JP3841665B2 - OFDM signal reception quality evaluation apparatus and its reception quality evaluation method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an OFDM signal reception quality evaluation apparatus and a reception quality evaluation method thereof capable of confirming frequency selective interference occurring in an OFDM signal in a digital transmission system using an OFDM (Orthogonal Frequency Division Multiplexing) scheme.
[0002]
[Prior art]
As a next-generation digital broadcasting system, terrestrial digital broadcasting using the current TV band using terrestrial waves has been put into practical use. As the transmission method, the OFDM (Orthogonal Frequency Division Multiplexing) method is adopted as a standard method in Japan and Europe.
[0003]
In the transmission system of digital broadcasting using OFDM, a number of orthogonal carriers forming an OFDM signal are digitally modulated, and frequency interleaving is performed after error correction coding in order to reduce transmission path errors.
[0004]
In addition, the transmission system of terrestrial digital broadcasting in Japan has a structure in which a 6 MHz transmission band 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.
[0005]
The purpose of such a hierarchical division transmission system is to select transmission parameters mainly in accordance with the reception mode. For example, the fixed reception layer is set to 64QAM and a coding rate of 7/8 with an emphasis on the information transmission rate, and the mobile reception layer is a DQPSK and code that can be received in a poor propagation environment such as fading. Hierarchical combinations such as a conversion rate of 2/3 are possible.
[0006]
[Problems to be solved by the invention]
By the way, when the above-mentioned digital broadcasting is put into practical use, a method for evaluating reception quality has become a problem. In other words, in the case of the analog transmission method, the degradation of the reception quality can be visually and audibly confirmed depending on the degree of the deterioration of the video signal and the audio signal, but in the case of the digital transmission method, such confirmation is possible. I can't do it. For this reason, it is difficult to investigate the cause when a problem occurs, which is an issue for the start of digital broadcasting.
[0007]
In particular, in the case of the OFDM transmission method, if there is a frequency selective interference such as spurious or co-channel interference in the OFDM band, the interference component is masked by the OFDM spectrum. For this reason, with a normal method based on confirmation of the frequency spectrum, even if the received signal is distorted due to interference, it is not possible to confirm the status of interference.
[0008]
Conventionally, an evaluation method for displaying the demodulated IQ axis symbol position in XY (constellation) is considered. However, in this method, only the carrier of the frequency that has been disturbed deteriorates. I cannot confirm the state of interference. When all the carriers are displayed, it is possible to confirm the state of being disturbed, but the disturbing frequency cannot be specified, and the cause cannot be determined.
[0009]
An object of the present invention is to solve the above-described problems, easily confirm frequency-selective interference that occurs in the transmission band of an OFDM signal, and contribute to investigation of the cause of reception quality degradation. An object of the present invention is to provide an evaluation apparatus and a reception quality evaluation method thereof.
[0010]
[Means for Solving the Problems]
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 configuration.
[0011]
(1) A display means for taking in the demodulated signal of the OFDM signal and displaying the waveform on the 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 provided. It is judged and evaluated as an interference component of frequency selectivity.
[0012]
That is, when the demodulated signal of the OFDM signal is viewed in units of one symbol, an originally flat amplitude waveform can be obtained by the equalization process, but if the frequency selective interference is received, it cannot be equalized by the interference component during demodulation. The waveform amplitude of the part expands. Therefore, it is possible to determine that the portion protruding from the display waveform with the above configuration is subjected to interference with frequency selectivity. At the time of display, by displaying the frequency on the time axis according to the transmission band of the signal frequency, it is possible to easily determine the frequency region receiving the interference.
[0013]
(2) In (1), there is provided signal selection means for selectively taking in the IQ-axis effective component, I-axis signal, and Q-axis signal of the demodulated signal before the display means.
[0014]
In this way, 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, either display is possible even if a dip occurs in part of the transmission band due to interference. Since the amplitude is enlarged and displayed, it can be discriminated.
[0015]
(3) In (1), when the OFDM signal is subjected to frequency interleaving, a signal before frequency deinterleaving is input to the display means.
[0016]
That is, after the frequency deinterleaving is performed, the frequency relationship with the OFDM signal is lost in the waveform display. Therefore, the frequency is taken out before the frequency deinterleaving and is evaluated.
