JP3389123B2 - OFDM receiver - Google Patents

Description

Detailed Description of the Invention

[0001]

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

[0002]

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

Regarding the structure of the OFDM receiver, "O
Development of FDM modulator / demodulator "Technical Report Vol.21, No.44, PP.13-18 of the Institute of Image Information and Television Engineers. Regarding the automatic gain control circuit (AGC circuit) of the conventional OFDM receiver, see “Japanese Patent Laid-Open No. 9-135137”.
Japanese Patent Publication ”.

[0004] Here, the conventional OF described in the above document.
In the DM receiving apparatus, the input OFDM modulated wave is tuned by the tuner, converted into an IF signal, and then converted into a digital signal to obtain a baseband OFDM modulated signal. Then, the in-phase component (I signal) and the quadrature component (Q signal) are converted, the carrier frequency error is canceled by frequency conversion, the average power of the received signal is detected, the AGC control signal is obtained from the result, and the tuner is calculated. By feeding back, the amplitude of the tuning output is made constant. Further, the frequency conversion output is converted into frequency axis data by FFT (Fast Fourier Transform) calculation, data demodulation is performed, and the data is output.

Here, the OFDM receiving apparatus includes two synchronous reproduction circuits. In the first synchronous reproduction circuit, a timing signal necessary for each circuit is generated from the frequency conversion output and clock reproduction is performed, and the reproduction clock is used 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 addition output is used as a control signal for canceling the carrier frequency error in the frequency conversion processing. Further, 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 FFT calculation result, and the detected frequency error signal is detected by the first synchronous reproduction circuit. Output to.

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

Under the above environment, since the analog TV signal has energy 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 has , A peak appears in the amplitude near the video carrier and 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 operation after the FFT processing is performed, if the AGC control is performed as described above, the amplitude level of the FFT processing output will be the video carrier and audio. It becomes smaller except near the carrier.

Therefore, there is a problem in that the calculation accuracy in the data demodulation circuit and the second synchronization reproducing circuit after the FFT processing deteriorates, and the reception synchronization performance as the OFDM receiving apparatus deteriorates. Also, in order to secure the calculation accuracy, FF
Although 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 synchronization reproduction circuit,
This increase in circuit scale poses a problem for cost reduction of the OFDM receiver.

[0009]

As described above, in the configuration of the conventional OFDM receiving apparatus, the AGC control of the FFT processing output is performed at the average amplitude level of the input reception signal to perform the FF.
Although the average amplitude level of the T-processed input is kept constant, the amplitude of the FFT-processed output is generated when there is co-channel interference or adjacent channel interference of the analog TV signal on the transmission path where digital broadcasting and analog broadcasting coexist. A peak appears near the video carrier and the audio carrier, and the amplitude level of the data carrier other than near the video carrier and the audio carrier becomes small. For this reason,
There is a problem that the calculation accuracy in the data demodulation circuit and the second synchronization reproduction circuit after the FFT processing deteriorates, and the reception synchronization performance as the OFDM receiving apparatus deteriorates. Further, in order to secure 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 synchronization reproduction circuit, which is a problem for the cost reduction of the OFDM receiver. Becomes

Therefore, the present invention provides an OFDM receiving apparatus which suppresses the deterioration of the calculation accuracy in the data demodulation and the synchronous reproduction after the FFT processing while suppressing the increase of the circuit scale so as not to cause the deterioration of the reception synchronization performance. The purpose is to do.

[0011]

In order to achieve the above object, an OFDM receiver according to the present invention receives an OFDM modulated signal, maintains its average power at a predetermined level by AGC, and performs quadrature detection to obtain a complex signal. after conversion into a baseband signal, the carrier frequency error is corrected by the frequency conversion means, converted from the time domain to the frequency domain by the processing of the discrete Fourier transform means, the amplitude level control means, Ru is output from the discrete Fourier transform means predetermined Of the subcarrier
The average amplitude is detected using only the signal, and the output of the discrete Fourier transform means is adjusted so that the average amplitude becomes a predetermined level.
The amplitude level of the force is controlled, and the symbol data transmitted to each subcarrier is demodulated from the output of the amplitude level control means.

That is, in the OFDM receiver having the above-mentioned configuration, the amplitude level control means detects the average amplitude using only the signal of the predetermined subcarrier of the discrete Fourier transform output so that the average amplitude becomes the predetermined level. In addition, since the amplitude level of the subcarrier is controlled, the deterioration of the calculation accuracy in the data demodulation is suppressed, and thereby the deterioration of the reception synchronization performance is eliminated.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a block diagram showing the structure 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 an A / D conversion circuit 104.
Is supplied to. The A / D conversion circuit 104 converts the OFDM modulated signal from the tuner 102 into a digital signal by the reproduction clock supplied from the first synchronous reproduction circuit 110, and its A / D conversion output is in the IQ conversion circuit 105. The in-phase component (I signal) and the quadrature component (Q signal) of the base band are converted, 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
Detects the average power of the received signal from the input OFDM modulated signal, obtains the AGC control signal from the result, and feeds it back to the tuner 102, thereby making the amplitude of the tuning output constant.

