JP2003143098A - Ofdm signal reception quality evaluating device and method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily confirm frequency selective disturbance within an OFDM transmission band. SOLUTION: In a reception quality evaluating device 31, an equalized demodulation signal is fetched from a demodulating part 24 of an OFDM receiver, and developed on a time axis by one symbol unit by a data processing part 311, and the waveform amplitude is displayed at a display device 312. A threshold setting part 313 is set for the display device 312, a set threshold is inputted to the display device 312, and an amplitude threshold for symbol waveform is displayed on the display screen, as necessary. The display device 312 is provided with the function for displaying components beyond the set threshold on the same screen in a histogram format. During displaying, the frequency display is executed according to the transmission band of the signal frequency. Although the undisturbed part becomes flat by the equalization processing, since the amplitude of the disturbed part is increased by a large amount, by displaying the frequency in matching to the transmission band of the signal frequency, the disturbance generating part can discriminated easily.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an OFDM signal reception quality evaluation apparatus and a reception quality evaluation method thereof for confirming frequency selective interference occurring in an OFDM signal in a digital transmission system based on an OFDM (Orthogonal Frequency Division Multiplexing) system. Regarding

[0002]

2. Description of the Related Art As a next-generation digital broadcasting system,
Practical application of terrestrial digital broadcasting using the current terrestrial TV band is in progress. As the transmission method,
The OFDM (Orthogonal Frequency Division Multiplexing) system has been adopted as a standard system in Japan and Europe.

In a digital broadcasting transmission system using OFDM, a large number of orthogonal carriers forming an OFDM signal are digitally modulated, and in order to reduce transmission line errors, frequency interleaving is performed after error correction coding.

In the terrestrial digital broadcasting transmission system in Japan, the transmission band of 6 MHz is divided into 13 segments, and an arbitrary modulation system and coding rate can be selected for each segment. A group of segments belonging to the same combination of modulation scheme and coding rate is called a hierarchy.

The reason why such a layer division transmission system is devised is mainly to select transmission parameters according to the reception form. For example, the layer for fixed reception is set to 64QAM and the coding rate is 7/8 with an emphasis on the information transmission speed, and the layer for mobile reception is DQPSK, which enables reception in a poor propagation environment such as fading. Conversion rate 2 /
Combinations of layers such as 3, are possible.

[0006]

By the way, when the above-mentioned digital broadcasting is put into practical use, a method of evaluating the reception quality becomes a problem. That is, in the case of analog transmission system,
Although the deterioration of the reception quality can be visually and audibly confirmed by the degree of deterioration of the video signal and the audio signal, such a confirmation method cannot be performed in the case of the digital transmission system. For this reason, it is difficult to investigate the cause when a defect occurs, which is a problem for the start of digital broadcasting.

Particularly, in the case of the OFDM transmission system, OF
If there is frequency selective interference such as spurious or co-channel interference in the DM band, the interference component is OFDM.
It is masked by the spectrum. For this reason, with the usual method of confirming the frequency spectrum, even if the received signal is distorted due to the disturbance, it is not possible to confirm the situation where the disturbance is received.

Conventionally, an evaluation method of displaying the demodulated IQ axis symbol position in XY (constellation) has been considered. However, in this method, only the carrier of the disturbed frequency deteriorates, and therefore some carriers It is not possible to confirm the state of the disturbance by checking. When all carriers are displayed, it is possible to confirm how interference is occurring, but it is not possible to identify the interference frequency, and it is not possible to investigate the cause.

The object of the present invention is to solve the above problems and
To provide an OFDM signal reception quality evaluation apparatus and a reception quality evaluation method thereof, which can easily confirm frequency selective interference occurring in the transmission band of an FDM signal and can contribute to investigation of the cause of reception quality deterioration. To aim.

[0010]

In order to solve the above problems, an OFDM signal reception quality evaluation apparatus and a reception quality evaluation method thereof according to the present invention have the following characteristic configurations.

(1) A display means for taking in the demodulated signal of the OFDM signal and displaying the waveform on a time axis in units of one symbol and displaying the frequency in accordance with the transmission bandwidth of the signal frequency for one symbol period is outstanding. It is characterized in that the waveform component is judged and evaluated as a frequency-selective interference component.

That is, when the demodulated signal of the OFDM signal is viewed in a unit of one symbol, an equalizing process gives an originally flat amplitude waveform. The waveform amplitude of the uncut portion expands. Therefore, it is possible to determine that the portion protruding from the display waveform having the above configuration is disturbed by the frequency selectivity. At the time of display, by displaying the frequency according to the transmission band of the signal frequency on the time axis, it is possible to easily determine the frequency region in which the interference is occurring.

