Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2002-09-27
2004-04-06
Choe, Henry (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S136000, C375S296000
Reexamination Certificate
active
06717464
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a distortion compensating apparatus, and in particular to a distortion compensating apparatus for adaptively compensating a non-linear characteristic of a circuit having a non-linear distortion.
Recently, a linear modulation method tends to be used for enhancing utilization efficiency of frequency which is a resource in a wireless communication including a mobile terminal. In this linear modulation method, a non-linear characteristic of an input/output power characteristic in a circuit having a non-linear distortion such as a power amplifier, and a low efficiency of power utilization caused thereby are the issues. In order to solve these issues, it is important to compensate the input/output characteristic of the power amplifier to provide a distortion-free linear characteristic.
2. Description of the Related Art
Various methods including a feed-forward type method have been proposed as a method of realizing a power amplifier having a linear characteristic, one of which is an adaptive predistorter type compensating method having a feedback loop.
FIG. 19A
shows an adaptive predistorter type distortion compensating apparatus
110
for compensating a non-linear characteristic of a power amplifier
300
by a feedback loop. This adaptive distortion compensating apparatus
110
is composed of an adaptive distortion compensating algorithm processor
130
and a multiplier
120
.
FIG. 19B
shows an input/output power characteristic of the power amplifier
300
, where a linear region A
1
indicating a proportional characteristic between an input power and an output power, and a non-linear region (portion indicated by a solid line) A
2
in which the output power approaches its saturation are shown.
The adaptive distortion compensating algorithm processor
130
operates an adaptive algorithm in order to minimize an error between a reference signal
500
which is an input signal
500
and a feedback signal
710
which is an output signal
600
of the power amplifier
300
, and calculates a distortion compensating coefficient
720
. The multiplier
120
provides to an input terminal of the power amplifier
300
a signal
510
obtained by multiplying the input signal
500
by the distortion compensating coefficient
720
.
Namely, the distortion compensating apparatus
110
preliminarily multiplies the input signal
500
by the distortion compensating coefficient
720
to be provided to the power amplifier
300
, thereby compensating the non-linear distortion of the power amplifier
300
. As a result, the utilization efficiency of the power amplifier
300
is enhanced.
FIG. 19C
shows an effect of a distortion compensation, where an abscissa indicates a frequency and an ordinate indicates an amplitude (radiant power or voltage). A frequency characteristic of a radiant power before the distortion compensation is shown by a solid line. Radiant powers B
1
and B
2
are generated in bands C
1
and C
3
outside a band C
2
of an input signal. The radiant powers (dashed lines) of the bands C
1
and C
3
after the distortion compensation are reduced to powers B
3
and B
4
.
In order to prevent the effect by the distortion compensation in such a prior art distortion compensating apparatus from being lost, it is necessary to reduce an influence by the frequency characteristic held by an analog circuit except the non-linear distortion. By means of an equalization with a filter having an inverse characteristic of the frequency characteristic of the analog circuit including the power amplifier
300
, the influence can be reduced to a certain degree.
Although an adaptive equalizer is required since the characteristic of the analog circuit substantially varies with temperatures and aging, an adaptive equalizer suitable for the adaptive predistorter type distortion compensating apparatus has not been developed so far.
SUMMARY OF THE INVENTION
It is accordingly an objective of the present invention to provide a distortion compensating apparatus for adaptively compensating a non-linear characteristic of a circuit having a non-linear distortion, whereby a frequency characteristic of the non-linear distortion circuit is adaptively equalized and also a computing amount required for this equalization is reduced.
In order to achieve the above-mentioned objective, a distortion compensating apparatus according to the present invention comprises: an adaptive distortion compensator for controlling an input signal to a circuit with a non-linear distortion by an adaptive algorithm so as to reduce an error between a reference signal and a feedback signal from the non-linear distortion circuit, thereby compensating the non-linear distortion; a digital filter connected between the adaptive distortion compensator and the non-linear distortion circuit, or to a pre-stage of the adaptive distortion compensator; a memory for preliminarily holding a filter coefficient group set for the digital filter; and an adaptive equalizing processor for adaptively selecting, from among the filter coefficient group set, a filter coefficient which makes an out-of-band power of the feedback signal reduced (to be set in the digital filter). (claim
1
)
Namely, an adaptive distortion compensator controls an input signal of a non-linear distortion power amplifier by an adaptive algorithm so as to reduce an error between a reference signal and a feedback signal from a non-linear distortion circuit (analog circuit), e.g. a non-linear power amplifier (hereinafter, a non-linear power amplifier is occasionally described as an example of a non-linear distortion circuit; also, a non-linear power amplifier represents one including an analog circuit such as a peripheral filter and a mixer), thereby compensating the non-linear distortion of the non-linear power amplifier.
An adaptive distortion compensator which has been proposed fits for this adaptive distortion compensator, and e.g. an adaptive predistorter type distortion compensating apparatus can be used for it.
A digital filter is connected either between the adaptive distortion compensator and the power amplifier in cascade, or to a pre-stage of the adaptive distortion compensator.
An adaptive equalizing processor adaptively selects, from among a filter coefficient group held in a memory, a filter coefficient which makes an out-of-band (radiant) power of the feedback signal reduced (to be set in a filter).
Thus, it becomes possible to adaptively equalize the frequency characteristic of the power amplifier. Also, by using the filter coefficient preliminarily held in the memory, the computing amount can be reduced.
Also, in the present invention according to the above-mentioned invention, the adaptive equalizing processor may comprise a Fourier transform operation processor for performing a Fourier transform to the feedback signal to output an amplitude spectrum of the feedback signal, and an adaptive equalizing algorithm processor for selecting, from among the filter coefficient group, a filter coefficient which makes the out-of-band power of the feedback signal reduced by the adaptive algorithm based on the amplitude spectrum (to be provided to the digital filter). (claim
2
)
Namely, a Fourier transform operation processor, e.g. a Fast Fourier Transform (hereinafter, occasionally abbreviated as FFT or FFT transform) operation processor performs a fast Fourier transform to the feedback signal to output an amplitude spectrum (hereinafter, occasionally referred to as amplitude characteristic) of the feedback signal.
An adaptive equalizing algorithm processor selects, from among the filter coefficient group, a filter coefficient which makes the out-of-band power of the feedback signal reduced by the adaptive algorithm based on the amplitude spectrum (to be provided to the digital filter).
Thus, it is possible to adaptively equalize the frequency characteristic of the analog circuit by the digital signal processing.
Also, in the present invention according to the above-mentioned invention, the adaptive equalizing pr
Fudaba Nobukazu
Hamada Hajime
Ishikawa Hiroyoshi
Kubo Tokuro
Nagatani Kazuo
Choe Henry
Fujitsu Limited
Katten Muchin Zavis & Rosenman
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