Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
1999-08-05
2001-08-14
Mottola, Steven J. (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
active
06275103
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a predistorter of an amplifier for mobile communication and the like, and an amplifying unit in which a predistorter is used, and more particularly to a predistorter applicable to a high-frequency power amplifier handling wideband signals.
2. Description of the Related Art
In mobile communication, a power amplifier having a low adjacent-channel leakage power (APC) is needed so as to give no disturbance to an adjacent channel thereof. Conventionally, in order to meet the need, an output back-off method by which output power of the amplifier is reduced to the extent of the adjacent-channel leakage power becoming small, or a predistortion method (Japan Application No. Hei09-2979297) by which a transmitting signal is in advance modified by the reverse characteristic of an input-to-output characteristic and then inputted into an amplifier, is employed to linearize the transmitting signal.
With respect to the amplifier, a larger output corresponds to a higher efficiency. However, since the output back-off method reduces the output of the amplifier, a high efficiency of the amplifier cannot be obtained. Hence, it is desirable to employ the predistortion method.
FIG. 15
shows a predistorter of the predistortion method. In this diagram, an input baseband signal is modified in a predistorter
21
and then the modified signal is inputted into a transmitter
23
via a D/A converter
22
. The transmitter
23
has a built-in power amplifier which can amplify an input signal power to become a transmitting power. In general, the power amplifier has a nonlinear characteristic f(x). The predistorter
21
is a circuit which modifies the input signal by a reverse characteristic f(x)
−1
of the nonlinear characteristic f(x). The input signal, which has been modified by the predistorter
21
, is further modified by the nonlinear characteristic f(x) of the power amplifier of the transmitter
23
. As a result, the input signal is linearized back to its initial linear state.
FIG. 16
shows a conventional example of the predistorter
21
. In this example, quadrature-modulation signals I and Q are inputted as baseband signals. In
FIG. 16
, an amplitude-value operating portion
1
functions to obtain an amplitude value of the baseband input signals I and Q according to a formula “I
2
+Q
2
”. A reverse-characteristic adding portion
2
functions to in advance modify the amplitude value (I
2
+Q
2
) by the reverse characteristic f(x)
−1
of the nonlinear characteristic f(x) of the power amplifier. Multiplying portions function to respectively multiply the input baseband signals I and Q by a signal which has had added thereto the reverse characteristic f(x)
−1
in the reverse-characteristic adding portion
2
. Thus, the signals I and Q become final predistortion signals Ipd and Qpd.
It is well known that distortion is generated based on the nonlinearity of the input-to-output characteristic of the amplifier. Further, in a case in which a transmitting signal becomes a wideband signal, the distortion is enlarged due to factors such as frequency characteristics of a bias circuit, frequency deviation, high-frequency load characteristic deviation and the like of the amplifier. These factors are other than the above-mentioned nonlinearity of the input-to-output characteristic. For this reason, in order to obtain an adjacent-channel leakage power characteristic which is the same as for a narrowband, output of the amplifier must be lowered. As a result, the amplifier works inefficiently.
FIGS. 9 and 10
are graphs for comparing a narrowband signal and a wideband signal in adjacent-channel leakage power.
FIG. 9
shows a spectrum of the adjacent-channel leakage power of the narrowband signal.
FIG. 10
shows a spectrum of the adjacent-channel leakage power of the wideband signal. As seen from the two graphs, in order to obtain the same adjacent-channel leakage power characteristic as that of the narrowband signal of
FIG. 9
, the wideband signal of
FIG. 10
should be lowered by about −6 dB.
However, only using the conventional linearization method cannot lower the adjacent-channel leakage power caused by the wideband.
Further, with respect to linear compensation, if output power, frequency, or temperature of the transmitter is varied, a stable reduction of the adjacent-channel leakage power cannot be obtained and the efficiency of the amplifier cannot be improved even in linear areas.
SUMMARY OF THE INVENTION
It is a general object of the present invention to provide a predistorter of an amplifier and an amplifying unit, in which the above disadvantages are eliminated.
A more specific object of the present invention is to provide a predistorter of an amplifier for mobile communication and the like and an amplifying unit using the predistorter, and more particularly to a predistorter suitable for a high-frequency power amplifier handling wideband signals.
The above objects of the present invention are achieved by a predistorter of an amplifier for lowering adjacent-channel leakage power of an output of the amplifier, the predistorter comprising:
a first part which modifies in advance an input signal to be inputted into the amplifier by a reverse characteristic of an input-to-output characteristic of the amplifier; and
a second part which determines correction coefficients corresponding to one or both of a differential and an integral of the modified input signal so as to further modify the input signal into a final predistortion signal based on the modified input signal and the correction coefficients.
With respect to distortion caused by factors such as frequency characteristics of a bias circuit, frequency deviation, high-frequency load characteristic deviation and the like of the amplifier in a case in which the signal is a wideband signal, by modifying the input signal according to the correction coefficients, the adjacent-channel leakage power generated by the wideband signal can be lowered.
The predistorter may be configured such that the correction coefficients are determined based on a difference signal instead of one or both of the differential and integral of the input signal, the difference signal being generated between a previous value and a present value in time series of the input signal.
Accordingly, it is possible to realize a memory-saving high-speed process.
Further the predistorter may be configured such that the correction coefficients is in advance determined so as to meet a pre-measured two-wavelength IMD characteristic, and stored in a memory.
It is difficult to analytically obtain the correction coefficients, but using the predistorter of the present invention, the correction coefficients can be simply obtained.
The above objects of the present invention may be achieved by an amplifying unit comprising:
an amplifier; and
a predistorter of the amplifier for lowering adjacent-channel leakage power of an output of the amplifier,
the predistorter comprising:
a first part which modifies in advance an input signal to be inputted into the amplifier by a reverse characteristic of an input-to-output characteristic of the amplifier; and
a second part which determines correction coefficients corresponding to one or both of a differential and an integral of the modified input signal so as to further modify the input signal into a final predistortion signal based on the modified input signal and the correction coefficients.
wherein coefficients are determined according to output power of the amplifier, and by arithmetically processing the input signal and the coefficients, the input signal can be modified according to the output power of the amplifier.
In the amplifying unit of the present invention, according to the size of the output power as a target, the input signal is multiplied by the coefficient so as to be modified, and thereby the adjacent-channel leakage power can be lowered with respect to a wide output range.
The above objects of the present invention may be achieved by
Fujitsu Limited
Helfgott & Karas, P C.
Mottola Steven J.
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