Pulse or digital communications – Transmitters – Antinoise or distortion
Reexamination Certificate
1999-10-20
2001-11-06
Chin, Stephen (Department: 2734)
Pulse or digital communications
Transmitters
Antinoise or distortion
C330S149000
Reexamination Certificate
active
06314142
ABSTRACT:
FIELD OF THE INVENTION
The present invention refers to modulation methods resulting in a non-constant envelope of a high-frequency carrier signal, and in particular to the predistortion for a non-linear channel in the high-frequency region.
BACKGROUND OF THE INVENTION
If in an e.g. wireless communication system modulation methods are used, which result in a non-constant envelope of the high-frequency carrier signal, all signal processing components after the modulator must have a sufficient linearity. This demand is difficult to fulfill, especially in power amplifier stages which should operate with high efficiency.
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.
Especially in transmission systems making use of a QPSK modulation (QPSK=Quatenary Phase Shift Keying), as is e.g. the case with mobile telephone systems, it is particularly important that the non-linear region of power amplifiers is utilized. In view of the fact that, e.g. in mobile telephones, the available power supply is limited, i.e. a larger storage battery makes the mobile telephone much heavier and much more expensive, the final stage amplifier must operate with the highest possible efficiency, but this is not possible in its linear amplification region. High-efficiency power amplifiers are therefore preferably operated in their non-linear region near saturation, and this causes non-linear distortions entailing the problems described.
Modulation methods requiring a linearization in addition to the PAM modulation, are, fundamentally, all multi-carrier methods (e.g. COFDM for digital broadcasting) and all pulse amplitude modulation methods which comprise, in addition to the QPSK modulation, also the QAM modulation. Further possibilities of use for the predistortion of a non-linear channel exist in the field of base stations of mobile telephone systems when several frequency channels are supplied to a final transmitter stage in parallel.
Hence, there is a need for a suitable linearization method based on predistortion. This method could be used whenever a non-linear channel is to be linearized in general. The linearization of a non-linear channel should additionally permit the frequency region of the input signal into the non-linear channel that is to be equalized to differ from the frequency region of the output signal from the non-linear channel. The linearization must therefore not be limited to mere amplifier stages, but it must also be possible to carry out frequency conversions within the non-linear channel.
DESCRIPTION OF PRIOR ART
A plurality of methods for linearizing high-frequency final stages already exist in the field of technology. The best-known methods of linearizing high-frequency final stages can be classified as follows.
When a signal to be transmitted is digitally predistorted, the digitally represented values of the signal are multiplied by suitably selected coefficients. Hence, the predistortion is carried out together with the digital generation of the control signal of the modulator.
A further known method is the analog predistortion. 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 high-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. Hence, the linearization does not take place in the high-frequency region, but in the baseband region, since the signals accessed are the IQ signals. Furthermore, the circuit described realizes a permanent negative feedback of the high-frequency final stage in the sense of a Cartesian loop.
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 predistorting the high-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 predistortion circuit for a power amplifier operated in the non-linear region. This predistortion 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 predistortion 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 osc
Gerhauser Heinz
Perthold Rainer
Chin Stephen
Fraunhofer-Gesellschaft zur Foerderung der Angewandten Forschung
Glenn Michael A.
Jiang Lenny
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