Amplifiers – Hum or noise or distortion bucking introduced into signal...
Reexamination Certificate
2002-04-29
2003-09-16
Choe, Henry (Department: 2817)
Amplifiers
Hum or noise or distortion bucking introduced into signal...
C330S129000, C330S136000, C375S297000
Reexamination Certificate
active
06621340
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to systems for conducting a pre-treatment, i.e., a pre-linearization and a pre-distortion, of an input signal to be transmitted via a non-linear amplifier, in particular a power amplifier, in order to compensate for level-dependent non-linearities of the gain of the amplifier.
The present invention can be used for conducting a pre-treatment of input signals formed using modulation methods which result in a non-constant envelope of the radio frequency carrier signal. Thus, the present invention preferably finds use in transmitters for digital broadcasting, which are fed by multi-carrier signals, such as OFDM signals (OFDM=orthogonal frequency division multiplex), for example. With such signals, the non-linearity of the amplifier causes unwanted frequency portions of the signal at the output of the amplifier. Such frequency portions interfere with adjacent-frequency channels.
In addition, the present invention is applicable to mobile communication systems using CDMA signals (CDMA=code division multiple access), for example. Additionally, the present invention can be advantageously used in satellite earth stations or base stations of mobile telephone systems.
When pulse amplitude modulation methods are used, the spectral efficiency will deteriorate due to the non-linearity of the amplifiers. The reason for this is to be seen in the non-linear amplitude output characteristics of an amplifier, which leads to an AM/AM conversion; the phase drift of an output signal of an amplifier relative to the phase of an input signal produces, in addition, intermodulation components, which is also referred to as AM/PM conversion. The AM/AM and the AM/PM conversion must, however, be prevented by suitable linearization methods. If this is not done, the spectral efficiency of the modulation method used as well as the signal-to-noise ratio will deteriorate. In digital transmission systems this can cause a substantial increase in the bit error rate during the transmission.
2. Description of Prior Art
A plurality of methods for linearizing radio frequency final stages in order to reduce spurious adjacent-channel emission at the output thereof already exist in the field of technology. The best-known methods of linearizing radio frequency final stages can be classified as follows.
When a signal to be transmitted is digitally pre-distorted, the digitally represented values of the signal are multiplied by suitably selected coefficients. Hence, the pre-distortion is carried out together with the digital generation of the control signal of the modulator.
A further known method is the analog pre-distortion. It makes use of non-linear components, such as Schottky diodes, so as to synthesize an equalizing characteristic which is complementary to the amplifier distortion characteristic.
The “Cartesian loop” represents an analog negative feedback of the radio frequency final stage, which is carried out in the baseband.
The forward coupling (also referred to as “feedforward” in the field of technology) constitutes a disturbance variable feedforward in the sense of control technology, the output signal of the final stage having added thereto a suitable correction voltage for compensating the distortion of the final stage.
WO 93/18581 describes a “cartesian loop” whose parameters are adjusted in accordance with various system parameters which are representative of the current operating condition of the system. A radio transmitting unit according to said WO 93/18581 comprises a power amplifier, a linearization means and a feedback means for feeding a signal back from an output of the power amplifier to the linearization means so as to guarantee the linearity of the output signal. The linearization means operates in the baseband, the IQ signals being controlled by a linear control means which is connected to a direct-access table storing predetermined loop linearization parameters. When the IQ signals have been processed in a suitable manner by the linearization means, these processed signals are subjected to up-conversion by an up-conversion mixer whereupon they are amplified by the power amplifier. The feedback means takes an output signal of the power amplifier, subjects it to down-conversion by means of a down-conversion mixer and feeds the down-converted signal into the linearization means.
GB 2240893 A discloses a circuit for linearizing the amplitude response and the phase response of an amplifier. An envelope detection circuit detects the envelope of an input signal to be transmitted, the output signal of said envelope detection circuit being inputted in a control circuit of the non-linear type as well as in a phase shift control circuit. The phase shift control circuit controls a phase shifter which precedes the power amplifier for pre-distorting the radio frequency signal with regard to its phase. The control circuit of the non-linear type feeds an input signal into a variable-voltage dc-dc converter which adjusts the bias voltage parameters, i.e. the operating point of the power amplifier in a suitable manner so as to compensate the distortion of the non-linear amplifier. The amplitude error of the amplifier is therefore compensated for by adjusting the operating point of said amplifier, a course of action which is disadvantageous insofar as the operating point parameters of the amplifier will have to be changed constantly, and this may it make much more difficult to adapt the amplifier to a load, since, normally, a changed operating point will automatically require a different (complex) transformation ratio of the output resistance.
U.S. Pat. No. 5,023,937 represents an analog pre-distortion circuit for a power amplifier operated in the non-linear region. This pre-distortion works by means of a negative feedback loop in which, in contrast to the Cartesian loop, not the IQ components of the output signal are controlled, but the amount and the phase of said output signal. An envelope detector detects the amplitude of the signal to be amplified, said amplitude being continuously compared, with regard to its feedback, with the envelope of the output signal of the power amplifier, the comparison result being applied to a variable attenuator which attenuates the input signal before the power amplifier in a suitable manner so as to produce an output signal which is as linear as possible. The phase pre-distortion is carried out by means of a phase locked loop receiving the signal to be amplified as an input signal. A part of the output signal of the amplifier is also inputted in the phase locked loop by means of a mixer, a local oscillator and a phase shift circuit, said phase locked loop supplying a local oscillator signal for a mixer preceding the power amplifier in order to suitably pre-distort the phase of the signal to be amplified. This circuit operates in a fully analogous manner and is based on a substantially continuous feedback, if the phase locked loop is locked.
Also U.S. Pat. No. 4,465,980 represents an analog pre-distortion circuit. A detector detects the envelope of a signal to be amplified and applies this signal to a field effect transistor with two gate terminals (“dual gate FET”). The signal to be amplified is applied to the other gate of the dual gate FET. The radio frequency input signal is suitably pre-distorted by controlling the operating point of this dual gate FET in a suitable manner so as to compensate the non-linear amplification of a power amplifier connected to the drain terminal of said dual gate FET via a matching network.
DE 3312030 A1 discloses an amplifier with pre-distortion compensation, which uses pre-distortion components produced by a power amplifying element, which is similar to the amplifying element, so as to achieve an effective linearization for substantially reducing all intermodulation distortion products. Furthermore, an additional feedback circuit can be provided so as to achieve a further reduction of non-linearities.
GB 8723874 discloses a linearity correction circ
Gerhaeuser Heinz
Perthold Rainer
Choe Henry
Dougherty Clements & Hofer
Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung
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