JPH03110930A - Automatic equalizer - Google Patents

Abstract

PURPOSE:To enhance a sufficient equalizing characteristic by adopting a variable type level adjustment means and a variable type expander and providing a companding rate controller controlling the compression rate of the variable level adjustment means and a expanding rate of the variable expander. CONSTITUTION:Absolute values of tap coefficients C-1, C1 are inputted to a companding rate controller 23 and comparators 35, 36 decide whether or not the absolute values of the tap coefficients are larger than a reference value CR respectively. When either or the absolute values of the tap coefficients C-1, C1 is larger than the reference value, an OR circuit 37 outputs its decision output to a variable attenuator 21 and a variable expander 22 as a control signal. Thus, when fading is larger than the reference value, an input base band signal is compressed and inputted to an A/D converter 12. Thus, no nonlinear distortion is generated at A/D conversion and a transversal filter applies correct waveform equalization.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an automatic equalizer, particularly in a demodulator for a digital wireless communication system.

Automatically removes waveform distortion in the propagation path due to fading and interference from other wireless systems. Regarding digital automatic equalizer.

[Background Art] In recent years, in digital wireless communication systems, multi-level modulation methods have been increasing in order to improve frequency usage efficiency. On the other hand, as the number of multi-values increases, the influence of waveform distortion of the propagation path due to fading, interference between cross-polarized waves (co-channel interference), interference from other wireless systems (FM interference, etc.) increases, and these linear distortions increase. Development of an automatic equalizer that removes interference is underway. As one of this automatic equalizer 5
A transversal equalizer that adaptively controls the tap coefficients of a transversal Φ filter has been put into practical use. An example of a conventional all-digital transversal equalizer is
As shown in the figure. As shown in Figure 2.

A conventional all-digital transversal equalizer uses compressor 1.
1. Analog-to-digital (A/D) converter 12, transversal filter 13. It consists of an expander 14 and a tap coefficient control circuit 15.

Input to terminal 1. An input base punt signal from a demodulator (not shown) is input to a compressor 11, compressed at a predetermined compression ratio, and then input to an A/D converter 12.

At this time, the compression ratio of the compressor 11 is set so that even if waveform distortion occurs due to fading in the propagation path, it does not exceed the allowable input level range (dynamic range) of the A/D converter. Here, the compression ratio is defined as (amplitude after compression/amplitude before compression). 8. If there is no waveform distortion due to fading, the A/D converter 12's dynamic
When the output of compressor 11 is set to the entire range,
When fading occurs, a signal exceeding the dynamic range is input manually to the A/D converter 'ri 12, so linear distortion occurs in the A/D converter valve, making it impossible to perform correct waveform equalization.

Next, the A/D converted digital signal is input to an all-digital transversal filter 13, automatically equalized by a tap coefficient control signal from a tap coefficient control circuit 15, and input to an expander 14. . In the decompressor 14, the original signal compressed by the compressor 11 is restored.

An output signal obtained by multiplying the input signal by an expansion rate equal to the reciprocal of the compression rate is output to terminal 2.

In order to control the tap coefficients of the transversal filter 13, the tap coefficient control circuit 15 generates an error signal E representing the difference between the estimated value of the transmission signal of the output signal of the expander 14 and the digital signal of the equalizer.

The judgment output signal of the A/D converter 12 is input.

The correlation between the error signal E and the judgment output signal is calculated, and the time average value is used as the tap coefficient for the transversal filter 1.
Output to 3. There are various embodiments of this adaptive automatic equalization algorithm, and they are described in literature (for example, "Digital Signal Processing" 7, edited by Institute of Electronics and Communication Engineers, 1975, 2003).
It is detailed in Table 11゜2) on page 41, so it will be omitted here.

For example, instead of the judgment output of the A/D converter.

There is also an algorithm in which the judgment output of the signal level conversion circuit is D.

Among the operations of the all-digital transversal equalizer described above, compression of the signal level in the compressor 11 and compression of the signal level in the decompressor 1 are performed.
The restoration of the signal level in step 4 will be explained below using an example with reference to FIG. As an example of human signal, 4
A value baseband signal is used. This is used as a baseband signal of 16-value orthogonal amplitude modulation g (16QAM).

