HK1066641A - Circuit and method for compensating for nonliner distortion of power ampllifier - Google Patents

Description

Circuit and method for compensating for non-linear distortion of power amplifier

Technical Field

The present invention relates to a circuit and method for compensating for nonlinear distortion of a power amplifier, and more particularly, to a transmission circuit having a nonlinear distortion compensation circuit.

Background

From the viewpoint of frequency utilization efficiency, digital microwave radio communication systems generally employ a quadrature amplitude modulation format, such as multilevel quadrature amplitude modulation. According to the quadrature amplitude modulation format, since a power amplifier for amplifying a transmission signal uses only a linear region of its input/output characteristics, it is desirable to have a sufficiently large back-off (back-off) (the back-off represents an operating point and is generally given as a difference between a maximum output amplitude level and a saturated output power level). However, if the compensation is large, a sufficiently large amount of transmission power cannot be obtained. Therefore, it is actually necessary to reduce the compensation of the power amplifier to use the nonlinear region of its input/output characteristics. As a result, there arises a problem that nonlinear distortion caused when the power amplifier uses a nonlinear region is superimposed on the transmission signal.

In order to solve the above problem, a process has been devised for compensating for nonlinear distortion generated when a transmission signal is amplified by: the inverse component (inversion) of the nonlinear distortion, which depends on the input signal power, is superimposed on the transmitted signal using a circuit called a predistorter. Heretofore, such predistorters have included analog circuitry for the RF band. However, the conventional predistorter is limited in its compensation accuracy due to variations in its components and is difficult to adjust. In recent years, the development of digital signal processing techniques has led to the emergence of predistorters configured as baseband digital circuits.

Transmission circuits using digital predistorters are generally classified into two types, open-loop transmission circuits and closed-loop transmission circuits. Fig. 1 of the drawings shows the arrangement of a typical open-loop transmission circuit (see patent document 1 (japanese laid-open patent publication No. 2001-22627)), and fig. 2 of the drawings shows the arrangement of a typical closed-loop transmission circuit (see patent document 2 (japanese laid-open patent publication No. 2000-228643)).

The open loop transmission circuit shown in fig. 1 includes an FIR filter 10, a predistorter 11, a modulator 12, and a power amplifier 13 connected in series. The input baseband digital signals (Ich data, Qch data) are supplied to a modulator 12 through an FIR filter 10 and a predistorter 11, and the modulator 12 quadrature-amplitude-modulates the signals. The modulated signal is then amplified by a power amplifier 13. Inverse characteristics (inverse characteristics) of the nonlinear distortion of the power amplifier 13 are predetermined and stored in the predistorter 11 to determine a compensation value for the power level of the input signal. The circuit arrangement shown in fig. 1 has the advantage of being simple and inexpensive. However, since the inverse characteristic held in the predistorter 11 has a fixed property, if the inverse characteristic held in the predistorter 11 and the actual inverse characteristic are different from each other for some reason, the open-loop transmission circuit cannot provide a sufficient nonlinear distortion compensation capability.

The closed loop transmit circuit shown in fig. 2 differs from the open loop transmit circuit shown in fig. 1 in that it has an adaptive predistorter 14 instead of the predistorter 11 shown in fig. 1, and also has a comparison/control circuit 15 and a demodulator 16. In operation, the modulated signal amplified by the power amplifier 13 is demodulated by the demodulator 16. The comparison/control circuit 15 compares the baseband digital signal (Ich data, Qch data) output from the FIR filter 10 and the demodulated signal from the demodulator 16 with each other, and adaptively changes the amount of compensation in the adaptive predistorter 14 so that the baseband digital signal and the demodulated signal are equalized with each other. The adaptive predistorter 14 can therefore always optimally compensate for nonlinear distortion.

As is generally known, the input/output characteristics of a power amplifier vary with its operating temperature. With the open loop setting, as described above, since the compensation amount is determined based only on the power of the input signal, if the temperature of the power amplifier varies, the compensation amount and the actual inverse characteristic become different from each other, resulting in an insufficient nonlinear distortion compensation capability. The closed loop arrangement does not have the above disadvantages because the characteristic changes due to temperature changes of the power amplifier are adaptively compensated. However, the closed loop transmission circuit is much more complex and expensive in circuit arrangement than the open loop transmission circuit, which requires a demodulator. Therefore, from the viewpoint of simpler and cheaper circuit arrangement, it is desirable to be able to realize temperature compensation in an open-loop transmission circuit.

