JP2000165343A - Ofdm receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent reception synchronization performance from being deteriorated by suppressing a deterioration in arithmetic accuracy in data demodulation and synchronization recovery after FFT processing while suppressing an increase in the circuit scale. SOLUTION: Under an environment where the same channel or adjacent channels disturbance is in existence on an analog TV signal on a transmission channel of a digital broadcast program, a peak of an output amplitude of an FFT circuit 107 appears around a video and audio carrier of an analog TV signal. When AGC control is applied to the input to the FFT circuit 107, an amplitude level of FFT processing output is smaller low in area other than the region around the video and audio carrier. Then a level control circuit 108 detects a mean amplitude of a data carrier, and the amplitude of the data carrier is controlled so that the mean amplitude reaches a processing amplitude. Accordingly, the signal level of the data carrier in the output of the FFT circuit 107 is kept to a prescribed level even on occurrence of channel disturbance or the like thereby ensuring arithmetic accuracy by a data demodulation circuit 109 and a 2nd synchronization recovery circuit 112.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) modulation method for receiving a transmission signal.
It relates to a DM receiver.

[0002]

2. Description of the Related Art In recent years, in the transmission of audio signals and video signals, digital modulation systems have been actively developed. In particular,
2. Description of the Related Art In digital terrestrial broadcasting, an orthogonal frequency division multiplexing (hereinafter, referred to as OFDM) modulation scheme having features such as resistance to multipath interference and high frequency use efficiency has been attracting attention. Hereinafter, a conventional technique related to the present invention will be described.

[0003] Regarding the configuration of the OFDM receiver, "O
Development of FDM Modulator / Demodulator ”, Technical Report of the Institute of Image Information and Television Engineers, Vol. 21, No. 44, PP. Regarding a conventional automatic gain control circuit (AGC circuit) of an OFDM receiver, see Japanese Patent Application Laid-Open No. Hei 9-135137.
No. Gazette.

Here, the conventional OF described in the above-mentioned document is described.
In the DM receiver, the input OFDM modulated wave is tuned by a tuner, converted into an IF signal, and then converted into a digital signal to obtain a baseband OFDM modulated signal. Then, the signal is converted into an in-phase component (I signal) and a quadrature component (Q signal), the carrier frequency error is canceled by frequency conversion, the average power of the received signal is detected, and the AGC control signal is obtained from the result. By feeding back, the amplitude of the channel selection output is made constant. Further, the frequency conversion output is converted into frequency axis data by FFT (Fast Fourier Transform) operation, and the data is demodulated and output.

[0005] Here, the above-mentioned OFDM receiver has two synchronous reproduction circuits. The first synchronous reproduction circuit generates a necessary timing signal in each circuit from the frequency conversion output and performs clock reproduction, and uses the reproduced clock as a sample clock for A / D conversion. Further, the frequency error of the carrier is detected and added to the frequency error signal from the second synchronous reproduction circuit, and the added output is used as a control signal for canceling the carrier frequency error in the frequency conversion processing. The second synchronous reproduction circuit detects a carrier frequency error by detecting a pilot signal inserted into a specific frequency slot and transmitted from the result of the FFT operation, and converts the detected frequency error signal into a first synchronous reproduction circuit. Output to

As described above, in the conventional OFDM receiving apparatus, AGC control is performed at the average amplitude level of the input received signal to control the average amplitude level of the input signal of the FFT operation to be constant. However, in Japan,
Until the spread of terrestrial digital broadcasting, analog broadcasting will continue to be performed. As described above, when digital broadcasting and analog broadcasting are mixed, the same channel interference or adjacent channel interference of the analog TV signal exists on the transmission path.

[0007] In the above environment, since the energy of the analog TV signal is concentrated near the video carrier and the audio carrier, when the OFDM modulated wave is received by the OFDM receiver, the received signal after the FFT processing is included in the received signal. , A peak appears in the amplitude near the video carrier and the audio carrier of the analog TV signal. Since overflow is not allowed in the FFT processing, when the FFT processing output is represented by a fixed point and the calculation after the FFT processing is performed, if the AGC control is performed as described above, the amplitude level of the FFT processing output becomes the video carrier and the audio. It becomes smaller except near the carrier.

