JP2773550B2 - Radar interference wave removal device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radar interference wave canceling apparatus, and more particularly to a decision feedback equalizer when a radar wave obtained by pulse-modulating a continuous carrier signal interferes with a digital transmission signal at an arbitrary D / U ratio. The present invention relates to a radar interference wave elimination device that can eliminate this interference wave by using the above-mentioned method, and can perform an elimination operation only when a radar pulse is present, thereby avoiding deterioration of the adaptive tracking characteristic to a desired wave signal.

[0002]

2. Description of the Related Art Conventionally, a radar interference wave eliminator using a decision feedback equalizer of this type has a delay element 30 having N delay times T (symbol periods) as shown in FIG.
1, N multipliers 302, adders 303, subtractors 30
4, decision unit 305, N delay times T (symbol period)
, A multiplier 307, an adder 308, a subtractor 309, and a tap coefficient modifier 310. Regarding this conventional technology, “Rejection of CW Interference in QPSK Systems Utilizing Decision” published by Lee and Milstein, IEE Transactions on Communication (Volcom 31 No. 4, issued April 1983). feedback
Filters ". According to this, in a narrow-band interference eliminator simply using a linear filter as a notch filter, a notch is formed with respect to the frequency of the interference wave to remove the interference wave, thereby affecting the spectrum of the desired wave itself. Therefore, the MMSE (Minimum Mean) that minimizes the root mean square of the error with only the linear filter is used.
As long as the Square Error method is used, good interference removal cannot be performed. On the other hand, when the decision feedback equalizer is used, the delay element 301, the multiplier 302, and the adder 303
Even if the front filter made of the deep filter makes a notch deep in the desired signal spectrum, the rear filter made up of the delay element 306, the multiplier 307 and the adder 308 compensates for the desired signal component cut off by the notch. It is stated that narrowband interference waves can be sufficiently removed without affecting the waves. Each tap coefficient of the forward filter and the backward filter is obtained from the LMS (Least Measurement) by the tap coefficient corrector 310 from the decision unit error signal and each tap signal.
n Square) algorithm.

According to this paper, the frequency of a CW interference wave is Ω, the interference wave power is J, the desired wave symbol period is T, the desired wave signal power is S, the receiver noise power is σ ² , the forward filter and the backward filter, respectively. Assuming that the number of taps is N, the tap coefficient w ¹ of the front filter and the tap coefficient f ¹ of the rear filter are expressed by the equations (1) and (2).

[0004]

The tap coefficients in equations (1) and (2) are ideal solutions derived from orthogonal conditions between the error signal and the signal at each tap. In order to minimize the root mean square value of the decision unit error signal in order to set the tap coefficient to the ideal solution, it is usually necessary to use LMS
The correction of the tap coefficient by an algorithm or the like has been performed. Equation (3) shows a correction equation based on the LMS algorithm.

[0006]

[0007] w _i ⁿ the value at time n of the i-th tap coefficient, ui is the signal in the i-th taps, mu
Denotes a correction coefficient, ε denotes a decision unit error signal, and * denotes a complex agreement. When the ideal solution of the tap coefficient is wopt,
The Wiener-Hoppler equation shown in the equation (4) holds.

Φ · wopt = v (4) Here, Φ is an N × N correlation matrix where N is the tap coefficient of the forward filter and the backward filter of FIG. v
Is a correlation vector between a signal component at each tap coefficient and a reference signal (here, a determination data signal when there is no error). The property indicating the transient response of the tap coefficient w when the tap correction is performed by the equation (3) is expressed by the equation (5).

[0009]

Here, λi represents the eigenvalue of the correlation matrix for the i-th tap. The recurrence formula of the formula (5) is a tap coefficient w _i
It shows that ⁿ exponentially approaches the ideal solution w _i opt. Here, as a condition for convergence, the condition shown in Expression (6) is required.

1−μλ _i <1 (6) That is, the exponential asymptotic characteristic depends on the correction coefficient and the input power that determines the eigenvalue. As described above, the CW interference wave elimination method using the decision feedback equalizer can be applied to radar interference wave elimination using CW as a signal source. Now, this characteristic will be described with reference to the waveform diagram of FIG.

In FIG. 2, a radar pulse waveform 201
Indicates that a radar wave is output only from t = 0 to τ, and is not output from t = τ to L. This intermittent radar pulse is repeated at a period L. Normally, an LMS algorithm or the like is used to correct tap coefficients, and the adaptive characteristics converge exponentially. Therefore, the time change of the tap coefficient with respect to the radar pulse 201 becomes like the tap coefficient 202. At time t = 0, a CW interference wave is added, and the tap coefficient of the LMS algorithm begins to rise as shown by 202 by tap correction, and then w
converges to opt. Since the interference wave is cut off at time t = τ, the tap coefficient starts returning from wopt to the initial value of zero here, but also returns to zero exponentially.

