JPH0746202A - Method and circuit for detecting fading - Google Patents

Abstract

PURPOSE:To improve the fading detection sensitivity in a 4 PSK receiver. CONSTITUTION:The circuit is provided with a phase identification circuit 24 identifying a phase of a base fond output of a 4PSK demodulator 10. A timing resulting from delaying an eye aperture timing obtained from a clock extract circuit 18 at a delay circuit 42 is used for a sampling timing for the phase identification circuit 24 by a sample-and-hold circuit 40. When the sampling timing (observed timing) by the sample-and-hold circuit 40 is shifted by the delay circuit 42, fading appears at an output of the phase identification circuit 24 as a rotation or torsion of a constellation. The phase identification circuit 24 receives no effect of level fluctuation of an input to the 4PSK demodulator 10 or deterioration in the modulation characteristic or the like.

Description

Detailed Description of the Invention

[0001]

The present invention relates to 4PSK (Phase Shif
t Keying), BPSK (Binary PSK), 16QAM (Qu
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fading detection method and circuit which are used in a receiver for receiving a modulated wave accompanied by a phase shift such as adrature Amplitude Modulation) and detect fading by evaluating waveform distortion.

[0002]

2. Description of the Related Art In a microwave digital radio communication system, reception is performed by space diversity (hereinafter abbreviated as SD). In SD reception, the receiving apparatus is configured so that good reproduction data quality can always be obtained by switching a plurality of receiving systems and these receiving systems. That is, the reception system is used while being switched according to the occurrence of fading or the like.

In this type of receiving apparatus, that is, in the switching type SD receiving apparatus, therefore, means for detecting the degree of deterioration of the receiving state due to fading is essential. This means is usually implemented as a fading detection circuit.

As a method for realizing the fading detection circuit, a plurality of methods have been conventionally known. First, the first
As a method of (1), a method of evaluating a decrease in the signal input level to the demodulator can be mentioned. This system focuses on the fact that the level of the signal input to the demodulator decreases when fading occurs in the transmission line. This method is the most simple method of constructing a fading detection circuit, but has the drawback that it is difficult to determine the input level for evaluation. That is, since the input level to the demodulator is reduced by factors other than fading (system gain reducing factor), it is difficult to determine how much the input level should be considered to be fading.

A second method is a method of evaluating a bit error rate of a reproduced code obtained through demodulation and signal value identification (code reproduction). This method has the advantages that the control algorithm is simple and the malfunction does not easily occur because the line quality related to microwave digital radio transmission can be directly evaluated. However, this method has a drawback that it is difficult to realize so-called hitless switching. That is, since fading is detected due to a decrease in the bit error rate, a reproduction code including a bit error is temporarily output when the receiving system related to SD reception is switched according to the detection. Therefore,
This method of temporarily generating such a poor quality condition is not suitable for applications that require switching (hitless switching) with a very low bit error rate.

A third method is a method of evaluating waveform distortion in the output (baseband signal) of the demodulator. This system focuses on the fact that the effect of fading appears as waveform distortion in the baseband output of the demodulator.

FIG. 18 shows the configuration of a fading detection circuit and a receiver including the same according to a conventional example. The fading detection circuit shown in this figure is a circuit configured based on the above-mentioned third method, and has a window comparator 20 and an integration determination circuit 22. The receiver shown in this figure is a system for demodulating a 4PSK modulated wave and reproducing data, and has a 4PSK demodulator 10, an identification circuit 12, and a clock extraction circuit 18 in addition to the fading detection circuit. The receiving device shown in this figure can be used as each receiving system which constitutes the switching type SD receiving device. At that time, using the judgment result that is output from the fading detection circuit of each receiving system and indicates the strength of fading, switch to one of the playback data output from multiple receiving systems (data that is played back in the receiving system). To be That is, the reception system with the best reproduced data quality without fading is selected.

The 4PSK demodulator 10 receives a 4PSK modulated wave received from a transmission device (not shown) via a transmission line (line), demodulates it, and outputs it. In 4PSK modulation, two mutually orthogonal carrier waves are respectively modulated by two series of modulated signals, so that 4PSK demodulator 1
The baseband signal, which is the demodulation output of 0, is
It will be 2 series of ch.

