JP4008804B2 - Reception margin detection apparatus and reception margin detection method for digital signal transmission system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is used in a digital signal transmission system that transmits a digital signal subjected to error correction coding processing, such as satellite digital broadcasting, terrestrial digital broadcasting, and digital FPU (Field Pickup Unit) transmission. The present invention relates to a reception margin detection apparatus and a reception margin detection method for detecting a reception margin (margin).
[0002]
[Prior art]
Recently, terrestrial digital broadcasting, which is scheduled to start operation, will be introduced simultaneously with analog broadcasting, so the channel placement situation of each channel will be complicated, and reception problems such as problems with SFN (Single Frequency Network) difficulty will occur. Is expected to occur. For this reason, it is essential to accurately grasp the reception quality at the reception location.
[0003]
In the case of conventional analog broadcasting, the degree of degradation of reception quality (transmission path degradation) can be performed by visually and audibly confirming the degree of degradation of video signals and audio signals. However, in the case of digital broadcasting, due to the characteristics of the cliff effect, the reception margin with respect to the reception limit point cannot be confirmed step by step according to the degree of deterioration of the reception quality. Even in a state where reception is possible, it may be a reception failure when the C / N is slightly degraded due to temporal fluctuations.
[0004]
As a method of detecting the reception margin, generally, noise is added to the received signal from a state where the C / N is infinite, the error rate is gradually increased, and the error rate is measured. A method of grasping the reception margin from the degree of change until the following is taken. However, this method requires trial and error adjustment in which noise is gradually increased until the error rate reaches a specified level by adding noise from an infinite C / N state. It takes too long to get. In particular, when it is necessary to check the reception margin for a large number of channels in antenna installation work for receiving terrestrial digital broadcasts, a considerable amount of work time is required, and improvement in work efficiency is required. Yes.
[0005]
As another conventional reception margin detection method, there is a method of detecting a reception margin from a change in error rate when the reception level is attenuated (see, for example, Patent Document 1). However, since this method also controls the attenuation amount by trial and error, it does not solve the above-mentioned problem.
[0006]
Note that it is already known that the reception quality is monitored by adding noise to a signal wave (for example, Patent Document 2). However, this reception quality is monitored by adding noise equivalent to the propagation loss to the end of the broadcast service area to the transmission signal in order to monitor the quality of the digital broadcast wave transmitted at the transmission station in real time. This is a technique different from detecting a reception margin at a reception point.
[0007]
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-016585
[Patent Document 2]
JP 2000-332702 A
[Problems to be solved by the invention]
As described above, in the digital signal transmission system, conventionally, the adjustment work required for detecting the reception margin with respect to the reception limit point is complicated and takes time.
[0010]
An object of the present invention is to provide a reception margin monitoring apparatus and a reception margin monitoring method for a digital signal transmission system that can solve the above-described problems and can easily detect a reception margin with respect to the reception limit point in a short time.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, the present invention comprises the following characteristic configuration.
[0012]
(1) Used in a digital signal transmission system that transmits a digital signal that has been subjected to error correction coding processing and has transmission parameters added as mode information, and the reception side of the received signal is detected when the reception side detects the reception margin of the received signal. A digital signal is demodulated from the signal, error correction processing is performed on the demodulated signal, the mode information is extracted from the demodulated received signal, and the C / N theory at the reception limit point is determined from the transmission parameters included in the mode information. A value is calculated, a C / N value before demodulation is detected, and the C / N of the received signal before demodulation is adaptively degraded based on the C / N detection value and the C / N theoretical value. C / N control is performed to create a state where the error rate obtained by the error correction unit reaches an allowable limit value, and a change amount of the C / N value before and after the C / N control is detected as a reception margin. .
[0013]
(2) In the configuration of (1), the C / N control generates noise to be added to the reception signal before demodulation, detects a C / N value of the reception signal before demodulation, and detects the C / N. The noise generation amount is adaptively controlled based on a difference value between a detection value and the C / N theoretical value so that an error rate obtained by the error correction becomes an allowable limit value.
