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| United States Patent |
6,288,610
|
|
Miyashita
|
September 11, 2001
|
Method and apparatus for correcting signals, apparatus for compensating for
distortion, apparatus for preparing distortion compensating data, and
transmitter
Abstract
An apparatus for correcting signals includes a unit for estimating
distortion characteristics in high-frequency circuit portion which
estimates distortion characteristics of a high-frequency circuit portion
concerning amplitude distortion or phase distortion impairing linearity.
The apparatus further includes an input signal processor for applying an
amplitude distortion correcting function or phase distortion concerning
function, which is calculated on the basis of the result of estimating the
distortion characteristics, to an input signal such as low-pass signals.
| Inventors:
|
Miyashita; Takumi (Kawasaki, JP)
|
| Assignee:
|
Fujitsu Limited (Kawasaki, JP)
|
| Appl. No.:
|
324044 |
| Filed:
|
June 2, 1999 |
Foreign Application Priority Data
| Aug 07, 1998[JP] | 10-224407 |
| Current U.S. Class: |
330/149; 375/296; 375/297 |
| Intern'l Class: |
H03F 001/26; H03F 001/30 |
| Field of Search: |
330/149
375/296,297
|
References Cited [Referenced By]
U.S. Patent Documents
Other References
Electronics Letters. Apr. 25, 1991, vol. 27, pp. 762-764 "Linearised
High-Efficiency Power Amplifier for PCN".
IEEE MTT-S Digest WE3F-7. 0-7803-3246-6/96 (pp. 835-838) "Adaptive Digital
Predistorter for Power Amplifiers with Real Time Modeling of Memoryless
Complex Gains".
|
Primary Examiner: Shingleton; Michael B
Attorney, Agent or Firm: Armstrong, Westerman, Hattori, McLeland & Naughton LLP
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This patent application is a continuation-in-part of patent application
Ser. No. 09/044,231 filed on Mar. 19, 1998 and now abandoned.
Claims
What is claimed is:
1. An apparatus for correcting signals, comprising:
a unit for estimating distortion characteristics in high-frequency circuit
portion which estimates distortion characteristics of a high-frequency
circuit portion concerning distortion impairing the linearity; and
an input signal processing unit for applying a distortion correcting
function, calculated on the basis of the result of estimating the
distortion characteristics, to a given input signal, and supplying the
input signal to said high-frequency circuit portion so that distortion
impairing linearity, and occurring in said high-frequency circuit portion,
can be corrected,
wherein the input signal is low-pass signals passed by low-pass filters
defined on the basis of the result of estimating the distortion
characteristics; after the frequencies of the low-pass signals are raised,
the low-pass signals are input to a first amplifier in said high-frequency
circuit portion; after frequencies relevant to the phase distortion
correction function to be applied to low-pass signals substantially
identical to the low-pass signals are raised, the low-pass signals are
input to a second amplifier in said high-frequency circuit portion; and
output signals of said first amplifier and second amplifier are added in
order to produce an output signal containing a small number of amplitude
distortion components impairing linearity.
2. An apparatus for correcting signals according to claim 1, wherein an
envelope transfer function g(x), whose variable x indicates an input
amplitude, is calculated in order to reproduce the distortion
characteristics of said high-frequency circuit portion; and as the
distortion correcting function, based on the envelope transfer function
g(x), an amplitude distortion correcting function h(x) used to correct
amplitude distortion impairing the linearity, and occurring in said
high-frequency circuit portion, is determined so that the relationship of
ax=g(h(x)) where a is a constant can be established, or a phase distortion
correcting function p(x) used to correct phase distortion impairing the
linearity, and occurring in said high-frequency circuit portion, is
determined so that the relationship of c=g(p(x)) where c is a constant can
be established.
3. An apparatus for correcting signals, comprising:
a unit for estimating distortion characteristic in high-frequency circuit
portion which estimates distortion characteristics of a high-frequency
circuit portion concerning at least one of amplitude distortion and phase
distortion impairing linearity; and
an input signal processing unit for applying at least one of an amplitude
distortion correcting function and a phase distortion correcting function,
which are calculated on the basis of the result of estimating the
distortion characteristics, to a given input signal, and supplying the
input signal to said high-frequency circuit portion so that at least one
of the amplitude distortion and the phase distortion impairing the
linearity, and occurring in said high-frequency circuit portion, can be
corrected,
wherein the input signal is low-pass signals passed by low-pass filters
defined on the basis of the result of estimating the distortion
characteristics; after the frequencies of the low-pass signals are raised,
the low-pass signals are input to a first amplifier in said high-frequency
circuit portion; after frequencies relevant to the phase distortion
correction function to be applied to low-pass signals substantially
identical to the low-pass signals are raised, the low-pass signals are
input to a second amplifier in said high-frequency circuit portion; and
output signals of said first amplifier and second amplifier are added in
order to produce an output signal containing a small number of amplitude
distortion components impairing linearity.
4. An apparatus for correcting signals according to claim 3, wherein an
envelope transfer function g(x), whose variable x indicates an input
amplitude, is calculated in order to reproduce the distortion
characteristics of said high-frequency circuit portion; and based on the
envelope transfer function g(x), an amplitude distortion correcting
function h(x) used to correct the amplitude distortion impairing the
linearity, and occurring in said high-frequency circuit portion, is
determined so that the relationship of ax=g(h(x)) where a is a constant
can be established, or a phase distortion correcting function p(x) used to
correct the phase distortion impairing the linearity, and occurring in
said high-frequency circuit portion, is determined so that the
relationship of c=g(p(x)) where c is a constant can be established.
5. An apparatus for correcting signals according to claim 3, wherein an
approximate expression of the envelope transfer function g(x), whose
variable x indicates an input amplitude, is defined as follows in order to
reproduce the distortion characteristics of said high-frequency circuit
portion:
g(x)=a.sub.0 +a.sub.1 x+g'(x)
where a.sub.0 and a.sub.1 are constants and g'(x) is a polynomial; and the
amplitude distortion correcting function is defined as x*(1-g'(x)/g(x)).
6. An apparatus for correcting signals according to claim 5, wherein the
constant a.sub.0 is set to 0.
7. An apparatus for correcting signals according to claim 5, wherein the
polynomial g'(x) in the envelope transfer function g(x)=a.sub.0 +a.sub.1
x+g'(x) is defined as g'(x)=.SIGMA..sub.i=2.sup.N a.sub.i x.sup.i where i
and N are positive integers equal to or larger than 2, and a.sub.i is a
constant.
8. An apparatus for correcting signals according to claim 7, wherein only
the terms of odd-numbered orders of the polynomial
g'(x)=.SIGMA..sub.i=2.sup.N a.sub.i x.sup.i are employed.
9. An apparatus for correcting signals according to claim 3, wherein the
phase distortion correcting function p(x) is defined as p(x)=-g(x).
10. An apparatus for correcting signals according to claim 4, wherein said
input signal is low-pass signals passed by low-pass filters defined on the
basis of the result of estimating the distortion characteristic, and
frequencies relevant to the phase distortion correcting function p(x) to
be applied to the low-pass signals are raised in order to produce an
intermediate-frequency or high-frequency signal for the purpose of
correcting the phase distortion impairing the linearity, and occurring in
said high-frequency circuit portion.
11. An apparatus for correcting signals according to claim 4, wherein, when
frequencies relevant to the phase distortion correcting function p(x) to
be applied to the input signal are raised by performing digital
modulation, signals passed by transmission filters exhibiting root Nyquist
characteristics are used as the input signal.
12. An apparatus for correcting signals according to claim 4, wherein, when
frequencies relevant to the phase distortion correcting function p(x) to
be applied to the input signal are raised by performing quadrature
modulation, a real-part signal and an imaginary-part signal constituting a
complex-number signal and passed by low-pass filters, that exhibit root
Nyquist characteristics and are defined on the basis of the result of
estimating the distortion characteristics, are used as the input signal.
13. An apparatus for correcting signals according to claim 3, wherein said
second amplifier causes substantially no distortion, and the phase
distortion impairing the linearity, and occurring in said first amplifier
alone, is corrected.
14. An apparatus for correcting signals according to claim 3, wherein the
phase distortion impairing the linearity, and occurring in said first
amplifier, is corrected and the phase distortion impairing the linearity,
and occurring in said second amplifier, is also corrected.
15. An apparatus for correcting signals according to claim 3, wherein the
input signal is low-pass signals passed by low-pass filters defined on the
basis of the result of estimating the distortion characteristics; after
the frequencies of the low-pass signals are raised, the low-pass signals
are input to a first amplifier in said high-frequency circuit portion;
after frequencies relevant to the amplitude distortion correcting function
to be applied to low-pass signals substantially identical to the low-pass
signals are raised, the low-pass signals are input to a second amplifier
in said high-frequency circuit portion; and output signals of said first
amplifier and second amplifier are added in order to produce an output
signal containing a small number of amplitude distortion components
impairing the linearity.
16. An apparatus for correcting signals according to claim 15, wherein said
second amplifier causes substantially no distortion, and the amplitude
distortion impairing linearity, and occurring in said first amplifier
alone, is corrected.
17. An apparatus for correcting signals according to claim 15, wherein the
amplitude distortion impairing the linearity, and occurring in said first
amplifier, is connected, and the amplitude distortion impairing the
linearity, and occurring in said second amplifier, is also corrected.
18. An apparatus for correcting signals according to claim 3, wherein the
input signal is low-pass signals passed by low-pass filters defined on the
basis of the result of estimating the distortion characteristics; an
amplitude distortion correcting function to be applied to said low-pass
signals is calculated; and frequencies relevant to a phase distortion
correcting function in the amplitude distortion correcting function are
raised in order to produce an intermediate-frequency signal or
high-frequency signal for the purpose of correcting the amplitude
distortion and the phase distortion impairing the linearity, and occurring
in said high-frequency circuit portion.
19. An apparatus for correcting signals according to claim 4, wherein the
envelope transfer function g(x) is a function including a table function;
at least one of the amplitude distortion correcting function h(x) and
phase distortion correcting function p(x) is an expansion composed of a
polynomial series; coefficients in the terms of orders of the expansion
are calculated; and thus at least one of the amplitude distortion
correcting function h(x) and phase distortion correcting function p(x) is
calculated.
20. An apparatus for correcting signals according to claim 4, wherein at
least one of the amplitude distortion correcting function h(x) and phase
distortion correcting function p(x) is fixed.
21. An apparatus for correcting signals according to claim 4, wherein, when
a plurality of high-frequency circuit portions are included, at least one
of the amplitude distortion correcting function h(x) and phase distortion
correcting function p(x), is fixed for each high-frequency circuit
portion, or for each group of high-frequency circuit portions produced
under the similar conditions for manufacturing a high-frequency circuit
portion.
22. An apparatus for correcting signals according to claim 4, wherein, when
said high-frequency circuit portion is in operation, the envelope transfer
function g(x) is calculated intermittently or all the time; and based on
the result of calculating the envelope transfer function g(x), at least
one of the amplitude distortion correcting function h(x) and phase
distortion correcting function p(x) is modified.
23. A method for correcting signals, including the steps of:
estimating distortion characteristics of a high-frequency circuit portion
concerning distortion impairing linearity;
applying a distortion correcting function, calculated on the basis of the
result of estimating the distortion characteristics, to a given input
signal; and
supplying the input signal to which the distortion correction function is
applied, to said high-frequency circuit portion for the purpose of
correcting the distortion impairing the linearity, and occurring in said
high-frequency circuit portion,
wherein the input signal is low-pass signals passed by low-pass filters
defined on the basis of the result of estimating the distortion
characteristics; after the frequencies of the low-pass signals are raised,
the low-pass signals are input to a first amplifier in said high-frequency
circuit portion; after frequencies relevant to the phase distortion
correction function to be applied to low-pass signals substantially
identical to the low-pass signals are raised, the low-pass signals are
input to a second amplifier in said high-frequency circuit portion; and
output signals of said first amplifier and second amplifier are added in
order to produce an output signal containing a small number of amplitude
distortion components impairing linearity.
24. A method for correcting signals, including the steps of:
estimating distortion characteristics of a high-frequency circuit portion
concerning at least one of amplitude distortion and phase distortion
impairing linearity;
applying at least one of an amplitude distortion correcting function and
phase distortion correcting function, calculated on the basis of the
result of estimating the distortion characteristics, to a given input
signal; and
supplying the input signal to which at least one of the amplitude
distortion correcting function and phase distortion correcting function is
applied, to said high-frequency circuit portion for the purpose of
correcting at least one of the amplitude distortion and the phase
distortion impairing the linearity, and occurring in said high-frequency
circuit portion,
wherein the input signal is low-pass signals passed by low-pass filters
defined on the basis of the result of estimating the distortion
characteristics; after the frequencies of the low-pass signals are raised,
the low-pass signals are input to a first amplifier in said high-frequency
circuit portion; after frequencies relevant to the phase distortion
correction function to be applied to low-pass signals substantially
identical to the low-pass signals are raised, the low-pass signals are
input to a second amplifier in said high-frequency circuit portion; and
output signals of said first amplifier and second amplifier are added in
order to produce an output signal containing a small number of amplitude
distortion components impairing linearity.
25. An apparatus for compensating for distortion to which an in-phase
component xi and an orthogonal component xq that have been filtered by a
low-pass filter, are supplied, having a function of previously distorting
the in-phase component and the orthogonal component so that non-linearity
of a power amplifier arranged on the downstream side can be improved, said
apparatus comprising:
an absolute value calculating unit for calculating an intensity of vector
(xi, xq); and
a predistortion unit in which a function h(x)/x obtained when a
predetermined predistortion function h(x) is divided by the above
intensity x is used or its reference table is used, for multiplying an
output xi, xq of the low-pass filter by h(x)/x.
26. An apparatus for compensating for distortion according to claim 25,
further comprising a phase pre-rotation unit in which a predetermined
function or its reference table is used, and the vector (xi, xq) or the
vector ((h/x)xi, (h/x)xq) generated in the above predistortion unit is
rotated by angle .phi.(x).
27. A digital signal processor characterized in that a function of the
apparatus for compensating for distortion described in claim 25 is
realized by a program.
28. A digital signal processor characterized in that a function of the
apparatus for compensating for distortion described in claim 26 is
realized by a program.
29. An apparatus preparing distortion compensation data comprising a
processor (2B), wherein, in order to improve nonlinearity of the power
amplifier, the processor (2B) is used for an apparatus for compensating
for distortion in which the amplitude x of a signal on the upstream side
of the power amplifier is previously distorted to the value of the
predistortion function h(x); when the output amplitude with respect to the
input amplitude x of the power amplifier is expressed by the function
g(x), the predistortion function h(x) is made to be approximate to a power
development of the amplitude x; the power development coefficients c1 to
cn are given initial values; the amplitude x is made to be an amplitude of
the inter-modulation wave of the base band signals, the angular
frequencies of which are .omega.1 and .omega.2; and the power development
coefficients c1 to cn of the predistortion function h(x) are determined so
that an absolute value .epsilon. of the ratio of the primary Fourier
coefficient of the function g(h(x)) with respect to the second Fourier
coefficient can be substantially a minimum.
30. An apparatus for preparing distortion compensation data according to
claim 29, wherein an amplitude of the intermodulation wave is determined
according to the maximum value of the input amplitude of the power
amplifier.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for correcting
signals used for a transmitter, an apparatus for compensating for
distortion, an apparatus for preparing distortion compensation data, and a
transmitter.
To be more specific, the present invention relates to a technique for
correcting amplitude distortion or phase distortion impairing linearity
and occurring in a high-frequency circuit employed in a transmitter or the
like for a portable telephone so as to maintain linearity for the purpose
of minimizing power leaking out of channels adjoining each transmission
channel, when various kinds of communication are carried out by using a
portable telephone, portable information equipment, and information
terminal within a communication system such as a local area network (LAN),
fiber-to-the-home (FTT), or the like or within a system within which
communication is carried out between base stations and mobile stations.
2. Description of the Related Art
A high-frequency circuit employed in a transmitter or the like for portable
telephones is required to save a frequency band occupied by each
transmission channel or transmission power as much as possible. This makes
it difficult to allow tolerance in the frequency band occupied by each
transmission channel. A signal of a narrow frequency band must be
transmitted over each communication channel for fear the spectrum of
signals propagating over communication channels may expand very widely,
due to distortion impairing linearity, and occurring in the high-frequency
circuit included in the transmitter. For supplying high enough power for
the high-frequency circuit including an amplifier to operate while
maintaining good linearity, an attempt has been made to correct distortion
characteristics of a high-frequency amplifier such as a microwave
amplifier, which is highly required to ensure especially good linearity,
or a characteristic thereof concerning the distortion impairing the
linearity, by utilizing a feed-forward method.
Referring to FIG. 1, which will be described in "Brief Description of the
Drawings", the configuration and operations of an apparatus for correcting
signals in accordance with the prior art will be described so that
problems underlying the apparatus for correcting signals can be easily
understood.
FIG. 1 is a circuit block diagram showing an example of the apparatus for
correcting signals utilizing the feed-forward method in accordance with
the prior art. FIG. 1 schematically shows the circuitry, of the apparatus
for correcting signals, that is connected to a high-frequency main
amplifier 130, of which the distortion characteristics are the subject for
correction, for the purpose of correcting the distortion characteristics
concerning the distortion impairing the linearity of an output signal Sout
relative to an input signal Sin.
