Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2001-02-15
2003-04-22
Mottola, Steven J. (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S136000
Reexamination Certificate
active
06552609
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a distortion compensating apparatus and method for compensating distortions of an amplifier that amplifies analog signals converted from digital input signals.
2. Description of the Related Arts
It is ideally desirable that amplifiers for amplification and output of input signals have a proportional relationship (linear relationship) between its input signal power (input power) and output signal power (output power) as indicated by an imaginary line (chain double-dashed line) in FIG.
15
.
The actual amplifiers however generically have input/output characteristics indicated by a solid line in
FIG. 15
, i.e., in the relatively small input power region (linear region) the input power is proportional to the output power whereas in the relatively large input power region (nonlinear region or saturated region) the input power is not proportional to the output power but the output power tends to become saturated with larger distortions accordingly as the input power increases.
In order to use the amplifiers with a high energy efficiency, use in the larger input power region is preferred and hence the amplifiers are typically used in the nonlinear region. Transmitters are thus provided with a distortion compensating apparatus arranged to compensate output signal distortions in the nonlinear region so as to allow the output signals to be proportional to the input signals.
FIG. 16
is a block diagram showing the configuration of a transmitter equipped with a conventional adaptive predistortor type distortion compensator which is one of the distortion compensating apparatuses.
This transmitter has input terminals S
i
and S
q
that receive an I channel (Ich) digital signal x
i
and a Q channel (Qch) digital signal x
q
, respectively, both in the form of base band signals. These signals are fed to a power calculating unit
100
and a multiplier
102
.
The power calculation unit
100
finds power values p of the fed digital signals x
i
and x
q
and feeds the power values p to a distortion compensation coefficient storage unit
101
. The distortion compensation coefficient storage unit
101
has distortion compensation coefficients which correspond to various input power values p and, when a power value p is received, it feeds to the multiplier
102
a distortion compensation coefficient h (h
i
corresponding to Ich and h
q
corresponding to Qch) corresponding to that power value p.
The multiplier
102
multiplies the digital signals x
i
and x
q
with the distortion compensation coefficient h and feeds the results of multiplication to digital-to-analog converters (hereinafter referred to as DACs)
103
i
and
103
q,
respectively. The DACs
103
i
and
103
q
convert input digital signals into analog signals and feed the analog signals to a modulation/amplification unit
104
.
The modulation/amplification unit
104
quadrature modulates the input analog signals and converts the base band signals into radio frequency (RF) band signals. The modulation/amplification unit
104
then amplifies the RF band signals for the output from an output terminal S
o
. Upon this amplification in the modulation/amplification unit
104
, the signals may suffer any distortions (amplitude distortions and phase distortions) by the amplifier.
An attenuation/demodulation unit
105
also receives the transmission signals in the form of feedback signals. The attenuation/demodulation unit
105
attenuates signals amplified by the modulation/amplification unit
104
, into signals having a pre-amplified power. The attenuation/demodulation unit
105
then quadrature demodulates them and converts RF band signals into base band signals. The signals converted by the attenuation/demodulation unit
105
are fed to analog-to-digital converter (hereinafter referred to as ADCs)
106
i
and
106
q
, for the conversion into digital signals.
The digital signals are fed to a distortion compensation coefficient update unit
108
and to one input terminals of subtractors
107
i
and
107
q.
The other input terminals of the subtractors
107
i
and
107
q
receive digital signals x
i
and x
q
, respectively, from the input terminals S
i
and S
q
, respectively. The subtractors
107
i
and
107
q
find difference (i.e., distortion upon the amplification in the modulation/amplification unit
104
) signals between the digital signals x
i
, x
q
and output signals of the ADCs
106
i,
106
q,
respectively, and feed the difference signals (error signals) to the distortion compensation coefficient update unit
108
.
The distortion compensation coefficient update unit
108
finds a new distortion compensation coefficient h′ based on output signals of the ADCs
106
i,
106
q,
output signals of the subtractors
107
i,
107
q
and distortion compensation coefficient h from the distortion compensation coefficient storage unit
101
. The unit
108
then updates the distortion compensation coefficient storage unit
101
by use of the new distortion compensation coefficient h′. This updated distortion compensation coefficient h′ is utilized for the subsequent input signal distortion compensations.
The above processings are iterated for each digital signal input.
In such a conventional transmitter, however, the feedback signals are converted by the ADCs
106
i
and
106
q
into digital signals. As described above, these feedback signals are obtained by subjecting the input signals to distortions upon the amplification and have the same level of amplitude value as the input signals. Due to the necessity to represent not merely the input signals but also the distortions, therefore, the ADCs
106
i
and
106
q
must have a high bit precision (i.e., a large bit number) and, because of the input signal handling, a high operation frequency. Accordingly as the input signals have a higher input bit rate, the conversion processing may possibly become too late. In the event that the input signals have an extremely large amplitude value and a high frequency as in the case of the CDMA system base station, this deficiency will become more prominent.
SUMMARY OF THE INVENTION
The present invention was conceived in view of such a situation. It is therefore an object of the present invention to provide a distortion compensating apparatus and a distortion compensating method which allow use of ADCs having a relatively low bit precision and having a relatively low operation frequency.
In order to achieve the above object there is provided a distortion compensating apparatus of the present invention for compensating distortions of an amplifier arranged to amplify analog signals converted from digital input signals, the distortion compensating apparatus converting the digital input signals into analog signals, the distortion compensating apparatus finding, in the analog region, distortion components of the amplifier from differences between the analog input signals and the analog output signals of the amplifier, the distortion compensating apparatus converting the distortion components into digital signals to thereby determine distortion compensation coefficients.
As a result, an analog-to-digital converter of the present invention handling only the distortion components for the conversion of analog signals into digital signals can be one having a lower bit precision and a relatively lower operation frequency than those of the conventional analog-to-digital converter for converting analog output signals from the amplifier into digital signals.
According to a first aspect of the present invention there is provided a distortion compensating apparatus for compensating distortions of an amplifier arranged to amplify analog signals converted from digital input signals, the distortion compensating apparatus comprising a distortion compensation coefficient application unit which holds distortion compensation coefficients for compensation of distortions of the amplifier, the distortion compensation coefficient application unit applying the held distortion compensation
Hamada Hajime
Ishikawa Hiroyoshi
Kubo Takuro
Nagatani Kazuo
Ode Takayoshi
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