Points A, B, C, and D in FIG. 3(a).

(denoted by *) represents the signal level of the four-level baseband signal before compression, and the table on the right represents the output of the A/D converter. Now, if we assume that the signal near point A1 has doubled due to waveform distortion due to fading, the signal level will exceed the allowable input range of the A/D converter, so the A/D converter output will be 1]- ...11 (all 1) and becomes saturated. To avoid this, 1 point A+, BL, C+, Dr.
If the signal level is compressed to 1/2 in advance so that each point is at each position of points A2, B2, C2, and D2, even if waveform distortion due to fading occurs, the A/D converter 12
The input signal level of is not saturated.

Next, since the signal level of one point A2, B2, C2, D2 must be the signal level of point A, B+, C1, D+, it will not give a correct judgment output, so the signal level conversion circuit 1
1 point A2, B2, C2 by 4.

The digital signal corresponding to D2 is converted as shown in the table of FIG. 3(b). Specifically, this conversion is performed in ROM (R
It can be realized by a digital multiplier, a logic circuit, etc.

[Problems to be Solved by the Invention] Since the above-mentioned conventional automatic equalizer has a fixed amplitude compression ratio, as long as there is no waveform distortion due to fading, the quantization accuracy of the A/D converter due to compression cannot be improved. This has the disadvantage that the error time is degraded due to the decrease in the error rate. Also, if large fading distortion occurs that exceeds initial expectations, it may be due to insufficient compression ratio.

This has the disadvantage that the A/D converter input is saturated and sufficient equalization characteristics cannot be obtained.

The present invention has been made in view of the above problems.

Even if the magnitude of waveform distortion due to fading changes.

It is an object of the present invention to provide an automatic equalizer that can exhibit sufficient equalization characteristics while effectively utilizing the dynamic range of an A/D converter.

[Means for Solving the Problems] An automatic equalizer used in a demodulator for a digital wireless communication system.

Level adjusting means for adjusting the level of a multi-level baseband signal from a demodulator of the demodulating device at a predetermined compression ratio, and converting the level-adjusted multi-level baseband signal output from the level adjusting means into a digital signal. Analog to convert
with digital converter.

a variable tap coefficient digital transversal filter that automatically equalizes the digital signal based on a tap coefficient control signal; an expander that expands the equalized digital signal at an expansion rate that is a reciprocal of the compression rate; and the analog - The judgment output of either the digital converter or the expander is used as the first input signal, and an error representing the error between the estimated value of the transmission signal and the equalized digital signal among the outputs of the expander An automatic equalizer comprising: a tap coefficient control circuit that generates and outputs the tap coefficient control signal based on the first input signal and the second input signal using a signal as a second input signal.

The level adjusting means and the expander are each variable type, and a compression/expansion small control device is provided for detecting the magnitude of fading and controlling the compression rate of the variable level adjusting means and the expansion rate of the variable expander. An automatic equalizer characterized by:

[Example] An example of the present invention will be described below with reference to the drawings.

FIG. 1 shows a transversal automatic equalizer according to a first embodiment of the present invention, in which a three-tap transversal filter is used for simplicity.

First, a multilevel baseband signal from a demodulator (not shown) is input to terminal 1 and is input to variable attenuator 21 . The variable attenuator 21 uses a compression ratio control signal (COMP) from a compression/expansion ratio control circuit 23 (to be described later) to change its output signal level to an analog/digital (A/D) signal even if waveform distortion due to fading occurs. ) The compression ratio X is automatically set so as not to exceed the allowable signal input level of the converter 12, and it has a function of accurately outputting the waveform information of the input multilevel baseband signal to the A/D converter 12. .

Next, the output signal of the variable attenuator 21 is input to the A/D converter 12, sampled and quantized, and converted into two transmission signal sequences, which are input to the transversal filter 13.