[0014]
(3) In the configuration of (1), the C / N control detects a C / N value of the received signal before demodulation and sets a difference value between the C / N detection value and the C / N theoretical value. On the basis of this, the received signal is adaptively attenuated so that an error rate obtained by the error correction becomes an allowable limit value.
[0015]
(4) Used in a digital signal transmission system that transmits a digital signal that has been subjected to error correction coding processing and has transmission parameters added as mode information, and the reception side detects the reception margin of the received signal. A digital signal is demodulated from the signal, error correction processing is performed on the demodulated signal, the mode information is extracted from the demodulated received signal, and the C / N theory at the reception limit point is determined from the transmission parameters included in the mode information. C / N value is calculated, C / N value before demodulation is detected, and C / N of the received signal before demodulation is deteriorated so that this C / N detection value becomes the C / N theoretical value. Control is performed, and the reception margin is converted from the number of error corrections before and after the C / N control.
[0016]
(5) In the configuration of (4), the C / N control generates noise to be added to the reception signal before demodulation, detects a C / N value of the reception signal before demodulation, and detects the C / N. The amount of noise generation is controlled so that the detected value matches the theoretical C / N value.
[0017]
(6) In the configuration of (4), the C / N control detects a C / N value of the received signal before demodulation, and the C / N detection value matches the theoretical C / N value. The reception signal before demodulation is attenuated.
[0018]
(7) In the configuration of (1) or (4), the transmission parameter includes at least a modulation scheme and a coding rate, and the C / N theoretical value is calculated from the modulation scheme and a coding rate. And
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0020]
In the following description, the case of terrestrial digital broadcasting will be described as an example of the digital signal transmission system according to the present invention. Here, in terrestrial digital broadcasting, OFDM (Orthogonal Frequency Division Multiplex) is adopted, and transmission parameter values such as modulation scheme and coding rate are set in the mode information in the TMCC (Transmission and Multiplexing Configuration Control) area. Has been added.
[0021]
(First embodiment)
FIG. 1 is a block diagram showing a configuration of a first embodiment of a reception margin detection apparatus according to the present invention. In FIG. 1, the received signal of the receiving antenna 11 that has received the digital broadcast wave is frequency-converted to a baseband signal in accordance with the selected channel by the tuner 12 and supplied to the adder 13. The adder 13 adds noise generated by the noise generation source 14 to the input baseband signal. The output is digitized by an A / D (analog / digital) converter 15 and then FFT (fast Fourier). A conversion circuit 16 converts the time domain signal into a frequency domain signal and demodulates and outputs it. This demodulated signal is subjected to phase / amplitude equalization processing by the equalization circuit 17, error correction processing and decoding processing by the error correction / decoding circuit 18, and returned to the original broadcast signal. The audio signal is decoded and reproduced by the monitor device 20.
[0022]
Of the demodulated signal of the FFT processing circuit 16, the TMCC signal is supplied to the C / N theoretical value calculation circuit 21. This circuit 21 reproduces mode information from the TMCC signal, discriminates a modulation method and a coding rate from a transmission parameter value included in the mode information, and calculates a C / N theoretical value from the discrimination result. This C / N theoretical value is supplied to the noise control circuit 22.
[0023]
On the other hand, the output of the adder 13 is also supplied to the C / N detector 23. The C / N detector 23 detects the C / N value of the reception signal output from the adder 13, and the C / N detection value obtained here is supplied to the noise control circuit 22.
[0024]
The noise control circuit 22 uses an error correction / decoding circuit 18 based on the C / N theoretical value obtained by the C / N theoretical value calculation circuit 21 and the C / N detection value obtained by the C / N detector 23. The noise generation amount of the noise generation source 14 is controlled so that the obtained error rate becomes an allowable limit value, and the C / N change amount before and after the control is obtained and output as a reception margin.
