP extraction circuit 108 extracts SP signals periodically arranged in the frequency direction and the time direction from the output of the FFT circuit 105.

Demodulation circuit 106 demodulates the output of FFT circuit 105 based on the SP signal, and outputs the demodulated data to output terminal 107.
The impulse response detection circuit 109 detects the impulse response by performing inverse Fourier transform on the SP signal extracted by the SP extraction circuit 108.
The peak detection circuit 110 detects the peak position of the impulse response from the impulse response detection circuit , and performs FFT as a peak detection signal (this is treated as a signal indicating a temporary main wave position because it does not consider the influence of aliasing). Output to the window control circuit 111.

The S / N detection circuit 112 detects the received S / N from the demodulated data from the demodulation circuit 106 and outputs it to the FFT window control circuit 111 as an S / N signal as a quality detection signal.
The FFT window control circuit 111 uses a peak detection signal (a signal indicating a temporary main wave position) and an S / N signal to detect a true main wave position from which the influence of aliasing has been eliminated, FFT window precision for determining the FFT window position where the S / N signal is the best by performing a search by changing the FFT window position stepwise within a predetermined range including the main wave position detected by the main wave search. Search.

The operation will be described below with reference to FIG.
An OFDM modulated signal is supplied to an input terminal 101 from an antenna (not shown), and a tuner 102 selects only a desired OFDM modulated signal. The output of the tuner 102 is converted into a digital signal by the A / D conversion circuit 103 and supplied to the quadrature detection circuit 104. A signal obtained by quadrature detection by the quadrature detection circuit 104 and converted into a baseband in-phase detection axis signal (I signal) and a quadrature detection axis signal (Q signal) is input to the FFT circuit 105. The FFT circuit 105 performs an FFT operation on a signal cut out by the FFT window signal from the FFT window control circuit 111 in the input OFDM modulation signal. The output of the FFT circuit 105 is branched, and one of the outputs is demodulated by the demodulation circuit 106 based on the SP signal and output from the output terminal 107 as demodulated data output.

  The other side from the FFT circuit 105 is supplied to the SP extraction circuit 108. The SP extraction circuit 108 extracts SP signals sparsely arranged in the frequency direction and the time direction as shown in FIG. The impulse response detection circuit 109 receives the extracted SP signal in the frequency domain and performs inverse Fourier transform operation to detect the impulse response in the time domain. The impulse response detection result is supplied to the peak detection circuit 110, and the peak position of the impulse response is detected. The detected peak position is supplied to the FFT window control circuit 111 as a peak detection signal.

  The output of the demodulation circuit 106 is supplied to the S / N detection circuit 112, and the received S / N is detected from the demodulated data constellation and supplied to the FFT window control circuit 111.

  First, the FFT window control circuit 111 obtains the true main wave position from which the influence of aliasing is eliminated by detecting the S / N signal while switching the plurality of window reference positions including the temporary main wave position in the FFT window. Is performed, and then the FFT window is refined while stepping the FFT window within a predetermined range of the obtained true main wave position, and the FFT window in which the S / N signal is the best is obtained. The position is detected and the FFT window signal is determined.

FIG. 2 shows an impulse response using (a) main wave and delay wave, and (b) SP signal. The delayed wave has a delay time τ with respect to the main wave which is a direct wave. In the FIG. 2 (a), the are drawn by narrowing the width of the delayed wave with respect to the main wave, the delayed wave represents the go low power compared to the main wave. One symbol includes an effective symbol period and a guard period in which the rear end portion of the effective symbol period is copied and connected to the front end of the effective symbol period. FIG. 2B shows the impulse response output (in other words, delay profile output) corresponding to the head of the effective symbol period of each of the main wave and the delayed wave on the time axis.

  As shown in FIG. 5, in the OFDM symbol format, SP carriers are inserted at a rate of one for every 12 carriers in the frequency direction in a state where the OFDM symbol is sent from the transmission side. SP carriers are inserted at a rate of one for every three carriers in the frequency direction by interpolating SP carriers in the time direction with respect to the SP carrier inserted for every four carriers in the direction. become. Therefore, when an SP signal inserted at a rate of one out of three by such interpolation is used, a delay wave having a delay time corresponding to 1/3 of the effective symbol length is delayed until the impulse response. The output can be expressed on the time axis. In other words, only a range of 1/3 (= ± 1/6) of the effective symbol length can be expressed by one impulse response.

