JPH09102774A - Automatic frequency control method and device - Google Patents

Abstract

(57) 【Abstract】 PROBLEM TO BE SOLVED: To provide an OFD having good transmission efficiency and frequency tracking capability.
It was necessary to provide an improved automatic frequency control device on the demodulation side for the M signal transmission system. SOLUTION: OFDM with guard interval added
By utilizing the similarity of the signals of the effective symbol period in the signal, the data of the partial period of the effective symbol period and the data of the partial period of the guard interval of one symbol period of the OFDM signal are respectively subjected to the discrete Fourier transform (8 , 9)
Then, the phase difference is calculated from the respective data subjected to the discrete Fourier transform (10, 11, 12) to detect the frequency shift, and the oscillation frequency of the local oscillator (15) is corrected.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an OFDM (Orthogo
nal frequency division multiplexing (AFC) transmission apparatus, in particular, AFC (Automatic Frequenx) of a digital demodulation apparatus that demodulates an OFDM transmission signal.
cy Control: Automatic frequency control).

[0002]

2. Description of the Related Art Conventionally, AFC has been used in an OFDM receiver.
In the frequency calibration of the reproduction carrier, the reference signal periodically time-division multiplexed at the transmission side is extracted at the reception side, and the reproduction carrier frequency is calibrated using the reference signal as a reference signal (for example, Reference: Ch. . Dosch et al., “First public d
emonstrations of COFDM / MASCAM. A milestone for the
future of radio broadcasting ", EBU Review-Technica
l, No. 232 (December 1988)).

[0003]

In general, reference symbols do not contribute to information transmission. In order not to reduce the transmission efficiency, it is necessary to increase the reference symbol interval. However, if the interval between the reference symbols is too large, it takes a long time to pull in the frequency and the frequency range to be pulled in becomes narrow. Therefore, an object of the present invention is to provide an automatic frequency control device for a digital demodulation device of an OFDM transmission signal, which has good transmission efficiency and frequency followability in consideration of these problems.

[0004]

In order to achieve this object, an automatic frequency control device of the present invention receives an OFDM signal having a guard interval, and the OFDM received signal is detected by a quadrature detector using a local oscillation signal in a baseband band. A first step of frequency-converting the complex-modulated signal into a digital signal, a second step of converting the frequency-converted complex-modulated signal into a digital signal, and a second step of performing discrete Fourier exchange on a part of the guard interval period of the digital signal. A third step, and a fourth step of performing a discrete Fourier transform of a part of the effective symbol period apart from the guard interval period of the digital signal by one effective symbol period,
The fifth step of complex division of the result of the third step and the result of the fourth step, the sixth step of phase conversion of the result of the fifth step, and the result of the sixth step. A seventh step of converting into an analog signal, an eighth step of filtering the analog signal resulting from the seventh step, and controlling the local oscillation signal according to the result of the eighth step. .

Further, the automatic frequency device of the present invention receives an OFDM signal signal having a guard interval and outputs the OF signal.
A quadrature detector that frequency-converts a DM reception signal into a baseband complex modulation signal by a local oscillation signal, and a first A / D that converts an output signal of the quadrature detector into a digital signal
A converter and a second A / D converter, and the first A / D
A first discrete Fourier transformer for performing a discrete Fourier transform on a part of the guard interval period of the output signal of the converter and the output signal of the second A / D converter; and the output signal of the first A / D converter And a part of the guard interval period of the output signal of the second A / D converter, a second discrete Fourier transformer for performing a discrete Fourier transform on a part of the effective symbol period separated by one effective symbol period, and Complex division means for performing complex division between the output signal of the first discrete Fourier transformer and the output signal of the second discrete Fourier transformer, and phase conversion means for converting the output signal of the complex division means into a signal corresponding to the phase. A D / A converter for converting the output signal of the phase conversion means into an analog signal, a loop filter for filtering the output signal of the D / A converter, and an oscillation frequency by the output signal of the loop filter. Those characterized by comprising a local oscillator number is controlled to output a local oscillation signal of the quadrature detector, a.