(2) In (1), a signal selecting means for selectively taking in the IQ axis effective component of the demodulated signal, the I axis signal, and the Q axis signal is provided in the preceding stage of the display means.

As described above, by switching the waveform display on the time axis between the IQ axis effective component or the I axis signal and the Q axis signal, even if a dip occurs in a part of the transmission band due to interference, either Since the amplitude is enlarged and displayed on one display, it is possible to determine this.

(3) In (1), when the OFDM signal is subjected to frequency interleaving, a signal before frequency deinterleaving is input to the display means.

That is, since the frequency relationship with the OFDM signal is broken in the waveform display after the frequency deinterleaving is performed, it is taken out before the frequency deinterleaving and used as an evaluation target.

(4) In (1), the display means is
As the frequency display, any one of a display corresponding to a radio frequency or an intermediate frequency and a display in which a waveform center is a center frequency of an OFDM signal is characterized.

As described above, if the frequency display is set according to the processing stage of the OFDM signal, the convenience in the evaluation according to the processing stage is enhanced.

(5) In (1), the display means further comprises threshold setting means for setting an arbitrary threshold on the result displayed on the time axis and displaying the result at the same time as the waveform. To do.

By thus setting the threshold value, it becomes possible to easily discriminate the protruding portion of the amplitude.

(6) In (5), the display means is
When the threshold value is set by the threshold value setting means, components exceeding the threshold value are displayed on the same screen in a histograph format.

As described above, by separately displaying the portion exceeding the threshold value in the histograph format, it is possible to quantitatively determine the interfering component.

(7) In (5), when the OFDM signal is coded at any of a plurality of coding rates,
The threshold setting means switches the threshold according to the coding rate.

That is, since the degree of spread of the symbol value changes according to the coding rate, it is possible to prevent accidental determination as an interference component by setting the threshold value in accordance with the coding rate. .

In (8) and (5), the threshold setting means variably controls the threshold based on the error detection result of the demodulated signal.

According to the above configuration, when the error detection amount is abnormally large, it is possible to easily determine the presence or absence of frequency selective interference by appropriately changing the threshold value.

[0027]

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

FIG. 1 is a block diagram showing the configuration in the case where the reception evaluation apparatus according to the present invention is applied to a digital broadcasting system based on the OFDM system.

First, on the transmitting side, after the broadcast data is subjected to correction coding by the error correction coding unit 11, the frequency interleave processing unit 12 performs frequency interleaving,
An IFFT (Inverse Fast Fourier Transform) processing unit 13 converts the frequency domain signal into a time domain signal, and an orthogonal modulation unit 14 performs orthogonal conversion to generate a baseband OFDM signal. This OFDM signal is converted into an RF signal by the frequency conversion unit 15,
The power is amplified by the power amplifier 16 and transmitted from the antenna 17 to the space.

On the receiving side, the received signal of the antenna 21 is tuned by the tuner 22, the OFDM signal of an arbitrary channel is extracted, the frequency conversion unit 23 converts it to the IF band, and the baseband OFDM signal is extracted and demodulated. Input to the section 24. In the demodulation unit 24, the OFDM signal is converted into a digital signal by the A / D conversion unit 241, and then the orthogonal demodulation unit 242 is used.
And demodulates the signals of the I and Q axes respectively, and the FFT processing unit 24
In 3, the signal on each axis is converted from a signal in the time domain into a signal in the frequency domain, and the equalization processing unit 244 performs equalization processing of signal amplitude and phase using a pilot signal and the like. In this way, the demodulation unit 2
The frequency deinterleave processing unit 25 performs frequency deinterleaving on the demodulated signal obtained in step 4, and the error correction decoding unit 26 performs error correction processing and Viterbi decoding on each symbol to extract broadcast data.

The reception quality evaluation apparatus 31 according to the present invention takes in the demodulated signal of the OFDM signal obtained by the demodulation section 24, and the data processing section 311 uses the I axis or the Q axis, or the I axis.
One of the demodulated signals of the Q-axis effective component is selected and expanded on the time axis in units of one symbol, and the waveform amplitude is displayed on the display device 312. A threshold value setting unit 313 is provided for the display device 312, and by inputting the threshold value set by the threshold value setting unit 313 to the display device 312, the amplitude threshold value for the symbol waveform can be appropriately displayed on the display screen. . The display device 312 has a function of displaying a component exceeding the set threshold value in a histograph format on the same screen. Further, the threshold setting unit 313 has a function of setting the threshold to a level registered in advance by giving the coding rate of the OFDM signal.

The processing contents and evaluation method of the reception quality evaluation device 31 having the above configuration will be described below.

