Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1999-11-10
2004-04-20
Chin, Stephen (Department: 2634)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S296000
Reexamination Certificate
active
06724831
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a technology for linearizing the input/output characteristic of a power amplifier.
2. Description of the Related Art
In mobile radio communications, since a signal in an unused band interferes with an adjacent channel, it is necessary to secure the linearity of a power amplifier in order to suppress the signal power in an unused band to a lower level. Although conventionally an output back-off method for suppressing an output power in such a way that out-of-band power becomes sufficiently low is used for this purpose, it is difficult to improve its linearity while securing the efficiency since this method has a trade-off between the efficiency and the linearity.
FIG. 1
shows the input/output characteristic of a typical power amplifier.
The output back-off method is described with reference to FIG.
1
.
As the output of a power amplifier becomes high, as shown in (1) of
FIG. 1
, the relationship between the input and output of the power amplifier deviates from a linearity relationship, as shown in (2) of FIG.
1
. Therefore, in the output back-off method, only the low input part of the characteristic curve (1) of an actual power amplifier in which the linearity is secured is used. Since as shown in
FIG. 1
, the low input part of the characteristic curve of a power amplifier yields an almost ideal characteristic curve (2), sufficient linearity can be obtained from the operation of a power amplifier, and the occurrence of an RF element due to the degradation of the waveform of a signal to be amplified, specifically the occurrence of noise in an adjacent channel can be suppressed to a low level.
However, since if the input of a power amplifier is low, the amplification efficiency of the power amplifier becomes low and the power amplifier cannot be used efficiently enough, which is a problem. In order to improve the amplification efficiency, the input must be increased, which leads to non-linearity in the power amplifier. Thus, since the output back-off method has a trade-off between the efficiency and the linearity, it is difficult for the high efficiency and linearity to exist together.
As a method for improving the linearity while securing the amplification efficiency of a power amplifier, a linearization by a pre-distortion method is attempted. For detailed information about this method, please see the specification of Japanese Patent Application 9-297297 previously applied for by this applicant.
FIGS. 2A and 2B
are drawings showing a pre-distortion method.
As shown in
FIG. 2A
, if a pre-distortion is performed for the input/output characteristic of a power amplifier (PA), a baseband I signal and a baseband Q signal are inputted to a baseband variable attenuator
180
, the power of baseband signals are adjusted, and the baseband signals are inputted to a pre-distortion circuit
181
. In the pre-distortion circuit
181
, the low input part of an inputted signal in which the linearity shown in the curve (1) of
FIG. 1
is pre-processed using a function adjusted in such a way that the inclination of the characteristic curve can be maintained, based on the inverse function of the input/output characteristic of a power amplifier
183
as shown in the curve of FIG.
1
. Specifically, no special treatment is made for the inputted baseband signals while the input value of the input baseband signals are within the range where the input/output characteristic of the power amplifier
183
shows linearity. However, if the input/output characteristic is out of the range where the input/output characteristic of a power amplifier
183
shows non-linearity, the power value of the inputted baseband signals are increased, and the degradation of the signal waveform due to the increase of the non-linearity of the power amplifier
183
is cancelled. The baseband signals pre-distorted by the pre-distortion circuit
181
are inputted to a modulator (in
FIG. 1
, a WPSK modulator
182
) and QPSK-modulated. By QPSK modulation, the pre-distorted signals are converted to RF signals, are amplified by the power amplifier
183
, and are transmitted from an antenna
184
. It is because a pre-distortion is performed by a digital signal process that the pre-distortion circuit
181
is installed immediately before the QPSK modulator. Specifically, if the pre-distortion circuit
181
is installed immediately after the QPSK modulator, signals are converted to RF signals and it becomes difficult to pre-distort the signals by a digital process.
If the pre-distortion method is realized using a digital circuit and if a wide power range is covered only by a digital signal process with a configuration as shown in
FIG. 2A
, the quantization error increases at a low power, and an adjacent channel leak power (ACP) characteristic significantly degrades. Specifically, the values of a function for applying a pre-distortion function, such as F
pd
(x) shown in
FIG. 4
are discretized, and if those values are stored in a table and a value corresponding to an inputted signal value is read from the table, the discretization against the inclination of the function of a signal corresponding to the low signal input part of the function with a gentle inclination becomes too steep, and a pre-distortion with a large error is applied against an input signal value changing like as analog signal.
FIG. 2B
shows an adjacent channel leak power (ACP) against the output of the power amplifier
183
. In
FIG. 2B
, the vertical axis and horizontal axis are ACP (unit: dB) and the output of the power amplifier
183
(unit: dBm), respectively.
If a pre-distortion is not performed, as indicated by 0 and + in
FIG. 2B
, it has been found that as the output power P
out
of the power amplifier increases, the adjacent channel leak power (ACP) increases. However, ACPs in the case where a pre-distortion is performed are indicated using a variety of lines (a dotted line, a one-point chained line, etc.,). Each line shown in
FIG. 2B
indicates with how many bits a pre-distortion function is digitized (discretized), specifically, cases of 7 bits, 8 bits, 9 bits, 10 bits, 11 bits and 12 bits are indicated from the top. As shown in
FIG. 2B
, as the number of bits to be used when a pre-distortion function is digitized increases, the ACP is further improved.
Thus, although increasing the number of input/output bits to be used when a pre-distortion function is discretized, is considered in order to improve an ACP, in that case, the circuit scale becomes large.
Although in the configuration shown in
FIG. 2B
, the attenuation characteristic of a variable attenuator (VATT)
180
is not taken into consideration, the attenuation of the variable attenuator further degrades the ACP, which is another problem.
FIGS. 3A and 3B
show the configuration and characteristic in the case where compensation is made by pre-distortion and where the attenuation characteristic of a variable attenuator is also taken into consideration.
According to the configuration shown in
FIG. 3A
, a pre-distortion is performed for baseband I and baseband Q signals in a pre-distortion circuit
190
, and the signals are modulated in a QPSK modulator
191
. Then, the signal power of the signals is attenuated in a variable attenuator
192
, is amplified in a power amplifier PA
193
and the RF signals are transmitted from an antenna
184
. At this time, an attenuation-degree control signal, VATT control provided to the variable attenuation
180
is inputted to the pre-distortion circuit
190
, and a pre-distortion is applied taking into consideration the attenuation characteristic of the variable attenuator
192
.
FIG. 3B
shows the relationship between an output value to a PA
193
and an ACP in the configuration shown in FIG.
3
A.
The meaning of each curve shown in
FIG. 3B
is the same as that shown in FIG.
2
B. As is clearly seen from
FIG. 3B
, if a pre-distortion is applied, an ACP characteristic is further improved than when the pre-distortion not applied. The greater the number of bits to b
Hasegawa Tsuyoshi
Maniwa Toru
Chin Stephen
Fujitsu Limited
Katten Muchin Zavis & Rosenman
Odom Curtis
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