[0025]
The processing operation of the above configuration will be described below with reference to FIG.
[0026]
When the received signal is in an ideal state, a certain theoretical curve characteristic is obtained between the C / N value and the error rate (BER) depending on the modulation method and coding rate of the digital broadcast signal. FIG. 2 shows theoretical value curves for coding rates (r) of 7/8 and 3/4 when the modulation method is 64QAM. This theoretical value curve varies depending on the modulation method. In digital terrestrial broadcasting, QPSK and 16QAM are scheduled as a modulation method in addition to 64QAM. Therefore, if the modulation method and coding rate are known on the receiving side, the C / N theoretical value at the error rate (2E-4 in FIG. 2) as the reception limit point can be obtained.
[0027]
Therefore, in the present embodiment, the C / N theoretical value calculation circuit 21 receives the TMCC signal from the FFT processing circuit 16, reproduces the mode information from the TMCC signal, and among the transmission parameter values included in the mode information, The modulation method and coding rate are discriminated, and the C / N theoretical value corresponding to the error rate at the reception limit point is calculated from the above theoretical value curve characteristics. For this calculation, a reference table of theoretical value curve characteristics corresponding to the possible values of the modulation method and coding rate is prepared in advance, and the corresponding reference table is selected from the determination result of the modulation method and coding rate. By calculating the corresponding value, the calculation process can be shortened.
[0028]
The noise control circuit 22 performs processing according to the control flow shown in FIG. First, the C / N theoretical value obtained by the C / N theoretical value calculation circuit 21 and the C / N detection value obtained by the C / N detection circuit 23 are fetched (step S11), and the difference value is obtained (step S12). A preset value (for example, +3 dB) is added to the difference value, and this value is set as a control initial value (step S13).
[0029]
Subsequently, a noise generation amount corresponding to the initial control value is obtained to control the noise generation amount of the noise generation source 14 (step S14). The error rate obtained by the error correction / decoding circuit 18 at this time is referred to (step S15), and compared with the allowable limit value, it is determined whether the allowable limit value has been reached (step S16). If the allowable limit value has not been reached, the noise generation amount is increased from the initial control value until the error rate reaches the allowable limit value (step S17). When the error rate reaches the allowable limit value in step S16, the C / N detection value is captured (step S18), the amount of change in the C / N detection value before and after noise control is obtained, and output as a reception margin (step S19). .
[0030]
As described above, in the present embodiment, the C / N of the received signal is deteriorated by adding noise, and is added to the digital broadcast wave when the degree of deterioration is changed until the error rate reaches the allowable limit value. The C / N theoretical value is obtained from the modulation method and coding rate of the transmission parameter, the control initial value is set in the vicinity of the error rate allowable limit value from the C / N theoretical value and the C / N detection value, and then the allowable limit The noise generation amount is increased to the value and the C / N value is lowered. For this reason, it is possible to detect the reception margin in a very short time as compared with the case where the amount of noise generation is controlled from the conventional C / N infinite state. In addition, the amount of noise generation is not adjusted by trial and error as in the prior art, but is automatically swept to the allowable limit value by the control loop, so that the reception margin can be detected very easily.
[0031]
In the above embodiment, the noise generation amount is gradually changed. However, the detection time can be further shortened by changing the noise generation amount with a constant step width. Also, the detection time can be shortened by increasing / decreasing the amount of noise generation around the C / N theoretical value to converge to the allowable limit value of the error rate.
[0032]
(Second Embodiment)
FIG. 4 is a block diagram showing the configuration of the second embodiment of the reception margin detection apparatus according to the present invention. However, in FIG. 4, the same parts as those in FIG. 1 are denoted by the same reference numerals, and redundant description is omitted here.