The apparatus for correcting signals shown in FIG. 1 includes a first
feed-forward amplifier 100 exhibiting substantially the same distortion
characteristic as the main amplifier 130 and requiring little power
consumption, and a second feed-forward amplifier 110 and third
feed-forward amplifier 130 for almost perfectly canceling out error signal
components generated by the first feed-forward amplifier 100.
To be more specific, in the apparatus for correcting signals shown in FIG.
1, the first feed-forward amplifier 100 intentionally produces a
distortion signal according to the distortion impairing the linearity and
likely to occur in the main amplitude 130. The distortion signal output
from the first feed-forward amplifier 100 is attenuated properly by a
first attenuating unit 310. A difference between a delayed input signal
produced by delaying the input signal Sin by means of a first delay unit
210 and the distortion signal attenuated by the first attenuating unit 310
is calculated and amplified by a second feed-forward amplifier 110.
Furthermore, a signal output from the second feed-forward amplifier 110 is
attenuated properly by a second attenuating unit 320. Thereafter, a
difference between a delayed input signal produced by delaying the input
signal Sin by means of a second delay unit and a distortion signal output
from the second attenuating unit 210 is calculated in order to cancel out
error signal components almost perfectly. Finally, the difference is
amplified by a third feed-forward amplifier 120, and a signal output from
the third feed-forward amplifier 120 is supplied to the main amplifier
130.
However, in this case, since it will not occur that the distortion
characteristics of a main amplifier are identical to that of a
feed-forward amplifier, when an attempt is made to almost perfectly cancel
out error signal components generated by the feed-forward amplifier, the
conditions for designing a circuit or the circuitry become complex.
Recently, the number of portable telephones or portable information
equipment put to use is increasing drastically, and the density of
frequencies of a spatial electromagnetic wave utilized by users becomes
higher. The frequency band occupied by each communication channel tends to
be restricted to a narrower band.
Furthermore, there is a tendency toward using a communication channel
permitting a relatively wide frequency band for the purpose of realizing
high-speed and large-capacity communication. The spectrum of signals
propagating over communication channels therefore expands, whereby the
number of intermodulation waves generated by the communication channels
increases. Furthermore, the possibility that each communication channel is
intercepted by adjoining channels increases.
Even in these circumstances, it is necessary to maintain low power
consumption in a transmitter or the like incorporated in a portable
telephone or portable information equipment, and to ensure a dynamic range
of a transmission output that is wide enough to control transmission power
so that reception power can be retained at a necessary minimum level all
the time.
In consideration of the recent trends in portable telephone or portable
information equipment, even when the method for correcting signals
utilizing the feed-forward method in accordance with the prior art is
adopted, the conditions for circuit designing or the circuitry required
for almost perfectly canceling out error signal components generated by a
feed-forward amplifier will become more and more complex. Besides, the
scale of a circuit for correcting distortion characteristics will become
larger. This poses a problem that power consumption required increases
drastically.
Aside from the aforesaid method of correcting a distortion characteristic
utilizing the feed-forward method, there are three methods of correcting a
distortion characteristics of a high-frequency circuit in accordance with
the prior art.
(1) Method Utilizing a Linearlizer
For improving linearity offered by a high-frequency circuit, distorted
components which impair the linearity and occur in an amplifier whose
distortion characteristics are the subject for correction, are canceled
out by using a nonlinear device exhibiting nonlinearity, such as a diode.
However, it is technologically difficult to design a circuit including
such a nonlinear device in such a manner that the circuit exhibits the
same characteristics as a circuit including an amplifier whose distortion
characteristics are the subject for correction. Advanced technologies are
therefore needed to design a circuit that does not need adjustment to
enable modification of the conditions for circuit operations or to cope
with the uncertainty in characteristics of a nonlinear device.
(2) Method Utilizing Cartesian Feedback
The linearity to be exhibited by an amplifier whose distortion
characteristics are the subject for correction is improved by demodulating
an output signal of the amplifier through quadrature demodulation, and
feeding back a resultant signal for producing a baseband signal. In this
case, the circuitry becomes rather complex.
(3) Method Utilizing a Two-dimensional Coefficient Table and a Predictor
While Providing a Learning Ability
While a two-dimensional coefficient table is corrected by detecting
distortion impairing linearity and occurring in an amplifier, a digital
signal processor (DSP) forming a predictor is used to pre-distort a
baseband signal. Thus, the baseband signal is corrected. This method
utilizing the two-dimensional coefficient table has been disclosed in, for
example, literature on a related art entitled "Improvement of Digital
Mapping Predistorters for Linearizing Transmitters" written by Qiming Ren
and Ingo Wolff (IEEE MTT-S Opening Forum 2, pp.0-7803-4603-6 of 2-52 in
CD-ROM, June 1997). The literature on the related art has reported a case
in which adjacent channel power (ACP) is reduced by 45 dB or more. In this
case, since the distortion impairing the linearity and occurring in an
amplifier, should be corrected with high accuracy, the storage capacity of
a memory containing the two-dimensional coefficient table gets relatively
large and the number of calculations performed by the DSP becomes larger.
This results in increased power consumption.
In any of the aforesaid methods of correcting a distortion characteristics
of a high-frequency circuit, in accordance with the prior art, the
aforesaid problems remain unsolved.
SUMMARY OF THE INVENTION
The present invention attempts to solve the foregoing problems. An object
of the present invention is to provide an apparatus for correcting signals
capable of, when various kinds of communication are carried out by using a
portable telephone, portable information equipment, or an information
terminal, reliably suppressing distortion impairing linearity and
occurring in a high-frequency circuit including an amplifier and
minimizing power leaking out of channels adjoining each communication
channel while maintaining low power consumption and having simpler
circuitry than a known apparatus, a transmitter having the apparatus for
correcting signals, and a method for correcting signals to be implemented
in the apparatus for correcting signals.
For solving the aforesaid problems, an apparatus for correcting signals in
accordance with the present invention comprises a means for estimating
distortion characteristics, in high frequency circuit portion, which
estimates distortion characteristics of a high-frequency circuit portion
or characteristics thereof concerning distortion impairing linearity; and
an input signal processing unit for applying a distortion correcting
function, calculated on the basis of the result of estimating the
distortion characteristics, to a given input signal, and supplying the
input signal to which the distortion correcting function is applied to the
high-frequency circuit portion so that the distortion impairing the
linearity, and occurring in the high-frequency circuit portion, can be
corrected.
Preferably, in an apparatus for correcting signals according to the present
invention, an envelope transfer function g(x), whose variable x indicates
an input amplitude, is calculated in order to reproduce the distortion
characteristics of the high-frequency circuit portion. Based on the
envelope transfer function g(x), as the distortion function, an amplitude
distortion correcting function h(x) used to correct amplitude distortion
impairing linearity, and occurring in the high-frequency circuit portion,
is determined so that the relationship of ax=g(h(x)) where a is a constant
can be established, or a phase distortion correcting function p(x) used to
correct phase distortion impairing linearity, and occurring in the
high-frequency circuit portion, is determined so that the relationship of
c=g(p(x)) where c is a constant can be established.
More preferably, an apparatus for correcting signals in accordance with the
present invention comprises a means for estimating distortion
characteristics in high-frequency circuit portion which estimates
distortion characteristics of a high-frequency circuit portion concerning
at least one of the amplitude distortion or the phase distortion impairing
the linearity, and occurring in the high-frequency circuit portion, and an
input signal processing unit for applying at least one of an amplitude
distortion correcting function and phase distortion correcting function,
which are calculated on the basis of the result of estimating the
distortion characteristics, to a given input signal, and supplying the
input signal to the high-frequency circuit portion so that at least one of
the amplitude distortion or phase distortion impairing linearity, and
occurring in the high-frequency circuit portion, can be corrected.
More preferably, in the apparatus for correcting signals according to the
present invention, an envelope transfer function g(x), whose variable x
indicates an input amplitude, is calculated in order to reproduce the
distortion characteristics of the high-frequency circuit portion. Based on
the envelope transfer function g(x), an amplitude distortion correcting
function h(x) used to correct the amplitude distortion impairing the
linearity, and occurring in the high-frequency circuit portion, is
determined so that the relationship of ax=g(h(x)) where a is a constant
can be established, or a phase distortion correcting function p(x) used to
correct the phase distortion impairing the linearity, and occurring in the
high-frequency circuit portion, is determined so that the relationship of
c=g(p(x)) where c is a constant can be established.
More preferably, in an apparatus for correcting signals according to the
present invention, an approximate expression of the envelope transfer
function g(x), whose variable x indicates an input amplitude and which
reproduces the distortion characteristics of the high-frequency circuit
portion, is defined as g(x)=a.sub.0 +a.sub.1 x+g'(x) where a.sub.0 and
a.sub.1 are variables and g'(x) is a polynomial. The amplitude distortion
correcting function is expressed as x.times.(1-g'(x)/g(x)).
More preferably, in an apparatus for correcting signals according to the
present invention, the constant a.sub.0 is set to 0 (zero).
More preferably, in an apparatus for correcting signals according to the
present invention, the polynomial g'(x) in the envelope transfer function
g(x)=a.sub.0 +a.sub.1 x+g'(x) is defined as g'(x)=.SIGMA..sub.i=2.sup.N
a.sub.i x.sup.i where i and N are positive integers equal to or larger
than 2 and a.sub.i is a constant.
More preferably, in an apparatus for correcting signals according to the
present invention, only terms of odd-numbered orders of the polynomial
g'(x)=.SIGMA..sub.i=2.sup.N a.sub.i x.sup.i are employed.
More preferably, in an apparatus for correcting signals according to the
present invention, the input signal is low-pass signals passed by low-pass
filters defined on the basis of the result of estimating the distortion
characteristics. Frequencies relevant to the phase distortion correcting
function p(x) to be applied to the low-pass signals are raised in order to
produce an intermediate-frequency or high-frequency signal for the purpose
of correcting the phase distortion impairing the linearity, and occurring
in the high-frequency circuit portion.
More preferably, in an apparatus for correcting signals according to the
present invention, when frequencies relevant to the phase distortion
correcting function p(x) to be applied to the input signal are raised by
performing digital modulation, signals passed by transmission filters
exhibiting root Nyquist characteristics are used as the input signal.
More preferably, in an apparatus for correcting signals according to the
present invention, when frequencies relevant to the phase distortion
correcting function p(x) to be applied to the input signal are raised by
performing quadrature modulation, a real-part signal and an imaginary-part
signal constituting a complex-number signal and passed by low-pass
filters, that exhibit root Nyquist characteristics and are defined on the
basis of the result of estimating the distortion characteristics, are used
as the input signal.
More preferably, in an apparatus for correcting signals according to the
present invention, the input signal is low-pass signals passed by low-pass
filters defined on the basis of the result of estimating the distortion
characteristics. After the frequencies of the low-pass signals are raised,
the low-pass signals are input to a first amplifier in the high-frequency
circuit portion. After frequencies relevant to the phase distortion
correcting function to be applied to low-pass signals that are
substantially identical to the low-pass signals are raised, the low-pass
signals are input to a second amplifier in the high-frequency circuit
portion. Output signals of the first amplifier and second amplifier are
added in order to produce an output signal containing a small number of
amplitude distortion components impairing the linearity thereof.
More preferably, in an apparatus for correcting signals according to the
present invention, the second amplifier substantially causes no
distortion. The phase distortion impairing the linearity, and occurring in
the first amplifier alone, is corrected.
More preferably, in an apparatus for correcting signals according to the
present invention, the phase distortion impairing the linearity, and
occurring in the first amplifier, is corrected, and the phase distortion
impairing the linearity, and occurring in the second amplifier, is also
corrected.
More preferably, in an apparatus for correcting signals according to the
present invention, the input signal is low-pass signals passed by low-pass
filters defined on the basis of the result of estimating the distortion
characteristics. After the frequencies of the low-pass signals are raised,
the low-pass signals are input to a first amplifier in the high-frequency
circuit portion. After frequencies relevant to the amplitude distortion
correcting function to be applied to low-pass signals that are
substantially identical to the low-pass signals are raised, the low-pass
signals are input to a second amplifier in the high-frequency circuit
portion. Output signals of the first amplifier and second amplifier are
added in order to produce an output signal containing a small number of
amplitude distortion components impairing the linearity thereof.
More preferably, in an apparatus for correcting signals according to the
present invention, the second amplifier substantially causes no
distortion. The amplitude distortion impairing the linearity, and
occurring in the first amplifier alone, is corrected.
More preferably, in an apparatus for correcting signals according to the
present invention, the amplitude distortion impairing the linearity, and
occurring in the first amplifier, is corrected, and the amplitude
distortion impairing the linearity, and occurring in the second amplifier
is also corrected.
More preferably, in an apparatus for correcting signals according to the
present invention, the input signal is low-pass signals passed by low-pass
filters defined on the basis of the result of estimating the distortion
characteristics. An amplitude distortion correcting function to be applied
to the low-pass signals is calculated. Frequencies relevant to a phase
distortion correcting function mated with the amplitude distortion
correcting function are raised in order to produce an
intermediate-frequency or high-frequency signal for the purpose of
correcting the eamplitude distortion and the phase distortion impairing
the linearity, and occurring in the high-frequency circuit portion.
More preferably, in an apparatus for correcting signals according to the
present invention, the envelope transfer function g(x) is a function
including a table function. At least one of the amplitude distortion
correcting function h(x) and phase distortion correcting function p(x) is
an expansion composed of a polynomial series. Coefficients in terms of
orders of the expansion are calculated, and thus at least one of the
amplitude distortion correcting function h(x) and phase distortion
correcting function p(x) is calculated.
More preferably, in an apparatus for correcting signals according to the
present invention, at least one of the amplitude distortion correcting
function h(x) and phase distortion correcting function p(x) is fixed.
More preferably, in an apparatus for correcting signals according to the
present invention, when a plurality of high-frequency circuit portions are
included, at least one of the amplitude distortion correcting function
h(x) and the phase distortion correction function p(x) is fixed for each
high-frequency circuit portion, or for each group of high-frequency
circuit portions produced under the resembling conditions for
manufacturing a high-frequency circuit portion.
More preferably, in an apparatus for correcting signals according to the
present invention, while the high-frequency circuit portion is in
operation, the envelope transfer function g(x) is calculated
intermittently or all the time. Based on the result of calculating the
envelope transfer function g(x), at least one of the amplitude distortion
correcting function h(x) and phase distortion correcting function p(x) is
modified.
According to the present invention, there is provided a transmitter
comprising an apparatus for correcting signals including a means for
estimating distortion characteristics in a high-frequency circuit portion
which estimates distortion characteristics of a high-frequency circuit
portion concerning distortion impairing linearity; and an input signal
processing unit for applying a distortion correcting function, calculated
on the basis of the result of estimating the distortion characteristic, to
a given input signal, and supplying the input signal to the high-frequency
circuit portion so that the distortion impairing the linearity, and
occurring in the high-frequency circuit portion, can be corrected. The
high-frequency circuit portion transmits a signal of a specified frequency
band having the distortion impairing the linearity thereof corrected.
More particularly, according to the present invention, there is provided a
transmitter comprising an apparatus for correcting signals including a
means for estimating distortion characteristics in high-frequency circuit
portion which estimates distortion characteristics of a high-frequency
circuit portion concerning at least one of amplitude distortion and phase
distortion impairing linearity; and an input signal processing unit for
applying at least one of an amplitude distortion correcting function and
phase distortion correcting function, calculated on the basis of the
result of estimating the distortion characteristics, to a given input
signal, and supplying the input signal to the high-frequency circuit
portion so that at least one of the amplitude distortion and the phase
distortion impairing the linearity, and occurring in the high-frequency
circuit portion, can be corrected. The high-frequency circuit portion
transmits a signal of a specified frequency band having at least one of
the amplitude distortion and the phase distortion impairing the linearity
thereof corrected.
More preferably, according to the present invention, there is provided a
transmitter, comprising an apparatus for correcting signals, including a
means for estimating distortion characteristics in high-frequency circuit
portion which estimates distortion characteristics of a narrow-band
amplifier concerning at least one of amplitude distortion and phase
distortion impairing linearity; and an input signal processing unit for
inputting, as an input signal, low-pass signals which are characterized to
have given low frequencies by means of digital filters defined on the
basis of the result of estimating the distortion characteristics, and
appending distortion components, which are reproduced according to the
distortion characteristics, to the input signal so that at least one of
the amplitude distortion and the phase distortion impairing the linearity,
and occurring in the narrow-band amplifier, can be corrected. The input
signal to which the distortion components are appended is multiplied by a
carrier wave for digital modulation, and then supplied to the narrow-band
amplifier. The narrow-band amplifier transmits a signal of a specified
frequency band, which has at least one of the amplitude distortion and the
phase distortion impairing the linearity thereof corrected.
More preferably, according to the present invention, there is provided a
transmitter, comprising an apparatus for correcting signals, including a
means for estimating distortion characteristics in high-frequency circuit
portion which estimates distortion characteristics of a narrow-band
amplifier concerning at least one of amplitude distortion and phase
distortion impairing linearity; and an input signal processing unit for
inputting, as an input signal, a real-part signal and imaginary-part
signal constituting a complex-number signal and characterized to have
given low frequencies by means of low-pass filters defined on the basis of
the result of estimating the distortion characteristics, and appending
distortion components including components, which are reproduced according
to the distortion characteristics and represent a function of an absolute
value of the complex-number signal or a function of a sum of squares of
the real-part signal and imaginary-part signal, to the input signal, so
that at least one of the amplitude distortion and the phase distortion
impairing the linearity, and occurring in the narrow-band amplifier, can
be corrected. The input signal to which the distortion components are
appended is subjected to digital quadrature modulation, and then supplied
to the narrow-band amplifier. The narrow-band amplifier transmits a signal
of a specified frequency band having at least one of the amplitude
distortion and the phase distortion impairing the linearity thereof
corrected.