[0017]
(4) In (1), the display means performs either the display corresponding to the radio frequency or the intermediate frequency or the display using the waveform center as the center frequency of the OFDM signal as the frequency display.
[0018]
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.
[0019]
(5) In (1), the display means further includes threshold setting means for setting an arbitrary threshold value for the result displayed on the time axis and displaying the result simultaneously with the waveform.
[0020]
In this way, by setting the threshold value, it is possible to easily determine the protruding portion of the amplitude.
[0021]
(6) In (5), when the threshold value is set by the threshold value setting means, the display means displays a component exceeding the threshold value on the same screen in a histogram format.
[0022]
In this way, by separately displaying the portion exceeding the threshold value in the histogram format, it becomes possible to quantitatively determine the disturbing component.
[0023]
(7) In (5), when the OFDM signal is encoded at any one of a plurality of encoding rates, the threshold setting means switches the threshold according to the encoding rate.
[0024]
That is, since the spread of the symbol value changes according to the coding rate, it is possible to prevent the symbol value from being inadvertently determined as an interference component by setting the threshold according to the coding rate.
[0025]
(8) In (5), the threshold value setting means variably controls the threshold value based on an error detection result of the demodulated signal.
[0026]
According to the above configuration, when the error detection amount is abnormally large, it is possible to easily determine the presence or absence of interference with frequency selectivity by appropriately changing the threshold value.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0028]
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 the OFDM system.
[0029]
First, on the transmission side, the broadcast data is subjected to correction coding by the error correction coding unit 11, then frequency interleaved by the frequency interleaving processing unit 12, and then processed from the frequency domain by the IFFT (Inverse Fast Fourier Transform) processing unit 13. The signal is converted into a region signal and orthogonally converted by the orthogonal modulation unit 14 to generate a baseband OFDM signal. This OFDM signal is converted into an RF signal by the frequency conversion unit 15, power amplified by the power amplifier 16, and transmitted from the antenna 17 to space.
[0030]
On the receiving side, the received signal of the antenna 21 is selected by the tuner 22, the OFDM signal of an arbitrary channel is extracted, converted into the IF band by the frequency conversion unit 23, and the baseband OFDM signal is taken out to the demodulation unit 24. input. In this demodulator 24, the OFDM signal is converted into a digital signal by the A / D converter 241, and then the orthogonal demodulator 242 demodulates the signals on the I and Q axes, and the FFT processor 243 converts the signals on each axis into time. The signal is converted from a domain signal to a frequency domain signal, and an equalization processing unit 244 performs a signal amplitude and phase equalization process using a pilot signal or the like. The demodulated signal obtained by the demodulator 24 is frequency deinterleaved by the frequency deinterleave processor 25, and the error correction decoder 26 performs error correction processing and Viterbi decoding of each symbol to extract broadcast data. .
[0031]
The reception quality evaluation apparatus 31 according to the present invention takes in the demodulated signal of the OFDM signal obtained by the demodulator 24 and selects either the I-axis or Q-axis or the IQ-axis effective component demodulated signal by the data processor 311 Then, it is 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 the threshold value set by the threshold value setting unit 313 is input to the display device 312, whereby 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 on the same screen in a histogram format. The threshold value setting unit 313 has a function of setting a threshold value to a pre-registered level by giving a coding rate of the OFDM signal.
[0032]
In the reception quality evaluation apparatus 31 having the above configuration, the processing contents and the evaluation method will be described below.
[0033]
FIG. 2 shows the state of 16QAM constellation. Each point on the IQ axis represents a symbol. Each symbol has a spread because its coordinate position changes due to transmission distortion or interference. This spread is converged to a substantially fixed position by the equalization process at the time of demodulation. However, when frequency interference is received in a specific area, the equalization process does not function and the range in which the symbols are located widens.
[0034]
Therefore, in the evaluation apparatus 31, the demodulated signal subjected to equalization is viewed from the I-axis direction (a) or the Q-axis direction (b), or the IQ-axis effective component is developed in units of one symbol on the time axis. Waveform amplitude is displayed so that changes in amplitude on the time axis can be visually recognized.
