Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2001-11-09
2003-06-10
Mottola, Steven J. (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S002000
Reexamination Certificate
active
06577192
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a predistortion-type distortion compensation amplifying apparatus suitable for use in a high-frequency circuit for mobile communication.
BACKGROUND ART
In mobile communication, a digital modulation system is used to increase the efficiency of frequency utilization, etc. In such case, distortion caused by non-linearity of characteristics of a power amplifier disturbs the adjacent channel, which is a problem. In order to prevent disturbance to the adjacent channel, a power amplifier having a low adjacent channel power (ACP: Adjacent Channel Power) is required. However, a use of the power amplifier within a linear operation region is not always advisable in view of the circuit scale and cost. Instead, a predistortion (hereinafter also referred as distortion compensation) is used in many cases.
Predistortion is a method of beforehand distorting an input signal to be amplified using a function representing a reverse characteristic of an input-output characteristic of the amplifier (power amplifier: hereinafter, the power amplifier being simply referred as an amplifier occasionally) when the signal is inputted to the amplifier. Namely, predistortion is a technique of beforehand distorting an input signal to be amplified and amplifying it, whereby a signal which has been linearized appears at the output of the amplifier.
FIG. 18
is a diagram showing an example of the radio transceiver using predistortion. In a radio transceiver
50
shown in
FIG. 18
, a baseband signal to be transmitted is modulated and predistorted in a digital signal processor (DSP: Digital Signal Processor)
50
a,
where a distortion compensation coefficient operating process is performed to estimate non-linear distortion of a power amplifier
50
c.
In a quadrature modulating-demodulating unit
50
b,
the processed baseband signal is up-converted into the RF (Radio Frequency) band. In the power amplifier
50
c,
a predetermined power is applied to the signal, and the signal is fed to an antenna
50
e
via a synthesizer
50
d
and transmitted.
On the other hand, the modulated signal fed back from a part of the signal outputted from the power amplifier
50
c
is down-converted into a baseband signal having a distorted component in the quadrature modulating-demodulating unit
50
b.
The converted signal is inputted to the digital signal processor
50
a,
where a distortion compensation coefficient operating process is performed. Accordingly, an RF signal free of distortion is outputted from the antenna
50
e
by this loop process. In the structure shown in
FIG. 18
, predistortion is performed in the baseband. Signal processing in the baseband will be now described with reference to
FIG. 19
, corresponding to equations.
FIG. 19
is a diagram showing an example of a known predistortion circuit (also referred as a predistorter). A quadrature modulator
60
shown in
FIG. 19
performs the distortion compensation coefficient operating process, which comprises memories
61
and
62
, multipliers
63
a,
63
b,
63
c
and
64
d,
and adders
64
a
and
64
b.
Baseband signals I and Q are undergone the distortion compensation operating process by referring to the respective memories
61
and
62
(memory regions denoted by reference characters
61
a,
61
b,
61
c
and
61
d,
and memory regions denoted by reference characters
62
a,
62
b,
62
c
and
62
d
) in a software process or the like. The signals processed in these memories
61
and
62
are multiplied in the multipliers
63
a
-
63
d,
added in the adders
64
a
and
64
b,
and I
pd
and Q
pd
are outputted.
When an output of an amplifier is P
o
(t), the output is expressed by a product of a function f(t) of amplitude and a function g(t) of phase as shown in Equation (1):
P
o
(
t
)
=f{M
i
(
t
)}·exp[(−
j·g{M
i
(
t
)})·exp(&ohgr;
t
) (1)
where, M
i
(t) is magnitude of amplitude of a modulated wave, &ohgr; is a center frequency, t is a time, and j is an imaginary unit which represents j
2
=−1.
When input signals to the quadrature modulator
60
are I(t) and Q(t), magnitude x(t) of amplitude of a modulated wave inputted to the amplifier are expressed by Equation (2) (operation in the memory regions
61
a
and
62
a
shown in FIG.
19
):
X
(
t
)={square root over ((
I
(
t
)
2
+Q
(
t
)
2
))} (2)
Magnitude y(t) of amplitude of a modulated wave component outputted from the amplifier is expressed, with G being a gain, by Equation (3):
y
(
t
)=
G·x
(
t
) (3)
FIG. 12
is a diagram showing an example of the input-output characteristic of the amplifier. A part denoted by B1 in
FIG. 12
is a non-saturation region, whereas a part denoted by B2 is a saturation region. The input-output characteristic changes its characteristic at a point denoted by A, which is a function having an upper limit because of the saturation characteristic of the amplifier. In order to compensate the distortion of an output signal amplified as expressed by Equation (3), distortion compensation using an inverse function f
−1
(t) of the function f(t) of amplitude within the quadrature modulator
60
is performed. An output P
pd
(t) of the amplifier undergone the distortion compensation is expressed by Equation (4):
P
pd
(
t
)=
f
−1
(
y
)·exp[
j·g{f
−1
(
y
)}]·exp(&ohgr;
t
) (4)
Namely, I
pd
(t) and Q
pd
(t) which are deformed I(t) and Q(t) (hereinafter abbreviated as I
pd
and Q
pd
, respectively) are expressed by Equations (5) and (6), respectively:
I
pd
={f
−1
(
y
)/
x}·[I
cos [
g{f
−1
(
y
)}]−
Q
sin [
g{f
−1
(
y
)}]] (5)
Q
pd
={f
−1
(
y
)/
x}·[Q
cos [
g{f
−1
(
y
)}]−
I
sin [
g{f
−
(
y
)}]] (6)
where x(t) and y(t) are abbreviated as x and y. Generally, input-output relationships of the Equations (5) and (6) are stored in the memories
61
and
62
as a reverse characteristic of the characteristic as shown in FIG.
12
. Values of I(t) and Q(t) are, for example, very frequently referred at sampling time intervals for digital signals, and the outputs I
pd
and Q
pd
are obtained.
Namely, y is determined from data in the memory region
61
b
shown in
FIG. 19
, then f
−1
(y)/x in the Equations (5) and (6) is calculated from y and data in the memory region
61
c.
Further, {f
−1
(y)/x}•I in the Equation (5) and {f
−1
(y)/x}•Q in the Equation (6) are calculated from data in the memory region
62
b,
and outputted. Similarly, y is determined form data in the memory region
62
b,
then g{f
−1
(y)} in the Equations (5) and (6) is calculated from y and data in the memory region
62
c.
Further, cos [g{f
−1
(y)}] and sin [g{f
−1
(y)}] in the Equations (5) and (6) are calculated from data in the memory region
62
d,
and outputted. These outputs are added in the multipliers
63
a
-
63
d,
after which, added in the adders
64
a
and
64
b,
and deformed I
pd
and Q
pd
are outputted. As this, distortion compensation is performed using a predistortion circuit as the quadrature modulator
60
to make the amplification characteristic of the amplifier linear, in general.
Incidentally, as a method for compensating a frequency characteristic in the vicinity of higher harmonic in bias circuits of the amplifier and the power circuit (not shown), and a matching circuit, there has been proposed a method of determining a coefficient from a differential value or an integral value of an amplitude quantity of an input signal, and multiplying the original signal by the coefficient to obtain a predistortion signal.
FIG. 20
is a diagram showing an example of a predistortion circuit using a differential value or an integral value of an input signal. In a predistortion circuit
70
shown in
FIG. 20
, m
Hasegawa Tsuyoshi
Kawasaki Yoshihiro
Maniwa Toru
Fujitsu Limited
Katten Muchin Zavis & Rosenman
Mottola Steven J.
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