Therefore, there is a problem that the calculation accuracy in the data demodulation circuit and the second synchronization reproduction circuit after the FFT processing is deteriorated, and the reception synchronization performance of the OFDM receiver is deteriorated. Also, in order to secure the calculation accuracy, FF
It is necessary to increase the number of bits of the T processing output and increase the circuit scale of the data demodulation circuit and the second synchronous reproduction circuit.
This increase in the circuit scale poses a problem for reducing the cost of the OFDM receiver.

[0009]

As described above, in the configuration of the conventional OFDM receiving apparatus, the AFT control of the FFT processing output is performed at the average amplitude level of the input received signal to perform the FF operation.
The average amplitude level of the T processing input is set to be constant. However, when the same channel interference or adjacent channel interference of the analog TV signal exists on the transmission line where digital broadcasting and analog broadcasting coexist, the amplitude of the FFT processing output is adjusted. , A peak appears near the video carrier and the audio carrier, and the amplitude level of the data carrier other than the vicinity of the video carrier and the audio carrier becomes small. For this reason,
The calculation accuracy in the data demodulation circuit and the second synchronization reproduction circuit after the FFT processing is deteriorated, and there is a problem that the reception synchronization performance of the OFDM receiver is deteriorated. Also, in order to ensure the calculation accuracy, it is necessary to increase the number of bits of the FFT processing output and increase the circuit scale of the data demodulation circuit and the second synchronous reproduction circuit, which is a problem for reducing the cost of the OFDM receiver. Becomes

Accordingly, the present invention provides an OFDM receiving apparatus which suppresses a deterioration in operation accuracy in data demodulation and synchronous reproduction after FFT processing while suppressing an increase in circuit scale, thereby preventing deterioration in reception synchronization performance. The purpose is to do.

[0011]

In order to achieve the above object, an OFDM receiving apparatus according to the present invention receives an OFDM modulated signal, maintains its average power at a predetermined level by AGC, performs quadrature detection and performs complex detection. After the conversion to the baseband signal, the carrier frequency error is corrected by the frequency conversion means, the time domain is converted to the frequency domain by the processing of the discrete Fourier transform means, and the output amplitude is detected by the amplitude level control means. Controlling the amplitude level of the output of the discrete Fourier transform means so that the detection result becomes a predetermined level; demodulating the symbol data transmitted to each subcarrier from the output of the amplitude level control means; The frequency error of the carrier is detected from the output of the means, and the error detection result is fed back to the frequency conversion means. So that is subjected to the correction of the carrier frequency error.

That is, in the OFDM receiving apparatus having the above configuration, since the amplitude of the data carrier is controlled to a predetermined level by the level control means, the deterioration of calculation accuracy in data demodulation and carrier frequency error detection is suppressed. Performance degradation is eliminated.

[0013]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a block diagram showing a configuration of an OFDM receiving apparatus according to the present invention. In FIG. 1, an OFDM modulated wave input to an input terminal 101 is tuned by a tuner 102, converted into an IF signal, and converted into an A / D conversion circuit 104.
Supplied to The A / D conversion circuit 104 converts the OFDM modulated signal from the tuner 102 into a digital signal using the reproduction clock supplied from the first synchronous reproduction circuit 110, and outputs the A / D conversion signal in the IQ conversion circuit 105. The signal is converted into a baseband in-phase component (I signal) and a quadrature component (Q signal), and the carrier frequency error is canceled in the frequency conversion circuit 106.

The output of the A / D conversion circuit 104 is AG
It is also supplied to the C circuit 103. This AGC circuit 103
Is to detect the average power of the received signal from the input OFDM modulated signal, obtain an AGC control signal from the result, and feed it back to the tuner 102 to make the amplitude of the channel selection output constant.

The output signal of the frequency conversion circuit 106 is
After being converted to frequency axis data by an FFT operation in an FFT (fast Fourier transform) circuit 107, the level control circuit 1
08. The level control circuit 108 detects the average amplitude of the data carrier and controls the amplitude of the data carrier so that the average amplitude becomes a predetermined value. The signal whose amplitude has been controlled by the level control circuit 108 is supplied to a data demodulation circuit 109, where the received data is demodulated and output from an output terminal 110.