Transversal filter 13 is delay circuit 131.1
32. It is a 3-tap type consisting of multipliers 133, 134, 135° and an adder 136, with one tap in addition to the main tap 138.
It has tap 137 and +1 tap 139. The respective tap output signals are input to multipliers 134, 133, and 135, respectively, and multiplied by tap coefficients C8+C-InCI.

It is input to adder 136. The output of the adder 136 is the one from which code amount interference due to fading has been removed, and becomes the output of the transversal filter 13. The output of this transversal filter 13 is sent to a variable expander 22.
The original signal level, which is inputted to , expanded at an expansion rate Y, and compressed by the variable attenuator 21, is restored. The expansion rate Y is generally set to the reciprocal of the compression rate X (Y-1/X).

As described above, the transversal reference filter 13 removes code amount interference, the variable expander 22 restores the signal level, and the output signal of the variable expander 22 is output to the terminal 2. On the other hand, an error signal E representing the error between the transmission signal and the equalized signal among the judgment output of the A/D converter 12 and the output of the variable expander 22 is input to the tap coefficient control circuit 15.

The phase difference between D and E is determined by digital calculation, and the time average value thereof is manually input to multipliers 133, 134, and 135 of the transversal filter 13 as tap coefficients. This is the same as explained in the conventional embodiment. Here, the tap coefficient C, (n is an integer) is.

C, -E I +D * +However, n -k -j-(1
) can be obtained. Note that represents the time average.

Since the details are detailed in the above-mentioned literature, they will be omitted here.

The magnitude of the tap coefficient changes depending on the magnitude of fading in the propagation path. Therefore, it is possible to estimate the magnitude of subsequent fading in the propagation path based on the change in the tap coefficient. Therefore, in this embodiment, the compression ratio X is determined using the tap coefficient value signals C-, , C, output from the tap coefficient control circuit 15.

When the compression/expansion rate control circuit 23 receives the tap coefficient value signals C-3 and C1, it determines the compression rate X as described later, and outputs the compression rate control signal COMP to the variable attenuator 21. At the same time, an expansion rate control signal EXP is outputted to the variable expander 22 by setting the reciprocal of the compression rate X to the expansion rate Y.

The relationship between fading and tap coefficients will be explained below, as well as a method for determining the compression ratio X.

Figure 4 shows the eye pattern of the four-level baseband signal input to terminal 1 and the corresponding transversal signal when frequency selective fading occurs due to two-wave interference of a direct wave and a reflected wave (delayed wave). Indicates the tap coefficients of the filter. In this case, the compression ratio of the variable attenuator 21 is set to 1.

Figure 4(a) shows the case where there is no fading at all.

At this time, the level of the eye pattern is within the dynamic range of the A/D converter, and the tap coefficients are 1 only for the main tap C6 and 0 for the others.

Next, when fading occurs, if the fading is small, waveform distortion will appear in the eye pattern as shown in Figure 4(b), but the signal level at the sampling point will exceed the dynamic range of the A/D converter. Moreover, the absolute values of tap coefficient C-1 and CI are relatively small.

As the fading increases further, the waveform distortion of the eye pattern becomes extremely large as shown in Figure 4 (C), and the signal level of the input bass punt signal at the sampling point exceeds the dynamic range of the A/D converter. At the same time, the tap coefficient also becomes extremely large and approaches 1. Therefore, in the case of FIG. 4(C), if the compression ratio is changed from 1 to, for example, 1/2, the input base punt signal is・It will fit within the range. At this time, the elongation rate is 2,
The output of the transversal filter 13 is sent to the variable expander 2.
2, it is expanded twice and output to terminal 2.

Variable attenuator 21. The variable expander 22 and the compression/expansion rate control circuit 23 are constructed as shown in FIG. 6, for example. Does the variable attenuator 21 include a 1/2 attenuator 31 and output the input signal from the demodulator as is?

or the output of the 1/2 attenuator 31 is output using the switch 32.
controlled by. Also, the variable expander 22 is connected to a digital multiplier 33 to which the output of the filter during transversal is connected.
, and the switch 34 selects whether to select the 9 multiplication coefficient 1 or 2.