According to one conventional process of realizing temperature compensation in an open-loop transmission circuit, a plurality of compensation values corresponding to a plurality of temperatures are stored in a table, and a predistorter acquires the compensation values from the table in accordance with the operating temperature of a power amplifier (see patent document 3 (japanese laid-open patent publication No. 2001-274851)).

With the arrangement disclosed in the above-mentioned patent document 3, it is necessary to store the temperatures in the table at small intervals to provide an accurate match between the compensation values retrieved from the table and the amount of nonlinear distortion generated in the power amplifier at the actual operating temperature. However, storing temperatures in a table at very small intervals requires that the memory holding the table be larger in circuit scale and therefore more expensive. Therefore, the arrangement disclosed in patent document 3 must store the temperatures at certain compromise intervals in the table. As a result, even with the arrangement disclosed in patent document 3, it is difficult to obtain an exact match between the retrieved compensation value and the amount of nonlinear distortion generated at the actual operating temperature and to provide a sufficient temperature compensation capability.

Disclosure of Invention

It is therefore an object of the present invention to provide a circuit and method for accurately compensating for nonlinear distortion of a power amplifier at the actual operating temperature of the power amplifier without causing unreasonable increases in circuit scale and cost.

It is another object of the present invention to provide a transmission circuit having a nonlinear distortion compensation circuit.

In order to achieve the above object, according to the present invention, there is provided a nonlinear distortion compensation circuit comprising: a power calculator for calculating a power value of the input signal; an operating point setting unit for calculating an apparent (apparent) power value from temperature information supplied from an external source and the power value calculated by the power calculator, the temperature information representing a measured temperature of the power amplifier, based on a relationship, which is provided in advance to the operating point setting unit, between an input/output characteristic of a power amplifier for amplifying the input signal and a temperature of the power amplifier; an inverse characteristic calculator for calculating an inverse component of the nonlinear distortion caused by the power amplifier from inverse characteristic data, which is supplied to the inverse characteristic calculator in advance and is related to the nonlinear distortion caused by the power amplifier, and the apparent power value calculated by the operating point setting unit; and a complex multiplier for superimposing the inverse component calculated by the inverse characteristic calculator on the input signal.

The non-linear distortion compensation circuit according to the present invention is based on the fact that the input/output characteristics of a power amplifier substantially shift with changes in its temperature. The operating point setting unit uniquely calculates an apparent power value Pin' from the power value Pin of the input signal and the temperature information of the power amplifier based on the temperature dependency of the input/output characteristic of the power amplifier. The inverse characteristic calculator calculates an inverse component of the nonlinear distortion caused by the power amplifier, which corresponds to the apparent power value Pin' input from the operating point setting unit, and the complex multiplier superimposes the inverse component calculated by the inverse characteristic calculator on the input signal. The thus superimposed inverse component (compensation amount) coincides precisely with the actual nonlinear distortion caused by the power amplifier.

The transmitting circuit according to the present invention includes: the nonlinear distortion compensation circuit described above; a modulator for modulating a signal output from the nonlinear distortion compensation circuit; a power amplifier for amplifying the modulated signal output from the modulator; and a thermometer for measuring a temperature of the power amplifier and providing temperature information representing the measured temperature to the nonlinear distortion compensation circuit. The transmitting circuit thus configured is capable of performing the function of the nonlinear distortion compensating circuit described above.

According to the invention, the method for compensating for non-linear distortions comprises the steps of (a) calculating a power value of an input signal, (b) measuring the temperature of a power amplifier used to amplify said input signal, (c) based on a predefined relationship, i.e., the relationship between the input/output characteristics of the power amplifier and the temperature of the power amplifier, an apparent power value is calculated from the power value calculated in step (a) and the temperature measured in step (b), (d) an inverse component of the nonlinear distortion caused by the power amplifier is calculated from inverse characteristic data provided in advance, which is related to the nonlinear distortion caused by the power amplifier, and the apparent power value calculated in step (c), and (e) superimposing the inverse component calculated in step (d) onto the input signal. The method is also capable of performing the functions of the nonlinear distortion compensation circuit described above.

According to the present invention, there is no need for a table having different compensation values at various temperatures, as in the table used in the conventional predistorter. Therefore, the nonlinear distortion compensation circuit can reduce the circuit scale and cost.

When the temperature of the power amplifier changes, the inverse component (compensation amount) superimposed on the input signal coincides precisely with the actual nonlinear distortion of the power amplifier. Therefore, the nonlinear distortion can be compensated more accurately than before.

