Feedforward amplifier and method of improving the performance thereof

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FEEDFORWARD AMPLIFIER AND METHOD
OF IMPROVING THE PERFORMANCE
THEREOF

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a feedforward amplifier, which is one of the leading devices of linearization systems 10 for mobile communications base stations, and a method of improving the performance thereof, and more particularly, to a feedforward amplifier using imperfect cancellation of a main signal and a method of improving the performance thereof, in which by including a predetermined amount of a ^ main signal in an error signal that is input to an error signal cancellation loop of the feedforward amplifier, more error components of a final output signal are removed such that the linearity and efficiency of the final output signal improve. 2o

2. Description of the Related Art

In long distance telecommunications such as satellite communications and mobile communications, a signal cannot be delivered in one direct transmission. Hence, relay stations or base stations are located at predetermined inter- 25 vals so that a signal can be amplified and transmitted again. A power amplifier used in the relay stations or base stations for this operates in a saturation region having a non-linear characteristic in order to obtain a maximum output. However, in this case, when multiple carriers (multi- 30 carriers) are input to the power amplifier, these multiple carriers cause intermodulation distortion (IMD) such that the performance of the amplifier is greatly degraded. The intermodulation distortion component is generated by interactions between two carrier waves when two or more carriers 35 are provided and pass a non-linear device.

It has been known that removal of these intermodulation distortion components with existing filters is very difficult and these components also work as a kind of noise greatly affecting the quality of calls. In order to reduce the non- 40 linear characteristic of the power amplifier, a variety of methods have been suggested and used. One of the leading methods is a linearization method using feedforward. Feedforward is a method by which the characteristics of a system improve by providing a part of input to an output end to 45 modify the output result, and linearization is to convert a device or a signal having a nonlinear characteristic to one having a linear characteristic.

Meanwhile, a linear amplifier is a kind of amplifier for 5Q providing power to a load. Linear amplifiers are classified as low frequency power amplifiers and high frequency power amplifiers. In a power amplifier, it is important to provide power efficiently with less distortion.

In ordinary feedforward linearization, only the intermodu- 55 lation distortion component is extracted from a signal intermodulation-distorted with a carrier wave obtained from the output of a power amplifier, and the extracted intermodulation distortion component is again coupled, with the phase of the component inversed, to the output of the 60 amplifier. By doing so, the intermodulation distortion signal is canceled and so that a carrier to intermodulation ratio, that is, the ratio of the power of the carrier to the power of the intermodulation distortion signal, improves.

FIG. 1 is a schematic diagram of the structure of an 65 ordinary feedforward amplifier. Referring to FIG. 1, the feedforward amplifier basically includes two cancellation

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loops 10 and 20. The cancellation loop 10 is a main signal cancellation loop and the cancellation loop 20 is an error signal cancellation loop. Each loop 10 and 20 comprises a power amplifier 12 and 22, a powercoupler 14 and 24, a vector modulator 16 and 26, and a delay line 18 and 28.

The main signal cancellation loop 10 of FIG. 1 extracts only an error component of the main amplifier 12, using the difference of the input signal of the entire system and a non-linear signal amplified by the main amplifier 12. The error signal cancellation loop 20 amplifies the error component, which is provided from the main signal cancellation loop 10, through an error amplifier 22, inverts the phase of the error component, and couples the result with the output signal of the main amplifier 12 so that only the error component of the output signal of the main amplifier 12 is removed.

As described above, after being amplified by the main amplifier 12 through the two loops 10 and 20, the non-linear signal becomes linearized. It has been known that this feedforward amplifier 1 has good performance in linearity improvement, covers a wide band at the same time, and has good stability.

The prior art feedforward amplifier 1 as shown in FIG. 1 usually uses an error signal with a perfectly canceled main signal, which is schematically shown at the top of FIG. 3. As shown at the top of FIG. 3, a control method for generating an error signal with a perfectly canceled main signal is realized by the main signal cancellation loop 10 and the error signal cancellation loop 20 detecting and controlling different signals, respectively, as shown in FIG. 2. That is, in the adaptive control method of the prior art feedforward amplifier 1, the detection unit 1 detects an error signal, the control unit 1 controls the main signal cancellation loop 10, the detection unit 2 detects the error component of the final output signal, and the control unit 2 controls the error signal cancellation loop 20, as shown in FIG. 2. Here, the main signal cancellation loop 10 is adaptively controlled so that the component of the main signal in the detected error signal is minimized. In this way, the main signal in the error signal is perfectly canceled.

However, since most systems use a modulated signal as an input signal, an error signal with an imperfectly canceled main signal, instead of an error signal with a perfectly canceled main signal, can show optimal performance.

SUMMARY OF THE INVENTION

To solve the above problems, it is an object of the present invention to provide a feedforward amplifier and a method of improving the performance thereof, in which a predetermined amount of a main signal is made to be included in an error signal input to the error signal cancellation loop of a feedforward amplifier, considering that in the feedforward amplifier, an error signal with an imperfectly canceled main signal, instead of an error signal with a perfectly canceled main signal, can show optimal performance.

That is, it is an object of the present invention to provide a feedforward amplifier using imperfect cancellation of a main signal and a method of improving the performance thereof, in which by including a predetermined amount of a main signal in an error signal that is input to an error signal cancellation loop of the feedforward amplifier, more error components of a final output signal are removed such that the linearity and efficiency of the final output signal improve.

