Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1999-08-06
2001-05-29
Pham, Chi (Department: 2631)
Pulse or digital communications
Transmitters
Antinoise or distortion
C330S149000
Reexamination Certificate
active
06240144
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a mobile radio communication system, and more particularly to an apparatus and method of linearizing the characteristic of a power amplifier in the mobile radio communication system by compensating for the non-linear characteristic of active elements included in a transmitting stage of the system.
2. Description of the Related Art
A high-power amplifier which is used for transmitting analog data or digital data in mobile radio communication systems requires a high spectrum efficiency as well as a high power efficiency in order to construct a low-power consuming system in a limited frequency band. In order to meet such general requirements in the system, baseband data modulation methods such as QPSK and QAM having a high spectrum efficiency have been used. Also, a high-efficiency power amplifier such as a class C amplifier has been used to improve the power efficiency of a transmitter in the system. Such a high-efficiency power amplifier generally has strong non-linear characteristics, consequently producing a distortion phenomenon, such as a sidelobe reproduction, in its output spectrum. This phenomenon is especially prevalent in the case where a modulated signal such as a QPSK or QAM signal, which does not have a constant envelope characteristic, passes through the power amplifier with the non-linear characteristic.
Various methods have been proposed for preventing the distortion of the output spectrum of the power amplifier resulting from the non-linear characteristics of the power amplifier. One among them is a method of compensating for the non-linear characteristic of the high-power amplifier by adaptively tracking the non-linear characteristic of the power amplifier, predistorting the baseband data in a manner opposite to the distortion caused by the non-linear characteristic of the power amplifier.
FIG. 1
is a block diagram illustrating the construction of a conventional power amplifier employing the above-described adaptive predistortion method.
Referring to
FIG. 1
, K-bit data encoded by an encoder (not illustrated) is inputted to a shift register
10
and a modulation select read only memory (ROM)
52
. The shift register
10
has the length of an L-symbol span, its output has a size of LK bits. At this time, if there is no linear distortion caused by the filtering operation of a filter (not illustrated) existing in the system and the length of one symbol is enough for the length L of the shift register
10
, while if a linear distortion due to the system filtering exists, the length of the shift register
10
should be longer than one symbol.
The LK-bit output of the shift register
10
is inputted to a predistort RAM
12
. This predistort RAM
12
is stored with predistortion data mapped for data outputted from the shift register
10
. The predistortion data is updated in accordance with input error data. Upon receiving data from the shift register
10
, the predistort RAM
12
outputs predistortion data corresponding to the received data. That is, the predistort RAM
12
predistorts data outputted from the shift register
10
using error data having a phase opposite to a distortion of the transmission signal so that a radio frequency (RF) transmission section detects the distortion of the transmission signal and compensates for the detected distortion. The output of the predistort RAM
12
is converted to an analog signal for transmission by an I-channel digital-to-analog (D/A) converter
14
and a Q-channel digital-to-analog (D/A) converter
16
, respectively. The analog signals converted by the respective D/A converters
14
and
16
are low-pass-filtered through low-pass filters (LPFs)
18
and
20
, and then inputted to a quadrature modulator
22
. The analog signals inputted to the quadrature modulator
22
are mixed with an output of a first oscillator
32
, and then modulated to an intermediate frequency signal in the quadrature modulator
22
. The intermediate frequency signal modulated by the quadrature modulator
22
is determined by the first intermediate frequency (IF) oscillator
32
, and is mixed with an output of a second oscillator
28
by a mixer
24
to be converted to a final radio frequency (RF) transmission frequency. The RF frequency signal outputted from the mixer
24
is finally amplified by a power amplifier
26
, and then transmitted through an antenna.
A portion of the output of the power amplifier
26
is fed back to a mixer
30
by a signal coupler
54
, and the fed-back signal is mixed with the output of the second oscillator
28
by the mixer
30
to be converted to a first IF signal. The converted IF signal is then converted to baseband data by a quadrature demodulator
34
using a local oscillation signal outputted from an IF oscillator
32
.
The baseband signal converted by the quadrature demodulator
34
is compared with each output signal of D/A converters
48
and
50
, which is used as a reference signal for generating an error signal, by analog adders
40
and
42
, respectively. Here, output signals of the modulation select ROM
52
are inputted to the D/A converters
48
and
50
, and used as reference signals for comparing with the signals fed back to update the value of the predistort RAM
12
. The reference signals added in the analog adders
40
and
42
and the fed-back signals are respectively converted to digital signals, and then added to the digital signals of the predistort RAM
12
by digital adders
36
and
46
to update the value of the predistort RAM
12
. Error data outputted from the digital adders
36
and
46
are inputted to the predistort RAM
12
via a data bus, and then stored in addresses determined by the shift register
10
to complete the predistortion process with respect to the baseband data.
However, there are some disadvantages according to the conventional predistortion method shown in FIG.
1
. First, the shift register
10
for generating addresses and the modulation select ROM
52
for obtaining the reference signal required for generating the error signal must be employed. Also, the high-accuracy adders
40
and
42
for obtaining the error signal must be employed to update the value of the predistort RAM
12
. Constructing such high-accuracy analog adders is difficult, and is highly dependant on accuracy. In addition, it is generally known that the performance obtained by the predistortion method is lower than that obtained by a feedforward method.
SUMMARY OF THE INVENTION
Accordingly, the present invention overcomes the disadvantages of the prior art using two objectives. One object of the present invention is to provide an apparatus and method of linearizing the characteristic of a power amplifier in a mobile radio communication system by compensating for the non-linear characteristic of active elements included in a transmitting stage of the system.
Another object of the present invention is to provide an apparatus and method of compensating for the non-linear characteristic of a power amplifier in a mobile radio communication system by a predistortion and feed forward method.
In order to achieve the above objects, in accordance with the present invention, an apparatus for linearizing a power amplifier in a mobile radio communication system is provided, comprising: an error detector for detecting an error between input and output signals of the power amplifier by comparing the fed-back output signal with the input signal; a predistortion lookup table for storing predistortion data; a predistortion lookup table controller for updating the data of the predistortion lookup table which corresponds to a position of a present input data by adding an error signal from the error detector to an output of the predistortion lookup table; a feedforward lookup table for storing feedforward control data; a feedforward lookup table controller for outputting corresponding feedforward control data of the feedforward lookup table by detecting a size of the error signal; a linearizer for distorting the in
Corrielus Jean
Dilworth & Barrese LLP
Pham Chi
Samsung Electronics Co,. Ltd.
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