Dynamic drive for radio frequency power amplifiers with wideband active load modulation
Final Report Abstract
In this DynaDrive project approaches for a fast and efficient control and pre-distortion of a Dual Input DPA were investigated. For this purpose, a complex measuring station was set up to test and further develop the control and pre-distortion of Dual Input DPAs on real hardware demonstrators. In addition, the course was set for the integration of the drive function into the linearization structures of the signal processing and a dynamic drive of the amplifier cores. Due to the fact that two fully functional dual input DPAs were available at the start of the project, it was appropriate to set up a corresponding measurment setup in a first step. This was realized by a proprietary signal generation and acquisition platform with a bandwidth of more than 400 MHz, extended by appropriate preamplifiers. Signals before and after the preamplifiers and after the Dual Input DPA were decoupled by means of directional couplers in order to calibrate the complete setup so that only the results of the Dual Input DPA could be measured. In addition, this enabled the control and possible readjustment of the signal parameters during the current measurement. As a consequence, a first, simple algorithm for the identification of drive functions based on CW signals was developed. In this algorithm the main amplifier is characterized up to a certain compression criterion to identify the switch-on point of the peak amplifier. Based on this switch-on point, the relative phase between the main and peak input signals was rotated 360 degrees at low peak input power to identify the optimum relative phase for maximum efficiency. At this point various effects could be identified, which are also confirmed by the theoretical results. Then the input power of the main and peak amplifier was increased and the optimal relative phase was determined again until the maximum power was reached. This algorithm has been continuously improved during the project by reducing the number of points, implementing the quasi-outphasing transition and other functions. In addition to the very good efficiency values that could be achieved by the identified drive functions, the result was a relatively good-natured behaviour of the amplifier towards changes in the drive function. Thus, it was possible to identify drive functions relatively quickly and efficiently, which show very good efficiency values but hardly any problems with the compression of the individual amplifier cores. A further optimization of the efficiency by controlling the compression of the individual cores did not seem to be appropriate and the focus of the project was shifted towards a practical implementation of signal processing and predistortion. For this purpose a predistortion concept was developed, which extends the classical predistortion of an amplifier by a drive functions block and a crosstalk predistortion. By extending the predistortion with a model that models the crosstalk between the amplifier cores, the linearization result could be improved significantly. An additional extension of the concept by predistortion model of the single cores is under investigation. However, it must be clarified to what extent information about the distortion of the individual cores can be extracted from the common output signal. After the implementation of an efficient identification algorithm and the excellent results of the predistortion, the logical next step was the full integration of the static drive functions into the polynomial predistortion model. For this purpose, different approximations of the control functions were tested. Again, the tendency of the Dual Input DPA to behave well was shown. In the middle of the band, even very strong approximations hardly caused any loss in efficiency. Only at the edges of the band, a strong approximation produced an inappropriate load modulation, which in some cases led to considerable efficiency losses. However, this again leads to the possibility to serve different load scenarios with different approximations and thus to a dynamic adaptation of the predistortion to different signal statistics. Based on these fundamentals and the theory of the so-called ’vector-switched’ models, a concept for dynamic predistortion for dual input DPAs could be developed.
Publications
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Wideband Active Load Modulation in RF Power Amplifiers, Doktorarbeit, FAU Forschungen, Reihe B, FAU University Press, 2019
C. Musolff
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A Robust Digital Predistortion Algorithm for 5G MIMO: Modeling a MIMO Scenario With Two Nonlinear MIMO Transmitters Including a Cross-Coupling Effect,"in IEEE Microwave Magazine, vol. 21, no. 7, pp. 54-62, July 2020
T. Ackermann, J. Potschka, T. Maiwald, A. Hagelauer, G. Fischer and R. Weigel