FIG. 2 shows the state of 16QAM constellation, in which each point on the IQ axis represents a symbol. The coordinate position of each symbol changes due to transmission distortion or interference and has a spread. This spread is converged to a substantially fixed position by the equalization processing during demodulation. However, when frequency interference is received in a specific area, the equalization process does not work, and the range in which the symbol is located expands.

Therefore, in the evaluation device 31, the equalized demodulated signal is viewed from the I-axis direction (a) or the Q-axis direction (b), or the IQ-axis effective component is expanded on the time axis 1. The waveform amplitude of each symbol is displayed so that the amplitude change on the time axis can be visually recognized.

FIG. 3A shows a reception spectrum waveform of an OFDM signal transmitting 16QAM data. In the figure, the dotted line represents the frequency selective interference component due to the same analog broadcast signal generated in the OFDM signal. When such a received signal is demodulated and input to the evaluation device 31, the evaluation device 31 displays a rectangular waveform as shown in FIG. At this time, the display period of one symbol is OF
It corresponds to the transmission band of the DM signal. 3 (a), (b)
As can be seen by comparing, the equalization processing flattens the undisturbed portion, but the amplitude of the disturbed portion greatly increases.

Here, as shown in FIG. 3 (b), a threshold value is set on the waveform display screen, components exceeding the threshold value are integrated, and as shown in FIG. 3 (c), a histogram is displayed on the same screen. Display in format. As a result, it is possible to easily identify the portion where the symbol waveform is widened due to the interference.

Since the frequency-selective interference generation changes with time, it is desirable to respond to the temporal change of the interference generation by averaging and displaying one symbol period for display.

The threshold value may be set manually, but it is effective to automatically switch the threshold value in accordance with the coding rate as shown in FIG. 3 (b). That is, since the degree of spread of the symbol value changes according to the coding rate, setting the threshold value according to the coding rate can prevent inadvertent determination as an interfering component. Further, the threshold may be variably controlled based on the error detection result of the demodulated signal. If the error detection amount is abnormally large, it is possible to easily determine the presence or absence of frequency selective interference by appropriately changing the threshold value.

Further, in the above waveform display, OFDM
By displaying the frequency according to the transmission band of the signal, it becomes possible to easily grasp at which frequency the interference is occurring. At this time, it is desirable that the frequency display can be switched for each stage of signal processing. An example thereof is shown in FIG.

FIG. 4A shows the internal structure of the frequency conversion section 23 of the receiving device. First, in the tuner 22, RF of UHF 15 channel (485 MHz)
When the signal is selected, the first frequency converter 231 outputs 57
It is converted into a first intermediate frequency of MHz and is converted into a second intermediate frequency of 8 MHz by the second frequency converter 232. The transmission band for one channel is ± 3 MHz with respect to the center frequency
And the effective frequency region of the OFDM signal is approximately ± 2.6M.
Hz.

FIG. 4B shows the frequency display corresponding to the RF signal, where the left end is 482.4 MHz and the right end is 487.6 MHz with respect to the center frequency of 485 MHz.

FIG. 4C shows the frequency display corresponding to the first IF signal. In this case, the frequency correspondence relationship is inverted on the time axis from the frequency relationship with the local signal during the first frequency conversion. Therefore, in the frequency display of the evaluation device 31, the left end is 59.6 MHz with respect to the center frequency of 57 MHz.
z and the right end are displayed as 54.4 MHz.

FIG. 4D shows the frequency display corresponding to the second IF signal. In this case, the frequency relationship is inverted again on the time axis during the second frequency conversion. Therefore, in the frequency display of the evaluation device 31, the left end is displayed as 5.4 MHz and the right end is displayed as 10.6 MHz with respect to the center frequency of 8 MHz.

In the above description of the frequency display, the case where the RF signal and the IF signal are used is described, but the center of the waveform may be used as the center frequency of the OFDM signal.

By the way, in the terrestrial digital broadcasting system in Japan, as described above, hierarchical transmission is possible in segment units. Generally, as shown in FIG.
It is considered to transmit the central one segment by DQPSK and the six segments on both sides by 16QAM. When such an OFDM signal is frequency-displayed on the time axis by the evaluation device 31, a waveform display corresponding to the segment area of the same modulation method is performed as shown in FIG. Therefore, when the frequency selective interference is received, it is possible to determine in which segment area the interference occurs.

Therefore, by using the evaluation device having the above-mentioned configuration, it becomes possible to easily confirm the frequency selective interference occurring in the transmission band of the OFDM signal, which can be useful for the early investigation of the cause of the reception quality deterioration. it can.

