JP2569903B2 - Interference wave canceller - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an interference wave canceling apparatus, and more particularly to adaptive equalization of waveform distortion due to fading and elimination of an interference wave when a strong interference wave exists in a multipath fading channel. The present invention relates to an interference wave removing device for performing

(Prior Art) Conventionally, interference from an FM line to a digital microwave line using PSK or QAM may be a problem. Especially when digital transmission is performed at high speed, FM interference waves are regarded as narrowband interference waves. In addition, adaptive equalization technology is indispensable in a strong multipath fading channel, and a matched filter (M
A receiver using F) and a decision feedback equalizer (DFE) is required. FIG. 4 shows an example of a conventional interference wave eliminator for removing a narrow-band interference wave in a multipath fading environment.

In FIG. 4, 41 is a transversal filter, 42
Is a matched filter (MF), 43 is a decision feedback equalizer (DFE)
Numerals 44 to 51 denote signal spectrums in respective parts of the conventional interference wave removing apparatus.

A transversal filter is usually used to remove the narrow band interference wave. This is intended to remove the interference wave by lowering the amplitude characteristic of the transversal filter in the frequency band of the interference wave. Control of the tap coefficient of the transversal filter is performed by an LMS algorithm, a Kalman algorithm, or the like such that the root mean square value of the error signal output from the determiner is minimized. However, the removal of the interference wave by the transversal filter is basically filtering in the frequency domain.If the interference wave is to be sufficiently removed, the same frequency component as the interference wave of the desired signal is removed. In addition to the generated multipath distortion, a strong waveform distortion is generated.

FIG. 4 (a) shows how interference waves are removed from a received signal in which an interference wave overlaps a desired signal. The received signal input to the transversal filter 41 is the desired signal S as shown at 44.
Is overlapped with the interference wave J. Transversal filter
41 causes a deep dip in the frequency band of the interference wave of the frequency characteristic of the filter in order to remove the interference wave J. Therefore, at the output of the transversal filter 41, the interference wave is removed as indicated by 45, but the frequency characteristic of the desired signal component is reduced. The occurrence of a notch on the frequency axis as in the signal spectrum 45 is equivalent to the impulse response being dispersed as in a two-wave model.
That is, this state is approximated to a multipath state. The matched filter 42 attempts to symmetrically disperse the dispersed impulse response and synthesize the delay-dispersed echo component into the main wave in phase.
Accordingly, the ratio of the echo wave to the main liquid is reduced, and the signal having a notch such as 45 passes through the matched filter 42 so that the notch becomes shallow as 46. Matched filter 42
Is input to the decision feedback equalizer 43 to remove multipath distortion, and the desired signal wave S is output as indicated at 47. As described above, in FIG. 4A, the interference wave included in the received signal is sufficiently removed, and the desired signal is accurately equalized.

FIG. 4B shows how interference waves are removed from a received signal in which an interference wave overlaps a desired signal in which multipath distortion has occurred.

In FIG. 4 (b), the received signal input to the transversal filter 41 is a signal in which multipath distortion has already occurred on the desired signal S on the transmission line, and the desired signal S is overlapped with the interference wave J. In order to remove the interference wave J, the transversal filter 41 generates a notch in the frequency band of the interference wave having the frequency characteristic of the filter. By the way, when the notch frequency due to the multipath already generated in the transmission path is different from the frequency of the interference wave, the signal input to the matched filter 42 has a plurality of notches in the signal band. That is, the frequency interval at which the notch is formed becomes shorter. Generally, a short notch interval corresponds to the presence of an echo wave having a large delay time difference in an impulse response. To remove distortion from the desired signal in this state, it is necessary to increase the number of taps of the matched filter 42 and the decision feedback equalizer 43. However, the number of taps is set to a value corresponding to the delay dispersion generated in the propagation path, and is usually set to the minimum necessary number of taps. Therefore, the occurrence of new distortion in the transversal filter 41 other than the distortion generated in the propagation path as shown in 49 increases the load on the matched filter 42. In particular, if the distribution region of the impulse response exceeds the number of taps of the matched filter 42, not only the impulse response cannot be made symmetrical, but also the distribution may be broadened, and a desired effect cannot be obtained. Although the output 50 of the matched filter 42 is input to the decision-type equalizer 43, the state of the impulse response exceeds the equalization capability, and the equalization is not sufficiently performed, and distortion is left as shown at 51.

