Instantaneous power variance and radio frequency to dc conversion efficiency of wireless power transfer systems

This study compares the maximum rectifier radio frequency (RF)–dc conversion efficiency for various input signals with random modulation. The instantaneous power variance (IPV) is proposed as an easy to compute parameter to classify the effect of modulation on the obtained rectifier efficiency. A prototype ultra high frequency (UHF) rectifier is used to simulate as well as measure the performance of randomly modulated signals with binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), 8 phase-shift keying (PSK) and 64 quadrature amplitude modulation (QAM) modulations. In addition to considering different modulation formats, the roll-off factor of the baseband filter is also varied due to the fact that it has a strong impact on the time-varying nature of the signal envelope. For the given rectifier, it is shown that the peak RF–dc conversion efficiency versus the output load is shifted to larger load values for signals with higher IPV. Furthermore, by comparing signals with different time-varying envelope characteristics, it is shown that IPV represents a more accurate signal characteristic than peak-to-average power ratio in terms of properly characterising the effect of modulation on the rectifier efficiency.