Waveform Impact on RF Wireless Power Transfer Efficiency of Low Power Harvesting Devices

Abstract

The paper is dedicated to the study of the impact waveform parameters: peak-to-average power ratio (PAPR) and spectral characteristics, on the performance of low power harvesting device during wireless power transfer (WPT) under real life scenario. In the current study a low power RF harvesting device, consisting of a classic voltage doubler circuit for RF-DC conversion and a DC-DC converter based on Texas Instruments manufactured chip BQ25504, is chosen. Moreover, as harvesting device consists of RF-DC converter and DC-DC converter, the study of the impact of load resistance on conversion efficiency for RF-DC rectifier and the whole harvesting device is performed. For the study of conversion efficiency and harvesting device performance three types of waveforms are used: single tone, multicarrier signals with low PAPR and multicarrier signal with high PAPR level; the number of subcarriers is varied from 4 to 16. To generate high PAPR signals all subcarriers with the same amplitude are summed in-phase, while to generate low PAPR signals, subcarriers with the same amplitude but specially selected phases are summed together. All signals are transmitted in an unlicensed sub-gigahertz ISM band with the same bandwidth of 5 MHz, while single tone frequency corresponds to the carrier frequency of multi-carrier signals. For baseband signal generation MATLAB/Simulink software is used, and for transferring on carrier frequency a software defined radio (SDR) USRP B210 is employed. During WPT directional antennas are used and the distance between them is varied up to 1.8 m. To evaluate performance of harvesting device and efficiency of conversions the average voltages at the input and output of the RF-DC converter as well as at the output of the DC-DC converter are measured.