JPS6352821B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a technique for compensating for nonlinear distortion caused by a traveling wave tube amplifier (hereinafter abbreviated as TWT).

Digital communication in the microwave band is based on the satellite system,
Regardless of the terrestrial method, it will be mandatory to operate with a higher density transmission method from the perspective of effective use of frequency bands. That is, in 1979 [International Conference on
"Characteristics of a High"(ICC'79) conference record, pages 48.4.1 to 48.4.6.
Capacity 16 QAM Digital Radio System on
a Multipath Fading Channel” and also 1979
National Telecommunications of the Year
“Distortion” described on pages 35.4.1 to 35.4.3 of the conference record of the Conference (NTC’79)
As can be seen in ``Analysis of 64QAM'', multilevel quadrature amplitude modulation (QAM) will be used.

At this time, the problem is the nonlinear distortion of 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 output phase rotation θ(x) characteristics (AM/PM conversion) corresponding to input level x.

It is therefore generally possible to compensate for this type of distortion to a considerable extent with relatively simple circuits.

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 in the past, but an automatic tracking type circuit that automatically guides the circuit to the most desirable operating state is proposed by the Communication Systems Research Group of the Institute of Electronics and Communication Engineers. Material CS78-201 “Study of TWT nonlinear compensation using automatic tracking complex synthesis predistortion”
There is no precedent for this. This example is for microwave band
Since it was developed for SSB communication, it is not very suitable for digital transmission.

An object of the present invention is to provide a nonlinear distortion compensation control system suitable for digital transmission.

According to this invention, the output of a digital modulator is input, passed through an odd function input/output characteristic circuit and a variable complex coefficient circuit, and added to the input to generate a complex composite distortion characteristic. In a control device for a nonlinear correction circuit provided to cancel generated nonlinear distortion, a distortion component due to the nonlinear circuit is detected from an output signal of the nonlinear circuit and at least one of an input signal or an output signal of the digital modulator. An error detector is provided, and the nonlinear distortion of the nonlinear circuit is removed by increasing or decreasing the complex coefficient of the variable complex coefficient circuit according to the correlation value between one of the input signal or output signal of the digital modulator and the output of the error detector. An automatic follow-up predistorter characterized by the following features can be obtained, and it is possible to provide a nonlinear distortion removal circuit that allows the nonlinear compensation circuit to operate in the most convenient condition for any TWT.

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

FIG. 1 is a block diagram of a conventionally commonly used nonlinear correction circuit, and FIG. 2 is a diagram for explaining the operation of the circuit.

The circuit shown in FIG. 1 comprises an odd function input/output characteristic circuit 10 (for example, a cubic nonlinear element), a variable phase shifter 11, a variable attenuator 12, and an adder 13 forming 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 r 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 θa 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 f(x) is output as f(x)=x+η·|x| ² ·x due to nonlinear distortion of TWT for input x. This is the actual
It is a good approximation of TWT nonlinear distortion. Here, η is a complex number such that η=α・e ^j 〓. Also, the second term of f(x) is the first term
Normally, |x|≫|η・|x| ²・x|

When digital signal X is input to TWT, its output is f(x)=X+η・|x|
²・X Before inputting the signal X to the TWT, if ^it passes through a nonlinear ^distortion correction circuit with the following characteristics:
^The TWT output C ₀ is: ^C ₀ ⁼
・(X+ξ｜ ^X ｜ ²・X) ^}
) |X| ²・X Therefore, the error E of C ₀ with respect to the original signal value X
subtracts the original value X from C ₀ and becomes E=C ₀ −X=( ^ξ +η) | From this formula, by setting ξ=-η, E
=0.

Then, the control of ξ (dξ/dt) is performed as follows: dξ/dt=∂|E| ² /∂ξ×(minimal coefficient), so that ∂|E|
² /∂ξ=0 and becomes stable.

∂｜E｜ ² /∂ξ=∂｜E｜ ² /∂ξ _R
+j∂｜E｜ ² /∂ξ _I =2(ξ+η)｜X｜ ⁶ (ξ _R =Real(ξ) ξ _I =Im(ξ)) Here, the value of the above equation is not subjected to such partial differentiation. Think about how to find it. First of all, the following quantities
Get C ₁ .

C ₁ = E × [digital modulator input signal (
= complex conjugate ^of (original value of X)] = (ξ+ ^η ) |
η) |X| ²・|X ^{| 2} so ^C ₁ = ¹ / | This means that we have found ² /∂ξ.

The above is the operating principle of this invention.

FIG. 4 is a block diagram showing an embodiment of the present invention, and in the figure, 1 is the nonlinear correction circuit of FIG. ), 2 is a transmitting TWT amplifier that works as a nonlinear circuit, and 5 is a digital
QAM modulator with base signal from input terminal 500.
Convert band QAM signal to carrier modulation wave. However, the sending filter is not included. Note that 6 is a transmitting antenna. 3 is an error detector that observes E described earlier, and a synchronous detector 24 that reproduces the original baseband signal from a part of the output of the TWT amplifier.
and a subtracter 22 that detects the difference between the baseband signal and the original baseband signal. The synchronous detector 24 is a general type consisting of a reference signal generator 27, a multiplier 26, and a low frequency filter 25. 4 is nonlinear correction circuit 1
This is a circuit that supplies a complex coefficient ξ to dξ/dt=-E*X ^* , and its configuration embodies ξ=∫ ^t _-∞ -E*X ^* dt.

First, 41 is the complex conjugate of the digital modulator input
An imaginary part polarity inversion circuit inverts only the polarity of the imaginary part ^{of X to obtain X* ,} 44 is a complex multiplier that obtains the product of E. It is an integrator that changes according to the value of X ^* .

As is clear from the above explanation, the control according to the present invention is based on the maximum slope method, and even if the distortion of the nonlinear circuit is not within the range of distortion generated by the nonlinear distortion correction circuit, the control becomes unstable. The best solution can be obtained without any problems.

In addition to this example, E can be found in the following ways:
It can also be obtained by taking the difference between the digital modulator output in the IF band and the signal of the TWT output downconverted to the IF band, and the correlation with
If it is desired to obtain the signal in the IF band, the complex conjugate signal of X can be obtained by inputting it into a digital modulator, and the correlation can be obtained with the signal E in the IF band mentioned above.

[Brief explanation of the drawing]

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.
The figure is a block diagram showing one embodiment of the present invention. DESCRIPTION OF SYMBOLS 1...Nonlinear distortion correction circuit, 2...Nonlinear circuit, 3...Error detector, 4...Complex coefficient supply circuit, 5...Digital modulator.

Claims (1)

[Claims] 1 The output of the digital modulator is input, passed through an odd function input/output characteristic circuit and a variable complex coefficient circuit, and added to the input to generate a complex composite distortion characteristic, which eliminates the nonlinear distortion generated in the nonlinear circuit at the subsequent stage. A control device for a nonlinear correction circuit provided for cancellation, comprising an error detector that detects a distortion component due to the nonlinear circuit from an output signal of the nonlinear circuit and at least one of an input signal or an output signal of the digital modulator. , the nonlinear distortion of the nonlinear circuit is removed by increasing or decreasing the complex coefficient of the variable complex coefficient circuit according to the correlation value between one of the input signal or output signal of the digital modulator and the output of the error detector. Features an automatic follow-up pre-distorter.