In a method for correcting signals in accordance with the present
invention, a distortion characteristic of a high-frequency circuit portion
concerning the distortion impairing the linearity is estimated, a
distortion correcting function, calculated on the basis of the result of
estimating the distortion characteristics, is applied to a given input
signal, and the input signal to which the distortion correcting function
is applied is supplied to the high-frequency circuit portion in order to
correct the distortion impairing the linearity, and occurring in the
high-frequency circuit portion.
Furthermore, in a method for correcting signals in accordance with the
present invention, distortion characteristics of a high-frequency circuit
portion concerning at least one of the amplitude distortion and the phase
distortion impairing the linearity is estimated, at least one of an
amplitude distortion correcting function and phase distortion correcting
function, calculated on the basis of the result of estimating the
distortion characteristics, is applied to a given input signal, and the
input signal to which at least one of the amplitude distortion correcting
function and phase distortion correcting function is applied is supplied
to the high-frequency circuit portion in order to correct at least one of
the amplitude distortion and the phase distortion impairing the linearity,
and occurring in the high-frequency circuit portion.
On the other hand, the apparatus for compensating for distortion of the
present invention to which an in-phase component and an orthogonal
component that have been filtered by a low-pass filter, are supplied,
having a function of previously distorting the in-phase component and the
orthogonal component so that non-linearity of a power amplifier arranged
on the downstream side can be improved. The above apparatus comprises an
absolute value calculating unit for calculating an intensity of vector;
and a predistortion unit in which a function obtained when a predetermined
predistortion function is divided by the above intensity of the vector is
used or its reference table is used, for multiplying an output of the
low-pass filter by the function.
Further, the apparatus for preparing distortion compensation data of the
present invention is used for an apparatus for compensating for distortion
by which an input amplitude of a signal on the upstream side of the power
amplifier is previously distorted to a value of the predistortion function
so as to improve non-linearity of the power amplifier. When an output
amplitude with respect to an input amplitude of the power amplifier is
expressed by a function, the predistortion function is made to be
approximate to a power development of the amplitude of the input, and the
power development coefficient is given an initial value, so that the
amplitude of the input is an amplitude of an intermodulation wave of a
base band signal of an angular frequency. The power development
coefficient of the predistortion function is determined so that an
absolute value of a ratio of the secondary Fourier coefficient of the
function with respect to the primary Fourier coefficient can be
substantially a minimum. Further, the apparatus for preparing distortion
compensation data of the present invention comprises a processor.
In summary, according to the present invention, the fact that distortion
characteristics of a high-frequency circuit portion concerning at least
one of the amplitude distortion and the phase distortion impairing the
linearity, and occurring in the high-frequency circuit portion, can be
reproduced using low-pass signals produced by a simple circuit, is
utilized effectively. A signal correcting circuit whose high-frequency
circuit portion is smaller than that in a known signal correcting circuit
and whose adjustment portion is also smaller is used to calculate a
distortion correcting function based on the distortion characteristics.
For carrying out various kinds of communication using a portable
telephone, portable information equipment, or an information terminal,
with low power consumption maintained, despite the circuitry being much
simpler than the conventional circuitry, the distortion impairing the
linearity, and occurring in the high-frequency circuit, can be reliably
suppressed, and power leaking out of channels adjoining each communication
channel can be minimized. Eventually, highly reliable communication can be
achieved.
BRIEF DESCRIPTION OF THE DRAWINGS
The above object and features of the present invention will be more
apparent from the following description of the preferred embodiments with
reference to the accompanying drawings, wherein:
FIG. 1 is a circuit block diagram showing an example of an apparatus for
correcting signals in accordance with the prior art;
FIG. 2 is a block diagram showing the configuration of a fundamental
embodiment based on the principles of the present invention;
FIG. 3 is a circuit block diagram showing a first preferred embodiment of
the present invention;
FIG. 4 is a circuit block diagram showing a second preferred embodiment of
the present invention;
FIG. 5 is a circuit block diagram showing a third preferred embodiment of
the present invention;
FIG. 6 is a circuit block diagram showing a fourth preferred embodiment of
the present invention;
FIG. 7 is a block diagram showing a low-pass model used to realize a method
for correcting signals in accordance with the present invention;
FIG. 8 is a graph showing the results of simulation performed using the
low-pass model shown in FIG. 7;
FIG. 9 is a graph indicating how amplitude distortion components are
canceled out in a high-frequency range in the preferred embodiments of the
present invention;
FIG. 10 is a graph showing a predistortion function used in the preferred
embodiments of the present invention;
FIG. 11 is a graph showing the results of simulation in which a baseband
signal is pre-distorted;
FIG. 12 is a graph indicating changes in spectrum caused by canceling out
high-frequency amplitude distortion components and applying predistortion
components;
FIG. 13 is a block diagram showing an example of a transmitter including an
apparatus for correcting signals in accordance with the present invention;
FIG. 14 is a block diagram showing an example of a transmitter in
accordance with the prior art which is presented for comparison with the
transmitter shown in FIG. 13;
FIG. 15 is a graph showing a relationship between an input amplitude and an
output amplitude of a power amplifier;
FIG. 16 is a schematic illustration for explaining circumstances of
distortion compensation of a power amplifier;
FIG. 17 is a block diagram showing another transmitter of the present
invention;
FIG. 18 is a flow chart showing a method of finding a predistortion
function h(x) for moderately correcting non-linearity of a power
amplifier;
FIG. 19 is a diagram showing an output waveform of a dividing unit in FIG.
17;
FIG. 20 is a characteristic diagram of a root-raised cosine filter, the
roll-off coefficient of which is 0.2;
FIG. 21 is a view showing a waveform of a base band signal which has passed
through a root-raised cosine filter;
FIG. 22 is a waveform diagram of an output of a predistortion unit;
FIG. 23 is a waveform diagram of an output of phase pre-rotation unit;
FIG. 24 is a diagram showing a waveform of an input of a power amplifier,
wherein the frequency of carrier waves is reduced;
FIG. 25 is a waveform diagram of an output of a power amplifier;
FIG. 26 is a diagram in which predistortion functions h(x) are respectively
shown by a short dotted line, long dotted-line and solid line, wherein the
short dotted line is a case in which predistortion is not conducted at all
(h=x), the long dotted line is a case in which g(h(x)) is made linear, and
the solid line is a case in which .vertline.IM(2)/IM(1).vertline. is
locally minimized by the method shown in FIG. 18;
FIG. 27 is a diagram in which amplitudes of outputs of a power amplifier
are respectively shown by a short dotted line, long dotted-line and solid
line in the three cases described in FIG. 26;
FIG. 28 is a diagram showing a line spectrum of amplitude g(h(x)) of the
output of a power amplifier in the case in which the predistortion is not
conducted at all when the intermodulation wave amplitude x is
2A.vertline.(cos((.omega.1-.omega.2)t/2).vertline.;
FIG. 29 is a diagram showing a line spectrum of amplitude g(h(x)) of the
output of a power amplifier in the case in which g(h(x)) is made linear
when the intermodulation wave amplitude x is
2A.vertline.(cos((.omega.1-.omega.2)t/2).vertline.;
FIG. 30 is a diagram showing a line spectrum of amplitude g(h(x)) of the
output of a power amplifier in the case in which
.vertline.IM(2)/IM(1).vertline. is locally minimized by the method shown
in FIG. 18 when the intermodulation wave amplitude x is
2A.vertline.(cos((.omega.1-.omega.2)t/2).vertline.; and
FIG. 31 is a diagram showing a relation between average transmission power
and adjacent channel power ratio ACPR in three cases shown in FIG. 26.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the appended drawings (FIGS. 2 to 31), a fundamental
embodiment and preferred embodiments of the present invention will be
described below.
FIG. 2 is a block diagram showing the configuration of a fundamental
embodiment based on the principles of the present invention. The
fundamental embodiment according to which distortion characteristics of a
high-frequency circuit, which has a high-frequency circuit portion 1
including a high-frequency amplifier, or characteristics thereof
concerning distortion impairing the linearity of an output signal Sout
relative to an input signal Sin is corrected will be described below.
As shown in FIG. 2, an apparatus for correcting signals in accordance with
the present invention comprises a means for estimating distortion
characteristics in high-frequency circuit portion which estimates
distortion characteristics of the high-frequency circuit portion 1
concerning the distortion impairing the linearity, and an input signal
processing unit 3 for applying a distortion correcting function, which is
calculated on the basis of the result of estimating the distortion
characteristics, to a given input signal, and supplying the input signal
Sin', to which the distortion correcting function is applied, to the
high-frequency circuit portion so that the distortion impairing the
linearity, and occurring in the high-frequency circuit portion, can be
corrected.
Preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, an envelope transfer
function g(x), whose variable x indicates an input amplitude, is
calculated in order to reproduce the distortion characteristics of the
high-frequency circuit portion. As the distortion correcting function,
based on the envelope transfer function g(x), an amplitude distortion
correcting function h(x) used to correct amplitude distortion impairing
the linearity and occurring in the high-frequency circuit portion is
determined so that the relationship of ax=g(h(x)) where a is a constant
can be established, or a phase distortion correcting function p(x) used to
correct phase distortion impairing the linearity, and occurring in the
high-frequency circuit portion, is determined so that the relationship of
c=g(p(x)) where c is a constant can be established.
More preferably, an apparatus for correcting signals in accordance with the
fundamental embodiment of the present invention comprises a means for
estimating distortion characteristics in high-frequency circuit portion
which estimates distortion characteristics of a high-frequency circuit
portion concerning at least one of the amplitude distortion and the phase
distortion impairing the linearity, and an input signal processing unit
for applying at least one of an amplitude distortion correcting function
and phase distortion correcting function, which are calculated on the
basis of the result of estimating the distortion characteristics, to a
given input signal, and supplying the input signal to the high-frequency
circuit portion so that at least one of the amplitude distortion and the
phase distortion impairing linearity, and occurring in the high-frequency
circuit portion, can be corrected.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, an envelope transfer
function g(x), whose variable x indicates an input amplitude, is
calculated in order to reproduce the distortion characteristics of the
high-frequency circuit portion. Based on the envelope transfer function
g(x), an amplitude distortion correcting function h(x) used to correct the
amplitude distortion impairing the linearity, and occurring in the
high-frequency circuit portion, is determined so that the relationship of
ax=g(h(x)) where a is a constant can be established, or a phase distortion
correcting function p(x) used to correct the phase distortion impairing
the linearity, and occurring in the high-frequency circuit portion, is
determined so that the relationship of c=g(p(x)) where c is a constant can
be established.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, an approximate expression
of the envelope transfer function g(x), whose variable x indicates an
input amplitude, is defined as g(x)=a.sub.0 +a.sub.1 x+g'(x) where a.sub.0
and a.sub.1 are constants and g'(x) is a polynomial. The amplitude
distortion correcting function is defined as x.times.(1-g'(x)/g(x)).
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, the constant a.sub.0 is
set to 0.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, the polynomial g'(x) in
the envelope transfer function g(x)=a.sub.0 +a.sub.1 x+g'(x) is defined as
g'(x)=.SIGMA..sub.i=2.sup.N a.sub.i x.sup.i where i and N are positive
integers equal to or larger than 2 and a.sub.i is a constant.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, only terms of
odd-numbered degrees of the polynomial g'(x)=.SIGMA..sub.i=2.sup.N a.sub.i
x.sup.i are employed.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, the input signal is
low-pass signals passed by low-pass filters defined on the basis of the
result of estimating the distortion characteristics. Frequencies relevant
to the phase distortion correcting function p(x) to be applied to the
low-pass signals are raised in order to produce an intermediate-frequency
or high-frequency signal for the purpose of correcting the phase
distortion impairing the linearity, and occurring in the high-frequency
circuit portion.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, when frequencies relevant
to the phase distortion correcting function p(x) to be applied to the
input signal are raised by performing digital modulation, signals passed
by transmission filters exhibiting root Nyquist characteristics are used
as the input signal.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, when frequencies relevant
to the phase distortion correcting function p(x) to be applied to the
input signal are raised by performing quadrature modulation, a real-part
signal and an imaginary-part signal constituting a complex-number signal
and passed by low-pass filters, that are defined on the basis of the
result of estimating the distortion characteristics and exhibit root
Nyquist characteristics, are used as the input signal.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, the input signal is
low-pass signals passed by low-pass filters defined on the basis of the
result of estimating the distortion characteristics. After the frequencies
of the low-pass signals are raised, the low-pass signals are input to a
first amplifier in the high-frequency circuit portion. After frequencies
relevant to the phase distortion correcting function to be applied to
low-pass signals that are substantially identical to the low-pass signals
are raised, the low-pass signals are input to a second amplifier in the
high-frequency circuit portion. Output signals of the first amplifier and
second amplifier are added in order to produce an output signal containing
a small number of amplitude distortion components impairing the linearity.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, the second amplifier
causes substantially no distortion. The phase distortion impairing the
linearity and occurring in the first amplifier alone is corrected.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, phase distortion
impairing linearity and occurring in the first amplifier is corrected, and
phase distortion impairing linearity and occurring in the second amplifier
is also corrected.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, the input signal is
low-pass signals passed by low-pass filters defined on the basis of the
result of estimating the distortion characteristics. After the frequencies
of the low-pass signals are raised, the low-pass signals are input to a
first amplifier in the high-frequency circuit portion. After frequencies
relevant to the amplitude distortion correcting function to be applied to
low-pass signals that are substantially identical to the low-pass signals
are raised, the low-pass signals are input to a second amplifier in the
high-frequency circuit portion. Output signals of the first amplifier and
second amplifier are added up in order to produce an output signal
containing a small number of amplitude distortion components impairing the
linearity.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, the second amplifier
causes substantially no distortion, and the amplitude distortion impairing
the linearity, and occurring in the first amplifier alone, is corrected.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, the amplitude distortion
impairing the linearity, and occurring in the first amplifier, is
corrected, and the amplitude distortion impairing the linearity, and
occurring in the second amplifier, is also corrected.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, the input signal is
low-pass signals passed by low-pass filters defined on the basis of the
result of estimating the distortion characteristics. An amplitude
distortion correcting function to be applied to the low-pass signals is
calculated. Frequencies relevant to a phase distortion correcting function
mated with the amplitude distortion correction function are raised in
order to produce an intermediate-frequency or high-frequency signal for
the purpose of correcting the amplitude distortion and the phase
distortion impairing the linearity, and occurring in the high-frequency
circuit portion.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, the envelope transfer
function g(x) is a function including a table function. At least one of
the amplitude distortion correcting function h(x) and phase distortion
correcting function p(x) is an expansion consisting of a polynomial
series. Coefficients in the terms of orders of the expansion are
calculated, and thus at least one of the amplitude distortion correcting
function h(x) and phase distortion correcting function p(x) is calculated.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, at least one of the
amplitude distortion correcting function h(x) and phase distortion
correcting function p(x) is fixed.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, when a plurality of
high-frequency circuit portions are included, at least one of the
amplitude distortion correcting function h(x) and phase distortion
correcting function p(x) is fixed for each high-frequency circuit portion
or for each group of high-frequency circuit portions produced under
similar conditions for manufacturing a high-frequency circuit portion.
More preferably, in an apparatus for correcting signals according to the
fundamental embodiment of the present invention, when the high-frequency
circuit portion is in operation, the envelope transfer function g(x) is
calculated intermittently or all the time. Based on the result of
calculating the envelope transfer function g(x), at least one of the
amplitude distortion correcting function h(x) and phase distortion
correcting function p(x) is modified.
According to the fundamental embodiment of the present invention, in an
apparatus for correcting signals, a transmitter having the apparatus for
correcting signals, and a method for correcting signals using the
apparatus for correcting signals, based on the result of estimating
distortion characteristics of a high-frequency circuit including the
high-frequency circuit portion 1, an amplitude distortion correcting
function or phase distortion correcting function effective in correcting
the amplitude distortion or the phase distortion occurring in the
high-frequency circuit can be easily calculated by using low-pass filters
or the like defined by the means for estimating distortion characteristics
in high-frequency circuit portion 2. For example, an envelope transfer
function g(x) which reproduces the distortion characteristics of the
high-frequency circuit concerning the amplitude distortion impairing the
linearity and of which variable x indicates an input amplitude can be
provided using the approximate expression (1):
##EQU1##
where N is an integer equal to or larger than 2.
To be more specific, a first method for correcting signals to be
implemented in an apparatus for correcting signals in accordance with the
present invention is such that nonlinear distortion components provided by
the polynomial (1), that is, by the second-order and succeeding terms of
the polynomial are subtracted from an output signal of a high-frequency
circuit. As a more practical method, there is a method where the
frequencies of the distortion components alone are raised (or
up-converted) then, if necessary, passed by an amplifier separate from a
main amplifier whose distortion characteristics are the subject for
correction, and then subtracted from an output signal of the main
amplifier.