X-c output from the 4PSK demodulator 10
The h and Y-ch baseband signals are input to the two comparators 14 and 16 that form the identification circuit 12, respectively. The comparators 14 and 16 compare the corresponding baseband signal with a predetermined value according to the sampling clock supplied from the clock extraction circuit 18.
Thereby, the signal value of each baseband signal is identified.
The reproduction code obtained as a result is output from the identification circuit 12 as reproduction data.

Further, since the Nyquist type band limitation is usually applied in the radio equipment, the baseband signal output from the 4PSK demodulator 10 exhibits a well-known eye pattern as shown in FIG. 19 or FIG. .

FIG. 19 shows an eye pattern when fading does not occur. As shown in this figure, a space portion called an eye appears in the output of the 4PSK demodulator 10. In the center of the eye, interference from the codes before and after it (so-called inter-code interference) is minimized. Therefore, when a code is reproduced from the output of the 4PSK demodulator 10, good reproduction code quality can be obtained by using the timing at the center of the eye (eye opening timing) as the sampling timing. In view of this point, the clock extraction circuit 18 extracts the eye opening timing,
It is supplied to the identification circuit 12 as a sampling clock.

Further, FIG. 20 shows an eye pattern in the case where fading having a large first-order inclination has occurred. That is, it is an eye pattern in the case where the transmission frequency characteristic has a primary slope due to fading. In the eye pattern of this figure, the waveform of the baseband signal that is the demodulated output is distorted due to the occurrence of fading, and the area of the eye opening (eye opening ratio) is reduced. Therefore,
The eye opening ratio can be used as a measure of line quality. The fading detection circuit shown in FIG. 18 directly monitors the eye opening ratio. That is, the decrease in the eye opening due to the occurrence of fading means that the demodulation output level of the 4PSK demodulator 10 has a smaller absolute value (absolute level). Therefore, fading can be detected by evaluating the absolute level of the demodulation output of the 4PSK demodulator 10.

More specifically, in FIG. 18, a window is set corresponding to the eye opening. When the absolute level of the demodulation output of the 4PSK demodulator 10 does not pass through this window, the waveform distortion of the baseband signal due to fading is not so great,
It has a range that can be considered significant when passing. The window comparator 20 inputs one of the baseband signals (Y-ch in the figure),
This is compared with such a window according to the sampling clock supplied from the clock extraction circuit 18, in other words, at the eye opening timing. The result of the discrimination based on such an absolute level is counted by the integration determination circuit 22, is integrated and averaged for a predetermined time, and is output as a fading determination result. This output represents the strength of fading at that time.

21 to 25 show changes in the eye opening width depending on the fading intensity. Each of these figures is a spatial signal constellation chart showing the constellation of signal points with the horizontal axis representing X-ch and the vertical axis representing Y-ch, that is, a constellation. In these figures, the frequency of the sampling clock CLK output from the clock extraction circuit 18 is 10 MHz, and the line C / N is 50 d.
It was obtained as B.

First, when fading does not occur, the signal points have an ideal arrangement as shown in FIG. 21, and each signal point has no spread. When fading occurs, each signal point starts spreading, as shown in FIGS. For example, as shown in FIG. 22, τ = 3 nsec, dip frequency Fdip = + 5
Mz, interference wave amplitude ρ = 0.5 (notch depth 30 dB)
When the 2-wave model fading occurs, the eye opening width (in the case of this conventional example, since the window identification is performed for the Y-ch, the interval between signal points in the vertical direction in the figure) is
When the case of FIG. 21 is set to 0 dB, it becomes −0.125 dB. Fading becomes stronger and dip depth ρ =
When it becomes 0.7, the eye opening width is -0.511 dB.
(FIG. 23), when ρ = 0.9, -3.205.
dB (FIG. 24), ρ = 0.97 (notch depth 30 dB)
In this case, the value is −5.491 dB (FIG. 25).

Therefore, how to set the window is important in the configuration of FIG. That is, if the window is set wide, fading can be detected with high sensitivity, and if the window is set narrow, the sensitivity is low.