On the other hand, the OFDM receiver includes a first synchronous reproduction circuit 111 and a second synchronous reproduction circuit 112. The first synchronous reproduction circuit 111 receives the output signal of the frequency conversion circuit 106, reproduces necessary timing signals in each circuit and performs clock reproduction, and supplies the reproduced clock to the A / D conversion circuit 104 as a sampling clock. , And a carrier frequency error. Further, the second synchronous reproduction circuit 112 receives the output signal of the level control circuit 108, detects a pilot signal inserted into a specific frequency slot and transmitted, and thereby detects a carrier frequency error. First and second synchronous reproduction circuits 111, 1
The frequency error signal detected at 12 is added by the adder 113 and supplied to the frequency conversion circuit 106, where the signal is used for correction of the carrier frequency error in the frequency conversion circuit 106.

The operation of the above configuration will be described below.

As described above, when digital broadcasting and analog broadcasting coexist, the same channel interference or adjacent channel interference of the analog TV signal exists on the transmission path. Since energy is concentrated near the carrier and the voice carrier, when the OFDM receiving apparatus receives the OFDM modulated wave, as shown in FIG. 2, the vicinity of the video carrier and the voice carrier of the analog TV signal of the received signal after the FFT processing is performed. A peak appears at the amplitude of Since overflow is not allowed in the FFT processing, when the FFT processing output is represented by a fixed point and the calculation after the FFT processing is performed, A
When the GC control is performed, the amplitude level of the output of the FFT processing becomes small except in the vicinity of the video carrier and the audio carrier.

Therefore, in this embodiment, the average amplitude of the data carrier is detected by the level control circuit 108,
The amplitude of the data carrier is controlled so that the average amplitude becomes a predetermined value. In other words, the FFT circuit 107 receives the same channel interference of the analog TV signal.
Even if the signal level of the data carrier becomes smaller at the output, the amplitude control ensures the calculation accuracy in the data demodulation circuit 109 and the second synchronous reproduction circuit 112 at the subsequent stage.

Therefore, in the OFDM receiver having the above configuration, the level of the data carrier is controlled to a predetermined level by the level control circuit 108, so that the data demodulation circuit 109,
Deterioration in calculation accuracy in the second synchronous reproduction circuit 112 is suppressed, and thereby deterioration in reception synchronization performance is eliminated.

Hereinafter, a specific configuration of the level control circuit 108 which is a feature of the present invention will be described.

FIG. 3 is a block diagram showing a first configuration example of the level control circuit 108. In FIG. 3, an output signal of the FFT circuit 107 is supplied from an input terminal 201,
The signal is supplied to one input terminal of the multiplier 202. Multiplier 2
The level control signal from the smoothing circuit 206 is input to the other input terminal 02. The result of the multiplication by the multiplier 202 is supplied to the clipping circuit 203. The clipping circuit 203 shifts the level of the input signal to extract and clip a signal larger than a predetermined level.

The output of the clipping circuit 203 is branched into two, one of which is output from an output terminal 207 as a level-controlled output signal. The other is an amplitude detection circuit 20
4 is supplied. The amplitude detection circuit 204 detects the amplitude of the input signal, and the result of the amplitude detection is supplied to the error detection circuit 205. This error detection circuit 205
An error signal is generated by comparing the amplitude detection result with a predetermined level. This error signal is smoothed by the smoothing circuit 206 and supplied to the multiplier 202 as a level control signal, whereby a feedback loop for amplitude control is formed.

By the feedback loop having the above structure, F
The control is performed so that the amplitude level of the FT output signal becomes a predetermined level.

FIG. 4 is a block diagram showing a second configuration example of the level control circuit 108. In FIG. 4, an output signal of the FFT circuit 107 is supplied from an input terminal 301,
The signal is supplied to one input terminal of the multiplier 302. Multiplier 3
The level control signal from the smoothing circuit 306 is input to the other input terminal 02. The result of the multiplication by the multiplier 302 is supplied to the clipping circuit 303. The clipping circuit 303 shifts the level of the input signal to extract and clip a signal larger than a predetermined level.

