Patent Application
20050008096
This invention relates to a circuit and a method for non-linear compensation of an amplifier used in e.g. the mobile communication. More particularly, it relates to a compensation circuit and a compensation method in case the amplifier has a characteristic termed a memory effect.
In digital mobile communication of these days, the CDMA (Code Division Multiple Access) communication system is widely used to enhance the ability to withstand interference. With the CDMA communication system, in which the instantaneous power is increased as compared to the average power, it is retained to be necessary to maintain the linearity of the high output power amplifier of a transmitter to an extremely high output level in order to reduce cross-talk power between neighboring channels. The result is that the amplifier is increased in size and cost, while the power consumption is increased.
Thus, the technique of pre-distortion, in which an inverse characteristic of the non-linearity in the base-band unit is applied to use a non-linear amplifier, is now being searched briskly.
As the pre-distorter for the amplifier, reference is made to the following Patent Documents 1 to 3. The Patent Document 1 discloses a pre-distorter for pre-modifies an input signal to the amplifier by the inverse characteristic of the input/output characteristic of the amplifier, in order to reduce the cross talk power in neighboring channels of amplifier outputs, in which a correction coefficient for the differential and/or the integration of the input signal is determined, and in which the input signal is modified based on the so determined correction coefficient to output the so modified signal as a final pre-distortion signal. The Patent Document 2 shows the structure of a non-linear compensation circuit of the adaptive pre-distortion system, which assures high-speed high-precision compensation despite the reduced circuit size. In addition, the Patent Document 3 discloses a method for generating compensation data (the inverse of the input/output characteristics of power amplifying means taking into account the non-linear components of power amplifying means) for compensating the non-linear distortion in the power amplifying means.
[Patent Document 1]
Japanese Patent Kokai Publication No. JP-P2000-78037A (page 2, FIGS. 15 and 16)
[Patent Document 2]
Japanese Patent Kokai Publication No. JP-P2001-268150A (page 2, FIGS. 2 and 3)
[Patent Document 3]
Japanese Patent Kokai Publication No. JP-P2001-284977A (pages 2 and 3, FIGS. 10 and 12)
Meanwhile, in connection with the non-linear characteristic of the amplifier 303, there are cases where the amplifier, in particular the high-power amplifier (HPA) exhibits a characteristic termed a memory effect or memory distortion. This is a case where the non-linear distortion of the amplifier 303 exhibits frequency dependency against the modulation frequency of the input signal or discrete harmonic frequencies of plural waves in multi-carrier amplification.
With the conventional circuit structure, explained with reference to
Accordingly, it is an object of the present invention to provide an apparatus and a method whereby, in compensating the non-linear characteristic of the amplifier, in a baseband unit, it is possible to reduce distortion ascribable to the memory effect of the amplifier.
The circuit in accordance with one aspect of the present invention, which provides an inverse characteristic of an input/output characteristic, termed ‘a gain characteristic’, of an amplifier, to an input signal supplied to the amplifier, to compensate the non-linear characteristic of the amplifier, includes first means for deriving an inverse gain as an inverse characteristic of the gain of the amplifier with respect to the input signal, and for outputting a reciprocal of the inverse gain, a filter supplied with the reciprocal for filtering the reciprocal to output a filtered output, second means for finding a reciprocal of the filtered output to output the reciprocal of the filtered output, and a multiplier for multiplying the input signal and an output of the second means.
A circuit in accordance with another aspect of the present invention includes first means for deriving a gain of an amplifier with respect to an input signal to output the resulting gain, a filter supplied with the gain for filtering the gain to output a filtered output, second means for finding a reciprocal of the filtered output to output the reciprocal of the filtered output, and a multiplier for multiplying the input signal with an output of the second means.
A method in accordance with another aspect of the present invention includes:
a step of deriving an inverse characteristic of a gain of an amplifier with respect to an input signal, and for outputting a reciprocal of the inverse characteristic;
a step of filtering the reciprocal;
a step of finding a reciprocal of the output of the filtering; and
a step of multiplying the input signal by the reciprocal of the output of the filtering.
A method in accordance with still another aspect of the present invention includes:
a step of deriving a gain of an amplifier with respect to an input signal;
a step of filtering the gain to output a filtered output;
a step of finding a reciprocal of the filtered output; and
a step of multiplying the input signal by the output of the filtering.
According to the present invention, it is possible to compensate the distortion ascribable to the memory effect of the amplifier.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description in conjunction with the accompanying drawings wherein only the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out this invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.
The preferred embodiment for carrying out the present invention is now described. Referring to
In the conventional adaptive pre-distorter, explained with reference to
According to the present invention, a reciprocal of the inverse characteristic of the gain of an amplifier (303 of
[Embodiment 1]
A pre-distorter according to a first embodiment, shown in
Referring to
The complex baseband signal r(i)(=rI (i)+jrQ (i), where rI (i) is an in-phase component, rQ (i) is a quadrature component and j2=−1, is converted by the amplitude converter 102 into an amplitude (={rI (i)2+rQ (i)2}1/2). The amplitude is supplied as an address to the memory 103 for pre-distortion and an inverse characteristic (an inverse gain) associated with the address is output.