In contrast, a second method for correcting signals, which is to be
implemented in an apparatus for correcting signal in accordance with the
present invention, is such that, assuming that a function of a certain
input signal is h(x) (that is, an amplitude distortion correcting
function), the amplitude distortion correcting function h(x) is calculated
so that the function g(h(x)) defined by the polynomial (1) can reproduce
given distortion characteristics. The phrase "the amplitude distortion
correcting function h(x) is calculated so that the function g(h(x))
defined by the polynomial (1) can reproduce given distortion
characteristics" means that, for example, when adjacent channel power ACP
poses a problem, the function g(h(x)) is used to carry out simulation so
as to measure the adjacent channel power ACP, and an approximate
expression of the function h(x) minimizing the adjacent channel power ACP
is calculated. Another meaning is that the function h(x) minimizing (that
is, nearly minimizing) b(x)-g(h(x)) where b(x) indicates a target linear
relationship of the variable x is calculated. Herein, b(x) can be modified
properly. Since the application of the amplitude distortion correcting
function h(x) is accompanied by predistortion of an input signal for
correcting distortion characteristics of a high-frequency circuit, the
amplitude distortion correcting function h(x) may be referred to as a
predistortion amplitude function.
In this case, a harmonic-balance method, modeling of a frequency range
using a Volterra series, or time domain simulation can be adopted as a
method of carrying out simulation for measuring adjacent channel power
ACP. For calculating approximate values of the amplitude distortion
correcting function h(x), the adoption of a diving method based on the
least mean squares (LMS) method or a delta rule would prove convenient.
According to the fundamental embodiment of the present invention, the fact
that distortion characteristics of a high-frequency circuit concerning the
distortion impairing the linearity can be reproduced by using low-pass
signals or the like produced by a simple circuit, is utilized effectively.
A signal correcting circuit, whose high-frequency circuit portion and
adjustment portion are smaller than those of a known signal correcting
circuit is used to calculate a distortion correcting function based on the
distortion characteristics. For carrying out various kinds of
communication using a portable telephone, portable information equipment,
or an information terminal, with low power consumption maintained and with
circuitry much simpler than the known circuitry, the distortion impairing
the linearity, and occurring in the high-frequency circuit, can be
suppressed reliably and power leaking out of channels adjoining each
communication channel can be minimized. Eventually, highly reliable
communication can be achieved.
According to another preferred embodiment of the present invention, as
shown in FIG. 17 described later, an apparatus for compensating for
distortion comprises: an absolute value calculating unit (30) to which an
in-phase component xi and an orthogonal component xq, which have been
filtered by a low-pass filter, are supplied, having a function of
previously distorting the in-phase component and the orthogonal component
so that non-linearity of a power amplifier arranged on the downstream side
can be improved, the absolute value calculating unit (30) calculating an
intensity of vector (xi, xq); and a predistortion unit (31, 32) in which a
function h(x)/x obtained when a predetermined predistortion function h(x)
is divided by the above intensity x is used or its reference table is
used, for multiplying the output xi, xq of the low-pass filter.
According to the above apparatus for compensating for distortion, the
following effects can be provided. When function h(x)/x is multiplied by
the in-phase component xi and the orthogonal component xq in accordance
with the intensity x of the vector of the in-phase component xi and the
orthogonal component xq, predistortion can be easily and accurately
conducted.
It is preferable that the above apparatus for compensating for distortion
comprises a phase pre-rotation unit (22) in which, for example, as shown
in FIG. 15, a predetermined function .phi.(x) or its reference table is
used, and the vector (xi, xq) or the vector ((h/x)xi, (h/x)xq) generated
in the above predistortion unit is rotated by angle .phi.(x).
According to the above apparatus for compensating for distortion, the
following effects can be provided. When the vector (the in-phase
component, the orthogonal component) is rotated by .phi.(x) according to
the intensity x of the vector of the in-phase component xi and the
orthogonal component xq, phase pre-rotation can be easily and accurately
conducted.
Further, in the digital signal processor of the present invention, the
above function of the apparatus for compensating for distortion is
realized by a program.
As shown in FIG. 17 described later, the transmitter of the present
invention comprises: an apparatus (22, 31, 32) for compensating for
distortion described in claim 30 or 31; an orthogonal modulation circuit
(70) for orthogonally modulating carrier waves by the in-phase component
and the orthogonal component which have passed through the apparatus for
compensating for distortion; and a power amplifier (10) for amplifying
orthogonally modulated waves sent from the orthogonal modulation circuit.
Further, the transmitter of the present invention comprises: an amplifying
characteristics detecting unit for detecting a relation between an input
amplitude and an output amplitude of the power amplifier; and a unit for
preparing distortion compensation data for determining the function h(x)/x
or the reference table according to the relation.
Further, the apparatus for preparing distortion compensation data of the
transmitter of the present invention, the function h(x)/x or the reference
table is determined so that the function g(h(x)) can be proportional to x,
wherein the output amplitude with respect to the input amplitude x of the
power amplifier is expressed by the function g(x).
According to the above transmitter, it is possible to determine an accurate
function h(x) for conducting distortion compensation.
Further, the section for preparing distortion compensation data of the
present invention comprises a processor characterized in that: when the
output amplitude with respect to the input amplitude x of the power
amplifier is expressed by the function g(x), the predistortion function
h(x) is made to be approximate to a power development of the amplitude x;
the power development coefficients c1 to cn are given initial values; the
amplitude x is made to be an amplitude of the intermodulation wave of the
base band signals, the angular frequencies of which are .omega.1 and
.omega.2; and the power development coefficients c1 to cn of the
predistortion function h(x) are determined so that an absolute value
.epsilon. of the ratio of the primary Fourier coefficient of the function
g(h(x)) to the second Fourier coefficient can be substantially locally
minimum.
According to the transmitter of the present invention, the following
effects can be provided. When the correction of nonlinearity of the power
amplifier is appropriately conducted, the predistortion function h(x) for
increasing the average transmitting power so that leakage power (power of
the side lobe) to an adjacent channel cannot exceed an allowable value,
can be easily and effectively obtained.
It is preferable that the unit for preparing distortion compensation data
determines the amplitude of the intermodulation wave according to the
maximum value of the input amplitude of the power amplifier.
According to the above transmitter, it is possible to obtain the
predistortion function h(x) capable of preventing the clipping of the
output of the power amplifier and also capable of further increasing the
output of the power amplifier.
In the transmitter of the present invention, a step in which an absolute
value .epsilon. of the ratio is calculated and the coefficients c1 to cn
are changed by the method of steepest descent is repeated until the
absolute value .epsilon. of the ratio is decreased to a value lower than a
predetermined value.
In the transmitter of the present invention, the amplifier characteristics
detection unit detects a slippage of the phase of the output from that of
the input when the input amplitude of the power amplifier is x, and the
unit for preparing distortion compensation data further determines a
function .phi.(x) for expressing a slippage of the phase or determines its
reference table.
The apparatus for preparing distortion compensation data of the present
invention comprises a processor characterized in that: for example, as
shown in FIG. 17 described later, in order to improve nonlinearity of the
power amplifier, the processor is used for a distortion compensation
device in which the amplitude x of a signal on the upstream side of the
power amplifier is previously distorted to a value of the predistortion
function h(x); when the output amplitude with respect to the input
amplitude x of the power amplifier is expressed by the function g(x), the
predistortion function h(x) is made to be approximate to a power
development of the amplitude x; the power development coefficients c1 to
cn are given initial values; the amplitude x is made to be an amplitude of
the intermodulation wave of the base band signals, the angular frequencies
of which are .omega.1 and .omega.2; and the power development coefficients
c1 to cn of the predistortion function h(x) are determined so that an
absolute value .epsilon. of the ratio of the primary Fourier coefficient
of the function g(h(x)) to the second Fourier coefficient can be
substantially a local minimum.
In the apparatus for preparing distortion compensation data of the present
invention, an amplitude of the intermodulation wave is determined
according to the maximum value of the input amplitude of the power
amplifier.
FIG. 3 is a circuit block diagram showing the first preferred embodiment of
the present invention. Shown, as an example, is the circuitry in which the
amplitude distortion or the phase distortion impairing the linearity, and
occurring in a main amplitude 10 exhibiting nonlinearity serving as a
high-frequency circuit whose distortion characteristics are the subject
for correction, is corrected. Hereinafter, the same reference numerals
will be assigned to components identical to those described previously.
In FIG. 3, the means for estimating distortion characteristics in
high-frequency circuit portion 2 (See FIG. 2) which estimate distortion
characteristics of the main amplifier 10 concerning the phase distortion
impairing the linearity is realized by a central processing unit (CPU) 41
included in a computer system. In the CPU 41, an approximate function of
an envelope transfer function g(x) whose variable x indicates an input
amplitude is defined as g(x)=a.sub.0 +a.sub.1 x+g'(x) (where a.sub.0 and
a.sub.1 are constants) in order to reproduce the distortion
characteristics of the main amplifier 10. By the way, g'(x) is a
polynomial. An amplitude distortion correcting function used to correct
the amplitude distortion impairing the linearity, and occurring in the
main amplifier 10, is given as x.times.(1-g'(x)/g(x)). It should be noted
is that when the variable x is a complex number, the input amplitude is
given as .vertline.x.vertline. (absolute value of x). The CPU 41 may
include a digital signal processor (DSP) shown in FIG. 13. In this case,
preferably, after the constant a.sub.0 is set to 0 and an offset is set to
0, initialization is carried out.
In FIG. 3, there are shown a first low-pass signal generating unit 4-1 and
second low-pass signal generating unit 4-2 which are realized with
low-pass filters having a filter coefficient set by the CPU 41. The first
low-pass signal generating unit 4-1 and second low-pass signal generating
unit 4-2 are preferably formed with root raised cosine filters (RRCOS
filters) exhibiting root Nyquist characteristics. The first low-pass
signal generating unit 4-1 and second low-pass signal generating unit 4-2
pass a real-part signal I and imaginary-part signal Q constituting a
complex-number signal generated by the CPU 4 and containing distortion
components reproduced according to the distortion characteristics of the
main amplifier, and thus exert the ability to reproduce nonlinear
distortion components occurring in the main amplifier.
To be more specific, when signals are passed by the RRCOS filters, the
polynomial g'(x) in the envelope transfer function g(x)=a.sub.0 +a.sub.1
x+g'(x) is expressed as g'(x)=.SIGMA..sub.i=2.sup.N a.sub.i x.sup.i (where
N is a positive integer equal to or larger than 2, and a.sub.i is a
constant). In this case, preferably, only the terms of odd-numbered orders
of the polynomial g'(x)=.SIGMA..sub.i=2.sup.N a.sub.i x.sup.i, which
express distortion components, are employed. In FIG. 3, there is shown an
absolute value-of-complex-number signal calculating unit (ABS) 30 for
calculating an absolute value of a complex-number signal consisting of the
real-part signal I and imaginary-part signal Q that are low-pass signals
output from the RRCOS filters or a sum of squares of the real-part signal
I and imaginary-part signal Q. Installed on the output stage of the
absolute value-of-complex-number signal calculating unit 30 are a phase
distortion correcting unit 20 formed with a rotator (ROT) that is a kind
of phase modulator, and a first amplitude distortion component generating
unit 31 (that is, a first amplitude modulator AM) and second amplitude
distortion component generating unit 32 (that is, a second amplitude
modulator AM).
In the first preferred embodiment, a phase distortion correcting function
p(x) to be applied to the complex-number signal consisting of the
real-part and imaginary-part signal that are substantially identical to
low-pass signals of basebands output from the first and second low-pass
signal generating units 4-1 and 4-2 is calculated in advance. The phase of
the complex-number signal is shifted by an angle given by reversing the
sign of the phase distortion correcting function p(x), whereby the phase
distortion impairing the linearity is corrected. In this case, the phase
distortion correcting function p(x) is expressed as p(x)=-g(x). The first
amplitude distortion component generating unit 31 and second amplitude
distortion component generating unit 32 do not correct the amplitude
distortion impairing the linearity, but modulate in amplitude the
complex-number signal so as to raise the frequency band of the
complex-number signal.
In FIG. 3, there are shown a first quadrature modulation unit 5-1 for
modulating signals having the phase distortion impairing the linearity
thereof corrected, and producing a desired intermediate-frequency or
high-frequency signal; and a second quadrature modulation unit 5-2 for
modulating signals containing amplitude distortion components, and
producing a desired intermediate-frequency or high-frequency signal.
The first quadrature modulation unit 5-1 includes a first in-phase carrier
wave multiplier 51 for multiplying the real-part signal I having phase
distortion impairing linearity thereof corrected by a carrier wave Ci that
is in phase with the real-part signal, a first 90.degree. out-of-phase
carrier wave multiplier 52 for multiplying the imaginary-part signal Q by
a carrier wave Cq that is 90.degree. out of phase with the imaginary-part
signal, and a first adder 53 for adding up an in-phase quadrature
modulation signal output from the first in-phase carrier wave multiplier
51 and a 90.degree. out-of-phase quadrature modulation signal output from
the second 90.degree. out-of-phase carrier wave multiplier 52.
The second quadrature modulation unit 5-2 includes a second in-phase
carrier wave multiplier 54 for multiplying the real-part signal I
containing amplitude distortion components by a carrier wave Ci that is in
phase with the real-part signal, a second 90.degree. out-of-phase carrier
wave multiplier 55 for multiplying the imaginary-part signal Q by a
carrier wave Cq that is 90.degree. out of phase with the imaginary-part
signal, and a second adder 56 for adding up an in-phase quadrature
modulation signal output from the second in-phase carrier wave multiplier
54 and a 90.degree. out-of-phase quadrature modulation signal output from
the second 90.degree. out-of-phase carrier wave multiplier 55.
Furthermore, a quadrature modulation signal having the phase distortion
impairing the linearity thereof corrected is output from the first
quadrature modulation unit 5-1, and then supplied to the main amplifier 10
(first amplifier) exhibiting nonlinearity. A quadrature modulation signal
containing amplitude distortion components is output from the second
quadrature modulation unit 5-2, and then supplied to an auxiliary
amplifier 11 (second amplifier) causing substantially no distortion and
offering good linearity.
In FIG. 3, an amplitude distortion component subtractor 12 formed with an
adder is installed on the output stage of the main amplifier 10 and
auxiliary amplifier 11. The amplitude distortion component subtractor 12
subtracts an output signal of the auxiliary amplifier 11 from an output
signal of the main amplifier 10, whereby an output signal Sout having the
amplitude distortion components thereof canceled out and exhibiting good
linearity can be produced.
When frequencies relevant to the phase distortion correcting function p(x)
to be applied to low-pass signals are raised by performing digital
modulation, signals passed by transmission filters exhibiting root Nyquist
characteristics should preferably be used as the low-pass signals.
Furthermore, when frequencies relevant to the phase distortion correcting
function p(x) to be applied to low-pass signals are raised by performing
quadrature modulation, real-part and imaginary-part signals constituting a
complex-number signal and passed by low-pass filters exhibiting the root
Nyquist characteristics defined according to the distortion characteristic
of the main amplifier 10 should preferably be used as the low-pass
signals.
Furthermore, in the first preferred embodiment, even when the linearity
offered by the auxiliary amplifier 11 is not very good, if the distortion
characteristics of the auxiliary amplifier are estimated and a phase
distortion correcting function is calculated in advance, the phase
distortion impairing the linearity, and occurring in the auxiliary
amplifier, can also be corrected.
FIG. 4 is a circuit block diagram showing the second preferred embodiment
of the present invention. Shown, as an example, is the circuitry in which
the amplitude distortion or phase distortion impairing the linearity, and
occurring in the main amplifier 10 offering nonlinearity and serving as a
high-frequency circuit, whose distortion characteristics are the subject
for correction, is corrected.
The configurations and abilities of the CPU 41, first low-pass signal
generating unit 4-1, and second low-pass signal generating unit 4-2 shown
in FIG. 4 are identical to those shown in FIG. 2. The detailed description
of the components will be omitted.
Even in FIG. 4, as in FIG. 3, there is shown an absolute
value-of-complex-number signal calculating unit 30 for calculating an
absolute value of a complex-number signal consisting of real-part and
imaginary-part signals that are low-pass filters output from the RRCOS
filters or calculating a sum of squares of the real-part signal I and
imaginary-part signal Q. Installed on the output stage of the absolute
value-of-complex-number signal calculating unit 30 are a phase distortion
correcting unit 20 formed with a rotator, a first amplitude distortion
component generating unit 31 and second amplitude distortion component
generating unit 32 for modulating in amplitude a complex-number signal,
and an amplitude distortion correcting unit 21 formed with a rotator.
In the second preferred embodiment, a phase distortion correcting function
p(x) which is to be applied to a complex-number signal consisting of a
real-part signal and imaginary-part signal substantially identical to
low-pass signals of basebands, is calculated in advance. The phase of the
complex-number signal is shifted by an angle given by changing the sign of
the phase distortion correcting function p(x), by means of the phase
distortion correcting unit 20, whereby the phase distortion impairing the
linearity is corrected. An amplitude distortion correcting function to be
applied to the complex-number signal is calculated in advance. The
amplitude of the complex-number signal is adjusted by the amplitude
distortion correcting unit 21, whereby the amplitude distortion impairing
the linearity is corrected.
In FIG. 4, there are shown a first quadrature modulation unit 5-1 for
modulating signals having the phase distortion impairing the linearity
thereof corrected, and producing a desired intermediate-frequency or
high-frequency signal; and a second quadrature modulation unit 5-2 for
modulating signals having the amplitude distortion impairing the linearity
thereof corrected, and producing a desired intermediate-frequency or
high-frequency signal.
The first quadrature modulation unit 5-1 includes, like the one shown in
FIG. 2, a first in-phase carrier wave multiplier 51, a first 90.degree.
out-of-phase carrier wave multiplier 52, and a first adder 53 for adding
up an in-phase quadrature modulation signal output from the first in-phase
carrier wave multiplier 51 and a 90.degree. out-of-phase quadrature
modulation signal output from the second 90.degree. out-of-phase carrier
wave multiplier 52.
