RF Wireless Power and Data Transfer : Experiment-driven Analysis and Waveform Design

The brisk deployment of the fifth generation (5G) mobile technology across the globe has accelerated the adoption of Internet of Things (IoT) networks. While 5G provides the necessary bandwidth and latency to connect the trillions of IoT sensors to the internet, the challenge of powering such a multitude of sensors with a replenishable energy source remains. Far-field radio frequency (RF) wireless power transfer (WPT) is a promising technology to address this issue. Conventionally, the RF WPT concepts have been deemed inadequate to deliver wireless power due to the undeniably huge over-the-air propagation losses. Nonetheless, the radical decline in the energy requirement of simple sensing and computing devices over the last few decades has rekindled the interest in RF WPT as a feasible solution for wireless power delivery to IoT sensors. The primary goal in any RF WPT system is to maximize the harvested direct current (DC) power from the minuscule incident RF power. As a result, optimizing the receiver power efficiency is pivotal for an RF WPT system. On similar lines, it is essential to minimize the power losses at the transmitter in order to achieve a sustainable and economically viable RF WPT system. In this regard, this thesis explores the system-level study of an RF WPT system using a digital radio transmitter for applications where alternative analog transmit circuits are impractical. A prototype test-bed comprising low-cost software-defined radio (SDR) transmitter and an off-the-shelf RF energy-harvesting (EH) receiver is developed to experimentally analyze the impact of clipping and nonlinear amplification at the digital radio transmitter on digital baseband waveform. The use of an SDR allows leveraging the test-bed for the research on RF simultaneous wireless information and power transfer (SWIPT); the true potential of this technology can be realized by utilizing the RF spectrum to transport data and power together. The experimental results indicate that a digital radio severely distorts high peak-to-average power ratio (PAPR) signals, thereby reducing their average output power and rendering them futile for RF WPT. On similar lines, another test-bed is developed to assess the impact of different waveforms, input impedance mismatch, incident RF power, and load on the receiver power efficiency of an RF WPT system. The experimental results provide the foundation and notion to develop a novel mathematical model for an RF EH receiver. The parametric model relates the harvested DC power to the power distribution of the envelope signal of the incident waveform, which is characterized by the amplitude, phase and frequency of the baseband waveform. The novel receiver model is independent of the receiver circuit’s matching network, rectifier configuration, number of diodes, load as well as input frequency. The efficacy of the model in accurately predicting the output DC power for any given power-level distribution is verified experimentally. Since the novel receiver model associates the output DC power to the parameters of the incident waveform, it is further leveraged to design optimal transmit wave-forms for RF WPT and SWIPT. The optimization problem reveals that a constant envelope signal with varying duty cycle is optimal for maximizing the harvested DC power. Consequently, a pulsed RF waveform is optimal for RF WPT, whereas a continuous phase modulated pulsed RF signal is suitable for RF SWIPT. The superior WPT performance of pulsed RF waveforms over multisine signals is demonstrated experimentally. Similarly, the pulsed phase-shift keying (PSK) signals exhibit superior receiver power efficiency than other communication signals. Nonetheless, varying the duty-cycle of pulsed PSK waveform leads to an efficiency—throughput trade-off in RF SWIPT. Finally, the SDR test-bed is used to evaluate the overall end-to-end power efficiency of different digital baseband waveforms through wireless measurements. The results indicate a 4-PSK modulated signal to be suitable for RF WPT considering the overall power efficiency of the system. The corresponding transmitter, channel and receiver power efficiencies are evaluated as well. The results demonstrate the transmitter power efficiency to be lower than the receiver power efficiency

Efficiency–Throughput Trade-off of Pulsed RF Waveforms in Simultaneous Wireless Information and Power Transfer