In order to reduce the influence of noise, in the automatic frequency control device of the present invention, the complex division means is composed of a plurality of complex dividers, and the output signals of the plurality of complex dividers are complex. It is characterized by comprising a complex adder for adding, and applying an output signal of the complex adder to the phase conversion means. Further, preferably, the automatic frequency control device of the present invention is characterized in that the phase conversion means is constituted by a ROM. Further, it is possible to make a further change by using one discrete Fourier transform means and a storage means instead of using two discrete Fourier transform means. It is also possible to execute by subtracting only the phase component without using the complex divider.

The present invention has two areas in which the discrete Fourier transform is performed. One is at the trailing edge of the guard interval period and the other is at the trailing edge of the effective symbol period. The guard interval period is to add the rear part of the effective symbol period directly to the front of the effective symbol period and transmit it from the transmitter.Therefore, if the receiver demodulates at the correct demodulation carrier and correct timing, the guard interval period The area of the trailing edge and the area of the trailing edge of the effective symbol period have the same waveform. Therefore, the frequency data resulting from the discrete Fourier transform is naturally the same. On the other hand, if the demodulation carrier has a frequency deviation, a phase change amount proportional to the frequency deviation is obtained as a result of two discrete Fourier transforms. Therefore, it is possible to realize automatic frequency control by calculating the frequency deviation from the phase difference of the discrete Fourier transform result in the area of the trailing edge of the guard interval period and the area of the trailing edge of the effective symbol period and controlling the oscillator of the regenerated carrier. Is.

In the present invention, the data of a part of the effective symbol period and the data of a part of the guard interval of one symbol period of the OFDM signal are discrete Fourier transformed, preferably Fourier transformed. Since the phase difference is calculated from each data to detect the frequency shift and the oscillation frequency of the local oscillator is corrected, even if transmission distortion occurs, the two partial discrete Fourier transform regions originally have the same waveform ( Since almost equal distortion is generated (due to one effective symbol interval), the influence of the frequency shift calculated from the discrete Fourier transform data of the two regions on the detection signal is small, and the transmission distortion is strong.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on embodiments with reference to the accompanying drawings. FIG. 1 is a configuration block diagram of a first embodiment of the present invention. In FIG. 1, the OFDM signal input to the input terminal 1 is quasi-coherently detected by the quadrature detector 2 and frequency-converted into a baseband complex modulation signal. Here, the quadrature detector 2 includes a first multiplier 3, a second multiplier 4, and a phase shifter 5 for shifting the local oscillation signal by π / 2.

The signals frequency-converted by the quadrature detector 2 into the baseband complex modulation signals are respectively the first A
It is converted into a digital signal by the / D converter 6 and the second A / D converter 7. The output signals of the A / D converters 6 and 7 are
It is input to the first discrete Fourier transformer (DFT) 8 and the second discrete Fourier transformer (DFT) 9. For example, the first discrete Fourier transformer 8 performs a discrete Fourier transform on a part of data in the guard interval, and the second discrete Fourier transformer 9 performs a discrete Fourier transform on a part of data in the effective symbol period. To do. Here, the transform region of the first discrete Fourier transformer and the transform region of the second Fourier transformer are set at positions shifted by one effective symbol period. In FIG. 2, the first region 21 and the second region 22 correspond to the transform regions of the first discrete Fourier transformer 8 and the second discrete Fourier transformer 9, respectively.

In this example, for example, the number of samples in the effective symbol period is 1024 and the guard interval period is 25.
There are 6 and 1280 data in total in one symbol period, and the first discrete Fourier transformer 8 and the second discrete Fourier transformer 9 are 8-point discrete Fourier transformers.
Here, the output signals of the A / D converters 6 and 7 will be described by adding 1 to 1280 as sample numbers. If the first discrete Fourier transformer 8 is set to perform the discrete Fourier transform on the samples 245 to 252, the first discrete Fourier transformer 8 and the second discrete Fourier transformer 9 are placed at positions separated by one effective symbol period. As such, the second discrete Fourier transformer 9 is set to discrete Fourier transform samples 1269 to 1276.

