Digital predistortion linearization methods for RF power amplifiers

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Doctoral thesis (monograph)
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TKK dissertations, 136
Even though high linearity is crucial in modern mobile communications, it is not desirable to use the most linear power amplifier types due to their poor efficiency. Predistortion is a commonly used, fairly simple and robust method for improving linearity of power amplifiers (PA). This thesis will investigate digital RF and baseband PA predistortion methods. A digital RF predistortion system uses an analog predistortion element prior to the power amplifier that is controlled by digital circuitry to compensate for the PA nonlinearity. One problem with RF predistorter is its sensitivity to delays between the control signals generated by the digital circuitry and the RF signal. This thesis presents a delay compensation method that can be implemented with digital circuitry, thus the delay being much smaller than the previously used analog methods. Another implementation issue that affects the performance of the RF predistorter, are analog envelope detectors that are required for generating the control signals for the digital circuitry. Three commonly used detection methods, power, linear diode and logarithmic detector, are compared. The linear diode detector was shown to be the most versatile. By using a lookup table, the power and logarithmic detectors can be linearized so that their performance comes close to the linear diode, but the biasing is easier. Design of an RF predistorter and the measurement results are presented. The designed RF predistorter was implemented to linearize a class AB PA with 22 dB gain and a 18 kHz 16QAM signal at 420 MHz. The digital algorithm is implemented with an FPGA. The predistorter was able to achieve 10 dB improvement in the ACP. The thesis also investigates the design of a baseband predistorter, that is implemented using the complex gain predistortion method. The effects of nonlinear quadrature modulator errors on the predistortion are discussed. Simulation and measurement results of the predistorter are presented. The designed baseband predistorter was implemented to linearize a class AB PA with 50 dB gain and an 18 kHz 16QAM signal at 400 MHz. The digital algorithm is implemented with an FPGA. The predistortion improves the ACP by 15 dB. Finally, predistortion function generation methods applicable to both RF and baseband predistorter implementations are discussed. Some improvements to these methods are suggested and simulations with and without the suggested improvements are presented. The simulations show that the suggested improvements are able to improve the ACP and reduce the time required for convergence.
digital predistortion, power amplifiers, linearization, FPGA, radio frequency
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