Predistorter

Details Predistorter Predistorter

1999-08-20

2003-07-01

Chin, Stephen (Department: 2634)

Pulse or digital communications

Transmitters

Antinoise or distortion

C375S295000, C375S297000, C330S149000, C330S151000

Reexamination Certificate

active

06587513

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a predistorter and particularly, to the predistorter which has a variable gain circuit for power control.
2. Description of the Prior Art
A predistorter is conventionally used for the purpose of preliminarily compensating, at a baseband, a distortion caused in a transmission power amplifier of a radio communication apparatus, and thereby reducing a spurious radiation caused by a non-linearity of the transmission power amplifier. In the case where a predistorter is used in the radio communication apparatus, a transmission power amplifier can operate in a non-linear region and accordingly, the power consumption during transmission of the radio communication apparatus can be decreased. If the radio communication apparatus is a portable phone which uses a battery as its power source in such a case, a talk time can be elongated.
Predistorters as explained above have been developed actively heretofore. The theory and structure of a predistorter is explained in detail in “Quantization Analysis and Design of a Digital Predistortion Linearizer for RF Power Amplifier”, Sundstrom L., Faulkner M., Johansen M.; Vehicular Technology, IEEE Trans. Vol. 45 4, pages 707-719.
A conventional predistorter will be explained hereunder while making reference to the above article.
In conventional mobile communications, it has been adopted a constant envelope modulation system such as FM system and GMSK system, in which an amplitude of a modulated signal is constant and information is carried on phase variation of the modulated signal. The reason for the adoption is that the constant amplitude enables the use of a C- or F-class non-linear amplifier which has a high power efficiency as a transmission power amplifier. In the case where a transmission power amplifier with a high power efficiency is used, the power consumption during a transmission is reduced and, if a portable phone is used, a talk time can be elongated.
Heretofore, however, increase in traffic has become a problem in mobile communication.
Hence, a modulation system with a high efficiency in frequency use also has been used. For example, PDC (personal Digital Cellular) and PHS (Personal Handyphone System) in Japan adopt &pgr;/4 shift QPSK. In addition, CDMA (Code Division Multiple Access) system adopts QPSK or offset QPSK.
These modulation systems are referred to as linear modulation systems, in which information is carried on amplitude as well as on phase. In such systems, a non-linear amplifier can not be adopted because the amplitude of a modulated signal varies within wide range. That is, it is needed to adopt an amplifier with a linearity as precise as possible.
FIG. 3
shows an example of an output spectrum in the case here an amplifier with a poor linearity is used.
A bold line in
FIG. 3
represents an original spectrum in a linear modulation system. In the case where a modulated signal passes through an amplifier with a poor linearity, spurious signals with a bandwidth, for example, three times as wide as the original signal (or third order distortion components) represented by a broken line are generated because of the influence of the third order distortion of the amplifier.
In such case where the spurious signals are generated, ACPR (Adjacent Channel Power Ratio) becomes worse and accordingly, efficiency in frequency use is lowered in spite of adoption of a linear modulation system with high efficiency in frequency use. This phenomenon is referred to as Spectrum Re-generation.
In order to avoid this phenomenon, it is necessary to cause an amplifier to operate in a region where it's linearity is sufficiently precise.
However, the better the linearity of an amplifier is, the more the amplifier consumes a power and the lower the power efficiency thereof becomes.
Linealizers such as predistorter have been developed as means for alleviating such problem. That is, a countermeasure to this problem is to use an amplifier with high power efficiency and poor linearity and a predistorter for compensating the poor linearity, thereby suppressing spurious powers to adjacent channels.
FIG. 4
shows an example of a distortion of a transmission power amplifier.
As shown in
FIG. 4
, as an input level increases, a gain gradually decreases and input-output characteristics gradually deviate from a straight line and eventually saturate. In addition, phase characteristics deviate from a certain constant from a point close to where the input-output characteristics begin to deviate from the straight line.
An input signal is a complex number which has an amplitude and an angle (or phase) and represented as:
S
r
=(
I
r
+jQ
r
)exp(
j
2
&pgr;f
c
t
),
where
I
r
: an in-phase component of a baseband signal
Q
r
: a quadrature component of the baseband signal
f
c
: carrier frequency.
Here, in the case where S
r
is regarded as representing a baseband signal, the exponential part is omitted and the baseband signal is represented as:
S
r
=I
r
+jQ
r
In the case where the signal S
r
is inputted to an amplifier, the output level becomes |S
o
|′ because of the distortion, though the output level should be |S
o
| originally. In addition, the phase shifts by &thgr;
1
. This relation generates the Spectrum Re-generation, which deteriorates ACPR.
A predistorter is effective to avoid the above. In order to cause the amplifier to output the output level S
o
, it is necessary to supply to the amplifier the input S
p
which has been preliminarily calculated instead of the input S
r
.
FIG. 5
shows a complex plane for explaining the compensation of a distortion carried out by the predistorter.
As shown in
FIG. 5
, the predistorter needs to generate the signal S
p
which has an amplitude made higher than that of signal S
r
in consideration of the saturation of the characteristics of the amplifier and has a phase &thgr;
2
in a direction for canceling the phase rotation.
FIG. 6
is a diagram for explaining the formulation of the compensation of the distortion as shown in FIG.
5
.
In
FIG. 6
, G represents the input-output characteristics of the amplifier and F represents the input-output characteristics of a predistorter. Both G and F include phase and are represented with complex numbers.
It is obvious from
FIG. 6
that the signal S
p
which has been predistorted is represented as:
S
p
=F
(|
S
r
|)·
S
r
It is obvious that F is determined by only the amplitude of S
r
or the absolute value of S
r
. The output S
o
of the amplifier is represented as:
S
o
=G
(|
S
p
|)·
S
p
=G{|F
(|
S
r
|)·
S
r
|}·F
(|S
r
|)·
S
r
In order to establish a linear relationship between S
o
and S
r
, it is necessary to satisfy the following equation:
G{|F
(|
S
r
|)·
S
r
|}·F
(|
S
r
|)=
C
=constant
If an objective value of C, the characteristics G of the amplifier, and the input signal S
r
are determined, then the predistortion function F is determined by using the above equation. Here, the function F is represented with a complex number or with a real part Re and an imaginary part Im.
FIG. 7
shows the structure of an example of general adaptive predistorters. Here, an input signal (or a source signal) can be treated as a complex signal which has, in a real part, an in-phase component I
r
with respect to a carrier and, in an imaginary part, a quadrature component Q
r
with respect to the carrier.
This conventional predistorter comprises: look-up table
23
in which a real part Re and imaginary part Im of a predistortion function is stored and which outputs the function value F in response to the amplitude of the input signal used as an address thereof; complex multiplier
20
which multiplies the function value F with the input signal by complex multiplication; digital-to-analog converter
21
which converts the result of the multiplication in the complex multiplier
20
to an ana

Affiliated with

Also associated with

Rating

Predistorter does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Predistorter, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Predistorter will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3014007