Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2001-07-03
2004-09-21
Choe, Henry (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S002000, C330S304000
Reexamination Certificate
active
06794936
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to communications, and, more particularly, to a system and method for a distortion reduction system using predistortion.
2. Description of Related Art
An ideal power amplifier amplifies an input signal with no waveshape alteration. The ideal power amplifier is therefore characterized as having a transfer function (input signal vs. output signal) which is linear with no transfer function discontinuities. In practice, however, a power amplifier has a transfer function with nonlinear and “linear” regions. Whether the power amplifier is operating in a linear or nonlinear region depends in part on the amplitude of the input signal. For the power amplifier to achieve as near to linear operation as possible, the power amplifier is designed to operate within its linear region given the range of possible input signal amplitudes. If the input signal has an amplitude which causes the power amplifier to operate outside the linear region, the power amplifier introduces nonlinear components or distortion to the signal. When the input signal possesses peak amplitudes which cause the amplifier to compress, to saturate (no appreciable increase in output amplitude with an increase in input amplitude) or to shut-off (no appreciable decrease in output amplitude with a decrease in input amplitude), the amplifier is being overdriven, and the output signal is clipped or distorted in a nonlinear fashion. Generally, an amplifier is characterized as having a clipping threshold, and input signals having amplitudes beyond the clipping threshold are clipped at the amplifier output. In addition to distorting the signal, the clipping or nonlinear distortion of the input signal, generates spectral regrowth or adjacent channel power (ACP) that can interfere with an adjacent frequency.
In wireless communications systems, high power amplification of signals for transmission are commonly encountered with very large peak to average power ratios (PAR). For example, in a time division multiple access (TDMA) system, such as Global System for Mobile Communications (GSM) or North American TDMA, when multiple carrier signals are combined for amplification with a power amplifier, the resulting PAR is about 9-10 dB for a large number of carriers. In a code division multiple access (CDMA) system, a single loaded 1.25 Mhz wide carrier can typically have a PAR of 11.3 dB. For orthogonal frequency division multiplexing (OFDM), multicarrier signals can have a PAR of up to 20 dB. These signals have to be amplified fairly linearly to avoid generating ACP.
Unfortunately, efficiency of the base station amplifier is inversely related to its linearity. To achieve a high degree of linearity, the amplifiers are biased to operate in the class A or “slight” class AB (meaning class AB operation that is closer to class A than to class B). Maximum AC to DC efficiency achievable for class A operation is 50%, whereas that of a class AB amplifier is between 50 and 78.5% (the latter representing the maximum efficiency of a class B amplifier). The closer the particular class AB operation is to class A, the lower the maximum efficiency. For amplifiers employing field effect transistors, the class of operation is set in accordance with the gate voltage applied, which controls the quiescent (idle) drain current. For class A operation, the gate voltage is set so that the idle drain current is approximately in the middle of the range between cutoff and saturation. Class B amplifiers are biased near cutoff, resulting in a rectified drain current waveform. Class AB amplifiers are biased in between the bias points of classes A and B.
Typically, strict linearity requirements in modern wireless communication systems dictate the use of the relatively inefficient class A or slight class AB modes. As a result, significant DC power is dissipated by the amplifiers, thereby generating heat which must be controlled to avoid degrading amplifier performance and reliability. Hence, the use of elaborate heat sinks and fans become a necessary by-product of the high linearity system. Naturally, these measures add to the cost, size and weight of the base station equipment. As the number of wireless communications users continues to grow, so do the number of base stations and the need to keep them small, light and inexpensive. Thus, a great deal of research has focused on the quest to improve amplifier efficiency in these and other systems.
Various linearization methods are used to enable the use of more cost-effective and more power efficient amplifiers while maintaining an acceptable level of linearity. Feed-forward correction is routinely deployed in modern amplifiers to improve the linearity of the main amplifier with various input patterns. The essence of the feed-forward correction is to isolate the distortion generated by the main amplifier on a feed forward path. The distortion is provided to a correction amplifier on the feed forward path which amplifies the distortion. The distortion on the feed forward path is combined with the distortion on the main signal path to cancel the distortion on the main signal path.
Predistortion techniques distort the input signal prior to amplification by taking into account the transfer function characteristics for the amplifier. As such, the desired amplified signal is achieved from the predistorted input signal by intentionally distorting the signal before the amplifier, so the non-linearity of the amplifier can be compensated.
FIG. 1
shows a block diagram of an adaptive power amplifier predistortion system
10
. The baseband digital input signal u
n
on a main signal path
12
is input into the predistortion function
14
(A(.)) to produce a predistorted signal x
n
where n is the time index. After digital to analog conversion by digital to analog (D/A) converter
16
, the resulting baseband analog signal is frequency up-converted in an up-conversion process
18
to radio frequency (RF). The analog RF signals are amplified by power amplifier
20
for transmission over the air using antenna
22
. A replica of the amplified analog RF signals is coupled off the main signal path
12
onto a predistortion feedback path
24
. The amplified analog RF signals on the predistortion feedback path
24
are down-converted by a down-conversion process
26
.
The down-converted analog signals on the predistortion feedback path
24
are provided to an analog to digital (A/D) converter
28
for conversion into the digital domain. The resulting digital feedback signal, which represents the output of the amplifier
20
, is provided to an amplifier characteristics estimation block
30
along with the digital baseband signal x
n
which represents the corresponding input to the amplifier
20
. In this embodiment, a delay adjuster
32
equalizes the delays between the digital baseband signal x
n
and the baseband digital feedback signal y
n
such that the digital signal x
n
is received by the amplifier characteristics estimation block
32
at the same time as the corresponding baseband digital feedback signal y
n
resulting from the output of the amplifier
20
. Given the digital signal x
n
prior to amplification and the digital signal y
n
resulting from the amplification of the analog and frequency converted versions of the digital signal x
n
, the amplifier characteristics estimation block
30
can determine the characteristics or a model function of the amplifier
20
. Once the model of the amplifier
20
is estimated, a predistortion calculation process
34
determines the predistortion function as the inverse of the amplifier characteristics function, and the predistortion function
14
(A(.)) applied to the input signal un is updated based on the predistortion calculation process
34
. For example, the predistortion function is determined by updating coefficients of the predistortion function or model based on the results of the amplifier characteristics estimation.
The up-conversion and down-conversion processes
18
and
26
require mixers and filters in the
Hsu Liang
Kim Jae-hyeong
Konstantinou Kyriaki
Choe Henry
Garceran Julio A.
Lucent Technologies - Inc.
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