The transform regions of the first discrete Fourier transformer 8 and the second discrete Fourier transformer 9 may be set at positions separated by one effective symbol period, but the closer to the beginning of the guard interval period, the more the influence of ghost. Easy to receive, so first
It is desirable that the area 21 of is set to be longer than the effective symbol period. The first discrete Fourier transformer 8 has F1 (0) to F
The converted output data up to 1 (7) is output. F1 (0) and F2 (0) are input to the complex divider 10
Calculate (0) / F2 (0). Similarly, using the remaining seven complex dividers, F1 (1) / F2 (1), F1
(2) / F2 (2), ..., F1 (7) / F2 (7)
Is calculated. These complex divider output signals are added by the complex adder Σ11.

F1 (0) / F2 (0) is a complex number x + jy
It can also be output in the form of, converted into polar coordinates, and expressed by f (R, θ). In the present invention, since the absolute value R of f (R, θ) is not required, only θ needs to be obtained. That is, only the angle θ needs to be obtained by the phase conversion means 12 that converts the signal into a signal corresponding to the phase. In addition, F1 (0) / F2
(0), F1 (1) / F2 (1), ..., F1 (7)
The present invention can be implemented with only one of / F2 (7), but in order to reduce the influence of disturbance such as noise, F1 (0) / F2 (0), F1 (1) / F2
The signals of (1), ..., F1 (7) / F2 (7) are added by the complex adder 11 and averaged.

The output signal of the complex adder 11 obtains an angle by the phase converting means 12 for converting it into a signal corresponding to the phase, converts it into an analog signal by the D / A converter 13, and further passes through the loop filter 14 to obtain a high frequency signal. Attenuate the components and control the local oscillator LO15. The output signal of the local oscillator 15 is input to the quadrature detector 2 as a local oscillation signal. In this embodiment, the phase conversion means 12 for converting into a signal corresponding to the phase calculates in advance from x + jy to θ and stores it in the ROM, and outputs θ by addressing x and y. Of course, the digital signal processing may be used for sequential calculation.

The first discrete Fourier transformer 8 and the second discrete Fourier transformer 9 may be discrete Fourier transformers having several to several tens of points, and the scale of hardware can be reduced. Although an example of the OFDM signal transmission format according to the present invention is shown in FIG. 3, the sections 31, 32, 33, 34,
Reference numerals 35, 36 and 37 correspond to the null symbol, the synchronization symbol, the symbol 1, the symbol 2, the symbol 3, the symbol 4 and the symbol N, respectively.
2, 3, 4, ..., N each have a guard symbol period (hatched portion) and an effective symbol period.
Further, it should be noted that the same signal as that of the rear end of the effective symbol period is inserted in the guard interval period in the OFDM signal, and the effective symbol period is generally preceded by the guard interval period.

Further, FIG. 4 shows a second embodiment when implemented by a complex multiplier. Since it is known that complex division is realized by multiplying complex conjugates, it is obvious that the present invention can be implemented without being limited to division (* in the figure indicates to take complex conjugates).

FIG. 5 shows a third embodiment realized by one Fourier transformer and a storage device without using two discrete Fourier transformers. The result of calculating a part of the guard interval period is stored in a storage device, and when the calculation of a part of the effective symbol period which is one effective symbol period away from the guard interval period is completed, a part of the guard interval period is stored. The complex division is performed by reading from the device.