The second quadrature modulation unit 5-2 includes, like the one shown in
FIG. 2, a second in-phase carrier wave multiplier 54, a second 90.degree.
out-of-phase carrier wave multiplier 55, and a second adder 56 for adding
up an in-phase quadrature modulation signal output from the second
in-phase carrier wave multiplier 54 and a 90.degree. out-of-phase
quadrature modulation signal output from the second 90.degree.
out-of-phase carrier wave multiplier 56.
A quadrature modulation signal having the phase distortion impairing the
linearity thereof corrected is output from the first quadrature modulation
unit 5-1, and then supplied to a phase distortion correcting signal
amplifier 13 offering good linearity. A quadrature modulation signal
having amplitude distortion components thereof corrected is output from
the second quadrature modulation unit 5-2, and then supplied to an
amplitude distortion correcting signal amplifier 14 offering good
linearity.
Referring to FIG. 4, an amplitude distortion/phase distortion correcting
signal adder 15 formed with an adder is installed on the output stage of
the phase distortion correcting signal amplifier 13 and amplitude
distortion correcting signal amplifier 14. The amplitude distortion/phase
distortion correcting signal adder 15 adds up two kinds of signals each
having the amplitude distortion and the phase distortion components
thereof corrected, and supplies a resultant signal to the main amplifier
10 having nonlinearity. Finally, an output signal Sout, having the
amplitude distortion and the phase distortion components thereof, which
impair the linearity thereof, canceled out, is output through the output
terminal of the main amplifier 10.
Furthermore, in the second preferred embodiment, even when the linearity
offered by the amplitude distortion correcting signal amplifier 14 and
phase distortion correcting signal amplifier 13 is not very good, if the
distortion characteristics of the amplifiers are estimated and an
amplitude distortion correcting function and phase distortion correcting
function are calculated in advance, the distortion impairing the
linearity, and occurring in the amplitude distortion correcting signal
amplifier 14 and phase distortion correcting signal amplifier 13, can be
corrected.
FIG. 5 is a circuit block diagram showing the third preferred embodiment of
the present invention. Shown, as an example, is the circuitry in which the
amplitude distortion or phase distortion impairing the linearity, and
occurring in the main amplifier 10 that has nonlinearity and serves as a
high-frequency circuit whose distortion characteristics are the subject
for correction, is corrected.
The configurations and abilities of the CPU 41, the first low-pass signal
generating unit 4-1, and the second low-pass signal generating unit 402
shown in FIG. 5 are identical to those shown in FIGS. 2 and 3. A detailed
description of the components will be omitted.
In FIG. 5, as in FIGS. 3 and 4, there is shown an absolute
value-of-complex-number signal calculating unit 30 for calculating an
absolute value of a complex-number signal consisting of a real-part signal
and imaginary-part signal that are low-pass signals output from the RRCOS
filters or the like, or for calculating a sum of squares of the real-part
signal I and imaginary-part signal Q. Installed on the output stage of the
absolute value-of-complex-number signal calculating unit 30 are a first
amplitude distortion component generating unit 31 and second amplitude
distortion component generating unit 32 for modulating in amplitude the
complex-number signal, and an amplitude distortion/phase distortion
correcting unit 22 formed with a common rotator.
In the third preferred embodiment, an amplitude distortion correcting
function and phase distortion correcting function to be applied to a
complex-number signal consisting of a real-part signal and imaginary-part
signal that are substantially identical to low-pass signals of basebands
are calculated in advance. The signs of values of the correction functions
are reversed by the amplitude distortion/phase distortion correcting unit
22. Thus, both amplitude distortion and phase distortion impairing the
linearity are corrected at the same time. More specifically, distortion
components expressed by the below-mentioned polynomial (2) are appended to
the complex-number signal, and amplitude distortion correction and phase
distortion correction are carried out at the same time. This results in a
simpler circuit.
Assuming that
##EQU2##
the signs of converted values of inputs that are distortion components are
reversed as follows.
##EQU3##
When it is said that "the signs of converted values of inputs that are
distortion components are reversed," this means that distortion components
contained in an output signal of the main amplifier are canceled out.
In FIG. 5, there is shown a common quadrature modulation unit 5-3 for
modulating signals, each of which has the amplitude distortion and the
phase distortion impairing the linearity thereof corrected simultaneously,
and producing a desired intermediate-frequency or high-frequency signal.
The common quadrature modulation unit 5-3 includes a common in-phase
carrier wave multiplier 57, a common 90.degree. out-of-phase phase carrier
wave multiplier 58, and a common adder 59. The common adder 59 has the
ability to add up an in-phase quadrature modulation signal and 90.degree.
out-of-phase quadrature modulation signal output from the common in-phase
carrier wave multiplier 57 and 90.degree. out-of-phase carrier wave
multiplier 58, and the ability to add up two kinds of signals each having
the amplitude distortion and the phase distortion components thereof
corrected.
Furthermore, a quadrature modulation signal, having the amplitude
distortion and the phase distortion impairing the linearity thereof
corrected, is output from the common quadrature modulation unit 503, and
then supplied to the main amplifier 10 having nonlinearity. Finally, an
output signal Sout having the amplitude distortion and the phase
distortion components thereof, which impair the linearity thereof,
canceled out is output through the output terminal of the main amplifier
10.
In the first to third preferred embodiments, the first method for
correcting signals to be implemented in an apparatus for correcting
signals in accordance with the present invention is adopted. Distortion
components alone occurring in a main amplifier that has nonlinearity are
subjected to quadrature modulation, and then subtracted from an output
signal of the main amplifier.
Furthermore, in the third preferred embodiment, distortion components
expressed by the below-mentioned polynomial (3) below may be appended to a
complex-number signal, and then amplitude distortion correction and phase
distortion correction may be carried out.
Assuming that
##EQU4##
when the signs of converted values of inputs that are distortion components
are reversed:
##EQU5##
where the denominator of the polynomial (3) is equivalent to g(x).
Distortion components expressed by the polynomial (3) includes "curved"
components of the complex-number signal. Amplitude distortion correction
and phase distortion correction can therefore be carried out with
relatively high precision. In this case, a circuit block diagram will be
identical to FIG. 5. Distortion components expressed by the polynomial (3)
can be readily reproduced merely by modifying software (i.e., software
program) in the CPU.
FIG. 6 is a circuit block diagram showing the fourth preferred embodiment
of the present invention. Shown as an example is the circuitry in which
the amplitude distortion or the phase distortion impairing the linearity,
and occurring in the main amplifier 10, that offers nonlinearity and
serves as a high-frequency circuit whose distortion characteristics are
the subject for correction, is corrected.
The configurations and abilities of the CPU 41, first low-pass signal
generating unit 4-1, and second low-pass signal generating unit 4-2 are
identical to those shown in FIGS. 3 to 5. The distortion characteristics
of the main amplifier 10 estimated by the CPU 41 are considerably
different from those of the main amplifiers shown in FIGS. 3 to 5.
In the fourth preferred embodiment, an envelope transfer function g(x),
whose variable x indicates an input amplitude, is calculated in order to
reproduce the distortion characteristics of the main amplitude. Based on
the envelope transfer function g(x), an amplitude distortion correcting
function h(x) (that is, a predistortion amplitude function) used to
correct the amplitude distortion impairing the linearity, and occurring in
the main amplifier, is calculated so that the relationship of ax=g(h(x))
where a is a constant can be established, or a phase distortion correcting
function p(x) used to correct the phase distortion impairing the
linearity, and occurring in the main amplifier, is calculated so that the
relationship of c=g(p(x)) where c is a constant can be established. That
is to say, according to a second method for correcting signals to be
implemented in an apparatus for correcting signals in accordance with the
present invention, the amplitude distortion correcting function h(x) is
calculated so that the function g(h(x)) can reproduce given distortion
characteristics.
For calculating the distortion characteristic correcting function h(x)
according to the second method for correcting signals, the number of
calculations is relatively large. Preferably, digital low-pass signals
passed by low-pass transmission filters exhibiting root Nyquist
characteristics are processed at high speed by means of a digital signal
processor (DSP).
In FIG. 6, as in FIGS. 3 to 5, there is shown an absolute
value-of-complex-number signal calculating unit 30 for calculating an
absolute value of a digital complex-number signal consisting of a
real-part signal and imaginary-part signal that are digital low-pass
signals output from the RRCOS filters or the like, or a sum of the squares
of the real-part signal I and imaginary-part signal Q.
Referring to FIG. 6, installed on the output stage of the absolute
value-of-complex-number signal calculating unit 30 is a predistortion
function generating unit 33 for calculating an amplitude distortion
correcting function h(x) to be applied to a complex-number signal that is
an input signal. The predistortion function generating unit 33 calculates
the predistortion amplitude function that is applied for distorting the
complex-number signal in advance so as to correct the distortion
characteristics of the main amplifier 10.
Referring to FIG. 6, a predistortion function absolute value calculating
unit 34 for calculating an absolute value of the predistortion function,
and an amplitude distortion/phase distortion correcting unit 23 for
simultaneously carrying out amplitude distortion correction and phase
distortion correction for an input signal are installed on the output
stage of the predistortion function generating unit 33. Herein, in
consideration of the fact that after the predistortion amplitude function
is applied to the complex-number signal, the amplitudes of the
complex-number signal vary, the predistortion function absolute value
calculating unit 34 calculates the absolute value of the complex-number
signal so that amplitude distortion correction and phase distortion
correction can be achieved with high precision.
In FIG. 6, there are shown digital-to-analog converters 24 and 25 for
converting a digital complex-number signal, which has the amplitude
distortion and the phase distortion impairing the linearity thereof
corrected by the amplitude distortion/phase distortion correcting unit 23,
into an analog form, and a common quadrature modulation unit 5-4 for
modulating analog complex-number signals output from the digital-to-analog
converters 24 and 25 and producing a desired intermediate-frequency or
high-frequency signal.
The common quadrature modulation unit 5-4 includes a common in-phase
carrier wave multiplier 57p, a common 90.degree. out-of-phase carrier wave
multiplier 58p, and a common adder 59p. The common adder 59p has the
ability to add up an in-phase quadrature modulation signal and 90.degree.
out-of-phase quadrature modulation signal output from the common in-phase
carrier wave multiplier 57p and 90.degree. out-of-phase carrier wave
multiplier 58p, and the ability to add up two kinds of signals each having
amplitude distortion and phase distortion components thereof corrected.
Furthermore, a quadrature modulation signal having the amplitude distortion
and the phase distortion impairing the linearity thereof corrected is
output from the common quadrature modulation unit 5-3, and then supplied
to the main amplifier 10 offering nonlinearity. Finally, an output signal
Sout having amplitude distortion and phase distortion components thereof,
which impair the linearity thereof, canceled out is output through the
output terminal of the main amplifier 10.
Furthermore, in the fourth preferred embodiment, the envelope transfer
function g(x) is a function including a "table" function. The amplitude
distortion correcting function h(x) or phase distortion correcting
function p(x) is an expansion consisting of a polynomial series.
Coefficients in terms of all orders of the expansion are calculated,
whereby the amplitude distortion correcting function h(x) or phase
distortion correcting function p(x) is calculated.
Furthermore, in the fourth preferred embodiment, the amplitude distortion
correcting function h(x) or phase distortion correcting function p(x)
should preferably be fixed.
Furthermore, in the fourth preferred embodiment, when a plurality of
amplifiers offering nonlinearity are present, the amplitude distortion
correcting function h(x) or phase distortion correcting function p(x)
should preferably be fixed for each amplifier or each group of amplifiers
produced under similar manufacturing conditions.
Furthermore, in the fourth preferred embodiment, while the amplifier is in
operation, the envelope transfer function g(x) is calculated
intermittently or all the time. Based on the result of calculating the
envelope transfer function g(x), the amplitude distortion correcting
function h(x) or phase distortion correcting function p(x) is modified.
In a variant of the fourth preferred embodiment, an amplitude distortion
correcting function defined by the below-mentioned polynomial (4) below
may be applied to a complex-number signal for predistortion.
##EQU6##
A coefficient ci in the polynomial (4) is a constant in the amplitude
distortion correcting function which is newly defined for predistortion.
In this case, a circuit block diagram will be the same as that of FIG. 6.
The coefficient c.sub.i in the amplitude distortion correcting function
defined by the polynomial (4) can be calculated for a relatively high
order merely by modifying software in the DSP.
Using any of the apparatuses for correcting signals in accordance with the
first to fourth preferred embodiments of the present invention, a method
for correcting signals in which: distortion characteristics of a
high-frequency circuit including an amplifier concerning the amplitude
distortion and the phase distortion impairing the linearity is estimated;
at least one of an amplitude distortion correcting function and phase
distortion correcting function calculated on the basis of the result of
estimating the distortion characteristics is applied to low-pass signals;
a signal based on the low-pass signals to which the amplitude distortion
correcting function or phase distortion correcting function is applied is
supplied to the high-frequency circuit; and thus at least one of the
amplitude distortion and the phase distortion impairing the linearity, and
occurring in the high-frequency circuit is corrected, can be easily
implemented.
A technique for reducing distortion, which occurs during intermodulation
according to distortion characteristics of a microwave amplifier, and the
conclusion of a discussion on the technique will be explained in detail.
Discussion on Reduction of Distortion Stemming from Intermodulation and
Occurring in a Microwave Amplifier due to Predistortion
(A) Summary
It is generally known that adjacent channel power ACP can be reproduced by
expressing the linearity offered by a microwave amplifier using a function
of an input amplitude, that is, two functions of an amplitude and phase
indicated by an envelope, expressing a carrier wave in the form of an
"instantaneous" transfer function, and employing a discrete model or a
model of discrete circuits. When aliasing caused by digital processing can
be suppressed and a frequency resolution is sufficient, even if a low-pass
model or a model of circuits each including a low-pass filter which has a
simpler configuration than the discrete model is employed, the same
results are obtained. Simulation was carried out using the low-pass model
in order to reproduce adjacent channel power ACP. It was confirmed that
the adjacent channel power ACP could be reproduced. It was then discussed
whether or not the distortion impairing linearity, and occurring in a
narrow-band high-frequency circuit, could be corrected on the basis of
this fact (baseband predistortion).
A signal of a personal digital cellular form was taken as an example.
Measured data concerning a hetero-junction bipolar transistor (HBT) power
amplifier was used to carry out simulation for reproducing adjacent
channel power ACP. Whichever of the discrete model and low-pass model was
used, measured values and the results of simulation agreed with each other
with a difference of 2 dB or smaller between them as far as significant
parts of the values and results were concerned. The aforesaid two methods
for correcting signals will be discussed as a method for correcting
distortion that occurs during intermodulation according to distortion
characteristics of a microwave amplifier. For confirmation, the two
methods for correcting signals will be reiterated briefly.
(1) Distortion components are removed on the output stage of a
high-frequency circuit. Adjacent channel power ACP is reduced by 40 dB or
more. It has been demonstrated that the phase distortion impairing the
linearity can be nullified by applying the inverse of the phase distortion
impairing the linearity.
(2) On the basis of an envelope amplitude distortion function employed in a
simulation for reproducing adjacent channel power ACP, a prediction
function (for example, a prediction amplitude function) is calculated
according to the LMS method. The prediction function is applied for
distorting the real-part signal and imaginary-part signal I and Q of a
complex-number signal in advance, whereby adjacent channel power ACP can
be reduced by about 10 dB.
(B) Estimating Adjacent Channel Power ACP
In a known discrete model, a PDC signal is used to estimate adjacent
channel power ACP generated by an amplifier according to a method
described below. A thirteenth-order polynomial is used to express the
amplitude distortion impairing the linearity and the phase distortion
impairing the linearity as a distortion model or a model of expressions
representing distortion components occurring in an amplifier, and applied
to a PDC up signal (QPSK modulation, .alpha.=0.5). When an input amplitude
x is expressed by a complex number, the amplitude and phase of an output
signal of the amplitude can be regarded as functions of an absolute value
of the amplitude .vertline.x.vertline. of the high-frequency input signal
of the amplifier, and are therefore given as an envelope amplitude
function and envelope phase function g(.vertline.x.vertline.) and
p(.vertline.x.vertline.) respectively. Specifically, the envelope
amplitude function and envelope phase function g(.vertline.x.vertline.)
and p(.vertline.x.vertline.) are defined by the expressions (5) and (6)
below. A signal whose output amplitude is determined by the envelope
amplitude function g(.vertline.x.vertline.) is given by the expression (7)
below.
##EQU7##
V.sub.osr =g(.vertline.x.vertline.)e.sup.i(.angle.p(x)) (7)
When the signal V.sub.osr given by the expression (7) is passed by a
low-pass filter, a signal F.sub.os resulting from Fourier transform and a
signal V.sub.os resulting from inverse Fourier transform are produced as
defined by the expression (8) and expression (9).
F.sub.os =fft(V.sub.osr)H.sub.lpf (8)
V.sub.os =ifft(F.sub.os) (9)
here Hlpf denotes a frequency characteristic of a low-pass filter for
passing frequencies within a bandwidth that is six times larger than the
frequency band handled by quadrature modulation, fft denotes Fourier
transform, and ifft denotes inverse Fourier transform. The narrower the
bandwidth of the low-pass filter is, the smaller the adverse effect of
aliasing noise stemming from digital signal processing is. A minimum
bandwidth necessary for predicting adjacent channel power ACP2 leaking out
of channels adjoining adjacent channels is therefore set to 6.times.21
kHz.apprxeq.100 kHz+10.5 kHz. When the absolute value of an input
amplitude of a carrier wave, .vertline.x.sub.c.vertline., is defined as
.vertline.x.sub.c.vertline.=(2+L Pin).times.2, if the amplitude of the
carrier wave is regarded as an instantaneous amplitude function
g'(.vertline.x.sub.c.vertline.) the carrier wave V.sub.c can be extracted
by passing a signal V.sub.cr through a narrow-band filter exhibiting
frequency characteristics of Hnbf. The instantaneous amplitude function
g'(.vertline.x.sub.c.vertline.), signal V.sub.cr supplied to the
narrow-band filter, and carrier wave V.sub.c passed by the narrow-band
filter are defined as the expressions (10), (11), and (12) below.