[0033]
4 is different from the configuration shown in FIG. 1 in that a noise generation additional portion is omitted, and a variable attenuator 24 is provided in the antenna output unit instead, and the attenuation control circuit 25 controls the attenuation of the variable attenuator 24. It is in that. The variable attenuator 24 functions to result in C / N by attenuating the received signal. The attenuation control circuit 25 corresponds to the noise control circuit 22 of the first embodiment, and the C / N theoretical value obtained by the C / N theoretical value calculation circuit 21 and the C / N detector obtained by the C / N detector 23. Based on the detected value, the attenuation of the variable attenuator 24 is controlled so that the error rate obtained by the error correction / decoding circuit 18 becomes an allowable limit value, and the C / N change amount before and after the control is obtained to obtain the reception margin. Output as.
[0034]
The control flow of the attenuation control circuit 25 is shown in FIG. 5, and the processing operation of this embodiment will be described.
[0035]
First, the C / N theoretical value obtained by the C / N theoretical value calculation circuit 21 and the C / N detection value obtained by the C / N detection circuit 23 are taken in (step S21), and the difference value is obtained (step S22). A preset value (for example, +3 dB) is added to the difference value, and this value is set as the control initial value (step S23).
[0036]
Subsequently, an attenuation amount corresponding to the initial control value is obtained to control the attenuation amount of the variable attenuator 24 (step S24). The error rate obtained by the error correction / decoding circuit 18 at this time is referred to (step S25), and compared with the allowable limit value, it is determined whether the allowable limit value has been reached (step S26). If the allowable limit value has not been reached, the amount of attenuation is increased from the initial control value until the error rate reaches the allowable limit value (step S27). When the error rate reaches the allowable limit value in step S26, the C / N detection value is taken in (step S28), the amount of change in the C / N detection value before and after attenuation control is obtained, and output as a reception margin (step S29). ).
[0037]
As described above, in the present embodiment, when the variable attenuator 24 attenuates the received signal, the C / N of the received signal is degraded, and the degree of degradation is changed until the error rate reaches the allowable limit value. The C / N theoretical value is obtained from the modulation method and coding rate of the transmission parameter added to the digital broadcast wave, and the control initial value is near the error rate allowable limit value from the C / N theoretical value and the C / N detection value. After that, the amount of attenuation is increased to an allowable limit value and the C / N value is decreased. For this reason, it is possible to detect the reception margin very easily and in a short time as compared with the conventional case where the control is performed by trial and error from an infinite C / N state.
[0038]
In the above embodiment, the detection time can be further shortened by changing the attenuation amount with a constant step width. Also, the detection time can be shortened by increasing or decreasing the attenuation around the C / N theoretical value to converge to the allowable limit value of the error rate.
[0039]
(Third embodiment)
In each of the above embodiments, the C / N change amount before and after the C / N control is obtained for the reception margin. However, according to the following method, the reception margin can be obtained easily.
[0040]
Since the configuration of the reception margin detection apparatus of the third embodiment is the same as that of FIG. 1, the configuration diagram is omitted here, and the control flow of the noise control circuit 22 which is a characteristic part is shown in FIG. A method for detecting the control contents and the reception margin will be described.
[0041]
In FIG. 6, first, the C / N theoretical value obtained by the C / N theoretical value calculation circuit 21 and the C / N detection value obtained by the C / N detection circuit 23 are fetched (step S31), and the difference value is obtained (step S31). Step S32). A noise generation amount corresponding to the difference value is obtained to control the noise generation amount of the noise generation source 14 (step S33). The error correction number obtained by the error correction / decoding circuit 18 at this time is referred to (step S34), the theoretical value curve characteristic is compared with the error correction number, and the C / N difference up to the allowable limit value and the C / N before and after the control are compared. The reception margin is converted from the N value (step S35).
[0042]
As described above, in this embodiment, the C / N of the received signal is deteriorated so that the C / N detection value becomes the C / N theoretical value by adding noise, and the reception margin is converted from the number of error corrections before and after this control. Therefore, processing can be performed with only one point of noise addition, the number of control / calculation steps can be greatly reduced, and a great effect can be obtained in shortening the detection time.