The output of the clipping circuit 303 is branched into two, one of which is output from an output terminal 307 as an output signal whose level is controlled. The other is supplied to a signal extraction circuit 308. In this example, the signal of the frequency slot of the data carrier excluding the vicinity of the video carrier and the audio carrier of the analog TV signal is extracted. Therefore, even if the co-channel interference of the analog TV signal exists, the O
The amplitude level of the FDM signal can be accurately detected.

The output signal of the signal extracting circuit 308 is supplied to an amplitude detecting circuit 304, where the amplitude of the input signal is detected. The amplitude detection result is supplied to the error detection circuit 305, and is compared with a predetermined level to generate an error signal.
The error signal is smoothed by the smoothing circuit 306 and supplied to the multiplier 302 as a level control signal, thereby forming a feedback loop for amplitude control.

By the feedback loop having the above configuration, F
The control is performed so that the amplitude level of the FT output signal becomes a predetermined level.

FIG. 5 is a block diagram showing a third configuration example of the level control circuit 108. In FIG. 5, an output signal of the FFT circuit 107 is supplied from an input terminal 401,
The signal is supplied to one input terminal of the multiplier 403 via the delay circuit 402 and is also supplied to the signal extracting circuit 404. The signal extracting circuit 404 extracts a signal excluding the vicinity of a video carrier and an audio carrier of an analog TV signal from an input signal.
05.

The amplitude detection circuit 405 detects the amplitude of the signal extracted by the signal extraction circuit 404.
The amplitude detection result is supplied to an error detection circuit 406, and is compared with a predetermined level to generate an error signal. This error signal is supplied to the smoothing circuit 407. This smoothing circuit 4
Reference numeral 07 is for calculating, for example, a section average of an input signal in one OFDM symbol period.

The error signal subjected to the section averaging process by the smoothing circuit 407 is supplied to the other input terminal of the multiplier 403 as a level control signal, whereby a feedforward loop is formed. The multiplier 403 is a FFT whose timing has been adjusted with the error signal by the delay circuit 402.
The output signal is multiplied by the level control signal from the smoothing circuit 407. The result of the multiplier is clipped by the clipping circuit 408 and output from the output terminal 409.

The level control circuit 108 having the above configuration can cope with the analog TV signal interference and the case where the OFDM signal level fluctuates with time by the sequential control of one symbol unit by the feedforward loop. OFD
In the M receiving apparatus, since it is conceivable that the amplitude of the data carrier fluctuates in units of one symbol, it is effective to perform the level control in units of one symbol. This is the same in the case of the level control circuit of the first or second configuration example.

In the third configuration example, the error detection circuit 406 compares the amplitude detection result of the amplitude detection circuit 405 with a predetermined level to detect the error, and then detects the error.
Although the level control signal is generated by smoothing at step 07, the arrangement of the error detection circuit 406 and the smoothing circuit 407 may be interchanged. In feedforward control,
Since the amplitude detection result is extremely low, it is considered advantageous to detect the level error after smoothing first.

[0035]

As described above, according to the present invention, the amplitude level of the output of the FFT circuit is detected and controlled so that the amplitude level becomes a predetermined value. It is possible to provide an OFDM receiver that does not cause deterioration in accuracy, in other words, does not cause deterioration in reception synchronization performance or can suppress an increase in circuit scale.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of an OFDM receiving apparatus according to the present invention.

FIG. 2 shows an OFDM receiving apparatus according to the embodiment;
FIG. 9 is a frequency distribution diagram illustrating a state in which peaks appear in amplitudes near a video carrier and an audio carrier of an analog TV signal of a reception signal after FT processing.

FIG. 3 is a block diagram showing a first configuration example of a level control circuit used in the embodiment;

FIG. 4 is a block diagram showing a second configuration example of the level control circuit used in the embodiment;

FIG. 5 is a block diagram showing a third configuration example of the level control circuit used in the embodiment;

[Explanation of symbols]