[0013]

As described above, the conventional radar interference wave removing apparatus has a configuration in which the time constant of the exponential change is faster than the convergence speed determined by the time constant of the change of the radar pulse period. , Tap coefficient is ideal solution Wop
The radar pulse is cut off before it can converge on t,
If the tap coefficient returns to zero again, there is a disadvantage that the time ratio for holding the ideal solution is reduced, and the CW interference wave removal characteristic of the decision feedback equalizer is significantly reduced.

An object of the present invention is to provide a radar interference wave removing apparatus which removes a good radar wave without disturbing adaptive convergence characteristics due to continuation of radar pulse modulation.

[0015]

A radar interference wave removing apparatus according to the present invention inputs a desired wave signal and an interfering wave signal such as an intermittent radar pulse, and a notch is formed in a portion of the interference wave signal by a transversal filter. A forward filter that removes the interfering wave, a rear filter composed of a transversal filter that compensates for the drop of the desired wave signal by the notch, and a difference between output data of the front filter and output data of the rear filter. A subtractor that takes, a determiner that determines a subtraction result of the subtractor, a subtractor that outputs an error signal before and after the determiner, and a plurality of tap coefficients of the front filter and the rear filter from the error signal. A radar interference wave canceller having a tap coefficient correcting circuit for correcting the input data and the final output data of the forward filter. A complex correlator that takes a correlation with the data of the tap, when the amplitude of the complex correlator becomes smaller than a certain threshold value, interrupts the tap coefficient correction operation of the tap coefficient correction circuit to change each tap coefficient. Keep the value before the interruption.

[0016]

Next, the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram of one embodiment of the present invention. FIG. 2 is an explanatory diagram for explaining the operation of this embodiment. 1, the same reference numerals as those in the conventional example of FIG. 3 have the same functions. That is, in the present embodiment, a plurality of correlators 109 for correlating the input and output of the forward filter and the tap coefficient correction circuit 111 are provided.

The features of the present invention will be described first. As described above, in the related art, when the state where the CW interference wave is added is disconnected, there is no need to remove the CW interference wave, so that the tap coefficient returns to the original zero as long as the MMSE control is performed. Although the present invention employs a circuit system that retains the tap coefficient of CW interference wave removal before the interruption even when the CW interference wave is interrupted, the effect of the retention is particularly important. Is not at all. This will be described using a simple model. {A} =... A ₋₁ , a ₀ , a ₁ , a ₂ ... (7) It is assumed that the transmission symbols of equation (7) are sequentially transmitted as transmission symbols.
It is assumed that the propagation path has no multipath distortion and is simply multiplied by a transmission multiplier. When this multiplier is normalized to 1,
The received signal is given by equation (8).

R (t) = a _n + √Jexp (jΩt) + n (t) (8) where n (t) is the receiver noise, J is the interference wave power, and Ω is the angular frequency of the CW interference wave. It is. When the judgment unit output is a ₀ ,
The reference tap u ₀ of the front filter is included as a desired signal component of a ₀ is the received signal. That is, the received signal distributed to each tap of the forward filter is represented by the equations (7) to (9).

[0019]

By the way, the tap coefficient of CW interference wave removal in the above-mentioned prior art is shown by the equation (1). However, when the SN ratio is relatively high as in digital microwave transmission, it is shown by the equation (11). Can be approximated as follows.

[0021]

The backward filter tap coefficient in the equation (2) described in the prior art is a solution obtained under the condition that the backward filter output is added to the forward filter output. Accordingly, if the subtractor 304 in FIG. 1 is configured to subtract the rear filter output from the front filter output, the expression (2) becomes

[0023]

Equation (12) is obtained. For example, the number of taps N = 2
In the case of, the tap coefficient is as shown in Expression (13).

W ₁ = f ₁ ≒ (− ／) exp (jΩIT) (l = 1,2) (13) In this case, the forward filter output y (t) is (7), (8),
From equations (9) and (13), the following is obtained.

[0026]

In the forward filter output shown in equation (14), the CW radar interference wave is eliminated. in case of a desired reference signal a _0, intersymbol interference is to be outputted from the front filter from one symbol before and 2 symbols earlier than the symbol. Meanwhile, the symbols corresponding to these intersymbol interference is already distributed as decision data signal in the rear filter f ₁ and f ₂ taps. Therefore, the subtractor 30 is obtained from the equations (12) and (14).
The four outputs z (t) are as shown in equation (15).

Z (t) = y (t) −f ₁ · a ₋₁ −f ₂ · a ₋₂ = a ₀ + W ₁ · n (t−T) + W ₂ · (t−2T) (15) That is, intersymbol interference accompanying CW interference removal is removed by the backward filter, and only noise components remain. This noise component is multiplied by the tap coefficient of the forward filter. When the tap coefficient increases, an adverse effect called a noise amplification effect is exerted. However, if the S / N ratio on the line is sufficient, there is a particular problem. Does not.

To summarize the above CW interference elimination, linear synthesis is performed so that CW interference waves are canceled between each tap of the front filter. However, the intersymbol interference related to the desired wave component caused by the cancellation is removed by the backward filter.

