JP2001345775A - Ofdm receiving device and data demodulation method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control FFT time window without a dedicated FFT circuit with no effect on demodulation, while a specified subcarrier of OFDM signal is not required to be amplitude 0. SOLUTION: Using a time division control circuit 113, the process of an FFT circuit 105 is time-divided into a data demodulation period and an FFT correlation window control period, and S/N is detected from an FFT output while phase of FFT correlation window signal is shifted during the correlation window control period. The FFT correlation window signal is matched with a phase at which S/N detection result becomes minimum, so that the phase of correlation window signal during data demodulation period is set. The phase of time window signal is set based on reception quality of FFT output, no specified subcarrier is required to be transmitted at amplitude 0. Since an FFT for controlling time window signal phase is also used in the FFT circuit 105 used for data demodulation, for smaller size and lower cost. A time division process does not affect data demodulation.

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.
DM receiver, especially FFT in OFDM demodulation
(Discrete Fourier Transform) Time window control technology.

[0002]

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

[0005] The FFT time window control circuit of the OFDM receiver is described in detail in Japanese Patent Application Laid-Open No. 07-46217. In the conventional example described in the above publication,
OFDM with predetermined subcarriers with amplitude 0 from the transmitting side
Send a signal. On the receiving side, the demodulator detects the amplitude of the predetermined subcarrier and controls the position of the FFT time window so that the amplitude is minimized, that is, the intersymbol interference is minimized. At this time, the demodulator uses a dedicated FFT circuit for FFT time window control.

As described above, the FF dedicated to the FFT time window control is used.
By using the T circuit, the phase at which the intersymbol interference is minimized can be detected while shifting the FFT window without affecting data demodulation. With this method, it is possible to control the FFT time window even under reception conditions where multipath interference with a long delay time exists, such as when SFN (single frequency network) is implemented.

[0005] However, in the above example, a dedicated FFT circuit is used for FFT time window control, and two FFT circuits are used. This indicates an increase in the circuit scale of the OFDM receiver, which is a problem when the size and cost of the OFDM receiver are considered.

In addition, since the FFT time window control uses a predetermined subcarrier transmitted with an amplitude of 0, the OFDM transmission system for transmitting the predetermined subcarrier with an amplitude of 0 needs to be used. It is not preferable to transmit the amplitude of a predetermined subcarrier at 0, since this reduces the data transmission capacity.

[0007]

As described above, the conventional OFDM receiving apparatus is premised on receiving an OFDM transmission signal in which predetermined subcarriers are transmitted at an amplitude of 0 for FFT time window control. Has become. Therefore, in order to control the sum of the absolute values or the sum of the squares of the subcarriers transmitted at the amplitude 0 to be minimized, data cannot be transmitted on the subcarrier transmitted at the amplitude 0.
As a result, the data transmission capacity is reduced.

Further, the FFT without affecting the data demodulation is performed.
In order to perform FFT time window control so as to detect the optimal FFT time window signal while changing the time window, an FFT circuit dedicated to FFT time window control is used, and miniaturization and cost reduction of the receiving device are considered. Then it becomes a problem.

Therefore, the present invention solves the above-mentioned problem, and it is not necessary for a predetermined subcarrier to be transmitted with an amplitude of 0, that is, there is no need to reduce the data transmission capacity, and it is not necessary to reduce the SF.
Even when multipath interference with a long delay time is present, as in the case where N is implemented, control of the FFT time window without affecting data demodulation without having a dedicated large-scale FFT circuit. It is an object of the present invention to provide an OFDM receiving apparatus capable of performing the following.

[0010]

According to the present invention, there is provided an OFDM receiving apparatus for receiving an OFDM (Orthogonal Frequency Division Multiplexing) signal.
Discrete Fourier transform means for receiving a DM signal, performing discrete Fourier transform based on a time window signal defining a range on a time axis for performing discrete Fourier transform, and transforming from the time domain to the frequency domain, and the discrete Fourier transform means And a data demodulation unit that reproduces data, and a reception quality signal that indicates reception quality is detected from the output of the discrete Fourier transform unit, and the time is set so that the reception quality is improved based on the reception quality signal. Time window control means for controlling the phase of the window signal, and switching control means for operating the discrete Fourier transform means while switching to processing for the data demodulation means and processing for the time window control means in a time division manner. Wherein the time window control means sets the time window in the processing for the data demodulation means to the discrete Fourier transform means, And setting the phase obtained by the control means.