Further, if the number of interference waves is two or more, filtering on the frequency axis becomes complicated, and it is necessary to considerably increase the number of taps of the transversal filter. If there are a plurality of frequency locations from which the interference wave is to be removed, the desired signal spectrum will be greatly reduced, the desired signal will not be accurately equalized, and the adverse effect will be large.

(Problems to be Solved by the Invention) In the above-described conventional interference wave elimination apparatus, since the received signal is subjected to distortion accompanying the processing of eliminating the interference wave in addition to the distortion due to multipath fading, a load on adaptive equalization is increased. There is a disadvantage that the waveform distortion is not sufficiently removed.

Therefore, an object of the present invention is to provide an interference wave removing device capable of effectively removing both multipath distortion and interference waves.

(Means for Solving the Problems) A first interference wave removing apparatus according to the present invention comprises: a first subtractor for generating a signal representing a difference between a reception signal composed of a desired signal and an interference wave and an estimated interference wave; The impulse response of the transmission system is estimated from the difference signal and the determination signal output from the first subtractor to obtain an estimated impulse response, and the time-inversion of the estimated impulse response is a complex conjugate response and the difference signal. And a decision feedback equalizer that removes multipath distortion included in the matching signal output from the matching filter to obtain the decision signal and the error signal of the decision result. A convolution unit that convolves the estimated impulse response and the determination signal to obtain a desired estimation signal; a delay element that applies a predetermined delay to the received signal; an output signal of the delay element and the desired estimation signal; The difference A second subtractor for extracting the interference wave component, a correlator for obtaining a correlation value by taking a correlation between the interference wave component and the error signal, and multiplying the interference wave component by the correlation value for the estimation. And a multiplier for obtaining an interference wave.

Further, a second interference wave removing apparatus according to the present invention comprises: a first subtractor for generating a signal representing a difference between a reception signal composed of a desired signal and an interference wave and an estimated interference wave; and an output from the first subtractor. The impulse response of a transmission system is estimated from the difference signal and the reference signal to obtain an estimated impulse response, and a matched signal obtained by convolving the complex conjugate response with the difference signal by time inversion of the estimated impulse response is obtained. A decision filter for performing decision feedback equalization on the matched signal using the matched filter and the reference signal to remove multipath distortion included in the matched signal to obtain a decision signal and a decision result error signal; A shape signal equalizer, a training signal generator for generating a training signal representing an assumed transmission symbol sequence, and selecting one of the decision signal and the training signal in accordance with the error signal to form the base signal. A switch for outputting as a quasi signal, a convolution unit for convolving the estimated impulse response with the reference signal to obtain an estimated desired signal, a delay element for giving a predetermined delay to the received signal, and an output of the delay element A second subtractor for extracting an interference wave component by taking a difference between the signal and the estimation desired signal; a correlator for taking a correlation between the interference wave component and the error signal to obtain a correlation value; A multiplier for multiplying the component by the correlation value to obtain the estimated interference wave.

(Example) Next, the present invention will be described with reference to the drawings. First
FIG. 1 is a block diagram showing the configuration of an embodiment of the first interference wave removing apparatus according to the present invention. FIG. 2 shows a second embodiment according to the present invention.
FIG. 2 is a block diagram showing an embodiment of the interference wave removing device of the present invention.

In FIG. 1, 1 is a matched filter (MF), 2 is a decision feedback equalizer (DFE), 3 is a convolution unit, 4 is a multiplier, 5
Is a correlator, 6 is a delay element having a delay time of τ, and 7 and 8 are subtractors.

