Mingyu Lu

Wireless Power Delivery to Low-Power Mobile Devices Based on Retro-Reflective Beamforming

This letter presents an experimental study on wireless power delivery to low-power mobile devices based on retro-reflective beamforming. In our scheme, a pilot signal composed of an impulse train is broadcast from a mobile device. The pilot signal is received and analyzed by a wireless charger that includes four antenna elements. Then, according to the analysis outcome, the wireless charger constructs a focused power beam onto the mobile device. In our experiments, the total power transmitted from the wireless charger is approximately 1 Watt; 14 mW of power is harvested by the mobile device 50-cm distance away. Our experimental results successfully demonstrate that the power beam in reaction to the pilot signal is capable of tracking the mobile devices location dynamically. In addition, since the pilot signals spectrum covers a certain frequency range, the carrier of wireless power in our scheme can be configured among multiple frequencies.

On the Internal Resonant Modes in Marching-on-in-Time Solution of the Time Domain Electric Field Integral Equation

Internal resonant modes are always observed in the marching-on-in-time (MOT) solution of the time domain electric field integral equation (EFIE), although “relaxed initial conditions,” which are enforced at the beginning of time marching, should in theory prevent these spurious modes from appearing. It has been conjectured that, numerical errors built up during time marching establish the necessary initial conditions and induce the internal resonant modes. However, this conjecture has never been proved by systematic numerical experiments. Our numerical results in this communication demonstrate that, the internal resonant modes amplitudes are indeed dictated by the numerical errors. Additionally, it is shown that in a few cases, the internal resonant modes can be made “invisible” by significantly suppressing the numerical errors. These tests prove the conjecture that the internal resonant modes are induced by numerical errors when the time domain EFIE is solved by the MOT method.

Experimental Study on the Impact of Soil Conductivity on Underground Magneto-Inductive Channel

This letter presents our experimental endeavor to characterize underground magneto-inductive channels. Specifically, a range of experiments are conducted in realistic underground environments, and the measurement data are compared with channel models established in prior publications. The channel model incorporates the impact of soil conductivity, is found in better agreement with the measurement data than models without soil conductivity included. It is therefore concluded that soil conductivity plays a significant role in underground magneto-inductive channels and must be modeled accurately.

On the Static Loop Modes in the Marching-on-in-Time Solution of the Time-Domain Electric Field Integral Equation

When marching-on-in-time (MOT) method is applied to solve the time-domain electric field integral equation, spurious internal resonant and static loop modes are always observed in the solution. The internal resonant modes have recently been studied by the authors; this letter investigates the static loop modes. Like internal resonant modes, static loop modes, in theory, should not be observed in the MOT solution since they do not satisfy the zero initial conditions; their appearance is attributed to numerical errors. It is discussed in this letter that the dependence of spurious static loop modes on numerical errors is substantially different from that of spurious internal resonant modes. More specifically, when Rao-Wilton-Glisson functions and Lagrange interpolation functions are used as spatial and temporal basis functions, respectively, errors due to space-time discretization have no discernible impact on spurious static loop modes. Numerical experiments indeed support this discussion and demonstrate that the numerical errors due to the approximate solution of the MOT matrix system have dominant impact on spurious static loop modes in the MOT solution.

Wireless charging to multiple electronic devices simultaneously in enclosed box

This paper investigates the feasibility of wireless charging to multiple electronic devices in enclosed box. Specifically in our scheme, multiple receivers deployed within a box with conducting walls receive wireless power simultaneously. Simulations and measurements are conducted in a fully-enclosed cubic box with dimensions 1 meter by 1 meter by 1 meter for three scenarios: “1 transmitter and 1 receiver,” “1 transmitter and 2 receivers,” and “1 transmitter and 3 receivers.” In all the three scenarios, high power transmission efficiencies are demonstrated. Particularly in the third scenario, each of the three receivers receives more than 25% of the power from the transmitter around 410 MHz.