FIG. 4 is a diagram for explaining impulse response detection according to the present embodiment.
Assuming that the peak position (provisional main wave position) detected by the peak detection circuit 110 is the time axis reference 0, there is one impulse response peak at the position of the reference 0 in one impulse response as shown in the figure. In this case, it can be said that it is the main wave, but as the output of the impulse response, only a time range corresponding to 1/3 length of the effective symbol length can be expressed, so the position of 0 on the time axis is the center. There may be a delayed signal or a preceding signal as shown in the figure at a distance of ± 1/3.

  As a result, for example, a delayed signal at a position that is 1/3 of the effective symbol length from the position of the reference 0 on the time axis may be folded back at the position of +1/6 and looked at the position of 0. Alternatively, the preceding signal at a position separated by −1/3 of the effective symbol length from the position of the reference 0 may be turned back at the position of −1/6 and looked at the position of 0. As described above, in the embodiment, since it is known that folding is performed at ± 1/6, in principle, when the FFT window is set to a position of 0 (range of ± 1/6 centered on the position of 0) +1/3 position (± 1/6 centered on +1/3 position) and -1/3 position (± 1/6 centered on -1/3 position) When the FFT is performed by sequentially switching the FFT window position, for example, when the reception quality (for example, reception S / N) is the best, the main wave position is assumed. After finding the wave, the FFT window position is changed while changing the FFT window position in a predetermined range including the main wave position (this range may be narrower than the illustrated impulse response detection range) by a predetermined delay amount. FFT window position with the best reception quality (for example, reception S / N) The to perform a process of detecting (ok).

FIG. 3 shows a configuration of the FFT window control circuit 111 according to the embodiment of the present invention.
In FIG. 3, the FFT window control circuit 111 includes a main wave detection circuit 201, a window search circuit 202, and an FFT window setting circuit 203.

  The main wave detection circuit 201 is a circuit for detecting the true main wave position using the peak detection signal from the peak detection circuit 110 and the S / N signal from the S / N detection circuit 112. The window search circuit 202 performs a main wave search for detecting the position of the true main wave in consideration of the influence of aliasing, while based on the main wave position detection signal (main wave detection completion flag) from the main wave detection circuit 201. The FFT window signal with the best reception quality is selected while performing an accurate search for the FFT window signal within a predetermined delay time range. The FFT window setting circuit 203 combines the window offset signal output from the window search circuit 202 and representing the offset from the main wave position with the window reference signal representing the main wave position from the main wave detection circuit 201 and outputs the FFT window signal. Generated and supplied to the FFT circuit 105.

The operation will be described below with reference to FIG.
The main wave detection circuit 201 is supplied with a peak detection signal from the peak detection circuit 110 and an S / N signal from the S / N detection circuit 112. Since the SP signals are arranged at intervals of three carriers in the frequency direction by SP carrier interpolation in the time direction performed by the receiver with respect to the SP carrier arrangement at the time of transmission shown in FIG. 5, the impulse response using the SP signal Can only represent a time up to 1/3 of the effective symbol length, that is, ± 1/6. Therefore, as shown in FIG. 4, the main wave detection circuit 201 has an S / N signal obtained when the FFT window position is shifted by ± 1/3 of the effective symbol length and an S / N obtained when the time length is 0. The signal is compared with the peak at the FFT window position where the S / N signal is the best, and the true main wave is determined. The main wave detection circuit 201 supplies the true main wave position to the FFT window setting circuit 203 and supplies a main wave detection completion flag to the window search circuit 202. After the main wave is detected, the window search circuit 202 determines that the S / N signal is the best while changing the FFT window signal stepwise on the time axis within a predetermined range with the position of the true main wave as the window reference. It operates to detect the FFT window signal. The FFT window setting circuit 203 combines the window offset signal for the main wave position output from the window search circuit 202 and the window reference signal from the main wave detection circuit 201 by the FFT window setting circuit 203 to generate an FFT window signal. , And supplied to the FFT circuit 105.

FIG. 6 is a flowchart for explaining the operation according to the first embodiment of the present invention. The operation of setting the FFT window position will be described with reference to FIG.
First, an SP signal included in the OFDM signal is extracted (step S1), and an impulse response is detected based on the extracted SP signal (step S2). Impulse response is synonymous with transmission line response or delay profile. In the next step S3, the largest peak of the impulse response detection results is detected as the main wave, and this is used as the main wave candidate (temporary main wave) to enter the FFT window reference control mode (main wave search) in step S4. .