We study the receiver efficiency–throughput trade-off in a realistic radio frequency (RF) simultaneous wireless information and power transfer (SWIPT) system. Based on the energy harvesting receiver characteristics, we propose a continuously phase-modulated pulsed RF waveform to achieve maximum receiver efficiency at any input RF power level. We study the impact of varying the duty cycle of a pulsed RF waveform on the receiver efficiency of wireless power transfer along with the throughput of information transfer, and the trade-off thereof. The experiments confirm that a phase-shift keying (PSK) modulated pulsed RF waveform yields superior receiver efficiency than other digital baseband modulations as well as multisine signals despite they are designed particularly for power transfer. However, the optimal efficiency is attained at the expense of a significant loss in throughput due to pulsed transmission, depending on the average input RF power level.Peer reviewe

Impact of Input Impedance Mismatch on the Receiver Efficiency of RF Wireless Power Transfer

The receiver efficiency of a radio-frequency (RF) wireless power transfer (WPT) system is determined by the RF-to-direct current (DC) conversion efficiency of the rectifier and the effectiveness of the preceding impedance matching network. The impedance mismatch at the rectifier's input, especially, hampers the power efficiency of RFWPT. In this paper, we experimentally examine the dependency of the rectifier input impedance of an off-the-shelf RF energy harvester on the average input RF power and the frequency of the incident RF signal as well as the load resistance. We develop a data-driven model of the RF energy harvester to study the receiver efficiency in terms of the matching efficiency and the rectifier efficiency. The results reveal that the matching efficiency varies significantly with the average input RF power and the load resistance. Moreover, the input impedance mismatch reduces the peak receiver efficiency of our RF WPT test-bed by around 1S%.acceptedVersionPeer reviewe

Practical Waveform-to-Energy Harvesting Model and Transmit Waveform Optimization for RF Wireless Power Transfer Systems

The received radio-frequency (RF) power in far-field RF wireless power transfer (WPT)—with or without simultaneous information transfer—is minuscule due to large propagation loss in wireless media. In such scenarios, adapting to the receiver characteristics by transmit waveform optimization is essential for maximizing the harvested direct current (dc) and, thus, the end-to-end efficiency of an RF WPT system. The receiver efficiency in RF WPT is governed by the RF-to-dc efficiency of the rectifier as well as the impedance mismatch at the antenna and load. In this article, we study the receiver efficiency for any fixed load and, subsequently, present a novel rectifier model that relates the average harvested dc power to the distribution, that is, the histogram, of the instantaneous power levels of the RF signal’s envelope over time. The proposed waveform-to-energy harvesting (EH) model enables us to anticipate the average harvested dc power for any waveform, including communication signals as well, given the knowledge of the power-level distribution. Consequently, we conduct rigorous waveform optimization to maximize the average harvested dc power and determine the digital baseband signal at the transmitter that does so, namely prove that a pulsed tone at appropriate frequency is optimal for RF WPT. We present a multiband test-bed for determining the receiver efficiency for any digital baseband waveform. The efficacy of the proposed model is corroborated through experiments as well as simulations, which confirm that it is operational as well as accurate in practice and that single-sine pulses yield higher efficiency than basic multisine waveforms, while a pulsed phase shift keying (PSK) is preferable for simultaneous wireless information and power transfer (SWIPT).Peer reviewe

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

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.Comment: Accepted for publication in IEEE Transactions on Microwave Theory and Technique

Joint impact of input power, papr, and load resistance on the receiver efficiency of multisine waveforms in RF energy harvesting

The receiver efficiency in radio-frequency (RF) energy harvesting (EH) is affected by numerous parameters such as the average RF input power and peak-to-average power ratio (PAPR) of the waveform, the EH circuit components, and the load. In this paper, we experimentally study the dependence of RF-to-DC efficiency on these parameters. We present a test-bed, comprising a signal generator and an off-the-shelf energy harvesting receiver with a diode-based rectifier, for evaluating the RF-to-DC efficiency for varying input RF power, load resistance and a number of co-phased baseband multisine waveforms (viz. PAPR). The experimental results suggest that the RF-to-DC efficiency of multisine waveforms in RF EH is significantly affected by the average input RF power and the load resistance. The high-PAPR multisine waveforms are preferable for RF EH only for a certain range of average input RF power and load resistance. In particular, for the considered rectifier, a single-sinusoid waveform yields the highest RF-to-DC efficiency for low-resistance and high average input RF power region, while high-PAPR baseband multisines are optimal for high-resistance and low average input RF power region.acceptedVersionPeer reviewe

