Amplifiers – With amplifier condition indicating or testing means
Reexamination Certificate
1998-03-05
2001-01-16
Shingleton, Michael B (Department: 2817)
Amplifiers
With amplifier condition indicating or testing means
C330S149000, C455S126000
Reexamination Certificate
active
06175270
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to signal amplification and, in particular, to intentionally induced distortion techniques utilized prior to and in conjunction with signal amplification.
BACKGROUND OF THE INVENTION
The function of a linear power amplifier is to amplify a signal with as little waveshape alteration as is practical. The ideal amplifier is therefore characterized as having a transfer function (input signal compared to output signal) which is completely linear with no transfer function discontinuities. Unfortunately, physical processes are seldom ideal and signal amplifiers are no exception. Amplifiers are specifically designed to operate as linearly as possible within their “linear region,” but amplifier nonlinearities are a reality in any amplifier design. Additionally, amplifiers which are “overdriven” deliver a clipped output signal. An amplifier is overdriven, and therefore the output signal is clipped, when the input signal possesses peak amplitudes which cause the amplifier to saturate (no appreciable increase in output amplitude with an increase in input amplitude) or to shut-off (no appreciable decrease in output amplitude with a decrease in input amplitude). Generally, an amplifier is characterized as having a “clipping threshold.” Input signals having amplitudes beyond the clipping threshold are clipped at the amplifier output.
One method of ameliorating the effects of nonlinear amplifier performance within the amplifier “linear region” is to intentionally distort the preamplified RF signal to anticipate and complement the recognized nonlinear trait of the amplifier. The device which performs this function is known as a signal predistorter (hereinafter referred to as a “predistorter”). Since the departure from linearity of an amplifier operating in its “linear region” is known (as characterized by a deviation of its transfer function in the “linear region” from that of an ideal amplifier's transfer function), a predistorter intentionally distorts a preamplified signal by compensating for the known nonlinearity in a complementary fashion. Thus, when the intentionally distorted preamplified signal is amplified, the nonlinearity of the amplifier causes the amplified version of the intentionally distorted signal to more closely resemble the waveshape of the original signal (the signal prior to amplification and predistortion). Predistortion is thus an effective method for compensating for amplifier nonlinearities within the “linear region” of an amplifier, and as such is frequently referred to as an amplifier linearization circuit.
However, conventional amplifier linearization (predistortion) techniques do not compensate for the resultant adverse effects when the amplified signal nonlinearity is the result of clipping. In a wireless RF transmitter, the presence of signal clipping at the power amplification stage presents an especially onerous problem. Specifically, clipping of a RF signal typically results in significant quantities of spectral regrowth (emission of RF signal energy outside the intended frequency band). In a wireless RF environment, where a high priority is placed upon effective and efficient utilization of limited bandwidth, the production of spectral regrowth causes RF interference emissions outside of the intended (or allocated) frequency band and therefore noise (interference) within unintended (nonallocated) frequency bands. The severity of these out-of-band emissions is proportional to the shortness and abruptness with which the clipping of the RF signal waveform occurs.
In the prior art, compensation for clipping within a power amplifier involved generating a compensating window to apply to a RF signal and multiplying a predetermined windowing function with the RF signal to be amplified. This method, disclosed in U.S. Pat. Nos. 5,287,387 and 5,638,403, is operable to reduces, but does not eliminate, spectral regrowth. Therefore, there exists a need for an improved method and device with which to further minimize the deleterious effects of clipping and other nonlinearities which occur during signal amplification.
SUMMARY OF THE INVENTION
Accordingly, the present invention is a device and a method for intentionally distorting a signal in a “tailored” manner, prior to the amplification of the signal, in order to prevent the deleterious effects of amplifier nonlinearities when the signal is amplified. “Tailoring a signal” involves altering the inherent waveshape of a signal in order to: (i) avoid unwanted output signal characteristics that may be induced during amplification, and/or (ii) induce desirable output signal characteristics during amplification. The manner and type of signal tailoring which is performed is dependent upon the specific amplifier application. For example, the present invention is especially useful when used to ameliorate the effects of clipping distortion upon a radio frequency (RF) signal, but should not be construed as limited to such applications. When utilized in conjunction with a RF system, and used to ameliorate the deleterious effects associated with RF signal clipping during amplification, the present invention monitors the amplitude of a RF signal prior to amplification. A modifying signal waveform is generated whenever the amplitude of the monitored RF signal is greater than a threshold value, chosen to correspond to the amplifier's clipping threshold. The RF signal is delayed to account for signal processing time associated with the generation of the modifying signal waveform. The generated modifying signal waveform is then summed with the delayed RF signal, thereby forming a intentionally modified (distorted) version of the original RF signal which no longer exceeds the amplifier clipping threshold. The modified signal is then conveyed to a RF signal amplifier for amplification. Since, amplifier clipping is prevented in this manner, the amplification process minimizes or prevents out-of-band emissions which would otherwise have occurred. The tailored distortion process which comprises the present invention is compatible with known predistortion techniques and may used in conjunction with, or in lieu of conventional signal predistortion.
The generation and use of a tailored distortion pulse in accordance with the present invention is advantageous since control over the power spectrum for the distorting pulse is maintained in a system which adds the tailored distortion pulse to the signal waveform to be modified. By doing so, the additive pulse is matched to the power spectrum of the signal waveform, thus reducing or preventing out-of-band emissions.
An alternative embodiment of the present invention tailors the intentional distortion in a pre-determined manner so as to prevent clipping of the RF signal during the subsequent amplification process, but does so not to eliminate out-of-band emissions, but rather to control the degree and severity of such out-of-band emissions and to predetermine the spectral composition of the emissions. This scheme is utilized in those various amplifier applications in which it is acceptable to emit power in specified bands outside the signal bandwidth. For example, a cellular base station may transmit in cellular channels not currently utilized, or in channels that are utilized by another base station that is sufficiently distant so as not to be adversely affected by the interference caused by out-of-band emissions.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete understanding of the present invention may be obtained from consideration of the following description in conjunction with the drawings in which:
FIG. 1
is a simplified block diagram illustrating a signal amplifier without a predistorter and without a tailored distorter, incorporated within a wireless RF transmission system;
FIG. 2
is a simplified block diagram illustrating a signal amplifier with a predistorter, but without a tailored distorter, incorporated within a wireless RF transmission system;
FIG. 3
is a graph illustrating a comparison between the transfer functions
Gibbons Del Deo Dolan Griffinger & Vecchione
Lucent Technologies - Inc.
Shingleton Michael B
LandOfFree
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