That is, in the OFDM receiving apparatus having the above configuration, the discrete Fourier transform period is time-divided into a data demodulation period and a time window control period, and a reception quality signal representing reception quality is output from the discrete Fourier transform output during the time window control period. Detect
The phase of the time window signal is controlled based on the time quality signal so that the reception quality is improved, and the time window of the data demodulation period is set to the phase.

In this case, since the phase of the time window signal is set based on the reception quality of the discrete Fourier transform output, it is not necessary to transmit a predetermined subcarrier with an amplitude of 0, that is, the data transmission capacity is reduced. No need. Also, since the discrete Fourier transform for time window signal phase control is also used by the discrete Fourier transform means used for data demodulation,
There is no need to have a dedicated discrete Fourier transform means with a large circuit scale. Further, since each discrete Fourier transform for data demodulation and time window control is processed by time division, the data demodulation is not affected.

[0013]

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

First, an OFDM signal used in the present embodiment will be described. FIG. 2 shows the transmission format. In FIG. 2, in one symbol period of an OFDM signal, a certain period is added to a signal converted into a time domain after an inverse FFT operation process in a transmitting apparatus in order to prevent intersymbol interference due to multipath interference, a so-called guard. A period is provided. The signal at the end of the effective symbol period is copied in this guard period. The OFDM receiver cuts out a sample period for an effective symbol period (for an FFT size) that is less affected by multipath interference or the like in a received one symbol period, performs an FFT operation, and demodulates data. In FIG. 2, the FFT operation is performed by cutting out the portion where the FFT input gate pulse is “H”.

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

In FIG. 1, an OFDM modulated wave input to an input terminal 101 is converted into a digital signal by an A / D conversion circuit 102. The output of the A / D conversion circuit 102 is converted into a complex signal I signal and Q signal by an IQ demodulation circuit 103 and supplied to a frequency conversion circuit 104.

The frequency conversion circuit 104 has a carrier &
The input signal is frequency-converted into a complex baseband signal by a carrier frequency control signal from the clock recovery circuit 112. Here, the frequency-converted OFDM signal is distributed to three systems, and the FFT circuit 105, the carrier &
The clock recovery circuit 112 is supplied to the memory circuit 106 for controlling the FFT time window.

The FFT circuit 105 converts the input complex baseband OFDM signal into a signal in the frequency domain by an FFT operation. Further, the carrier & clock recovery circuit 112 detects a precise carrier frequency error from the input complex baseband OFDM signal, adds it to a coarse frequency error signal from a carrier frequency error detection circuit 111 described later, and adds a frequency control signal. Is generated, output to the frequency conversion circuit 104, clock recovery is performed from the input signal, and the recovered clock is supplied to each circuit. Also,
The memory circuit 106 receives the input complex baseband O
The FDM signal is temporarily stored and used for FFT time window control.

The output of the FFT circuit 105 is supplied to a signal switching circuit 107. The signal switching circuit 107 switches an output destination between data demodulation and FFT time window control in accordance with a control signal from the time division control circuit 113. And supplied to the frequency error detection circuit 111,
The FFT output signal for controlling the FT time window is supplied to the S / N detection circuit 114.

The data demodulation circuit 108 demodulates I data and Q data from the FFT output signal, and outputs
9 and 110. The frequency error detection circuit 11
Numeral 1 detects a coarse frequency error of a carrier by using a pilot carrier (unmodulated carrier) or a phase-modulated data carrier such as BPSK which is inserted into a predetermined subcarrier in advance from an FFT output signal and transmitted. The frequency error signal detected here is supplied to the carrier & clock recovery circuit 112, and is used for carrier recovery.