In FIG. 2, 21 is a matched filter (MF), 22 is a decision feedback equalizer (DFE), 22a is a forward equalizer (FE), 22b is a backward equalizer (BE), 22c is a determiner, 22d and 22e are subtractors, 23 is a convolution unit, 24 is a multiplier, 25 is a correlator, 26 is a delay element having a delay time of τ, 27 and 28 are subtractors, 29 is a switch, and 30 is a training signal. The generator, 31 is a controller.

In the embodiment shown in FIG. 1, the transmission symbol sequence is represented by a _n (n =
-∞ ... + ∞), when the discrete value h _n of the impulse response of the transmission system to be input to the MF1, the received signal 102 discrete values r
_n is Indicated by Here, J indicates an interference wave in a band narrower than the frequency band of the desired signal S.

MF1 is the judgment signal a _n 106 and the reception signal 10 shown in the equation (1).
By taking a correlation with 2, the impulse response of the transmission system is estimated. The following equation shows the correlation process.

The MF 1 performs an operation of convolving the received signal 102 with the complex conjugate h _−n ^{* by} time inversion of the estimated impulse response.

On the other hand, the convolution unit 3 calculates the impulse response h estimated by MF1.
convolving the decision signal a _n 106 of the output of the _n 103 and DFE2. That is, when the determination signal a _n 106 is equal to the transmission symbol sequence a _n is
That is, the convolution unit 3 estimates the desired reception signal S. Because of the processing time required to generate the desired signal replica (reproduced waveform) to be estimated, the desired signal replica is delayed by τ from the desired signal component at the input terminal 11. Therefore, the delay of the input signal 101 is adjusted by the delay element 6, and the desired signal replica of the output of the convolution unit 3 is subtracted from the output signal of the delay element 6 by the subtractor 8. Therefore, the subtractor 8 outputs the interference wave J delayed by τ. Here, as compared with the desired signal spectrum, narrow-band interference waves such as FM waves are targeted, so if the center frequency of the interference waves is ω, the delay of τ is exp
It can be approximated by giving a phase shift of (−jωτ) to the interference wave J. That is, the output of the subtractor 8 is J · exp (−jωτ).

A correlation value W1 which is an output of the correlator 5 is added to the output of the subtractor 8.
07 is multiplied by the multiplier 4 and supplied to the subtractor 7. If the output of the multiplier 4 is given by: = W · J · exp (−jωτ) (3), the output of the subtractor 7 is (S + J−). DF
A signal obtained by multiplying the output of the subtractor 7 by the transfer function H of MF1 and DFE2 is input to the decision unit in E2. Ie DF
The error signal ε output from the decision unit in E2 includes an error component due to the interference wave J. In particular, when intersymbol interference (ISI) due to multipath fading is present at the same time, the determinator error signal ε includes both an error component due to the ISI and an error component due to the interference wave J. However, the error signal ε is used to control DFE2, but the interference wave component in the error signal ε is ISI
Since it is independent of the component due to, it does not affect the correction of the tap coefficient of DFE2. That is, if the ISI is removed by the MF / DFE receiver consisting of MF1 and DFE2, the error signal ε
Is approximated as

ε = H · (J−) (4) The evaluation function あ る, which is the root-mean-square value of the error signal ε, is expressed as follows when the speed of fading and the fluctuation of the interference wave J is lower than the desired signal speed. .epsilon..epsilon. ^* ] = HH ^* (J-). (J-) ^* .

ξ is a function of the tap coefficient W of the multiplier 4, and W at which ξ is minimum is W ^opt = exp (jωτ) (6) using equation (3).

By the way, to adaptively follow the ideal value of W,
The gradient method is used for the evaluation function ξ. The gradient of the ξ plane at a certain W value is shown as follows, omitting the average value display E.

That is, if the tap coefficient W is corrected by the following equation, it converges to the ideal value of equation (6).

W ⁿ = W ^n-1 −μ · ε · {Jexp (−jωτ)} ^* (8) where μ is a correction coefficient.

The tap correction operation described above is performed by the correlator 5 in FIG.
Done by The correlator 5 outputs the signal Jexp (−
By taking the average of the product of the complex conjugate of (jωτ) and the error signal ε105 from DFE2, the tap correction represented by equation (8) is realized.