[0017]

However, when setting the window, it is necessary to consider input level fluctuations and deterioration of modulation characteristics. That is, if the input level to the demodulator fluctuates, or if the modulation characteristic in the transmitter deteriorates, the absolute level of the demodulated output also fluctuates
The window must be set with some margin to eliminate these effects. Taking this margin, a so-called erroneous judgment prevention margin into consideration, the reference amplitude of 75
Normally, the window should be about%. This window corresponds to fading with a dip depth ρ = 0.97.

Therefore, when fading is detected by using a window set in anticipation of such an erroneous determination prevention margin and switching is performed in the switching type SD receiver, in the worst case, τ = 3 nsec, ρ = 0. 97, Fdip =
Even when fading such as +5 MHz has arrived, the switching is not performed. As a result, the line quality deteriorates and bit errors occur at a rate of about 10 ⁻⁷ to 10 ⁻⁸ .

The present invention has been made to solve the above problems, and a fading detection method for detecting fading by evaluating waveform distortion of a demodulated signal without performing absolute level discrimination and a fading detection method. The purpose is to provide a circuit. Moreover, it aims at suppressing an erroneous determination margin by this and improving a fading detection sensitivity.

[0020]

In order to achieve such an object, the fading detection method of the present invention uses the phase of a demodulated signal obtained by demodulating a modulated wave with a phase shift,
Based on the phase at which the signal point should be placed, and at the observation timing that is temporally shifted with respect to the eye opening timing at which the signal value is discriminated, the observed phase is aggregated and the fading It is characterized in that fading that appears as an odd function transmission waveform distortion with respect to time is detected by determining the presence or absence or the degree thereof.

Further, the fading detection circuit of the present invention is
Observe the phase of the demodulated signal obtained by demodulating the modulated wave, with the observation timing shifted temporally with respect to the eye opening timing at which the signal value is identified, with reference to the phase at which the signal point should be placed. And a phase observing means for determining the presence or absence of fading or the degree of fading based on the result, and detecting fading appearing as a transmission waveform distortion of an odd function with respect to time. Is characterized by.

The receiving device of the present invention identifies the demodulation means for demodulating the above-mentioned modulated wave and the signal value of the demodulated signal obtained by the demodulation at the eye opening timing, and outputs the result as reproduced data. Means and a fading detection circuit of the present invention are provided.

[0023]

In the present invention, 4PSK, BPSK, 16
The phase of a demodulated signal obtained by demodulating a modulated wave with a phase shift such as QAM is observed. This observation is performed with reference to the phase at which the signal points should be placed. The observation timing is set to a timing that is temporally shifted with respect to the eye opening timing at which the signal value is identified. Further, the observed phases are aggregated, and the presence or absence of fading or the degree thereof is determined based on the result.

Here, the waveform distortion that appears in the demodulated signal due to fading is an odd function distortion with respect to time, such as the distortion that appears due to the primary slope of the transmission frequency characteristic. Such distortion causes the constellation to rotate or twist at a timing that is temporally shifted from the eye opening timing. The rotation or twist of the constellation, i.e. the difference in the actual phase with respect to the phase where the signal point should originally be located, can be detected by phase observation as described above.

As described above, in the present invention, the rotation or twist of the constellation that appears when the observation timing is shifted is detected in place of the conventional amplitude displacement detection, so the results are aggregated (for example, the time axis). It is possible to determine the presence or absence of fading or its intensity by performing the integration along the line. Further, the rotation or twist of the constellation is hardly affected by the fluctuation of the input level to the demodulator and the deterioration of the modulation characteristic of the transmitter. Therefore, it is not necessary to consider the erroneous determination margin, and the fading detection sensitivity can be improved.

Further, such a fading detection method and circuit can be used in a receiving system such as a switching type SD receiver. In this case, the signal value of the demodulated signal obtained by demodulating the modulated wave is identified at the eye opening timing, while fading detection according to the present invention is performed, and switching is performed based on the result. At that time, since the fading detection sensitivity is improved, the line quality is secured and the bit error is reduced.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings. The same components as those of the conventional example shown in FIGS. 18 to 25 are designated by the same reference numerals and the description thereof will be omitted.

FIG. 1 shows the configuration of a fading detection circuit and a 4PSK receiver having the same according to an embodiment of the present invention. The receiving device shown in this figure can be used as any one of a plurality of receiving systems constituting the switching type SD receiving device.

