Wireless Information and Power Transfer: Nonlinearity, Waveform Design and Rate-Energy Tradeoff (1607.05602v3)

Abstract: The design of Wireless Information and Power Transfer (WIPT) has so far relied on an oversimplified and inaccurate linear model of the energy harvester. In this paper, we depart from this linear model and design WIPT considering the rectifier nonlinearity. We develop a tractable model of the rectifier nonlinearity that is flexible enough to cope with general multicarrier modulated input waveforms. Leveraging that model, we motivate and introduce a novel WIPT architecture relying on the superposition of multi-carrier unmodulated and modulated waveforms at the transmitter. The superposed WIPT waveforms are optimized as a function of the channel state information so as to characterize the rate-energy region of the whole system. Analysis and numerical results illustrate the performance of the derived waveforms and WIPT architecture and highlight that nonlinearity radically changes the design of WIPT. We make key and refreshing observations. First, analysis (confirmed by circuit simulations) shows that modulated and unmodulated waveforms are not equally suitable for wireless power delivery, namely modulation being beneficial in single-carrier transmissions but detrimental in multi-carrier transmissions. Second, a multicarrier unmodulated waveform (superposed to a multi-carrier modulated waveform) is useful to enlarge the rate-energy region of WIPT. Third, a combination of power splitting and time sharing is in general the best strategy. Fourth, a non-zero mean Gaussian input distribution outperforms the conventional capacity-achieving zero-mean Gaussian input distribution in multi-carrier transmissions. Fifth, the rectifier nonlinearity is beneficial to system performance and is essential to efficient WIPT design.

• The paper proposes a tractable nonlinear model for WIPT rectifiers, departing from traditional linear models to accurately capture behavior and optimize waveform design.
• The research shows that exploiting rectifier nonlinearity with optimized multisine waveforms adapted via channel state information can enhance energy conversion efficiency, particularly in multicarrier and multi-antenna systems.
• This approach suggests combining deterministic multisine signals with data-carrying modulated signals to achieve dual benefits and provides a framework for future WIPT research in multi-user or relay scenarios.

Overview of Wireless Information and Power Transfer: Nonlinearity, Waveform Design, and Rate-Energy Tradeoff

The paper by Bruno Clerckx addresses significant challenges in the domain of Wireless Information and Power Transfer (WIPT), proposing solutions that depart from traditional linear models often employed in the design of wireless power systems. It builds upon the foundational concept of WIPT, which aims to simultaneously transmit data and energy using wireless signals, and introduces nonlinear considerations that change the landscape of system design. This research is situated within a broader context where WIPT applications have garnered substantial interest, given their potential to revolutionize telecommunications efficiency and integration.

Nonlinear Modeling in WIPT Systems

A critical contribution of this paper is the departure from the conventional linear model, which oversimplifies the behavior of energy harvesters. The authors present a tractable nonlinear model of the rectifier, fundamental to designing efficient WIPT architectures. The nonlinear model accounts for the intrinsic nonlinearity observed in practical rectenna operations, particularly when exposed to multisine and modulated waveforms. The paper meticulously details how multicarrier unmodulated waveforms (multisine signals) are optimized in conjunction with modulated waveforms to maximize both data rate (R) and energy conversion (I_{DC}). The traditional approaches fail to capture the complexities of diode rectification, whereas this model provides a more realistic portrayal that influences waveform design.

Benefits of Nonlinearity in WIPT Design

Clerckx's findings propose that harnessing rectifier nonlinearity can be beneficial, showing improved system performance when multisine waveforms are adapted to the channel state information (CSI). The analysis indicates that modulation, while offering benefits in single-carrier systems due to higher-order statistical moments, is less advantageous in multicarrier systems compared to optimized multisine strategies. This discrepancy arises because nonlinear effects in the energy harvester can be exploited to achieve constructive interference among multicarrier signals, enhancing energy conversion efficiency—a phenomenon that linear models overlook.

Optimization and Design Strategy

The paper presents algorithms optimized for maximizing energy transfer efficiency while satisfying data rate constraints, underlining the importance of adaptive waveform designs that leverage CSI. Algorithmically, the optimization problem is not standard as it fits within the category of posynomial maximization, tackled via reversed geometric programming approaches. The results underscore that multi-antenna systems benefit from this adaptation, showing gains in both rate and energy tradeoffs. Additionally, the paper suggests that deterministic multisine signals, when superposed with data-carrying modulated signals, can enhance energy harvesting without compromising communication efficiency—ushering in dual benefits.

Implications and Future Research

This research poses potential implications for future development in WIPT systems, offering a framework that can expand to multi-user setups, relaying scenarios, and potentially influence backscatter communication designs where similar nonlinear characteristics are prevalent. The paper invites further theoretical and experimental work to explore the boundaries of nonlinear modeling in wireless communication networks, suggesting routes for enhancing WIPT standards and implementing practical solutions for emerging wireless technologies.

In summary, Clerckx's paper is a pivotal work that opens avenues for a deeper understanding and practical application of nonlinear models in WIPT systems. It suggests a paradigm shift from linear simplifications to comprehensive nonlinear rectifier models that better equip engineers and researchers to optimize wireless power and data transfer technologies effectively.