Drawings

The above and other objects, features and advantages of the present invention will become more apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention, in which:

FIG. 1 is a block diagram of a typical open loop transmit circuit;

FIG. 2 is a block diagram of a typical closed loop transmit circuit;

FIG. 3 is a block diagram of an exemplary arrangement of a digital predistorter with temperature compensation in accordance with the present invention;

FIG. 4 is a block diagram of a schematic arrangement of a transmit circuit including the digital predistorter with temperature compensation shown in FIG. 3;

fig. 5 shows how the operating point of a power amplifier changes due to a change in its temperature in the input/output characteristics of the power amplifier; and

fig. 6 is a block diagram of an arrangement of a complex multiplier having the temperature compensation function shown in fig. 3 and the digital predistorter of the modulator shown in fig. 4.

Detailed Description

Embodiments of the present invention will be described below with reference to the accompanying drawings.

Fig. 3 shows in block diagram form a schematic arrangement of a digital predistorter with temperature compensation functionality according to the present invention, while fig. 4 shows in block diagram form a schematic arrangement of a transmit circuit incorporating the digital predistorter with temperature compensation functionality shown in fig. 3.

The transmission circuit shown in fig. 4 will be described first. The transmission circuit is a circuit used in a digital microwave radio communication system, and includes an FIR (finite impulse response) filter 1, a digital predistorter 2 having a temperature compensation function, a modulator 3, a power amplifier 4, and a thermometer 5. It is assumed that the transmit circuit is provided with an input signal in a quadrature amplitude modulation format, such as a multilevel quadrature amplitude modulation format.

The FIR filter 1 has an input terminal to which the baseband digital signals (Ich data) are supplied and another input terminal to which the baseband digital signals (Qch data) are supplied, and limits the frequency band of these baseband digital signals (I/Qch data). The output (I/Qch data) of the FIR filter 1 is supplied to a digital predistorter 2 having a temperature compensation function. The baseband digital signal (Ich data) includes a series of real part values (corresponding to the I axis), and the baseband digital signal (Qch data) includes a series of imaginary part values (corresponding to the Q axis).

The digital predistorter 2 having a temperature compensation function superimposes the inverse component of the nonlinear distortion caused by the power amplifier 4 on the output baseband digital signal (I/Qch data) from the FIR filter 1 to compensate for the nonlinear distortion. Based on the temperature information input from the thermometer 5, the digital predistorter 2 having a temperature compensation function can change the compensation amount in accordance with a change in the input/output characteristics of the power amplifier 4.

The modulator 3 is supplied with a baseband digital signal (I/Qch data) which has been compensated for nonlinear distortion from the digital predistorter 2 having a temperature compensation function, and the modulator 3 outputs a modulation signal (transmission signal) generated by applying quadrature amplitude modulation to the supplied baseband digital signal (I/Qch data). The modulated signal output from the modulator 3 is supplied to the power amplifier 4.

The power amplifier 4 is for amplifying the modulated signal input from the modulator 3, and it can automatically control the level of the input signal so that its average output level is constant. The thermometer 5 measures the operating temperature of the power amplifier 4 and supplies the measured operating temperature as temperature information to the digital predistorter 2 having a temperature compensation function.

With the above-described transmission circuit, the digital predistorter 2 having a temperature compensation function can provide a high degree of nonlinear distortion compensation capability even if the input/output characteristics of the power amplifier 4 are changed due to a change in temperature.

The specific details of the digital predistorter 2 having a temperature compensation function will be described below with reference to fig. 3.

As shown in fig. 3, the digital predistorter 2 having a temperature compensation function includes a power calculation circuit 6, an operating point setting circuit 7, a complex multiplier 8, and an inverse characteristic calculation circuit 9. The power calculation circuit 6 calculates the power P of the baseband digital signal (I/Qch data) input from the FIR filter 1 according to the equation shown below, and supplies the calculated power P to the operating point setting circuit 7.

P＝I²+Q²

The operating point setting circuit 7 has been provided with data on the relationship between the input/output characteristics of the power amplifier 4 and its temperature. The operating point setting circuit 7 calculates an apparent power value P 'from the power value P input from the power calculation circuit 6 and the temperature information from the thermometer 5, and supplies the calculated power value P' to the inverse characteristic calculation circuit 9. The apparent power value P' changes according to the operating temperature of the power amplifier 4.

The inverse characteristic calculating circuit 9 holds inverse characteristic data of the nonlinear distortion of the power amplifier 4. The inverse characteristic calculating circuit 9 calculates the phase rotation and gain of the inverse component corresponding to the nonlinear distortion from the inverse characteristic data and the apparent power value P' input from the operating point setting circuit 7, and supplies their real part and imaginary part amounts to the complex multiplier 8.