##EQU8##
V.sub.cr =g'(.vertline.x.sub.c.vertline.)e.sup.i.angle.xc (11)
V.sub.c =ifft(fft(A.sub.cr).times.H.sub.nbf) (12)
where .xi. denotes a scaling factor determined for filtering coefficients
.alpha..sub.i included in a Bessel series on the basis of measured values
indicating amplitude modulation-amplitude modulation (AM-AM)
characteristics, and is provided by the expression below.
.xi.=.pi./2max(.vertline.x.vertline.) (13)
An output signal V.sub.o of the amplifier is defined by the expression (14)
below on the assumption that the amplitude of input power Pin is given as
(2+L P.sub.in +L ).
V.sub.o =V.sub.os V.sub.c e.sup.i{-p(.vertline.x.vertline.)} /g(2+L
P.sub.in +L ) (14)
Coefficients of finite orders are determined according to amplitude
modulation-amplitude modulation (AM-AM) characteristics and amplitude
modulation-frequency modulation (AM-PM) characteristic which are actually
measured by inputting a single tone to an amplifier. Thus, approximate
values of the functions are calculated. According to this method, adjacent
channel power ACP can be calculated using a PDC signal by varying input
power. Only when a discrete model is used, adjacent channel power ACP2
leaking out of channels adjoining adjacent channels can be reproduced
successfully. This is because the frequency band of a signal to which
amplitude distortion components impairing the linearity are appended is
limited. Even when a low-pass model relating to the present invention is
used, if similar filtering is carried out, adjacent channel power ACP1
leaking out of adjacent channels and adjacent channel power ACP2 leaking
out of channels adjoining the adjacent channels can be reproduced with
high accuracy (Refer to the expression (6)). As far as the low-pass model
employed this time is concerned, a signal V.sub.osr and signal V.sub.os
are defined by the expressions (15) and (16) below by employing the
aforesaid envelope amplitude function and envelope phase function
g(.vertline.x.vertline.) and p(.vertline.x.vertline.).
V.sub.osr =g(.vertline.x.vertline.)e.sup.i.angle.x (15)
V.sub.os =ifft(fft(V.sub.osr).times.H.sub.lpf) (16)
A final output signal of the amplifier is provided as the expression (17)
below including multiplication of 2 sin(.omega..sub.c t) expressing a
carrier wave.
V.sub.o =2 sin(.omega..sub.c
t).times.V.sub.os.times.e.sup.i-p(.vertline.x.vertline.) (17)
FIG. 7 is a block diagram showing a low-pass model used to implement a
method for correcting signals in accordance with the present invention by
carrying out a procedure described below; and FIG. 8 is a graph showing
the results of simulation performed using the low-pass model.
In the low-pass model shown in FIG. 7, a low-pass signal 6-1 is used as an
input signal, and an envelope amplitude function generating unit 6-2
calculates an envelope amplitude function g(.vertline.x.vertline.). A
signal representing the envelope amplitude function
g(.vertline.x.vertline.) is passed through a low-pass filter (LPF) 6-3. An
envelope phase function generating unit 6-4 calculates an envelope phase
function (exp(ip(.vertline.x.vertline.))). An amplitude distortion
component amplifier 6-5 multiplies the value of the signal passed by the
low-pass signal by 2 sin(.omega..sub.ct) expressing a carrier wave.
Eventually, an output signal V.sub.o of an amplifier is produced by
carrying out simulation.
As a result of carrying out simulation using the low-pass model shown in
FIG. 7, the graph of FIG. 8 is plotted to indicate the relationship
between output power P.sub.out of an amplifier and adjacent channel power
ACP. In the graph, two solid lines indicate the results of measuring
adjacent channel power ACP1 leaking out of adjacent channels and the
results of measuring adjacent channel power ACP2 leaking out of channels
adjoining the adjacent channels. Two dashed lines indicate, as results of
simulation, adjacent channel power leaking out of the adjacent channels on
both sides. Two dot-dash lines indicate, as results of simulation,
adjacent channel power ACP2 leaking out of channels adjoining the two
adjacent channels on both sides.
As is apparent from the graph of FIG. 8, as long as output power P.sub.out
of an amplifier falls within a range of levels within which the power is
actually consumed, the adjacent channel power ACP1 leaking out of adjacent
channels and the adjacent channel power ACP2 leaking out of channels
adjoining the adjacent channels are reproduced with high accuracy.
(C) Reducing Adjacent Channel Power ACP Using a Low-pass Model
As mentioned above, it has been demonstrated that distortion stemming from
intermodulation and occurring in a high-frequency circuit can be
reproduced using a low-pass model. A method of reducing adjacent channel
power ACP using the low-pass model will be discussed.
(C-1). Correcting Amplitude Distortion Impairing Linearity and Occurring in
a High-frequency Circuit
Using the configuration of the first embodiment shown in FIG. 3, simulation
is carried out on the assumption that the amplitude distortion impairing
the linearity, and occurring in a high-frequency circuit, will be
corrected. In this case, the amplitude of low-pass signals output from the
RRCOS filters is worked out by calculating an absolute value of a
complex-number signal composed of the real-part signal I and
imaginary-part signal Q. The terms of second and higher orders of an
envelope amplitude function used to estimate adjacent channel power ACP
express distortion components occurring in the high-frequency circuit. The
signs of the terms of second and higher orders are reversed in order to
calculate the amplitudes of the distortion components. Frequencies
relevant to the amplitudes are raised and subtracted from an output of the
high-frequency circuit. Thus, the adjacent channel power ACP can be
expected to be reduced.
FIG. 9 is a graph showing how amplitude distortion components are canceled
out in a high frequency range according to any of the embodiments of the
present invention. Shown is the relationship between output power Pout and
adjacent channel power ACP attained when simulation is carried out using
the configuration of the first embodiment (FIG. 3) of the present
invention in order to correct the amplitude distortion impairing the
linearity and occurring in an amplifier.
In the graph of FIG. 9, two solid lines indicate the results of measuring
adjacent channel power ACP1 leaking out of adjacent channels, and the
results of measuring adjacent channel power ACP2 leaking out of channels
adjoining the adjacent channels. For the measurement, the amplitude
distortion impairing the linearity, and occurring in the amplifier, is not
corrected. Two dashed lines indicate adjacent channel power ACP1 leaking
out of the adjacent channels after amplitude distortion components
occurring in the amplifier are canceled out. Two dot-dash lines indicate
adjacent channel power ACP2 leaking out of the channels adjoining the
adjacent channels after amplitude distortion components occurring in the
amplifier are canceled out.
As apparent from the graph of FIG. 9, the adjacent channel power ACP1
leaking out of adjacent channels and the adjacent channel power ACP2
leaking out of channels adjoining the adjacent channels are reduced by 40
dB or more, and adjacent channel power ACP can thus be reduced drastically
using the low-pass model. Even when the output power Pout of an amplifier
is 32 dBm or more, the effect of reducing adjacent channel power can be
exerted. However, in reality, the degree of reduction will be restricted
by the ability of an amplifier for correction. The phase distortion
impairing the linearity can be corrected readily by shifting the phase of
a signal supplied to a main amplifier by an angle given by
-p(.vertline.x.vertline.).
(C-2). Predistortion Function and Its Utilization
Distortion components of an output signal of an amplifier can be diminished
by distorting an input signal of the amplifier in advance using an
amplitude distortion correcting function, that is, a predistortion
amplitude function h(.vertline.x.vertline.). An envelope transfer function
g(h(.vertline.x.vertline.)) (g(h) in the polynomial (18) to be described
later) and the predistortion amplitude function h(.vertline.x.vertline.)
are expressed as the polynomials (18) and (19) below.
##EQU9##
When an output signal of an amplifier is defined in relation to an input
amplitude .vertline.x.vertline. using the polynomials (18) and (19), the
polynomial (20) below is obtained.
##EQU10##
In this case, the polynomial (18) and coefficients c.sub.i in the
polynomial (19) are determined so that the envelope transfer function
g(.vertline.x.vertline.) will approach the amplitude,
b.multidot..vertline.x.vertline., of an output provided with a gain b and
undergoing no distortion. Herein, the amplitude
b.multidot..vertline.x.vertline. of the output of the amplifier that
produces the gain b and causes no distortion is regarded as a target
envelope transfer function, and the coefficients c.sub.i are calculated
using A method of steepest descent based on the LMS method so that a sum
of errors between the square of the envelope transfer function and the
square of the output amplitude b.multidot..vertline.x.vertline. will be
minimized (LMS method). Assuming that a cumulative error between the
square of the envelope transfer function g(.vertline.x.vertline.) and the
square of the output amplitude b.multidot..vertline.x.vertline. that is
the target envelope transfer function is .epsilon., updated coefficients
.delta..sub.i are calculated by differentiating the envelope transfer
function g(.vertline.x.vertline.) with respect to the coefficients
c.sub.i. A procedure of calculating the updated coefficients .delta..sub.i
is a procedure of calculating the expressions (21) to (24) below.
##EQU11##
.delta..sub.i
=-.theta..epsilon..multidot.(.differential.g(.vertline.x.vertline.)/
.differential.c.sub.i) (22)
##EQU12##
c.sub.i +=.delta..sub.i (24)
While a step gain .eta. is adjusted, the calculation of the expressions
(21) to (24) is repeated approximately 1000 times. Eventually, a set of
coefficients c.sub.i of thirteen orders are calculated.
FIG. 10 is a graph showing a predistortion function employed in the
embodiments of the present invention. In the graph of FIG. 10, an input
amplitude .vertline.x.vertline. is simplified to x.
The method of steepest descent based on the LMS method employed in the
fourth embodiment of the present invention is used to calculate a
predistortion amplitude function h(.vertline.x.vertline.) (dashed line in
FIG. 10). An envelope transfer function g(.vertline.x.vertline.) (solid
line in FIG. 10) is modified using the predistortion amplitude function
h(.vertline.x.vertline.) for the purpose of predistortion. This makes it
possible to correct amplitude distortion components impairing the
linearity which are reproduced by the envelope transfer function
g(.vertline.x.vertline.). consequently, a function
g(h.vertline.x.vertline.) exhibiting good linearity can be defined over a
relatively large range of input amplitudes (dot-dash line in FIG. 10).
FIG. 11 is a graph showing the results of simulation in which a baseband
signal is pre-distorted, FIG. 12 is a graph showing changes in spectrum
caused by canceling out high-frequency amplitude distortion components and
by carrying out predistortion.
In the graph of FIG. 11, two solid lines indicate the results of measuring
adjacent channel power ACP1 leaking out of adjacent channels, and the
results of measuring adjacent channel power ACP2 leaking out of channels
adjoining the adjacent channels. For the measurement, the amplitude
distortion impairing the linearity, and occurring in an amplifier, is not
corrected. Two dashed lines indicate adjacent channel power ACP1 leaking
out of the adjacent channels when simulation is carried out by
pre-distorting a baseband signal. Two dot-dash lines indicate adjacent
channel power ACP2 leaking out of the channels adjoining the adjacent
channels when simulation is carried out by pre-distorting the baseband
signal.
As is apparent from the graph of FIG. 11, the adjacent channel power ACP1
leaking out of the adjacent channels and the adjacent channel power ACP2
leaking out of the channels adjoining the adjacent channels are reduced by
20 dB to 40 dB, and thus adjacent channel power ACP can be reduced
drastically by pre-distorting the baseband signal.
FIG. 12 shows spectra of frequencies (1/T) of integrated power having a
bandwidth of 21 kHz, that is, spectra attained when an average output
power level is 30.5 dBm. In the graph of FIG. 12, a spectrum representing
frequency characteristics of a main amplitude and not including
frequencies of corrected amplitude distortion components is indicated with
a solid line, a spectrum including frequencies of pre-distorted components
is indicated with a dot-dash line, and a spectrum including frequencies of
components remaining after high-frequency amplitude distortion components
alone are amplified, removed from an output signal of the main amplifier,
and then canceled out, is indicated with a dashed line.
In this case, when frequency characteristics of a main amplifier are
reproduced and when predistortion is carried out, power of 18 dBm is
input. When high-frequency amplitude distortion components alone are
amplified and removed from an output signal of the main amplifier, power
of 16.2 dBm is input. Each spectrum is integrated by passing the frequency
components through an "ideal filter" having a bandwidth of 21 kHz. As far
as frequency bands having a difference of 50 kHz (.+-.2.38/T) (that is,
adjacent channel power ACP1 leaking out of adjacent channels) between them
are concerned, the spectrum including frequencies of pre-distorted
components is lowered by approximately 10 dB compared with the spectrum
representing the frequency characteristics of the main amplifier. The
spectrum including frequencies of components remaining after
high-frequency amplitude distortion components alone are amplified and
removed from the output signal of the main amplifier is lowered
approximately 40 dB compared with the spectrum representing the frequency
characteristics of the main amplifier.
(D) Summary of Two Methods for Correcting Signals
As mentioned above, two methods for correcting signals which are intended
to improve distortion characteristics of a microwave amplifier, which
impairs the linearity, have been discussed in detail. The first method for
correcting signals is such that nonlinear distortion components alone are
amplified by an amplifier offering good linearity, and are subtracted from
an output signal of a main amplifier whose output signal is the subject
for correction. An envelope amplitude function defined by a
thirteenth-order polynomial was used to carry out simulation. It was
demonstrated that adjacent channel power ACP was reduced by 40 dB or more.
Likewise, the sign of a polynomial serving as an approximate expression of
an envelope phase function is reversed in order to pre-distort in phase an
input signal of the main amplifier. However, in this case, another
amplifier is needed for correcting the distortion characteristics of the
main amplifier. The feasibility of adapting this method to portable
telephones and portable equipment is poor.
The second method for correcting signals is such that an input signal is
pre-distorted so that an output amplitude of an amplifier will have a
linear relationship to an input amplitude. The sign of a phase transfer
function concerning the distorted signal is reversed, and then a resultant
signal is input to the amplifier.
According, especially, to the second method for correcting signals, when a
predistortion function was calculated and simulation was carried out, it
was revealed that adjacent channel power ACP was reduced by about 10 dB.
As long as a digital signal processor (DSP) has an extra ability, only a
modification to firmware is required.
If the relationship between a modulation signal and high-frequency output
signal can be expressed in the same form as an envelope function
concerning a narrow-band amplifier which was employed in the simulation,
the distortion characteristics of a modulator or multiplier concerning the
distortion impairing the linearity can be improved. When a portable
telephone or portable equipment is in operation, if linearity offered by
an amplifier or adjacent channel power ACP is measured, coefficients in a
predistortion function can be updated for keeping up with a change in
characteristics of a circuit by executing a smaller number of processing
steps than the number of processing steps required conventionally.
FIG. 13 is a circuit block diagram showing an example of a transmitter
including an apparatus for correcting signals in accordance with the
present invention; and FIG. 14 is a circuit block diagram showing an
example of a transmitter in accordance with the prior art which is
presented for comparison with the transmitter shown in FIG. 13.
The transmitter shown in FIG. 13 comprises, in addition to the apparatus
for correcting signals according to the third preferred embodiment of the
present invention, a narrow-band signal generating unit 74, a front-end
amplifier 75, a main amplifier 76, and an antenna A, and thus transmits a
signal of a specific frequency band having the amplitude distortion and
the phase distortion impairing the linearity thereof corrected.
Referring to FIG. 13, the means for estimating distortion characteristics
in high-frequency circuit portion 2 (FIG. 2), which estimates distortion
characteristics of the main amplifier 76 concerning the amplitude
distortion and the phase distortion impairing the linearity, is composed
of a digital signal processor (DSP) 7 and a controller 8 formed with a CPU
for controlling the DSP 7. The DSP 7 defines a digital filter for
calculating an amplitude distortion correcting function and phase
distortion correcting function, which are used to correct the amplitude
distortion and the phase distortion occurring in the main amplifier 76, on
the basis of a data signal DATA such as a complex-number signal according
to the distortion characteristics of the main amplifier 76 and a power
control signal Spc output from the controller 8.
In FIG. 13, there is shown a first low-pass signal generating unit 41 and
second low-pass signal generating unit 42 which are formed with low-pass
filters characterized to pass desired low frequencies by means of the
digital filter defined by the DSP 7. The first low-pass signal generating
unit 41 and second low-pass signal generating unit 42 are preferably
formed with RRCOS filters exhibiting a root Nyquist characteristic. The
first low-pass signal generating unit 41 and second low-pass signal
generating unit 42 have the ability to reproduce nonlinear distortion
components occurring in the main amplifier by passing a real-part signal I
and imaginary-part signal Q constituting a complex-number signal, which
has components reflecting the distortion characteristics of the main
amplifier, and being produced by the DSP 7.
Referring to FIG. 13, a quadrature modulation unit 70 for modulating the
real-part and imaginary-part signals of the complex-number signal, which
are produced by the RRCOS filters and have components representing an
amplitude distortion correcting function and phase distortion correcting
function, and producing a desired intermediate-frequency or high-frequency
modulation signal is installed on the output stage of the first low-pass
signal generating unit 41 and second low-pass signal generating unit 42.