By the above operation, the input signal 10
The estimated value of the interference wave J in 1 is obtained. This estimated interference wave J
Is subtracted from the input signal S + J101 using the subtractor 7 to remove the interference wave J.

Even if the interference wave fluctuates in the subsequent operation, the interference wave is adaptively removed. On the other hand, waveform distortion due to multipath fading is removed by the MF / DFE receiver independently of interference removal.

Next, the above operation will be described with reference to a vector diagram and a spectrum shown in FIG. The case where the narrow-band interference wave J exists in the desired signal spectrum that has undergone multipath distortion in the propagation path is shown in the spectrum column of FIG. The vector function is shown in the column of the vector diagram. A notch occurs in the spectrum of the desired signal S due to multipath distortion. The output of the convolution unit 3 makes a replica of the desired signal S. Since this replica is delayed by τ from the desired signal S at the input terminal 11, the replica is shifted in phase from the S vector in FIG. b) as shown in the S vector.
(C) shows the result obtained by subtracting the desired signal replica of (b) from the result obtained by delaying (a) by τ. That is, the subtractor 8
Means that the interference wave J is estimated. This is multiplied by a tap coefficient W output from the correlator 5 to obtain (d)
Is the output of the multiplier 4. By subtracting the estimated interference wave J of (d) from the input of (a), it is possible to extract only the desired signal component that has undergone multipath distortion as shown in (e), and the MF / DFE receiver obtains ( Adaptive equalization is performed as in f).

In the above operation, distortion due to interference wave elimination other than multipath distortion is not given to the desired signal S. As described above, in the present embodiment, the interference wave is removed without performing the filtering in the frequency domain. Therefore, even if there are a plurality of interference waves, the interference wave can be removed.

By the way, when starting up the MF / DFE receiver, if there is already a strong interference wave with a negative D / U ratio, the MF / DFE receiver cannot output a correct determination signal. If the determination signal is incorrect, the MF cannot perform a correct impulse response estimation. Thus, the desired signal replica is no longer correct. In this case, if it is left as it is, it will not be possible to stand up forever.

The embodiment shown in FIG. 2 solves this initial pull-in.

In FIG. 2, 21, 22, 23, 24, 25, 26, 27 and 28 respectively correspond to 1, 2, 3, 4, 5, 6, 7 and 8 in FIG. The components in the figure perform the same operations as the components in FIG. Until interference J is removed, to output the same training signal and the transmission symbol sequence a _n a training signal generator 30, switching unit 29 the training signal
, The convolution unit 23 and the MF 21
And DFE22. In this case, MF21 estimates the impulse response of the transmission system by taking the correlation between the received signal 102 shown in training sequence a _n and (1). At this time, since the correlation value between the interference wave J and the determination signal _n 106 becomes zero, the estimated impulse response has a correct value even if the interference wave J has not been removed yet. Further, since the convolver 23 using training sequence a _n, the output is correct desired signal replica. If the desired signal replica has the correct value,
The subtractor 28 extracts the interference wave component Jexp (-jωτ). On the other hand, since the training signal is supplied to the subtractor 22e in the DFE 22, a decision unit error signal ε105 that correctly includes the interference wave component is obtained. Also, if a wrong decision signal is used in the backward equalizer (BE) 22b in the DFE 22,
Since an incorrect signal is fed back to the subtractor 22d, the error signal ε105 is disturbed. Therefore, in this embodiment, BE22b
The output signal of the switch 29 is also supplied. As described above, at the time of the initial pull-in, the correlation value of the correlator 25 is controlled so that the error signal ε105 is minimized by using the training signal. That is, a correct estimated value of the interference wave J is obtained at the output of the multiplier 24, and the interference wave can be removed by the subtracter 27.

