JPH07118688B2 - Cross polarization interference canceller - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a cross polarization interference canceling device used in an orthogonal polarization communication system that uses two polarizations that are orthogonal to each other, and is particularly a main part of the device. The present invention relates to an interference prevention technique at the time of resetting a cross polarization interference cancellation circuit that is a component.

(Prior Art) As is well known, in a microwave digital radio communication system, a multilevel digital modulation method is adopted from the viewpoint of effective use of frequencies, and two polarized waves (eg, horizontal polarization) of the same frequency orthogonal to each other are used. Polarization and vertical polarization, right-hand circular polarization and left-hand circular polarization, etc.) are used for the orthogonal polarization communication method.

By the way, when the orthogonal polarization communication system is adopted, cross polarization interference may occur between both polarization signals due to anisotropy of a medium such as raindrops during rainfall. Therefore, on the demodulation side in this type of digital wireless communication system, there is a conventional method such as the third method.
A cross polarization interference canceller as shown in the figure is provided.

The cross polarization interference removing operation may be performed in the intermediate frequency band or in the base band (see, for example, JP-A-55).
-133156, Japanese Patent Laid-Open No. 59-77734), and the cross polarization interference canceling circuit 25 shown in FIG. AGC circuits (automatic gain control circuits) 22 and 23 are placed in front of the cross polarization interference cancellation circuit 25, respectively, and the reception signals applied to these input terminals (20, 21) are caused by variations in the reception electric field. Even if the level changes, the input signal (1, 2) of the cross polarization interference cancellation circuit 25 is supplied with a constant level of the received signal.

Of the reception signals of the two orthogonal polarizations, one reception signal is demodulated by the demodulator 11 via the input terminal 1, and the other reception signal is demodulated by the demodulator 12 via the input terminal 2. The input signal of the demodulator 11 is obtained by combining the output signal of the transversal filter 5 with one received signal, and the input signal of the demodulator 12 is combined with the output signal of the transversal filter 6 with the other received signal. It has been made. Here, the transversal filter 5 receives the branch reception signal 19 which is a part of the other reception signal as an input signal, and the transversal filter 6 receives the branch reception signal 18 which is a part of the one reception signal as an input signal. Control circuit
The same operation is performed under the control of 13.

That is, if one reception signal is called a main polarization signal and the other reception signal is a different polarization signal, the transversal filter 5
A signal of the same amplitude and opposite phase as the different polarization component leaking into the main polarization signal is generated based on the branch reception signal 19 which is the different polarization signal, and it is converted to the main polarization signal at the input stage of the demodulator 11. Combine,
The interference component is suppressed. At this time, the coupling degree is determined based on the control signal 9 from the control circuit 13.

The control circuit 13 uses the demodulator on the main polarization side (demodulator 11 in the above example) and the error signal and the reproduced data signal obtained by the demodulator on the other polarization side (demodulator 12 in the above example). The control signal (9, 10) is formed and output based on the above, and the corresponding transversal filter (5, 6) is controlled. For specific control methods, see, for example, Japanese Patent Laid-Open No. 59-112739.

By the way, in the demodulator, when the input signal level is extremely lowered (this occurs due to fading or the like), or when noise is increased, carrier synchronization loss occurs and an abnormality occurs in the demodulation output. Then, the cross polarization interference cancellation circuit does not operate normally and, on the contrary, results in injecting interference into the main polarization signal. Therefore, the control circuit
13 has a function of resetting the cross polarization interference cancellation circuit in such a case. That is, the control circuit 13 obtains the asynchronous detection signal from the demodulators (11, 12), and when one or both of the demodulators 11 and 12 are asynchronous, sets the tap coefficient of the corresponding transversal filter (5, 6). It is set to zero to prevent the recovery delay to the normal control operation due to the divergence of the transversal filter. The original purpose of this reset is to prevent the demodulator on the different polarization side from getting out of synchronization and not disturb the demodulator on the main polarization side that is not out of synchronization. Can be said. It should be noted that the above-mentioned JP-A-59-77734 discloses an example of the reset method, which should be referred to.

(Problems to be Solved by the Invention) In this way, in the cross polarization interference canceling circuit, each tap coefficient of the transversal filter is set to zero when the demodulator is not synchronized, but this is the original purpose of the reset. From this, it means that the attenuation amount (isolation amount) of each tap of the transversal filter is set to infinity, and it is necessary that the output signal level be at least a predetermined attenuation amount that can be regarded as zero. However, in reality, it is difficult to obtain the required amount of attenuation required for the system due to variations in the scale and characteristics of the transversal filter hardware. In this case, there is a problem that the main polarization side demodulation system is interfered with and the characteristics are deteriorated.