Next, when there is no CW interference wave, the above CW
A description will be given of what operation is performed when a tap coefficient for removing an interference wave is set for each tap. In this case (1
The calculation can be performed assuming that the interference wave component does not exist in the second, third and fourth rows of the equation (4), and the result matches the fifth row of the equation (14). That is, as long as the decision feedback equalizer takes the tap coefficient given by equation (13) regardless of the presence or absence of the CW interference wave,
Intersymbol interference of the desired wave component due to the forward filter tap coefficient is completely removed by the backward filter. From the above description, the radar wave composed of CW has the tap coefficient 201 shown in FIG.
When the pulse modulation as described above is performed, the tap coefficient for removing the CW interference wave of the decision feedback equalizer is held as the tap coefficient 203 in FIG. 2 even when the radar wave is cut off. During this time, there is no radar wave interference, but there is no effect on the desired wave. Even if the radar wave interference is input again at t = L, the tap coefficient at t = τ is retained, so that the interference elimination can be performed by the ideal value of the tap coefficient without waiting for the adaptive convergence time as in the related art. You can start. Above was in order to control the the above, it is necessary to detect the continuation of the interference wave by the radar pulse, u ₀ and the reference tap by the complex correlator 110 shown in FIG. 1 u
Take the correlation between the received signals of _N taps. Complex correlator 110
Is as shown in equation (16).

[0031]

Here, E [] indicates an expected value operation, and * indicates a complex agreement. As is apparent from equation (16), complex correlator 110 has an output value only when a CW interference wave exists. Accordingly, the tap correction circuit 111 updates the tap coefficient by the LMS algorithm only when the amplitude η of the correlation value is equal to or greater than a certain threshold γ. If η is smaller than γ, it is determined that there is no CW interference wave and LMS
The tap correction by the algorithm is interrupted, and the value before the interruption is retained. The correction formulas for the i-th tap coefficient of the tap correction circuit 111 can be summarized as in Expressions (18) to (21).

[0033]

Even if the pulse modulation period is faster than the adaptation speed, if the time constant of the expected value operation of the complex correlator 110 is made faster, the adaptation characteristics will not be disturbed by the tap correction method. In general, the transfer coefficient for radar waves fluctuates with time, which corresponds to fading in wireless communication, and has a frequency component slower than the pulse modulation cycle. The decision feedback equalizer can adaptively follow such a change during the time τ during which the radar pulse is output.

As described above, the pulse-modulated CW radar interference wave can be removed by the present circuit system.

[0036]

As described above, according to the present invention, the presence or absence of a radar interference pulse is detected by the correlation means between the forward filter taps of the decision feedback equalizer, and when radar interference occurs, tap correction by a normal adaptive algorithm is performed. The tap correction is interrupted when the radar pulse is interrupted, and the tap coefficient before the interruption is retained, so that the adaptive tracking characteristic of the decision feedback equalizer due to the high-speed pulse modulation is not deteriorated, There is an effect that good radar interference waves can be removed.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an embodiment according to the present invention.

FIG. 2 is an explanatory diagram illustrating the operation of the present embodiment.

FIG. 3 is a configuration diagram of a conventional radar interference wave removing apparatus.

[Explanation of symbols]

 110 Complex correlator 111 Tap correction circuit 301, 306 Delay element 302, 307 Multiplier 303 Adder 304, 310 Subtractor 305 Judge

────────────────────────────────────────────────── (5) Continuation of the front page (56) References JP-A-5-95298 (JP, A) JP-A-63-67811 (JP, A) JP-A-3-284014 (JP, A) JP-A-3-28401 284012 (JP, A) JP 8-31820 (JP, B2) LI AND L. B. MILS TEIN, "REJECTION OF CW INTERFERENCE IN QPSK SYSTEMS USI NG DECISION-FEEDBA CK FILTERS", IEEE TRANS. COMMUN. VOL. COM-31, NO. 4, PP. 473-483, APR. 1983 (58) Field surveyed (Int.Cl. ⁶ , DB name) G01S 7/36 H03H 17/00-17/06 H04H 7/005

Claims (1)

(57) [Claims] 1. A forward filter for receiving a desired wave signal and an interfering wave signal such as an intermittent radar pulse, forming a notch in the part of the interfering wave signal by a transversal filter, and removing the interfering wave. A rear filter composed of a transversal filter for compensating for the drop of the desired wave signal itself, a subtractor for obtaining a difference between the output data of the front filter and the output data of the rear filter, and determining a subtraction result of the subtractor Interference, a subtractor that outputs an error signal before and after the determiner, and a tap coefficient correction circuit that corrects a plurality of tap coefficients of the front filter and the rear filter from the error signal. The apparatus comprises a complex correlator for correlating input data of the forward filter with data of a final output tap, A radar interfering wave for interrupting the tap coefficient correcting operation of the tap coefficient correcting circuit when the amplitude of the complex correlator becomes smaller than a certain threshold value and holding each tap coefficient at a value before the interruption; Removal device.