JPH041534B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a nonlinear distortion removal circuit, and particularly to a nonlinear distortion removal circuit using a traveling wave tube amplifier (hereinafter abbreviated as TWT).

Microwave band communications, whether satellite or terrestrial, must be operated using higher-density transmission methods from the perspective of effective use of frequency bands.

That is, in 1979, the International Conference on Communications
"Characteristics of a High Capacity QM Digital Radio System on a Multipath" described on pages 48.4.1 to 48.4.6 of the conference record of ICC '79
Feedeng Channel (Characteristics of a
High Capacity 16 QAM Digital Radio
“System on a Multipath Fading Channel)”
and the 1979 National Telecommunications Conference.
Telecommunications Conference) (NTC'79)
As can be seen in "Distortion Analysis of 64 QAM" on pages 35.4.1 to 35.4.3 of the conference record, multilevel quadrature amplitude modulation (QAM) will be used.At this time, The problem is nonlinear distortion in the transmission amplifier (TWT), which distorts the QAM signal.
The nonlinear distortion of TWTs differs slightly depending on each TWT, but they form one category. In other words, the amplitude saturation characteristic (AM/AM conversion) and the output phase rotation θ(x) characteristic (AM/PM
transformation). Therefore, it is generally possible to compensate for this type of distortion to a considerable extent with relatively simple circuitry.

Now, if we consider the case where band limiting of the transmitted signal is not performed in front of the TWT, we can separate the nonlinear effects from the band limiting effects, so we can accurately observe this distortion on both the transmitting and receiving sides. can do.

Based on this idea, various preset type nonlinear distortion compensation circuits have been proposed.
An automatic tracking circuit that automatically guides this circuit to the most desirable operating state is described in "Study of TWT nonlinear compensation using automatic tracking complex synthesis predistortion" in material CS78-201 of the Communication Systems Study Group of the Institute of Electronics and Communication Engineers. Just look at the precedent.

The present invention uses the perturbation method to control the parameters of the nonlinear correction circuit, similar to those described in the previous literature.
However, by detecting the effect of the perturbation by correlating it with the perturbation signal, it becomes possible to use a very low level perturbation signal. Therefore, the above parameter control can be performed without the perturbation signal disturbing the signal that should originally be received.

According to this invention, a nonlinear distortion correction circuit that passes an input signal through an odd function input/output characteristic circuit and a variable complex coefficient circuit and adds it to the input signal to generate a complex composite distortion characteristic is placed before or after the nonlinear element. , in a method for canceling nonlinear distortion, a change extraction circuit extracts a change from the original frequency characteristic of the input signal after passing through the nonlinear distortion correction circuit and the nonlinear element; and the variable complex coefficient. and a correlator that correlates the change extraction circuit output with the oscillator output, and by changing the variable complex coefficient in a direction opposite to the correlator output polarity, the nonlinear element There is obtained a nonlinear distortion removal circuit characterized in that the distortion of the nonlinear distortion is canceled out by the nonlinear distortion correction circuit.

Next, the present invention will be explained in detail with reference to the drawings.

Figure 1 shows a block diagram of a conventionally commonly used nonlinear distortion correction circuit, and Figure 2 shows a block diagram of a conventional nonlinear distortion correction circuit.
FIG. 3 is an explanatory diagram of the operation of the circuit shown in the figure.

FIG. 1 shows an odd function input/output characteristic circuit 10 (for example, 3
The variable position shifter 11, the variable attenuator 12, and the adder 13 constitute a variable complex coefficient circuit.

Let the input x to the input terminal 100 be the vector 200 in FIG. The output of the variable attenuator 12 is changed into vectors 201 and 20, for example, by the phase shift amount φ [rad] of the phase shifter 11, as shown in the vector 208 in FIG.
2 and 203. Since the output γ of the adder 13 is the vector sum 207 of the vector 200 and the vector 208, each vector 20
4,205 and 206. The relative length of the vector 208 with respect to the vector 200 depends on the characteristic f(x) of the odd function input/output characteristic circuit 10, but generally f(x)={ax+bx ³ +cx ⁵ ...}α
If it has the form (b, c≠0), the vector 208 will relatively extend as the input x becomes larger. Therefore, the input/output phase difference θ _a increases, and the relative output amplitude increases in the range of 0<φ<π/2. FIG. 3 shows the input/output characteristics of the circuit shown in FIG. 1, where a curve 301 shows the amplitude characteristics and a curve 302 shows the phase characteristics. This characteristic is the inverse of the input/output characteristic of TWT.