The output signal of the frequency conversion circuit 106 is
The FFT (Fast Fourier Transform) circuit 107 converts the data into frequency axis data by FFT calculation, and then 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 is controlled by the level control circuit 108 is supplied to the data demodulation circuit 109, where the received data is demodulated and output from the output terminal 110.

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

In the above structure, the operation of the characteristic part of the present invention will be described below.

As described above, when digital broadcasting and analog broadcasting are mixed, there is co-channel interference or adjacent channel interference of the analog TV signal on the transmission path. Under such an environment, the analog TV signal is a video image. Since energy is concentrated near the carrier and voice carrier, when the OFDM modulated wave is received by the OFDM receiver, as shown in FIG. 2, near the video carrier and voice carrier of the analog TV signal of the received signal after FFT processing. A peak appears in the amplitude of. Since overflow is not allowed in the FFT processing, if the FFT processing output is represented by a fixed point and the operation 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 near the video carrier and the audio carrier.

Therefore, in this embodiment, the level control circuit 108 detects the average amplitude of the data carrier,
The amplitude of the data carrier is controlled so that the average amplitude becomes a predetermined value. That is, due to co-channel interference of analog TV signals, FFT circuit 107
Even if the signal level of the data carrier becomes low at the output of, the amplitude control ensures the calculation accuracy in the data demodulation circuit 109 and the second synchronization reproduction circuit 112 in the subsequent stage.

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

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

FIG. 3 is a block diagram showing a first configuration example of the level control circuit 108. In FIG. 3, the output signal of the FFT circuit 107 is supplied from the input terminal 201,
It 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 of 02. The multiplication result of the multiplier 202 is supplied to the clip circuit 203. The clipping circuit 203 level-shifts the input signal, extracts a signal larger than a predetermined level, and clips it.

The output of the clipping circuit 203 is branched into two, and one of them is output from the output terminal 207 as a level-controlled output signal. The other is the amplitude detection circuit 20.
4 is supplied. The amplitude detection circuit 204 detects the amplitude of the input signal, and the amplitude detection result 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 configuration, F
The amplitude level of the FT output signal is controlled to a predetermined level.

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

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

The output signal of the signal sampling circuit 308 is supplied to the amplitude detection circuit 304, where the amplitude of the input signal is detected. The amplitude detection result is supplied to the error detection circuit 305 and 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, whereby a feedback loop for amplitude control is formed.

By the feedback loop having the above configuration, F
The amplitude level of the FT output signal is controlled to a predetermined level.

FIG. 5 is a block diagram showing a third configuration example of the level control circuit 108. In FIG. 5, the output signal of the FFT circuit 107 is supplied from the input terminal 401,
The signal is supplied to one input terminal of the multiplier 403 via the delay circuit 402 and also to the signal sampling circuit 404. The signal extracting circuit 404 extracts a signal excluding the vicinity of the video carrier and the audio carrier of the analog TV signal from the input signal, and the output thereof is the amplitude detecting circuit 4.
It is supplied to 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 the error detection circuit 406 and compared with a predetermined level to generate an error signal. This error signal is supplied to the smoothing circuit 407. This smoothing circuit 4
In 07, for example, the section average of the input signal for one symbol period of OFDM is obtained.

The error signal subjected to the interval averaging 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 an FFT whose timing is 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-described configuration can cope with an analog TV signal interference or a case where the OFDM signal level fluctuates with time by the sequential control of the symbol in a feedforward loop. OFD
In the M receiving apparatus, it is considered that the amplitude of the data carrier fluctuates in a unit of one symbol, and thus it is effective to perform the level control in a unit of one symbol. This also applies to 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 the smoothing circuit 4
Although the level control signal is generated by smoothing at 07, the arrangement of the error detection circuit 406 and the smoothing circuit 407 may be exchanged. With feedforward control,
Since the amplitude detection result is extremely low, it is considered more advantageous to first smooth and then detect the level error.

[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, whereby the arithmetic operation in the data demodulation circuit and the synchronous reproduction circuit is performed. It is possible to provide an OFDM receiving apparatus 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 drawings]

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

FIG. 2 is a diagram showing an F receiver in the OFDM receiver of the same embodiment.
FIG. 4 is a frequency distribution diagram showing a state where a peak appears in the amplitude near the video carrier and the audio carrier of the analog TV signal of the received signal after FT processing.

