Waveform Design for Wireless Power Transfer

Abstract

Abstract-Far-field Wireless Power Transfer (WPT) has attracted significant attention in recent years. Despite the rapid progress, the emphasis of the research community in the last decade has remained largely concentrated on improving the design of energy harvester (so-called rectenna) and has left aside the effect of transmitter design. In this paper, we study the design of transmit waveform so as to enhance the DC power at the output of the rectenna. We derive a tractable model of the non-linearity of the rectenna and compare with a linear model conventionally used in the literature. We then use those models to design novel multisine waveforms that are adaptive to the channel state information (CSI). Interestingly, while the linear model favours narrowband transmission with all the power allocated to a single frequency, the non-linear model favours a power allocation over multiple frequencies. Through realistic simulations, waveforms designed based on the non-linear model are shown to provide significant gains (in terms of harvested DC power) over those designed based on the linear model and over non-adaptive waveforms. We also compute analytically the theoretical scaling laws of the harvested energy for various waveforms as a function of the number of sinewaves and transmit antennas. Those scaling laws highlight the benefits of CSI knowledge at the transmitter in WPT and of a WPT design based on a non-linear rectenna model over a linear model. Results also motivate the study of a promising architecture relying on large-scale multisine multi-antenna waveforms for WPT. As a final note, results stress the importance of modeling and accounting for the non-linearity of the rectenna in any system design involving wireless power.

Large-Scale Multiantenna Multisine Wireless Power Transfer

“…Although most off-the-shelf rectifier models in the context of microwave theory are able to provide insights into the accurate rectification process, non-closed forms or highly complex structures in these models [9], [10] make it hard to derive efficient algorithms for WPT. By contrast, [11] built a simplified (but inaccurate [8], [12], [13]) model by truncating the Taylor expansion of the Shockley diode equation to the 2nd-order term, where the rectenna DC output current is linearly proportional to the average incident RF power. This model is referred to as a linear model.…”

“…Nevertheless, the linear model may not properly describe the rectenna behaviour. In the scenario where multiple frequency components can be utilized for WPT, the linear model favours a waveform strategy where the transmit power is allocated to a single frequency component [8], [13]. Unfortunately, this contradicts experimental results that show the benefits of a power allocation over multiple frequency components to boost the RF-to-DC conversion efficiency [2]- [5].…”

“…The reason is that the 4th-order truncation is necessary for characterizing the basic diode rectification process [5]. To balance the accuracy and complexity of the optimization, [13] truncates the Taylor expansion of the diode equation to the n o th-order term (n o ≥ 4), yielding a nonlinear rectenna model. Based on this nonlinear model, [13] developed waveform optimization methods for multi-antenna multi-sine WPT.…”

“…To balance the accuracy and complexity of the optimization, [13] truncates the Taylor expansion of the diode equation to the n o th-order term (n o ≥ 4), yielding a nonlinear rectenna model. Based on this nonlinear model, [13] developed waveform optimization methods for multi-antenna multi-sine WPT. Circuit simulations in [13] validate the newly developed nonlinear model, highlight the inaccuracy of the linear model and confirm the superiority of optimized waveforms over various baselines.…”

“…Based on this nonlinear model, [13] developed waveform optimization methods for multi-antenna multi-sine WPT. Circuit simulations in [13] validate the newly developed nonlinear model, highlight the inaccuracy of the linear model and confirm the superiority of optimized waveforms over various baselines. It is shown that the linear model favours narrowband transmission, while the nonlinear model favours a power allocation over multiple frequencies.…”