The switches 32 and 34 are controlled by the output of the compression/expansion rate control circuit 23. The compression/expansion rate control circuit 23 receives the tap coefficients C-+ and the absolute value C-1 of C8 (and IC11), and comparators 35 and 36 determine whether the absolute value of the tap coefficient is larger or smaller than the reference value CR. and outputs the results to the OR circuit 37.The OR circuit 37 determines that the fading is sufficiently large if either Ic-+I or IC11 exceeds the reference value.
The determination output is outputted to the variable attenuator 21 and variable expander 22 as a switching control signal. Thus, when fading is greater than the reference value.

Since the input base punt signal is compressed and input to the A/D converter 12, nonlinear distortion does not occur during A/D conversion.
Correct waveform equalization is performed with a transversal filter.

Although the compression/expansion rate control circuit 23 described above changes the compression rate and expansion rate in two stages, it is also possible to continuously change the compression rate and expansion rate for more fine-grained control. Variable attenuator 21 in this case. A second embodiment of the variable expander 22 and compression/expansion rate control circuit 23 is shown in FIG.

The variable attenuator 21 has an analog multiplier 41 .

The baseband signal input to the output terminal 1 of the amplifier circuit is multiplied by the compression ratio control signal COMP and output to the A/D converter 12. The variable expander 22 also has a digital multiplier 33, which multiplies the output digital signal of the transversal filter 13 by an expansion rate control signal EXP and outputs the result. The compression ratio control signal COMP and the expansion ratio control signal EXP are output from the compression/expansion small side m circuit 23. The compression/expansion rate control circuit 23 inputs the tap coefficients C-+ and C4, and the coefficient conversion circuit 42 responds to changes in the tap coefficients. Compression ratio X is obtained continuously. The coefficient conversion circuit 42 is, for example, a ROM (Read Only Memory).
This can be easily realized using a memory element such as y).

Since the compression ratio determination signal X output from the coefficient conversion circuit 42 is a digital signal, before inputting it to the analog multiplier 41,
The signal is input to a digital-to-analog converter (D/A converter) 43, converted into an analog signal compression ratio control signal, and input to an analog multiplier 41.

On the other hand, the digital multiplier 33 in the variable expander 22 receives the compression rate determination signal
-X-1 and is converted into a digital expansion rate control signal EX
Output as P.

With the above configuration, the variable compressor 21 and the variable expander 22
is continuously controlled by changes in fading, so smooth operation can be expected.

In the example of FIG. 7, it is also possible to replace the variable attenuator 21 with a so-called automatic gain control circuit (AGC). A third embodiment of the present invention using this method is shown in FIG. The difference from the second embodiment is that the variable attenuator 2 in FIG.
1, an automatic gain circuit (AGC) 24 is used to control the level of the input base punt signal, and the AGC
24 is input to the A/D converter 12.

and the control signal COMP of the variable attenuator 21 in FIG.

The D/A converter 43 that generates COMP is deleted.

Instead, an AGC control circuit 45 that controls the AGC circuit 24 is used.

In this embodiment, the AGC control operates so that the output signal of the variable expander 22 is constant.

When the fading becomes large and the expansion rate of the variable expander 22 becomes greater than 1, the gain of the AGC circuit 24 decreases below 1 to prevent the signal level input to the A/D converter 12 from being saturated. Conversely, if fading becomes smaller, the expansion ratio of the variable expander 22 approaches 1, and the gain of the AGC circuit 24 approaches 1, so the dynamic gain of the A/D converter 12 decreases.
A/D converter 1 has an input level that allows full use of the range.
2, the S/N ratio of A/D conversion is improved, and deterioration of short error time is less likely to occur.

Although the automatic equalizer of the present invention has been described above with respect to a specific embodiment, the transversal filter of the present invention may have any configuration, and the tap coefficient control algorithm may also be the same as the zero and It may be a forcing method or a minimum error method, and is not influenced by the algorithm.

Further, in the above embodiment, the magnitude of fading is determined based on the tap coefficient of the transversal 0 filter, but of course, the magnitude of fading may be determined using other information, such as detecting the amount of notches in fading using a spectrum. The thing is clear.

