Coding efficiency optimization for multilevel PWM based switched-mode RF transmitters

Switched-mode power amplifier (SMPA) techniques with pulse-width modulation (PWM) encoding are one of the most favored techniques for the highly efficient radio frequency (RF) transmitters. Unfortunately, SMPAs can only provide good efficiency for maximum output power and signals with low peak-to-average power ratios (PAPRs). With multilevel PWM based transmitters, a superior electrical efficiency can be provided for a large signal dynamic. In this paper, the coding efficiency, which is the upper bound of the electrical efficiency, is investigated for multilevel pulsed transmitter architectures. It is shown that the optimum coding efficiency can be obtained by changing the input threshold according to the probability density function (pdf) of the signal. Closed-form expressions for determining the optimum threshold as well as the maximum coding efficiency are derived for given signal statistics. Two examples are presented to demonstrate the derived equations.