  In steps S4 to S7, an S / N detection is performed by setting an FFT window in the impulse response detection range (1) as the main wave position (eg, 0) detected in step S3 (step S5), and then steps S6 and S7. And sequentially execute each step. For example, after the main wave position 0, the FFT window is set in the impulse response detection range (2) as the main wave position (-1/3) and S / N detection is performed, and then the main wave position (+ 1 / As 3), S / N detection is performed by setting an FFT window in the impulse response detection range (3) (steps S6 and S7). When all the searches in the impulse response detection ranges (1) to (3) are completed, the FFT window having the maximum S / N as the search result is used as the FFT window reference to shift to the next FFT window precision search (step S8).

  In the FFT window precision search, first, a predetermined range is set so that the true main wave position (any one of 0, −1/3, and +1/3) detected by the main wave search in the previous stage is included. By changing the FFT window stepwise back and forth (for example, any range of the impulse response detection ranges (1) to (3), but may be a narrower range) (step S9), S / N detection is performed (step S10). Then, S / N detection is performed for all precision steps in which the FFT window in the predetermined range (time width) is changed stepwise back and forth (step S11), and thereafter, S / N values detected in all precision steps are detected. The maximum S / N FFT window position is finally selected and determined (step S12).

  According to the first embodiment, it is possible to control the FFT window at a position where the reception quality is the best even when there is a delayed wave exceeding 1/3 of the effective symbol length.

[Second Embodiment]
FIG. 7 shows an OFDM receiving apparatus according to the second embodiment of the present invention.
In FIG. 7, an OFDM receiver 100A includes an OFDM modulated signal input terminal 101, a tuner 102, an A / D conversion circuit 103, an orthogonal detection circuit 104, an FFT circuit 105, a demodulation circuit 106, and a demodulated signal. An output terminal 107, an SP extraction circuit 108, an impulse response detection circuit 109A, a peak detection circuit 110, an FFT window control circuit 111, and an S / N detection circuit 112A are provided.

  A difference in configuration from the first embodiment is an impulse response detection circuit 109A and an S / N detection circuit 112A. Instead of the configuration for performing S / N detection based on the output of the demodulation circuit 106 in the first embodiment, the configuration for performing S / N detection based on the output of the impulse response detection circuit 109A in the second embodiment. It is what. In addition, the same code | symbol is attached | subjected to the component similar to FIG. 1, and description is abbreviate | omitted.

  The impulse response detection circuit 109A supplies the impulse response detection result to the peak detection circuit 110 and also to the S / N detection circuit 112A. The S / N detection circuit 112A calculates the power of a signal having a level equal to or lower than a predetermined threshold value from the impulse response detection output. Since this value increases when the S / N is bad, for example, the reciprocal of this value can be obtained and used as the S / N signal.

  According to the second embodiment, since the demodulated data is not used in the S / N signal detection, the FFT window control can be performed before demodulation, and there is an advantage that the response is excellent.

[Third Embodiment]
FIG. 8 is a block diagram showing an OFDM receiving apparatus according to the third embodiment of the present invention.
In FIG. 8, an OFDM receiving apparatus 100B includes an OFDM modulated signal input terminal 101, a tuner 102, an A / D conversion circuit 103, an orthogonal detection circuit 104, an FFT circuit 105A, a demodulation circuit 106A, and a demodulated signal. an output terminal 107, an FFT circuit 105, a demodulation circuit 106, an SP extraction circuit 108, an impulse response detection circuit 109, the peak detection circuit 110, an FFT window control circuit 111, and the S / N detecting circuit 112A, It has.

In the first and second embodiments, the configuration of performing FFT window control by sharing the FFT circuit and the demodulation circuit, which are part of the main line system, has been described. The FFT window control may be processed in parallel.
The difference from the first and second embodiments in the configuration is that a main line FFT circuit 105A and a demodulation circuit 106A are separately provided. Each of the FFT circuit 105A and the demodulation circuit 106A may be the same circuit as the FFT circuit 105 and the demodulation circuit 106 in the FFT window control processing system.

FIG. 9 is a flowchart for explaining the operation according to the third embodiment of the present invention.
The difference from the flowchart of the second embodiment shown in FIG. 6 is that the FFT window position determined in step S12 is returned to step S1 and the FFT window search is repeatedly performed continuously, and the determination in step S12 is completed. The FFT window signal is output to the main line FFT circuit 105A. The other steps are the same as those in FIG.

  According to the third embodiment, the FFT window control processing system is provided in parallel with the main line data demodulation, and the processing is performed in parallel, whereby the FFT window search can be repeatedly and continuously operated. If the situation changes, you can follow it.

  As described above, according to the present invention, it is possible to control the FFT window so that the reception quality is the best even when a delayed wave exceeding 1/3 of the effective symbol length exists.