When the interference wave is removed, the determinator error signal ε105 becomes smaller. The controller 31 receives the determinator error signal ε105 as an input and monitors the root-mean-square value 、. DF is controlled by
The determination signal 106 from E22 is selectively output. Thereafter, the convolution unit 23, MF21 and DFE22 are supplied with the decision signal 106 and
The same interference wave removing operation as described in the illustrated embodiment is continued. There are two methods for inserting a training signal on the transmitting side. One is a method of periodically inserting the training signal into a transmission signal sequence in a burst form, and the other is a method of inserting or releasing the signal based on a signal from the receiving side.

According to the embodiment shown in FIG. 2, it is possible to remove the narrow band interference wave in which the initial pull-in is solved.

(Effects of the Invention) As described above, the present invention extracts interference components by subtracting a desired signal replica from a received signal without performing filtering on the frequency axis, and determines the interference component of the decision feedback equalizer. Since the interference wave is removed by weighting the extracted interference wave to minimize the error signal and subtracting the interference component from the received signal, the desired signal subjected to multipath distortion is affected by the interference wave cancellation. In addition, even if there are a plurality of interference waves, there is an effect that both the multipath distortion and the interference waves can be effectively removed.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an embodiment of a first interference wave canceling apparatus according to the present invention, and FIG. 2 is a block diagram showing a configuration of an embodiment of a second interference wave removing apparatus according to the present invention. FIG. 3 is a diagram for explaining the operation of the embodiment shown in FIGS. 1 and 2, and FIG. 4 is a diagram showing a conventional interference wave elimination device and its interference wave elimination. 1,21 Matched filter (MF), 2,22 Decision feedback equalizer (DFE), 3,23 Convolution, 4,24 Multiplier, 5,25
... a correlator, a delay element having a delay time of 6, 26 ... τ,
7, 8, 27, 28 ... subtracter, 11 ... input terminal, 12 ... output terminal, 22a ... forward equalizer (FE), 22b ... backward equalizer (B
E), 22c: decision unit, 22d, 22e: subtractor, 29: switching unit, 30: training signal generator, 31: controller.

Claims (2)

(57) [Claims] 1. A first subtractor for generating a difference signal between a received signal composed of a desired signal and an interference wave and an estimated interference wave, and the difference signal and the determination signal output from the first subtractor. A matched filter that obtains an estimated impulse response by estimating the impulse response of the transmission system from, and obtains a matched signal obtained by convolving the complex conjugate response and the difference signal by time inversion of the estimated impulse response, and an output from the matched filter. A decision feedback equalizer that removes multipath distortion included in the matched signal to obtain the decision signal and an error signal of the decision result; and convolves the estimated impulse response with the decision signal to perform estimation. A convolution unit for obtaining a signal, a delay element for giving a predetermined delay to the received signal, a second subtractor for extracting an interference wave component by taking a difference between an output signal of the delay element and the desired signal for estimation. , The interference wave It said correlator for obtaining a correlation value by taking the correlation between the error signal, the interference wave removal apparatus characterized by comprising a multiplier to obtain the estimated interference wave by multiplying the correlation value to the interference wave component. 2. A first subtractor for generating a difference signal between a received signal comprising a desired signal and an interference wave and an estimated interference wave, and the difference signal and the reference signal output from the first subtractor. A matched filter that obtains an estimated impulse response by estimating the impulse response of the transmission system from, and obtains a matched signal obtained by convolving the complex conjugate response and the difference signal with time inversion of the estimated impulse response, and using the reference signal. A decision feedback equalizer that performs decision feedback equalization on the matched signal to remove multipath distortion included in the matched signal to obtain a decision signal and a decision result error signal; A training signal generator that generates a training signal representing a column, and a switch that selects one of the determination signal and the training signal according to the error signal and outputs the selected signal as the reference signal. A convolution unit for convolving the estimated impulse response with the reference signal to obtain a desired signal to be estimated; a delay element for giving a predetermined delay to the received signal; and a difference between an output signal of the delay element and the desired signal to be estimated. A second subtractor that extracts the interference wave component by taking a correlation, a correlator that obtains a correlation value by taking a correlation between the interference wave component and the error signal, and multiplies the interference wave component by the correlation value. An interference wave removing device comprising: a multiplier for obtaining an estimated interference wave.