The fading detection circuit shown in this figure comprises a phase identification circuit 24, a sample hold circuit 40, a delay circuit 42 and an integration decision circuit 22.
The phase identification circuit 24 includes comparators 26 to 32 and an EO.
It is composed of R34 to R38.

The phase identification circuit 24 is a 4PSK demodulator 10.
It is a circuit for inputting the X-ch and Y-ch demodulated signals output from the circuit and discriminating their phases. That is, the phase identification circuit 24 uses the phase of the 4PSK modulated wave input to the 4PSK demodulator 10 as a reference on the spatial signal arrangement chart (constellation) in which the horizontal axis represents X-ch and the vertical axis represents Y-ch. It discriminates whether the phase is ahead or behind and outputs the result one after another. In addition, since the modulated wave to be handled is a 4PSK modulated wave, the phase identification is performed so that each of the first to fourth quadrants on the spatial signal arrangement chart has rotational symmetry in consideration of the phase uncertainties peculiar to the 4PSK modulated wave. , The point (phase angle) where the constellation should originally exist is used as a reference.

That is, the phase identification circuit 24 is shown in FIG.
Phases are identified by the logics shown in (a) to (g). 2A to 2G show the phase identification circuit 24.
Each member (comparators 26 to 32 and EOR
34 to 38) are represented on the constellation, and the open circles indicate the positions where the signal points should originally exist. Further, the diagonal lines indicate the regions where the comparison is established or the output is 1 in the corresponding members.

First, FIGS. 2A to 2D correspond to the comparators 26 to 32, respectively. Comparator 2
6 inputs the X-ch demodulated signal output from the 4PSK demodulator 10, and compares this with 0. Further, the comparator 28 outputs Y-c output from the 4PSK demodulator 10.
The demodulated signal of h is input and compared with 0. Therefore,
The areas where the outputs of the comparators 26 and 28 are 1 are the spatial areas indicated by the diagonal lines in FIG. 2A or 2B, respectively. As a result, the output of EOR34 is 1
The area that becomes is the area indicated by the diagonal lines in FIG.

Further, the comparator 30 compares the sum of the demodulated signal of X-ch and the demodulated signal of Y-ch with 0. Therefore, the area where the output of the comparator 30 becomes 1 is shown in FIG.
In (c), the area is shown by the diagonal lines. In addition, the comparator 32 outputs the demodulated signal of X-ch and Y-c
The difference of the demodulated signal of h is compared with 0. Therefore, the area where the output of the comparator 32 is 1 is the area shown by the diagonal lines in FIG. Output of EOR36 is 1
Therefore, the area that becomes is the area indicated by the diagonal lines in FIG.

The EOR 38 outputs the output of the EOR 34 and the EO.
Input the output of R36. Therefore, the area where the output of the EOR 38 becomes 1 is the area shown by the diagonal lines in FIG.

As described above, in this embodiment, the comparators 26 to 32 and the EO which constitute the phase identifying circuit 24.
By R34-38, on the constellation,
The original phase (reference phase) of the signal point and the phase of the signal point related to the demodulated signal output from the 4PSK demodulator 10 are compared. The result is sampled by the sample hold circuit 40 provided at the subsequent stage of the phase identification circuit 24.

The sampling timing in the sample hold circuit 40 is given by the delay circuit 42. The delay circuit 42 is a circuit for shifting the sampling timing (observation timing) in the sample hold circuit 40 with respect to the eye opening timing extracted by the clock extraction circuit 18. The phase discrimination result sampled by the sample hold circuit 40 and further held is integrated and averaged by the integration determination circuit 22 over a predetermined time. This result is supplied to, for example, the SD switching means as a determination result indicating the strength of fading.

As described above, in the present embodiment, when detecting fading, the amplitude absolute level of the demodulated signal output from the 4PSK demodulator 10 is not detected, but is detected.
The phase of the demodulated signal is identified. Such phase discrimination is performed by the fluctuation of the input level to the 4PSK demodulator 10,
It is hardly affected by the deterioration of the modulation characteristic in the transmitter. Therefore, the fading detection in the present embodiment essentially does not require consideration such as providing an erroneous determination margin to cope with these factors.