Impact of Software-Defined Radio Transmitter on the Efficiency of RF Wireless Power Transfer

In this paper, we experimentally examine the impact of a digital radio transmitter on radio-frequency (RF) wireless power transfer (WPT). The test-bed for this purpose consists of a software-defined radio (SDR) transmitter, an external power amplifier (PA), a variable attenuator to emulate the high losses in wireless propagation, and an RF energy harvesting receiver. The test-bed facilitates determining the transmitter efficiency and overall DC-DC efficiency of different test waveforms for RF WPT. We assess the performance of multisine signals as well as an information-bearing quadrature phase-shift keying (QPSK) signal for RF WPT, under different channel conditions, by determining the transmitter, DC-DC and receiver efficiency. The experiments reveal that high peak-to-average power ratio constraints the maximum RF output power for multisine signals. Even with similar input average power, single sinusoid RF waveforms yield the highest DC-DC and receiver efficiency. Moreover, a QPSK signal with transmit filtering provides good efficiency for WPT and seems to be a waveform suitable for simultaneous wireless information and power transfer applications.acceptedVersionPeer reviewe

Rate and Power Throttling for Traffic Asymmetry in Reverse TDD HetNets

In this paper, sum link capacity expressions for successive interference cancellation (SIC) and regarding interference as noise (IAN) in reverse time-division duplexing (RTDD) hetero-geneous cellular network are derived. The considered RTDD network always operates in a synchronized fashion such that if the macro tier is in the uplink (UL), then the small tier will be in the downlink (DL) and vice-versa. Rate and power throttling are used in the uplink (UL) for both IAN and SIC to consider an asymmetric traffic network (DL≫UL). System-level simulations are performed to compare the overall system throughput of IAN and SIC for different DL/UL ratios. It is observed that rate or power-throttled SIC performs better than rate-throttled IAN and worse than power-throttled IAN.publishedVersio

Software-Defined Radio Test-Bed for Measuring Efficiency of Multisine Waveforms in RF Power Transfer

In this paper, we investigate the efficacy of multisine waveforms for radio-frequency (RF) wireless power transfer (WPT). We present a test-bed developed for experimentally assessing the end-to-end efficiency of waveforms in RF WPT. The test-bed primarily comprises a software-defined radio (SDR) transmitter and an energy harvesting receiver. We examine how the transmitter distorts digital multisine waveforms with high peak-to-average power ratio (PAPR), and measure the effect of varying the PAPR of clipped multisines on the end-to-end efficiency. The results reflect the poor performance of high-PAPR multisines in RF power transfer and hint towards low-PAPR signals being ideal for RF WPT.publishedVersio

Experimenting Waveforms and Efficiency in RF Power Transfer

In this paper, we present a prototype test-bed for radio-frequency (RF) wireless power transfer (WPT) comprising a software-defined radio (SDR) transmitter and an energy harvesting receiver with a diode-based rectifier. The test-bed allows us to study the end-to-end efficiency of RF WPT when employing different co-phased multisine waveforms. In particular, we analyze the clipping and non-linear behaviours of the transmitter by experimentally evaluating how they affect the performance of waveforms. The experimental results indicate that transmitting impulse-like signals is actually not optimal for RF WPT in practice despite they would be ideal in terms of rectifier efficiency. Instead, the results highlight the superior performance of single-tone signals over co-phased multisines in terms of both end-to-end WPT efficiency and spectral purity.acceptedVersionPeer reviewe