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

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

FIG. 5 is a block diagram showing a third configuration example of the level control circuit used in the same 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 sync reproduction circuit, 112 ... Second sync 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
8 ... Signal sampling circuit, 402 ... Delay circuit, 403 ... Multiplier, 404 ... Signal sampling circuit, 405 ... Amplitude detection circuit, 406 ... Error detection circuit, 407 ... Smoothing circuit, 408
… Clip circuit.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Makoto Sato 3-3-9 Shimbashi, Minato-ku, Tokyo Inside Toshiba Abu E., Ltd. (72) Inventor Yuji Ohashi 3-3, Shimbashi, Minato-ku, Tokyo No. 9 within Toshiba Abu E., Ltd. (56) Reference JP-A-5-227059 (JP, A) JP-A-10-51415 (JP, A) JP-A-8-317012 (JP, A) ) JP-A-9-135137 (JP, A) JP-A-10-294764 (JP, A) JP-A-10-126374 (JP, A) Akira Kisoda, Tomohiro Kimura, Kenichiro Hayashi, Shigeru Soga, Sadaka Kageyama. , Seiji Sakashita, Hitoshi Mori, Development of OFDM modulator / demodulator, Technical Report of The Institute of Image Media, Japan, The Institute of Image Media, August 26, 1997, VOL. 21, NO. 44, p. 13-18 (58) Fields investigated (Int.Cl. ⁷ , DB name) H04J 11/00

Claims (8)

(57) [Claims] 1. Receiving means for receiving an OFDM (Orthogonal Frequency Division Multiplexing) modulated signal and maintaining its average power at a predetermined level, and quadrature detection of the OFDM modulated signal obtained by this means to form a complex baseband signal. Quadrature detection means for transforming, frequency transforming means for correcting carrier frequency error of the output of this means, discrete Fourier transforming means for transforming the output of this means from time domain to frequency domain by discrete Fourier transform, and this discrete Fourier transform predetermined subkey that it will be output from the unit
The average amplitude is detected using only the carrier signal, and the discrete Fourier transform is performed so that the average amplitude becomes a predetermined level.
OFDM receiver of the amplitude level control means for controlling the amplitude level of the output means, characterized by comprising a data demodulation means for demodulating symbol data being transmitted in each sub-carrier from the output of the amplitude level control means apparatus. 2. The amplitude level control means controls the amplitude of the output of the discrete Fourier transform means according to an amplitude level control signal, and clips the output of the amplitude control means at a predetermined amplitude level. Clipping means and a signal of a predetermined subcarrier from the output of this clipping means
A signal extracting means for extracting only the signal, an amplitude detecting means for detecting an amplitude from the output of the signal extracting means , an amplitude error detecting means for detecting an error between the detection result of the amplitude detecting means and a predetermined amplitude level, and the amplitude detecting means. the output of the error detecting means to smooth said a smoothing means for outputting to the amplitude control means as said amplitude level control signal, wherein the output of said clipping means to an output of said amplitude level control means according Item 2. The OFDM receiver according to Item 1. 3. The amplitude level control means, signal extraction means for extracting only a signal of a predetermined subcarrier from the output of the discrete Fourier transform means , and amplitude detection means for detecting the amplitude from the 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 and smoothed. Amplitude level control signal generating means for generating an amplitude level control signal by detecting an error between the amplitude level and a predetermined amplitude level, and the output of the discrete Fourier transform means according to the amplitude level control signal generated by this means. Amplitude control means for controlling the amplitude, and clipping means for clipping the output of the amplitude control means at a predetermined amplitude level are provided. 2. The OFDM receiver according to claim 1, wherein the output of the lip means is the output of the amplitude level control means. 4. The signal extracting means is the OFDM.
4. The OFDM receiver according to claim 2 , wherein signals of subcarriers other than the vicinity of the video carrier and the audio carrier of the analog television signal existing in the transmission band of the (orthogonal frequency division multiplexing) modulated signal are extracted. Wherein said level control means, OFDM receiving apparatus according to any one of claims 1 to 4, characterized in applying a level controlled by a symbol unit of the OFDM modulated signal. 6. The output of the amplitude level control means
Detecting a frequency error of the carrier from, OFDM of the error detection result according to any one of claims 1 to 5, characterized in that it comprises a frequency error detecting means for providing a correction of the carrier frequency error is fed back to the frequency converter
Receiver. 7. The receiving means receives the OF from the input signal.
A tuner for selecting a DM modulation signal; and an automatic gain control means for detecting an average power of a tuning output of the tuner and performing feedback control of the gain of the tuner so that the detection result becomes a predetermined level. The OFDM receiver according to any one of claims 1 to 6 , which is characterized in that. 8. A receiving step of 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 in this step to form a complex baseband signal. A quadrature detection step for conversion, a frequency conversion step for correcting the carrier frequency error of the output of this step, a discrete Fourier transform step for transforming the output of this step from the time domain to the frequency domain by the discrete Fourier transform, and this discrete Fourier transform predetermined service from step Ru output
The average amplitude is detected using only the carrier signal, and the discrete Fourier transform is performed so that the average amplitude becomes a predetermined level.
An OFDM characterized by comprising an amplitude level control step for controlling the amplitude level of the output of the converting means, and a data demodulation step for demodulating the symbol data transmitted to each subcarrier from the output of the amplitude level control step. Receiving method.