According to an aspect of the present invention, there is provided a feedforward amplifier comprising a main signal 3

cancellation loop which comprises a main amplifier and a vector modulator and extracts an error component of the main amplifier; an error signal cancellation loop which comprises an error amplifier and a vector modulator and removes the error component in the output signal of the main 5 amplifier; a final error signal detection unit which detects an error component of a final output signal of the feedforward amplifier; a main signal cancellation loop control unit which controls the main signal cancellation loop based on the error component detected by the final error signal detection unit; and an error signal cancellation loop control unit which controls the error signal cancellation loop based on the error component detected by the final error signal detection unit, wherein the error signal generated by the main signal cancellation loop is optimized in the direction linearizing the final output signal and the main signal cancellation loop is adaptively controlled by the main signal cancellation loop control unit.

It is preferable that the error signal output from the main signal cancellation loop has an imperfectly canceled main 20 signal.

It is preferable that the output signal of the main signal cancellation loop control unit is input to the vector modulator of the main signal cancellation loop, and the output signal of the error signal cancellation loop control unit is 25 input to the vector modulator of the error signal cancellation loop.

According to another aspect of the present invention, there is provided a method for improving the performance of a feedforward amplifier comprising a main signal cancella- 30 tion loop and an error signal cancellation loop, the method comprising detecting the error component of a final output signal of the feedforward amplifier; and controlling the main signal cancellation loop and the error signal cancellation loop at the same time using the detected error component. 35

It is preferable that the method further comprises, before controlling the loops, detecting the error signal of the main signal cancellation loop and using the error signal, controlling the main signal cancellation loop; and detecting the error component of a final output signal of the feedforward 40 amplifier and controlling the error signal cancellation loop.

BRIEF DESCRIPTION OF THE DRAWINGS

The above objects and advantages of the present invention will become more apparent by describing in detail preferred 45 embodiments thereof with reference to the attached drawings in which:

FIGS. 1 and 2 are schematic diagrams of the structures of feedforward amplifiers according to the prior art;

FIG. 3 is a conceptual diagram of a method of improving 50 the performance of a feedforward amplifier according to the present invention;

FIG. 4 is a diagram of the structure of a feedforward amplifier according to the present invention;

FIG. 5 is a flowchart of the steps performed in the method 55 of improving performance of a feedforward amplifier according to the present invention; and

FIGS. 6a and 6b are spectrum graphs comparing the feedforward amplifier according to the present invention with the prior art feedforward amplifier, and more 60 specifically, FIG. 6a is a graph of an error signal spectrum and FIG. 6b is a graph of a final output signal spectrum.

DESCRIPTION OF THE PREFERRED

EMBODIMENTS 65

FIG. 3 is a conceptual diagram of a method for improving the performance of a feedforward amplifier according to the

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present invention, and FIG. 4 is a diagram of the structure of a feedforward amplifier according to the present invention. FIG. 5 is a flowchart of the steps performed in the method of improving the performance of a feedforward amplifier according to the present invention, and FIGS. 6a and 6b are spectrum graphs comparing the feedforward amplifier according to the present invention with the prior art feedforward amplifier.

Referring to FIG. 4, the feedforward amplifier 100 according to the present invention comprises a main signal cancellation loop 110 which comprises a main amplifier 112 and a vector modulator 116 and extracts an error component of the main amplifier 112; an error signal cancellation loop 120 which comprises an error amplifier 122 and a vector modulator 126 and removes an error component in the output signal of the main amplifier 112; a final error signal detection unit 152 which detects the error component of a final output signal of the feedforward amplifier 100; a main signal cancellation loop control unit 161 which controls the main signal cancellation loop 110 based on the error component detected by the final error signal detection unit 152; and an error signal cancellation loop control unit 162 which controls the error signal cancellation loop 120 based on the error component detected by the final error signal detection unit 152. Here, the main signal cancellation loop 110 is adaptively controlled by the main signal cancellation loop control unit 161 so that the error signal generated by the main signal cancellation loop 110 can be optimized in the direction linearizing the final output signal. Also, in the feedforward amplifier 100 of the present invention, the error signal output from the main signal cancellation loop 110 has an imperfectly canceled main signal, as shown at the bottom of FIG. 3.

Meanwhile, in the feedforward amplifier of the present invention, the output signal of the main signal cancellation loop control unit 161 is input to the vector modulator 116 of the main signal cancellation loop 110 and the output signal of the error signal cancellation loop control unit 162 is input to the vector modulator 126 of the error signal cancellation loop 120.

Reference numbers 118 and 128 of FIG. 4 indicate delay compensation lines that perform the same functions as the prior art delay compensation lines 18 and 28.

Referring to FIG. 5, the method of improving the performance of a feedforward amplifier according to the present invention comprises step S200 in which the error component of the final output signal of the feedforward amplifier 100 is detected and, using the detected error component, the main signal cancellation loop 110 and the error signal cancellation loop 120 are controlled at the same time. Here, the simultaneous control step S200 comprises step S22 for detecting the error component of the final output signal and controlling the error signal cancellation loop 120, step S24 for determining whether or not convergence of the step S22 is accomplished, step S32 for controlling the main signal cancellation loop using the error component of the final output signal, and step S34 for determining whether or not convergence of the step S32 is accomplished.

Meanwhile, the method of improving the performance of a feedforward amplifier of the present invention comprises a preceding step S100 which is performed preceding the simultaneous control step S200 that is the major characteristic of the method of the present invention. The preceding step S100 comprises step S12 for detecting the error signal of the main signal cancellation loop 110 and controlling the main signal cancellation loop 110, step S14 for determining