Abstract-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.
Wireless communication and/or wireless power transmission are highly desired in some of the practical environments fully enclosed by conducting walls. In this paper, a semi-analytical modal analysis is conducted for the purpose of characterizing wireless channels in a fully-enclosed space. The modal analysis is based upon an integral equation method. The cavity Green's function in the spectral domain (that is, expressed in term of cavity modes) is employed in the integral equation. The analysis results indicate that when a transmitter and a receiver are symmetric to each other with respect to a certain cavity mode, the load of the receiver could be coupled to the transmitter with little dispersion, leading to excellent wireless channels with the potential of accomplishing efficient wireless communication and/or wireless power transmission. A cubic cavity with a side length of 1 meter is analyzed as a specific example, and the modal analysis results are verified by experiments. Measurement data agree with the theoretical analysis very well. As predicted by the theoretical analysis, excellent wireless channels associated with the TM 220 mode (with a bandwidth of 40 MHz), TM 310 mode (with a bandwidth of 10 MHz), and TM 311 mode (with a bandwidth of 20 MHz) are demonstrated inside a cubic box with a side length of 1 meter.
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