[0043]
(Fourth embodiment)
Since the configuration of the reception margin detection apparatus of the fourth embodiment is the same as that of FIG. 4, the configuration diagram is omitted here, and the control flow of the attenuation amount control circuit 22 which is a characteristic part is shown in FIG. The control contents and how to detect the reception margin will be described.
[0044]
In FIG. 7, first, the C / N theoretical value obtained by the C / N theoretical value calculation circuit 21 and the C / N detection value obtained by the C / N detection circuit 23 are taken in (step S41), and the difference value is obtained (step S41). Step S42). An attenuation amount corresponding to the difference value is obtained and the attenuation amount of the variable attenuator 24 is controlled (step S43). The error correction number obtained by the error correction / decoding circuit 18 at this time is referred to (step S44), the theoretical value curve characteristic is compared with the error correction number, and the C / N difference up to the allowable limit value and the C / N before and after the control are compared. The reception margin is converted from the N value (step S45).
[0045]
As described above, in the present embodiment, the C / N of the received signal is deteriorated so that the C / N detection value becomes the C / N theoretical value due to the attenuation of the received signal, and the reception margin is calculated from the number of error corrections before and after this control. As in the third embodiment, processing can be performed with only one point of noise addition, and the number of control / calculation steps can be greatly reduced, greatly reducing detection time. Effects can be obtained.
[0046]
As a result, it is possible to improve the efficiency of the reception margin confirmation work in the work for adjusting the direction of the transmission path and the receiving antenna.
[0047]
In the above embodiment, the terrestrial digital broadcasting system employing the OFDM method has been described as an example, but the present invention is not limited to this. For example, the present invention can be applied to a digital transmission system that employs a multiplexing method such as a CDM method. In particular, the present invention is also applicable to a digital signal transmission system that transmits a digital signal that has been subjected to error correction coding processing and added transmission parameter information, such as satellite digital broadcasting and digital FPU (Field Pickup Unit) transmission.
[0048]
In the above embodiment, the case where the modulation method and the error rate are used as the transmission parameter information has been described. However, any parameter can be used as long as the theoretical value curve is a different parameter depending on the type.
[0049]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a reception margin monitoring apparatus and a reception margin monitoring method for a digital signal transmission system that can easily detect a reception margin with respect to a reception limit point in a short time.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a first embodiment of a reception margin detection apparatus according to the present invention.
FIG. 2 is a characteristic diagram showing an example of a theoretical value curve characteristic of C / N value versus error rate used in the first embodiment.
FIG. 3 is a flowchart showing control contents of a noise control circuit used in the first embodiment.
FIG. 4 is a block diagram showing a configuration of a second embodiment of a reception margin detection apparatus according to the present invention.
FIG. 5 is a flowchart showing the control content of an attenuation control circuit used in the second embodiment.
FIG. 6 is a flowchart showing the control contents of a noise control circuit used in the third embodiment.
FIG. 7 is a flowchart showing control contents of an attenuation control circuit used in the fourth embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... Reception antenna 12 ... Tuner 13 ... Adder 14 ... Noise source 15 ... A / D converter 16 ... FFT processing circuit 17 ... Equalization circuit 18 ... Error correction / decoding circuit 19 ... Decoder 20 ... Monitor device 21 ... C / N theoretical value calculation circuit 22 ... noise control circuit 23 ... C / N detector 24 ... variable attenuator 25 ... attenuation control circuit

Claims (22)

In a reception margin detection device that performs error correction coding processing and is used in a digital signal transmission system that transmits a digital signal with transmission parameters added as mode information, and detects a reception margin of a reception signal on the reception side.