102: tuner, 103: AGC circuit, 104: A /
D conversion circuit, 105: IQ conversion circuit, 106: frequency conversion circuit, 107: FFT circuit, 108: level control circuit, 109: data demodulation circuit, 111: first synchronous reproduction circuit, 112: second synchronous reproduction circuit, 113 ... adder, 20
2 Multiplier, 203 Clip circuit, 204 Amplitude detection circuit, 205 Error detection circuit, 206 Smoothing circuit, 30
2 Multiplier, 303 Clip circuit, 304 Amplitude detection circuit, 305 Error detection circuit, 306 Smoothing circuit, 30
Reference numeral 8: signal extraction circuit, 402: delay circuit, 403: multiplier, 404: signal extraction circuit, 405: amplitude detection circuit, 406: error detection circuit, 407: smoothing circuit, 408
... clip circuit.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takafumi Seki 8-8 Shinsugita-cho, Isogo-ku, Yokohama-shi, Kanagawa Prefecture Inside of Toshiba Multimedia Technology Research Institute (72) Inventor Makoto Sato 3-3-9, Shimbashi, Minato-ku, Tokyo No. Toshiba Abu E Co., Ltd. (72) Inventor Yuji Ohashi 3-3-9, Shimbashi, Minato-ku, Tokyo Toshiba Abu E Co., Ltd. F-term (reference) 5K022 DD01 DD19 DD33 DD34 DD42 DD43

Claims (6)

[Claims] 1. A receiving means for receiving an OFDM (Orthogonal Frequency Division Multiplexing) modulated signal and maintaining its average power at a predetermined level, and orthogonally detecting the OFDM modulated signal obtained by this means to obtain a complex baseband signal. Quadrature detection means for converting, frequency conversion means for correcting the carrier frequency error of the output of the means, discrete Fourier transform means for converting the output of this means from the time domain to the frequency domain by discrete Fourier transform, and this discrete Fourier transform Amplitude level control means for detecting the output amplitude of the means and controlling the amplitude level of the output of the discrete Fourier transform means so that the result of the amplitude detection becomes a predetermined level; and each subcarrier from the output of the amplitude level control means. Data demodulation means for demodulating the symbol data transmitted to the A frequency error detecting means for detecting a frequency error and feeding back the error detection result to said frequency converting means for correction of a carrier frequency error.
DM receiver. 2. An amplitude level control means for controlling an amplitude of an output of the discrete Fourier transform means in accordance with an amplitude level control signal, and clipping an output of the amplitude control means at a predetermined amplitude level. Clipping means; amplitude detecting means for detecting an amplitude from an output of the clipping means; amplitude error detecting means for detecting an error between a detection result of the amplitude detecting means and a predetermined amplitude level; and an output of the amplitude error detecting means. 2. An OFDM apparatus according to claim 1, further comprising: a smoothing means for smoothing the signal and outputting the amplitude level control signal to the amplitude control means.
Receiver. 3. The amplitude level control means, comprising: an amplitude control means for controlling an amplitude of an output of the discrete Fourier transform means according to an amplitude level control signal; and clipping an output of the amplitude control means at a predetermined amplitude level. A clipping means, a signal extracting means for extracting only a signal of a predetermined subcarrier from an output of the clipping means, an amplitude detecting means for detecting an amplitude from an output of the signal extracting means, a detection result of the amplitude detecting means and a predetermined An amplitude error detecting means for detecting an error from the amplitude level, and a smoothing means for smoothing an output of the amplitude error detecting means and outputting the output to the amplitude control means as the amplitude level control signal. 2. The OFDM receiver according to 1. 4. An amplitude level control means, comprising: a signal extraction means for extracting a signal of a predetermined subcarrier from an output of the discrete Fourier transform means; an amplitude detection means for detecting an amplitude from an output of the signal extraction means; An error between the detection result of the amplitude detection means and a predetermined amplitude level is detected, and the error detection result is smoothed to generate an amplitude level control signal, or the output of the amplitude detection means is smoothed. Amplitude level control signal generation means for detecting an error between an amplitude level and a predetermined amplitude level to generate an amplitude level control signal; and an output amplitude of the discrete Fourier transform means according to the amplitude level control signal generated by the means. And a clipping means for clipping the output of the amplitude control means at a predetermined amplitude level. OFDM receiving apparatus according to claim 1,. 5. The OFDM signal extracting means according to claim 1, wherein
5. The OFDM receiver according to claim 3, wherein a signal of a data carrier other than a video carrier and a voice carrier of an analog television signal existing in a transmission band of the (orthogonal frequency division multiplexing) modulated signal is extracted. 6. The OFDM receiver according to claim 1, wherein said level control means performs level control in units of one symbol of said OFDM modulated signal.