A reconfigurable scheme of wireless power transmission in fully-enclosed box

This paper proposes a reconfigurable scheme of wireless power transmission in fully-enclosed box. The proposed scheme includes one transmitting element and multiple parasitic elements. Field distribution inside the box is reconfigured by switching the parasitic elements termination between open and short, with the aim of delivering wireless power efficiently to a receiver whose location is not fixed. Experimental results demonstrate that, via tuning nine parasitic elements at frequency 425 MHz it is possible to achieve power transmission efficiency greater than 70% when a wireless power receiver changes its location within a 0.4 m by 0.6 m region inside a 1-cubic-meter cubic box.

Retro-directive beamforming versus retro-reflective beamforming for wireless power transmission

This paper studies the difference between retro-directive beamforming technique and retro-reflective beamforming technique in the context of wireless power transmission applications. In all of our studies, a wireless power receiver broadcasts continuous-wave pilot signal, the wireless power transmitter receives and analyzes the pilot signal, and finally the wireless power transmitter transmits continuous-wave power with phase profile conjugate to that of the received pilot signal. Our study demonstrates that, a linear equi-spaced array configuration employed by the wireless power transmitter behaves as a retro-directive beamformer when the wireless power receiver resides in the far-zone of the wireless power transmitter, whereas it behaves as a retro-reflective beamformer when the wireless power receiver is not in the far-zone. Our studies further show that, far-zone gain could be reduced by adjusting the arrays geometrical configuration without affecting focusing in the near-zone. All the conclusions drawn in this paper are supported by numerical results as well as experimental results.

A Reconfigurable Scheme of Wireless Power Transmission in Fully Enclosed Environments

This letter proposes a reconfigurable scheme of wireless power transmission in fully enclosed environments. The proposed scheme includes one transmitting element and multiple parasitic elements. Field distribution in the fully enclosed environment is reconfigured by switching the parasitic elements’ termination between open and short, with the aim of delivering wireless power efficiently to a receiver whose location is not fixed or not known. Experimental results demonstrate that, via tuning nine parasitic elements at frequency 425 MHz, it is possible to achieve power transmission efficiency greater than 70% when a wireless power receiver changes its location within a 0.4 m by 0.6 m region inside a 1-cubic-meter cubic box.

Retro-Directive Beamforming Versus Retro-Reflective Beamforming with Applications in Wireless Power Transmission

This paper studies the difference between retro-directive beamforming technique and retroreflective beamforming technique in the context of wireless power transmission applications. In all of our studies, a wireless power receiver broadcasts continuous-wave pilot signal; the wireless power transmitter receives and analyzes the pilot signal; finally, the wireless power transmitter transmits continuous-wave power with phase profile conjugate to that of the received pilot signal. Our study demonstrates that a linear equi-spaced antenna array configuration employed by the wireless power transmitter behaves as a retro-directive beamformer when the wireless power receiver resides in the far-zone of the wireless power transmitter, whereas it behaves as a retro-reflective beamformer when the wireless power receiver is not in the far-zone. This paper further investigates two types of array configurations other than linear equi-spaced array when the wireless power transmitter behaves as a retro-reflective beamformer. One is a V-shaped array, which is obtained by deforming the linear equi-spaced array to a “V” shape. The other is termed “perturbed array:” on the basis of linear equi-spaced array, all the elements’ locations are perturbed randomly. It is particularly interesting to compare the equi-spaced array and perturbed array. When the wireless power receiver resides 5 or 6 wavelengths away, a 6-element equi-spaced array and a 6-element perturbed array produce the same power level at the near-zone focal point, but the maximum far-zone gain associated with the perturbed array is 1 dB lower than the equi-spaced array. All the conclusions drawn in this paper are supported by numerical results as well as experimental results.

A distributed retro-reflective beamforming scheme for wireless power transmission

An experimental verification of wireless power transmission based on distributed retro-reflective beamforming is presented. The wireless power transmitter includes eight antennas well separated in space; they receive pilot signals broadcasted from a mobile device and then jointly deliver radio frequency power to the mobile device. Our experimental results demonstrate that, the distributed retro-reflective beamformer is able to track the mobile devices location within a certain region and focus radio frequency power onto the devices location.