The quadrature modulation unit 70 includes an in-phase carrier wave
multiplier 71, a 90.degree. out-of-phase carrier wave multiplier 72, and
an adder 73. The adder 73 has the ability to add up an in-phase quadrature
modulation signal and 90.degree. out-of-phase quadrature modulation signal
output from the in-phase carrier wave multiplier 71 and 90.degree.
out-of-phase carrier wave multiplier 72 respectively, and the ability to
add up two kinds of signals having the amplitude distortion and the phase
distortion thereof corrected.
Furthermore, a quadrature modulation signal having the amplitude distortion
and the phase distortion thereof corrected simultaneously is output from
the quadrature modulation unit 70, and then supplied to the narrow-band
signal generating unit 74 including a narrow-band filter. The narrow-band
signal generating unit 74 produces a signal having frequencies thereof
passed with respect to a specified bandwidth. The signal is then supplied
to the front-end amplifier 75 offering good linearity. Finally, an output
signal having amplitude distortion components and phase distortion
components thereof canceled out is output through the output terminal of
the main amplifier 76 offering nonlinearity.
On the other hand, in the transmitter in accordance with the prior art
shown in FIG. 14, the controller 8 does not have a facility for estimating
the distortion characteristics of the main amplifier 76. The first
low-pass signal generating unit 41 and second low-pass signal generating
unit 42 cannot therefore produce the real-part and imaginary-part signals
of a complex-number signal, which contain components representing an
amplitude distortion correcting function and phase distortion correcting
function, by reproducing nonlinear distortion components occurring in the
main amplifier.
Instead of the facility, the transmitter in accordance with the prior art
has a gain control amplifier 80 on the input stage of the main amplifier
76. A gain produced by the gain control amplifier 80 is adjusted properly
according to a gain control signal supplied from the controller 8 to the
gain control amplifier 80 via a digital-to-analog converter 81. The
influence of the distortion impairing the linearity and occurring in the
main amplifier upon the transmitter, is thus limited. Unlike the
transmitter in accordance with the present invention, the transmitter
having this configuration has drawbacks that it is difficult to adjust a
gain to be produced by an analog gain control amplifier with high accuracy
and that the circuitry is complex.
Preferably, in the transmitter in accordance with the present invention
shown in FIG. 13, after distortion components reproduced according to the
distortion characteristics of the narrow-band amplifier are appended to
low-pass signals, the low-pass signals are multiplied by a carrier wave
for digital modification. The low-pass signals undergoing this digital
modulation are supplied to the narrow-band amplifier. The narrow-band
amplifier then transmits a signal of a specified frequency band having the
amplitude distortion or the phase distortion impairing the linearity
thereof corrected.
More preferably, in the transmitter in accordance with the present
invention shown in FIG. 13, a complex-number signal, including real-part
and imaginary-part signals, which are characterized to have desired low
frequencies by means of low-pass filters, is used as an input signal. The
input signal containing components representing a function of an absolute
value of the complex-number signal or a function of a sum of squares of
the real-part and imaginary-part signals is subjected to digital
quadrature modulation. A signal resulting from the digital quadrature
modulation is supplied to the narrow-band amplifier. The narrow-band
amplifier then transmits a signal of a specified frequency band having the
amplitude distortion and the phase distortion impairing the linearity
thereof corrected.
Next, a method by which nonlinearity of the power amplifier 10 is changed
into linearity will be explained below.
FIG. 15 is a graph showing a relationship between the input amplitude of
the power amplifier 10 and the output amplitude, and FIG. 16 is a diagram
for explaining a state of distortion compensation of the power amplifier
10.
A relation of the output amplitude to the input amplitude of RF power
amplifier, the electric power efficiency of which is high, is nonlinear in
a region in which the input amplitude is large as shown in FIG. 15. For
example, when the input amplitude is x1, the output amplitude becomes
g(x1). When x1 is changed to hi, a point determined by (x1, g(h1)) is on a
straight line in FIG. 15. Therefore, a relation between x and g(h(x)) can
be made linear as follows. A relationship between x=x1 and h=h1 is
previously found with respect to an arbitrary value of x. When the input
amplitude is x, this value is previously distorted to a value of the
predistortion function h(x) on the upstream side of the power amplifier
10. Due to the foregoing, the relationship between x and g(h(x)) can be
made linear.
In other words, as shown in FIG. 16, when the original input to the power
amplifier 10 is x.multidot.cos(.omega..multidot.t), the input amplitude is
previously distorted from x to h as described above, and
h.multidot.cos(.omega..multidot.t) is inputted into the power amplifier
10. Then, an output of the power amplifier 10 becomes
h.multidot.cos(.omega..multidot.t-.phi.-.phi.0). In this case, .phi.0 is a
slippage of the phase that is not caused by h, and .phi. is a slippage of
the phase that is caused by h. In order to correct the slippage .phi. of
the phase, the phase of h.multidot.cos(.omega..multidot.t) is previously
sifted by .phi., and h.multidot.cos(.omega..multidot.t+.phi.) is inputted
into the power amplifier 10. Due to the foregoing, the amplitude and phase
of the output of the power amplifier 10 can be compensated.
FIG. 17 is a block diagram showing another transmitter of the present
invention.
In this transmitter, the digital signal processor (DSP) 7A conducts
processing until S is divided into two orthogonal components and subjected
to predistortion. The function in DSP 7A is realized by a program. In FIG.
17, this function is shown by blocks.
Base band signal S in series to be transmitted is made to be a 2-bit
parallel signal in which the in-phase component is I and the orthogonal
component is Q, and the bit continuation time is twice as long as the time
of signal S. FIG. 19 is a diagram showing a specific example of the
waveforms of signals I and Q. In FIG. 19, bit "0" is made to correspond to
-0.7, and bit "1" is made to correspond to 0.7. The reason why the
waveform is inclined between "0" and "1" is to reduce the side lobe of the
base band signal.
Referring to FIG. 17 again, signals I and Q are respectively supplied to
the root-raised-cosine filters 4-1 and 4-2, the characteristics of which
are the same. The reason why the root-raised-cosine filter is used as a
low-pass filter is that the side lobe is cut as short as possible in order
to prevent interference between the adjoining channels and further prevent
interference between marks on the receiving side.
The pass characteristics of the root-raised-cosine filter, the roll-off
coefficient of which is 0.2, are shown in FIG. 20. FIG. 21 is a diagram
showing the waveforms of signals xi and xq obtained when signals I and Q
shown in FIG. 19 have passed through filters 4-1, 4-2, the characteristics
of which are shown in FIG. 20. The time axis of FIG. 21 is shifted from
that of FIG. 19. Therefore, FIG. 21 does not correspond to FIG. 19.
Referring to FIG. 17 again, the in-phase component xi is supplied to the
absolute value calculating unit 30 and the predistortion unit 31, and the
orthogonal component xq is supplied to the absolute value calculating unit
30 and the predistortion unit 32. In the absolute value calculating unit
30, an intensity x of vector (xi, xq) is calculated and supplied to the
predistortion units 31, 32 and the phase pre-rotation unit 22. In FIG. 17,
in order to facilitate understanding, the predistortion unit is divided
into two predistortion units 31, 32. However, the two predistortion units
31, 32 function as one predistortion unit. The predistortion units 31, 32
are provided with parameters or reference tables as distortion
compensation data, wherein the parameters or reference tables determine an
approximate expression of the function h(x)/x in which the predistortion
function h(x) is divided by x. When the predistortion units 31, 32 are
provided with reference tables, the predistortion units 31, 32 designate
the address x for the memory and read out the data h(x)/x. Then, the
product h(x).multidot.(xi/x) of this value h(x)/x and xi is calculated,
and also the product h(x).multidot.(xq/x) of this value h(x)/x and xq is
calculated.
In the case in which values of the function h(x)/x are accurately
calculated using a power development expression, its calculation circuit
may be provided outside DSP 7A.
When the phase angle of the signals xi and xq is represented by x.theta.,
the outputs of the predistortion units 31, 32 can be respectively
expressed by h(x).multidot.cos x.theta. and h(x).multidot.sin x.theta..
The phase pre-rotation unit 22 rotates the phase angle of the input vector
(h(x)cos(x.theta.), h(x).multidot.sin(x.theta.)) by .phi.(x), and supplies
the in-phase component h(x)cos(x.theta.+.phi.) and the orthogonal
component h(x).multidot.sin(x.theta.+.phi.) respectively to D/A converters
24, 25. In the same manner as that of the predistortion units 31, 32, the
phase pre-rotation unit 22 is provided with parameters or reference tables
as distortion compensation data, wherein the parameters or reference
tables determine an approximate expression of the function .phi.(x).
The in-phase component and the orthogonal component of the output of the
phase pre-rotation unit 22 are respectively made to be analogue signals by
D/A converters 24, 25 arranged outside DSP 7A and supplied to the
orthogonal modulation circuit 70.
In the orthogonal modulation circuit 70, the carrier wave
cos(.omega..multidot.t), which is outputted from the cosine wave
generation circuit 91, is multiplied and modulated by the output of D/A
converter 24 in the multiplying circuit 71. The phase of the output of the
cosine wave generation circuit 91 is shifted by .pi./2 in the phase
shifting circuit 92, and the carrier wave sin (.omega..multidot.t) is
generated, which is multiplied and modulated by an output of D/A converter
25 in the multiplying circuit 72. In the composite circuit 73, both
modulation waves are added to each other, and a QPSK wave is formed and
supplied to the power amplifier 10. This QPSK wave becomes
h(x).multidot.cos(x.theta.+.phi.).multidot.cos(.omega..multidot.t).multido
t.
h(x).multidot.sin(x.theta.+.phi.).multidot.sin(.omega..multidot.t)=h(x).mu
ltidot.cos(.omega..multidot.t+x.theta.+.phi.). That is, h(x) becomes the
input amplitude of the power amplifier 10.
Electric waves are transmitted from antenna A by the output of the power
amplifier 10.
When the absolute value calculating unit 30, the predistortion units 31, 32
and the phase pre-rotation unit 22 are not used, h(x)=x, and .phi.=0.
Therefore, an output of the composite circuit 73 becomes
x.multidot.cos(.omega..multidot.t+x.phi.), and x becomes an input
amplitude of the power amplifier 10. That is, by the compensation of the
predistortion units 31, 32, an output of the composite circuit 73 becomes
as follows. As shown in FIG. 14(B), x.multidot.cos(.omega..multidot.t)
becomes h.multidot.cos(.omega..multidot.t). Further, by the compensation
of the phase pre-rotation unit 22, this becomes
h.multidot.cos(.omega..multidot.t+.phi.). Accordingly, the output of the
power amplifier 10 becomes g(h).multidot.cos(.omega..multidot.t-.phi.0) in
which distortion caused by nonlinearity has been corrected.
The above functions or tables, which are used in the predistortion units
31, 32 and the phase pre-rotation unit 22, are determined as follows.
The amplifier characteristics detecting unit 2A outputs a signal, which has
already been known, to DSP 7A, and sampling is conducted on the output of
the power amplifier 10 via the orthogonal restoring circuit 93 and D/A
converter 94. The amplitude g(h) and slippage of .phi.(h)+.phi.0 of the
phase are measured, and the results are supplied to the unit 2B for
preparing distortion compensation data. The input amplitude h of the power
amplifier 10 is an intensity h of the output vector of DSP 7A, and the
digital value h is received from DSP 7A. Due to the foregoing,
consideration can be given not only to the nonlinearity of the power
amplifier 10 but also to the nonlinearity of the circuit formed between
the DSP 7A and the power amplifier 10.
The unit 2B for preparing distortion compensation data is composed of a
processor, for example, one chip microcomputer or DSP. This unit 2B for
preparing distortion compensation data conducts processing as follows
according to a schematic illustration shown in FIG. 18.
(1) The measured waveform of g(x) is made to be approximate to a power
development expression of x. A coefficient aj (j=1 to N) of the degree j
of x in the power development expression can be determined by the method
of least squares.
(2) Next, h(x) is made to be approximate to a power development expression
of x. A coefficient of the degree j of x is cj (j=1 to N) in the power
development expression. As shown in FIG. 15, the equation of
g(h(x))=a1.multidot.x is established. Therefore, the coefficient of j (j=1
to N) is determined so that .vertline.g(h(x))-a1.multidot.x1.vertline. can
be minimum. As can be seen in FIG. 15, c1=1.
(3) When the predistortion units 31, 32 find h(x)/x by a polynomial
calculation, the coefficient cj (j=1 to N) is supplied to the
predistortion units 31, 32. For example, N=10. In this case, it is
possible to reduce a necessary amount of distortion correction data. When
the predistortion units 31, 32 use a reference table, a table on which
h(x)/x is made to correspond to x is made, and this table is stored in
memories used by the predistortion units 31, 32.
(4) The measured waveform of .phi.(x) is stored as a reference table in the
memory used by the phase pre-rotation unit 22. Alternatively, this
waveform is made to be approximate to a power development expression of x,
and the coefficient of development is supplied to the phase pre-rotation
unit 22.
According to the transmitter having the above structure, when the function
h(x)/x is multiplied by the in-phase component xi and the orthogonal
component xq in accordance with an intensity x of the vector of the
in-phase component xi and the orthogonal component xq, predistortion can
be easily and accurately conducted.
In FIG. 16, when the amplitude x before conducting the predistortion is
large, the amplitude h after the predistortion becomes larger. When the
apparatus shown in FIG. 17 is used for a portable telephone, it is
necessary to extend life of a battery. Therefore, an upper limit of the
amplitude at which no signal is saturated is relatively low. For the above
reasons, when the amplitude x is large, the circuit between DSP 7A and the
power amplifier 10 shown in FIG. 17 is saturated, and an output of the
composite circuit 73 is subjected to clipping. As a result, an output of
the power amplifier 10 is distorted compared with a case in which
distortion compensation is not conducted. Accordingly, a leakage of
electric power to the adjoining channel exceeds an allowable value. In
other words, an average transmitting electric power of the transmitter is
limited.
In order to solve the above problems, it is possible to take the following
countermeasures. When nonlinearity of the power amplifier 10 is
appropriately corrected, the leakage of electric power (electric power of
the side lobe) to the adjoining channel is suppressed so that it cannot
exceed the allowable value.
Next, a method will be explained as follows, by which the predistortion
function h(x) for conducting the above correction can be easily and
effectively obtained.
For example, in wide band CDMA, the band width of a base band signal is
4,096 MHz with respect to the frequency 1.9 GHz of carrier waves. In order
to simplify the explanation, the base band signal x is a multiple signal
A(sin(.omega.1.multidot.t)+sine(.omega.2.multidot.t)) of the angular
frequency .omega.1 and the angular frequency .omega.2. This signal can be
expressed by
2A(cos(((.omega.1-.omega.2)t/2).multidot.sin((.omega.1+.omega.2)t/2), and
the amplitude x can be expressed by
2A.vertline.(cos((.omega.1-.omega.2)t/2).vertline..
When the characteristics of the power amplifier 10 are linear, distortion
of the amplitude 2A.vertline.(cos((.omega.1-.omega.2)t/2).vertline., which
has been subjected to intermodulation, is not outputted from the power
amplifier 10. However, the characteristics of the power amplifier 10 are
actually nonlinear. Therefore, a signal of intermodulation of the degree
m(m.multidot.2, 3, 4, . . . ), the angular frequency of which is
m(.omega.1-.omega.2), is outputted from the power amplifier 10. In
accordance with the deterioration of linearity of g(x), the degree m,
which can not be neglected, is increased.
In the determination of h(x), if the intermodulation wave amplitude
x=2A.vertline.(cos((.omega.1-.omega.2)t/2).vertline. is used and the
secondary intermodulation distortion of g(h(x)) can be minimized, g(h(x)),
the linearity of g(x) of which is improved, can be easily and effectively
obtained without giving consideration to the intermodulation distortion,
the degree of which is not less than tertiary.
This predistortion function h(x) is found in the unit 2B for preparing
distortion compensation data shown in FIG. 15 by the method shown in FIG.
16. Development coefficients of g(x) and h(x) are expressed by the same
characters in the following explanations. Marks in parentheses are the
step identification marks shown in FIG. 18.
(S1) As described above, g(x) is made to be approximate to an expression of
the degree N of x, and the power development coefficients a.sub.1 to aN
are determined.
(S2) Initial values are given to the power development coefficients c1 to
cN when h(x) is made to be approximate to an expression of N degree of x.
When g(x) is linear, h=x. Therefore, it is most natural that the initial
values are set at c1=1, cj=0(j=2 to N).
(S3) A waveform of h(x(t)) with respect to the time t is found by numerical
calculation. In this case,
x(t)=2A.vertline.(cos((.omega.1-.omega.2)t/2)).vertline..
(S4) A waveform of g(x(t)) with respect to the time t is found by numerical
calculation.
(S5) The primary Fourier coefficient IM(1) and the secondary Fourier
coefficient IM(2) in the coefficients IM(m) of the Fourier term
cos(m(.omega.1-.omega.2)t/2) of the waveform of g(h(x(t))) are calculated.
Since IM(m) is used when .vertline.IM(2)/IM(1).vertline. is found in the
next step S6, the multiplication by a constant and the mark may be
arbitrarily determined. In the mathematical calculation, g(x) may be set
at g(-x)=-g(x) so that it can be symmetrical with respect to the origin
and the definition region of x may be extended to a negative region. That
is, it is possible to set to be
g(-cos((.omega.1-.omega.2)t/2))=-g(cos((.omega.1-.omega.2)t/2)). Then, for
example, IM(k) is found by the following expression.
