Effect of Power Amplifier

In today's high-speed wireless communication applications, the simultaneous demand for bandwidth-efficient modulation schemes such as M-QAM and high power-efficiency of electronic components in the system imposes a contradicting tradeoff. On one hand, with the RF power amplifier (PA) at the transmitter antenna being the major power "hungry" block in many such systems, high power-efficiency for the system is obtained when the PA consumes most of the little supplied dc power (e.g. from a battery) as RF power, which is the power delivered to the load. On the other hand, at high powers, the PA becomes nonlinear, which becomes problematic when the input to the PA has a time-varying envelope. More precisely, when an M-QAM modulated signal (which has varying envelope and phase) is transmitted, the PA nonlinearity characteristics adversely affect the system performance, such as degradation of the Bit Error Rate (BER). Although linearization and compensation techniques can be used to minimize the above mentioned degradation in system performance, such techniques are mostly not suitable for low-cost and small-size integrated-circuit implementations. As a result, to obtain an optimal solution to this linearity/efficiency tradeoff, the objective of this thesis is to qualitatively and quantitatively characterize the effect of the PA nonlinearity on the system performance metric, BER. To gain an intuitive understanding of the effect of the PA nonlinearity, a system simulation is developed. In addition, to achieve a simple relationship between this circuit nonlinearity and system performance, an appropriate model (AM-AM/AM-PM) to characterize the PA nonlinearity is found. In fact, the results of this thesis in characterizing the effect of the PA nonlinearity on system performance can be useful to PA designers to design cost-effective and efficient PAs by finding a PA operating point that lies in the nonlinear region as much as possible while still meeting the system performance requirements. Thesis Supervisor: Charles G. Sodini Title: Professor of Electrical Engineering and Computer Science