The complex multiplier 8 performs complex multiplication on the baseband digital signal (I/Qch data) input from the FIR filter 1 and the inverse component of the nonlinear distortion input from the inverse characteristic calculation circuit 9. Thus, the inverse component of the nonlinear distortion of the power amplifier 4 is superimposed on the baseband digital signal (I/Qch data) input from the FIR filter 1. The complex multiplier 8 provides its output to the modulator 3.

The operation principle of the digital predistorter 2 with a temperature compensation function for compensating nonlinear distortion will be described below.

In general, the input/output characteristics of the power amplifier 4 vary according to its operating temperature. Fig. 5 schematically shows how the operating point of the power amplifier 4 changes due to a change in its temperature. In fig. 5, an input/output characteristic curve a represents the input/output characteristic of the power amplifier 4 at a temperature for which a compensation amount (inverse characteristic) is reserved in the predistorter, and an input/output characteristic curve B represents the input/output characteristic of the power amplifier 4 at an actual temperature (shifted from the input/output characteristic a).

According to the input/output characteristic B, when the input level is Pin, the output level is Pout'. The power amplifier 4 has a function of keeping the output level Pout constant and attenuates the input signal to Pin' to generate the output level Pout. This means that the actual operating point of the power amplifier 4 is the point at which the input level Pin' and the input/output characteristic B intersect each other. In such an operation of the power amplifier 4, when the predistorter superimposes the compensation amount for the input level Pin directly on the input signal of the power amplifier 4, there arises a problem that the compensation amount and the actual amount of nonlinear distortion caused by the power amplifier 4 are different from each other. The digital predistorter 2 having a temperature compensation function according to the present invention solves the above-described problems according to the following compensation processing:

as can be seen from fig. 5, the input/output characteristics of the power amplifier 4 substantially shift with changes in its temperature. When the operating temperature of the power amplifier 4 changes, the operating point setting circuit 7 can uniquely calculate the apparent power value Pin' from the power value Pin of the input signal and the temperature information from the thermometer 5 based on the temperature dependency of the input/output characteristic of the power amplifier 4. The inverse characteristic calculating circuit 9 holds inverse characteristic data of the nonlinear distortion of the power amplifier 4, and it can calculate a phase rotation and a gain, which correspond to the inverse component of the nonlinear distortion caused by the power amplifier 4, from the apparent power value Pin' input from the operating point setting circuit 7.

With the digital predistorter 2 having the temperature compensation function, as described above, the inverse characteristic calculating circuit 9 does not calculate the compensation amount of the power level Pin but calculates the compensation amount of the power level Pin'. The digital predistorter 2 is therefore able to compensate for the non-linear distortion in dependence on the actual operating point of the power amplifier 4. Since the digital predistorter 2 does not need to calculate the compensation amount like the temperature data table used in the conventional predistorter, the increase in the circuit scale of the digital predistorter 2 is small.

With the transmission circuit shown in fig. 4, the digital predistorter 2 having a temperature compensation function compensates for nonlinear distortion of the output of the power amplifier 4 by: the inverse component of the nonlinear distortion caused by the power amplifier 4 is superimposed on the baseband digital signal input to the modulator 3. The signal output from the digital predistorter 2 is supplied to a modulator 3, and the modulator 3 performs quadrature amplitude modulation on the signal. The modulated signal is then provided to a power amplifier 4, which amplifies the modulated signal. Since the inverse characteristic depending on the operating temperature of the power amplifier 4 has been superimposed on the modulation signal, the power amplifier 4 can produce an output signal free from nonlinear distortion.

Details of the complex multiplier 8 and the modulator 3 will be described below with reference to fig. 6. As shown in fig. 6, the complex multiplier 8 includes 4 multipliers 80a to 80d, and 2 adders 81a, 81 b. The inputs of multipliers 80a, 80c are supplied with signal i (t) corresponding to the baseband digital signal (Ich data) from FIR filter 1, respectively, and the inputs of multipliers 80b, 80d are supplied with signal q (t) corresponding to the baseband digital signal (Qch data) from FIR filter 1, respectively. The multipliers 80a, 80d have further input terminals, respectively, to which the real part α of the compensation value depending on the apparent power value is supplied from the inverse characteristic calculation circuit 9, and the multipliers 80b, 80c have further input terminals, respectively, to which the imaginary part β of the compensation value depending on the apparent power value is supplied from the inverse characteristic calculation circuit 9. The adder 81a has an input terminal a ("+" terminal) to which the output from the multiplier 80a is supplied and another input terminal B ("-" terminal) to which the output from the multiplier 80B is supplied, and outputs the sum (a-B) of the supplied inputs to the modulator 3. The adder 81B has an input terminal a to which the output from the multiplier 80c is supplied and another input terminal B to which the output from the multiplier 80d is supplied, and outputs the sum (a + B) of the supplied inputs to the modulator 3.