  In addition, although some embodiment of this invention was described, these embodiment is shown as an example and is not intending limiting the range of invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 100, 100A, 100B ... OFDM receiver, 102 ... Tuner, 103 ... A / D conversion circuit, 104 ... Quadrature detection circuit, 105 ... FFT circuit (Fourier transform means), 106 ... Demodulation circuit (data demodulation means), 108 ... SP extraction circuit (pilot extraction means), 109 ... impulse response detection circuit (impulse response detection means), 110 ... peak detection circuit (peak detection means), 111 ... FFT window control circuit (cutout window control means), 112 ... S / N detection circuit (quality detection means).

Claims (5)

In an OFDM receiver that receives an OFDM signal including pilot signals periodically arranged in a frequency direction and a time direction,
Fourier transform means for cutting out the OFDM signal from the time domain signal by a cutout window signal and converting it to a frequency domain signal by Fourier transform;
Data demodulating means for demodulating the output of the Fourier transform means to obtain demodulated data;
Pilot extraction means for extracting pilot signals periodically arranged in the frequency direction and time direction from the output of the Fourier transform means;
Impulse response detection means for detecting an impulse response from the output of the pilot extraction means;
Peak detection means for detecting a peak position from the output of the impulse response detection means;
Quality detection means for detecting the reception quality of the OFDM signal;
A main wave search for detecting a true main wave position from which the influence of aliasing is eliminated using the peak detection signal from the peak detection means and the quality detection signal from the quality detection means, and detection by this main wave search A precise search for the cutout window for determining the Fourier transform cutout window position where the quality detection signal is the best by performing a search by stepping the Fourier transform cutout window position within a predetermined range including the main wave position. Cutting window control means for performing
Equipped with,
The clipping window control means uses the peak position detected by the peak detection means as a temporary main wave position, and determines the position of the main wave search clipping window relative to the main wave position by an impulse response using a pilot signal. Compare each reception quality obtained when switching while shifting back and forth by a predetermined expressible delay time, and compare the reception quality obtained at the temporary main wave position with the cutout window position with the best reception quality. A main wave search for detecting a peak of the impulse response as a true main wave position, and the Fourier transform cut-out window position within a predetermined range with the true main wave position by the main wave search as a window reference While changing the delay amount by a predetermined amount, a clipping window precision search is performed to detect the position of the Fourier transform clipping window with the best reception quality. OFDM receiving apparatus and outputs a square frame signal obtained by Chi said Fourier transform means. The clipping window control means includes
In the main wave search,
The range of the clipping window for main wave search is a range of ± 1/6 of the effective symbol length of the OFDM signal, and when the temporary main wave position is set to 0,
A quality detection signal obtained when the center position of the main wave search clipping window is set to a position of 0;
A quality detection signal obtained when the center position of the cut-out window for main wave search is changed from 0 to a position of 1/3 of the effective symbol length of the OFDM signal;
Compared with the quality detection signal obtained when the center position of the clipping window for main wave search is changed from 0 to -1/3 of the effective symbol length of the OFDM signal,
Detecting the peak position of the impulse response at the time of the main wave search cutout window with the best reception quality as the true main wave position;
In the cutting window precision search,
The position of the Fourier transform cutout window having the predetermined range is changed by a predetermined delay amount so as to include the detected true main wave position, and the position of the Fourier transform cutout window having the best reception quality is detected. Do
The OFDM receiver according to claim 1 . The clipping window control means includes
A main wave detection circuit for detecting the position of the true main wave using the peak detection signal from the peak detection circuit and the quality detection signal from the quality detection means;
While performing a main wave search to detect the position of the true main wave that eliminates the effects of aliasing, the position of the Fourier transform cutout window is determined within a predetermined delay time range based on the main wave detection completion signal from the main wave detection circuit. A window search circuit for determining a Fourier transform cutout window position with the best reception quality while performing a precise search;
A cutout window signal is generated by combining a window offset signal that is output from the window search circuit and that represents an offset from the main wave position and a window reference signal that represents the main wave position from the main wave detection circuit, A clipping window setting circuit for outputting to the conversion means;
The OFDM receiver according to claim 1 or 2, further comprising: The quality detection means uses the reception S / N calculated from the output of the data demodulation means as the reception quality, and the clipping window control means determines the position of the clipping window that maximizes the reception S / N. The OFDM receiving apparatus according to claim 1, wherein a search is performed . The quality detection means uses the received S / N calculated from the output of the impulse response detection means as the reception quality, and the clipping window control means determines the position of the clipping window that maximizes the received S / N. The OFDM receiver according to claim 1, wherein the OFDM receiver is searched .

Images