Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2001-05-22
2002-12-03
Pascal, Robert (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C375S297000, C330S107000, C330S136000
Reexamination Certificate
active
06489846
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a distortion compensating device, and particularly to a distortion compensating device applicable to a high-frequency power amplifier.
2. Description of the Related Art
With an increasing speedup and capacity in telecommunication in recent years, there is a demand for a stringent linearity in a transmission high-frequency power amplifier in the field of digital radio communication equipment. At the same time, this trend also hinders improvement in power efficiency in the high-frequency power amplifier. On the other hand, digital mobile phones are already wide spread in the market, and there is an ever increasing demand for a longer operation time available for a continuous telephone call on these phones. When a new digital wireless communication device is introduced into the market, the length of usable time is becoming an un-negligible factor in view of product competition. Given such a background as the above, there is an increasing trend for improving the efficiency by means of distortion compensation technology represented by such a device as a predistorter. However, introduction of such a technology as the above will require an extremely large sized circuit, making difficult adaptation thereof to the mobile phone that is sold on advantages of light weight and compactness.
In general, the high-frequency power amplifier for use in the telecommunication has a problem of increased non-linearity in an input-output characteristic that results in distorted output signal if efficiency in power utility is to be improved. In an attempt to solve this problem, a predistorter has been used. Specifically, a compensating device having a reverse characteristic to the characteristic of the high-frequency power amplifier is connected to a prestage of the high-frequency power amplifier, in order to cancel the distortion.
Further, in general, the nonlinear characteristic of an amplifier changes over time, temperature and so on. For this reason, a compensating characteristic of the compensator must be changed in accordance with the characteristic change in the amplifier. This is called adaptive compensation.
An example of such a predistorter is “MEMORYLESS POLYNOMIAL ADAPTIVE PREDISTORSION” IEEE. Int. conf. Acoust speech process 1995. No5. pp981-985.
FIG. 12
is a system diagram of a principal portion of a distortion compensation circuit shown in the above document.
FIG. 13
is a detailed system diagram of a portion in
FIG. 12
directly related to the compensation.
Referring now to
FIG. 12
, digital modulation yields a couple of base-band signals, i.e. an orthogonal component Q and an in-phase component I (hereinafter called I signal and Q signal). These signals are fed to input terminals T
1
and T
2
, then receive pulse shaping performed by pulse filters
1
a
and
1
b
, and then inputted to a distortion compensator
2
.
The distortion compensator
2
shown in detail in
FIG. 13
includes at least a voltage amplitude calculator
2
a
and a phase calculator
2
b
, and outputs the I″ signal and Q″ signal for compensation of distortion in amplitude and phase of a signal generated by a high-frequency power amplifier (hereinafter called HPA)
5
. The I″ and Q″ signals receive analog conversion in digital-analog converters (hereinafter called DAC)
3
a
,
3
b
respectively, and then are fed to accumulators
4
a
,
4
b
in a quadrature modulator
4
, where a carrier cos &ohgr; t and sin &ohgr; t from an oscillator are multiplied. An output as a result of the accumulations is applied via an adder
4
c
to the amplitude characteristic and the phase characteristics, to compensate for nonlinear input-output characteristic of the HPA
5
. Then an output from the HPA
5
is supplied to an output terminal T
3
and a restoring calculator
6
.
A base band signal demodulated by the quadrature demodulator
6
is inputted to the distortion compensator
2
via an analog-digital converter (hereinafter, referred to ADC)
7
.
The distortion compensator
2
shown in
FIG. 12
is constituted as shown in FIG.
13
. Description will now cover
FIG. 13
briefly. The two base-band signals I, Q generated by the digital modulation are fed to a voltage amplitude calculator
2
a
for calculation of a voltage amplitude (I
2
+Q
2
). A phase difference calculator
2
b
calculates a phase difference &phgr;+arctan (I/Q) between the I and Q signals. The voltage amplitude value is inputted to an amplitude compensator
2
d
for conversion into a signal that compensates for the amplitude distortion. An I-Q restoring calculator
2
e
calculates, from the phase difference &phgr; sent from the phase difference calculator
2
b
and a compensating voltage amplitude v′ sent from the amplitude compensator
2
d
, a signal I′=v′ cos &phgr; and signal Q′=v′ sin &phgr;. A shift synthesizing calculator
2
f
calculates a compensating phase value &thgr; that corresponds to the compensating voltage amplitude v′. By using this phase value &thgr; and the signals I′ and Q′, calculation of I″=I′ cos &thgr;−Q′·sin &thgr; and Q″=Q′·cos &thgr;+I′·sin &thgr; are made. Outputs from these calculations are fed to DAC's
3
a
,
3
b
in FIG.
12
.
According to the prior art distorter described above, a higher degree polynomial expression is used for the calculation of the compensating amplitude voltage v′ for the compensation of the amplitude compensator
2
d
and for determining &thgr; necessary for the phase compensation.
This means an increase in the number of variables that must be used for obtaining the compensating value for the compensator
2
. The increase in the number of the variables leads to an increase in the size of circuit, eventually leading to an increase in power consumption in actual utilization of the distortion compensator
2
. Further, if the amplitude compensating function is a multinomial, then a solution cannot be obtained as a mathematically exact solution. In other words, the obtained value is nothing more than an accidental discovery of the compensating function. With such a method as this, there is definitely a limitation to the effect of the compensation.
In “ADAPTIVE DIGITAL PREDISTORTER FOR POWER AMPLIFIERS WITH REAL TIME MODELING OF MEMORYLESS COMPLEX GAINS” by Ernesto G. Jeckeln el. al, IEEE. MTT-S digest 1996, pp835-838 it is disclosed that a broken-line approximation is used in the calculation of v′ for performing the amplitude distortion compensation, and calculation of &thgr; necessary for the phase distortion compensation.
In such a case as this, in which broken-line approximation is used as the method for the compensation calculation and a broken-line approximation equation is used as a compensating function, the compensation does not truthfully reflect the actual characteristics of the amplifier, and so there is a limitation to the effect of the compensation.
The prior art distortion compensation is an adaptive compensation based on the base-band signal (in other words, the compensation in a predetermined digital computing portion) For this reason, a quadrature demodulator is essential. Further, the need for the quadrature demodulator means a significant increase in the size of circuit, leading to a problem of increased power consumption in the entire compensator.
Further, although the significance of the distortion compensation is to improve on the overall efficiency of the power amplifier by reducing the distortion, such an increase in power consumption as described above rather creates a problem of decreased efficiency. Further, the increase in the size of circuit goes against the fundamental advantage of the light weight and compactness of the mobile phone in terms of the constitution.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a distortion compensation device in which coefficients necessary for computing a compensation po
Maioli Jay H.
Nguyen Khanh Van
Pascal Robert
Sony Corporation
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