Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2003-01-15
2004-06-01
Shingleton, Michael B. (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S151000
Reexamination Certificate
active
06744315
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a transmission amplifier and, more particularly, to a transmission amplifier, which is equipped with a distortion compensating function, so adapted as to be capable of both stand-alone operation and parallel operation.
In wireless communications in recent years, there is growing use of high-efficiency transmission using digital techniques. In instances where multilevel amplitude modulation is applied to wireless communications, a vital technique is one which can suppress non-linear distortion by linearizing the amplification characteristic of the power amplifier on the transmitting side and reduce the leakage of power between adjacent channels. Also essential is a technique which compensates for the occurrence of distortion that arises when an attempt is made to improve power efficiency by using an amplifier that exhibits poor linearity.
FIG. 20
is a block diagram illustrating an example of a transmitting apparatus in a radio according to the prior art. Here a transmit-signal generator
1
transmits a serial digital data sequence and a serial/parallel (S/P) converter
2
splits the digital data sequence alternately one bit at a time to convert the data to two sequences, namely an in-phase component signal (also referred to as an “I signal”) and a quadrature component signal (also referred to as a “Q signal”). A DA converter
3
converts the I and Q signals to respective analog baseband signals and inputs these to a quadrature modulator
4
. The latter multiplies the input I and Q signals (the transmit baseband signals) by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90· and sums the results of multiplication to thereby perform quadrature modulation and output the modulated signal. A frequency converter
5
mixes the quadrature-modulated signal and a local oscillation signal to thereby effect a frequency conversion, and a transmission power amplifier
6
power-amplifies the carrier output from the frequency converter
5
. The amplified signal is released into space from an antenna
7
.
In mobile communications based upon W-CDMA, etc., the transmission power of the transmitting apparatus is a high ten watts to several tens of watts, and the input/output characteristic [distortion function f(p)] of the transmission power amplifier
6
is non-linear, as indicated by the dotted line in (a) of FIG.
21
. Non-linear distortion arises as a result of this non-linear characteristic, and the frequency spectrum in the vicinity of a transmission frequency f
0
develops side lobes, as shown in (b) of
FIG. 21
, leakage into the adjacent channel occurs and this causes interference between adjacent channels. More specifically, owing to non-linear distortion, there is an increase in power that causes transmitted waves to leak into the adjacent frequency channel, as shown at (b). ACPR (Adjacent Channel Power Ratio), which indicates the magnitude of leakage power, is the ratio between the power of the channel of interest, which is the area of the spectrum between the one-dot chain lines A and A′ in FIG.
21
(
b
), and the adjacent leakage power, which is the area of the spectrum between the two-dot chain lines B and B′, that leaks into the adjacent channel. Such leakage power constitutes noise in other channels and degrades the quality of communication of these channels. Such leakage must be limited to the utmost degree.
Leakage power is small in the linear region [see (a) in FIG.
21
] of the power amplifier and large in the non-linear region. Accordingly, it is necessary to broaden the linear region in order to obtain a transmission power amplifier having a high output. However, this necessitates an amplifier having a performance higher than that actually needed and therefore is inconvenient in terms of cost and apparatus size. Accordingly, a transmission apparatus that has come to be adopted is equipped with a distortion compensating function that compensates for distortion ascribable to non-linearity of the power amplifier.
FIG. 22
is a block diagram of a transmitting apparatus having a digital non-linear distortion compensating function that employs a DSP (Digital Signal Processor). Here digital data (a transmit signal) sent from the transmit-signal generator
1
is converted to the two sequences of I and Q signals by the S/P converter
2
. These signals enter a distortion compensator
8
constituted by a DSP. The distortion compensator
8
includes a distortion compensation coefficient memory
8
a
for storing distortion compensation coefficients h(pi) (i=0~1023) conforming to power levels pi of a transmit signal x(t); a predistortion unit
8
b
for subjecting the transmit signal to distortion compensation processing (predistortion) using a distortion compensation coefficient h(pi) that is in conformity with the power level of the transmit signal; and a distortion compensation coefficient calculation unit
8
c
for comparing the transmit signal x(t) with a demodulated signal (feedback signal) y(t), which has been obtained by demodulation in a quadrature detector described later, and for calculating and updating the distortion compensation coefficient h(pi) in such a manner that the difference between the compared signals will approach zero.
The transmit signal that has been subjected to predistortion processing by the distortion compensator is input to the DA converter
3
. The latter converts the input I and Q signals to analog baseband signals and applies the baseband signals to the quadrature modulator
4
. The latter multiplies the input I and Q signals by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90· and sums the results of multiplication to thereby perform quadrature modulation and output the modulated signal. The frequency converter
5
mixes the quadrature-modulated signal and a local oscillation signal to thereby effect a frequency conversion, and the transmission power amplifier
6
power-amplifies the carrier signal that is output from the frequency converter
5
. The amplified signal is released into space from the antenna
7
.
Part of the transmit signal is input to a frequency converter
10
via a directional coupler
9
so as to undergo a frequency conversion and then be input to a quadrature detector
11
. The latter multiplies the input signal by a reference carrier wave and a signal that has been phase-shifted relative to the reference carrier by 90° to thereby perform quadrature detection, reproduces the I, Q signals of the baseband on the transmitting side and applies these signals to an AD converter
12
. The latter converts the applied I and Q signals to digital data and inputs the digital data to the distortion compensator
8
. By way of adaptive signal processing using the LMS (Least Mean Square) algorithm, the distortion compensator
8
compares the transmit signal before the distortion compensation thereof with the feedback signal demodulated by the quadrature detector
11
and proceeds to calculate and update the distortion compensation coefficient h(pi) in such a manner that the difference between the compared signals will become zero. By subsequently repeating this operation, non-linear distortion of the transmission power amplifier
6
is suppressed to reduce the leakage of power between adjacent channels.
FIG. 23
is a diagram useful in describing distortion compensation processing by an adaptive LMS. A multiplier
15
a
(which corresponds to the predistortion unit
8
b
in
FIG. 22
) multiplies the transmit signal x(t) by a distortion compensation coefficient h
n
(p). A DA converter
15
b
converts the distortion-compensated signal to an analog signal, which is applied to a power amplifier
15
c
having a distortion function f(p). A feedback loop
15
d
feeds back the output signal y(t) from the power amplifier and digital converter
15
e
converts the analog feedback signal to a digital signal. A power calculation unit
15
f
calculates the power
Katten Muchin Zavis & Rosenman
Shingleton Michael B.
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