Further, fading can be detected in this embodiment because the delay circuit 42 shifts the observation timing of the phase identification result with respect to the eye opening timing. Next, the principle of fading detection in this embodiment will be described.

FIGS. 3 to 7 show changes in the constellation that occur when the observation timing is shifted with respect to the eye opening timing in the situation where fading does not occur. These figures show the clock extraction circuit 18
This is a constellation when the frequency of the sampling clock CLK output from the device is 10 MHz and the C / N is 50 dB.

First, when the observation timing of the phase identification result is not shifted with respect to the eye opening timing, as shown in FIG. 5, each signal point on the constellation has a dot shape without spread. . From this state,
When the observation timing of the phase identification result is shifted to the eye opening timing by -10% and -20% (that is, to the past side), as shown in FIG. 4 and FIG. The points spread evenly. On the contrary, even when the observation timing of the phase identification result is shifted to the eye opening timing by + 10%, + 20% (that is, toward the future side), as shown in FIGS. , The signal points spread evenly.

On the other hand, when fading occurs, the signal points rotate or twist as the observation timing of the phase identification result shifts with respect to the eye opening timing. 8 to 12 show the situation of occurrence of such rotation or twist. These figures show τ = 3 nsec, ρ = 0.9, Fdip = + 5
The constellation in the case where fading of MHz occurs is shown, and the frequency and C / N of the clock CLK are the same values as in FIGS. 3 to 7.

First, when the observation timing of the phase identification result is not shifted with respect to the eye opening timing, the phase shift of the actual signal point with respect to the phase where the signal point should originally exist is as shown in FIG. It is 0 °. On the other hand, when the observation timing of the phase identification result is shifted by −10% and −20% with respect to the eye opening timing, as shown in FIG. 9 and FIG.
Phase shifts of 3.2 ° and −5.8 ° occur. vice versa,
When the observation timing of the phase identification result is shifted by + 10% and + 20% with respect to the eye opening timing, as shown in FIG. 11 and FIG. The phase displacement of the actual signal point with respect to the reference phase) increases as + 3 ° and + 5.5 °.

As shown in FIGS. 13 to 15, the signal point arrangement when fading occurs is rotated or twisted with respect to the original signal point arrangement, as shown in FIGS. 13 to 15. Because it is an odd function with respect to time. That is, when a first-order inclination occurs in the frequency characteristic of the transmission band due to fading, FIG.
As indicated by the broken line in Fig. 3, distortion is superimposed on the 1-bit transmission waveform (depending on the basic response characteristic) indicated by the solid line, in the positive direction on the future side and in the negative direction on the past side with the eye opening timing as the center. It becomes a waveform.

Such distortion causes displacement of the constellation. That is, the interference wave that causes fading is, as shown by the alternate long and short dash line in FIG.
It can be expressed as an instant vector arrow. The presence of such an interference wave causes the constellation to rotate or twist from the original constellation indicated by ∘ in FIG. 14 to the constellation indicated by ∘.

When such transmission waveform distortion and rotation or twist of the constellation resulting therefrom are three-dimensionally represented along the time axis, for example, an image as shown in FIG. 15 is obtained. In this figure, it is shown that the dash-dotted line constellation including the signal points indicated by ○ is rotated or twisted along the time axis t as indicated by the arrow due to the distortion of the transmission waveform indicated by the broken line. Has been done.

As described above, when the transmission band frequency characteristic is temporarily inclined due to fading, the transmission waveform itself is distorted, and as a result, the constellation is rotated or twisted. When the phase identification result is observed at the timing shifted to the future side with respect to the eye opening timing, the constellation rotates counterclockwise on the spatial signal arrangement chart due to the positive waveform distortion as shown in FIG. The signal point arrangement is as shown in FIG. 11 or 12. On the contrary, when the observation timing of the phase identification result is shifted to the past side with respect to the eye opening timing, waveform distortion in the negative direction as shown in FIG. 13 occurs, and therefore, as shown in FIG. 8 or FIG. Clockwise rotation or twisting occurs.

In this embodiment, since the observation timing is shifted with respect to the eye opening timing by the delay circuit 42, the occurrence of fading and its intensity are
It can be known from the output of the phase identification circuit 24.