On the other hand, S / N detection circuit 114 detects a received S / N in symbol units according to FFT time window control using predetermined subcarriers of the FFT output signal. Here, the predetermined subcarrier is, for example, the aforementioned pilot carrier or a data carrier modulated by BPSK, and does not require a dedicated signal for FFT time window control.
The S / N signal detected by the S / N detection circuit 114 is FFT
The FFT time window signal, which is supplied to the time window control circuit 115 to optimize the reception S / N, is generated and used for data demodulation.

FIG. 3 is a block diagram showing a specific configuration of the S / N detection circuit 114. Input terminals 301, 30
The output signal of the signal switching circuit 107 is supplied to 2, and the amplitude is detected by the amplitude detection circuit 303. The amplitude detection result is 2
One is supplied to one input terminal of a multiplier 304, and the other is supplied to a delay circuit 305. Delay circuit 30
5 is passed through a coefficient generation circuit 306 to the multiplier 304.
Is supplied to the other input terminal.

The delay circuit 305 delays the input amplitude detection result by the multiplier 304 so as to be multiplied by the previous coefficient of the same subcarrier. Also, the coefficient generation circuit 3
Reference numeral 06 compares the input signal with a predetermined amplitude value, and outputs the reciprocal of the ratio with the predetermined amplitude value as a coefficient.

The output of the multiplier 304 is input to a subtractor 307, and a difference from the above-mentioned predetermined amplitude value is obtained.
The output of the subtracter 307 is input to an integration circuit including an adder 308 and a register 309, and only a pilot carrier or a data carrier modulated by BPSK is integrated, and a smoothed signal is output as an S / N detection signal. Is done. Therefore, the smaller the S / N detection signal, the smaller the inter-symbol interference due to multipath interference, and the FF that takes the minimum value
The T time window signal is used for data demodulation.

As described above, since only signals for the effective symbol period (for the FFT size) of one symbol period are used for data demodulation, the time for the guard period is FFT.
The circuit 105 has a time during which no signal is input, and that time is an idle time for the FFT circuit 105.

Therefore, the control signal from the time-division control circuit 113 allows the period to be set separately from the data demodulation by the FFT.
FFT calculation is performed while shifting the phase of the FFT time window signal for time window control. Reception S / N of the FFT operation output
The movement of the FFT time window is stopped by detecting the time point when becomes optimal.

The FFT is performed using the time of the guard period.
When the FFT calculation for the time window control is performed, if the FFT calculation for one symbol is not completed within the time corresponding to the guard period of one symbol, the calculation result of the halfway is stored in the memory circuit 113 and the next symbol is vacated. FF following the time
T operation is performed. This makes it possible to perform the FFT operation for one symbol over several symbols.

The above processing is executed while shifting the phase of the FFT time window, and the reception S / N at that time is detected. Then, an FFT operation for data demodulation is performed using the FFT time window signal having the phase at which the S / N detection signal is minimized.

FIG. 4 is a flowchart for explaining the specific processing operation of the FFT time window control circuit 115. Less than,
The detailed operation will be described with reference to FIG.

First, in step 401, it is determined whether the operation mode of the FFT operation is the data demodulation operation mode or the FFT time window control operation mode. In the FFT operation, data demodulation and FFT time window control are time-divisionally processed.

In step 402, when the FFT operation is not in the data demodulation operation mode, the FFT operation of time window control is performed. In step 403, it is determined whether or not two symbols have been completed in the FFT operation. Return to The time until the completion of FFT varies depending on how much time other than the time required for data demodulation. Therefore, when the FFT operation speed is constant, the time required for the FFT varies depending on the guard period length.

After the FFT is completed, at step 404, the S / N
The detection signal is calculated and stored in, for example, a register. In step 405, it is determined whether the search of the FFT time window has been completed. If the search of the search range of the predetermined FFT time window has not been completed, the search is continued by shifting the phase of the FFT time window signal by the predetermined step. If the FFT time window search has been completed, in steps 407 and 408, it is determined at which phase within the search range the S / N detection signal is minimum, and the FFT time window signal is subjected to data demodulation FFT. Used for calculation.