A feature of the present invention is that the compression ratio of the variable attenuator or the gain of the automatic gain circuit and the expansion ratio of the variable expander are automatically controlled depending on the magnitude of fading.
The key is to use the range effectively.

[Effects of the Invention] As explained above, according to the present invention, by detecting the magnitude of waveform distortion due to fading, the variable attenuator or the
By controlling the GC circuit and variable expander, when fading is large, the baseband signal is compressed and applied to the A/D converter, thereby preventing nonlinear distortion of the A/D converter due to waveform distortion and automatic etc. Demonstrates sufficient equalization ability as a stopper,
When fading is small, applying the baseband signal to the A/D converter without compressing it prevents the quantization accuracy of the A/D converter from decreasing, improves the S/N of the A/D output, and reduces errors. It is possible to prevent sexual deterioration. In either case, the output signal level of the equalizer remains constant regardless of the compression ratio due to the automatically controlled variable expander.
The error signal can be easily extracted, not only the transversal filter can be controlled, but also the demodulator's automatic gain control and automatic phase control can be easily performed, and various signal processing of the subsequent reproduced data signal can also be performed easily.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a first embodiment of the automatic equalizer of the present invention, FIG. 2 is a block diagram showing a conventional automatic equalizer,
Figures 3 (a) and (b) are diagrams for explaining signal level conversion; (a) shows before compression, (b) shows after compression;
Figures (a), (b), and (c) are diagrams for explaining the magnitude of waveform distortion due to fading and the absolute magnitude of the tap coefficient, where (a) shows no fading, (b) shows small fading, (C) shows a case where fading is large; FIG. 5 is a diagram for explaining the effect of baseband signal compression at the time of waveform distortion. Figure 6 shows a variable attenuator used in the automatic equalizer of Figure 1;
A concrete block diagram of a variable expander and a compression/expansion rate control circuit, FIG. 7 is a block diagram showing a second embodiment of the present invention,
FIG. 8 is a block diagram showing a third embodiment of the present invention. 11... Compressor, 12... A/D converter, 13...
・Transversal filter, 14...Extender, 1
5... Tap coefficient control circuit, 21... Variable attenuator,
22... Variable expander, 23... Compression/expansion rate control circuit, 24... Automatic gain circuit (AGC), 45... A
GC control circuit. 4 [baseband signal 1 before compression (G) 3rd 4fi baseband signal after 1 compression 2-1 2-22-3---2-n (b) 5z sample point sample point (a) Compression rate ( b) Compression ratio i Figure 6 procedural amendment (voluntary) December 11, 1999

Claims (1)

[Claims] 1. An automatic equalizer used in a demodulator of a digital wireless communication system, which adjusts the level of a multilevel baseband signal from a demodulator of the demodulator at a predetermined compression rate. and an analog baseband signal for converting the level-adjusted multilevel baseband signal output from the level adjustment means into a digital signal.
a digital converter; a variable tap coefficient digital transversal filter that automatically equalizes the digital signal based on a tap coefficient control signal; and an expansion that expands the equalized digital signal at an expansion rate that is the reciprocal of the compression rate. and the determination output of either the analog-to-digital converter or the expander is used as a first input signal, and the estimated value of the transmission signal among the outputs of the expander and the equalized digital signal are combined. automatic equalization comprising: a tap coefficient control circuit that generates and outputs the tap coefficient control signal based on the first input signal and the second input signal using an error signal representing an error as a second input signal; In the device, the level adjustment means and the expander are each variable type, and compression/expansion rate control is performed to detect the magnitude of fading and control the compression ratio of the variable level adjustment means and the expansion rate of the variable expander. An automatic equalizer characterized by being provided with a device. 2. The automatic equalizer according to claim 1, wherein a variable attenuator is used as the variable level adjustment means. 3. The automatic equalizer according to claim 1, wherein an automatic gain control circuit is used as the variable level adjustment means. 4. The automatic equalizer according to claim 2, wherein the variable attenuator is a multiplier.