JPS6352820B2 - - 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).

Microwave band digital communications, whether satellite or terrestrial, are important from the perspective of effective use of the frequency band.
It will be mandatory to operate using a higher density transmission method.

That is, in 1979 [International
From page 48.4.1 of the conference record of Conference on Communications〕 (ICC'79)
“Characteristics” listed on page 48.4.6
of a High Capacity 16 QAM Digital Radio
System on a Multipath Fading Channel”
and also the 1979 National
“Distortion Analysis of 64 QAM” on pages 35.4.1 to 35.4.3 of the Telecommunications Conference (NTC'79) conference record
As you can see, 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. TWT nonlinear distortion differs slightly depending on each TWT, but forms one category. That is, it is characterized by amplitude saturation characteristics (AM/AM conversion) and output phase rotation θ(x) characteristics (AM/PM conversion) corresponding to input level x. 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. This example was developed for microwave band SSB communication, so 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, a digital modulation signal subjected to nonlinear distortion is received as an input signal, passed through an odd function input/output characteristic circuit and a variable complex coefficient circuit, and added to the input signal to generate a complex composite distortion characteristic. The nonlinear distortion correction circuit includes a discriminator that outputs an estimated value of a transmission code from the output signal of the nonlinear distortion correction circuit, and an error detector that detects a difference between input and output signals of the discriminator, and The nonlinear distortion of the digital modulation input is offset by the nonlinear characteristic of the nonlinear distortion correction circuit by increasing or decreasing the complex coefficient of the variable complex coefficient circuit according to the correlation value between the digital modulation signal and the input/output signal difference. It is possible to obtain a nonlinear distortion removal circuit characterized by the following, and it is possible to provide a nonlinear distortion removal circuit that allows a 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.

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
(multiplicative 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 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(b,
c≠0), the vector 208 relatively extends 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. Figure 3 shows the input/output characteristics of the circuit in Figure 1, with curve 301
indicates the amplitude characteristic, and curve 302 indicates the phase characteristic. 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 digital signal X is input to the TWT, its output is f(x)=X+η・｜x ^| α・e ^j 〓) When passed through a nonlinear distortion correction circuit with the following characteristics, its output C ₀ is C ₀ =X+η|X| ²・X+ξ{|X+ ^η |X ^|
・(X+η | ^X | ²・X)} ^X +η |
) |X| ²・X Therefore, the error E for the original signal value X of C ₀
subtracts the estimated value of X=X^ and becomes E=C ₀ −X^=(η+ξ)|X| ²・X. From this formula, by setting η=-ξ, E
=0.

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

∂｜E｜ ² /∂ξ=∂｜E｜ ² /∂ξ _R +j∂｜E
^{| 2} /∂ξ _{I =} 2 ₍ η + ξ) |

C ₁ = E × (complex conjugate of the ^estimate of the original digital code X from ^C ₀ ) = X^ ^* = (η + ξ) |
ξ) |X| ²・|X ^{| 2} so ^C ₁ = ¹ /| This means that we have found ² /∂ξ.

Next, consider the quantity C ₂ .

C ₂ = E × (nonlinear distortion correction circuit input) = E × (X + α | X | ^{2 ・}X) ^* E × X ^* = (η + ξ) | _X | ² _| It can be seen that ∂|E| ² /∂ξ can be obtained approximately.

Based on the above considerations, the complex coefficient ξ of the nonlinear distortion removal circuit is firstly dξ/dt=−∂|E| ² /∂ξ=−E×X^ ^* Secondly, ∂ξ/dt=−∂|E | ² /∂ξ=-E×(nonlinear distortion correction circuit input signal) By controlling with two control methods, ξ=-η and E=0.

The above is the operating principle of this invention.

FIG. 4 is a block diagram showing an embodiment of the first invention, in which block 1 is composed of a cube circuit 10 which functions as an odd function input/output characteristic circuit, a complex multiplier 14 and an adder 13 which function as variable complex coefficient circuits. This is a nonlinear distortion correction circuit consisting of a nonlinear distortion correction circuit whose output corresponds to C ₀ described above. Block 2 is a discriminator that estimates the correct value X from the distorted digital input signal 〓X, and its output corresponds to X^ (=X) described earlier.
Block 3 is an error detector that detects the difference between the input and output signals of this discriminator, and its output corresponds to E in the previous explanation. Block 4 is a circuit that supplies the complex coefficient ξ to the variable complex coefficient circuit described above, and its configuration embodies ξ=∫ ^t _-∞ −E・X^ ^* dt from dξ/dt=−E・X^ ^* . This is what I did.

In block 4, 41 is ^the complex conjugate of X
An imaginary part polarity ^inversion circuit inverts only the polarity of the imaginary ^part of X to obtain
is an integrator that changes according to the value of .

FIG. 5 is a block diagram showing an embodiment of the second invention, in which each component is the same as the one with the same reference numeral in FIG. 4, and the difference from FIG.
4 is the input signal of the nonlinear distortion correction circuit 1 instead of the output of the discriminator 2. Therefore, the output of the multiplier 44 in this case corresponds to _C2 in the previous explanation.

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 form of distortion of the input signal is outside the range of the form of distortion generated by the nonlinear distortion correction circuit, the control according to the present invention is possible. There is no instability and the best solution is obtained.

[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.
FIG. 5 is a block diagram showing a first embodiment of the present invention. FIG. 5 is a block diagram showing a second embodiment of the present invention. 1... Nonlinear distortion correction circuit, 2... Discriminator, 3
...Error detector.

Claims (1)

[Claims] 1. A digital modulation signal subjected to nonlinear distortion is received as an input signal, passed through an odd function input/output characteristic circuit and a variable complex coefficient circuit, and added to the input signal to generate a complex composite distortion characteristic. A nonlinear distortion correction circuit comprising: a discriminator that outputs an estimated value of a transmission code from the output signal of the nonlinear distortion correction circuit; and an error detector that detects a difference between input and output signals of the discriminator; The nonlinear distortion of the digital modulation input is offset by the nonlinear characteristics of the nonlinear distortion correction circuit by increasing or decreasing the complex coefficient of the variable complex coefficient circuit according to the correlation value of the input/output signal difference. Nonlinear distortion removal circuit. 2. In a nonlinear distortion correction circuit that receives a nonlinearly distorted digital modulation signal as an input signal, passes it 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. , comprising a discriminator that outputs an estimated value of the transmission code from the output signal of the nonlinear distortion correction circuit, and an error detector that detects the difference between the input and output signals of the discriminator, A nonlinear distortion removal circuit characterized in that the nonlinear distortion of the digital modulation input is offset by the nonlinear characteristics of the nonlinear distortion correction circuit by increasing or decreasing the complex coefficient of the variable complex coefficient circuit according to the correlation value. .