FIG. 6 shows that the arithmetic operation is performed without using complex division.
By extracting and subtracting only the required phase component,
It is a fourth embodiment in the case of controlling the partial oscillation frequency. You
That is, the output signal of the first discrete Fourier transformer and the second
The output signal of the discrete Fourier transformer and the polar coordinates f (R, θ)
To the form (however, it is not necessary to find R), and θ
₁(0), θ₁(1), ... θ₁(7) and θ
_Two(0), θ_Two(1), ... θ _TwoFind (7). Next
Input the calculated signal to the subtractor and₁(0)
−θ_Two(0), θ₁(1) -θ_Two(1), ... θ
₁(7) -θ_Two(7) is obtained from the output of the subtractor. these
The signal of is averaged by an adder. In this example, θ₁
(0) -θ_Two(0), θ₁(1) -θ_Two(1), ...
θ₁(7) -θ_TwoAny one of (7) (eg θ
₁(0) -θ_Two(0)) can be implemented but noise
Θ to reduce the influence of disturbances such as₁(0)-
θ_Two(0), θ₁(1) -θ_Two(1), ... θ
₁(7) -θ_TwoThe signal of (7) is averaged by an adder
You.

[0019]

As described above, since the automatic frequency control device of the present invention can detect the frequency deviation in each short period of one symbol period, it is possible to reproduce the stable reference carrier wave and the frequency tracking property. Can be improved. Also,
Even if automatic frequency control is temporarily lost due to pulse noise or fading, it is possible to quickly pull in. Further, since it is not necessary to add the reference signal on the transmitting side, the transmission efficiency does not decrease due to the addition of the reference signal.

[Brief description of the drawings]

FIG. 1 is a configuration block diagram of a first embodiment of the present invention.

FIG. 2 is a diagram for explaining a region where a partial discrete Fourier transform is performed in the present invention.

FIG. 3 is an example of a transmission format of an OFDM signal according to the present invention.

FIG. 4 is a configuration block diagram of a second embodiment of the present invention.

FIG. 5 is a configuration block diagram of a third embodiment of the present invention.

FIG. 6 is a configuration block diagram of a fourth embodiment of the present invention.

[Explanation of symbols]

 1 Input Terminal 2 Quadrature Detector 3,4 Multiplier 5 Phase Shifter 6,7 A / D Converter 8,9 Discrete Fourier Transformer 10 Complex Division Means 11 Complex Adder 12 Phase Transforming Means 13 D / A Converter 14 Loop filter 15 Local oscillator 16 Complex multiplication means 17 Storage device 18 Subtraction means 21 First discrete Fourier transform region 22 Second discrete Fourier transform region 31 Null symbol 32 Synchronization symbol 33 Symbol 1 34 Symbol 2 35 Symbol 3 36 Symbol 4 37 Symbol N

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kazuhiko Shibuya 2-2-1 Jinnan, Shibuya-ku, Tokyo Inside the Japan Broadcasting Corporation Broadcasting Center (72) Morihiro Otsuka 2-2-1 Jinnan, Shibuya-ku, Tokyo Japan Broadcasting Corporation Broadcast Center (72) Inventor Tadaaki Tanaka 2-2-1 Jinnan, Shibuya-ku, Tokyo Japan Broadcasting Corporation Broadcast Center (72) Inventor Hiroshi Okamura 2-2-1 Shinnan, Shibuya-ku, Tokyo Within Japan Broadcasting Corporation Broadcasting Center

Claims (8)