The problem is how to choose parameters α and φ
The question is whether it is possible to approximate the inverse characteristic of the TWT characteristic.

Now, suppose that for an input x, the nonlinear distortion of the TWT outputs f(x) as follows: f(x)=x+η·|x| ² ·x. This is the actual
It is a good approximation of TWT nonlinear distortion. Here, η is a complex number η=α′.e ^j 〓′. Also, the second term of f(x) is the first term
Normally, |x|≫|η・|x| ²・x|

When the signal X is input to the TWT ^, the output is f( ^X ) =・^e ) ^j 〓 When passed through a nonlinear distortion correction _circuit with the characteristic
| ²・X) ²・(X+η|X| ²・X)} X+η|X| ²・X+ξ|X| ²・X=X
⁺ ( _{η +} ξ) ^| By setting η=-ξ according to this equation, E=0.

Therefore, let's rewrite ξ as follows.

ξ≡−η±ξd ξd is a parameter indicating how far ξ is currently deviated from the optimal value. Since this error E is usually a complex number, its absolute value |E| is minimized when optimization is performed. As mentioned earlier, ξd=0
When |E|=0, otherwise |E|≠0
Therefore, in the vicinity of ξd=0, the characteristics are as shown in FIG. 6a. If ξd can be directly observed, η=−ξ±
E can be minimized by setting ξd. However, we can only observe |E|.
At this time, if we can somehow find out whether ξd currently exists on the right or left side of aξd=0 in Figure 6, we can control η in a direction to eliminate it.
The principle is simple. In other words, when it is on the right side, δ|E|/δξd>0, and on the other hand, when it is on the left side, δ|E|/
δξd<0. In order to find the polarity of this differential coefficient, we can actually add a perturbation △ξd to ξ and directly check whether the resulting change in |E| increases or decreases.

Normally, Δξd cannot be made large enough to cause disturbance to the communication signal itself, so it becomes a very small signal. Therefore, △|E|/△ξd is also a very small signal and is buried in noise. For this purpose, over and over again△
It is necessary to add ξd and detect the resulting change in |E| cumulatively. For that, △ξd and △｜E
An effective method is to observe the correlation with |. That is, if the correlation value is R, find R such that R=△||・△, and the polarity is exactly △|E|/△ξd
The polarity becomes . Because sign(△|E|/△ξd)=sign(△|E|・△ξd)
This is because. As perturbation △ξd, ξ ₀・sinω ₀ t
Using the sine wave (shown in Figure 6b), we get |E|
Due to this perturbation, approximately |E||E ₀ |+
It changes in the form sign(ζd)・β・sinω ₀ t (Figure 6b). Here, β is a constant introduced to satisfy the approximate expression.

To find R in the above equation, R=1/2To∫ ^To _-Tp |E|・ξ ₀・sinω ₀ tdt=ξ
0／2To∫ ^To _-Tp (｜E｜＋sign(ξd)・βsinω ₀ t)・s
inω ₀ tdt = ξ0／2To2Tosign(ξd)β・π (However, To
2π/ω ₀ )=sign(ξd)ξ ₀ π・β Therefore, from the above equation, the polarity of R is the polarity of ξd sign(ξd)
It turns out that it is the same as This situation is shown in FIG. If ξd reaches the optimum value ξopt as shown in FIG. 6b, R=0. Therefore, to control ξ, it is sufficient to increase or decrease it in the opposite direction to the polarity of the correlation value R determined by the above equation. That is, dξ/dt=-α・R: α=minimal coefficient Integrating both sides of the above equation by t gives ξ=-α∫Rdt=-α∫1/2To∫｜E｜
・ξ ₀・sinω ₀ tdt・dt Here, ξ is a double integral, and in this R
Since the integral related to is simply a process of extracting an average value for obtaining a correlation value, the effect does not change even if it is substituted with an integral attached to the outside. Therefore, ξ may be controlled in the following manner.

ξ=−α∫|E|・ζ0・sinω ₀ t・dt The above is the principle of the present invention.

