Study of space condition effect and analyzing digital techniques for improving RF power amplifier's linearity and efficiency for small satellites

Abstract

Objective of modern small satellite communication is to provide the end users with higher data rate downlink capability, in addition to, reliability and system efficiency. A significant device, with respect to power consumption and influence on system linearity, use in the transmission chain of small satellites, is power amplifier. The power amplifier tends to add distortion and non-linearity in the transmitted signal, when operating close to saturation point. For avoiding the non-linearity addition by the PA, it should be operated in linear region which causes the degradation in power efficiency. Therefore, for having the maximum power efficiency and improving linearity, the predistortion should be performed before inputting the signal to power amplifier. For compensating non-linear distortion, linearization scheme based on digital predistortion is used, which requires a feedback path for adaptation and extraction of new coefficients for DPD. Hence for making the DPD adaptive, ADC is required to add in the system. As a consequence of performing digital predistortion, the spectral regrowth occurs which causes the increase in bandwidth up to five times of original signal. Due to this reason, digital to analog converter has to sample the signal at five times of nyquist frequency which increases the cost and power consumption of DAC. This master thesis presents the methodology implementation for compensating the non-linear distortion in PAs, applicable for small satellite communication, with a cooperative technique of digital and analog predistortion. This thesis provides with the solution for the increased sampling rate of signal at analog to digital converter with a use of combination of digital and analog predistortion. The predistortion (digital and analog) is design to focus on maximizing the linearity and minimizing the distortion and spectral regrowth. While the adaptive scheme of combined digital predistortion and analog predistortion (simulated) is designed, with an ideal low-pass filter between them, for implementation ease of the digital-to-analog converter and decrease in signal sampling rate. The results provided in the thesis have shown that same linearity and efficiency can be achieved at amplifier's output by implementing the above mentioned solution with a benefit of reducing the signal sampling frequency at DAC. A comparative analysis of power amplifier behavior in various configuration is presented in the dissertation.