According to the above processing, F demodulation is performed for data demodulation.
Since there is no influence of the FFT operation for the FT time window control,
It is possible to always operate the FFT time window control to follow the state of the transmission path.

FIG. 5 shows an OF according to a second embodiment of the present invention.
FIG. 3 is a block diagram illustrating a configuration of a DM receiving device. FIG.
In the figure, the parts performing the same functions and operations as those in FIG. 1 are denoted by the same reference numerals, and the description here will focus on the differences from the first embodiment.

In this embodiment, the FFT circuit 501 includes
For example, one that can perform the FFT operation twice (two symbols) in one symbol period is used. When the output of the frequency conversion circuit 104 is supplied to the FFT circuit 501,
The FFT circuit 501 performs an FFT operation for data demodulation once and an FFT operation for FFT time window control once.

At this time, the memory circuit 511 operates as an input signal holding circuit for performing the FFT operation twice. The output of the signal switching circuit 502 is switched between the S / N detection circuit 509 and the time expansion circuit 504 by a control signal from the time division control circuit 503. Here, the signal switching circuit 5
02 outputs an FFT operation output for data demodulation to the time expansion circuit 504, and outputs an FFT operation for FFT time window control.
The calculation output is output to the S / N detection circuit 509.

The control signal of the time division control circuit 503 is supplied to each circuit and used for switching between a data demodulation operation and an FFT time window control operation.

The time expansion circuit 504 includes a signal switching circuit 5
The signal, which has been time-compressed by the FFT operation by a factor of two and is supplied from 02, is time-expanded. The processing source is supplied to the data demodulation circuit 505. Data demodulation circuit 505
Are demodulated to output terminals 506 and 507.

The output of the signal switching circuit 502 is supplied to a frequency error detection circuit 508, which uses a pilot signal transmitted on a predetermined subcarrier or a phase-modulated data carrier such as BPSK to generate a frequency error of the carrier. Is detected.

On the other hand, the FFT operation output for FFT time window control is sent from the signal switching circuit 502 to the S / N detection circuit 50.
9 and an optimal S / N is detected. This circuit 5
The S / N detection signal obtained in step 09 is supplied to the FFT time window control circuit 510. In this S / N detection circuit 509,
A variation in the amplitude of a phase-modulated data carrier such as a pilot signal or BPSK transmitted on the predetermined subcarrier is detected.

That is, if inter-symbol interference occurs due to the influence of multipath interference, the S / N deteriorates and the variation in amplitude increases. Therefore, if the amplitude simply changes due to the multipath interference, the intersymbol interference component of the multipath interference is detected by correcting using the amplitude of the same subcarrier one symbol before. In FFT time window control circuit 510, the phase at the time when the S / N detection signal is minimized when the phase of the FFT time window signal is shifted within a predetermined range is used for the FFT operation for data demodulation.

As described above, also in this embodiment, the FF for controlling the FFT time window besides the data demodulation.
Since the T operation is performed, FFT time window control can be performed while shifting the FFT time window without affecting data demodulation. Therefore, since it can be operated at all times,
It can follow changes in the state of the transmission path.

In the second embodiment, the example in which the FFT circuit 501 can perform the calculation at double speed is shown. However, the present invention is not limited to this, and the FFT circuit 501 operates at N / M double speed (N> M). By using the one that performs the FFT operation, it is possible to perform the FFT operation for the FFT time window control other than the data demodulation,
Needless to say, the same effect can be obtained.

In the second embodiment, the time expansion circuit 504 is disposed immediately after the signal switching circuit 502. However, the operation is also changed when the time expansion circuit 502 is disposed after the data demodulation circuit 505. Needless to say, there is no.

[0045]

As described above, according to the present invention, the reception S / N can be reduced even in a transmission path in which multipath interference with a long delay time exists, for example, when SFN (single frequency network) is implemented. The FFT time window to be optimized can be detected. In addition, since a dedicated pilot signal or the like is not required for FFT time window control and control can be performed using a phase-modulated data carrier, there is an effect that the data transmission capacity can be increased.