FIG. 4 is an equivalent baseband block diagram of one embodiment of the present invention, which is a concrete example of the above principle. Block 1 in the figure is the same as the nonlinear distortion correction circuit shown in FIG. In this example, for simplicity,
It is assumed that the variable phase shifter 11 has already been optimally set, and the attenuation amount ξ of the remaining variable attenuator 12 is
This is the control and control over. Block 4 is
It is a nonlinear element such as TWT.

Block 2 is a change extraction circuit that detects the degree of deformation E of the input signal. Specifically, a bandpass filter 20 and a full-wave rectifier circuit 21 detect the power in a band where the input signal originally does not exist. This means that the out-of-band radiation generated by passing through the nonlinear element 4 is detected, and by appropriately selecting the value of the variable attenuator 12 of the nonlinear distortion correction circuit 1, this out-of-band radiation power can be becomes smaller. 31 is the perturbation ξ ₀・
An oscillator that generates sinω ₀ t, 33 an integrator for control, and 34 a polarity inversion circuit. Adder 32 is for adding a perturbation signal. 30 is a correlator for determining the correlation value R between the perturbation signal and the output of the change extraction circuit, and is composed of a multiplier 300 and an integrator 301. However, this integrator 301 may be omitted.

In this embodiment, the switch 38 has two operating modes. First, when the switch 38 is turned to the (a) side, the operation follows the principle explained earlier. When tilted to the (b) side, the perturbation amount ξ ₀ sinw ₀ t is the error signal |E| obtained from the terminal 103 and the attenuation amount δ
The value δ obtained by passing through the attenuator 35 of
It is multiplied by |E|. As a result, the amount of perturbation added to the adder 32 becomes δ||ζ0sinw ₀ t. This corresponds to making the perturbation amplitude ξ ₀ proportional to ξ′ ₀ =δ||ξ ₀ and ||, and reducing it by giving damping. As a result, when the control progresses and |E|→0, the amount of perturbation decreases accordingly, and no disturbance is inadvertently given to the signal.

FIG. 5 is a block diagram of another embodiment of the present invention, and the nonlinear distortion correction circuit 1 in this embodiment has five
It corrects up to the third-order distortion, and the parameters to be controlled are a variable attenuator 12 for third-order distortion, and a phase shifter 1.
1. Variable attenuator 15 for fifth-order distortion, phase shifter 1
6 of 4. For this purpose, four of the same control circuits 3 as shown in FIG. 4 are used. That is, they are 3, 3', 3'', and 3. However, since the output of the change extraction circuit 2 is used in common, the output signals of the oscillator 31 in the block 3 must be made into four orthogonal functions. These may be, for example, {sinω ₀ t, cosω ₀ t, sin2ω ₀ t, cos2ω ₀ t} or {sinω ₀ t, sin2ω 0 t, _{sin3ω 0 t} , sin4ω ₀ t}.

Similarly, by increasing the number of control circuits, good high-order nonlinear compensation can be performed automatically and easily.

As explained above, according to the present invention, nonlinear distortion caused by TWT or the like can be automatically and accurately compensated for using a high-order nonlinear compensation circuit.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an example of a conventional nonlinear distortion correction circuit, Fig. 2 is an explanatory diagram of the operation of the circuit shown in Fig. 1, Fig. 3 is an input/output characteristic diagram of the circuit shown in Fig. 1, and Fig. 4 is a block diagram showing an example of a conventional nonlinear distortion correction circuit.
5 is a block diagram of an embodiment of the present invention, and FIG. 6 is a block diagram of an embodiment of the present invention.
Figures a, b and Figure 7 are diagrams for explaining the principle of the present invention. 1... Nonlinear distortion correction circuit, 2... Change extraction circuit, 3... Control circuit, 4... TWT, 30... Correlator, 31... Oscillator.

Claims (1)

[Claims] 1. A nonlinear distortion correction circuit that passes an input signal through an odd function input/output characteristic circuit and a variable complex coefficient circuit and adds it to the input signal to generate a complex composite distortion characteristic is placed before or after the nonlinear element to eliminate nonlinear distortion. The canceling method includes: a change extraction circuit that extracts a change from the original frequency characteristic of the input signal after passing through the nonlinear distortion correction circuit and the nonlinear element; and a change extraction circuit that perturbs the variable complex coefficient. an oscillator; and a correlator that takes a correlation between the output of the change extraction circuit and the output of the oscillator, and the distortion of the nonlinear element is adjusted to the nonlinear A nonlinear distortion removal circuit characterized in that the distortion is canceled by a distortion correction circuit.