The inverse gain (complex signal) 1/a(t), output from the memory 103 in keeping with the amplitude from the amplitude converter 102, is converted by the first reciprocal converter 104 into a reciprocal a(t). The reciprocal a(t) of the inverse gain is supplied to the FIR filter 105 from which a signal w(i) is output. An output signal w(i) of the FIR filter 105 is expressed, as convolution of filter coefficients (impulse response) h(0), h(1), h(2), . . . , h(n), where N is the number of orders of the filter, and the signal a(i), by the following equation (1):
w(i)=ΣNj-0h(j)·a(i−j) (1).
An output signal w(i)(=wI (i)+jwQ (i), where wI (i) is an in-phase component, jwQ (i) is a quadrature component and j2=−1), is again converted by the second reciprocal converter 106 into a reciprocal 1/w(i)(={wI (i)−jwQ (i)}/{wI (i)2+wQ (i)2}) which is supplied to the complex multiplier 101. The complex multiplier 101 executes complex multiplication of the input signal r(i) and 1/w(i) to output a resulting signal.
In connection with the FIR filter 105, the memory effect of the amplifier is now explained.
In an amplifier, devoid of the memory effect, the product of the complex input signal x(t) to the amplifier and the gain a(t) of the amplifier represents an output signal v(t) of the amplifier, as indicated by the following equation (2)
v(t)=a(t)·x(t) (2)
On the other hand, in an amplifier having the memory effect, a product with the input signal x(t) of a signal w(t), obtained on filtering the gain a(t) of the amplifier devoid of the memory effect, represents an output v(t) of the amplifier.
The signal w(t), filtered by a filter 203 (linear phase filter) having a characteristic corresponding to the memory effect, is represented by an integration over continuous time of a(t) and h(τ), over continuous time, as an impulse response of the filter 203 (convolutional integration), as indicated by the equation (3)
w(t)=∫h(τ)·a(t−τ)dτ (3)
The output w(t) of this filter 203, multiplied (by complex multiplication) by the input signal x(t), as indicated by the equation (4):
v(t)=w(t)·x(t)=x(t)·{∫h(τ)·a(t−τ)dτ} (4)
represents an output v(t) of the amplifier.
In the pre-distorter 301, the input signal x(t) supplied to the amplifier needs to be distorted at the outset so that the output v(t) will be equal to the input signal r(t). From the above equation (4), it is sufficient to perform the following processing:
x(t)=r(t)/w(t)=r(t)/{∫h(τ)·a(t−τ)dτ} (5)
The processing of the equation (5) may be implemented by the structure shown in
Referring again to
r(i)/w(i)=r(i)/{τNj-0h(j)·a(i−j)} (6)
That is, the signal output from the complex multiplier 101 by the processing shown in
Referring to
The amplitude of the complex baseband signal is obtained by the amplitude converter 102 (step S1).
The inverse gain, corresponding to the amplitude, is then found from the memory 103, having stored therein an inverse characteristic of the input/output characteristic of the amplifier (inverse gain characteristic) (step S2).
The reciprocal of the inverse gain is then obtained by the first reciprocal converter 104 (step S3).
The reciprocal of the inverse gain is obtained by the FIR filter 105 (step S4).
The second reciprocal converter 106, supplied with the filtered reciprocal, finds a reciprocal thereof (step S5).
The complex multiplier 101 performs complex multiplication of the complex baseband signal and the reciprocal found in the step S5 (step S6).
[Embodiment 2]
The second embodiment of the present invention is hereinafter explained. Referring to
In the present embodiment, the amplitude of the complex baseband signal is found by the amplitude converter 102. The gain a(i), corresponding to the amplitude, is output from the memory 103′. The FIR filter 105 filters the gain a(i). The second reciprocal converter 106 finds the reciprocal of the filtered gain w(t). The complex multiplier 101 multiplies the complex baseband signal with the reciprocal output from the second reciprocal converter 106.
Referring to
The amplitude of the complex baseband signal is obtained by the amplitude converter 102 (step S11).
The gain of the amplifier, associated with the amplitude, is then obtained from the memory 103′, having stored therein the input/output characteristic (gain characteristic) of the amplifier (step S12).
The FIR filter 105 filters the gain from the memory 103′ (step S13).
The second reciprocal converter 106, supplied with the filtered reciprocal, finds its reciprocal (step S14).
The complex multiplication of the complex baseband signal and the reciprocal found in the step S14 is carried out (step S15).
The pre-distorter of each of the above-described embodiments is used as the pre-distorter 301 of
In the above embodiments, the amplitude converter 102, memory 103 (103′) and the first reciprocal converter 104 may, of course, be replaced by other calculating circuits performing equivalent processing operations.
The amplitude converter 102 and the memory 103 (103′) may, of course, be similarly replaced by other calculating circuits performing equivalent processing operations.
Moreover, in the above embodiments, the filter coefficient h(i) and/or the number of filter orders of the FIR filter may, of course, be varied. Although the present invention has so far been explained with reference to the above-described embodiments, the present invention is not limited to these embodiments and may comprise various modifications or corrections that may readily occur to those skilled in the art within the scope of the invention.
According to the present invention, described above, it is possible to compensate non-linear distortion of the amplifier in case such distortion exhibits frequency dependency against the modulation frequency of the input signal. Thus, the apparatus and the method of the present invention may conveniently be used for e.g. a transmission apparatus of the mobile communication system.
It should be noted that other objects, features and aspects of the present invention will become apparent in the entire disclosure and that modifications may be done without departing the gist and scope of the present invention as disclosed herein and claimed as appended herewith.
Also it should be noted that any combination of the disclosed and/or claimed elements, matters and/or items may fall under the modifications aforementioned.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-272181 | Jul 2003 | JP | national |