Therefore, a predetermined subcarrier has an amplitude of 0
, That is, there is no need to reduce the data transmission capacity, and also when there is a multipath interference with a long delay time such as when SFN is implemented,
Without having a dedicated FFT circuit with a large circuit scale,
An OFDM receiver capable of controlling an FFT time window without affecting data demodulation can be provided.

[Brief description of the drawings]

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

FIG. 2 is an exemplary view for explaining a transmission format of an OFDM signal used in the embodiment;

FIG. 3 is a block diagram showing a specific configuration of an S / N detection circuit used in the embodiment.

FIG. 4 is a flowchart showing specific processing contents of an FFT time window control circuit used in the embodiment.

FIG. 5 is a block diagram showing a second embodiment of the OFDM receiving apparatus according to the present invention.

[Explanation of symbols]

102: A / D conversion circuit, 103: IQ demodulation circuit, 10
4. Frequency conversion circuit, 105 FFT circuit, 106 memory circuit, 107 signal switching circuit, 108 data demodulation circuit, 111 frequency error detection circuit, 112 carrier & clock reproduction circuit, 113 time division control circuit, 114
... S / N detection circuit, 115 ... FFT time window control circuit.

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 5C025 AA13 AA15 AA18 AA19 BA25 DA01 5C063 AA01 AB03 AB07 AB11 AB15 AC01 CA12 5K022 AA26 DD33

Claims (6)

[Claims] 1. An OFDM receiving apparatus for receiving an OFDM (orthogonal frequency division multiplexing) signal, wherein the received OFDM signal is input and based on a time window signal defining a range on a time axis for performing a discrete Fourier transform. Discrete Fourier transform means for performing a discrete Fourier transform to convert from the time domain to the frequency domain; data demodulating means for demodulating an output of the discrete Fourier transform means to reproduce data; and receiving quality from an output of the discrete Fourier transform means. A time window control means for controlling the phase of the time window signal so that the reception quality is improved based on the reception quality signal, and the discrete Fourier transform means, And a switching control unit that operates while switching in a time-sharing manner to a process for the time window control unit. Mamado control means, the relative discrete Fourier transform means, processing OFDM receiving apparatus, characterized in that the time window is set to the phase obtained in said time window control means in for the data demodulation means. 2. The time window control means detects an amplitude of a signal component phase-modulated on a specific subcarrier of the OFDM signal in an output of the discrete Fourier transform means, and calculates the amplitude of the signal component from a previously detected amplitude. A correction coefficient is calculated, the calculated correction coefficient is multiplied by the newly detected amplitude, a result obtained by comparing the multiplication result with a predetermined amplitude value is detected as the reception quality signal, and based on the reception quality signal, 2. The OFDM receiving apparatus according to claim 1, wherein a phase of a time window signal for the discrete Fourier transform is controlled so as to improve reception quality. 3. The switching control unit assigns a time corresponding to a guard period of the OFDM signal to the discrete Fourier transform unit as a period used for the time window control. OFDM described
Receiver. 4. The discrete Fourier transform means includes:
4. The OFDM receiver according to claim 1, wherein a discrete Fourier transform process is performed on the DM signal at N / M times speed (N> M). 5. When the phase control of the time window signal is not completed within the period allocated by the switching control unit, the time window control unit performs phase control following the next allocation period, and completes the phase control. 5. The OFDM receiving apparatus according to claim 1, wherein a phase in which the reception quality is within an allowable range is searched later. 6. A time axis that is used in an OFDM receiving apparatus that receives an OFDM (orthogonal frequency division multiplexing) signal, receives the received OFDM signal, and performs a discrete Fourier transform within one transmission symbol of the OFDM signal. The discrete Fourier transform is performed on the basis of the time window signal that defines the above range, and a time period for converting from the time domain to the frequency domain is time-divided into a data demodulation period and a time window control period, and the discrete Fourier transform is performed in the time window control period. A reception quality signal representing the reception quality is detected from the converted output, the phase of the time window signal is controlled based on the reception quality signal so that the reception quality is improved, and the time window of the data demodulation period is set to the phase. A data demodulation method for an OFDM receiving apparatus.