Waveform Design for Wireless Power Transfer (1604.00074v2)

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.

• The paper develops an analytical non-linear rectenna model using higher-order Taylor series to capture diode behavior beyond traditional linear assumptions.
• The paper formulates an adaptive multisine waveform optimization strategy leveraging perfect CSIT to maximize the DC output current under practical PAPR constraints.
• The paper demonstrates that using wideband transmission and scaling laws significantly enhances energy transfer efficiency compared to conventional narrow-band approaches.

An Overview of Waveform Design for Wireless Power Transfer

The paper "Waveform Design for Wireless Power Transfer" by Bruno Clerckx and Ekaterina Bayguzina addresses a critical and previously underexplored aspect of wireless power transfer (WPT)—the optimization of input waveforms to enhance the efficiency and effectiveness of power delivery. The authors emphasize the need for exploiting the multisine waveform design tailored to channel conditions, a departure from mainstream strategies concentrated largely on improving energy harvester designs.

Key Contributions and Findings

The paper makes several contributions to the domain of WPT, primarily focusing on the non-linearity inherent in energy harvesting rectennas and proposing an adaptive waveform design strategy:

1. Non-linear Rectenna Modeling: The authors develop an analytical model considering the non-linearity of the rectenna, departing from the conventional linear model. They derive expressions through higher-order Taylor series expansions of the diode's I-V characteristics, which unveil the importance of non-linear effects in system design.
2. Optimization of Multisine Waveforms: Assuming perfect channel state information at the transmitter (CSIT), the paper formulates an optimization problem for designing multi-antenna multisine waveforms. This approach aims to maximize the DC output current by adaptively adjusting waveform weights based on channel conditions.
3. Comparison between Linear and Non-linear Models: The authors demonstrate that while linear models suggest narrow-band transmission, non-linear models advocate for a wideband power allocation strategy. This empirical finding underscores the potential inefficiencies in waveform designs that neglect rectenna non-linearity.
4. Scaling Laws and Non-Linear Effects: The paper presents scaling laws revealing that for frequency-flat channels, DC power increases linearly with the number of sinewaves when non-linear models are used. It stresses that the gain from CSIT in frequency-selective scenarios further amplifies the benefits illustrated by these scaling laws.
5. PAPR Constraints and Applications: The paper also considers practical constraints such as Peak-to-Average Power Ratio (PAPR) limits, linking waveform design objectives to real-world implementation constraints. This analysis aligns with observed behavior that higher PAPR waveforms can improve RF-to-DC conversion efficiency.

Theoretical and Practical Implications

The work significantly shifts perspectives on WPT system design by illustrating that the architecture of the input signal plays a crucial role in maximizing energy transfer efficiency. By enhancing waveform design strategies through non-linear modeling:

• Theoretically, the results introduce a new dimension in the WPT literature, encouraging further exploration of non-linear models in waveform optimization.
• Practically, they suggest that future WPT systems, especially those used in wireless sensor networks and IoT applications, could achieve higher efficiencies with appropriately designed transmitter-side waveforms.

Speculation on Future Developments:

The paper opens pathways for advancing WPT technologies towards large-scale implementations, akin to the innovations seen in massive MIMO for communication networks. It suggests that by scaling the number of sinewaves and antennas while employing non-linear models, wireless power delivery could be made more robust and efficient. Future research could delve into automatic adaptation mechanisms for waveform design in real-time, integrating machine learning algorithms for dynamic CSIT estimation and resource allocation based on environmental changes.

In summary, Clerckx and Bayguzina’s work demonstrates the tangible impact of accounting for rectenna non-linearity in WPT, suggesting a paradigm shift in designing such systems. This paper strongly motivates further investigation into multisine waveform optimization and positions these strategies as foundational for the next generation of wireless energy systems.