Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2002-04-04
2003-12-02
Choe, Henry (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C375S297000, C455S063300
Reexamination Certificate
active
06657493
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a distortion compensation apparatus and a distortion compensation method that compensate distortion of a power amplifier amplifying a transmission signal of a radio base station or the like by controlling an adjacent channel leakage power ratio to be small.
2. Description of the Related Art
In mobile communications systems to which W-CDMA (Wideband Code Division Multiple Access), PDC (Personal Digital Cellular) or the like is applied, it is necessary to control transmission power of a radio base station in a scope from approximately 10 mW to approximately 20 W, for example. As such a control means, transmission power control (TPC) such as an inner loop method, an open loop method, a closed loop method or the like is performed so as to control the power amplifier to obtain a desired transmission signal.
It is desirable to use the power amplifier that amplifies the transmitting signal in a linear region so as to make an amplification distortion small. However, in this case, power added efficiency is lowered to several percent and consumption of transmission power becomes large. The power added efficiency represents a ratio of a difference between the input power and output power to the input power. For example, in
FIG. 1
, an example of relationship among the output power [dBm], the power added efficiency [%] and the input power [dBm] is shown. A horizontal axis represents the input power [dBm], a left vertical axis represents the output power [dBm] and a right vertical axis represents the power added efficiency [%]. In
FIG. 1
, tendency of relationship between the output power and the power added efficiency is shown. That is, it is realized that the power added efficiency is very low when using only the linear region of the output power characteristics. Thus, a means has been employed in which the power added efficiency is improved by enabling the power amplifier to be used in a nonlinear region.
The amplification distortion becomes large when simply operating the power amplifier in the nonlinear region, causing leakage power to an adjacent channel to be large. Accordingly, a problem occurs in that an adjacent channel is interfered with. Thus, using a power amplifier having characteristics of wide linear region may be conceived. However, it is necessary to prepare the power amplifier having more capacity than required, thus an economical problem arises. Therefore, a structure using a linealizer (distortion compensation apparatus) compensating the distortion of the power amplifier has been in practical use.
For example, as shown in
FIG. 2
, when the distortion compensation is not performed, the transmission power characteristic is illustrated by a continuous curving line, and the leakage power to an adjacent channel between a one-dot chain line and a two-dot chain line becomes large. However, as shown by dotted lines, it is possible to reduce the leakage power to the adjacent channel by performing the distortion compensation.
In this case, an ACLR (Adjacent Channel Leakage Power Ratio) of the power which leaks to the adjacent channel to the transmitting power of the transmitting channel is equivalent to a ratio of an area of a spectrum between the one-dot chain lines representing the power of the transmission channel in
FIG. 2
to an area of the spectrum between the one-dot chain line and the two-dot chain line representing the leakage power to the adjacent channel. This leakage power is strictly regulated so as to use frequency bands effectively since the leakage power becomes a noise component to the adjacent channel. Further, ACLR is the same as the ACPR (Adjacent Channel Power Ratio) used commonly.
Additionally, power of the channels adjacent to the transmission channel and power of the channels adjacent to the channels adjacent to the transmission signals are also regulated strictly. For example, in
FIG. 3
, P
1
represents transmission power of a transmission band, PH
1
represents leakage power to an adjacent channel having a higher frequency, PH
2
represents leakage power to a channel having a further higher frequency, PL
1
represents leakage power to an adjacent channel having a lower frequency, and PL
2
represents the leakage power to a channel having a further lower frequency. A vertical axis represents power and a horizontal axis represents frequency. An adjacent channel leakage power ratio ACLR
1
and the next adjacent channel leakage power ratio ACLR
2
can be obtained by formulas as follows:
ACLR
1
=
PH
1
(or
PL
1
)/
P
1
ACLR
2
=
PH
2
(or
PL
2
)/
P
1
In this case, with regard to ACLR
1
, it is possible to take an average of PH
1
and PL
1
for a numerator of P
1
. Similarly, with regard to ACLR
2
, it is possible to take an average of PH
2
and PL
2
for a numerator of P
1
. In the following, ACLR
1
and ACLR
2
are referred to as ACLR other than a case where it is required to differentiate ACLR
1
from ACLR
2
.
FIG. 4
is a schematic diagram showing a basic structure of the linealizer (distortion compensation apparatus) for performing the distortion compensation of the power amplifier. The linealizer includes a multiplier
110
which structures a pre-distortion part, an adaptive distortion compensation control part
111
, a subtractor
112
, and a power amplifier
113
.
Additionally, f(p) represents a distortion function of the power amplifier
113
. Illustrations of a directional coupler, a cymoscope and the like for branching a part of an amplified output signal of the power amplifier
113
are omitted.
The adaptive distortion compensation control part
111
receives a difference e(t) between the transmission signal x(t) and the amplified output signal. Then, the adaptive distortion compensation control part
111
inputs a distortion compensation signal to the multiplier
110
. The distortion compensation signal thereof makes the difference e(t) become zero and corresponds to an amplitude or power of the transmission signal x(t). Thereby, a distortion in an opposite direction, that is, pre-distortion is given to the transmission signal x(t) so that the amplified output signal of the power amplifier
113
does not include a distortion component.
Additionally, the linealizer shown in
FIG. 5
includes a multiplier
120
, a distortion compensation signal memory
121
, a distortion compensation signal generation part
122
, a power amplifier
123
and a subtractor
124
. The same as the basic structure shown in
FIG. 4
, the multiplier
120
gives the pre-distortion corresponding to a distortion function f(p) of the power amplifier
123
to the transmission signal x(t). Besides, the distortion compensation signal memory
121
stores a distortion compensation coefficient corresponding to a level or power of the transmission signal x(t). The distortion compensation signal generation part
122
receives the difference e(t) between the transmission signal x(t) and the amplified output signal so as to generate a distortion compensation signal, and updates the distortion compensation coefficient of the distortion compensation signal memory
121
.
Furthermore, a structure of a linealizer (distortion compensation apparatus) shown in
FIG. 6
is proposed in Japanese Laid-Open Patent Application No. 09-069733. In
FIG. 6
, the linealizer includes a multiplier
130
, a distortion compensation table
131
, a power calculation part (|x(t)|
2
)
132
, a power amplifier
133
, a subtractor
134
, a complex number converter (conjg)
135
, multipliers
136
through
138
, an adder
139
and a directional coupler
140
. In addition, f(p) represents a distortion function of the power amplifier
133
, x(t) represents the transmission signal, e(t) represents a difference between the transmission signal and a signal which is branched from the amplified output signal by the directional coupler
140
, &mgr; represents a step size parameter and y(t) represents an output signal of the power amplifier
133
Ishikawa Hiroyoshi
Kubo Tokuro
Nagatani Kazuo
Ode Takayoshi
Oishi Yasuyuki
Choe Henry
Fujitsu Limited
Katten Muchin Zavis & Rosenman
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