In FIG. 16, when the observation timing of the phase identification result is shifted with respect to the eye opening timing,
The result of calculating how much the phase of the signal point is displaced with respect to the reference phase (phase displacement amount) is shown. This diagram is a diagram in the case where the frequency of the clock CLK extracted by the clock extraction circuit 18 is 10 MHz and the C / N is 50 dB, τ = 3 nsec, and the dip frequency Fd.
It shows a case where fading of ip = + 5 HMz occurs. Further, the amplitude ρ of the interference wave is represented as a parameter.

As shown in this figure, the amplitude ρ of the interference wave
= 0.5, that is, even with a relatively shallow fading such as a notch depth of 6 dB, a phase shift of 1 ° is observed due to an observation timing shift of about 20%. This amount of phase displacement can be sufficiently detected by the circuit configuration as shown in FIG. That is, in the present embodiment, even shallow fading can be preferably detected, and the fading detection sensitivity is improved as compared with the conventional example.

Also, fading generally appears as a notch coming from outside the transmission band. In FIG. 17,
The horizontal axis represents the position of the notch related to fading, and the amount of phase displacement is represented. In this figure, the frequency of the clock CLK extracted by the clock extraction circuit 18 is 10 MHz, C / N is 50 dB, τ = 3 nsec, ρ
= 0.9 and the observation timing shift with respect to the eye opening timing is set to 10%.

In the example shown in this figure, the amount of phase displacement reaches a peak when the notch position related to fading reaches the band edge (5 MHz), and the amount of phase displacement becomes 0 at the center of the band (0 MHz). Therefore, according to the present embodiment, the fading is detected by identifying the phase difference (phase displacement) of the actual signal point with respect to the reference phase where the signal point should originally exist. It is possible to detect fading that arrives at an early stage.

Although the above description is based on the circuit configuration shown in FIG. 1, the configuration of the present invention is not limited to the configuration shown in FIG. In addition, the present invention is a switching type S
It is not limited to the fading detection circuit used in each reception system of the D reception device. In addition, the invention is generally applicable to PSK modulation. That is, 4P
Not only SK but also modulated waves with phase shift such as BPSK and 16QAM can be applied.

[0053]

As described above, according to the present invention,
Since the rotation or twist of the constellation associated with fading is identified by the time shift of the observation timing and the results are tabulated, relatively shallow fading can be detected, and the detection sensitivity can be improved.

Further, at that time, it is not necessary to consider the erroneous decision margin because it is not affected by the fluctuation of the input level to the demodulator and the deterioration of the modulation characteristic in the transmitter. Furthermore, fading that generally arrives as a notch from outside the band can be detected early. As a result, when the present embodiment is applied to each receiving system of the switching type SD receiving device, for example, it is possible to switch between the receiving systems without hitting, and it is possible to realize remarkably high line quality.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a 4PSK receiver including a fading detection circuit according to an embodiment of the present invention.

2A and 2B are spatial signal arrangement charts showing the operation of the phase identification circuit according to this embodiment. FIG. 2A shows the comparator 26, FIG. 2B shows the comparator 28, and FIG.
2C shows the comparator 30, FIG. 2D shows the comparator 32, FIG. 2E shows the EOR 34, and FIG.
FIG. 2G of the OR 36 is a diagram showing the output of the EOR 38.

FIG. 3 is a spatial signal constellation chart showing a constellation obtained when the observation timing with respect to the eye opening timing is shifted by −20% under the condition that no fuzzing occurs.

FIG. 4 is a spatial signal constellation chart showing a constellation obtained when the observation timing with respect to the eye opening timing is shifted by -10% under the condition that fading does not occur.

FIG. 5 is a spatial signal constellation chart showing a constellation obtained when the observation timing with respect to the eye opening timing is shifted by 0% under the condition that fading does not occur.

FIG. 6 is a spatial signal arrangement chart showing a constellation obtained when the observation timing with respect to the eye opening timing is shifted by + 10% under the condition that fading does not occur.

FIG. 7 is a spatial signal constellation chart showing a constellation obtained when the observation timing with respect to the eye opening timing is shifted by + 20% under the condition that fading does not occur.

FIG. 8: τ = 3 nsec, ρ = 0.9, Fdip = + 5
9 is a spatial signal constellation chart showing a constellation obtained when the observation timing is shifted by -20% with respect to the eye opening timing under the condition that MHz fading occurs.