[0035]
FIG. 3A shows a received spectrum waveform of an OFDM signal that transmits 16QAM data. In the figure, a dotted line represents a frequency selective interference component generated in the OFDM signal by the same analog broadcast 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 made to correspond to the transmission band of the OFDM signal. As can be seen by comparing FIGS. 3 (a) and 3 (b), the part that has not been disturbed becomes flat by equalization processing, but the amplitude of the part that has been disturbed greatly increases.
[0036]
Here, as shown in FIG. 3 (b), a threshold value is set on the waveform display screen, and components exceeding the threshold value are integrated, and as shown in FIG. To do. As a result, it is possible to easily determine the portion where the symbol waveform is expanded due to interference.
[0037]
It should be noted that the occurrence of interference with frequency selectivity changes with time, so it is desirable to cope with the temporal change of occurrence of interference by averaging and displaying and integrating one symbol period.
[0038]
The threshold value may be set manually, but it is effective to automatically switch the threshold value according to the coding rate as shown in FIG. That is, since the spread of the symbol value changes according to the coding rate, it is possible to prevent the symbol value from being inadvertently determined as an interference component by setting the threshold according to the coding rate. Further, the threshold value may be variably controlled based on the error detection result of the demodulated signal. When the amount of error detection is abnormally large, it is possible to easily determine the presence or absence of interference with frequency selectivity by appropriately changing the threshold value.
[0039]
Further, in the waveform display, by performing frequency display in accordance with the transmission band of the OFDM signal, it becomes possible to easily grasp at which frequency the interference is received. At this time, it is desirable that the frequency display can be switched for each stage of signal processing. An example is shown in FIG.
[0040]
FIG. 4A shows the internal configuration of the frequency conversion unit 23 of the receiving apparatus. First, in the tuner 22, when an RF signal of UHF 15 channel (485 MHz) is selected, the first frequency conversion unit 231 converts it to a first intermediate frequency of 57 MHz, and the second frequency conversion unit 232 converts the second intermediate signal of 8 MHz. Converted to frequency. The transmission band of one channel is ± 3 MHz with respect to the center frequency, and the effective frequency region of the OFDM signal is approximately ± 2.6 MHz.
[0041]
FIG. 4B shows the frequency display corresponding to the RF signal, and the left end is displayed as 482.4 MHz and the right end is displayed as 487.6 MHz with respect to the center frequency of 485 MHz.
[0042]
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 displayed as 59.6 MHz and the right end as 54.4 MHz with respect to the center frequency of 57 MHz.
[0043]
FIG. 4D shows a 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 as 10.6 MHz with respect to the center frequency of 8 MHz.
[0044]
In the above description of the frequency display, the case where the RF signal and the IF signal are supported has been described. However, the display may be made with the waveform center as the center frequency of the OFDM signal.
[0045]
By the way, in the Japanese terrestrial digital broadcasting system, as described above, hierarchical transmission is possible in segment units. In general, as shown in FIG. 5A, it is considered that one center segment is transmitted by DQPSK and each of the six segments on both sides is transmitted 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 region of the same modulation method is made as shown in FIG. For this reason, it is also possible to determine in which segment area the interference occurs when the frequency selective interference is received.
[0046]
Therefore, by using the evaluation apparatus having the above-described configuration, it becomes possible to easily confirm the frequency selective interference that occurs in the transmission band of the OFDM signal, which can be used for early investigation of reception quality degradation.
[0047]
In the above embodiment, the demodulated signal is extracted from the receiving apparatus and input to the evaluation apparatus. However, the present invention can be similarly implemented even if the evaluation apparatus itself has a reception function. Further, the waveform display process of the data processing unit can be implemented by either a hardware configuration by logic or a software configuration by waveform data processing. For this reason, for example, by incorporating a reception quality evaluation software program into a general portable personal computer and mounting a signal input interface, a portable evaluation device capable of evaluating reception quality at any point can be easily realized. Can do.
[0048]
【The invention's effect】
As described above, according to the present invention, an OFDM signal reception quality evaluation apparatus capable of easily confirming frequency selective interference occurring in the transmission band of an OFDM signal and contributing to investigation of the cause of reception quality degradation, The reception quality evaluation method can be provided.
[Brief description of the 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 16QAM constellation.
3A is a received spectrum waveform of an OFDM signal that transmits 16QAM data, FIG. 3B is a waveform amplitude display example on the time axis in the evaluation apparatus, and FIG. 3C is a histogram graph corresponding to FIG. FIG.