A demodulator that demodulates a digital signal from the received signal;
An error correction unit that performs error correction processing on the digital signal demodulated by the demodulation unit;
A C / N theoretical value calculator that extracts the mode information from the demodulated received signal and calculates a C / N theoretical value at a reception limit point from a transmission parameter included in the mode information;
The error correction unit detects a C / N value before demodulation and adaptively degrades the C / N of the reception signal before demodulation based on the C / N detection value and the C / N theoretical value. C / N control means for creating a state in which the error rate obtained by the step becomes an allowable limit value;
C / N change amount detecting means for detecting a change amount of the C / N value before and after the control of the C / N control unit as a reception margin;
A reception margin detection apparatus for a digital signal transmission system. The C / N control means includes
A noise generation source that generates noise to be added to the received signal before demodulation, and whose noise generation amount is variable;
A C / N detector for detecting a C / N value of the received signal before demodulation;
Based on the difference value between the C / N detection value obtained by the C / N detection unit and the C / N theoretical value, the noise generation source so that the error rate obtained by the error correction unit becomes an allowable limit value. A noise control unit that adaptively controls the amount of noise generation,
The reception margin detection apparatus of the digital signal transmission system according to claim 1, further comprising: The C / N control means includes
A variable attenuator that attenuates the received signal before demodulation and makes the attenuation variable;
A C / N detector for detecting a C / N value of the received signal before demodulation;
Based on the difference value between the C / N detection value obtained by the C / N detection unit and the C / N theoretical value, the attenuation of the variable attenuator so that the error rate obtained by the error correction unit becomes an allowable limit value. An attenuation control unit for adaptively controlling the amount;
The reception margin detection apparatus of the digital signal transmission system according to claim 1, further comprising: In a reception margin detection device that performs error correction coding processing and is used in a digital signal transmission system that transmits a digital signal with transmission parameters added as mode information, and detects a reception margin of a reception signal on the reception side.
A demodulator that demodulates a digital signal from the received signal;
An error correction unit that performs error correction processing on the digital signal demodulated by the demodulation unit;
A C / N theoretical value calculator that extracts the mode information from the demodulated received signal and calculates a C / N theoretical value at a reception limit point from a transmission parameter included in the mode information;
C / N control means for detecting the C / N value before demodulation and degrading the C / N of the received signal before demodulation so that the C / N detection value becomes the C / N theoretical value;
Reception margin conversion means for converting a reception margin from the number of error corrections of the error correction unit before and after C / N degradation by the C / N control means;
A reception margin detection apparatus for a digital signal transmission system. The C / N control means includes
A noise generation source that generates noise to be added to the received signal before demodulation, and whose noise generation amount is variable;
A C / N detector for detecting a C / N value of the received signal before demodulation;
A noise control unit that controls the noise generation amount with respect to the noise generation source so that a C / N detection value obtained by the C / N detection unit matches the C / N theoretical value;
5. The reception margin detection apparatus for a digital signal transmission system according to claim 4, further comprising: The C / N control means includes
A variable attenuator that attenuates the received signal before demodulation and makes the attenuation variable;
A C / N detector for detecting a C / N value of the received signal before demodulation;
An attenuation amount control unit for controlling the attenuation amount of the variable attenuator so that the C / N detection value obtained by the C / N detection unit matches the C / N theoretical value;
5. The reception margin detection device for a digital signal transmission system according to claim 4, further comprising: The transmission parameters include at least a modulation scheme and a coding rate,
5. The reception margin detection apparatus for a digital signal transmission system according to claim 1, wherein the C / N theoretical value calculation unit calculates a C / N theoretical value from the modulation scheme and coding rate. 5. The digital according to claim 1, wherein the C / N theoretical value calculation unit includes a reference table indicating a correspondence relationship between a possible value of the transmission parameter and a corresponding C / N theoretical value in advance. Reception margin detection device for signal transmission system. 2. The reception margin detecting apparatus for a digital signal transmission system according to claim 1, wherein the C / N control means starts control from a C / N initial value obtained by adding a set value to the C / N theoretical value. 10. The reception margin detection apparatus for a digital signal transmission system according to claim 9, wherein the C / N control means degrades the C / N value with a predetermined step width from the C / N initial value. 10. The reception margin of the digital signal transmission system according to claim 9, wherein the C / N control means converges the C / N value back and forth from the C / N initial value around the C / N theoretical value. Detection device. In a reception margin detection method in which an error correction coding process is performed and a transmission parameter is used in a digital signal transmission system that transmits a digital signal added as mode information, and a reception margin of a reception signal is detected on the reception side.