FIG. 9: τ = 3 nsec, ρ = 0.9, Fdip = + 5
9 is a spatial signal constellation chart showing a constellation obtained when the observation timing is shifted by -10% with respect to the eye opening timing under the condition that fading of MHz occurs.

FIG. 10: τ = 3 nsec, ρ = 0.9, Fdip = +
7 is a spatial signal constellation chart showing a constellation obtained when the observation timing is shifted by 0% with respect to the eye opening timing under the condition where fading of 5 MHz occurs.

FIG. 11: τ = 3 nsec, ρ = 0.9, Fdip = +
9 is a spatial signal arrangement chart showing a constellation obtained when the observation timing is shifted by + 10% with respect to the eye opening timing under the condition where fading of 5 MHz occurs.

FIG. 12: τ = 3 nsec, ρ = 0.9, Fdip = +
9 is a spatial signal arrangement chart showing a constellation obtained when the observation timing is shifted by + 20% with respect to the eye opening timing under the condition where fading of 5 MHz occurs.

FIG. 13 is a timing diagram showing a change in transmission waveform due to fading.

FIG. 14 is a spatial signal arrangement chart showing the rotation or twist of the constellation due to the fading-induced interference wave.

FIG. 15 is a diagram showing an image of twist of a constellation viewed in the time axis direction.

FIG. 16 is a diagram showing calculated values of constellation phase displacement with respect to observation timing shift amount.

FIG. 17 is a diagram showing a calculated value of a constellation phase displacement amount with respect to a position of a fading notch frequency.

FIG. 18 is a block diagram showing a configuration of a 4PSK receiver including a fading detection circuit according to a conventional example.

FIG. 19 is 4 when fading does not occur
It is a figure which shows a PSK demodulator output.

FIG. 20 is a diagram showing an output of a 4PSK demodulator in the case of fading having a large first-order gradient distortion.

FIG. 21 is a spatial signal constellation chart showing a constellation obtained when fading does not occur.

FIG. 22: τ = 3 nsec, ρ = 0.5, Fdip = +
6 is a spatial signal constellation chart showing a constellation obtained in a situation where 5 MHz fading occurs.

FIG. 23: τ = 3 nsec, ρ = 0.7, Fdip = +
6 is a spatial signal constellation chart showing a constellation obtained in a situation where 5 MHz fading occurs.

FIG. 24: τ = 3 nsec, ρ = 0.9, Fdip = +
6 is a spatial signal constellation chart showing a constellation obtained in a situation where 5 MHz fading occurs.

FIG. 25: τ = 3 nsec, ρ = 0.97, Fdip =
6 is a spatial signal constellation chart showing a constellation obtained in a situation where +5 MHz fading occurs.

[Explanation of symbols]

 10 4PSK demodulator 12 discrimination circuit 14, 16, 26, 28 30, 32 comparator 18 clock extraction circuit 22 integration determination circuit 24 phase discrimination circuit 34, 36, 38 EOR 40 sample hold circuit 42 delay circuit

Claims (3)

[Claims] 1. A phase of a demodulated signal obtained by demodulating a modulated wave accompanied by a phase shift is temporally based on a phase at which a signal point is to be arranged and an eye opening timing at which a signal value is identified. Detecting fading that appears as an odd-function transmission waveform distortion with respect to time by observing at the observation timing shifted to, and observing the phases and determining the presence or absence of fading or its degree based on the result. A fading detection method characterized by: 2. The phase of a demodulated signal obtained by demodulating a modulated wave with a phase shift is temporally based on the phase at which the signal point is to be arranged and with respect to the eye opening timing at which the signal value is identified. It is equipped with a phase observation means for observing at the observation timing shifted to, and a judgment means for accumulating the observed phases and judging the presence or absence of fading or the degree of fading based on the result. A fading detection circuit characterized by detecting fading that appears as distortion. 3. Demodulating means for demodulating a modulated wave accompanied by phase shift, and signal value identifying means for identifying a signal value of a demodulated signal obtained by demodulation at eye opening timing and outputting the result as reproduced data. A fading detection circuit according to Item 2, and a receiver.