IM(k)=g(h).multidot.s.multidot.EXP(i.multidot.k(.omega.1-.omega.2)t/2)dt.
In this case, i is a unit of an imaginary number, s is a code of
cos((.omega.1-.omega.2)t/2), and a range of integration is from
-2.pi./(.omega.1-.omega.2) to 2.pi./(.omega.1-.omega.2).
(S6) The secondary intermodulation distortion ratio
.epsilon.=.vertline.IM(2)/IM(1).vertline. is calculated.
In the following steps S7 to S12, processing is repeated by the method of
steepest descent until .epsilon. becomes a local minimum, and the power
development coefficients c1 to CN of h(x) are determined.
(S7) The increment dc1 of c1 is given an initial value, and the increment
dcj of cj (j=2 to n) is found in the order of j=2 to N by the expression
dcj=.epsilon./dck. In this case, k=j-1.
(S8) .epsilon.p.rarw..epsilon.
(S9) cj .rarw.cj+dcj with respect to each of j=1 to N
(S10) The above steps S3 to S6 are carried out with respect to h(x)
determined by c1 to cN obtained in step S9.
(S11) dc1.rarw.-.eta..multidot..epsilon.(.epsilon.-.epsilon.p)/dc1
In this case, dc2 to dcN are renewed in the same manner as that shown in
Step S7.
(S12) When the expression of
.vertline..epsilon.-.epsilon.p.vertline..gtoreq..DELTA. is satisfied, the
program returns to step S8. When the expression of
.vertline..epsilon.-.epsilon.p.vertline..gtoreq..DELTA. is is not
satisfied, the program proceeds to step S13. In this case, .DELTA. is a
minute value that has been previously given.
(S13) As described above, the predistortion units 31, 32 shown in FIG. 17
are supplied with c1 to cN which are renewed values of compensation data.
Alternatively, a table on which h(x)/x is made to correspond to x is made
and supplied.
A table of .phi.(x) with respect to the phase pre-rotation unit 22 is found
by the same method as that shown in FIG. 17.
The above processing is conducted, for example, at predetermined periods of
time or according to a changeover of a ratio of amplification or a change
in temperature. In a case immediately after the electric power source has
been turned on, that is, when a change in the characteristics of the power
amplifier 10 are so quick that the unit for preparing distortion
compensation data can not follow the change, an output of the power
amplifier 10 is limited so that the side lobe is decreased.
In this connection, h(x) depends upon a difference (.omega.1-.omega.2) in
the angular frequency in the expression of the amplitude x. Also, h(x)
depends upon the amplitude 2A. However, when the value of .omega.1 and the
value of .omega.2 are somewhat different from each other, h(x) seldom
depends upon the value of (.omega.1-.omega.2). Therefore, for example,
(.omega.1-.omega.2)/(2.pi.) is made to be a bandwidth of the base band
signal. When the value 2A of the amplitude x is determined by the maximum
value of the input amplitude of the power amplifier 10 actually used, the
predistortion function h(x) capable of enhancing the average transmitting
power is determined. In the case of a portable telephone, an input
amplitude of the power amplifier 10 is adjusted by a control signal sent
from a base station. Therefore, the value 2A of the amplitude according to
that is used. This value of the amplitude is described later in reference
to simulation.
A result of simulation will be explained as follows.
FIGS. 22 to 25 are views showing cases in which the predistortion function
h(x) found by the method shown in FIG. 18 is used. FIG. 22 is an output
waveform diagram of the predistortion units 31, 32 shown in FIG. 17; FIG.
23 is an output waveform diagram of the phase pre-rotation unit 22 shown
in FIG. 17; and FIGS. 24 and 25 are respectively an input and an output
waveform diagram of the power amplifier 10. However, in FIGS. 24 and 25,
the carrier wave frequency is made to be 672 kHz, which is considerably
lower than the actual carrier wave frequency, so that the waveform can be
confirmed.
In FIG. 26, a short-dotted-line expresses the predistortion function h(x)
in the case in which the predistortion is not conducted at all, a
long-dotted line expresses the predistortion function h(x) in the case
where g(h(x)) is made linear as shown by a one-dotted chain line in FIG.
15, and a solid line expresses the predistortion function h(x) in the case
in which the method shown in FIG. 18 is used. In this case, N=12. FIG. 26
shows h(x) which has been multiplied by a constant.
In FIG. 27, the values of g(h(x)) in the above three cases are respectively
shown by a short dotted line, long dotted line and solid line.
FIGS. 28, 29 and 30 respectively shows a line spectrum of the output
amplitude g(h(x)) of the power amplifier 10 in the above three cases when
the intermodulation amplitude x is
x=2A.vertline.cos((.omega.1-.omega.2)t/2).vertline. as described above.
The horizontal axis represents an angular frequency ratio
2.omega./(.omega.1-.omega.2).
FIG. 31 expresses a relation between the average transmitting electric
power (average electric power of the output of the power amplifier 10) of
a transmitter and the adjoining channel electric power ratio ACPR
(adjacent channel power ratio) in the above three cases. ACPR is found by
calculating (total of electric power of the side lobe section in a
predetermined range)/(total of electric power of the main lobe section)
with respect to the frequency spectrum of the output of the power
amplifier 10, that is, ACPR shows a degree of interference of the
adjoining channel. In each case, a solid line shows a case in which the
side lobe is a lower side lobe, and a dotted line shows a case in which
the side lobe is an upper side lobe. However, they are substantially
symmetrical to each other with respect to the carrier wave frequencies of
both side lobes. Therefore, both curves are substantially equal to each
other.
As described above, in the case in which the method shown in FIG. 18 is
used, h(x) relies on the amplitude 2A of the intermodulation wave
amplitude x=2A.vertline.cos((.omega.1-.omega.2)t/2).vertline., and h(x) is
found in each of the cases in which electric power (2A) is 6.1, 7.1 and
9.1 dBm, and ACPR is calculated.
In order to prevent interference with the adjoining channel, it is
necessary that ACPR is kept at a value, for example, not higher than -40
dBc. As can be seen in FIG. 31, only when the method shown in FIG. 18 is
used, it is possible to use the transmitter, at the average transmission
power having a value which is in the proximity of 26 dBm. As can be seen,
it is preferable to use the predistortion function h(x) in the case where
the electric power (2A) is 7.1 dBm. As described above, it is possible to
find the average transmitting electric power of the transmitter and the
amplitude 2A of the intermodulation wave amplitude x by means of
simulation.
As described so far, in the apparatuses for correcting signals according to
several preferred embodiments of the present invention, first, the fact
that a distortion characteristics of a high-frequency circuit concerning
the distortion impairing the linearity can be reproduced using low-pass
signals or the like, is noted. A simple circuit designed for processing
the low-pass signals is used to estimate the distortion characteristics so
as to calculate a distortion correcting function. With low power
consumption maintained, and despite circuitry simpler than the
conventional circuitry, the distortion impairing linearity, and occurring
in a high-frequency circuit, can be suppressed reliably and power leaking
out of adjacent channels can be minimized.
In the apparatuses for correcting signals according to several preferred
embodiments of the present invention, second, as a distortion correcting
function concerning a high-frequency circuit, an amplitude distortion
correcting function h(x) is determined so that the relationship of
ax=g(h(x)) can be established, or a phase distortion correcting function
p(x) is determined so that the relationship of c=g(p(x)) can be
established. Amplitude distortion components or phase distortion
components which impair the linearity and occur in the high-frequency
circuit, can be reliably canceled out.
In the apparatuses for correcting signals according to several preferred
embodiments, third, an approximate expression of an envelope transfer
function is defined as g(x)=a.sub.0 +a.sub.1 x+g'(x), and an amplitude
distortion correcting function is defined as x.times.(1-g'(x)/g(x)). It is
therefore guaranteed that the amplitude distortion impairing the
linearity, and occurring in a high-frequency circuit, can be corrected
relative to a wide range of input amplitudes. Consequently, power leaking
out of adjacent channels can be minimized.
Furthermore, in the apparatuses for correcting signals according to several
preferred embodiments of the present invention, fourth, a polynomial g'(x)
in the envelope transfer function g(x)=a.sub.0 +a.sub.1 x+g'(x) is defined
as
##EQU13##
If necessary, therefore, coefficients in the polynomial g'(x) having
relation to an amplitude distortion correcting function can be readily
modified by using a digital signal processor (DSP) or the like.
Furthermore, in the apparatuses for correcting signals according to several
preferred embodiments of the present invention, fifth, the phase
distortion correcting function p(x) is defined as p(x)=-g(x). Using the
simple circuitry for shifting a phase according to a phase distortion
correcting function, the phase distortion impairing the linearity, and
occurring in a high-frequency circuit, can be reliably suppressed, and
power leaking out of adjacent channels can be minimized.
Furthermore, in the apparatuses for correcting signals according to several
preferred embodiments of the present invention, sixth, low-pass signals
passed by low-pass filters defined on the basis of the result of
estimating the distortion characteristics of a high-frequency circuit are
used to calculate a phase distortion correcting function. Despite the
simple circuitry, the phase distortion impairing the linearity, and
occurring in the high-frequency circuit, can be reliably suppressed and
power leaking out of adjacent channels can be minimized.
Furthermore, in the apparatuses for correcting signals according to several
preferred embodiments of the present invention, seventh, signals passed by
transmission filters exhibiting root Nyquist characteristics are used to
calculate a phase distortion correcting function p(x). If necessary,
therefore, coefficients set to realize the transmission filters can be
readily modified. Consequently, phase distortion correction can be carried
out quickly and reliably.
Furthermore, in the apparatuses for correcting signals according to several
preferred embodiments of the present invention, eighth, after low-pass
signals passed by low-pass filters are modulated, a phase distortion
correcting function to be applied to low-pass signals substantially
identical to the low-pass signals is input to one amplifier. An output
signal of the one amplifier and an output signal of the other amplifier
are added up. Consequently, the phase distortion impairing the linearity,
and occurring in a high-frequency circuit, can be reliably suppressed, and
power leaking out of adjacent channels can be minimized.
Furthermore, in the apparatuses for correcting signals according to several
preferred embodiments of the present invention, ninth, an amplifier
causing no distortion is used to correct the phase distortion impairing
the linearity, and occurring in a high-frequency circuit. Despite the
simple circuitry, the phase distortion impairing the linearity, and
occurring in the high-frequency circuit, can be reliably suppressed, and
power leaking out of adjacent channels can be minimized.
Furthermore, in the apparatuses for correcting signals according to several
preferred embodiments of the present invention, tenth, the phase
distortion impairing the linearity, and occurring in a plurality of
amplifiers, can be corrected simultaneously. Despite the simple circuitry,
the phase distortion impairing the linearity, and occurring in the
plurality of amplifiers, can be corrected quickly and accurately.
Furthermore, in the apparatuses for correcting signals according to several
preferred embodiments of the present invention, eleventh, after low-pass
signals passed by low-pass filters are modulated, an amplitude distortion
correcting function to be applied to low-pass signals substantially
identical to the low-pass signals is input to one amplifier. Output
signals of the one amplifier and the other amplifier are added up.
Consequently, the amplitude distortion impairing the linearity, and
occurring in a high-frequency circuit, can be reliably suppressed, and
power leaking out of adjacent channels can be minimized.
Furthermore, in the apparatuses for correcting signals according to several
preferred embodiments of the present invention, twelfth, low-pass signals
passed by low-pass filters defined on the basis of distortion
characteristics of a high-frequency circuit are used to calculate an
amplitude distortion correcting function and phase distortion correcting
function. Despite the simple circuitry, the amplitude distortion and the
phase distortion impairing the linearity, and occurring in the
high-frequency circuit, can be reliably suppressed, and power leaking out
of adjacent channels can be minimized.
Furthermore, in the apparatuses for correcting signals according to several
preferred embodiments of the present invention, thirteenth, a common
circuit is used to correct the amplitude distortion and the phase
distortion impairing the linearity of low-pass signals. Despite the very
simple circuitry, the amplitude distortion and the phase distortion
impairing the linearity, and occurring in a high-frequency circuit, can be
reliably suppressed, and power leaking out of adjacent channels can be
suppressed almost perfectly.
Furthermore, in the apparatuses for correcting signals according to several
preferred embodiments of the present invention, fourteenth, at least one
of an amplitude distortion correcting function h(x) and phase distortion
correcting function p(x) is an expansion consisting of a polynomial
series. Despite the simple circuitry, approximate values of amplitude
distortion components or phase distortion components impairing the
linearity, and occurring in a high-frequency circuit, can be calculated.
Consequently, the amplitude distortion or the phase distortion impairing
the linearity, and occurring in an amplifier, can be corrected quickly and
accurately.
Furthermore, in the apparatuses for correcting signals according to several
preferred embodiments of the present invention, fifteenth, at least one of
an amplitude distortion correcting function h(x) and phase distortion
correcting function p(x) is fixed. Despite the simple circuitry,
approximate values of amplitude distortion components or phase distortion
components impairing the linearity, and occurring in a high-frequency
circuit, can be calculated. Consequently, the amplitude distortion or the
phase distortion impairing the linearity, and occurring in an amplifier,
can be corrected quickly and accurately.
Furthermore, in the apparatuses for correcting signals according to several
preferred embodiments of the present invention, sixteenth, even when a
plurality of high-frequency circuits are present, at least one of an
amplitude distortion correcting function h(x) and phase distortion
correcting function p(x) is fixed for each high-frequency circuit or for
each group of high-frequency circuits produced under the similar
conditions for manufacturing a high-frequency circuit portion. Approximate
values of amplitude distortion components and phase distortion components
impairing the linearity, and occurring in a high-frequency circuit, can be
calculated. Consequently, the amplitude distortion or the phase distortion
impairing the linearity, and occurring in an amplifier, can be corrected
quickly and accurately.
Furthermore, in the apparatuses for correcting signals according to several
preferred embodiments of the present invention, seventeenth, an envelope
transfer function g(x) is calculated. If necessary, at least one of an
amplitude distortion correcting function h(x) and phase distortion
correcting function p(x) is modified. Consequently, the amplitude
distortion or the phase distortion impairing linearity, and occurring in
an amplifier, can be corrected quickly and accurately.
In the transmitter according to a preferred embodiment of the present
invention, first, a distortion characteristic of a high-frequency circuit
is estimated by a simple circuit designed for processing low-pass signals.
A distortion correcting function is then calculated. With low power
consumption maintained, and despite circuitry simpler than the
conventional circuitry, the distortion impairing the linearity, and
occurring in the high-frequency circuit, can be reliably suppressed, and
power leaking out of adjacent channels, can be minimized. Consequently,
highly reliable communication can be achieved.
Furthermore, in the transmitter according to a preferred embodiment,
second, low-pass signals passed by low-pass filters defined on the basis
of the result of estimating the distortion characteristics of a
narrow-band amplifier are arranged to have the amplitude distortion or the
phase distortion, which impairs the linearity thereof and occurs in the
narrow-band amplifier, corrected. With low power consumption maintained,
and despite circuitry simpler than the conventional circuitry, the
distortion impairing the linearity, and occurring in a high-frequency
circuit, can be reliably suppressed, and power leaking out of adjacent
channels can be minimized. Consequently, highly reliable communication can
be achieved.
Furthermore, in the transmitter according to a preferred embodiment, third,
real-part and imaginary-part signals constituting a complex-number signal
and passed by low-pass filters defined on the basis of the result of
estimating distortion characteristics of a narrow-band amplifier are
subjected to digital quadrature modulation. The amplitude distortion and
the phase distortion impairing the linearity, and occurring in the
narrow-band amplifier, are then corrected simultaneously. The phase
distortion correction can therefore be carried out quickly and reliably.
Consequently, highly reliable communication can be achieved.
According to a method for correcting signals utilizing any of the preferred
embodiments of the present invention, a simple circuit is used to estimate
a distortion characteristics of a high-frequency circuit so as to
calculate a distortion correcting function. With low power consumption
maintained, according to a procedure simpler than a conventional one, the
distortion impairing linearity, and occurring in a high-frequency circuit,
can be reliably suppressed and power leaking out of adjacent channels can
be minimized.
In short, when a class-AB, B, or C amplifier exhibiting good power
efficiency is used to correct a distortion characteristics, an apparatus
for correcting signals in accordance with the present invention can be
used for transmission in a high frequency band. The apparatus for
correcting signals in accordance with the present invention has a
high-frequency circuit portion that is smaller than that in a conventional
linearizer, and can therefore be realized readily. Moreover, an adjustment
portion of the apparatus is small and may sometimes be eliminated.
Consequently, the apparatus is advantageous in the aspects of reliability,
mass-productivity, cost, and power consumption.
An envelope transfer function g(x) can also reproduce distortion components
impairing the linearity, and occurring in a modulator,
intermediate-frequency circuit, and low-pass filters succeeding distortion
characteristics correcting unit. Distortion components occurring in all
these circuits can be corrected. Linearity is measured to be followed or
predicted, whereby a change in characteristics of a high-frequency circuit
whose distortion characteristics are the subject for correction can be
coped with. Many processing sequences can be carried out by means of a DSP
incorporated in a transmitter. Even when coefficients must be modified, if
the modification is carried out intermittently, the increase in the amount
of processing is negligible. When the coefficients cannot be modified in a
timely fashion, for example, immediately after the power supply is turned
on, if a facility for limiting transmission output to such an extent that
a magnitude of interception will not exceed a permissible range is
included, highly reliable communication can be achieved.
* * * * *
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