4A is a configuration example of a frequency conversion unit in a receiving apparatus, FIG. 4B is a frequency display example corresponding to an RF signal, FIG. 4C is a frequency display example corresponding to a first IF signal, and FIG. The figure which shows the example of a frequency display corresponding to 2IF signal.
5A is a diagram illustrating an example of a hierarchically transmitted OFDM signal, and FIG. 5B is a diagram illustrating an example in which a demodulated output of the OFDM signal of FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... Error correction encoding part 12 ... Frequency interleaving process part 13 ... IFFT (inverse fast Fourier transform) process part 14 ... Orthogonal modulation part 15 ... Frequency conversion part 16 ... Power amplifier 17 ... Antenna 21 ... Antenna 22 ... Tuner 23 ... Frequency Conversion unit 24 ... demodulation unit 241 ... A / D conversion unit 242 ... orthogonal 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 (8)

In an apparatus that evaluates the reception quality of an OFDM signal that is used in a digital transmission system using an OFDM (Orthogonal Frequency Division Multiplexing) scheme and is encoded at any one of a plurality of coding rates ,
Display means for capturing a demodulated signal of the OFDM signal, matching one symbol period to the transmission bandwidth of the OFDM signal, and displaying a waveform of the frequency of the demodulated signal on a time axis in units of one symbol;
A threshold setting means for switching the display means on the display waveform on the time axis according to the coding rate and simultaneously displaying the threshold on the display means;
An OFDM received signal quality evaluation apparatus characterized by judging and evaluating a protruding waveform component as a frequency selective interference component. In an apparatus for evaluating the reception quality of an OFDM signal used in a digital transmission system by OFDM (Orthogonal Frequency Division Multiplexing) system,
Display means for capturing a demodulated signal of the OFDM signal, matching one symbol period to the transmission bandwidth of the OFDM signal, and displaying a waveform of the frequency of the demodulated signal on a time axis in units of one symbol;
A threshold setting unit configured to set a threshold value for the display waveform displayed on the time axis and display the display unit simultaneously with the waveform, and to variably control the threshold value based on an error detection result of the demodulated signal; And
An OFDM received signal quality evaluation apparatus characterized by judging and evaluating a protruding waveform component as a frequency selective interference component. 3. The OFDM signal reception quality evaluation according to claim 1, further comprising a signal selection unit that selectively takes in an IQ-axis effective component, an I-axis signal, and a Q-axis signal of the demodulated signal before the display unit. apparatus. 3. The OFDM signal reception quality evaluation apparatus according to claim 1, wherein when the OFDM signal is subjected to frequency interleaving, a signal before frequency deinterleaving is input to the display means. 3. The OFDM signal according to claim 1, wherein the display means performs any one of a display corresponding to a radio frequency or an intermediate frequency, and a display using a waveform center as a center frequency of the OFDM signal as the frequency display. Reception quality evaluation device. The display means, when the threshold is set by the threshold value setting means, OFDM signal receiving quality evaluation apparatus according to claim 1 or 2, wherein the displaying in Hisutogurafu format on the same screen the component exceeding the threshold. In a method for evaluating the reception quality of an OFDM signal used in a digital transmission system using an OFDM (Orthogonal Frequency Division Multiplexing) scheme and encoded at one of a plurality of coding rates ,
The demodulated signal of the OFDM signal is taken in , one symbol period is matched with the transmission bandwidth of the OFDM signal, and the frequency of the demodulated signal is displayed on the time axis in one symbol unit. An OFDM signal reception quality evaluation method , wherein a threshold value is switched and set on a display waveform on an axis in accordance with the coding rate and simultaneously displayed, and a protruding waveform component is determined and evaluated as a frequency selective interference component. In 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,
The demodulated signal of the OFDM signal is taken in, and a transmission period of the OFDM signal for one symbol period The frequency of the demodulated signal is displayed on the time axis in units of symbols according to the bandwidth, and a threshold is set for the display waveform displayed on the time axis in relation to the display and displayed simultaneously with the waveform. An OFDM received signal quality evaluation method, wherein the threshold value is variably controlled based on an error detection result of the demodulated signal, and a protruding waveform component is determined and evaluated as a frequency selective interference component.

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