In the above embodiment, the demodulated signal is extracted from the receiving device and input to the evaluation device. However, the evaluation device itself may be provided with a receiving function, and the same operation can be performed. Further, the waveform display processing of the data processing unit can be implemented by either a hardware configuration by logic or a software configuration by waveform data processing.
Therefore, for example, by incorporating a reception quality evaluation software program into a general portable personal computer and mounting a signal input interface, it is possible to easily realize a portable evaluation device capable of evaluating reception quality at any point. You can

[0048]

As described above, according to the present invention, OFDM
It is possible to provide an OFDM signal reception quality evaluation device and a reception quality evaluation method thereof that can easily confirm frequency selective interference occurring in a signal transmission band and contribute to investigating the cause of reception quality deterioration. .

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration when a reception evaluation apparatus according to the present invention is applied to a digital broadcasting system based on an OFDM system.

FIG. 2 is a diagram showing a state of constellation of 16QAM.

FIG. 3A is an OF for transmitting 16QAM data.
FIG. 6 is a waveform diagram showing a received spectrum waveform of a DM signal, (b) a waveform amplitude display example on the time axis in the evaluation device, and (c) a histograph format display example corresponding to (b).

FIG. 4A is a configuration example of a frequency conversion unit in a receiving device, FIG. 4B is a frequency display example corresponding to an RF signal,
(C) is a frequency display example corresponding to the first IF signal, (d)
FIG. 4 is a diagram showing a frequency display example corresponding to a second IF signal.

5A is a diagram showing an example of an OFDM signal to be hierarchically transmitted, and FIG. 5B is a diagram showing an example when a demodulation output of the OFDM signal of FIG.

[Explanation of symbols]

11 ... Error correction coding unit 12 ... Frequency interleave processing unit 13 ... IFFT (Inverse Fast Fourier Transform) processing unit 14 ... Quadrature modulator 15 ... Frequency converter 16 ... Power amplifier 17 ... Antenna 21 ... Antenna 22 ... Tuner 23 ... Frequency converter 24 ... Demodulation unit 241 ... A / D converter 242 ... Quadrature demodulation unit 243 ... FFT processing unit 244 ... Equalization processing unit 25 ... Frequency deinterleave processing unit 26 ... Error correction decoding unit 31 ... Reception quality evaluation device 311 ... Data processing unit 312 ... Display device 313 ... Threshold setting unit

Claims (9)

[Claims] 1. An apparatus used in a digital transmission system based on an OFDM (Orthogonal Frequency Division Multiplexing) system for evaluating the reception quality of an OFDM signal, wherein a demodulated signal of the OFDM signal is taken in and waveforms are taken on a time axis in units of one symbol An OFDM received signal quality evaluation device comprising display means for displaying and displaying a frequency of one symbol period in accordance with a transmission bandwidth of a signal frequency, and judging and evaluating a salient waveform component as a frequency selective interference component. . 2. The I of the demodulated signal is provided before the display means.
2. The OFDM signal reception quality evaluation device according to claim 1, further comprising signal selection means for selectively capturing the Q-axis effective component, the I-axis signal, and the Q-axis signal. 3. The signal before frequency deinterleaving is input to the display means when the OFDM signal is subjected to frequency interleaving.
The described OFDM signal reception quality evaluation device. 4. The display means displays the frequency,
2. The OFDM signal reception quality evaluation device according to claim 1, wherein the display is performed corresponding to a radio frequency or an intermediate frequency, and the center of the waveform is set to the center frequency of the OFDM signal. 5. The threshold value setting means for causing the display means to set an arbitrary threshold value for a result displayed on a time axis and display the result at the same time as the waveform. OFDM signal reception quality evaluation device. 6. The OFDM signal reception quality according to claim 5, wherein said display means, when the threshold value is set by said threshold value setting means, displays components exceeding the threshold value in a histograph format on the same screen. Evaluation device. 7. The threshold value setting means switches the threshold value according to the coding rate when the OFDM signal is coded at any of a plurality of coding rates. OFDM signal reception quality evaluation device. 8. The OFDM signal reception quality evaluation apparatus according to claim 5, wherein the threshold value setting means variably controls the threshold value based on an error detection result of the demodulated signal. 9. A method for evaluating the reception quality of an OFDM signal used in a digital transmission system using an OFDM (Orthogonal Frequency Division Multiplexing) method, wherein a demodulated signal of the OFDM signal is displayed as a waveform on a time axis in a unit of one symbol. An OFDM signal reception quality evaluation method is characterized in that the frequency of one symbol period is displayed in accordance with the transmission bandwidth of the signal frequency, and the protruding waveform component is evaluated as a frequency selective interference component.