For example, as described above, the cross polarization interference cancellation circuit 25
The AGC circuit (22,23) is placed in front, but for maintenance etc., disconnect the input terminal 21 of the AGC circuit 23 on the different polarization side, and reconnect the input terminal 21 after the maintenance work to reconnect the signal. Consider the operation when applied.

FIG. 4 shows the transient response characteristics of the AGC circuit 23 in such a case by the change of the envelope, but as shown in the figure, in the AGC circuit 23,
When the input signal is applied at time T ₀ , the output signal level rapidly increases and reaches the maximum value level A rapidly, then gradually decreases to the steady level at time T ₁ , and thereafter maintains the normal level. .

Therefore, it goes without saying that the demodulator 12 on the different polarization side is in the asynchronous state when the input signal is disconnected before time T ₀ , but at time T ₀
Since the input signal level is high during the period from to T ₁ , the demodulator 12 is still in the asynchronous state during this period, and synchronization is established for the _first time at T ₁ . Therefore, the transversal filter 5 is set to the reset state before the time T ₁ including the time when the input signal is cut off, and is switched to the automatic control mode after the time T ₁ .

Here, the problem is the operation in the period from time T ₀ to time T ₁ . That is, in this time domain, the transversal filter 5 is in the reset state because the demodulator 12 is asynchronous, but the maximum attenuation level A is input because the tap attenuation is finite due to variations in hardware as described above. Then, the signal is leaked to the output of the transversal filter 5 without being attenuated, and is coupled to the input of the demodulator 11 as an interference wave, and the characteristic of the main polarization signal is temporarily deteriorated. It occurs.

The present invention has been made in view of such a conventional problem, and an object thereof is to reduce the reception signal level in a cross polarization interference canceling device in which an automatic gain control circuit is placed before the cross polarization interference canceling circuit. It is an object of the present invention to provide a cross-polarization interference canceller capable of suppressing the transient response of the automatic gain control circuit that occurs when a sudden change occurs, and thus preventing temporary deterioration of the cross-polarization interference cancel characteristics.

(Means for Solving the Problem) In order to achieve the above object, the cross polarization interference canceling apparatus of the present invention has the following configuration.

That is, the cross polarization interference canceling apparatus of the present invention includes two series of automatic gain control circuits for performing automatic gain control on received signals of each of two orthogonal polarizations to make the output signal level constant; Cross-polarization elimination configured to receive the output of the control circuit and remove the heteropolarization side signal of the other signal leaking into the main polarization side signal of one signal centering on the transversal filter A cross polarization interference canceller comprising: a circuit; and a detection signal designating this state when the input signal level of each of the two series of automatic gain control circuits falls below a prescribed value including a level break. And at least two level detectors for transmitting over a predetermined time including from the time when the recovery of the input signal level is detected to the time when the normal state is reached; The control circuit fixes the gain of a corresponding one of the two series of level detectors to a predetermined low gain while receiving the detection signal, and temporarily deteriorates the cross polarization interference cancellation characteristics when the input signal level suddenly changes. Is designed to suppress the above.

(Operation) Next, the operation of the cross polarization interference canceling apparatus of the present invention configured as described above will be described.

When the received signal level on the different polarization side falls below the specified value,
For example, when the input is cut off, the automatic gain control circuit on the different polarization side originally operates to increase the gain, but in the present invention, conversely operates to decrease the gain.

Therefore, when the level of the received signal on the side of the different polarization recovers to the specified value after being "disconnected", the processing of the recovered received signal is performed in the low gain control state, so that the transient characteristics as shown in FIG. Generation is suppressed, and the input on the different polarization side of the cross polarization interference cancellation circuit does not become an excessive level.

Thus, the temporary deterioration of the cross polarization interference cancellation characteristics can be prevented.

Embodiment An embodiment of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows a cross polarization interference canceller according to an embodiment of the present invention. The same components as those of the conventional device are designated by the same reference numerals. Hereinafter, a portion according to the present invention will be described.

In FIG. 1, 7 and 8 are main intermediate frequency amplifiers, and 16 and 17 are AGC.
It is a loop amplifier, and the loop circuit of 7 (8) and 16 (17) is basically the same as the AGC circuit 22 (23) of the conventional example device. In order to facilitate understanding of the invention, FIG. It is shown in subdivided form.

That is, in the present invention, the level detector (3, 4) for detecting a change in the signal level of the input signal of the AGC circuit, specifically, the input signal of the main intermediate frequency amplifier (7, 8) is provided, The AGC loop amplifier 16 (17) is the level detector 3
By changing the content of the AGC voltage control signal output to the main intermediate frequency amplifier 7 (8) in response to the output of (4), the gain of the main intermediate frequency amplifier 7 (8) can be changed and controlled. is there.

