Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
2000-12-22
2004-12-28
Chin, Stephen (Department: 2634)
Pulse or digital communications
Transmitters
Antinoise or distortion
C375S297000
Reexamination Certificate
active
06836517
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a distortion compensating apparatus and, more particularly, to a distortion compensating apparatus for compensating for a distortion of a transmission power amplifier, using the correlation between a transmission signal and a feedback signal, and adjusting a timing of each element of the distortion compensating apparatus on the basis of a delay time obtained.
Frequency resources have become strained in recent years and greater reliance is being placed upon highly efficient digital transmission in radio communication. In a case where multivalued amplitude modulation is applied to radio communication, an important technique used on the transmission side is to linearize the amplification characteristic of a power amplifier to suppress a nonlinear distortion thereby and reduce the leakage power between adjacent channels. In a case where an amplifier having an inferior linearity is used and an attempt is made to improve the power efficiency, a technique for compensating for the distortion produced by this amplifier is essential.
FIG. 23
is a block diagram of an example of a transmitter in a conventional radio apparatus. A transmission signal generator
1
sends a group of serial digital data, and a serial/parallel (S/P) converter
2
alternately separates the group of digital data bit by bit into an in-phase component (I) signal and a quadrature component (Q) signal. A D/A converter
3
converts each of the I signals and Q signals into analog baseband signals and inputs the base band signals into a quadrature modulator
4
. The quadrature modulator
4
multiplies the input I and Q signals (transmission baseband signals) by a reference carrier wave and signal phase-shifted from the reference carrier by 90°, respectively, adds the two products, thereby performing a quadrature conversion, and outputs the result. A frequency converter
5
mixes the signal subjected to quadrature modulation and a local oscillation signal to perform a frequency conversion, and a transmission power amplifier
6
amplifiers the power of the carrier wave output by the frequency converter
5
and radiates the amplified signal into space from an antenna
7
.
In such a transmitter, the input/output characteristic (distortion function f(p) of the signal is a non-linear characteristic, as indicated by the dashed line in FIG.
24
A. This nonlinear characteristic produces a nonlinear distortion, and a frequency spectrum centered about a transmission frequency f
0
possesses side lobes, as indicated by the dashed line in FIG.
24
B. This results in a leakage of the signal into the adjacent channels and causes interference on the adjacent channels. To prevent this, a Cartesian loop method, a polar loop method, etc. have been proposed as a technique for compensating a distortion of a feedback system, and a distortion of a power amplifier is suppressed.
FIG. 25
is a block diagram of a transmission apparatus having a function for digitally compensating for a nonlinear distortion by using a DSP (Digital Signal Processor). The group of digital data (transmission signals) sent from the transmission signal generator
1
is converted into I signals or Q signals by the S/P converter
2
, and input into a distortion compensator
8
which is constituted by a DSP. The distortion compensator
8
as shown in the functional diagram of
FIG. 26
, is provided with a distortion compensation function memory
8
a
for storing the distortion compensation coefficient h(pi) (i=0~1023) which corresponds to the power level 0~1023 of a transmission signal, a pre-distortion unit
8
b
which applies a distortion compensation processing (pre-distortion) to the transmission signal by using the distortion compensation coefficient h(pi) which corresponds to the level of the transmission signal, and a distortion compensation coefficient arithmetic unit
8
c
which compares a transmission signal with a (feed back) signal demodulated by a later-described quadrature detector, and which calculates and updates the distortion compensation coefficient h(pi) by using the error.
The distortion compensator
8
applies a pre-distortion processing to the transmission signal by using the distortion compensation coefficient h(pi) which corresponds to the level of the transmission signal, and inputs the processed signal to the DA converter
3
. The DA converter
3
converts the input I and Q signals into analog baseband signals and inputs the baseband signals into the quadrature modulator
4
. The quadrature modulator
4
multiplies the input I and Q signals by a reference carrier wave and a signal phase-shifted from the reference carrier by 90°, respectively, adds the two products, thereby performing a quadrature conversion, and outputs the result. The frequency converter
5
mixes the signal subjected to quadrature modulation signal with a local oscillation signal to perform a frequency conversion, and the transmission power amplifier
6
amplifiers the power of the carrier wave output from the frequency converter
5
and radiates the amplified signal into space from the antenna
7
. A part of the transmission signals are input into a frequency converter
10
via a directional coupler
9
. The frequency converter
10
converts the frequency of the signals and inputs them into a quadrature detector
11
. The quadrature detector
11
multiplies each of the input signal by the reference carrier wave and by the signal phase-shifted from the reference carrier by 90°, thereby performing quadrature detection and reproducing the baseband I, Q signals from the transmission side, and inputs these signals into an AD converter
12
. The AD converter
12
converts the input I and Q signals into digital signals and inputs them into the distortion compensator
8
. The distortion compensator
8
compares the transmission signal before a distortion compensation processing with the feedback signal which is demodulated by the quadrature detector
11
by an adaptive algorithm using the LMS (Least Means Square) method, and calculates and updates the distortion compensation coefficient h(pi) by using an error between the transmission signal and the feedback signal. The transmission signal to be sent next is then subjected to a pre-distortion processing using the updated distortion compensation coefficient h(pi), and the processed signal is output. The above-described operation is repeated thenceforce, thereby suppressing the nonlinear distortion of the transmission power amplifier
6
and reducing the power leaked to the adjacent channels.
FIG. 27
is an explanatory view of the distortion compensation processing based on an adaptive algorithm using an LMS method. The reference numeral
21
a
denotes a multiplier (pre-distortion unit) for multiplying a transmission signal (input baseband signal) x(t) by a distortion compensation coefficient h
n
(p),
21
b
a DA converter for converting the transmission signal subjected to a distortion compensation processing into an analog signal,
21
c
a device (transmission power amplifier) in which a distortion generates and which has a distortion function f(p),
21
d
a feedback system for feeding back a signal y(t) output from the transmission power amplifier,
21
e
an AD converter for converting a feedback signal into a digital signal, and
21
f
a distortion compensation coefficient memory for storing the distortion compensation coefficient h
n
(p) in correspondence with the power p (=|x(t)|
2
) of the transmission signal x(t). The distortion compensation coefficient memory
21
f
updates the distortion compensation coefficient h
n
(p) by a distortion compensation coefficient h
n+1
(p) which is obtained by an LMS algorithm.
The reference numeral
21
g
denotes an arithmetic unit for calculating the power p (=|x(t)|
2
) of the transmission signal x(t) and outputs the power as a reading address,
21
h
a delay circuit for generating a writing address of the distortion compensation coefficient memory
21
f.
It takes a predeterm
Kubo Tokuro
Nagatani Kazuo
Ode Takayoshi
Oishi Yasuyuki
Chin Stephen
Katten Muchin Zavis & Rosenman
Williams Lawrence B.
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