Demodulate a digital signal from the received signal,
Apply error correction to this demodulated signal,
Extracting the mode information from the demodulated received signal, calculating a C / N theoretical value at a reception limit point from a transmission parameter included in the mode information;
The error correction unit detects a C / N value before demodulation and adaptively degrades the C / N of the reception signal before demodulation based on the C / N detection value and the C / N theoretical value. C / N control is performed to create a state where the error rate obtained in (1) becomes an allowable limit value,
A reception margin detection method for a digital signal transmission system, wherein a change amount of a C / N value before and after the C / N control is detected as a reception margin. The C / N control generates noise to be added to the reception signal before demodulation, detects a C / N value of the reception signal before demodulation, and detects the C / N detection value and the C / N theoretical value. 13. The reception margin of the digital signal transmission system according to claim 12, wherein the noise generation amount is adaptively controlled so that an error rate obtained by the error correction becomes an allowable limit value based on a difference value of Detection method. The C / N control detects a C / N value of the received signal before demodulation, and an error rate obtained by the error correction based on a difference value between the C / N detection value and the C / N theoretical value. 13. The reception margin detection method of the digital signal transmission system according to claim 12, wherein the reception signal is adaptively attenuated so that becomes an allowable limit value. In a reception margin detection method in which an error correction coding process is performed and a transmission parameter is used in a digital signal transmission system that transmits a digital signal added as mode information, and a reception margin of a reception signal is detected on the reception side.
Demodulate a digital signal from the received signal,
Apply error correction to this demodulated signal,
Extracting the mode information from the demodulated received signal, calculating a C / N theoretical value at a reception limit point from a transmission parameter included in the mode information;
C / N value before the demodulation is detected, and C / N control is performed to degrade the C / N of the received signal before the demodulation so that the C / N detection value becomes the C / N theoretical value,
A reception margin detection method for a digital signal transmission system, wherein a reception margin is converted from the number of error corrections before and after the C / N control. The C / N control generates noise to be added to the reception signal before demodulation, detects a C / N value of the reception signal before demodulation, and this C / N detection value is the same as the C / N theoretical value. 16. The reception margin detection method of the digital signal transmission system according to claim 15, wherein the generation amount of the noise is controlled so as to match. The C / N control detects a C / N value of the reception signal before demodulation, and attenuates the reception signal before demodulation so that the C / N detection value matches a C / N theoretical value. 16. The reception margin detection method of the digital signal transmission system according to claim 15, wherein the reception margin is detected. The transmission parameters include at least a modulation scheme and a coding rate,
16. The reception margin detection method for a digital signal transmission system according to claim 12, wherein the C / N theoretical value is calculated from the modulation scheme and coding rate. 16. The digital signal transmission according to claim 12, wherein the C / N theoretical value is obtained in advance from a look-up table indicating a correspondence relationship between a possible value of the transmission parameter and a corresponding C / N theoretical value. System reception margin detection method. 13. The reception margin detection method for a digital signal transmission system according to claim 12, wherein the C / N control starts from a C / N initial value obtained by adding a set value to the C / N theoretical value. 21. The reception margin detection method for a digital signal transmission system according to claim 20, wherein in the C / N control, the C / N value is deteriorated by a predetermined step width from the C / N initial value. 21. The reception margin detection of a digital signal transmission system according to claim 20, wherein the C / N control converges the C / N value from the initial value of C / N around the theoretical value of C / N so as to converge. Method.

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