[Claims] 1. A first step of receiving an OFDM signal having a guard interval, frequency-converting the OFDM received signal into a baseband complex modulation signal by a quadrature detector using a local oscillation signal, and the frequency conversion. The second step of converting the complex modulated signal into a digital signal, the third step of performing a discrete Fourier transform of a part of the guard interval period of the digital signal, and the part of the guard interval period of the digital signal are A fourth step of performing a discrete Fourier transform on a part of the effective symbol period apart from the effective symbol period, a fifth step of performing complex division of the result of the third step and the result of the fourth step, and the fifth step. A sixth step of phase-converting the result of the step of (4) and converting the result of the sixth step into an analog signal 7. An automatic frequency control method comprising: a seventh step; an eighth step of filtering an analog signal resulting from the seventh step; and controlling the local oscillation signal according to the result of the eighth step. 2. A quadrature detector that receives an OFDM signal having a guard interval and frequency-converts the OFDM reception signal into a complex modulation signal of a baseband by a local oscillation signal, and an output signal of the quadrature detector is a digital signal. To a first A / D converter and a second A / D converter, an output signal of the first A / D converter, and the second A / D converter
A first discrete Fourier transformer for performing a discrete Fourier transform on a part of the guard interval period of the output signal of the / D converter;
The output signal of the first A / D converter and the second A / D converter
A second discrete Fourier transformer for performing a discrete Fourier transform of a part of the effective symbol period separated by one effective symbol period from the part of the guard interval period of the output signal of the converter; and the first discrete Fourier transformer. Complex division means for performing complex division between the output signal and the output signal of the second discrete Fourier transformer, phase conversion means for converting the output signal of the complex division means into a signal corresponding to the phase, and output of the phase conversion means. A D / A converter for converting a signal into an analog signal, and the D / A converter
A loop filter for filtering the output signal of the A / A converter, and a local oscillator for outputting the local oscillation signal of the quadrature detector whose oscillation frequency is controlled by the output signal of the loop filter are provided. Frequency control device. 3. The complex division means is composed of a plurality of complex dividers, and a complex adder for complex-adding output signals of the plurality of complex dividers is provided, and an output signal of the complex adder is provided. The automatic frequency control device according to claim 2, wherein the automatic frequency control device is applied to the phase conversion means. 4. The automatic frequency control device according to claim 2, wherein the phase conversion means is composed of a ROM. 5. A quadrature detector that receives an OFDM signal having a guard interval and frequency-converts the OFDM reception signal into a complex modulation signal of a baseband by a local oscillation signal, and an output signal of the quadrature detector is a digital signal. To a first A / D converter and a second A / D converter, an output signal of the first A / D converter, and the second A / D converter
Discrete Fourier Transform for discrete Fourier transforming a part of the guard interval period of the output signal of the / D converter and a part of the effective symbol period separated from the guard interval period by one effective symbol period, and a part of the guard interval period Storage means for storing the output of a discrete Fourier transformer for performing a discrete Fourier transform of the signal, an output signal of the storage means, and an output signal of a discrete Fourier transformer for performing a discrete Fourier transform on a part of the effective symbol period separated by one effective symbol period. A complex division means for performing complex division, a phase conversion means for converting an output signal of the complex division means into a signal corresponding to a phase, and a D / A converter for converting an output signal of the phase conversion means into an analog signal, A loop filter for filtering the output signal of the D / A converter, and an oscillation frequency depending on the output signal of the loop filter It was equipped with a local oscillator for outputting a local oscillation signal of the quadrature detector is your automatic frequency control apparatus according to claim. 6. The complex division means is composed of a plurality of complex dividers, and a complex adder for complex-adding output signals of the plurality of complex dividers is provided, and an output signal of the complex adder is provided. The automatic frequency control device according to claim 5, wherein the automatic frequency control device is applied to the phase conversion means. 7. The automatic frequency control device according to claim 5, wherein the phase conversion means is constituted by a ROM. 8. A quadrature detector that receives an OFDM signal having a guard interval and frequency-converts the OFDM reception signal into a complex modulation signal of a baseband by a local oscillation signal, and an output signal of the quadrature detector is a digital signal. To a first A / D converter and a second A / D converter, an output signal of the first A / D converter, and the second A / D converter
A first discrete Fourier transformer for performing a discrete Fourier transform on a part of the guard interval period of the output signal of the / D converter;
The output signal of the first A / D converter and the second A / D converter
A second discrete Fourier transformer for performing a discrete Fourier transform of a part of the effective symbol period separated by one effective symbol period from the part of the guard interval period of the output signal of the converter; and an output of the first Fourier transformer. First phase conversion means for phase-converting a signal, second phase conversion means for phase-converting an output signal of the second Fourier transformer, output signal of the first phase conversion means, and the second A subtractor for subtracting the output signal of the phase conversion means, a D / A converter for converting the output signal of the subtractor into an analog signal, a loop filter for filtering the output signal of the D / A converter, and the loop. An automatic frequency control device comprising: a local oscillator that controls an oscillation frequency by an output signal of a filter and outputs a local oscillation signal of the quadrature detector.