As shown in fig. 6, the modulator 3 is a quadrature modulator, and includes 2 multipliers 30a, 30b and 1 adder 31. The multiplier 30a has an input terminal to which the output from the adder 81a is supplied and another input terminal to which a carrier signal cos (2 pi ft) is supplied, and multiplies the supplied inputs. The multiplier 30b has an input terminal to which the output from the adder 81b is supplied and another input terminal to which a carrier signal that delays the carrier signal cos (2 pi ft) by pi/2 is supplied, and multiplies the supplied inputs. The adder 31a has an input terminal a ("+" terminal) to which the output from the multiplier 30a is supplied and another input terminal B ("-" terminal) to which the output from the multiplier 30B is supplied, and outputs the sum (a-B) of the supplied signals to the power amplifier 4.

In the above embodiments, the input signal has a quadrature amplitude modulation format, for example a multilevel quadrature amplitude modulation format. However, the present invention is not limited to this modulation format, but is applicable to any modulation type in the case where the circuit according to the present invention superimposes the inverse component of the nonlinear distortion of the power amplifier on the transmission signal.

Although preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that various changes and modifications may be made without departing from the spirit or scope of the appended claims.

Claims (6)

1. A nonlinear distortion compensation circuit comprising:

a power calculator for calculating a power value of the input signal;

an operating point setting unit for calculating an apparent power value from temperature information supplied from an external source and the power value calculated by the power calculator, the temperature information representing a measured temperature of the power amplifier, based on a relationship previously supplied to the operating point setting unit, that is, a relationship between an input/output characteristic of the power amplifier for amplifying the input signal and a temperature of the power amplifier;

an inverse characteristic calculator for calculating an inverse component of nonlinear distortion caused by the power amplifier from inverse characteristic data, which is supplied to the inverse characteristic calculator in advance and is related to the nonlinear distortion caused by the power amplifier, and the apparent power value calculated by the operating point setting unit; and

a complex multiplier for superimposing the inverse component calculated by the inverse characteristic calculator on the input signal.

2. The nonlinear distortion compensation circuit as claimed in claim 1, wherein the input signal comprises a modulation signal generated by quadrature amplitude modulating a first baseband signal comprising a series of real values and a second baseband signal comprising a series of imaginary values, and wherein the power calculator calculates the power value from the first and second baseband signals, the inverse characteristic calculator supplies real and imaginary parts of inverse components of the nonlinear distortion to the complex multiplier, and the complex multiplier performs complex multiplication on the first and second baseband signals and the real and imaginary parts supplied from the inverse characteristic calculator.

3. A transmit circuit, comprising:

the nonlinear distortion compensation circuit as recited in claim 1;

a modulator for modulating a signal output from the nonlinear distortion compensation circuit;

a power amplifier for amplifying the modulated signal output from the modulator; and

a thermometer for measuring a temperature of the power amplifier and providing temperature information indicative of the measured temperature to the nonlinear distortion compensation circuit.

4. The transmission circuit of claim 3, wherein the modulator comprises a modulator for performing a quadrature amplitude modulation format.

5. A method for compensating for nonlinear distortion, comprising the steps of:

(a) calculating a power value of the input signal;

(b) measuring a temperature of a power amplifier for amplifying the input signal;

(c) calculating an apparent power value from the power value calculated in step (a) and the temperature measured in step (b) based on a relationship given in advance between the input/output characteristic of the power amplifier and the temperature of the power amplifier;

(d) calculating an inverse component of the nonlinear distortion caused by the power amplifier from inverse characteristic data given in advance, which is related to the nonlinear distortion caused by the power amplifier, and the apparent power value calculated in step (c); and

(e) superimposing the inverse component calculated in step (d) onto the input signal.

6. The method of claim 5, wherein the input signal comprises a modulated signal generated by quadrature amplitude modulating a first baseband signal comprising a series of real values and a second baseband signal comprising a series of imaginary values, and wherein step (a) comprises the step of calculating the power value from the first and second baseband signals, step (d) comprises the step of calculating real and imaginary parts of the inverse of the nonlinear distortion, and step (e) comprises the step of performing complex multiplication on the first and second baseband signals and the real and imaginary parts calculated in step (d).