FIG. 2 shows a configuration example of the AGC loop amplifier (16, 17). AG
The C loop amplifier is basically a main intermediate frequency amplifier 7
A detector 31 for detecting the output signal of (8) (that is, the input signal of the input terminal 1 (2) of the cross polarization interference cancellation circuit 25);
It is composed of an amplifier 32 that amplifies this detection output and outputs an AGC voltage control signal to the main intermediate frequency amplifier 7 (8). However, in the present invention, the switch 33 is interposed between the detector 31 and the amplifier 32. The DC power supply 34 to the other switching terminal of the switch 33.
And switch control of switch 33 to level detector 3
The detection signal of (4) is used.

In the normal state where the received signal is at the normal level, the switch 33 selects one of the switching terminals and inputs the output of the detector 31 to the amplifier 32, and normal AGC loop control is performed.

Then, when the received signal level suddenly changes and becomes a disconnection state, for example, the switch 33 selects the other switching terminal and the DC power source 34
The predetermined value plus the voltage is input to the amplifier 32. As a result, the AGC voltage control signal, which is the output of the amplifier 32, becomes a positive voltage, and the internal variable attenuator is controlled so that the main intermediate frequency amplifier 7 (8) which receives this has a minimum gain. When the received signal recovers from the disconnection state to the normal level, the level detector 3 (4) outputs the disconnection detection signal until a predetermined time elapses from the recovery detection time. The DC power supply 34 is controlled to be selected until a predetermined time has elapsed.

Then, in the initial stage when the received signal is restored to the normal level, originally, the transient response as shown in FIG. 4 is originally shown, but since the main intermediate frequency amplifier 7 (8) is operating with the minimum gain, the output level is still. Low level, input terminal 1
A signal having a level higher than the normal level is not input to (2).

In short, according to the present invention, an excessive signal is not applied to the input terminal 2 on the side of the different polarization, so that the demodulation system on the side of the main polarization of the cross polarization interference canceling circuit 25 is temporarily operated as before. It is possible to prevent characteristic deterioration.

Since the switch 33 selects the output of the detector 31 after the lapse of the predetermined period, the output level of the AGC circuit is controlled from the minimum level to the normal level according to the response speed of the AGC loop.

(Effects of the Invention) As described above, according to the cross polarization interference canceller of the present invention, when the received signal level becomes equal to or lower than the specified value, the automatic gain control circuit reduces the gain to a constant low gain. Since this state is kept sufficiently longer than the transient response time when the received signal level is restored, when the received signal level is restored to the specified value, the transient response of the automatic gain control circuit can be suppressed. As a result, it is possible to prevent excessive signals from being input to the cross polarization interference cancellation circuit, and temporary deterioration of cross polarization interference cancellation characteristics, especially interference of the main polarization side signal when the cross polarization side signal suddenly changes. This has the effect of fundamentally preventing temporary deterioration of the removal characteristics.

[Brief description of drawings]

FIG. 1 is a configuration block diagram of a cross polarization interference canceller according to an embodiment of the present invention, and FIG. 2 is an AGC loop amplifier (16, 17).
FIG. 3 is a block diagram of the configuration of the conventional example device, and FIG. 4 is a transient response characteristic diagram of the different polarization side AGC circuit. 1,2,20,21 …… Reception signal input terminal, 3,4 …… Level detector, 5,6 …… Transversal filter, 7,8 …… Main intermediate frequency amplifier, 11,12 …… Demodulator , 13 …… Control circuit, 14, 15
...... Output terminal, 16,17 …… AGC loop amplifier, 25 …… Cross polarization interference canceller, 31 …… Detector, 32 …… Amplifier, 33 ・ ・ ・
… Switch, 34 …… DC power supply.

Claims (1)

[Claims] 1. A two-series automatic gain control circuit for automatically controlling gains of received signals of two orthogonal polarizations to make an output signal level constant; receiving outputs of the two-series automatic gain control circuits, respectively. A cross polarization elimination circuit configured to perform a removal operation of a signal on the other polarization side of the other signal leaking into the signal on the main polarization side of one signal, and a cross polarization elimination circuit configured to: In the interference canceller; when the input signal level of each of the two series of automatic gain control circuits becomes equal to or less than a predetermined specified value including a level break, a detection signal designating this state is generated to recover at least the input signal level. Two levels of level detectors for transmitting over a predetermined time including from the time of detection to the time of reaching a normal state are provided; The gain of the corresponding one of the detectors is fixed to a predetermined low gain while receiving the detection signal to prevent temporary deterioration of the cross polarization interference cancellation characteristics when the input signal level suddenly changes. A cross polarization interference canceller characterized by: