Waveforms and End-to-End Efficiency in RF Wireless Power Transfer Using Digital Radio Transmitter

Abstract: We study radio-frequency (RF) wireless power transfer (WPT) using a digital radio transmitter for applications where alternative analog transmit circuits are impractical. An important paramter for assessing the viability of an RF WPT system is its end-to-end efficiency. In this regard, we present a prototype test-bed comprising a software-defined radio (SDR) transmitter and an energy harvesting receiver with a low resistive load; employing an SDR makes our research meaningful for simultaneous wireless information and power transfer (SWIPT). We analyze the effect of clipping and non-linear amplification at the SDR on multisine waveforms. Our experiments suggest that when the DC input power at the transmitter is constant, high peak-to-average power ratio (PAPR) multisine are unsuitable for RF WPT over a flat-fading channel, due to their low average radiated power. The results indicate that the end-to-end efficiency is positively correlated to the average RF power of the waveform, and that it reduces with increasing PAPR. Consequently, digital modulations such as phase-shift keying (PSK) and quadrature amplitude modeulation (QAM) yield better end-to-end efficiency than multisines. Moreover, the end-to-end efficiency of PSK and QAM signals is invariant of the transmission bit rate. An in-depth analysis of the end-to-end efficiency of WPT reveals that the transmitter efficiency is lower than the receiver efficiency. Furthermore, we study the impact of a reflecting surface on the end-to-end efficiency of WPT, and assess the transmission quality of the information signals by evaluating their error vector magnitude (EVM) for SWIPT. Overall, the experimental observations of end-to-end efficiency and EVM suggest that, while employing an SDR transmitter with fixed DC input power, a baseband quadrature PSK signal is most suitable for SWIPT at large, among PSK and QAM signals.